The optical properties of the finest fraction of lunar soil: Implications for space weathering     

Sarah K. NOBLE  Carl M. PIETERS  Lawrence A. TAYLOR  Richard V. MORRIS  Carlton C. ALLEN  David S. McKAY  Lindsay P. KELLER 《Meteoritics & planetary science》2001,36(1):31-42

Abstract— The fine fraction of lunar soils (<45 μm) dominates the optical properties of the bulk soil. Definite trends can be seen in optical properties of size separates with decreasing particle size: diminished spectral contrast and a steeper continuum slope. These trends are related to space weathering processes and their affects on different size fractions. The finest fraction (defined here as the <10 μm fraction) appears to be enriched in weathering products relative to the larger size fractions, as would be expected for surface correlated processes. This <10 μm fraction tends to exhibit very little spectral contrast, often with no distinguishable ferrous iron absorption bands. Additionally, the finest fractions of highland soils are observed to have very different spectral properties than the equivalent fraction of mare soils when compared with larger size fractions. The spectra of the finest fraction of feldspathic soils flatten at longer wavelengths, whereas those of the finest fraction of basaltic soils continue to increase in a steep, almost linear fashion. This compositional distinction is due to differences in the total amount of nanophase iron that accumulates in space weathering products. Such ground‐truth information derived from the <10 μm fraction of lunar soils provides valuable insight into optical properties to be expected in other space weathering environments such as the asteroids and Mercury.  相似文献   

The development of a historical digital cadastral database†     

GARY J. HUNTER  IAN P. WILLIAMSON 《International journal of geographical information science》2013,27(2):169-179

With an ever-increasing number of land information systems being developed, there is a growing demand by users for systems which support the use of temporal data for time-based studies. System managers, however, may have difficulty responding to these calls because of the lack of available research to guide them in their task. This paper examines the simple method of storing and processing temporal graphical data by the addition of time-encoding attributes to data elements as required. To illustrate the technique, a historical digital cadastral database has been created which al lows users to display and report on the cadastral parcellation of a region at any time in the past. The method can also be widely applied to other data to assist a broad range of space/time-related studies.  相似文献   

Counterpoint     

P.A. ALLEN  ROBERT CAMPBELL 《Geology Today》1985,1(3):74-74

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Metamorphic evidence for an inverted crustal section, with constraints on the Main Karakorum Thrust, Baltistan, northern Pakistan   总被引：1，自引：0，他引：1  

T. ALLEN  C. P. CHAMBERLAIN 《Journal of Metamorphic Geology》1991,9(4):403-418

Abstract The Shyok Suture Zone separates rocks in the Asian plate from rocks in the Kohistan-Ladakh island arc. In Baltistan, this suture has been reactivated by the late 'break-back'Main Karakorum Thrust (MKT). The P-T histories of metamorphic rocks both north and south of the MKT have been determined in an effort to place constraints on the tectonic history of this zone. The terranes north and south of the MKT have different, unrelated metamorphic histories. Rocks from the Kohistan-Ladakh island arc south of the MKT have undergone a static low- P (2–4 kbar, c. 500° C) thermal metamorphism. The P-T paths and metamorphic textures of these rocks are consistent with metamorphism due to emplacement of plutonic rocks into the island arc. This metamorphism pre-dates folding and deformation of these rocks. Rocks in the Karakorum Metamorphic Complex, north of the MKT, have experienced a complex deformational and metamorphic history. Prograde metamorphic isograds have been deformed by subsequent south-verging folding and by gneiss dome emplacement. However, decompression metamorphic reactions occurred during nappe emplacement. Higher pressure rocks are associated with higher level nappes, creating an inverted pressure metamorphic sequence (8–9-kbar rocks over 5–6-kbar rocks). There is little variation in temperature with structural level (550–625° C). These two different terranes have been juxtaposed after metamorphism by the late south-directed MKT.  相似文献   

