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The Role of Buoyancy Reversal in Turbidite Deposition and Submarine Fan Morphology: Examples From Flume Experiments, Quaternary Deposits, and the Ancient Rock Record


River-derived hyperpycnal currents and turbidity currents initiated in relatively shallow water that travel into deeper and colder water commonly contain interstitial fluid that is less dense than the surrounding ambient water. These currents are initially ground-hugging due to high suspended sediment concentrations. However, as sediment settles from suspension, bulk current density decreases and may become less dense than the surrounding ambient water, at which point the current becomes buoyant and rises from the basin floor. This process of buoyancy reversal, or lofting, affects both the internal architecture of turbidites and their overall morphology. Here, we present results from field studies of Holocene fans on the continental shelf of Southern California and Jurassic hyperpycnites in the Neuquén Basin of Argentina, in combination with three-dimensional flume experiments, to demonstrate the ways in which lofted-current turbidites differ from classic turbidite models. Experimental results and Quaternary examples show that lofting currents are width-limited and generate narrower, more elongate deposits than ground-hugging currents. Factors such as steeper basin floor gradients and higher suspended-sediment concentrations push the lofting point farther basinward and ultimately result in wider deposits. Most importantly, the use of a 3-dimensional experimental tank allows for the first detailed analysis of the lofting process and its effects on length-to-width ratios of turbidite lobes. Cores collected from shelf fans in the Santa Barbara Channel provide grain-size trends, radiocarbon dates, and overall stratigraphic architecture of lofted-current deposits. The currents deposited slightly graded, structureless fine- to medium-grained sand beds. These beds became well-sorted through the stripping of fine-grained material in suspension at the point of lift-off. Similar to our experimental results, the lobes created by lofted flows are narrower than other non-lofted deposits. We compare our experimental results and Quaternary deposits with upper-slope, river-derived turbidites of the Mid-Jurassic Los Molles Formation of the Neuquén Basin in Western Argentina. Their well-sorted nature and narrow geometry suggest that some of these beds may be another example of deposition from currents with reversing buoyancy.