Fluvial Sequence Stratigraphy: The Wapiti Formation, West-Central Alberta, Canada

Abstract

Abstract Outcrop exposures and high-resolution subcrop data allow the description and mapping of four stratigraphic discontinuities in the fully nonmarine strata of the Campanian–Maastrichtian Wapiti Formation (Western Interior foreland basin, Alberta, Canada). This framework defines four unconformity-bounded depositional sequences (A–D), based on sequence stratigraphic methodology, well-log patterns, and facies analysis. In ascending order, sequence A records the transition from the underlying marine facies of the Puskwaskau Formation into the Wapiti fluvial facies, and consists of strongly progradational and aggradational stacking patterns. Sequences B and C show a similar pattern of basal amalgamated channel-fill deposits that grade upwards into floodplain-dominated strata. Finally, sequence D is dominated by channelized sediments and extensive overbank facies. An aggradational stacking pattern suggests deposition under high-accommodation conditions. Maximum flooding surfaces are interpreted within fine-grained deposits in the upper portions of sequences C and D. They are tied to regionally extensive coals that accumulated more than 250 km away from the coeval shoreline. This study provides new evidence that major coal seams may represent the extension of marine maximum flooding surfaces into the downstream-controlled fluvial realm. Furthermore, the new stratigraphic framework of subaerial unconformities and maximum flooding surfaces provides the means of subdividing the previously undifferentiated fully nonmarine Wapiti Formation into depositional sequences and component systems tracts.