Papers by Dick van der Wateren
Three Pleistocene tills can be distinguished in a coastal cli! section near Heiligenhafen, northe... more Three Pleistocene tills can be distinguished in a coastal cli! 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 "ne 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 #int) and autochtonous components (local Eocene siltstone and meltwater sediments). This is paralleled by a trend towards increasing deformation ("nite 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 glacio#uvial 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 "nally overridden by the Late Weichselian Young Baltic advance, eroding the folded LT and MT and depositing the UT.
In this paper, we review ideas on the geomorphological history of the southwest African passive m... more 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 Ž. fission track AFT and cosmogenic isotope studies and numerical models of coupled tectonic-surface processes in the same Ž. area suggest that long-term denudation rates of this passive margin after a period of significant post-rift denudation have been very low, ; 5 mrmy. 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 cosmogenic 21 Ne dating of pediment surfaces, river terraces and river cut bedrock benches. Correlation of terrace sediment bodies defining ancient river courses is supported by a sediment provenance study using heavy mineral analyses. Our results generally confirm previous work, adding new insights in the late Neogene landscape evolution of the central Ž. Namib. They indicate that denudation rates since at least 5 Ma have been even lower of the order of 0.5–1 mrmy than those calculated by previous studies of inselbergs. In the central Namib, inselbergs appear to be located in or near areas of PliorPleistocene incision and badland erosion. These inselbergs therefore are not the best sites to obtain representative passive margin denudation rates. We obtained a 21 Ne age of 5.18 " 0.18 Ma from a resistant quartz vein projecting above pediment unaffected by recent incision, the oldest exposure age from the Namib so far. The pediment surface stretching from the base of the escarpment to the ocean, on average, probably has undergone less than 10 m of surface lowering since the mid Miocene. Our results also indicate that a late Neogene episode of accelerated denudation in the central Namib Desert and incision of deep canyons started around 2.81 " 0.11 Ma ago. We interpret this as a signal of increasingly wet conditions in this region, in response to PliorPleistocene global cooling. As aggradation of the river systems is followed by degradation, their drainage patterns change from braided to meandering. Evidence of, yet undated, earlier incision episodes suggests that the denudation history of this passive margin is one of slow long-term denudation—as other workers concluded on the basis of AFT and cosmogenic nuclide analyses—interrupted by pulses of rapid denudation. q
Modern and Past Glacial Environments, 2002
During a detailed structural geological and geomorphological survey of ice-pushed ridges around t... more During a detailed structural geological and geomorphological survey of ice-pushed ridges around the Gel- Cj derse Vallei (centre of the Netherlands) several questions arose about the origin of these landforms. The Gelderse Vallei is a Saalian glacial basin filled with younger sediments, 40 km long and up to 20 km wide, running NNW-SSE. Thrust sheets which build up the ice-pushed ridges on either side of the valley were transported away from the centre of the basin. They are up to 25 m thick and are pushed to a level about 100 m above the décollement. The thrust sheets, mainly consisting of coarse sand layers, moved as rigid masses, while only the finegrained basal layers deformed by heterogeneous simple shear.
Boreas, 2008
A three–till section in eastern Overijssel, eastern Netherlands, represents the most complete til... more A three–till section in eastern Overijssel, eastern Netherlands, represents the most complete till sequence of the Older Saalian glaciation presently known from The Netherlands and adjacent Germany. On the basis of structural analyses, four different glacial events within one continuous glacial cover can be distinguished: (1) deposition of till 1 (lowest till) by ice probably moving in a southerly direction, (2) glaciotectonic dislocation of till 1 under a compressive strain regime acting towards the west, (3) deposition of till 2 by ice moving in a westerly direction, and (4) deposition of till 3 by southeasterly moving ice. During deposition of tills 2 and 3, the underlying older tills were subjected to a strong extensional tectonic regimc, leading among other things to the development of characteristic sheath folds. On the basis of directional and compositional characteristics, the three tills can be correlated with the till stratigraphy of the northern Netherlands.
Two main mechanisms have been proposed for the retreat of marine ice sheets such as the West Anta... more Two main mechanisms have been proposed for the retreat of marine ice sheets such as the West Antarctic ice sheet, one related to changes in the backpressure from abutting ice shelves, the other relating to an ice-stream sliding model Both models are becoming increasingly untenable on account of new observations and theoretical considerations. We present a coupled marine ice-sheet/lithosphere model (self-gravitating visco-elastic) which investigates coupling between marine ice sheet and earth dynamics. Coupling lithosphere and ice dynamics does not facilitate retreat, and we propose an alternative mechanism based on ice stream fluctuations. We consider optimal ways of (a) Modelling the retreat of the West Antarctic Ice Sheet in order to estimate the present deglaciation signal and (b) Estimating the balance of the WAIS using the heterogeneous data sources available. The best constraints of time-dependent ice-sheet behaviour come from the dated marginal retreat sequences following the last glacial maximum. A digital database of absolute dates relevant to ice-sheet geometry has been established at BAS. This will be used to constrain models of the Holocene retreat of the Antarctic ice sheet. Constraint is proposed using the Kalman Filter. Some significant dated deposits are from lakes dammed by valley glaciers, themselves damned by the Ross Ice Sheet. The least age of these deposits is somewhat later than the retreat of the Ross Ice Sheet from the locality on account of the lagged resposne of the glacier. We present a model of the Hatherton glacier where this response time is estimated. We consider ways of obtaining optimal estimates of the current state of the ice-sheet. A methodology is proposed which takes account of satelltite observations of elevation, change in elevation and surface velocities. The principle of these methods should extend to assimilating lithospheric data. All these data will both be needed for the optimal use of data from gravity missions.
During a detailed structural geological and geomorphological survey of ice-pushed ridges around t... more During a detailed structural geological and geomorphological survey of ice-pushed ridges around the Gel- Cj derse Vallei (centre of the Netherlands) several questions arose about the origin of these landforms. The Gelderse Vallei is a Saalian glacial basin filled with younger sediments, 40 km long and up to 20 km wide, running NNW-SSE. Thrust sheets which build up the ice-pushed ridges on either side of the valley were transported away from the centre of the basin. They are up to 25 m thick and are pushed to a level about 100 m above the décollement. The thrust sheets, mainly consisting of coarse sand layers, moved as rigid masses, while only the finegrained basal layers deformed by heterogeneous simple shear.
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Papers by Dick van der Wateren