Diagenetic history of Ordovician and Devonian oil ... - Geology Ontario

17
nodules enclosed within mudstone-wackestone are characteristic of the upper Trenton
Group, whereas the interbeded and fining-upward bioclastic carbonates are more
characteristic of the middle Trenton Group. Bioclasts in both the Trenton and Black River
groups include fragments of echinoderms, brachiopods, bryozoans, and to a lesser extent,
trilobites, nautiloids, corals, gastropods and bivalves.
Lake Simcoe region outcrop equivalents to the Trenton Group are described by
Brett and Brookfield (1984) and Brookfield and Brett (1988) as an overall deepeningupward
succession from open shelf to deep shelf with a shallowing-upward sequence in
the Verulam Formation (Sherman Fall Formation equivalent). Brett and Brookfield
(1984) and Brookfield and Brett (1988) suggest the shelf had topographic highs and
deposition was commonly influenced by storms.
Common to both the Trenton and Black River groups are altered volcanic ash
beds (K-bentonites) and submarine exposure surfaces (hardgrounds). Trevail (1988a)
indicated the altered volcanic ash beds are located mainly near the contact between the
Trenton and Black River groups and range from 5 to 35 cm in thickness. Middle
Ordovician hardgrounds in outcrop have been described in detail by Wilkinson et al.
(1982) and Brett and Brookfield (1984).
Overall, the Black River and Trenton groups form a deepening-upward sequence
of supratidal through shallow subtidal environments recording deposition in an epeiric
sea. This deepening-upward sequence is overlain by the deep subtidal shales of the Blue
Mountain Formation.