ABSTRACT: Differential Tectonism as a Cause for a Disconformity in the Upper Judith River Formation of Southern Alberta

EBERTH, DAVID A., Royal Tyrrell Museum, Drumheller, Alberta, Canada, and ANTHONY P. HAMBLIN, Geological Survey of Canada, Calgary, Alberta, Canada

The upper one-half of the nonmarine Judith River Formation of southern and southeastern Alberta is divisible into two lithostratigraphic units separated by a regionally extensive and diachronous disconformity. Differences in the origin, depositional history, and geographic distribution of both clastic units largely reflect a differential tectonic history between the central and southern Canadian Cordillera.

The lower unit (Oldman member) represents the remains of a low-sinuosity fluvial system that originated in the southern Cordillera of Canada and northern Montana and flowed northeastward, perpendicular to the axis of the Alberta basin. The upper unit (Dinosaur Park member) represents the remains of a high sinuosity, fluvial-to-estuarine system that originated in the north-central Cordillera and flowed east-southeastward, subparallel to the axis of the Alberta basin.

During Judith River "time" (middle to late Campanian), active thrusting, tectonic loading, and subsidence, with attendant fractionation of coarse-grained clastics in the foredeep and intervals of sediment starvation in the distal basin, comprised the long-term and dominant tectono-sedimentary pattern in the southern Cordillera and adjacent foreland basin. In contrast, isostatic rebound during tectonic quiescence, followed by cannibalization and widespread southeastward dispersal of foredeep sediments, was the long-term and dominant tectono-sedimentary pattern in the northern Cordillera and adjacent foreland basin. These differential tectonic patterns resulted in a southeasterly tilt along the axis of the Alberta basin that allowed the Dinosaur Park member to prograde slowly southeastw rd as the basin reached equilibrium.

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