Beds comprising debrite sandwiched within co-genetic turbidite: origin and widespread occurrence in distal depositional environments

摘要

Co-genetic debrite-turbidite beds occur in a variety of modern and ancient turbidite systems. Their basic character is distinctive. An ungraded muddy sandstone interval is encased within mud-poor graded sandstone, siltstone and mudstone. The muddy sandstone interval preserves evidence of en masse deposition and is thus termed a debrite. The mud-poor sandstone, siltstone and mudstone show features indicating progressive layer-by-layer deposition and are thus called a turbidite. Palaeocurrent indicators, ubiquitous stratigraphic association and the position of hemipelagic intervals demonstrate that debrite and enclosing turbidite originate in the same event. Detailed field observations are presented for co-genetic debrite-turbidite beds in three widespread sequences of variable age: the Miocene Marnoso Arenacea Formation in the Italian Apennines; the Silurian Aberystwyth Grits in Wales; and Quaternary deposits of the Agadir Basin, offshore Morocco. Deposition of these sequences occurred in similar unchannellized basin-plain settings. Co-genetic debrite-turbidite beds were deposited from longitudinally segregated flow events, comprising both debris flow and forerunning turbidity current. It is most likely that the debris flow was generated by relatively shallow (few tens of centimetres) erosion of mud-rich sea-floor sediment. Changes in the settling behaviour of sand grains from a muddy fluid as flows decelerated may also have contributed to debrite deposition. The association with distal settings results from the ubiquitous presence of muddy deposits in such locations, which may be eroded and disaggregated to form a cohesive debris flow. Debrite intervals may be extensive (> 26 10 km in the Marnoso Arenacea Formation) and are not restricted to basin margins. Such long debris flow run-out on low-gradient sea floor (< 0·1°) may simply be due to low yield strength ( 50 Pa) of the debris-water mixture. This study emphasizes that multiple flow types, and transformations between flow types, can occur within the distal parts of submarine flow events.