SEDIMENTARY AND TECTONIC ORIGIN OF A TRANSGRESSIVE SURFACE OF EROSION- VIKING FORMATION, ALBERTA, CANADA

The upper Albian Viking Formation in the subsurface of Alberta contain s at least four major erosional surfaces, or bounding discontinuities, The highest of these surfaces, Viking Erosion surface 4 (VE4), can be correlated basinwide. There is an easily recognized well log marker i n the shales 10-50 m above VE4, known as the Base of Fish Scales (BFS) , An isopach map of the BFS-VE4 interval has been constructed using 99 87 wells over an area of about 85,800 km(2), The depositional surface represented by BFS was probably smooth and almost flat during depositi on, and the isopach map can be interpreted as showing the topography o f the VE4 surface, The surface is stratigraphically deepest in the nor theast, and rises southwestward. Its topography is characterized by a broad east-west-trending arch and two linear steps that trend northwes t-southeast, The VE4 surface is overlain by a coarse lag a few centime ters to about 5 m thick, with clasts up to 15 cm proximally (SW) and 7 mm distally (NE), Topographic relief on VE4 is at least 40 m, VE4 is interpreted as having formed initially during a major lowering of rela tive sea level, with modification of the subaerial topography and remo val of all evidence of subaerial exposure during the ensuing southwest ward marine transgression-thus VE4 is a transgressive surface of erosi on, Several well log markers onlap the VE4 surface, They represent coa rse to granule sandstones with extremely sharp bases, These tongues of sandstone can be traced up to 20 km basinward from their points of on lap and are interpreted as thin lower shoreface sandstones that formed during minor forced regressions superimposed on the overall transgres sion. The initial regression that immediately preceded VE4 implies a f all of relative sea level of at least 40 m, and a northeastward moveme nt of the shoreline of at least 250 km, These two figures imply a basi nward gradient of about 0.00016. Structure contour maps of the top of the Mannville Group (Albian) and the top of the Wabamun (Upper Devonia n) show smooth unbroken surfaces in the study area, with strikes diffe rent from those of the arch and steps. Thus there appear to be no olde r structural elements that influence Viking trends, The broad east-wes t-trending arch cannot be identified at Wabamun, Mannville, or older V iking horizons, and had disappeared before the BFS horizon was deposit ed, It is at least 170 km wide and 6 m high, and formed shortly before the VE4 transgression, Thus there may have been structural control co nfined to the Viking that influenced changes in relative sea level at the end of Viking deposition.