Concept and styles of reciprocal stratigraphies: Western Canada foreland system

The late Cretaceous-Palaeocene Western Canada foreland system provides a re cord of high-frequency cycles of reciprocal flexural tectonics superimposed on longer term changing dynamic subsidence. Initial dominance of dynamic s ubsidence during the deposition of the Bearpaw marine succession resulted i n continual subsidence with differential rates across the flexural hinge li ne. Subsequent dominance of flexural tectonics resulted in opposite base-le vel changes between the proximal and distal reaches of the foreland system during the deposition of the post-Bearpaw nonmarine succession. In both cas es, the contrasting base-level changes generated out-of-phase stratigraphic sequences, which defines the concept of reciprocal stratigraphies. Two sty les of reciprocal stratigraphies have been identified in relation to the pa ttern of base-level changes across the foreland system. The Bearpaw style c onsists of a conformable succession of correlative transgressive and regres sive systems tracts, suggesting continuous basin-wide sedimentation with th e rates within the range of Variation of the rates of base-level rise. The post-Bearpaw style shows sequences correlative to age-equivalent sequence b oundaries related to coeval rising and falling base-level, respectively. Th e succession of Bearpaw and post-Bearpaw sequences corresponds to a cycle o f marine to nonmarine foreland sedimentation controlled by changing ratios between dynamic subsidence and flexural tectonics. The amount of sediment s upply derived from the orogen to the foreland system may also reflect the r ates of dynamic subsidence, as a decrease in dynamic loading may lead to ac celerated denudation of the sediment source areas.