Paleoecological analysis of molluscan assemblages in two marl deposits in Waterloo Region, southwestern Ontario, Canada

Mollusc assemblages were studied in two marl deposits at Kitchener and Camb ridge, in southwestern Ontario. Lacking datable terrestrial plant material, a chronology was determined by pollen analysis. Similarities in mollusc as semblages and changes allowed five equivalent mollusc assemblage zones to b e identified in the two deposits, spanning approximately 12 to 7 ka. In tot al, 34 mollusc taxa were identified, with 19 common to both sites. Overall, the most abundant taxa were Valvata spp., Gyraulus parvus, Pisidium casert anum, and P. ferrugineum. Terrestrial molluscs (four taxa) were rare. Overa ll, cool, shallow water with abundant vegetation and quiet conditions were indicated. At around 9 ka, a marked decrease in mollusc abundance and diver sity, but relatively stable percentages, is noted at both sites. We specula te this was caused by an increase in sedimentation rate, perhaps caused by a brief warming. Inconsistencies in assemblages at marl sites may be partly attributable to sampling, but a chance factor in dispersal is also suggested. A minimum of two sampling sites at a given sedimentation basin, nearshore and mid-basin, are recommended to maximize assessment of assemblages. More detailed data from living molluscs are needed to enhance paleoenvironmental interpretatio ns.