Volume 7, numéro 1, march 1980

EN :

Much interest and activity is presently centred on the stratigraphie use of tephra. Distinctivetephra layers constitute important time-parallel markers, which if widespread, offer the potential for reliable correlation over long distances. Furthermore, they provide valuable geochrono-logical control for their age can be determined by several radiometric methods. Confident correlations require a multiple criteria approach to tephra characterisation; samples should only be considered equivalent if their stratigraphie, palaeontologic, palaeomagnetic, and radiometric age relations are compatible and the physico-chemical properties ol their glass shards and phenocrysts agree. Special attention should be given to the possibility of resedimentation into a younger stratigraphie position. Because grain-discrete methods of analysis are sensitive to contamination effects, they are to be preferred over those methods that require use of bulk separates.

Coarse, proximal tephra can be reliably dated by the K-Ar method as pure mineral separates can be readily isolated, but distal ash-grade tephra is better dated by the fission-track method in which ages are based on tracks counted in individual grains so that detrital contaminants can be easily recognised and avoided. The recent successful application of the fission-track method to distal tephra has resulted in a greatly improved understand-i ng of the late Cenozoic geochronology of areas remote from volcanic centres.

Tephrochronology is a useful tool in many areas of Quaternary research; in particular, it will undoubtedly continueto play an important role in the connection of marine and continental sequences, refinement of the palaeomagnetic chronology, calibration of hominid evolution in eastern Africa and other areas, and age-definition of Antarctic and Greenland ice cores.

EN :

Interdisciplinary studies have endeavoured to understand the geomorphology, sediments, soils, vegetation, and wildlife of the Ontario coast of James Bay, which is unique for its fast emergence (70 to 100 cm/century) related to post-glacial isosta-tic rebound. From a geomorphological-sedimentological point of view, the coast has many similarities to other mesotidal settings, although typical features due to ice-rafting and scouring of arctic and subarctic areas are recorded in thin (3 m) sedimentary sequences. Gleysolic soils form in its low-lying wetlands, and Regosolic soils evolve into Podzolic soils on sandy and gravelly beach ridges as they emerge and become forested.

The vegetation of the marshes is typical of subarctic areas. The most common colonizing plants of intertidal zones are Puccinellia phryganodes in salt marshes, and Hippurus vulgaris in brackish marshes. Associated with large rivers, inverted marshes occur with brackish zones near the shoreline, and saltier zones inland due to evaporation of waters brought in by storm surges. The fauna of this coast ranges from numerous mosquitoes and other insects, to invertebrates, to migratory birds. The shore has irreplaceable international importance because it contains the breeding and feeding grounds of the migratory avifauna of central and eastern America. Tens of thousands of shorebirds and waterfowl feed on specific parts of the ecosystem depending on their food requirements and their anatomical limitations imposed by depth of burrowing of infauna, depth of water in marsh pools, and height of vegetation indifferent marshes at different times of the year.

Whereas studies of this virgin, rapidly changing environment can help in understanding and perhaps managing the wildlife resource, and in predicting the response of the coastal zone to future human activities, such as regulation and diversion of rivers, all these processes leave their imprints in the sedimentary and soil sequences. In sedimentological terms the study of this coast will lead to a definition of a recognizable model of a cold, brackish, shallow, regressive, inland sea similar to those that have occupied this area during Pleistocene times, or other more ancient, emergent, glacial, arctic basins.

EN :

Several Quaternary sequences in North America have been analysed for fossil insects. Most of the insect remains are from beetles, and these are rapidly becoming recognised as very reliable indicators of climatic and vegetational changes. Like pollen they are present in most Quaternary deposits, but unlike most pollen grains they can be identified specifically. Also, beetles appear to respond rapidly to climatic change illustrating thermal variations which are often not detected by other conventional indicators. This paper briefly summarises some of the techniques and results of work in Canada, parts of the United States and England.