A New Look At The Regional Sequence Stratigraphic Framework of the Classic Greenhorn T-R Cycle Across Northeastern Colorado, Southeastern Wyoming, and South Western Nebraska

A detailed regional sequence stratigraphic study of the Cretaceous Greenhorn Transgressive/ Regressive (T6/R6) Cycle within the Denver Basin was conducted using a grid of well-log cross sections, as well as geochemical data from three key cores. This classic T-R cycle in the Denver Basin traditionally includes the Muddy “J” Sandstone, Mowry Shale, Graneros, Greenhorn, and Carlile Formations. In this study, fourteen (14) depositional sequence boundaries and thirteen (13) petrophysically and geochemically distinct depositional sequences were defined within the classic Greenhorn (T6/R6) Cycle within northeastern Colorado, southeastern Wyoming, and southwestern Nebraska. Of these fourteen sequence boundaries, the most significant is a major regional (Mid-Early Cenomanian) angular unconformity (K600sb) located at the base of the Middle Member of the Graneros Formation. Regionally from north to south in the Denver Basin, this angular unconformity sequentially truncates the Lower Graneros, and then the Mowry Shale (D Sand/Huntsman). Thus, in the southern Denver Basin, near Pueblo Colorado, the Middle Graneros is juxtaposed on the Muddy Formation. With the recognition of this angular unconformity within the Graneros Formation, it becomes clear that the Muddy, Mowry (D Sand/Huntsman), and Lower Graneros strata are genetically unrelated to the overlying Middle and Upper Graneros, Greenhorn, and Carlile strata, and should be placed within a separate older (Mowry) T-R Cycle, not within, or part of, any younger (Greenhorn) T-R Cycle, as traditionally interpreted. Within the Mowry Cycle, two third-order composite sequences (M1 and M2) were defined. Two composite sequences were also defined within the updated Greenhorn T-R Cycle (G1 and G2). Within the composite sequence framework for the updated Greenhorn Cycle, the Bridge Creek Limestone represents the basal portions of a trans-gressive sequence set genetically related to the overlying Carlile Formation, and not the underlying portions of the traditional Greenhorn Formation. In the four composite sequences defined within this study, the maximum flooding surfaces (mfs) are placed at the base of siliciclastic-rich mudstones in the Mowry Shale, Hartland Shale, and Blue Hill Shale and within the siliciclastic-rich Muddy J-1 sand. This interpretation differs markedly from previous studies, where maximum flooding surfaces were traditionally placed within carbonate-rich units within T-R cycles (sequences).