Lower Palaeozoic rocks of New Zealand

Abstract

Lower Palaeozoic rocks of New Zealand are confined to the West Nelson-Westland Region and Fiordland Region. Fiordland rocks are largely intermediate to high-grade metamorphics, and New Zealand's early Palaeozoic stratigraphy is established on the sequence of West Nelson and Westland. Within West Nelson and Westland, Lower Palaeozoic strata form three main belts — the Western, Central, and Eastern Sedimentary Belts, separated by major faults, the Anatoki Thrust and Devil River Fault. The lithostratigraphic succession of each belt is outlined. The Western Sedimentary Belt is composed of a quartz-rich sequence of sandstone and mudstone, largely of turbidite origin, and graptolitic black shale; age range is Late Cambrian (or earlier) to Late Ordovician. In contrast, the Central Sedimentary Belt is characterised by volcanics, volcanogenic sediments and carbonates and ranges from Vendian or Early Cambrian to Late Ordovician in age. The Eastern Sedimentary Belt is composed of carbonates followed by quartz-rich clastics and is of Early Ordovician to Early Devonian age. The sediments are thought to represent a western non-volcanic flysch trough (or prism) assemblage and an eastern volcanic arc-carbonate assemblage, brought into juxtaposition by thrust faulting during or prior to the earliest Devonian. A western, continental, source area is inferred for the non-volcanic trough assemblage whereas sediments in the volcanic arc assemblage are largely authigenic or are derived from older units of the same assemblage. Fossils represent the Vendian, Middle and Upper Cambrian, Lower to Upper Ordovician, Middle to Upper Silurian, and Lower Devonian. A relatively complete graptolite succession in Ordovician rocks has enabled recognition of 11 graptolite zones. The zones are correlated with those of Victoria, Australia, and the Victorian stages are used for the New Zealand sequence. Faunal affinities lie most closely with faunas of Australia. A major volcanic episode in the Middle and ?Early Cambrian is represented by a thick pile of basic to intermediate volcanics and volcanogenic sediments (Devil River Volcanics) in the Central Sedimentary Belt. Volcanism continued sporadically in the Upper Cambrian and lower Lower Ordovician. A small ultramafic body of peridotite, orthopyroxenite, websterite, and gabbro was emplaced within Devil River Volcanics in the? Lower or Middle Cambrian. Widespread granites of the Tuhua and Karamea belts appear to be largely of Middle Devonian or younger age. Many mineral ages in these rocks have been reset by a major thermal event associated with further granite emplacement in the mid Cretaceous. Rocks of all three belts have suffered polyphase deformation, and structure is complex. An early phase of recumbent folding (F1) is recognised in the Central Sedimentary Belt and has been strongly overprinted by north-south trending tight folds (F2) with steeply-east-dipping axial planes. Faulting is severe, particularly in the Central Sedimentary Belt where thrust faulting and nappe formation has been recognised. At least four deformational events can be recognised; the Haupiri Disturbance (late Middle to Early Cambrian), the Greenland Event (late Ordovician to Silurian), the “sub-Baton” event (early Early Devonian), and the main phase of the “Tuhua Orogeny” (Middle to Late Devonian). The problem of regional structure is discussed within the context of two alternative models. The sedimentary, igneous and metamorphic-tectonic history of the Fiordland Region is poorly known. The most recent work suggests a. basement complex of retrograded granulite facies rocks overthrust by a metasedimentary cover sequence (possibly including equivalents of the West Nelson-Westland formations) and now metamorphosed to amphibolite facies. Sparse isotopic age data suggest a Devonian age for the amphibolite facies metamorphism.