FLUVIAL FACIES AND RECONSTRUCTION OF THE CRETACEOUS CEDAR MOUNTAIN FORMATION IN DINOSAUR NATIONAL MONUMENT, COLORADO AND UTAH

This study, prompted by a recent sauropod discovery in Dinosaur National Monument, investigates fluvial systems that deposited the Lower Cretaceous Cedar Mountain Formation. Volumetrically, the majority of the Cedar Mountain Formation is composed of fine-grained shales, mudstones, and siltstones, which show distinctive color variegation and mottling typical of paleosol horizons, and contain extensively developed pedogenic calcretes. Coarse-grained clastic facies of the Cedar Mountain Formation occur most commonly in single-story channel sandstones.

Composition of the basal channel conglomerates is bimodal, including chert pebbles and calcareous pebbles and cobbles. Most clasts are intraformational, eroded from calcrete horizons in floodplain soils. Minor chert pebbles and lithic sands may have been derived from unroofing of Paleozoic strata in the Sevier fold and thrust belt to the west, or more locally from erosion of the unconformably underlying Morrison Formation.

Regionally, paleocurrents measured from multiple ribbon sands indicate transport toward the north, northeast, and southeast. Within individual sand ribbons, paleocurrent analysis shows low variance, suggesting relatively straight channels. The observed pattern of clast lithologies and relatively straight channels is interpreted to result from channel incision into floodplain calcretes, which inhibited lateral migration.

Based on measurements of channel dimensions and clast sizes, we tentatively estimate that Cedar Mountain Formation river systems experienced minimum basal shear stresses of 8 N/m2 and minimum velocities of 0.4 m/s.

The observation of abundant highly polished chert pebbles throughout the Cedar Mountain Formation has led previous workers to suggest that they may be dinosaur gastroliths. These chert pebbles occur both in the bases of channel conglomerates and sandstones, and as isolated clasts within fine-grained overbank mudstones and calcretes. In nearly all occurrences, the highly polished chert clasts are anomalously large compared to their surrounding matrix. We demonstrate that invoking ordinary clastic fluvial processes to transport these clasts does not adequately explain their occurrence and distribution, and explore alternative mechanisms including transport by lithophagic vertebrates.