Petrology of the moenkopi formation (Early Triassic), Uinta Mountain Area, Northeastern Utah

The Moenkopi Formation (Early Triassic) in northeastern Utah is a red bed sequence composed of red and drab-colored rocks. The abundance of carbonate rocks, representing about 65 percent of the formation, has been understated by previous workers and therefore their importance has been overlooked. Sandy and silty sparite, allochem-rich sparite and siltstone are the most abundant rock types present. Compositional gradations between carbonate and terrigenous rocks are common. Many thin beds of gypsum are present. Terrigenous rocks are well to poorly sorted, and typically lack maturity. The Moenkopi is composed of a wide range of carbonate and terrigenous constituents in both carbonate and terrigen­ ous rocks. Allochemical constituents include oolites and intraclasts, plus lesser amounts of pellets and almost no fossils. Orthochemical constituents present include sparry carbonate, microcrystalline carbonate and authigenic silica in order of abundance. In most cases, sparry carbon­ ate acts as a pore filler although some spar had a neomorphic origin. Intergranular forces generated from precipitation and crystallization of sparry carbonate caused many rocks to be cement-supported. Terrigenous constituents are dominated by the abundance of quartz and feldspar grains,and lesser amounts of chert, quartzite, granitic rock fragments, hematite, magnetite, ilmenite and other rock fragments. Quartz grains are generally subequant in shape, display nonundulatory extinction, and are subangular, angular and subrounded. Feldspar minerals include orthoclase, Na-plagioclase, microcline and perthite in order of abundance. Mica and clay minerals present are muscovite, biotite, sericite, illite , chlorite, mixed-layered illite-montmorillon ite and kaolinite. Diagenesis has caused substantial effects in carbonate and terrigenous rocks of the Moenkopi Formation. Recrys­ tallization of micrite to microspar to spar has occurred in some rocks. Dolomitization of sparry carbonate and allochems along with associated dissolution, has increased porosities in many rocks. Post-depositional alteration of iron-rich minerals to form hematite stain in the cement and matrix was responsible for red pigmentation in many rocks. Dissolution in red beds of iron-rich materials by carbonated solutions was probably responsible for drab colors within red beds (Picard, 1965, p. 478). The Moenkopi was deposited in shallow marine shelf depositional environments. Paleocurrent data indicate that a general northeast-southwest trending shoreline existed during deposition in northeastern Utah. Textural and petrographic observations suggest that the major source area was the Ancestral Rockies, which furnished detritus from acid igneous, metamorphic and older sedimentary terrains It is believed that Moenkopi sediment was deposited in or very close to a carbonate environment because of the abun­ dance of alloehems in both carbonate and terrigenous rocks. Tectonic episodes in the source area provided floods of detritus at various times, and was probably responsible for formation of terrigenous rocks in or near a carbonate environment. Tectonism in the depositional environment did not play a significant role during deposition. Climate in the source area and depositional environment was semi-arid to arid.

PETROLOGY OF THE MOENKOPI FORMATION (EARLY T R I A S S I C ) , UINTA MOUNTAIN AREA, NORTHEASTERN UTAH by B r u c e R o b e r t D e v e n t er t h e s i s s u b m i t t e d t o t h e f a c u l t y t U n i v e r s i t y U t a h i n p a r t i a l f u l f i l l m e n t h e r e q u i r e m e n ts f o r t h e d e g r e e of M a s t e r S c i e n ce i n G e o l o g y D e p a r t m e n t G e o l o g y G e o p h y s i cs U n i v e r s i t y Utah u n e 1 9 7^ PETROLOGY OF THE MOENKOPI FORMATION (EARLY TRIASSIC). UINTA MOUNTAIN AREA . NORTHEASTERN UTAH by Bruce Robert Van Deventer A thesis submitted to the faculty of t he Un i versity of Utah in partial fulfillment of t he r equirements for the degree of Master of Science in Geology Department of Geology and Geophysics Un i versity of Utah J une 1974 UNIVERSITY OF UTAH GRADUATE SCHOOL S U P E R V I S O R Y C O M M I T T E E A P P R O V AL of a thesis submitted by B r u c e R o b e r t B e v e n t er I have read this thesis and have found it to be of satisfactory quality for a master's degree. Date M, P i c a rd Chairman, Supervisory Committee I have read this thesis and have found it to be of satisfactory quality for a master's degree. ^, y^P ^ Date Wm, Lee S t o l i es Member, Supervisory Committee I have read this thesis and have found it to be of satisfactory quality for a master's deggrreeee.. / Raymond C, W i l s on Member, Supervisor)' Committee UNWERSfiY Or b i w LiBRAIWS* SUPERVISORY COMMITTEE APPROVAL Bruce Robert Van Deventer 1::.-/7 /71 D~te ~_~_.~_ _( _ C-(_ J__ ' M. Dane Picard Chainnan, ~~~ Wm. l,ee Stokes ~Z7 1; I ! ZL/ Date 2. degree. 1M ~7--:j=J-ru Date R "4 h 1,1l<q elL . Wi ~ (l~ Rayrn~d C. Wilson Member, Supervisory Committee UNIVERSITY OF UTAH GRADUATE SCHOOL F I N A L R E A D I N G A P P R O V AL To the Graduate Council of the University of Utah: I have read the thesis of R o b e r t D e v e n t e r in i t s final form and have found that (1) its format, citations, and bibliographic style are consistent and acceptable; (2) its illustrative materials including figures, tables, and charts are in place; and (3) the final manuscript is satisfactory to the Supervisory Committee and is ready for submission to the Graduate School. D a t e P i c a rd Member, Supervisory Committee Approved^for the Major Department I t a n l e y H. Ward Chairman/Dean Approved for the Graduate Council S t e r l i n g M. M c M u r r in Death of the Graduate School FINAL READING APPROVAL I have read the thesis of __ Bruce Robert Van Deven~~ __ in its eonsistent tahles, charts are in place; and (3) the final manuscript is satisfactory to the Supervisory Committee and is ready for submission to the Graduate School. __ 1?1 . J ~ __ /!U J- Date M. Dane Picard Member, Supervisory Committee Chairman/Dean AL~&'L1JL)jf.,_~ Ster~M. McMurrin DeVn of the Graduate School ACKNOWLEDGEMENTS The a u t h o r e x p r e s s e s h i s s i n c e r e a p p r e c i a t i o n t o Dr. P i c a r d f o r s u g g e s t i n g t h e p r o b l e m a n d f o r h is g u i d a n c e a n d a s s i s t a n c e d u r i n g a l l p h a s e s o f t h e s t u d y. T h a n k s a r e a l s o e x p r e s s e d t o D r . W i l l i a m S t o k e s a n d Dr. W i l s o n c r i t i c a l l y r e v i e w e d t h e m a n u s c r i pt a n d o f f e r e d i m p r o v e m e n t s t o i t s f o r m a n d c o n t e n t . Dr. J o n a t h o n G o o d w i n , a s w e l l a s g r a d u a t e s t u d e n t s, s u g g e s t i o n s w h i c h a i d e d i n i n t e r p r e t a t i o n p e t r o ­g r a p h y , d i a g e n e s i s , X - r a y a n a l y s i s s a m p l e s . Dr. W i l l i a m P . a s s i s t e d i n t a k i n g p h o t o m i c r o g r a p h s. F i g u r e s w e r e d r a f t e d O l s o n. P a r t i a l f i n a n c i a l s u p p o r t f o r t h i s s t u d y was p r o v i d ed b y t h e S p e c i a l C o m m i t t e e of G e o l o g i c F u n d s , D e p a r t m e n t of G e o l o g i c a l G e o p h y s i c a l S c i e n c e s , U n i v e r s i t y U t a h. ACKNOWLEDGEMENTS The author expresses his sincere appreciation to Dr. M. Dane Picard for suggesting the problem and for his guidance and assistance during all phases of the study. Thanks are also expressed to Dr. William L. Stokes and Dr. Raymond C. Wilson who critically reviewed the manuscript and offered improvements to its form and content. Dr. Jonathon H. Goodwin, as well as many graduate students, made suggestions which aided in interpretation of petro­graphy, diagenesis, and X-ray analysis of samples. Dr. William P. Nash assisted in taking photomicrographs. Figures were drafted by D. L. Olson. Partial financial support for this study was provided by the Special Committee of Geologic Funds, Department of Geological and Geophysical Sciences, University of Utah. CONTENTS P a g e ILLUSTRATIONS vii ABSTRACT ix INTRODUCTION. . . . . 1 P u r p o s e of s t u d y . , 1 P r e v i o u s s t u d i e s , 2 M e t h o d s , . . . . . . . . . . 3 GENERAL STRATIGRAPHY. , « 7 SEDIMENTARY STRUCTURES. . . . . R i p p l e m a r k s . . . . . . . 12 L i n e a r a s y m m e t r i c a l a n d s y m m e t r i c al r i p p l e m a r k s 12 S i n u o u s a s y m m e t r i c a l r i p p l e m a r k s 14 C u s p a t e r i p p l e m a r k s 16 S e c o n d a r y i n t e r f e r e n c e r i p p l e m a r k s . . . . 16 O t h e r s e d i m e n t a r y s t r u c t u r e s 16 S t r a t i f i c a t i o n t y p e s 17 H o r i z o n t a l p a r a l l e l s t r a t i f i c a t i o n , . . . . 17 R i p p l e s t r a t i f i c a t i o n 17 C r o s s - s t r a t i f i c a t i o n 18 C o n v o l u t e s t r a t i f i c a t i o n 13 B i o t u r b a t e d s t r a t i f i c a t i o n 20 TEXTURE OF TERRIGENOUS ROCKS 2.1 G r a i n s i z e 21 S o r t i n g 23 R o u n d i n g . 24 M a t u r i t y 28 ALLOCHEMICAL CONSTITUENTS 31 O o l i t e s . • 31 P e l l e t s . ...... 33 F o s s i l s . • • « . . . . . . • « • • • • • • • * * 33 A l g a e 3^ G a s t r o p o d s 3^ I n t r a c l a s t s . . . . . . . . • • 35 ORTHOCHEMICAL CONSTITUENTS * 37 M i c r o c r y s t a l l i n e c a r b o n a t e « 37 S p a r r y c a r b o n a t e . . . . . . . . . . 38 ILLUSTRATIONS . ABSTRACT. INTRODucrr ION. Purpo se of study . Previous studies . Me thod s . GENERAL STRATIGRAPHY. SEDIMENTARY STRUCTURES. CONTENTS Ripple marks • . .•• •• • • . • Linear a symmetrical and symmetrical ripple marks . Sinuous asymmetrical ripple Inarks . Cuspate ripple marks. Se condary interference ripple marks • Other sedimentary structures • Stratification types • Horizontal parallel stratification. Ripple stratification • Cross-stratification. • Convolute stratification. Bioturbated stratification. TEXTURE OF TERRIGENOUS ROCKS. Grain size • Sorting. Rounding • Maturity . ALLOCHEMICAL CONSTITUENTS . Oolites. Pellets. Fossils. Al gae . Gastro pods. Intraclasts. OR'j'HOCHEriIICAL CONSTITUENTS . • M' crocrystalline ca rbonate . Sparry carbonate . • Page vii ix 1 I 12 12 12 14 16 16 16 17 17 17 18 18 20 21 21 23 24 28 31 31 33 33 34 34 35 37 37 38 A u t h i g e n i c s i l i ca TERRIGENOUS CONSTITUENTS Q u a r t z F e l d s p a r R o c k f r a g m e n ts M i c a a n d c l a y m i n e r a ls PETROGRAPHY C l a s s i f i c a t i o n D e s c r i p t i o n of c a r b o n a t e r o c k s . . . . M i c r i t e and m i c r o s p a r i t e . . . . S p a r i t e . . D e s c r i p t i o n of t e r r i g e n o u s r o c k s . • . V e r y f i n e - g r a i n e d s a n d s t o n e and s i l t s t o n e S i m i l a r i t i e s i n v e r y f i n e - g r a i n ed s a n d s t o n e and s i l t s t o n e , , D i f f e r e n c e s i n v e r y f i n e - g r a i n ed s a n d s t o n e s i l t s t o n e . . GYPSUM DIAGENESIS S e q u e n c e of d i a g e n e t i c c h a n g e s . . . . O r i g i n of r e d p i g m e nt O r i g i n a l c o l o r of M o e n k o p i s e d i m e n t . PALEOCURRENT ANALYSIS M e t h o d s R e s u l t s I n t e r p r e t a t i o n PROVENANCE . . . . DEPOSITIONAL ENVIRONMENT SUMMARY REFERENCES CITED VITA v i Authigenic silica • TERRIGENOUS CONSTITUENTS . Quartz. Feldspar. Rock fragments ••••• Mica and clay minerals. PETROGRAPHY. Classification. Description of carbonate rocks. Micrite and microsparite Sparite. Description of terrigenous rocks. Very fine-grained sandstone and siltstone Similarities in very fine-grained sandstone and siltstone. Differences in very fine-grained sandstone and siltstone GYPSUM • DIAGENESIS • Sequence of diagenetic changes. Origin of red pigment • Original color of Moenkopi sediment • PALEOCURRENT ANALYSIS. Methods • Results. Interpretation. PROVENANCE • DEPOSITIONAL ENVIRONMENT • SUMMARY. REFERENCES CITED • VITA • vi 44 44 45 47 47 49 49 50 50 55 58 58 60 60 64 67 67 70 73 75 75 76 76 83 86 89 91 98 ILLUSTRATIONS F i g u r e Page 1 . L o c a t i o n a n d o u t c r o p map f o r M o e n k o p i F o r m a t i on i n n o r t h e a s t e r n U t a h 4 2 . C o m p o s i t e s t r a t i g r a p h i c s e q u e n c e i n n o r t h e a s t e rn U t a h 8 3 . Moenkopi F o r m a t i o n n e a r F l a m i n g G o r g e R e s e r v o i r 9 4 . R e l a t i v e a b u n d a n c e of s e d i m e n t a r y s t r u c t u r e s 13 5 . R e l a t i v e a b u n d a n c e of s t r a t i f i c a t i o n t y p e s 13 6 . L i n e a r a s y m m e t r i c a l r i p p l e m a r k s 15 7 . H o r i z o n t a l p a r a l l e l s t r a t i f i c a t i o n 19 8 . C o n v o l u t e s t r a t i f i c a t i o n i n c a l c a r e o u s s i l t s t o n e 19 9 . R e l a t i v e a b u n d a n c e of g r a i n s i z e s i n t e r r i g e n o us r o c k s 22 1 0 . R e l a t i v e a b u n d a n c e of t e r r i g e n o u s r o c k t y p e s 22 1 1 . Well s o r t e d s i l t s t o n e w i t h r e d p i g m e n t i n c e m e n t 25 1 2 . G r a i n - m a t r i x - c e m e n t d i a g r a m f o r t e r r i g e n o u s r o c k s 26 1 3 . R o u n d n e s s of g r a i n s i n s i l t s t o n e 27 1 4 . R o u n d n e s s of g r a i n s i n v e r y f i n e - g r a i n e d s a n d s t o n e 27 1 5 . S u p e r m a t u r e , t i g h t l y - p a c k e d v e r y f i n e - g r a i n ed s a n d s t o n e 3° 1 6 . S p a r r y o o l i t e s 32 1 7 . M i c r i t i c i n t r a c l a s t s 32 1 8 . Group of s p a r c r y s t a l s w i t h u n i f o r m o p t i c al o r i e n t a t i o n 39 1 9 . R e c r y s t a l l i z a t i o n of m i c r i t e t o m i c r o s p a r 39 ILLUSTRATIONS Figure Page 1. Location and outcrop map for Moenkopi Formation in northeastern Utah 4 2. Composite stratigraphic sequence in northeastern Utah 8 3. Moenkopi Formation near Flaming Gorge Reservoir 9 4. Relative abundance of sedimentary structures 13 5. Relative abundance of stratification types 13 6. Linear asymmetrical ripple marks 15 7. Horizontal parallel stratification 19 8. Convolute stratification in calcareous siltstone 19 9. Relative abundance of grain sizes in terrigenous rocks 22 10. Relative abundance of terrigenous rock types 22 11. Well sorted siltstone with red pigment in cement 25 12. Grain-matrix-cement diagram for terrigenous rocks 26 13. Roundness of grains in siltstone 27 14. Roundness of grains in very fine-grained sandstone 27 15. Supermature, tightly-pac~ed very fine-grained sandstone 30 16. Sparry oolites 32 17. Micritic intr,aclasts 32 18. Group of spar crystals with uniform optical orientation 39 19. Recrystallization of micrite to microspar 39 F i g u r e Page 2 0 . A b u n d a n c e of a u t h i g e n i c s i l i c a i n r o c k s 43 2 1 . S i l i c a i n f i l l i n g f r a c t u r e s a n d b u r r o w s 43 2 2 . R e l a t i v e a b u n d a n c e of f e l d s p a r t y p e s i n t e r r i g e n o us r o c k s 46 2 3 . T e r r i g e n o u s - a l l o c h e m i c a l - o r t h o c h e m i c a l c o n s t i t u e nt d i a g r a m f o r M o e n k o p i r o c k s 52 2 4 . M i c r i t e - a l l o c h e m - s p a r r y c a r b o n a t e d i a g r a m f or c a r b o n a t e r o c k s 53 2 5 . Modal a n a l y s e s of a l l o c h e m s i n c a r b o n a t e r o c k s 5^ 2 6 . S i l t y m i c r i t e 56 2 7 . G r a i n c o m p o s i t i o n f o r t e r r i g e n o u s r o c k s 6l 2 8 . C e m e n t - s u p p o r t e d c a l c a r e o u s s i l t s t o n e 62 2 9 . A b u n d a n c e of r e d p i g m e n t i n a n d i m m e d i a t e l y n e ar i r o n - r i c h g r a i n s 62 3 0 . Gypsum o c c u r r i n g a s v e r t i c a l c r o s s - c u t t i n g v e i n s 66 3 1 . P a l e o c u r r e n t p a t t e r n f o r e n t i r e M o e n k o p i F o r m a t i on i n n o r t h e a s t e r n U t a h 77 3 2 . P a l e o c u r r e n t p a t t e r n s f o r M o e n k o p i on n o r t h and s o u t h f l a n k s of t h e U i n t a M o u n t a i n s i