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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 55 (1971)

Issue: 8. (August)

First Page: 1391

Last Page: 1391

Title: Environments of Deposition of Moenkopi Formation in North-Central Arizona: ABSTRACT

Author(s): Joan Baldwin

Article Type: Meeting abstract


In north-central Arizona, the Moenkopi Formation of Triassic age consists of generally unfossiliferous red mudstone, siltstone, gypsum, and sandstone that contain abundant sedimentary structures, such as ripple marks, cross-stratification, ripple laminae, salt-crystal casts, mudcracks, sole marks, parting lineation, and core-and-shell structures.

Three informal members established for this study were the "lower member," the "lower massive sandstone," and the "upper member."

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 are the suite of sedimentary features that are interpreted to indicate a tidal-flat environment during deposition of the "lower member."

The very fine-grained "lower massive sandstone" can be divided into four subunits, which were formed by a transgression-regression of the sea. Wavy-ripple laminated beds in subunit 1 were probably deposited in very shallow water. Medium-scale wedge planar and trough sets of cross strata with average southeastward dip make up subunit 2, and indicate megaripples formed by longshore drift. Subunit 3 consists of lenticular, wavy, pod-shaped beds that formed in water shallower than that for subunit 2. Continuous, even, large-scale, low-angle cross strata and medium-scale wedge-planar and trough sets of cross strata characterize subunit 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, an increase in the number and thickness of sandstone and siltstone beds compared with the "lower member," plus vertebrate bones, tracks, and plant impressions. This suite of features indicates a floodplain environment.

Early in Moenkopi deposition, north-central Arizona was a tidal flat and sabkha. The sea on 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 floodplain. In the northern, southern, and central parts of the region, sabkhas existed for a time during regression. At the end of Moenkopi deposition, the entire area was a floodplain.

The association of redbeds and evaporites, the absence of fossils in the "lower member" and the "lower massive sandstone," and the paleowind directions indicate that the climate during lower and middle Moenkopi deposition was probably hot and arid. The additional sandstones, the presence of Calamites (?) impressions, and trackways and bones of amphibians in the "upper member" suggest that the climate became more humid at the end of Moenkopi deposition.

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Copyright 1997 American Association of Petroleum Geologists