Morphology and origin of an erosion surface cut into the Bad Heart Formation during major sea-level change, Santonian of west-central Alberta, Canada

Marine sandstones of the early Santonian Bad Heart Formation are up to 50 m thick and are underlain and overlain by marine mudstones of the Muskiki and Puskwaskau Formations, respectively. Our examination of 395 well logs in this stratigraphic interval allowed the recognition of many distinctive well log markers. Correlations of these markers at and below the base of the Bad Heart indicates a basinward depositional thinning of about 10 cm/km. The Bad Heart itself thins from 50 to 2 m northeastward, over a distance of 100 km. Although part of this thinning is probably depositional, the truncation of gamma ray and resistivity log markers within the Bad Heart indicates that there is at least 38 m of erosional topography. A paleotopographic map and three-dimensional mesh diagram show 1) a relatively smooth, high-level "terrace," ending seaward at 2) a main erosive "bevel" trending NW-SE across the area. The bevel grades seawards into 3) a topography of "bumps and hollows," which fades into 4) a relatively smooth "basin plain." Most of the erosion was probably due to erosional shoreface retreat through wave-scour processes, during a major relative lowering of sea level. The upper parts of the erosion surface were probably subaerially exposed during maximum lowstand of sea level. In outcrop, the erosion surface is marked by a thin pebble lag, which is in turn overlain by marine mudstones. The contact between the conglomerate and underlying sandstone is sharp and locally irregular but gives no hint of the presence of deep regional erosion, which can only be demonstrated by using the subsurface data.