ORIGIN OF THE CARNEGIE QUARRY SANDSTONE, MORRISON FORMATION (JURASSIC), AT DINOSAUR NATIONAL MONUMENT, UTAH

The Carnegie Museum dinosaur quarry at Dinosaur National Monument was developed in a sandstone unit locally referred to as the "Quarry Sandstone” bed of the Morrison Formation. The Quarry Sandstone is a localized, discontinuous, heterolithic unit that is present within the Brush Basin Member, a thick succession of greenish gray mudstone containing thin limestone beds and nodules. These fine-grained strata have been interpreted as being deposited in low-relief paludal and lacustrine environments. Some authors have attributed the genesis of the Quarry Sandstone to a variety of river channel deposits, including meandering and braided. Detailed study of numerous measured sections through, and along, the Quarry Sandstone's 1.8-km outcrop belt indicates that this sandstone is actually an assemblage of a number of separate sandstone bodies. The lowest part of the Quarry Sandstone is a laterally continuous, tabular unit composed of graded sandstone and conglomerate that overlies lacustrine mudstones of the Brushy Basin Member. This sandstone unit contains re-sedimented carbonate nodules from the surrounding Brushy Basin, and a sharp basal contact. Overlying the tabular basal unit are numerous, discontinuous sandstone bodies. Some of these units exhibit erosional bases and fine upsection, while others display flat bases and coarsen upsection. These discontinuous sandstone bodies interfinger with reddish-brown, rooted, mudstones containing calcic paleosols. The heterolithic character and rapid vertical and lateral facies variations exhibited by the Quarry Sandstone indicate that this unit was deposited as a local crevasse splay that formed during multiple channel avulsion events. The Carnegie Quarry was opened within one of the distributary channel sandstones of this splay complex.