NON-MARINE TO MARGINAL-MARINE TRANSGRESSIVE SEQUENCE OF THE CRETACEOUS DAKOTA FORMATION IN NORTHEAST UTAH

This study reports on sedimentary structures and vertebrate trace fossils in the Late Cretaceous (Cenomanian) Dakota Formation in northeast Utah, between Vernal and Dinosaur National Monument. The formation is a laterally continuous cliff or ridge-former, with thicknesses ranging from 30 to 45 meters. Dakota Formation channels scour deeply into the underlying Cedar Mountain Formation. The lower beds (lowest ~10 meters) of the Dakota Formation consist of coarse-grained, trough cross-bedded sandstones in complex multi-story channels, representing a braided fluvial environment. In the next interval (~10-15 m), trough cross bedding is commonly overturned or recumbent. Recumbent cross bed sets are immediately overlain by upper-flow-regime planar-laminated beds, indicative of high bed shear stresses. The middle fluvial facies passes upward into shallow marine facies, indicated by wave-rippled and linguoid-rippled fine-grained sandstones, mud drapes in dune-scale and ripple-scale foresets (i.e., flaser bedding) and very low-angle cross stratification characteristic of beach deposits. Paleocurrent orientations further suggest tidal influence on deposition; most trough cross-beds in the middle and upper intervals indicate westward currents, directly opposite to the direction expected if Dakota Formation sandstones were deposited strictly by fluvial systems flowing eastward from the Sevier fold and thrust belt. We interpret this vertical sequence of sedimentary structures as resulting from a marine regression, which resulted in channel incision into underlying strata, followed by a gradual transgression that resulted in a tide-dominated delta. Trace fossils in the uppermost 2 meters of the Dakota Formation include tracks of a three-toed vertebrate, most likely a bird; bird tracks in the Dakota Formation have been reported elsewhere in the Colorado Plateau. Tracks are 2.3 cm in length and 3.0 cm in width, with a >90° angle between first and third toes distinguishing them from tracks typical of a small theropod. The bird tracks occur on the surface of a fine-grained, current-rippled sandstone bed. A bed within the same interval of uppermost Dakota Formation contains additional traces of three closely-spaced parallel slashes, interpreted scratch marks made by a similar three-toed organism.