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Geological Society, London, Special Publications; 1984; v. 15; p. 331-342;
DOI: 10.1144/GSL.SP.1984.015.01.22
© 1984 Geological Society of London

Hemipelagites and Associated Facies of Slopes and Slope Basins

A late Miocene and early Pliocene upper slope-to-shelf sequence of calcareous fine sediment from the Pacific margin of New Zealand

P. F. Ballance, M. R. Gregory, G. W. Gibson, G. C. H. Chaproniere1, A. P. Kadar & T. Sameshima

Geology Department, Auckland University, Auckland, New Zealand

Upper Miocene and lower Pliocene sediments at East Cape, North Island, New Zealand, comprise an upwards-coarsening and shallowing sequence. Calcareous sandy siltstones, with coccoliths and about 10% clay, were deposited in upper bathyal depths. Upwards they became sandier, with an increasing ratio of benthic to planktic foraminifera and increasing macrofauna, and grade into fossiliferous very fine silty sands, with many coccoliths and a few percent clay; the sands were deposited in shelf depths of 50 to 100 m. Comparison with modern equilibrium shelves suggests that the shelf sediments were deposited on a storm-dominated, wind- and wave-driven ocean margin characterized by long-period swells and normal wave heights of 1 to 3 m. Clay-size sediment seems to have been winnowed out of all the sediments and transferred to greater depths, apart from a few percent trapped between framework grains. The prolific trace fossil fauna contains several ichnotaxa common to all the sediments; however, Teichichnus, a Zoophycos-type and a compound Nereites type, along with minor forms and non-calcareous tube fossils (cf. Terebellina) are confined to the bathyal facies, while Asterosoma extends from the shelf facies part way into the bathyal facies. Coccoliths comprise a restricted flora of temperate-water aspect, lacking both deep water and tropical forms. Macrofauna of some 50 species in the bathyal sediments is dominated by thin-shelled Nuculid and Nuculanid bivalves, while some 80 or more species in the shelf sediments are dominated by the thick-shelled bivalve Cucullea. Debris-flow deposits in the bathyal facies include both bathyal and shelf sediments; they suggest a shelf-slope topography. Numerous interbeds of graded volcanic sand occur in all the facies studied. They were deposited by air-fall and turbidity currents, are mostly rhyolitic in composition, and were derived from the active New Zealand magmatic arc.


1 Present address: Bureau of Mineral Resources, P.O. Box 378, Canberra City, ACT 2601, Australia