Abstract

Large-scale muddy clinoform stratification is rare in Cretaceous marine strata of the Western Canada Foreland Basin; most offshore mudstone units show near-parallel to gently diverging stratification that reflects differential basin subsidence. The early to middle Cenomanian Dunvegan Formation, of broadly deltaic character, is unusual in that it comprises a spectacular set of clinothems. These clinothems have been mapped over a 950 km dip transect, and downlap from NW to SE onto a phosphatic condensed section termed the FSU marker. Our regional mapping shows that the FSU marker is of basin-wide extent and marks the abrupt cessation of deposition for up to about 2 My in the south. In the northwest, the Dunvegan Formation forms a prominent wedge over 300 m thick, interpreted to fill a high-accommodation area. Muddy clinothems in this high-accommodation area are relatively tall (~ 80 m; 115 m decompacted) and extend about 80 km from the delta front to a zero edge at the downlap surface. Wave-rippled and hummocky cross-stratified fine sandstones of the upper delta front record the effects of storms, and extend about 30 m (~ 42 m decompacted) below contemporaneous beach facies, suggesting that effective wave base for sand transport was at about 40 m water depth; mud was transported in deeper water, presumably by storm-driven flows. Towards the SE, the Dunvegan Formation gradually changes from a wedge to a tabular shape, interpreted to indicate deposition in a low-accommodation area. In the transition from high- to low-accommodation areas, clinothems become progressively less tall (50 to about 20 m; 70 to about 29 m decompacted) but longer (150 to > 250 km). The most down-dip “clinothems” are essentially tabular and > 400 km in extent. In the high-accommodation area, offshore mud dispersal is inferred to have been limited by episodic geostrophic flows, and possibly by permanent along-shelf flows that swept mud (probably suspended, flocculated, and pelleted) into a shore-parallel prism, producing relatively short, steep clinothems. Muddy clinothems downlap onto sandy condensed facies of the FSU marker, the latter interpreted to have been deposited in shallow water prior to Dunvegan deposition, but which subsided below wave base in response to loading by the advancing deltaic clinothems. In the southern, low-accommodation area, tabular Dunvegan mudstones suggest that the sea floor was able to aggrade to effective wave base (i.e., about 40 m or less), at which point, wave resuspension of mud became frequent, resulting in efficient off- and along-shore dispersal of mud by geostrophic currents. The initiation of clinothem deposition is interpreted to coincide (a) with a phase of accelerated flexural subsidence in the northwest, related to renewed tectonic loading, and (b) possibly with the onset of large-scale anticlockwise marine circulation at the inception of the Greenhorn Seaway in the early Cenomanian. Together, these tectonic and oceanographic events formed an effective sediment trap along the western margin of the seaway. Only after the western sediment trap had filled, in late Dunvegan time, was mud again dispersed widely across the basin by storm processes in water less than about 40 m deep.

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