ABSTRACT: Shoaling Upwards Cycles (parasequences) in Middle Ordovician Carbonates of Southern Ontario, Canada

Muftah S. El Gadi, and Michael E. Brookfield, Guelph University, Guelph, Ontario, Canada

The shelf carbonates of the Black River and Trenton Groups form a thin (average 150 metres) transgressive systems tract of a Middle to Upper Ordovician depositional sequence. Within this tract, various upward shoaling cycles bounded by flooding surfaces (parasequences) can be used for local correlation.

The Black River contains symmetrical and asymmetrical low energy 'lagoonal'-supratidal cycles within a generally deepening (backstepping) succession. Flooding surfaces are marked by various condensed 'glauconitic' horizons with a marine, though somewhat low-diversity, fauna. The Black River-Trenton boundary is a major flooding surface separating a 'lagoonal'-tidal flat succession (Black River) from an open marine succession (Trenton Group). This change is practically synchronous from Lake Simcoe to Kingston and marks either a relatively rapid and significant rise in relative sea level, or an erosion surface caused by shelf reworking between depositional shoreline and deep shelf facies (the shaved shelf depositional model of James et al., 1994).

The Trenton Group also contains asymmetrical and symmetrical cycles (exactly like the Jurassic Klupfel cycles of western Europe), whose resistant capping grainstone form persistent and mappable units over much of southern Ontario. Like Klupfel cycles, the Trenton cycles become more symmetrical and complete from shelf to basin (from western Ontario to central New York). Furthermore, each cycle contains distinctive biofacies and nektonic/pelagic faunas.

Though eustatic sea-level changes probably determined the grosser cyclicity of these Middle Ordovician carbonates, tectonic effects caused by the ongoing emplacement of the Taconic allochthons to the east also affected sedimentation.

Search and Discovery Article #90907©2000 AAPG Eastern Section Meeting, London, Ontario, Canada