ALTERNATIVES TO HYPERPYCNAL FLOWS

A river approaching the ocean is hyperpycnal when the density of the sediment-laden water is heavier than the density of sea water. A hyperpycnal flow can plunge to create a continuous turbidity current that delivers large amounts of sediment to the deep sea. With the exception of the Yellow River, however, nearly all modern hyperpycnal flows are associated with active margins. In the case of large rivers on passive margins such as the Mississippi, Congo and Amazon Rivers, however, a modern hyperpycnal flow is a near-impossibility. Conditions in this regard were not likely to have been much different during the last low stand, or the period of falling stand preceding it. Yet turbidity currents can and do form on passive margins, creating deep submarine canyons and magnificent meandering channels on submarine fans. Mechanisms for the formation of such turbidity currents are explored. It is shown that a non-hyperpycnally generated turbidity current and its deposit can mimic many of the features that are presently associated with a turbidity current and its deposit generated by hyperpycnal flows.