Spatial and temporal variability of point bar architecture in the tidal-fluvial transition of Sittaung River, Myanmar

Abstract

Inclined heterolithic stratification (IHS) constitutes major architectural component of point bar deposits along the 100-150 km long tidal-fluvial transition zone of Sittaung River, Myanmar. In the fluvial and tide-influenced fluvial channels where unidirectional flows retard and accelerate due to tidal backwater effect and tidal drawdown, IHS consists of sand beds formed during river floods and mud beds formed during waning floods. Tide-modulated features can be preserved within cross beddings of dunes at the lower part of IHS. Mud beds, representing prolonged low discharge period or dry season (inter-floods), are not extensive and confined to the trough of the dunes because of distinct seasonality in discharge. In the tide-influenced fluvial channels where current reversal occurs, IHS is composed of sand and mud beds, reflecting floods and inter-floods, respectively. Tidal signatures such as bidirectional ripples and subtle rhythmic laminations can be preserved in the mud beds of IHS. The mud beds tend to be more extensive than those in the upstream location. In the seaward tide-dominated channels where strong rectilinear currents prevail, IHS consists of predominantly tide-generated beddings. Seasonal discharge fluctuations may be cryptic and limited to sand beds in which high discharge events are reflected by coarser-grained and ebb-dominated facies. The thickest and most extensive mud beds occur within turbidity maximum zone, corresponding to the location of greatest channel sinuosity. The continuity of IHS are influenced by many factors including but not limited to the magnitude of seasonal discharge fluctuation, waves and rainfall. IHS fines upward regardless of tidal influence. Downstream fining is notable along the meander bend of a river-dominated channel, especially where the channel migrates by downstream translation due to its proximity to a resistant valley flank. The downstream-fining trend becomes indistinct seaward as flood currents become stronger with increasing tidal influence. Instead, fining toward the bend apex is obvious at the tighter meander bend of tide-dominated channel where flow separation takes place over the shoaling area by mutually evasive tidal currents. Facies and stratigraphic architecture of IHS varies along the tidal-fluvial transition zone of Sittaung River in response to relative importance of river and tidal currents, wind-induced waves, the intensity of rainfalls and antecedent basement topography.