Sedimentary Facies of an Arid Zone Delta: Gascoyne Delta, Western Australia

Abstract

The Gascoyne delta is forming on a semi-arid, wave-dominated coast at the northern end of Shark Bay, Western Australia. The Gascoyne River flows intermittently. The delta has a flattened lobate form with a peripheral strandplain of elongate lagoons and tidal flats separated by beach spits and low beach ridges. Three major stratigraphic components form the Gascoyne delta: basal sheet, deltaic sequence, and plain sequence. The basal sheet is a thin transgressive unit deposited during the Holocene sea-level rise. Offshore it is shelly quartz sand with a relict, shallow-water sandflat fauna. Inshore it contains sediment admixed from contemporary seagrass communities or overlying units. The deltaic sequence is a regressive terrigenous wedge overlying the basal sheet. It consists of the bar unit, the delta bay unit, and the strandplain sheet. The bar unit is an upward-coarsening sand body being deposited on the distributary mouth bars and the shoreface; it grades laterally into the delta bay unit, a muddy sand body accumulating in the protected embayment to the south of the delta. The strandplain sheet overlies both units and is composed of sand and muddy sand with minor mud. It is being deposited in beach, beach ridge, tidal flat, and supratidal flat environments. The plain sequence consists of three units: the channel and levee units and the plain sheet. The channel unit is coarse sand and gravel; the levee unit is fine sand, silt, and mud. The channel and levee units form a narrow belt of bedded fluvial deposits normal to the strandline. The plain sheet is homogeneous, red-brown mud and sandy mud deposited by sheetwashing. The marine deltaic sequence is a laterally extensive, upward-coarsening sand body typical of other wave-dominated deltas (e.g., Sao Francisco and Rhone). However, the subaerial sequence, a narrow, axial belt of channel sands and levee silts flanked by extensive red-brown muds, forms a different cap. In contrast both the Sao Francisco and Rhone deltas have sandy deltaic plain sequences. Other arid zone deltas (Colorado and New River) differ markedly in their process style. Neither are wave-dominated. Arid climate in the Gascoyne does not markedly influence the marine sequence but is reflected in subaerial units by extensive red-brown, desiccated muds and confinement of marsh deposits to the narrow coastal, tidal zone. The Gascoyne is a potential redbed delta, particularly the subaerial sequences. Reddening of marine sequences is theorized as being dependent on the incorporation of organic matter, and possibly on inshore productivity.