Characteristics of Hyperpycnal Flow and its Deposits as an Innovative Factor for the Turbidite Paradigm

Abstract

Hyperpycnal flow is one type of turbidity current, which is generated at a river mouth when the suspended sediment concentration of river discharge is so high that the density of the ef- fluent exceeds the water of the receiving basin. Generations of this type of flow have become a common phenomenon recently. A lot of hyperpycnal flow deposits are expected to be preserved naturally in stratigraphic records. Hyperpycnal flow deposits （hyperpycnites） might become an innovative factor for turbidite paradigm considering the particularity of the flow, and bring facies analysis towards the next step. In this review paper, the criteria for identifying hyperpycnal flow deposits from strata are proposed by summarizing studies on hyperpycnal flow and its deposits. Typical characteristics of hyperpycnal flow include : （1） vertical succession composed of 2 parts, inversely-graded lower part and normally-graded upper part, （2） internal scour surface, （3） re- petitive alternation of fine-grained and coarse-grained layers, or laminated and massive layers, （4） abrupt pinch-out of beds, and （5） inclusion of terrestrial materials such as leaves. Hyperpy- cnal flow gradually waxes and then wanes to terminate in response to flood conditions, resulting in （1）. The internal scour surface （2） developed in accordance with the degree of waxing. Fluc- tuations of flow velocity and sediment concentration due to changes of river discharge and/or in- ternal waves can occur during the flow event, and lead to repetitive alternation of （3）. Because of the fresh interstitial water, the marine hyperpycnal flow might start to lift off after losing sus- pended sediments to the degree that the density of the flow is exceeded by ambient saline water. This results in （4） at the lifting point. Terrestrial materials referred to in （5） are the result of the fact that the hyperpycnal flow originated from terrestrial floods. A much wider variety of deposits, which form in one history of a turbidity current, than that expected from the Bouma sequence model, is shown. Exploration of the variety has just started.