ABSTRACT: Foraminiferal Assemblages and Facies of the Cretaceous Colorado Group, Alberta

SCHRODER-ADAMS, C. J., Schroder Paleo Consulting, Calgary, Alberta, Canada, and J. BLOCH and D. A. LECKIE, Geological Survey of Canada, Calgary, Alberta, Canada

Foraminifera from top of the Albian Viking Formation to top of the Turonian Second White Speckled Shale (SWS) were studied from the Colorado Group in southeastern Alberta. Integration of fauna, lithology, and geochemistry results in the identification of four shale units.

The Albian foraminiferal assemblage of Shale 1 consists of benthic, agglutinated species related to the Miliammina manitobensis Zone. High faunal abundance and species diversity coincide with increased bioturbation. Assemblages are related to local rather than regional changes in sedimentation and thus are facies controlled and cannot serve for regional subzones. The agglutinated tests texture corresponds to lithological variations.

The lowermost Cenomanian Fish Scale Zone (FSZ) is barren and represents dominantly anoxic deposition. Shale 2, which gradationally overlies FSZ, is characterized by a poorly developed benthic fauna that is attributed to sulfate reduction which results in disaggregation of agglutinated tests. Faunal content of Shale 2 compares with the Verneuilinoides perplexus Zone. In the western Alberta Foothills and Peace River Plains, a rich Cenomanian assemblage is present in correlative strata.

The overlying SWS represents maximum sea level during the Cretaceous of western Canada. A pelagic fauna, consisting of planktonic foraminifera and coccoliths, represents the lower Turonian Hedbergella loetterlei Zone. Benthic foraminifera are absent or rare in the SWS, which is characterized by marine organic matter and minimal bioturbation. The basal SWS biofacies boundary can be sharp in coinciding with the lithofacies boundary, or be gradational due to a gradual influx of planktonic foraminifera. In the latter case, the lithofacies and biofacies boundaries do not coincide.

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