Eelgrass (Zostera marina L.) as an indicator organism of trace metals in the Limfjord,Denmark

The concentrations of Pb, Cu, Cd and Zn were determined in above- and belowground parts of eelgrass (Zostera marina L.) at forty stations in a shallow, brackish water area (the Limfjord, Denmark). The concentrations of the trace metals were significantly elevated near the cities of Aalborg (Pb, Cu) and Struer (Cd). Trace metal concentrations in above and belowground parts of eelgrass were log-normal distributed and the concentrations of Cd, Cu and Zn in aboveground parts were significantly higher than in belowground parts. Furthermore, a significant correlation between trace metal concentrations in above- and belowground parts was found. The background level of trace metal concentrations in eelgrass in the Limfjord was estimated.The application of eelgrass as a monitoring organism is discussed; it is suggested that the concentration of some trace metals in above- and belowground parts of eelgrass may be used as a measure of the bioavailable fraction of these trace metals in ambient and interstitial water (sediment), respectively.     

Pre-Loessic Faulting in the Nanking Hills

During the last year I had ample opportunity for geologicalexcursions in the surroundings of Nanking,both with the students of theDepartment of Geology of National Central University,and in connectionwith fieldwork for the Geological Research Institute of the AcademiaSinica.From casual observations on these trips,I came to some     

Some aspects on the origin of fold-type fabrics — theory,experiments and field applications

An applicable interpretation of fabrics should be based mainly on geometrical considerations in order to cover available field data. Therefore a theory on the formation of foldtype fabrics including congruent and concentric flexural-slip folds as well as kink bands in materials subjected to rhombic and different monocline strain types is derived by means of particle motion fields for homogeneous and isotropic bodies. The analysis of experimental results and their comparison with field observations largely confirms the theory and contributes to its improvement. Some trends can be established: With increasing monocline character of the strain type, the probability of concentric and congruent flexural-slip folding is reduced. It is substituted by kink band formation. While the portion of monocline strain is enlarged with depth, rhombic fold symmetry indicates, in the realm of elastic-plastic behaviour, the proximity of the surface of the earth or of a detachment surface. With gradual increase of the rock anisotropy, the development of shear faults, kink folds and, finally, congruent and concentric flexural-slip folds is favoured.     

Some aspects on the origin of fold-type fabrics — theory,experiments and field applications

An applicable interpretation of fabrics should be based mainly on geometrical considerations in order to cover available field data. Therefore a theory on the formation of foldtype fabrics including congruent and concentric flexural-slip folds as well as kink bands in materials subjected to rhombic and different monocline strain types is derived by means of particle motion fields for homogeneous and isotropic bodies. The analysis of experimental results and their comparison with field observations largely confirms the theory and contributes to its improvement. Some trends can be established: With increasing monocline character of the strain type, the probability of concentric and congruent flexural-slip folding is reduced. It is substituted by kink band formation. While the portion of monocline strain is enlarged with depth, rhombic fold symmetry indicates, in the realm of elastic-plastic behaviour, the proximity of the surface of the earth or of a detachment surface. With gradual increase of the rock anisotropy, the development of shear faults, kink folds and, finally, congruent and concentric flexural-slip folds is favoured.     

The N Variscan margin of the Ruhr coal district (Western Germany): structural style of a buried thrust front?

