Holocene evolution and depositional model of a bayhead delta, Lake Illawarra, Australia

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Extensive research into the evolution of wave-dominated estuaries has been documented since the 1960s. However, there has only been limited research on the prograding bay-head deltas that are the primary drivers for the rate and stage of estuary evolution. This paper presents the findings of a high-resolution spatial study into the evolution of the Macquarie Rivulet bayhead delta in the Lake Illawarra barrier estuary. The delta’s evolution has been established based on sedimentological analysis of 74 cores, two 14C ages and 45 amino acid racemization ages. This study intersected two Pleistocene and 10 Holocene sedimentary facies associations representing both the retrogradational and progradational phases of delta development. The distribution of bayhead delta facies associations in shallow barrier estuaries is initially controlled by the antecedent morphology, but, in contrast to deeper estuaries, this influence decreases as sedimentation proceeds. Changing relative sea level also has a major control on facies distribution with transgressive facies deposited as relative sea-level rises being replaced by prograding deltaic facies during the highstand and subsequent minor relative sea-level fall. Fluvial sediment supply and river flood events affect the rate of delta progradation, and produce low and high flow sediment deposits within the overall deltaic sequence. The low tidal flux in barrier estuaries, the muddy cohesive nature of the sediments and the prevalence of bioturbation means that primary sedimentary structures are rarely preserved in these bayhead delta facies. The depositional model of bayhead delta evolution shown by the Macquarie Rivulet delta would be widely applicable to other similar barrier estuary settings.

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