MULTIPLE GLACIAL DEVELOPMENT OF UPPER GEORGES BANK

A scenario for multiple glacial development of the top of Georges Bank has been worked out by reanalysis of 2900 km of high-resolution seismic reflection data augmented by AMCOR and COST wells. Four seismic units (D1-4) represent deposits of four glaciations that lie on a high mid-Bank coastal plain remnant, flanked to the west and east by lowlands first carved deeply by late Tertiary rivers during marine regressions and later modified by glacial erosion.

Extensive lower Pleistocene glaciodeltaic deposits prograded southward to completely fill the coastal-plain lowland underlying Great South Channel and the western side of Northeast Channel. Beneath the western Bank the oldest unit D1 contains a 130-m thick sequence of deltaic fluvial–foreset strata extending from -90 m to -240 m altitude. Sand from unit D1 in core hole 6013 contains fresh erratic rock fragments and diatoms of early Pleistocene age and correlates with similar coarse pebbly sand at -153– -163 m in core hole 6014. The early Pleistocene D1 glaciomarine delta plain extended to glacioeustatic sea level at -90 m; bottomset beds reached the edge of the Continental Slope at -548 m. Upper deltaic beds of seismic unit D2 overlie the older strata above a wide ravinement of the succeeding transgression. The D2 plain extends to below -90 m. The areas of the D1 and D2 plains are similar and comparable to the area of Connecticut, which indicates glacial maxima events of considerable duration.

Upper Pleistocene deposits (D3-4) averaging 25m in thickness unconformably blanket the top of the entire Bank. Above another marine unconformity, extensive unit D3 glaciofluvial deposits, and delta foreset strata at <-90 m altitude, are correlated with Illinoian beds on Nantucket, and major glacial erosion and overdeepening of depressions in the Gulf of Maine. When the late Wisconsinan ice sheet halted at the northern subaerial cuesta of Georges Bank, unit D4 Bank-top fluvial outwash deposits were deposited on a regional ravinement. The ice margin extended southeastward as a tongue or shelf into deep marine waters of Northeast Channel. To the west, the South Channel ice lobe advanced south past the Nantucket moraine to its lowland terminus at 40.8^o N latitude; wood and marine shells in drift and carbon in till matrix indicate that the ice front arrived about 24 ka cal.