The Troubled oceans of the Devonian-Mississippian of Western Canada

The Devonian-Mississippian Exshaw Formation is one of the main hydrocarbon-contributing source rocks that supplied the massive oil sands and heavy oil accumulations of Alberta. It is sometimes referred to as the “Alberta Bakken’ play as it is stratigraphically equivalent to the prolific oil-producing Bakken strata in the Williston Basin. The Exshaw mudrocks are widely distributed across the subsurface of Alberta and outcrop in the Rocky Mountain Front Ranges. These strata are sandwiched between thick carbonate ramp successions.

The formation of organic-rich mudrocks has intrigued researchers for decades. Unique conditions prevailed in these ancient epicontinental seas of western North America to justify such rich accumulations of organic matter on the sea floor and subsequent preservation in the rock record.

This presentation takes us on a journey to the ancient Exshaw seas where prevailing water-column conditions were hostile to most life-forms. I will suggest conditions responsible for generating such a severe perturbation to the palaeoenvironment; a palaeoenvironment that had been stable and conducive to ambient carbonate production both pre- and post-Exshaw deposition. These strata were studied from an interdisciplinary approach incorporating sedimentology, ichnology, stratigraphy, inorganic geochemistry, organic geochemistry and palaeoceanography. Evidence indicates that the water column experienced eutrophication; development of massive phytoplanktonic blooms causing a ‘greening’ of the euphotic zone. Consequently, consumption exceeded supply of free oxygen in the water column which led to oxygen-deficient conditions not only on the sea-floor but also in the water column due to copious production of organic matter.

Severe perturbations to palaeoenvironmental conditions of the water column resulted in catastrophic reduction in the carbonate production factory. Conditions lethal to the continued production of carbonate deposits led to ‘drowning’ of carbonate ramps and their subsequent replacement by these organic-rich mudrocks. Later, the water column returned to ‘blue’ oligotrophic conditions and the resumption of carbonate production.

Green Eutrophic Seas: Formation of the Organic-Rich Exshaw Mudrocks

Mark Caplan, PhD
Director, CSPG
Tuesday, March 15 – 5:30 pm – University of British Columbia
Wednesday, March 16 – 5:00 pm – Mount Royal University
CSPG University Outreach Lecture Tours