A PRELIMINARY PALEOMAGNETIC STUDY OF THE GLACIAL LACUSTRINE/CHAMPLAIN SEA TRANSITION IN CORES FROM THE NORTHERN LAKE CHAMPLAIN VALLEY, NEW YORK

Glacial Lake Vermont formed in front of the retreating Laurentide Ice Sheet in the Lake Champlain Valley during the last deglaciation. Lake Vermont discharged through the Hudson Valley into the North Atlantic until the retreating ice margin exposed a lower elevation route through the Gulf of St. Lawrence, and the lake dropped about 40 m to contemporary sea level. Marine waters then entered the isostatically depressed valley forming the Champlain Sea. Several cores through the sedimentary sequence of this transition were taken near Plattsburgh, New York using a truck mounted vibra-corer. The glacial lacustrine sediments are primarily gray clay varves with red clay and silt clasts near the top of the sequence. The transition is marked by a massive gray clay unit with sand partings below laminated gray clay containing small black organic flecks. The initial marine section is massive gray clay, also containing black organic flecks. The glacial lacustrine sediments contain only freshwater ostracodes, while the transition sediments contain both freshwater ostracodes and brackish-water foraminifera. The marine sediments contain marine foraminifera and ostracodes.

A pilot paleomagnetic study was conducted on a 150-cm long u-channel sample collected from the center of a split core containing the upper glacial lacustrine and transitional sediments. We estimate this to represent a duration of less than 200 years. The u-channel sample was subjected to stepwise alternating field demagnetization at a 1-cm measurement interval using an automated long-core cryogenic magnetometer. The sediments exhibited a strong and stable magnetization with magnetite as the predominant magnetic carrier. Although the coring was done with a vibra-corer, the paleomagnetic directions were coherent and did not show significant changes over the length of the u-channel, which is consistent with the estimated short duration. The mean inclination of about 48 degrees is also consistent with published results from contemporaneous varve sections in the region. Because the core was not azimuthally oriented, absolute declination values could not be determined. These results indicate that paleomagnetic studies of longer cored intervals can provide records of paleosecular variation that can be used for correlation and dating of these sediments.