Tidal and seasonal controls on the stratigraphic architecture of blind tidal channel deposits in the fluvial-tidal transition of the macrotidal Sittaung River estuary, Myanmar

Abstract

The morphodynamics and sedimentology of tide-dominated estuaries are controlled by the interaction of tidal currents and river processes. Decoupling tidal and river processes is challenging and rarely documented for the distal part of the estuary, where river influences are cryptic due to the large tidal prism. Dendritic blind tidal channels drain the inshore tidal flats and saltmarshes in the distal part of the fluvial-tidal transition zone (FTZ) of the macrotidal Sittaung River estuary, Myanmar. The present study details the sedimentology and stratigraphic architecture of point bars of the blind tidal channels to evaluate the relative importance of tidal and seasonal river processes in the distal part of the FTZ. The point bars consist of silty tidal rhythmites that display hierarchical tidal cycles manifested in the layer thicknesses. The rhythmites formed during the dry season are slightly muddier and more thinly laminated with accentuated neap-spring tidal cycles, whereas those formed during the wet season exhibit anomalously thick layers with indistinct tidal cycles and abundant subcritical climbing ripples. The wet-season rhythmites are commonly associated with bank-normal, high-relief truncation surfaces presumably formed by heavy rainfall at low tides during river floods. Chevron-like upbuilding of the tidal rhythmites overlies the surfaces in the upper part of tidal point bars, attesting to the rapid removal of accommodation space by sedimentation during the river floods with sustained high sediment loads. A time series of digital elevation models exhibited a rapid infill of the blind tidal channels, suggesting high sediment supply onto the tidal flats throughout the year. Despite being located in the seaward end of the FTZ of the estuary, where tidal amplification occurs, the blind tidal channels evolve in response to seasonal river floods as well as tidal currents, accounting for the dynamic geomorphic changes of the inshore tidal flats and saltmarshes in the Sittaung River estuary.