Lithofacies of Leda Clay in the Ottawa Basin of the Champlain Sea

In an attempt ta establish the lithological variation of sediments in a selected part of the Champlain Sea, closely spaced core samples have been used in a number of stratigraphic borings that penetrate the entire suite of fine sediments. Texture, structure, and colour banding in the sediments reveal a suite of lithologies that reflect the environmental changes from immediately post-last­glacial (iate Wisconsinan) ta late Holocene, excluding active sedimentation of modern alluvium, but including some sediments of relatively young abandoned terraces. Two trends in grain-size gradation occur: 1) a vertical coarsening upwards within the sediment body as a whole and within each lithic unit and 2) a radial fining from the margin towards the centre of the basin. Offlap of the sea produced the following suite of sedimentary environments, from oldest to youngest: Prodelta deposits - an early varve-like rhythmite unit that rapidly grades upwards into a massive blue-grey marine clay; this in turn grades upwards into thick bedded, rhythmically or cyclically stratified fine sediments; Delta front deposits - rhythmically or cyclically stratified fine sediments (clay-silt) in which frequency and quantity of coarse sediment (silt-sand) increase upwards in the suite; they exhibit features of channel erosion and deposition, including mass wasting phenomena; Delta top deposits - mainly coarse sediment in the silt-sand range, exhibiting structures of open channel and interfluve environments of the emergent delta; associated beds of stratified silt probably represent slackwater conditions or pondings; bodies of these sediments are tabular and extensive; Fluvial deposits - sand and silt derived mainly from erosion and redeposition of aider basin sediments have typical fluvial features and occur in abandoned channels and terraces of the Holocene proto-Ottawa drainage system. Comparisons of sedimentary suites show that Ottawa valley sediments have many similarities to the Mississippi Delta and Black Sea sediments, while revealing significant differences that relate to the particular glacio-isostatic conditions of the study region.