Olav B. Lian

On the Interpretation of Subglacial Till Fabric

ABSTRACT The modal distribution of stone long-axis fabrics and their respective eigenvalues can be used to infer the genesis of subglacial till. In this paper we offer a two-axis diagram that compares fabric modality to fabric isotropy (S3/S1) and addresses the problem of eigenvectors falling between the modes of some well-developed till fabrics with low eigenvalues. Our simple five-fold scheme of modality categories includes: (1) unimodal clusters, (2) spread unimodal, (3) bimodal clusters, (4) spread bimodal, and (5) polymodal to girdle-like fabrics, and requires the analyst to study equal-area, lower-hemisphere (Schmidt) plots of the fabric data. After assigning the fabric to a morality category, isotropy is calculated and both results are plotted on the graph, whic helps to separate two main fields of subglacial till: (1) lodgement and subglacial meltout tills, and (2) deformation fill. On the basis of selected published fabrics from tills at modern glaciers, as well as our own Pleistocene till data, lodgement and subglacial meltout tills tend to have unimodal or bimodal fabrics. In contrast, deformation tills and tills that experienced multiple processes tend to have polymodal to girdle-like fabrics. Some overlap occurs between fields be. cause of the complex nature of till formation (i.e., because pure end-member till facies are rare and most tills are hybrids). We strongly recommend that Schmidt plots be visually analyzed and used in conjunction with eigenvalues when studying till. However, fabric data alone is not enough. Multiple criteria including structural, lithologic, and stone morphologic data from the till must also be considered before drawing conclusions on till genesis. Furthermore, if eigenvectors fall between fabric modes, then they cannot be used to indicate former ice movement directions. Finally, our new modality-isotropy diagram may have wider applications.

Reinterpretation of the glacial chronology of South Westland, New Zealand

Abstract The glacial chronostratigraphy for South Westland established by Almond in 1996 was based on the soil stratigraphy of loess coverbeds on glacial landforms in and around Saltwater Forest. Loess sheets and morphologically identified soil features were correlated with global climatic events established by the marine oxygen isotope record. Pollen analysis and few radiocarbon ages supported the chronostratigraphy. In this study, we have used total element, mineral and phytolith analysis, and luminescence dating of loess to test the current correlation of loess/soil stratigraphic units with climatic events. Data from new sections in Okarito Forest have been incorporated. Luminescence dating was found to be of limited value. Thermoluminescence dating yielded unacceptable and low‐precision ages that were anomalously old, and in strati‐graphically reversed sequence. Optical (IRSL) dating removed some of these anomalies but uncertainty about accuracy remained a problem. Some optical ages were significantly younger than associated radiocarbon ages. We suspect the highly weathered nature of the sediments being dated in combination with sediment provenance may be the cause of these effects. Routine optical dating which targets potassium feldspars or quartz cannot be applied in Westland. Loess sheets Lla, Lib, L2, L3, and L4 are now thought to be of Last (Otira) Glacial age. L 1a accumulated in marine oxygen isotope stage (OIS) 2 before 16 ka, but after 24 ka, and Lib also accumulated in OIS 2 but between 24 and c. 36 ka. L2 accumulated during OIS 3 from 45 to c. 50 ka. Loess sheets L3 and L4 are now correlated with OIS 4. Soil alteration in loess sheet L5 indicates significant weathering consistent with deposition before the last interglacial, probably during OIS 6. On the basis of the revised loess stratigraphy, glacial advances M5 and M6, and associated landforms, which have no loess cover, both occurred during OIS 2. Two glacial advances, M4a and M4b, are both assigned to OIS 3, and are thought to have occurred between 45 and 50 ka and c. 24 and 36 ka, respectively. Moraines associated with advances M2 and M3 are conformably overlain by L4 and are correlated to OIS 4. Moraine Ml, which is draped by loess sheets L1‐L5, is thought to be of Waimea Glaciation age (OIS 6). Our present work is consistent with recent studies in Fiordland, which suggest a more complex glacial stratigraphy for the Otiran than is recognised in North Westland.

Sheet-gravel evidence for a late Holocene tsunami run-up on beach dunes, Great Barrier Island, New Zealand

Abstract A semi-continuous sheet of granule to cobble-size clasts forms a distinctive deposit on sand dunes located on a coastal barrier in Whangapoua Bay, Great Barrier Island, New Zealand. The gravel sheet extends from the toe of the foredune to 14.3 m above mean sea level and 200 m landward from the beach. Clasts are rounded to sub-rounded and comprise lithologies consistent with local bedrock. Terrestrial sources for the gravel are considered highly unlikely due to the isolation of the dunes from hillslopes and streams. The only source for the clasts is the nearshore to inner shelf of Whangapoua Bay, where gravel sediments have been previously documented. The mechanism for transport of the gravel is unlikely to be storm surge due to the elevation of the deposit; maximum-recorded storm surge on this coast is 0.8 m above mean high water spring tide. Aeolian processes are also discounted due to the size of clasts and the elevation at which they occur. Tsunami is therefore considered the most probable mechanism for gravel transport. Minimum run-up height of the tsunami was 14.3 m, based on maximum elevation of gravel deposits. Optical ages on dune sands beneath and covering the gravel allow age bracketing to 0–4.7 ka. Within this time frame, numerous documented regional seismic and volcanic events could have generated the tsunami, notably submarine volcanism along the southern Kermadec arc to the east-southeast of Great Barrier Island where large magnitude events are documented for the late Holocene. Radiocarbon ages on shell from Maori middens that appear to have been reworked by tsunami run-up constrain the age of this event to post ca. 1400 AD. Regardless of the precise age of this event, the well-preserved nature of the Whangapoua gravel deposit provides for an improved understanding of the high degree of spatial variability in tsunami run-up.

