Pedogenic Transformation of Fractured Granitic Bedrock, Southern California

Abstract

Joint fractures in granitic bedrock are known to influence patterns of weathering and landform evolution, but their influence on pedogenic processes has not been thoroughly studied. This research was designed to elucidate dominant pedogenic processes and the role of joint fractures in the transformation of a soil–weathered bedrock profile in the San Jacinto Mountains of southern California. The granitic bedrock is sufficiently weathered to meet paralithic materials criteria, but is not so intensively weathered to be considered saprolite. A lateral sequence of five morphologic zones exists within each weathered bedrock horizon: the matrix, matrix rind, fracture rind, fracture coating, and fracture fill. Distributions of pedogenic clay and citrate‐bicarbonate‐dithionite extractable Fe (Fed), and micromorphologic observations of mineral grain alteration indicate that the effects of chemical weathering increase upward in the profile and laterally towards joint fractures. Illuvial clay was detected throughout the profile, but is most abundant in the shallowest weathered bedrock horizons and in morphologic zones near fracture margins. Shrink–swell activity of clay deposited in planar voids, microcracks, and root channels is responsible for mineral grain fragmentation and localized disruption of rock‐controlled fabric. Stresses generated by swelling clay are also responsible for fabric reorganization and development of incipient subsoil structure in the shallowest weathered bedrock horizon. Illuviation and shrink–swell processes play critical roles in the pedogenic transformation of this granitic bedrock.