STRATIGRAPHY AND GEOCHEMISTRY OF THE BRUSHY BASIN MEMBER OF THE MORRISON FORMATION NEAR THE AARON SCOTT QUARRY (UTAH) INDICATING AN ENVIRONMENTAL CHANGE FROM LACUSTRINE TO FLUVIAL DEPOSITIONAL SYSTEM

The Aaron Scott Quarry (Brushy Basin Member, Morrison Formation, Jurassic) was deposited on the margin of a large lake with a delta along the eastern margin and flow to the west. Rare earth element analysis of bones from the middle rip-up clast layer indicate fluvial deposition, while bones of an associated Barosaurus skeleton indicate lacustrine deposition. Micro-scale sequence stratigraphy in the area of the quarry shows a prograding delta westward into the lake. Geochemistry of the bones in the bone beds compared to sediments in the bone beds and the overall stratigraphy of the Morrison Formation are used for interpretation of the depositional environment of the bone beds. The bone beds are within a thick calcite-cemented siltstone and contain abundant barite nodules throughout the layers and along bones. The presence of calcite cementation and barite nodules is consistent with organic matter decay. Stratgiraphically, the Aaron Scott Quarry is in the lower Brushy Basin in dominantly green mudstone. The upper Brushy Basin in this area is red and green paleosols, indicating that the lake had filled with sediment and the region became floodplain deposition.