PALAEOPROTEROZOIC GLACIOMARINE SEDIMENTATION IN AN EXTENSIONAL TECTONIC SETTING - THE HONKALA FORMATION, FINLAND

The Honkala Formation of the Honkajarvi Group in the Kainuu Schist Bel t of northern Finland (Central Fennoscandian Shield) provides evidence of Palaeoproterozoic glaciation on the fractured surface of the crato n. The diamictites and associated rocks of the Honkala Formation are s ome 190 m thick in the studied area. They consist of a remnant of the glacigenic sequence deposited during an incipient stage of crustal ext ension. The rocks rest unconformably on the Late Archaean basement and conformably on rift-related volcanics. The volcanics were erupted dur ing a period of general fragmentation and rifting of the Late Archaean basement similar to 2.4-2.3 Ga ago. At this time, shallow intracraton ic basins and depressions started to form on a continental scale. Subs equently, many of these basins subsided below sea level. The lower par t of the Honkala Formation consists of turbiditic, non-glacial mudston es and sandstones, up to 70 m thick. Glacial advance is indicated by i ce-rafted deposits forming a succession of some 10 to 50 m. Deposition took place in a glaciomarine rift-basin extending towards the interio r of the Archaean Craton. Waning glacial influence is indicated by som e 70 m of sandstones. The uppermost sandstones were influenced by inte rmittent chemical weathering indicating that these deposits were subje cted to subaerial conditions and warmer climate. Correlation on a glob al scale is possible. During the early Palaeoproterozoic, the Fennosca ndian and North American cratons were probably situated close together at a high latitude and both were affected by a major continental glac iation. The Fennoscandian Craton then shifted to lower latitudes, wher eas the North American cratonic margin recorded further glaciations at high latitudes.