The Ruhr coal district represents part of an external fold and thrust belt of the Variscan orogen in central Europe. Extensive mining exposed molasse-type, coal-bearing clastic sequences of Upper Carboniferous age, deformed by folding and thrusting.Thrusts range in scale from decameters to 40 kilometers along strike but displacements are below 2.5 km. They are distributed in various stratigraphic levels and no regional detachment horizon is exposed. Displacement is mainly towards NW (towards the foreland) but also towards SE. The thrusts are bound to stockwerk-tectonics and die out towards depth frequently antithetic to the exposed strata.Folds vary in wavelength from decameters to more than 10 kilometers. Transitions from cylindrical or concentric to chevron-type folds are common. They have also been modelled in experiments. There, varying fold types compensate volume problems, created (e.g.) during concentric folding, without a major detachment.Another mechanism of compensating excess volume during folding is evident, where thrusts transport volume from fold hinge areas to fold limbs. Folds generally deform both footwall and hangingwall of thrusts. Numerous observations prove a contemporary development of thrusts in mechanic relation to the folding-process.Overall orogenic shortening decreases slowly from SE to NW, towards the foreland without a major break. The presented structural observations and experimental data point to the fact that orogenic shortening in the marginal fold and thrust belt of the Ruhr district may well be compensated by folding and minor thrusting without major detachment. Therefore, the buried tip line of a regional Variscan detachment horizon may be situated towards the internal parts of the orogen, southeast of the Ruhr district.

---

Zusammenfassung Das Ruhrkarbon ist Teil des externen Falten- und Überschiebungsgürtels der Varisziden in Mitteleuropa. Ausgedehnte Bergbauaufschlüsse geben hier Einblick in die gefaltete und zerblockte, molasseartige Schichtenfolge des Oberkarbons.Die aufgeschlossenen Überschiebungen erreichen hier eine streichende Länge von maximal 40 km, die Schubweiten aber allerhöchstens 2.5 km. Die Überschiebungen treten in unterschiedlichen stratigraphischen Niveaus auf; ein regionaler Abscherhorizont ist nicht aufgeschlossen. Die Schubbewegungen sind vorwiegend nach NW in Richtung auf das Vorland gerichtet, es treten aber auch zahlreiche Überschiebungen auf, die nach SE gerichtet sind. Das Auftreten der Überschiebungen ist an ein bestimmtes tektonisches Stockwerk gebunden. Sie laufen zur Tiefe hin häufig antithetisch zur Schichtung aus.Die Spannweite der Falten reicht von Zehnermetern bis zu über 10 km. Übergänge zwischen zylindrischen oder konzentrischen Falten zu Knickfalten sind allgemein verbreitet. Sie lassen sich auch experimentell nachbilden, wobei die verschiedenen Faltentypen Volumenprobleme lösen, die bei konzentrischer Faltung auftreten, ohne daß eine größere Abscherung erfolgt.Eine andere Möglichkeit Volumenüberschüsse abzubauen, ist dort verwirklicht, wo Überschiebungen Material von den Faltenkernen zu den Faltenflanken transportieren. Im allgemeinen setzen sich die Faltenachsen über die Überschiebungen hinweg fort, so daß Hangend- und Liegendscholle gleichartig verfaltet sind. Dies und zahlreiche andere Beobachtungen zeigen, daß die Überschiebungen zeitgleich mit und in mechanischer Abhängigkeit von der Faltung entstanden sind.Die generelle orogene Einengung nimmt im Ruhrkarbon allmählich von SE nach NW, d. h. in Richtung auf das Vorland ab, ohne daß eine sprunghafte Änderung dieses Gradienten eintritt.Die dargelegten Beobachtungen und experimentellen Ergebnisse zur Strukturgeologie deuten darauf hin, daß die orogene Einengung des Ruhrkarbons allein durch Faltung und untergeordnete Überschiebungstektonik erfolgt sein dürfte, ohne daß die Annahme eines größeren Abscherhorizontes erforderlich wäre. Aus diesem Grunde kann mit dem Auftreten großregionaler varistischer Abscherhorizonte im Untergrund erst südlich vom Ruhrgebiet, also näher am internen Bereich des Orogens, gerechnet werden.

