CAMBRIAN–ORDOVICIAN STRATIGRAPHY OF WEST TEXAS AND SOUTHERN NEW MEXICO

Paleoenvironmental analysis of the Bliss Formation and lower part of the overlying El Paso Group in western Texas and southern New Mexico allow reconstruction of the depositional history of Cambrian–Ordovician transgression onto this part of Laurentia. The Bliss consists largely of quartz- and carbonate-cemented, very fine sandstone to pebble conglomerate. Radical changes in thickness over short distances reflect deposition over a highly irregular Precambrian basement. Widespread glauconitic and hematitic marker beds are interpreted as products of regional geochemical events brought about by relative sea level changes. The upper Bliss is a complex, mixed siliciclastic–carbonate unit dominated by glaucarenite, dolomite, and bioclastic grainstone with minor amounts of fine sandstone and shale. Storm-generated structures such as hummocky cross-stratification (HCS) and flat-pebble conglomerate are common. This upper Bliss facies yielded numerous trilobites, brachiopods and graptolites that confirm that the formation spans the Cambrian–Ordovician boundary in south-central New Mexico. The basal (30-40) meters of the overlying El Paso Group (Sierrite Limestone of Caballo Mountains), consists almost entirely of amalgamated parallel laminated and HCS grainstone beds. The lower part of the Sierrite is referable to the Rossodus manitouensis conodont zone, but carbon isotope stratigraphy and recovery of the trilobites Leiostegium and Kainella from the highest 10-15m, suggest that it ranges into the overlying Low Diversity Interval.. The top of the Sierrite represents a major shift in depositional style, marked by the appearance of thrombolitic and stromatolitic mounds within meter- to decameter-scale depositional cycles. The reefal interval gives way upward to bioturbated lime mudstone and fine grainstone that are in turn overlain by the dark, bioturbated, oolite-rich grainstone of the “Jose Member” of the Hitt Canyon Formation. This interval in west Texas is thicker and lighter in color, but includes thin units of a deeper water, organic-rich, ribbon and nodular limestone mudstone and black shale facies. Our stratigraphic data provide for detailed correlation across the study area and to sections in the central and northern Rockies.