Abstract: Ice-Drifted Boulders as Paleoclimatic Indicator, South Shore of Lower St. Lawrence Estuary

J. C. Dionne

Ice-drifted boulders are at the surface of the postglacial marine-clay terraces and in raised beaches on the south shore of the St. Lawrence estuary from the present shoreline up to the highest Goldthwait Sea shoreline. They are mainly Precambrian crystalline rocks which were drifted from the north shore of St. Lawrence across a distance of 30 to 55 km. Proportions of crystalline boulders concentrated in modern and ancient beaches and tidal flats range from 60 to 90 percent, but variation is less than 5 percent in neighboring till deposits.

Ice-drifted boulders at the different levels formerly occupied by the Goldthwait Sea show that floating and shore ice were present in the St. Lawrence from the early phases of postglacial submergence until the present.

Mean annual air temperature today in the area considered is 3.3°C at Trois-Pistoles and 2.7° at Pointe-au-Pere, 300 km northeast of Quebec City, with 5 months with a mean temperature below 0°, mean temperature for January being respectively - 11.8 and - 10.8°. Shore and floating ice are present from December to April.

Since the retreat of the Laurentides ice sheet the climate in the Lower St. Lawrence Estuary probably was similar to the present one with possibly some colder and warmer phases; however, it was cold enough to allow freezeup and ice drifting of boulders from the north shore to the south shore of the estuary. In absence of other evidence in sediments, ice-drifted boulders could be a valuable indicator of a temperature climate with cold winter.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA