Stratigraphy, chronology, and correlation of the Plio-Pleistocene (c. 2.2-0.8 Ma) Kauroa Ash sequence, western central North Island, New Zealand

Abstract

The Kauroa Ash beds (K-beds) comprise a 12-20 m-thick sequence of extremely weathered, clay-rich (40-95% <4 μm clay) beds of tephra and loess, and associated paleosols. Found in isolated remnants throughout the western central North Island, the sequence comprises 15 defined members, with as many as 44 constituent macroscopic beds. The type site, ‘Woodstock’, near Raglan, is the most comprehensive sequence known, but other sites (e.g. Papakura Creek and Tiritirimatangi Peninsula) contain units not found or poorly defined at Woodstock. Field properties as well as magnetic susceptibility measurements and particle-size analysis characterise the facies in the sequence. Field properties (in particular colour, consistence, macrofabric) describe the lithostratigraphy. The sequence contains five interpretive (i.e. genetic) ‘facies’: paleosols, primary tephra, very weathered tephra (possibly composite beds), loess and ‘tephric loess’ beds. At least seven loess beds are (newly) identified in the sequence: K4a, K5, K6ai, K8ai, K8bi, K10a and K14ai. Mass-specific susceptibility and frequency-dependent susceptibility results partly conform to established models (developed mostly on Chinese loess-paleosol deposits) of susceptibility enhancement in paleosols and depletion in loess. Many parts of the sequence do not appear to conform to this model and the results more closely resemble the inverse relationship found on Alaskan loess-paleosol beds. Frequency-dependent susceptibility is reliable in delineating paleosols by their ‘ultrafine’ ferrimagnetic mineral content, and citrate-bicarbonate-dithionite treatments successfully remove all iron oxides so that remeasurement of susceptibility isolates a strictly ‘pedogenic’, rather than lithogenic, fraction. Laser diffraction particle-size analysis shows that the Kauroa Ash beds are texturally reasonably homogenous. They have bimodal particle-size distributions with the most dominant mode at around 11.25 ɸ inferred to be the product of intense and prolonged weathering. Other modes are variously centred on 7-8.5 ɸ and, despite weathering and pedogenesis, have some relationship to the original depositional particle-size distributions because trends between facies (i.e. genetic units) are delineated. Principal components analysis objectively characterises these modes as (Wentworth size classes) ‘very fine clay’ and ‘coarse silt’, although there is no proportional relationship between them, supporting a post-depositional origin for the former mode. The chronology of the sequence, previously poorly defined, is greatly improved by a combination of tephrochronologic correlations, fission-track dating, and paleomagnetism. Five zircon fission-track dates provide independent age ‘spikes’ and range from 2.24 ± 0.29 Ma in the basal member, K1, to 1.28 ± 0.11 Ma for the distal ignimbrite unit K12a. Paleomagnetism is invaluable in providing additional age information. The top of the sequence, member K15, is dated as >0.78 Ma (Brunhes-Matuyama boundary) because of its reversed polarity; two episodes of normal polarity are found in beds K14b and K2b and are inferred to represent the Jaramillo (1.07-0.99 Ma) and Olduvai (1.95-1.79 Ma) subchrons, respectively. Beds underlying the Kauroa Ash sequence are also of normal polarity, indicating that they were deposited in the Gauss Chron (>2.6 Ma). Identification and correlation of tephras by conventional means (fingerprinting by their lithological or geochemical properties) is impossible in the Kauroa Ash sequence because the beds have no remaining volcanic glass, which has instead been altered to an assemblage of authigenic phases (clays) by weathering and pedogenesis. However, a new technique analysing fresh glass found as melt inclusions in quartz grains is successful in circumventing this problem. Inclusions represent samples of non-degassed magma that became entrapped during phenocryst growth prior to eruption. The glass has remained unaltered because it is hermetically sealed in a chemically resistant phenocryst, which has protected it from weathering processes. Electron microprobe analysis of the glass inclusions yield results which are wholly reasonable for glass (totals ranging from 93-97%; low standard deviations of <1 %), and a number of provisional correlations are established by comparing the major element composition of Kauroa Ash tephra beds with those of proximal deposits. The Kauroa Ash sequence may contain deposits correlated with at least seven major TVZ eruptions, in many cases expanding the known extent of the (distal) deposit and, for the first time answering the question as to the origin of the Kauroa Ash beds. These correlations, together with an improved chronology, enable the Kauroa Ash sequence to be placed in a regional stratigraphic framework alongside other New Zealand Plio-Pleistocene sequences such as those in the Wanganui Basin, Wairarapa, Cape Kidnappers and Port Waikato. Using paleosols as chrono- and climatostratigraphic entities (correlated to warm periods in global climate), the sequence can also be placed alongside a global reference, the marine oxygen isotope stratigraphy. A further correlation to the Chinese loess-paleosol record suggests that large parts of the Kauroa Ash sequence were deposited in an incremental manner akin to deposition of loess, so that the sequence is not only a record of TVZ volcanism, but also of Plio-Pleistocene paleoclimate