ABSTRACT: A stratigraphic approach to the study of the late-glacial Champlain Sea deposits with the use of Foraminifera

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

The Champlain Sea is a glacial-isostatic sea that occupied the present lowlands of the St. Lawrence River, southern Quebec, Canada, after the retreat of the late Wisconsinan ice sheet, between approximately 12800 and 9500 years BP. The inundated area received a cover of clay, silt and sand of up to a few tens of meters in thickness. 353 surface and subsurface samples of Champlain Sea sediments were processed, of which 207 yielded about 90 species of foraminifera, plus many ostracoda. Comparison between quantitative foraminifera analyses, performed at different localities and stratigraphic depths showed the existence of two sequences of microfossil assemblages correlatable over most of the area: a deep-water and a shallow-water sequence. At its base, the deep-water sequence starts with oligohaline conditions which quickly become near-normal marine with salinities of up to ca. 30 ppt. Afterwards, salinities decrease gradually until oligohaline to freshwater conditions are reached. During the high-salinity phase, the climate was cold and Arctic. It may have improved in the later part of the deep-waer sequence, but data from these levels are poor. The shallow-water sequence records only a cold, high salinity phase succeeded by a low-salinity phase showing a definite improvement in temperature. The early oligohaline phase corresponds to an early retreat of the ice coupled with a limited influx of seawater. Although poorly dated, the phase of maximum salinity of the deep-water sequence is correlated with the high-salinity phase of the shallow-water sequence. This probably coincides with a standstill in the ice retreat (the St. Narcisse episode) during which near-normal marine conditions existed over most of the basin except near the ice front. The subsequent decrease in salinity is related to accelerated melting of ice following the end of the St. Narcisse episode. This phase may have also known low oxygenation in the deeper parts of the basin. The final, brackish to freshwater phase is probably due to isostatic uplift progressively isolating the basin from the sea and transforming it into a lake (Lake Lampsilis).

Bibliography of Canadian Geomorphology