Sedimentology and sequence stratigraphy of the ordovician Black River and lower Trenton Groups, Lake Simcoe area, Ontario

The Black River and Trenton Limestone Groups of southern Ontario are a relatively thin (average 150 metres) carbonate succession in the Lake Simcoe area, 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 facies in the Lake Simcoe area form repetitive cycles which can then be traced laterally into adjacent areas, complicated by the effects of sea-floor topography and possibly by synsedimentary faulting. Both minor and major cycles are dominantly coarsening upwards, and normally bounded by sharp non-depositional surfaces representing marine transgressions. Both minor and major cycles are compared to those of other ancient successions, and those inferred from cores in the modern ramps used for the microfacies and facies comparison. Their development can be related to both autocylic and allocyclic processes.; The facies architecture of the cycles across southern Ontario show that they are the basic architectural elements of the Black River-Trenton transgressive systems tract. The major cycles form five backstepping parasequences extending across southern Ontario from Manitoulin Island to Kingston. Periodic uplift and subsidence of the area, fluctuation in the position of the relative sea level, local tectonism, and sea-water temperatures were responsible for development of the succession.