Abandonment and rapid infilling of a tide-dominated distributary channel at 0.7 ka in the Mekong River Delta

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).

Late Holocene stratigraphic evolution and sedimentary facies of an active to abandoned tide‐dominated distributary channel and its mouth bar

“…It is also possible that a partial connection to the deltaic system was still present, but this had likely been reduced to the point that only the mud fraction was being supplied from upstream. Instead, it is suggested that most of the sediment infilling this abandoned channel would be supplied from its mouth by the action of tidal currents, after being transported to the shelf mainly by channels located north‐eastward of the Ba Lai palaeochannel (Tamura et al ., 2012; Gugliotta et al ., 2021). This is consistent with the general dynamics of the present‐day MRD, with sediment being exported by the channels during high flow and partially reimported from the shelf during low flow (Nowacki et al ., 2015; Gugliotta & Saito, 2019; Jiang et al ., 2020), and with a longshore net sediment transport towards the south‐west (Fang et al ., 2006; Tamura et al ., 2010).…”

“…This is consistent with the general dynamics of the present‐day MRD, with sediment being exported by the channels during high flow and partially reimported from the shelf during low flow (Nowacki et al ., 2015; Gugliotta & Saito, 2019; Jiang et al ., 2020), and with a longshore net sediment transport towards the south‐west (Fang et al ., 2006; Tamura et al ., 2010). The consistent ages and high sediment accumulation rates for the abandoned‐channel fill suggest that the channel was abandoned abruptly at approximately 0.7 ka and infilled rapidly in the order of decades to a few centuries (Gugliotta et al ., 2021).…”

“…At 0.7 ka, the channel was abruptly abandoned and rapidly infilled with sediment (Fig. 11E) as suggested by the nearly coeval ages and high accumulation rates from the abandoned‐channel fill (Gugliotta et al ., 2021). During this phase, the channel became disconnected or only partly connected to the deltaic network and mainly with acting tidal processes, whereas river processes were absent or strongly reduced (Gugliotta et al ., 2021).…”

“…The palaeochannel was estimated to be up to 11 m thick, approximately 3 km wide and 50 km long, with a volume of 1.3 km 3 (Gugliotta et al, 2021).…”

“…Age-depth models and accumulation rates for five of the cores based on AMS 14 C and OSL (Optically-Stimulated-Luminescence) ages. Model of core BL2 is modified fromGugliotta et al (2021).…”