QUATERNARY FORAMINIFERAL STRATIGRAPHY IN SEDIMENTS OF THE EASTERN CHAMPLAIN SEA BASIN, QUEBEC

Sediments of the late-glacial Champlain Sea basin have been sampled at 18 sites in the Trois-Rivieres and Quebec City regions. Two successio ns of foraminiferal ecozones (deep and shallow water) comparable to th ose previously reported from the western Champlain Sea are recognized. Faunal composition indicates an early phase (pre-A) of hyposaline wat ers with consistent freshwater evidence only at the southwestern limit of the area. Salinity control in pre-A could be the result of advance or retreat of an ice lobe in the Quebec City region. It is followed b y zone A (salinity: 25-32 parts per thousand) which is in part synchro nous with the formation of the St-Narcisse Moraine. In the easternmost Champlain Sea, zone A lasted approximately from 11.3 until after 10.6 ka BP, possibly up to near 10.2 ka BP. In the western part of the are a, it is progressively replaced by the less saline conditions of zones B (10-25 parts per thousand) and C (2-10 parts per thousand). Near ma jor freshwater inlets, thick delta sequences barren of foraminifera su bstitute for zones B and C. East of Trois-Rivieres, zone B is probably discontinuous and does not reach Quebec City. Zone C is present at on ly one site, west of Trois-Rivieres. The shallow water zones EH and EA , contemporaneous with zones A and B, record relatively high salinitie s (annual maximum of up to 30 parts per thousand for EH, somewhat less for EA), zone EA suggesting also warmer summers. The paleoecological results concerning salinity do not allow detection of the diversion of Lake Agassiz outflow. The discharge envisaged by some authors may not have been large enough to detectably change salinity in the deep, lat e Champlain Sea. In shallow waters, the effect could have been signifi cant but would be difficult to date because of error due to recycled o ld carbonate.