Sequence stratigraphy of the siliciclastic East Puolanka Group, the Palaeoproterozoic Kainuu Belt, Finland

Abstract

The 2300–2600 m thick Palaeoproterozoic East Puolanka Group within the central Fennoscandian Shield records four major transgressions on the cratonic margin within the approximate time period 2.25–2.10 Ga. Stacking of siliciclastic facies in parasequences and parasequence sets provides data to evaluate oscillation of relative sea-level and subsidence on different temporal scales. The lowermost part of the passive margin prism is characterized by alluvial plain to shallow marine sediments deposited in incised valleys. The succeeding highstand period is recorded by ca. 250 m of progradational parasequence sets of predominantly rippled and horizontally laminated sandstones, representing stacked wave-dominated shoreline units in sequence 1, capped by a hiatus or, in some places, by a subaerial lava. As relative sea-level rose again, sand-rich barrier-beach complexes developed with microtidal lagoons and inlets, corresponding to a retrogradational parasequence set. This was followed by a highstand period, with aggradation and progradation of alluvial plain and coastal sediments grading up into wave-tide influenced shoreline deposits in sequence 2. In sequence 3, the succeeding mudstones represent tidal flat deposits in a back-barrier region. With continued transgression, the parasequences stacked retrogradationally, each flooding episode being recorded by increasingly deeper water deposits above low-angle cross-bedded sandstones of the swash zones. The succeeding highstand progradation is represented by alluvial plain deposits. The next transgressive systems tract, overlying an inferred erosional ravinement surface, is recorded by a retrogradational parasequence set dominated by low-angle cross-stratified swash zone deposits in sequence 4. The large-scale trough cross-bed sets in these parasequences represent sand shoals and sheets of the inner shelf system. The overall major transgression recorded in the lowermost part of the Palaeoproterozoic cratonic margin succession was related to first- to second-order sea-level changes, probably due to increasing regional thermal subsidence of the lithosphere following partial continental breakup. The stratigraphic evolution can be related to changes of relative sea-level with a frequency of ca. 25 million years, probably propagated by episodic thermal subsidence. The parasequences identified here are related to high-frequency cycles of relative sea-level change due to low-magnitude eustatic oscillations.