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Lower Ordovician conglomerates and sandstones of the Cap des Rosiers Formation occur in a sequence of fine-grained sandstone and siltstone turbidites near Grosses Roches in Quebec. The conglomerates lie on eroded turbidites, and the presence of rip-up structures, channeling of underlying sediments, grading, poor sorting, very large blocks, and some chaotic fabric in the conglomerates suggests that they were formed by deposition from gravity-controlled slides or flows.
There are 3 lithologies in the conglomeratic facies: (1) polymict conglomerate, (2) medium- to coarse-grained quartzose sandstone, and (3) fine- to medium-grained dark-gray sandstone with slump balls. Most of the conglomerate beds and many of the coarse-grained sandstones show extensive internal layering. The layering is defined by thin sandstone lenses in the conglomerates and by pebbles in the sandstone. Detailed logging of a 100-m thick section shows that layers are traceable for distances of up to 80 m along strike; but most layers are of extremely limited lateral extent. Field evidence suggests that their extent is an original depositional feature rather than the result of erosion by succeeding layers. Erosive structures such as those at the bases of conglomerate beds are not com on between layers within beds. Thin turbidite beds and thick sandstone beds are present between conglomerate beds but not between layers within beds. This pattern suggests that deposition of each conglomerate bed occurred as a series of events which were closely related in time and space, and the beds therefore can be considered composite. An origin by some type of progressive failure at source is suggested.
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