Sedimentology of the Dakota Formation (Cretaceous), Uinta Mountains, Northeastern Utah

Beds of the Dakota Formation in northeastern Utah represent a fluvial environment in which deposition occured in two basic types of streams: large meandering streams and smaller alluvial plain streams. The large meandering streams were characterized by lateral migration and point-bar deposits. These deposits are relatively thick have high sand/shale ratios ranging from 2.3/1 to 13/1 and have laterally continuous individual channels. Also, the outcrop itself is laterally continuous over several hundred feet forming thick, unbroken exposures of sandstone. Deposition was cyclical with as many as six vertically adjacent channel deposits. The alluvial plain streams were characterized by channel avulsion and abundant overbank material resulting in an en echelon arrangement of ancient channels. These deposits are thinner and have lower sand/shale ratios that are about 1/1. Individual channels, as well as the entire outcrop, are less extensive laterally than in the larger meandering stream deposits. text;ural parameters, especially grain size, are useful in distinguishing sandstone and siltstone originating in both types of streams and overbank areas. Petrographic differences are more limited in usefulness. The significant differences occurred in sandstone and siltstone that were deposited in different flow conditions within the large meandering stream channels. The contrasts are in the percents of matrix and cement and in the relationship of quartz to matrix and cement. All of the Dakota channel deposits are quartz-rich and are quartzarenite, sublitharenite and subarkose. Evaluation of paleocurrent azimuths taken from cross-stratified sedimentary structures showed that the azimuths are fairly well concentrated in the channels for both types of streams. The similarity of paleocurrent information suggests that the alluvial plain streams were also meandering. The net regional direction of sediment movement was to the north with the ultimate site of deposition possibly being west-central Wyoming. In studying the overall dispersal system, it was found that the large meandering streams evolved through two stages of development. First, this stream system was characterized by degrading streams in disequilibrium with the surroundings. Later, (in Dakota time) this stream system reached "grade" and equilibrium with the surroundings. Both stages of stream development are registered in all outcrops of large meandering stream deposits. A new upper contact is proposed at the base of the "coarsegrained unit". This unit is a pebbly sandstone and conglomerate heretofore included within the Dakota. It is believed, however, that this unit is genetically related to the transgressing Mowry sea, represents a transitional environment between continental and marine conditions, and was strongly affected by tidal currents. The unit is therefore considered part of the Mowry Formation. The source area for Dakota sediment was probably the Meso- cordilleran geanticline. The composition of the source terrane was was strongly dominated by sedimentary rocks. The environment of deposition probably was a low-lying alluvial, coastal plain. The first stage of development of the large meandering streams was farthest from the sea. The second stage and the alluvial plain streams were evidently much closer to the sea.