--> Abstract: The Complex Sedimentary and Ecological Environments of a Drowned River Valley: Port Stephens, SE Australia, by Claudia Schroder-Adams and Tanya Tran; #90124 (2011)

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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

The Complex Sedimentary and Ecological Environments of a Drowned River Valley: Port Stephens, SE Australia

Claudia Schroder-Adams1; Tanya Tran1

(1) Earth Sciences, Carleton University, Ottawa, ON, Canada.

Estuaries are complex sedimentary and ecological systems, where variable controlling factors are dependent on wave versus tidal dominance and fluvial processes. Paleoenvironmental reconstruction of ancient counterparts in the form of coastal valley deposits in the subsurface or outcrop requires a multidisciplinary approach. Microfossils can play an integral part in identifying estuarine subenvironments. Foraminifera can be abundant in modern estuaries and resemble characteristics of brackish ichnofaunal communities featuring low species diversity but high abundances of opportunistic species, different feeding strategies and common infaunal species. Sediment distribution is highly controlled by energy regimes, whereas foraminifera respond to salinity and tidal exposure in addition to substrates. Whereas individual taxa can range widely bathymetrically, the combined occurrence of specific taxa becomes diagnostic for estuarine environments.

Port Stephens, located in SE Australia, is a shallow, drowned river valley with two basins separated by a narrow channel. Foraminiferal biofacies distribution reveals a complex ecosystem. The flood-tide delta forms a transition between the wave-dominated open coast and the microtide-dominated estuary. Sand wave dynamics cause a shifting substrate resulting in allochthonous assemblages of calcareous shelf species transported into the bay through a wide entrance. The increasingly muddy central basin is heavily bioturbated, and allows for an in-situ, mixed calcareous and agglutinated assemblage of a progressively more estuarine character. Burial of organic matter permits infaunal species to increase in abundance. The bayhead delta is characterized by lithic-rich substrates that are associated with coarse-grained agglutinated species. Tidal cycles, combined with low river-flux, provide nearly fully marine conditions far up the Karuah River. Marginal regions, from sandy/muddy tidal flats to high marsh, show distinct assemblages of opportunistic species that respond to tidal exposure, substrate variations, organic matter and salinities. Foraminiferal biofacies analysis in combination with the brackish-water ichnofossil model is a powerful tool for recognizing ancient marginal marine environments. Early diagenetic taphonomic loss, however, needs to be considered using microfossil assemblages. Port Stephens, with its environmental complexity, offers a valuable modern analogue for coastal valley deposits of the geological past.