Early Triassic (Induan) Radiolaria and carbon-isotope ratios of a deep-sea sequence from Waiheke Island, North Island, New Zealand

Abstract

This study examines a Triassic deep-sea sequence consisting of rhythmically bedded radiolarian cherts and shales and its implications for early Induan radiolarian fossils. The sequence, obtained from the Waipapa terrane, Waiheke Island, New Zealand, is composed of six lithologic Units (A–F) and, based on conodont biostratigraphy, spans at least the interval from the lowest Induan to the Anisian. Unit A (the basal unit) consists of black chert and shale beds containing fine pyrite minerals; this corresponds to the oceanic anoxic event described at Arrow Rocks further north in New Zealand. The δ 13C org values of Unit A show a pronounced negative shift between the pale-green chert and black shale/chert, which may represent the negative excursion across the Permian–Triassic boundary that has been documented worldwide. The black cherts, which give minimum C-isotopic ratios (around −30‰), are early Induan, and contain a rich radiolarian fauna characterized by Entactinosphaera? crassispinosa Sashida and Tonishi, E.? spoerlii Takemura and Aono, Bistarkum martiali Feng, Entactinia cf. itsukaichiensis Sashida and Tonishi, Ellipsocopicyntra? sp., and rare Nassellaria. A new Induan nassellarian species, Tripedocorbis? blackae n. sp., from the black chert bed, is described herein. Its presence indicates that Triassic-type Nassellaria had already appeared in the early Induan in the pelagic realms of southern hemisphere Panthalassa.