MINERALOGY, CHEMISTRY AND PARTICLE-SIZE INTERRELATIONSHIPS IN SOME POSTGLACIAL MARINE DEPOSITS OF THE ST-LAWRENCE LOWLANDS

To better characterize the nature of the source material of the presen t-day suspended load of the St. Lawrence River, we, have examined the mineralogy and chemistry of the clay and colloid-size fractions of sub aerially exposed Champlain Sea deposits and of submerged Goldthwait Se a deposits under the Upper St. Lawrence Estuary. Similar analyses were obtained on the lower St. Lawrence River and estuarine suspended sedi ments. The mineralogical composition, obtained from semi-quantitative X-ray diffraction studies, changes with particle size. Feldspar and qu artz are predominant in the clay-size fraction, while illite and Fe-ri ch chlorite are abundant in the colloid-size fraction. The mineralogic al composition of the clay-sits material in the Champlain and Goldthwa it Sea deposits is similar to that found in suspension. The marine dep osits of the St. Lawrence Lowlands (Champlain and Goldthwait Sea depos its) contain the lowest mineral maturity (M(M)) and chemical maturity (Ch(M)) values. When compared to some of the major world rivers, it ap pears that mineralogically and chemically immature material is still b eing supplied by the St. Lawrence River under present climatic conditi ons. An increase in illite content, M(M), Ch(M) and a decrease in feld spar in the colloid-size fraction with increasing depth in some of the profiles investigated suggests the possibility of post-glacial digene tic alteration. Higher chemical index of alteration (CIA) values in se diments which have been subaerially exposed as compared to those which have remained submerged may be due to ground water removal of alkalis .