Abstract

Between 1992 and 1995, approximately 2000 km of seismic-reflection profiles were collected along three transects across the province of Alberta, as part of Canada's ongoing Lithoprobe geoscience project. Using these data, a 1450 km long structural seismic section has been assembled to image the stratigraphy of a large segment of the Western Canada Sedimentary Basin (WCSB) between Fort St. John, British Columbia and Pincher Creek, Alberta. The procedure for compiling this section from the individual profiles included application of static corrections using a floating datum and smoothly varying replacement velocity in order to resolve significant mis-ties between the lines. The large extent of the resulting composite section enables the interpretation of regional reflection characteristics spanning several basement tectonic domains, as well as interregional comparisons of key structural elements in the WCSB. As an alternative to mapping reflection geometries within the uppermost basement, which are largely obscured by multiple reverberations, we focus instead on seismic attributes (instantaneous frequency and amplitude) of the top of basement reflection in order to gain a better understanding of the nature of the crystalline basement and its contact with the overlying sedimentary strata. Seismic attribute analysis was also carried out for the Base of Fish Scales (BFS) reflector, an Upper Cretaceous seismic marker that serves here as a reference reflection. Within the Lacombe domain, a basement domain composed of weakly metamorphosed volcanic and sedimentary rocks, the top of basement reflection is characterized by anomalously high amplitude and frequency. Comparison of Carboniferous normal faulting in the Peace River Arch (PRA) region with Late Cretaceous (or younger?) faults in southern Alberta shows a number of common characteristics, including drape of sedimentary layers over deeper brittle detachments. These features in the Alberta Basin exhibit a strong similarity to previously published laboratory models of extensional forced folds. In structures of this type, brittle basement offset gives way upwards to a broader region of deformation within which extensive fracturing, mostly at a sub-seismic scale, is present.

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