Original paper

Holocene evolution of the Minnamurra River estuary, southeast Australia: foraminiferal evidence

Haslett, S.K.; Davies-Burrows, R.; Panayotou, K.; Jones, B.G.; Woodroff, C.D.

Abstract

The Holocene evolution of estuaries along the coast of New South Wales, southeastern Australia, has been the subject of conceptual models based primarily on amino acid racemisation- and radiocarbon-dated lithostratigraphic studies. This study examines the effectiveness of foraminiferal biostratigraphic indicators in support of the model of barrier estuary evolution. Four modern assemblage zones in the Minnamurra River estuary are clearly distinguished through cluster analysis characterising salt marsh, mangrove, pneumatophore (mangrove roots), and unvegetated tidal flat environments. This forms the basis for palaeoenvironmental reconstruction of depositional environments in an 8.55 m deep radiocarbon-dated sediment core, taken 4 km upstream of the present estuary mouth. Foraminifera are relatively abundant and well preserved in the initial marine transgressive sand sheet that represents a tidal flat at the site by around 7 ka. Sub-tidal conditions then develop, implying a ria-like environment with restricted tidal access to the estuary basin before c. 5 ka. Between c. 4.6 and 0.96 m depth a shelly sand with a diverse marine foraminiferal assemblage that includes extra-estuarine taxa, indicates significant sediment infilling from marine-sourced deposition. AMS radiocarbon dating indicates rapid deposition during the interval 3.9-2.1 m (between 5274-4709 and 4535-3893 cal. yrs BP, respectively), which coincides with deposition of a sand sheet with similar characteristics in the adjacent Dunmore coastal basin. The foraminiferal assemblages in both the Minnamurra and Dunmore deposits indicate basin-wide high-energy barrier overwashing, possibly due to tsunami. Sediments between 0.96-0 m are barren of foraminifera and are interpreted as floodplain deposits. Foraminiferal evidence indicates that mangrove and salt marsh environments are entirely lacking from the core sequence.

Keywords

foraminiferacoastal evolutionminnamurra river estuarynew south walesaustraliaholocenetsunami