LITHOPROBE BASIN-SCALE SEISMIC PROFILING IN CENTRAL ALBERTA - INFLUENCE OF BASEMENT ON THE SEDIMENTARY COVER

Crustal-scale seismic reflection data, acquired in 1992 by Lithoprobe, have yielded a continuous seismic cross-section across central Albert a and provide new constraints on the nature and extent of basement con trols over Phanerozoic depositional and diagenetic processes in this p art of the Western Canada Sedimentary Basin (WCSB). In particular, we recognize three styles of basement influence that have distinct seismi c expressions: passive topographic effects, indicated by onlap, drape and infill of basin sediments over relief on the basal unconformity; f aulting of the basement and overlying Cambrian strata; and abrupt late ral facies changes of uncertain origin, vertically overlying a deep-se ated tectonic boundary. Taken together, examples from the seismic prof ile clearly demonstrate that basement structures have exerted subtle, but significant, influence over Phanerozoic paleogeography in this par t of the WCSB. Overall basement relief in this region is dominated by flexural effects associated with the Alberta Foreland Basin. However, older and more localized paleotopographic elements, with maximum verti cal relief of 175 m and horizontal scales ranging from tens of metres to more than 50 km, are superimposed on the smooth basement surface. T he most prominent basement high underlies the Leduc southern Alberta s helf margin on the western edge of the Precambrian Loverna Block; on t he basis of onlapping relationships and systematic traveltime-differen ce minima with respect to the Second White Specks reflection, it appea rs to have been an area of positive relief from the Late Cambrian to D evonian. In the same region, the basement surface is broken up by smal l vertical offsets along a number of faults that appear to extend upwa rd to the top of the Middle Cambrian Earlie Formation. One of the most striking linear features of the basin is the Rimbey-Meadowbrook reef chain, which overlies the ca. 1825 Ma Rimbey magmatic arc. In map view there is an imperfect alignment of the Rimbey-Meadowbrook trend with the aeromagnetic fabric of the basement, suggesting that if basement s tructures had controlled the reef orientation, the control must have b een indirect in nature. However, juxtaposition of seismic images from opposite sides of the reef trend reveals a series of abrupt facies cha nges within Middle Cambrian to Middle Devonian strata that directly ov erly the Rimbey are. In addition, the top-of-base ment reflection (TBR ) is less coherent and preceded by a high-amplitude, localized basemen t precursor (BP) event in the sediments. Deep-seated controls over the orientation of the reef foundation, and possibly over coeval fluid-ci rculation patterns in the basin, could explain the coincidence of thes e elements in the seismic profile.