Tabularity of individual turbidite beds controlled by flow efficiency and degree of confinement

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Submarine lobes have various geometries and stacking patterns, whose differences are likely to be the result of variations in flow efficiency and degree of confinement. This study examines four contrasting units with differing flow efficiency and confinement, to evaluate their effects on bed geometries and stacking patterns. Three of these units occur in the Late Palaeozoic Paganzo Basin, north-west Argentina: the Las Lajas system is developed in a 0·8 km wide palaeofjord; the Cerro Bola system (of which two different units were studied) was deposited in a larger sub-basin, at least 20 km wide. The Paine C system of the late Cretaceous Magallanes Basin in Chile is confined by an incision surface 3 km wide. Seventy-eight individual beds in the four units have been chosen to calculate flow efficiency and degree of confinement. Individual flow efficiency has been estimated semi-quantitatively using the outcrop cross-sectional area of the bed (as a proxy for flow volume) and percentage of mud in the beds (as a relative estimate for that in the flows). The degree of confinement experienced by the flows was assessed semi-quantitatively by dividing the flow efficiency by the maximum preserved basin dimension. It is found that: (i) degree of confinement (efficiency divided by maximum preserved basin dimension) influences individual bed geometry, highly confined flows having a higher tabularity (smaller thinning rate); (ii) in highly confined settings, individual beds stack vertically, whereas in unconfined systems, they stack compensationally; (iii) highly confined or high efficiency flows have higher tabularity (smaller thinning rate), which implies that truly sheet-like systems are only developed in highly confined and high efficiency systems. The generic model of architecture of submarine lobes and turbidite sheet systems, as a function of flow efficiency and degree of confinement, could be applied widely to sheet-like systems both at outcrop and in the subsurface.

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