Turbid Flows and Their Deposits on Slopes with Minibasins : A Modelling Approach

Abstract

Passive continental margins display a great diversity of seafloor bathymetries induced by gravity driven extensional faulting and compressional folding, as well as diapiric movements of salt or mud. In many diapirically controlled settings, slope bathymetries are complicated and characterized by numerous ridges, trenches and minibasins such as in the Gulf of Mexico and offshore West Africa. These bathymetries play a significant role in controlling turbidity current behavior, the resulting sediment distribution and the internal architecture. Numerous researchers have investigated the influence of pre-existing or developing minibasins on the behaviour of turbidity currents and the resulting depositional systems using seismic data, analogue field outcrops, and laboratory and numerical experiments. The classic fill-spill model was proposed to describe the depositional process in linked intraslope minibasins in the Gulf of Mexico. However, due to the inherent limitations of present-day geophysical techniques and the limited exposure of field outcrops, the small-scale internal architecture and stacking pattern of such confined or semi-confined turbidite systems are still not well understood. The objective of this thesis is to better understand the interaction between flow, sediment and topography, and attempt to develop conceptual models for the changes in sediment dispersal and stacking patterns in diapirically controlled minibasins on passive margins. In order to achieve this, we combine laboratory analogue modelling of intraslope minibasins with numerical flow simulations of multi-event turbidity currents. Previous studies on salt tectonics show that minibasins can be bounded by fold-and-fault systems or are sitting above allochthonous or autochthonous salt bodies. Gravity gliding explains well the typical structural zones (extensional, transitional and compressional) of passive margins, and therefore, in our studies, we conducted analogue tectonic sandbox experiments in which the deformations are driven gravity gliding. Sand and silicone putty are used to represent the prekinematic sediment and salt respectively. The experimental results from different setups show that three types of minibasins are formed and distinguished according to their boundary contact relationships: MB1 (no contact with the silicone layer), MB2 (the silicone layer as the basin base) and MB3 (the silicone diapir as the basin flank). The resulting topographies are scanned with a laser beam from which a digital elevation model is obtained. One topography that is considered most realistic is selected and upscaled to dimensions that typically occur in nature. Furthermore, a channel is added on the shelf and the shelf break to serve as point source for the flows. Subsequently, a numerical flow simulation software (“FanBuilder”, Groenenberg et al., 2009) is employed to model multi-event low-density turbidity currents that flow from the incised channel down into the minibasins on the continental margin. A series of sets of parameters within ranges expected to occur in nature were compiled from literature study and used for the flow simulation experiments. Multiple flow events (non-equilibrium and equilibrium flows) from the same point source were run whereby the deposits were stacked on top of its predecessor. The resulting stratigraphy is then analyzed in 3-D, typically in a series of strike and dip sections. The experimental results of a series of numerical simulations are compared and discussed in terms of flow evolution, flow-deposit interaction, and internal architecture and stacking patterns. In our models, the turbidity currents show a behaviour that can be divided into three phases: the ponding, the fill-and-spill, and the trapping stages. A significant grain-size partitioning happens at the early fill-and-spill stage, with the coarser grains getting trapped in the up-dip minibasin and finer grains transported by the spillover flows further downslope. Significant deposition in the minibasin takes place on the counterslope after the first minibasin depression. The flow pathway and evolution depend much on the flow volume reaching the up-dip minibasin, the remaining accommodation space, and the topography geometry and gradient. The deposits can smooth the gradient of the counterslope, allow more spillover, but they can also make the bounding ridge grow and move upstream and thereby restrict the flows to the minibasin. Overall, the turbidite system undergoes a sequence of progradation followed by aggradation and retrogradation. A sequence of coarsening- and thickening-upward trends is dominant in the down-dip minibasins, while the upper minibasin shows different sequences at different locations. The group depocenters in three minibasins all migrate towards upstream longitudinally and to the minibasin center laterally, which results in a back-filling stacking pattern. Some supportive evidence from published literature has been found to validate our main results. Recommendations for future research include seismic or outcrop studies, syn-tectonic sedimentation experiments, and numerical simulations of high-density gravity flows.