Concept and styles of reciprocal stratigraphies: Western Canada foreland system

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The late Cretaceous–Palaeocene Western Canada foreland system provides a record of high‐frequency cycles of reciprocal flexural tectonics superimposed on longer term changing dynamic subsidence. Initial dominance of dynamic subsidence during the deposition of the Bearpaw marine succession resulted in continual subsidence with differential rates across the flexural hinge line. Subsequent dominance of flexural tectonics resulted in opposite base‐level changes between the proximal and distal reaches of the foreland system during the deposition of the post‐Bearpaw nonmarine succession. In both cases, the contrasting base‐level changes generated out‐of‐phase stratigraphic sequences, which defines the concept of reciprocal stratigraphies. Two styles of reciprocal stratigraphies have been identified in relation to the pattern of base‐level changes across the foreland system. The Bearpaw style consists of a conformable succession of correlative transgressive and regressive systems tracts, suggesting continuous basin‐wide sedimentation with the rates within the range of variation of the rates of base‐level rise. The post‐Bearpaw style shows sequences correlative to age‐equivalent sequence boundaries related to coeval rising and falling base‐level, respectively. The succession of Bearpaw and post‐Bearpaw sequences corresponds to a cycle of marine to nonmarine foreland sedimentation controlled by changing ratios between dynamic subsidence and flexural tectonics. The amount of sediment supply derived from the orogen to the foreland system may also reflect the rates of dynamic subsidence, as a decrease in dynamic loading may lead to accelerated denudation of the sediment source areas.