Exploratory assessment of sedimentary provenance and pedogenesis: implications for sedimentary landscape
Description
In humid tropical climates, the evolution of sedimentary landscapes is primarily controlled by intense chemical weathering and pedogenesis, often obscuring the boundary between sedimentary provenance and soil formation. A key question persists regarding the origin of sandy surface layers: is it a sedimentary cover from the Quaternary or an in-situ product of intense pedogenesis? To investigate this interplay, an integrated pedological and sedimentological assessment was conducted along a catena on the Rio Claro Planation Surface, where four soil profiles were sampled from a slope adjacent to an isolated topographic depression. The approach here proposed sought to elucidate the genesis of sandy material capping hilltops and conduct an exploratory assessment along a flattened sedimentary landscape. Analytical methods included soil macromorphology, X-ray diffraction (XRD), X-ray fluorescence (XRF), grain-size analysis, and quartz grain microtextural analysis. The profiles are dominated by kaolinite, gibbsite, and Fe-Al oxides, with quartz as the primary residual mineral. High chemical weathering indices (CIA, CIW > 97%) and a low Desilication Index (DI) attest to an advanced stage of chemical alteration. Quartz grains in the surficial sandy layer display microtextures indicative of both mechanical transport (impact, abrasion) and, more critically, intensive chemical dissolution, with the latter becoming more pronounced with depth and overprinting primary sedimentary features. These findings confirm that the sandy material capping the landscape is a pedogenetic residue from the prolonged and intense lateritic weathering of the Piramboia Formation sandstones. This autochthonous model of landscape evolution, driven by prolonged pedogenesis under tectonic stability and a humid tropical climate, best explains the thorough homogenization of the original sedimentary sequence.
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During the sampling efforts, about 100 g of soil from each representative horizon were collected. The samples were selected for the analysis of quartz grain microtextures, utilizing according to their morphological similarities. To remove carbonates, iron oxides, and organic matter from these grains, approximately 20 g of the dried sample were treated by diluted HCl solution (Hossain et al., 2020). Clay, silt, and other small fractions attached to the sand grain surfaces were eliminated by wet sieving using a 125 µm sieve. The quartz grains (ca. 250–420 µm sizes) were randomly handpicked under a stereomicroscope for SEM analyses. Quartz grains of this size range (250–420 µm) usually present some depositional features (e.g., grooves, pits, fractures, polishing features) indicative of the transport and depositional processes to which it was subjected (Krinsley and Doornkamp, 1973). A total of 24 quartz grains in each sample were mounted on carbon tape and coated with carbon. Subsequently, they were imaged using a JEOL JSM-6010LA Scanning Electron Microscope (SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS) at the Scanning Electron Microscopy Laboratory (MEV), located in the Departament of Geology, São Paulo State Universirty (UNESP), Rio Claro, São Paulo, Brazil. A total of 20 different microtextures were identified and classified according to their origin as follows: (i) mechanical (craters, V-shaped, conchoidal fractures, bulbous edges, crescent percussion marks, subrounded outline, rounded outline, upturned plates, parallel striations and abrasion fatigue); (ii) mechanical-chemical (adhering particles, elongated depressions and low, medium, high relief), (iii) chemical origin (solution pits, scaling, silica globules, flowers and pellicle). These features were grouped into: (i) abundant (A; microtextures identified in >75% of the grains), (ii) common (C; 50–75%), (iii) present (P; 25–50%), (iv) sparse (S; 5–25%), (v) rare (R; <5%), and (vi) absent (AB; 0%) (Hossain et al., 2020). The quartz grains morphological classification (sub-angular, sub-rounded, rounded, and well-rounded), were performed using the scale of Powers (1953). Microtextures were also classified into different categories according their abundance and the inferred transport and depositional processes. The abundance values of each microtexture per horizon were obtained from their frequency of occurrence among the 24 quartz grains analyzed per sample.
Institutions
- Universidade Estadual Paulista Julio de Mesquita Filho Instituto de Geociencias e Ciencias Exatas Campus de Rio ClaroSP, Rio Claro
- Universidade Federal da Bahia Instituto de GeocienciasBahia, Salvador