Paul Augustinus

Glacial valley cross-profile development: the influence of in situ rock stress and rock mass strength, with examples from the Southern Alps, New Zealand

Abstract The evolution of the glacial valley cross-profile form is commonly attributed primarily to the glaciological variables that control the erosion of the channel. However, studies in the New Zealand Southern Alps suggest that the rock mass strength (RMS) of the eroded rock mass is a major control on slope stability, and hence on the final form of the trough. RMS of the slope rock will alter with time and erosional excavation and oversteepening of the valley slopes. The in situ rock stress field induced by: (1) the extreme topography in the axial ranges of the New Zealand Southern Alps, and (2) tectonically by collision of the Australian and Pacific crustal plates, may play a role in valley slope development by controlling the location of rock failure and reducing RMS. This provides weakened rock that may provide sites for selective glacial erosion of the rock mass. Hence, valley form evolution models should also take into account the RMS and the in situ stress field acting in the eroded rock mass. This study has implications for the development and modification of alpine glacial troughs in similar tectonic settings elsewhere.

Bunger Hills, East Antarctica: Ice free at the Last Glacial Maximum

Optically stimulated luminescence dating of glaciofluvial and glacial-lake shoreline sediments indicates that the Bunger Hills area, in coastal East Antarctica, was largely ice free by the Last Glacial Maximum (LGM). Deglaciation commenced as early as 30 ka, and the southern hills were completely exposed by 20 ka. The sediments do not record evidence of an LGM readvance. Previous reconstructions of LGM ice limits for the area are incompatible with this new evidence.

A long late-Quaternary record from Lake Poukawa, Hawke's Bay, New Zealand

Abstract The Lake Poukawa Basin is a large co-seismic depression located at 20 m above mean sea level in Hawke’s Bay in eastern North Island, New Zealand. We present a detailed environmental history of the basin for the last c. 60 ka based on analyses of the top 105 m of a 200-m core record. Dating control is provided by radiocarbon, optically stimulated luminescence (OSL) and U/Th disequilibrium ages. The chronology is supported by nine tephras of inferred age including marker tephras, Kawakawa (22 590±230 yr BP at −18.25 m core datum), Tahuna (c. 35–43 ka) at −33.1 m core datum, and Rotoehu (45–50 ka) at −39.1 m core datum. Disagreements between some of the older tephra ages and the numerical ages from the OSL and U/Th dating mean that more than one age model can be applied. Three major lithostratigraphic units are identified: a basal calcareous silt with lignitic peats between 105.28 and 98.58 m of marine oxygen isotope stage (MIS) 3 age; an extended sequence of detrital shelly sands and silts, between 98.58 and 8 m of MIS 3 and 2 age; and a Holocene peat unit (MIS 1) from 8 to 0 m. Alternatively, but less likely, the basal unit may represent stage MIS 5a and the detrital shelly sands would then contain an amalgam of MIS 4, 3, and 2 deposits. We propose a notably moist phase represented by the peat which our numerical dating model places near the start of isotope stage 3. This suggests the existence of mild conditions during an interstadial in central New Zealand at c. 55–50 ka when a podocarp–beech–broadleaf forest of near-interglacial affinity surrounded the basin. The interstadial is marked by both lake and peat formation in the basin. After 50 ka a thermal decline set in, though the climate remained moist initially. Under these conditions, the Poukawa Basin was rapidly infilled by alluvial fan deposits from the surrounding hills. The floor of the basin was occupied by grasses and sedges, responding to both the highly disturbed environment and swampy conditions in the basin. After the deposition of the Rotoehu Ash, effective precipitation declined markedly and woody shrubs expanded across the previously swampy basin floor. The data suggest an apparent thermal decline of c. 6–7°C for much of MIS 2 and the latter half of MIS 3. The Holocene was marked by the establishment of fen and lake environments on the basin floor. Prior to human disturbance, podocarp–broadleaf forest surrounded the basin.

