Multi-pulsed turbidity current dynamics and geological implications

Deposits of turbidity currents - turbidites - commonly exhibit upward-fining grainsize profiles, reflecting deposition from flows with simple rapidly-waxing then progressively-waning velocity structures. However, turbidites with patterns of multiple cycles of inverse-to-normal grading are not uncommon. Such deposits are interpreted as being deposited under the influence of repeated waxing-waning velocity cycles within multi-pulsed turbidity currents and are termed "multi-pulsed turbidites". Multi-pulsed flow can be initiated by sequences of retrogressive submarine failures in which each slumping episode can form a pulse in the velocity structure, or may arise due to the combination of multiple flows at downstream confluences; separate flows may even run into each other over long distances. In the first case, it has been inferred that multi-pulsed deposits might carry signals of flow initiation, with each slump linked to a seismic impulse, and further, that such signals can be recognised in the vertical grading structures of distal turbidites. The focus of this research has been to establish i) how multi-pulsed flow dynamics and associated deposits vary along flow pathways and ii) the degree to which grading structures in turbidites deposited by multi-pulsed flows permit inference of flow initiation mechanisms. Initial experiment modelling of single- and multi-pulsed solute-driven gravity flows shows that internal pulses are necessarily advected forward, eventually merging with the flow head such that multi-pulsed flows transition from being cyclically waxing-waning to waxing on arrival then monotonically waning. This finding implies that initiation signals should be distorted then lost in any deposits along the flow pathway. Accordingly, an interpretational template for the spatial variation in turbidite character along flow pathways was developed, accounting for both pulse merging and flow combination at confluences. Further experiments were conducted to support a scaling analysis to estimate merging lengths; these lengths are shorter than those documented from prototype settings, and may reflect a limitation in the scope of application, arising from experimental constraints. Experiment modelling of single- and multi-pulsed sediment-driven gravity flows confirms the occurrence of the pulse merging phenomenon in turbidity currents. Analysis of associated deposits confirms the downstream spatial transition from multi- to uni-pulsed turbidites, albeit with the point of transition being more proximal in the laboratory deposit than the point of pulse merging. However, the spatial persistence of the complex velocity structure up to the point of merging need not be reflected in the associated deposit. Beyond the merging point, single-pulsed turbidites must always be deposited. Such deposits cannot be used to infer flow initiation mechanisms.