The initiation and development of metamorphic foliation in the Otago Schist, Part 2: evidence from quartz grain-shape data

Shape, size and orientation measurements of quartz grains sampled along two transects that cross zones of increasing metamorphic grade in the Otago Schist, New Zealand, reveal the role of quartz in the progressive development of metamorphic foliation. Sedimentary compaction and diagenesis contributed little to the formation of a shape‐preferred orientation (SPO) within the analysed samples. Metamorphic foliation was initiated at sub‐greenschist facies conditions as part of a composite S1‐bedding structure parallel to the axial planes of tight to isoclinal F1 folds. An important component of this foliation is a pronounced quartz SPO that formed dominantly by the effect of dissolution–precipitation creep on detrital grains in association with F1 strain. With increasing grade, the following trends are evident from the SPO data: (i) a progressive increase in the aspect ratio of grains in sections parallel to lineation, and the development of blade‐shaped grains; (ii) the early development of a strong shape preferred orientation so that blade lengths define the linear aspect of the foliation (lineation) and the intermediate axes of the blades define a partial girdle about the lineation; (iii) a slight thinning and reduction in volume of grains in the one transect; and (iv) an actual increase in thickness and volume in the survivor grains of the second transect. The highest‐grade samples, within the chlorite zone of the greenschist facies, record segregation into quartz‐ and mica‐rich layers. This segregation resulted largely from F2 crenulation and marks a key change in the distribution, deformation and SPO of the quartz grains. The contribution of quartz SPO to defining the foliation lessens as the previously discrete and aligned detrital quartz grains are replaced by aggregates and layers of dynamically recrystallized quartz grains of reduced aspect ratio and reduced alignment. Pressure solution now affects the margins of quartz‐rich layers rather than individual grains. In higher‐grade samples, therefore, the rock structure is characterized increasingly by segregation layering parallel to a foliation defined predominantly by mica SPO.     

High‐resolution stratigraphic architecture and lithological heterogeneity within marginal aeolian reservoir analogues

Marginal aeolian successions contain different lithological units with variable geometries, dimensions and spatial distributions. Such variations may result in considerable heterogeneity within hydrocarbon reservoirs developed in successions of this type, which poses a high risk to their efficient development. Here, such heterogeneity is described and characterized at inter‐well (<1 km) scales using two well‐exposed outcrop analogues of ‘end member’ marginal aeolian deposits from the Permian Cedar Mesa Sandstone and Jurassic Page Sandstone of south‐central Utah, USA. The sedimentology and stratigraphic architecture of the Cedar Mesa Sandstone was studied in a 1·2 km² area in the Indian Creek region of southern Utah, where the interval consists of interbedded fluvial and aeolian deposits representative of a fluvial‐dominated erg margin. The Page Sandstone was studied in a 4·3 km² area near Escalante, close to the Utah‐Arizona border, where it consists of interbedded sabkha and aeolian deposits representative of a transitional‐marine erg margin. The three‐dimensional stratigraphic architectures of both reservoir analogues have been characterized, in order to establish the dimensions, geometries and connectivity of high‐permeability aeolian sandstones. Facies architecture of the aeolian‐sabkha deposits is characterized by laterally continuous aeolian sandstone layers of relatively uniform thickness that alternate with layers of heterolithic sabkha deposits. Aeolian sandstones are thus likely to form vertically unconnected but laterally widespread flow units in analogous reservoirs. Facies architecture in the aeolian‐fluvial deposits is more complex, because it contains alternating intervals of aeolian sandstone and fluvial heterolithic strata, both of which may be laterally discontinuous at the studied length‐scales. Aeolian sandstones encased by fluvial heterolithic strata may form small, isolated flow units in analogous reservoirs, although the limited continuity of fluvial heterolithic strata results in vertical connectivity between successive aeolian sandstones in other locations. These architectural templates may be used to condition zonation schemes in models of marginal aeolian reservoirs.     

ESTABLISHING LICHENOMETRIC AGES FOR NINETEENTH- AND TWENTIETH-CENTURY GLACIER FLUCTUATIONS ON SOUTH GEORGIA (SOUTH ATLANTIC)

