ABSTRACT: Facies Recognition in Cretaceous Colorado Group Shales of Alberta

BLOCH, J., and D. A. LECKIE, Geological Survey of Canada, Calgary, Alberta, Canada, and C. SCHROEDER-ADAMS, Schroeder Paleoconsulting, Calgary, Alberta, Canada

A composite section comprising two cores (10-35-45-2W4 and 11-12-6 16W4) is representative of the subsurface Albian to Turonian interval of the Cretaceous Colorado Group in Alberta. Four distinct shale units have been identified by integrating lithostratigraphy with micropaleontology and geochemical data. These units are similar to and correlative, in part, with the Shaftesbury, Dunvegan, and Blackstone formations that crop out in the Alberta Foothills.

The lowermost shale (Shale 1) conformably overlies the Albian Viking Formation and equivalent rocks and is disconformably overlain by the Fish Scale Zone (FSZ). The basal contact of the FSZ comprises a major boundary (sequence boundary?) that corresponds to the Albian-Cenomanian boundary. Above the basal contact, the FSZ comprises a condensed section that is a radioactive, organic-rich mudstone containing dominantly Type II organic matter (OM). The FSZ grades upward into Shale 2 that has a higher silt and sand content and contains dominantly Type III OM. The Second White Speckled Shale (SWS) abruptly overlies Shale 2 and is characterized by a high bioclastic carbonate content, dominantly Type II OM and minimal bioturbation. A regionally extensive bentonite bed up to 30 cm thick marks he base of the SWS and coincides closely with the Cenomanian-Turonian boundary.

The interval from the top of the Viking Formation to the top of the SWS represents deposition during a eustatic sea level rise of perhaps up to 300 m. In the Western Canada sedimentary basin, this worldwide sea level rise was punctuated by at least one lowstand event. Evidence of discrete progradational events is present within Shales 1 and 2. The distinct characteristics of the individual shale units indicate widespread anoxic conditions and a stratified water column persisted in the basin during basal Shale 1, FSZ, and SWS deposition. Dysaerobic to oxic bottom conditions were persistent during the deposition of upper Shale 1 and Shale 2.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)