SEQUENCE STRATIGRAPHY OF THE MID-CRETACEOUS DAKOTA FORMATION AND MOWRY SHALE, UINTA BASIN, UTAH AND COLORADO

The Dakota Formation and Mowry Shale of the eastern Uinta Basin comprise an overall transgressive interval consisting of continental, marginal marine, and marine strata. However, complex stratigraphic relationships within the Dakota Formation and Mowry Shale, and between both older and younger strata, have led to different interpretations regarding the stratigraphic architecture of the interval. In order to provide a better understanding of the Albian-Cenomanian rocks of the region, palynological samples from the Dakota Formation, Mowry Shale, and overlying units were collected from the northern and southern margins of the Uinta Basin. When combined with previous stratigraphic studies, these data can be used to construct a unifying sequence-stratigraphic model for Dakota and Mowry deposition. Results of this study indicate the Dakota Formation contains three, unconformity-bound depositional sequences. The first Dakota sequence is late Albian in age and consists of TST and HST fluvial and estuarine facies deposited above a regional LST unconformity. The second Dakota sequence was deposited above a second LST unconformity that formed during latest Albian time. In the northern Uinta Basin, TST fluvial and tidal deposits of second Dakota sequence are late Albian in age. Late Albian-Early Cenomanian HST marine deposits of the overlying Mowry Shale comprise the upper parts of the sequence. In the southern Uinta basin, TST and HST deposits of the second Dakota sequence are dominantly fluvial in origin. A middle Cenomanian decrease in sea level resulted in the development of a regional LST unconformity throughout the study area. In the southern Uinta Basin, incised TST fluvial channel complexes of the third Dakota sequence lie above the unconformity. The third Dakota sequence in this region is capped by the middle Cenomanian-Turonian Mancos Shale Tununk Member. In the northern Uinta basin, deposits of the third sequence are absent and the unconformity is situated at the erosional contact between the Mowry Shale and the Tununk Member.