Martin P. Crundwell

New Zealand Geological Timescale NZGT 2015/1

An updated age calibration of the New Zealand Geological Timescale, NZGT 2015/1, is presented. This incorporates local stage reassessments and recent international chronostratigraphic calibrations. Of the 54 locally defined New Zealand stages, the basal ages of 17 have undergone changes in age of 1% or more since NZGT 2004 (three since NZGT 2012). Some intervals of Permian and Cretaceous time remain unrepresented by local New Zealand stages.

Middle Miocene paleoclimate change at Bryce Burn, southern New Zealand.

Abstract New Zealands isolated position in the Southwest Pacific and the quality of its marine and terrestrial sedimentary record make it a valuable source of information for studies of global climate change. A section at Bryce Burn in Southland, South Island, provides a record of middle Miocene global cooling associated with buildup of an extensive semi‐permanent ice sheet on Antarctica. The section yielded δ18O, δ13C, lithological, clay mineral, foraminiferal, and pollen data that have been linked to a high‐resolution age model based on magnetostratigraphy and foraminiferal dating. The sequence exhibits evidence for a δ18O c. 0.6‰ positive baseline shift consistent with the age and magnitude of the E3/ Mi3 isotope excursion around 13.9 Ma, carbon maxima 4–6 δ13C excursions, probable orbitally controlled changes in clay mineralogy, and short‐term changes in terrestrial vegetation. This is the first time these stable isotopic events have been recognised in outcrop sequences in New Zealand. There appears to be no clear, long‐term shift in the data associated with middle Miocene oceanic cooling, apart from the stable isotope evidence, though Milankovitch‐scale, short‐term fluctuations are present.

A new high‐resolution, middle Miocene magnetostratigraphy from western Southland, New Zealand

Abstract We present an astrochronologically constrained magnetostratigraphic age model for a middle Miocene (16–13 Ma) mudstone succession from the Bryce Burn, western Southland, New Zealand. Fourteen magnetic polarity zones were identified from magnetic remanence measurements. Eleven of the polarity zones can be correlated with the geomagnetic polarity time‐scale using four biostratigraphic tie points. The magnetostratigraphically determined sedimentation rate was verified by conducting spectral analysis of an orbitally paced magnetic susceptibility record which contains obliquity and eccentricity cycles. In the Bryce Burn succession, the Lillburnian/Clifdenian stage boundary (defined at the lowest occurrence of Orbulina suturalis) occurs within Chron C5ADn and is assigned an age of 14.450 ± 0.2 Ma, which is some 650 kyr younger than the current estimate (15.1 ± 0.34 Ma). At least two hiatuses are indicated near the base of the studied succession (between 180 and 280 m; 15.3–14.3 Ma) and these are interpreted to indicate scouring by invigorated ventilation of the deep Pacific during the cryospheric expansion at the beginning of the middle Miocene climatic deterioration.

Refined depositional history and dating of the Tongaporutuan reference section, north Taranaki, New Zealand: new volcanic ash U–Pb zircon ages, biostratigraphy and sedimentation rates

ABSTRACT This study presents new radiometric ages from volcanic ash beds within a c. 1900 m thick, progradational, deep-water clastic slope succession of late Miocene age exposed along the north Taranaki coast of the North Island, New Zealand. The ash beds yield U–Pb zircon ages ranging from 10.63 ± 0.65 Ma to 8.97 ± 0.22 Ma. The new ages are compatible with and provide corroboration of New Zealand Tongaporutuan Stage planktic foraminiferal and bolboformid biostratigraphic events identified in the same section. The close accord between these two age datasets provides a stratigraphically consistent and coherent basis for examining margin evolution. The arrival of a prograding clastic wedge and ensuing upward shoaling is recorded by sedimentation rates c. 2000 m/Ma–1 that are an order of magnitude higher than sedimentation rates on the precursor deep basin floor. This outcrop study provides new constraints for interpreting analogous subsurface deposits in Taranaki Basin and complements the regional late Miocene biostratigraphic dating framework.