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Location of Dolomitized Grainstone Reservoirs in the Upper Ordovician Black River and Trenton Limestone Groups in Ontario: a Persian Gulf/West Florida and Northern Afghanistan Comparison.

Michael E. Brookfield and M. El Gadi, Land Resource Science, Guelph University, Guelph, Ontario N1G 2W1, Canada. ([email protected])

Dolomitized bioclastic grainstones form the main hydrocarbon reservoirs in the Ordovician of Ontario. These are controlled by depositional environment, tectonics and climate. Environmental position on the ramp determined where the contributing organisms lived and what sediments were deposited. Tectonics not only determined the configuration of the ramp and hence where grainstones developed, but also movements on faults which controlled the dolomitization.

By analogy with carbonate ramp models based on modern environments (the most applicable ones being cool Persian Gulf/West Florida models), bioclastic grainstones occur in two main settings: in shallow shelf shoal areas around normal wave base (brachiopod-coralgal), and in deeper shelf areas affected by contour currents (pelmatozoan-bryozoan). The grainstones form the tops of coarsening (and possibly shallowing) upwards cycles (parasequences) within a generally transgressive succession.

By analogy with tectonic models based on Cenozoic collisions (the most applicable one based on the indentation and rotation of Afghanistan by NW India), Ordovician dolomitization and reservoir formation occurred along lateral transtensional and transpressional faults related to collision of the Taconic arc with the Ordovician carbonate ramp. Differential shear can be related to the rheological contrast between the Canadian shield and the areas to the south (with complex basin/plateau structure) - the dividing line roughly running from Anticosti Island down the St Lawrence into southern Ontario and Ohio.

The Ordovician succession in Ontario and New York is now known to be at least partially controlled by synsedimentary tectonics. Such successions, especially in the absence of a reliable chronostratigraphy, should not be automatically assumed to consist of blankets of uniform and easily correlated facies controlled by relative changes of sea-level; nor can simple sequence models be applied to such successions. For example, the differential shelf edge erosion towards the top of the Trenton can plausibly be attributed to the development and uplift of a forebulge during emplacement of Taconic allochthons to the east. In this case, relative sea-level changed divergently in closely adjacent areas lead to incompatible sequence descriptions.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York