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.

Estratigrafía, paleoambientes y correlación del Paleozoico Superior en la localidad Río Francia, Cuenca Paganzo, Argentina

The Tupe and Patquía formations are known for their marine, deltaic, river, and eolian deposits that record different Late Paleozoic events in the Paganzo Basin. However, in the area of the Río Francia (Province of San Juan, Argentina), these units are relevant because they are located in a strategic place of connection with the southernmost sedimentary sequences in the Paganzo Basin on the Central Precordillera. The Tupe Formation (which by palynological data would represent the late Serpukhovian-Gzhelian interval) has a minimum thickness of 140 m of bioturbed mudstones with a minor percentage of coarse to fine sandstones intercalated. In this work they are grouped into 3 facies associations: I lagoon; II barrier island and III shoreface. The Patquía Formation (from palynological data, in this locality, of Cisuralian in age), it is a typical succession of red beds with deposits of fine -grained sandstones with cross-bedding, and alternation of sandstones with mudstones. 3 facies associations were differentiated: IV anastomosed fluvial; V eolian and VI playa lake. A correlation analysis between these formations and their lithostratigraphic equivalents provided possible scenarios for the western sector of the Paganzo Basin, showing a lineament of units with coastal environment features for the Tupe Formation and equivalents at these latitudes. On the other hand, the Patquía Formation and their equivalents display a progressive continentalization from north to south.