MICROSTRATIGRAPHY OF AN UPPER TRIASSIC-LOWER JURASSIC DINOSAUR QUARRY

The Upper Triassic - Lower Jurassic Nugget Sandstone was deposited within a great sand sea that once covered much of the western US. In 2008, the Saints and Sinners Quarry was discovered near Dinosaur National Monument (DINO) in northeastern Utah. So far, the quarry has produced thousands of theropod dinosaur bones from a bone bed horizon about a meter thick. Outcrop scale relationships of the interdunal sedimentary layers that contain the bonebed and data from quarry maps and studies of the bone occurrences have been used to constrain hypotheses on how this deposit formed. However, smaller scale features of the sediments of the bone bed are not apparent to the naked eye or at levels of magnification possible in the field.

Oriented samples were collected from several locations distributed laterally and vertically throughout the quarry and the adjacent interdunal sediments, and the locations of the samples have been mapped. Thin sections have been prepared from the samples for further microscopic analysis. Initial results show that the sediment consists of quartz grains that are subangular to subrounded in shape with generally low sphericity. Most of the sediments appear to be moderately to well sorted with a few samples more poorly sorted. No preferred orientation of the quartz grains is apparent, but overall bedding orientation might be determined by the cement and iron oxides in the samples. Ongoing analysis of these samples continues in order to identify trends in how the sediment varies within the quarry.