A felsic dyke swarm formed under the sea: the Archean Hunter Mine Group,south-central Abitibi Belt,Quebec, Canada

An extensive rhyolitic dyke swarm has intruded subaqueous pyroclastic deposits, iron-formations, hyaloclastite breccias and lava flows of the 2730 Ma Hunter Mine Group (HMG) in the south-central part of the Archean Abitibi belt, Quebec. The dyke swarm has a minimum width of 500 m and can be traced perpendicular to the section for 2.4 km. Based on crosscutting relationships, chilled margins, quartz content and colour, five distinct dyke generations have been established. Each dyke generation has several magmatic pulses as indicated by parallel rows of columnar joints. Absence of brecciation between parallel rows suggests extremely brief intervals between magma pulses. The central parts of most dykes display inverted V-shaped patterns of columnar-joint convergence, inferred to indicate differential cooling during the late stages of dyke propagation. The dykes commonly display delicate spherulites suggesting rapid cooling, solidification temperatures between 400 and 600°C and penecontemporaneous devitrification. Quartz-feldspar aggregates in the groundmass have locally developed microgranophyric textures. Large spherulites near the chilled margins probably formed at temperatures below 400°C. Percolation of abundant water throughout the dyke complex is suggested by ubiquitous prominent chilled dyke margins. Development of a chilled margin 500 m along one dyke suggests that water percolated at least 500 m below the water/rock interface. Because the dykes intruded subaqueous pyroclastic deposits of similar composition, dyke emplacement below the sea floor is inferred. Interstratification of pillowed flows and brecciated pillowed flows containing rhyolite fragments at the top of the 4–5-km-thick sequence indicates that the central felsic complex probably never emerged during its evolutionary history, supporting the contention that the felsic dyke complex was emplaced beneath the Archean sea floor.     

International geographical union commission on the coastal environment

The International Geographical Union Reports

International geographical union commission on the coastal environmentNewsletter No. 26 — Editor: Norbert P. Rusty — (abridged by GeoJournal)     

The petrology and geochemistry of volcanic rocks from the Northern Harşit river area, Pontid Volcanic Province, Northeast Turkey

Two volcanic cycles can be distinguished, in the Pontid magmatic arc. They comprise an Upper Cretaceous, Lower Volcanic Cycle of which only the waning stage contains abundant pyroclastic rocks. The latter show spatial association to the fault pattern and are closely related to mineralisation. The overlying, early Tertiary, Upper Volcanic Cycle shows evidence of explosive vulcanicity in the Upper Basic Series. Dacites and rhyodacites are only locally developed and again show spatial association with the faulting.Comparison of the major and minor element chemistries of the two cycles demonstrates the clear separation into a lower tholeiitic and an upper calc-alkaline cycle. The rocks show similar chemistry to volcanic suites from island arcs in other areas.The origin of the tholeiitic magma is ascribed to melting of “dry” amphibolite during early subduction of Tethyan ocean floor beneath “Pontian Land”. This resulted in low K abundances and K/Rb ratios, and some Fe enrichment in the tholeiitic basalts.The calc-alkaline magma is thought to be derived from a later stage in the subduction process when melting of amphibole was joined by melting of biotite or phlogopite. The Upper Volcanic Cycle is thus enriched in K and shows no Fe enrichment due to a probable higher water content. The higher Cr and Ni contents of the Upper Volcanic Cycle, together with K, may imply some melting of lherzolite overlying the subducted slab.     

The Mid-Atlantic Ridge at 33°N: the Hayes Fracture Zone

A geophysical study was conducted over the Mid-Atlantic Ridge between 32–39°N and 30–40°W. A particularly deep fracture was observed which offset the ridge crest 110 km in the vicinity of 33°N. A pole of relative motion between the North American and African plates was deduced from this fracture zone as being at 63.1°N, 17°W.     

"THE HEART OF YORKSHIRE": SURVEYING THE WEST WINDOW OF YORK MINSTER

The authors describe the method adopted to prepare a photogrammetric record drawing of both the interior and the exterior stonework tracery of the west window of York Minster.     

Hydrologic equilibrium status of a disturbed Eastern North Carolina watershed

Evaluation of the impacts of artificial drainage systems on wetlands requires an assessment of the hydrologic equilibrium status of the surface hydrologic system. Hydrologic equilibrium is characterised by a high correlation between surplus moisture (as calculated by Thornthwaite's methodology) and stream discharge. A qualitative stability analysis shows that deterioration of artificial drainage channels is the process allowing the surface hydrologic system to adjust to the disturbances. The model is tested by examining the correlation between surplus and discharge for a small watershed altered by forestry drainage which is periodically rehabilitated. With hydrologic equilibeium arbitrarily defined as a surplus-discharge correlation coefficient 0.8, it is shown that the original, largely undisturbed watershed was in hydrologic equilibrium. Disequilibrium was induced by artificial drainage-ways, with a return to equilibrium over time. Subsequent renovations of ditches and canals were associated with a recurrence of disequilibrium. The surface hydrologic system is metastable, and cannot be expected to return to previously-existing states following a disturbance. Consistently maintained artificial drainage system appears to preclude any return to any type of hydrologic equilibrium.     

Nonpoint source pollution control effectiveness of riparian forests along a coastal plain river

A detention-time model of water quality buffer zones is used to evaluate the nonpoint source pollution control effectiveness of riparian forests in a two-county area of the lower Tar River basin, North Carolina. Soil map units, which represent specific combinations of soil, topography, and vegetation characteristics, are compared in terms of their relative ability to filter nitrate in agricultural runoff. All typical riparian forests provide significant water quality protection, but there is a wide variation in buffer effectiveness. This suggests a need for flexibility in determining buffer widths. A range of 15–80 m is appropriate for the soil-landform-vegetation complexes found in riparian zones within the study area. Buffer widths of 60 m — and often much less — are generally adequate on the soils likely to be used for agricultural production.     

Extinct mountain goat (Oreamnos harringtoni) in Southeastern Utah

The extinct Harrington's mountain goat (Oreamnos harringtoni Stock) is predominantly known from dry cave localities in the Grand Canyon, Arizona, in addition to two sites in the Great Basin, Nevada, and from San Josecito Cave, Nuevo Leon, Mexico. A dry shelter in Natural Bridges National Monument, on the central Colorado Plateau, southeastern Utah, preserves numerous remains of the extinct mountain goat in addition to pack rat middens. Remains from a 100-cm stratigraphic profile indicate that O. harringtoni lived on the plateau >39,800 yr B.P., the oldest directly dated find of extinct mountain goat. Plant macrofossils indicate that Engelmann's spruce (Picea engelmannii), limber pine (Pinus flexilis), rose (Rosa cf. woodsii), and Douglas fir (Pseudotsuga menziesii) grew during the late Pleistocene where a riparian and a pinyon-juniper (Pinus edulis-Juniperus osteosperma) community now predominates; Douglas fir are found only in mesic, protected, north-facing areas. Limber pine, Douglas fir, bark, and grasses were the major dietary components in the dung. A springtime diet of birch (Betula) is determined from pollen clumps in dung pellets.