Abstract

Petrographie counts of more than 84 500 debris along eskers and in the adjacent till were used to study the dispersion of rock debris derived from local marker outcrops (Opemiska Pluton and Chibougamau Formation) and from distant indicator rocks of Mistassini Basin (Albanel Formation dolomites and Papaskwasati quartzitic sandstones). The frequency distributions are compared for the following granulometric classes:0.8–1.6 cm (fine gravels), 1.6–5.0 cm (small pebbles), 5.0–15.0 cm (large pebbles), and 15.0–45.0 cm (blocks).In the till, the minor content of debris derived from the Chibougamau Formation, downstream from the Waconichi Lake source-area, suggests that glacial erosion in this area was insignificant. Nevertheless, meltwaters transported erratic clasts of this formation for distances of more than 55 km. But the currents that deposited the sediments of the big eskers, which represent a succession of subaquatic outwash deposits, were restrained by the existing Ojibway Glacial Lake. The apparent greater displacement of debris in these eskers, as compared to that in the neighbouring till, is the result of an abundant incorporation of intraglacial materials in the tunnels. Rock debris from the Albanel and Papaskwasati formations were dispersed over distances of more than 250 km.The lack of a significant increase of fine debris at the expense of larger fragments suggests that there was limited crushing at the glacier base, or else limited disintegration in the meltwater currents. The decrease in the frequency of a petrographie marker is mainly controlled by the progressive sedimentation of fragments derived from the indicator outcrop, and by the rate of incorporation of nonmarker debris. Thus, the nature of the eroded rocks, and the size of the marker debris acquired at the source, are determining factors influencing the clastic dispersion. The variable rate of incorporation of rocks issued directly from the dispersion area are in large part responsible for the irregularities in the curves representing the concentration of marker debris. [Journal Translation]

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