Mineral deposit formation in Phanerozoic sedimentary basins of north-east Africa: the contribution of weathering

The intra- and epicontinental basins in north-east Africa (Egypt, Sudan) bear ample evidence of weathering processes repeatedly having contributed to the formation of mineral deposits throughout the Phanerozoic.The relict primary weathering mantle of Pan-African basement rocks consists of kaolinitic saprolite, laterite (in places bauxitic) and iron oxide crust. On the continent, the reaccumulation of eroded weathering-derived clay minerals (mainly kaolinite) occurred predominantly in fluvio-lacustrine environments, and floodplain and coastal plain deposits. Iron oxides, delivered from ferricretes, accumulated as oolitic ironstones in continental and marine sediments. Elements leached from weathering profiles accumulated in continental basins forming silcrete and alunite or in the marine environment contributing to the formation of attapulgite/saprolite and phosphorites.The Early Paleozoic Tawiga bauxitic laterite of northern Sudan gives a unique testimony of high latitude lateritic weathering under global greenhouse conditions. It formed in close spatial and temporal vicinity to the Late Ordovician glaciation in north Africa. The record of weathering products is essentially complete for the Late Cretaceous/Early Tertiary. From the continental sources in the south to the marine sinks in the north, an almost complete line of lateritic and laterite-derived deposits of bauxitic kaolin, kaolin, iron oxides and phosphates is well documented.     

Grenzen quantitativer Untersuchungen bei tektonischen Experimenten

Quantitative experimental and theoretical investigations, except basic research, should focus only to such parameters which can be measured in the field. This is shown by three examples.

1. A model of a discontinuum composed of particles, between which friction occurs during shear is presented. Even at simple shear external and internal stresses are in equilibrium; no rigid body rotation occurs. The theory is examinated experimentally.
2. In deformation experiments with clay and sand a development of anisotropy is observed, which can cause shear fractures. The orientation of the normal stress directions can be discovered from the shear sets, their angular distribution, and the rotation of the individual blocks.
3. Most folds can be traced back to the following three types, a) Similar folds, b) concentric folds, and c) box folds. b) and c) occur only near the surface and near shear planes, as shown by experiments and field observations. In all other cases the more energy-consuming similar folds are formed. Kink bands are presumably transitional between folds and shear fractures.

     

Density control in Potamogeton perfoliatus L. and Potamogeton pectinatus L.

The effect of density on the growth, recruitment of new ramets, biomass allocation, rhizome spacer length and rhizome branching of the submersed macrophyte Potamogeton perfoliatus L. was experimentally evaluated in a mesocosm with three different initial shoot densities. The findings suggest that the number of primary shoots available in the beginning of the season can strongly influence patterns of growth and clonal reproduction. In contrast to many studies that found decreasing production parameters with plant density, ramet and biomass production of Potamogeton perfoliatus plants were highest at medium plant density. This is probably due to negative feed-back through crowding at high density and unclear negative effects at low density. Shoot allocation tended to increase with density, rhizome allocation tended to decrease with density and root biomass remained unchanged. At low density Potamogeton perfoliatus produced longer rhizome spacers and more branchings than at higher densities. Shorter rhizome spacers at high plant density probably restrict patch expansion and cause discrete patch shapes. A likely mechanism for plastic changes in clonal architecture is increasing competition for light at increasing density, but other density-dependent factors may also contribute.Investigations on propagule numbers of tubers and turions show that self-regulation of ramet number was associated with self-regulation of propagule number in both Potamogeton perfoliatus and Potamogeton pectinatus, with higher densities producing less propagules per plant.     

Experimental and theoretical bond critical point properties for model electron density distributions for earth materials

Generalized X-ray scattering factor model experimental electron density distributions and bond critical point, bcp, properties generated in recent studies for the earth materials stishovite, forsterite, fayalite and cuprite with high energy single crystal synchrotron X-ray diffraction data and those generated with high resolution diffraction data for coesite and senarmonite were found to be adequate and in relatively good agreement, ~5% on average, with those calculated with quantum chemical methods with relatively robust basis sets. High resolution low energy single crystal diffraction data, recorded for the molecular sieve AlPO₄-15, were also found to yield model electron density distribution values at the bcp points for the AlO and PO bonded interactions that are in relatively good to moderately good agreement with the theoretical values, but the Laplacian values of the distribution at the points for the two bonded interactions were found to be in relatively poor agreement. In several cases, experimental bcp properties, generated with conventional low energy X-ray diffraction data for several rock forming minerals, were found overall to be in poorer agreement with the theoretical properties. The overall agreement between theoretical bcp properties generated with computational quantum methods and experimental properties generated with synchrotron high energy radiation not only provides a basis for using computational strategies for studying and modeling structures and their electron density distributions, but it also provides a basis for improving our understanding of the crystal chemistry and bonded interactions for earth materials. Theoretical bond critical point properties generated with computational quantum methods are believed to rival the accuracy of those determined experimentally. As such the calculations provide a powerful and efficient method for evaluating electron density distributions and the bonded interactions for a wide range of earth materials.Dedicated to Professor Robert F. Stewart of Carnegie Mellon University on his retirement for his brilliant and original work modeling electron density distributions.     

Evolution and structure of the Upper Rhine Graben: insights from three-dimensional thermomechanical modelling

The evolution and geometry of the Tertiary Upper Rhine Graben were controlled by a continually changing stress field and the reactivation of pre-existing crustal discontinuities. A period of WNW-ESE extension in the late Eocene and Oligocene was followed by lateral translation from the early Miocene onwards. This study utilizes 3D finite element techniques to simulate extension and lateral translation on a lithospheric scale. Brittle and creep behaviour of lithospheric rocks are represented by elastoplasticity and thermally activated power-law viscoplasticity, respectively. Contact elements allocated with cohesion and frictional coefficients are used to describe pre-existing zones of weakness in the elastic-brittle field. Our results suggest that (1) extension is accommodated along listric border faults to midcrustal depth of 15–16 km. Beneath, pure shear stretching occurs without a need for localized shear zones in lower crust and upper mantle. (2) Ductile flow at midcrustal depth across the graben accounts for the pronounced halfgraben morphology. Thereby, the shape of the border faults, their frictional coefficients, and sedimentary loads have profound effects on the rate of ductile flow across the graben. (3) Horizontal extension of 8–8.5 km and sinistral displacement across the rift of 3–4 km are needed to accommodate the observed sediment thickness.     

Beiträge zur Dynamik der Biomasse in Oxydationsteichen,Teil 4: Untersuchungen an einer hochbelasteten abflußlosen Abwasserteichanlage (Fäkaldeponie Ückeritz)

The system consists of three preliminary clarifiers and two ponds operating in series. It drains 100,000 to 120,000 m³ highly loaded wastewater per year over the ground. In dependence on load, in the pond 1 a midsummer-time bacterial maximum of up to 800 m³/l occurs. The algal biomass, too, shows an annual curve with one peak; the maximum is independent of the ice cover and varies with the seasons. The BOD₅ shows a spring maximum and a summer maximum independent of the biomasses and their variations. In the second pond only slight changes are found from June till October; here prevail algae and a maximum of flagellates. Although a stage of filtrators and consumers is lacking, the system can be regarded as a sewage pond system with a reduction of BOD₅ by 73%.