Glaciers in small mountain cirques on South Georgia respond rapidly and sensitively to changes in South Atlantic climate. The timing and rate of their deglaciation can be used to examine the impact that nineteenth- and twentieth-century climate change has had on the glacial dynamics and terrestrial ecosystems of South Georgia. As part of a reconnaissance study in Prince Olav Harbour (POH), South Georgia, we measured the size of lichens ( Rhizocarpon Ram. em Th. Fr. subgenus. Rhizocarpon group) on ice-free moraine ridges around two small mountain cirques. Our aims were twofold: first, to provide age estimates for lichen colonization, and hence, deglaciation of the moraine ridges, and second, to examine the potential of applying lichenometry more widely to provide deglacial age constraints on South Georgia. In the absence of lichen age-size (dating) curves for South Georgia, we use long-term Rhizocarpon lichen growth-rates from recent studies on sub-Antarctic Islands and the western Antarctic Peninsula to calculate likely age estimates. These data suggest ice retreat from the two outermost moraines occurred between the end of the 'Little Ice Age' (post c. 1870) and the early twentieth century on South Georgia. Lichen colonization of the innermost moraines is probably related to glacier retreat during the second half of the twentieth century, which has been linked to a well-defined warming trend since c. 1950. Patterns of possible nineteenth- and twentieth-century glacial retreat identified in POH need to be tested further by establishing species- and site-specific lichen age-size (dating) curves for South Georgia, and by applying lichenometry to other mountain cirques across South Georgia.     

Development of a cliff-top dune indicated by particle size and geochemical characteristics: Rubjerg Knude, Denmark

Particle size and geochemical data have been used to investigate the development of a large cliff-top dune at Rubjerg Knude, located on the western coast of Jutland, Denmark. Textural parameters and geochemical ratios provided useful indicators of the dune sediment provenance and mode of evolution of the dune. The dune sediments themselves showed no significant spatial particle size trends and reflect a number of processes, including grainfall, wind-ripple migration and avalanching (grainflow), which formed a high proportion of the deposits on both the stoss and lee sides of the present dune. Fine grainfall sediments, which have accumulated to form a sandplain in the lee of the dune, show fining and improved sorting with distance, and extend more than 2 km downwind of the dune crest. Comparison of the textural and geochemical data from Rubjerg Knude and other locations on the Jutland coast indicates that, although there is a contribution of sand to the dune from local marine sources, the main source of sand to the cliff-top dune and sand plain sediments has been provided by the wind erosion of the underlying cliffs, which are composed of Weichselian age sandy glaciofluvial and glaciolacustrine deposits. Optically stimulated luminescence dating indicated an apparent age for the sand at the base of 274 ± 14 years. If this date is reliable, it suggests that accumulation of the aeolian sand in this area began within approximately the last 300 years. Map and photographic evidence indicate that the modern high dune only began to form after 1885, apparently associated with an acceleration in the rate of coastal cliff retreat.     

RACE,IMMIGRANTS, AND RESIDENCE: A NEW RACIAL GEOGRAPHY OF WASHINGTON,D.C.*

ABSTRACT. This study examines the residential patterns of immigrant newcomers within a relatively new immigrant destination, Washington, D.C. Particular attention is given to how these patterns are shaped by the newcomers' race and ethnicity. Our analysis is based primarily upon data on immigrant flows from the U.S. Immigration and Naturalization Service for the 1990–1998 period. Evidence from our study suggests a racial and ethnic hierarchy to the residential location of Africans, Asians, Europeans, and Latin Americans, providing some support for the pattern expected in the place‐stratification model. When their residential patterns are compared with those of non‐Hispanic blacks, we find that African newcomers are more likely to settle among blacks than are other regional‐origin groups. However, Africans are also found to reside in multiethnic zip codes where large numbers of newcomers are concentrated. The findings suggest that race is an important but not exclusive factor in explaining the residential choices of recent immigrants.     

The initiation and development of metamorphic foliation in the Otago Schist, Part 1: competitive oriented growth of white mica

The 3D shape, size and orientation data for white mica grains sampled along two transects of increasing metamorphic grade in the Otago Schist, New Zealand, reveal that metamorphic foliation, as defined by mica shape‐preferred orientation (SPO), developed rapidly at sub‐greenschist facies conditions early in the deformation history. The onset of penetrative strain metamorphism is marked by the rapid elimination of poorly oriented large clastic mica in favour of numerous new smaller grains of contrasting composition, higher aspect ratios and a strong preferred orientation. The metamorphic mica is blade shaped with long axes defining the linear aspect of the foliation and intermediate axes a partial girdle about the lineation. Once initiated, foliation progressively intensified by an increase in the aspect ratio, size and alignment of grains, although highest grade samples within the chlorite zone record a decrease in aspect ratio and reduction in SPO strength despite continued increase in grain size. These trends are interpreted in terms of progressive competitive anisotropic growth of blade‐shaped grains so that the fastest growth directions and blade lengths tend to parallel the extension direction during deformation. The competitive nature of mica growth is indicated by the progressive increase in size and resultant decrease in number of metamorphic mica with increasing grade, from c. 1000 relatively small mica grains per square millimetre of thin section at lower grades, to c. 100 relatively large grains per square millimetre in higher grade samples. Reversal of SPO intensity and grain aspect ratio trends in higher grade samples may reflect a reduction in the strain rate or reduction in the deviatoric component of the stress field.