LONG-TERM CHEMICAL WEATHERING RATES IN THE PERIGLACIAL PIEDMONT PHYSIOGRAPHIC PROVINCE OF SOUTHEASTERN PENNSYLVANIA: WATERSHED GEOCHEMICAL MASS-BALANCE, SAPROLITIZATION RATES, AND EVIDENCE FOR A MODERN SAPROLITE

The occurrence of saprolite in the periglacially-influenced Piedmont Physiographic Province of the mid-Atlantic United States has been explained by one of three scenarios: (1) The saprolite is entirely relict, having formed during the Paleocene to early Miocene when the region's climate was relatively warm and wet; (2) The saprolite is forming today at rates comparable with long-term (geologic timescale) average rates; and (3) The saprolite is forming today, but at substantially higher rates than during glacial times.

The small (21.4 ha) Brubaker Run watershed is located in periglacial southeastern Pennsylvania and contains saprolite developed on a muscovite-chlorite schist of the Lower Paleozoic Octoraro Formation. Long-term (10³-10⁵ year) mineral weathering rates have been calculated for the Brubaker Run watershed using total denudation rates derived from 10-beryllium data. The long-term average rates at which the weathering front has penetrated the bedrock (the “saprolitization” rate) are 4.5 m/My and 6.1 m/My for chlorite and muscovite, respectively. These measured long-term average saprolitization rates compare very favorably with published theoretical values for the nearby northern Maryland Piedmont which range from 2.2 to 5.3 m/My. Furthermore, at such saprolitization rates, the entire 3 m thickness of saprolite found in the Brubaker Run watershed could have formed within the last 700,000 years.

Preliminary present-day elemental stream solute flux values for Brubaker Run are approximately a factor of two higher than the long-term average losses. These observations support the hypothesis that the saprolite is forming at the present time, but at a higher chemical weathering rate relative to that of glacial times. Therefore, saprolite of southeastern Pennsylvania is not likely a relict weathering profile of Tertiary age.