Gallery of Geology: The Pennsylvanian section at Bishop Cap, Do�a Ana County, New Mexico

Abstract

Located about 19 km (12 mi) southeast of Las Cruces in Doña Ana County, New Mexico, Bishop Cap is a rugged peak at the southern end of the Organ Mountains. At an elevation of 1,651 m (5,419 ft), Bishop Cap rises about 335 m (1,100 ft) above the alluvial fans at its base. The miter-shaped peak looks like a bishop’s cap, hence the name. Bishop Cap is on a westward-tilted fault block between the Organ Mountains of New Mexico and the Franklin Mountains of West Texas. Its western side is a monocline-like flexure that dips to the west beneath Quaternary alluvial-fan deposits. The oldest strata exposed at Bishop Cap are along the eastern base of the peak. They are Mississippian marine rocks capped by the Late Mississippian (Chesterian) Helms Formation. A limestone-dominated Pennsylvanian section about 256 m thick makes up most of Bishop Cap, and these strata rest with evident disconformity on olive-gray shale of the Helms Formation. During the early 20th Century, fluorite was mined at Bishop Cap and in the surrounding hills, and published descriptions of the mining geology and stratigraphy first appeared in the 1920s (Seager, 1973, 1981). The lithostratigraphic nomenclature long applied to the Pennsylvanian strata at Bishop Cap has been that of Nelson (1940), names he introduced at Vinton Canyon in the northern Franklin Mountains to the south. Nelson (1940) coined the names (ascending order) La Tuna, Berino, and Bishop’s Cap members of the Magdalena Formation. At Vinton Canyon, these strata and an overlying unnamed interval of Pennsylvanian strata comprise a stratigraphic section about 823 m thick (Harbour, 1972). At Bishop Cap, Seager (1973, 1981) mapped the Pennsylvanian strata as the La Tuna and Berino formations of the Magdalena Group (ironically, Nelson’s “Bishop’s Cap Member” is not exposed at Bishop Cap, having been removed by