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

Abstract

To better understand the sedimentary facies of tide-dominated deltas, a core dataset from the Ba Lai palaeochannel in the Mekong River Delta was obtained and studied. Nine sedimentary facies were identified and interpreted as representing the Late Holocene evolution of the Ba Lai palaeochannel, including its pre-abandonment and post-abandonment phases, as well as pre-channel phases. The channel formed at 2.6 ka as a distributary channel connected to the deltaic network and was abruptly abandoned and rapidly infilled with sediment at 0.7 ka. The channel deposits are up to ca 11 m thick and overlie shelfal shell layers, which, in turn, overlie Mid-Holocene and Pleistocene deposits. The active-channel fill and mouth-bar deposits consist of sand and mud with cyclical patterns, bidirectional lenses and abundant mud layers, suggesting deposition mainly driven by river and tidal processes. The abandoned-channel fill consists entirely of organic-rich mud, suggesting a predominance of tidal processes. Other sedimentary facies include tidal-flat and marsh deposits; they mostly consist of mud and formed in shallow to subaerial areas near the channel margins or on barforms. Depending on the exact location of the core in this depositional setting, three possible stratigraphic successions and facies models are presented herein. Near channel margins, the deposits show an upward gradual change from heterolithic to mud with a well-developed fining-upward trend. Away from the channel margins, the deposits show an upward sharp change from heterolithic to mud due to the channel abandonment. The mouth-bar-area facies model shows an upward gradual change from heterolithic to heterolithic/ sand to mud deposits with a coarsening-upward to fining-upward trend. Although differences among systems likely exist, the authors suggest that the sedimentary facies described in this study and the resulting facies models should be used to better understand tide-dominated deltaic systems and to improve their interpretation in the geological record.

Reevaluating the process regime in the Sego Sandstone: Sedimentological and ichnological evidence for an underemphasised fluvial signature

“…By contrast, Brown and Hale (2021) report that in deposits of the modern tide‐dominated Ganges–Brahmaputra delta, regular reworking by tides obliterates any potential signatures of seasonal riverine variations. Similar observations were also made by Gugliotta et al (2022) for the tide‐dominated and wave‐dominated Mekong delta, where the monsoonal signature is thoroughly overprinted by tidal processes in the tide‐dominated part of the system. The deposits of modern wave‐dominated deltas and strandplains also show more intense marine reworking and reduced preservation of interbedded patterns, although some of these beds are present locally (Khan et al, 2021; Qiaola et al, 2022; Tamura et al, 2008).…”

“…It was also reported that the palaeoseaward‐dipping bedding of the interpreted tidal bars was different from the ones of modern tidal bars, and instead were more similar to the mouth bars of fluvial‐dominated deltas or inlet deltas (see p. 503 of Willis & Gabel, 2001). Furthermore, the high sandstone–mudstone ratios in the Sego Sandstone contrast with the much lower sand–mud ratios the typify the deposits of modern tide or mixed tide‐dominated and wave‐dominated deltas, such as the Mekong River delta (Gugliotta et al, 2017, 2019, 2022), which has been proposed as an analogue for the studied units (Legler et al, 2014; van Cappelle et al, 2016). The reinterpretation of the Sego Sandstone/Buck Tongue system as a mixed process, fluvial‐dominated delta would better explain the accretion and palaeocurrent patterns observed and would reconcile the discrepancy regarding the sand–mud ratios.…”

“…In agreement with this recent literature, it is believed that the interbedded motif of the Sego Sandstone with up to decimetre‐scale beds is difficult to explain by tidal processes alone. In fact, tidal rhythmites typically show thinner laminae, generally residing at the millimetre to centimetre‐scale and showing more gradual transitions between sandier and muddier intervals associated with neap‐spring or other cycles (Gugliotta et al, 2022; Longhitano et al, 2012). In deposits of the distal delta front of the Buck Tongue (the secondary target of the study), the pattern is more cryptic, owing to stronger reworking by waves and more common presence of associated wave‐generated sedimentary structures.…”

“…The sandstone beds, however, remain largely unburrowed or display rare, top-down bioturbation, consistent rapid event deposition and physico-chemically stressed conditions, likely related to strong river processes. In contrast, modern tidedominated deltas show more persistent tidal and saline conditions throughout the year, at least near their river mouths and within a portion of their channels, resulting in more pervasive presence of marine facies and/or bioturbations (Dalrymple et al, 2003;Gugliotta et al, 2017Gugliotta et al, , 2022Wolanski et al, 1995Wolanski et al, , 1996.…”