THE CRUST BENEATH THE INTRACRATONIC WILLISTON BASIN FROM GEOPHYSICAL DAT A

It is generally acknowledged that the nature of the crust beneath sedimentary basins, particularly intracratonic ones, has a significant impact on their structural evolution, sedimentary histories and hydrocarbon enrichment. Deep seismic profiling, in conjunction with other available information, will certainly play a major role in imaging deep structures relevant to hydrocarbon exploration. Although the Williston Basin has been the subject of study for many years, its origin and the character of the underlying Precambrian basement have been the subject of considerable controversy. In recent years, a number of studies have led to new views on the nature of the crust in this region. The structural character of the Precambrian basement is revealed in recent images of gravity and magnetic data. The interpretation of several seismic refraction profiles collected by the Canadian COCRUST group suggested the presence of a lower crustal high velocity (>7. 0 km / s) layer overlying a relatively flat Moho with a minor 3 to 4 km thinning of the crust towards the centre of the basin in Canada. Recent COCORP seismic reflection data collected in Northern Montana, just south of the refraction lines and closer to the depocentre, suggest that the high velocity layer may be coincident with a zone of strong reflectivity at ·the base of the crust. Whether this anomalous lower crustal layer is associated with the Phanerozoic origin of the Williston Basin or with the evolution of the underlying Early Proterozoic Trans-Hudson orogen, or both, has yet to be determined. A major crustal electrical conductivity anomaly (the North American Central Plain anomaly NACP) spatially associated with the Trans-Hudson orogen has been found to coincide with a N-S trending high heat flow anomaly. These basement structures and anomalies have influenced or controlled the development of important oil-bearing structures within the Williston Basin