RECIPROCAL ARCHITECTURE OF BEARPAW T-R SEQUENCES, UPPERMOST CRETACEOUS, WESTERN CANADA SEDIMENTARY BASIN

The Bearpaw Formation consists of marine shale, siltstone and minor sa ndstone, and represents the final widespread marine unit in the Wester n Canada Foreland Basin. It is of late Campanian-early Maastrichtian a ge 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 s equences have been delineated, These T-R sequences are bounded by suba erial unconformities and/or conformable transgressive surfaces. The re cognition of a maximum flooding surface allows each sequence to be sub divided into a transgressive systems tract (TST) and a regressive syst ems tract (RST). Two distinct types of third-order T-R sequences were recognized in the Bearpaw Formation and each is found in a geographica lly restricted area. Type A sequences are characterized by a thin TST and thick RST, and they occur in the southwestern part of the study ar ea, 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 s ector. The line of demarcation between the proximal and distal sectors is termed a hinge line. Correlative bentonite beds and biostratigraph ic data indicate that the conformable transgressive surfaces of the pr oximal sequences correlate with the maximum flooding surfaces of the d istal sequences and vice-versa. This reciprocal architecture for the s equences 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. Duri ng tectonic loading the proximal area would experience increased subsi dence and transgression (TST), whereas the distal area would undergo r educed subsidence and uplift (RST), During the following quiescence st age the proximal region would subside less and in part be uplifted (RS T), 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 de position. This is consistent with the tectonic regime of dextral trans gression 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.