Paper No. 6
Presentation Time: 9:30 AM
PALEOZOIC K-BENTONITES IN DRILL CORES FROM THE SILJAN IMPACT STRUCTURE, SWEDEN
New drill cores from the largest known impact structure in Europe, the relict of the late Devonian Siljan meteorite crater in central Sweden, provide new possibilities to reconstruct Early Paleozoic marine environments and ecosystems, and to document changes in sedimentary facies, sea level and paleoclimate in Baltoscandia. Two core sections recording K-bentonites, Mora 001 and Solberga 1, have been analyzed. The volcanic record, expressed by Ordovician and Silurian K-bentonites, may be compared to occurrences of ash layers in other parts of Baltoscandia which serve as time-lines in a detailed stratigraphic framework including litho-, bio-, and sequence stratigraphic parameters. This report is focused on the mineralogy and geochemistry of these ash beds with the intent to explore possible correlations with previously described K-bentonites throughout Baltoscandia. The Solberga 1 core has a series of K-bentonites in the Dalby Ls (Sandbian) to Freberga Fm (Katian) interval as well as the Motala Fm (Rhuddanian-Aeronian) to Kallholn Fm (Aeronian-Telychian) interval. Biotite from the SO32-SO35 samples was also analyzed by EDAX. In addition, these samples contain euhedral zircon and apatite phenocrysts. The Sandbian clay mineralogy consist of mixed-layer I/S with 80% illite while the Llandovery and Wenlock samples vary from I/S to dominantly illite and chlorite, but some with substantial amounts of kaolinite, particularly in the Telychian sequence SO16-SO20. The Middle Ordovician section at Röstånga in Scania contains eighteen K-bentonite beds ranging from 1-67 cm in thickness, and all occur within the D. multidens graptolite biozone. Several beds correlate equally well with the Kinnekulle bed and thus argue strongly for the composite nature of what is called the Kinnekulle K-bentonite. We suggest the same for the OS32-OS35 sequence. And similarly, the succession SO16-SO20 in the Lower Telychian Kallholn Fm is both kaolinite-rich and has a much higher proportion of smectite in the I/S phase compared with the Kinnekulle samples. This succession is consistent both stratigraphically and mineralogically with the position and composition of the Osmundsberg K-bentonite that is known throughout the Baltic, and thus we suggest that these samples represent a composite succession of the Osmundsberg K-bentonite.