Hyperpycnal (over density) flows and deposits

Carlos Zavala

doi:10.1186/s42501-020-00065-x

Abstract

A hyperpycnal flow forms when a relatively dense land-derived gravity flow enters into a marine or lacustrine water reservoir. As a consequence of its excess of density, the incoming flow plunges in coastal areas, generating a highly dynamic and often long-lived dense underflow. Depending on the characteristics of the parent flow (flow duration and flow rheology) and basin salinity, the resulting deposits (hyperpycnites) can be very variable.According to flow duration, land-derived gravity flows can be classified into short-lived or long-lived flows. Short-lived gravity flows last for minutes or hours, and are mostly related to small mountainous river discharges, alluvial fans, collapse of natural dams, landslides, volcanic eruptions, jökulhlaups, etc. Long-lived gravity flows last for days, weeks or even months, and are mostly associated with medium- to large-size river discharges.Concerning the rheology of the incoming flow, hyperpycnal flows can be initiated by non-Newtonian (cohesive debris flows), Newtonian supercritical (lahars, hyperconcentrated flows, and concentrated flows) or Newtonian subcritical flows (pebbly, sandy or muddy sediment-laden turbulent flows). Once plunged, non-Newtonian and Newtonian supercritical flows require steep slopes to accelerate, allow the incorporation of ambient water and develop flow transformations in order to evolve into a turbidity current and travel further basinward. Their resulting deposits are difficult to differentiate from those related to intrabasinal turbidites. On the contrary, long-lived Newtonian subcritical flows are capable of transferring huge volumes of sediment, freshwater and organic matter far from the coast even along gentle or flat slopes. In marine settings, the buoyant effect of interstitial freshwater in pebbly and sandy hyperpycnal flows can result in lofting due to flow density reversal. Since the excess of density in muddy hyperpycnal flows is provided by silt-clay sediments in turbulent suspension, lofting is not possible even in marine/saline basins. Muddy hyperpycnal flows can also erode the basin bottom during their travel basinward, allowing the incorporation and transfer of intrabasinal sediments and organic matter. Long-lived hyperpycnal flow deposits exhibit typical characteristics that allow a clear differentiation respect to those related to intrabasinal turbidites. Main features include (1) composite beds with gradual and recurrent changes in sediment grain-size and sedimentary structures, (2) mixture of extrabasinal and intrabasinal components, (3) internal and discontinuous erosional surfaces, and (4) lofting rhythmites in marine/saline basins.

Highlights

River discharges are largely the most efficient mechanism for transferring sediments from production areas into related marine and lacustrine basins. Syvitski et al (2003) demonstrated that present rivers contribute 89% (2020) 9:17A hypopycnal flow (Fig. 1a) forms when the river discharge is less-dense respect to the density of the water in the reservoir (Dr < Dw)
2 Methods This paper explores the different types of land derived sediment gravity flows and the characteristics of their related hyperpycnal flows and deposits when entering marine and lacustrine basins
Of special interest for the initiation of hyperpycnal flows are mixtures of water and sediment since they can achieve the required bulk density to exceed that of the receiving water body at coastal areas

Summary

Introduction

A hypopycnal flow (Fig. 1a) forms when the river discharge is less-dense respect to the density of the water in the reservoir (Dr < Dw). In this situation the incoming flow experienced a rapid deceleration and loss of confinement at the river mouth, with the consequent accumulation of coarse-grained fractions in mouth bars. Freshwater, suspended fine-grained materials (silt/ clay) and plant remains form a buoyant plume that can extend some distance from the coastline. The collapse of these materials composes a fine grained prodelta. The typical result of hypopycnal flows are marine and eventually lacustrine littoral deltas

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Conclusion