Detailed grain size analysis of recent Marine sediments and post-glacial history of Port au Port Bay, west Newfoundland

Shearer, James Moxley (1970) Detailed grain size analysis of recent Marine sediments and post-glacial history of Port au Port Bay, west Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Grain size analysis of about 130 bottom grab samples taken from the recent marine sediments of Port au Port Bay have led to the distinction of three sediment types with different, although characteristic, cumulative grain size distribution curves. The three types encountered are: 1) unimodal sands found in areas of high wave and tidal current energies, 2) bimodal silts and clays deposited in the low energy basin areas, and 3) bimodal gravels in areas of medium to high energies. The coarse component of Type No. 3 is often a residual deposit from a time of even higher energy associated with the lower post-glacial sea level. -- Based on this analysis, it is proposed that the characteristic "break" to the finer grades in many fine sand and silty sediments is due to the inherent scarcity in the natural environment of material with diameters between 10 and 40 microns. This size range represents the gap between a fine sand and a clay population, each of which is representative of a different mode of formation. This is in opposition to many workers who believe that this "break" is due to a different mode of transport. -- A correlation between particle diameter and mode of transport is nevertheless attempted. It is based on the assumption that tidal currents do not affect the areas of the bay below the deepest sill which, in turn, is deduced from studies of the faunal distribution and the physical oceanographic properties. Deposition in the basin areas, then, is thought to be from suspension only with the maximum particle diameter in these sediments, after removal of the ice-rafted fraction, being around 0.125 mm. -- The sea level at the time of the last ice retreat from Port au Port Bay (around 13500 yrs. B.P.) was at the "marine limit", more than 100 feet above the present datum. Isostatic recovery of the land area, as a result of glacial unloading, was apparently occurring faster than the accompanying eustatic rise of the sea, a consequence of the returning glacial meltwaters. Following the ice withdrawal sea level fell rapidly until the isostatic component had diminished to a rate equal to that of the eustatic rise, when the lowest post-glacial sea level was registered. This lowest level calculated from the grain size variations within one of the basin cores was around -35 and -45 feet from the present datum. This is in close agreement with the lowest level calculated from the difference between the theoretical rebound curve and the world-wide eustatic curve. Subsequently, with a further decrease in the isostatic rebound rate, the eustatic component was greater and slow submergence of the land area took place.

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