n n o r t h e a s t e rn U t a h 78 3 3 . I n t e r p r e t e d s h o r e l i n e o r i e n t a t i o n f r o m n o r t h ­e a s t e r n U t a h t o w e s t - c e n t r a l Wyoming d u r i ng d e p o s i t i o n of M o e n k o p i a n d Red P e a k F o r m a t i o n 82 T a b l e Page 1 . F r e q u e n c y of o c c u r r e n c e of M o e n k o p i r o c k t y p e s 50 2 . C l a s s i f i c a t i o n of t e r r i g e n o u s r o c k s 59 3 . Moenkopi p a l e o c u r r e n t d a t a , U i n t a M o u n t a i n a r e a, n o r t h e a s t e r n U t a h 79 v i i i Figure Page 20. Abundance of authigenic silica in rocks 43 21. Silica infilling fractures and burrows 43 22. Relative abundance of feldspar types in terrigenous rocks 46 23. Terrigenous-allochemical-orthochemical constituent diagram for Moenkopi rocks 52 24. Micrite-allochem-sparry carbonate diagram for carbonate rocks 53 25. Modal analyses of allochems in carbonate rocks 54 26. Silty micrite 56 27. Grain composition for terrigenous rocks 61 28. Cement-supported calcareous siltstone 62 29. Abundance of red pigment in and immediately near iron-rich grains 62 30. Gypsum occurring as vertical cross-cutting veins 66 31. Paleocurrent pattern for entir(; Moenkopi Formation in northeastern Utah 77 32. Paleocurrent patterns for Moenlcopi on north and south flanks of the Uinta Mountains in northeastern Utah 78 33. Interpreted shoreline orientation from north­eastern Utah to west-central Wyoming during deposition of Moenkopi and Red Peak Formation 82 Table Page 1. Frequency of occurrence of Moenkopi rock types 50 2. Classification of terrigenous rocks 59 3. Moenkopi paleocurrent data, Uinta Mountain area, northeastern Utah 79 viii ABSTRACT M o e n k o p i F o r m a t i o n E a r l y T r i a s s i c ) i n n o r t h e a s t e rn U t a h i s a r e d b e d s e q u e n c e c o m p o s e d o f r e d a n d d r a b - c o l o r ed r o c k s . a b u n d a n c e c a r b o n a t e r o c k s , r e p r e s e n t i ng a b o u t 65 p e r c e n t t h e f o r m a t i o n , h a s b e e n u n d e r s t a t ed b y p r e v i o u s w o r k e r s t h e r e f o r e t h e i r i m p o r t a n c e h a s b e en o v e r l o o k e d . a n d s i l t y s p a r i t e , a l l o c h e m - r i c h s p a r i te a n d s i l t s t o n e a r e t h e m o s t a b u n d a n t r o c k t y p e s p r e s e n t. C o m p o s i t i o n a l g r a d a t i o n s b e t w e e n c a r b o n a t e t e r r i g e n o us r o c k s a r e t h i n b e d s o f g y p s um a r e p r e s e n t. T e r r i g e n o u s r o c k s a r e w e l l t o p o o r l y s o r t e d , t y p i c a l ly l a c k m a t u r i t y. T h e M o e n k o p i i s c o m p o s e d w i d e r a n g e o f c a r b o n a te a n d t e r r i g e n o u s c o n s t i t u e n t s i n b o t h c a r b o n a t e a n d t e r r i g e n ­o u s r o c k s . A l l o c h e m i c a l c o n s t i t u e n t s i n c l u d e o o l i t es a n d i n t r a c l a s t s , p l u s l e s s e r a m o u n t s p e l l e t s a n d a l m o st n o f o s s i l s . O r t h o c h e m i c a l c o n s t i t u e n t s p r e s e n t i n c l u de s p a r r y c a r b o n a t e , m i c r o c r y s t a l l i n e c a r b o n a t e a u t h i g e n ic s i l i c a i n o r d e r a b u n d a n c e . m o s t c a s e s , s p a r r y c a r b o n ­a t e a c t s a s p o r e f i l l e r a l t h o u g h s p a r h a d n e o m o r p h ic o r i g i n . I n t e r g r a n u l a r f o r c e s g e n e r a t e d f r om p r e c i p i t a t i on a n d c r y s t a l l i z a t i o n s p a r r y c a r b o n a t e c a u s e d r o c ks t o c e m e n t - s u p p o r t e d . T e r r i g e n o u s c o n s t i t u e n t s a re d o m i n a t e d t h e a b u n d a n c e q u a r t z a n d f e l d s p a r g r a i n s, ABSTRACT The Moenkopi Formation ( Early Triassic) in northeastern Utah is a red bed sequence composed of red and drab-colored rocks. The abundance of carbonate rocks, representing about 65 percent of the formation, has been understated by previous workers and therefore the~r importance has been overlooked. Sandy and silty sparite, allochem-rich sparite and siltstone are the most abundant rock types present. Compositional gradations between carbonate and terrigenous rocks are common. Many thin beds of gypsum are present. Terrigenous rocks are well to poorly sorted, and typically lack maturity. The Moenkopi is composed of a wide range of carbonate and terrigenous constituents in both carbonate and terrigen­ous rocks. Allochemical constituents include oolites arid intraclasts, plus lesser amounts of pellets and almost no fossils. Orthochemical constituents present include sparry carbonate, microcrystalline carbonate and authigenic silica in order of abundance. In most cases, sparry carbon­ate acts as a pore filler although some spar had a neomorphic origin. Intergranular forces generated from precipitation and crystallization of sparry carbonate caused many rocks to be cement-supported. Terrigenous constituents are dominated by the abundance of quartz and feldspar grains, a n d l e s s e r a m o u n t s of c h e r t , q u a r t z i t e , g r a n i t i c r o ck f r a g m e n t s , h e m a t i t e , m a g n e t i t e , i l m e n i t e and o t h e r r o ck f r a g m e n t s . Q u a r t z g r a i n s a r e g e n e r a l l y s u b e q u a n t i n s h a p e, d i s p l a y n o n u n d u l a t o r y e x t i n c t i o n , and a r e s u b a n g u l a r , a n g u l ar a n d s u b r o u n d e d . F e l d s p a r m i n e r a l s i n c l u d e o r t h o c l a s e , Na-p l a g i o c l a s e , m i c r o c l i n e and p e r t h i t e i n o r d e r of a b u n d a n c e. M i c a a n d c l a y m i n e r a l s p r e s e n t a r e m u s c o v i t e , b i o t i t e, s e r i c i t e , i l l i t e , c h l o r i t e , m i x e d - l a y e r e d i l l i t e - m o n t m o r i l l on i t e k a o l i n i t e. D i a g e n e s i s h a s c a u s e d s u b s t a n t i a l e f f e c t s i n c a r b o n a te a n d t e r r i g e n o u s r o c k s of t h e M o e n k o p i F o r m a t i o n . R e c r y s ­t a l l i z a t i o n of m i c r i t e t o m i c r o s p a r t o s p a r h a s o c c u r r e d in some r o c k s . D o l o m i t i z a t i o n of s p a r r y c a r b o n a t e a n d a l l o c h e ms a l o n g w i t h a s s o c i a t e d d i s s o l u t i o n , h a s i n c r e a s e d p o r o s i t i es i n many r o c k s . P o s t - d e p o s i t i o n a l a l t e r a t i o n of i r o n - r i ch m i n e r a l s t o f o rm h e m a t i t e s t a i n i n t h e c e m e n t a n d m a t r ix was r e s p o n s i b l e f o r r e d p i g m e n t a t i o n i n many r o c k s. D i s s o l u t i o n i n r e d b e d s o f i r o n - r i c h m a t e r i a l s c a r b o n a t ed s o l u t i o n s p r o b a b l y r e s p o n s i b l e f o r d r a b c o l o r s w i t h in r e d b e d s P i c a r d , 1 9 6 5 . p . ^ 7 8 ). M o e n k o p i d e p o s i t e d i n s h a l l o w m a r i n e s h e lf d e p o s i t i o n a l e n v i r o n m e n t s . P a l e o c u r r e n t d a t a i n d i c a t e t h at a g e n e r a l n o r t h e a s t - s o u t h w e s t t r e n d i n g s h o r e l i n e e x i s t ed d u r i n g d e p o s i t i o n i n n o r t h e a s t e r n U t a h . T e x t u r a l and p e t r o g r a p h i c o b s e r v a t i o n s s u g g e s t t h a t t h e m a j o r s o u r ce a r e a t h e A n c e s t r a l R o c k i e s , w h i c h f u r n i s h e d d e t r i t us f r om a c i d g n e o u s , m e t a m o r p h i c o l d e r s e d i m e n t a r y t e r r a i ns and lesser amounts of chert, quartzite, granitic rock fragments, hematite, magnetite, ilmenite and other rock fragments. Quartz grains are generally subequant in shape, display nonundulatory extinction, and are subangular, angular and subrounded. Feldspar minerals include orthoclase, Na­plagioclase, microcline and perthite in order of abundance. Mica and clay minerals present are muscovite, biotite, sericite, illite, chlorite, mixed-layered illite-montmorillon~ ite and kaolinite. Diagenesis has caused substantial effects in carbonate and terrigenous rocks of the Moenkopi Format ion. Recrys­tallization of micrite to microspar to spar has occurred in some rocks. Dolomitization of sparry carbonate and allochems, along with associated dissolution, has increased porosities in many rocks. Post-depositional alteration of iron-rich minerals to form hematite stain in the cement and matrix was responsible for red pigmentation in many rocks. Dissolution in red beds of iron-rich materials by carbonated solutions was probably responsible for drab colors within red beds ( Picard, 1965, p. 478). The Moenkopi was deposited in shallow marine shelf depositional environments. Paleocurrent data indicate that a general northeast-southwest trending shoreline existed during deposition in northeastern Utah. Textural and petrographic observations suggest that the major source area was the Ancestral Rockies, which furnished detritus from acid i gneous, metamorphic and older sedimentary terrains. x I t i s b e l i e v e d t h a t M o e n k o p i s e d i m e n t was d e p o s i t e d i n or v e r y c l o s e t o a c a r b o n a t e e n v i r o n m e n t b e c a u s e of t h e a b u n ­d a n c e of a l l o e h e m s i n b o t h c a r b o n a t e and t e r r i g e n o u s r o c k s. T e c t o n i c e p i s o d e s i n t h e s o u r c e a r e a p r o v i d e d f l o o d s of d e t r i t u s a t v a r i o u s t i m e s , a n d was p r o b a b l y r e s p o n s i b l e f or f o r m a t i o n of t e r r i g e n o u s r o c k s i n o r n e a r a c a r b o n a te e n v i r o n m e n t . T e c t o n i s m i n t h e d e p o s i t i o n a l e n v i r o n m e n t d id n o t p l a y a s i g n i f i c a n t r o l e d u r i n g d e p o s i t i o n . C l i m a t e in t h e s o u r c e a r e a d e p o s i t i o n a l e n v i r o n m e n t s e m i - a r id t o a r i d. x i It is believed that Moenkopi sediment was deposited in or -very close to a carbonate environment because of the abun­dance of allochems in both carbonate and terrigenous rocks. Tectonic episodes in the source area provided floods of detritus at various times, and was probably responsible for formation of terrigenous rocks in or near a carbonate environment. Tectonism in the depositional environment did not playa significant role during deposition. Climate in the source area and depositional environment was semi-arid to arid. xi INTRODUCTION The M o e n k o p i F o r m a t i o n o f E a r l y T r i a s s i c a g e i s a r e d b e d s e q u e n c e d e p o s i t e d o v e r w i d e s p r e a d a r e a s of t he R o c k y M o u n t a i n r e g i o n . Red b e d s a n d t h e i r o r i g i n h a ve a r o u s e d t h e i n t e r e s t g e o l o g i s t s f o r y e a r s ( K e l l e r, 1 9 2 9 ; B a r t r a m , 1 9 3 0 ; H e a t o n , 1933? W i l l i a m s , 1 9 ^ 5 ; Thomas a n d K r u e g e r , 1 9 4 6 ; 1 9 5 ^ • 1 9 6 4 ; S h o e m a k e r a n d 1 9 5 9 ; C l a r k , 1 9 6 2 ; H o u t e n , 1 9 6 4 , 1 9 6 8 ; P i c a r d , I 9 6 5. 1 9 6 7 ) . S e v e r a l i d e a s h a v e b e e n a d v a n c e d c o n c e r n i n g o r i g in o f r e d b e d s r a n g i n g f r om c h e m i c a l p r e c i p i t a t i o n i r o n, e r o s i o n a n d r e d e p o s i t i o n o l d e r r e d m a t e r i a l , p o s t - d e p o s i t i o n a l c h a n g e s i n o r i g i n a l n o n - r e d s e d i m e n t . The i n t e n s e i n t e r e s t i n r e d b e d s h a s p r o b a b l y t h e m of t h e m o s t s t u d i e d r o c k t y p e s i n t h e w o r l d. P u r p o s e of S t u dy The p u r p o s e of t h i s s t u d y i s t o d e t e r m i n e a s much i n f o r m a t i o n a s p o s s i b l e a b o u t t h e d e p o s i t i o n a l e n v i r o n m e nt o f t h e M o e n k o p i F o r m a t i o n i n n o r t h e a s t e r n U t a h . S e v e r al s p e c i f i c t o p i c s a r e s t u d i e d i n o r d e r t o a c c o m p l i s h t h i s . P e t r o g r a p h i c a n d t e x t u r a l a n a l y s e s of t h i n s e c t i o n s p r o v i de t h e b a s i c d a t a f o r r o c k c o m p o s i t i o n , g r a i n s i z e g r a in r e l a t i o n s h i p s w h i c h s h o u l d be a d i r e c t r e s u l t of t h e d e p o ­s i t i o n a l e n v i r o n m e n t . S t u d i e s of s e d i m e n t a r y s t r u c t u r es INTRODUCTION The Moenkopi Formation of Early Triassic age is a red bed sequence deposited over widespread areas of the Rocky Mountain region. Red beds and their origin have aroused the interest of geologists for many years (Keller, 1929; Bartram, 1930; Heaton, 1933; Williams, 1945; Thomas and Krueger, 1946; McKee, 1954, 1964; Shoemaker and Newman, 1959; Clark, 1962; Van Houten, 1964, 1968; Picard, 1965, 1967). Several ideas have been advanced concerning origin of red beds ranging from chemical precipitation of iron, erosion and redeposition of older red material, and post­depositional changes in original non-red sediment. The intense interest in red beds has probably made them one of the most studied rock types in the world. Purpose of Study The purpose of this study is to determine as much information as possible about the depositional environment of the Moenkopi Formation in northeastern Utah. Several specific topics are studied in order to accomplish this. Petrographic and textural analyses of thin sections provide the basic data for rock composition, grain size and grain relationships which should be a direct result of the depo­sitional environment. Studies of sedimentary structures a n d p a l e o c u r r e n t d i s t r i b u t i o n y i e l d i n f o r m a t i o n a b o u t t he e n v i r o n m e n t of f o r m a t i o n and p o s s i b l e c u r r e n t p a t t e r ns w i t h i n t h e d e p o s i t i o n a l e n v i r o n m e n t . Study of d i a g e n e s is r e v e a l s p o s t - d e p o s i t i o n a l c h a n g e s i n M o e n k o p i s e d i m e nt w h i c h w e r e d i r e c t l y i n f l u e n c e d by t h e d e p o s i t i o n a l e n v i r o n ­m e n t , c l i m a t e and c h e m i c a l c o n d i t i o n s. P r e v i o u s S t u d i es S i m i l a r t y p e s of s t u d i e s of t h e M o e n k o p i o r i ts e q u i v a l e n t s h a v e b e e n p e r f o r m e d i n c e n t r a l and s o u t h e rn U t a h a n d A r i z o n a ( B a l d w i n , 1 9 7 3 ; S t e w a r t , P o o l e , and W i l s o n, 1 9 7 2 ; S t e w a r t a n d o t h e r s , 1 9 5 9 ; S c h u l t z , 1 9 6 3 ; C a d i g a n, 1 9 7 1 ) . a n d Wyoming ( P i c a r d , 1 9 6 6 ; P i c a r d a n d H i g h , 1 9 6 8 ). P o r t i o n s t h e s t u d i e s S t e w a r t , P o o l e , a n d W i l s o n ( 1 9 7 2) a n d C a d i g a n ( 1 9 7 1 ) t o u c h b r i e f l y on t h e M o e n k o p i F o r m a t i on i n n o r t h e a s t e r n U t a h , b u t n o t i n g r e a t d e t a i l. P r e v i o u s w o r k e r s ( U n t e r m a n n a n d U n t e r m a n n , 19^9» 195^» 1 9 5 5 . 1 9 6 4 , 1 9 6 9 a , 1 9 6 9 b ; H a n s e n , 1 9 5 5 . 1 9 5 6 , 1 9 6 5 ; McKee, 1 9 5 ^ ; K i n n e y , 1 9 5 5 . W i l l i a m s , 1 9 ^ 5 ; Thomas a n d K r u e g e r, 1 9 4 6 ; M a c L a c h l a n , - 1 9 5 7 ; S c o t t , 1 9 5 9 ; R i t z m a , 1959? S c h e l l, 1 9 6 9 ; S t e w a r t , P o o l e , a n d W i l s o n , 1 9 7 2 ) h a v e s t u d i e d t he M o e n k o p i s t r a t i g r a p h y a n d h a v e s t r e s s e d t h e a b u n d a n c e of t e r r i g e n o u s r o c k s . However, i n t h i s s t u d y a s i g n i f i c a nt p r o b l e m a r o s e when p e t r o g r a p h i c a n a l y s e s r e v e a l e d t h a t most o f t h e r o c k s a r e a c t u a l l y c a r b o n a t e s . B e c a u s e of t h i s , l i t t l e h e l p c o u l d o b t a i n e d f r om t h e s e s t u d i e s c o n c e r n i ng c o n d i t i o n s i n n o r t h e a s t e r n U t a h . However, p e t r o g r a p h ic a n d d i a g e n e t i c s t u d i e s i n n e a r b y a r e a s ( P i c a r d, 2 ·and paleocurrent distribution yield information about the environment of formation and possible current patterns within the depositional environment. Study of diagenesis reveals post-depositional changes in Moenkopi sediment which were directly influenced by the depositional environ­ment, climate and chemical conditions. Previous Studies Similar types of studies of the Moenkopi or its equivalents have been performed in central and southern Utah and Arizona (Baldwin, 197), Stewart, Poole, and Wilson, 1972; Stewart and others, 1959; Schultz, 196); Cadigan, 1971), and Wyoming (Picard, 1966; Picard and High, 1968). Portions of the studies of Stewart, Poole. and Wilson (1972) and Cadigan (1971) touch briefly on the Moenkopi Formation in northeastern Utah, but not in great detail. Previous workers (Untermann and Untermann. 