Search

The remote-sensing study of an exhumed upper Paleozoic surface in the southern Gaspé Peninsula of Quebec has provided indirect evidence for the presence of a previously unidentified post-Acadian (post—Middle Devonian orogeny) graben-fill unit. Subsequent work on stratigraphic sections in that area confirms that the graben-fill unit, the new Saint-Jules Formation, is indeed petrographically distinct and older than previously documented upper Paleozoic strata in that area, which were collectively assigned to the Viséan (Upper Mississippian) Bonaventure Formation. Paleogeographic reconstruction from sedimentary facies, paleocurrent, and provenance studies concurs with the geometric detail that was provided by the preliminary geomorphic study. The present paper therefore presents the case study of an upper Paleozoic stratigraphic unit that was first inferred by geomorphic constraints prior to being recognized in the field through the study of stratigraphic sections. The study underlines the stratigraphic relevance of analyzing exhumed paleosurfaces at the margins of ancient sedimentary basins. [ABSTRACT FROM AUTHOR]

Although widely distributed throughout Arctic and subarctic regions, thermokarst ponds and lakes remain relatively unexplored regarding geomorphological changes in their catchments and their internal properties in relation to climate change over the past decades. This study synthesizes recent landscape evolution and modern sedimentology of limnologically diverse thermokarst ponds near southeastern Hudson Bay, Canada. Spatio-temporal analysis of permafrost mounds, thermokarst ponds, and vegetation surface areas over the past five decades revealed that the recent climate-induced decrease of permafrost-affected areas was not primarily compensated by thermokarst pond development, but rather by a remarkable increase in vegetation cover. These changes appeared to be modulated by topographical and hydrological gradients at the study site, which are associated with east-ward increasing thickness of postglacial marine deposits. At a more contemporary timescale, physico-chemical measurements made on sedimenting materials (sediment traps) and freshly deposited lacustrine sediments of selected thermokarst ponds revealed striking differences both among ponds and between the oxic epilimnion and the oxygen-depleted hypolimnion. These findings underscore the major influence of local landscape properties and oxycline development on pond sedimentology and geochemistry, such as the transport of detritic particles and the concentration of redox-sensitive elements. [ABSTRACT FROM AUTHOR]

Marine magnetic and bathymetric data have been acquired in a 6.5 × 10 km area centred on the Ile Rouleau impact structure, Quebec, Canada. The deeply eroded crater is emplaced in weakly magnetic shaley dolomites of the Palaeoproterozoic Mistassini Group. Subaerial exposure of the structure consists of an island (Ile Mintunikus Misaupinanuch) in Lac Mistassini, which hosts shatter cones. Short-wavelength and linear NNE-trending magnetic anomalies that pervade the survey area are inferred to reflect the presence of magnetic glacial landforms on the lake floor. The most prominent magnetic feature is a high-amplitude anomaly that wraps round the western side of the island. The anomaly correlates with an abrupt change in bathymetry but is not consistently circular about the island and its relationship to the impact is unclear. To the east, a disjointed arcuate anomaly lies parallel to the island 100 m from shore. Its source may be interpreted to be either (1) a locally thickened layer of magnetic glaciofluvial deposits, or (2) post-impact chemically induced magnetization along faults. The abrupt variations in glaciofluvial thickness needed to account for the anomaly under scenario (1) are, however, hard to explain geologically and hypothesis (2) is therefore favoured. Regardless of which model is correct, both suggest that the arcuate anomaly coincides with the fault-bounded perimeter of a central peak. Two possible reconstructions for the original crater diameter, estimated from the curvature of the magnetic anomaly and using morphometric relationships, are 4.4 km and 6.8 km. The former is more realistic with regards to the topography and the lack of conspicuous impact-related deformation on adjacent Ile Manitounouc. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Geographer is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Giant circular patterns of low tree density in black spruce (Picea mariana) stands were investigated in the Abitibi region of Quebec. We used dendrochronological techniques to test the hypotheses that ring patterns of low tree density are caused either by radial changes in spruce mortality or productivity. Seven circles were sampled. We found no gradient in the age of spruce along circle radii suggesting that rings of low tree density do not expand radially, that is, they are not spatially dynamic entities. The results indicate, however, that spruce trees were less dense and productive within the rings due to excessive moisture in the soil. Measurements of surface elevation, thickness of the organic layer and elevation of the mineral substrate across the circles revealed that a depression in the mineral soil beneath the rings traps the surface water and this area of poor drainage seems to prevent the establishment of black spruce within the rings. The origin of the ring-shaped depressions was attributed to geological or geomorphological causes.Key words: black spruce, Picea mariana, mortality, productivity, rings, geomorphology.Des formations circulaires géantes caractérisées par une densité arborescente faible ont été étudiées dans des peuplements d'épinettes noires (Picea mariana) de l'Abitibi au Québec. Les hypothèses liées à un processus de mortalité radiale ou à une baisse de productivité dans l'anneau ont été documentées dans sept cercles à l'aide de techniques dendrochronologiques. L'absence d'un gradient d'âge des épinettes ne supporte pas une expansion radiale des anneaux. Par contre, les résultats indiquent que la productivité et la densité des épinettes sont plus faibles dans l'anneau à cause de l'humidité excessive du sol à cet endroit. Des relevés topographiques de la surface du sol, de l'épaisseur de la couche organique et de l'altitude du substrat minéral ont démontré l'existence d'une dépression dans le sol minéral sous l'anneau. Cette dépression, dans un environnement comme celui-ci où la nappe phréatique est près de la surface, piège les eaux de surface et maintient l'humidité excessive du sol prévenant ainsi l'établissement de l'épinette noire. Les anneaux sont donc statiques et l'origine de la dépression annulaire relève de causes géologiques ou géomorphologiques.Mots clés : épinette noire, Picea mariana, mortalité, productivité, anneau, géomorphologie. [ABSTRACT FROM AUTHOR]

The morphometry of shore platforms is determined by their morphogenic environments. Gaspá, Quábec is a mesotidal storm wave environment. Shore platforms are cut into steeply dipping flysch, consisting of shale, graywackes, and argillites. The platforms are quasihorizontal, with a low-tide cliff or low-tide ramp at their seaward termini, and a high-tide ramp at the cliff base. Platform gradient is determined by tidal range, mean platform elevation by midtide level, and the cliff platform junction by level of the high tides; lithological and structural factors play a secondary role, Platform width is a function of wave intensity and rock hardness. Most elements of platform morphometry are determined by the elevational distribution of wave action, particularly during storms. [ABSTRACT FROM AUTHOR]