BIOFACIES INTERPRETATION OF A SHELF MARGIN SUCCESSION: SAN ANDRES FORMATION (GUADALUPIAN, MIDDLE PERMIAN), LAST CHANCE CANYON, NEW MEXICO

Gradient analysis of marine fossil assemblages often identifies an environmental gradient in faunal composition that varies with depth. This interpretation assumes that fossils are generally not transported out of habitat. In depositional settings dominated by gravity-driven sediment transport, skeletal material can be transported to environments far removed from where the species lived, complicating gradient analysis interpretations. The San Andres Formation in Last Chance Canyon is a mixed carbonate-siliciclastic, shelf margin succession dominated by gravity-driven sediment transport. We examine fossiliferous shell beds of the San Andres Formation in Last Chance Canyon through a single fourth-order depositional sequence to characterize biofacies changes in this shelf margin succession. Fossil occurrences and abundance were mapped onto the north face of Last Chance Canyon using a combination of measured sections, GPS, and GigaPan images. Four distinct assemblages were found within the sequence. Molluscan dominated assemblages were found below the maximum flooding surface (MFS) within a channelized peloidal sandstone facies. Fusulinid-echinoid assemblages are found directly above the MFS at the base of a massive sandstone facies. Fossil abundance decreases markedly higher in the massive sandstone facies where assemblages are dominated by brachiopods and sponges. Fusulinid dominated assemblages are found within the thin-bedded sandstone and carbonate facies above the massive sandstone. Graded bedding, soft sediment deformation, disarticulated fossils, and orientation of fusulinid tests within the channelized peloidal sandstone and the base of the massive sandstone indicates high energy environments that transported skeletal elements of the molluscan and fusulinid-echinoid assemblages down ramp and deposited fossils in outer ramp and basin turbidite fans. The stratigraphically higher brachiopod-sponge and fusulinid assemblages do not show evidence of transport and are not within high energy lithofacies. Although Detrended Correspondence Analysis of fossil assemblages shows a gradient of molluscan to fusulinid to brachiopod-sponge dominated assemblages, biofacies variation in this shelf margin succession reflects primarily fossil transport in a turbidite fan complex.