A GEOPHYSICAL, SEDIMENTOLOGICAL, AND MICRO-PALEONTOLOGICAL STUDY OF PLEISTOCENE LAKE DEPOSITS IN WESTPORT, NEW YORK

Early summer heavy rains in upstate New York caused a landslide (rotational slump block) that revealed a large deposit of Pleistocene clay along the Lake Champlain shoreline in Westport, New York. The slump is 100-120 meters long (parallel to the shore), 40-50 meters wide and 7-10 meters high. The location lies within the limits of late-glacial lacustrine and highest marine shorelines so these sediments are likely from the Champlain Sea or glacial Lake Vermont. A hammer seismic survey, conducted at several locations above the slide indicates that more than 25 meters of surficial material overlies bedrock.

Drilling near the head scarp of the slide revealed that about 24 meters of clay overlie at least 1 meter of glacial till. High-plasticity clay was encountered at a depth of about 13 m, which corresponds to the level of Lake Champlain. This horizon is the likely zone of failure for the landslide. A two-meter thick layer of nonplastic gray silt occurs at about 18 meters below land surface, which in turn overlies stiff lacustrine clay. Microfaunal analysis of samples collected at 10 cm intervals from 0.8 to 2.5 meters yielded only the freshwater ostracode Candona subtriangulata. The absence of any marine microfossils suggests that these deposits are solely lacustrine (glacial Lake Vermont) in origin and that any evidence for later Champlain Sea deposits or fossils was not preserved at this location.