INTRA-TILL LACUSTRINE DEPOSITS IN THE CONNECTICUT VALLEY OF SOUTHERN NEW HAMPSHIRE AND VERMONT: EVIDENCE OF SUBGLACIAL CAVITATION DRIVEN BY SUPRA-GLACIAL MELTWATER DRAINAGE TO THE GLACIAL BED?

In southern NH and VT lenses of laminated mud, <2.5 m thick, occur within massive and fissile till. Regionally till is stony with a clayey silt matrix. It is drumlinized in small N-S valleys and dissected till benches that have streamlined upper surfaces on the sides of E-W valleys. The intra-till lacustrine units differ from proglacial units in that they have dropstones, abundant diamicton pellets, and graded and laminated diamicton units. Intra-till units never have nearly pure clay beds (i.e. winter layers) or the trace fossil Cochlichnus isp., which is common in local proglacial deposits. Intra-till units are only found on the lee sides of hills within thick till (up to 40 m). The tops of the units are horizontally sheared and lack shearing below as indicated by undisturbed laminations and non-linear (highly oblate) fabrics (anisotropy of magnetic susceptibility measurements that have nearly equal maximum and intermediate susceptibility axis values). Intra-till lenses are formed in subglacial, subaqueous pockets resulting from cavitation. It is hypothesized that they represent cavities inflated by meltwater draining to areas of reduced glacial bed pressure. It is further hypothesized that meltwater drained down from the glacier surface as occurs today when surface melt pools drain to the bed of the Greenland Ice Cap. The pools form in the summer just down ice from the equilibrium line and 40-100 km in from the ice cap margin. A test of this hypothesis would be to determine whether sediment in the intra-till cavities is the same age as ice-front recessional positions 40-100 km to the south, but there are no available dating methodsfor determining the ages of the intra-till units. However, the hypothesis predicts that intra-till units must have paleomagnetic declinations similar to times when corresponding ice fronts were still 40-100 km south. The hypothesis passed this declination test at three sites where remanent declination could be measured. Secular variation of declination is cyclic and the declination test can only disprove the hypothesis. A more rigorous test is needed with a greater number of sites that span more time. Intra-till lens formation beneath the lower 100 km of the glacier implies that all till found above these deposits, and likely some below, was deposited in the ablation zone during ice recession.