Abstract

Field data from fracture sets, folds and faults affecting generally flat lying Lower Ordovician to Lower Silurian strata on western Anticosti Island, Québec, are integrated with remote sensing and seismic reflection data. Spatial and temporal relationships among different structural features observed on the Island and in the Gulf of St. Lawrence are assessed with the aim of understanding the role of these structural features for petroleum migration through the Anticosti Platform. The tectonic impact of Appalachian orogenic events on Anticosti Platform is considered, together with Jurassic and Quaternary tectonics.

Four fracture sets are identified in Lower Silurian rocks: 1) a NS-EW orthogonal fracture system (Sets I and II); 2) a N40–70 set (Set III); 3) and a N135–165 set (Set IV). Some irregular, randomly oriented fractures are also observed. The orthogonal system may be related to Early Acadian events, which may have induced extensional stresses within the Platform during Late Silurian-Early Devonian time. Normal faults observed in the Lower Silurian succession may be related to this extensional event. The N135–165 and N40–70 sets may have developed either during the Middle Devonian Acadian orogeny or later in the Jurassic. The irregular fractures could have formed during the final stages of fracture development in response to erosion of the Lower Silurian and younger succession, or in response to isostatic rebound following glaciation.

Shuttle Radar Digital Elevation Model lineaments are consistent either with observed fracture set orientations, or subsurface Taconian fault orientations. We infer some Taconian faults and Silurian fracture zones were reactivated, probably during the opening of the Atlantic Ocean, as indicated by the intrusion of two Jurassic dikes.

In the subsurface of the Island, major SW-dipping normal faults and extensional fractures affect the Lower to Upper Ordovician succession most. These probably formed in response to an extensional stress field in front of the Taconian deformation front during Late Ordovician time.

WSW-trending typically open folds occur on southern Anticosti Island in Chicotte Formation and in the Gulf of St. Lawrence. They involve Lower Silurian to Devonian rocks, and may have formed in a compressive regional setting during the Middle Devonian Acadian orogeny.

Thermal maturation and petroleum generation on Anticosti Platform are controlled mainly by the burial history. However, the structures described in this study, in particular Taconian and Early Acadian fractures and faults systems, may have played an important role in petroleum migration and in potential reservoir development.

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