SEQUENCE STRATIGRAPHY OF THE ALBIAN-CENOMANIAN NATURITA FORMATION AND LOWER MANCOS SHALE, EASTERN UTAH AND WESTERN COLORADO

The late Albian-Cenomanian Naturita Fm. and lower Mancos Shale in eastern Utah and western Colorado are interpreted as containing three unconformity-bound depositional sequences. The first Naturita sequence (Kn1) consists of fluvial channel conglomerates/sandstones and overbank mudstones deposited above a regional erosion surface above the Albian Ruby Ranch Member of the Cedar Mountain Fm. The second sequence (Kn2) contains lithologies similar to the Kn1. The Kn2 was deposited above an unconformity surface that was incised into the underlying Kn1 during latest Albian time. In places, this unconformity eroded most of the Kn1 interval and Kn2 deposits rest directly on the Cedar Mountain Fm. Above the Kn2 unconformity, the interval consists of fluvial and tidally-influenced sandstones, mudstone, coal, and bentonites. These non- and marginal-marine strata can be correlated with coeval shelf and shoreface deposits of the early Cenomanian Mowry Shale in the subsurface of Uinta and Piceance basins to the north. This same marine interval outcrops in western Colorado where Kn2 deposits are overlain by a transgressive surface of erosion and upward-coarsening beds of mudstone-sandstone in the lower Mancos Shale that are interpreted as Kn2 marine TST and HST deposits. Near Delta, Colorado this interval is overlain by pebbly, medium-coarse grained, trough cross-stratified, tidal-channel sandstones. The erosion surface at the base these deposits is interpreted as sequence-bounding unconformity that corresponds to the base of fluvial-channel sandstones that are erosionally incised into the top of Kn2 sequence fluvial deposits in eastern Utah. These fluvial/tidal-channel deposits represent LST and TST deposits of the third Naturita depositional sequence (Kn3). A transgressive surface of erosion and TST/HST marine shelf deposits of the Kn3 sequence overlies the basal interval. Palynomorphs, U-Pb zircon and ⁴⁰Ar/³⁹Ar sanidine ages of intercalated bentonite deposits, and age-diagnostic marine mollusks from the study interval indicate a late Albian depositional age for the Kn1 sequence, a latest Albian-early Cenomanian age for the Kn2 sequence, and a middle to late Cenomanian age for the Kn3 sequence.