The Dutung-Thaktote extensional fault zone and nappe structures documented by illite crystallinity and clay-mineral paragenesis in the Tethys Himalaya between Spiti river and Tso Morari, NW India

Numerous measurements by XRD of the Scherrer width at half-peak height (001 reflection of illite), coupled with analyses of clay-size assemblages, provide evidence for strong variations in the conditions of low temperature metamorphism in the Tethyan Himalaya metasediments between the Spiti river and the Tso Morari. Three sectors can be distinguished along the Spiti river-Tso Morari transect. In the SW, the Takling and Parang La area is characterised by a metamorphism around anchizone-epizone boundary conditions. Further north, in the Dutung area, the metamorphic grade abruptly decreases to weak diagenesis, with the presence of mixed-layered clay phases. At the end of the profile towards the NE, a progressive metamorphic increase up to greenschist facies is recorded, marked by the appearance of biotite and chloritoid. The combination of these data with the structural. observations permits to propose that a nappe stack has been crosscut by the younger Dutung-Thaktote extensional fault zone (DTFZ). The change in metamorphism across this zone helps to assess the displacements which occurred during synorogenic extension. In the SW and NE parts of the studied transect, a burial of 12 km has been estimated, assuming a geothermal gradient of 25 degrees C/km. In the SW part, this burial is due to the juxtaposition of the Shikar Beh and Mata nappes and in the NE part, solely to burial beneath the Mata nappe. In the central part of the profile, the effect of the DTFZ is to bring down diagenetic sediments in-between the two aforesaid metamorphic zones. The offset along the Dutung-Thaktote normal faults is estimated at 16 km.

Illite "crystallinity'' revisited

The Kubler Index (KI) is defined as the full width at half-maximum height (FWHM) of the 10-Angstrom X-ray diffraction peak of illite-smectite interstratified (I-S) clay minerals, The only parameters controlling the Kubler index are assumed to be the mean number of layers (N) in the coherent scattering domains (CSD). the variance of the distribution of the number of layers of the CSB, the mean percentage of smectite layers in I-S (%S), and the probability of layer stocking (Reichweite). The Kubler-Index measurements on ah-dried (KIAD) and ethylene-glycolated (KIEG) samples were compared to N and %S using the NEWMOD computer program to simulate X-ray diffraction patterns. Charts of KIAD versus KIEG corrected for instrumental broadening were made and isolines were mapped for constant N and CLS. Isolines allow a direct and rapid determination of N and RS from KI measurements. The method allows quantification of the metamorphic anchizone limits by considering mean thickness of fundamental particles in MacEwan crystallites. The transition from diagenesis to the anchizone and from the anchizone to the epizone of low-grade metamorphism corresponds to thicknesses of 20- and 70-layer fundamental particles, respectively.

Illite and hydrocarbon exploration

Illite is a general term for the dioctahedral mica-like clay mineral common in sedimentary rocks, especially shales. Illite is of interest to the petroleum industry because it can provide a K-Ar isotope date that constrains the timing of basin heating events. It is critical to establish that hydrocarbon formation and migration occurred after the formation of the trap (anticline, etc.) that is to hold the oil. Illite also may precipitate in the pores of sandstone reservoirs, impeding fluid flow. Illite in shales is a mixture of detrital mica and its weathering products with diagenetic illite formed by reaction with pore fluids during burial. K-Ar ages are apparent ages of mixtures of detrital and diagenetic end members, and what we need are the ages of the end members themselves. This paper describes a methodology, based on mineralogy and crystallography, for interpreting the K-Ar ages from illites in sedimentary rocks and for estimating the ages of the end members.

Hydrothermal control on organic matter alteration and illite precipitation, Mt Isa Basin, Australia

Abundant illite precipitation, in Proterozoic rocks from Northern Lawn Hill Platform, Mt Isa Basin, Australia, occurred in organic matter-rich black shales rather than in sandstones, siltstones and organic matter-poor shales. Sandstones and siltstones acted as impermeable rocks, as early diagenetic quartz and carbonate minerals reduced the porosity-permeability. Scanning and transmission electron microscopy (SEM and TEM) studies indicate a relation between creation of microporosity-permeability and organic matter alteration, suitable for subsequent mineral precipitation. K-Ar data indicate that organic matter alteration and the subsequent illite precipitation within the organic matter occurred during the regional hydrothermal event at 1172 +/- 150 (2sigma) Ma. Hot circulating fluids are considered to be responsible for organic matter alteration, migration and removal of volatile hydrocarbon, and consequently porosity-permeability creation. Those rocks lacking sufficient porosity-permeability, such as sandstones, siltstones and organic matter poor shales, may not have been affected by fluid movement. In hydrothermal systems, shales and mudstones may not be impermeable as usually assumed because of hydrocarbons being rapidly removed by fluid, even with relatively low total organic carbon.

Soil mineralogy of mangrove forests from the state of Sao Paulo, Southeastern Brazil

The minerals of the clay fraction in estuarine plains are mainly detrital being a mixture of marine and continental sediments, but can also be authigenic. Because of the importance of mangrove ecosystems in tropical estuarine areas and the relatively few existing studies of the mineralogical composition of soils in these environments, the aim of this study was to determine the mineralogical assemblage and identify potential contrasts along the coast of the State of Sao Paulo. Soils from I I mangroves distributed along the coastal plain of the State of Sao Paulo were sampled at depths of 0 to 20 and 60 to 80 cm, and samples of suspended sediments from the Ribeira do Iguape River were collected for analysis. Mineralogical analyses were performed on the clay and silt fractions by x-ray diffraction (XRD) and transmission electron microscopy, and fresh soil samples were analyzed by scanning electron microscopy-energy dispersive spectrometry and suspended sediments by XRD. The silt fraction contained quartz, feldspars, gibbsite, kaolinite, illite, and vermiculite, and the clay fraction contained smectite, kaolinite, illite, gibbsite, quartz, and feldspars. Locally, vermiculite, biotite, anatase, halloysite, and goethite may occur because of recent transport of sediments to the system. Pyrite was identified in fresh samples. The allochthonous minerals found either were terrestrial and transported by rivers or had originated from the continental platform by past transgressive events. We suggest that the neoformation of smectite and kaolinite occurs in mangrove soils. Different geomorphological settings along the Sao Paulo coast appear to regulate mineral distribution in mangrove soils.