Eruptions and deposition of volcaniclastic rocks in the Dunedin volcanic complex, Otago Peninsula, New Zealand

Abstract

The Otago Peninsula is part of the Dunedin Volcanic Complex and the Dunedin Volcanic Group, of Miocene age. Tephra and a suite of alkali basalt magma began erupting in the Sandfly Bay area.

The Dunedin Volcanic Complex built up on a continental shelf Activity commenced with the occasional eruption of basaltic tephra and lava. The earliest pyroclastic deposits are accompanied by peperites that formed due to interaction of magma with unconsolidated marine sands. These peperites together with pillow lavas, hyaloclastite and turbidites are evidence of subaqueous activity.

Activity from various small vents over several million years has resulted in a complex stratigraphy and produced a wide range of pyroclastic deposits, including tephra from submarine, emergent, and fully subaerial vents. Most of the small volcanoes on the Otago Peninsula seem to represent small short-lived eruption sites but locally there are also eruptive centres that were active for quite a substantial time or were reactivated. A new volcanological map of the Otago Peninsula shows the distribution of these tephra deposits and the eruption sites.

Rocks of the Dunedin Volcanic Complex rest on Cretaceous and Tertiary sedimentary rocks, which are underlain by pre-Cretaceous basement of quartzofeldspathic schist (Haast Schist Group). The sedimentary rocks comprise marine sandstones, mudstones and limestones, which were deposited on the continental shelf, and a non-marine-marine basal unit.

A loosely constrained stratigraphy is based on relationships of volcanic rocks to these marine sedimentary rocks, over- and underlying unit contacts, and correlation with the best available global sealevel curve. Documentation and age dating of features, such as correlative conformities, in seismic, well-log, and outcrop data, in marine outcrops in different parts of the world have led to a new generation of Cenozoic sea level cycle charts with greater event resolution than that obtainable from seismic data alone. This sea level curve has been used together with other available data (analyses of bedding in tephra, fossil content, pillow lavas as indicator for subaqueous environment) to support interpretations of depositional environments, and to infer ages of the deposits by correlation to the sea level curve. Such a correlation assumes that there has been no or only little uplift or subsidence of the Dunedin Volcanic Complex.

The history of the Otago Peninsula includes six stages of volcanic activity, of which four took place in a subaqueous to emergent environment. This result indicates that subaqueous volcanic activity took place over quite a long period of time during the early history of the Dunedin Volcanic Complex. The tephra ranges in composition from basanite and basalt, to phonolite and trachyte, typical for intraplate volcanism. Microprobe glass analyses and whole rock XRF analyses indicate a process of fractionation crystallisation with fractionation trends resulting from processes operating on mantle derived magmas at various levels in the crust.