Late Cenozoic geomorphology and surficial geology of central New Jersey

Surficial deposits and landforms in central New Jersey show two episodes of valley incision, one in the late Miocene-early Pliocene and one in the early Pleistocene, and two periods of planation and fluvial deposition. The two incisions occurred in response to sustained glacioeustatic declines in the late Miocene and in the late Pliocene. The periods of planation and fluvial deposition occurred in response to rising sea-level trends between the late Oligocene and middle Miocene and in the early Pliocene. The stratigraphic record indicates that the oldest landforms in the study area are no older than late Miocene, that planation in coastal regions of low-relief passive margins can be achieved in less than 20 m.y., and that these surfaces can be incised and extensively dissected in less than 5 m.y.; The age and erosional history of the upland erosion surface, the oldest landform in the study area, is tested using inventories of atmospherically produced cosmogenic 10Be in regolith. Two such inventories yield minimum exposure ages of 227,000 and 195,000 years. Relict fluvial gravel lags and surficial deposits of known original elevation and age indicate that surface erosion and runoff have produced deficits in the 10Be inventories. Plausible estimates of erosion after inventory buildup (up to 10 m) and runoff bypassing (up to 50% of total precipitation) yield exposure ages of as much as 6.4 and 11.5 Ma at the two sites. These assumptions also yield upland denudation rates that are consistent with those indicated by the preservation of old surficial deposits on the uplands.; Paleotopography of a 2800 km2 part of the study area is reconstructed at five times from the late Miocene to the Holocene. Denudation rates and patterns are calculated by subtracting successive topographies, with a horizontal spatial resolution of 60 m. The overall late Miocene to Holocene denudation rate of 10 m/m.y. is comparable to other estimates of long-term denudation in the region. The overall rate resolves spatially into a wide range of local rates, varying from a minimum of about 0.2 m/m.y. on residual uplands to as high as 600 m/m.y. in incising stream channels. The distribution of local rates shows that denudation is accomplished by sustained stream incision followed by slope retreat into residual uplands from the incised channels. Slope retreat replaces the uplands with pediments and straths. Pediment landforms and sediments indicate that groundwater seepage from the base of residual uplands is an important mechanism of slope retreat in the Coastal Plain. This process is self-limiting because as the volume of residual upland is reduced, seepage diminishes and slope retreat slows until small residuals experience little further reduction. Thus, relict topography is preserved even as most of the landscape erodes.