Till fabric associated with deformable beds

This paper investigates the development of till fabric associated with glaciers overlying deformable beds. First, the till fabrics from both modern and ancient, soft bed and hard bed subglacial environments are investigated, and then soft bedded sites with specific strain histories are examined. The results indicate that lodgement tills had a strong fabric, whilst there is a wide range of much weaker fabric strengths associated with soft bed tills. It is suggested from a theoretical study of soft bed till deposition that longitudinal and vertical processes in the subglacial deforming layer are important in fabric development. Field sites were chosen to isolate these two factors, and fabrics investigated in areas of known relative shear strain, and known relative thickness of the deforming layer. The results indicate that: (1) for a fixed deforming layer, fabric strength does increase with increasing deformation (except at very high strains where the development of a transverse fabric can lead to a reduction in the overall fabric strain); and (2) lodgement tills and soft bed tills associated with a thin deforming layer have a high fabric strength, whilst soft bed tills associated with a thick deforming layer have a low fabric strength. Where the deforming layer is thin, clasts are oriented parallel with the glacier direction either by lodging against a hard bed or ploughing against a soft bed. However, where the deforming layer is thicker and shear strains are high, melt-out processes encourage a parallel fabric, but the increased shear strain and changes in the deforming layer thickness cause the clasts to develop both a parallel and transverse fabric. These processes combine to reduce the overall fabric strength. Till fabric studies are a further sedimentological technique to separate the depositional processes associated with a homogeneous till.     

Fluid heterogeneities and hornblende stability in interlayered graphitic and nongraphitic schists (Tauern Window,Eastern Alps)

The assemblage hornblende+white mica occurs in graphite-free schists at two localities in the southwest corner of the Tauern Window, Eastern Alps. In interbedded graphitic layers (1 mm to 1 m thick), however, hornblende is typically replaced by pseudomorphs of biotite+plagioclase +epidote±chlorite+staurolite in the presence of white mica. Garnets adjacent to these pseudomorphs have pronounced growth discontinuities near their rims, in contrast to the continuously zoned garnets in nongraphitic layers. These observations imply that reactions of the type hbl+white micagar+bio+plag+epid±chl±staur +H₂O occurred in the graphitic samples, but that hbl+white mica remained stable in graphite-free layers.Calculation of the equilibrium constants for solid phases in five dehydration equilibria at locality 1 indicates thata(H₂O) in the nongraphitic layers was 6 to 11 times greater thana(H₂O) in the graphitic layers. Similar calculations involving six dehydration equilibria at locality 2 show no difference ina(H₂O) between layers at the conditions of final equilibration. Initial differences in fluid composition maintained between the graphitic and nongraphitic layers caused the hbl+white mica reaction to occur at differentP-T conditions in different horizons of the schists.These data indicate that systematic differences in fluid composition were generated during metamorphism of the interlayered graphitic and non-graphitic schists but were subsequently homogenized at locality 2. The heterogeneities could initially have been produced while the rocks were in theP-T field of CO₂-H₂O immiscibility. Development of a penetrative, layer-parallel shear foliation at this time would have prevented subsequent mixing of the fluids across layers after temperatures exceeded the consolute temperature in the CO₂-H₂O system. Late-stage homogenization of fluids at locality 2 is thought to reflect loss of the buffer capacity of the mineral assemblage in response to total consumption of hornblende.     

Mineralogical changes occurring during the retrogression of Archaean gneisses from the Lewisian complex of NW Scotland

Gneisses, metamorphosed at granulite facies ca 2.7 Ga, were subsequently retrogressed to amphibolite facies during a prolonged period of retrogression, perhaps lasting as long as 200 m.y. The Scourie dykes were emplaced towards the end of this event. Localised Laxfordian shear zones further modified the mineral assemblages. The retrogression caused the production of a uniform $\text{plagioclase-hornblende}\text{- ±}\text{quartz}\text{±}\text{biotite}$ assemblage. A study of hornblende composition shows that it depends on metamorphic grade, host rock composition and paragenesis. The sequence of mineral assemblages suggests that retrogression took place on a falling temperature path, beginning at about $\text{650±50°}\text{C}$. Post-tectonic muscovite indicates that temperatures were still in excess of 500°C after the formation of Laxfordian shear zones. This indicates that the Lewisian complex was uplifted and cooled extremely slowly.     

