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Department of Earth Sciences, University of Western Ontario, London, ON, N6A SB7
The early to middle Cenomanian Dunvegan Formation represents a major deltaic complex that, within the study area, prograded 400 km from NW to SE, parallel to the Cordillera, over about 2 m.y. The formation ranges from about 90 to 270 m thick, being thickest in the NW, in the vicinity of Chetwynd B.C. The formation has been divided into ten allomembers, broadly comparable to transgressive-regressive sequences, labeled J to A in ascending order. Updip, allomembers are bounded by erosional valleys and interfluve surfaces whereas downdip, they are bounded by regionally-traceable marine ravinement and flooding surfaces. The geometry and facies distribution of the ten allomembers has been mapped over an area of about 80 000 km2 using 2340 well logs and 63 outcrop sections. Twelve log cross-sections summarize regional facies distribution and thickness, and key surfaces are correlated into 28 outcrop sections around the northern and western margins of the basin. Allomembers are composed of "shingles" that represent individual upward-shoaling delta lobes. The updip, sandy parts of shingles are attributed to the lithostratigraphic Dunvegan Formation, whereas muddy downdip parts are assigned to the Shaftesbury Formation. Allomembers downlap onto a prominent condensed section, rich in fish scales (the radioactive, "Fish Scales Upper" log marker), that in this study defines the base of an informal Dunvegan alloformation. A prominent marine to lagoonal flooding surface on top of Dunvegan allomember A, traceable throughout the study area, is considered to mark the top of the alloformation. Marine flooding surfaces that bound allomembers in downdip areas can be traced updip into surfaces that define the tops of interfluve paleosols that separate 20–35 m deep valleys. Because they provide evidence of alternating deposition and erosion, allomembers can be interpreted as transgressive-regressive sequences reflecting relative sea level oscillations. Only two of the allomembers display a recognizable downward shift in facies and onlap that typifies classical Exxon-type sequences. Allomembers J-F have a sigmoidal-prismatic geometry, thickest in the marine delta-front area and thinning up- and down-dip. They show no significant thickening along-strike towards the Cordillera, suggesting no differential subsidence in that direction. In contrast, allomembers E to A show progressive updip thickening of alluvial facies in the NW, adjacent to the Cordillera, and concomitant decrease in volume of marine deltas downdip. This probably reflects progressive sediment partitioning in response to renewed updip flexural subsidence. Paleogeographic maps for allomembers J-C reveal a series of lobate to cuspate deltas indicating significant fluvial influence. Local facies successions are, however, dominated by wave-formed structures which could, in the absence of paleogeographic information, be misinterpreted to represent linear coastal systems. A summary paleogeographic map of the basal 15–50 m of the overlying Kaskapau Formation shows a 400 km shoreline backstep from Dunvegan allomember C, with the formation of a broad embayment fringed by low-energy, shoal-water deltas in the north. Coeval, wave-dominated sandbodies to the SE are isolated in shelf mudstone and might represent lowstand shoreline deposits. Within the lower Kaskapau Formation, an unconformity, termed "Kl", locally truncates several tens of m of strata from W to E. In the east, the Kl unconformity is correlative with a Glossifungites firmground, overlain by up to 1 m of ooidal ironstone. Mudstone-encased SCS (swaley cross stratification) and HCS (hummocky cross stratification) sandstones within this interval, including the Doe Creek and Pouce Coupe sandstones are partially truncated by the Kl unconformity; these sands pass westward into mud-stones and may have been shed from a rising forebulge in the east. Stratigraphically-equivalent marine sandstones on the western margin of the basin have no physical connection with Doe Creek and Pouce Coupe sandstones. The Howard Creek and other unnamed sandstones overlie the Kl surface; they appear to be associated with another erosion surface at the base of the highly radioactive Second White Speckled Shale that approximates the Cenomanian-Turonian boundary.
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