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Mass-Balance Constraints on Stratigraphic Interpretation of Linked Alluvial-Coastal-Shelfal Deposits: Cretaceous Western Interior Basin, Utah and Colorado, USA


The Upper Cretaceous Blackhawk Formation, Castlegate Sandstone, and related strata exposed in the Book Cliffs of east-central Utah are widely used as an archetype for the sequence stratigraphy of marginal-marine and shallow-marine strata. Stratigraphic architectures in these strata are classically interpreted to reflect forcing by relative sea level. However, key aspects can instead be attributed to variations in sediment flux, which are characterised using a mass-balance framework that captures the rate of upsystem-to-downsystem loss of sediment (mass) and the spatial distribution of accommodation. Facies partitioning and sediment budgets are estimated for eight stratigraphic intervals, in order to compare temporal dynamics of the sediment routing system from erosional source to depositional sink. Mapping of each stratigraphic interval and its constituent segments, from upsystem to downsystem, was achieved along a representative, dip-oriented 2D cross section over a distance of c. 350 km. The cross section provides time-averaged estimates of the spatial distribution of deposition in each interval. Sediment supply characteristics for each of the eight stratigraphic intervals are constrained by total facies proportions in each interval. Comparison of the downsystem mass-balance characteristics of the eight stratigraphic intervals suggests that there were depositional gains and losses of shallow-marine shale in the five youngest intervals, which can be attributed to along-strike sediment transport. This result is consistent with increased interaction through time with vigorous wave- and tide-driven circulation in the seaway, as the sediment-routing system advanced out of a sheltered embayment in response to decreasing tectonic subsidence rate. In the youngest stratigraphic interval, the upstream-unconformable base of the Castlegate Sandstone is marked by a pronounced increase in the sand- to gravel-grade mass fraction of the fluvially supplied depositional volume. This marked increase can be attributed to hinterland unroofing and/or cannibalization of wedge-top basins, leading to export of coarse-grained sediment into the Castlegate fluvial system. Our results demonstrate the value of analyzing downsystem sediment loss within a mass-balance framework as a simple and practical tool to quantify the relationship between accommodation and sediment supply, and thus to decode past external forcing mechanisms from stratigraphic architecture.