Pennsylvanian stratigraphy in the Fra Cristobal and Caballo Mountains, Sierra County, New Mexico
Spencer G. Lucas, Karl Krainer, and Justin A. Spielmann


Pennsylvanian strata exposed in the Fra Cristobal and Caballo Mountains of central Sierra County, New Mexico, are assigned to the (ascending order) Red House, Gray Mesa, Bar B and Bursum formations. The terms Magdalena Group, Madera Group and Nakaye Formation are rejected for these rocks. The Morrowan-Atokan Red House Formation is 29-93 m thick and is a slope-forming succession of shale, limestone and minor conglomerate and sandstone. It unconformably overlies rocks of Proterozoic to Mississippian age. The Atokan-Desmoinesian Gray Mesa Formation conformably overlies the Red House Formation and is 118-210 m of mostly ledge- and cliff-forming beds of cherty limestone. In the southern Caballo Mountains (Green Canyon), the Gray Mesa Formation consists of three members, Elephant Butte, Whiskey Canyon and Garcia, but elsewhere in the Caballo and Fra Cristobal Mountains the formation is not subdivided. The Desmoinesian-Virgilian Bar B Formation conformably overlies the Gray Mesa Formation, is 72-107 m thick and is mostly a slope-forming unit of shale with relatively thin limestone beds. It is unconformably overlain by the lower Wolfcampian (Newwellian) Bursum Formation or by red beds of the middle Wolfcampian Abo Formation. Conodont and fusulinid biostratigraphy establishes the ages of the Pennsylvanian strata in the Fra Cristobal and the Caballo Mountains. These strata differ significantly from the Pennsylvanian section (especially Upper Pennsylvanian strata) in the nearby Mud Springs Mountains. Pennsylvanian tectonism – differential subsidence and tectonic activity of the Caballo uplift – best explains these differences.


  1. Lucas, Spencer G.; Krainer, Karl; Spielmann, Justin A., 2012, Pennsylvanian stratigraphy in the Fra Cristobal and Caballo Mountains, Sierra County, New Mexico, in: Geology of the Warm Springs region, Lucas, Spencer G.; McLemore, Virginia T.; Lueth, Virgil W.; Spielmann, Justin A.; Krainer, Karl, New Mexico Geological Society, Guidebook, 63rd Field Conference, pp. 327-344.

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