Sequence stratigraphy of the Karelian formations (2.4–2.0 Ga) of the Fennoscandian Shield – Significance of major unconformities

Established sequence stratigraphy for the Palaeoproterozoic sedimentary sequences of the Karelian formations of the Fennoscandian Shield illustrate first- and second-order base-level cycles within a synrift to cratonic margin transition. The stacking of siliciclastic facies in sequences and parasequence sets in the East Puolanka Group provide data to evaluate changes of relative sea-level and basin subsidence in four third-order sequences within a second-order cycle. In the Kainuu Belt, the sedimentary sequences of the autochthonous part of the belt are up to 4700 m in total thickness. The sedimentary sequences show the effects of very long duration first-order cyclicity of sea-level, with a duration of ca 400 Ma, as part of the progressive development of a rifted Neoarchaean continent. Attenuation of continental crust during rifting and breakup of the continent raised relative sea-level. From the autochthonous part of the Kainuu Belt, three major basin evolution stages are recognized (synrift, transitional and initial passive margin), each expressed by unconformity-bounded second-order cyclicity of duration more than 100 Ma. Basal mafic volcanics and alluvial to glacial siliciclastics, some 400–700 m thick, of the Kurkikylä and Honkajärvi groups represent synrift deposits of a second-order basin-fill sequence related to a continental-scale fragmentation of the crust ca 2.4–2.3 Ga ago. Many of the shallow intra-cratonic basins subsided below sea-level, some being affected by a major continental glaciation. The subsequent peneplanation was dominated by an intensive period of subaerial chemical weathering, apparently of the extent of an entire craton and identified as the next major unconformity. The following second-order sequence of deposits from quartz-arenitic alluvial fans and broad braided alluvial plains of the Korvuanjoki Group, up to 1400 m thick, was deposited in half-grabens, still in a relatively stable cratonal setting and corresponding to the incipient stage in the development of a continental margin. Stacked highstand sequences were then encountered and extensive erosion and deep incised valleys were formed during the falling stage of relative sea-level. Within the next unconformity-bounded second-order, 2300–2600 m thick, sequence of a passive-margin prism of the East Puolanka Group (ca 2.25–2.10 Ga), four third-order cycles were identified, defined by variable system tracts and sequence boundaries formed during a falling stage of relative sea-level. These sequences record lowstand and transgressive systems tracts followed by pronounced highstand systems tracts. The sparse parasequence sets identified in these sequences are related to high-frequency cycles of relative sea-level change. Deposition was related to the increasing tectonic flexure and episodic regional subsidence of the cratonic margin, followed later by distal shelf mudstones, carbonates and deeper water deposition that rimmed much of the continental margin.