Lithoprobe Crustal Seismic Profiles in Central Alberta: New Reflections on the Evolution of the Albe

The crystalline basement beneath WCSB (Western Canadian Sedimentary Basin) is composed of crustal domains delineated on the basis of aeromagnetic signatures and U-Pb zircon geochronology of drill core recovered by the petroleum industry. Using analogies from the Canadian Shield, the mosaic of crustal domains is inferred to have formed during continental collision and accretionary events during the interval of 1.78-2.0 Ga. A long-standing uncertainty in basin evolution and formulation of exploration strategies has been the degree of control of sedimentary and diagenetic patterns by antecedent basement structures. In order to examine this problem and elucidate the Precambrian tectonic evolution of western Canada, LITHOPROBE acquired more than 500 km of crustal scale (18 sec) seismic reflection data along a transect through Alberta. The profile extends from the Alberta-Saskatchewan border northwestwards into west of Edmonton and crosses a number of Phanerozoic features including hydrocarbon production trends such as the Pembina field, Rimbey-Meadowbrook trend and Bashaw reef trend.

Prominent reflections have been observed throughout the entire thickness of the crust and sole into a reflective lower crust, the base of which is characterized by a dramatic loss in reflectivity (12-14 sec.) which is interpreted as a reflection from the Moho. Many of the reflections are geometrically suggestive of compressional deformation and delineate a broad region of crustal-scale thrust imbrication. The eastern 300 km shows evidence for a west-north west-verging thrust belt that corresponds to an increase in depth to Moho and metamorphic grade into the inferred hinterland region. A prominent positive step in the gravity field in the western part of the transect is associated with an inferred reversal in vergence and possibly a crust-penetrating offset of the Moho formed during late stages of collision along the Snowbird Tectonic Zone. Available geochronologic constraints suggest contemporaneity of crustal imbrication observed in the Alberta basement with that documented in the Trans-Hudson Orogen to the east (1.78-1.81 Ga). This implies that a region of continental crust was undergoing shortening across strike for more than 1000 km during assembly of this part of the Canadian Shield.

Consistent acquisition and processing parameters, coupled with the unusual length and continuity of this transect, provide a unique regional seismic perspective from which to address the question of basement control on basin evolution. Three distinct styles of basement influence have been observed: 1) sedimentary drape over paleogeographic highs on the basal unconformity; 2) intersections of strong dipping reflections (faults?) with the base of the sedimentary section and 3) seismic sedimentary facies transitions that are spatially associated with basement domain boundaries. In general the long wavelength relief of the basement surface is the result of Laramide flexure but is punctuated by local paleotopographic features that range in scale from tens of metres to 50 kilometres. A long wavelength basement high near Stettler, Alberta underlies the Leduc southern Alberta shelf edge and was an area of positive relief from Upper Cambrian to as late as Devonian. Evidence for the second class of interaction is provided by a series of strong, east-dipping reflections in the uppermost basement beneath central Alberta that are continuous with regional compressional reflection fabric in the basement and thus may represent faults in the basement. Relationships with the sedimentary cover suggest that, within the limits of seismic resolution, these structures affected Middle Cambrian or older strata. An example of the third class of basement influence is particularly evident beneath the Rimbey-Meadowbrook reef trend, where lower Paleozoic reflections (pre-Cooking Lake) exhibit an abrupt lateral change in amplitude and continuity that appears to coincide with a major basement suture zone.