Mineralogy, Chemistry and Particle Size Interrelationships in some Post-Glacial Marine Deposits of the St. Lawrence Lowlands

Abstract

To better characterize the nature of the source material of the present-day suspended load of the St. Lawrence River, we have examined the mineralogy and chemistry of the clay and colloid-size tractions of subaerially exposed Champlain Sea deposits and of submerged Goldthwait Sea deposits under the Upper St. Lawrence Estuary. Similar analyses were obtained on the lower St. Lawrence River and estuarine suspended sediments. The mineralogical composition, obtained from semi-quantitative X-ray diffraction studies, changes with particle size. Feldspar and quartz are predominant in the clay-size fraction, while illite and Fe-rich chlorite are abundant in the colloid-size fraction. The mineralogical composition of the clay-size material in the Champlain and Goldthwait Sea deposits is similar to that found in suspension. The marine deposits of the St. Lawrence Lowlands (Champlain and Goldthwait Sea deposits) contain the lowest mineral maturity (M_M) and chemical maturity (Ch_M) values. When compared to some of the major world rivers, it appears that mineralogically and chemically immature material is still being supplied by the St. Lawrence River under present climatic conditions. An increase in illite content, M_M, Ch_M, and a decrease in feldspar in the colloid-size fraction with increasing depth in some of the profiles investigated suggests the possibility of post-glacial digenetic alteration. Higher chemical index of alteration (CIA) values in sediments which have been subaerially exposed as compared to those which have remained submerged may be due to ground water removal of alkalis.