Mechanism of generation hyperpycnal flow and the features of hyperpycnite
Speaker: Ming-Wei Liao
Abstract
Mulder and Syvitski(1995) defined a hyperpycnal flow as a negatively buoyant flow that flows along the basin floor due to density in excess of surrounding density of the standing water-body, as the result of the particle load that it carries. This means that (1) hyperpycnal currents will transport distally only particles finer than medium sands and (2) sediment might be transported through a very long distance. Hyperpycnal flows form in the marine environment when river discharge enters the ocean with suspended concentrations in excess of 36 kg mˇV3 due to buoyancy considerations. They form at a river mouth during floods of small to medium size rivers Hyperpycnites differ from other turbidites because of their well-developed inversely graded facies and intrasequence erosional contacts. These observations lead to a complete redefinition and interpretation of fine-grained turbidites.
Despite the historical assumption that the bulk of marine ˇ§shelfˇ¨ mud is deposited by gradual fallout from suspension in quiet water, recent studies of modern muddy shelves and their associated rivers show that they are dominated by hyperpycnal fluid mud. This has not been widely applied to the interpretation of ancient sedimentary fluvio-deltaic systems, such as dominate the mud-rich Cretaceous Western Interior Seaway of North America. The associated prodeltaic mudstones match modern hyperpycnite facies models, and suggest a correspondingly hyperpycnal character. Physical sedimentary structures include diffusely stratified beds that show both normal and inverse grading,indicating sustained flows that waxed and waned. They also display low intensities of bioturbation, which reflect the high physical and chemical stresses of hyperpycnal environments. Distinct ˇ§mantle and swirlˇ¨ biogenic structures indicate soupground conditions, typical of the fluid muds that represent the earliest stages of deposition in a hyperpycnal plume.
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Janok P. Bhattacharya,James A.Maceachern (2009) Hyperpycnal rivers and PRodeltaic Shelves in the Cretaceous Seaway of North America. Journal of Sedimentary Research, v. 79, p.184ˇV209.