Quantitative subsidence analysis of the Western Canada foreland basin with implications for short-term facies changes

Abstract

This paper presents a quantitative study of Jurassic-Lower Tertiary basement subsidence in the Western Canada foreland basin, corrected for local sediment loading and paleobathymetrical changes. Subsidence patterns have been investigated for the effects of erosion induced uplift by means of analytical estimations. It is demonstrated that phases of rapid basin shallowing may be related to periods of dominant erosion of the Cordilleran fold-and-thrust belt, leading to 200-20 m flexural uplift of the lithosphere within distances of 300 km of the belt. The amount of sediment that has been derived from the orogenic belt in such a period, can be sufficient for a shelf edge progradation of at least 300 km Myr −1. The history of the Western Canada foreland basin has been subdivided in five stages, 200-154.7 Ma (sI), 154.7-124.5 Ma (sII), 124.5-104 Ma (sIII), 104-77.7 Ma (sIV), and 77.7-60 Ma (sV), recording polyphase lateral and temporal changes in the loading history. The subdivision between stage I and stages II–V is based on a transition from significant vertical motions in the Williston basin in the southeast and minor sedimentation to the west during sI, to continued vertical motions in the Williston basin and major subsidence and sediment accumulation to the west during sII–sV. This major change forms the expression of the transition from passive to convergent margin conditions (cf. Davies and Poulton, 1986). The sII–sIII transition is marked by the expansion of the area of tectonic activity towards the north due to a large scale plate reorganization (cf. Chamberlain and Lambert, 1985). Stage IV is characterized by generally continuous subsidence with deposition of shales, interrupted by phases of rapid uplift with deposition of sandy shallow water units, while the subsequent stage V is defined by similar tectonic processes, but sedimentation conditions changed from mainly shale accumulation to mainly sand accumulation. These latter two stages probably reflect lateral and temporal changes in the rate of progradation of the accretionary wedge.