Planform geometry, stacking pattern, and extrabasinal origin of low strength and intermediate strength cohesive debris flow deposits in the Marnoso-arenacea Formation, Italy

The Miocene Marnoso-arenacea Formation (Italy) is the only ancient sequence where deposits of individual submarine density flow deposits have been mapped in detail for long (>100 km) distances, thereby providing unique information on how such flows evolve. These beds were deposited by large and infrequent flows in a low-relief basin plain. An almost complete lack of bed amalgamation aids bed correlation, and resembles some modern abyssal plains, but contrasts with ubiquitous bed amalgamation seen in fan-lobe deposits worldwide. Despite the subdued topography of this basin plain, the beds have a complicated character. Previous work showed that a single flow can commonly comprise both turbidity current and cohesive mud-rich debris flows. The debris flows were highly mobile on low gradients, but their deposits are absent in outcrops nearest to source. Similar hybrid beds have been documented in numerous distal fan deposits worldwide, and they represent an important process for delivering sediment into the deep ocean. It is therefore important to understand their origin and flow dynamics. To account for the absence of debrites in proximal Marnoso-arenacea Formation outcrops, it was proposed that debris flows originated within the study area due to erosion of mud-rich seafloor; we show that this is incorrect. Clast and matrix composition show that sediment within the cohesive debris flows originated outside the study area. Previous work showed that intermediate and low strength debris flows produced different downflow-trending facies tracts. Here, we show that intermediate strength debris flows entered the study area as debris flows, while low strength (clast poor) debris flows most likely formed through local transformation from an initially turbulent mud-rich suspension. New field data document debrite planform shape across the basin plain. Predicting this shape is important for subsurface oil and gas reservoirs. Low strength and intermediate strength debrites have sub