The tectonic history of the Pre-Saharan depression (Morocco) a geomorphological interpretation

During the Cretaceous and Tertiary periods the Pre-Saharan depression between Ouarzazate and Errachidia was an area of deposition in a spatially and chronologically highly differentiated pattern. The variable structural and tectonic history of this region in also reflected in its denudational development. Until the late Pliocene/early Pleistocene the western part (Ouarzazate basin) was a depositional basin for lacustrine and alluvial sediments. Pediment formation did not start before the Pleistocene period. In the central part of the depression (between Boumalne and Tinerhir) denudational activity on cuesta scarps started already in the late Miocene to early Pliocene as can be deduced from a new dating of the Foum el Kous volcano (2.9 my) and from Djebl Sarhro gravels on the crest of the Paleogene scarp. In the easternmost part of the depression (between Goulmima and Errachidia) there are no Neogene sediments, and the calculations of rates of scarp retreat demonstrate that scarp backwearing must have begun in the late Eocene. Thus the shift from depositional to erosional activity in the Pre-Saharan depression ranges from the late Eocene to the Pleistocene. This is at the same time an expression of its complex tectonic history.

---

Zusammenfassung Die präsaharische Senke zwischen Ouarzazate und Errachidia war während der Kreide und des Tertiärs ein Sedimentationsgebiet mit zeitlich und räumlich hoch differenzierten Fazies- und Mächtigkeitsabfolgen. Die variable strukturelle und tektonische Entwicklung dieser Zeit spiegelt sich wider in der nachfolgenden Abtragungsgeschichte. Der westliche Teil der Senke, das Becken von Ouarzazate, war bis zum späten Pliozän/frühen Pleistozän ein Ablagerungsbecken für limnische und alluviale Sedimente, bis im frühen Pleistozän die Genese von Pedimenten einsetzte. Im zentralen Teil der präsaharischen Senke zwischen Boumalne und Tinerhir begann die erosive Reliefentwicklung an Stufenhängen bereits im späten Miozän bis frühen Pliozän, wie sich aus einer absoluten Datierung des Ankaratrits des Foum el Kous (2,9 my) ableiten läßt, der sich in die zu dieser Zeit bereits existente Stufenvorlandzone ergoß. Eine untere Zeitmarke für die Entstehung der Schichtstufe stellen die von Süden vom Djebl Sarhro in die Senke hereintransportierten Schotter von Timadriouine dar. Im Ostteil der präsaharischen Senke zwischen Goulmima und Errachidia fehlen neogene Sedimente, und die Ermittlung von Stufenrückwanderungsraten für die Hamada de Meski zeigt, daß die Abtragung hier schon im späten Eozän eingesetzt haben muß. Der Wechsel von Sedimentations- zu subaerischen Abtragungsbedingungen reicht in der präsaharischen Senke vom frühen Pleistozän im Westen bis zum Eozän im Osten. Das ist auf geomorphologischer Grundlage ein Hinweis auf die komplexe strukturelle und tektonische Entwicklung dieser Region.

---

Résumé Au cours du Crétacé et du Tertiaire, la dépression présaharienne entre Ouarzazate et Errachidia a été un domaine de sédimentation marqué par de fortes variations de faciès et d'épaisseurs dans le temps et dans l'espace. L'histoire structurale et tectonique mouvementée de cette période se reflète également dans la dénudation qui a suivi. La partie ouest de la dépression (bassin d'Ouarzazate) a constitué jusqu'à la limite Plio-Pléistocène une aire de dépôt de sédiments lacustres et alluviaux; la formation de pédiments n'y a débuté qu'au Pléistocène. Dans la partie centrale de la dépression, entre Boumalne et Tinerhir, l'érosion en cuestas a débuté dès le Miocène supérieur/Pliocène inférieur, comme l'indique une datation récente à 2,9 Ma de l'ankaratrite du volcan Foum el Kous qui s'est écoulée en avant d'une cuesta qui existait déjà à ce moment. Une limite inférieure pour la formation des cuestas est fournie par les graviers de Timadriouine transportés dans la dépression depuis le sud à partir du Djebel Sarhro.Dans la partie orientale de la dépression présaharienne, entre Goulmina et Errachidia, les sédiments néogènes font défaut et l'évaluation de la vitesse de recul des fronts de cuestas montre que ce processus a dû commencer à l'Eocène supérieur. Ainsi, le passage de la sédimentation à l'activité érosive dans la dépression pré-saharienne varie du Pléistocène inférieur à l'ouest, à l'Eocène à l'est. C'est là, à partir de données géomorphologiques, une indication de l'histoire structurale et tectonique complexe de cette région.

