--> Abstract: High-Resolution Mapping of Depositional Trends over the Vulcan Structure and Bow Island Arch: Cardium Formation (Turonian-Coniacian), Southern Alberta, by Joel Shank and Guy Plint; #90124 (2011)

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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

High-Resolution Mapping of Depositional Trends over the Vulcan Structure and Bow Island Arch: Cardium Formation (Turonian-Coniacian), Southern Alberta

Joel Shank1; Guy Plint1

(1) Department of Earth Sciences, University of Western Ontario, London, ON, Canada.

Interest in the Turonian-Coniacian Cardium Formation of Alberta has traditionally centered on large hydrocarbon reserves hosted by shallow marine sandstone and conglomerate facies. Muddy strata, which comprise the bulk of the formation, have been neglected; such rocks nevertheless provide the most detailed record of subsidence.

This analysis incorporates over 3500 well logs, integrated with sedimentological logs of 12 outcrop and 12 core sections. An allostratigraphic framework based on the correlation of marine flooding surfaces provides a proxy chronostratigraphy that allows patterns of subsidence to be discerned as they evolved through time. Because the Cardium Formation was deposited in shallow water on a sea-floor that effectively lacked topography, isopach maps can be interpreted to represent contemporaneous subsidence, rather than depositional topography.

The Vulcan structure is a basement lineament has previously been interpreted as a collision zone between the Precambrian Medicine Hat and Loverna Blocks (Eaton et al, 1999). Thickness changes of Cardium allomembers across the Vulcan structure indicate that the structure was reactivated during the Late Cretaceous, with pulses of both subsidence and uplift.

The Bow Island Arch of southern Alberta is related to a Jurassic flexural forebulge. A regional erosion surface at the top of the Cardium Formation truncates over 20 m of underlying strata, indicating that the Arch formed a contemporaneous topographic high. Localized erosional “bulls-eyes” show a similar degree of erosional relief and may be due to syn-depositional doming, perhaps caused by plutonism, followed by marine erosion. The Arch has previously been shown to control depositional trends in Santonian strata (e.g. Schröder-Adams et al.,1998).

The recognition of pulses of basement-controlled subsidence and uplift suggests that tectonic controls are not necessarily slow and steady, but may be intermittent (c.f. Vakarelov and Bhattacharya, 2009). Detailed stratigraphic studies permit the recognition of rapidly shifting depocentres. An appreciation of the complexity of such depocentres is important for prediction of potential reservoirs and reservoir heterogeneity.