ABSTRACT: The Jurassic-Cretaceous Boundary in West-Central Alberta

KRAMERS, JOHN, Alberta Geological Survey, Alberta Research Council, Edmonton, Alberta, Canada, GRAHAM DOLBY, G. Dolby and Associates, Calgary, Alberta, Canada, and DENNIS NIKOLS and RUDY STROBL, Alberta Geological Survey, Alberta Research Council, Edmonton, Alberta, Canada

The Jurassic-Cretaceous boundary in west-central Alberta is an unconformable contact between similar lithologies, thus making it difficult to delineate. Because the age of the deepest producing horizon determines the depth to which a lease extends, this enigmatic boundary leads to problems in lease administration.

The Jurassic in west-central Alberta was deposited under dominantly marine to nearshore conditions with an easterly, mature sediment source. As mountain building was initiated to the west, Lower Cretaceous sediments were deposited in nonmarine conditions with an immature sediment source. A combination of methods are being applied to define the boundary. Palynology can be used to differentiate the Jurassic strata from the Lower Cretaceous strata. Where palynology cannot provide the answer, lithologic attributes such as sand mineralogy and environment of deposition can point to the boundary location.

Regional cross sections have been constructed in west-central Alberta to show the location and style of the Jurassic-Cretaceous contact. Over a large portion of the study area (a highland), the boundary can be shown to be an angular unconformity, where westwardly dipping Jurassic strata subcrop against the lowest Cretaceous sediments. In the area of the Gilby, Medicine River, and Sylvan Lake fields, a major valley system cuts through the highland and various ages of valley fill make delineation of the boundary less certain.

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