Channel incision into a submarine landslide: an exhumed Carboniferous example from the Paganzo Basin, San Juan, Argentina

Emplacement of submarine landslides, or mass transport deposits, can radically reshape the physiography of continental margins, and strongly influence subsequent sedimentary processes and dispersal patterns. The irregular relief they generate creates obstacles that force reorganisation of sediment transport systems. Subsurface and seabed examples show that channels can incise directly into submarine landslides. Here, we use high-resolution sedimentological analysis, geological mapping and photogrammetric modelling to document the evolution of two adjacent, and partially contemporaneous, sandstone-rich submarine channel-fills (NSB and SSB) that incised deeply (>75 m) with steep lateral margins (up to 70°) into a 200 m thick debrite. The stepped erosion surface mantled by clasts, ranging from gravels to cobbles, points to a period of downcutting and sediment bypass. A change to aggradation is marked by laterally-migrating sandstone-rich channel bodies that is coincident with prominent steps in the large-scale erosion surface. Two types of depositional terrace are documented on these steps: one overlying an entrenchment surface, and another located in a bend cut-off. Above a younger erosion surface, mapped in both NSB and SSB, is an abrupt change to partially-confined tabular sandstones with graded caps, interpreted as confined lobes. The lobes are characterised by a lack of compensational stacking and increasingly thick hybrid bed deposits, suggesting progradation of a lobe complex confined by the main erosion surface. The incision of adjacent and partially coeval channels into a thick submarine landslide, and sand-rich infill including development of partially confined lobes, reflects the complicated relationships between evolving relief and changes in sediment gravity flow character, which can only be investigated at outcrop. The absence of channel-fills in bounding strata, and the abrupt and temporary presence of coarse sediment infilling the channels, indicates that the submarine landslide emplacement reshaped sediment transport systems, and established conditions that effectively separated sand- from mud-dominated deposits.