Potholes and shear zones deformation in sandstones: Central Amazon basin

Abstract

A pothole is a form related to morphogenetic processes that occur in rock outcrops (such as granites), which are particularly susceptible to water physical and chemical weathering processes, generally in humid tropical climates. In the center of the Amazon basin, waterfalls, rapids streams, and knickzones with the development of potholes, are formed in the beds of several rivers (Urubuí and their tributaries) that erode sandstones from the Silurian Nhamundá Formation. To elucidate the origin of these potholes, there are highlights on analyzing the susceptibility to erosion processes, variations in texture, mineral composition, and strain deformation recorded in quartzarenite to subarkosic sandstones. Panholes are observed and associated with knickzones, where ruptile dextral simple shear zone systems and their conjugate structures influence the drainage network. This search shows that the potholes are spatially positioned at the intersections of tectonic structures which, in addition to the dissolution processes of the sandstone framework, allow the development of large-scale potholes, such as in the Mutum waterfall. They were formed by morphodynamic processes in river valleys carved out of the Nhamundá Formation.