---

Résumé Le Carbonifère de la Ruhr fait partie de la zone externe plissée et charriée du tectogène hercynien en Europe occidentale. La série stratigraphique du Carbonifère supérieur, de type molassique, plissée et fracturée, est bien connue grâce aux nombreuses exploitations minières.Les chevauchements peuvent y être suivis en direction sur 40 km au maximum, mais leurs rejets ne dépassent pas 2.5 km. Ces failles se rencontrent dans des niveaux stratigraphiques différents. Aucun décollement d'importance régionale n'a été décelé. Le transport est surtout dirigé vers l'avant-pays (vers le nord-ouest), mais il existe aussi de nombreux chevauchements vers le sud-est. La présence des chevauchements est liée à un certain étage tectonique. Vers la profondeur, ils se terminent souvent dans une position antithétique par rapport à la stratification.Les plis sont décamétriques à plurikilométriques (> 10 km). Des transitions entre des plis isopaques et des plis aigus de type «en chevron» sont fréquentes. La coexistence de ces diverses formes de plis, réalisée aussi expérimentalement, permet d'éliminer les problèmes de volume soulevés par un plissement uniquement isopaque, sans faire intervenir un décollement important. Un autre mécanisme d'élimination des excédents de volume est représenté par des chevauchements qui transportent des matériaux depuis les zones de charnières vers les flancs des plis. En général, les surfaces axiales se poursuivent à travers ces failles dont le toit et le mur ont été plissés de la même façon. Cette disposition, jointe à d'autres observations, indique que les chevauchements ont été contemporains du plissement et en interdépendance mécanique avec celui-ci.Dans le Carbonifère de la région de la Ruhr, le raccourcissement orogénique diminue du sud-est vers le nord-ouest, c'està-dire vers l'avant-pays, de manière progressive, sans variation brusque.En conclusion, le raccourcissement orogénique du Carbonifère de la région de la Ruhr a été réalisé par les plis et les chevauchements mineurs associés, sans décollement d'importance régionale. Pour cette raison, l'existence de tels décollements en profondeur ne peut être envisagée qu'au sud de la région de la Ruhr, donc dans une zone plus interne de l'orogène.

---

. , , . 40 , — , — 2,5 . . . . . NW, SE . - . , . 10 . , . , , . , . , , . , . SE NW, . . . , , , . , . . .

     

Stability Relations of the Ferruginous Biotite, Annite

Annite, KFe₃AISi₃O₁₀(OH)₂ a member of the iron biotites andthe ferrous analogue of phlogopite, has been synthesized andits phase relations have been determined as functions of temperature,fugacity of oxygen (fo₂), and total pressure (P_totalPH₂O+PH₂).A method for controlling fo₂at high total pressures is described,and data for the ‘oxygen buffers’ used are given.Buffers range from quartz+iron+fayalite assemblages (low fo₂)to magnetite-hematite assemblages (high fo₂). Optical propertiesand unit-cell dimensions of synthetic annites depend on theconditions of synthesis. By recalculating published analyses of natural iron-rich biotitesit can be shown that one cannot assume a constant hydrogen contentfor such biotites. Oxidation may have occurred by drying at115?C. Octahedral occupancy therefore cannot be calculated fromsuch data. Phase relations of annite are presented in 2,070 and 1,035 barsections. Depending on fo₂-T values annite was found to decomposeto one of the following assemblages: hematite+ sanidine, magnetite+sanidine,fayalite+leucite+kalsilite, iron+sanidine. All decompositionsare dehydration and redox reactions and are sensitive to changesin fH₂0 and fo₂ (or fH₂0 and fH₂). At 2, 070 bars total pressureannite+magnetite+sanidine can coexist between 425?C and 825?C, depending upon the magnitude of fo₂. In the presence of quartz the stability field of annite is morerestricted. Phase equilibria in the system KAlSiO₄–SiO₂–Fe–O₂–H₂have been summarized schematically. Wherever possible, thermodynamic extrapolations are made totest the internal consistency of the data. Enthalpies of formationare calculated for both annite and phlogopite. Ranges of fo₂values in nature as well as mechanisms for changes in fo₂ areinvestigated. It is useful to distinguish between assemblageswhich are internally buffered with respect to fo₂changes andthose which are not buffered. The applications of individualreactions involving annite to specific geologic problems arediscussed with respect to igneous, metamorphic, and sedimentaryrocks.