Investigation of high-level glaciofluvial terraces and re-evaluation of the established soil stratigraphy for Early and Middle Pleistocene surfaces, central Yukon, Canada

Palaeosols developed on the highest Yukon River glaciofluvial terraces were investigated in order to reconstruct the Plio-Pleistocene evolution of the river valley beyond Late Pleistocene glacial limits. A record of at least two pre-Reid (> 311 kyr) glaciations is chronicled by the presence of two populations of glaciofluvial terraces within the study area. The populations of pre-Reid terraces were identified based on their degree of soil development and elevation. Pre-Reid terraces 200–250 m above river level have preserved morphological and mineralogical features of the Wounded Moose palaeosol, a palaeosol previously associated with pre-Reid surfaces in central Yukon. Clay mineralogy and colour indicate that the Wounded Moose palaeosol developed in part during warm and sub-humid as well as temperate and humid interglacials. A second set of pre-Reid terraces between 110 and 30 m above river level are characterized by the presence of the less-developed Diversion Creek palaeosol, a palaeosol previously associated with only Reid-aged (< 311 kyr) surfaces in central Yukon. In contrast to the Wounded Moose palaeosol, the Diversion Creek palaeosol developed during comparatively cool and humid interglacial conditions. The presence of Diversion Creek palaeosols on pre-Reid outwash terraces suggests that a transition from dominantly warmer to cooler interglacial conditions occurred prior to 311 kyr in Yukon Territory. In addition, the presence of a Diversion Creek palaeosol cannot be used to differentiate stable Reid and stable pre-Reid surfaces across central Yukon.     

An (U‐Th)/He age for the shallow‐marine Wetumpka impact structure,Alabama, USA

Abstract– Single crystal (U‐Th)/He dating was applied to 24 apatite and 23 zircon grains from the Wetumpka impact structure, Alabama, USA. This small approximately 5–7.6 km impact crater was formed in a shallow marine environment, with no known preserved impact melt, thus offering a challenge to common geochronological techniques. A mean (U‐Th)/He apatite and zircon age of 84.4 ± 1.4 Ma (2σ) was obtained, which is within error of the previously estimated Late Cretaceous impact age of approximately 83.5 Ma. In addition, helium diffusion modeling of apatite and zircon grains during fireball/contact, shock metamorphism, and hydrothermal events was undertaken, to show the influence of these individual thermal processes on resetting (U‐Th)/He ages in the Wetumpka samples. This study has shown that the (U‐Th)/He geochronological technique has real potential for dating impact structures, especially smaller and eroded impact structures that lack impact melt lithologies.     

MIT's Computer Resource Lab:

Abstract

The paper describes the curriculum on geographic information systems developed in the Department of Planning at the Massachusetts Institute of Technology for those studying for the Master in City Planning degree. Particular emphasis is given to the part played by the Computer Resources Lab in serving the needs of the architecture and planning departments, the interactions between teaching and research, and those between the course and professional practice. The required courses are outlined and the wide range of recommended courses, drawn from a number of departments in the Institute, are listed- The authors emphasize the necessity of their being at the forefront of research if they are lo maintain the flow of hardware and software from contacts and donations on which their research and teaching depend.     

菲律宾Zambales蛇绿岩套的再研究：对其特提斯来源的启示

A number of geological studies have already been conducted on the Zambales Ophiolite Complex (ZOC), a north-south trending complete ophiolite sequence exposed in the western portion of Central Luzon, Philippines. Previous works recognized the ZOC as being made up of two blocks, the Acoje and the Coto, acting as an arc-back arc pair sometime during the Eocene.     

Dynamic stiffness of foundation embedded in layered halfspace based on wave propagation in cones

An Erratum has been published for this article in Earthquake Engineering & Structural Dynamics 33(6) 2004, 793. The dynamic stiffness of a foundation embedded in a multiple‐layered halfspace is calculated postulating one‐dimensional wave propagation in cone segments. In this strength‐of‐materials approach the sectional property of the cone segment increases in the direction of wave propagation. Reflections and refractions with waves propagating in corresponding cone segments occur at layer interfaces. Compared to rigorous procedures the novel method based on cone segments is easy to apply, provides conceptual clarity and physical insight in the wave propagation mechanisms. This method postulating one‐dimensional wave propagation in cone segments with reflections and refractions at layer interfaces is evaluated, calculating the dynamic stiffness of a foundation embedded in a multiple‐layered halfspace. For sites resting on a flexible halfspace and fixed at the base, engineering accuracy (deviation of ±20%) is achieved for all degrees of freedom with a vast parameter variation. The behaviour below the cut‐off frequency in an undamped site fixed at its base is also reliably predicted. The accuracy is, in general, better than for the method based on cone frustums, which can lead to negative damping. Copyright © 2003 John Wiley & Sons, Ltd.     

