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The AAPG/Datapages Combined Publications Database

CSPG Bulletin


Bulletin of Canadian Petroleum Geology
Vol. 44 (1996), No. 1. (March), Pages 1-13

Seismic Interpretation of the Rocky Mountain Thrust Front Near the Crowsnest Deflection, Southern Alberta

N.J. Begin, D.C. Lawton, D.A. Spratt


Interpretation of seismic profiles in the southwestern Alberta Foothills near the Crowsnest Deflection delineates along- and across-strike variations of orogenic structures in the triangle zone. The overall structural geometry is dominated by an east-verging allochthonous wedge of Mesozoic rocks, overlain by west-verging thrust sheets of Upper Cretaceous and Tertiary rocks, and underlain by undeformed Paleozoic strata. The Big Coulee Fault, which rides within the Bearpaw Formation, represents the boundary between east- and west-verging structures and is the main upper detachment of the triangle zone. Strike seismic lines reveal evidence of major lateral ramps and folding of the Big Coulee Fault. Backthrusts above the upper detachment (e.g. Co-op, Waldron and Callum Creek) die out southward near Crowsnest River and folding of these faults decreases eastward. This suggests an eastward progression in the temporal development of west-verging faults above the triangle zone. In the vicinity of the Crowsnest Deflection, a major lateral ramp occurs in the lower detachment, as it cuts up-section to the north from the Blairmore Group to the Alberta Group. This is evidence for southward thickening of the allochthonous wedge, in addition to its eastward-tapering geometry documented along the entire fold and thrust belt.


L'interpretation de profils de sismique reflexion dans les contreforts rocheux du sud-ouest de l'Alberta defini les variations laterales et longitudinales des structures orogeniques de la zone triangulaire. La geometrie structurale est dominee par un biseau allochtone de roches Mesozoiques, vergeant a l'est, recouvert de nappes de retro-charriage vergeant a l'ouest, composee de roches du Cretace superieur et du Tertiaire, et le tout empile au-dessus de roches Paleozoiques. La faille Big Coulee s'est developpee dans la formation de Bearpaw. La faille repre la limite entre les structures vergeant a l'est et celles vergeant a l'ouest, et est identifiee comme le decollement superieur de la zone triangulaire. Les lignes sismiques paralleles aux structures majeures montrent des evidences de rampes laterales et de plissement de la faille Big Coulee. Les retrocharriages sus-jacents au decollement superieur (i.e. Co-op, Waldron et Callum Creek) se terminent vers le sud, pres de la riviere Crowsnest, et sont moins plisses a l'est. Ceci suggere une progression vers l'est quant a l'age de formation des failles a vergence ouest. Pres de la deflexion de Crowsnest, une rampe laterale majeure est presente dans le decollement inferieur, recoupant vers le nord du Groupe de Blairmore au Groupe de L'Alberta. Ceci documente un epaississement vers le sud du biseau allochtone pres de la deflexion, en plus de son amincissement vers l'est tout le long de la ceinture de pli et charriage.

Traduit par Normand J. Begin.

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