Structural geology and sedimentology of Saalian tills near Heiligenhafen, Germany.

Three Pleistocene tills can be distinguished in a coastal cliff section near Heiligenhafen, northern Germany, on the basis of structural and petrographic characteristics. The Lower and Middle Tills had previously been ascribed to the Saalian, and the Upper Till to the Late Weichselian. The former two tills are folded, and unconformably overlain by the Upper Till. In this paper, structural and sedimentological observations are used to investigate whether the Lower and Middle Tills belong to one glacial advance, or two separate (Saalian) advances, as was suggested in earlier studies based on fine gravel stratigraphy. From the contact with local rocks to the top of the MT there is a steady increase in allochtonous components (Scandinavian rocks) and decrease in parautochtonous (chalk and flint) and autochthonous components (local Eocene siltstone and meltwater sediments). This is paralleled by a trend towards increasing deformation (finite strain) from the bedrock to the top of the section. The most obvious aspect of this latter trend is the massive appearance of the MT which can be interpreted as the result of homogenization by repeated folding and attenuation of sediment lenses which have been incorporated into the till. This interpretation is supported by macroscopic and microscopic observations of structures in both tills. The structural analysis of the tills is based on the marked contrast in symmetry between sections parallel and perpendicular to the shear direction. Structures on all scales in the LT as well as in the MT indicate E-W (dextral) shearing, except in the western part of the section, where this is overprinted by W-E (sinistral) shearing. The sediment inclusions in the chalk-rich LT are mainly fragments of one or more strongly extended glaciofluvial delta bodies with a depositional direction towards WSW. Locally these delta sediments rest on Eocene siltstone and contain numerous angular fragments of this local bedrock. Boudins and lenses of sorted sediments are incorporated into the till and occur as 'islands of low strain' in a high strain homogeneous matrix. It is concluded that the LT and MT do not belong to two stratigraphically separate Saalian advances. The section is alternatively interpreted as one subglacial shear zone (deformation till) with upward increasing strain and allochtonous component content. It probably formed during the Younger Saalian (Warthe) westward advance from the Baltic region. Folding of the two diamicts occurred due to lateral compression near the Late Saalian ice margin. The section was finally overridden by the Late Weichselian Young Baltic advance, eroding the folded LT and MT and depositing the UT

Late Neogene passive margin denudation history: cosmogenic isotope measurements from Central Namib desert.

In this paper, we review ideas on the geomorphological history of the southwest African passive margin, focusing on the central Namib sector and presenting new evidence on the late Neogene landscape evolution of this region. The hyperarid central Namib Desert occupies the 100-150-km-wide pediment at the foot of the Great Escarpment and forms part of the southwest African passive margin, which formed after breakup in the South Atlantic at around 118 Ma. Previous apatite fission track (AFT) and cosmogenic isotope studies and numerical models of coupled tectonic-surface processes in the same area suggest that long-term denudation rates of this passive margin (after a period of significant post-rift denudation) have been very low, ∼5 m/my. Aridity of the Namib Desert is generally assumed to have started with the onset of Benguela upwelling in the SE Atlantic at 10-15 Ma and to have prevailed ever since. It has been implied that during this period, the landscape has undergone only marginal change. Here, we present new evidence from in situ cosmogeni

Feedbacks of lithosphere dynamics and environmental change of the Cenozoic West Antarctic Rift System.

This special issue of Global and Planetary Change contains 11 contributions dealing with various aspects of the Cenozoic West Antarctic Rift System. During the last two decades, investigations of the interplay of tectonics and climate greatly improved understanding of Cenozoic global change. Major plate reorganizations in the Southern Hemisphere, starting with the breakup of Gondwana in the Mesozoic, were a key factor in the thermal isolation of Antarctica and the initiation of the East and West Antarctic Ice Sheets, which, in their turn, influenced lower latitude and Northern Hemisphere climates with the oceans as intermediaries. Recent studies are increasingly drawing attention to the tectonic effects in glaciated rifts and rifted margins. During the Pleistocene, the rifted margins around the North Atlantic repeatedly were the core regions for continental ice sheet expansions. However, as demonstrated by recent work, Cenozoic uplift of the Norwegian margin started as early as 30 Ma. Similarly, from the early Oligocene onwards, topographic uplift of the Transantarctic Mountains, the western flank of the Cenozoic West Antarctic Rift System, triggered the formation of local ice centres which merged to form large ice sheets. This process was repeated a number of times until, in the late Pliocene, the present-day East and West Antarctic Ice Sheets were established. Throughout the evolution of the Antarctic Ice Sheet, lithosphere dynamic processes and environmental change have been strongly linked, as indicated by: (1) the great variation in landscape evolution histories along the Transantarctic Mountains rift flank, as a result of differential topographic uplift of fault blocks under late Neogene transtension in the western Ross Embayment; (2) the influence of the rising rift flank on ice sheet dynamics which differs for regions in the central Transantarctic Mountains and more peripheral regions; (3) the routing of major ice streams draining the West Antarctic Ice Sheet along the Cenozoic rift, which, by its high geothermal heat flux, directly influences the dynamics of these ice streams; and (4) the late Neogene evolution of the continental shelves of the Ross Sea and Weddell Sea, from shallow seaward sloping to overdeepened landward sloping shelves, influencing the production and dynamics of the oceans' abyssal currents. To unravel the interplay of tectonics and environmental change requires data constraining the rates of both surface processes and underlying lithospheric processes. The parallel evolutions of one of the world's largest riff systems and the world's largest ice sheets make the West Antarctic Rift System a key region for studies quantifying the effects of these processes on regional and global environmental change. An integrated approach is required to elucidate the fine structure of the interaction of tectonics, surface processes and climate in glaciated rift systems. (C) 1999 Elsevier Science B.V

Sublimation of ice through sediment in Beacon Valley, Antarctica

The time-dependent physics of ice sublimation through thin layers of till is considered, to determine whether sublimation could be sufficiently slow to permit the preservation of ice for 8 Ma in the Dry Valleys, Antarctica. This could only happen if the ice had been very thick, but other evidence (crystal size, dating of other ice-cored moraines) is not consistent with this possibility. Steady-state models suggest that sublimation is rate-controlled by vapor transport. A time-dependent model coupling vapor concentration, air pressure, temperature and ice concentration is formulated, and the resulting equations solved non-linearly. No transient coupling between vapor concentration, air temperature and pressure that substantially slows down sublimation was found in the numerical experiments. This means either that vapor transport is being slowed down by some unconsidered physical process or that the ice is much younger than 8 Ma