Testing etching hypothesis for the shaping of granite dome structures beneath lateritic weathering landsurfaces using ERT method

Abstract

Granite domes, boulders and knobs buried within saprolite have been detected beneath lateritic weathering landsurfaces using 2D electrical resistivity tomography (ERT). This technique provides a valuable means of mapping the bedrock topography and the regolith structures underneath landsurfaces, as it is intrinsically very sensitive to the electrical properties of superimposed pedological, hydrological and geological layers, allowing the determination of their relative geometry and spatial relationships. For instance, 2D inverse electrical resistivity models including topographic data permit the definition of lithostratigraphic cross-sections. It shows that resistive layers, such as the more or less hardened ferruginous horizons and/ or the bedrock, are generally well differentiated from poorly resistive layers, such as saprolite, including water-saturated lenses, as has been corroborated by past and actual borehole observations. The analysis of the 2D geometrical relations between the weathering front, i.e. the bedrock topography, and the erosion surface, i.e. the landsurface topography, documents the weathering and erosion processes governing the development of the landforms and the underlying structures, thus allowing the etching hypothesis to be tested. The infiltration waters are diverted by bedrock protrusions, which behave as structural thresholds compartmentalizing the saprolite domain, and also the regolith water table, into distinct perched saturated subdomains. The diverted waters are thus accumulated in bedrock troughs, which behave like underground channels where the saprolite production rate may be enhanced, provided that the water drainage is efficient. If the landsurface topography controls the runoff dynamics, the actual bedrock topography as depicted by ERT imaging influences the hydrodynamics beneath the landsurface. In some way, this may control the actual weathering rate and the shaping of bedrock protrusions as granite domes and knobs within thick saprolite, before their eventual future exposure.

Sensitivity analysis of pediment development through numerical simulation and selected geospatial query

“…Climatic variability: effects of high moisture availability on deep regolith production As expected, increasing d 1 , which would accelerate bedrock-weathering processes at depth, leads to the development of deeper regolith mantles on the piedmont and loss of diagnostic pediment morphology. This represents cases, for example, in which a portion of underlying bedrock may be composed of biochemical rocks such as limestone (Eppes et al, 2002;Pearce et al, 2004) and be subject to dissolution; or cases in which granitic or other types of rock may be subject to higher potential groundwater residence times in more humid environments (Beauvais et al, 2003). As d 1 is increased from 0.5 to 6.0 m, piedmont regolith thickness escalates from a piedmont-averaged 3.02 ± 0.49 to 26.00 ± 6.37 m (Figs.…”

Application of field geophysics in geomorphology: Advances and limitations exemplified by case studies

“…Through the multitude of possible electrode arrays, 2-D resistivity is suitable for many very different tasks (Beauvais et al, 2003). However, the two main groups of current applications in geomorphology comprise the detection and characterization of permafrost and the investigation of landslides.…”

Lateritic weathering of trachyte, and bauxite formation in West Cameroon: Morphological and geochemical evolution

“…The loose and heterogeneous structures of the lateritic materials from the downslope profile with lower IOL values than in the plateau profiles typify a reworked allochthonous formation resulting from erosion and redistribution of various sized fragments of bauxites and/or Fe lateritic crusts on gently sloping surfaces (Beauvais et al, 2003;Boulangé, 1984;Chardon et al, 2018;Eno Belinga, 1972;Grimaud et al, 2015;Momo Nouazi et al, 2012).…”