Eodiagenetische Vorgänge bei der Bildung von Salinargesteinen

Zusammenfassung Chemische Präzipitate bleiben als Sedimente nur so lange erhalten, als sie mit der darüberstehenden Lösung, dem Bodenwasser des Salinarbeckens, im chemischen Gleichgewicht stehen. Die in dem Laugenkörper solcher Meeresbecken bestehenden, horizontalen Dichtegrenzen, die auf Konzentrationsunterschieden beruhen, können sich durch klimatische, meeresgeographische u. a. Veränderungen vertikal verschieben. Dadurch ist das Sediment (der Bodenkörper) Umbildungen ausgesetzt. Verdünnung der Lauge bringt entweder totale Wiederauflösung oder selektive Rücklösung der leichtestlöslichen Sediment-Anteile mit sich: durch NaCl-Auflösung bei Haliten ein tonig-anhydritisches Residuum (hardground), Sylvinitisierung (bis Halitisierung) durch Ent-Carnallitisierung (MgCl₂-Wegführung) bei Kalisalzen. Tiefgreifende Umkristallisationen und Entstehung von polygonalen Pfeilern erheblicher Tiefenerstreckung sprechen für Stillstand der Sedimentation und für subaquatische Verkarstung. Auf mechanische Umformungen durch Wellenschlag gehen wohl Augensalze und Knollenanhydrite zurück (Tempestite). An paläogeographisch geeigneten Positionen entstanden durch Gleitbewegungen (slumping) Breccienbänke und Falten erheblichen Ausmaßes in Anhydriten und Kalisalzen. Alle diese Vorgänge werden in das früheste Stadium der Diagenese eingeordnet.Zur Vermeidung der aus zwei verschiedenen Sprachen zusammengesetzten Wortkombination Frühdiagenese wird die BezeichnungEodiagense hiermit in Vorschlag gebracht.

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Chemical deposits are only preserved as sediments when they are in chemical equilibrium with the lowermost part of the brine basin. The brine bedding, caused by density, or concentration respectively, can be disturbed by changes of climate, by currents a. s. o. When the brine boundaries are shifted in vertical direction the bottom sediment may be transformed. A thinning of the brine will give rise either to total re-dissolution or to any selective removal of the highest soluble substance, f. i. NaCl from rock salt, by leaving anhydritic or marly residuals (hardground), or MgCl₂ from Carnallite, by leaving Sylvite etc. Breaks within the progressive sedimentation produce polygonal structures reaching several meters down into the pre-existing halite sediment. And thatone may be recrystallized in big scale. Physical influence, as movement in the waterbody by wind and waves, gave rise to so-called augensalz, fragmental anhydrites a. s. o. Brecciated anhydrites as well as huge creeping folds in anhydrites and in potash beds have to be explaned by slumping.All these transformations are caused by early diagenesis. For avoiding the term early diagenesis≓ which is composed by 2 words of different languages, the wordEodiagenesis is proposed herewith.

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Résumé Les sédiments chimiques ne sont conservés qu'aussi longtemps qu'ils sont en équilibre chimique avec la partie basale de la saumure qui se trouve au-dessus d'eux. Les plans de stratification, dûs aux différences de densité liées à la concentration en sels dans la saumure peuvent se déplacer verticalement sous l'action d'influences climatiques, océaniques, tectoniques etc., ce qui expose le sédiment déposé à des transformations. Une dilution de la saumure peut provoquer une redissolution totale ou une dissolution sélective des sels suivant leur solubilité. Par example: dans les halites normales, la solubilisation du NaCl laissera comme résidu un »hardground« composé d'une substance argiloanhydritique; dans les sels potassique, par la dissolution sélective du MgCl₂, la carnallitite sera transformée en sylvinite. — Dans nombre de gisements de halite des structures en pilliers polygonaux de quelques mètres de puissance, où la halite peut Être complètement récrystallisée sont probablement à attribuer à un arrÊt de la sédimentation et à une karstification.Des textures »conglomératiques« observées dans les anhydrites et dans les halites (»Knollenanhydrit«, »Augensalz«), sont à rapporter à l'action des vagues et pourraient donc Être des »tempestites«. Des bancs bréchoides et les plis d'ampleur considérable dans les anhydrites et les sels potassiques ne peuvent Être expliqués que par des glissements subaquatiques.Tous ces événements sont à classer dans un stade très précoce de la diagénèse. Pour éviter le terme de »Frühdiagenese« qui provient de deux langues différentes, il est proposé ici, de désigner ces changements diagénétique précoces sous le nom de »éodiagénèse«.