SEDIMENT SIZE DISTRIBUTION PROFILE ON THE CONTINENTAL SHELF OFF NEW JERSEY1     

JESSIE G. DONAHUE  ROBERT C. ALLEN  BRUCE C. HEEZEN 《Sedimentology》1966,7(2):155-159

A series of samples from the continental shelf off New Jersey reveals a continuous surface layer of fine sand overlying a variety of sediment type. Ripples and shell debris suggest the fine sand is reworked and is in adjustment with the present shelf environment.  相似文献   

Features of cross-stratified units due to random and other changes in bed forms   总被引：1，自引：0，他引：1  

J. R. L. ALLEN 《Sedimentology》1973,20(2):189-202

Because cross-stratified units depend upon the movement of bed forms, any change in the shape, size and direction of travel of the forms is reflected in the geometry of the units, notably in their relative length, breadth and thickness, mode of termination upstream and downstream, and internal discontinuities. Most models of cross-stratification so far published are unsatisfactory because they ignore the fact that real bed forms are subject to change. The changes are thought to occur at two levels of detail independently. Those at the coarser level depend on the essential non-uniformity, unsteadiness and multi-directionality of natural flows, when assessed on a suitably large scale. At the finer level, change is related to the random behaviour of individual bed forms as they interact with the adjacent flow, and it proceeds even when the flow is an equilibrium one overall. Flume experiments on current ripples show that many features of cross-stratified units can be explained by the random behaviour of bed forms. The finite streamwise length of such units, and their upstream and downstream erosional termination, is governed by the life-span (finite) of individual ripples and by the extent of net deposition on the bed. Internal discontinuities, closely resembling features described as reactivation structures, were also found to depend on the relative motion of ripples, no change of flow discharge and stage being involved. The degree of relative motion in the ripple assemblages was substantial, as measured by the fluctuating component of the ripple celerity.  相似文献   

ETHNIC RESIDENTIAL CONCENTRATIONS IN UNITED STATES METROPOLITAN AREAS     

JAMES P. ALLEN  EUGENE TURNER 《Geographical review》2005,95(2):267-285

ABSTRACT. Although residential concentrations of immigrant ethnic groups in cities were common a century ago, it is not clear to what extent members of more recently arrived groups live near each other. We attempt to determine how common such clustered settlement is today, using 2000 census data to measure concentrations of Asians, Hispanics, and their larger ethnic subgroups in fifteen large metropolitan areas. The percentage of an ethnic group that is residentially concentrated correlated significantly with the group's proportion in an area. With metropolitan areas weighted equally, 38 percent of Hispanics and 13 percent of Asians were concentrated. However, when we analyzed eight specific nationality groups, the residentially concentrated proportion ranged from 14 to 59 percent. Level of cultural assimilation appears to explain group differences in level of concentration. Although ethnic concentrations were more pronounced in the largest metropolitan areas, important concentrations were also found in many of the smaller areas in our study.  相似文献   

Sedimentology and evolution of the Martinsburg Formation (Upper Ordovician) fine-grained turbidite depositional system, central Appalachians   总被引：1，自引：0，他引：1  