---

- Ouarzazate Errachidia , , , . . , Ouarzazate / ; . Boumalne Tinerhir () , , Foum el Kous (2,9 Ma), . Timadriouine Djebl Sarhro . Goulmina Errachidia Hamada de Meski , , , . , . .

     

Regional distribution and temporal trends of some contaminants in the water of the Baltic Sea

During a cruise on board RV Gauss in May/June 1988, joint investigations into organochlorine compounds, dissolved trace metals, petroleum hydrocarbons and basic hydrography were carried out at representative stations of the Baltic Monitoring Programme (BMP). The aim of the cruise was to study distribution patterns and — using previous data — to establish temporal trends if at all discernible.Each group of contaminants investigated showed specific characteristics, with differences even between compounds within the same group. The differences are due to:

Mean-field effects in the Galloway–Proctor flow

In the framework of mean-field electrodynamics the coefficients defining the mean electromotive force in Galloway–Proctor flows are determined. These flows show a two-dimensional pattern and are helical. The pattern wobbles in its plane. Apart from one exception a circularly polarized Galloway–Proctor flow, i.e. a circular motion of the flow pattern is assumed. This corresponds to one of the cases considered recently by Courvoisier, Hughes & Tobias. An analytic theory of the α effect and related effects in this flow is developed within the second-order correlation approximation and a corresponding fourth-order approximation. In the validity range of these approximations there is an α effect but no γ effect, or pumping effect. Numerical results obtained with the test-field method, which are independent of these approximations, confirm the results for α and show that γ is in general non-zero. Both α and γ show a complex dependency on the magnetic Reynolds number and other parameters that define the flow, that is, amplitude and frequency of the circular motion. Some results for the magnetic diffusivity  η_t  and a related quantity are given, too. Finally, a result for α in the case of a randomly varying linearly polarized Galloway–Proctor flow, without the aforementioned circular motion, is presented. The flows investigated show quite interesting effects. There is, however, no straightforward way to relate these flows to turbulence and to use them for studying properties of the α effect and associated effects under realistic conditions.     

Onset of aridity in southern Western Australia—a preliminary palaeomagnetic appraisal

The timing of the onset of full arid conditions in southern Western Australia during the late Cenozoic remains uncertain. The playas and associated sedimentary sequences preserved as part of the Tertiary palaeodrainage networks, which are widely developed in Western Australia, provide the stratigraphic evidence necessary to resolve this issue. Lake Lefroy forms part of a chain of playas that occur in the eastern Yilgarn Craton. These lake chains are the remnants of a once external palaeodrainage system, developed in pre-Eocene times. Eocene non-marine to marginal marine sequences were deposited in the palaeodrainage as channel infills. The low relief area of the palaeodrainage featured a permanent to semi-permanent lacustrine environment during post-Eocene times, and fine-grained red–brown clastic clay up to 10 m in thickness was deposited over an extensive area. A significant hydrological transition, as inferred by the litho-sedimentary change from freshwater clay to evaporitic gypsum-dominated sedimentation, took place in the late Cenozoic. The extensive freshwater system changed to the saline/deflation playas that characterises this landscape today. A detailed palaeomagnetic study was carried out on the lacustrine clay unit and the overlying evaporitic gypsum unit in Lake Lefroy. Results from drill core and pit wall exposures have provided the first time constraints for these sequences. Age estimates, based on extrapolation from the Brunhes/Matuyama geomagnetic boundary, suggest that the gypsum-dominated sedimentation and by inference, full arid conditions in Lake Lefroy, commenced within the Brunhes Normal Polarity Chron, probably within the last 500 Ka. This age is considerably younger than previously thought, but appears to bear some correspondence to similar claims to the age of the onset of aridity in southeast and central Australia. Evidence emerging from the inland dune field to the surrounding oceans suggests a trend of increasing aridity during the Quaternary in Australia. The onset of full aridity may well indicate that the impact of global glacial–interglacial cycles on Australian climate, especially the large scale glacial ‘dryness' resulted from the 100 Ka astronomic variations reached beyond its threshold.     

The significance of scarp retreat for cenozoic landform evolution on the Colorado Plateau,U.S.A.

