Deep weathering in the semi-arid Coastal Cordillera, Chile

Laura V Krone,Armin Dielforder,Rómulo Oses-Pedraza,Lars Ganzert,Casey Bryce,Jeffrey Paulo H Perez,Dirk Wagner,Santiago Aldaz,Ferdinand J Hampl,Ute Weckmann,Kirstin Übernickel,Friedhelm Von Blanckenburg,Pablo Sanchez-Alfaro,Axel Kitte,Christopher Schwerdhelm

doi:10.1038/s41598-021-90267-7

Abstract

The weathering front is the boundary beneath Earth’s surface where pristine rock is converted into weathered rock. It is the base of the “critical zone”, in which the lithosphere, biosphere, and atmosphere interact. Typically, this front is located no more than 20 m deep in granitoid rock in humid climate zones. Its depth and the degree of rock weathering are commonly linked to oxygen transport and fluid flow. By drilling into fractured igneous rock in the semi-arid climate zone of the Coastal Cordillera in Chile we found multiple weathering fronts of which the deepest is 76 m beneath the surface. Rock is weathered to varying degrees, contains core stones, and strongly altered zones featuring intensive iron oxidation and high porosity. Geophysical borehole measurements and chemical weathering indicators reveal more intense weathering where fracturing is extensive, and porosity is higher than in bedrock. Only the top 10 m feature a continuous weathering gradient towards the surface. We suggest that tectonic preconditioning by fracturing provided transport pathways for oxygen to greater depths, inducing porosity by oxidation. Porosity was preserved throughout the weathering process, as secondary minerals were barely formed due to the low fluid flow.

Highlights

Rock weathering, the conversion of coherent rock through contact with atmospheric gases, water, or organisms into weathered rock and mobile soil is a fundamental geologic process
Between 10 and 36 m, we found more consolidated saprolite featuring fractures, red-stained parts, and a moderate porosity (2.8 ± 2%; average ± 1 standard deviation (SD)) in zone III
A key observation is that the depletion of the most soluble major elements is concomitant with high fracture density, high porosity, and low bulk regolith density

Summary

Introduction

The conversion of coherent rock through contact with atmospheric gases, water, or organisms into weathered rock and mobile soil is a fundamental geologic process. The weathering zone and associated reaction fronts are much thicker and found deeper than in mafic lithologies[5,6] This observation requires the advance of the weathering front at depth to be coupled to erosion at the surface through a feedback[5,7]. Non-weathering-related processes generate pathways resulting from tectonic pre-fracturing[19,20], which often involves the development of planar faults and macrofractures on the metre-scale[21]. Such macroscale structures are distinct from weathering-induced fractures that are typically developed on nano- to micro-scale within mineral grains and along grain b oundaries[22]. Tectonic pre-fracturing causes microfractures in minerals[23]

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