Abstract

The Bearpaw Formation consists of marine shale, siltstone and minor sandstone, and represents the final widespread marine unit in the Western Canada Foreland Basin. It is of late Campanian-early Maastrichtian age and forms the core of a second-order transgressive-regressive (T-R) sequence. The area of study covers the southern parts of Alberta and Saskatchewan. Within the Bearpaw Formation, up to 11 third-order T-R sequences have been delineated. These T-R sequences are bounded by subaerial unconformities and/or conformable transgressive surfaces. The recognition of a maximum flooding surface allows each sequence to be subdivided into a transgressive systems tract (TST) and a regressive systems tract (RST). Two distinct types of third-order T-R sequences were recognized in the Bearpaw Formation and each is found in a geographically restricted area. Type A sequences are characterized by a thin TST and thick RST, and they occur in the southwestern part of the study area, proximal to the orogenic belt. Type B sequences are characterized by a thick TST and a thin RST and occur to the northeast in a distal sector. The line of demarcation between the proximal and distal sectors is termed a hinge line. Correlative bentonite beds and biostratigraphic data indicate that the conformable transgressive surfaces of the proximal sequences correlate with the maximum flooding surfaces of the distal sequences and vice-versa. This reciprocal architecture for the sequences is best explained by a tectonic control on the foreland basin stratigraphy through the flexural compensation of the lithosphere in response to successive cycles of orogenic loading and quiescence. During tectonic loading the proximal area would experience increased subsidence and transgression (TST), whereas the distal area would undergo reduced subsidence and uplift (RST). During the following quiescence stage the proximal region would subside less and in part be uplifted (RST). At this time the distal area would experience increased subsidence and decreased sediment influx (TST). The hinge line migrated a short distance closer to the orogen and slightly northward during Bearpaw deposition. This is consistent with the tectonic regime of dextral transgression in the Canadian Cordillera during the latest Cretaceous. The third-order sequences of the Bearpaw Formation provide solid evidence for the dominance of tectonic control for sequence boundary generation and indicate that the flexural tectonic model for foreland basins is valid.

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