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St. John's 2001
Technical Programme

GS8:  Sedimentology 
Organizers / Organisateurs:  Programme Committee
Room / Salle:  SN2018

Date:  29/05/2001
Time:  8:40 AM
Presenter:  Lisa M. Nadjiwon


Facies and dolomitization of the Middle Devonian Dunedin and Keg River Formations, northeastern British Columbia

NADJIWON¹, L.M., lmnadjiw@scimail.uwaterloo.ca, MORROW², D.W. and CONIGLIO¹, M., ¹University of Waterloo, Waterloo, ON, N2L 3G1, ²Geological Survey of Canada, Calgary, AB, T2L 2A7

Some of the largest carbonate reservoirs in the Devonian of the Western Canada Sedimentary Basin were deposited on the Macdonald Shelf, which was flanked by deep troughs to the west (Kechika Trough) and north (Meilleur River Embayment).  Middle Devonian strata from the shelf succession were examined from seventeen localities in the Rocky Mountains and twenty cores from the adjacent subsurface to the east of the outcrop belt.  These strata were deposited during a marine transgression, during which Lower Devonian strata were progressively onlapped from north to south.

The Dunedin Formation, which occurs in the northern part of the study area, is a transgressive unit composed of peritidal and subtidal platform carbonates. In the furthest northern portion of the study area, crinoid shoals protected the extensive shelf, where tidal flats, ponds, and tidal channels developed during the early part of Dunedin deposition.  These peritidal environments progressively shifted southward due to marine transgression, and are overlain by deeper subtidal and open marine carbonates, followed by shales of the Besa River Formation.

In the central part of the study area, Dunedin strata pass laterally into more bioclastic carbonates of the Keg River Formation.  The basal beds of the Keg River Formation are sandstones and dolomitic siltstones, overlain by subtidal and intertidal platform carbonates.  Overlying patch reefs and mud mounds eventually coalesced to form a north- and west-facing barrier reef system composed of reef-core stromatoporoid and coral boundstones and rudstones, fore-reef bioclast-rich debris flow beds, and back-reef lagoonal strata containing amphiporid thickets.

The Dunedin and Keg River formations have similar diagenetic facies, in which the original limestone was altered to coarsely crystalline replacive dolomite and white sparry dolomite cement. Late stage precipitates in some larger pore spaces include coarsely crystalline calcite and prismatic quartz crystals.  This diagenetic assemblage is most commonly found in the Keg River Formation within reef and reef-associated facies. Similar styles of dolomitization occur in equivalent strata in northern Alberta, the Yukon and the N.W.T. where tectonic compression or  topographic recharge dolomitization models have been invoked. In the present study, fluid inclusion microscopy, stable carbon and oxygen isotope geochemistry, and petrographic evidence indicate dolomitization occurred through hydrothermal convection at moderate burial depths, and was followed much later by cooler and fresher fluids precipitating calcite and quartz.  Understanding the dolomitization mechanism for such coarsely crystalline dolomites has important implications for predicting the geometry and extent of potential reservoir rocks.