MASS-BALANCE CONSTRAINTS ON STRATIGRAPHIC INTERPRETATION OF LINKED ALLUVIAL-COASTAL-SHELFAL DEPOSITS FROM SOURCE TO SINK: EXAMPLE FROM CRETACEOUS WESTERN INTERIOR BASIN, UTAH AND COLORADO, U.S.A.

摘要

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.