Sediment dynamics of estuarine Holocene incised-valley fill deposits recorded by Siphonichnus (ancient Red River, Gulf of Tonkin)

Andreas Wetzel,Daniel Unverricht

doi:10.1016/j.palaeo.2020.110041

Andreas Wetzel, Daniel Unverricht

Open Access

https://doi.org/10.1016/j.palaeo.2020.110041

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Abstract

The Holocene muddy infill of the valley incised by the ancient Red River during Pleistocene sea-level lowstand records a change from fluvial to estuarine and finally marine depositional conditions. Siphonichnus, a mainly vertical trace fossil produced by burrowing bivalves, documents otherwise unrecorded episodes of enhanced erosion, bypass and deposition of, on average, rapidly aggrading (> 1 m kyr−1), mainly completely bioturbated sediments within the fluvial-marine transition zone (FMTZ), supposedly within the polyhaline domain. The producers of Siphonichnus moved nearly 1 m downward or upward in response to erosional or depositional phases, respectively. The fill of Siphonichnus burrows, in addition, records erosion and/or bypass of sediment: The traces were produced in mud, but many of them are filled with coarse silt and sand that is otherwise not present in the studied intervals in distinct layers. Obviously, this material was available when Siphonichnus was produced and connected to the sediment surface. In addition to seasonal erosion and deposition during freshet and its waning stage, Siphonichnus records exceptional events that displace the sediment surface by >0.5 m. Such events occurred episodically at an estimated frequency of 1 event per several hundreds of years. The complete bioturbation of the deposits suggests apparent continuous accumulation, but in fact they represent a pile of stacked event deposits resulting from depositional phases and intermittent severe erosion.

Highlights

During times of considerably lowered sea level, rivers incised val leys into exposed shelf areas (e.g., Zaitlin et al, 1994; Boyd et al, 2006)
Turbidity and sediment dynamics affect the bi valves burrowing in fluvial-marine transition zone (FMTZ) deposits, but a study taking into account all these factors has not yet been accomplished to our knowledge
Rapid sea-level changes affected the low-gradient shelf and the coastal region of the Gulf of Tonkin: Sea-level fall was so fast that the valley system traversing the present Gulf of Tonkin did not reach geo morphic equilibrium and remained prone to erosion

Summary

Introduction

During times of considerably lowered sea level, rivers incised val leys into exposed shelf areas (e.g., Zaitlin et al, 1994; Boyd et al, 2006). In the Mekong River, chemical data indicate an upstream extent of saline water for 50 km during dry season, whereas a brackish fauna occurs up to 160 km inland the river mouth (Gugliotta et al, 2017) that, can be seen as a function of larval recruitment (e.g., Gingras et al, 2012) The record of such hydraulic processes by physical sedimentary structures is limited (see La Croix and Dashtgard, 2015) and this is particular true in the Holocene fill of the Red River incised valley that is completely mud dominated and strongly bioturbated (Wetzel et al, 2017; see below). Bioturbational structures document the environmental si tuation better than shells because traces reflect the general adaptation

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