Analysis of cyclical sedimentation in the lower Permian Mchuchuma coalfield (South-West-Tanzania)

A statistical approach by a modified Markov process model was used to prove that the lower Permian coal measures of the Tanzanian Mchuchuma basin developed distinct cyclicities during deposition. From results the transition path of lithologic states typical for this coal sequence is as follows: (A) coarse sandstone, (B) medium sandstone, (C) fine-very fine sandstone, (D) shale, (E) mudstone, (F) coal or (E+F) and again (A) coarse sandstone. The majority of cycles is are asymmetric (ABCDEF-ABC.)but symmetrical cycles are present as well (ABCDEFED). The statistical results concur with observed sedimentological evidence of depositional environment. Fining upward cycles correspond to the development of basal channel bars, changing into levées, then into back swamps which are topped by coal swamps. Frequent interbedding of coal and mudstone in the upper part of the coal measures calls for periodic flooding and flushing of fine clastics into coal-forming swamps causing interruption of peat formation. Cyclical deposition is explained by wandering channels in response to varying discharge and rate of deposition, their lateral frequent shifting caused the formation of asymmetrical cycles in this fluviodeltaic depositional model.

---

Zusammenfassung Das kohleführende untere Perm des Mchuchuma Beckens (SW-Tanzania) wurde statistisch mit Hilfe der Markov Analyse auf Zyklizität innerhalb der Abfolge untersucht. Die statistischen Daten zeigen, daß die bevorzugte und für dieses Kohlebecken typische Sequenz folgende lithologische Abfolge umfaßt: (A) grober Sandstein, (B) mittlerer Sandstein, (C) feiner Sandstein, (D) Siltstein, (E) Tonstein, (F) bzw. (E+F) Kohle und erneut (A) grober Sandstein. Es existieren meist asymmetrische Zyklen (ABCDEF-ABCD...) aber symmetrische Zyklen (ABCDEFED) treten ebenfalls auf. Die Markov Analyse ergänzt sehr gut die Beobachtungen bezüglich des Ablagerungsmilieus in den Aufschlüssen. »Fining up« Zyklen beginnen mit basalen Gerinnesanden, überlagert von levée Ablagerungen, welche ihrerseits übergehen in »back swamps« und Kohlesümpfe. Der obere Teil der Kohleabfolge ist durch häufige Wechsellagerungen von Kohle mit Tonstein charakterisiert, die auf periodische Überflutungen und Verfrachtungen von Feinklastika in die Kohlesümpfe zurückzuführen sind. Wandernde Flußgerinne, ausgelöst durch rasch wechselnde Sedimentfracht und Ablagerungsrate, führten zu zyklischen Ablagerungen. Häufig auftretende laterale Gerinneverlagerungen erklären die asymmetrischen Zyklen in diesem fluvio-deltaischen Ablagerungsmodell.

---

Résumé Des analyses statistiques ont été effectuées par la méthode de Markov sur le Permien inférieur charbonnier du bassin de Mchuchuma (Tanzanie du sud-ouest) dans le but de montrer l'existence de cycles sédimentaires. Les résultats font apparaître une séquence, typique de ce bassin, qui s'ordonne ainsi de bas en haut: (A) grès grossier, (B) grès moyen, (C) grès fin à très fin, (F) siltite, (E) pélite, (F) charbon ou (E+F) et de nouveau (A) grès grossier. Les cycles sont pour la plupart assymétriques (ABCDEF-ABC...) mais il en existe aussi de symétriques (ABCDEFED).Les résultats obtenus par la méthode de Markov concordent avec les observations sédimentologiques relatives au milieu de dépôt. Les cycles granoclassés débutent par des sables fluviatiles surmontés par des dépôts de levées suivis à leur tour de »back swamps«, et se terminent par des dépôts de marais tourbeux. La partie supérieure de la série est caractérisée par une stratification alternée de charbon et de pélite due à des inondations périodiques amenant dans les marais des produits clastiques fins qui interrompaient la formation de la tourbe. La sédimentation cyclique s'explique par la migration de chenaux, provoquée par les variations de l'apport sédimentaire et de la vitesse de dépôt. Les fréquents déplacements latéraux des chenaux expliquent l'asymétrie des cycles, dans ce modèle fluvio-deltaïque.

---

- Mchuchuma (-). , : (A) ; (B) ; (C) - ; (D) ; (E) ; (F) , (E + F) (A) . (ABCDEF-ABCD), (ABCDEFED). . « » , , , . (). , , , . / .

     

Physical weathering of building stones induced by freeze–thaw action: a laboratory long-term study

Damages to natural building stones induced by the action of frost are considered to be of great importance. Commonly, the frost resistance of building stones is checked by standardised freeze–thaw tests before using. Corresponding tests normally involve 30–50 freeze–thaw action cycles. In order to verify the significance of such measurements, we performed long-term tests on four selected rocks over 1,400 freeze–thaw action cycles. Additionally, numerous petrophysical parameters were analysed to compare the behaviour of rocks in the weathering tests according to the current explanatory models of stress formation by growing ice crystals in the pore space. The long-term tests yield more information about the real frost sensibility of the rocks. A clear deterioration cannot be determined in most cases until 50 weathering cycles have been completed. In the freeze–thaw tests, the samples are also stressed by changing temperature and moisture, indicating that different decay mechanisms can interfere with each other. Thus, thermohygric and moisture expansion are important damage processes.     

