New Mexico Geological Society Annual Spring Meeting — Abstracts

The “Boquillas Formation” (= Mancos Shale) is shale-dominated strata of Late Cretaceous (middle Cenomanian) age known in New Mexico from outcrops on the northern flank of Cerro de Cristo Rey, Doña Ana County. We report newly discovered outcrops of the Boquillas exposed by excavation of an unpaved road just south of Flag Hill in the Franklin Mountains of El Paso County, Texas (NM Museum of Natural History locality 7730). This outcrop exposes about 4.5 m of Boquillas strata that consist of: (1) thinly-laminated (mm-scale) mudstone in which gray mudstone laminae alternate with slightly coarser, silty, brownish mudstone laminae; (2) some thin gypsum layers; (3) fine-grained, laminated carbonate beds that are composed of mudstone containing locally abundant planktic globigerinid foraminifers such as Hedbergella, peloids, a few small bivalve shells, gastropods, ostracods and small echinoderm fragments. These carbonate beds are up to 15 cm thick and contain abundant shells on bed tops. The carbonate beds laterally lense out. Some of the carbonate beds form lenses less than 1 m wide, probably representing the fills of paleotopographic lows. The mudstone also contains ammonites, bivalves, gastropods and other fossils that are partly fragmented, indicating that the fossils have been transported. These strata are very similar to the Boquillas Formation at Cerro de Cristo Rey, where the exposed thickness is 18.6 m.

Fossils from the Boquillas Formation at the newly discovered outcrop represent the following taxa: unidentified gastropods; the bivalves Ostrea beloiti Logan, Inoceramus arvanus Stephenson, I. praefragilis Stephenson and Pinna sp.; the ammonoids Acanthoceras amphibolum Morrow, Desmoceras sp./Moremanoceras sp., and Idiohamites sp. These taxa are also found at Cerro de Cristo Rey and support assignment to the middle Cenomanian Acanthoceras amphibolum zone. In southern New Mexico and West Texas, the name Mancos Formation is best applied to this shale-dominated interval at the base of the Greenhorn cycle of deposition. More parochial stratigraphic names such as Boquillas, Chispa Summit and Ojinaga are best abandoned to unify the stratigraphic terminology so that it better reflects the regional distribution of this lithosome throughout southern New Mexico, West Texas and northern Chihuahua.