Cryptic Migrating Depocentres in a Mudstone-Dominated Succession: Coniacian Muskiki and Marshybank Members, Central Alberta Plains and Foothills, Western Canada Foreland Basin

Abstract

Marine mud- and siltstone-dominated Coniacian (86.3-89.8 mya) strata comprising the Muskiki and Marshybank members of the Wapiabi Formation are distributed widely across the foredeep of the Western Canada Foreland Basin. These rocks record a regional transgression following an early Coniacian lowstand. To investigate the stratal architecture and depositional history of this ∼100 m thick package of rock, a detailed allostratigraphic framework was constructed. In this framework, 14 outcrops and 14 cores are linked into a regional grid of 1002 wireline logs that are distributed over ∼45,000 km². The basic stratigraphic signature of the rocks comprises siltier- and sandier-upward successions (parasequences) of the order of 5-15 m thick. Regional allostratigraphic correlations show that packages of parasequences are bounded by subtle, typically mud-on-mud, disconformities that define five, strongly wedge-shaped allomembers. Thickness trends of intra-allomember parasequences reveal both where and when deposition and erosion occurred. In some cases, sheet-like parasequences are progressively beveled off, indicating uniform subsidence followed by regional tilting and erosion. In other cases, parasequences show progressive thinning towards regions of erosion, implying contemporaneous sedimentation and tilting. Parasequences are also found to onlap onto underlying erosional topographic highs, implying relief on the sea-floor. The allomember wedges also exhibit differential thickness trends, with some thickening into the foredeep and others thinning. This implies that ‘normal’ flexural subsidence of the foreland basin was discontinuous in both space and time. Three allomembers pinch out along distinct linear trends, which define regions of hiatus. In general, the erosional hiatuses are confined to the eastern, more distal part of the foredeep, whereas the west accommodated a more complete record of Coniacian time. The linear erosional trends suggest differential uplift and subsidence, or differential compaction of underlying Paleozoic rocks. However, it is presently difficult to relate patterns of Cretaceous erosion to definite features in the underlying rocks, such as faults or major facies changes. This study demonstrates that apparently uniform mudstone successions may be punctuated by major, cryptic discontinuities that separate genetic units. Where units have contrasting lithological and reservoir properties, this could affect attempts to exploit gas resources.

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