OSL dating of sediments from a mountainous river in southeastern Brazil: Late Cenozoic tectonic and climatic implications

Abstract

The upper Das Velhas River basin is located in the central portion of the Quadrilátero Ferrífero in southeastern edge of São Francisco Craton, southeastern Brazil. Its drainage network has a strong lithostructural influence and mountainous river segments crossing different tectonic blocks. The neotectonic activity in this cratonic edge is associated with epirogenetic Brazilian Shield uplift and with local and regional differential movements of crustal blocks due to WNW migration of South-American plate. This process cause an intraplate transfer of tectonic efforts, forcing the reactivation of old faults (NW-SE and NE-SW) and the generation of recent ones (E-W). Using optically stimulated luminescence (OSL) dating and fluvial deposit data, this paper focuses on the late Cenozoic evolution of this mountainous river basin, relating it to tectonic and climatic forcing. Even with few records of ancient alluvial deposits as a result of intense dissection, three levels of river terraces were identified in some sites along the valley: a higher terrace level (T3), a cut-in-bedrock terrace of ~ 48 ka; an intermediate terrace level (T2) of ~ 7.5 ka; and a lower terrace level (T1), of ~ 1 ka. The T2 is found along the valley as a cut-in-bedrock terrace or as a fill terrace, and is found T1 as a cut-in-bedrock terrace or as a nested fill terrace or even overlapping T2 sediments. The diverse relation between these terraces may be related to unequal fluvial downcutting. We identified three main downcutting periods: (i) before T3; (ii) from T3 to T2, related to Late Pleistocene regional tectonics; and (iii) from T2 to T1, which was a response to tectonics and climate change in the early Holocene. These stages of more intense downcutting should be a response to uplift pulses. In the most active blocks, cut-in-bedrock terraces show a greater unevenness in relation to lower levels, while in the most stable blocks, T1 is found as a nested fill terrace or overlapping T2.