Fluid flow in cataclastic thrust fault zones in sandstones, Sub-Andean Zone, southern Bolivia

The Bolivian Sub-Andean Zone (SAZ) corresponds to a Neogene thrust system that affects an about 10-km thick Palaeozoic to Neogene siliciclastic succession. The analysis of macro and microstructures and cement distribution in thrust fault zones shows that they are sealed by quartz at depths >3 km, due to local silica transfer by pressure-solution/precipitation activated at temperatures >70–90 °C. At shallower depths, faults have remained open and could be preferential drains for lateral flow of carbonate-bearing fluids, as shown by the occurrence of carbonate cements in fractures and their host-sandstone. Due to decreasing burial, resulting from foothill erosion during fault activity, critically buried fault segments can be affected by non-quartz-sealed structures that post-date initial quartz-sealed structures. The integration of textural, fluid inclusion and isotopic data shows that carbonates precipitated at shallow depth (<3 km), low temperature (<80 °C) and relatively late during the thrusting history. Isotopic data also show that precipitation occurred from the mixing of gravity-driven meteoric water with deeper formation water bearing carbonate carbon derived from the maturation of hydrocarbon source rocks (Silurian and Devonian shales). The combined microstructural and isotopic analyses indicate that: (i) fluid flow in fault zones often occurred with successive pulses derived from different or evolving sources and probably related to episodic fault activity, and (ii) at a large-scale, the faults have a low transverse permeability and they separate thrust sheets with different fluid histories.     

Mass Spectrometry and reverse phase HPLC techniques for the identification of degraded fossil pigments in lake sediments and their application in palaeolimnology

Accurate identification of fossil pigments is essential if they are to be used as biomarker compounds in palaeolimnological studies. In recent years High Performance Liquid Chromatography (HPLC) has greatly enhanced the efficiency with which fossil pigments can be characterised and quantified. Using HPLC, undegraded pigments are typically identified through retention times, absorbance spectra and co-chromatography with authentic reference standards. However, lake sediments may also contain degraded pigments for which there are often no standards, and which may be difficult to identify using HPLC alone. In this study, we submitted HPLC fractions of fossil pigments and pigment derivatives collected from a meromictic lake in south west Tasmania, to a combination of Mass Spectrometry (MS) techniques including Electron Impact (EI) and static Liquid Secondary Ion MS (LSIMS) to identify their molecular ion characteristics and organic chemical composition. Mass Spectrometry permitted the detection of specific mass ions which were used to verify the identity of pigments and their derivatives. These included five carotenoids, chlorophyll a and derivatives, three previously described bacteriochlorophyll c derivatives with molecular weights of 770, 784, and 802, and two undescribed derivatives of bacteriochlorophyll c with molecular weights of 766 and 788. With these improved identifications we speculate on the pathways and modes of pigment degradation in the lake and asses the value of the degraded pigments as biomarkers. The use of MS permitted the identification of a greater number of signature pigments of algal and bacterial communities thus increasing the palaeolimnological value of the sediments. These methods are best applied in fossil pigment studies where there are a large number of unknown pigments and pigment degradation products, and where there are no authentic standards for co-chromatography. Practical suggestions for pigment MS are included in the discussion.     

Reconceptualising local community: environment, identity and threat

Summary Economic 'development' driven by global economic forces produces specific expressions of 'community' in places where large new economic projects are to be located. This paper draws on contemporary geopolitical literature to theorise community identity as partly formulated in response to external 'threats'. A comparative study of community mobilisation in response to proposals to locate coastal superquarries on the Isle of Harris, Outer Hebrides, Scotland, and Cape Breton, Nova Scotia, Canada, suggests the applicability of this theoretical framework for extending geographical analysis of community identity and the politics of place.     

Use of magnetic and radiometric measurements to investigate erosion and sedimentation in a British upland catchment

In response to the potential shortcomings of single-technique strategies in the investigation of erosion and sedimentation, a combined magnetic and radiometric (¹³Cs and ²¹⁰Pb) approach has been undertaken in the upland, watershed-lake system of Howden Reservoir, Derbyshire. By combining these techniques, some assessment of sediment sources and the erosion status of the catchment has been achieved. Alone, each approach would have been unable to determine unequivocally sediment provenance. Furthermore, the parallel use of these measurements has highlighted limitations and/or uncertainties in both the magnetic and ¹³⁷Cs techniques. These problems reflect the particular soil characteristics and drainage conditions of this upland catchment. Despite the documented severity of peat erosion in the region, Howden Reservoir has a mixture of sediment sources and a relatively moderate rate of sedimentation. Sediment yields (total 127·7 t km^?2 yr^?1 including organic fraction 31·3 t km^?2 yr^?1) are, however, higher than in other British upland areas.     

