Influence of the Pre-Cretaceous Unconformity on Deposition of Lower Mannville Clastic Sequence, Drumheller, Alberta: ABSTRACT

The paleotopography of the Pre-Cretaceous unconformity and distribution of the overlying sediments has been examined in a 576 mi2 (1,491 km2) area of south-central Alberta using all available cores, geophysical logs, selected well cuttings, micropaleontology, and seismic sections. Control averages one well per square mile. The Cretaceous strata in this area lie unconformably on Mississippian carbonates and shales. The relief is controlled, in part, by the lithology of the subcropping Paleozoic formations. The basal sandstones of the lower Mannville are valley-fill deposits ranging in thickness from 0 to more than 100 ft (30 m). They are separated from the underlying Paleozoic carbonate rocks by a detrital assemblage consisting of variegated, poorly sorted, cherty rocks. This thick basal quartz sandstone assemblage can be subdivided into a number of distinct lithic units which are recognizable on the basis of core studies, but are not readily identifiable on geophysical logs. Three different lithic units have been mapped integrating all available data. A depositional model has been constructed which clarifies the relation between the unconformity and the occurrence of basal quartz hydrocarbon reservoirs. End_of_Article - Last_Page 634------------