Structural and stratigraphic study of the Laurentian Basin, offshore eastern Canada

Fagan, Alphonsus J. (2010) Structural and stratigraphic study of the Laurentian Basin, offshore eastern Canada. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Laurentian Basin is a large Mesozoic rift basin located off the south coast of the island of Newfoundland and directly east of the Scotian Shelf. In spite of being adjacent to hydrocarbon discoveries of the Scotian Shelf the basin has remained essentially unexplored because of a moratorium that has only recently been lifted. Only one exploration well has been drilled in the central part of the basin and has been reported as a dry hole in a press release from the operator. Detailed results of this well are still proprietary and therefore unavailable for this study. However, several wells located near the eastern and western margins of the basin and further along strike in adjacent basins with a similar structural and depositional history were suitable for seismic stratigraphic correlation to the study area. -- The basin was previously studied by MacLean and Wade (1992), of the Geological Survey of Canada (GSC), whose primary dataset consisted of 3072 km of 2D reflection seismic data acquired for the GSC in 1984 and 1987. The current study used the same core dataset but with the benefit of modern reprocessing by Arcis of Calgary. This reprocessing, which was carried out in 2005, resulted in significant improvements to the data, largely because of better multiple suppression and pre-stack time migration. The GSC seismic dataset was supplemented by approximately 14,000 km of 2D reflection seismic data provided by geophysical contractors GSI, GX Technology and Western Geco. The seismic data were integrated with well data, the GSC's regional gravity and magnetic data, and a recent high resolution magnetic survey donated to this project by Fugro. The study area covered approximately 87,000 km² of which 51,000 km² was located off-shelf on the continental slope and rise. -- The Laurentian Basin has also been subjected to recent tectonism. In 1929 a magnitude 7.2 earthquake with an epicentre close to the geographic centre of the basin triggered an immense turbidite flow known as the Grand Banks Slide. The mass transport event caused a tsunami which resulted in significant damage and loss of life on the south coast of Newfoundland. The regional geological framework includes two major fault systems converging in the study area, namely: the Cobequid - Chedabucto (CC) system running east-west along the northern boundary of the basin; and the Newfoundland Fracture Zone (NFZ) which projects into the basin from a southeast direction. The CC fault system also corresponds to a hinge line that divides the basin in two. North of the hinge the pre-Mesozoic basement is overlain by a thin Mesozoic section. Crossing the hinge line to the south we enter a deep complexly structured Mesozoic basin that has been formed by extensional tectonics during the rifting of the Scotian - Grand Banks margin. The Mesozoic sequences are overlain by a Cenozoic section that reaches thicknesses of up to 3000 m and thins landward to a zero edge. To varying degrees during its development the Mesozoic - Cenozoic section in this area has been deformed by localized compressional strike slip movement, oblique extension and salt tectonism. One of the goals of the study was to investigate the role of the CC and NFZ fault systems in the basin's evolution since the start of rifting in the Late Triassic. -- The major effort in defining the tectono-structural setting of the basin went into mapping the Pre-Rift horizon (basement), which presented significant challenges due to: (a) thick sedimentary cover resulting in poor energy penetration; and (b) the presence of complex salt features in the overburden. The resulting Pre-Rift basement map is tenuous in some areas, but reveals a southeast trending en echelon ridge and fault system in the northern part of the basin. Further south, near the modern shelf edge, a large ridge or perhaps series of coalescing smaller ridges trends in a roughly east - west direction. The Pre-Rift section drops off to much greater depths under the continental slope, and ultimately falls below the seven second recording time for much of the seismic data. Certain regional seismic lines running further seaward and recorded to thirteen seconds show the Pre-Rift horizon fading below complex salt structures and re-emerging seaward in approximate continuation with a strong rough textured reflector interpreted to be transitional crust. -- Time and depth maps were also constructed for the Break-up Unconformity (Pleinsbachian), the Avalon Unconformity (approximate base Cretaceous to Albian), the Base Tertiary Unconformity, Late Oligocene(?) Unconformity and the Mid Miocene(?) Unconformity. Structural mapping shows two largely independent fault systems: (1) older rift related faults which have been re-activated from time to time by recurring tectonic events; and (2) younger gravitationally driven faults affecting the Tertiary, Cretaceous and Late Jurassic sediments. The rift related faults seldom penetrate the shallower sedimentary section. However, in the northwestern corner of the basin, extensional faults propagate upwards and show inversion caused by transtension and convergence of the NFZ and CC faults. Compressional folds are also observed in this area. Additionally, a series of shallow seated faults penetrating the seabed are observed to correlate to short wavelength magnetic lineaments running parallel and sub-parallel to the NFZ, which are an indication of recent movement on the NFZ and its imbricates. -- Dip seismic lines indicate that the basin experienced periods of significant seaward tilting during the Jurassic and Tertiary. Jurassic tilting was caused by differential thermal subsidence that followed the start of seafloor spreading in the Pleinsbachian. A thickened Jurassic section is observed in the Laurentian Basin as compared to the Scotian Shelf. One explanation as to why Jurassic subsidence was greater in the Laurentian Basin than in the Scotian Shelf basins to the west, is that continued activity along the NFZ at this time may have introduced additional heating and cooling episodes to the Laurentian Basin. Basinward tilting during the Tertiary is the result of gradual cooling / subsidence as the spreading centre moved further away. Similar basinward tilting during the Tertiary is observed in all of the Atlantic Canada Mesozoic - Cenozoic basins. -- The Laurentian basin is expected to have a similar petroleum system to the Scotian Shelf basins, because of its proximity and similar Mesozoic - Cenozoic geological evolution. Seismic lines show numerous direct hydrocarbon indicators such as gas chimneys and amplitude anomalies, which are often coincident with potential stratigraphic and structural traps. -- Dramatic erosional features associated with eustatic sea level changes, the arrival of the paleo St. Lawrence River / Estuary(?), and numerous canyons related to recent de-glaciation periods are observed throughout the Tertiary section.

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