How do rocks of very different properties erode at the same rate: Erosion rates of the Quadrilatero Ferrifero escarpment, Brazil, derived from cosmogenic nuclides

In a landscape in which the channel network is cutting through varying rock types it is common for hillslopes to take on very different form depending on the rock properties. “Hard” rocks may form barren outcrop cliffs while “soft” rocks draining to the same channel may have modest slopes with a soil mantle. Such observations stimulated the pioneering work of G. K. Gilbert and John Hack on the process of “equal action” and the notion of dynamic equilibrium. We still have limited ability, however, to quantitatively define what “hard” and “soft” mean and to predict how those properties result in such different landforms. One way to make progress on this problem is to seek field settings where distinctly different rock types border on opposite sides of a common channel. In such a case, and over sufficient time and for some range of channel incision rates it seems possible that landscapes on the two very different rock types could erode at the same rate, and if so, their morphologic and weathering differences would then shed vital clues on how material property drive processes. In this study we test this hypothesis exploiting ideal settings in the escarpment topography of SE Brazil, by using in-situ-produced cosmogenic radionuclides 10Be, 26Al and 36Cl, to determine erosion rates over a period of 10^3 to 10^6 years. The schists, phyllites, banded iron formations and quartzites of this cratonic inland region form distinctly different landscapes, with the schists and phyllites producing convex soil mantled hillslopes and the quartzite yielding only steep, barren poorly dissected uplands. The banded iron formation hillslopes but mostly soil mantled. Samples have been collected from rock outcrops, (local rates of erosion) and fluvial sediments, providing average erosion rates of the watersheds associated with these different rock types. Our first test is to determine if such widely different landforms are in fact eroding at the same rate (where the share the same channel at their base). If this proves to be the case, it opens the door to quantify how material properties drive landforms.