Studies on the Macrourus and Slandrom Formations I Shell Fragment Frequencies of the Macrourus Formation and Adjacent Strata at Fjäcka. Gräsgård, and File Haidar

Abstract

The author has studied the role of shell fragments in the formation of some calcareous rocks belonging to the Middle Ordovician Ludibundus and Macrourus formations, and the Upper Ordovician Slandrom formation in Sweden.

The material has been collected in the central and southeastern parts of Sweden, at Fjäcka in Dalarne and (from erratic blocks) at Gräsgård on the island of Öland. These localities are redescribed and the fossils found at each place are listed. In addition, some samples obtained from core drilling at File Haidar, island of Gotland, have been analyzed.

The samples were analyzed with regard to the total amount of shell fragments larger than 0.1 mm, the proportions of different phyla in this fraction, and the total amount of the insoluble residue which was mainly clay and silt. These values were obtained by dissolving samples in diluted hydrochloric acid, and by measurements of thin sections according to the Rosiwal method. The mean values of these constituents and the standard deviations are given for the principal rock types. The total amount of shell fragments in these rocks is below 20 %; they are thus calcilutites according to Jaanusson (1952).

The total amount of shell fragments and the proportion of Pelmatozoa seem to be independent of the amount of the insoluble residue, whereas the proportion of Arthropoda decreases, and that of Brachiopoda increases with an increasing amount of insoluble residue.