Early Cambrian and latest Proterozoic stratigraphy,Desert Syncline,southern Georgina Basin

In the Desert Syncline of the southern Georgina Basin there is an Early and Middle Cambrian sequence unconformably overlying late Proterozoic sediments. Stratigraphic drilling and subsequent palaeontological studies have allowed the documentation of the sequence across the Proterozoic‐Cambrian unconformity. Earliest Cambrian green shales are bioturbated and contain distinctive acritarchs. These are overlain, probably unconformably, by sandstone with Diplocraterion burrows, in turn succeeded by archaeocyathan dolostone. Ordian and Templetonian (Middle Cambrian) shales and carbonates unconformably overlie the Early Cambrian sequence. The stratigraphic sequence is very similar to that in the Amadeus Basin and the Adelaide Geosyncline.     

Stable isotope,petrological, and fluid inclusion studies of minor mineral deposits from the McArthur Basin: Implications for the genesis of some sediment‐hosted base metal mineralization from the Northern Territory

The geology, stable isotopes and fluid inclusions from mineralized and unmineralized Middle Proterozoic sequences of the McArthur Basin, Northern Territory, have been studied at Eastern Creek, Bulman Mines, Beetle Springs, and other localities in the McArthur Basin where disseminated sulphides in unmineralized black shales were available from drill core. At Eastern Creek, galena and minor chalcopyrite (δ³⁴S+3.6 to +11.2%o) occur in an evaporitic sedimentary sequence. Barite (δ³⁴S+18.4 to +24.7%o) also occurs, and saline brines are trapped along healed fractures in the barite. Pressure‐corrected trapping temperatures in the barite (95–138°C), and in vein dolomite (158–168°C) agree with temperature estimates from the degree of maturation of the sedimentary organic matter. The δ¹⁸O and δ¹³C_Co2 values of the mineralizing fluid were calculated to be +3.5 to +4.5%o and ‐2.7%o, respectively. Sedimentary dolomite has restricted δ¹³C and δ¹⁸O ranges, within the reported ranges for non‐mineralized Middle Proterozoic dolomite. An ore formation model developed for Eastern Creek, in which a basinal fluid at about 200°C carrying base metals and sulphide was released from underlying sediments during local fault movement, may be applicable to a number of other deposits. The mineralization deposited from these fluids occurs only below the pre‐Roper Group unconformity, implying that it may be older than the basal Roper Group. The δ³⁴S values of iron sulphides in fine grained black dolostones (not associated with mineral deposits) from the McArthur Basin were assessed in the light of the values found for sulphides in modern organic‐rich sedimentary environments. The data so obtained suggest that the considerable concentration of iron sulphide in the mineral deposits formed, at least in part, from heated basinal waters and that disseminated iron sulphides remote from mineralization also formed from a similar source.     

Chemical weathering of a marine terrace chronosequence, Santa Cruz, California. Part II: Solute profiles, gradients and the comparisons of contemporary and long-term weathering rates

The spatial and temporal changes in hydrology and pore water elemental and ⁸⁷Sr/⁸⁶Sr compositions are used to determine contemporary weathering rates in a 65- to 226-kyr-old soil chronosequence formed from granitic sediments deposited on marine terraces along coastal California. Soil moisture, tension and saturation exhibit large seasonal variations in shallow soils in response to a Mediterranean climate. These climate effects are dampened in underlying argillic horizons that progressively developed in older soils, and reached steady-state conditions in unsaturated horizons extending to depths in excess of 15 m. Hydraulic fluxes (q_h), based on Cl mass balances, vary from 0.06 to 0.22 m yr⁻¹, resulting in fluid residence times in the terraces of 10-24 yrs.As expected for a coastal environment, the order of cation abundances in soil pore waters is comparable to sea water, i.e., Na > Mg > Ca > K > Sr, while the anion sequence Cl > NO₃ > HCO₃ > SO₄ reflects modifying effects of nutrient cycling in the grassland vegetation. Net Cl-corrected solute Na, K and Si increase with depth, denoting inputs from feldspar weathering. Solute ⁸⁷Sr/⁸⁶Sr ratios exhibit progressive mixing of sea water-dominated precipitation with inputs from less radiogenic plagioclase. While net Sr and Ca concentrations are anomalously high in shallow soils due to biological cycling, they decline with depth to low and/or negative net concentrations. Ca/Mg, Sr/Mg and ⁸⁷Sr/⁸⁶Sr solute and exchange ratios are similar in all the terraces, denoting active exchange equilibration with selectivities close to unity for both detrital smectite and secondary kaolinite. Large differences in the magnitudes of the pore waters and exchange reservoirs result in short-term buffering of the solute Ca, Sr, and Mg. Such buffering over geologic time scales can not be sustained due to declining inputs from residual plagioclase and smectite, implying periodic resetting of the exchange reservoir such as by past vegetational changes and/or climate.Pore waters approach thermodynamic saturation with respect to albite at depth in the younger terraces, indicating that weathering rates ultimately become transport-limited and dependent on hydrologic flux. Contemporary rates R_solute are estimated from linear Na and Si pore weathering gradients b_solute such that

