Hugh E. G. Morgans

Global dinoflagellate event associated with the late Paleocene thermal maximum

The late Paleocene thermal maximum, or LPTM (ca. 55 Ma), represents a geologically brief time interval (∼220 k.y.) characterized by profound global warming and associated environmental change. The LPTM is marked by a prominent negative carbon isotope excursion (CIE) interpreted to reflect a massive and abrupt input of 12 C-enriched carbon to the ocean-atmosphere reservoir, possibly as a result of catastrophic gas-hydrate release, on time scales equivalent to present-day rates of anthropogenic carbon input. The LPTM corresponds to important changes in the global distribution of biota, including mass extinction of marine benthic organisms. The dinoflagellate cyst record indicates that surface- dwelling marine plankton in marginal seas also underwent significant perturbations during the LPTM. We report on the dramatic response of representatives of the genus Apectodinium from two upper Paleocene–lower Eocene sections in the Southern (New Zealand) and Northern (Austria) Hemispheres, where the dinoflagellate records are directly correlated with the CIE, benthic foraminifera extinction event, and calcareous nannofossil zonation. The results indicate that the inception of Apectodinium -dominated assemblages appears to be synchronous on a global scale, and that the event is precisely coincident with the beginning of the LPTM. Apectodinium markedly declined in abundance near the end of the LPTM. This Apectodinium event may be associated with (1) exceptionally high global sea-surface temperatures and/or (2) a significant increase in marginal-marine surface-water productivity. Such a globally synchronous acme of dinoflagellate cysts is unprecedented within the dinoflagellate cyst fossil record.

Tropical sea temperatures in the high-latitude South Pacific during the Eocene

Sea-surface temperature (SST) estimates of ~30 °C from planktic foraminifera and archaeal membrane lipids in bathyal sediments in the Canterbury Basin, New Zealand, support paleontological evidence for a warm subtropical to tropical climate in the early Eocene high-latitude (55°S) southwest Pacific. Such warm SSTs call into question previous estimates based on oxygen isotopes and present a major challenge to climate modelers. Even under hypergreenhouse conditions (2240 ppm CO2), modeled summer SSTs for the New Zealand region do not exceed 20 °C.

Middle Eocene climate cyclicity in the southern Pacific: Implications for global ice volume

We use a multiproxy approach to determine surface and bottom water temperatures off the coast of New Zealand during the middle Eocene and to constrain the δ18O of seawater. We use these data to place constraints on the size and variability of global ice sheets at that time. The Hampden Section in South Island is characterized by exceptionally well preserved micro fossils and clear sedimentary cyclicity, providing a remarkable window into conditions at paleo latitude ~55°S in the Pacific Ocean. The cyclicity was studied in detail over a ~4 m section corresponding to an interval of ~70 k.y., ca. 41.7 Ma. The sedimentary cycles are defined by fluctuations in the sand (>63 μm) component, occurring on a wavelength of ~1 m, corresponding to Milankovitch-scale frequency. Analyses of foraminifer oxygen isotopic (δ18 O) and Mg/Ca composition, combined with TEX86 analyses from organic carbon, are used to generate records of seawater temperature and oxygen isotopic composition (δ18OSW). These indicate bottom water temperatures of ~11–13 °C and sea surface temperatures of ~23–25 °C with good agreement between the proxies. Temperature cyclicity with a magnitude of ~1.5 °C occurs in both surface and bottom waters, approximately in phase with the sedimentary cycles. Estimates of δ18OSW have a mean value of −1.2‰ throughout the study section. Taken together, the data suggest a largely ice-free world with orbital-scale cycles expressed as temperature and hydrological variation with little or no ice volume change.

Quantitative biostratigraphy of the Taranaki Basin, New Zealand: A deterministic and probabilistic approach

A quantitative biostratigraphic analysis of the Paleocene to lower Miocene of the Taranaki Basin has enabled high precision in correlation, zonation, and assessment of depositional history. Biostratigraphic range-end events, based on 493 taxa in cuttings samples from eight wells, representing foraminifera, nannofossils, dinoflagellates, and miospores, were culled to 87 range-top events that were then analyzed by deterministic (constrained optimization [CONOP]) and probabilistic (ranking and scaling [RASC]) techniques. All except 16 of the events are found to have relatively good biostratigraphic reliability. The RASC probable sequence and probabilistic zonation give the best estimate of the sequence of events and zones to be encountered in any new well in the basin and a precise biostratigraphic scale for future exploration. The CONOP composite section, which matches well with that derived by conventional graphic correlation (GRAPHCOR), is readily related to previous zonations based on maximum ranges of taxa but gives an order-of-magnitude greater precision. CONOP provides a precise correlation framework and reveals marked variation in thickness of stages across the basin. When the composite section is calibrated against the time scale, basinwide changes in depositional rate are revealed. The upper Eocene and Oligocene mark an interval of slow deposition, whereas the Miocene marks a sharp increase in deposition. The time-calibrated composite section enables unconformities and changes in depositional rate found in individual wells to be precisely estimated. Many new unconformities are indicated, particularly in the Paleocene and Eocene.

