Late Quaternary tephras of Coromandel Peninsula, North Island, New Zealand: A mixed peralkaline and calcalkaline tephra sequence

Abstract

A mixed peralkaline and calcalkaline late Quaternary tephra sequence, previously known as the Whangamata and Waihi Ashes, occurs on the Coromandel Peninsula, North Island, New Zealand. The tephra sequence is up to 2 m thick and is subdivided into 4 field classes which consist of a mixture of 1 peralkaline eruptive from Mayor Island and at least 5 calcalkaline tephras from the Central Volcanic Region.

Class 1, Rotoehu Ash (c. 42 000 years), is characterised by a pure, shower-bedded fine ash basal component and an upper component which occurs throughout the mixed tephra sequence, decreasing in concentration towards the modern soil. Class 2 comprises a mixture of Hauparu Tephra (c. 37000 years) and Rotoehu Ash, forming a bright brown, fine ash paleosol. Class 3 is a yellowish brown fine ash paleosol formed on one or more unidentified fine-grained tephras mixed with Rotoehu Ash. The unidentified tephras are thought to consist chiefly of Rotorua Subgroup tephras aged between c. 10000 and 20000 years, especially the Waiohau, Rotorua and Okareka Ashes (c. 11300, 13500, 17000 years respectively). Class 4 is dominated by Tuhua Tephra (c. 6200 years), a peralkaline medium pumiceous lapilli grading to coarse ash. Other tephras mixed within class 4 include variable amounts of the underlying calcalkaline tephras and the younger tephras Taupo Pumice and Kaharoa Ash.

Marker beds are traced from the Te Puke area in the Bay of Plenty region north to Coromandel Peninsula, and individual tephras identified by stratigraphic position and field lithologies, together with laboratory “fingerprinting” techniques including multicomponent methods (ferromagnesian mineral assemblages) and single component (single particle) methods (titanomagnetite analysis by electron microprobe).

A model of tephra deposition and postdepositional modification is presented and examines the interrelationships between thickness of tephra deposits and their cover beds, soil formation, and mixing processes (pedoturbation). It is suggested that the activity of soil fauna and vegetation through the physical effect of root growth and decay and tree-fall (wind-throw), are primarily responsible for the extensive mixing of tephras found in the sequence. South of Coromandel Peninsula, mixing of the individual tephras detailed above is much less evident, probably because of increased tephra thicknesses, thicker cover beds, and a different paleovegetation pattern south of Katikati.

Tuhua Tephra Formation (new definition) is a rhyolitic peralkaline eruptive which blankets the Whangamata-Thames region with 60 cm of medium pumiceous lapilli at Whangamata, reducing to approximately 20 cm of coarse pumiceous ash at Thames. It is characterised by a unique mineral assemblage containing anorthoclase feldspar, quartz, aegirine, cossyrite, riebeckite, and tuhualite and is derived from the peralkaline rhyolite volcano Mayor Island. This assemblage provides an unusual opportunity to examine physical mixing with other tephras, and to map its distribution even where it is only a few millimetres in thickness. Five unpublished radiocarbon dates, obtained by the University of Waikato Radiocarbon Dating Laboratory, of 6280 ± 70 years B.P. (Wkl06), 6070 ± 80 years B.P. (Wk241), 6440 ± 80 years B.P. (Wk242), 6060 ± 80 years B.P. (Wk244), and 6710 ± 80 years B.P. (Wk243) establish the age of Tuhua Tephra at approximately 6200 years old.

Keywords:

• pyroclastics
• tephrochronology
• late Quaternary
• radiocarbon dating
• pedoturbation
• peralkaline
• Coromandel Peninsula
• Tuhua Tephra Formation
• Whangamata Ash
• Waihi Ash