A Younger Dryas (Loch Lomond Stadial) jökulhlaup deposit, Fort Augustus, Scotland

This study provides an insight into the impact of probably the largest flood ever to have been identified in mainland Britain by examining new sedimentary evidence from the Auchteraw terrace, Fort Augustus, Scotland. Study of three sections reveals a succession consisting of: (1) sheet gravels; (2) large trough-shaped depressions infilled with cross-stratified sands and gravels; (3) smaller-scale, finer-grained cross-strata; and (4) sheet-like, occasionally channelized, bimodal sand and gravel beds. This study shows that both the sedimentology and morphology of the Auchteraw terrace are consistent with jökulhlaup deposition and reveal a greater variety of lithofacies types than identified in previous studies of jökulhlaups from ice-dammed lakes. The fine-grained nature of the sediment discussed in this study emphasizes the importance of sediment supply for the formation of distinctive jökulhlaup successions. The sedimentary evidence recorded here provides a valuable tool for the interpretation of the magnitude and frequency of proglacial meltwater flows associated with Pleistocene ice sheets worldwide.     

De-bugging the 'millipede' porphyroblast microstructure: a serial thin-section study and 3-D computer animation

ABSTRACT Seventy-five spatially orientated, serial thin sections cut from a single rock containing ‘millipede’ porphyroblast microstructure from the Robertson River Metamorphics, Australia, reveal the three-dimensional (3-D) geometry of oppositely concave microfolds (OCMs) that define the microstructure. Electronic animations showing progressive serial sections of the 3-D microstructure are made available via the World Wide Web. The OCM amplitudes decrease regularly from a maximum in near-central sections to a minimum in near-marginal sections, whereas the OCM interlimb angles increase regularly from a minimum in near-central sections to a maximum in near-marginal sections. These observations illustrate that the OCMs are noncylindrical surfaces with culminations located in near-central sections. Because the porphyroblast cores appear to have been present before significant development of the syn-OCM foliation, all of the OCMs were formed by heterogeneous extension around these cores. The overall geometry of the OCMs described here reflects the strain state, and cannot be used to constrain deformation paths.     

Last Interglacial stratigraphy at Ristinge Klint, South Denmark

Past environmental changes in the Baltic area are discussed on the basis of foraminifera and ostracods as well as pollen and spores in marine sediments in cliff sections at Ristinge Klint, Langeland, southern Denmark. The sediment succession represents Jessen & Milthers' (1928) pollen zones d-g or Andersen's (1961, 1975) zones E2-E5, and a correlation with the annually laminated Bispingen sequence indicates that the sequence spans about 3400 years. Marine conditions seem to have occurred at c. 300-365 years after the beginning of the Eemian Interglacial, close to fully marine conditions developing by c. 2500 years. This early date of the marine ingression pre-dates that of most previous studies in the region by several hundred years, but it post-dates the initial marine ingression in the easternmost Baltic. A marked change in salinity at c. 650 years after the beginning of the Eemian was presumably caused by an opening of the Danish Belts. An indication of a major alteration in current activity is registered at c. 3000 years after the beginning of the interglacial. The recognition of the relative timing of these events may be significant for the understanding of the opening of connections between the North Sea, the Baltic and the White Sea.     

Sheet slides and rotational failures on a convergent margin: the Kidnappers Slide, New Zealand

The Kidnappers Slide, on the upper continental slope of the convergent margin off eastern North Island, New Zealand, has been re-examined using a grid of high-resolution seismic reflection profiles. The slide is not a single feature but a complex of sheet slides and rotational failures ranging from 20 to 140 m thick, and covering a total area of 720 km². Failures occurred in several phases, on slopes of 1–5°, in late Quaternary, muddy, shelf-edge clinoforms that have prograded into an accretionary, trench-slope basin. Piston cores and seismic stratigraphy show that the main failure probably occurred in early Holocene times but that movements ranged from mid last glacial to late Holocene times. The sheet slides exhibit tensional collapse via numerous listric normal faults that sole out on glide planes; there is no clear evidence of compressional structures anywhere within the complex. The glide planes occur at progressively deeper stratigraphic levels towards the northeastern end of the complex, and near the steep slope that defines the seaward edge of the trench-slope basin. There is retrogressive failure at the top of the slope. The surficial slides are being deformed by growth of active tectonic faults and folds associated with the convergent plate margin. This type of slope failure may be partially related to metastable sandy layers within the last glacial age progradational sequence, and possibly to formation of bubble phase gas at shallow depths. Failure was probably triggered by earthquake loading of sediments in this highly seismic region.     

