Sedimentological and Sequence Stratigraphic Organization of a Foreland Clastic Wedge, Mannville Group, Western Canada Basin

Abstract

ABSTRACT The Aptian-Albian Mannville Group of the Western Canada foreland basin shows longitudinal paleoflow parallel to the basin axis, and therefore its sequence stratigraphic organization differs from that of either a passive margin or a transversely supplied foreland. A detailed isopach map of the lower transgressive systems tract shows that over most of the basin, the greater part of this unit was not deposited in accommodation generated primarily by flexural subsidence In the west, the basal Cadomin Conglomerate was shed during a period of erosion of the overthrust load and uplift of the foreland, and does not mark the initiation of overthrusting. The Cadomin was preserved in disequilibrium with respect to any sea-level-related base level. To the east, basal Mannville sediments were deposited in valleys forming a major drainage system cut into the underlying Paleozoic rocks or in a large but shallow cratonic basin overlying a thick Paleozoic salt undergoing solution. The valley fills are nested, cut into one another because of the lower rates of ormation of accommodation and also the narrow topographic depressions on the basal unconformity. None of these sediments were deposited in accommodation resulting from flexural subsidence. The upper highstand systems tract includes a number of unconformities, indicated in nonmarine to marginal-marine areas by incised valleys or abrupt, discontinuous facies shifts, and in shallow marine areas by falling to lowstand shoreface sandstones. In the area of higher subsidence rates near the overthrust belt, subaerial unconformities do not extend onto marine shelves; conformable surfaces with no evidence of subaerial exposure (Type 2 sequence boundaries) occur here. Eastward (parallel to the shoreline) in areas of lower subsidence rates, these pass into subaerial unconformities (Type 1 sequence boundaries) cutting across marine shelves, with falling and lowstand shoreface deposits at their distal edges. Traced much farther east into areas with the lowest subsidence rates, the Ty e 1 unconformities amalgamate, with the result that the gaps in the section are larger stratigraphically and represent longer time periods. The proportion of falling, lowstand, and transgressive sea-level deposits compared to highstand deposits is also greater cratonward. These stratigraphic and sedimentologic patterns are interpretable as the result of fluctuations in sea level generated by some unspecified extrabasinal mechanism, possibly eustasy, superimposed on the laterally varying foreland subsidence gradient.