Modern soil aggregates–colluvium generated by overland flow – stratigraphy and physical experiments

Abstract

ABSTRACTIn tropical and subtropical zones, sandy colluviums composed of soil aggregates are highly significant for the understanding of depositional processes in the natural realm. The formation processes of soil aggregates–colluviums (pedosediments in other climate zones) are poorly defined. Deeper knowledge on the origin of soil aggregates–colluviums from low‐density or high‐density overland flows is essential for improved understanding of colluviums found in modern, Quaternary and ancient stratigraphic records. The understanding of depositional processes of soil aggregates–colluviums has been integrated into this study by combining macrostratigraphy and microstratigraphy and laboratory experimental techniques. The actualism principle was applied using a sedimentological description of a modern soil aggregates–colluvium deposit generated by overland flows. In addition, physical experiments were performed to simulate the natural sedimentation processes of overland flows composed of various sediment concentrations. The results indicate the maximum grain size (granules and pebbles) of the soil aggregates–colluvium beds originating from high competence overland flows involving large volumes of local rainfall (storms). Overland flows are responsible for laminated beds of soil aggregates–colluviums; however, the laminations are heterogeneous sequences compared to those of low‐density flows, which have homogeneous sequences. The microstratigraphy of the natural, modern soil aggregates–colluviums shows varied structures in overland flow laminations, implying several types of flow density. The internal organization of the laminae suggests that the accretion of the colluvium deposit occurred through high‐density flows and high to low‐transitional density flows. Low‐density flows were less frequent and may constitute either single or more diluted tail flows. Finally, overland flow can generate facies with similar end members to mass movements, being distinguished only by bed thickness. Linking field observations with experimental results, as in the present study, leads to deeper understanding of overland flow sedimentation processes, especially in tropical and subtropical zones.