TUNNEL CHANNELS AND LACUSTRINE FANS IN NEW JERSEY

Most valleys in New Jersey were occupied by glacial lakes during the late Wisconsinan glaciation, either because they drained toward, and so were dammed by, the glacier margin, or because they were dammed by previously deposited moraine or ice-contact deltaic deposits. Sedimentologic, geomorphic, and stratigraphic evidence indicates that much of the meltwater sediment laid down in these lakes was deposited from subglacial tunnel channels. Sedimentologic evidence includes lacustrine-fan deposits consisting of massive cobble gravel in thick subhorizontal beds with sharp erosional basal contacts, interbedded with plane-bedded coarse sand and pebbly sand in subhorizontal beds. Gravel composition in these deposits is enriched in local valley-bottom bedrock lithologies compared to the bedrock of the bordering uplands, and to the gravel composition of the local till, indicating a local subglacial source. Geomorphic evidence includes linear strings of knolls of sand and gravel that trend along valley axes (“beaded eskers’) and, less commonly, smaller continuous single-ridge eskers, in large lake basins. In places, current-scoured and potholed bedrock directly underlies these deposits. Stratigraphic evidence from wells and borings, and rare exposures, shows that, in narrow valleys and in deeper parts of broader valleys, these esker-like deposits are covered by deltaic and lake-bottom sediments, and typically (although not everywhere) lie on bedrock in the deepest part of the valley. In places, these deposits occur at the proximal base of ice-contact deltas and are overlain by, and deformed beneath, thin, compact, matrix-supported diamictons that are likely basal tills. These features indicate that the deposits were laid down at the mouths of, or within, subglacial tunnels beneath active ice, and, where the ice margin was stable for a time, aggraded to the lake surface and then prograded into the lake as deltas. The extent of the deposits and their relation to dated recessional positions indicate that the tunnels may have extended for 50 km and persisted for more than a thousand years. These deposits are important aquifers, especially where they are confined by overlying silt-clay lake-bottom deposits. Understanding their genesis and geometry is thus important for aquifer exploration and modeling.