Seaward Changing Tidal Signatures in a Palaeoproterozoic Estuary, Västervik Basin, SE Sweden

Lena Sultan¹, Piret Plink-Björklund¹, Lennart Björklund¹, and Stefan Claesson². (1) Department of Geology, Göteborg University, Earth Sciences Centre, Box 460, Gothenburg, 405 30, Sweden, phone: +46 31 773 28 16, fax: +46 31 773 28 49, lena@gvc.gu.se, (2) Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, Stockholm, 104 05, Sweden

The Palaeoproterozoic Västervik Basin in southeastern Sweden has good exposures of well-preserved primary sedimentary features in spite of metamorphic overprint of greenschist to lower amphibolite facies. The Västervik sedimentary succession consists of continental to offshore deposits. This paper focuses on tidal signatures in fluvial to estuarine facies. Detailed exposures of Palaeoproterozoic tidal features, suitable for understanding of Palaeoproterozoic tidal cyclicity are rare. The Västervik fluvial to estuarine succession exemplifies landwards decreasing tidal influence. The most landwards expression of the tidal influence are reactivation surfaces within the otherwise fluvial cross sets. Further seawards occur rhythmical double mud-drapes, sigmoids, bi-directional cross-stratification and –lamination, multiple reactivation surfaces, lenticular bedding, and compound cross-stratification. The sedimentary structures together with the axial tidal channels, sandy tidal bars, and marginal tidal flats indicate that deposition occurred in a tidally dominated estuary. In the tidal flats desiccation cracks and synaeresis cracks are commonly occurring, suggesting subaerial exposure and changes in salinity during the tidal cycle.

U/Pb ion microprobe zircon dates show that Västervik Basin contains Palaeoproterozoic as well as Archaean detritus with ages not well represented in the presently exposed crust. We suggest the occurrence of a hitherto unknown continental terrane supplying material at the time of deposition.

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