ABSTRACT: The Transition from Foredeep to Extensional Basin Subsidence in the Bowser Basin Overlap Assemblage, Northern British Columbia

RICKETTS, BRIAN D., Geological Survey of Canada, Vancouver, British Columbia, Canada, V6B 1R8

The Early Jurassic demise of Stikinian arc volcanism in northern Bowser basin was followed by regional transgression and shale deposition. Accumulation of a late Toarcian-Aalenian condensed stratigraphic unit and subsequent downlapping of basin shale (upper Spatsizi Group) took place in response to thrusting of Cache Creek oceanic rocks over Stikinian basement (King Salmon Thrust). This event coincides with the closing of Cache Creek ocean and accretion of Stikinia to Quesnel-North America, and reflects initial flexural

subsidence of Stikinia beneath the Cache Creek accretionary prism. The sediment-starved Bowser foredeep lasted until the Middle Bajocian or Bathonian (about 10 m.y.) when Cache Creek rocks were subaerially exposed.

A prodigious influx of chert-dominated sediment into northern Bowser basin from Bathonian to Oxfordian signaled a major change in the tectonic regime. Thick successions of stacked, south-southwest prograding fan deltas fed sand and gravel through submarine canyons to base-of-slope submarine fans. Criteria such as the sedimentary facies and stratigraphic architecture suggest that Mid-Upper Jurassic sedimentation took place in discrete fault-bound basins. Tectonic subsidence histories are compatible with sedimentation in basins under extension, or perhaps transpression. Increased subsidence, sedimentation, and local uplift in fault blocks in the Early Oxfordian may also be related to initial subsidence of Western Canada foreland basin.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)