CGRG Bibliography of Canadian Geomorphology
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Author : Duxfield, A.; Hughes Clarke, J.; Parrott, D.R.; Wildish, D.; and Fader, G.B.J.
Date : 2003.
Title : An examination of the relationship between linear chains of pockmarks, shallow seismic structures, and the glacial geology in Passamaquoddy Bay, New Brunswick.
Publication : Joint Annual Meeting of the Canadian Quaternary Association and the Canadian Geomorphology Research Group. Halifax, Nova Scotia, June 8-12, 2003.
Issue :
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Abstract
The original description and discussion on the regional distribution of pockmarks in Passamaquoddy Bay was made by Miller and Fader (1989) and Pecore and Fader (1989). These studies were based on reconnaissance sidescan data and some shallow seismic reflection data. Subsequently denser multibeam coverage of nearly the entire bay floor has now confirmed their distribution and provided much more detail on their specific geometry. In the Bay these pockmarks are loosely confined within two roughly circular fields. The highest densities occur between Navy and Deer Islands where the pockmarks frequently occur as linear chains. The trend of these linear chains is predominantly NW-SE, which matches the alignment of large scale glacial fluting apparent in the provincial topographic relief. The detailed bathymetry of the bay floor has also identified glacial deposits that have the same trend. A grid of sub-bottom profiler transects was constructed to investigate the relationship between the surface expression of the pockmarks and the shallow sedimentary structure. Interpretation of these profiles show that the pockmarks are all developed in an upper acoustically-transparent unit, the Holocene Clays, with deepest pockmark penetration extending just to the base of this unit. No fault offsets, associated with the linear chains of pockmarks, were observed at the base of the unit. This surface unit overlies an erosionalunconformity, Pleistocene in age. Below the unconformity two contrasting acoustic facies define a strong structural relief in the order of10-30 m high, some of the highs are truncated at the unconformity. The facies with high structural relief are interpreted as glacial deposits and eroded bedrock, with an infill of glaciomarine sediments. The predominant structural orientation of the highs is NW-SE. The chains of pockmarks preferentially appear along the buried boundaries of these structural highs. In Passamaquoddy Bay it appears that the glacial deposits are controlling the formation of the pockmarks. The sequence of glacial events combined with the lacustrine and marine phases in the Bay provide a probable source for the gas creating the pockmarks in the form of post-glacial organic sediments being deposited in lake beds.
Bibliography of Canadian Geomorphology