Identification and correlation of thinly bedded late quaternary tephras of Coromandel Peninsula, New Zealand

Abstract

The Coromandel Peninsula of North Island, New Zealand is covered by up to 2.5 m of thinly bedded Late Quaternary tephras. On the basis of their field characteristics, the tephras are divided into five field classes: the Recent bed (field class I) - a black, friable sandy loam; the Pumiceous bed (field class II) - reddish brown coarse lapilli and ash; the Silty bed (field class III) - a yellowish brown silt loam; the Lumpy bed (field class IV) - a brown sandy loam; and the Shower-bedded class ( field class V) - shower bedded and massive sands and loamy sands. The properties of the field classes were examined at 17 stratigraphic sections on the peninsula and indicate that classes I to IV each contain a mixture of at least two tephras. Individual tephras were characterised in the laboratory by multicomponent methods of analysis (particle size parameters, mineral assemblages), single component methods of analysis (chemical analysis of titanomagnetites by XRF), and single particle methods of analysis (microprobe analysis of titanornagnetite grains). The Recent bed is composed of at least three tephras, including the Kaharoa Tephra (930 yrs B.P.), Taupo Pumice (1,819 yrs B.P.), and the fine upper portion of the Whangamata Tephra (6,280 yrs B.P.). The Pumiceous bed contains a thorough mixture of the coarser, basal component of the Whangamata Tephra and the finer grained Mamaku Ash (7,050 yrs B.P.) and Rotoma Ash (c. 9,000 yrs B.P.). The Silty bed, Lumpy bed and Shower-bedded class all contain a common tephra, the Rotoehu Ash (41,700 yrs B.P.). The Shower-bedded class consists of pure Rotoehu Ash; the Lumpy bed is a composite of the upper part of the Rotoehu Ash and the Hauparu Tephra (30,000-40,000 yrs B.P.); the Silty bed is also a composite, but contains in addition to the upper part of the Rotoehu Ash and Hauparu Tephra, some unidentified hypersthene-bearing tephras of 20,000-11,000 yrs B.P. age, named “X” tephras. The Whangamata Taphra Formation (symbol Wg) is dated at 6,280 ± 70 yrs B.P. (Wk 106), and is a brown, graded tephra deposited in two lobes, a coarse lobe of pumiceous ash and lapilli, up to 60 cm thick, extending between Whangamata and Thames on the Coromandel Peninsula, and a fine ash lobe, less than 10 cm in thickness, extending to the south and southwest of the Peninsula, in the Bay of Plenty and Waikato regions. The pumiceous lapilli have a peralkaline mineralogy with phenocrysts of anorthoclase, quartz, aegirine, cossyrite, olivine, riebeckite, and tuhualite, which identifies Mayor Island as the source volcano for the tephrae. It is suggested that the processes that have mixed the “X”, Hauparu and Rotoehu tophras in the Katikati region (latitude 37° 30’s) are related to the vegetation pattern, in the Late Otiran when the tephras became mixed under a forest cover north of about latitude 37° 30’s, - but remained unmixed under scrubland south of this latitude, in the Katikati-Tauranga area.