Drowning history of a Miocene carbonate platform (Zhujiang Formation, South China Sea)

The identification and interpretation of drowning events in the geologic record can aid significantly to the reconstruction of the depositional, tectonic and eustatic history of a study area and often improve reservoir and seal prediction in carbonate rocks. The differentiation between drowned platforms showing a record of continuous deepening and those with a record of exposure followed by rapid deepening remains, however, problematic. The Zhujiang carbonate platform (Liuhua 11-1 field, South China Sea) study shown here provides an example of an integrated approach combining high-resolution geochemistry, microfacies analyses and foraminiferal biostratigraphy in order to improve the reconstruction of environmental conditions prior, during and after platform demise and drowning. The Zhujiang carbonate platform displays the following vertical succession of four facies types i) skeletal grain facies with a miogypsinid/lepidocyclinid-dominated fauna deposited in a moderately deep (< 50 m), oligotrophic back-reef setting; ii) in situ corals in patch-reef facies in an oligotrophic lagoon (< 10 m); iii) rhodoid facies with in situ red algal crusts, dominated by Heterostegina sp. and spiroclypeids, possibly capped by a subaerial exposure surface. Well-rounded rhodoids representing a mesotrophic lagoon dominate the upper portions of the rhodoid facies; iv) pelagic marine shales of the Hanjiang Formation burying the carbonate platform after drowning. This facies succession, in combination with geochemical evidence suggests a deepening-upward trend. This trend might have been interrupted by transient subaerial exposure but no evidence for meteoric diagenesis was found at the drowning unconformity topping the carbonate platform. Instead, microfacies analyses suggest that platform demise may be related to progressive changes in environmental conditions, including increasing nutrient-levels and/or decreasing temperature up-core towards the drowning unconformity. These findings are of significance for those concerned with Miocene carbonate factories and, more specifically, the demise of carbonate platforms in general.     

Stochastic Modelling of Mineral Exploration Targets

Mathematical Geosciences - Mineral deposits are metal enrichment anomalies, occurring as local manifestations of the interplay between various geological processes that operate at a wide range of...     

The wetting ability of Si-bearing liquid Fe-alloys in a solid silicate matrix—percolation during core formation under reducing conditions?

The wetting characteristics of liquid Fe–Si alloys in a matrix of the respective predominating stable silicate mantle mineral (forsterite or silicate perovskite) at pressures of 2–5 and 25 GPa and temperatures of 1600–2000 °C were studied by determining the liquid metal–solid silicate contact angles. The median angle values from texturally equilibrated samples were found to be independent of pressure, temperature, silicate mineralogy and the Si content in the metal fraction and range between 130° and 140° which is far above the critical wetting boundary of 60°. This shows that within the studied range of conditions dissolved Si does not lower the surface energies between Fe-rich liquids and silicate mantle grains. As a consequence, under reducing conditions the presence of Si in the metal phase of planetary bodies would not have enhanced percolative flow as an effective metal–silicate separation process.     

Mössbauer and molecular orbital study of chlorites

The different Fe²⁺ lattice sites in iron-rich chlorites have been characterized by Mössbauer spectroscopy and molecular orbital calculations in local density approximation. The Mössbauer measurements were recorded at 77?K within a small velocity range (±3.5?mm?s^?1) to provide high energy resolution. Additionally, measurements were recorded in a wider velocity range (±10.5?mm?s^?1) at temperatures of 140, 200, and 250?K in an applied field (7?T) parallel to the γ-beam. The zero-field spectra were analyzed with discrete Lorentzian-shaped quadrupole doublets to account for the Fe²⁺ sites M1, M2, and M3 and with a quadrupole distribution for Fe³⁺ sites. Such a procedure is justified by the results obtained from MO calculations, which reveal that different anion (OH^?) distributions in the first coordination sphere of M1, M2, and M3 positions have more influence on the Fe²⁺ quadrupole splitting than cationic disorder. The spectra recorded in applied field were analyzed in the spin-Hamiltonian approximation, yielding a negative sign for the electric field gradient (efg) of Fe²⁺ in the M1, M2, and M3 positions. The results of the MO calculations are in quantitative agreement with experiment and reveal that differences in the quadrupole splittings (ΔE _Q ), their temperature dependence and in the isomer shifts (δ) of Fe²⁺ in M1, M2, and M3 positions can theoretically by justified. Therefore, the combined Mössbauer and MO investigation shows that the three Fe²⁺ lattice sites in the chlorites investigated here can be discriminated according to their ΔE _Q -δ parameter pairs. With the calculated average iron-oxygen bond strength, the MO study provides an explanation for the observed trend that the population of the three lattice sites by Fe²⁺ increases according to the relation M1?相似文献