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Exposure age and erosional history of an upland planation surface in the US Atlantic Piedmont
Author(s) -
Stanford Scott D.,
Seidl Michele A.,
Ashley Gail M.
Publication year - 2000
Publication title -
earth surface processes and landforms
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.294
H-Index - 127
eISSN - 1096-9837
pISSN - 0197-9337
DOI - 10.1002/1096-9837(200008)25:9<939::aid-esp108>3.0.co;2-0
Subject(s) - geology , fluvial , erosion , saprolite , denudation , geomorphology , aggradation , pleistocene , surface runoff , deposition (geology) , glacial period , neogene , geochemistry , paleontology , physical geography , sediment , weathering , tectonics , structural basin , geography , ecology , biology
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 structure in diabase, basalt, sandstone, mudstone and gneiss. These flats define a low‐relief regional surface that corresponds in elevation to residual hills in the adjacent Coastal Plain capped by a fluvial gravel of late Miocene 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 surface. 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 depths for atmospherically produced cosmogenic 10 Be. The measured inventory, assuming a deposition rate of 1·3 × 10 6 atoms cm −2 a −1 , yields a minimum exposure age of 227 000 years, or, assuming continuous surface erosion, a constant erosion rate of 10 m Ma −1 . Because the sample site lies about 60 m above the aggradation surface of the Pliocene fluvial deposit, and itself supports a pre‐Pliocene fluvial gravel lag, this erosion rate is too high. Rather, episodic surface erosion and runoff bypassing probably have produced an inventory deficit. Reasonable estimates of surface erosion (up to 10 m) and bypassing (up to 50 per cent of total precipitation) yield exposure 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 erosion, and (2) exposure ages based on 10 Be inventories are highly sensitive to surface erosion and runoff bypassing. Copyright © 2000 John Wiley & Sons, Ltd.