ABSTRACT: Palinspastically Restored Belly River Delta in the Crowsnest Embayment of the Wapiabi (Colorado) Sea: Southern Foothills of the Canadian Cordillera

JERZYKIEWICZ, TOMASZ, Geological Survey of Canada, Calgary, Alberta, Canada, and DONALD K. NORRIS, Kelowna, British Columbia, Canada

Strata of the Belly River delta were deposited in the Crowsnest Embayment of the Wapiabi Sea between the south- and southeast-trending legs of the Western Canada miogeocline. Structurally, the delta lies along the eastern margin of the foreland thrust and fold belt, where the whole of the Cordilleran Orogen east of the Rocky Mountain Trench changes strike from south to southeast (Crowsnest Deflection). The Laramide Orogeny superposed upon the delta a family of thrust faults and buckle folds that contracted, thickened and repeated the delta succession, allowing access to its several members. In the vicinity of Crowsnest River, where exposures are optimal, the deformed delta can be restored palinspastically to its pre-Laramide configuration within the supracrustal wedge, horizontal shor ening there is on the order of 45%, and the allochthonous part of the delta beneath the Lewis Thrust plate has been displaced eastward some 70 km relative to the autochthon of the Alberta Syncline. Two members of the Belly River formation, the "transitional" and the "fluvial," form a complete delta sequence that developed in response to the regression of the Wapiabi (Colorado) sea from the Alberta foredeep in the lower Campanian. The subaqueous delta plain, represented by the "transitional member," consists of shallow water turbidites, distal bar sediments, subaqueous slumps, and distributary mouth bar sandstone. The subaerial delta plain represented by the "fluvial member" consists of distributary channel fill and interdistributary bay, levee, and marsh sediments. The distributary

mouth sand-bodies consist of well-sorted and stratified sandstone with a high quartz content, diagnostic of a high wave energy delta shoreline. Paleocurrent readings from cross-stratified sandstones of the distributary mouth sand-bodies and the directions of the distributary channels indicate northeastern progradation of the Belly River delta into the sea.

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