DISTRIBUTION AND ABUNDANCE OF BENTHIC MARINE TAXA IN SHELF MARGIN DEPOSITIONAL SEQUENCES: SAN ANDRES FORMATION (GUADALUPIAN, PERMIAN), LAST CHANCE CANYON, NEW MEXICO

The San Andres Formation in Last Chance Canyon is a mixed carbonate-siliciclastic succession consisting of two fourth-order depositional sequences, both of which contain multiple fifth-order sequences. Outstanding exposures of these shelf-margin sequences provide a framework to test for controls of fossil occurrences and abundance relative to sequence architecture. We report preliminary observations on the occurrence and abundance of fossils within one fourth-order depositional sequence. The lowstand systems tract is characterized by fossiliferous sandy peloidal grainstone. The highstand systems tract is characterized by massive bioturbated very-fine-grained sandstone, which overlies and downlaps onto the sandy peloidal grainstone. Micritic dolomite and sandy peloidal packstone interfingers with the bioturbated sandstone near the top of the sequence and terminate against the overlying Grayburg Formation sequence boundary in a toplap relationship. Fossil occurrences and abundance were mapped onto the north face of Last Chance Canyon using a combination of measured sections, GPS, terrestrial lidar scans, and Gigapan images. Bivalves and gastropods were found just beneath the downlap surface within the sandy pelloidal grainstone. Fusulinids (Parafusulina) and echinoid spines dominate fossil assemblages directly above the downlap surface within the very-fine-grained sandstone. Fossil abundance decreases markedly above the downlap surface with only the occasional sponge or brachiopod recorded. Near the top of the sequence, sponges and brachiopods increase in abundance. The very-fine-grained sandstone is capped by shellbed concentrations of Parafusulina, sponges, and brachiopods. Fossil occurrences and abundance in Last Chance Canyon are concentrated around the fourth order downlap and toplap surfaces suggesting that condensation, reflecting low net accumulation of sediment, is the primary control on fossil occurrence and abundance.