The Thaynes formation of the Moenkopi group, North-Central Utah (Thesis and maps)

Very detailed study of the Thaynes Formation in six sections necessitated consideration of subjacent and superjacent strata also, The Moenkopi Group of the study area includes the Dinwwody Formation; Woodside Shale; Lower Member of the Thaynes Formation; a tongue of the Ankareh Shale ("Middle Red"); Upper Member of the Thaynes Formation and the Ankareh Shale. It overlies the Permian Park City and is overlain by the Gartra Formation. The entire sequence is (from the base): Dinwoody Formation-silt-shale and calcareous siltstone; present in three sections only; fauna similar to Thaynes. Woodside Shale-red siltstone and silt-shale; ripple marks and mudcracks common; Early Triassic, Griesbachian and/or Dienerian Stage. Lower Member of the Thaynes Formation-gray siltstone with some biosparite; ammonoids in restricted beds. Anasibirites bed is time datum; very poorly preserved fossils abundant; Smithian Stage. Tongue of Ankareh Shale-dark red and brownish red silt-shale and siltstone; boundaries, time-transgressive; probably lower Spathian. Upper Member of the Thaynes Formation-gray siltstone with shale and limestone; considerable lithologic variation; ""Pentacrinus"" columnals present; bed containing terebratulid brachiopods, and inferred time datum, permit subdivision for facies study; Early-?Middle Triassic; ?Spathian Stag. Ankareh Shale-red siltstone and silt-shale; Early and/or Middle Triassic. Clastic rocks, with silt-grade elastics divided into eight particle grades, are dominant in all sections. Carbonate rocks (mostly biosparite) constitute a relatively small percentage of the Thaynes. Laminae and thin laminae are dominant primary structures. Processes and environment of formation of ""'low-angle, draped cross laminae"" are discussed. . Lithofacies variations in distribution of sizes of clasts, total thickness and clastic-carbonate percentages are discussed for each subdivision for which close probable time equivalence is postulated. Probable correlations of each of the six units with the Dinwoody, Woodside, Thaynes and Ankareh strata in sections in south­ eastern Idaho, northwestern Utah, Nevada, the Uinta Mountains, and the Colorado Plateau are made. Small pelecypods are the dominant faunal elemen. All gastropod fossils, abundant in many beds, are internal molds. Small fragments of echinoderms, also frequently abundant, are crinoid columnals, small echinoid fragments and rare stelleroids. Lyssakid siliceous sponges are present locally. Inarticulate brachiopods (linguloid type) occur in the Lower Member; small articulate terebratulid brachiopods form a marker bed in the Upper Member. Cephalopods occur in restricted horizons in Lower Member; are rare in the Upper Member. Distribution of three vaguely defined faunal assemblages is of minor significance. The Thaynes Formation probably was deposited in a mildly subsiding miogeosyncline. Periods of open-marine transgression were interrupted by regressive intervals. Sediment source was to the east (Uncompahgre uplift area); paleoslope gradually deepened northwesterly. Environment was uniform-only slight shifting in basins occurred; climate, warm; paleocurrents, weak; energy, low. A section on the leading edge of the Strawberry thrust was compared to autochothonous Deer Creek area section. Lithofacies maps and isopach maps indicate that the section has probably been displaced approximately ten to twelve miles. Probably detailed knowledge of restricted areas, such as this study, will in the future be of great aid in contributing to our understanding of a region. However closer horizontal control should be available in order to provide optimum consideration of this much vertical detail.