Abstract

The Carnegie Quarry at Dinosaur National Monument was excavated in the Quarry Sandstone bed of the Brushy Basin Member of the Morrison Formation (Jurassic). The Quarry Sandstone is a discontinuous, lenticular, heterolithic unit that rests upon drab, greenish-gray claystones and is capped by reddish-brown mudstones. The underlying drab claystones are interpreted as being deposited in an overbank interfluvial wetland or lacustrine environment. By tracing the sandstone and studying detailed measured stratigraphic sections along its 1.8-km outcrop belt, this interval can be shown to be a complex of intergrading sandstone bodies and mudstone units. The Quarry Sandstone interval is reinterpreted as being deposited within a crevasse splay complex. The lowest part of this deposit is a laterally continuous, tabular unit composed of polymictic sandstone and granule conglomerate that directly overlies lacustrine claystones. This unit contains re-sedimented carbonate clasts from the surrounding Brushy Basin, displays a gradational to sharp basal contact, and is interpreted as the initial crevasse splay deposit. Overlying the tabular basal unit are numerous, laterally continuous, upward-coarsening sandstone bodies interpreted as distributary mouth bars. The interpreted distributary mouth bar deposits are cut by lenticular, trough cross-bedded, upward fining sandstone units exhibiting erosional bases. These sandstone lenses are interpreted as distributary channels within the crevasse splay. They, in turn, are overlain by discontinuous sandstone bodies that interfinger with, and are overlain by reddish brown and variegated, rooted mudstones containing calcic paleosols. The Carnegie dinosaur quarry was excavated in one of the main distributary channels of this splay complex.

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