Migrating, Short-Lived Depocentres Revealed through High-Resolution Allostratigraphy

Albian-Cenomanian clastic units in the western Canada foreland basin were deposited in distal offshore, nearshore, shoreface, delta-front to coastal plain environments. Allostratigraphic analysis shows that on time-scales of a few m.y. to a few hundred k.y., subsidence took place along limited portions of the orogen, forming broadly arcuate flexural depocentres. Coastal plain and bay deposits of the Late Albian Paddy alloformation fill a semi-circular depocentre of ~200 km radius centered on ~Lat. 55 N. The succeeding marine mudstone of the Joli Fou alloformation is more tabular but thickens into a depocentre S of 49 N. The overlying sandy, shallow-marine Viking alloformation is broadly tabular but is truncated in the N. The uppermost Viking allomember, VD, fills depocenters in the S and N, but thins to zero over an arch centered at 53 N. Late Late Albian rocks of the Westgate and Fish Scales alloformations are dominated by offshore mudstone and record the rapid subsidence a north-western depocentre, centered at 55 N. The top of the Fish Scales alloformation is a phosphatic condensed section that forms the downlap surface beneath a thick pile of deltaic deposits of the Early to early Late Cenomanian Dunvegan alloformation. Lower Dunvegan rocks indicate deposition in up to ~100 m of water, due to initially rapid subsidence towards a depocentre in the NW at ~56 N. Middle to upper Dunvegan rocks show a new depocentre developing in the SW at ~54 N. Late Cenomanian marine mudstones of the Lower Kaskapau alloformation indicate a northward migration of the depocenter to ~55 N, followed by subsidence over a broad front in latest Cenomanian and early Turonian time. The subsidence patterns suggest that individual thrust sheets advanced for a period of a few hundred ky over a front of perhaps 100-300 km before movement shifted laterally to another sheet.