1949, 1954, 1955. 1964, 1969a, 1969b; Hansen, 1955, 1956, 1965; McKee, 1954: Kinney, 1955; Williams, 1945, Thomas and Krueger. 1946, MacLachlan; 1957; Scott, 1959; Ritzma, 1959, Schell, 1969, Stewart, Poole, and Wilson, 1972) have studied the Moenkopi stratigraphy and have stressed the abundance of terrigenous rocks. However, in this study a significant problem arose when petrographic analyses revealed that most of the rocks are actually carbonates. Because of this, little help could be obtained from these studies concerning conditions in northeastern Utah. However, petrographic and diagenetic studies in nearby areas of Wyoming (Picard. 3 1 9 ^ 5 * 1 9 6 6 ) on a M o e n k o p i e q u i v a l e n t t h e r e , a s w e l l a s o t h er s t u d i e s (Van H o u t e n , 1 9 6 1 , 1 9 6 4 , I 9 6 8 ) , h a v e b e e n q u i te u s e f u l f o r i n t e r p r e t a t i o n of c o n d i t i o n s i n n o r t h e a s t e rn U t a h . I t i s n o t t h e i n t e n t of t h i s s t u d y t o d e t e r m i n e t he M o e n k o p i s t r a t i g r a p h y i n g r e a t d e t a i l a l t h o u g h p r e v i o us s t u d i e s i n n o r t h e a s t e r n U t a h a r e p o s s i b l y i n a c c u r a t e. T h i s w o u l d h a v e b e e n a t t e m p t e d h a d t h e u n s u s p e c t e d a b u n d a n ce o f c a r b o n a t e r o c k s b e e n d i s c o v e r e d by u s e of t h i n s e c t i o ns much e a r l i e r i n t h e f i e l d w o r k . The s t r a t i g r a p h y was s t u d i e d o n l y e n o u g h t o become f a m i l i a r w i t h t h e s t r a t i g r a p h ic s e q u e n c e f r om o u t c r o p t o o u t c r o p . No m e m b e r s of t h e Moenkopi F o r m a t i o n h a v e y e t b e e n r e c o g n i z e d i n n o r t h e a s t e r n U t a h, a n d no a t t e m p t was made t o do so i n t h i s s t u d y. M e t h o d s A l l f i e l d work i n v o l v e d i n t h i s s t u d y was a c c o m p l i s h ed d u r i n g t h e m o n t h s o f S e p t e m b e r t h r o u g h N o v e m b e r , 1 9 7 3. s e c t i o n t h e M o e n k o p i F o r m a t i o n a t B r u s h C r e e k was m e a s u r e d t o d e t e r m i n e t h e g e n e r a l s t r a t i g r a p h i c s e q u e n c e. F i e l d d e s c r i p t i o n s l i t h o l o g i e s , s e d i m e n t a r y s t r u c t u r es a n d s t r a t i f i c a t i o n t y p e s w e r e r e c o r d e d a t 7 s e c t i o n s ; on t h e n o r t h f l a n k of t h e U i n t a M o u n t a i n s , a n d 5 o n t h e s o u th f l a n k of t h e U i n t a M o u n t a i n s ( f i g u r e 1 ) . O n e - h u n d r ed f i f t e e n r o c k s a m p l e s w e r e c o l l e c t e d f o r b i n o c u l a r m i c r o s c o p ic e x a m i n a t i o n , t h e p u r p o s e b e i n g t o d e t e r m i n e c o l o r , modal g r a i n s i z e , g r o s s m i n e r a l o g y , s o r t i n g a n d r o u n d i n g g r a i ns a n d g e n e r a l g r a i n r e l a t i o n s h i p s . c o l o r d e s c r i p t i o ns 3 1965, 1966) on a Moenkopi equivalent there, as well as other studies (Van Houten, 1961, 1964, 1968), have been quite useful for interpretation of conditions in northeastern utah. It is not the intent of this study to determine the Moenkopi stratigraphy in great detail although previous studies in northeastern Utah are possibly inaccurate. This would have been attempted had the unsuspected abundance of carbonate rocks been discovered by use of thin sections much earlier in the field work. The stratigraphy was studied only enough to become familiar with the stratigraphic sequence from outcrop to outcrop. No members of the Moenkopi Formation have yet been recognized in northeastern Utah, and no attempt was made to do so in this study. Methods All field work involved in this study was accomplished during the months of September through November, 1973. One section of the Moenkopi Formation at Brush Creek was measured to determine the general stratigraphic sequence. Field descriptions of lithologies, sedimentary structures and stratification types were recorded at 7 sections; 2 on the north flank of the Uinta Mountains, and 5 on the south flank of the Uinta Mountains (figure 1). One-hundred fifteen rock samples were collected for binocular microscopic examination, the purpose being to dete~mine color, modal grain size, gross mineralogy, sorting and rounding of grains and general grain relationships. All color descriptions STRATIGRAPHIC SECTIONS L Sheep Creek Canyon 2. West Flaming Gorge 3. Red Mountain I 4. Red Mountain 2 5. Brush Creek 6. Dinosaur Quarry I 7. Dinosaur Quarry 2 LOCATION AND OUTCROP MAP OF THE MOENKOPI FORMATION NORTHEASTERN UTAH Seal* in Milit Figure I • ~~ MANILA/) ~~ ~ 0 U N T A INS I .'0111111' r--J STUDY AREA I Li-· I UT4. ! .OL""400 I I L.. _ __ l __ ~ \ U \ • VERNAL IN o I Figure 10 I o o SECnONS l Sheep Creek Canyon 2. Wesl Flaming Gorge 3. Red Mounlain I 4. Red Mounlain 2 5. Brush Creek 6. Dinosaur Ouarry I 7. Dinosaur Ouarry 2 4 5 w e r e made u s i n g t h e t e r m i n o l o g y of G o d d a r d a n d o t h e r s ( 1 9 4 8 ). One h u n d r e d s a m p l e s w e r e t a k e n f r om r e p r e s e n t a t i ve l i t h o l o g i e s f o r t h i n s e c t i o n a n a l y s i s . N i n e t y - s e v e n t h in s e c t i o n s w e r e s t u d i e d by m o d a l a n a l y s i s w i t h 200 p o i nt c o m p o s i t i o n s c o u n t e d i n e a c h . Each m o d a l a n a l y s i s was p e r f o r m e d u n d e r h i g h m a g n i f i c a t i o n (X 3 2 0 ) w i t h a m a n u a l l y - o p e r a t e d m e c h a n i c a l s t a g e . D e r i v e d d a t a w e r e u s e d in c l a s s i f i c a t i o n , t e x t u r a l a n d m i n e r a l o g i c a l s t u d i e s. T w e n t y - t h r e e s a m p l e s u s e d i n m a k i n g t h i n s e c t i o n s were a l s o s t u d i e d by X - r a y a n a l y s i s . T e r r i g e n o u s s a m p l e s w e re a n a l y z e d t o d e t e r m i n e c l a y m i n e r a l o g y a n d o t h e r m i n e r a ls n o t n o t e d by m o d a l a n a l y s i s . C a r b o n a t e s a m p l e s w e r e s t u d i ed t o d e t e r m i n e t h e p r e s e n c e and r e l a t i v e a b u n d a n c e of d o l o m i t e. A l l s a m p l e s w e r e r u n a t 40 KV a n d 20 MA w i t h Cu K - a l p ha X - r a d i a t i o n . T h i r t y - t w o p o l i s h e d s l a b s w e r e s t a i n e d t o d e t e r m i ne t h e a b u n d a n c e a n d r e l a t i o n s h i p s of c a l c i t e a n d d o l o m i t e. I n some c a s e s s t a i n i n g h e l p e d t o b e t t e r d e l i n e a t e c a r b o n a te a l l o c h e m i c a l s t r u c t u r e s. A s p e c i a l e f f o r t was made i n t h e f i e l d t o f i n d f o s s i ls b e c a u s e v e r y few p r e v i o u s l y h a d v e e n f o u n d . E x a m i n a t i o n of f i e l d o u t c r o p s , b r e a k i n g o p e n o f many r o c k s a n d d i s a g g r e g a t i on o f 4 s a m p l e s i n t h e l a b o r a t o r y a n d t h e i r s t u d y u n d e r a b i n o c u l a r m i c r o s c o p e y i e l d e d no f o s s i l s s e e n . Only o n e t h in s e c t i o n o u t of 100 was f o u n d t o c o n t a i n a n y f o s s i l m a t e r i a l. S e d i m e n t a r y s t r u c t u r e s w e r e s t u d i e d i n t e r m s o f t h e ir t y p e , a b u n d a n c e and s t r a t i g r a p h i c l o c a t i o n ; and t h e i r mode o f f o r m a t i o n was i n t e r p r e t e d . M e a s u r e m e n t s o f some 5 were made using the terminology of Goddard and others (1948) . One hundred samples were taken from representative lithologies for thin section analysis. Ninety-seven thin sections were studied by modal analysis with 200 point compositions counted in each. Each modal analysis was performed under high magnification (X 320) with a manually­operated mechanical stage. Derived data were used in classification. textural and mineralogical studies. Twenty-three samples used in making thin sections were also studied by X-ray analysis. Terrigenous samples were analyzed to determine clay mineralogy and other minerals not noted by modal analysis. Carbonate samples were studied to determine the presence and relative abundance of dolomite. All samples were run at 40 KV and 20 MA with Cu K-alpha X-radiation. Thirty-two polished slabs were stained to determine the abundance and relationships of calcite and dolomite. In some cases staining helped to better delineate carbonate allochemical structures. A special effort was made in the field to find fossils because very few previously had veen found. Examination of field outcrops. breaking open of many rocks and disaggregation of 4 samples in the laboratory and their study under a binocular microscope yielded no fossils seen. Only one thin section out of 100 was found to contain any fossil material. Sedimentary structures were studied in terms of their type. abundance and stratigraphic location; and their mode of formation was interpreted. Measurements of some 6 s t r u c t u r e s w e r e made t o d e t e r m i n e t h e p r o p e r t e r m i n o l o gy i n d e s c r i b i n g t h e m . C l a s s i f i c a t i o n s y s t e m s of McKee and W e i r ( 1 9 5 3 ) » P o t t e r a n d P e t t i j o h n ( I 9 6 3 ) a n d B l a t t , M i d d l e t on a n d M u r r a y ( 1 9 7 2 ) w e r e u s e d. A t o t a l of 175 p a l e o c u r r e n t m e a s u r e m e n t s w e r e t a k en t o d e t e r m i n e c u r r e n t d i r e c t i o n s d u r i n g d e p o s i t i o n of t he f o r m a t i o n . F i f t y - t w o m e a s u r e m e n t s w e r e made a t 2 s e c t i o ns o n t h e n o r t h f l a n k of t h e U i n t a M o u n t a i n s , a n d 1 2 3 w e re made a t 5 s e c t i o n s on t h e s o u t h f l a n k . A d d i t i o n a l p r o c e d u r es u s e d i n p a l e o c u r r e n t a n a l y s i s a r e p r e s e n t e d i n a l a t e r s e c t i o n of t h i s t h e s i s. structures were made to determine the proper terminology in describing them. Classification systems of McKee and 6 Weir (1953), Potter and Pettijohn (1963) and Blatt, Middleton and Murray (1972) were used. A total of 175 paleocurrent measurements were taken to determine current directions during deposition of the formation. Fifty-two measurements were made at 2 sections on the north flank of the Uinta Mountains, and 123 were made at 5 sections on the south flank. Additional procedures used in paleocurrent analysis are presented in a later section of this thesis. GENERAL STRATIGRAPHY A g e n e r a l d e s c r i p t i o n of M o e n k o p i F o r m a t i o n r o c k u n i ts i n n o r t h e a s t e r n U t a h i s p r e s e n t e d h e r e . All i n f o r m a t i on i s b a s e d f i e l d , b i n o c u l a r a n d p e t r o g r a p h i c e x a m i n a t i on o f t h e r o c k s. The M o e n k o p i i s c o m p o s e d of r e d , r e d d i s h - b r o w n and l i g h t g r e e n t o g r e e n i s h - g r a y s a n d y a n d s i l t y c a r b o n a t e, c a l c a r e o u s v e r y f i n e - g r a i n e d s a n d s t o n e a n d s i l t s t o n e , and g y p s u m . Green a n d g r a y r o c k s a r e r e f e r r e d t o a s d r a b. C a l c a r e o u s s i l t s t o n e , v e r y f i n e - g r a i n e d s a n d s t o n e and s i l t y c a r b o n a t e b e d s a r e g r a d a t i o n a l a n d a r e d i f f i c u l t to d i f f e r e n t i a t e i n t h e f i e l d . At B r u s h C r e e k , t h e M o e n k o pi F o r m a t i o n i s 9^1 f e e t t h i c k w i t h m o s t b e d s m a i n t a i n i ng u n i f o r m t h i c k n e s s o r t h i n n i n g g r a d u a l l y t o t h e e a s t i n t he s t u d y a r e a . l o w e r p a r t t h e f o r m a t i o n i s u s u a l ly c o v e r e d w i t h a l l u v i u m b e c a u s e n o n r e s i s t a n t l i t h o l o g i es t h e r e , g e n e r a l d e s c r i p t i o n o f t h e s t r a t i g r a p h i c s e q u e n ce i s p r e s e n t e d b e l o w a n d i n f i g u r e 2, F i g u r e 3 shows t h e M o e n k o p i F o r m a t i o n n e a r F l a m i n g G o r g e R e s e r v o i r. The l o w e r m o s t 167 f e e t of t h e M o e n k o p i F o r m a t i o n is c o m p o s e d m o s t l y of d r a b n o n r e s i s t a n t m i c r i t e , s i l t y s p a r i te a n d c a l c a r e o u s s i l t s t o n e . R e d d i s h - b r o w n b e d s a r e a b s e nt a l t h o u g h s m a l l p a t c h e s of r e d a r e l o c a l l y p r e s e n t on b e d d i ng s u r f a c e s , a r e l e s s t h a n 3 f e e t t h i c k a n d a r e p o o r ly GENERAL STRATIGRAPHY A general description of Moenkopi Formation rock units in northeastern Utah is presented here. All information is based on field, binocular and petrographic examination of the rocks. The Moenkopi is composed of red. reddish-brown and light green to greenish-gray sandy and silty carbonate. calcareous very fine-grained sandstone and siltstone, and gypsum. Green and gray rocks are referred to as drab. Calcareous siltstone. very fine-grained sandstone and silty carbonate beds are gradational and are difficult to differentiate in the field. At Brush Creek, the Moenkopi Formation is ,941 feet thick with most beds maintaining uniform thickness or thinning gradually to the east in the study area. The lower part of the formation is usually covered with alluvium because of nonresistant lithologies there. A general description of the stratigraphic sequence is presented below and shown in figure 2. Figure) shows the Moenkopi Formation near Flaming Gorge Reservoir. The lowermost 167 feet of the Moenkopi Formation is composed mostly of drab nonresistant micrite, silty sparite and calcareous siltstone. Reddish-brown beds are absent although small patches of red are locally present on bedding surfaces. Beds are less than) feet thick and are poorly THICKNESS UTHOLOGY GENERAL LITHOLOGIC DESCRIPTION 900' 800' 700' 600' 500' 400' 300' 200' 100' 1111S 'i' i 'i 'i 'i 'i 1/ axs^a m w m . 11111111 Massive beds of resistant calcareous siltstone,very fine-groined sandstone 8 silty sparite. Sedimentary structures abundant. Thin, resistant and nonresistant beds of sparite, micrite, gypsum, colcareous siltstone, and very fine-grained sandstone. Less gypsum present in this interval Thin, mostly nonresistant beds of silty or sandy sparite, calcareous siltstone and very fine-grained sandstone, silty or sandy micrite, and gypsum. wsTilhitthsict okthn-ibene dbadnedaeds v oerfre ysg fiysiptnasuenitn g ,rmcaoinilcc rsaitaeren o&duss tso pnaer ite. Thin, resistant and nonresistant beds of silty and sandy sparite, calcareous siltstone, silty or sandy micrite, and gypsum. Some litholpgies poorly exposed. Thin nonresistant beds of micrite, silty or sandy sparite and cal­careous siltstone. No gypsum beds present. This interval commonly covered with alluvium. LEGEND sandstone calcareous sandstone siltstone calcareous siltstone rJ HI sparite F i g u r e 2. G e n e r a l c o m p o s i t e s t r a t i g r a p h i c s e q u e n c e of t he M o e n k o p i F o r m a t i o n i n n o r t h e a s t e r n U t a h. LITHOLOGY 900' 800' 700' 600' r.:-:.L":"'". :"'":-7".:-::-.L.7".7 :-"7:.""::" ":•L•:: :::- -.- - - - --M;sivebed~fresistan-;- r:-~~i;::~~~ calcareous siltstone,very fine- 1-._ grained sandstone 8 silty sparite. ~. ~. ·5· ·5·.: .5 ·:5· ·5·.: ).1. :.1 ]·5.5 ·:E·L ______S edimenta~ _st_ructu_re_s ~bundant. ~:":"5 1-- • • ':" · =:-:~ '-I ·" .