Last Interglacial Marine Deposits at Mary Ann Bay, Tasmania

It is always dangerous to comment on a famous geologic site without undertaking a detailed field study. But I must indulge myself on this occasion as the site is one of the most important in the Quaternary history of southeastern Australia. I referring to Mary Ann Bay on the South Arm Peninsula of the Derwent Estuary, south of Hobart, Tasmania.

The site has been studied by quite a few Quaternary geologists and geomorphologists over the years. It was first reported by Van de Geer, Colhoun and Bowden in 1979 in their exciting paper published in Geologie en Mijnbouw (v. 58) on evidence and problems of interglacial marine deposits in Tasmania. This paper, along with field work by Bowden in northeast Tasmania alerted me to the differences in elevation of possible Last Interglacial (LIG) deposits in Tasmania compared to those on the mainland. Colin Murray-Wallace subsequently undertook more detailed study of this and other Tasmanian sites, first summarised in a paper with Albert Goede in Zeitschrift fur Geomorphologie (1991,v.35), and later revisited in a more comprehensive paper in the Journal of Quaternary Research (2002, v.17) on sea-level highstands and neotectonism of the southern Australian passive continental margin. However, there are a set of papers in Quaternary Australasia that really provide the substance of the existing debate on the Mary Ann Bay deposits.

In 1990, Murray-Wallace, Goede and Picker published a note in the newsletter of the Australasian Quaternary Association on what they interpreted as LIG coastal sediments at Mary Ann Bay and their neotectonic significance (v. 8 (2); see also Murray-Wallace and Goede, 1995, Australian Journal of Earth Sciences, v. 42). Radiocarbon, amino acid racemisation and electron spin resonance dating techniques were applied to shell material collected around 22m above present sea level. They wanted other workers to use this publication to document important Quaternary reference sections.

In 2012, Slee, McIntosh, Price and Grove revisited the sections at Mary Ann Bay and published in Quaternary Australasia (v.29) an alternative explanation for the deposits. They did not dispute the age of the shells as LIG but argued that their position and stratigraphy could be best explained by reworking and transport of a nearby accumulation of LIG shells by strong westerly winds. Thermoluminescence dates on sands from the section dated around 30,000 years ago. There was subsequent debate in this journal between Murray-Wallace et al. and McIntosh et al. (Quaternary Australasia v.30) which leaves the interpretation of marine versus aeolian unresolved. McIntosh et al., in conclusion, reminds us of the importance of this site and welcomes further sedimentological study noting that “dating of sands and shells by a variety of methods would be especially useful”. But there is no mention of the need for a more detailed stratigraphic and geomorphologic study of the lands adjoining the section.

Following C2C conference in April, Chris Sharples kindly took my Irene and me to Mary Ann Bay. It was a revelation to see such interbedded sands and shells of many species at such a high elevation. On the mainland I am not used to seeing LIG marine deposits more than 4-5m above present sea level. I would encourage all interested in Quaternary sequences to visit this site because if Murray-Wallace et al. are correct it has enormous tectonic implications that Bowden and others first postulated in the late 1970s. My concern was that there had been no accompanying morphostratigraphic analysis by drilling and dating in the area. The cleared landscape is just waiting for a drill rig to uncover its secrets. There appears to be considerable cover of dune sand across the South Arm Peninsula. There are also exposures of shells with rounded dolerite rocks closer to present sea level that also may be Pleistocene in age. I hope that there are opportunities for more detailed work to be undertaken here in the near future.

Words by Prof Bruce Thom. Please respect the author’s thoughts and reference appropriately: (c) ACS, 2018, posted 4 May 2018, for correspondence about this blog post please email austcoastsoc@gmail.com 

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