Tectonic and sequence stratigraphic implications of the Morrison Formation-Buckhorn Conglomerate transition, Cedar Mountain, east-central Utah

by Roca, Xavier.

The contact between the Morrison Formation and the Buckhorn Conglomerate has been interpreted as a low order unconformity bridging the Jurassic and Cretaceous Periods in the northwestern Colorado Plateau. However, sedimentologic and stratigraphic data gathered at Cedar Mountain, the Buckhorn type section in east-central Utah, provide new evidence of the conformable transition of the two stratigraphic units. This evidence consists of gutter casts, mud injection, and dinosaur tracks at the base of the Buckhorn as well as interfingering of the Buckhorn with Morrison mudstones. The Buckhorn braided river deposit is interpreted to be the result of the exhumation, reworking, and eastern propagation of older synorogenic coarse materials of a late Jurassic flexural foredeep located in western Utah. This interpretation, when combined with accommodation rates inferred from Late Jurassic fluvial styles, defines a Morrison depositional sequence. Evidence of the Jurassic onset of Buckhorn deposition precludes its chronostratigraphic correlation with similar Lower Cretaceous conglomeratic deposits found throughout the North American Western Interior.