High rates of erosion and rapid weathering in a Plio-Pleistocene mudstone badland, Taiwan

The badlands of southwest Taiwan are characterized by sharp ridges, and are located in a tropical monsoon area that experiences contrasting rainy and dry seasons. Surface erosion predominates and subsurface erosion by piping is rather limited to accumulation of mud in valley bottoms. Erosion rates on Plio-Pleistocene mudstone slopes, measured using erosion pins over a period of 4 years, averaged up to 9 cm/y, which is considerably faster than the rates of a few cm/y typical in arid or semi-arid areas. Mudstone sample cores recovered from the slopes in the dry season (April), and in the early rainy season (July), prior to the extensive erosion that occurs later in the rainy season, suggested the following mechanism for weathering and erosion. Near-surface layers (< 10 cm deep) on these slopes develop lower bulk densities, larger void ratios, and higher salt contents during the early rainy season, and this degraded near-surface layer is rapidly removed by slaking and erosion during the intense precipitation of the main rainy season. This slaking may be caused by the electric repulsion and dispersion of grains. Following the period of erosion, the newly exposed rock dries during the subsequent dry season, and the near-surface layer then follows the same cycle of development and rapid removal during the next dry and early rainy seasons.