Variable Subsidence Patterns in an Upper Cretaceous Foreland Basin Wedge, Alberta

Aditya Tyagi¹ and A. Guy Plint^2
1 University of Western Ontario, London, ON
² Department of Earth Sciences, University of Western Ontario, London, ON

The Blackstone Formation ranges in age from Late Albian to Middle Turonian and constitutes a fine-grained succession that forms a wedge, thinning from about 550 m in the foredeep to about 130 m in the NE over 300 km. Analysis of the Cenomanian-Turonian portion of the Blackstone Fm. is based on about 2000 wells and18 outcrop sections, allowing division into 12 allostratigraphic units bounded by marine flooding surfaces and major regional bentonites. Log-markers show extreme lateral continuity, and the thickness of allomembers changes very gradually. These observations suggest that sea floor formed an essentially planar surface, sloping very gradually to the E and SE. We therefore interpret changes in isopach patterns between successive allomembers to primarily reflect changes in subsidence pattern. In general terms, sediments thicken towards the orogen, with the depocentre forming a broadly arcuate ‘moat' with a typical radius of about 300 km. However, between successive allomembers, the centre of the depocentre may shift parallel to the orogen by up to 200 km. These changes are tentatively interpreted to reflect lateral shifts in the locus of thrust loading on timescales of about 0.75-0.5 Ma. In addition to these principal trends, two allomembers also show E-W isopach trends in the east that approximately coincide with the boundary between the Precambrian Thorsby and Rimbey terranes; differential flexure across the terrane boundary might be inferred. Finally, one map shows an elongate depocentre parallel to, but displaced 100-150 km outboard from the deformation front; at present, this pattern remains unexplained!

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005