DEPOSITIONAL ENVIRONMENTS AND SEQUENCES OF THE PERMIAN CEDAR MESA SANDSTONE -- TOPOGRAPHY, VEGETATION AND WATER TABLE INFLUENCES

Across the exposure of the Early Permian(Wolfcampian) Cedar Mesa Sandstone on the Monument Upwarp in Southeastern Utah, eleven 80- 125 m stratigraphic sections on the San Juan River are integrated with subsurface data, and satellite imagery to form a more complete understanding of how the dune field interacted with icehouse eustatic changes and variation in climate. The Cedar Mesa Sandstone can be divided into sequences capped by paleosols. These sequences are subdivided by other surfaces marking incursions of extradunal facies into the dune sea. Sequences in the Cedar Mesa form through an interplay of eolian topography and fresh water availability in the dune sea. Cedar Mesa units can be correlated across 50 km of the 80 km width of the erg and therefore represent true sequences, with unconformities represented by vegetated stabilization surfaces. Complete sequences begin with sand sheets in the downwind ⅔ of the dune sea and dunes in the upwind ⅓ of the dune sea. The sandsheets exhibit paleosols, with abundant insect feeding traces forming halos around rhizoliths. Sand sheets may contain 1 to 3 stacked paleosols marking episodic aggradation and stabilization. Dunes are preserved as migrating large dunes, at least 20 m high or as descending sets migrating down the fronts of larger forms. In the bases of the sequences, extradunal facies form thin layers and lenses and mark floods into the dune field, as shown by paleosols and rare fluvial channel fills. Interdune pond carbonates are evident in the lower portions of the sequences indicating that the water table intersected the surface at times. The upper parts of complete sequences are dunes, exhibiting extensive paleosols in their tops. Preserved topography is evident with vegetated sand sheets onlapping upwind and downwind flanks of the dunes. Possibly resulting from climate change that results in stabilization and an increase in vegetation and decrease in sand flux that preserved each sequence.