New Mexico Geological Society Annual Spring Meeting — Abstracts

Sedimentary rocks of Pennsylvanian age were among the first strata studied by geologists working in New Mexico, beginning with Marcou’s 1853 observations that were among the earliest studies of New Mexico geology by a trained geologist. In the subsequent 150 years, Pennsylvanian strata have been identified in most of New Mexico’s mountain ranges, as well as in some other outcrop areas and in the subsurface. Given their broad distribution across New Mexico, and because of the isolated locations of extensive outcrops in various mountain ranges and the lithologic heterogeneity of the Pennsylvanian strata, a complex lithostratigraphic nomenclature has arisen for them. In 1998, we began to work on the Pennsylvanian strata across much of New Mexico. This work has created a substantial new understanding of their lithostratigraphy, petrography and microfacies, depositional environments and biostratigraphy.

The Pennsylvanian section across much of northern and central New Mexico 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 unit with apparently cyclical stacking patterns and very few beds of coarse-grained siliciclastics, the type 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 clastic and carbonate strata, including various beds of coarse-grained siliciclastics, of late Desmoinesian-Virgilian age, the Alamitos, Guadalupe Box, Atrasado, Bar-B, Beeman and Holder formations. Morrowan strata (Osha Canyon Formation in the Jemez Mountains) are rare in northern New Mexico, and at most outcrops Atokan strata rest on Mississippian strata or Proterozoic basement.

However, the three-part architecture of the Pennsylvanian section in northern New Mexico is not evident in the Pennsylvanian sections in southern New Mexico south of about the latitude of Silver City west of the Rio Grande and south of about the latitude of Las Cruces east of the Rio Grande. Instead, these sections are almost entirely limestone with some calcareous shale interbeds and lack any significant beds of coarse-grained siliciclastic composition. 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. 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 siliciclastic 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.

We assign the southern Pennsylvanian lithosome strata to the Horquilla Formation. Not only is the type Horquilla Formation of southeastern Arizona of similar lithology—limestone dominated, few coarse-grained siliciclastic beds--but it is correlative based on fusulinid biostratigraphy. The Magdalena Group (Formation, Limestone) of Gordon is based on a section in the Magdalena Mountains of Socorro County, New Mexico, of the northern lithosome—it includes coarse clastic beds of the Sandia Formation at its base, overlain by limestone-dominated strata of the Gray Mesa Formation capped by an incomplete section of the mixed siliciclastic-carbonate Atrasado Formation. Applying the name Magdalena to the southern New Mexico-West Texas limestone-dominated lithosome, as has been done for decades, thus is not justified. Magdalena Group (Limestone, Formation) is a term that should be banished from the New Mexican lithostratigraphic lexicon. It certainly should not be applied to Pennsylvanian strata in southern New Mexico.

Distinct lateral and vertical changes in thickness and facies in the northern lithosome indicate that sedimentation during the Pennsylvanian was mainly controlled by tectonic processes related to the ancestral Rocky Mountain orogeny (ARM), although climate and eustatic sea-level fluctuations also influenced sedimentation. In central New Mexico (Sierra – Bernalillo shelf) three tectonic pulses of the ARM can be identified that occurred during the Atokan, Missourian, and late Virgilian. The Sandia Formation and equivalent strata mark the onset of ARM tectonics, ARM tectonics is also evident in the Orogrande basin where a broad and tectonically stable ramp with dominantly carbonate deposits was developed on the western margin of the basin (Robledo shelf), whereas the eastern margin (Sacramento shelf) was a narrow, fault-bounded shelf margin on which siliciclastic and carbonate sediments with marked vertical and lateral facies changes accumulated. Tectonic pulses in the sedimentary succession of the Sacramento shelf can be identified in the Morrowan-Atokan, Missourian and late Virgilian-Wolfcampian.

The Pennsylvanian-lower Permian succession exposed in the Robledo Mountains represents the deposits of the western shelf of the Orogrande basin. This succession differs strongly in facies and thickness from the Pennsylvanian-lower Permian of the Sacramento shelf. At Robledo Mountain, the Pennsylvanian-lower Permian succession is only about 418 m thick and entirely composed of limestone and intercalated shale/covered intervals. The Pennsylvanian-Lower Permian of the Pedregosa basin, well exposed and well studied in the Big Hatchet Mountains of Hidalgo County, is a thick succession (~ 1 km thick) of shelf carbonates with some shale intervals and lacking coarser-grained siliciclastics. This succession demonstrates that tectonic processes of the ARM had little effect on local sedimentation processes; the tectonic pulses that are recognized in northern New Mexico and in the Sacramento Mountains are not observed in the Pedregosa basin.