Abstract

The isopach map is a standard tool for geologists, used for interpreting the development of stratigraphic sequences within sedimentary basins and the structural history of those basins. However, compaction distorts the original depositional “geometries” of sediments. By “decompacting” the sediments to their original thicknesses, the subsidence of the crust below a basin can be estimated. Subsidence values and isopach values can be statistically compared by creating product maps, which may delineate structurally related features that do not appear on present-day isopach maps. These features can then be compared with sandstone maps to determine if they exerted control on lithofacies distribution.

The Lower Cretaceous interval in the Deep Basin of northwestern Alberta can be divided into three main intervals: the Bullhead Group and Bluesky Formation, the Spirit River Formation, and the Peace River Formation. Subsidence analyses have been carried out for each of these intervals.

Product mapping, which compares different maps of the same geological interval, shows that some apparently thick accumulations indicated on isopach maps occur in areas that had lower than average subsidence. Alternatively, some thin accumulations occur in areas that had higher than average subsidence. The product map for the Bullhead Group to Bluesky Formation interval suggests that a central region, between Townships 69 and 72, Ranges 8 to 14W6, was more structurally stable than the surrounding regions. This feature reappears on product maps for the Falher Member, suggesting some underlying basement control from late Barremian to late Early Albian. Sandstone maps indicate that the highest percentage of sand and conglomerate are found in this area. Since the feature was stable relative to the more rapidly subsiding areas to the north over the Peace River Arch (Clearwater shale basin), the Clearwater sea could not move farther south. Consequently, regressive Falher shoreline sands and conglomerates were stacked on top of each other. A product map for the Paddy Member shows that the occurrence of postulated barrier bar sands (north of Township 78) appear to have been controlled by relatively greater subsidence over the Arch.