Navarino Island is composed mainly of a sequence of geosynclinal sediments-the Yahgan Formation-at least 3,000 metres thick and of probable Early Cretaceous age. The main sediments are greywackes, argillites, and cherts. Calcareous concretions are characteristic of some horizons but are nowhere abundant. Many coarser-grained sediments contain abundant andesitic to basaltic debris, indicating widespread contemporaneous volcanic activity. Fossils are rare and comprise poorly preserved Radiolaria, fragmentary plant remains, and imprints of Inoceramus. Recrystallisation is evident in many rocks; in particular, secondary prehnite is characteristic, especially in the coarser sediments. The formation includes several thick quartz dolerite sills. The largest of these forms a conspicuous east-west chain of serrated peaks, the so-called Dientes de Navarino. The south-west part of the island and the nearby Bertrand Island are composed of altered basic igneous rocks, believed to be contemporaneous with the Yahgan Formation sediments.Dioritic rock, belonging to the Andean intrusive series (Patagonian Batholith) and considered to be Upper Cretaceous, cuts the folded Yahgan Formation, more especially in the west and south parts of the island.The sediments are moderately to strongly folded and dip mainly towards the south. In many places, however, minor thrusts and asymmetric folds are present, and their attitudes suggest that orogenic stress was towards the north.

No abstract

Seven oil-prospective basins are recognised in New Zealand, and their stratigraphy is reviewed. Potential oil formations, which range in age from Upper Cretaceous to Pliocene, are discussed for each basin. Their particular position in the stratigraphic column is largely controlled by the local tectonic history.In anyone basin, possible source and reservoir rocks are in about the same stratigraphic position and generally closely related to each other. In five of the seven basins, which are characterised as epicontinental, unstable shelf areas, they generally occur near the base of the whole sedimentary sequence; this is shown to be inherent to the paleogeographic setting and particular tectonic evolution, and is consistent with the pattern found under similar conditions in other parts of the world. The characteristic lithologic assemblage of possible source sequences found here is of the shale-sandstone-coal type, and its environment is a transitional one of the near-shore marine and deltaic or estuarine-brackish to freshwater zone.Additional potential reservoir formations occur in some of the basins higher up in the section. They are formed by sandstone as well as carbonate rocks, and their presence is discussed in relation to the local tectonic history.

Continuous seismic reflection profiling has shown that morphological highs off the East Coast, North Island, are structure-controUed and may have older sedimentary rock exposed on the sea bottom. Samples with plentiful microfossils taken simultaneously with seismic profiling across Ariel Bank prove a mid-Miocene and possibly older age for the core of a tight anticlir,e, and an age around the PlioPleistucene boundary for outward-dipping beds on its western and eastern flank. Correlation between bottom samples and seismic profiles suggest that two wellmarked unconformities in the syncline west of Ariel Bank correspond to (1) a regional unconformity between the Miocene and the Pliocene, and (2) an unconformity of more local lharacter in the uppemost Pliocene. Thick Pleistocene deposits in this syncline may have been derived from the east. Acoustic basement in seismic sections lies within the sedimentary sequence around the Lower Miocene-Lower Tertiary in.terval. Very thick sediments known to exist in the Eocene to Aptian sequence and also their basement (probably Jurassic), are effectively concealed from seismic detection. It is suggested that seismic energy is dissipated and, therefore, no deeper penetration achieved, because of the presence of a thick series of undercompacted Maastrichtian to Eocene shales, which are under high pressure because of their sealed-in water and gas content.

The absence of a trench and the existence of a separate, western chain of islands in the central New Hebrides island arc are a consequence of earlier tectonic evolution, and not the result of subduction of the d'Entrecusteaux Zone as was previously suggested. Because of tectonic consolidation in the western islands prior to present subduction, a resistant block was created opposing subduction, and a trench nevcr did form here. The d'Entrecasteaux Zone is responsible only locally for additional deformation of the subducting plate, in a way that can be regarded as an initial stage of obduction.