Abo Formation (early Permian) Sacramento Mountains, New Mexico : a dry alluvial fan and associated basin-fill
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Outcrops of the Abo Formation (Wolfcampian to early Leonardian age) in the Sacramento Mountains of south-central New Mexico record the evolution of a dry alluvial fan system as it was deposited off of the Pedernal Uplift into the Orogrande Basin. A mud-rich, upward-fining basin-fill sequence characterized by stratigraphic thickness variations of as much as 450 m (1480 ft) and contacts that range from conformable to unconformable relationships is observed in the outcrop area. Five distinctive lithologic intervals are delineated in the Sacramento Mountains, three of which are studied in detail. These three units are informally designated the lower, middle, and upper Abo intervals and are all fluvial in origin. The remaining two intervals, the Lee Ranch Tongue of the Abo Formation and the Pendejo Tongue of the Hueco Limestone, are marginal marine in origin and are restricted to the southern portion of the range. Two types of lithofacies are present in the lower, middle, and upper Abo intervals. Matrix facies represent fine-grained overbank sediments, whereas framework facies represent coarser streamflow and sheetflow deposits. Lower Abo framework facies consist of middle-fan to fan-fringe stream-channel, braided stream, and sheetflood deposits which are arranged in one or more stacked, assymetrical, upward-fining megasequences 10-50 m (33-165 ft) in thickness. Middle and upper Abo framework facies were deposited basinward of the fans and represent deposits of incised, low to moderate sinuosity ephemeral streams and associated overbank splays. Middle Abo streams exhibited an anastomosed channel pattern whereas upper Abo channels were characterized by a distributive pattern. Pedogenic and sedimentologic evidence suggests that Abo deposition was strongly influenced by a long term, semi-arid to arid climate. Lateral and vertical facies relationships indicate that the five Abo intervals were part of a northeast to southwest transverse facies tract consisting of 1) proximal alluvial fans, 2) medial anastomosed streams, and 3) distal low-gradient mud-dominated floodbasins characterized by either distributary streams or tidal flats with associated marine carbonates. The overall Abo basin-fill sequence reflects the migration of the facies tract in response to the final stages of tectonic activity and the eventual quiescence and long-term erosion, retreat (pedimentation), and onlap of the Pedernal Uplift.