Nature, chemistry, and origin of late Cenozoic megascopic tephras in Legs 89 and 90 Cores from the Southwest Pacific

Several thin (1-10 cm) megascopic vitric tephras occur in the late Cenozoic calcareous oozes on Lord Howe Rise in the Tasman Sea and off eastern South Island, New Zealand. Of the 18 tephras analyzed 15 are silicic (75-78% SiO2) with abundant clear glass shards and a biotite ± hypersthene ± green hornblende ferromagnesian mineralogy. The Neogene silicic tephras were derived from the now-extinct Coromandel volcanic area in New Zealand, and the Quaternary ones from the presently active Central Volcanic Region of New Zealand. On the basis of glass chemistry and age, several of the Quaternary tephras are probably correlatives, and at least two can be matched to the major on-land Mt. Curl tephra (-0.25 m.y.). The occurrence of correlative silicic tephras both northwest and southeast of New Zealand may result from particularly violent eruptions, the ash below and above an altitude of -20 km being dispersed in opposite directions toward the Pacific Ocean and Tasman Sea, respectively. Ash drifting eastward into the southeasterly trade wind belt off northeastern New Zealand could also be carried into the central and northern Tasman Sea. Three megascopic tephras consist of altered basic shards and common labradorite crystals. They record Neogene explosive basaltic to andesitic activity from nearby ocean island or ridge sources in the Ontong-Java Plateau and Vanuatu regions. The megascopic tephras are a very incomplete and biased record of late Cenozoic explosive volcanism in the southwest Pacific because the innumerable, thin, green argillaceous layers in the cores (Gardner et al., this volume) probably represent devitrified intermediate to basic tephras derived mainly from oceanic arc volcanism along the Pacific/Australia plate boundary. In contrast to the New Zealand-derived silicic glass shards, the preservation potential of these more basic shards in Leg 90 calcareous sediments was low.

Silicoflagellates of DSDP Leg 90 sites

Silicoflagellates are described from Sites 588 (middle Eocene), 591 (middle Miocene to lower Pliocene), and 594 (middle Miocene to Quaternary) in the southwest Pacific. At Sites 591 and 594 a detailed silicoflagellate zonation is possible, although there are some obvious differences arising from the latitudinal position of the sites in the silicoflagellate assemblages. Comparison between the sequences recovered at Sites 591 and 206 (Leg 21) revealed two hiatuses in the latter, but helped to establish a zonation for this area from the lower Miocene to the Pleistocene and a correlation to standard nannoplankton zones. The stratigraphic implications of the taxonomy used by various authors and the species concept presented here are discussed in detail. Special reference is made to types described by Ehrenberg and to later synonyma, because the Ehrenberg collection is the base for all subsequent descriptions and evaluations of silicoflagellate taxa. Two new genera (Neonaviculopsis, Paramesocena), two new subspecies (Dictyocha fibula subsp. asymmetrica, Neonaviculopsis neonautica subsp. praenautica), and three new forms (Dictyocha perlaevis f. pentaradiata, Distephanus speculum subsp. speculum f. nonarius, and Mesocena ? hexalitha f. heptalitha) are described from the southwest Pacific Neogene and Pleistocene. Associated sponge spicules were noted and will be described in detail in a later paper, but some are documented on Plate 13.