MUDROCK CHEMOSTRATIGRAPHY OF THE GRANEROS SHALE - PRELIMINARY CHEMOFACIES ANALYSIS AND SEQUENCE STRATIGRAPHIC INTERPRETATION

Quantitative chemostratigraphic analyses of mudrock successions reveal these units record significant facies shifts resulting from changing environmental and depositional conditions. Variations in these chemostratigraphic profiles can be used to identify key stratigraphic surfaces and construct sequence stratigraphic frameworks within apparently homogeneous mudrock successions. An outcrop of Graneros Shale, located near Morrison, CO, exposes ~132’ of mudrock and allows preliminary chemostratigraphic analysis of the Western Interior Cretaceous Seaway (WIKS).

Detrital sediment input correlates with Ti, Zr, Al, and K. The degree of basin restriction correlates with redox sensitive trace metal concentrations, barring certain mineralogical affinities. The quartz fraction is estimated by plotting the Si/Al ratio. Sequence stratigraphic frameworks can be developed using the following general criteria. Progradational packages record increasing concentrations of Ti, Zr, Al, and K. Retrogradational packages record declining trends in these elements, indicating the Transgressive Systems Tract (TST). Lowstands Systems Tract (LST) will transition from a progradational to aggradational packages, while Highstands Systems Tract (HST) transition from aggradational to progradational packages.

The Graneros Shale, an argillaceous mudrock unit in the Denver Basin, was deposited during the incursion of the WIKS in the Cenomanian. The Si/Al ratio tends to follow the detrital indicators, suggesting the quartz component is primarily detrital in this area. The base of the section (0-15’) preserves chemofacies indicative of weak to moderate bottom water restriction with several zones containing sand-rich sediment gravity flows and high phosphorous concentrations. The basal Graneros Shale also records a progradational chemostratigraphic trend and is interpreted as LST. The middle zone (15-80’) preserves chemofacies indicating moderate to high restriction. This zone shows a general retrogradational trend (TST). The uppermost zone (80-132’) preserves chemofacies indicating a general decline in restriction with punctuated instances of high restriction near the top of the exposed section. This zone shows an aggradational trend with a slight shift to progradation near the top of the section (HST).