A Tool for Forecasting the Arrival Time of a Tracer or a Pollutant at a Karst Spring

Karst aquifers are a groundwater resource of global importance. Protection and contamination of karst water resources are often tackled by means of land-use planning and monitoring of physico-chemical and microbial water properties coupled to early-warning systems. However, these solutions of management do not integrate observational data on karst hydrodynamic, for example, tracer tests. Thus, this technical note aims at introducing a straightforward method for forecasting the arrival time of a tracer or a pollutant at a karst spring. Deliverables from the proposed method can assist professionals in the case of a pollution involving a swallow hole of the karst network. The approach is based on repeated tracer testing, gauging of spring flow rate and mathematical curve fitting. Tracer tests and gauging of spring flow rates are repeated under low, middle, and high flow periods. Arrival times of tracer from a given swallow hole of the network are plotted as a function of measured flow rates at the karst spring. The observational data are finally fitted by a mathematical function. Flow rate monitoring at high-temporal resolution at the karst spring and the mathematical functions allow forecasting the arrival of a tracer or a pollutant at the spring in the case of an accident involving a swallow hole of the network or estimating the duration of a pollution if hydrodynamic conditions do not change abruptly. The applicability of the method is documented by three study sites in Switzerland.     

Anorthositic Rocks of the Duluth Complex: Examples of Rocks Formed from Plagioclase Crystal Mush

Anorthositic rocks compose 35–40% of the Middle Proterozoic(Keweenawan; 1?1 Ga) Duluth Complex—a large, compositemafic body in northeastern Minnesota that was intruded beneatha comagmatic volcanic edifice during the formation of the Midcontinentrift system. Anorthositic rocks, of which six general lithologictypes occur in one area of the complex, are common in an earlyseries of intrusions. They are characterized on a local scale(meters to kilometers) by nonstratiform distribution of rocktypes, variably oriented plagioclase lamination, and compositeintrusive relationships. Variably zoned, subhedral plagioclaseof nearly constant average An (60) makes up 82–98% ofthe anorthositic rocks. Other phases include granular to poikiliticolivine (Fo_66–38), poikilitic clinopyrox-ene (En'_73–37),subpoikilitic Fe-Ti oxides, and various late-stage and secondaryminerals. Whole-rock compositions of anorthositic rocks are modelled bymass balance to consist of three components: cumulus plagioclase(70–95 wt.%), minor cumulus olivine (0–5%), anda gabbroic postcumulus assemblage (5–27%) representinga trapped liquid. The postcumulus assemblage has textural andcompositional characteristics which are consistent with crystallizationfrom basaltic magma ranging from moderately evolved olivinetholeiite to highly evolved tholeiite (mg=60–25). Sympatheticvariations of mg in plagioclase and in mafic minerals suggestthat cumulus plagioclase, though constant in An, was in approximateequilibrium with the variety of basaltic magma compositionswhich produced the postcumulus assemblages. Standard models of mafic cumulate formation by fractional crystallizationof basaltic magmas in Duluth Complex chambers, although ableto explain the petrogenesis of younger stratiform troctoliticto gabbroic intrusions, are inadequate to account for the field,petrographic, and geochemical characteristics of the anorthositicrocks. Rather, we suggest an origin by multiple intrusions ofplagioclase crystal mushes—basaltic magmas charged withas much as 60% intratelluric plagioclase. The high concentrationsof cumulus plagioclase (70–95%) estimated to compose theanorthositic rocks may reflect expulsion of some of the transportingmagma during emplacement or early postcumulus crystallizationof only plagioclase from evolved hyperfeldspathic magma. Althoughthe evolved compositions of anorthositic rocks require significantfractionation of mafic minerals, geophysical evidence indicatesthat ultramafic rocks are, as exposure implies, rare in theDuluth Complex and implies that plagioclase crystal mushes werederived from deeper staging chambers. This is consistent withinterpretations of olivine habit and plagioclase zoning. Moreover,plagioclase could have been segregated from coprecipitatingmafic phases in such lower crustal chambers because of the buoyancyof plagioclase in basaltic magmas at high pressure. The geochemicaleffects of plagioclase suspension in basaltic magmas are consistentwith observed compositions of cumulus plagioclase in the anorthositicrocks and with the geochemical characteristics of many comagmaticbasalts. The petrogenesis of the anorthositic rocks and theoverall evolution of Keweenawan magmas can be related to thedynamics of intracontinental rift formation.