Geological evolution of the Holocene Yarra Delta and its relationship with Port Phillip Bay

ABSTRACT

The Holocene and pre-Holocene sediments and stratigraphy of the Yarra Delta have been examined using nearly 600 geotechnical bores. The oldest Holocene unit is the Coode Island Silt that has two depocentres, each up to 20.0–25.0 m thick, separated by a NW–SE belt of older pre-Holocene units. The northern depocentre represents estuarine infill to the Yarra and Maribyrnong, a river system, whereas the southern depocentre appears to be an offshore bay facies. The youngest unit is the Port Melbourne Sand, which is largely restricted to the area south of the present Yarra River. It is between 5.0 and 28.0 m thick, and is diachronous with the underlying Coode Island Silt. New ¹⁴C shell dates from the Coode Island Silt and Port Melbourne Sand have shown an age range between 8341 and 2760 yrs BP. These sediments infill former swamplands covering low-stand river valleys of the Yarra and Maribyrnong rivers across West Melbourne, Fishermans Bend and South Melbourne. After ca 2760 yrs BP active sedimentation in the delta ceased as base-levels fell, and Yarra and Maribyrnong river sediments bypassed the delta because of falling bay levels. The Yarra and Maribyrnong river courses also shifted progressively westwards behind growing beach barriers of the Port Melbourne Sand. A comparable stratigraphy exists between the Yarra Delta and the adjacent Port Phillip Bay, i.e. marine and lagoonal shelly sediments of the Coode Island Silt and barrier sands of the Port Melbourne Sand infill last-glacial channels cut into the middle Pleistocene Fishermans Bend Silt.

KEYWORDS:

• Yarra Delta
• stratigraphy
• paleogeography
• radiocarbon dating
• Port Melbourne Sand
• Coode Island Silt

Acknowledgements

We acknowledge the help and encouragement of Trevor O'Shannessy of Golders for the cross-section (Figure 8) and facilitating access to public bore databases and shell material for ¹⁴C dating. John Nielsen and Warren Peck provided very helpful advice concerning data access and geological evolution of the area. The newer ¹⁴C shell dating was carried out at Waikato University, NZ. We also acknowledge use of the LiDAR and SWATH imagery of Port Phillip Bay, as provided by the Port of Melbourne Corporation. Our thanks also go to the detailed comments and improvements to the text by the reviewers.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary paper

Borehole location and stratigraphy database for the Yarra Delta.