PALEONTOLOGY AND SEDIMENTOLOGY OF A TURONIAN (LATE CRETACEOUS) LAGOON, SOUTHWESTERN UTAH

A preliminary study of fossils recovered from UMNH (Utah Museum of Natural History) locality IP24 in Bryce Canyon National Park in the Smoky Hollow Member of the Straight Cliffs Formation (middle Turonian, Late Cretaceous) revealed previously unreported vertebrate and invertebrate taxa. The most fossiliferous horizon is in a carbonaceous siltstone immediately above one of the lignite layers about 15.5 meters above base of the member. Fossils were recovered by wet screen-washing, a technique traditionally not applied to invertebrate localities. Many of these taxa (including numerous brackish water gastropods and bivalves including oysters, and several possibly fresh water taxa including ostracods, pycnodontid fish, crocodilians, myledaphid rays, shark, amiids, and gars) have not been previously reported from brackish water localities in the Smoky Hollow Member. The recovered taxa suggest an intermingling of fresh and brackish water taxa as would be expected in a lagoonal environment. Stratigraphic sections from multiple sites were measured and depositional features analyzed. The Smoky Hollow Member overlies the regressive marginal marine sandstones of the Tibbet Canyon Member. The stratigraphic sequence in the lower part of the Smoky Hollow Member represents a continuation of that regressive sequence and is composed mostly of fine-grained sandstones and a highly variable sequence of organic rich siltstone-lignite couplets ~12.5 to 32 m above the base of the member, each about one meter thick. These couplets probably represent autocyclic or possibly Milakovitch driven cycles. As interpreted here, the sandstones represent the barrier beach environments, the organic rich siltstone the lagoonal environment, and the lignites the adjacent swamps. These facies are highly variable both laterally and vertically, suggesting local controls such as channel and bar migration. The absence of marine or brackish water fossils below the lignite and the lack of sedimentological structures suggest reworking of sediment by wave-action. The lignite and organic rich siltstone layers along with taxonomic associations of oyster beds, other brackish water taxa as well as fresh water taxa suggest a coastal lagoon setting with adjacent barrier beaches and swamps.