Abstract Oceanic plates are geologically young, forming at mid-ocean ridges and becoming deeper and older with distance away from these spreading centres, to be subducted into ocean trenches. Most of the islands that occur on these oceanic plates are basaltic, formed at hot spots, and carried into deeper water as the plate migrates. In tropical reef-forming seas, volcanic islands are usually protected by coral reefs, and undergo transition from fringing reefs, to barrier reefs, to atolls, as envisaged by Darwin. Linear island chains comprise volcanic islands at successive stages in the progression from volcano through coral reefs to seamounts and guyots. Erosion occurs rapidly in the early stages once eruption has ceased, and older islands are conspicuously dissected by fluvial action, as observed by Dana. Many are subject to submarine slumping. In the absence of coral reefs, marine abrasion truncates islands, producing near-vertical cliffs, and islands may be entirely bevelled; Balls Pyramid in the southern Pacific appears to be at the penultimate stage of this planation with a broad shelf around it. Coral reefs protect the shoreline, which is usually deeply embayed, with progressive subsidence until volcanic residuals are all that remain on ‘almost-atolls’. Reef limestones indicate earlier phases of reef formation, and there are limestone cliffs around many tropical islands composed of Last Interglacial limestone often veneering older reef terraces. In some cases, the morphology of these limestone coasts contains prominent notches or surf benches reflecting different degrees of exposure to wave energy, or subtle flexure and vertical displacement. Islands provide discrete examples of rocky coasts, with contrasts between adjacent islands, or islands of different ages, providing many insights into the evolutionary stages and the morphodynamics of bold coasts.

Abstract Deltas in the shallow marine epicontinental Gulf of Carpentaria have developed in the slowly subsiding Karumba Basin and are influenced by monsoonal fluvial discharge and a diurnal tidal regime. The McArthur delta is protected from wave action by offshore islands, and the restricted width of the delta is a function of bedrock outcrops near the delta mouth. The upper delta plain is characterized by fluvial lateral-accretion point-bar deposits. In the lower delta plain, progradation has resulted in a buildup of shelly delta-front sands overlain by muddy intertidal and supratidal deposits. The latter dry mudflats are areas of wind deflation and may equate to emergent surfaces in older analogues. Fluvially active distributary channels have a relatively uniform width whereas abandoned channels adopt a tapering tide-dominated form. This delta shows progressive influence of riverine processes downstream onto the lower delta plain. Although the tide-dominated channels have a high sinuosity, their patterns reflect former fluvial channels. The Gilbert River, by contrast, is not bedrock-controlled and contains a prograding coastal wedge of Holocene sediment extending laterally for a distance of 125 km along the coast. The subaerial portion of the delta has prograded 15–20 km during the past 6,500 years, and the subsurface facies show prodelta mud overlain successively by delta-front and subtidal sands, strandline beach and chenier ridges, and intertidal to supratidal mudflats. Thin floodplain deposits cover the inner portion of the delta. Changes in the locus of sedimentation result from fluvial avulsions and account for age variations in the local subtidal sand and beach-ridge accumulations. Deltas in the northern Australian region are not adequately defined by simple morphological classifications. They drain from geologically mature landscapes and illustrate complex morphological patterns that develop in response to specific tidal and fluvial regimes. No major incision would have occurred around the Gulf of Carpentaria during the last low stand of sea level because the onshore and offshore gradients are equivalent, and the Holocene deposits have built out as temporary wedges of sediment (up to 30 km wide and 10 m thick) adjacent to the present shoreline.