Bogdan L. Varban¹, A. Guy Plint²

(1) University of Western Ontario, Department of Earth Sciences, London, ON
(2) University of Western Ontario, Department of Earth Sciences

ABSTRACT: The Enigma Of Forced Regressions In A High-Subsidence Foreland Basin Margin: Upper Cretaceous Kaskapau Formation, NE British Columbia, Canada

The upper part of the L-M Turonian Kaskapau Fm. thins eastward from <600 m to >50 m over 600 km. Although dominated in subsurface by mudstone and minor fine sandstone of offshore aspect, the upper Kaskapau undergoes a dramatic westward transition, over <15 km, into outcrop sections that display stacked nearshore and shoreface sandstones and lagoonal mudstones; marine shales are confined to the lowest 100 m of the section. Bioturbated to swaley shoreface sandstones form sharp-based, sometimes gutter-casted units, 5-10 m thick. Sandstone successions may be separated: i) by a few dm of marine mudstone, ii) by only a pebble-veneered surface, iii) by a rooted beach overlain by up to 40 m of lagoonal deposits, including laminated mudstones, oyster reefs and tidal channel sandstones. The great thickness of the study section, and its location close to the active margin of the foreland basin both suggest that overall accommodation rate was high. However, marine sandstones usually have sharp, erosive bases, resting on thin marine mudstones or lagoonal deposits. Obviously transgressive marine deposits are thin or absent. The presence of about 25-30 successions spanning about 3 my, suggest that accommodation was alternately generated and removed, resulting in repeated forced regression of the shoreline, with concomitant erosion or non-deposition of transgressive and highstand deposits. Although high-frequency eustatic excursions are possible causes, is perhaps more likely that flexure and unflexure of the plate, related to episodic movement of thrust sheets, may have been responsible for the observed deepening and shallowing events.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado