CAMBRIAN–ORDOVICIAN STRATIGRAPHY OF WEST TEXAS AND SOUTHERN NEW MEXICO

The Bliss Formation and lower part of the overlying El Paso Group in western Texas and southern New Mexico contain a record of Cambrian–Ordovician transgression onto the southwest margin of Laurentia. The quartz- and carbonate-cemented, very fine sandstone to pebble conglomerate of the Bliss Formation displays radical changes in thickness over short distances, which reflect deposition over a highly irregular Precambrian basement. Trilobites, brachiopods and graptolites 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 in the Caballo Mountains), consists almost entirely of amalgamated parallel laminated and HCS grainstone beds. This formation strongly resembles, in thickness and character, the lower Manitou Formation in western and central Colorado indicating that much of the inner detrital belt of western Laurentia was transformed into a high-energy, storm-dominated carbonate shelf early during deposition of the Rossodus manitouensis conodont Zone. Assignment of the highest 10-15 of the Sierrite to the Leiostegium-Kainella trilobite Zone on the recovery of the eponymous genera and supportive carbon isotope data indicates that this style of deposition continued into the overlying Stairsian Stage (Low Diversity Interval in conodont biostratigraphy). 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 are overlain by the dark, bioturbated, oolitic grainstone of the “Jose Member” of the Hitt Canyon Formation. In west Texas, this member contains thin intervals of deeper water, organic-rich, ribbon and nodular lime mudstone with black shale interbeds. The succeeding highstand deposits contain depositional cycles with microbial mounds capped by planar microbial laminites and scattered terrigenous sand. A prominent sequence boundary and the Pistol Range Sandstone cap these cycles. The lower El Paso Group is a single thick depositional sequence that paradoxically contains a prominent oolite unit as its most distal facies.