New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

Pennsylvanian strata are present on the surface and in the subsurface across most of New Mexico and can be divided into two broad lithosomes. The northern lithosome crops out across much of northern and central New Mexico and has an overall three part stratigraphic architecture of: (1) a basal, siliciclastic-dominated unit, which includes quartzose, coarse-grained sandstone and conglomerate of Morrowan (locally), Atokan (mostly) and early Desmoinesian (locally) age, the Sandia, Red House and Gobbler formations; (2) a medial, limestone-dominated, apparently cyclical unit with very few coarse-grained siliciclastic intercalations, the Porvenir Formation and the Gray Mesa Formation (= Nakaye Formation, = most of the Lead Camp Limestone, = Bug Scuffle Member of Gobbler Formation), primarily of Desmoinesian age; and (3) an upper interval of mixed siliciclastic and carbonate strata, including various beds of coarse-grained clastics, of late Desmoinesian-Virgilian age, the Alamitos, Atrasado, Bar-B, Beeman and Holder formations. This three part architecture is not evident in the Pennsylvanian strata of southern New Mexico, which are cyclically bedded (apparently) limestone with calcareous shale interbeds and lack any significant coarse-grained siliciclastic intercalations. Age data (primarily from fusulinids) indicate that these strata encompass essentially all of Pennsylvanian time where they are thick and well exposed, as in the Big Hatchet Mountains of Hidalgo County. Thus, these southern New Mexico strata are broadly equivalent temporally to the northern strata. We assign the southern lithosome strata to the Horquilla Formation because the type Horquilla Formation of southeastern Arizona is of similar lithology—limestone dominated, few coarse-grained clastic beds--and it is correlative based on fusulinid biostratigraphy.

In a simple sense, we can thus view the Pennsylvanian strata of New Mexico as comprising two lithosomes, a northern New Mexico lithosome with coarse-grained clastic beds in its lower and upper strata, and a southern New Mexico lithosome almost entirely made of limestone with very few coarse-grained siliciclastic beds. The northern lithosome crops out as far south as the Black Range, Caballo Mountains and Derry Hills of Sierra County and in the Sacramento Mountains of Otero County. It reflects Ancestral Rocky Mountain tectonics with the local onset of tectonic movements in late Morrowan-Atokan time, and a second interval of elevated tectonism during the Late Pennsylvanian. Glacio-eustasy was a minor driver of deposition of the northern lithosome, whereas it was a more important driver of the cyclically bedded Horquilla strata of southern New Mexico that were evidently deposited with less influence from local/regional tectonism.