Abstract: 

Experimental work suggests that the rate of upstream-to-downstream loss of sediment from an active depositional system to permanent storage exerts a fundamental control on stratigraphic architecture. This rate of sediment (mass) loss is determined by the spatial distribution of tectonic subsidence and rate of sediment supply. The character of input sediment (grain-size distribution and composition) is the third parameter that affects stratigraphic architecture. We apply this concept in a mass-balance framework to linked alluvial–coastal–shelfal deposits of the Upper Cretaceous Castlegate Sandstone, Blackhawk Formation, Star Point Sandstone, and Mancos Shale (Western Interior Basin, Utah and Colorado, USA). 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 using extensive outcrop exposure and densely spaced subsurface wells. The cross section provides time-averaged estimates of the spatial distribution of deposition. Grain-size data show that there is limited downsystem fining of any particular facies within the Castlegate Sandstone, but that the proportion of facies changes systematically downsystem to accommodate an overall fining trend. Therefore, it is reasonable as a first approximation to use facies proportions as a “textural replacement” for grain size. Sediment supply characteristics for each of the eight stratigraphic intervals are constrained by total facies proportions in each interval. For each stratigraphic interval, we assess the level of interaction between alluvial and coastal-to-shelfal segments of the routing system.

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. 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 import of coarse-grained sediment into the Castlegate fluvial system. Our results demonstrate the value of analyzing downsystem sediment loss (i.e., downsystem mass extraction) 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.

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