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An Allostratigraphic Correlation of a Mudstone-Dominated, Syn-Tectonic Wedge: The Puskwaskau Formation (Santonian-Campanian) in Outcrop and Subsurface, Western Canada Foreland Basin
The Santonian-early Campanian Puskwaskau Formation is a prismatic wedge <70 to >340 m thick that occupies the foredeep of the Western Canada foreland basin. It is dominated by muddy, offshore sediments deposited by storms in water probably no more than a few tens of metres deep. The Puskwaskau rocks are organized into siltier-upward and sandier-upward successions, bounded by regionally-mappable marine flooding surfaces that permit definition of 14 informal allomembers (labeled A to N in ascending order). These rocks span early Santonian to? early Campanian time and probably represent approximately 2.5 Ma. Only allomember L contains swaley-stratified sandstone bodies (correlative with the Chungo Member in outcrop) that represent at least 40 km of north-eastward shoreface progradation. The upper surface of allomember L is a major, pebble-veneered erosion surface with 15 m of relief that cuts down towards the NE, implying significant relative sea-level fall, followed by subsequent transgressive ravinement. Overlying transgressive mudstone of allomember M onlaps south-westward and appears to fill bathymetric relief cut into the top of allomember L. Allomembers A to L can be grouped into three larger stratigraphic packages, informally termed units, each inferred to represent approximately 750 ka. Each unit is typified by an overall upward-fining succession, followed by an upward-coarsening succession that is capped by a transgressive or flooding surface. Within each unit, facies and small-scale parasequence stacking patterns suggest that accommodation was generated relatively rapidly in the lower part, but accommodation rate diminished towards the top of each unit, possibly even becoming negative. Units 1, 2 and 3 fill discrete arcuate depocentres, the cores of which show abrupt lateral displacement along the orogen by 200–350 km. This is interpreted to record abrupt lateral shifts in the locus of active thrust-sheet advance in the contemporaneous fold-and-thrust belt. These discrete depocentres cannot, however, be discerned in a formation-scale isopach map. In the NW, the base of allomember L is a beveling unconformity that progressively truncates allomembers K to H from the SE towards the NW. Beveling implies gradual tilting and uplift in the NW, prior to and possibly during deposition of allomember L. The initiation of new flexural depocentres took place geologically instantaneously, across the boundaries of individual allomembers (average duration approximately 190 ka); this observation implies that thrust sheets advanced independently, and were separated by major high strain zones.
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