THE BELL CANYON/CASTILE FORMATIONAL CONTACT: IMPLICATIONS FOR THE GUADALUPIAN/LOPINGIAN SERIES BOUNDARY (MIDDLE/UPPER PERMIAN) IN THE APACHE MOUNTAINS, WEST TEXAS

The Bell Canyon Formation (Middle Permian, Guadalupian) is comprised of six members named for strata in the Guadalupe Mountains of West Texas and can be recognized biostratigraphically in the Apache Mountains on the southwestern side of the Delaware Basin. Some member names, such as Lamar and Rader, have been carried to this area even though the lithofacies display different characteristics and their use is questionable. Bed by bed sampling of several sections in the uppermost Bell Canyon equivalent has recently documented these strata as coeval with the upper part of the Reef Trail Member (uppermost Bell Canyon in the Guadalupe Mountains), and both contain the conodonts Jinogondolella altudaensis, Clarkina crofti, C. postbitteri hongshuiensis, and the fusulinaceans Paraboultonia, and Conodonofusiella (Lantschichites).

The precise placement of the lithostratigraphic boundary between the Bell Canyon Formation and the overlying Castile Formation presents a most interesting problem because of a one to two meter transitional interval. This interval conformably overlies two thin superposed limestone beds containing foraminifers, conodonts, and radiolarians. The transitional zone begins with a laminated barren gypsiferous limestone overlain by a pinkish limestone with silica filled vugs from the dissolution of gypsum and the unit contains sparse specimens of the conodont Clarkina postbitteri hongshuiensis and the fusulinaceans Paraboultonia and Codonofusiella (Lantschichites). A second barren limestone at the top of the transitional interval is also laminated, and is overlain by typical varved and brecciated strata of the Castile Formation. At some sections, there are a few thin beds of radiolarian limestone interbedded with siltstone before the typical Castile deposits. The placement of part or the entire transitional interval in the uppermost Bell Canyon has been suggested by various authors. The Bell Canyon/Castile contact could also be placed at the base of the first laminated limestone, as suggested by King in 1942, which would imply that the lowermost few meters of the Castile Formation would fall into the uppermost Guadalupian. The presence of the conodont Clarkina postbitteri postbitteri, which defines the base of the Lopingian Series, has yet to be documented in the Permian of West Texas.