Stratigraphic Effects of Precambrian Basement Influence on Marine to Non-Marine Facies Distribution in the Western Canada Sedimentary Basin, Upper Cretaceous, Southern Alberta

Abstract

Thick marine mudstone-dominated strata of Santonian to early Campanian age form the upper part of the Wapiabi Formation in the Western Canada Foreland Basin. These rocks span ∼2 myr and represent a phase of active flexural subsidence. A regional high-resolution allostratigraphic framework based on 2200 wireline logs, 7 outcrop sections and 7 cores distributed over 95,000 km² of SW Alberta allowed investigation of spatial and temporal variations in facies and subsidence. Fourteen allomembers, of regional to sub-regional extent are recognized. In the eastern, offshore part of the study area, the succession comprises stacked, siltier- and sandier-upward parasequences, a few to ∼12 m thick, grouped into allomembers bounded by regionally-recognizable flooding surfaces. Many parasequences can be traced for hundreds of km, thinning gradually eastward towards the forebulge, with rare lap-out suggesting the absence of significant bathymetric relief on the basin floor. Sedimentary structures indicate deposition on a featureless marine ramp from decelerating combined flows above storm wave base. Towards the south and west, marine mudstones pass laterally into nearshore and coastal plain facies. Subsurface mapping of nearshore sandstone indicates a NW-trending shoreline. South of latitude 50°N, log correlations indicate a southward lateral facies change from marine to nonmarine strata that thicken steadily southward. Spatially, this facies transition corresponds closely with the Vulcan structure in the Precambrian basement, which marks a boundary between two basement terranes. The restriction of the southward-thickening wedge of nonmarine strata to the region south of the Vulcan structure suggests that the latter acted as a hinge with differential subsidence to the south during the early Campanian. The recognition of this lateral facies change provides an explanation for a longstanding lithostratigraphic enigma, namely the lateral relationship between the Chungo sandstone of the Wapiabi Fm. in the north and the sandstone Virgelle Fm. of the Milk River Group in the south. The Chungo Mbr. represents a diachronous northward extension of both the nonmarine Deadhorse Coulee Fm. and the marine Virgelle Fm. of the Milk River Gp. Subsidence across the Vulcan structure created a local high accommodation zone in the south, allowing the deposition of an aggradational-progradational stack of alluvial sediments and a coeval, northward-prograding reservoir-quality sandy shoreface.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016