Weichselian Convolution Phenomena and processes in fine sediments.

In three excavations in the southern Netherlands and northern Belgium, the Weichselian convolutions in fine sand, silt and clay deposits were investigated, and an attempt was made towards a semi-quantitative approach to some sediment parameters. This, together with a detailed field survey, enabled interpretation of the deformations as forms of periglacial loadcasting. They originated from the time of permafrost degradation when large quantites of water were available. The amplitude of the involutions can be related to the depth of the permafrost table at that time. During the weichselian, two periods occurred in which this involution process could have been active     

REMARQUES SUR L'INTERPRETATION DES SONDAGES DE POLARISATION PROVOQUEE ESSAI DE THEORIE QUANTITATIVE *

The transient phenomena associated with induced polarization are very complex and it is necessary to have both a proper understanding of the physical processes and a good supply of mathematical aids in order to make any advance in the quantitative interpretation of I.P. surveys. The analogy between the classical electrical sounding method and the induced polarization sounding method has led a number of workers to interpret the results of the latter like the results of the former. To clarify this idea, it seems very interesting to calculate a mathematical model in order to know the form of the anomaly that can be probably expected. The case of a thin horizontal polarizable layer, with the same uniform and isotropic conductibility as the homogeneous background, has been studied in this paper. The result of the calculation for the case of a single current electrode and a single potential electrode is given. From these data, the theoretical curve of IP sounding using the Wenner electrode array is computed. The master curve is compared with experimental curves obtained on scale models. A good correlation between these curves is noticed. In addition, it can be concluded that the mathematical model is proving the validity of the experiments on interpretation scale models.     

DELINEATION OF SHALLOW SALT DOMES AND SURFACE FAULTS BY TEMPERATURE MEASUREMENTS AT A DEPTH OF APPROXIMATELY 2 METRES*

The feasibility of using temperature measurements at a depth of about 2 m for locating and delineating salt domes and faults has been investigated both theoretically and in experimental field surveys. It is shown that measurable temperature anomalies in the soil are to be expected over shallow salt domes. In a field survey over a salt-dome area bordering the Groningen gas field, a large number of temperature measurements were made in small holes (2 m deep, 3 cm in diameter) within a relatively short time (some weeks). The results clearly indicate several temperature anomalies with differential temperatures of about 1°C. Comparison of our thermal contour map with interpretations of available seismic or gravity data, or with direct evidence from wells, showed an excellent correlation. Seismic data even support the shape of the thermal contours. Results in similar agreement with gravity or well data were obtained over salt ridges in a tropical area. Experiments showed that the technique worked as well in lakes and marshes as on dry land. In addition, some experimental evidence collected so far over shallow and surface faults is presented. In several cases, strong thermal anomalies coincided with known surface faults. A thermal model for a surface-fault zone is suggested which accounts satisfactorily for the observed thermal data. It suggests some diagnostic value for the fault's geometry. For shallow faults, however, lack of knowledge of subsurface detail prevented any unambiguous correlation with observed thermal anomalies. Accordingly any geological use of thermal analysis over shallow faults remains debatable. The field technique is simple, needs little correction and can, where useful, easily be included in routine gravity work to provide additional local information.     

ON A FOSSIL FRESHWATER CRAYFISH FROM EASTERN MONGOLIA

1.—Introduction. Father E. Licent, S. J., during one, of his travels in Eastern Mongolia, discovered, in the Lycoptera and Ephemeropsis shales, the remnants of three Decapod Crustaceans. They belong to a new species, which shall be designated as Astacus licenti, nov. sp., in honour of that indefatigable explorer, that is Father Licent.     

A modified binary space partitioning tree for geographic information systems

We present a reactive data structure, that is, a spatial data structure with detail levels. The two properties, spatial organization and detail levels, are the basis for a geographic information system (GIS) with a multi-scale database. A reactive data structure is a novel type of data structure catering to multiple detail levels with rapid responses to spatial queries. It is presented here as a modification of the binary space partitioning tree that includes the levels of detail. This tree is one of the few spatial data structures that does not organize space in a rectangular manner. A prototype system has been implemented. An important result of this implementation is that it shows that binary space partitioning trees of real maps have O(n) storage space complexity in contrast to the theoretical worst case O(n² ), with n the number of line segments in the map.     

Understanding and managing the morphology of branches incising into sand-clay deposits in the Dutch Rhine Delta

In the Rhine-Meuse delta in the south-western part of the Netherlands,the morphology of the river branches is highly dependent on the erodibility of the subsoil.Erosion processes that were initiated after closure of the Haringvliet estuary branch by a dam(in 1970),caused a strong incision of several connecting branches.Due to the geological evolution of this area the lithology of the subsoil shows large variations in highly erodible sand and poorly erodible peat and clay layers.This study shows how the geological information can be used to create 3D maps of the erodibility of the sub-soil, and how this information can be used to schematize the sub-soil in computational models for morphological simulations.Local incisement of sand patches between areas with poorly erodible bed causes deep scour holes,hence increasing the risk on river-bank instability(flow slides) and damage to constructions such as groynes,quays,tunnels, and pipelines.Various types of mathematical models,ranging from 1D(SOBEK) to quasi-3D(Delft3D) have been applied to study the future development of the river bed and possible management options.The results of these approaches demonstrate that models require inclusion of a layer-bookkeeping approach for sub-soil schematization, non-uniform sediment fractions(sand-mud),tidal and river-discharge boundary conditions,and capacity-reduction transport modeling.For risk-reducing river management it has been shown how the development of the river bed can be addressed on a large scale and small scale.For instance,the use of sediment feeding and fixation of bed can be proposed for large-scale management,while monitoring and interventions at initiation of erosion can be proposed as response to small-scale developments that exceed predefined intervention levels.     

