Tensor magnetotelluric (MT) soundings were carried out to delineate the geometry of an extended shallow conductor detected by airborne electromagnetic method in the mining camp of Chibougamau (Canada). Careful distortion analysis of the impedance tensors from eleven sites indicate that the subsurface is reasonably 2-D within the 0.0025–10s range. The recovered regional impedances are corrected for static shifts employing 2-D inversion for minimum structure. The modeled resistivity cross‐sections show a far more complex conductivity structure than anticipated. The sheet‐like shallow conductor lies near the boundary between the conductive southern section located in the Abitibi subprovince and the resistive northern section located in the Opatica subprovince. It extends to a depth of 250 m. Two other limited conductive zones are imaged respectively between 0.4 km and 3 km and between 1.5 km and 4 km. We propose that the conductors are associated with sulfide mineralization and graphite in shear zones. They are delimited by steeply south dipping thrust faults. The northern resistive section does not show clear resistivity changes up to a depth of 15 km. Gravity data collected along the MT profile have provided additional constraints. Both MT and gravity models are consistent with one another and with the known regional geologic structure. They indicate that the Abitibi greenstone belt is 3.5–4.4 km thick in the survey area and that the steeply dipping geology at the surface is flattening at depths of 4 km approximately.

The Chapais region has been extensively studied during the last 40 years. Recent work in this area has provided information that can be used to assess the various models proposed for similar volcano-sedimentary belts. By using the known surface geology and measured rock densities, gravity interpretation can define the structure of the geological formations. Aeromagnetic data have been used to improve surface mapping in areas of few or no outcrop. We conclude that the Chibougamau syncline has a depth extension of about 4 km. The depth extension of the granitic intrusives cannot be accurately determined because of our lack of knowledge of their density variation with depth. Any geological model must satisfy potential field data constraints, especially those given by gravity data. In the present case, we have shown that only those models indicating a limited thickness of volcano-sedimentary rocks are acceptable.

The extensive hydraulic piston coring activity on Leg 90 provided hundreds of meters of good quality sediment from which to conduct a comprehensive physical properties examination of nannofossil ooze and chalk. Most sites exhibit a well-defined transitional zone between the soft ooze and the indurated chalk, and a clear correlation is established between the onset of this region and sharp increases in sediment shear strength. Distinct increases in the values of compressional velocity and thermal conductivity across the ooze-to-chalk transformation are less evident. In a few instances, the porosity-versus-depth profiles depict zones where sediment porosity remains relatively constant across substantial depth intervals. Porosity is seemingly unaffected by large increases in overburden pressure, and no significant compaction occurs across several hundred meters of the sediment column. This behavior is attributed to a combination of several environmental factors. It is postulated that remarkably high sedimentation rates, a high diagenetic potential, and, perhaps, an appropriate grain size distribution all have had some relative influence in promoting sediment rigidity and retarding compaction.

Dans la région de Trois-Pistoles, l’action des vagues est moins importante en raison de la prédominance que prennent le glaciel et la marée dans le transport des sédiments littoraux de cette région. La disparité et le mauvais triage des sédiments de surface montrent que plusieurs processus sédimentologiques contribuent à la répartition des sédiments récents. L’influence de la marée dont le marnage atteint 5 m et celle des glaces flottantes dont l’activité dure environ 100 jours par année sont des processus relativement bien connus. L’analyse de l’action des vagues est basée sur le modèle de mise en mouvement des sédiments mis au point par KOMAR et MILLER (1975) en utilisant les données de vagues enregistrées au large de Trois-Pistoles. L’action prédominante des vagues consiste davantage à brasser les sédiments qu’à les trier à cause du balancement de la marée. La formation de nombreuses flèches dans la région montre cependant que les vagues d’intensité plus forte réparties sur de longues périodes contribuent à la construction des formes d’accumulation dans la région. Le rôle que jouent les vagues à l’interface entre les sédiments récents et les argiles de la mer de Goldthwait reste à déterminer.In the Trois-Pistoles area wave action is less important because of the predominance of tides and drifting ice as sediment transport processes. The heterogeneity and poor sorting of surface sediments are indicative of the action of many sedimentological processes contributing to the distribution of surficial sediments. The influence of the tide (5m range) and of the drifting ice, whose activity lasts for some 100 days yearly, are relatively well known processes. The analysis of wave action is based on a model for threshold of sediment motion under waves developed by KOMAR and MILLER (1975) using wave data recorded offshore Trois-Pistoles. The wave action serves more to stir surficial sediments than to sort them because of the range of the tide. The formation of numerous spits in the area shows however that waves of higher intensity are contributing over long periods to the formation of the littoral zone. The action of waves at the interface between recent sediments and the underlying Goldthwait clays remains to be evaluated