Towards robust tephra correlations in early and pre-Quaternary sediments: A case study from North Island, New Zealand

Abstract

Abstract Tephra deposits are used by many disciplines of the natural sciences, not only for gaining important insight into volcanism, but also for chronological purposes, especially the dating of sedimentary sequences throughout geological time. Use of these deposits as isochronous tie points relies on the ability to accurately correlate tephra from one site to another including more diffuse, distal sites, as well as to possibly identify the source eruption. However, the ability to uniquely identify tephra deposits, and thus correlate them with certainty, is not without its complications. Here we present a case study from North Island, New Zealand, to highlight the complications and intricacies of achieving robust tephra deposit correlations. We highlight three key issues including: 1) the large uncertainties associated with direct dating techniques for older (≥1 Ma) tephra horizons; 2) the lack of/or minimalist amount of published and accessible geochemical data; and 3) the lack of data pertaining to the source regions themselves. We report new age and geochemical data for tephra deposits from a region that has been the depositional site of many eruptions since at least 12 Ma. Of the tephra horizons sampled in this region, six contained sufficient zircons for fission-track dating. Ages of the horizons range from 1.4 to 5.7 Ma, in good agreement with previously assigned age constraints based on biostratigraphic and/or paleomagnetic markers. However, the precision of these ages (2σ uncertainties of ±0.4–1.6 Ma) is insufficient to correlate the tephra horizons to either their specific source (eruption event) or alternative tephra deposits using geochronology alone. Therefore, in addition to the new (and existing) age data, we present the results of major and trace element analysis of glass shards from 18 tephra horizons. For most horizons the geochemical signatures of the glass shards are internally homogeneous for both major and trace elements, suggesting a single source eruption. In a few cases, however, the tephra horizons exhibit heterogeneous geochemical signatures, which, when coupled with the deposit morphology, suggest evidence for reworking, post-depositional devitrification, and a more complex source history. We discuss correlation of the tephra horizons to onshore and offshore counterparts and also to source. Our results emphasise the importance of combining geochronological constraints with geochemical fingerprinting and additional parameters including stratigraphy to provide the most accurate correlations. As a result, we encourage the use of electronic storage files in publications to give access to full geochemical (including secondary standard data) and chronological datasets. This will not only allow complete characterisation of the tephra, but also facilitate rigorous statistical assessment to produce accurate correlation to other deposits in future studies.