// ./ . .·/./../ A ~ .... . · 1 (" ._- ._:J.:: --·c-3- · .. .. . · I"'t-. Thin, resistant and nonresistant beds of sporite, micrite , gypsum, calcareous siltstone, and very fine- grained sandstone. Less gypsum present in this interval f.l: ····· ·v 1- . - . _..;> 0::::t..:-..:..,c:-y ____________ ----' ___ _ .b . . · "/x Thin, mostly nonresistant beds of silty or sandy sparite, calcareous siltstone and very fine- grained sandstone, silty or sandy micrite, and gypsum. 400' 300' 200' "100' o I"f' •••• • • • ~ •• •• • !i..:. - •• 7 . · . :.1; • • • x .. --:1 - - - • _.J . - . _ .---{ ...... . . ·J..·· 1 "//////1 • • •.. . . y . ....... . ' .L- • -:-=....:.. . .=-:\ · . _ ... .. " ,-- . . _/ .- .-!:.. . ~{ - • •• - •• J -.u:: . ::.J.:.. · .. ........ .. . · ... .. . - - - --Thick-beddedresi:;tantcplcareous siltstone and very fine groin sandstone _____ wil!L!h~eds ~...!!..l!licrit.L8~orile . Thin, resistant and nonresistant beds of silty and sandy sparite, calcareous siltstone, silty or sandy lithologies poorly exposed. Thin nonresistant beds of micrite, silty or sandy spa rite and cal­coreoussiltstone. No gypsum beds present. This interval commonly covered with alluvium. r:-::::J J.:.:::..:j ~ sparite or micrite ~ I.::.:::-..:J § [E] calcareous siltstone JiIilm ~ ~ sandyor silty spa rite or micrite gypsum Figure 2. General composite stratigraphic sequence of the Moenkopi Formation in northeastern Utah. 8 F i g u r e 3 . F o r m a t i o n n e a r F l a m i n g G o r g e R e s e r v o i r, n o r t h e a s t e r n U t a h. 9 Figure 3. Moenkopi Formation near Flaming Go rge Re servoir, northeastern Utah. 10 e x p o s e d . T h i s p a r t of t h e f o r m a t i o n f o r m s v a l l e y f l o o r s or g e n t l e s l o p e s. O v e r l y i n g t h i s s e q u e n c e a r e a b o u t 205 f e e t of r e d d i s h - b r o w n b e d s of c a l c a r e o u s s i l t s t o n e a n d s i l t y o r s a n dy s p a r i t e w i t h d r a b i n t e r b e d s of s i l t y m i c r i t e , s p a r i t e and c a l c a r e o u s s i l t s t o n e . Many t h i n b e d s of gypsum a r e p r e s e n t, a l w a y s a s s o c i a t e d w i t h d r a b r o c k s . L i t h o l o g i e s a r e t h i n ly b e d d e d a n d s o m e t i m e s p o o r l y e x p o s e d . T h i s p a r t of t he f o r m a t i o n f o r m s g e n t l e s l o p e s a n d r e s i s t a n t r o c k s a re p r e s e n t w h e r e c h a n g e s i n s l o p e o c c u r. Above t h i s s e q u e n c e a r e kZ f e e t of t h i c k - b e d d e d, r e d d i s h - b r o w n c a l c a r e o u s s i l t s t o n e , v e r y f i n e - g r a i n e d s a n d ­s t o n e a n d s i l t y s p a r i t e , a l o n g w i t h t h i n d r a b b e d s of m i c r i t e a n d s p a r i t e a s s o c i a t e d w i t h g y p s u m . T h i s i n t e r v al i s r e s i s t a n t a n d i s r e c o g n i z e d by a b u n d a n t r i p p l e m a r k s, r i p p l e s t r a t i f i c a t i o n a n d some c o n v o l u t e b e d d i n g. O v e r l y i n g t h i s a r e a b o u t 180 f e e t of r e d t o r e d d i s h - b r o w n b e d s of s i l t y a n d s a n d y s p a r i t e , c a l c a r e o u s s i l t s t o ne a n d v e r y f i n e - g r a i n e d s a n d s t o n e . Drab b e d s of s i l t y m i c r i t e, s p a r i t e , s i l t s t o n e and g y p s um a r e a l s o p r e s e n t . T h i s p a rt o f t h e f o r m a t i o n f o r m s a g e n t l e s l o p e a n d r e s i s t a n t b e ds m a r k c h a n g e s i n s l o p e. Above t h i s s e q u e n c e a r e a p p r o x i m a t e l y 255 f e e t of t h in b e d s of r e d d i s h - b r o w n s p a r i t e , c a l c a r e o u s s i l t s t o n e and v e ry f i n e - g r a i n e d s a n d s t o n e w i t h d r a b b e d s of s a n d y m i c r i t e and s i l t y s p a r i t e . Gypsum i s l e s s common i n t h i s i n t e r v a l. T h i s i n t e r v a l f o r m s s t e e p s l o p e s w i t h r e s i s t a n t l i t h o l o g i es f o r m i n g l e d g e s. exposed. This part of the formation forms valley floors or gentle slopes. Overlying this sequence are about 205 feet of reddish­brown beds of calcareous siltstone and silty or sandy sparite with drab interbeds of silty micrite, sparite and calcareous siltstone. Many thin beds of gypsum are present, always associated with drab rocks. Lithologies are thinly bedded and sometime ~ poorly exposed. This part of the formation forms gentle slopes and resistant rocks are present where changes in slope occur. Above this sequence are 42 feet of thick-bedded, reddish-brown calcareous siltstone, very fine-grained sand­stone and silty sparite, along with thin drab beds of micrite and sparite associated with gypsum. This interval is resistant and is recognized by abundant ripple marks, ripple stratification and some convolute bedding. Overlying this are about 180 feet of red to reddish­brown beds of silty and sandy sparite, calcareous siltstone and very fine-grained sandstone. Drab beds of silty micrite, sparite, siltstone and gypsum are also present. This part of the formation forms a gentle slope and resistant beds mark changes in slope. Above this sequence are approximately 255 feet of thin beds of reddish-brown sparite, calcareous siltstone and very fine-grained sandstone with drab beds of sandy micrite and silty sparite. Gypsum is less common in this interval. This interval forms ste.ep slopes with resistant lithologies forming ledges. A p p r o x i m a t e l y 93 f e e t of r e d d i s h - b r o w n , m a s s i v e l y - b e d d e d c a l c a r e o u s s i l t s t o n e , v e r y f i n e - g r a i n e d s a n d s t o ne a n d s i l t y s p a r i t e c a p t h e M o e n k o p i F o r m a t i o n . T h i s i n t e r v al i s r e s i s t a n t a n d i s c h a r a c t e r i z e d by v e r t i c a l s l o p e s or c l i f f s and a b u n d a n t r i p p l e m a r k s. The M o e n k o p i F o r m a t i o n c o n f o r m a b l y o v e r l i e s t he Dinwoody F o r m a t i o n , w h i c h i s c o m p o s e d of d r a b , n o n r e s i s t a nt c l a y e y a n d s i l t y c a r b o n a t e a n d c a l c a r e o u s c l a y s t o n e and s i l t s t o n e . Some w r i t e r s ( U n t e r m a n n a n d U n t e r m a n n , 1969a? R i t z m a , 1 9 5 9 ) c o n s i d e r t h e s e b e d s t o b e l o n g i n t h e u p p er p a r t of t h e P e r m i a n P a r k C i t y F o r m a t i o n . Dinwoody t e r m i n o ­l o g y i s u s e d h e r e b e c a u s e of s i m i l a r i t i e s i n c o l o r , l i t h o l o gy a n d s e q u e n c e t o b e d s of t h e D i n w o o d y F o r m a t i o n i n Wyoming ( P i c a r d , 1973» p e r s o n a l c o m m u n i c a t i o n ) . The Dinwoody commonly f o r m s v a l l e y f l o o r s i n n o r t h e a s t e r n U t a h. The G a r t r a F o r m a t i o n u n c o n f o r m a b l y o v e r l i e s t h e M o e n k o pi a n d i s c o m p o s e d o f m e d i u m - t o - c o a r s e - g r a i n e d s a n d s t o n e and c o n g l o m e r a t e . The u n c o n f o r m i t y i n n o r t h e a s t e r n U t a h is e r o s i o n a l , b u t a c c o r d i n g t o P o o l e a n d S t e w a r t ( 1 9 6 4 , p . D33) i t may b e a n g u l a r b e c a u s e i t r e s t s on p r o g r e s s i v e l y o l d er b e d s t o t h e e a s t . The G a r t r a i s c o n s i d e r e d t o h a v e f o r m ed d u r i n g a b o u t t h e same t i m e a s t h e S h i n a r u m p C o n g l o m e r a te o f c e n t r a l U t a h ( M c C o r m i c k a n d P i c a r d , I 9 6 9 . p . 1 7 6 - 1 7 7 ). Approximately 93 feet of reddish-brown, massively­bedded calcareous siltstone, very fine-grained sandstone 11 and silty sparite cap the Moenkopi Formation. This interval is resistant and is characterized by vertical slopes or cliffs and abundant ripple marks. The Moenkopi Formation conformably overlies the Dinwoody Formation, which is composed of drab, nonresistant clayey and silty carbonate and calcareous claystone and siltstone. Some writers (Untermann and Untermann, 1969a; Ritzma, 1959) consider these beds to belong in the upper part of the Permian Park City Formation. Dinwoody termino­logy is used here because of similarities in color, lithology and sequence to beds of the Dinwoody Formation in Wyoming (Picard, 1973, personal communication). The Dinwoody commonly forms valley floors in northeastern Utah. The Gartra Formation unconformably overlies the Moenkopi and is composed of medium-to-coarse-grained sandstone and conglomerate. The unconformity in northeastern Utah is erosional, but according to Poole and Stewart (1964, p. D33) it may be angular because it rests on progressively older beds to the east. The Gartra is considered to have formed during about the same time as the Shinarump Conglomerate of central Utah (McCormick and Picard, 1969, p. 176-177). SEDIMENTARY STRUCTURES S e d i m e n t a r y s t r u c t u r e s i n t h e M o e n k o p i F o r m a t i o n in n o r t h e a s t e r n U t a h a r e a b u n d a n t a n d e a s i l y o b s e r v a b l e. S e v e r a l s t u d i e s h a v e b e e n d o n e i n t h e M o e n k o p i o r e q u i v a ­l e n t s i n o t h e r a r e a s 1 9 5 ^ 1 B a l d w i n , 1 9 7 3 ; P o o l e, 1 9 6 1 s B l a k e y , 1973» P i c a r d , 1 9 6 7 ; P i c a r d a n d H i g h , 1 9 6 8 ), b u t l i t t l e h a s b e e n d o n e i n n o r t h e a s t e r n U t a h . P r o p er i n t e r p r e t a t i o n s e d i m e n t a r y s t r u c t u r e s c a n a i d i n e v a l u a t i on o f t h e d e p o s i t i o n a l e n v i r o n m e n t a n d a s s o c i a t e d c u r r e nt d i r e c t i o n s s t r e n g t h d u r i n g d e p o s i t i o n . D i s c u s s i o n of s e d i m e n t a r y s t r u c t u r e s f o u n d i n t h e f o r m a t i o n i s p r e s e n t ed h e r e . T h e i r r e l a t i v e a b u n d a n c e i s i n f i g u r e k. R i p p l e Marks R i p p l e m a r k s a r e t h e m o s t a b u n d a n t s e d i m e n t a ry s t r u c t u r e s i n t h e f o r m a t i o n , c o n s i s t i n g g r e a t e r t h an p e r c e n t a l l s t r u c t u r e s o b s e r v e d . a r e a b u n d a nt i n t e r r i g e n o u s r o c k s s a n d y a n d s i l t y s p a r i t e. S e v e r a l t y p e s a r e p r e s e n t * l i n e a r a s y m m e t r i c a l and s y m m e t r i c a l , s i n u o u s a s y m m e t r i c a l , c u s p a t e a n d s e c o n d a ry i n t e r f e r e n c e r i p p l e m a r k s. L i n e a r A s y m m e t r i c a l a n d S y m m e t r i c a l R i p p l e M a r k s . - - T h e s e r i p p l e m a r k s a r e by f a r t h e m o s t a b u n d a n t , w i t h l i n e ar a s y m m e t r i c a l r i p p l e s much more common t h a n s y m m e t r i c a l. SEDIMENTARY STRUCTURES Sedimentary structures in the Moenkopi Formation in northeastern Utah are abundant and easily observable. Several studies have been done in the Moenkopi or equiva­lents in other areas (McKee, 1954, Baldwin, 19731 Poole, 1961, Blakey, 1973. Picard, 1967. Picard and High, 1968), but little has been done in northeastern Utah, Proper interpretation of sedimentary structures can aid in evaluation of the depositional environment and associated current directions and strength during deposition. Discussion of sedimentary structures found in the formation is presented here. Their relative abundance is shown in figure 4. Ripple Marks Ripple marks are the most abundant sedimentary structures in the formation, consisting of greater than 90 percent of all structures observed. They are abundant in some terrigenous rocks and sandy and silty sparite. Several types are present, linear asymmetrical and symmetrical, sinuous asymmetrical, cuspate and secondary interference ripple marks. Linear Asymmetrical and Symmetrical Ripple Marks.-­These ripple marks are by far the most abundant, with linear asymmetrical ripples much more common than symmetrical. 70- i 60 H Ul O 50- 1 Z 40 Ld a: 3 0 UJ *• 20 LEGEND LA Linear asymmetrical ripple marks SA Sinuous asymmetrical ripple marks LS Linear symmetrical ripple marks 0 Other sedimentary structures S I Secondary interference ripple marks CR Cuspate ripple marks 10 - SA LS CR F i g u r e 4 . R e l a t i v e a b u n d a n c e s e d i m e n t a r y s t r u c t u r es s e e n i n t h e M o e n k o p i F o r m a t i o n. 40-i Ld o L^U IOHn OL LEGEND HP Horizontal parallel stratification RS Ripple stratification TC Trough cross-statification PC Planar cross-stratification CL Convolute lamination BS Bioturbated stratification HP RS TC PC CL BS F i g u r e 5. R e l a t i v e a b u n d a n c e s t r a t i f i c a t i o n t y p es s e e n i n t h e M o e n k o p i F o r m a t i o n. 60- LEGEND L&J LA (!) 50 ;! SA Z 40 LS LLJ 0 U 0:: 30 51 La&.J CR npple • 0 • LA SA LS 0 51 CR Figure 4. Relative abundance of sedimentary structures seen in the Moenkopi Formation. LEGEND HP stratification RS stratification 40 TC cross-(L&!J) PC cross- ~ 30- CL Z 20- BS stratification LLJ U I 0:: 10 LLJ a. I 0 HP RS TC PC CL BS Figure 5. Relative abundance of stratification types seen in the Moenkopi Formation. 13 L i n e a r a s y m m e t r i c a l r i p p l e s a r e c h a r a c t e r i z e d by t he s t r a i g h t n e s s of r i p p l e c r e s t s , and one " s i d e " i s i n c l i n ed s t e e p e r t h a n t h e o t h e r ( f i g u r e 6 ) . R i p p l e s y m m e t r y i n d i c es o f 30 r i p p l e m a r k s r a n g e f r om 1 . 1 5 t o 1 . 9 0 , t h e mean v a l ue b e i n g 1 . 4 3 . A c c o r d i n g t o T a n n e r ( 1 9 6 7 * p . 9*0 » r i p p l e m a r ks w i t h r i p p l e s y m m e t r y i n d i c e s l e s s t h a n 3 . 0 0 a r e f o r m ed u s u a l l y i n wave o r s w a s h z o n e s . The d o w n c u r r e n t d i r e c t i on i s d e t e r m i n e d by n o t i n g i n w h i c h d i r e c t i o n t h e s t e e p e r s i de o f t h e r i p p l e p o i n t s , a n d i s m e a s u r e d p e r p e n d i c u l a r t o t he s t r i k e of t h e c r e s t i n t h i s d i r e c t i o n. L i n e a r s y m m e t r i c a l r i p p l e m a r k s a l s o h a v e s t r a i g h t - t r e n d i n g c r e s t s b u t i t i s v i r t u a l l y i m p o s s i b l e t o d e t e r m i ne t h e s t e e p e r s i d e of r i p p l e s f r o m e x t e r n a l g e o m e t r y. R i p p l e s y m m e t r y i n d i c e s of s y m m e t r i c a l r i p p l e s a r e e q u a l to 1 . 0 0 . When t h e r i p p l e s a r e g e o m e t r i c a l l y s y m m e t r i c a l , it i s n e c e s s a r y t o i n s p e c t t h e o r i e n t a t i o n of t h e i n t e r n al f o r e s e t l a m i n a e t o d e t e r m i n e t h e d i r e c t i o n of f l o w . Very f ew s y m m e t r i c a l r i p p l e s w e r e f o u n d t h a t d i d n o t y i e l d a f l o w d i r e c t i o n f r o m t h i s t e c h n i q u e . T r u l y s y m m e t r i c al r i p p l e s a r e r a r e a l t h o u g h a p p e a r t o be common a t f i r st g l a n c e . S i n u o u s A s y m m e t r i c a l R i p p l e M a r k s . - S i n u o u s a s y m m e t r i ­c a l r i p p l e m a r k s a r e s i m i l a r i n a p p e a r a n c e t o l i n e a r r i p p l es e x c e p t t h a t t h e c r e s t s a r e c u r v e d i n t o a n e l o n g a t e d " S" s h a p e . Commonly t h e c r e s t s a r e b r o k e n a n d s l i g h t l y o f f s e t, a n d o c c a s i o n a l l y s i n u o u s r i p p l e c r e s t s g r a d e l a t e r a l l y i n to l i n e a r c r e s t s . S i n u o u s a s y m m e t r i c a l r i p p l e s a r e commonly 14 Linear asymmetrical ripples are characterized by the straightness of ripple crests, and one "side" is inclined steeper than the other (figure 6). Ripple symmetry indices of 30 ripple marks range from 1.15 to 1.90, the mean value being 1.43. According to Tanner (1967, p. 94), ripple marks with ripple symmetry. indices less than 3.00 are formed usually in wave or swash zones. The downcurrent direction is determined by noting in which direction the steeper side of the ripple points, and is measured perpendicular to the strike of the crest in this direction. Linear symmetrical ripple marks also have straight­trending crests but it is virtually impossible to determine the steeper side of ripples from external geometry. Ripple symmetry indices of symmetrical ripples are equal to 1.00. When the ripples are geometrically symmetrical, it is necessary to inspect the orientation of the internal foreset laminae to determine the direction of flow. Very few symmetrical ripples were found that did not yield a flow direction from this technique. Truly symmetrical ripples are rare although appear to be common at first glance. Sinuous Asymmetrical Ripple Marks.