Early Miocene thin‐skinned tectonics and wrench faulting in the Pongaroa district, Hikurangi margin, North Island, New Zealand

Abstract The Pongaroa‐Akitio area, Northern Wairarapa, North Island, New Zealand, is part of the exposed East Coast Deformed Belt at the obliquely convergent plate boundary of the Hikurangi margin. The sedimentary succession includes an allochthonous unit of Early Cretaceous greywacke basement resting on latest Cretaceous rocks. Since the units basal contact is subparallel to the bedding of the strata it overlies, the allochthon is inferred to be an unrooted gliding nappe similar to allochthonous outliers described in Northland and the Raukumara Peninsula. The southward emplacement of this “Greywacke Nappe” is supported by structural markers in the body of the nappe and is well dated as earliest Miocene by the youngest rocks involved, which are earliest Miocene (Waitakian; c. Aquitanian), and because Otaian‐Altonian (c. Burdigalian) faults postdate nappe emplacement. This thin‐skinned tectonic phase immediately preceded inception of dextral strike‐slip faulting along northeast‐trending Otaian‐Altonian (B...

Early Paleogene evolution of terrestrial climate in the SW Pacific, Southern New Zealand

[1] We present a long-term record of terrestrial climate change for the Early Paleogene of the Southern Hemisphere that complements previously reported marine temperature records. Using the MBT′-CBT proxy, based on the distribution of soil bacterial glycerol dialkyl glycerol tetraether lipids, we reconstructed mean annual air temperature (MAT) from the Middle Paleocene to Middle Eocene (62–42 Ma) for southern New Zealand. This record is consistent with temperature estimates derived from leaf fossils and palynology, as well as previously published MBT′-CBT records, which provides confidence in absolute temperature estimates. Our record indicates that through this interval, temperatures were typically 5°C warmer than those of today at such latitudes, with more pronounced warming during the Early Eocene Climate Optimum (EECO; ∼50 Ma) when MAT was ∼20°C. Moreover, the EECO MATs are similar to those determined for Antarctica, with a weak high-latitude terrestrial temperature gradient (∼5°C) developing by the Middle Eocene. We also document a short-lived cooling episode in the early Late Paleocene when MAT was comparable to present. This record corroborates the trends documented by sea surface temperature (SST) proxies, although absolute SSTs are up to 6°C warmer than MATs. Although the high-calibration error of the MBT′-CBT proxy dictates caution, the good match between our MAT results and modeled temperatures supports the suggestion that SST records suffer from a warm (summer?) bias, particularly during times of peak warming.

An integrated sequence stratigraphic, palaeoenvironmental, and chronostratigraphic analysis of the Tangahoe Formation, southern Taranaki coast, with implications for mid-Pliocene (c. 3.4-3.0 Ma) glacio-eustatic sea-level changes

Abstract Sediments of the mid‐Pliocene (c. 3.4–3.0 Ma) Tangahoe Formation exposed in cliffs along the South Taranaki coastline of New Zealand comprise a 270 m thick, cyclothemic shallow‐marine succession that has been gently warped into a north to south trending, low angle anticline. This study examines the sedimentologic, faunal, and petrographic characteristics of 10 Milankovitch‐scale (6th order), shallow‐marine depositional sequences exposed on the western limb of the anticline. The sequences are recognised on the basis of the cyclic vertical stacking of their constituent lithofacies, which are bound by sharp wave cut surfaces produced during transgressive shoreface erosion. Each sequence comprises three parts: (1) a 0.2–2 m thick, deepening upwards, basal suite of reworked bioclastic lag deposits (onlap shellbed) and/or an overlying matrix supported, molluscan shellbed of offshore shelf affinity (backlap shellbed); (2) a 5–20 m thick, gradually shoaling, aggradational siltstone succession; and (3) a 5–10 m thick, strongly progradational, well sorted “forced regressive” shoreline sandstone. The three‐fold subdivision corresponds to transgressive, highstand, and regressive systems tracts (TSTs, HSTs, and RSTs) respectively, and represents deposition during a glacio‐eustatic sea‐level cycle. Lowstand systems tract sediments are not recorded because the outcrop is situated c. 100 km east of the contemporary shelf edge and was subaerially exposed at that time. Well developed, sharp‐ and gradational‐based forced regressive sandstones contain a variety of storm‐emplaced sedimentary structures, and represent the rapid and abrupt basinward translation of the shoreline on to a storm dominated, shallow shelf during eustatic sea‐level fall. Increased supply of sediment from north‐west South Island during “forced regression” is indicated from petrographic analyses of the heavy mineralogy of the sandstones. A chronology based on biostratigraphy and the correlation of a new magnetostratigraphy to the magnetic polarity timescale allows: (1) identification of the Mammoth (C2An.2r) and Kaena (C2An. 1r) subchrons; (2) correlation of the coastal section to the Waipipian Stage; and (3) estimation of the age of the coastal section as 3.36–3.06 Ma. Qualitative assessment of foraminiferal census data and molluscan palaeoecology reveals cyclic changes in water depth from shelf to shoreline environments during the deposition of each sequence. Seven major cycles in water depth of between 20 and 50 m have been correlated to individual 40 ka glacio‐eustatic sea‐level cycles on the marine oxygen isotope timescale. The coastal Tangahoe Formation provides a shallow‐marine record of global glacio‐eustasy prior to the development of significant ice sheets on Northern Hemisphere continents, and supports evidence from marine δ18O archives that changes in Antarctic ice volume were occurring during the Pliocene.

