GROUNDWATER SEEPAGE, LANDFORMS, AND LANDSCAPE EVOLUTION IN THE NEW JERSEY PINE BARRENS

The New Jersey Pine Barrens consist of dry uplands and wet lowlands formed on highly permeable Miocene quartz sand. Uplands are 15 to 30 m above valley bottoms and are capped with veneers of fluvial quartz and chert gravel of late Miocene through early Pleistocene age. Lowlands contain broad terraces of quartz sand and gravel, including a middle Pleistocene upper terrace up to 10 m above Holocene wetlands on valley bottoms, and a late Pleistocene lower terrace up to 3 m above the wetlands. There is no upland surface runoff, and few slopes are steeper than 5 degrees. This geology creates a geomorphic system dominated by groundwater seepage. Seepage erosion shaped a landscape framed by four events: 1) a permanent sea-level drop of 50 m in the middle and late Miocene, 2) a second sustained sea-level drop of 20 m in the late Pliocene and early Pleistocene, 3) diversion of the Hudson-Pensauken river during early Pleistocene glaciation, and 4) several periods of permafrost in the middle and late Pleistocene. The sea-level drops led to river incision, which provided relief to drive groundwater flow and initiate seepage erosion. The river diversion led to deepened fluvial and marine erosion on the northeast margin of the Barrens, causing westward divide migration and stream capture. Permafrost, by forming an impermeable layer in the subsurface, acted as an accelerant to seepage erosion, and elevated the position of seepage in the landscape.

Seepage today occurs at the base of scarps between terraces and valley-bottom wetlands, and in pediment-floored hollows in headwater areas. Paleoseeps are marked by similar hollows higher in the terrain that are dry today. Seepage erosion causes scarp retreat, starting at incising stream channels and working back into uplands, leaving terraces and pediments capped with a thin sand and gravel veneer. The maximum overall denudation rate from this erosion in the Barrens since 10 Ma is 5 m/my. Accelerated seepage occurs on the low side of asymmetric divides, particularly where aided by clay beds beneath the divide. Long-term baseflow measurements in several asymmetric basins on the Atlantic margin of the Barrens show that gaining basins have double to quadruple the groundwater feed of the losing basins. This additional feed drives divide migration, at a minimum rate of 10 km/my since the early Pleistocene.