Sedimentary facies and biofacies of the T orinosu L imestone in the T orinosu area, K ochi P refecture, J apan

• Published in: The island arc Vol. 22; no. 2; pp. 150 - 169
• Main Authors: Ohga, Hiromichi, Kołodziej, Bogusław, Nose, Martin, Schmid, Dieter U., Takayanagi, Hideko, Iryu, Yasufumi
• Format: Journal Article
• Language: English
• Published: 01.06.2013
• ISSN: 1038-4871; 1440-1738
• DOI: 10.1111/iar.12009

Abstract The T orinosu L imestone represents carbonate platform deposits in a foreland basin, the sedimentary setting of which is highly different from those of well‐known L ate J urassic reefs in the western T ethys that developed on shelf areas of continental margins and intra‐ T ethyan platforms. Sedimentological and paleontological analyses were conducted on a 55.5 m‐thick U pper J urassic– L ower C retaceous ( T ithonian– B erriasian) carbonate sequence ( T orinosu L imestone) at the E astern H itotsubuchi Q uarry, K ochi P refecture, S outhwest J apan. The carbonate sequence is composed of two sections that are separated by a subaerial exposure surface. Two and three depositional units have been defined in the lower and upper sections, respectively, based on changes in lithology and the biotic composition of the carbonates; they are numbered from 1 to 5, in ascending order. Calcified demosponges (stromatoporoids and a chaetetid C haetetopsis crinita ) are abundant in three units (2, 3, and 5), in which microencrusters (mostly L ithocodium aggregatum and B acinella irregularis ) and microbialites are also common to abundant. Although most of them are para‐allochthonous, in‐situ branching stromatoporoids are found on and above the subaerial exposure surface (unit 3). Corals are less common, poorly diverse, and primarily represented by the family M icrosolenidae. Siliciclastic grains occur in all units, but they are particularly common in units 1 and 4. The co‐occurrence of the L ithocodium – B acinella association, which is typical of oligotrophic or moderately mesotrophic shallow‐water environments, with microsolenids, which are indicative of high nutrient levels and/or low‐light intensity due to high turbidity, suggests repeated changes in nutrient levels associated with terrigenous input. Based on lithology, biotic composition, and succession, we infer that sea‐level changes and related terrigenous input controlled the sedimentary environment of the studied carbonate sequence.