New Mexico Geological Society Annual Spring Meeting — Abstracts

Deposited on the inactive margin of Pray's (1959) Orogrande basin, the Robledo shelf (Meyer, 1966), the Bar B Formation represents deposition during a transition from shallow marine to nonmarine conditions at the end of Pennsylvanian time. The Bar B Formation is
correlative with the Panther Seep Formation of the San Andres Mountains (Kottlowski et aI., 1956), which represents deposition within the most rapidly subsiding axis of the Orogrande basin, and the Holder Formation of the Sacramento Mountains (Pray, 1961). which
represents deposition along the active margin of the Orogrande basin. Thick Panther Seep sections also occur in oil wells drilled in the Jornado del Muerto between the Caballo and San Andres Mountains.

Within its western exposures, the Bar B Formation ranges from 50 to 60 m thick, and is composed of slope-forming, calcareous shales intercalated with thin-bedded to nodular limestones. Limestones and shales are interbedded repetitively throughout the Bar B Formation. Thin conglomerate beds, composed of Pennsylvanian-aged clasts, cap most of the sections. The Bar B Formation thickens eastward to 210 m along the eastern escarpment of the Caballo Mountains where laterally-discontinuous, upward-coarsening conglomerates, also composed of Pennsylvanian-aged clasts, persist throughout the entire section.

A diminished number of repeated carbonate-shale sequences. lack of a capping conglomerate, and the presence of caliche nodules within the Bar B Formation indicate prolonged subaerial exposure late in Bar B deposition in the southwestern Caballo Mountains and the presence of a north-south trending ridge located on the Robledo shelf. This ridge was periodically exposed during latest Pennsylvanian time and provided a source for the Pennsylvanian clasts observed within both sets of conglomerates.

The conglomerates located throughout the section along the eastern escarpment of the Caballo Mountains are interpreted to represent offshore bars, presumably concentrated along the edge of the Robledo shelf. The eastward thickening of the Bar B Formation and its continued dramatic thickening to the east into the Panther Seep Formation beneath the Jornado del Muerto indicate a rapid west-east subsidence gradient. The hingeline separating the Robledo shelf from the Orogrande basin is thus Interpreted to have been located very near the eastern escarpment of the present day Caballo Mountains.