From red to grey and back again : a detailed study of the lower Kundan (Middle Ordovician) 'Täljsten' interval and its enclosing strata in Västergötland, Sweden

Abstract: Lithologic and faunal characteristics of the anomalous lower Kundan (Darriwilian, Middle Ordovician) 'Täljsten' limestone interval and its enclosing strata have been studied at Kinnekulle (Västergötland, south-central Sweden). A 2.6 m thick rock succession has been sampled, resulting in thin-sections and acid-insoluble residues. Both lithology and fauna change markedly throughout the succession, as is visible at both macro- and microscopic level. Stratigraphically ascending, the lithology changes from red color in the Lanna Limestone to grey throughout most of the 'Täljsten' interval and then back to red again in the Holen Limestone. These changes are accompanied by a gradual transition from wacke- to packstones, and a reorganization of microfaunal components takes place in the 'Täljsten'. This faunal reorganization is most pronounced in the relative abundance of brachiopods and echinoderms. The former taxon first dominates among identified skeletal grains, but then notably declines in the lower half of the 'Täljsten', whereas the latter taxon increases and ultimately becomes dominant. Echinoderms are also abundant at a macroscopic scale in the 'Täljsten', in beds dominated by diploporite cystoids ('Sphaeronites beds'). A stabilization of faunal component relations is seen in the upper half of the 'Täljsten' and the superjacent Holen Limestone, where a new faunal association appears to have been established. These observations fit well with previous studies of the rock succession, and seem to be in concordance with prevailing views that the 'Täljsten' represents a regressive event.