Regional precipitation and temperature scenarios for climate change

Abstract

To investigate the consequences of climate change on the water budget in small catchments, it is necessary to know the change of local precipitation and temperature. General Circulation Models (GCM) cannot provide regional climate parameters yet, because of their coarse resolution and imprecise modelling of precipitation. Therefore downscaling of precipitation and temperature has to be carried out from the GCM grids to a small scale of a few square kilometres. Daily rainfall and temperature are modelled as processes conditioned on atmospheric circulation. Rainfall is linked to the circulation patterns (CPs) using conditional probabilities and conditional rainfall amount distribution. Both temperature and precipitation are downscaled to several locations simultaneously taking into account the CP dependent spatial correlation. Temperature is modelled using a simple autoregressive approach, conditioned on atmospheric circulation and local areal precipitation. The model uses the classification scheme of the German Weather Service and a fuzzy rule-based classification. It was applied in the Aller catchment for validation using observed rainfall and temperature, and observed classified geopotential pressure heights. GCM scenarios of the ECHAM model were used to make climate change predictions (using classified GCM geopotential heights); simulated values agree fairly well with historical data. Results for different GCM scenarios are shown.     

Spatial variability of watermass conditions within the European Epicontinental Seaway during the Early Jurassic (Pliensbachian–Toarcian)

In order to constrain spatial variability in watermass conditions within the European Epicontinental Seaway prior to, during and after the Toarcian Oceanic Anoxic Event, carbon (δ¹³C_bel, δ¹³C_carb) and oxygen (δ¹⁸O_bel, δ¹⁸O_carb) isotope records were obtained from three sections in the Grands Causses Basin (southern France). These data were then compared with similar records along a north–south transect across the European Epicontinental Seaway. As the conclusions reached here strongly depend on the reliability of belemnite calcites as archives of palaeoceanographic changes, an attempt was made to improve the understanding of isotope signals recorded in belemnite calcite. Intra‐rostral carbon and oxygen‐isotope data from six belemnite specimens belonging to the genus Passaloteuthis were collected. Intra‐rostral carbon‐isotopes are influenced by vital effects, whereas oxygen‐isotopes reflect relative changes in temperature and salinity. Palaeotemperatures calculated from δ¹⁸O_bel‐isotope records from the Grands Causses Basin confirm relatively low temperatures throughout the Late Pliensbachian. Similar cool water conditions have previously been shown in Germany, England, Spain and Portugal. A temperature increase of up to 6 °C is observed across the Pliensbachian–Toarcian boundary. A pronounced negative shift of at least ?3‰ (Vienna‐Pee Dee Belemnite) is recorded in bulk carbonate carbon during the lower Harpoceras serpentinum zone, typical of the Toarcian Oceanic Anoxic Event. Before and after the Toarcian Oceanic Anoxic Event, a good correlation between δ¹³C_carb and δ¹³C_bel exists, indicating well‐ventilated bottom‐waters and normal marine conditions. Instead, data for the Toarcian Oceanic Anoxic Event indicate the development of a strong north–south gradient in salinity stratification and surface‐water productivity for the Western Tethyan realm. This study thus lends further support to a pronounced regional overprint on carbon and oxygen‐isotope records in epicontinental seaways.     

Diagenetic formation of gypsum and dolomite in a cold‐water coral mound in the Porcupine Seabight,off Ireland

Authigenic gypsum was found in a gravity core, retrieved from the top of Mound Perseverance, a giant cold‐water coral mound in the Porcupine Basin, off Ireland. The occurrence of gypsum in such an environment is intriguing, because gypsum, a classic evaporitic mineral, is undersaturated with respect to sea water. Sedimentological, petrographic and isotopic evidence point to diagenetic formation of the gypsum, tied to oxidation of sedimentary sulphide minerals (i.e. pyrite). This oxidation is attributed to a phase of increased bottom currents which caused erosion and enhanced inflow of oxidizing fluids into the mound sediments. The oxidation of pyrite produced acidity, causing carbonate dissolution and subsequently leading to pore‐water oversaturation with respect to gypsum and dolomite. Calculations based on the isotopic compositions of gypsum and pyrite reveal that between 21·6% and 28·6% of the sulphate incorporated into the gypsum derived from pyrite oxidation. The dissolution of carbonate increased the porosity in the affected sediment layer but promoted lithification of the sediments at the sediment‐water interface. Thus, authigenic gypsum can serve as a signature for diagenetic oxidation events in carbonate‐rich sediments. These observations demonstrate that fluid flow, steered by environmental factors, has an important effect on the diagenesis of coral mounds.