Sedimentological and ichnological implications of rapid Holocene flooding of a gently sloping mud‐dominated incised valley – an example from the Red River (Gulf of Tonkin)

Abstract

The Gulf of Tonkin coastline migrated at an average rate of ca 60 m year−1 landward during Holocene sea‐level rise (20 to 8 ka). Due to a combination of rapid coastline migration and undersupply of sand, neither coastal barriers nor tidal sand bars developed at the mouth of the Red River incised valley. Only a 30 to 80 cm thick sandy interval formed at the base of full‐marine deposits. Thus, the river mouth represented a mud‐dominated open funnel‐shaped estuary during transgression. At the base of the valley fill, a thin fluvial lag deposit marks a period of lowered sea‐level when the river did not reach geomorphic equilibrium and was thus prone to erosion. The onset of base‐level rise is documented by non‐bioturbated to sparsely bioturbated mud that occasionally contains pyrite indicating short‐term seawater incursions. Siderite in overlying deposits points to low‐salinity estuarine conditions. The open funnel‐shaped river mouth favoured upstream incursion of seawater that varied inversely to the seasonal strongly fluctuating discharge: several centimetres to a few tens of centimetres thick intervals showing marine or freshwater dominance alternate, as indicated by bioturbational and physical sedimentary structures, and by the presence of Fe sulphides or siderite, respectively. Recurrent short‐term seawater incursions stressed the burrowing fauna. The degree of bioturbation increases upward corresponding to increasing marine influence. The uppermost estuarine sediments are completely bioturbated. The estuarine deposits aggraded on average rapidly, up to several metres kyr−1. Siphonichnidal burrows produced by bivalves, however, document recurrent episodes of enhanced deposition (>0·5 m) and pronounced erosion (<1 m) that are otherwise not recorded. The slope of the incised valley affected the sedimentary facies. In steep valley segments, the marine transgressive surface (equivalent to the onset of full‐marine conditions) is accentuated by the Glossifungites ichnofacies, whereas in gently sloped valley segments the marine transgressive surface is gradational and bioturbated. Marine deposits are completely bioturbated.

Sensitivity study on the main tidal constituents of the Gulf of Tonkin by using the frequency-domain tidal solver in T-UGOm

“…In the deeper part of the basin, near the boundary, two deeper branches (in light red) are distinguishable. These latter could correspond to the location of the ancient river bed of the Red River during the last glacial time, which split in two around 18 • N-108 • E (Wetzel et al, 2017). The biggest differences compared to GEBCO are observed in the central part of the region and in the Hainan Strait ( Fig.…”

“…The GoT is subjected to the Southeast Asian subtropical monsoon climate (Wyrtki, 1961), and therefore largely influenced by seasonal water discharges from the Red River (Vietnam) and by many smaller rivers such as the Qinjiang, Nanliu, and the Yingzai rivers (China). The Red River, which brings in average 3500 m 3 s −1 (Dang et al, 2010) of water along 150 km of coastline, was ranked as the ninth river in the world in terms of sediment discharge in the 1970s with 145-160 Mt yr −1 (Milliman and Meade, 1983).…”

“…With the development of numerical models, the discrepancies have been significantly reduced by improving the accuracy of tides and tidal currents prediction. Wyrtki (1961) was the first to identify the main tidal constituents in the SCS (O 1 , K 1 , M 2 , and S 2 ), and Ye and Robinson (1983) were the first to successfully simulate the tides in the area. Until recently, only a few numerical studies have focused on the GoT (Fang et al, 1999;Manh and Yanaki, 2000;van Maren and Hoekstra, 2004) and on the Hainan Strait (Chen et al, 2009).…”

Holocene evolution of tidal systems in The Netherlands: Effects of rivers, coastal boundary conditions, eco-engineering species, inherited relief and human interference

“…Most present-day estuaries and tidal embayments formed early in the middle Holocene (8500-7500 yr BP) when sea level rose rapidly, continental shelfs were flooded and shifting coast lines were approaching their current positions (Stanley et al, 1994;Smith et al, 2011;Hijma et al, 2012). Pre-Holocene topography determined the initial planform of many estuaries in that low-lying glacial and river valleys were inundated first (e.g., Dalrymple et al, 1992;Hori and Saito, 2007;Rossi et al, 2011;Tanabe et al, 2015;Wetzel et al, 2017). The subsequent evolution of estuaries and tidal embayments is mainly determined by the balance between the formation and infilling of accommodation space (e.g., Nichols, 1989).…”

Why and How Do We Study Sediment Transport? Focus on Coastal Zones and Ongoing Methods

“…With a 120-m sea level rise between −17,000 and −5000 BC at a rate of about one meter per century [26], the bed of old rivers became canyon (e.g., [27]), sedimentary plains and new deltas have formed, while melting glaciers tore up tremendous quantities of particles, transported downstream.…”