Map of Port Stephens showing the location of shell bioherms (green shading) forming shallow areas in the modern estuary. Source: NSW Geological Survey Report GS1974/093

Last week I was invited to give a talk at Salamander Bay on the geology and geomorphology of the Port Stephens area. It was part of a series involving community education sponsored by Port Stephens Council. One of the groups attending were members of the local Ocean and Coastal Care Initiative (OCCI), a community organisation with a passionate interest in the environmental health of their coast.

In presenting this talk, I was reminded of many years of field studies in this area which culminated in the monograph published by ANU Press in 1992. This study had a long list of authors, many of whom undertook all or part of their PhD research in the area. It also included work by Peter Roy and a colleague from the NSW Geological Survey, Chris Peat. Part of their work was on some remarkable features, called bioherms, which they published in Quarterly Notes, NSW Geological Survey (1975).

Bioherms occur as thick accumulations of shell on the bed of Port Stephens below present sea level. I located a map showing their general location in the early 1960s. This map was prepared by BHP and Sulphide Corporation during World War II for possible use as a source of lime. The NSW Geological Survey undertook more detailed seismic profiling and drilling as part of the survey of the stratigraphy of Port Stephens. Samples for shell identification and radiocarbon dating were obtained by SCUBA diving. I vividly recall scrambling down the face of a mound of these shells on one of our dives in the Port in the 70s.

The bioherms occur within the mud basin area of Port Stephens. The deposits are primarily composed of shell with less than 50% calcareous sand and mud. The large mud oyster, Ostrea angasi, is the major shell species present. The deposits form massive reef-like accumulations scattered throughout parts of the Port below present sea level. They appear to be inactive today being veneered with mud or muddy calcareous sand. Thicknesses of up to 20m are estimated with sections of densely packed dead oysters being observed on the flanks. The bioherms are part of the Holocene sequence of sediment infill in the estuary and are mostly confined to the plateau-like tops of the eroded Pleistocene sediment sequence. One radiocarbon date of c. 1600 years BP from just beneath the surface of one bioherms gave what could be interpreted as the minimum age for the shell deposits, but clearly more dating is required.

Peat and Roy postulated that the oysters began their reef-like accumulation on the top of topographic highs as the sea commenced its drowning of the Port around 8000 to 9000 years ago. With continued drowning the mounds or reefs accumulated vertically, faster than the rate of mud accumulation in surrounding areas. Why the oysters stopped growing is unclear as growth must have continued through the period that sea level has been around its present level (last 6-7000 years). Perhaps they reached a critical water depth in relation to wave action during this so-called stillstand sea level period. But more recently other growth forms have taken over and the surface of the mounds has been colonised by other molluscs, bryozoa, sponges and sea grasses.

I am not aware of similar mounds of oyster shells in estuaries in Australia. Perhaps others can let us know if they exist. Similar features but of less thickness have been described in lagoons in Texas. What excites me is: first, that they were discovered at a time of great threat to Australia and the need to maintain a steel industry; second, their magnitude and position within the Quaternary sediment sequence of Port Stephens is quite incredible; and third, that this remarkable estuary can offer such interesting sedimentary units about which we now know something but more remains to be discovered.

Words by Prof Bruce Thom. Please respect Bruce Thom’s thoughts and reference where appropriately: (c) ACS, 2017, posted 27th February 2017, for correspondence about this blog post please email