TECTONIC INFLUENCE ON DEPOSITION AND EROSION IN A RAMP SETTING - UPPER CRETACEOUS CARDIUM FORMATION, ALBERTA FORELAND BASIN

This paper presents the results of a combined outcrop and subsurface i nvestigation of the Upper Cretaceous Cardium Formation in northwestern Alberta and adjacent British Columbia, Canada. More than 100 measured core and outcrop sections combined with more than 1100 well logs were used to examine stratigraphic relationships in an area of over 45,000 km2. The Cardium consists of stacked regressive-transgressive cycles deposited during shoreline progradation along the western margin of a foreland basin occupied by the Western Interior seaway. In general, th ese cycles consist of shoaling upward successions (highstand systems t racts composed of shoreface sandstones and conglomerates and heterolit hic offshore deposits) capped by transgressive erosion surfaces. Linea r incised lowstand coarse-grained shoreface deposits are present at so me stratigraphic levels. A single, thick (locally more than 20 m thick in the study area) nonmarine wedge that overlies shoreface sandstones is interpreted to represent deposition during rising sea level rather than accumulation behind a prograding strandline. Preservation of the incised lowstand shoreface deposits is associated with topographic br eaks. In places, trends of isopachs, facies transitions, erosional bev els on transgressive erosion surfaces, synsedimentary faults, and mode rn production trends can be directly superimposed and closely correspo nd to basement structural trends. This suggests that basement structur es were being remobilized during Cardium deposition, possibly in respo nse to thrusting in the orogenic belt.