Abstract

A Sequence of Lower Ordovician conglomerates and sandstones outcrops on the south shore of the St. Lawrence River, northeast of the village of Grosses Roches, Quebec. Detailed measurement of a section 100 meters thick shows that the conglomerate facies, which includes conglomerates, conglomeratic sandstones, and sandstones, occurs above a sequence of claystones, and alternates with a sandstone-shale turbidite facies. The conglomerates erode both the claystones and the turbidites. Rip-up structures, channeling of underlying sediments, grading, poor sorting, very large clasts, and chaotic clast fabric in some conglomerates suggest formation by deposition from gravity-controlled slides or flows. The conglomerates display two types of bedding: (1) Beds with no marked internal textural changes from top to bottom. These are termed simple beds in the present study. (2) Beds with several thin discontinuous sandstone layers in them. These layers can be traced up to 80 m along strike, but most layers are of limited lateral extent. These conglomerate beds are termed compound beds. Sandstone layers within compound conglomerate beds generally have flat tops and irregular bases. Where they wedge out, they invariably thin from the base upwards. They are not eroded by overlying conglomerate layers, although erosive features are common at the bases of both simple and compound beds. Thin fine-grained turbidites and thick medium to coarse-grained quartzose sandstone beds occur between compound conglomerate beds but not between layers within compound beds. This suggests that deposition of individual compound beds occurred as a series of events which were closely related in time and space, i.e., the beds are composite . It is likely that the development of such a series of events is related to the mechanism of initiation of a slide or flow rather than to the transport or depositional mechanism. This implies some type of progressive failure of the sediment mass at source. Recent advances in the field of experimental soil mechanics, and observations during recent earthquakes, suggest that the phenomenon of progressive liquefaction may account for relatively rapid but discontinuous failure of unconsolidated sediment. Considering the grain size of the material involved and the structures in the conglomerates, it is suggested that the composite beds in the Grosses Roches section result from deposition from a submarine slide or flow which was caused by progressive liquefaction of sandy layers in a series of unconsolidated sands and gravels, perhaps during an earthquake.

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