Wenlock metabentonites from Gotland, Sweden: geochemistry, sources and potential as chemostratigraphic markers

Abstract

Two metabentonite suites occur within the Wenlock limestones and marls of Gotland, one within the Slite Formation (M. belophorus to the C. ellesae biozones) and the other in the Mulde Formation (G. nassa–M. dubius Biozone). Their geochemical characteristics based on rare-earth element (REE) distributions in apatite crystals separated from the metabentonites suggest origins from three separate volcanic sources. One of these sources has an alkaline affinity, reflected in relatively high levels of Th, Nb and Zr, which suggests that it may have lain over thickening continental crust during the waning stages of the closure of the Tornquist Sea, while the other two represent calc-alkaline magmas. The source of the volcanic ash is placed some 400 km to the southwest of Gotland, on the Tornquist–Teysseyre Zone, which is the northernmost expression of the Trans-European Suture Zone. Distinctive differences in REE distribution in apatite from these metabentonites should assist in future correlation studies in Wenlock stratigraphy, both within the Baltic region and further afield.

Carbonate platform evolution and conodont stratigraphy during the middle Silurian Mulde Event, Gotland, Sweden

“…These are in growth position, low to highly dome-shaped, and are draped by overlying marls. Eight bentonites, none more than 5 cm thick, crop out at Djupvik 1 (Odin et al 1986;Batchelor & Jeppsson, 1999). None of these bentonites have yet been identified at localities NE of Djupvik 1 where outcrops are scarce, less fresh and smaller.…”

Multiphase Silurian bentonites in the Baltic Palaeobasin

“…Within these compositional limits it is difficult to assume that the composition of deposited ash had a specific control on the development of different authigenic assemblages. Nevertheless, the geochemical composition of the altered volcanic ash and the tectonic context suggest that the Llandoverian ash beds were derived from calc-alkaline volcanism , whereas the pyroclastics of Wenlockian bentonites of the same region might have originated from an alkaline type of volcanics (Batchelor and Jeppsson, 1999). This change in source material coincides with the change from kaolinitic to illite-smectite dominated composition of bentonites in both drill-cores (Fig.…”

“…The geochemical composition of the altered volcanic ash and the tectonic context suggest that the volcanism which produced the Llandovery ash beds, was of calcalkaline type and related to volcanic activity at the subduction zone of a closing Iapetus Ocean . However, pyroclastics of Wenlock bentonites might have originated from alkaline type volcanic activity on another tectonically active margin of the Baltica plate now known as Tornquist-Teysseyre Zone (Batchelor and Jeppsson, 1999). The most prominent Silurian bentonite from the Baltic region, the Osmundsberg bentonite occurs in Llandovery sediments in the southwestern part of the Estonian mainland and Baltic Sea islands Huff et al, 1998;.…”

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

“…Contemporaneous bentonites from the Slite Group and the distal portions of the Halla Formation (Mulde Brick-clay Member; cf. Batchelor & Jeppsson 1999) indicate dacitic affinities. The export of silica and other elements and compaction of the initial ash layer significantly decrease the thickness of the bentonite.…”