Interactions between geomorphology and ecosystem processes in travertine streams: Implications for decommissioning a dam on Fossil Creek, Arizona

Travertine deposits of calcium carbonate can dominate channel geomorphology in streams where travertine deposition creates a distinct morphology characterized by travertine terraces, steep waterfalls, and large pools. Algae and microorganisms can facilitate travertine deposition, but how travertine affects material and energy flow in stream ecosystems is less well understood. Nearly a century of flow diversion for hydropower production has decimated the natural travertine formations in Fossil Creek, Arizona. The dam will be decommissioned in 2005. Returning carbonate-rich spring water to the natural stream channel should promote travertine deposition. How will the recovery of travertine affect the ecology of the creek? To address this question, we compared primary production, decomposition, and the abundance and diversity of invertebrates and fish in travertine and riffle/run reaches of Fossil Creek, Arizona. We found that travertine supports higher primary productivity, faster rates of leaf litter decomposition, and higher species richness of the native invertebrate assemblage. Observations from snorkeling in the stream indicate that fish density is also higher in the travertine reach. We postulate that restoring travertine to Fossil Creek will increase stream productivity, rates of litter processing, and energy flow up the food web. Higher aquatic productivity could fundamentally shift the nature of the stream from a sink to a source of energy for the surrounding terrestrial landscape.     

The influence of geological fabric and scale on drainage pattern analysis in a catchment of metamorphic terrain: Laceys Creek, southeast Queensland, Australia

The relationship between geological fabric and drainage patterns in the 81.8 km² Laceys Creek sub-catchment of the North Pine River catchment, southeast Queensland, Australia, is analysed using a new channel–ordination system. The Laceys Creek catchment is situated on the South D'Aguilar Block, which underwent metamorphism, faulting and uplift from the Late Carboniferous to Late Triassic. The catchment drains exposures of two main rock units, the Neranleigh–Fernvale Beds and the Bunya Phyllite. Both units are composed of metamorphosed deep-sea sediments that accumulated as an accretionary wedge during late Palaeozoic subduction of the palaeo-Pacific plate under the eastern margin of the Australian craton. The new channel ordination system used in this study allows improved classification of stream segments of equal prominence or rank in comparison to previous schemes. A 10 m contour digital elevation model (DEM) was produced within which drainage channels were digitised. Planar geological features, including bedding, faults, joints and cleavage, were mapped in the field and collated with data from previous geological mapping programs.Regional and local trends of geological fabric are reflected in the variable orientation of channels of different rank in the catchment. Cleavage and fractures are the dominant planar features of the Bunya Phyllite and these correlate most closely with the orientation of middle-order incised stream segments. In contrast, middle-order channels on the Neranleigh–Fernvale Beds most closely correlate with bedding, which dominates the fabric of this unit. Although anthropogenic factors exert local influence and climatic processes exert broad influence on the catchment, this study focuses on structural and lithological fabrics, which are the apparent dominant controls on middle-order channel orientations. Identification of congruent patterns between bedrock fabric and channel ranks is variable, depending on the scale and number of channels included in the analysis. Many low-rank channels correspond closely to the orientation of fine-scale bedding and foliation and these influences may not be detected by coarse-scale mapping. Understanding the extent of geological controls on the morphology of a catchment may assist geo-hazard identification, land-use planning and civil-engineering projects.     

Why rehabilitate urban river systems?

This paper addresses the philosophical question: 'why rehabilitate urban river systems?' within an Australian context. Rehabilitation of river systems has become an important objective of many local, state and national governments around the world, who allocate substantial investment into various river projects. An understanding of the various factors influencing stream condition and potential rehabilitation options is essential in order to determine how the process is undertaken, and how success is measured. This paper examines the triple bottom line (economic, social and environmental) factors that influence decisionmaking with respect to urban stream rehabilitation and management and considers their relative value and importance.     

Social vulnerability and participation in disaster recovery decisions: public housing in Galveston after Hurricane Ike

Natural Hazards - In September 2008, Hurricane Ike caused massive damages to Galveston Island’s residential structures including four public housing developments. These developments were...