Platform evolution and sequence architecture of an ancient (Pennsylvanian: Desmoinesian−Missourian) laterally mixed shelf, Sacramento Mountains, New Mexico

Pennsylvanian-aged mixed carbonate-siliciclastic deposits from the southwestern United States contain records of high-amplitude, high-frequency sea-level changes associated with the late Paleozoic ice age that influenced deposition of sediments over the backdrop of Ancestral Rocky Mountains orogenesis. These ancient mixed systems are natural laboratories for understanding vertical and lateral facies transitions and offer insight into extrinsic controls on sediment partitioning during global icehouse periods and active tectonism. This study of >75 km of nearly continuous Desmoinesian−Missourian outcrops in the Sacramento Mountains of New Mexico documented contemporaneous carbonate and siliciclastic deposition on a narrowing shelf bordered to the east by the actively uplifting granitic Pedernal massif and to the west by the actively subsiding Orogrande Basin. Data for this study came from more than 30 measured sections (>5000 m), one core, fusulinid and conodont biostratigraphy, outcrop photomosaic mapping, paleocurrent analysis, and extensive hand sample and petrographic observations. The result is a well-constrained outcrop example of an ancient platform-scale laterally mixed system, with only minor evidence for reciprocal deposition, that developed during the early stages of the late Paleozoic ice age. A wave-swept carbonate shelf (Bug Scuffle Member) dominated by nutrient-tolerant, filter-feeding heterotrophs and limited photoautotrophs is bisected by a siliciclastic fan delta (Gobbler Formation). Reconstructions of facies distributions and evolving platform-to-basin orientation demonstrate a dynamic coastal setting that transitioned from a south-dipping siliciclastic-dominated shelf in the Atokan to a west-facing mixed carbonate-siliciclastic shelf by the late Desmoinesian driven by rejuvenated unroofing of the Pedernal uplift and coincident subsidence of the Orogrande Basin. By comparison with coeval mixed carbonate-siliciclastic deposits from the Paradox Basin, this study demonstrates the importance of local climate, tectonics, and siliciclastic routing in cycle development and carbonate factory composition.