Pennyslvanian strata and Ancestral Rocky Mountain tectonism in Sierra County, New Mexico

Pennsylvanian strata are well exposed in the San Andres, Fra Cristobal, Caballo and Mud Springs Mountains in Sierra County, New Mexico. The oldest of these strata are the Morrowan-Atokan Red House Formation, a mixed siliciclastic-carbonate unit (shale, sandstone and limestone) as much as 50 m thick. The overlying Pennsylvanian lithosome is dominated by cyclically-bedded cherty limestone, nodular limestone and shale, the Atokan-Desmoinesian Gray Mesa Formation (Nakaye Formation and Lead Camp Limestone are unnecessary synonyms) as much as 388 m thick. Member-level subdivisions of the Gray Mesa Formation Elephant Butte, Whiskey Canyon and Garcia members can be recognized in some sections, but not in others, due to substantial lateral facies changes. The youngest Pennsylvanian lithosome is more regionally complex, and is the relatively thin (up to ~63 m at Bar-B Draw) Bar-B (Missourian-Virgilian) and (up to 120 m) Bursum (lower Wolfcampian) formations to the west and the very thick (up to 1000 m) Panther Seep Formation (Missourian-early Wolfcampian) to the east. This lithosome is mixed clastic and carbonate strata within which the Bursum base has a demonstrably disconformable contact on older strata. A comprehensive analysis of lithostratigraphy, facies and biostratigraphy allows us to construct a tectonostratigraphy of the Pennsylvanian strata in Sierra County that identifies three pulses of the ancestral Rocky Mountain (ARM) orogeny: (1) onset of the ARM at about the end of Morrowan time with initial synorogenic Red House clastics deposited unconformably on older Paleozoic strata; (2) a pulse close to the beginning of the Missourian, indicated by basal Missourian conglomerates and/or the relatively thin (locally absent) Missourian section; and (3) a latest Pennsylvanian pulse indicated by the sub-Bursum unconformity. Although eustasy can be invoked to explain some of the stratigraphic architecture of the Pennsylvanian System in Sierra County, much more of it is explicable by ARM tectonism, particularly by a local Caballo uplift (centered near the Derry Hills?) present and tectonically active intermittently during the Pennsylvanian.