Structural and stratgraphic study of the Laurentian Basin, offshore Eastern Canada.

摘要

The Laurentian Basin has 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.;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. (Abstract shortened by UMI.);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.