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Synthesis and thermal stability of 2 \frac{1}{2}-octahedral sodium mica,NaMg2.5 (OH)2∫i4O10]

A new 2 \(\frac{1}{2}\) -octahedral sheet silicate, NaMg_2.5 Si₄O₁₀ (OH)₂, has been synthesized from oxide mixtures in the temperature range 500–600°C at pressures between 1 and 5 kb. The lattice parameters are a ₀ = 5.298 Å, b ₀ = 9.047 Å, c ₀ = 9.479 Å and ß=99.55°. X-ray data are given in the text. At temperatures above 605° C/1 kb and 630° C/5 kb, it decomposes to magnesiorichterite plus quartz.     

Strain determination from the measurement of pebble shapes

Cross-sections of pebbles or other near-ellipsoidal bodies in rocks can be measured in three planes, conveniently chosen orthogonal to each other, but otherwise oriented arbitrarily. The three sets of measurements, referred to a single Cartesian coordinate system, can in principle be used to calculate the average pebble shape. Some of these measurements, however, are necessarily redundant and, generally, not compatible without adjustments for the distribution of an error. The calculation of the average ellipsoid is developed, as is that of the strain which transforms an original sphere into such an ellipsoid. This calculation involves the distribution by a weighted least-squares method of the error due to redundancy and leads to the assignment of an estimated error to each of the second-rank tensor components of the strain. The assumption is made that, after being referred to the common coordinate system, the measured values for each tensor component have a normal distribution. This assumption may not always be correct but could be tested before this method is used.     

Thermal metamorphism of primitive meteorites—IX. On the mechanism of trace element loss from Allende heated up to 1400° C

We report data for Ag, Bi, Cd, Co, Cs, Ga, In, Se, Te, Tl and Zn determined by neutron activation in Allende samples heated for one week at 100° increments in the 1000–1400°C range in a low pressure (initially 10^?5atmH₂) environment using an apparatus of novel design. In the extremes, concentrations of these trace elements—initially present at ppm-ppb levels—in unheated material are lowered even farther by factors of 10^?4–10^?5 over a broad temperature span. Loss of some elements above 1000°C extends trends evident below 1000°C; loss of others is even more extreme. On Arrhenius diagrams some elements exhibit but one apparent activation energy over the entire temperature span of loss while others exhibit 2 or 3, each operative in a particular temperature region. These discontinuities seem related to mineralogic/petrologic alteration and probably reflect differences in diffusion mechanism rather than siting differences. The extension of previous experiments into the temperature regime postulated for chondritic differentiation should lead to a better understanding of the evolution of meteoritic parent bodies.     

Location and height of intertidal bars on macrotidal ridge and runnel beaches

Previous studies devoted to the morphology and hydrodynamics of ridge and runnel beaches highlight characteristics that deviate from those initially postulated by King and Williams (Geographical Journal, 1949, vol. 113, 70–85) and King (Beaches and Coasts, 1972, Edward Arnold). Disagreements on the morphodynamics of these macrotidal beaches include the position of the ridges relative to the mean neap and spring tide levels, the variation in the height of the ridges across the intertidal profile and, most importantly, whether the ridges are formed by swash or surf zone processes. The morphological characteristics of ridge and runnel beaches from three locations with varying wave, tidal and geomorphic settings were investigated to address these disagreements. Beach profiles from each site were analysed together with water‐level data collected from neighbouring ports. It was found that the ridges occur over the entire intertidal zone. On one site (north Lincolnshire, east England), the ridges are uniformly distributed over the intertidal beach, whereas on the two other sites (Blackpool beach, northwest England, and Leffrinckoucke beach, north France) there is some indication that the ridges appear to occur at preferential locations. Most significantly, the locations of the ridge crests were found to be unrelated to the positions on the intertidal profile where the water level is stationary for the longest time. It was further found that the highest ridges generally occur just above mid‐tide level where tidal non‐stationarity is greatest. These findings argue against the hypothesis that the ridges are formed by swash processes acting at stationary tide levels. It is tentatively suggested that the ridges are the result of a combination of swash and surf zone processes acting across the intertidal zone. Elucidation of the morphodynamic roles of these two types of processes, and other processes such as strong current flows in the runnels, requires further comprehensive field measurements complemented by numerical modelling. Copyright © 2001 John Wiley & Sons, Ltd.