Testing Erosion Potential of Heterogeneous Lithologies to Understand Atypical Valley Morphology in the Buffalo National River Watershed, Arkansas

Abstract

The effect of heterogeneous lithologies on valley morphology in tectonically stable landscapes is an unresolved question in fluvial geomorphology. The upper reaches of the Buffalo National River incise the Mississippian Boone Formation, a cherty limestone, forming a wide gently sloping valley. As the Boone Formation gives way to the Ordovician Everton Formation, an interbedded dolostone, limestone, and sandstone, a significant narrowing of the valley is observed. This pattern is repeated in the middle reaches of the river when a fault brings the Boone Formation back to the surface. To tease out what processes are driving the atypical valley morphology, laboratory tests were conducted to analyze the mechanical and chemical weathering characteristics of the primary cliff-forming lithostratigraphic units in the Buffalo River watershed. Specifically, mechanical weathering characteristics were evaluated using a vertical abrasion mill, and chemical weathering characteristics were evaluated using dissolution experiments. The abrasion mill experiments were conducted on samples from four different lithologies - the Everton Formation (dolostone), the St. Joe Limestone, the Boone Formation (limestone), and the Batesville Sandstone - with abrasive aggregate sampled from modern gravel bars in the watershed. The final erosion rates of the Everton, St. Joe, Boone, and Batesville samples were 10.2 g/hr, 18.9 g/hr, 9.3 g/hr, and 30.9 g/hr respectively. Fragments of the samples were crushed and dissolved in a hydrochloric acid solution. The two limestones, the Boone and St. Joe, dissolved almost completely at 99% and 90% respectively. 37% of the Everton dissolved and only 2% of the Batesville dissolved reflecting much lower carbonate content of these samples. The abrasion mill results indicate that there is iii minor variability in mechanical resistance of the lithologies tested. Therefore, the primary control on valley morphology within the watershed is the variability in the chemical competence of the lithologies. Similar patterns in valley morphology have been documented in watersheds that transition from hard crystalline rock to soft sedimentary rock. This experiments suggests that even small variation in rock characteristics can influence valley evolution in tectonically stable landscapes.