Greg H. Browne

The stratigraphic signature of the late Cenozoic Antarctic Ice Sheets in the Ross Embayment

A 1284.87-m-long sediment core (AND-1B) from beneath the McMurdo sector of the Ross Ice Shelf provides the most complete single section record to date of fluctuations of the Antarctic Ice Sheets over the last 13 Ma. The core contains a succession of subglacial, glacimarine, and marine sediments that comprise ∼58 depositional sequences of possible orbital-scale duration. These cycles are constrained by a chronology based on biostratigraphic, magnetostratigraphic, and 40 Ar/ 39 Ar isotopic ages. Each sequence represents a record of a grounded ice-sheet advance and retreat cycle over the AND-1B drill site, and all sediments represent subglacial or marine deposystems with no subaerial exposure surfaces or terrestrial deposits. On the basis of characteristic facies within these sequences, and through comparison with sedimentation in modern glacial environments from various climatic and glacial settings, we identify three facies associations or sequence “motifs” that are linked to major changes in ice-sheet volume, glacial thermal regime, and climate. Sequence motif 1 is documented in the late Pleistocene and in the early Late Miocene intervals of AND-1B, and it is dominated by diamictite of subglacial origin overlain by thin mudstones interpreted as ice-shelf deposits. Motif 1 sequences lack evidence of subglacial meltwater and represent glaciation under cold, “polar”-type conditions. Motif 2 sequences were deposited during the Pliocene and early Pleistocene section of AND-1B and are characterized by subglacial diamictite overlain by a relatively thin proglacial-marine succession of mudstone-rich facies deposited during glacial retreat. Glacial minima are represented by diatom-bearing mudstone, and diatomite. Motif 2 represents glacial retreat and advance under a “subpolar” to “polar” style of glaciation that was warmer than present, but that had limited amounts of subglacial meltwater. Sequence motif 3 consists of subglacial diamictite that grades upward into a 5- to 10-m-thick proglacial retreat succession of stratified diamictite, graded conglomerate and sandstone, graded sandstone, and/or rhythmically stratified mudstone. Thick mudstone intervals, rather than diatomite-dominated deposition during glacial minima, suggest increased input of meltwater from nearby terrestrial sources during glacial minima. Motif 3 represents Late Miocene “subpolar”-style glaciation with significant volumes of glacially derived meltwater.

Facies development and sequence architecture of a late Quaternary fluvial-marine transition, Canterbury Plains and shelf, New Zealand: implications for forced regressive deposits

