Microstructures Related to Soft-Sediment Deformation Within Mass-Transport Deposits

Mass transport deposits (MTDs) represent a valuable opportunity for the microstructural investigation of soft-sediment deformation, as they comprise a diverse array of deformation features impacting rocks at varying stages of lithification. In this study, we investigate microstructural aspects of Permian-Carboniferous MTDs from the Paraná Basin, southern Brazil. The studied deposits range from heterogeneous, layered sandstones and mudstones to poorly sorted, homogeneous diamictites disposed in up to 100 m thick MTD units. Our analysis is grounded in classical structural analysis, revealing that the structures identified, originating from the same deformation event, can be categorized as either brittle or ductile. The microstructural examination provides evidence that deformation resulted in the reorganization of grains through a granular flow mechanism, without any signs of grain fragmentation or comminution. Within shear features, a distinct realignment of grains into smears is evident, occurring both on fault planes and within zones exhibiting S–C patterns. The granular flow is observed on poorly deformed layers to highly deformed deposits within continuous processes that promoted disaggregation and homogenization. Notably, the deformation exhibits a close relationship between structural styles, layer thickness, and grain size. Fine-grained intervals (mud to fine sand) appear to display a greater degree of cohesiveness when contrasted with intervals featuring thicker layers and larger grain sizes (medium to coarse sand). The findings from this study highlight the direct influence of grain size and sorting on consolidation degree and, consequently, on the structural styles imposed by deformation.