Trace gas exchange at the air/water interface: Measurements of mass accommodation coefficients

A liquid jet of 90 m diameter and variable length has been utilized to determine absorption rates and, hence, mass accommodation coefficients , of atmospheric trace gases. The compounds investigated are HCl (0.01), HNO₃ (0.01), N₂O₅ (0.005), peroxyacetyl nitrate (>0.001), and HONO (0.005). It is concluded that the absorption of these trace gases by liquid atmospheric water is not significantly retarded by interfacial mass transport. The strengths and limitations of the liquid jet technique for measuring mass accommodation coefficients are explored.     

Zur Theorie einfacher und mehrfacher räumlicher Mittelbildungen

Zusammenfassung Die Arbeit stellt einen Beitrag zur Theorie der einfachen und mehrfachen räumlichen Mittelbildung einer Feldfunktion dar. Es werden Formeln für die verschiedenen Mittelbildungen angegeben. Ferner wird gezeigt, daß eine zweifache räumliche Mittelbildung durch eine einfache mit einer entsprechend anderen Gitterdistanz ersetzt werden kann. Weiter wird die Genauigkeit der Approximation des Differentialoperators der Mittelung durch einen Differenzenoperator an Hand von Rossby-Wellen diskutiert. Dabei wird nachgewiesen, daß nur, wenn die Gitterdistanz der räumlichen Mittelbildung gegenüber der Wellenlänge von Rossby-Wellen klein verbleibt, eine hinreichend genaue Näherung erzielt werden kann.

---

Summary The paper is a contribution to the theory of single and multiple space means of a field function. Formulae for the derivation of various types of means are given. It is shown that a double space mean can be replaced by a single one, provided an appropriate grid distance is used. For the case ofRossby waves, the degree of approximation of the mean attained by using an operator with finite increments and the accurate differential operator is discussed. Sufficient approximation can only be obtained when the grid distance is small compared to the length of theRossby waves.

---

Résumé Ce mémoire est une contribution à la théorie de l'établissement dans l'espace des moyennes simples et multiples d'une fonction de champ. On y indique des formules pour l'établissement des diverses moyennes. On montre ensuite qu'une moyenne spaciale double peut être remplacée par une moyenne simple par la seule modification de la distance séparant les mailles du réseau. On discute au moyen d'ondes deRossby la précision de l'approximation obtenue par les moyennes issues d'un opérateur différentiel et d'un opérateur de différences. On démontre ainsi que l'approximation ne peut être suffisante que si la grandeur des mailles du réseau servant à l'établissement des moyennes reste petite par rapport à la longueur des ondes deRossby.

---

---

Mit 4 Textabbildungen     

Tying platform drowning to perturbations of the global carbon cycle with a δ13COrg-curve from the Valanginian of DSDP Site 416

The carbonate carbon isotope record of the Phanerozoic is marked by repeated high-amplitude excursions towards more positive values. Although the interpretation of C-isotope events remains controversial, they are regarded as a proxy of the global C-cycle. Using δ¹³C_Org-measurements of samples from DSDP Site 416, it is shown that a classic example of platform drowning coincided with the onset of the mid-Valanginian carbon-cycle excursion. Widespread carbonate platform drowning would have contributed to the observed positive shift in the C-isotope record. As choking of carbonate production was compensated by increased organic production, both processes favoured a shift in marine carbon partitioning from the oxidized to the reduced carbon reservoir. This would have resulted in an increased flux of CO₂ from the atmosphere into the marine and sedimentary carbon reservoir.     

Korallenriffe—Bedrohungen,Schutzkonzepte und Rehabilitationsmaßnahmen

Ocean Dynamics - Korallenriffe sind der artenreichste Lebensraum im Meer und zugleich der wirkungsvollste Schutz tropischer Küsten. Die hermatypischen Organismen konvertieren durch die...     

The soil hydrologic response to forest regrowth: a case study from southwestern Amazonia

As a large and dynamic land‐use category, tropical secondary forests may affect climate, soils, and hydrology in a manner different from primary forests or agricultural areas. We investigated the saturated hydraulic conductivity K_sat of a Kandiudult under different land uses in Rondonia, Brazil. We measured K_sat at four depths (12·5, 20, 30 and 50 cm) under (a) primary forest, (b) a former banana–cacao plantation (SF1), and (c) an abandoned pasture (SF2). At 12·5 cm, all three land uses differ significantly (α = 0·1), but not at the 20 and 30 cm depths. At 50 cm, K_sat was significantly greater in the former pasture than in other land uses. Lateral subsurface flow is expected during intense rainfall (about 30 times per year) at 30 cm depth in SF1 and at 50 cm depth in the forest, whereas the relatively low permeability at shallow 12·5 cm in the SF2 may result not only in lateral subsurface flow, but also saturation overland flow. For modelling purposes, recovering systems seem to have K_sat values distinct from primary forest at shallow depths, whereas at deeper layers (>20 cm) they may be considered similar to forests. Copyright © 2002 John Wiley & Sons, Ltd.     

14C age and vertical mixing of deep-sea sediments

Vertical mixing models applied to the radiocarbon age stratigraphy of sediments have to make allowance for the presumably non-vanishing zero-age of the sedimentary particulates freshly arriving at the sediment surface. The appropriate modifications of the simple box model recently presented by Berger and Johnson are outlined. For seven deep-sea cores, zero-age values are found varying between 0.1 and 1.4 kyr. A high zero-age is likely to be indicative for redistribution processes occurring at the sea floor. The box model has been extended to make allowance for a change of sedimentation rate in the depositional interval under study.