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Cyclic Marine Sedimentation in the Lower Cretaceous Luscar Group and Spirit River Formation of the Alberta Foothills and Deep Basin
Outcrop studies in the marine to nonmarine transition of the Moosebar and Gates formations, in the Cadomin and Grande Cache areas (western, deformed Deep Basin), together with studies of the subsurface Spirit River Formation (undeformed Deep Basin), have identified several transgressive-regressive cycles in an overall prograding shoreline succession. Six marine to brackish water cycles are recognized in outcrop in the Grande Cache area, the lower three associated with possible wave dominated prograding deltas and/or strandlines, the upper three having a more brackish (lagoonal, tidal channels, etc.) association. Only the lower four cycles are present in the Cadomin area. These cycles can be correlated with the Falher and Wilrich cycles in the Deep Basin.
Sedimentological examination of the lower three cycles in the outcrop areas shows a progression of offshore to shoreface to foreshore environments, culminating in the deposition of coal seams. A variety of physical and biogenic sedimentary structures are present, such as hummocky cross-stratification and Skolithos ichnofacies, indicating shallow marine conditions. Interpretation of the upper three cycles is based primarily on limited trace fossils and foraminifera.
The six cycles observed in this study and their stratigraphic architecture can be explained by making the following assumptions: a constant rate of sediment supply, an overall 2nd order eustatic sea-level rise throughout the Early Albian, and minor changes in the rates of local Deep Basin subsidence. The cycles cannot be directly related to global 3rd or 4th order sea-level curves.
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