Riverine—tidal interactions in the monsoonal Gilbert River fandelta, northern Australia

Abstract

The Gilbert River fandelta drains into the Gulf of Carpentaria, northern Queensland, providing a marginal deltaic facies in a stable intracratonic epeiric sea. The Holocene deltaic facies unconformably overlies an irregular Pleistocene surface and forms a lensoidal body up to 8 m thick and 25 km wide. Progradation of this facies has resulted from a high rate of sedimentation in an area with a low regional gradient. Channel avulsion in the mid to upper fluvial reaches of the fandelta has controlled sediment supply and the rate of progradation along the delta front. A succession of strandline sand ridges marks prior positions of the prograding delta but the rate of progradation has been enhanced by a slight decrease in sea level over the last 6 ka. The sand ridges, in turn, restrict the position in which distributary channels can reach the coast. Sediment movement within the delta region is controlled by a combination of tidal and monsoonal river hydrodynamics. Sand only reaches the delta front during monsoonal river flow conditions and as the latter decrease sand plugs are deposited within the river channel due to tidal retardation of river flow. During the remainder of the year channels in the tidal portion of the delta redistribute mud derived from wave reworking of the delta front deposits. This alternating bedload and suspended load depositional system is recorded by distinctive annual sand and silt couplets deposited on point bars and vertical accretion on the interchannel supratidal mudflats. Channel morphology in the tidally influenced portion of the Gilbert River fandelta can be divided into a lower sinuous tide-dominated segment and a landward river-dominated tidal channel section. These morphological features are similar to channel planforms shown in other northern Australian deltas with differences being attributed to variation in tidal range and annual river discharge.