Abstract

The Dewey Lake Formation (Late Permian?) was studied, using surface and subsurface techniques, to more clearly define its depositional environment and tectonic/paleogeographic setting. The Formation contains numerous wide, shallow channels filled with horizontally laminated siltstone and flanked by laterally thinning “wings” of micro-cross laminated siltstone. This morphology indicates that the Dewey Lake was deposited in a fluvial plain dominated by localized, ephemeral flows. The “wings” represent distally thinning floodplain deposits, formed as the channel filled with sediment and the water overflowed onto the surrounding plain.

The Dewey Lake is generally restricted to the Late Permian; however, the supporting evidence is weak. Lateral thickness variations in the Dewey Lake reflect syndepositional subsidence (i.e., the Delaware Basin was still active), not post-depositional erosion. Ash beds in the lower Quartermaster Formation (lateral equivalent), with ages near the Permo-Triassic boundary, date the initiation of sedimentation, not its duration. These observations suggest that although deposition of Dewey Lake sediments may have begun in the latest Permian, it continued through the Early Triassic.

The silts and fine grained sands comprising the Dewey Lake were derived from an uplift, to the south/southeast, encompassing the Pennsylvanian Ouachita/Marathon thrust belt and fore-deep basins. Triassic uplift of this region is indicated by an erosional unconformity between Late Permian and Cretaceous strata in several fore-deep basins and Triassic conglomerates in the Glass Mountain and Llano Uplift regions interpreted as alluvial fans. Timing and location of this uplift suggest that it may have originated as a pre-rift bulge. Lithologic, stratigraphic, and sedimentologic similarities between the Dewey Lake and Moenkopi Formation suggest that the alluvial plain north of this uplift may have extended throughout the southwestern United States.