--Sinuous asymmetri­cal ripple marks are similar in appearance to linear ripples except that the crests are curved into an elongated "S" shape. Commonly the crests are broken and slightly offset, and occasionally sinuous ripple crests grade laterally into linear crests. Sinuous asymmetrical ripples are commonly F i g u r e 6, P h o t o g r a p h s h o w i n g l i n e a r a s y m m e t r i c a l r i p p le m a r k s n e a r t h e t o p of t h e Moenkopi F o r m a t i o n a t B r u sh C r e e k , Figure 6. Photograph showing linear asymmetrical ripple marks near the top of the Moenkopi Formation at Brush Creek. 15 16 s e e n i n s e v e r a l b e d s of s p a r i t e and s i l t s t o n e n e a r t he m i d d l e of t h e f o r m a t i o n. C u s p a t e R i p p l e M a r k s . - C u s p a t e r i p p l e m a r k s a r e r a re a n d a r e r e s t r i c t e d t o t h e u p p e r m o s t r e s i s t a n t s i l t s t o n e and v e r y f i n e - g r a i n e d s a n d s t o n e b e d s . T h e i r c r e s t s a r e t y p i c a l ly "CH s h a p e d w i t h t h e c o n v e x s i d e a n d t h e i n t e r n a l f o r e s et l a m i n a e p o i n t i n g d o w n c u r r e n t . w e r e o b s e r v e d o n l y at t h e S h e e p C r e e k s e c t i o n. S e c o n d a r y I n t e r f e r e n c e R i p p l e M a r k s . - S e c o n d a ry i n t e r f e r e n c e r i p p l e m a r k s a r e n o t common. In m o s t c a s es t h e i r p r e s e n c e i s m a r k e d o r m o r e s e t s l i n e ar a s y m m e t r i c a l r i p p l e s c o n v e r g i n g u p o n e a c h o t h e r a t a c u te a n g l e s . S o m e t i m e s i t i s d i f f i c u l t t o d e t e r m i n e i f , i n f a c t, t h e s e a r e i n t e r f e r e n c e r i p p l e s b e c a u s e a l l r i p p l e s n ot b e f r om t h e s t r a t i g r a p h i c l a m i n a . s u s p e c t ed o c c u r r e n c e s w e r e l a t e r f o u n d t o s e p a r a t e d s t r a t i g r a p h i c a l ly b y a s l i t t l e a s 0 . 5 i n c h e s . i n m o s t c a s e s i t is a p p a r e n t t h a t r e f r a c t i o n r i p p l e c r e s t s t h e r e s u l t of i n t e r f e r e n c e c u r r e n t s. O t h e r S e d i m e n t a r y S t r u c t u r es S e v e r a l o t h e r s e d i m e n t a r y s t r u c t u r e s a r e p r e s e n t b ut i n r a r e a m o u n t s . P a r t i n g l i n e a t i o n i s o c c a s i o n a l l y a s s o c i a t ed w i t h i n t e r v a l s w h e r e h o r i z o n t a l s t r a t i f i c a t i o n i s p r e s e n t. B a l l - a n d - p i l l o w s t r u c t u r e s a r e l o c a l l y p r e s e n t i n t h e c l i f f - f o r m i n g s p a r i t e and t e r r i g e n o u s r o c k s n e a r t h e t o p of t he f o r m a t i o n . P o l y g o n a l s h r i n k a g e c r a c k s , f o u n d t o be common ~een in several beds of sparite and siltstone near the middle of the formation. cuspate Ripple Marks.--Cuspate ripple marks are rare 16 and are restricted to the uppermost resistant siltstone and very fine-grained sandstone beds. Their crests are typically "C" shaped with the convex side and the internal foreset laminae pointing downcurrent. They were observed only at the Sheep Creek Canyon section. Secondary Interference Ripple Marks.--Secondary interference ripple marks are not common. In most cases their presence is marked by two or more sets of linear asymmetrical ripples converging upon each other at acute angles. Sometimes it is difficult to determine if, in fact, these are interference ripples because all ripples may not be from the same stratigraphic lamina. Some suspected occurrences were later found to be separated stratigraphically by as little as 0.5 inches. However, in most cases it is apparent that refraction of ripple crests was the result of interference of currents. Other Sedimentary Structures Several other sedimentary structures are present but in rare amounts. Parting lineation is occasionally associated with intervals where horizontal stratification is present. Ball-and-pillow structures are locally present in the cliff­forming sparite and terrigenous rocks near the top of the formation. Polygonal shrinkage cracks, found to be common 17 i n t h e M o e n k o p i i n o t h e r a r e a s , a r e p r e s e n t a t o n l y 2 o f t he s t u d i e d s e c t i o n s i n n o r t h e a s t e r n U t a h. S t r a t i f i c a t i o n T y p es H o r i z o n t a l P a r a l l e l S t r a t i f i c a t i o n . - H o r i z o n t al p a r a l l e l s t r a t i f i c a t i o n i s t h e m o s t s t r a t i f i c a t i on t y p e s e e n i n t h e M o e n k o p i F o r m a t i o n f i g u r e s 5» 7 ) . I t is p r e s e n t i n s i l t s t o n e , v e r y f i n e - g r a i n e d s a n d s t o n e and s p a r i t e b e d s t h r o u g h o u t t h e f o r m a t i o n . p a r a l l el l a m i n a e a r e a l i g n e d h o r i z o n t a l l y e x t e n d l a t e r a l ly s e v e r a l f e e t . s e t s a r e l e s s t h a n i n c h e s t h i c k. H o r i z o n t a l p a r a l l e l s t r a t i f i c a t i o n i s m o s t a b u n d a n t in c l i f f - f o r m i n g r o c k s n e a r t h e t o p t h e f o r m a t i o n . The e x a c t h y d r o l o g i c c o n d i t i o n s r e s u l t i n g i n i t s f o r m a t i o n a re n o t known, h o w e v e r P i c a r d a n d H i g h ( 1 9 7 3 » P» 1 4 5 - 1 4 6) s u g g e s t t h a t b o t h l o w e r - f l o w a n d u p p e r - f l o w r e g i m e c u r r e n ts r e s p o n s i b l e. R i p p l e S t r a t i f i c a t i o n . - R i p p l e s t r a t i f i c a t i o n i s t he m o s t v i s i b l e a n d s e c o n d - m o s t a b u n d a n t s t r a t i f i c a t i o n t y pe o b s e r v e d . T h i s t y p e i s t h e r e s u l t c o m p l e t e p r e s e r v a t i on o f r i p p l e f o r m s (McKee, 1957» p . 1 7 1 8 ) . T h i c k n e s s e s of s e v e r a l s e t s o b s e r v e d r a n g e f r o m 2 , 5 i n c h e s t o g r e a t e r t h an 12 f e e t . R i p p l e s t r a t i f i c a t i o n i s p a r t i c u l a r l y a b u n d a nt i n v e r y f i n e - g r a i n e d s a n d s t o n e a n d s i l t s t o n e i n t h e m i d d le a n d u p p e r p a r t s of t h e f o r m a t i o n . A c c o r d i n g t o McKee ( 1 9 6 5* p . 7 ^ - 7 5 ) » r i p p l e s t r a t i f i c a t i o n i s t h e r e s u l t of r a p id d e p o s i t i o n i n a r e a s of h i g h s e d i m e n t i n p u t. 17 in the Moenkopi in other areas, are present at only 2 of the studied .sections in northeastern Utah. Stratification Types Horizontal Parallel Stratification.--Horizontal parallel stratification is the most common stratification type seen in the Moenkopi Formation ( figures 5, 7). It is present in many siltstone, very fine-grained sandstone and some sparite beds throughout the formation. Thin parallel laminae are aligned horizontally and extend laterally several feet. Most sets are less than 3 inches thick. Horizontal parallel stratification is most abundant in cliff-forming rocks near the top of the formation. The exact hydrologic conditions resulting in its formation are not known. however Picard and High (1973. p. 145-146) suggest that both lower-flow and upper-flow regime currents may be responsible. Ripple Stratification.--Ripple stratification is the most visible and second-most abundant stratification type observed. This type is the result of complete preservation of ripple forms (McKee. 1957. p. 1718). Thicknesses of several sets observed range from 2.5 inches to greater than 12 feet. Ripple stratification is particularly abundant in very fine-grained sandstone and siltstone in the middle and upper parts of the formation. According to McKee (1965. P. 74~75). ripple stratification is the result of rapid depOSition in areas of high sediment input. C r o s s - s t r a t i f i c a t i o n , - T w o t y p e s of c r o s s - s t r a t i f i c a t i on a r e p r e s e n t , t h e m o s t a b u n d a n t b e i n g s m a l l - a n d - m e d i u m - s c a le t r o u g h c r o s s - s t r a t i f i c a t i o n , and l e s s e r a m o u n t s of medium-s c a l e p l a n a r c r o s s - s t r a t i f i c a t i o n ( t e r m i n o l o g y of McKee a n d W e i r , 1 9 5 3 ) . Both a r e f o u n d i n o n l y a f e w b e d s of s a n d y s p a r i t e a n d t e r r i g e n o u s r o c k s . None w e r e s e e n in m i c r i t e o r m i c r o s p a r i t e , A c c o r d i n g t o B l a t t , M i d d l e t o n and M u r r a y ( 1 9 7 2 , p . 1 1 7 - 1 1 8 ) , m o s t c r o s s - s t r a t i f i c a t i o n is f o r m e d by a v a l a n c h i n g of t e r r i g e n o u s g r a i n s down l e e s l o p es o f r i p p l e s , d u n e s o r b a r s . T h i s s u g g e s t s f o r m a t i o n in l o w e r - f l o w r e g i m e c u r r e n t s . A l s o , B l a t t , M i d d l e t o n and M u r r a y ( 1 9 7 2 , p , 1 2 7 ) s u g g e s t e d t h a t m o s t s m a l l - s c a le t r o u g h c r o s s - s t r a t i f i c a t i o n c a n r e s u l t f r o m m i g r a t i on o f c u s p a t e r i p p l e s \ and t h a t m e d i u m - a n d - l a r g e - s c a l e t r o u gh c r o s s - s t r a t i f i c a t i o n r e s u l t s f r om m i g r a t i o n of s i n u o us r i p p l e s o r d u n e s. C o n v o l u t e S t r a t i f i c a t i o n , - C o n v o l u t e s t r a t i f i c a t i on i s p r e s e n t i n a 15 f o o t i n t e r v a l of v e r y f i n e - g r a i n e d s a n d ­s t o n e n e a r t h e m i d d l e o f t h e f o r m a t i o n . It i s c h a r a c t e r i z ed b y m a r k e d c r u m p l i n g a n d i n t r i c a t e f o l d i n g of l a m i n a e w i t h in a n e s s e n t i a l l y u n d e f o r m e d b e d ( f i g u r e 8 ) . Most l a m i n ae a r e c o n t i n u o u s up t o a b o u t t o f e e t l a t e r a l l y , o c c a s i o n a l ­l y o f f s e t by m i n o r s l u m p s t r u c t u r e s . C o n v o l u t e d l a m i n ae f o rm s m a l l - s c a l e a n t i c l i n e s a n d s y n c l i n e s w i t h some r e c u m ­b e n t f o l d s s e e n . c o n v o l u t e d i n t e r v a l i s b o u n d e d a b o ve a n d b e l o w by r i p p l e - s t r a t i f i e d s i l t s t o n e a n d s p a r i t e, p o s s i b l y s u g g e s t i n g t y p e r e l a t i o n s h i p b e t w e e n t he 18 Cross-stratification.--Two types of cross-stratification are present, the most abundant being small-and-medium-scale trough cross-stratification, and lesser amounts of medium­scale planar cross-stratification (terminology of McKee and Weir, 1953). Both are found in only a few beds of sandy sparite and terrigenous rocks. None were seen in micrite or microsparite. According to Blatt, Middleton and Murray (1972, p. 117-118), most cross-stratification is formed by avalanching of terrigenous grains down lee slopes of ripples, dunes or bars. This suggests formation in lower-flow regime currents. Also, Blatt, Middleton and Murray (1972, p. 127) suggested that most small-scale trough cross-stratification can result from migration of cuspate ripples; and that medium-and-large-scale trough cross-stratification results from migration of sinuous ripples or dunes. Convolute Stratification.--Convolute stratification is present in a 15 foot interval of very fine-grained sand­stone near the middle of the formation. It is characterized by marked crumpling and intricate folding of laminae within an essentially undeforrned bed (figure 8). Most laminae are continuous up to about 6 to 7 feet laterally, occasional­ly offset by minor slump structures. Convoluted laminae form small-scale anticlines and synclines with some recum­bent folds seen. The convoluted interval is bounded above and below by ripple-stratified siltstone and sparite, possibly suggesting some type of relationship between the F i g u r e P h o t o g r a p h i l l u s t r a t i n g c o n v o l u t e l a m i n a t i on i n c a l c a r e o u s s a n d s t o n e. Figure 7. Photograph illustrating horizontal parallel stratification in calcareous siltstone. ~igure 8. Photograph illustrating convolute lamination In calcareous sandstone. 19 .two b e d f o r m s . A c c o r d i n g t o D z u l y n s k i a n d ' S m i t h ( 1 9 6 3 , p. 6 2 3 ) » c o n v o l u t e s t r a t i f i c a t i o n i s f o r m e d by p l a s t i c move­ment of mud i n t e r b e d d e d w i t h w a t e r - s a t u r a t e d s a n d , c a u s i ng d i f f e r e n t i a l movement b e t w e e n l a m i n a e a n d t h e i r u l t i m a te d e f o r m a t i o n . B i o t u r b a t e d S t r a t i f i c a t i o n . - I n t h e M o e n k o p i F o r m a t i on t h e r e a r e s e v e r a l i n t e r v a l s t h a t v i r t u a l l y s t r a t i ­f i c a t i o n c o u l d r e f e r r e d t o a s m a s s i v e l y - b e d d e d. T h i s w r i t e r s p e c u l a t e s t h a t t h e o r i g i n a l s t r a t i f i c a t i o n d e s t r o y e d b u r r o w i n g o r g a n i s m s . b u r r o w i n g s t r u c t u r es w e r e s e e n a t o u t c r o p s b u t t h i n s e c t i o n e v i d e n c e r e v e a ls t h e i r p r e s e n c e . A c c o r d i n g t o B l a t t , M i d d l e t o n a n d M u r r ay 1 9 7 2 , p . 1 1 8 ) , t h e a c t i o n b u r r o w i n g o r g a n i s m s c a n i n t e n s e e n o u g h t o d e s t r o y a l l o r i g i n a l e v i d e n c e of s t r a t i f i c a t i o n . two bedforms. According to Dzulynski and ' Smith (196), p. 62), convolute stratification is formed by plastic move-ment of mud interbedded with water-saturated sand, causing differential movement between laminae and their ultimate deformation. Bioturbated Stratification.--In the Moenkopi Formation there are several intervals that show virtually no strati­fication and could be referred to as massively-bedded. This writer speculates that the original stratification was destroyed by burrowing organisms. Few burrow~ng structures were seen at outcrops but thin section evidence reveals their presence. According to Blatt, Middleton and Murray ( 1972, p. 118), the action of burrowing organisms can be intense enough to destroy all original evidence of stratification. 