Abstract The Canterbury Plains, South Island, New Zealand, comprise a c. 7500 km2 coarse-grained, braidplain that accumulated during Quaternary glacio-eustatic, sea-level fluctuations. The adjacent Canterbury Bight shelf covering c.13,000 km2, comprises coeval shelf–slope deposits, that are punctuated by advances of the braidplain onto the shelf during periods of sea-level fall. This study examines the sedimentological and stratal characteristics of outcropping last glacial braidplain deposits, and then traces oscillations in the position of the fluvial-marine transition over several late Quaternary sea-level cycles using high-resolution seismic reflection profiles of the Canterbury shelf and slope. Outcropping last glacial Burnham Formation sediments display numerous, aggradationally stacked massive and cross-stratified gravel deposits with minor intercalated sand and mud. The gravels accumulated as longitudinal bars and channel fills within an extensive braidplain succession, with some evidence of frozen ground conditions during deposition based on sedimentological features. High-frequency (3.5 kHz) seismic reflection data of the subsurface Canterbury shelf identify up to seven unconformity-bound, Milankovitch-duration depositional sequences. These sequences are inferred to correlate with successive 100-ka, sea-level cycles spanning Oxygen Isotope Stages 16 to 1 (last c. 700 ka). Each sequence displays a distinctive stratigraphic motif comprising four recurring seismic units: 1. Basinward of the glacial maximum shoreline, wedge-shaped units displaying steeply dipping clinoforms that onlap the continental slope are interpreted as “perched lowstand deltas” belonging to the lowstand prograding wedge systems tract (LST). 2. Irregular hummocky units up to 10 m thick, containing high-amplitude discontinuous reflectors, are interpreted as representing stranded coastal deposits of the transgressive systems tract (TST). 3. Low-amplitude seismic units which offlap and downlap onto the TST, infilling local paleotopography, and interpreted as comprising fine-grained marine sediments of the highstand systems tract (HST). 4. Basinward thickening units (up to 40 m thick), containing a strongly progradational series of offlapping, inclined (0.5–1.0°), high-amplitude reflectors, that downstep towards the basin are interpreted as coarse-grained, fluvio-deltaic sediments, similar to the last glacial Burnham Formation, deposited during glacio-eustatic sea-level fall, or forced regression. We assign this unit to the regressive systems tract (RST), which displays a gradational lower boundary overlain by a sharp planar regionally extensive sequence boundary or ravinement surface. 2-D forward stratigraphic modelling, constrained by outcrop and seismic data, indicates that rivers of the Canterbury region did not incise during eustatic sea-level fall. This may be the case elsewhere, too, where a coastal plain is flanked by a lower gradient shelf. On the Canterbury shelf, fluvial incision did not occur during Quaternary forced regressions, but instead, subaerial accommodation was created and filled in by thick, fluvio-deltaic deposits, as contemporary rivers graded to the glacial maximum shoreline. Incision was restricted to three zones: (1) The lowstand shelf break, where canyons of limited extent formed by nickpoint retreat; (2) the transgressive coastline, where rivers incised due to coastal erosion; and (3) the inner braidplain adjacent to the Southern Alps, where degradation was caused by tectonic uplift.

Early and middle Miocene Antarctic glacial history from the sedimentary facies distribution in the AND-2A drill hole, Ross Sea, Antarctica

In 2007, the Antarctic Geological Drilling Program (ANDRILL) drilled 1138.54 m of strata ~10 km off the East Antarctic coast, includ ing an expanded early to middle Miocene succession not previously recovered from the Antarctic continental shelf. Here, we pre sent a facies model, distribution, and paleoclimatic interpretation for the AND-2A drill hole, which enable us, for the fi rst time, to reconstruct periods of early and middle Miocene glacial advance and retreat and paleo environmental changes at an ice-proximal site. Three types of facies associations can be recognized that imply signifi cantly different paleoclimatic interpretations. (1) A diamictite-dominated facies association represents glacially dominated depositional environments, including subglacial environments, with only brief intervals where ice-free coasts existed, and periods when the ice sheet was periodically larger than the modern ice sheet. (2) A stratifi ed diamictite and mudstone facies association includes facies characteristic of open-marine to iceberg-infl uenced depositional environments and is more consistent with a very dynamic ice sheet, with a grounding line south of the modern position. (3) A mudstone-dominated facies association generally lacks diamictites and was produced in a glacially infl uenced hemipelagic depositional environment. Based on the distribution of these facies associations, we can conclude that the Antarctic ice sheets were dynamic, with grounding lines south of the modern location at ca. 20.1‐19.6 Ma and ca. 19.3‐18.7 Ma and during the Miocene climatic optimum, ca. 17.6‐15.4 Ma, with ice-sheet and sea-ice minima at ca. 16.5‐16.3 Ma and ca. 15.7‐15.6 Ma. While glacial minima at ca. 20.1‐19.6 Ma and ca. 19.3‐18.7 Ma were characterized by temperate margins, an increased abundance of gravelly facies and diatomaceous siltstone and a lack of meltwater plume deposits suggest a cooler and drier climate with polythermal conditions for the Miocene climatic optimum (ca. 17.6‐15.4 Ma). Several periods of major ice growth with a grounding line traversing the drill site are recognized between ca. 20.2 and 17.6 Ma, and after ca. 15.4 Ma, with evidence of cold polar glaciers with ice shelves. The AND-2A core provides proximal evidence that during the middle Miocene climate transition, an ice sheet larger than the modern ice sheet was already present by ca. 14.7 Ma, ~1 m.y. earlier than generally inferred from deep-sea oxygen isotope records. These fi ndings highlight the importance of high-latitude ice-proximal records for the interpretation of far-fi eld proxies across major climate transitions.

