Abstract The lithological characteristics of wave- and/or storm-dominated delta-front deposits are fundamentally similar to those of facies deposited on the wave-formed shorefaces of strandplain settings. Differentiating ancient shoreface deposits from those that record deposition in proximity to contemporaneous wave-dominated deltas, therefore, is challenging, especially where the facies represent deposits that are intermediate between end-member strandplains and delta fronts. To date, archetypal facies models are inadequate to describe and distinguish between such deposits. The challenge is further accentuated where studies are limited entirely to core and other subsurface data. Depositional processes typical of deltaic settings influence infaunal organisms in subtle but significant ways. The resulting ichnological signatures clearly reflect the innate differences in physicochemical conditions and paleoenvironmental stresses operating in these settings, such as variations in sedimentation rates, substrate consistencies, oxygenation, salinities, energy conditions, increased turbidity levels, and episodic deposition associated with river floods. Lower Permian successions of the Wasp Head, Pebbley Beach, and Snapper Point Formations of the southern Sydney Basin in southeastern Australia are spectacularly exposed in extensive coastal outcrops. The preserved lithologies and many of the primary sedimentary structures are virtually identical to those characteristic of offshore and strandplain shoreface deposits. Integration of the lithological, sedimentological, and subtle ichnological differences, however, demonstrate that these units were deposited under the influence of paleoenvironmental stresses. There is also considerable evidence of very cold climatic conditions and concomitant effects on the depositional environment from ice rafting, which imposed additional paleoenvironmental stresses. For the most part, fair-weather beds closely resemble strandplain shoreface deposits, with trace-fossil suites that are very diverse and contain a mixture of structures that reflect a variety of feeding strategies characteristic of the Cruziana and Skolithos Ichnofacies. Variations in the ichnological signatures, and departures from the archetypal ichnofacies expressions, in the form of sporadic bioturbation levels, reduced assemblage diversities, and reductions in ichnogenera sizes compared to their unstressed counterparts, suggest intermittent physicochemical stresses. Associated storm deposits display many of the sedimentological and ichnological characteristics associated with river influx and deltaic conditions, including: soft-sediment deformation structures and sediment-gravity-flow deposits, recording rapid sediment emplacement; mudstone drapes that are characteristic of hyperpycnally emplaced fluid muds and rapidly flocculated muds that are produced along the zone of mixing at the base of a hypopycnal (buoyant) mud plume; unbioturbated, carbonaceous mudstone interbeds with synaeresis cracks consistent with freshet-induced salinity fluctuations; an abundance of phytodetrital material, and allochthonous wood and large logs; and sandstone beds with “stressed” trace-fossil suites attributable to the Cruziana Ichnofacies, where ordinarily suites representative of the Skolithos Ichnofacies would be expected. These characteristics suggest that fair-weather beds reflect ambient wave shoaling, but during and immediately following storms, increased river discharge strongly influenced the depositional environment and thus the characteristics of the resultant event beds. Overall, the successions are therefore interpreted as wave- and storm-dominated prodelta to proximal delta-front deposits. Variations in storm signature throughout the successions reflect temporal and spatial variations in the preservation and, therefore, abundance of fair-weather beds. Such variations may represent changing storm climates, climatic seasonality, fluctuations in river discharge, increased amalgamation of beds by persistent storm activity, subtle changes in storm tracks with respect to delta-front orientation, and subtle shallowing or deepening along the delta front.

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.