Since the beginning of the Cenozoic period several hundreds of metres of the sedimentary cover have been removed from the Colorado Plateau. Palaeoclimatic considerations show that the Colorado Plateau has been dominated by dry climates throughout the Cenozoic with the possible exception of the early Palaeocene. Today in the still prevailing arid climate, which strongly accentuates differences in rock resistance, the relief shows a structurally controlled cuesta scarp topography in the slightly deformed strata of alternating resistance. In examining whether the denudational efficiency of scarp retreat was sufficient to account for the wide erosional gaps in the sedimentary cover, rates of scarp retreat were determined by using the information of dated volcanic material and by applying a new method, which calculates the amount of retreat from the width of beheaded valleys of known age. Rates of retreat range from 0·5 to 6·7 km my^?1. The results show that the rates of retreat are controlled by the thickness and resistance of the caprocks. A model of Cenozoic scarp retreat demonstrates that the rates of recession calculated for the scarps in Upper Cretaceous rocks were sufficient to bring them into their present positions from the centre of the Monument Uplift on the central Colorado Plateau. The late Eocene positions of the cliffs in the Early Tertiary formations give an indication of their maximal extent. After the Upper Cretaceous sediments had been removed from the uplifts, erosion cut through successively older rocks, and activated scarps in stratigraphically lower positions. Scarp retreat can operate simultaneously and independently at different levels, which enabled this erosional mechanism to remove great proportions of the sedimentary cover of the Colorado Plateau during the Cenozoic.     

Regional variation of mechanical and chemical denudation,upper Colorado River Basin,U.S.A.

The variation of mechanical and chemical denudation is investigated using discharge and sediment yield data from the Upper Colorado River System. Annual precipitation ranges from approximately 150 mm to 1500 mm. Mean specific yield ranges from 0-2 1/s km² ( = 6 mm p a) to 151/s km² ( = 475 mm p a). The hydrological-geomorphological system adjusts itself to these varying climatic conditions; in some areas, however, the effects of lithology or land use seem to override the climatic controls. It is demonstrated that the increase in the absolute and particularly the relative amount of suspended sediment is closely related to a decrease in annual runoff and to an increase in the importance of high magnitude/low frequency events. This indicates that in areas of low annual runoff and high runoff variability, soluble rocks are more resistant than in more humid areas. During high magnitude/low frequency events, suspended sediment concentrations and loads are very high in semiarid areas due to sparse vegetation cover and dominance of direct runoff. Events of moderate magnitude and frequency, which in more humid areas transport most of the dissolved load, seldom occur. The trend towards increasing mechanical denudation is even observed in areas of very low runoff (0-221/s km² = 7 mm p a). The peak of sediment yield in dry areas seems to approximate the point of no runoff very closely. Mechanical and chemical denudation are of equal importance at a runoff of about 300 mm per year.     

Spectral reflectance measurements of estuarine waters

Shapes and variations of reflectance spectra in estuarine water were investigated for the purpose of monitoring chlorophyll in situ by optical means. A survey undertaken in an estuarine environment, using reflectance measurements between 400 and 850 nm with a full-width half maximum (FWHM) of about 2 nm, revealed that the first derivative in the neighbourhood of the chlorophyll absorption band shows a defined spectral region which can be used to estimate chlorophyll concentrations. Correlation between chlorophyll and the first derivative was found to be low, but a good relationship exists between the ratio of the reflectance R680/R670 and chlorophyll concentrations. Based on dissolved organic carbon measurements, it is assumed that chromophoric dissolved organic compounds mask the absorption band of chlorophyll in the blue part of the spectrum, resulting in a low correlation coefficient in that spectral range. Therefore, the use of the red bands is an alternative for measuring photosynthetic pigments in coastal water at longer wavelengths. Results presented here demonstrate that the spectral locations of bands in the visible are not adequate and that hyperspectral data are required for positioning the very narrow bands for measuring chlorophyll at longer wavelengths.     

Interpretation of carbon dioxide diffusion behavior in coals

Storage of carbon dioxide in geological formations is for many countries one of the options to reduce greenhouse gas emissions and thus to satisfy the Kyoto agreements. The CO₂ storage in unminable coal seams has the advantage that it stores CO₂ emissions from industrial processes and can be used to enhance coalbed methane recovery (CO₂-ECBM). For this purpose, the storage capacity of coal is an important reservoir parameter. While the amount of CO₂ sorption data on various natural coals has increased in recent years, only few measurements have been performed to estimate the rate of CO₂ sorption under reservoir conditions. An understanding of gas transport is crucial for processes associated with CO₂ injection, storage and enhanced coalbed methane (ECBM) production.A volumetric experimental set-up has been used to determine the rate of sorption of carbon dioxide in coal particles at various pressures and various grain size fractions. The pressure history during each pressure step was measured. The measurements are interpreted in terms of temperature relaxation and transport/sorption processes within the coal particles. The characteristic times of sorption increase with increasing pressure. No clear dependence of the characteristic time with respect to the particle size was found. At low pressures (below 1 MPa) fast gas diffusion is the prevailing mechanism for sorption, whereas at higher pressures, the slow diffusion process controls the gas uptake by the coal.