ABSTRACT: Ichnologic and Paleoecologic Trends in Successive Offshore-Shoreface Successions within the Upper and Middle Mannville Subgroups of East-Central Alberta

MATTISON, BLAIR W., Mattison Geoconsulting, Calgary, Alberta, Canada

Siliciclastic strata within the Upper and Middle Mannville subgroups of the Cold Lake Oil Sands area of east-central Alberta are divisible into seven unconformity-bounded stratigraphic units. These units reflect seven transgressive-regressive cycles imposed upon a longer term regressive trend. This punctuated regression ultimately saw the withdrawal of the Early Albian boreal Clearwater Sea from western Canada.

Ichnologic studies of offshore shales in stacked offshore-shoreface coarsening-upward successions within this interval indicate a gradual trend from open marine conditions in offshore shales of the basal stratigraphic units towards paleoecologically restricted, brackish water conditions in offshore shales of the upper stratigraphic units. This reduced paleosalinity trend is indicated by changes in ichnofaunal makeup, in smaller size of individual ichnogenera, and in decreased intensity of bioturbation.

These changes mirror not only shallowing of the Clearwater Sea resulting from the overall long-term regressive trend, but also increases in freshwater influx to the basin caused by increases in fluvial drainage. Salinity stratification resulting from increased freshwater influx may have promoted bottom water anoxia and a consequent reduction in biologic activity.

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