Abstract

In north-central Arizona, the Moenkopi Formation of Triassic age consists of generally unfossiliferous red mudstones, siltstones, gypsum, and sandstones that contain abundant sedimentary structures such as ripple marks, cross-stratification, ripple laminae, salt crystal casts, mud cracks, sole marks, parting lineation and core-and-shell structures. Three informal members were established for this study: the lower member, the lower massive sandstone, and the upper member. The lower member is interpreted to represent deposition in a tidal-flat environment based upon the following: flaser, wavy and lenticular bedding, bimodal distribution of ripple-laminae dips, parallel ripple marks dominant over cuspate ripple marks, gypsum beds and veins, salt crystal casts, and lack of channel deposits. The very fine grained lower massive sandstone can be divided into four units, which were formed by a transgression-regression of the sea. Wavy and ripple laminated beds in unit one were probably deposited in very shallow marine water. Medium-scale wedge-planar and trough sets of cross strata with mean dip directions to the southeast make up unit 2 and indicate megaripples formed by long-shore drift. Unit 3 consists of lenticular, wavy, podshaped beds that were created in water shallower than for unit 2. Continuous, large-scale, low-angle cross strata of uniform thickness and medium-scale wedge-planar and trough sets of cross strata characterize unit 4 and are typical of beach deposits. The significant sedimentary features in the upper member are unimodal distribution of ripple laminae dips, cuspate ripple marks dominant over parallel ripple marks, channel deposits with shallow-trough cross strata, increase in the number and thickness of sandstone and siltstone beds compared with the lower member; and vertebrate bones, tracks, and plant impressions. This suite of features indicates a flood-plain environment which was a continuation of the regression that started during lower massive time. Early in Moenkopi time, north-central Arizona was a tidal flat and sabkha. The sea to the west fluctuated east and west and finally transgressed over the entire area. As the sea regressed, a beach formed, and rivers flowing from the east deposited sediment on a westward-prograding flood plain. In some portions of the region, sabkhas existed for a short time following the regression. At the end of Moenkopi time, the entire area was a flood plain.

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