Abstract

In the lower Proterozoic Sariolian Group of eastern Finland, the metasedimentary Urkkavaara Formation possesses the attributes of a glaciogenic deposit. Within the formation, four members have been established—a lower siltstone-argillite member, a graded sandstone member, an upper siltstone-argillite member, and a diamictite member. The lowest three members contain lonestones. The upper siltstone-argillite member passes gradually over 1 m into a massive diamictite.

Many lonestones in the siltstone-argillite members are clearly dropstones. These members, and their association with diamictite, constitute strong evidence for a glaciogenic origin for the formation. The vertical transition of siltstone-argillite into diamictite indicates a similar “raining-down” mechanism of deposition for the diamictite. The thickness of the formation, 30 to 60 m (minimum), suggests deposition from icebergs, rather than from seasonal ice, for the laminated members and the diamictite. The graded sandstone member is interpreted to be a result of turbidity current deposition near a glacial terminus. A glaciomarine environment is suggested.

Lonestone-bearing units and diamictites in the Soviet Union just east of Finland have recently been reinterpreted as glaciogenic deposits. There appears, therefore, to have been an early Proterozoic continental glaciation about 2,500 to 2,300 m.y. ago on the Baltic Shield, approximately the same time that early Proterozoic glaciation occurred in North America.

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