DATING A MID-PLIOCENE AGGRADATIONAL EPISODE AND SUBSEQUENT RIVER INCISION IN THE SHENANDOAH VALLEY WITH COSMOGENIC 26AL AND 10BE

The Shenandoah Valley has played a pivotal role in the development of numerous geomorphological concepts, particularly the Hackian model of dynamic equilibrium. Hack stated that the area represented an equilibrium landscape, wherein all surfaces should be eroding continuously through time. One consequence of the constant erosion assumption is that all surficial deposits in the Shenandoah Valley should be relatively young, likely Pleistocene or younger in age.

Recent cosmogenic nuclide studies have demonstrated that the Shenandoah Valley is more geomorphologically complex than previously thought. Erosion rates derived from in-situ ¹⁰Be in bedrock and detrital sands along the South Fork of the Shenandoah River exhibit a potential dependence on lithology and slope, which may indicate that the Shenandoah area is not fully equilibrated. This apparent state of disequilibrium along the South Fork of the Shenandoah River may be in part due to the propagation of a knickpoint presently located at ~300 m asl near Lynnwood, VA.

A corresponding knickpoint is not visible on the North Fork of the Shenandoah River, which exhibits a concave profile consistent with an equilibrated landscape. To test Hack’s hypotheses that (1) the North Fork watershed is equilibrated and (2) that the surficial deposits in such a landscape are constantly being deposited and eroded, we dated a fluvial terrace in the upper watershed of the North Fork near Timberville, VA. The terrace dated to 3.34 ± 0.15 My and yielded a paleoerosion rate of ~14 m/My. Given the terrace’s elevation of 10 m above river level, an average incision rate of 3 m/My since the Mid-Pliocene may be inferred. The age of the terrace coincides with that of a dissected gravel deposit 40 km to the southwest at Briery Branch, VA, which dated to 3.00 ± 0.37 My and also had a paleoerosion rate of ~14 m/My. These results demonstrate that even in apparently equilibrated landscapes, unconsolidated deposits may be preserved for millions of years. This concurs with recent ⁴⁰Ar/³⁹Ar measurements that revealed the preservation of supergene manganese oxide deposits dating through the Eocene in the Shenandoah Valley. Our results are consistent with a gradually incising landscape whose history is punctuated by brief aggradational events likely driven by fluctuations in climate and/or sea level.