Geochemical variations within the mid-Silurian Grötlingbo Bentonite (Gotland, Sweden): discriminating between magmatic composition, ash transport fractionation and diagenetic effects

This paper reports on the geochemistry of the mid-Silurian Grötlingbo Bentonite, a ca. 0.1–0.4-m-thick and regionally important bentonite bed in Sweden and the East Baltic area. A series of eight samples, spaced by 5 cm, were taken from the Hunninge-1 drillcore in Gotland, Sweden, and were analysed in order to establish the vertical element composition and variation in the bentonite. The results show that the Götlingbo Bentonite originates from one source magma and from one single eruption. The lowermost 0.1–0.15 m of the bentonite (compacted) was deposited from air-transport fall-out, whereas the upper portions have been reworked in a shallow-marine environment and re-deposited. Vertical differences in Ti and Zr within the Grötlingbo Bentonite mean that regional correlation of the bed may need several samples at each locality to be reliable