--> Tide-Modulated Fluvial Sedimentation in a Highly Meandering Channel Near the Dry-Season Tidal Limit of Sittaung River, Myanmar

AAPG ACE 2018

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Tide-Modulated Fluvial Sedimentation in a Highly Meandering Channel Near the Dry-Season Tidal Limit of Sittaung River, Myanmar

Abstract

Tide-modulated sedimentation in the backwater zone is crucial to infer the landward limit of tidal influence. Tidal signals in otherwise fluvial deposits are increasingly recognized from ancient outcrops. Despite its sedimentologic and stratigraphic significance, however, tide-modulated sedimentation is scarcely documented from modern rivers where process-product relationships are straightforward. Tide-modulated sedimentation was identified from the point bars of a highly sinuous channel in the proximal backwater zone near dry-season tidal limit of Sittaung River in Myanmar. Prior to 2016 when the channel was cutoffed, dunes on the point bars display a cyclic pattern in foreset dipping angles, bottomset thickness and amount of organic debris, representing alternation between acceleration and retardation of flows regulated by tidal cycles. Up to 20 cm thick mud drapes on the dune troughs record cyclic alternation of daily tides in rhythmically interlaminated very fine sands and muds, whereby very fine sands formed during falling tides and muds represent sedimentation during rising tides. Rhythmic deposition in the mud drapes appears to be facilitated during spring tides when river flood retardation by rising tides becomes more accentuated. In the newly abandoned channel, where channel width reduced drastically by 40% in a year due to extreme sediment influx after neck cutoff event in 2016, the evidence of tide-modulated sedimentation associated with dunes is confined to the proximal location to the neck cutoff. No dunes are developed in the distal location from the neck cutoff, where rippled fine to very fine sands alternating repeatedly with faintly to interlaminated muds constitute a >1 m thick unit that represents rapid sedimentation during waning river floods after the neck cutoff event. Tide-modulated sedimentation is very cryptic and appears to be recorded in the interlaminated muds and the upper part of the rippled sands. The preferential occurrence of tide-modulated sedimentation in the mud layers in the dune trough and the uppermost part of dune cosets in the lower point bar indicates that tidal backwater becomes effective towards the late stage of waning river floods. The presence of rhythmically interlaminated muds in the upper point bar, however, suggests that discharge fluctuation caused by local backwater effects of tributaries and oxbow lakes may lead to non-tidal rhythmic sedimentation during the early stage of waning river floods.