Stratigraphy, age and correlation of middle Pleistocene silicic tephras in the Auckland region, New Zealand: A prolific distal record of Taupo Volcanic Zone volcanism
Bibliographic record
Abstract
Abstract Coastal sections in the Auckland region reveal highly carbonaceous and/or highly weathered clay‐dominated cover‐bed successions with numerous discrete distal volcanic ash (tephra) layers, fluvially reworked siliciclastic (tephric) deposits, and two widely distributed pyroclastic density current (PDC) deposits generated from explosive silicic volcanism within the Taupo Volcanic Zone (TVZ). The younger of the two PDC deposits (informally named Waiuku tephra) is glass‐isothermal plateau fission‐track (ITPFT) dated at 1.00 ± 0.03 Ma and occurs in a normal polarity interval interpreted as the Jaramillo Subchron. Waiuku tephra is correlated with Unit E sourced from the Mangakino Volcanic Centre of the TVZ. Waiuku tephra can be subdivided into two distinctive units enabling unequivocal field correlation: a lower stratified unit (dominantly pyroclastic surge with fall component) and an upper massive to weakly stratified unit (pyroclastic flow). At many sites in south Auckland, Waiuku tephra retains basal “surge‐like” beds (<1.4 m thickness). This provides clear evidence for primary emplacement and is an exceptional feature considering the c. 200 km this PDC has travelled from its TVZ source area. However, at many other Auckland sites, Waiuku tephra displays transitional sedimentary characteristics indicating lateral transformation from hot, gas‐supported flow/surge into water‐supported mass flow and hyperconcentrated flow (HCF) deposits. The older PDC deposit is dated at 1.21 ± 0.09 Ma, is enveloped by tephras that are ITPFT‐dated at 1.14 ± 0.06 Ma (above) and 1.21 ± 0.06 Ma (below), respectively, and occurs below a short normal polarity interval (Cobb Mountain Subchron) at c. 1.19 Ma. This PDC deposit, correlated with Ongatiti Ignimbrite sourced from the Mangakino Volcanic Centre of TVZ, has laterally transformed from a gas‐supported, fine‐grained pyroclastic flow deposit at Oruarangi, Port Waikato, into a water‐supported volcaniclastic mass flow deposit farther north at Glenbrook Beach. The occurrence of Ongatiti Ignimbrite in Auckland significantly extends its northward distribution. Large numbers of post‐ and pre‐Ongatiti rhyolitic tephra layers, ranging in age from c. 1.31 to 0.53 Ma, are also recognised in the region, with some up to 0.5 m in compacted fallout thickness. Although some tephras can be attributed to known TVZ eruptions (e.g., Ahuroa/Unit D), many have yet to be identified in proximal source areas and remain uncorrelated. However, some can be reliably correlated to tephra layers occurring in marine to nearshore sequences of Wanganui Basin and deep‐sea cores retrieved east of North Island. The identification of previously unrecognised mid‐Pleistocene TVZ‐sourced tephra deposits in the Auckland region, and their correlation to the offshore marine record, represent an advance in the construction of a higher resolution history for the TVZ where, close to eruptive source, the record is fragmentary and obscured by deep burial, or erosion, or both.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.002 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".