New Mexico Geological Society Annual Spring Meeting — Abstracts

In New Mexico, the contact between Middle and Upper Pennsylvanian strata is often reported to coincide closely with the top of the Gray Mesa Formation, a widespread Middle Pennsylvanian lithostratigraphic unit in the central to south-central part of the state. This perception is based partly on the fusulinacean biostratigraphy of D.A. Myers (USGS), who worked in the Manzano Mountains during the 1960s through the 1980s. Myers’ monograph on the distribution of fusulinids from the Manzano Mountains (USGS Prof. Paper 1446) identifies a single locality where age-diagnostic fusulinids allegedly indicate that deposition of the Gray Mesa Formation lasted throughout Desmoinesian and into earliest Missourian time. In particular, the fusulinid genera Eowaeringella, which is restricted to strata near the base of the Missourian Stage, was reported at the top of the Gray Mesa Formation (what Myers initially called the “lower member of the Madera Formation,” and, in later maps and reports, the ”Los Moyos Limestone”). However, more recent biostratigraphy of Pennsylvanian rocks in the southern Manzano Mountains and in the hills east of Socorro suggest that the Missourian base in those areas is higher in the Pennsylvanian succession; that is, in the lower part of the overlying Atrasado Formation. Consequently, we revisited Myers’ locality in order to assess the fusulinid biostratigraphy and the lithostratigraphy near the contact between the Gray Mesa and Atrasado formations at that site. The locality is in Gotera Canyon in the Manzano Mountains, approximately 30 km east of Isleta Pueblo. It was originally documented by Myers during geologic mapping of the Escabosa 7.5' quadrangle (USGS geologic map GQ-795), which included a measured stratigraphic section through the interval containing Eowaeringella. Myers’ measured section was located and a 70+ m thick interval comprising the uppermost Gray Mesa Formation and the overlying Bartolo, Amado and lowermost Tinajas members of the Atrasado Formation was examined and measured. We were able to locate distinct intervals, including sandstone beds, shown on Myers’ measured section, and the Eowaeringella-bearing horizon was indeed present, together with additional fusulinid-bearing beds several meters higher containing species of thin-walled, cylindrical Triticites, indicating an early to middle Missourian age. The Eowaeringella horizon is approximately 50 m above the top of the Gray Mesa Formation, near the contact between the Amado and the overlying Tinajas members of the Atrasado Formation. Interestingly, Myers’ graphic section places the Eowaeringella zone at the top of the Gray Mesa Formation, but the geologic map itself places the top well below that level, in agreement with the obvious lithostratigraphic boundary evident on outcrop. Indeed, the contact between the Gray Mesa and Atrasado formations at the Gotera Canyon locality, similar to other exposures in the Manzano-Manzanita Mountains, is marked by a relatively thick basal sandstone unit (Coyote Sandstone Bed), followed by a succession of mostly covered (probably shale) intervals that comprise the main part of the Bartolo Member of the Atrasado Formation.

The Pueblo of Isleta granted permission to examine the Gotera Canyon section, and their assistance with gaining access is gratefully acknowledged.