52 Provenance of the Eastern North American Margin: From Triassic Rifting to Modern Sedimentation

Sunday, 22 March 2020
8:00 AM - 12:00 PM
Lake Fairfax

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

Paper number: 52-12
Program: Theme Sessions
Time: 11:20 AM – 11:40 AM

Authors

Abstract

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 20th 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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 2

doi: 10.1130/abs/2020SE-344774

© Copyright 2020 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.