BASIN EVOLUTION OF THE PALEOPROTEROZOIC KARELIAN SUPERGROUP OF THE FENNOSCANDIAN (BALTIC) SHIELD

Abstract

The peneplaned Archean craton of the Fennoscandian Shield served as a platform upon which a continental margin assemblage, the Karelian Supergroup, was deposited between ~2.45 and ~1.9Ga. Major subaerial unconformities separate five sedimentary-volcanic groups of the supergroup — the Sumian, Sariolian, Jatulian (Lower and Upper), Ludicovian, and Kalevian. Second-order depositional sequences are implied. Early extension (~2.45Ga) resulted in localized rifts that were likely areas of later subsidence as well; they received thicker accumulations of sediments and volcanic rocks than did the adjacent platform. It is in these rifts and perhaps other downwarped areas that the sediments that were once more widespread were preserved, leading to interpretations of separate depositional basins by some workers. Seas transgressed onto the craton at least three times — in Sariolian time as evidenced by interpreted glaciomarine deposits, in Jatulian time as evidenced by widespread orthoquartzites (including tidalites) and stromatolitic carbonates, and in Ludicovian time as evidenced by organic-rich shales and turbidites. The tectonic-magmatic history is complex. Three episodes of mafic volcanism were widespread at 2.45, 2.2, and 2.1Ga. Island arcs formed to the south of the craton and collided at ~1.9–1.85Ga (the Svecofennian orogeny). This collision resulted in northeastward thrusting (e.g. the Outokumpu nappe) and folding and metamorphism of the Karelian Supergroup. The primary paleoclimatic indicators are (1) glaciogenic rocks near the base of the Paleoproterozoic succession indicating ice-house conditions; (2) remnants of a major paleosol on the glaciogenic rocks, indicative of deep weathering under greenhouse conditions (subtropical or tropical?); and (3) carbonate pseudomorphs after evaporite minerals in stromatolitic dolomites, perhaps indicative of aridity. Similarities in magmatism, tectonics, sedimentary rock types and sequences, and paleoclimatic indicators have led to hypotheses that the Fennoscandian Shield and North America may have been part of the same supercontinent during Neoarchean and Paleoproterozoic time.