Hermatypic Coral Assemblages: Growth Form, Spatial Distribution, and Paleobathymetry

Abstract

Assemblages of hermatypic corals occur in the sediments investigated either in the form of widespread biostromes or as individual, minor bioherms. The most widespread coral biostromes are those in the Liesberg Member with a matrix which was primarily a mixture of calcareous and argillaceous mud. Coral biostromes in a matrix of primarily pure calcareous mud occur at the basinward margin of the St-Ursanne Formation and in the Olten Member at the basinward rim of the Balsthal Formation. Coral colonies in the lowermost part of biostromes are almost exclusively plate-like microsolenids or Isastraea with probably a low diversity. The initially substantial content of clay minerals in the muddy matrix at the base of the Liesberg Member diminished in the course of sedimentation upward in the member. The upper boundary of the member to the limestone of the St-Ursanne Formation is, therefore, transitional. Growth of coral assemblages at the basinward margin of the carbonate platform of the St-Ursanne Formation was uninterrupted from base to top. The growth form of these coral assemblages was probably a biostrome at all stages of growth. The best place to see this is section RG 404 in the gorge of the Scheulte River 1 km southeast of the village of Mervelier in Canton Jura, which is shown on Pl. 36 in Gygi (2000a). These biostromes probably grew at a water depth of more than 6 m below the bathymetric range in which calcareous ooids were accreted in the coeval Tiergarten Member. Most of the Olten Member at the basinward margin of the carbonate platform of the Balsthal Formation was a coral biostrome growing in deeper water off the wide oolitic sand bank of the Steinibach Member. The Olten Member is intersected in a natural outcrop in the small gorge north of the village of Wangen west of Olten and above the eastern end of the bridge across the Aare River between Olten and Winznau.