Deep Structural Control on Valley Form Geometry in a Low Accommodation Foreland

Basin Settings: An Example from the Lower Cretaceous Basal Quartz

of Southern Alberta

By

Brian A. Zaitlin, Marian J. Warren, and Dan Potocki

EnCana Energy Corporation, Calgary, AB

Lorne Rosenthal

Mannville Group Inc, Calgary, AB

Ron Boyd

University of Newcastle, Callaghan, Australia

The Lower Cretaceous Basal Quartz (BQ) of the Southern Alberta Basin was deposited in an overall accommodation-limited setting. The BQ displays multiple, closely spaced unconformities, and changing paleodrainage, depositional style and incised valley organization. The dominant control is interpreted as the interplay between eustasy and heterogeneous basement subsidence across the Vulcan aeromagnetic Low, a major suture between two Archean crustal blocks, which appears to have controlled accommodation through time.

Braided to coarse meandering "sheet" deposits dominate the south and low accommodation areas northeast of the Low. Meandering deposits are associated with the younger low accommodation and higher accommodation units. Fluvial-estuarine deposits are associated with units northwest of the Vulcan Low in higher accommodation setting.

Valley organization varies from high width:depth forms in the south, and/or base of cycles, to more deeply incised low width:depth forms towards the northwest and/or top of cycles. Tributaries are absent with older high width:depth valley forms south of the Low. Narrow, thin tributaries are associated with intermediate width:depth systems in younger units south of the Low. Deeply-cut complex tributary systems occur across the Low, whereas linear deep tributaries are mapped north of the Low with lower width:depth valley forms. The approximate position of the main interfluve per unit can be demonstrated to migrate approximately 60 km eastward through time.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming