The paleoecology of the estuarine Pomahaka Formation, West Otago

Gard, Henry J. L.

The upper Oligocene (Duntroonian) Pomahaka Formation, one of the few paleo-estuarine deposits known from New Zealand, crops out along the Pomahaka River and Waikoikoi Stream near Tapanui, West Otago. Sediments include lignite seams interbedded with fossiliferous clays, muds, silts and occasionally sands. A drill core indicates that the entire sequence may be up to 90m thick. The Pomahaka Formation rests unconformably on Caples Terrane basement of low metamorphic grade and is overlain by the glaucony-rich Chatton Formation. Unusual well-preserved fossils not found elsewhere are present in the clay-rich, muddy and silty sediments. Shellbeds are up to 1 m thick and different molluscan-dominated assemblages occur at different horizons. Bivalves include Hormomya, ?Barbatia, two oysters (?Crenostrea and Crassostrea), Hinemoana acuminata, cf. Potamocorbula, , Tellinota n.sp., ?Eumarcia n.sp. and two new species of venerids. Gastropods include the small neretid Clithon(?) pomahakaensis with extraordinary colour markings, Batillaria pomahakaensis, Melanopsis pomahaka, Batillona amara, Maoricrypta, Pomahakia aberrans, together with new species of Potamopyrgus and the freshwater genus Melonoides. Small drillholes, possibly made by the predatory gastropod ?Xymene, are present in many molluscs. A species of a lobster-like decapod and trace fossils were found in concretions. Vertebrate fossils include a cheloniid-like (turtle) xiphiplastron, shark teeth of the family Odontaspididae (sand tigers), and bony fish remains. Plant fossils include numerous seeds, amber, leaves and in situ tree stumps up to 53 cm in diameter. Foraminifera include Elphidium excavatum, and species of Trochammina and Haplophragmoides, unlike any modern New Zealand species: they suggest that most of the molluscs lived in a sheltered, brackish, mid-high tidal estuarine paleoenvironment. The neretid Clithon(?) pomahakaensis and the turtle plastron plate suggest that sea temperatures were at least marginally subtropical.

vi, 104 pages A4. Map in back pocket. ~1m

2014Gard

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