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.