Abstract

Shoreface sediments exposed in raised deltaic deposits along the Hudson Bay coast are characterized by a thick succession of predominantly parallel-laminated and small-scale cross-laminated fine sand. Ripple forms, where preserved, include both combined-flow and wave-ripple geometries. Hummocky cross stratification is present in lower parts of the section. Similar facies have been found in cores from the present-day shoreface. Measurements of waves, currents, and suspended sediment concentrations in 10 m water depth, conducted over a 15-day period in the fall of 1999, captured four different meteorological periods. These included an intense four-day storm which sustained significant wave heights over 3 m for 15 hours and a storm surge of over 1 m height. Fair-weather conditions produce weak alongshore tidal flows, but oscillatory motions generated by small local waves predominate. During storms, orbital motions dominate the flow but a strong inertial flow generated by wind forcing is also present. Comparison with a published combined-flow bedform-stability diagram suggests that the predominant bedforms at 10 m depth would be small 2-D wave-dominated ripples and small, weakly asymmetrical 3-D ripples under fair-weather conditions and large 3-D ripples and upper plane bed under storm wave conditions. The predominantly interbedded parallel-laminated and small-scale cross-laminated facies is therefore interpreted to be characteristic of the upper shoreface, where the seabed experiences intense oscillatory motions under all storms and even some fair-weather conditions. Combined-flow ripples would form during storm events because of the stronger wind-driven mean residual current. Hummocky cross-stratification forms in deeper water during storms where the ratio of maximum orbital velocity to mean residual current velocity remains for longer periods in the large 3-D ripple field during the waning phase of storms. A numerical bed predictor model, using the 15-day data set as input, also suggests bimodal conditions fluctuating between wave ripples and upper plane bed at 10 m water depth. This model further indicates that combined-flow ripples would form rarely, but tends to underestimate the threshold of sediment suspension.

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