20 TEXTURE OF TERRIGENOUS ROCKS T e x t u r a l p a r a m e t e r s of g r a i n s i z e , s o r t i n g , r o u n d i ng a n d m a t u r i t y a r e b r i e f l y d e s c r i b e d h e r e . M i c r o s c o p ic e x a m i n a t i o n o f f i e l d s a m p l e s a n d t h i n s e c t i o n s p r o v i d e t he d a t a n e e d e d f o r i n t e r p r e t a t i o n of t e x t u r e . A l l s a m p l es w e r e c o m p a r e d t o s a m p l e s of known o r c a l c u l a t e d g r a i n s i ze a n d c l a s s i f i e d a c c o r d i n g t o W e n t w o r t h ( 1 9 2 2 ). G r a i n S i ze S i l t i s t h e d o m i n a n t g r a i n s i z e , r e p r e s e n t i n g 72 p e r c e n t of a l l g r a i n s o b s e r v e d ( f i g u r e 9 ) . The o r d e r of a b u n d a n c e g r a i n s i z e s i s : c o a r s e s i l t , v e r y f i n e s a n d, s i l t , f i n e s i l t , c l a y , f i n e s a n d , a n d v e r y f i n e s i l t. O n l y t h r e e g r a i n s i z e s a r e a b u n d a n t e n o u g h t o j u s t i f y r o ck n a m e s f i g u r e 1 0 ) . C o a r s e - g r a i n e d s i l t s t o n e i s t h e most a b u n d a n t r o c k t y p e , f o l l o w e d v e r y f i n e - g r a i n e d s a n d s t o ne a n d m e d i u m - g r a i n e d s i l t s t o n e. B e d s o f s i l t s t o n e and v e r y f i n e - g r a i n e d s a n d s t o n e a re common i n t h e m i d d l e a n d u p p e r p o r t i o n s of t h e f o r m a t i o n. G r a i n s i z e s w i t h i n t h e s e b e d s v a r y , and c o a r s e r g r a i n s a re more common i n t h e e a s t e r n s e c t i o n s of t h e s t u d y a r e a . Two r i p p l e - s t r a t i f i e d b e d s of v e r y f i n e - g r a i n e d s a n d s t o n e at D i n o s a u r Q u a r r y become c o a r s e - g r a i n e d s i l t s t o n e a t S h e ep C r e e k C a n y o n . S i m i l a r d e c r e a s e s i n g r a i n s i z e c a n a l s o be TEXTURE OF TERRIGENOUS ROCKS Textural parameters of grain size, sorting, rounding and maturity are briefly described here. Microscopic examination of field samples and thin sections provide the data needed for interpretation of texture. All samples were compared to samples of known or calculated grain size and classified according to Wentworth (1922). Grain Size Silt is the dominant grain size, representing 72 percent of all grains observed (figure 9). The order of abundance of grain sizes is: coarse silt, very fine sand. medium silt. fine silt, clay. fine sand, and very fine silt. Only three grain sizes are abundant enough to justify rock names ( figure 10). Coarse-grained siltstone is the most abundant rock type. followed by very fine-grained sandstone and medium-grained siltstone. Beds of siltstone and very fine-grained sandstone are common in the middle and upper portions of the formation. Grain sizes within these beds vary, and coarser grains are more common in the eastern sections of the study area. Two ripple-stratified beds of very fine-grained sandstone at Dinosaur Quarry become coarse-grained siltstone at Sheep Creek Canyon. Similar decreases in grain size can also be 60-. UJ 40- o z txJ 30 o LU Q. 20 10- I r 1/8 1/512 G R A I N S I Z E (mm) F i g u r e 9. R e l a t i v e a b u n d a n c e of g r a i n s i z e s o b s e r v e d in t e r r i g e n o u s r o c k s of t h e M o e n k o p i F o r m a t i o n. 60-i txi o z UJ 30 O 01 UJ a. 20- 1/64 G R A I N S I Z E ( m m) F i g u r e 1 0 , H i s t o g r a m s h o w i n g r e l a t i v e a b u n d a n c e of t e r r i g ­e n o u s r o c k t y p e s i n t h e M o e n k o p i F o r m a t i o n. 60 50 Lt&!)J 40 - ~ Z L&J 30 - 0 0:: L&J Q.; 20 - 10 - I I • I o 1/4 lIS 1/16 1/32 1/64 1/128 1/256 1/512 GRAI N SIZE (mm) Figure 9. Relative abundance of grain sizes observed in terrigenous rocks of the Moenkopi Formation. 60 50- LaJ 40- t!) ~ Z L&J 30 o 0:: L&J Q.; 20 10- o~---L------------~ 1/4 1/8 1/16 1/32 1/64 GRAIN SIZE (mm) 22 Figure 10. Histogram showing relative abundance of terrig­enous rock types in the Moenkopi Formation. 23 s e e n f o r t e r r i g e n o u s g r a i n s i n c a r b o n a t e r o c k s . This s u g g e s t s t h a t t h e e a s t e r n s e c t i o n s w e r e s i t u a t e d c l o s er t o a t e r r i g e n o u s s o u r c e a r e a d u r i n g d e p o s i t i o n . S i m i l ar r e s u l t s w e r e o b t a i n e d by C a d i g a n ( 1 9 7 1 . p . 4 0 ). G r a i n s i z e a f f e c t s t h e a m o u n t of c a r b o n a t e cement a n d h e m a t i t e s t a i n i n t h e c e m e n t . As t h e t e r r i g e n o u s g r a in s i z e d e c r e a s e s , t h e a m o u n t s o f c a r b o n a t e c e m e n t a n d h e m a t i te s t a i n i n c r e a s e . These r e l a t i o n s h i p s a r e p o s s i b l y r e l a t ed t o d i f f e r e n c e s i n p a c k i n g a n d s u r f a c e a r e a s of g r a i n s in s i l t s t o n e a n d v e r y f i n e - g r a i n e d s a n d s t o n e . Very f i n e ­g r a i n e d s a n d s t o n e i s s l i g h t l y b e t t e r p a c k e d a n d i n t h in s e c t i o n s t h e r e a r e more g r a i n c o n t a c t s p e r g r a i n . I t is p o s t u l a t e d t h a t r e c r y s t a l l i z a t i o n e f f e c t s of c a r b o n a t e cement ( d i s c u s s e d i n a l a t e r s e c t i o n of t h i s t h e s i s ) w e r e b e t t er a b l e t o p u s h s i l t g r a i n s a p a r t b e t t e r t h a n s a n d g r a i n s, t h u s a c c o u n t i n g f o r p o o r e r p a c k i n g i n s i l t s t o n e . S i n c e t he r o c k s d i s c u s s e d h e r e a r e m o s t l y c e m e n t - s u p p o r t e d , i t is a p p a r e n t t h a t s i l t s t o n e s h o u l d h a v e more c a r b o n a t e c e m e nt t h a n v e r y f i n e - g r a i n e d s a n d s t o n e . The f a c t t h a t h e m a t i te s t a i n i s more a b u n d a n t i n s i l t s t o n e i s p r o b a b l y r e l a t ed t o t h e a b i l i t y of h e m a t i t e t o s t a i n f i n e r - g r a i n e d m a t e r i al more h e a v i l y , a s s e e n i n t h i n s e c t i o n s. S o r t i n g T h i n s e c t i o n s w e r e e x a m i n e d t o d e t e r m i n e s o r t i n g of t e r r i g e n o u s g r a i n s , a n d e a c h r o c k was c l a s s i f i e d by v i s u al c o m p a r i s o n t o a s o r t i n g c h a r t p r e p a r e d by Compton ( 1 9 6 2, P . 2 1 4 ) . F o u r t e e n t h i n s e c t i o n s a r e c l a s s i f i e d a s p o o r ly seen for terrigenous grains in carbonate rocks. This Suggests that the eastern sections were situated closer to a terrigenous source area during deposition. Similar results were obtained by Cadigan (1971, p. 40). Grain size affects the amount of carbonate cement 2) and hematite stain in the cement. As the terrigenous grain size decreases, the amounts of carbonate cement and hematite stain increase. These relationships are possibly related to differences in packing and surface areas of grains in siltstone and very fine-grained sandstone. Very fine­grained sandstone is slightly better packed and in thin sections there are more grain contacts per grain. It is postulated that recrystallization effects of carbonate cement (discussed in a later section of this thesis) were better able to push silt grains apart better than sand grains, thus accounting for poorer packing in siltstone. Since the rocks discussed here are mostly cement-supported, it is apparent that siltstone should have more carbonate cement than very fine-grained sandstone. The fact that hematite stain is more abundant in siltstone is probably related to the ability of hematite to stain finer-grained material more heavily, as seen in thin sections. Sorting Thin sections were examined to determine sorting of terrigenous grains, and each rock · was classified by visual comparison to a sorting chart prepared by Compton (1962, P. 214). Fourteen thin sections are classified as poorly 2k s o r t e d , 11 a s m o d e r a t e l y s o r t e d , and 9 a s w e l l s o r t e d. F i g u r e 11 i l l u s t r a t e s a w e l l s o r t e d s i l t s t o n e. I n g e n e r a l , v e r y f i n e - g r a i n e d s a n d s t o n e i s s l i g h t ly b e t t e r s o r t e d t h a n c o a r s e - o r - m e d i u m - g r a i n e d s i l t s t o n e. H o w e v e r , t h e d i f f e r e n c e i n s o r t i n g c h a r a c t e r i s t i c s may be d u e t o a c l a s s i f i c a t i o n p r e j u d i c e and n o t d e p i c t any t r a n s p o r t a t i o n a l d i f f e r e n c e s . The a r b i t r a r y c u t o f f f or s a n d g r a i n s i s l / l 6 mm, a n d a l l m a t e r i a l i n v e r y f i n e ­g r a i n e d s a n d s t o n e l e s s t h a n t h i s s i z e i s c o n s i d e r e d as m a t r i x . Many t h i n s e c t i o n s of v e r y f i n e - g r a i n e d s a n d s t o ne h a v e a b u n d a n t m a t e r i a l l e s s t h a n l / l 6 mm i n s i z e , a s c an b e s e e n i n f i g u r e 1 2 . When a l l m a t e r i a l c o a r s e r t h an 1 / 2 5 6 mm i s c o n s i d e r e d i n a l l r o c k s , t h e r e i s r e l a t i v e ly l i t t l e d i f f e r e n c e i n s o r t i n g c h a r a c t e r i s t i c s b e t w e e n v e ry f i n e - g r a i n e d s a n d s t o n e a n d s i l t s t o n e . T h i s s u g g e s t s t h at b o t h r o c k t y p e s w e r e d e p o s i t e d u n d e r e s s e n t i a l l y t h e same p h y s i c a l c o n d i t i o n s w i t h i n t h e d e p o s i t i o n a l e n v i r o n m e n t. R o u n d i n g Two t h o u s a n d g r a i n s i n v e r y f i n e - g r a i n e d s a n d s t o n e and s i l t s t o n e w e r e m e a s u r e d t o d e t e r m i n e r o u n d i n g q u a l i t i e s . A l l g r a i n s w e r e c o m p a r e d v i s u a l l y w i t h P o w e r s ' ( 1 9 5 3 ) g r a in r o u n d i n g s c a l e . r e s u l t s a r e i n f i g u r e s a n d 1 4. S u b a n g u l a r , s u b r o u n d e d a n d a n g u l a r g r a i n s a r e t h e most a b u n d a n t t y p e s o b s e r v e d . Very f i n e - g r a i n e d s a n d s t o n e g r a i ns a r e s l i g h t l y b e t t e r r o u n d e d t h a n g r a i n s i n s i l t s t o n e , m a i n ly b e c a u s e of more r o u n d e d a n d l e s s v e r y a n g u l a r and a n g u l ar g r a i n s i n v e r y f i n e - g r a i n e d s a n d s t o n e . t he sorted. 11 as moderately sorted. and 9 as well sorted. Figure 11 illustrates a well sorted siltstone. In general. very fine-grained sandstone is slightly better sorted than coarse-or-medium-grained siltstone. However. the difference in sorting characteristics may be due to a classification prejudice and not depict any transportational differences. The arbitrary cutoff for sand grains is 1/16 mm. and all material in very fine­grained sandstone less than this size is considered as matrix. Many thin sections of very fine-grained sandstone have abundant material less than 1/16 mm in size, as can be seen in figure 12. When all material coarser than 1/256 mm is considered in all rocks. there is relatively little difference in sorting characteristics between very fine-grained sandstone and siltstone. This suggests that both rock types were deposited under essentially the same physical conditions within the depositional environment. Rounding 24 Two thousand grains in very fine-grained sandstone and siltstone were measured to determine rounding qualities. All grains were compared visually with Powers' (1953) grain rounding scale. The results are shown in figures 13 and 14 . Subangular, subrounded and angular grains are the most abundant types observed. Very fine-grained sandstone grains are slightly better rounded than grains in siltstone. mainly because of more rounded and less very angular and angular grains in very fine-grained sandstone. However, the F i g u r e 1 1 . P h o t o m i c r o g r a p h s h o w i n g t y p i c a l w e l l - s o r t ed s i l t s t o n e w h i c h h a a b u n d a n t r e d p i g m e n t i n o p a q u e g r a i n s, c e m e n t , m a t r i x . P h o t o m i c r o g r a p h d i a m e t e r - 1 . 6 6 P l a n e l i g h t. Figure 11. Photomicrograph showing a typical well-sorted siltstone which ha s abundant red pigment in opaque grains, cement, and matrix. Photomicrograph diameter = 1.66 mm. Plane light. F i g u r e 1 2 . G r a i n - m a t r i x - c e m e n t d i a g r a m f o r t e r r i g e n o us r o c k s i n t h e M o e n k o p i F o r m a t i o n. LEGEND c. Very Fine-Grained Sandstone o Well Sorted Siltstone D Poorly Sorted Siltstone N=31 10 GRAINS 50 90 Figure 12. Grain-matrix-cement diagram for terrigenous rocks in the Moenkopi Formation. 26 40-1 UJ 30 o 01 UJ ^ a. IO T r F i g u r e 1 3 . H i s t o g r a m s h o w i n g r o u n d n e s s o f t e r r i g e n o us g r a i n s i n s i l t s t o n e. 40-i UJ 30- 5 20H o UJ CL KH F i g u r e 1 4 , H i s t o g r a m s h o w i n g r o u n d n e s s of t e r r i g e n o us g r a i n s i n v e r y f i n e - g r a i n e d s a n d s t o n e. 40 IJJ 30 (!) t! z 20 IJJ (.) 0:: IJJ Q. 10 - • - - - Figure 13. Histogram showing roundness of terrigenous grains in siltstone. 40 ,- IJJ 30I - (!) ~ Z 20 IJJ I- (.) 0:: IJJ ,- Q. 10 I I Figure 14. Histogram showing roundness of terrigenous grains in very fine-grained sandstone. 27 2 8 d i f f e r e n c e s i n r o u n d i n g q u a l i t i e s a r e s m a l l e n o u g h t h a t it i s i n t e r p r e t e d t h a t b o t h g r a i n s i z e s w e r e p r o b a b l y t r a n s ­p o r t e d a n d d e p o s i t e d u n d e r much t h e same p h y s i c a l c o n d i t i o n s. T h i s i s s u p p o r t e d by s i m i l a r i t i e s i n r o u n d i n g of g r a i n s, o v e r a l l s o r t i n g a n d m i n e r a l o g y of g r a i n s i n v e r y f i n e ­g r a i n e d s a n d s t o n e a n d s i l t s t o n e. M a t u r i t y T h i n s e c t i o n a n a l y s i s of v e r y f i n e - g r a i n e d s a n d s t o ne a n d s i l t s t o n e p r o v i d e s t h e i n f o r m a t i o n n e e d e d f o r t h e s t u dy o f t e x t u r a l m a t u r i t y . 1 9 6 8 , p , s u g g e s t e d t h at t e x t u r a l m a t u r i t y i s i n d i c a t i v e t h e e f f e c t i v e n e s s t he e n v i r o n m e n t i n w i n n o w i n g , s o r t i n g a n d a b r a d i n g t h e d e t r i t us f u r n i s h e d t o i t . F o l k ' s t e x t u r a l m a t u r i t y c l a s s i f i c a t i on s y s t e m 1 9 6 8 , p , 1 0 2 - 1 0 3 ) i s u s e d h e r e . s y s t e m is b a s e d t h e a m o u n t d e t r i t a l c l a y p r e s e n t a n d t h e s o r t i ng a n d r o u n d i n g g r a i n s . T h i s s y s t e m i n t e n d e d t o an i n d e x o r i g i n a l s e d i m e n t o l o g i c a l c o n d i t i o n s , t h e r e f o re a l l e f f e c t s d i a g e n e s i s a r e i g n o r e d. A p p r o x i m a t e l y 64 p e r c e n t of t h e t e r r i g e n o u s r o c k s a re c l a s s i f i e d a s s u b m a t u r e . They h a v e l e s s t h a n 5 p e r c e n t d e t r i t a l c l a y , g r a i n s a r e m o d e r a t e l y s o r t e d o r w o r s e , and g r a i n s h a v e i n t e r m e d i a t e g r a d e s of r o u n d i n g . About 2? p e r c e n t of t h e r o c k s s t u d i e d a r e i m m a t u r e , h a v i n g more t h an 5 p e r c e n t d e t r i t a l c l a y a n d p o o r s o r t i n g . M a t u r e r o c k s, r e p r e s e n t i n g 6 p e r c e n t , h a v e l e s s t h a n 5 p e r c e n t d e t r i t al c l a y , h a v e w e l l s o r t e d g r a i n s , and s u b a n g u l a r a n d a n g u l ar g r a i n s a r e a b u n d a n t . s a m p l e , r e p r e s e n t i n g 3 p e r c e n t , 28 differences in rounding qualities are small enough that it is interpreted that both grain sizes were probably trans­ported and deposited under much the same physical conditions. This is supported by similarities in rounding of grains, overall sorting and mineralogy of grains in very fine­grained sandstone and siltstone. Maturity Thin section analysis of very fine-grained sandstone and siltstone provides the information needed for the study of textural maturity. Folk ( 1968, p. 102) suggested that textural maturity is indicative of the effectiveness of the environment in winnowing, sorting and abrading the detritus furnished to it. Folk's textural maturity classification system ( 1968, p. 102-10) is used here. His system is based on the amount of detrital clay present and the sorting and rounding of grains. This system was intended to be an index of original sedimentological conditions, therefore all effects of diagenesis are ignored. Approximately 64 percent of the terrigenous rocks are classified as submature. They have less than 5 percent detrital clay, grains are moderately sorted or worse, and grains have intermediate grades of rounding. About 27 percent of the rocks studied are immature, having more than 5 percent detrital clay and poor sorting. Mature rocks, representing 6 percent, have less than 5 percent detrital clay, have well sorted .grains, and subangular and angular grains are abundant. One sample, representing) percent, i s c l a s s i f i e d a s s u p e r m a t u r e . T h i s s a m p l e h a s v i r t u a l ly no d e t r i t a l c l a y a n d h a s w e l l r o u n d e d a n d s o r t e d g r a i ns ( f i g u r e 1 5 ). T e x t u r a l i n v e r s i o n s w e r e n o t o b s e r v e d i n a n y r o c k s. S e v e r a l r o c k s d i s p l a y p a t c h e s o f l i m e mud ( m i c r i t e ) l i n i ng f r a c t u r e s i n w e l l - s o r t e d s i l t s t o n e , b u t t h i s was p r o b a b ly b r o u g h t i n t o t h e r o c k s by b u r r o w i n g o r g a n i s m s . However, t h e d e t e r m i n a t i o n of t h e a b u n d a n c e of d e t r i t a l c l a y was d i f f i c u l t b e c a u s e of t h e p r e s e n c e of r e d p i g m e n t i n t he c e m e n t a n d m a t r i x. F i n e n e s s a n d g e n e r a l p o o r s o r t i n g of g r a i n s s u g g e st t h a t l o w e n e r g y l e v e l s w e r e m a i n t a i n e d i n t h e d e p o s i t i o n al e n v i r o n m e n t . D e p o s i t i o n p r o b a b l y o c c u r r e d r a p i d l y , t h us l i m i t i n g t h e a m o u n t of r e w o r k i n g a n d r o u n d i n g of g r a i n s. I t i s b e l i e v e d t h a t m i n o r t e c t o n i c p u l s e s i n t h e s o u r ce a r e a c a u s e d d e t r i t a l m a t e r i a l t o be s u p p l i e d i n g r e a t a m o u n ts a t v a r i o u s t i m e s t o t h e d e p o s i t i o n a l e n v i r o n m e n t . Other c a l m e r t e c t o n i c p e r i o d s c a u s e d l i m i t e d c o n t r i b u t i o n s of d e t r i t u s . T h i s e x p l a n a t i o n h e l p s a c c o u n t f o r t h e t e x t u r al f e a t u r e s o b s e r v e d i n t h i n s e c t i o n s , a n d h e l p s i n t h e u n d e r ­s t a n d i n g o f p r o c e s s e s i n v o l v e d i n d e p o s i t i o n of t h e w i de v a r i e t y of r o c k t y p e s f r o m v e r y f i n e - g r a i n e d s a n d s t o n e to m i c r i t e i n t h e M o e n k o p i F o r m a t i o n. is classified as supermature. This sample has virtually no detrital clay and has well rounded and sorted grains {figure 15). Textural inversions were not observed in any rocks. Several rocks display patches of lime mud (micrite) lining fractures in well-sorted siltstone, but this was probably brought into the rocks by burrowing organisms. However, the determination of the abundance of detrital clay was difficult because of the presence of red pigment in the cement and matrix. Fineness and general poor sorting of grains suggest that low energy levels were maintained in the depositional environment. Deposition probably occurred rapidly, thus limiting the amount of reworking and rounding of grains. It is believed that minor tectonic pulses in the source 29 area caused detrital material to be supplied in great amounts at various times to the depositional environment. Other calmer tectonic periods caused limited contributions of detritus. This explanation helps account for the textural fea~ures observed in thin sections, and helps in the under­standing of processes involved in deposition of the wide variety of rock types from very fine-grained sandstone to micrite in the Moenkopi Formation. F i g u r e 1 5 . P h o t o m i c r o g r a p h s h o w i n g t h e o n l y t i g h t l y - p a c k e d r o c k s e e n i n t h e M o e n k o p i F o r m a t i o n . T h i s r o ck c l a s s f i e d s u p e r m a t u r e . P h o t o m i c r o g r a p h d i a m e t e r = 0 . 4 4 mm. G r o s s e d n i c o l s Figure 15. Photomicrograph showing the only tightly­packed rock seen in the Moenkopi Formation . This rock is class i fied as supermature . Photomicrograph diameter = 0.44 mm . Crossed nicols . 30 ALLOCHEMICAL CONSTITUENTS A l l o c h e m i c a l c o n s t i t u e n t s a r e t h o s e c o n s t i t u e n t s t h at h a v e f o r m e d by c h e m i c a l o r b i o l o g i c a l p r e c i p i t a t i o n w i t h in t h e b a s i n o f d e p o s i t i o n t h a t h a v e b e e n o r g a n i z e d i n t o a g g r e ­g a t e b o d i e s a n d h a v e u n d e r g o n e l i m i t e d t r a n s p o r t a t i o n to t h e d e p o s i t i o n a l s i t e F o l k , 1 9 6 8 , p . 1 5 3 ) . t y p e s of a l l o c h e m s a r e r e c o g n i z e d * o o l i t e s , p e l l e t s , f o s s i l s and i n t r a c l a s t s . O o l i t e s O o l i t e s a r e r o u n d t o o v a l - s h a p e d b o d i e s t h a t h a ve e i t h e r c o n c e n t r i c o r r a d i a l i n t e r n a l s t r u c t u r e ( f i g u r e 1 6 ). D i a m e t e r s of o o l i t e s r a n g e f r o m a b o u t 0 . 1 0 t o 0 . 3 0 mm w i th u n i f o r m s i z e s w i t h i n a g i v e n r o c k . Most o o l i t e s d i s p l ay 2 c o n c e n t r i c l a y e r s of m i c r o c r y s t a l l i n e c a r b o n a t e o r s p a r ry c a r b o n a t e w i t h t e r r i g e n o u s g r a i n s a s n u c l e i i . In some c a s es m i c r o c r y s t a l l i n e c a r b o n a t e a c t s a s n u c l e i i and c o a r s er c a r b o n a t e l a y e r s r i m t h e m . Some p o r t i o n s of i n t e r n al s t r u c t u r e h a v e b e e n d e s t r o y e d , b u t n o t e n o u g h t o h i n d er t h e i r i d e n t i f i c a t i o n a s o o l i t e s . R a r e l y , o o l i t e s a r e rimmed w i t h t h i n r e d h e m a t i t e c o a t i n g s. The o r i g i n of o o l i t e s i s p r o b a b l y t h e r e s u l t of i n o r g an i c p r e c i p i t a t i o n s i m i l a r t o t h e p r o c e s s d e s c r i b e d by C a y e ux ( I 9 7 O , p , 2 3 4 - 2 3 5 ) . An o r g a n i c o r i g i n i s n o t s u p p o r t ed ALLOCHEMICAL CONSTITUENTS A1lochemical constituents are those constituents that have formed by chemical or biological precipitation within the basin of deposition that have been organized into aggre­gate bodies and have undergone limited transportation to the depositional site ( Folk, 1968, p. 153). Four types of allochems are recognized. oolites, pellets, fossils and intraclasts. Oolites Oolites are round to oval-shaped bodies that have either concentric or radial internal structure (figure 16). Diameters of oolites range from about 0.10 to 0.30 mm with uniform sizes within a given rock. Most oolites display 2 concentric layers of microcrystalline carbonate or sparry carbonate with terrigenous grains as nucleii. In some cases, microcrystalline carbonate acts as nucleii and coarser carbonate layers rim them. Some portions of internal structure have been destroyed, but not enough to hinder their identification as oolites. Rarely. oolites are rimmed with thin red hematite coatings. The origin of oolites is probably the result of inorgan­ic precipitation similar to the process described by Cayeux (1970, p. 234-235). An organic origin is not supported F i g u r e 1 6 . P h o t o m i c r o g r a p h s p a r r y o o l i t e s , h a v i ng m i c r i t i c c o r e s . c a s e s t h e i n t e r n a l s t r u c t u r e has b e e n p a r t i a l l y d e s t r o y e d . P h o t o m i c r o g r a p h d i a m e t e r = 0 . 44 mm. C r o s s e d n i c o l s. F i g u r e 1 7 . P h o t o m i c r o g r a p h m i c r i t i c i n t r a c l a s t s in i n t r a m i c r i t e . h a v e b e e n d e p o s i t e d a s o o l i t e s or p e l l e t s . P h o t o m i c r o g r a p h d i a m e t e r = 1 . 6 6 mm. P l a n e l i g h t. Figure 16. Photomicrograph of sparry oolites, some having micritic cores. In some cases the internal structure has been partially destroyed . Photomicrograph diameter = 0.44 mm. Crossed nicols. Figure 17. Photomicrograph of micritic intraclasts in intramicrite. Some may have been deposited as oolites or pellets. Photomicrograph diameter = 1 . 66 mm . Plane light. 32 33 b e c a u s e no o r g a n i c m a t t e r was s e e n w i t h i n o o l i t e s u n d er c o n v e r g e n t l i g h t. P e l l e t s P e l l e t s a r e r a r e i n t h e M o e n k o p i F o r m a t i o n a n d a re o b s e r v e d i n o n l y 2 t h i n s e c t i o n s . They o c c u r a s r o u n d ed a g g r e g a t e s of m i c r o c r y s t a l l i n e c a r b o n a t e , h a v i n g no i n t e r n al s t r u c t u r e and no t e r r i g e n o u s g r a i n s a s n u c l e i i . They a re d a r k b r o w n u n d e r c o n v e r g e n t l i g h t , i n d i c a t i n g t h e a b u n d a n ce o f o r g a n i c m a t e r i a l w i t h i n t h e m . P e l l e t s a r e u s u a l l y s m a l l ­e r t h a n o o l i t e s i n t h e same r o c k s . O c c a s i o n a l l y t h e y h a ve b e e n r e c r y s t a l l i z e d t o s p a r r y a g g r e g a t e s , t h e a b u n d a n ce o f o r g a n i c m a t t e r w i t h i n t h e m i s t h e o n l y m e a n s of r e c o g n i t i o n . S e v e r a l w o r k e r s ( B e a l e s , 1965f p . 51J F o l k , 1 9 6 2 a, p . 65$ C a y e u x , 1 9 7 0 , p . 250) h a v e s u g g e s t e d t h a t p e l l e ts a r e t h e r e s u l t of o r g a n i c o r i n o r g a n i c a g g l u t i n a t i o n f o l l o w ed b y t r a n s p o r t a t i o n a n d a b r a s i o n . The o r i g i n o f p e l l e t s in t h e M o e n k o p i F o r m a t i o n i s p r o b a b l y s i m i l a r t o t h a t d e s c r i b ed b y F o l k ( 1 9 6 2 b , p . 5 ^ 7 ) i t h a t i s , t h e y may r e p r e s e n t f e c al m a t e r i a l a s shown b y t h e i r h i g h o r g a n i c c o n t e n t a n d u n i f o rm s i z e . F o s s i l s T h e v i r t u a l a b s e n c e of f o s s i l s i n t h e M o e n k o p i is c o n f i r m e d b e c a u s e o n l y t h i n s e c t i o n o u t c o n t a i n ed a n y f o s s i l m a t e r i a l . f o s s i l m a t e r i a l i s d i f f i c u l t to i d e n t i f y b e c a u s e i t i s b a d l y w o r n , p o o r l y p r e s e r v e d , is because no organic matter was seen within oolites under convergent light. Pellets 33 Pellets are rare in the Moenkopi Formation and are observed in only 2 thin sections. They occur as rounded aggregates of microcrystalline carbonate, having no internal structure and no terrigenous grains as nucleii. They are dark brown under convergent light, indicating the abundance of organic material within them. Pellets are usually small­er than oolites in the same rocks. Occasionally they have been recrystallized to sparry aggregates, and the abundance of organic matter within them is the only means of recognition. Several workers (Beales, 1965, p. 51; Folk, 1962a, p. 65; Cayeux, 1970, p. 250) have suggested that pellets are the result of organic or inorganic agglutination followed by transportation and abrasion. The origin of pellets in the Moenkopi Formation is probably similar to that described by Folk (l962b, p. 547); that is, they may represent fecal material as shown by their high organic content and uniform size. Fossils The virtual absence of fossils in the Moenkopi is confirmed because only 1 thin section out of 100 contained any fossil material. The fossil material is difficult to identify because it is badly worn, poorly preserved, and is 3k p r e s e n t u s u a l l y a s f r a g m e n t s . Only 2 g r o u p s of f o s s i ls w e r e i d e n t i f i e d i n t h i s t h i n s e c t i o n , and a r e d e s c r i b ed b e l o w . A l g a e . - C o m p a r i s o n of a l g a l f r a g m e n t s t o t h o se d i s c u s s e d by R e z a k ( 1 9 5 9 ) r e v e a l t h a t t h e f a m i ly D a s y c l a d a c e a e i s r e p r e s e n t e d . I d e n t i f i c a t i o n t o g e n u s l e v el i s d u f f i c u l t , b u t s i m i l a r i t i e s i n s h a p e a n d i n t e r n al t e x t u r e i n d i c a t e t h a t E p i m a s t o p o r a s p . a n d M i z z i a s p . a re p r e s e n t R e z a k , 1959» f i g u r e s 3 , 4 ) . E p i m a s t o p o r a is c h a r a c t e r i z e d o v a l , o b l o n g t o r o u n d e d b o d i e s m i c r o - c r y s t a l l i n e c a r b o n a t e w i t h s m a l l l o c a l a r e a s m i c r o s p ar w i t h i n t h e m . d i a m e t e r s r a n g e f r o m 0 . 1 4 t o 0 , 2 5 f r a g m e n t d i s p l a y e d t h e s e c h a r a c t e r i s t i c s p l u s s e g m e n t ­a t i o n w i t h i n t h e b o d i e s . M i z z i a h a s r o u n d t o o v a l b o d i es o f l i g h t - t e x t u r e d m i c r o c r y s t a l l i n e c a r b o n a t e a n d n u m e r o us s m a l l i n c l u s i o n s d a r k - t e x t u r e d m i c r i t e . b o dy d i a m e t e r s r a n g e f r o m a b o u t 0 , 1 2 t o 0 . 3 0 A c c o r d i n g to R e z a k 1 9 5 9 . p . 9 ) » r o c k s c o n t a i n i n g f o s s i l s t h e f a m i ly D a s y c l a d a c e a e p r o b a b l y w e r e d e p o s i t e d i n v e r y s h a l l ow m a r i n e w a t e r s. G a s t r o p o d s . - O n e p o o r l y p r e s e r v e d g a s t r o p o d was s e en i n t h i n s e c t i o n b u t i d e n t i f i c a t i o n t o g e n u s l e v e l c o u l d n ot b e a c c o m p l i s h e d . The f o s s i l was b a d l y w o r n f r om t r a n s p o r t a - t i o n a l o r d i a g e n e t i c p r o c e s s e s , b u t t h e o v e r a l l o u t l i n e was d i s c e r n a b l e . present usually as fragments. Only 2 groups of fossils were identified in this thin section. and are described b"elow. 34 Algae.--Comparison of algal fragments to those discussed by Rezak (1959) reveal that the family Dasycladaceae is represented. Identification to genus level is dufficult. but similarities in shape and internal texture indicate that Epimastopora £2. and Mizzia ~. are present ( Rezak. 1959. figures 3.4). Epimastopora is characterized by oval, oblong to rounded bodies of micro­crystalline carbonate with small local areas of microspar within them. Body diameters range from 0.14 to 0.25 mm. One fragment displayed these characteristics plus segment­ation within the bodies. Mizzia has round to oval bodies of light-textured microcrystalline carbonate and numerous small inclusions of dark-textured micrite. Long body diameters range from about 0.12 to 0.30 mm. According to Rezak ( 1959. p. 9). rocks containing fossils of the family Dasycladaceae probably were deposited in very shallow marine waters. Gastropods.--One poorly preserved gastropod was seen in thin section but identification to genus level could not be accomplished. The fossil was badly worn. from transporta­tional or diagenetic processes. but the overall outline was discernable. I n t r a c l a s t s 3 5 I n t h i s s t u d y an i n t r a c l a s t i s c o n s i d e r e d t o be any a l l o c h e m i c a l c o n s t i t u e n t t h a t f o r one r e a s o n o r a n o t h er c o u l d n o t be i d e n t i f i e d a s a n o o l i t e , p e l l e t o r f o s s i l. T h i s u s e o f i n t r a c l a s t a s a g e n e r a l t e r m was n e c e s s a ry b e c a u s e r e c r y s t a l l i z a t i o n o r r e p l a c e m e n t o b s c u r e d or a l t e r e d t h e i n t e r n a l s t r u c t u r e of many a l l o c h e m s , m a k i ng p r e c i s e i d e n t i f i c a t i o n d i f f i c u l t . i n t r a c l a s t s " h a v e b e e n d e p o s i t e d o r i g i n a l l y a s o o l i t e s o r p e l l e t s , b ut a r e c l a s s i f i e d a s i n t r a c l a s t s b e c a u s e d e s t r u c t i o n of p a r t s o r a l l t h e i n t e r n a l s t r u c t u r e a n d p o s s i b l e o r g a n ic m a t e r i a l . T h i s u s e i n t r a c l a s t i s j u s t i f i e d p r o b a b le s i m i l a r o r i g i n a l l a l l o c h e m s , e x c e p t f o s s i l s , t he s i m i l a r l y s m a l l a m o u n t s t r a n s p o r t a t i o n i n v o l v e d b e f o re d e p o s i t i o n . M i c r i t i c i n t r a c l a s t s a r e common i n m i c r i t e , m i c r o s p a r i te a n d some s p a r i t e i n t h e M o e n k o p i F o r m a t i o n . These i n t r a c l a s t s o r i g i n a t e d f r om t h e e r o s i o n of i n c o m p l e t e ly c o n s o l i d a t e d s e d i m e n t i n t h e s h a l l o w m a r i n e d e p o s i t i o n al e n v i r o n m e n t . L e n g t h s i n t r a c l a s t s r a n g e f r om 0 . 0 8 t o 1.4 w i t h q u a r t z o r f e l d s p a r g r a i n s a s n u c l e i i a r o u n d w h i ch t h e i n t r a c l a s t s w e r e f o r m e d . i n t r a c l a s t s a p p a r e n t ly f o r m e d i n p l a c e b e c a u s e s e l e c t i v e r e c r y s t a l l i z a t i o n of s u r r o u n d i n g m i c r o c r y s t a l l i n e c a r b o n a t e t o m i c r o s p a r , l e a v i ng p a t c h e s m i c r i t e u n a f f e c t e d . O t h e r i n t r a c l a s t s a r e p r e s e nt i n some f r a c t u r e s and b u r r o w s i n r o c k s w h e r e l a t e r i n f i l l i ng b y s p a r r y c a r b o n a t e o r s i l i c a o c c u r r e d . i n t r a c l a s ts Intraclasts In this study an intraclast i s considered to be any allochemical constituent that for one reason or another could not be identified as an oolite, pellet or fossil. 