Sequence Stratigraphy of the Lower Cretaceous Mannville Group of East Central Alberta

Abstract

A review of the stratigraphic record through the correlation of geophysical well log data indicates there are at least nine 3rd order cycles of relative sea-level rise that flooded the region. The lower three flooding events subdivide a retrogradation al succession, while the upper six subdivide a dominantly progradational succession. The transitio n from retrogradation to progradation coincides with the top of the McMurray Formation and the Cummings Member. The initial flooding event caps fluvial deposits of the Dina member and the mid - lower McMurray Formation. The second flooding event caps a widespread upward-coarsening sheet-sand of the upper McMurray Formation, incised along the axis of the Prairie Evaporite Salt Scarp. The third flooding event caps a similar sheet-sand forming the top of the McMurray Formation and a series of stacked sheet-sands of the Cumming Member. The upper six flooding surfaces subdivide similar successions, each consisting of a thin upward-coarsening sheet-sand in the south and a thick southward tapering sandstone wedge in the north. The thin sheet-sand is associated with rapid progradation of the shoreline during relative stillstand. The thicker sandstone wedge is associated with relative sea-level lowering followed by sea-level rise. Associated with the sandstone wedge are incised valleys which cut into the thin sheet-sand to the south. The Lloyd, Rex, and General Petroleum Members are the initial phase of shoreline progradation culminating in thick lowstand wedges included in the Clearwater Formation. The Sparky, Waseca, McLaren, and Colony Members are the initial phase of shoreline progradation culminating in thick lowstand wedges included in the Grand Rapids Formation. The mapping of each cycle of relative sea-level rise is based on the correlation of thin, distinctive marine shale beds in the northern part of the region, and in the south, by less distinct marine to brackish shale beds that commonly overlie distinctive regional coal beds (Figure 1). Ambiguity in the correlation of each flooding event, especially above the McMurray-Cummings level, occurs between the thin sheet-sand to the south and the thicker wedge to the north. In this zone neither marine shale beds nor coal beds are well developed. In these areas widespread erosional surfaces provide a more reliable means of correlation between genetic untis to the north and south. Regarde d on a 2nd order level, the Mannville Group consists of a lower Transgressive Systems Tract and an upper Highstand Systems Tract. The transition between the two systems tracts is based on stratal geometry, reflecting a change from retrogradation to progradation. Third to 4th order transgressive/regressive cycles are superimposed on both of these systems.