The Pennsylvanian section at Bell Hill in the southern San Mateo Mountains of southern Socorro County, New Mexico (sec. 16, T8S, R4W and vicinity), has long been considered the thickest Pennsylvanian section (~ 800 m thick) in south-central New Mexico and the focal point of a late Paleozoic San Mateo depositional basin. Kottlowski (1960) first described the Paleozoic section at Bell Hill (his Eaton Ranch section) as 808 m of Pennsylvanian strata on Proterozoic basement and overlain by the lower Permian Abo Formation. Furlow (1965;Kelley and Furlow, 1965), however, interpreted the section differently, as ~ 91 m of Cambro-Ordovician strata resting on the basement, overlain by ~732 m of Pennsylvanian strata beneath the Abo Formation. We restudied the Pennsylvanian section at Bell Hill and determined that the Pennsylvanian section is thinner than previously reported, only 495 m thick. At Bell Hill, the Pennsylvanian strata overlie a thin (~ 70 m thick) lower Paleozoic section that consists of the Cambro-Ordovician Bliss Formation and the Ordovician El Paso Group (Sierrite Member of the Hitt Canyon Formation) and Montoya Formation (Cable Canyon and Upham members). The Middle Pennsylvanian Red House Formation unconformably overlies the Montoya Formation, and is at least 31 m thick (structural complications prevent a certain estimate). It is overlain by the Gray Mesa Formation, which is 158 m thick and divisible into the Elephant Butte (at least 18 m thick), Whiskey Canyon (53 m thick) and Garcia (87 m thick) members. The overlying Bar B Formation is 306 m thick, substantially thicker than to the south, and accounts for most of the relatively great thickness of the Bell Hill Pennsylvanian section. The Bar B Formation is overlain by the Bursum Formation, ~ 18 m thick, which is overlain by the Abo Formation. Fusulinid and conodont biostratigraphy indicate that at Bell Hill the Red House Formation is Atokan, the Gray Mesa Formation is Desmoinesian and the Bar B Formation is Desmoinesian-Virgilian. The new estimate of the thickness of the Pennsylvanian section at Bell Hill requires redrawing the late Paleozoic San Mateo basin as an east-west-oriented trough instead of a single, localized depocenter.
North of Rancho de Chaparral in the Nacimiento Mountains of Sandoval County (sec. 2, T19N, R1E), Middle Pennsylvanian (Atokan) strata of the Sandia Formation rest on Precambrian basement. Poorly and incompletely exposed, the Sandia Formation is at least 10 m thick and consists of interbedded greenish gray/brown shale and coarse-grained to conglomeratic, quartzose sandstone. Overlying cherty limestones ("Gray Mesa Formation") contain the fusulinacean Wedekindellina, indicative of a Desmoinesian age. In a 0.7-m-thick bed of conglomeratic sandstone that is ~ 4 m below the top of the Sandia Formation, we recovered an isolated bone that is the first tetrapod fossil from the Sandia Formation and New Mexico's oldest fossil tetrapod. This bone is columnar, incomplete and ~ 40 mm long with a flat articular end that is 9.5 mm wide. The shaft is slightly bowed on its long axis, shallowly concave on one side, shallowly convex on the other side and widens toward the less complete articular end. It closely resembles the fibula or possibly a presacral rib of a primitive temnospondyl amphibian such as Greererpeton. However, we only tentatively identify the fossil as temnospondyl. Most other Pennsylvanian tetrapod records from New Mexico are of Late Pennsylvanian age (Bursum, El Cobre Canyon and Atrasado formations, most notably the Kinney Brick quarry), and the oldest previously reported record was a captorhinomorph bone from the Desmoinesian Flechado Formation in Taos County. The occurrence of tetrapod bone in the Sandia Formation thus pushes back New Mexico's fossil record of tetrapods into the Atokan.
The Pennsylvanian strata exposed on the northern flank of the Oscura Mountains are a classic section because M. L. Thompson used it as a key reference section in his 1942 monographic synthesis of the Pennsylvanian in New Mexico. This section is ~ 334 m thick, and we assign it to the Sandia, Gray Mesa, Atrasado and Bursum formations. The Sandia Formation is thin (~ 13 m), overlies Precambrian granite and is mostly shale and sandy limestone. The lowest cherty limestone ledge marks the base of the overlying Gray Mesa Formation, which is ~ 162 m thick. Most of the Gray Mesa is limestone (63% of the section), the remainder is covered slopes (shale?), most of the limestones are cherty and numerous fusulinid packstones are present. The Atrasado Formation is ~ 124 m thick and includes (in ascending order) the Veredas, Hansonburg and Keller groups of Thompson, names that should be abandoned. The facies ranges from siliciclastic sandstone and fine-grained conglomerate to bedded fossiliferous limestone of an open marine shelf to massive algal limestone of a mound facies formed just below wave base in quiet water in the photic zone. The formations named by Thompson work as local lithostratigraphic units at the member or bed rank. Thus, we recognize (in ascending order) the Adobe Member ( 33.5 m), Council Springs Bed (4.8 m), Burrego Member (16.4 m), Story Member (18.3 m), Del Cuerto Member (34.8 m) and Moya Member (16.5 m) of the Atrasado Formation. The Bursum Formation overlies the Atrasado Formation and is 35 m of marine limestone and red-bed clastics. Abo Formation red-bed siliciclastics overlie the Bursum Formation and are of presumed earliest Permian age. Fusulinids indicate that the DesmoinesianMissourian boundary is in the upper Gray Mesa Formation, the Missourian-Virgilian boundary is ~ base of Del Cuerto Member and the Wolfcampian (Newwellian) base is ~ base of Bursum. pp. 33,
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.