Composition and spatial distribution of microencrusters and microbial crusts in upper Jurassic?lowermost Cretaceous reef limestone (Torinosu Limestone, southwest Japan)

Abstract

Tethyan microencrusters and microbial crusts, most of them previously unknown in Japanese Mesozoic biotas, are present in the uppermost Jurassic–lowermost Cretaceous Torinosu Limestone distributed in southwestern Japan. They construct reefal facies together with reef-forming metazoans. Bacinella irregularis and Lithocodium aggregatum are quantitatively most important, while subordinate constituents include Thaumatoporella parvovesiculifera, Koskinobullina socialis , Iberopora bodeuri , Girvanella sp. and “ Tubiphytes “ morronensis. They are especially common in the shallow-water reefal facies, but appear micritic in outcrops. Microencrusters and microbial crusts can only be recognized in thin sections, and they grow around the reef building metazoans and form bindstone. Each microencruster exhibits some specific spatial distribution associated with its paleoecology. Similarities with the taxonomic composition of the upper Jurassic Tethyan microencruster association imply that the community extended geographically at least to the Tethyan gateway where the Japanese Island Arc was located.