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An Allostratigraphic Framework for a Retrogradational Delta Complex: The Uppermost Dunvegan Formation (Cenomanian) in Subsurface and Outcrop, Alberta and British Columbia
Allomembers A and B, re-defined herein, are the uppermost components of the Dunvegan Formation, and are of ?late Middle to early Late Cenomanian age. They record the final drowning of the huge Dunvegan delta complex that occupied the western margin of the Cretaceous foreland basin in Alberta and British Columbia. During deposition of allomembers A and B, the delta-front back-stepped north-westward by about 220 km relative to allomember C, on a timescale estimated at 105–440 k.y. Allomembers A and B are now divided into three sequences A1–A3 and B1–B3, bounded by marine transgressive surfaces; each sequence is estimated to span approximately 18–72 k.y. Sequences shoal upward and represent wave-influenced delta, prodelta, and coastal plain environments. Sequence B3 is incised by an extensive system of narrow (much less than 1 km) sandstone-filled paleo-valleys, up to 16 m deep. Although sequences rarely exceed 10 m thick, they are mappable for hundreds of km and appear to be primarily of allogenic (?eustatic) origin. Sequences were probably deposited in water no more than approximately 15 m deep and each records at least several metres of relative sea-level fall. Eustatic changes on a vertical scale of several metres, over a few tens of thousands of years are potentially attributable to thermal expansion/contraction of seawater, water storage on land, and glacio-eustasy. All three mechanisms may have contributed to the observed sea-level changes, and were probably driven by orbitally-modulated climate cycles.
A regional isopach map of allomember C shows negligible updip (westward) thickening, suggesting that contemporaneous thrust loading of the basin margin was minimal. In contrast, allomembers A and B collectively show dramatic thickening to the west, suggestive of renewed loading in that direction. Although all sequences in allomembers A and B thicken westward, this trend is most prominent in sequences B3 to A3, suggesting that the rate of tilting accelerated over time. The transgressive limit of the shoreline mapped for successive sequences shows about 20 km of backstep per sequence, with the principal transgressive direction being towards the NW. This is oblique to the pattern of flexural subsidence inferred from isopach mapping and suggests that the depositional surface (coastal plain and prodelta) was inclined down to the SE, despite maximum tectonic subsidence being down to the west. This implies that the rate of sediment supply exceeded that of subsidence, and that depositional systems were able to maintain a slope inclined from NW to SE. The overall transgression recorded by allomembers A and B appears to have been strongly influenced by an accelerating rate of flexural subsidence, and, probably by slow eustatic rise during the Greenhorn transgression. However, this trend was modulated by higher-rate eustatic oscillations of at least several metres that caused corresponding shoreline excursions of several tens of kilometres within an overall transgressive regime.
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