     

THE COMPOSITION OF THE CHASSIGNY METEORITE

Spark source mass spectrometric analysis of the Chassigny meteorite has given the following data (in ppm): Rb 0.4, Sr 7.2, Y 0.64, Zr 1.5, Nb 0.32, Ba 7.1, La 0.39, Ce 1.12, Pr 0.13, Nd 0.54, Sm 0.11, Eu 0.038, Gd 0.11, Tb 0.02, Dy 0.12, Ho 0.03, Er 0.09, Yb 0.10, Pb 1.0, Th 0.057, U 0.021. These data, in conjunction with major element composition and mineralogical and textural features, indicate that this meteorite is an olivinerich cumulate, possibly genetically related to the nakhlites.     

The distribution of Fe2+ and Fe3+ in iron-bearing tourmalines: A Mössbauer study

Summary Room-temperature Mössbauer spectra of five iron-bearing tourmalines were measured and analyzed. The Fe²⁺/Fe³⁺ ratio and the iron occupancy of the Y and Z positions could be assigned to all samples, with the help of two previously well characterized samples, from Mexico and Madagascar. Ferric or ferrous ions or both partially occupy the Z as well as the Y octahedra. This fact of observation is interpreted as the chemical response, during crystal growth, to the requirement of size matching for the edge-sharing Y and Z oxygen octahedra. It accounts for the inexistence of solid solution between the Mg and (Li, Al) tourmalines.

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Die Verteilung von Fe²⁺ und Fe³⁺ in eisenhaltigen Turmalinen: Eine Mössbauer-Untersuchung

Zusammenfassung Mössbauer-Spektren von fünf eisenhaltigen Turmalinen wurden gemessen und analysiert. Das Verhältnis Fe²⁺/Fe³⁺ und die Eisenverteilung konnten mit Hilfe von zwei gut identifizierten Turmalin-Kristallen von Mexiko und Madagascar für die Y-und Z-Lagen aller Exemplare bestimmt werden. Zweiwertiges sowie dreiwertiges Eisen findet sich sowohl in der Z-als auch in der Y-Lage. Da sich die Y-und Z-Oktaeder in einer gemeinsamen Kante treffen, wird diese Beobachtung als chemische Antwort des Kristalles auf die erforderte Größenanpassung der Y-und Z-Oktaeder während seines Wachstums erklärt. Die Abwesenheit der festen Lösung zwischen Dravit und Elbait kann somit erklärt werden.

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With 4 Figures     

Dévitrification et altération supergène d'une rhyolite (bassin du Paraná, Brésil) : mécanismes et bilans géochimiques

The weathering of a rhyolitic lava flow of the Paraná Basin (Brazil) developed, on few centimeters only, simplified petrographic features. The initial meteoric fluid pass-way is marked by celadonite-to-nontronite transformation. The second step is the complete dissolution of plagioclases, and the last one is the massive precipitation of halloysite. The geochemical mass balances according to the alteration features demonstrate the progressive leaching of major and trace elements fractionated by plagioclases, glass and opaques, and the sorption of Cu>As>Sb in the halloysite matrix. To cite this article: M. Bernard et al., C. R. Geoscience 336 (2004).     

Numerical simulation of dynamic soil–structure interaction in shaking table testing

This paper provides an insight into the numerical simulation of soil–structure interaction (SSI) phenomena studied in a shaking table facility. The shaking table test is purposely designed to confirm the ability of the numerical substructure technique to simulate the SSI phenomenon. A model foundation–structure system with strong SSI potential is embedded in a dry bed of sand deposited within a purpose designed shaking-table soil container. The experimental system is subjected to a strong ground motion. The numerical simulation of the complete soil–foundation–structure system is conducted in the linear viscoelastic domain using the substructure approach. The matching of the experimental and numerical responses in both frequency and in time domain is satisfying. Many important aspects of SSI that are apparent in the experiment are captured by the numerical simulation. Furthermore, the numerical modelling is shown to be adequate for practical engineering design purposes.