Exposure age and erosional history of an upland planation surface in the US Atlantic Piedmont

The upland planation surface in the Piedmont of central New Jersey consists of summit flats, as much as 130 km(2) in area, that truncate bedding and s tructure in diabase, basalt, sandstone, mudstone and gneiss. These flats de fine a low-relief regional surface that corresponds in elevation to residua l hills in the adjacent Coastal Plain capped by a fluvial gravel of late Mi ocene age. A Pliocene fluvial sand is inset 50 m below the upland features. These associations suggest a late Miocene or early Pliocene age for the su rface. To assess exposure age and erosional history, a 4.4 m core of clayey diabase saprolite on a 3 km(2) remnant of the surface was sampled at six d epths for atmospherically produced cosmogenic Be-10. The measured inventory , assuming a deposition rate of 1.3 x 10(6) atoms cm(-2) a(-1), yields a mi nimum exposure age of 227 000 years, or, assuming continuous surface erosio n, a constant erosion rate of 10 m Ma(-1). Because the sample site lies abo ut 60 m above the aggradation surface of the Pliocene fluvial deposit,and i tself supports a pre-Pliocene fluvial gravel lag, this erosion rate is too high. Rather, episodic surface erosion and runoff bypassing probably have p roduced an inventory deficit. Reasonable estimates of surface erosion (up t o 10 m) and bypassing (up to 50 per cent of total precipitation) yield expo sure ages of as much as 6.4 Ma. These results indicate that (1) the surface is probably of pre-Pleistocene age and has been modified by Pleistocene er osion, and (2) exposure ages based on 10Be inventories are highly sensitive to surface erosion and runoff bypassing. Copyright (C) 2000 John Wiley & S ons, Ltd.