Age relations of the Dunedin volcanic complex and some paleogeographic implications—Part II

Abstract

An attempt is made to determine the duration of the Dunedin volcanicity which was shown in Part I (Coombs, White & Hamilton, 1960) to have been initiated during Waiauan or early Tongaporutuan times. Paleobotanical data offer potential indicators of age since carbonaceous beds are widely distributed at various horizons within the volcanic sequence but present microfloral evidence is indecisive. It indicates an age from Waiauan to Opoitian for material deposited during the intervals between both the First and Second, and Second and Third Major Eruptive Phases. From the occurrence of Third Phase phonolite boulders in Late Pleistocene soliflual deposits, the deep weathering of some of these phonolites and the general lack of constructional landforms in the Dunedin district, it is suggested that volcanic activity was complete not later than early Pleistocene and probably earlier. If world-wide correlations on paleomagnetic data prove to be valid, then the lavas of the latter part of the Second Major Eruptive Phase having anomalous or reversed magnetisation may be assigned to a Late Miocene - basal Pliocene period of reversal recognised in France. The normal magnetisation of the Third Major Eruptive Phase lavas may likewise indicate that volcanicity ceased before a period of reversals in mid-Pliocene times. Earlier concepts of a Late Tertiary peneplain in Eastern Otago require modification. Although a pre-volcanic erosion surface of low relief occurs in the western part of the Dunedin region, the conformable relationship between Waiauan sediments and volcanics in the lower Harbour area rules out the possibility of a peneplain there.