GARY G. LASH 《Sedimentology》1988,35(3):429-447

The Upper Ordovician Martinsburg Formation of eastern Pennsylvania consists of mudstone, siltstone, and sandstone turbidites that accumulated in a tectonically active foreland basin. The mudstone-rich Bushkill Member, the stratigraphically lowest unit of the Martinsburg in this area, grades upward into approximately equal proportions of mudstone, siltstone, and sandstone of the Ramseyburg Member. Many of the turbidites of these units are arranged in small-scale (1–9 m) fining-upward sequences that are interpreted as reflecting the influence of external or allocyclic controls such as variations in the local rate of sea-level rise and/or variations in the intensity of tectonic activity in shelf/nearshore or hinterland areas rather than more commonly cited autocyclic mechanisms. The thick (approximately 2000 m) Bushkill-Ramseyburg coarsening-upward sequence records progradation of a muddy turbidite depositional system along the axis of the foreland basin. Although this sequence accumulated during a Caradocian eustatic rise in sea-level, sedimentation rates landward of the shoreline were apparently great enough to allow for long-term seaward progradation of the shelf source. The paucity of depositional lobe-like facies (coarsening-upward sequences) in the Bushkill Member allows for tentative comparison of the progradational Bushkill-Ramseyburg system with the active fan lobe of the Mississippi Fan. Progradation of the Bushkill-Ramseyburg system ceased abruptly when mudstone turbidites and laminated black shale of the upper unit of the Martinsburg, the Pen Argyl Member, accumulated. The great thickness of some mudstone turbidite beds of the Pen Argyl Member is interpreted to record topographic confinement of the central Appalachian foreland basin, which may have helped to preclude continued progradation of the Bushkill-Ramseyburg turbidite system.  相似文献   

A preliminary study of the dynamics of a modern draa, Algodones, southeastern California, USA   总被引：2，自引：0，他引：2  

KAREN G. HAVHOLM  GARY KOCUREK 《Sedimentology》1988,35(4):649-669

A transverse crescentic draa in the Algodones dune field, California, was monitored for a year using surface process mapping, aerial photography and supplemental wind measurement. The draa is oriented by the long-term resultant wind, whereas its superimposed features are in equilibrium with the bedform-modified secondary airflow. Surface airflow and the movement of superimposed bedforms is typically oblique or parallel to the draa brinkline, particularly on the lee slope. Comparison of measurements of draa movement and sand deposition on the lee slope, with expected rates calculated from wind data and draa size, confirm that there is a significant component of sand flow parallel to the draa brinkline. The internal structure being generated at the base of the draa lee slope is inferred from the surface processes active there. Within the space of a kilometer two types of compound cross-strata, separated by an area of simple cross-strata, are being produced. This has significant implications for interpretations of ancient aeolian strata. Variations in internal structure types found in lateral sequence may be generated by one complex bedform, and these cross-strata may be simple or compound. Second-order bounding surface orientations indicate resultant primary palaeowind directions; compound cross-strata dip directions indicate secondary flow conditions. The existence of cross-strata dip directions oblique or perpendicular to the second-order surface indicates longitudinal secondary flow on the lee face, but not necessarily a longitudinal or oblique draa. Without further detailed knowledge about various draa configurations and behaviour, stratification attributed to draas can be used only to interpret activity on the lower draa lee face.  相似文献   

Longitudinal petrographic variations in a Middle Ordovician trench deposit, Central Appalachian orogen     

GARY G. LASH 《Sedimentology》1987,34(2):227-235

Petrographic analysis of Middle Ordovician turbidite sandstones of the Greenwich slice of the Hamburg klippe (eastern Pennsylvania), inferred to be part of a fossil subduction complex, define three coeval petrofacies. The Jonestown petrofacies was derived from felsic plutonic and less abundant metasedimentary rocks, whereas the Windsor Township, the most extensive petrofacies, and Werleys Corner petrofacies were derived from sources characterized by various proportions of sedimentary/metasedimentary, plutonic, and volcanic rocks. The presence of minor but conspicuous extrabasinal carbonate and microlitic volcanic lithic fragments together with higher percentages of polycrystalline quartz, serve to distinguish the Werleys Corner from the Windsor Township petrofacies. It is conceivable that sandstones of the Greenwich slice were derived from microplates inferred to have existed to the southeast of the proto-North American plate in Early Palaeozoic time. The variations in sandstone composition along the length of the Greenwich slice may be explained by post-accretion tectonic juxtaposition of petrofacies derived from various sources. An equally plausible explanation involves transverse infilling of a channelized longitudinal transport system (Windsor Township petrofacies) by sediment derived from compositionally diverse source terranes orthogonal to the trench (Jonestown and Werleys Corner petrofacies).  相似文献