DEPOSITIONAL ENVIRONMENTS AND SHELL-BED GENESIS IN THE LATE ORDOVICIAN (KATIAN) VERULAM FORMATION AT GAMEBRIDGE QUARRY, ONTARIO

Laurentia during the Late Ordovician experienced wide-spread carbonate deposition in the warm Tippecanoe sea. Environmental changes associated with the Taconic Orogeny were restricted to southeastern Laurentia. This study is a detailed analysis of the sedimentology and depositional environments of an area near the region of environmental changes associated with the Taconic Orogeny but away from their influence. Gamebridge Quarry, located on the stable carbonate platform about two hours north of Toronto, Ontario, Canada is an important locality because it is easily accessible and the uppermost Bobcaygeon through Verulam are well-exposed. A previous study described just over 9m of the Verulam but additional quarrying now exposes a total of 30.5m including the uppermost Bobcaygeon.

The Verulam Formation contains subtle shifts in lithofacies mainly involving variations in grain size and shale content. Numerous gradational changes from shale-poor, dominantly coarse grainstone to more shale-rich, fine grain- to wackestone likely record shifts from more proximal to more distal ramp depositional environments.

Monotaxic shell beds are hashes of bryozoa, brachiopods or gastropods. These are more common proximally and typically cap grainstone beds. The fragmented and tumbled nature of the clasts suggests genesis through storm-winnowing. Diverse beds, more often occurring distally, contain a high abundance of brachiopod, bryozoan, gastropod, pelmetazoan and trilobite body fossils and are usually capped by laminated shale to lime mudstone. Abrasion and disarticulation indicate that they are time-averaged accumulations rather than obrution deposits. Encrustation of large bioclasts by bryozoa imply that these pavements were produced through episodic starvation.