Holocene vegetation, environment, and tephra recorded from Lake Pupuke, Auckland, New Zealand

Abstract Lake Pupuke provides a near‐complete, high‐resolution environmental record of the Holocene from northern New Zealand. Tephra beds constrain the timing of a range of proxy indicators of environmental change, and demonstrate errors in a radiocarbon chronology. Agathis australis forest progressively increases from c. 7000 yr BP and, in conjunction with indicators of reduced biomass productivity, support a model of long‐term climate change to drier conditions over the Holocene. However, except for Agathis, conifer‐hardwood forest dominated mainly by Dacrydium cupressinum shows little change throughout the pre‐human Holocene, suggesting environmental stability. Dramatic vegetation change occurred only within the last millennium as a result of large‐scale Polynesian deforestation by fire. This happened a short time before the local eruption of c. 638 cal. yr BP Rangitoto Tephra. The identification of two eruptions of tephra from Rangitoto volcano has implications for future hazard planning in the Auckland region, because the volcanoes were previously considered single event centres. Changes in atmospheric circulation since the Late Glacial, possibly causing lower frequency of distal ashfall in Auckland during the Holocene, complicates the use of long‐term records in hazard frequency assessment.

Tephrostratigraphy and geochronology of a ca. 120 ka terrestrial record at Lake Poukawa, North Island, New Zealand

Abstract A 198-m-long core was obtained from Lake Poukawa, Hawkes Bay, New Zealand for paleoclimatic analysis. A chronology extending back to ca. 120 ka has been developed using a combination of tephrostratigraphy, radiocarbon, optical, and U–Th disequilbrium dating. The core contains a new record of tephra beds, including temporal intervals poorly recorded elsewhere, and revises the dispersal for some known events. Thirty macroscopic tephra beds were identified, comprising 20 rhyolites with compositions consistent with previously studied tephra from Taupo and Okataina calderas, and 10 andesites–dacites compositionally similar to Tongariro and Egmont centre eruptions. Electron microprobe data provides evidence for a total of 24 rhyolite eruptions amongst the 20 macroscopic beds. Four widespread rhyolitic marker beds: Whakatane (4.6 ka), Kawakawa (22.6 ka), Tahuna (ca. 43 ka), and Rotoehu (ca. 50 ka) provide temporal constraints for the upper 40 m of the core. The occurrence of Opepe (9 ka) and Okaia (23 ka) tephra beds in this core extends their known dispersal to southern North Island. A previously unrecognised and chemically distinct rhyolite tephra (ca. 35 ka) was also found in the sequence. Twelve rhyolitic tephra occur in the interval 50–120 ka, a period in which the timing and nature of volcanic events is poorly understood at proximal sites of the Taupo Volcanic Zone. The Lake Poukawa core provides evidence for widely dispersed tephra-producing eruptions from Egmont volcano and Tongariro centre back to about 120 ka. The tephra are glassy, unlike many proximal deposits, and can be geochemically fingerprinted, thus providing an opportunity to develop a framework for eruptions not assessable in proximal localities. The pre-50 ka, high-K Egmont tephra are compositionally similar to younger (post-30 ka) proximal pyroclastics, but differ from contemporaneous low- and medium-K rocks that characterise the proximal ring-plain of the volcano. An average Holocene peat accumulate rate of 1.5 m/ka and an average post-50 ka sedimentation rate of 0.78 m/ka are implied from the ages of interbedded tephra. However, the depositional history of the core is complex because tephra at a depth of 40 m, and optical and U–Th disequilbrium ages at ca. 103 m are the same age within analytical uncertainties. This implies either rapid alluvial sedimentation or unrecognised problems in the dating methods. U–Th disequilbrium ages, together with paleoecological information, suggest that a peat interval at 143–146 m depth formed during the last interglacial maximum (oxygen isotope substage 5e).

Luminescence studies of dunes from North-Eastern Tasmania

Abstract Northern Tasmania has a geographically extensive cover of Quaternary aeolian features and although the morphology and stratigraphy of many of these have been studied it is difficult to assign a reliable chronology because of the lack of material suitable for radiocarbon dating. The dunes are primarily composed of quartz and hence are ideally suited for the application of luminescence dating. In this paper a variety of luminescence techniques are applied to samples from the area. Analysis of the multiple aliquot OSL measurements showed that a higher precision could be obtained using the signal from the initial 6 mJ.cm−2 of the optical stimulation rather than the total light sum. The initial, near exponential. decay of the OSL signal is highly reproducible from one sub-sample to another, but the later part of the OSL decay curve, which is dominated by retrapping effects, is highly variable. The multiple aliquot OSL results are consistent with single aliquot additive dose results. The luminescence ages show that the Ainslie Linear Dunes were initially deposited by 44 ka, were mobile again at 30 ka and have been subject to reworking within the last 100–200 years. It appears that the Quaternary history of aeolian activity in this area has been more complex than was previously thought.

GPR stratigraphy of a large active dune on Parengarenga Sandspit, New Zealand

The Parengarenga Sandspit (Figure 1) is New Zealands only coastal source of silica sand for glass manufacturing. Concerns about the environmental effects of sand extraction from the harbor mouth in the last two decades led the extraction companies to instigate a monitoring program. The Parengarenga beach-dune-monitoring program started in 1982; since that time cross-profiles have been surveyed twice a year from below chart datum (CD) to landward of the sparsely vegetated foredune system. This is the longest continuous record of beach profile measurement in New Zealand, and provides important information on the behavior of this coastal spit system (Parnell, 1997). The sedimentary material in Parengarenga Sandspit consists of 93.5% quartz and has a very uniform fine, sandy grain size (median 193 microns) from near shore to the beach and in the dunes.

Outlet glacier trough size-drainage area relationships, Fiordland, New Zealand

Abstract This paper examines relationships between the area supplying ice to fiord troughs and various measures of fiord size such as depth. width and length in Fiordland, New Zealand. The size of the outlet troughs was found to be adjusted to the discharge of ice, following a similar relationship to that encountered with fiords developed under essentially cold-based ice sheets in British Columbia. However, the Fiordland outlet troughs were substantially shorter and shallower than their counterparts in British Columbia. This difference may reflect contrast in mode of erosion in each case, although the major dimensions of the fiords undoubtedly reflect the controlling influence of the pre-glacial lanscape upon which ice-caps were superimposed with their rediating outlet glaciers.

Rock Slope Development in Mcmurdo Oasis, Antarctica, and Implications for Interpretations of Glacial History

The upper slopes of the Asgard and Olympus ranges, of the Transantarctic Mountains, have been assessed for their rock mass strength. A finite-element model of two peaks has shown that under gravitational loading they have a tendency to spread laterally as a result of internal stresses. Stress release joints and slab failures are a consequence of this loading. The characteristic slope forms are cliffs in strength equilibrium surmounting Richter denudationslopes. It is argued that the ubiquity of such forms, and their antiquity, make an hypothesis of high-level glacial overriding unverifiable.

GEOMORPHIC AND SEDIMENTARY EVIDENCE OF HUMAN IMPACT ON THE NEW ZEALAND COASTAL LANDSCAPE

The coastal landscape of New Zealand has been utilized heavily by humans for the last 600 to 800 years, first by Polynesian settlers who disturbed native forests through burning and later by Europeans who continued forest burning and introduced logging and grazing in the mid-19th century. Whangape Harbor and its catchment in Northland, North Island, is an example of a heavily used coastal landscape where the impacts of human use are clearly evident on the deforested and eroding slopes of the catchment, and in the harbor where siltation is contributing to expansion of mangrove forests and a deterioration in the quality and quantity of seafood stocks. This paper documents the physical condition of Whangape Harbor and its catchment and uses sedimentological data (grain size, magnetic susceptibility, pollen) to establish links between sediment sources, pathways, and sinks. Radiocarbon dating of in situ estuarine shells suggests that sedimentation rates in the estuary have increased by an order of magnitude during the period of human occupation. We argue that human impact on Whangape Harbor has caused an acceleration of the natural process of estuary infilling, but has not controlled the type of geomorphic processes operating in the system. [Key words: Estuary, erosion, sedimentation, Maori.]