STRATIGRAPHY AND STRUCTURE: THE BORREGO SECTION OF THE PENNSYLVANIAN-PERMIAN HORQUILLA FORMATION, BIG HATCHET MOUNTAINS, NEW MEXICO

In 1966, Zeller (NMBMMR Memoir 16) described the Borrego section of the PennsylvanianPermian Horquilla Fm in the Big Hatchet Mountains (SE¼ sec. 27, T31S, R15W, Hidalgo County). He interpreted the Virgilian-Wolfcampian interval here as a single, homoclinal section about 500 m thick divided into 5 units: (1) lower, limestone-dominated unit, ~120 m thick, late Virgilian-early Wolfcampian; (2) light gray “basinal” shale ~105 m thick; (3) middle limestone, ~60 m thick, early-middle Wolfcampian; (4) light gray “basinal” shale ~102 m thick; and (5) upper limestone, ~90 m thick, middle Wolfcampian. In contrast, Drewes (1991, USGS Map I-2144) mapped shingled thrust faults in which units 1-2 are Horquilla Fm (1) depositionally overlain by Earp Fm (2), separated by a thrust fault from units 3-4, which are Horquilla-Earp again, separated from another thrust fault from unit 5, which is Horquilla. Our study of the Borrego section indicates that Zeller’s identification of a single, homoclinal section is correct, though we differ from Zeller in identifying units 2 and 4 as very shallow marine facies of the Horquilla Fm (mostly crossbedded calcarenites, thinly laminated siltstones and grainstones), not a basinal facies. Several observations refute the thrust fault interpretation of the Borrego section: (1) regionally, the Horquilla-Earp contact is late Wolfcampian, so if unit 2 is Earp