A partial explanation of the dependency of hydraulic conductivity on positive pore water pressure in peat soils

Part of the relationship between positive pore water pressures and hydraulic conductivity in peat soils may be explained by accumulations of methane bubbles. We show how compression and expansion of gas bubbles with changes in pore water pressure could cause changes in hydraulic conductivity and thus help to explain some observations of dependency of hydraulic conductivity in peats on pore water pressure. Consideration is also given to the effect on hydraulic conductivities of methane gas going into solution with increase in pore water pressure.     

Spontaneous strain at the structural phase transition in NaNO3

The temperature dependence of the hexagonal c unit cell parameter of high-purity NaNO₃ shows an anomaly at 553 K corresponding to the orientational ordering transition. The a unit cell parameter is barely influenced by the transition. The single component spontaneous strain for this zone boundary instability is large (55×10^–3 at 295 K), and couples quadratically with the order parameter. The critical exponent is found to have the value 0.22 ± 0.01, which differs from that expected in the classical case. Below ca 450 K, crossover to tricritical behaviour is observed (=1/4). The temperature evolution of the macroscopic order parameter as revealed by the temperature dependence of the spontaneous strain follows a tricritical behaviour between 70 K and 450 K. At temperatures below 70 K order parameter saturation is observed. Combined with recent data from specific heat measurements, the critical exponents suggest that the three-dimensional, three-states Potts model may describe the transition.Precursor spontaneous strain above T _c is consistent with local ordering and may result from fluctuations associated with an antiordered NO₃ group pair configuration.     

Oxygen isotope geochemistry of the Mesozoic volcanics of the Etendeka Formation,Namibia

The Etendeka Formation volcanics consist of a bimodal association of basalts and quartz latites. Forty three new whole rock oxygen isotope analyses are reported for all the major magma types. All the rocks except a minor suite of dolerites have higher ¹⁸O values than normal mantle. The basic rocks (average of 29=8.8) have significantly different ¹⁸O to the acid rocks (average of 10=14.4) These data are apparently consistent with previously published petrogenetic models, which propose that the basalts were affected by crustal contamination and that the quartz latites are crustally derived. However, mineral oxygen data show that there is significant oxygen isotopic disequilibrium between phenocryst and whole rock, the latter being significantly higher in most cases. One of the basic magma types (the Tafelberg basalts) shows mutual positive correlations between ¹⁸O, SiO₂ and _Sr. If these correlations are due to crustal contamination, then as much as 45% contamination is required by material having a ¹⁸O value of 15 which is the maximum observed value in the Damaran basement rocks. In the absence of pyroxene phenocryst ¹⁸O data for the high _Sr Tafelberg basalts (they are aphyric), it is not possible to confirm that contamination has taken place. An alternative explanation is that the correlation between _Sr and SiO₂ resulted from assimilation coupled with fractional crystallization (AFC) (before emplacement). Post-eruption alteration resulted in a correlation between SiO₂ ¹⁸O because the material with the most Si-O bonds was able to concentrate ¹⁸O more effectively. The limited mineral data for the quartz latites suggests that there is some source heterogeneity. A pyroxene ¹⁸O value of 10% for a southern Etendeka quartz latite is consistent with a crustal source.     

A model of channel response in disturbed alluvial channels

Dredging and straightening of alluvial channels between 1959 and 1978 in West Tennessee caused a series of morphologic changes along modified reaches and tributary streams. Degradation occurred for 10 to 15 years at sites upstream of the area of maximum disturbance and lowered bed-levels by as much as 6·1 m. Following degradation, reaches upstream of the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and gradient reduction. Aggradation downstream of the area of maximum disturbance reached 0·12 m per year with the greatest rates occurring near the stream mouths. The adjustment of channel geometry and phases of channel evolution are characterized by six process-oriented stages of morphologic development—premodified, constructed, degradation, threshold, aggradation, and restabilization. Down-cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and angle of the bank material is exceeded (threshold stage). Channel widening continues through the aggradation stage as the ‘slough line’ develops as an initial site of lower-bank stability. The bank profile develops three dynamic elements (1) vertical face (70° to 90°), (2) upper bank (25° to 50°), and (3) slough line (20° to 25°). Alternate channel bars form during the restabilization stage and represent incipient meandering of the channel.