COMPLEX INTERACTIONS BETWEEN FLUVIAL CHANNELS AND SYN-DEPOSITIONAL PEDOGENIC CARBONATE IN THE LOWER CRETACEOUS CEDAR MOUNTAIN FORMATION AND DINOSAUR NATIONAL MONUMENT, UTAH AND COLORADO

This presentation summarizes findings of three seasons of undergraduate research projects on the Lower Cretaceous Cedar Mountain Formation within and surrounding Dinosaur National Monument. A middle member of the formation correlates to the Ruby Ranch Member defined by Kirkland and others in central Utah; that nomenclature is adopted here. Although the Ruby Ranch Member is primarily a clastic unit, thickly-developed palustrine carbonate horizons occur commonly within its fine-grained sequences.

Within the Ruby Ranch Member, architectural relationships between pebbly channel deposits and underlying and lateral carbonate horizons suggest that channel behavior was significantly constrained by already-indurated carbonates. Intraclastic cobbles of fine-grained carbonate, occurring within pebbly fluvial beds, were derived from undercutting and lateral erosion by stream channels. Incised channels with limited lateral mobility are characteristic of anastamosing river systems, an interpretation further supported by paleocurrent data and field observations of low-sinuosity channel geometries.

The architecture observed in the Cedar Mountain Formation is matched by flume experiments. Shallow flow over an even sand bed was established in an 8’ flume. After a braid plain was developed, flow was stopped and the sand surface was sprinkled with a 3-4 mm layer of bentonite clay. The clay absorbed water from the braid plain, and was allowed to partially dry and stiffen. When water flow was resumed, braided channels quickly incised through the bentonite layer, usually via nickpoint migration. Once the channels were established, lateral migration of channels was restricted by the bentonite layer.

These experiments, along with observation of modern anastamosing systems, show the development of deep pools at channel intersections. Pool formation and subsequent filling during flood stage suggests a mechanism for rapid burial of Abydosaurus mcintoshi in a thick sandy sequence within the Cedar Mountain Formation.