Late Neogene Radiolaria from the East Coast Deformed Belt, New Zealand

Abstract

Within the East Coast Deformed Belt there are a number of Late Neogene sedimentary basins with relatively deep-water sediments which, at places, contain abundant radiolarian skeletons. The region was subject to relatively open ocean circulation patterns during the Neogene which, combined with the input of rhyolitic glass shards, has enhanced the siliceous microfossil preservation. A short review of the silica budget is presented and discussed in relation to the preservation of siliceous microfossils in the New Zealand sequences. Techniques were developed to extract and quantitatively study fossil Radiolaria from some of the relatively barren shelf/slope sediments. One hundred and thirty-eight radiolarian taxa are described, most of which can be assigned at the generic level, but thirty-one of which can not be assigned specific names and may eventually prove to be new species. The radiolarian zonation presented is based on detailed analysis of 155 samples from 26 sections and sites ranging in age from basal Tongaporutuan (early Late Miocene) to middle Nukumaruan (early Pleistocene). Sediments of the Kapitean (uppermost Miocene) were generally deposited in shallow water environments or are missing in unconformities in the East Coast Deformed Belt, consequently the radiolarian zonation is based on very poor data in this time segment. Also upper Opoitian and Waipipian (middle Pliocene) sediments, although at places deposited in relatively deep water, generally lack siliceous tuffs, and radiolarian preservation is poor. Five major radiolarian zones can be recognised: Diartus hughesi Zone, Didymocyrtis sp. A Zone, Didymocyrtis sp. A Zone, Didymocyrtis tetrathalmus tetrathalmus Zone, Lamprocyrtis heteroporos Zone, and Lamprocyclas gamphonycha Zone. In samples with good radiolarian preservation six subzones can be identified. The Diartus hughesi Zone can be divided into the Heliodiscus umbonatum Subzone, Didymocyrtis laticonus Subzone, Heliodiscus asteriscus forma large pores Subzone, and Anthocyrtidium ehrenbergi pliocenica Subzone. Additionally the Didymocyrtis tetrathalmus tetrathalmus Zone can be divided into the Lychnocanium sp. aff. grande Subzone and Lamprocyrtis hannai Subzone. The bioevents that define the zonal boundaries are discussed along with other biostratigraphically useful radiolarian datums. These zones and zubzones are correlated to the foraminiferal zonation which in turn has been related, in part, to the paleomagnetic time scale. Correlation are then made with other radiolarian zonations in the north Pacific, tropics, and southern ocean. Points to emerge from these correlations include the apparent provincialism in the transition from Stichocorys delmontense to Stichocorys peregrine in the tropical Pacific. This transition has been reported to occur during approximately 1.5Ma but in New Zealand occurs over a time segment of at least 5.5Ma. The first appearance of Lamprocyclas gamphonycha appears to be an isochronous datum level in temperate radiolarian faunas of the northern and southern Pacific. The last appearance datum of Diartus hughesi at about 7.5Ma is in good agreement with its level in the tropics. The presence of this taxon in lower Gilbert Antarctic cores suggests either a grossly diachronous event between tropical/temperate areas and the southern ocean or, more probably, a misinterpretation of the paleomagnetic signature from key southern ocean piston cores. If the latter situation is the case then the real age estimates on the "Pre middle Gilbert" southern ocean diatom and silicoflagellate stratigraphies are questionable because they are based on the same key cores. Statistical faunal analysis shows that during the Miocene there was not much change in the radiolarian faunas with time and a major change, probably climatically controlled, took place across the Miocene/Pliocene boundary. Variability in preservation has probably affected the faunas to obscure more precise time variation although post-Miocene faunas indicate that some is present. In conclusion, the Radiolaria, although not as common in the fossil record as the foraminifera, definitely contribute to New Zealand Late Neogene integrated stratigraphy and suggest that our knowledge could be greatly enhanced by the study of other siliceous microfossil groups.