Relationship of grain-size modes to nearshore sedimentary environments, Lake Superior, Ontario

Abstract

Longshore-troughs and bars were studied at Batchawana Bay and Pancake Bay, Lake Superior, Ontario. The longshore-trough areas would appear to have been formed by plunging waves that winnowed out the finer sand grains which were then moved lakeward by the undertow to build up the adjacent longshore-bar areas. This process would appear to be confirmed by a grain-size analyses of 242 samples taken from along the axes of the longshore-troughs and bars. The longshore-trough samples had either a unimodal or bimodal grain-size distribution whereas the longshore-bar samples always had a unimodal grain-size distribution. The longshore-trough samples showed a strong tendency towards negative skewness as would be expected from winnowed sediments and would also appear to have a modified grain-size modality of the parent sediment. The longshore-bar samples were finer grained, well sorted and unimodal, and showed a strong tendency towards positive skewness as would be expected if the sediments had been winnowed out of the longshore-trough areas and then re-deposited along the longshore-bars.