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Oncholite (with quartz pebble)
Oncoids (a term used for rounded stromatolites that rolled about during growth) are represented at Vinegar Hill in a single bed. Fine grained laminated carbonate (calcite) composing the stromatolite incorporates small amounts of terrigenous sand and ooids.
Small-scale unconformities show that stromatolite growth was interrupted by current-rolling and abrasion, or periodic desiccation.
The presence of narrow (1-3mm) fissures containing coated sand-sized grains (ooids) subsequently healed by succeeding layers, suggests that stromatolites growth was affected by emergence and desiccation cracking.
Calcified cyanobacteria filaments in the oncoids (Lindqvist, 1994) provide evidence for the likely role of photosynthesis in carbonate precipitation.
GEOLOGICAL SOCIETY OF NEW ZEALAND
ANNUAL CONFERENCE 2003
FIELD TRIP 9
Paleobotany and sedimentology of Late Cretaceous –
Miocene nonmarine sequences in Otago and Southland
WHEN A 'STROMATOLITE' ISN'T A STROMATOLITE
Margaret A Bradshaw—Canterbury Museum
A wrongly identified specimen in Canterbury Museum highlights a very common mistake regarding 'Stromatolites'. A specimen of layered calcareous algae surrounding a clast of older quartzite, from the Tertiary of Lauder, Otago, has been cataloguesd as a Stromatolite. This, however, was wrong; it is simply a layered algal ball, which-when cut and polished like Nico Stufken's specimen currently on display in the Museum's 'Recently Acquired' case makes a very striking exhibit.
The term Stromatolite is reserved for laminated structures in sediments, often with bulbous, cabbage-like heads of considerable size, that are thought to have been created by algal action, but in which no organic structures are preserved.
The term is therefore a sedimentary one and not a fossil name, Strromatolites are thought to have been formed by algal nets entrapping sediment, then growing up through the layer to inadvertently trap more particles, and so on, until quite large structures developed. During the processes of lithification, the thin layers of dead algal cells disappear, but the layering of sediment resulting from their presence was left. These are particularly striking in Precambrian and Cambrian rocks in various parts of the world, for instance, the Flinders Range in South Australia.
The New Zealand calcareous algal balls, or ONCHOLITES as they are called scientifically, (perhaps Claytonolites? - ED) are quite different. CALCAREOUS ALGAE are organisms capable of absorbing Calcium Carbonate from water and re-depositing it in a calcareous skeletal layer over its growing surface.' Sediment is trapped at the same time, and a banded structure is created.
The Lauder algal balls are Miocene in age, (22.5 to 5 million) and occur in a single, closely packed bed that can be traced laterally for 40 metres. Maximum recorded size of the algal balls in 40 x 40 x 15 cm, and they are thought to have developed on a carbonate bank in a shallow embayment of a large, freshwater lake where sedimentation was very low. Muddy sediments above and below the algal ball bed contain bird bone, fish bone, plants, and freshwater bivalves, all consistent with a fresh water origin.
Most of the Lauder algal ball specimens show growth one side of a piece 'of older rock that 'seeded' the growth, though a sausage shaped algal ball from Vinegar Hill, near Lauder, donated to the Museum by Trevor Daikee, must have been rolled about the sea floor by currents for some time to allow an even, algal coating.
Recent work on the Lauder algal balls has shown that the pale calcareous layers still show algal cell walls and sometimes even fruiting bodies. This makes them quite different from their look-alikes, Stromatolites
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