Geology of Port Stephens

In past times this area was a plain which was uplifted 300 metres at the end of the Tertiary period, 10 million years ago.

The resulting plateau has been eroded down to it's present level, with the exception of the many hills such as Gan Gan, Glovers Hill etc. remaining at the original level.

Port Stephens became an estuary during the Late Pleistocene (70,000 years ago) when the sea level rose about 60 metres. The higher points remained as islands.

Since then an emergence of 5 metres has taken place raising some land above the current sea level, about 6000 years ago.

Port Stephens is a drowned valley. Before submergence a ridge at Soldiers Point to Middle Island and beyond divided the watershed of the Karuah and Myall Rivers. Before submergence the Karuah River flowed south to join up with the Hunter River system. At that time the Myall River alone flowed into what we now call Port Stephens.

Rocks in the Port Stephens area belong to the Carboniferous Kuttung Series. This comprises lava flows composed of andesite and rhyolite, separated by eroded weaker sedimentary strata.

A.
At Morna Point on the southern headland of One Mile Beach there is a 100m thick flow of rhyolite (phenocrysts of quartz, orthoclase and plagioclase). Erosion along joint planes has led to the formation of small stacks (pillars). Here the rhyolite has been intruded along the joint planes by fine grained dolerite dykes of Tertiary age (40 million years ago). These dykes vary from a few centimetres to 5 metres in width. In some places the dykes have been weathered away to form a chasm (deep ditch).

B.
Between Morna and Cemetery Points there are 40 or 50 dykes. This is called a dyke swarm. Some dykes run in a North-South direction and others East-West. A few of them have "jumped" from one joint plane to another.

C.
At Gan Gan there is an outcrop of toscanite, some of which has weathered to clay.

D.
Between Halifax Park and Nelson Head is another toscanite flow about 30 metres thick. Within this toscanite is a glassy phase 3-4 metres thick. Here some of the black glass is devitrifying into a stony type of toscanite. Also caught up in the flow are pebbles and xenoliths (broken fragments of the earlier cooled crust) as well as haematite staining. At Nelson Bay dolerite dykes also occur in the toscanite.

E.
At Halifax Park, along the foreshore is a raised pebbly beach indicating an emergence of the land of about 4 metres.

F.
Near the Fishermans Co-Op at Nelson Bay are two flows of hornblende-pyroxene andesite separated by coarse conglomerates. There are other similar flows between here and Corlette Point. The flow is 30 metres thick at sea level. The conglomerate is 10 metres thick. Boulders range up to one metre in diameter, and consist of granite, quartz porphyry and feldspar porphyry in a tuffaceous matrix.

G.
Tomaree consists of an outcrop of andesite at sea level overlain by toscanite.

H.
Similarly Yacaaba consists of andesite at sea level on the northern side, overlain by toscanite. These lavas are separated by a sedimentary fault of the Burindi Series, outcropping on the south. Burindi sediments consist of a bed of conglomerates over the top of a thin bed of tuffaceous limestones.

Rhyolite is formed when molten material called magma cools relatively slowly to form a hard-wearing rock of minerals and crystals. Rhyolite has the same mineral composition as granite but cools too quickly to form large crystals and ends up fine grained. Quartz, alkali feldspar and plagioclase feldspar are the three main minerals present.

Dolerite has the same chemistry as gabbro and basalt. It is medium-grained plagioclase and pyroxene, 10% quartz, olivine and magnetite. It cools more slowly than basalt.

Andesite. Fine grained, with 60% silica content. It is composed mostly of plagioclase feldspar along with pyroxene and biotite mica.