Turbidite stacking patterns in salt-controlled minibasins: insights from integrated analogue models and numerical fluid flow simulations

摘要

The sea floor of intraslope minibasins on passive contine ntal margins plays asignificant role in controlling turbidity current pathw ays and the resulting sed-iment distribution. To address this, laborat ory analogue modelling of intra-slope minibasin formation is combined with numeri cal flow simulations ofmulti-event turbidity currents. This approach permits an improved under-standing of evolving flow–bathymetry–deposit interactions and the resultinginternal stacking patterns of the infills of such minibasins. The bathymetryincludes a shelf to slope channel followed by an upper minibasin, which areseparated by a confining ridge from two lower minibasins that compares wellwith analogous bathymetries reported from natural settings. From a wider rangeof numerical flow experiments, a series of 100 consecutive flows is reported indetail. The turbidity currents are released into the channel and upon reachingthe upper minibasin follow a series of stages from short initial ponding, ‘fillingand spilling’ and an extended transition to long retrogradat ional ponding. Uponreaching the upper minibasin floor, the currents undergo a hydraulic jump andtherefore much sediment is deposited in the central part of the minibasin andthe counterslope. This modifies the bathymetry such that in the fill and spillstage, flow stripping and grain-size partitioning cause some finer sediment to betransported across the confining ridge into the lower mini basins. Throughoutthe basin infill pro cess, the sequences retrograde upstream, accompanied by lat-eral switching into locally formed depressions in the upper minibasin. After thefill and spill stage, significant deposition occurs in the channel where retrograd-ing cyclic steps with wavelengths of 1 to 2 km develop as a function of pulsatingflow criticality. These resul ts are at variance with conventional schemes thatemphasize sequenti al downstream minibasin filling through ponding domi-nated by vertical aggradation. Comparison of these results with published fieldand experimental examples provides support for the main conclusions.