Palaeoecology and post-Weichselian shore-level changes on the coast of Møre, western Norway

Pollen analysis, macrosubfossil determinations and radiocarbon datings from a 0.95 m thick peat deposit resting on sand and buried by a 1.3 m thick beach ridge at Haramsøy (an outer-arc island off the coast of Møre og Romsdal province), reveal changes in the local vegetation and in the groundwater level of the landward lagoon-like area. These are considered to reflect the relative shore-level changes between late Preboreal and early Atlantic times: an initial section with a high groundwater level reflecting the early Boreal high shore level, an intervening section with a low groundwater level, from the time of the Boreal regression minimum, and a final section, with a rising groundwater level, reflecting the late Boreal eustatic rise in sea level, which led to complete burial of the peat and the formation of an extensive Tapes beach ridge. Radiocarbon dates reveal that the basal sand (approx. 8 m above sea level) rose above sea level at approx. 9500 B. P. and that the top of the peat (approx. 9 m above sea level) was transgressed by the sea at approx. 7300 B. P. The spread of alder ( Alnus ) within the area may have been delayed by a thousand years compared with other regions in south Norway.     

Satellite remote sensing of water turbidity / Sonde de télémesure par satellite de la turbidité de l'eau

Abstract

Remote sensing instruments obtain an optical measure of water colour and turbidity. Colour increases the absorption of light in water and decreases the remotely sensed signal; turbidity increases the backscatter of light. For low concentrations of suspended materials, spectral reflectance is determined mostly by the absorptance characteristics of water; for higher concentrations, the absorptance characteristics of suspended particles are the most important factor. Remote sensing offers considerable advantages for the study of large areas, determination of current and circulation patterns, and monitoring of sedimentation, water productivity, and eutrophication.     

Mineralogy and Geochemistry of a Basanite to Phonolite Sequence at Hut Point Peninsula, Antarctica, based on Core from Dry Valley Drilling Project Drillholes 1, 2 and 3

The Dry Valley Drilling Project (DVDP) cored three holes (DVDP1, 2 and 3) at Hut Point Peninsula (HPP) on Ross Island, Antarctica.DVDP 1 reached 201 m and penetrated 40 stratigraphic units,mainly basanite and Ne-hawaiite lava flows and associated pyroclasticrocks. DVDP 2 and 3 are nearly identical. DVDP 3 reached 381m and penetrated 15 stratigraphic units. The lower unit is a214 m thick basanite hyaloclastite, this is overlain by flowsand pyroclastic rocks of basanite, Ne-hawaiite, and then alternatingNe-mugearite and Ne-benmoreite flows. Phonolite stratigraphicallyoverlies the drill site. Primary olivine (Fo₈₈-Fo₇₄) is confined to the basanites, althoughxenocrysts occur in other rock types. Calcium-rich clinopyroxeneis ubiquitous but shows little variation in terms of Ca, Mg,Fe²⁺. Kaersutite is common in all lavas except the basanitesand shows very little compositional variation. It is believedto have crystallized at temperatures below 1100 °C and atpressures between 1·5 and 10 kb. Rhönite, a titanosilicateis a common groundmass phase in some basanite lavas. Oxide mineralsshow a wide range of compositions; spinel and chromian spineloccur as inclusions in olivine in the basanites. Titano-magnetiteoccurs in most samples examined. Labradorite is the predominantfeldspar in the basanite, with andesine and oligoclase importantin all other rock types. DVDP and HPP samples show a well-defined sequence of basanite-Ne-hawaiite-Ne-mugearite-Ne-benmoreite-phonolite,termed the DVDP lava lineage. Differentiation of the lineagehas been modeled using major element least squares mass balancemodels. Over 75 per cent of the evolution from basanite to phonoliteoccurs in the initial step from basanite to Ne-hawaiite, whichrepresents a 43 per cent residual. The phonolite is only a 25per cent residual from a basanite parent; thus even though Ne-mugeariteand Ne-benmoreite are common, they represent only minor stepsin the differentiation sequence. The mass balance models indicatethat olivine, clinopyroxene, kaersutite, opaque oxides and feldsparare the dominant fractionated phases. Trace element (includingrare earth element) contents calculated using the Rayleigh equationconfirm all models. ^* Present address: Department of Geoscience, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801.