Abstract
The Ba Lai distributary channel of the Mekong River Delta was abandoned and infilled with sediment during the Late Holocene, providing a unique opportunity to investigate the sediment fill, timing and mechanisms of channel abandonment in tide-dominated deltaic systems. Based on analysis and age dating of four sediment cores, we show that the channel was active since 2.6 ka and was abandoned at 0.7 ka as marked by the abrupt disappearance of the sand fraction and increase in organic matter and sediment accumulation rate. We estimate that the channel might have been filled in a time range of 45–263 years after detachment from the deltaic network, with sediment accumulation rates of centimetres to decimetres per year, rapidly storing approximately 600 Mt of organic-rich mud. We suggest that the channel was abandoned due to a sediment buildup favoured by an increase in regional sediment supply to the delta. This study highlights that mechanisms for abandonment and infilling of tide-dominated deltaic channels do not entirely fit widely used models developed for fluvial-dominated environments. Their abandonment might be driven by autogenic factors related to the river-tidal and deltaic dynamics and favoured by allogenic factors (e.g., human impact and/or climate change).
Highlights
The mechanism of channel abandonment in paralic environments is often explained by meander cutoffs to form oxbow lakes or by avulsions, largely based on observations from fluvial e nvironments[1,2,3,4,5]
The Ba Lai is the only abandoned channel of the Mekong River Delta (MRD), which represents a unique case of a tide-dominated distributary channel that was abandoned in recent geological times
Channel deposits overlie shell layers dated at 3.4–3.2 ka and 2.3 ka, which were found in the two longest cores (BL1 and BL2) (Fig. 2); these layers consist of silty clay with abundant shells, suggesting a shelf environment prior to the full development of the Ba Lai channel
Summary
The mechanism of channel abandonment in paralic environments is often explained by meander cutoffs to form oxbow lakes or by avulsions, largely based on observations from fluvial e nvironments[1,2,3,4,5]. Deposition on shelf, basinward of river mouth active-channel phase comprises tidal-flat and marsh deposits dated between 1.8 ka and 1.3 ka (Fig. 2); they mainly consist of organic-rich structureless or laminated mud, which together with the location of the core (i.e., note the proximity of this core to the palaeochannel margin in Fig. 1B), suggests accumulation in shallow areas near the channel margin.
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