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Fossil bearing siltstone Cairnmuir Road Bannockburn
As you cross the Bannockburn Bridge continue up the hill to the township of Bannockburn. Near the top of the hill on the left hand side in the road cutting a ‘shaley’ mudstone material can be seen. When exploring this material and exposing the layers leaf fossils and other fos silerous material can be found. These sediments which contain fossils are between 19 – 16 million years old or early miocene.
This locality is famous for the preservation of Casuarinacean stems and compressed "cones" which occur in comparative abundance in thin-bedded siltstones of the Manuherikia Group (Early to Mid-Miocene) at several localities near Bannockburn, Central Otago.
The"cones" show the existence of a casuarinacean (sheoak) forest or scrub element in the Mid-Tertiary vegetation of Southland and Central Otago. They confirm a long-held notion, based on pollen evidence, that Casuarinaceae lived in New Zealand in Tertiary times. The presence of probable Gymnostoma is interesting. The genus is mainly tropical at the present-day with most species being found in New Caledonia and Papua New Guinea.
A second location for finding these fossils is in an area called Bannockburn Bay. As you come off the Bannockburn bridge turn left following Bannockburn Inlet (Cairnmuir Rd). Near the top of the Bay is a picnic area park here and cross the road. In amongst the quartzose sand and conglomerates can be found seams of fossil bearing siltstones along with lignite. Some seams are richer in fossils than others.
It is reported that fossil fish have been found at this locality. The fossil fish of Miocene age from the Manuherikia group, is described and identified as a galaxiid, probably genus Galaxias. The fish was large, estimated at 383 mm, and thus larger than all extant New Zealand Galaxias except G. argenteus, which is known to reach 580 mm. However,it appears to have been of quite slender form. There was, therefore, in Miocene times, a large, slender, perhaps lacustrine Galaxias in Palaeo-lake Manuherikia in Central Otago.
Casuarina avenacea cone/seed ?
Casuarina venacea cone
Mature Casuarina avenacea cone
Fossil Hyridella shell
Fossil Hyridella surrounded by plant material. Found in roadside cutting
Collection of fossil leaves from various plants types.
The lake has been described as a being of poorly drained muddy swamps, peat-forming herbaceous and forest swamps, that encroached over well drained swamps.
Lake manuherikia streched from Bannockburn to Alexandra lauder St Bathans. At St Bathans many discoveries have been found out aout New Zealands past inclunding the discovery of Crocodile bones and teeth, many fish types and the bones of water fowl.
More amazing are the finds of shell fragment from birds, including moa, and liards – like Tuatara.
Early-middle Miocene, the diverse flora included a number of sclerophyll (eucalypt/acacia) taxa such as Acacia, Beauprea,(Protea) Diplopeltis, (peperflower – Australia) Gyrostemon, Micrantheum (Box Micrantheum – Australia) as well as rainforest taxa such Ilex (Holly family), Mischocarpus (soapberry family Australia) and Symplocos (White Hazelwood Australia.) Co-existing with these now locally extinct angiosperms were araliads, Fuchsia, Gunnera, Metrosideros, Nothofagus fusca and menziesii types, Phormium, Ripogonum and Weinmannia, as well as many, as yet, unassigned plants. Note that more than ten conifer types are now known from the Bannockburn (Central Otago area), as well as at least three ferns and a cycad and countless types of pollens from different plant types from the Central Otago area.A species of Casuarina, C. avenacea from Bannockburn and Casuarina “cones” and Nothofagus leaves are moderately common in a siltstone unit in the roadcut below the Bannockburn Hotel.
The best location that I have found to locate fossils is at the top of the Bannockburn Rd as previously described and at Bannockburn Bay, where along the roadside large amounts of siltones are visible. amongst the lignite veins.
These fossils are part of the oldest terristal fossils found in New Zealand.
Careful separation of the silt layers may also reveal fossil water mollusks (Hyridella). These are sometimes difficult to spot. Figure 9 shows just one of these water mollusks surrounded by fossil leaves and ferns.
It is also possible to separate the mussel from the siltstone matrix as shown in fig 10. Be careful when carrying this out as the shell is extremely thin.
Nothofagus leaf
Acacia leaf Bannockburn
fossil leaves – browsed by insects ??
Nothofagus leaf
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