Abstract:
Thirteen new tephra formations are recognized for the time interval 20,000 years-42,000 years B.P. in the Bay of plenty and adjoining areas. The source of each is shown to be either the Okataina Volcanic Centre or the Taupo Volcanic Centre.
From the Okataina Volcanic Centre the new tephra formations from youngest to oldest are: Omataroa, Awakeri, Mangaone, Hauparu, Te Mahoe, Maketu, Tahuna, and Ngamotu. From the Taupo Volcanic Centre the new tephra formations from youngest to oldest are: Poihipi, Okaia, Tihoi, Waihora, and Otake. The Kawakawa Tephra Formation is created to include from the base, the Aokautere Ash, Scinde island Ash, and Oruanui Breccia members. Its lower contact represents the c. 20,000 years B.P. timeplane. The c. 42,000 years B.P. timeplane is recognized as the Rotoiti Breccia Formation or nearly correlative Earthquake Flat Breccia Formation.
Paleosols-the weathered upper part of each tephra-are shown to be best defined in the laboratory by their organic carbon contents.
An attempt was made to 'fingerprint' each of the late Pleistocene tephras from the Okataina Volcanic Centre by a combination of petrological and chemical criteria. They are mostly rhyolitic (greater than 70% SiO2 by weight in the residual glass), the Hauparu, Te Mahoe, and Maketu tephras are dacitic (less than 70% SiO2 by weight in the residual glass).
Major elements, rare earth elements, and several other minor and trace elements in the glass, are insufficient on their own to 'fingerprint' the tephras.
The relatively less weathered lower part of all the tephras comprise white to pale yellow volcanic glass and varying proportions and types of phenocrysis. Plagioclase is the most abundant phenocryst in all the tephras. Orthopyroxene (predominantly hypersthene), titanomagnetite, and ilmenite are invariably present. Amphibole, either calcic hornblende or cummingtonite, clinopyroxene (augite), and biotite are present in some tephras.
The relative proportions of the three common ferromagnesian minerals: orthopyroxene, clinopyroxene and calcic hornblende is the most useful method of 'fingerprinting'.
Equilibration temperatures are estimated from the iron-titanium oxides. Amphibole dominant tephras group in the temperature range 740-790°C, whereas clinopyroxene bearing tephras group in the temperature range 850-960°C. A continuous sequence of temperatures from c. 740°C to c. 950°C probably exists, the temperatures gap being attributed to inadequate sampling. Clinopyroxene is restricted to the higher temperature eruptions which also contain the most, Ca-rich, and Mg-rich orthopyroxene. The amphibole rich tephras contain relatively Ca-poor and Mg-poor orthopyroxene.
