Application de la scanographie à l'étude de la densité des sédiments et à la caractérisation des structures sédimentaires : exemple des sédiments déposés dans la rivière Saguenay (Québec, Canada) après la crue de juillet 1996

Abstract

Following the flood of July 1996, a significant quantity of sediment, coming from the catchment areas contiguous to the Saguenay fjord, was discharged in the Baie des Ha! Ha! and the North Arm. To characterize the mode of deposit and the potential for remobilization of these deposits, a series of cores was taken in the fjord using a box corer and a gravity corer during the summers of 1997 and 1998. These cores have been analyzed by the axial tomodensitometer at the Regional Hospital Center of Rimouski. The use of the tomodensitometer in geology allows a nondestructive study of the sample and a qualitative and quantitative evaluation of the density of the sediments, their mode of deposition, and their compaction. The study of the upper 45 cm of sediments shows the presence, in the uppermost layer, of several facies type characteristics of a gravity deposit. This layer rests on an erosional unconformity cut into the surface of the older deposits, themselves of gravity type. One year after the flood, no trace of reworking could be detected in the upper part of the cores. Nevertheless, cores taken in the summer of 1998 show a significant reworking of the superficial sediments in direct relation with bioturbation. No reworking of hydrodynamic origin was observed.Key words: scanography, flood, turbidity currents, sedimentary structures, compaction, bioturbation.