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.

Late Neogene sedimentation adjacent to the tectonically evolving North Island axial ranges: Insights from Kuripapango, western Hawke's Bay

Abstract Kuripapango‐Blowhard is a relatively small region in western Hawkes Bay, but one that displays a stratigraphically diverse Neogene sedimentary record. Earliest sedimentation on greywacke basement consisted of late Miocene (?Tongaporutuan‐Kapitean) conglomerate, sandstone, and calcareous sandstone of the Blowhard Formation (new). Deposition of siliciclastic and mixed carbonate sediments of the Mangatoro, Te Waka, Kaumatua, and Sentry Box Limestone Formations continued through to late Pliocene (Nukumaruan) time. These sediments are divided into a broadly transgressive to regressive succession that is up to 2 km thick. The lithologies indicate a strong link between sediment supply and active tectonism through time. An initial pulse of uplift is suggested during the late Miocene, followed by widespread subsidence and relative tectonic quiescence. A later, and temporally distinctive period of late Pliocene tectonic uplift in the region has continued to the present day. Contrasts in the sedimentary record in adjacent fault‐bounded blocks suggest different sedimentary histories related to whether the fault blocks were uplifted and eroded, or were subsiding and were depocentres for sediment.

Widespread compression associated with Eocene Tonga-Kermadec subduction initiation

Eocene onset of subduction in the western Pacific was accompanied by a global reorganization of tectonic plates and a change in Pacific plate motion relative to hotspots during the period 52–43 Ma. We present seismic-reflection and rock sample data from the Tasman Sea that demonstrate that there was a period of widespread Eocene continental and oceanic compressional plate failure after 53–48 Ma that lasted until at least 37–34 Ma. We call this the Tectonic Event of the Cenozoic in the Tasman Area (TECTA). Its compressional nature is different from coeval tensile stresses and back-arc opening after 50 Ma in the Izu-Bonin-Mariana region. Our observations imply that spatial and temporal patterns of stress evolution during western Pacific Eocene subduction initiation were more varied than previously recognized. The evolving Eocene geometry of plates and boundaries played an important role in determining regional differences in stress state.

Grain‐size characteristics for distinguishing basin floor fan and slope fan depositional settings: Outcrop and subsurface examples from the late Miocene Mount Messenger Formation, New Zealand

Abstract An outcrop section of late Miocene deep‐water sediments of the Mount Messenger Formation in Taranaki, New Zealand, displays distinctive physical sedimentary features that allow differentiation of basin floor and slope fan depositional units. Sandstone grain‐size characteristics have been examined in this study to differentiate these two types of deep‐water deposits. Outcrop data indicate that basin floor fan sandstones are relatively sand rich in comparison to silt‐rich slope fan sandstones. Both basin floor and slope fan sandstones show better sorting with increasing grain size, though cross‐plots show the nature of this relationship differs slightly for basin floor and slope fan samples. These relationships appear to hold for both outcrop and subsurface sandstone samples from the formation. This finding is unexpected given the c. 600 m stratigraphic thickness of the formation, representing several million years of depositional history, and implies a uniform sediment texture was supplied to the basin through time. The differentiation of basin floor fan and slope fan deposits is significant especially in subsurface settings involving petroleum well data. Hydrocarbon exploration strategies will vary markedly for basin floor fan versus slope fan reservoirs, making such differentiation of lithofacies types important to optimise hydrocarbon discovery. With subsurface data, the interpretation of these two reservoir sandstone lithofacies is often difficult to make. The grain‐size changes appear to mimic the contrasting depositional mechanisms operative in these two deep‐water settings.

