TECTONIC SETTING OF MIDDLE DEVONIAN TO LOWER CARBONIFEROUS ROCKS IN THE MAGDALEN BASIN

Seismic reflection data in the Gulf of St. Lawrence were used to const ruct an isopach map of the Horton Group (Middle Devonian-Early Carboni ferous) in the Magdalen Basin. The map shows that the Horton Group was deposited in basins that developed parallel to the offshore extension of structural trends in New Brunswick and the Gaspe Peninsula. Horton Group strata were deposited in narrow linear fault bounded basins, pr esently up to 8 km deep, and in broad sag basins up to 3 km deep. The sag basins may thin latterally to form sedimentary veneers less than 1 km thick. The fault bounded basins are mostly half-grabens and they d eveloped during an early crustal extension phase. The largest and deep est basin strikes northeast-southwest, almost parallel to the Moncton Basin in New Brunswick. Sag basins and thin veneers are not fault cont rolled and are widespread, underlying most of the Magdalen Basin. They probably formed later than the fault bounded basins in response to th e onset of a regional subsidence phase. Deformation of Horton Group ro cks in the Magdalen Basin is concentrated in fault zones up to 20 km w ide, and mainly affected deep subbasins, where thrust faults and possi ble flower structures are observed. An early deformation phase occurre d towards the end of Horton Group sedimentation (Late Tournaisian-Earl y Visean) and a later phase occurred, with less intensity, during the deposition of Visean to ?Namurian sediments. The first phase of deform ation affected most of the Magdalen Basin, whereas the second phase af fected mainly the southern areas.