LITHOFACIES AND PALEONENVIRONMENTS OF THE TYPE SECTION OF THE PENNSYLVANIAN OSHA CANYON FORMATION, JEMEZ MOUNTAINS, NEW MEXICO

Abstract

The Pennsylvanian (Morrowan) Osha Canyon Formation is 27.3 m thick at the type locality near Guadalupe Box in the Jemez Mountains. It is underlain by red and greenish shales of the Mississippian Log Springs Formation, and sharply overlain by coarse-grained, troughcrossbedded, quartzose fluvial sandstones of the Sandia Formation. At the type section, the Osha Canyon Formation is entirely of marine origin and composed of red, purple and greenish marly shale (80.6% of the type section) and interbedded light-gray and reddish limestone beds and mixed siliciclastic-carbonate sandstone (19.4%). Marly shales are poorly exposed, and contain brachiopods and small solitary corals, particularly in unit 10 of the section. Shales of unit 14 also contain small gray limestone nodules. The ledge-forming limestones are 0.1 to1 m thick, wavy bedded and composed of abundant large skeletal fragments derived mostly from brachiopods and crinoids, and subordinately from bryozoans, gastropods and other organisms. Limestones also contain siliciclastic grains, mostly quartz grains, and subordinate granitic rock fragments. The amount of siliciclastic grains increases towards the top of the section. The uppermost 2.5-m-thick interval is a fossiliferous, mixed siliciclastic-carbonate sandstone. The siliciclastic material is probably derived from the nearby Peñasco uplift. Sediments of the Osha Canyon Formation were deposited in a shallow marine shelf environment of normal salinity. The fossiliferous marly shales formed in a shallow-water, low-energy environment below the wave base, whereas the limestones and the mixed siliciclastic-carbonate sandstone on top reflect deposition in a shallow marine, high-energy environment above the wave base.