35 This use of intraclast as a general term was necessary because recrystallization or replacement obscured or altered the internal structure of many allochems, making precise identification difficult. Some " intraclasts" may have been deposited originally as oolites or pellets, but are classified as intraclasts because of dest~uction of parts or all of the internal structure and possible organic material. This use of intraclast is justified by a probabl~ similar origin of all allochems, except fossils, and the similarly small amounts of transportation involved before deposition. Micritic intraclasts are common in micrite, microsparite and some sparite in the Moenkopi Formation. These intraclasts originated from the erosion of incompletely consolidated sediment in the shallow marine depositional environment. Lengths of intraclasts range from 0.08 to 1.4 mm with quartz or feldspar grains as nucleii around which the intraclasts were formed. Some intraclasts apparently formed in place because of selective recrystallization of surrounding microcrystalline carbonate to microspar, leaving patches of micrite unaffected. Other intraclasts are present in some fractures and burrows in rocks where later infilling by sparry carbonate or silica occurred. These intraclasts a r e b e l i e v e d t o h a v e b e e n b r o u g h t i n t o r o c k s by b u r r o w i ng o r g a n i s m s b e c a u s e t h e y l i n e a n d c o a t c o n d u i t w a l l s a n d a re r a r e l y f o u n d e l s e w h e r e. O t h e r m i c r i t i c a n d s p a r r y i n t r a c l a s t s a r e a b u n d a n t in many c a r b o n a t e a n d t e r r i g e n o u s r o c k s of t h e M o e n k o p i. T h e s e may h a v e b e e n d e p o s i t e d o r i g i n a l l y a s p e l l e t s or o o l i t e s ( f i g u r e 17) b e c a u s e of t h e i r r o u n d t o o v a l s h a p es a n d e x c e l l e n t s o r t i n g w i t h i n a g i v e n r o c k . However, most r e m n a n t s of i n t e r n a l s t r u c t u r e o r o r g a n i c m a t e r i a l h a v e b e en d e s t r o y e d , so t h e y a r e c l a s s i f i e d a s i n t r a c l a s t s . D i a m e t e rs o f t h e s e i n t r a c l a s t s r a n g e f r o m 0 . 1 0 t o 0 . 2 0 mm w i t h q u a r tz o r f e l d s p a r g r a i n o c c a s i o n a l l y a s n u c l e i i . 36 are believed to have been brought into rocks by burrowing organisms because they line and coat conduit walls and are rarely found elsewhere. Other micritic and sparry intraclasts are abundant in many carbonate and terrigenous rocks of the Moenkopi. These may have been deposited originally as pellets or oalites (figure 17) because of their round to oval shapes and excellent sorting within a given rock. However, most remnants of internal structure or organic material have been destroyed, so they are classified as intraclasts. Diameters of these intraclasts range from 0.10 to 0.20 mm with quartz or feldspar grain occasionally as nucleii. ORTHOCHEMICAL CONSTITUENTS O r t h o c h e m i c a l c o n s t i t u e n t s i n c l u d e a l l p r e c i p i t a t es f o r m e d w i t h i n t h e b a s i n of d e p o s i t i o n o r w i t h i n t h e r o ck i t s e l f t h a t show l i t t l e t o no e v i d e n c e of t r a n s p o r t a t i on o r a g g r e g a t i o n i n t o c o m p l e x e n t i t i e s ( W i l l i a m s o n , 1 9 7 2 , / p . 21? F o l k , 1 9 6 8 , p . 1 5 3 ) . T h r e e o r t h o c h e m i c a l c o n s t i t u e n ts a r e v o l u m e t r i c a l l y a b u n d a n t ; m i c r o c r y s t a l l i n e c a r b o n a t e, s p a r r y c a r b o n a t e a n d a u t h i g e n i c s i l i c a. M i c r o c r y s t a l l i n e C a r b o n a te M i c r o c r y s t a l l i n e c a r b o n a t e i n c l u d e s a l l c a l c i t e and d o l o m i t e w i t h g r a i n s i z e s l e s s t h a n 20 m i c r o n s . The t e rm m i c r i t e i n c l u d e s g r a i n s i z e s l e s s t h a n 4 m i c r o n s , a n d m i c r o - s p a r r a n g e s f r o m 4 t o 20 m i c r o n s i n d i a m e t e r . The a d j e c t i v es H c a l c M a n d " d o l o " a r e u s e d i n d e s c r i b i n g t h e c h e m i c al c o m p o s i t i o n s g r a i n s t h e m s e l v e s. M i c r o c r y s t a l l i n e c a r b o n a t e i s c o m p o s e d of v a r y i ng a m o u n t s o f c a l c m i c r i t e a n d d o l o m i c r i t e , w i t h d o l o m i c r i te u s u a l l y more a b u n d a n t . T h e r e a p p e a r s t o be no s i g n i f i c a nt d i f f e r e n c e i n g r a i n s i z e b e t w e e n c a l c m i c r i t e and d o l o m i c r i te a l t h o u g h no q u a n t i t a t i v e s t u d y was m a d e . In t h i n s e c t i o n, m i c r i t e i s s u b t r a n s l u s c e n t a n d r a n g e s f r o m l i g h t t o d a rk g r e e n i n p l a n e l i g h t . M i c r o s p a r i s more c o a r s e l y c r y s t a l l i n e, t r a n s l u s c e n t , and commonly g r a d a t i o n a l w i t h m i c r i t e i n t h in ORTHOCHEMICAL CONSTITUENTS Orthochemical constituents include all precipitates formed within the basin of deposition or within the rock itself that show little to no evidence of transportation or aggregation into complex entities (Williamson, 1972, / p. 211 Folk, 1968, p. 153). Three orthochemical constituents are volumetrically abundantI microcrystalline carbonate, sparry carbonate and authigenic silica. Microcrystalline Carbonate Microcrystalline carbonate includes all calcite and dolomite with grain sizes less than 20 microns. The term micrite includes grain sizes less than 4 microns, and micro­spar ranges from 4 to 20 microns in diameter. The adjectives "calc" and "dolo" are used in describing the chemical compositions of grains themselves. Microcrystalline carbonate is composed of varying amounts of calcmicrite and dolomicrite, with dolomicrite usually more abundant. There appears to be no significant difference in grain size between calcmicrite and dolomicrite although no quantitative study was made. In thin section, micrite is subtransluscent and ranges from light to dark green in plane light. Microspar is more coarsely crystalline, transluscent, and commonly gradational with micrite in thin 38 s e c t i o n s . M i c r o s p a r i s c o n s i d e r e d t o be a n e o m o r p h ic p r o d u c t of m i c r i t e w h e r e r e c r y s t a l l i z a t i o n o r r e p l a c e m e nt h a s o c c u r r e d . E v i d e n c e s u p p o r t i n g t h i s i n t e r p r e t a t i o n in t h i n s e c t i o n s i n c l u d e : (1) p o c k e t s of m i c r o s p a r c o m p l e t e ly s u r r o u n d e d by m i c r i t e , and ( 2 ) a g r a d u a l g r a d a t i o n i n g r a in s i z e f r o m m i c r i t e t o m i c r o s p a r i n s e v e r a l a r e a s of t h in s e c t i o n s . Most s l i d e s c o n t a i n i n g m i c r o c r y s t a l l i n e c a r b o n a te s h ow t h e s e c h a n g e s i n g r a i n s i z e a t l e a s t l o c a l l y. S p a r r y C a r b o n a te S p a r r y c a r b o n a t e i s a n o r t h o c h e m i c a l c o n s t i t u e n t w i th g r a i n s i z e s g r e a t e r t h a n 20 m i c r o n s . Spar i s by f a r t he m o s t a b u n d a n t o r t h o c h e m i c a l c o n s t i t u e n t i n t h e M o e n k o p i. I n t h i n s e c t i o n s , s p a r i s t r a n s l u s c e n t and f o r m s m o s a i cs o f s u b e q u a n t a n d e q u a n t , r a n d o m l y a n d u n i f o r m l y o p t i c a l ly o r i e n t a t e d c a r b o n a t e c r y s t a l s . G r a i n s i z e s r a n g e f r o m 0 , 02 t o a b o u t 0 . 3 5 mm. Many l o c a l i z e d g r o u p s of s p a r c r y s t a ls h a v e a l l o c h e m s and t e r r i g e n o u s g r a i n s f l o a t i n g w i t h i n t h e m, a n d a l l s p a r c r y s t a l s h a v e t h e same o p t i c a l o r i e n t a t i on f i g u r e 1 8 ) . O t h e r g r o u p s s p a r c r y s t a l s c o n t a i n r a n d o m ly o r i e n t a t e d s p a r r y c a r b o n a t e c r y s t a l s. P u b l i s h e d c r i t e r i a w e r e u s e d t o d e t e r m i n e t h e o r i g in o f s p a r r y c a r b o n a t e ( F o l k , 1965» p . 4 0 - 4 5 ; S t a u f f e r , 1 9 6 2, p . 3 6 1 - 3 6 3 ; H a r b a u g h , I 9 6 I , p . 9 6 ; D a p p l e s , 1971» p . 1 9 7 ). F r om t h i n s e c t i o n a n a l y s i s , i t i s a p p a r e n t t h a t m o s t s p a r ry c a r b o n a t e was p r e c i p i t a t e d d i r e c t l y i n t o o p e n v o i d s . C r y s t a l l i z a t i o n s p a r l o c a l l y f o r c e f u l e n o u g h t o p u sh 38 ·sections. Microspar is considered to be a neomorphic product of micrite where recrystallization or replacement has occurred. Evidence supporting this interpretation in thin sections include. (1) pockets of microspar completely surrounded by micrite, and (2) a gradual gradation in grain size from micrite to microspar in several areas of thin sections. Most slides containing microcrystalline carbonate show these changes in grain size at least locally. Sparry Carbonate Sparry carbonate is an orthochemical constituent with grain sizes greater than 20 microns. Spar is by far the most abundant orthochemical constituent in the Moenkopi. In thin sections, spar is transluscent and forms mosaics of subequant and equant, randomly and uniformly optically orientated carbonate crystals. Grain sizes range from 0.02 to about 0.35 mm. Many localized groups of spar crystals have allochems and terrigenous grains floating within them, and all spar crystals have the same optical .orientation ( figure 18). Other groups of spar crystals contain randomly orientated sparry carbonate crystals. Published criteria were used to determine the origin of sparry carbonate (Folk, 1965, p. 40-45; Stauffer, 1962, p. 361-3631 Harbaugh, 1961, p. 96, Dapples, 1971, p. 197). From thin section analysis, it is apparent that most sparry carbonate was precipitated directly into open voids. Crystallization of spar was locally forceful enough to push F i g u r e 1 8 . P h o t o m i c r o g r a p h i l l u s t r a t i n g g r o u p s p a r ry c a r b o n a t e c r y s t a l s i n w h i c h c r y s t a l s h a v e u n i f o r m o p t i c al o r i e n t a t i o n . P h o t o m i c r o g r a p h d i a m e t e r = 1 . 6 6 mm. C r o s s ed n i c o l s . F i g u r e 1 9 . P h o t o m i c r o g a p h d i s p l a y i n g e v i d e n c e t h a t r e c r y s ­t a l l i z a t i o n m i c r i t e t o m i c r o s p a r h a s o c c u r r e d. P h o t o m i c r o g r a p h d i a m e t e r = 1 . 6 6 mm. P l a n e l i g h t. Figure 18. Photomicrograph illustrating a group of sparry carbonate crystals in which crystals have uniform optical orientation. Photomicrograph di a~eter = 1 . 66 mm . Crossed nicols. 39 Figure 19. Photomicrog r aph displaying evidence that recrys­tallization of micrite to microspar has occurred. Photomicrograph diameter 1. 66 mm . Plane light. 40 a l l o c h e m s and t e r r i g e n o u s g r a i n s a p a r t d u r i n g e a r l y d i a g e n e s i s, c r e a t i n g c e m e n t - s u p p o r t e d r o c k s . L a t e r , n e o m o r p h i s m of l o c a l g r o u p s of s p a r r y c a r b o n a t e c r y s t a l s o c c u r r e d , c a u s i ng r e o r i e n t a t i o n of c r y s t a l s w i t h i n t h e s e g r o u p s t o o b t a in u n i f o r m o p t i c a l e x t i n c t i o n . No g r a i n g r o w t h of s p a r c r y s t a ls n e e d h a v e o c c u r r e d d u r i n g t h i s p r o c e s s . E v i d e n c e s u p p o r t i ng t h i s i n t e r p r e t e d p r o c e s s i s a s f o l l o w s. 1 . T h e r e a r e many a r e a s o f a b u n d a n t s p a r r y c a r b o n a te w i t h a l l o c h e m s a n d t e r r i g e n o u s g r a i n s f l o a t i n g w i t h i n t h e m, a l o n g w i t h l o c a l a r e a s of t i g h t l y - p a c k e d g r a i n s w h e r e l i t t l e s p a r r y c a r b o n a t e i s p r e s e n t . T h i s s u g g e s t s t h a t c r y s t a l l i z a ­t i o n of s p a r r y c a r b o n a t e was l o c a l l y f o r c e f u l e n o u g h to p u s h g r a i n s a p a r t . V/here s p a r i s n o t a b u n d a n t , g r a i n s a re t i g h t l y - p a c k e d . 2 . Many l o c a l i z e d g r o u p s of s p a r r y c a r b o n a t e c r y s t a ls c o n t a i n c r y s t a l s w i t h t h e same o p t i c a l o r i e n t a t i o n a n d h a ve s i m i l a r f e a t u r e s d e s c r i b e d i n ( 1 ) a b o v e . T h i s s u g g e s ts t h a t r e o r i e n t a t i o n of o r i g i n a l s p a r r y c a r b o n a t e c r y s t a ls y i e l d i n g u n i f o r m o p t i c a l o r i e n t a t i o n h a s t a k e n p l a c e, 3 . O t h e r l o c a l i z e d a r e a s of s p a r r y c a r b o n a t e a re p r e s e n t i n w h i c h c a r b o n a t e c r y s t a l s h a v e r a n d o m o p t i c al o r i e n t a t i o n s . T h i s i n d i c a t e s t h a t c r y s t a l r e o r i e n t a t i on p r o c e s s e s a c t e d s e l e c t i v e l y a n d d i d n o t a f f e c t t h e se p a r t i c u l a r c r y s t a l s t o o b t a i n u n i f o r m o p t i c a l o r i e n t a t i o n. O t h e r l e s s c o m p l i c a t e d e x a m p l e s show t h a t r e c r y s t a l l i z a ­t i o n m i c r o s p a r t o s p a r h a s o c c u r r e d f i g u r e 1 9 ). G r a d a t i o n a l c h a n g e s i n g r a i n s i z e f r o m m i c r o s p a r t o s p a r a re 40 allochems and terrigenous grains apart during early diagenesis, creating cement-supported rocks. Later, neomorphism of local groups of sparry carbonate crystals occu~red, causing reorientation of crystals within these groups to obtain uniform optical extinction. No grain growth of spar crystals need have occurred during this process. Evidence supporting this interpreted process is as follows. 1. There are many areas of abundant sparry carbonate with allochems and terrigenous grains floating within them, along with local areas of tightly-packed grains where little sparry carbonate is present. This suggests that crystalliza­tion of sparry carbonate was locally forceful enough to push grains apart. Where spar is not abundant, grains are tightly-packed. 2. Many localized groups of sparry carbonate crystals contain crystals with the same optical orientation and have similar features described in (1) above. This suggests that reorientation of original sparry carbonate crystals yielding uniform optical orientation has taken place. 3. Other localized areas of sparry carbonate are present in which carbonate crystals have random optical orientations. This indicates that crystal reorientation processes acted selectively and did not affect these particular crystals to obtain uniform optical orientation. Other less complicated examples show that recrystalliza­tion of microspar to spar flas occurred ( figure 19). Gradational changes in grain size from microspar to spar are 4 1 d i a g n o s t i c of t h i s e v e n t . A l s o , l o c a l a r e a s of s p a r a re c o m p l e t e l y s u r r o u n d e d by m i c r o s p a r . Many o o l i t e s and i n t r a c l a s t s h a v e b e e n r e c r y s t a l l i z e d o r r e p l a c e d by s p a r ry c a r b o n a t e i n many M o e n k o p i r o c k s , a n d t h e a b u n d a n c e of b o th m i c r i t i c and s p a r r y a l l o c h e m s i n d i c a t e s t h a t n e o m o r p h i sm a c t e d r a n d o m l y. M i n o r r e p l a c e m e n t of q u a r t z a n d f e l d s p a r g r a i n s by s p a r r y c a r b o n a t e was o b s e r v e d i n o n l y a f e w t h i n s e c t i o n s. T e r r i g e n o u s g r a i n b o u n d a r i e s s h o w i n g t h i s r e p l a c e m e n t a re s l i g h t l y c o r r o d e d , a n d s o m e t i m e s i t i s d i f f i c u l t t o d e t e r ­m i n e w h e