New Mexico Geological Society Annual Spring Meeting — Abstracts

The Colorado Formation (Upper cretaceous) in southwestern New Mexico consists of approximately 200m of interbedded shale and sandstone and is exposed at several locations in Grant and Luna counties. Three informal members have been designated in each of eight stratigraphic sections. The lower member is composed of bioturbated, fossiliferous siltstone, shale, and very fine-grained sandstone, and was deposited in a shallow marine or prodelta environment. The middle membpr consists of massive or laminated sandstone, shale, and laminated and planar tabular-crossbedded sandstone that were deposited in a delta front environment. The upper member is composed of carbonaceous shale and lenticular, trough-crossbedded sandstone. Sedimentary structures and lithologies indicate a nonmarine depositional environment, perhaps within a lower delta plain.

The Colorado Formation was deposited in the southwesternmost part of the Western Interior Seaway. Paleocurrent data from delta plain sandstones indicate a northeastward paleoslope. Detrital modes of 22 sandstones imply derivation from sedimentary and volcanic source rocks that were probably part of the arc and back-arc orogenic belt in southern Arizona and northern Mexico.