Lithological influence on bedrock incision and transience: Insights from the Aparados da Serra Escarpment, southeast Brazil

Abstract

Bedrock strength is widely regarded to exert major control over fluvial incision and landscape evolution. Despite that, quantifying lithological effects on natural landscapes has been extremely difficult due to limited continuous exposure at a watershed scale, especially in vegetated environments. Recent numerical models emphasize the pivotal role of rock strength in steering long-term landscape evolution, causing deviations from steady-state conditions, the formation of knickpoints, and biased erosion records. Situated along the southeast coast of Brazil, the Aparados da Serra Escarpment (lat. 28–29°S) represents a prominent geomorphological feature (>1,500 m asl) in the passive margin section of South America. Marked by a sequence of relatively weak sedimentary units overlain by relatively strong volcanic rocks, the escarpment exhibits gradual changes in the strong/weak ratio along its length due to a regional south-dipping contact. The well-constrained stratigraphy (stacked sedimentary and volcanic rocks), climatic conditions (uniform rainfall and temperature), and tectonic setting (regionally uniform and low uplift rates) make this region an ideal laboratory for investigating the influence of rock strength on river profiles. This study conducts a comprehensive series of in-situ rock strength measurements at closely spaced intervals (15 m vertical intervals between each site) along three sections across the escarpment (Rocinha at 28.8°S, Rio do Rastro at 28.4°S, and Corvo Branco at 28.0°S), covering a total escarpment segment > 100 km along strike and providing near-continuous exposure from sea level to 1,500 m asl. To this end, we perform detailed mapping along each section and use a Schmidt hammer type N to record the compressive strength of each lithological unit in the area. In addition, we also record the weathering state and fracture spacing for each site, allowing us to build the first continuous rock strength suite for a major geomorphological feature. Our resulting dataset (> 200 sites with > 30 measurements for each site) allows us to examine a long-postulated but rarely documented relationship between rock strength and bedrock channel steepness. Comparative analysis of normalized river steepness (Ksn) from adjacent watersheds (<5 km away) reveals that: (i) Ksn closely follows rock strength in all escarpment sections, and most of the major knickpoints and steepened reaches can be explained by lithological effects alone, (ii) for the same lithology/geological unit, absolute Ksn and rock strength values are the same across all the sections, (iii) even modest rock strength differences, as little as 30%, can induce changes in Ksn values and the development of knickpoints, and (iv) sedimentary rocks exhibit an increase in rock strength with age. Our results have implications for both forward and inverse landscape evolution models since autogenic knickpoints and transient reaches have the potential to mask uplift signals, particularly for slow-uplifting areas such as passive margins and cratons. These findings largely support the parametrization of lithological heterogeneities in bedrock incision models, as well as a detailed mapping when conducting relatively small-scale (< 10,000 km2) landscape evolution studies.