Blackhawk Formation

Abstract: Three macroscopic diagenetic features can be recognized in the sandstones of the Upper Cretaceous Desert Member of the Blackhawk Formation and Castlegate Sandstone of the Mesaverde Group exposed in the Book Cliffs, Utah, each of which have distinctive form, geometry, and stratigraphic distribution. Diagenetic alterations are: (1) leached zones ("whitecaps"), up to 10 m thick, beneath coal beds; (2) large (up to 8 m) concretionary carbonate-cemented bodies in amalgamated shoreface and thin fluvial sandstones; and (3) thin (up to 2 m), laterally extensive carbonate-cemented horizons beneath major marine flooding surfaces. Each feature has distinct petrographic and geochemical signatures, and formed through discrete diagenetic processes. Large isolated carbonate-cemented bodies are composed of ferroan dolomite, most of which precipitated during early diagenesis. Field and petrographic data, coupled with stable-isotope data (early cements, 13C = -2.5 to +3.4o/oo VPDB; 18O = -7.8 to -12.0o/oo VPDB; 87Sr/86Sr = 0.7078; later cements, 13C = -3.1 to -5.7o/oo VPDB; 18O = -12.0 to -15.1o/oo VPDB; 87Sr/86Sr = 0.7093) suggest precipitation from meteoric fluids, input into sediments during times of relative sea-level fall. The source of carbonate for the dolomite cement was dissolution of detrital dolomite from beneath coals by organic acids and subsequent mobilization by meteoric fluids. Carbonate precipitation in laterally extensive cement horizons appears to have started as a result of hiatus in sediment accumulation during marine flooding events (relative sea-level rise). Cement precipitation in these horizons continued through sediment burial as a result of organic-matter oxidation reactions in overlying organic-rich mudstones. The results of this study show a link between sedimentation (related to changes in relative sea level) and diagenesis, leading to the potential for the development of process-based, predictive models of early diagenesis in depositional successions.

Abstract: The Spring Canyon Member of the Blackhawk Formation in Coal Creek Canyon, Utah, consists of four regressive nearshore-to-offshore sequences punctuated locally by hummocky cross-stratification. Collectively, nonstorm bedding units span lower offshore to middle shoreface lithofacies. Associated ichnofaunas tend to be diverse, distinctive, and diagnostic of original depositional gradients. Nevertheless, all resident ichnofaunas are referable to the Cruziana ichnocoenose. Ichnofaunas of hummocky beds, in contrast, mainly represent either a Skolithos ichnocoenose or a mixed Skolithos-Cruziana ichnocoenose. These post-storm ichnocoenoses evidently correspond to a sere of opportunistic pioneers or ensuing seres of resilient resident populations, although distal biocoenoses may have remained essentially beyond reach of prevalant nearshore opportunists. The predominance of domichnia over fodinichnia in initial post-storm ichnocoenoses probably reflects original larval settlement patterns tempered by gradients in hydraulic energy. Differences in ichnofaunas also are related to differences in rates of post-storm deposition: the slower the rate of sediment accumulation, the greater the degree of overprinting by burrows of subsequent seres or equilibrium communities.

Abstract: Parasequence architecture and the nature of parasequence boundaries in marine to nonmarine strata are well illustrated in the Spring Canyon Member of the Upper Cretaceous Blackhawk Formation. Parasequences and parasequence sets are stratal successions which are the building blocks of sequences. In marine strata, parasequences result from basinward progradation of the shoreline, and typically shallow and coarsen upward; in the nonmarine, parasequences show a distinct vertical facies succession which begins with lagoon-fill deposits and ends with freshwater coals. A flooding surface (parasequence boundary), indicating an abrupt increase in water depth, accompanied by minor submarine erosion and nondeposition separates individual parasequences within a parasequence set. The parasequence boundary is a continuous, single surface that can be traced from updip in the coastal plain to downdip in the distal shelf. The parasequence boundary has different physical expressions depending on where it is observed, and enables correlation of nonmarine/marginal marine rocks to coeval marine strata within the same parasequence. Parasequence evolution and depositional reconstruction is dependent on the application of sequence stratigraphic concepts. Outcrop examples from the Spring Canyon Member document parasequence expression. Both wave-dominated shoreface sandstone and river-dominated deltaic sandstone exist laterally in the marine portion of the same parasequence. Both are terminated by a flooding event marked by a rapid landward shift in facies, with no transgressive lag. A number of marginal marine and nonmarine subenvironments exist laterally within the same parasequence. The parasequence boundary provides a

Abstract: Nonmarine sequence stratigraphic models are based largely on studies of fluvial units in the Cretaceous Western Interior of North America, including the Blackhawk and Castlegate formations of central Utah. These models suggest that fluvial units should show a transition from mudstone-prone, isolated meander-belt deposits of the transgressive and highstand systems tract into amalgamated braided-stream sandstones of the lowstand systems tract. These mudstone-rich fluvial strata are largely assumed, rather than documented, to represent the deposits of isolated meander-belt deposits, despite recent work that suggests that there is no simple relationship between the proportion of preserved floodplain mud and fluvial style. There are numerous studies documenting the braided character of the sandstone-rich Castlegate Formation, but there are no corresponding studies that document the internal fluvial facies architecture within the Blackhawk Formation to support the interpretation that the Blackhawk comprises the deposits of a meandering-stream system. We test the validity of the sequence stratigraphic model by comparing dimensions and styles of associated bar and channel deposits, as well as cross-stratal thicknesses, between the Blackhawk and Castlegate formations along Salina Canyon in Utah. Data compiled from 24 measured sections and 6 photomosaics show that the Blackhawk Formation comprises isolated sandy channel-belt sheet sandstones, 5–8 m thick, contained within thick floodplain mudstones, whereas the Castlegate Formation consists of about 80 m of amalgamated sandy channel-belt sheets, 4–7 m thick, with only minor mudstone. The average height of cross-sets is 13 cm in the Castlegate Formation and 14 cm in the Blackhawk Formation and formative dune height is estimated to be 38 cm for the Castlegate Formation and 45 cm for the Blackhawk Formation. Average bar-accretion thicknesses are 2 m in both the Blackhawk and Castlegate formations. Corresponding water depths are estimated to be 2.5–4.1 m for the Blackhawk and 2.3–3.8 m for the Castlegate. The correlation of master bedding planes in the Blackhawk Formation shows overlapping lens-shaped bar deposits and channel fills. Bar tops appear to dip in several directions, indicating both lateral and downstream accretion of mid-channel braid bars. While there is some indication that Castlegate rivers were slightly shallower, the differences do not suggest a major change in fluvial style across the sequence boundary; both are braided. The lack of change in scale of channels also is not compatible with a hypothesized increase in aridity between the Blackhawk and the Castlegate. The difference in fluvial architecture between the two formations is interpreted to reflect changes in accommodation that were likely tectonic in origin. This supports that idea that preservation of thick successions of fine-grained overbank material is not a function of the plan-view channel geometry.