Microfacies Analysis and Depositional Model for the Black River -Trenton Carbonates (Ordovician) of Southern Ontario, Canada

This study documents the detailed microfacies depositional environment of the Ordovician Black River and Trenton limestone Groups in Lake Simcoe area of southern Ontario, resting unconformably on Precambrian basement. Local Precambrian 'highs' complicate the overall facies pattern and lithofacies patterns define local shoal, intershoal and basinal environments. Detailed microfacies and facies analysis in large quarries in the Lake Simcoe area are used to infer various carbonate environments by comparison with analogous modern and ancient ramps. Fifteen microfacies are grouped into six microfacies associations based on composition, grain size and texture. Relative energy levels, and depositional environments for these are then inferred. These environments resemble the Recent Arabian Shelf of the Persian Gulf for the Black River Group, and the Recent West Florida Shelf for the Trenton, though other Recent ramps like South Australia are also comparable. The vertical arrangement of facie\s in the Lake Simcoe area forms repetitive cycles, which can then be traced laterally into adjacent areas, complicated by the effects of sea-floor topography and possibly by synsedimentary faulting.