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.

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).

Palaeoenvironmental reconstruction from the last interglacial using phytolith analysis, southeastern North Island, New Zealand

Phytoliths extracted from loess resting conformably on lacustrine sediments in southeastern North Island, New Zealand, provide a nearly continuous vegetation–climate record spanning the time period from the last interglacial (Marine Oxygen Isotope Stage 5) to the present. Phytoliths reveal changes in vegetation patterns following changes in climate. Correlation between tree–shrub phytolith fluctuations and the SPECMAP oxygen isotope curve, between Marine Oxygen Isotope Stages 1 and 5, suggest that changes in the ratio of arboreal to non-arboreal phytoliths directly result from changes in climate. These data generally support the existing pollen and diatom record. This study confirms the usefulness of phytolith fossils for providing environmental and climate information from the Quaternary sedimentary record, especially in cases where the deposits contain no fossil pollen or diatoms. Copyright

Holocene Record of Gradual, Catastrophic, and Human-Influenced Sedimentation from a Backbarrier Wetland, Northern New Zealand

Abstract An infilled backbarrier wetland located behind a stationary-type coastal bay barrier is used to reconstruct a 6000-year paleo-environmental record that incorporates gradual, catastrophic, and human-influenced sedimentation styles on the New Zealand coast. The record is assembled from a range of proxy indicators (grain size, magnetic susceptibility, organic content, diatoms, and pollen) and is temporally constrained by tephrochronology, radiocarbon dating, and optical dating. Postglacial sea-level rise, volcanism, tsunami, and catchment clearance are all evident in the sediment record, either as artifacts or indirect indicators. Results from optical dating also provide insights into the process of sediment reworking and mixing from multiple sources during tsunami transport. We argue that backbarrier wetlands formed behind stationary-type (aggraded) bay barriers are of greater value (more sensitive) for longer-term paleo-environmental reconstruction than wetlands associated with prograded-type and receded-type barriers, where the sediment record is typically less complete.

Geomorphological Map of Ribbed Moraines on the Dubawnt Lake Palaeo-Ice Stream Bed: A Signature of Ice Stream Shut-down?

Abstract Please click here to download the map associated with this article. The beds of active ice streams in Greenland and Antarctica are largely inaccessible, hindering a full understanding of the processes that initiate, sustain and inhibit fast ice flow in ice sheets. Detailed mapping of the glacial geomorphology of palaeo-ice stream tracks is, therefore, a valuable tool for exploring the basal processes that control their behaviour. In this paper we present a map that shows detailed glacial geomorphology from a part of the Dubawnt Lake Palaeo-Ice Stream bed on the north-western Canadian Shield (Northwest Territories), which operated at the end of the last glacial cycle. The map (centred on 63° 55″ 42′N, 102° 29″ 11′W, approximate scale 1:90,000) was compiled from digital Landsat Enhanced Thematic Mapper Plus satellite imagery and digital and hard-copy stereo-aerial photographs. The ice stream bed is dominated by parallel mega-scale glacial lineations (MGSL), whose lengths exceed several kilometres but the map also reveals that they have, in places, been superimposed with transverse ridges known as ribbed moraines. The ribbed moraines lie on top of the MSGL and appear to have segmented the individual lineaments. This indicates that formation of the ribbed moraines post-date the formation of the MSGL. The presence of ribbed moraine in the onset zone of another palaeo-ice stream has been linked to oscillations between cold and warm-based ice and/or a patchwork of cold-based areas which led to acceleration and deceleration of ice velocity. Our hypothesis is that the ribbed moraines on the Dubawnt Lake Ice Stream bed are a manifestation of the process that led to ice stream shut-down and may be associated with the process of basal freeze-on. The precise formation of ribbed moraines, however, remains open to debate and field observation of their structure will provide valuable data for formal testing of models of their formation.

Dating techniques: Illuminating the past.

The technique of optical dating was first reported 30 years ago, and has since revolutionized studies of events that occurred during the past 500,000 years. Here, two practitioners of optical dating assess its impact and consider its future.

Palliser’s Triangle: Reconstructing the ‘central desert’ of the southwestern Canadian prairies during the late 1850s

Between 1857 and 1860 the British North American Expedition, led by Captain John Palliser, explored and surveyed the Canadian Prairies primarily to establish its suitability for agriculture and settlement. Historical and paleoclimate records indicate the Expedition coincided with below normal precipitation, leading to the perception of an arid or semi-arid region that would ‘forever be comparatively useless’ for agriculture. Today, this part of the Canadian Prairies is known as the Palliser Triangle, and is Canada’s productive dryland agricultural region. Here we present historic, geomorphologic, and chronometric evidence to reconstruct the landscape encountered by the Expedition. We contend that Palliser’s perception of the region was strongly influenced by his experience travelling through active sand dunes in the Middle Sand Hills of southeastern Alberta. At present, the dunes are entirely stabilized by vegetation, in contrast to Palliser’s report of ‘miles of burning sand’. Archival aerial photographs and optical ages of near-surface samples are used to reconstruct the landscape encountered by the Expedition in the Middle Sand Hills. Optical ages of presently stabilized sand dunes date primarily to between ad 1850 and 1934, peaking in c. ad 1925, and are indicative of a dune field undergoing reduction in activity, prior to the onset of 20th century droughts. Ages of interdune sand sheets further attest to regional dune activity occurring at least since ad 1750, concurrent with activity in other southern Canadian Prairie dune fields. Collectively, this evidence supports observations by Palliser of severe travelling due to bare sand conditions in 1857–1859. These conditions and Palliser’s inference of their extent influenced his perception of a ‘central desert’, thus delaying construction of the Canadian Pacific Railway along a southern route and postponing the westward colonization of Canada.