LONG-TERM DENUDATION RATES IN THE VIRGINIA VALLEY AND RIDGE ESTIMATED FROM 40AR/39AR DATING OF SUPERGENE MANGANESE OXIDE ORES

The southern and central Appalachian Mountains have played a key role in geomorphic thought, from the Davisian theory of peneplanation and punctuated uplift to the Hackian concept of dynamic equilibrium. It has recently been suggested on the basis of stream profiles, offshore sediment budgets, and seismic tomography that the southern Appalachians experienced Neogene uplift. However, few measurements of long-term erosion and river incision rates exist to test this hypothesis. Here we complement Plio-Pleistocene cosmogenic nuclide ages with ⁴⁰Ar/³⁹Ar data to estimate denudation rates across the Cenozoic.

Cosmogenic nuclide dating of fluvial sediments along the New, Green, Cumberland, Shenandoah, and Tennessee Rivers has demonstrated that much of the Appalachian Mountains were slowly eroding during the latest Miocene and Pliocene at <10 m/My. Gradual erosion was interrupted during the Pleistocene, when fluctuations in climate drove pulses of rapid incision in the Ohio and Potomac River drainages. These results indicate that denudation in the latest Cenozoic was likely driven by climate rather than tectonics; however, the half-lives of ²⁶Al and ¹⁰Be limit the ²⁶Al/¹⁰Be dating technique to the last ~5 million years.

⁴⁰Ar/³⁹Ar dating applied to K-bearing manganese oxides provides an opportunity to constrain denudation rates throughout the Cenozoic. Manganese oxides are supergene ore deposits that form within ~100 m of the surface through extended weathering. The deposits are scattered throughout the Valley and Ridge province and were extensively mined during the early 20^th century. Our previous work in the Shenandoah Valley has provided Eocene (48-36 Ma) and Oligocene (31-28 Ma) ages for manganese oxides. We present geochronology on manganese oxides from Virginia that reveals formation ages during the Eocene, Oligocene, Miocene and Pleistocene. These results demonstrate that the landscape preserves near-surface deposits for tens of millions of years. Given their shallow depth of formation, these ages suggest that parts of the Virginia Valley and Ridge have been gradually eroding for >40 My at rates not exceeding 10 m/My. Geochronology from manganese oxide deposits in Virginia, West Virginia, and Tennessee will provide additional insight into the long-term denudation rates of the Appalachian Mountains.