The composition of altered volcanic ash of the Late Ordovician Kinnekulle bed was studied in geological sections of the Baltic Paleobasin. The composition of altered ash varies with paleosea depth from northern Estonia to Lithuania. The ash bed in shallow shelf limestones contains an association of illite-smectite (I-S) and K-feldspar, with the K2O content ranging from 7.5 to 15.3%. The limestone in the transition zone between shallow- and deep-shelf environments contains I-S-dominated ash with K2O content from 6.0 to 7.5%. In the deep-shelf marlstone and shale, the volcanic ash bed consists of I-S and kaolinite with a K2O content ranging from 4.1 to 6.0%. This shows that authigenic silicates from volcanic ash were formed during the early sedimentary-diagenetic processes. The composition of the altered volcanic ash can be used as a paleoenvironmental indicator showing the pH of the seawater or porewater in sediments as well as the sedimentation rate.
Thirty suspected altered volcanic ash (bentonite) samples from the Nässumaa-825 and Orissaare-859 sections were analysed by the X-ray fluorescence method. Twenty of these samples revealed chemical signs of pure volcanogenic material, one was of mixed terrigenous-volcanogenic origin, and nine were classified as terrigenous claystones. Twenty of the bentonites were correlated, with variable confidence, with bentonites from earlier studied sections; one sample represents a formerly unknown eruption. New and earlier published bentonite correlations were used for tracing the diachronous nature of the Rumba-Velise formations boundary and for composing new isopach schemes of six Telychian bentonites.
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