Ottawa Embayment, Early Cambrian to Early Ordovician

Reconstruction of the position of the continents Middle Ordovician

Reconstruction of the position of the continents Late Ordovician

Ottawa Embayment, Middle Ordovician to Late Ordovician

At the beginning of the Paleozoic Era, Pangea split, with the continental fragments drifting apart and the proto-Atlantic ocean, called Iapetus Ocean opening between them. The eastern margin of the Grenville orogen rocks formed the east coast of the North American continent and were open to the ocean. At that time the Avalonian tectonic plate (Appalachian mountains) were still drifting at a distance from North America . The National Capital Area is located at the north-western end of a bay, called the Ottawa Embayment, that extended into the continental mass, bounded by the Frontenac Arch and Adirondack mountains.

Late Cambrian and Early Ordovician: Marine transgression.

The sea slowly transgressed into eastern Québec during the Early Cambrian, reaching the Ottawa area in the Late Cambrian. The first sedimentary deposits were derived from Precambrian quartzites and conglomerates to form the feldspathic conglomerates and sandstones of the Covey Hill Formation. This unit was formed near the margins of the invading ocean and correspond to alluvial fan and braided fluvial deposits. It outcrops only in a few locations (north-west of Rockland, Maps - GeoServ ) and its distribution is limited to topographic depressions on the Precambrian surface.

During the Late Cambrian to Early Ordovician, the first marine sediments were deposited in shallow water, close to shore, within the tidal zone. These early marine deposits, called Nepean Formation, consist of fine-to coarse-grained quartz sandstone of marine and terrestrial origin, indicating fluctuations of the sea level. The upper part of the formation contains dolomitic beds which are characteristic of marine transgression (increase in water level).

During the Early Ordovician, the sea continued to advance covering Precambrian knobs which protruded through the Nepean Formation, and deposited shallow marine carbonate and clastic sediments. The lower unit, the March Formation, consists of interbedded sandstones and dolostones, with boulder-and cobble-sized interclasts of quartzite where the unit is in contact with Precambrian rocks. As the depth of water was increased, the sediments became finer-grained, slowly grading into clay-size sediments and carbonate precipitates (CaMg) of the Oxford Formation (dolostone), characteristic of hypersaline environment. The last occurrence of sand corresponds to the transition between the March Formation and the Oxford Formation.

Middle to Late Ordovician: Fluctuating sea level.

In the Middle to Late Ordovician, the drifting Avalonian microcontinent was approaching North America and the convergent plates produced an orogenic belt (Appalachian Mountains) along the eastern margin of the Laurentian craton. Consequently, the continental shelf upon which the Late Cambrian and Lower Ordovician deposits accumulated was progressively deformed. The uplift of the continental shelf in early Middle Ordovician, caused a regression of the sea which resulted in subaerial exposure and erosion of a large part of the Early Ordovician rocks.

The erosional period was followed by a second period of marine transgression in the Ottawa Embayment that deposited sediments (Rockliffe Formation) derived from the surrounding uplifted Precambrian rocks . The rocks of the Rockliffe Formation grade from sandstone, with quartz-pebble conglomerate locally present, to shale and limestone with silty dolostone interbeds, indicating a gradual deepening of the water, with deeper water conditions to the east. The sedimentation period that produced the Rockliffe Formation was followed by a marine regression which exposed the sediments to erosion.

A third marine invasion followed the hiatus subsequent to the Rockliffe Formation deposition. The depositional sequence begins with nearshore sediments of the Shadow Lake Formation which disconformably overlies the Rockliffe Formation. This formation does not outcrop (at the bedrock surface) in the NCA area but can be observed between the Rockliffe and Gull River Formations in a section eroded by the Rideau River at Prince of Wales Falls (Hog's Back Park). The base of the formation consists of sandstone which grades upward to sandy shale interlayered with carbonate rocks.

The overlying succession of shales, dolostones and limestones, of the Gull River, Bobcaygeon, Verulam and Lindsay formations represent a near-continuous deposition on a deepening shelf.

A characteristic of the Middle Ordovician formations is the increasing presence of organic matter in the younger rocks. Although fossils are present in the March and Oxford formations, it is only with the Gull River and more so with the Bobcaygeon formations that the fossils are abundant enough to influence the characteristics of the rocks. The Verulam Formation is described as fossiliferous limestone with inter-beds of calcareous shale, whereas the Lindsay and Eastview formations are inter-bedded black, organic-rich limestone and highly calcareous shale. The increasing abundance of vegetation and marine life in the Ottawa Embayment indicates ideal conditions for the development of life, such as warm water temperature and protected shallow marine environment.

Late Ordovician: Deep water sedimentation and regression.

During the first part of the Late Ordovician the sea continued to rise, leading to deep shelf sedimentation conditions. During the second part, the continued northwestward migration of the Appalachian structural front caused an uplift of the shelf between the Laurentian and Avalonian cratons. The crustal uplift combined with a possible continental glaciation of the North African craton, lead to a drop in the sea level to a point that the Ottawa Embayment was isolated from the open sea and eventually dried up.

The Billings Formation consists of shale with thin interbeds of dark grey limestone, and is distinguished from the underlying Eastview Formation by its noncalcareous nature indicating deposition in a deep shelf environment. The sediments of the Billings Formation were therefore deposited during marine the high-stand of marine transgression. The Billings Formation grades into the Carlsbad Formation which consists of interbedded shale, fossiliferous calcareous siltstone, and silty bioclastic limestone, indicating that the conditions were evolving from deep sea to shallow sea. The depositional sequence indicates marine regression that continued during deposition of the Queenston Formation, reaching marginal marine to shallow marine environment. The Queenston Formation consisting of red to green-grey shales, siltstone and minor limestones, is the youngest preserved Paleozoic unit.