HYDROGEOLOGY OF GLACIO-LACUSTRINE DEPOSITS ON NANTUCKET ISLAND, MASSACHUSETTS

A 25-meter continuous core of Wisconsin-age glacio-lacustrine sediments from glacial lake Nantucket Sound was analyzed as part of a larger study on Pleistocene and Holocene hydrogeology of the Atlantic Continental Shelf. The sediment consists of approximately 30% clay, 60-75% silt, and 0-5% sand. Drop stones of >1 cm were also noted in the clay. The carbon content of the sediment was measured by Loss on Ignition. Carbon content within the finer sediment ranges from 1.5 - 2.4%, whereas the coarser sediment had values of <0.5% to almost 0%. The mineralogy of this deposit was determined using X-ray powder diffraction. The clay consists of major quartz, illite, chlorite and microcline with minor kaolinite, albite and actinolite. Permeability and porosity were measured using compression/consolidation tests and drying experiments. Radial diffusion cells were also used to measure the solute diffusivity for the clay. The permeability of the clay was measured to be about 10-17 m2, and the porosity and solute diffusivity from four 4” diameter samples were found to be approximately 0.3 and 10-10m2/s, respectively.

These low permeable sediments are believed to have been deposited in a pro-glacial lake between about 18-16 ka. Glacial melt waters from the lake must have been isotopically light (18O = -18 to -20‰). However, pore waters extracted from the sediment using a Manhiem squeezer under a confining pressure of ~18,000 psi showed no distinctive gradient and had a composition comparable to modern values. Numerical models of solution diffusion though the clay using measured diffusivity values suggest that some isotopically light pore fluids should remain in the center of the deposit after 18,000 years. We hypothesize that the lacustrine deposits must have drained during a period of sea level lowstand and were later replaced with isotopically modern water. Our findings suggest that large-scale water table fluctuations on the order of 100 m must have occurred on the continental shelf during the past 16-18 kyr.