New Mexico Geological Society Annual Spring Meeting — Abstracts

The Carboniferous/Permian boundary (CPB) has been defined by the first occurrence of the conodont species Streptognathodus isolatus and the fusulinid Sphaeroschwagerina (base of the Sphaeroschwagerina vulgaris – S. fusiformis Zone), with a poorly exposed type section located along Aidaralash Creek near Aktobe in the southern Ural Mountains (northern Kazakstan) within a succession of hemipelagic pelitic sediments and intercalated sandstone beds. In the Big Hatchet Mountains in southwestern New Mexico (Hidalgo County), the entire Pennsylvanian and part of the Lower Permian (Wolfcampian) is represented by a thick succession composed of shallow marine limestone of the Horquilla Formation.

At New Well Peak, the Horquilla Formation is approximately 1000 m thick and can be divided into three members. The CPB lies within the upper member, which consists of a succession of dmbedded gray to dark gray, predominantly micritic, fossiliferous limestone. Intercalated are indistinctly bedded to massive limestone intervals 3.3-4.5 m thick and limestone with chert nodules. The most common microfacies is bioclastic wackestone to packstone with a highly diverse fossil assemblage. Most abundant are fusulinids, smaller foraminifers, calcareous algae, echinoderms, brachiopods, bryozoans, gastropods and Tubiphytes. The limestone accumulated in a shallow, open marine environment with normal salinity. Carbonate sedimentation was periodically interrupted as indicated by red mudstone layers, resulting in a cyclic succession. Thin paleocaliche beds containing alveolar root structures at the top of the limestone intervals/base of the red mudstones indicate short periods of subaerial exposure. The Horquilla Formation was deposited on the stable shelf of the Pedregosa Basin. The cycles are interpreted to be caused by glacioeustatic sealevel fluctuations.

At New Well Peak, the CPB can be drawn with fusulinids (first appearance of Pseudoschwagerina) as well as with conodonts (first appearance of Streptognathodus isolatus). Accessibility, excellent outcrop quality, abundance of fossils (including fusulinids and conodonts) and the uniform facies across the C/P boundary favor the New Well Peak section as a potential new type section (GSSP) for the Carboniferous/Permian boundary.