ABSTRACT: Variations in Anatomy and Facies of the Foredeep: The Stratigraphic Response to Laramide Orogeny in the Southern Canadian Cordillera

JERZYKIEWICZ, TOMASZ, Geological Survey of Canada, Calgary, Alberta, Canada

Surface and subsurface mapping of the Laramide foredeep in Alberta has led to the recognition of five clastic wedges. Each wedge is underlain by a basin-wide break in the stratigraphic record and/or commences with an influx of coarse-grained detritus, heralding major events in the basin. These events were: (1) regressions of the Wapiabi and Bearpaw seas from the foredeep at approximately 80 and 75 Ma, respectively, and (2) changes in the paleodrainage patterns at approximately 68, 63, and 60 Ma. The wedges show significant variations in depositional geometry and facies distribution. The Campanian and Maastrichtian wedges were segmented by postdepositional thrusting. The autochthonous parts of these wedges show no depositional axes, forebulge, and proximal facies. They are characterist c of an overfilled foreland basin. The uppermost Maastrichtian/Paleocene and Paleocene clastic wedges display a triangular geometry mimicking the form of lithospheric flexure. The forebulge is marked by stratigraphic condensation of the Paleocene sections east of the axis of the Alberta Syncline. The change in the geometry of the basin in late Maastrichtian, which coincides with the first appearance of a pebble conglomerate and Kneehills Tuff, is interpreted in terms of a major Laramide event in the Cordillera. The foredeep migration from its most western, distal position in the Campanian to its Paleocene locus along the axis of the Alberta Syncline took approximately 20 m.y. and was punctuated by significant facies changes at approximately 80, 75, 68, 63, and 60 Ma. The sequence of ages for the lower boundaries of the five clastic wedges may indicate tectonic pulses in the orogenic front, although the emerging thrust slices capable of producing proximal sediments were probably separated from the foredeep by a piggyback basin, since the pebble-to-cobble conglomerates show features of multiple and high-energy water transportation.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)