HOW COMPLETE IS THE STRATIGRAPHIC RECORD OF THE CENOMANIAN-TURONIAN MARINE-SHALES IN THE WESTERN INTERIOR BASIN?

Studies documenting the presence of hiatuses in the marine fine-grained successions are rare. Hiatuses and missing sedimentary record are difficult to detect in fine-grained successions, often these hiatuses are beyond the biostratigraphic resolution, leading to the assumption that the stratigraphic record is complete. Regional stratigraphic study of the Cenomanian-Turonian Blackstone Formation in southern and central Alberta indicates that this assumption may not correct. This study demonstrates that over much of the plains of Alberta and Saskatchewan the stratigraphic record is full of hiatuses, and is not complete.

This study encompasses an area of 200,000 Km² and integrates data from 2500 wells, 17 outcrops in foothills, 3 cores, biostratigraphy, and radiometric dates. Beds of the Blackstone Formation represent dominantly fine-grained clastic wedge, and were deposited during the Greenhorn transgressive-regressive cycle of the Western Interior Basin. The thickness of this clastic wedge decreases dramatically from around 500 m in the foothills area to around 40 m in the east. The lower two members of the Blackstone Formation namely Sunkay and Vimy Members are the focus of this study. Towards north, the Blackstone Formation is split into lower and upper shale dominated packages by the sandy strata of Dunvegan Formation. The stratigraphic succession from top of the Dunvegan equivalent strata up to the top of Vimy Member is divided into VIII allostratigraphic units in ascending order. The duration of each unit is approximately in the range of 375-500 thousand years. The bounding surfaces of these units are flooding surfaces, and regional bentonite beds. These can be correlated over the entire study area and exhibit remarkable parallelism, suggesting that they are synchronous surfaces. The strata of Unit I are truncated towards the east by Middle Cenomanian forebulge unconformity. The strata of the Units II-VIII progressively onlap / downlap towards east on the underlying units, and are extremely thin or absent close to Alberta-Saskatchewan border.