Abstract

The Cap Chat Mélange crops out discontinuously for 200 km along the south shore of the St. Lawrence River in the Gaspé Peninsula. It is located just south of Logan's line, the northern limit of the Humber zone with the Taconian foreland basin. This mélange is composed of dismembered rocks of the adjacent formations, in particular the Lower Ordovician Rivière Ouelle and Middle Ordovician Tourelle formations, with lesser contributions by the Middle Ordovician Des Landes and the Cambrian Orignal formations. Blocks in the mélange vary in size from a few centimetres to several kilometres, with well-preserved internal stratigraphy in the larger blocks. The distribution of blocks is not uniform and the composition of the surrounding matrix changes with corresponding changes in block composition. Tectonic processes, mostly extensional and compressional faulting, are responsible for some of the chaotic aspects of the mélange. However, the main mechanism was as follows: (i) large-scale liquefaction of the mudstone-rich Rivière Ouelle Formation, (ii) sinking with consequent dismembering of the Tourelle Formation into this underlying weakened Rivière Ouelle Formation, and (iii) fluidization of the lowermost sand beds of the Tourelle Formation resulting in abundant sandstone sills and dikes in the Rivière Ouelle Formation.

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