Microstructures and deformation mechanisms of mass transport-related, soft-sediment folds

Soft-sediment deformation structures (SSDS) are the result of deformation in sedimentary material before significant diagenesis or lithification, and they can result from various triggers, including mass transport (slides, slumps and debris flows). There are several previous works on SSDS within mass-transport deposits (MTDs) and their deformation mechanisms, however most of them consider only macro-to mesoscale data (outcrops and hand specimens), resulting in a lack of microanalysis of deformation mechanisms. In this work we investigate microstructures aiming to interpret deformation mechanisms involved in the formation of folds commonly found in MTDs, aiming to create a compendium of these structures. Using samples from two evolved (highly homogeneized) MTDs in the Permo-Carboniferous Itararé Group, Paraná Basin, southern Brazil, we analyze structural styles in thin sections and discuss their occurrence, geometry and deformation mechanisms. Structural analysis reveals a diverse range of folds resulted from buckling-dominated and bending-dominated folding processes, whose formation was influenced by variations in sediment cohesion, layer thickness and rheology. The absence of cataclasis and early diagenetic minerals, along with grain alignment to fault planes and fold axial surfaces, indicates that deformation occurred under low confining stress prior to lithification, driven by granular flow. Local variations in cohesion within sand- and silt-rich layers are attributed to pore-fluid pressure fluctuations. The findings suggest that the primary deformation mechanism involves progressive shearing of water-saturated sediment, enhanced by liquefaction and fluidization processes.