Identification of a Waipawa Formation equivalent in the upper Te Uri Member of the Whangai Formation - implications for depositional history and age

Abstract Stable isotopes and biomarkers have identified a unit with similar organic geochemistry to the Waipawa Formation, in the upper Te Uri Member of the Whangai Formation, exposed in the Akitio River, at Tawanui, southern Hawkes Bay, New Zealand. At Tawanui, the uppermost greensand of the Te Uri Member contains a large positive 813C isotopic excursion from ‐27.0%o to ‐20%e and an increase in total organic carbon from 0.1% to 1.0%. Biomarker analyses demonstrate a similar C30 sterane fingerprint to other deposits of the Waipawa Formation. We propose that the uppermost greensand of the Te Uri Member at Tawanui is a condensed stratigraphic equivalent of the Waipawa Formation at nearby Angora Stream and other East Coast Basin localities. This correlation demonstrates that Waipawa Formation is middle Teurian (middle Paleocene) and precedes the late Paleocene thermal maximum event by c. 5 m.y. The likely upwelling event that resulted in deposition of the Waipawa Formation was geographically widespread but probably restricted to the outer shelf/upper slope. In places, biogenic activity prevented the preservation of organic carbon in equivalent condensed stratigraphic intervals. Localised restriction of upwelling and black shale deposition may be demonstrated by the occurrence of a thick black shale at Angora Stream only c. 10 km from the coeval greensands at Tawanui. Alternatively, Oligocene‐Miocene east‐west shortening and structural reorganisation in the East Coast Basin may have juxtaposed facies that were originally many tens of kilometres apart. Our correlation also implies that the Te Uri Member is diachronous. It may have been on the outermost shelf to upper slope during lowstand conditions, where it is oldest, to higher on the shelf during transgression and highstand conditions.

Integrated stratigraphy of the Waitakian‐Otaian Stage boundary stratotype, Early Miocene, New Zealand

Abstract The base of the type section of the Otaian Stage at Bluecliffs, South Canterbury, is recognised as the stratotype for the boundary between the Waitakian and Otaian Stages. Principal problems with the boundary are the restriction of existing bioevent proxies to shelf and upper slope environments and its uncertain age. These topics are addressed by a multidisplinary study of a 125 m section about the boundary, which examines its lithostratigraphy, depositional setting, biostratigraphy, correlation, and geochronology. The lower siltstone lithofacies (0–38.5 m) was deposited at upper bathyal depths (200–600 m) in a marginal basin which was partially sheltered from fully oceanic circulation by a submarine high and islands. The site was covered by cool‐temperate water and was probably adjacent to the Subtropical Convergence. This unit is succeeded by the banded lithofacies (38.5–106 m) and the upper siltstone lithofacies (basal 19 m studied). Paleodepth probably declined up‐sequence, but deposition at ...

Strontium isotope stratigraphy of the Oligocene‐Miocene Otekaike Limestone (Trig Z section) in southern New Zealand: Age of the Duntroonian/Waitakian Stage boundary

Abstract The Otekaike Limestone Trig Z sequence in southeastern South Island, New Zealand, is the type section for the local Waitakian Stage, and contains the Duntroonian/ Waitakian Stage boundary, previously thought equivalent to the Oligocene/Miocene Epoch boundary. Strontium isotope analysis of molluscan (pectinid) macrofossil fragments and mainly benthic foraminifera from the Otekaike Limestone has, together with foraminiferal and nannofossil bio‐stratigraphy, provided a chronology for the section and established a correlation with the global time‐scale. Strontium isotope ratios of macrofossil and foraminiferal extracts are identical and indicate an age for the section ranging from c. 25.5 Ma at the base to c. 23.7 Ma at the top, placing most of the section within the late Oligocene (Chattian). The Duntroonian/Waitakian boundary is set at c. 25.2 Ma (intra‐Chattian) and is not coeval with the Oligocene/Miocene boundary (23.8 Ma), which occurs near the top of the section.