Abstract: Dakota Sandstone of Eastern Edge of San Juan Basin, New Mexico

D. E. Owen

The Dakota Sandstone forms a north-south trending outcrop from the El Vado domes area to near the southern end of the Nacimiento uplift marking the eastern edge of the San Juan basin. Within the Dakota, a highly cross-bedded, fluvial lower sandstone unit, a paralic middle shale unit, and a marine upper sandstone unit are traceable on outcrop. Whether to include a white, noncarbonaceous, fluvial sandstone (Burro Canyon Formation?) with the overlying tan, carbonaceous, fluvial sandstone in the lower unit is a problem. Evidence of a more marine environment increases southward as, locally, the upper part of the middle shale unit contains oysters (Pycnodonte) and gastropods. The upper sandstone unit throughout the area contains a great abundance of marine trace fossils, especi lly Ophiomorpha. In the southern part of the area the threefold subdivision changes into intertonguing, cyclical-marine shale and marine-sandstone members. An overall southward thinning from as much as 225 ft, excluding the white, noncarbonaceous sandstone, north of the Nacimiento uplift to as little as 110 ft along the edge of the Nacimiento uplift is evident. This thinning is attributable mainly to decreasing thicknesses in the lower part of the formation. The marine-shale tongues produce a thickening in the southernmost part of the area. Interpretations of age, depositional environments, lateral stratigraphic changes, and paleocurrent patterns are critical in reconstruction of the initial Cretaceous marine transgression across the area.

AAPG Search and Discovery Article #90979©1975 AAPG – SEPM Rocky Mountain Sections Annual Meeting, Albuquerque, New Mexico