Anthony, Darlene M. Ph.D.

Landscape Evolution and Cave Development in Response to Episodic Incision of the Cumberland River, Tennessee and Kentucky, USA.

Episodic incision punctuated by periods of base level stability during the Plio-Pleistocene left the Upper Cumberland River in Tennessee and Kentucky deeply entrenched into the unglaciated Appalachian Plateaus. The relative chronology of episodic river incision and base level stability is well documented thanks to over a century of careful mapping of upland surfaces, inset straths, and terrace gravels. Constraining the timing of these incision events has been difficult, however, primarily due to a lack of suitable dating methods for terrace materials ranging from several hundred thousand to several million years of age, and reworking of upland gravels onto lower terraces. These problems are solved by dating the burial age of undisturbed cave sediments in place of terrace deposits, using the differential decay of cosmogenic 26Al and 10Be in quartz exposed to cosmic radiation at the surface. This study offers a new chronology of river incision beginning with initial incision into the Highland Rim after ~3.5 Ma; development of the Parker strath between ~3.5 and ~2 Ma; incision of the Parker strath at ~2 Ma; development of a major terrace beneath the Parker strath between ~2 and ~1.5 Ma; incision into this terrace at ~1.3 Ma; and the development of several discontinuous terraces above the modern flood plain between ~1.3 Ma and the present.
Large caves on tributaries of the Upper Cumberland River record a headward wave of incision in the Pliocene and Early Pleistocene. The passage of a knickpoint in the system is modeled as a perturbation to steady-state incision according to the stream power law, which is tested against the abandonment dates in seven caves. Model results for m/n = 0.68 are within previously published theoretical and empirical values of 0.5 to 1.0, but suggest that values for the drainage-area exponent m are several times higher
than previous studies. This may be caused by a stronger variance of discharge to drainage area in fluviokarst reaches compared with non-karst watersheds. Knickpoint migration rates in limestone bedrock channels of fluviokarst tributaries to the Cumberland River are calculated between 10-18 cm/year during the Plio-Pleistocene, with m = 1.91 and m/n = 0.79.