CLASTIC DISPERSION OF ROCK DEBRIS IN ESKE RS AND ADJACENT TILL (CHAPAIS-CHIBOUGAMAU REGION, QUEBEC, UPPER WISCONSINIAN)

Petrographic counts of more than 84 500 debris along eskers and in the adjacent till were used to study the dispersion of rock debris derive d from local marker outcrops (Opemiska Pluton and Chibougamau Formatio n) and from distant indicator rocks of Mistassini Basin (Albanel Forma tion 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 (lar ge 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 are a was insignificant. Nevertheless, meltwaters transported erratic clas ts of this formation for distances of more than 55 km. But the current s that deposited the sediments of the big eskers, which represent a su ccession of subaquatic outwash deposits, were restrained by the existi ng Ojibway Glacial Lake. The apparent greater displacement of debris i n these eskers, as compared to that in the neighbouring till, is the r esult of an abundant incorporation of intraglacial materials in the tu nnels. Rock debris from the Albanel and Papaskwasati formations were d ispersed over distances of more than 250 km. The lack of a significant increase of fine debris at the expense of larger fragments suggests t hat there was limited crushing at the glacier base, or else limited di sintegration in the meltwater currents. The decrease in the frequency of a petrographic marker is mainly controlled by the progressive sedim entation of fragments derived from the indicator outcrop, and by the r ate of incorporation of nonmarker debris. Thus, the nature of the erod ed rocks, and the size of the marker debris acquired at the source, ar e determining factors influencing the elastic dispersion. The variable rate of incorporation of rocks issued directly from the dispersion ar ea are in large part responsible for the irregularities in the curves representing the concentration of marker debris.