Reciprocal stratigraphy of the Campanian–Paleocene Western Interior of North America

Abstract

The Claggett, Bearpaw, and Cannonball successions (Campanian–Paleocene) of the Western Interior foreland system were deposited during a series of regional second- and third-order transgressions and regressions, locally modulated by fourth- and lower-order cycles. The reciprocal flexural behaviour of the foreland lithosphere in response to orogenic cycles of loading and unloading results in out of phase sequences on opposite sides of the basin, which defines the concept of “reciprocal stratigraphies”. The proximal and distal regions of the basin are separated by a few-kilometre wide “hinge zone” of proximal to distal facies change, with the mid point taken as a stratigraphic “hinge line”, which defines the boundary between coeval marine transgressive and regressive systems tracts, or between nonmarine sequences and correlative sequence boundaries. Hinge lines have been mapped in Alberta, Saskatchewan, Montana and Wyoming for consecutive time-slices during the Campanian–Paleocene interval. Areally the hinge line traces a semielliptical pattern outlining the region of maximum foreland basin subsidence. It migrated at ∼10 km/Ma to the north in response to Late Cretaceous–Paleocene orogenic dextral transpression. The hinge line also migrated eastward during the Campanian and Maastrichtian in response to thrust-sheet advance, and westward during the Paleocene in response to the redistribution of orogenic load associated with a transition from transpression to transtension, possibly accompanied by a visco-elastic deepening and narrowing of the foredeep.