Bannockburn Formation

The Manuherikia Group is a fluvial-lacustrine sedimentary fill in the Central Otago area of New Zealand, at the site of the prehistoric Lake Manuherikia. The area consists of a valley and ridge topography, with a series of schist-greywacke mountains at roughly ninety degrees to each other. The Manuherika Group occurs in the current basins, and occasionally on the mountains themselves.

History

One of the earliest geologists to work in the area, McKay, understood that the Manuherikia Group was probably originally continuous. Although some workers came to believe the sediments were deposited in a series of small, interconnected basins between the mountain ranges, e.g. Park, later workers, like Cotton argued that the sediments had been isolated by later mountain growth. DouglasDouglas, B. J. 1986: Lignite resources of Central Otago. New Zealand Energy Research and Development Committee Publication P104: 367. placed the Manuherikia Group sediments into a coherent genetic context. He drew attention to Manuherikia Group sediments on the top of mountain ranges, and to the observation that sequences in distinct basins were similar. His conclusion was that the Manuherikia Group is the result of sedimentation in a single (except for the earliest stages) very large basin that was later intruded by the growth of the mountain ranges. This basin ultimately grew into a single huge lake – Lake Manuherikia – that extended over some 5600 km².

Palynological work by Couper, Mildenhall, Mildenhall and Pocknall has indicated that the Manuherikia Group is basically Miocene in age. The plant macrofossils, common in the lower Manuherikia Group, are mostly Early Miocene, perhaps with some in the earliest Middle Miocene.

Stratigraphy

Following Douglas, the primary subdivision of the Manuherikia Group is into a lower, fluvial (commonly with coal) Dunstan Formation and an upper, lacustrine Bannockburn Formation. The oldest unit of the Manuherikia Group is the Saint Bathans Member of the Dunstan Formation, consisting of the braided-river fill of valleys incised into the basement rocks. Traces of three St Bathans paleovalleys are known – one at Blue Lake, adjacent to St Bathans, is the best exposed. The St Bathans paleovalleys eventually filled, sedimentation spread out over a broader area, and the fluvial character became dominantly meandering. This material is grouped as the Fiddlers Member. Extensive coal swamps developed between the rivers and their flood basins.

Individual flood basin lakes began to coalesce into a true lacustrine system – Lake Manuherikia. Its muddy and sandy sediments are grouped as the Bannockburn Formation. Around the lake margins, distinctive shoreline facies developed. The most widespread of these is the Kawarau Member which contains diverse plant fossils. Distinctive facies within the Kawarau Member have been termed the Cromwell Submember and the Ewing Submember. The Ewing is typically intercalated with the Lauder Member, an informal unit of the Bannockburn Formation. This contains a fossil fauna (the Saint Bathans Fauna) of birds, fish, and a crocodilian, New Zealand's first known terrestrial mammal, as well as bats. Stromatolites are also present.

At two locations around the depocenter, river deltas developed and large thicknesses of coal built up on upper delta plains. These are termed the Blackstone Delta and Teviot Delta. Fully lacustrine conditions ensued with the submergence of these deltas and for much of the mid Miocene, Lake Manuherikia was without known bounds. Rising mountains in the Late Miocene-Pliocene

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