FACIES ANALYSIS, DEPOSITIONAL ENVIRONMENTS, AND MICRO SEQUENCE STRATIGRAPHY OF SAINTS AND SINNERS DINOSAUR QUARRY STRATA, NUGGET SANDSTONE, NORTHEASTERN UTAH

The Saints and Sinners Quarry near Dinosaur National Monument in northeastern Utah preserves the only vertebrate body fossil fauna in the Nugget Sandstone and the most diverse fauna known from the Nugget-Navajo-Aztec sandstone erg. The fauna includes nine genera represented by more than 50 individuals, including theropods, sphenosuchians, sphenodontians, a drepanosaur and a pterosaur. Through a sedimentologic and stratigraphic study of the quarry strata, we understand the paleo-environment of this unique fauna assemblage in the Late Triassic Nugget Sandstone.

To determine facies, depositional environment, and development of quarry strata, we used field descriptions and petrographic analysis. Facies are grouped inton two depositional environments - dune and interdune. Large to small trough cross-stratified sandstones dominate the more than 50-m-thick Nugget Sandstone in the study area and are interpreted as dune facies. Massive sandstones are interpreted as bioturbated interdune facies. Wrinkly laminated sandstones intercalated with secondary iron-rich laminae are interpreted as damp interdune facies. Green silts and clays represent a flooded lacustrine interdune facies. The interdune facies are 10 m thick and ~90 m wide and include (1) bioturbated interdune, (2) damp interdune, and (3) flooded lacustrine interdune.

The spatial relationships of the various dune and interdune facies suggest the area fluctuated between arid and wet conditions. As the environment became less arid, large dunes gave way to smaller dunes, damp interdunes, and flooded interdunes. Water levels in the flooded interdune commonly fluctuated and led to interfingering of flooded interdune facies and bioturbated interdune facies along paleo-shorelines. This interfingering allows for application of sequence stratigraphic principles, and creates several interdune parasequences bounded by lacustrine flooding surfaces represented by mm-cm scale clay and silt drapes. Preliminary Results indicate that bones are preserved along paleo-shorelines in at least four decimeter-scale parasequences.