ABSTRACT: The Ottawa Embayment: A Giant, Structurally-Controlled Lagoon During Early Blackriveran Stage (Upper Ordovician), Eastern Ontario-Southwestern Quebec

SALAD HERSI, O., and LAVOIE, D., Quebec Geoscience Centre, Ste-Foy, Quebec, Canada

Chazyan (Middle Ordovician) shallow marine strata in the Quebec Reentrant thicken eastward from Ottawa through Montreal to Lake Champlain region, suggesting an uninterrupted, eastward deepening platform. Stratigraphic correlations and sedimentologic attributes of the lowest Blackriveran unit (the Pamelia Formation, lower Upper Ordovician) in Montreal, Ottawa and Kingston regions indicate that by the end of the Chazyan stage, regional tectonic activities altered the paleogeography of the reentrant. Uplifting of the Montreal region together with the paleo-high of the Adirondack mountains and the Laurentian Highs of the Canadian Shield delimited a semi-closed basin (lagoon) with intermittent connection to the Appalachian Sea across the present Frontenac Axis. Periods of lagoon - sea connection are marked by normal, shallow-marine sedimentation represented by subtidal to intertidal bioclastic and oolitic wackstone to grainstone lithofacies. Sedimentation in the lagoon then evolved into simultaneously aggrading sheets leaving behind extensive fine-grained, bioclastic intertidal lime wackstone to mudstone followed by fine crystalline, supratidal, sandy to pure dolostone lithofacies. Six rhythmic units of such subtidal to supratidal sheets form the Pamelia Formation of the Ottawa Embayment. Recognition of these units in the Pamelia Formation of Kingston and upstate New York, and the reduced thickness of the formation with the lack of rhythmic units in the Montreal region support that flooding of the lagoon was from the southwest. By the end of the Pamelia sedimentation, normal marine circulation throughout the regions breached the lagoon configuration of the embayment.

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