RATES AND PATTERNS OF LATE CENOZOIC DENUDATION IN CENTRAL NEW JERSEY

Topography of a 2800 km2 section of unglaciated central New Jersey is reconstructed for 5 times from the late Miocene to the Holocene (9 Ma, 4 Ma, 2 Ma, 125 ka, 15 ka). The reconstructions are based on mapping of terraced fluvial and colluvial deposits and associated straths and pediments at 1:24,000 scale. The deposits are dated by radiocarbon, palynostratigraphy, and by correlation to adjacent glacial and marine strata. Denudation rates and patterns are calculated by subtracting successive topographies, with 60 m horizontal resolution and 6 m vertical resolution. The overall denudation rate (=total volume eroded /total area) for the late Miocene to Holocene is 10 (+/-2) m/m.y., a rate similar to other estimates of long-term denudation in the region. The overall rate resolves spatially into a wide range of local rates (=volume eroded from grid cell in a timestep/area of cell), from a minimum of about 0.2 m/m.y. on upland remnants to as high as 600 m/m.y. in glacially rerouted river channels. The distribution of local rates shows that denudation is accomplished by stream incision in response to long-term baselevel drop in the late Miocene and early Pleistocene, followed by slope retreat into uplands that replaces the uplands with pediments and straths. The periods of baselevel drop correspond to long-term glacioeustatic drawdown from growth of Antarctic and northern hemisphere ice sheets. Denudation rates are largely insensitive to local climate change because most slopes are gentle (<4 degrees) and there was little deforestation except briefly under periglacial conditions.

Pediment landforms and sediments indicate that groundwater seepage from the base of uplands drives slope retreat, particularly in the Coastal Plain. This process is self-limiting because as the volume of upland is reduced, seepage diminishes and slope retreat slows until remaining uplands experience little or no further reduction, preserving small remnants of old topography.