Pedogenesis on Jurassic formations in the Araripe Basin, northeastern Brazil: Weathering and parent material

Abstract

The Araripe Basin is the largest sedimentary basin in the countryside of Northeast Brazil. Despite the wide diversity of lithostratigraphic, geomorphological, and paleontological studies, little is known about the pedogenesis and weathering. This study aimed to characterize the morphological, physical and chemical attributes, and the spectral behavior of soils in a toposequence over Jurassic formations of the Araripe Basin so as to identify the main pedogenetic processes, as well as their degree of weathering. A toposequence composed of four soil profiles was selected, described, and sampled in a semi-arid depression that is geologically composed of lacustrine (mudstones) and fluvial (sandstones) deposits. The variability of soil attributes such as color, texture, base saturation, clay activity, and spectral behavior (vis-NIR and MIR) reflects the predominant influence of the parent material on the relief factor. The values of Ki index (SiO2/Al2O3) and Fed/Fet ratio indicate a very low degree of weathering, conditioned by a dry climate. From the MIR spectra, absorption features associated with calcite, Mg-silicate, illite, and smectite-illite interstratifications were identified. These minerals were interpreted to have originated authigenically, formed from evaporation processes in lacustrine environments. Despite the smaller influence of the relief and the low action of pedogenetic processes, incipient evidence of clay translocation is observed in the upper third of the landscape.