The geological setting of the Darfield and Christchurch earthquakes

Abstract The 2010–2011 Canterbury earthquake sequence occurred near the southeastern margin of Neogene deformation associated with the Australia–Pacific plate boundary. Basement comprises indurated rocks of the Torlesse Composite Terrane, of Permian to Early Cretaceous age, overlain by 1–2 km of less-indurated Cretaceous–Neogene rocks and unconsolidated Quaternary sediments. Proximity to the subduction interface between Gondwana and the paleo-Pacific Ocean produced a Mesozoic-age structural grain in the basement rocks, aligned broadly east–west in the Canterbury to Chatham Rise areas. These structures provided an inherited weakness that was likely reactivated by present-day stress. Mid- to Late Cretaceous extension, marked by localised fault-bounded grabens, was followed by deposition of a Late Cretaceous to Paleogene passive-margin transgressive sedimentary sheet and minor intraplate basaltic volcanics. Mid-Cenozoic inception of the modern Australia–Pacific plate boundary heralded deposition of a regressive succession of Neogene sediments and further episodes of volcanism, most notably constructing the Late Miocene Banks Peninsula intraplate volcanoes. The east- to northeast-striking faults associated with the Darfield and Christchurch earthquakes are probably aligned with the Mesozoic structural grain within the Torlesse basement rocks.

An outcrop‐based study of the economically significant Late Cretaceous Rakopi Formation, northwest Nelson, Taranaki Basin, New Zealand

Abstract The Late Cretaceous Rakopi Formation (Pakawau Group) represents one of the most important petroleum source rock units and a potential reservoir unit in the highly prospective Taranaki Basin. This paper presents a predominantly outcrop‐based study of the sedimentology, petrography, stratigraphy, and depositional environment of the Rakopi Formation in the Paturau River and Pakawau areas of northwest Nelson, southern Taranaki Basin, together with some preliminary insights into the stratigraphie architecture of the Pakawau Group on a more basin‐wide scale. The Rakopi Formation is interpreted here as a terrestrial deposit, representing sedimentation in fluvial channels and their associated overbank and levee environments. However, the presence of dinoflagellates, glauconite, and elevated coal seam sulfur contents is evidence for periodic marine influence during deposition. This could be explained by a low‐gradient coastal plain paleogeography, crossed by a series of rivers and their associated floodplain deposits, episodically inundated by marine incursions during successive transgressions. A modern analogue setting from the present‐day Hauraki Graben, North Island, New Zealand, indicates that marine influence within coastal plain systems can extend several tens of kilometres inland. Given such a physiography, relatively small increases in relative sea level could potentially move the shoreline several kilometres or tens of kilometres farther inland, sufficient to introduce the type of marine influence on sedimentation that we suggest for the Rakopi Formation. The results from this study suggest a greater marine influence within the Rakopi Formation, northward into the greater Taranaki Basin, than has previously been recognised. This raises the possibility of both different reservoir facies as well as potentially a greater proportion of marine mudstones, which would have implications for both reservoir and trapping of hydrocarbons. In addition, marine‐influenced coaly rocks within the Rakopi Formation are expected to have greater petroleum generative potentials and to be more oil‐prone than their fully non‐marine counterparts.

Continental margin sedimentation to be studied in New Zealand

The National Science Foundations (NSF) MARGINS research program examines the processes governing continental margin evolution through four science initiatives. By promoting research strategies that redirect traditional approaches, these initiatives will help to stimulate coordinated, interdisciplinary research in a few focus areas over the next decade. For the MARGINS Source-to-Sink science initiative, New Zealand and New Guinea have been selected as the two primary focus areas (http://www.ldeo.columbiaedu/margins/SedStrat.html).