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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

Previous work has shown that large-volume (> 100 km3) turbidity currents are capable of depositing extensive (> 100 km) turbidites in non-channelized settings. This study of the Miocene Marnoso Arenacea Formation demonstrates that smaller-volume (< ~ 0.5 km3) turbidites can also be remarkably extensive. Some thin (< 40 cm) turbidites extend for 120 by 30 km within this non-channelized sequence. Flows that produced these thin turbidites were capable of traversing the basin plain in opposing directions, despite being too weak to cause significant substrate erosion. Some flows traveled upslope for at least 60 km. Long run-out distances result from the relatively large fraction of fine particles in these highly efficient flows. Sandstone intervals thin in an approximately exponential fashion, across the proximal 20 to 30 km of outcrop. Similar sandstone shapes are observed for flows that traversed the basin plain in opposite directions. This sandstone shape is a generic property of spatially depletive flows; it is consistently reproduced by laboratory and numerical models in which velocity, sediment concentration, and deposition rates decrease down-flow. Rates of sandstone thinning are orders of magnitude smaller than those observed in deposits from laboratory experiments. This discrepancy primarily reflects a low ratio of flow thickness to sediment settling velocity, and low rates of sediment suspension from the bed, for slower-moving laboratory flows. The distal part of the thin-bed sandstone interval is thinner (< 5 cm) and more tabular. Starved ripples suggest that this distal tabular sandstone shape results partly from bedload reworking. The thickest turbidite mudstone occurs in the same two locations, for flows that traveled across the basin in opposite directions. This suggests that substantial volumes of muddy fluid ponded within bathymetric lows. Abrupt lateral termination of two mudstone beds, which have a distinctly tabular shape, suggests that certain mudsto