Glass chemistry, paleomagnetism, and correlation of middle Pleistocene tuffs in southern North Island, New Zealand, and Western Pacific

Abstract

New Zealand middle Pleistocene (Castlecliffian) rhyolitic tuffs older than 0.5 Ma are characterised on the basis of glass chemistry and paleomagnetism for the purpose of correlation. Of the tuffs examined, Rewa Pumice (c. 0.74 Ma), Potaka Pumice (c. 0.64 Ma), and Lower Te Muna Tephra (c. 0.75 Ma) are particularly widespread marker horizons. Their proven proximity to the Brunhes-Matuyama paleomagnetic reversal provides a useful framework for correlation and absolute age control between regions in southern North IslandWanganui, Wairarapa, and Hawke's Bay-and with deep-sea cores from the Western Pacific. The tuffs allow correlation between a diverse range of sedimentary facies including marine, freshwater, and terrestrial, and provide absolute age control for thick sequences of unfossiliferous strata in Wairarapa and Hawke's Bay.The characterisation and correlation of the tuffs has a number of implications for Castlecliffian paleoclimatic, tectonic, and stratigraphic studies. The Rewa Pumice is contained in periglacial facies, and we assign it to oxygen isotope stage 20. Some tuffs in Hawke's Bay contain charcoal and are considered to be derived from ignimbrites. Their presence implies that the main axial ranges in Hawke's Bay were uplifted after 0.6 Ma. Previous lithostratigraphic correlations from the Castlecliffian stratotype along the Wanganui coast to sections across the Wanganui Basin, which contain radiometrically dated tuffs, are not sustainable, suggesting greater facies variations than hitherto assumed. The sequence of strata in the Rangitikei section containing the Rewa and Potaka Pumices is not a correlative of Kaimatira Pumice Sand at the coast section. Previous tuff correlations were made on the basis of age and mineralogy. Only the integrated use of glass chemistry and paleomagnetism allows correlation of tuffs in this time interval.

A widespread, early Pleistocene tephra (Potaka tephra, 1 Ma) in New Zealand: Character, distribution, and implications

“…The mean SiC> 2 content of about 77 wt% and a total alkali content of about 7.5 wt% indicate a calc-alkaline rhyolite. The glass composition is broadly similar to that of middle and late Pleistocene tephras erupted from the TVZ (Froggatt 1983;Nelson et al 1986;Shane & Froggatt 1991). The mean composition from each exposure is similar within variation limits (Table 2), consistent with them representing a common eruption.…”

“…5). Other tephras are homogeneous (e.g., Froggatt 1983;Shane & Froggatt 1991). also chemically distinctive, allowing correlation.…”

“…At the Mangatewaiiti section, where exposure allowed a site to be sampled just 20 cm above the tephra, it is shown to be immediately overlain by reversed polarity sediments. At (Seward 1976;Shane & Froggatt 1991) in the Rewa Hill section.…”

“…They have the potential to provide a numerical chronology and be a correlation tool for thick, high resolution Pleistocene sequences (Pillans 1992). Although the stratigraphy of eruptive events is well known for the last 50 ka (Froggatt & Lowe 1990), less is known about the tephras in the interval extending back to c. 2 Ma (e.g., Seward 1976;Nelson et al 1986;Shane & Froggatt 1991). This paper reports on the deposits of a large, rhyolitic ignimbrite-producing eruption, which occurred about 1 Ma ago.…”

Composition and Origin of Volcaniclastic Sediments in the Lau Basin (Southwest Pacific), Leg 135 (Sites 834-839)

“…ago (Smith et al, 1977), though farther away from the Lau Basin (about 1600 km). North Island is notorious for explosive eruptions of high silica rhyolites during the Pleistocene, producing widespread ash layers very similar in composition to the three fallout ashes in Hole 834A (e.g., Kyle and Seward, 1984;Shane and Frogatt, 1991). The striking chemical similarity of all three ashes erupted over a time interval of >0.6 m.y.…”