No change in fluvial style across a sequence boundary, Cretaceous Blackhawk and Castlegate Formations of Central Utah, U.S.A.

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 Black- hawk 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.