Muftah El Gadi¹, Michael Brookfield¹
(1) University of Guelph, Guelph, ON

Abstract: Faults control of facies patterns and reservoir rocks in temperate Middle Ordovician shelf carbonates (Black River and Trenton Limestone Groups) of South-Central Ontario, Canada

The Black River and Trenton Limestone Groups of southern Ontario are a relatively thin (average 150metres) transgressive carbonate succession deposited on a temperate carbonate platform. Local Precambrian basement “highs” complicate the overall facies pattern within this transgressive succession, and detailed lithofacies patterns define local shoal, intershoal and basinal environments. Most workers thought that the Precambrian basement “highs” were inherited from the Pre-Ordovician landscape. Our work now shows that the “highs” were produced by contemporary reactivation of Precambrian faults, with different sedimentation patterns on juxtaposed blocks. In the absence of an adequate biostratigraphy, both local and regional correlations are difficult or impossible.

Tectonic control has been proposed for the mid-Ordovician shelf margin in New York. Equivalent cratonic successions in Ontario are also partly controlled by tectonics. Such successions should not automatically be assumed to consist of blankets of uniform and easily correlated facies controlled by relative changes of sea level. These faults controlled the location of fracture dolomitization and bioclastic bar facies, both of which are reservoir rocks for oil and gas in the subsurface of Ontario. Study of facies related to faulting should help in locating these tectonically related reservoir facies.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana