The Evolution of Deltaic Depositional Systems in a Sequence Stratigraphic Framework; Upper Cretaceous Dunvegan Formation, Alberta Subsurface

BHATTACHARYA, JANOK, Alberta Geological Survey, Edmonton, Alberta, Canada

Sequence stratigraphic subdivision of the Dunvegan clastic wedge allows the recognition and delineation of discrete shoreline related depositional systems over an area of 30,000 sq km, using 500 well logs and 130 cores. These depositional systems are recognized at the parasequence scale and can be grouped (from oldest to youngest) into highstand, lowstand, and transgressive systems tracts.

Shoreline systems in the highstand and early lowstand systems tracts comprise the deposits of river-dominated deltaic systems. Distributary systems and lowstand channels associated with these deltas may be transformed into fluvial-dominated estuaries, especially where capped by a major marine flooding surface (commonly in the last parasequence in an offlapping parasequence set).

Depositional systems in the late lowstand and transgressive systems tract are characterized by a much greater degree of reworking by basinal processes and include wave-dominated deltas and regressive and transgressive barriers. They also contain a much greater number of marine unconformities. Estuaries associated with these systems are more marine-dominated than in the lower portions of the Dunvegan wedge.

The overall decrease in fluvial influence upwards is interpreted to result both from the position in the sequence and from an overall decrease in sedimentation rate produced by the progressive denudation of highlands to the northwest.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)