Regolith residence time and the concept of surface age of the Piedmont “Peneplain”

Abstract

The common upland weathering profile on foliated metamorphic rocks on the Appalachian Piedmont between Alabama and Maryland includes a 50 cm-1 m thick clay-rich soil B-horizon over 10–20 m of saprolite. The saprolite retains the volume, fabric and structure of the underlying parent rock, but has lost about one-third of the mass of the parent rock. Where the saprolite is thickest beneath upland drainage divides, those divides are of loow slope and relief. The flatness of the Piedmont drainage divides provides the appearance of a planation surface of accordant summits, or peneplain. I interpret this flatness to be the result of three active processes: volume reduction during compaction of saprolite into soil, loss of mass in dissolved solids draining the soil B-horizon, and loss of mass from the soil surface by erosion of clay-size particles. The cumulative effects of these upland processes are to convert saprolite into soil, to limit soil thickness and residence time, and to limit the age of the oldest residual soils on the surface. This results in a continual lowering of the Piedmont “peneplain” surface. This interpretation is corroborated by evidence of topographic inversion of fluvial gravels since the Miocene near the Fall Line. Study of baseflow dissolved solids draining Piedmont basins indicates a minimum rate of saprolite production of about 4 m Ma −1. This minimum rate of production near the Fall Line is less than the estimated rate of surface lowering and saprolite consumption during the late Tertiary and Quaternary periods of topographic inversion. Measurement of cosmogenic 10Be inventories in two 15–20 mores of Piedmont upland regolith indicate a minimum residence time of residual soil and saprolite of about 1 Ma. This indicates a possible maximum rate of saprolite production and erosion of about 20 m Ma −1. Using the best available evidence, therefore, the typical Piedmont upland regolith has a residence time of between 1 and 5 Ma. Residence times may not be the same everywhere and ages may vary systematically across the surface. The geomorphic evidence of topographic inversion, and similar regolith profiles in other humid temperate and tropical regions, strongly suggests that production and removal of Piedmont saprolite has been an ongoing Quaternary process and that most of the Piedmont regolith is significantly younger than 5 Ma. Independent geophysical evidence suggests that the rate of uplift, probably involving tilting, of parts of the Piedmont, Blue Ridge, and Valley and Ridge has averaged between 5 and 20 m Ma −1 through the Cenozoic.