Jahanna Hoehne

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1998-The Michael J. Keen Award

B.Sc. (Honours) Thesis

SEISMIC VELOCITIES AND REFLECTION SEQUENCES OF WISCONSINAN GLACIAL DEPOSITS IN EMERALD BASIN(SCOTIAN SHELF)

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Emerald Basin is a 2430 km2 depression located on the central Scotian Shelf approximately 40 kilometers off the coast of Nova Scotia. Sleeve-gun reflection data collected in 1994 and Ocean Bottom Seismometer data collected in 1995, were analyzed for this project in order to detail the velocity structures and the lithostratigraphic boundaries of the units present in Emerald Basin. Three units were identified based on their acoustic velocities. Namely, the Scotian Shelf Drift (1.9 km/s) the Mesozoic-Cenozoic strata (2.2 km/s) and the Cambro-Ordovician Meguma Group bedrock (6.1 km/s). Previous works were considered in order to establish a more complex velocity model and to have a velocity value for each lithological unit present in the basin. The velocities of the upper sediments namely, the LaHave Clay and Emerald Silt were not determinable from the data set analyzed due to limitations in resolution, therefore, values of 1.44 km/s and 1.46 km/s (determined by Moran et al. 1991) were utilized for the LaHave Clay and Emerald Silt respectively, for the purposes of mapping.

The boundaries and thicknesses of the LaHave Clay, the Emerald Silt and the Scotian Shelf Drift deposit were determined and mapped during this project. On first interpretation it looked as though the high velocity structure (basement) was present exclusively under the moraine, however when previous work was consulted (Louden 1994) it was determined that the Meguma Group bedrock does occur in the basin as well as under the moraine but it is found at a much greater depth within the basin. The resolution and depth penetration limitations in the data did not allow this unit to be seen and therefore the unit could not be mapped across the area.

The coincidental occurrence of the high velocity strata under the moraine complex led the author to infer that the position of the moraine was a result of this pre-exiting basement structure. When the complex history of the Scotian Shelf is considered it becomes a plausible situation. A major fault complex runs across the Scotian Shelf as a result of early rifting. This would explain the shallower existence of the bedrock under the moraine and the deeper existence in the basin. This idea was taken further to conclude that the shape of the basin and the occurrence of its lithological units and glaciogenic features are a result of how the Wisconsinan ice sheet behaved due to the pre-existing shape of the bedrock.

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Pages: 86
Supervisor: Keith Louden