PENNSYLVANIAN-PERMIAN DEPOSITIONAL BREAK IN THE CUTLER FORMATION, EL COBRE CANYON, NORTHERN NEW MEXICO

Two "megasequences" (ms) of the Cutler Formation exposed in El Cobre Canyon, Rio Arriba County, NM (Eberth & Miall, Sed. Geol.,1991) record two different, unconformity-separated, depositional events, one of Pennsylvanian (ms 1, more than 130 m thick) and the other (megasequence 2, 125 m thick) of Early Permian age. The main lithologies of both megasequences are polymict, poorly-sorted conglomerates and arkosic sandstones, usually trough-crossbedded, representing channel fills of a braided to anastomosed fluvial system, and overbank deposits of fine-grained sandstones, siltstones and claystones, and intercalated crevasse channel and splay deposits. Conglomerates contain abundant reworked caliche carbonate clasts, and paleocurrent analysis indicates flow directions to the S/SW. The boundary between the two megasequences is a major erosional surface that can be traced laterally for km, characterized by deeply (more than 7 m) incised channels. But, this boundary does not correspond to the sequence boundary of Eberth & Miall, which is ~ 25 m higher in the section. Abundant, well-preserved megaplant fossils occur in two levels in dark gray shale and silty shale at the base of exposed ms 1. This horizon is not below the Cutler Fm., as proposed by Smith et al. (1961, NMBMMR Bulletin 75). The plant fossils indicate a Pennsylvanian age, as does the tetrapod-fossil assemblage of ms 1, which includes Desmatodon and Limnoscelis. Tetrapods from ms 2 (Diadectes, Sphenacodon) indicate a Permian age and correlation to the Abo Formation to the south.