Jérôme Chablais, University of Geneva (Switzerland)
Rossana Martini, University of Geneva (Switzerland)
Sylvain Rigaud, University of Geneva (Switzerland)
Elias Samankassou, University of Fribourg (Switzerland)
Tetsuji Onoue, University of Kagoshima (Japan)
Sano Hiroyoshi, University of Kyushu (Japan)
The Sambosan Accretionary Complex (AC) represents an unmetamorphosed Late Jurassic to Early Cretaceous subduction-generated accretionary complex in south-western Japan. The Upper Triassic limestones of the Sambosan AC are remnants of typical seamount-capping atoll-type carbonates in the Panthalassan Ocean. They generally crop out as limestone breccia associated with basaltic rocks and ribbon-cherts or as limestone blocks and pebbles within a volcanoclastic matrix, intercalated in volcanic successions. The Upper Triassic limestones are characterised by shallow-marine fossils, including corals, sponges, bivalves, crinoids, foraminifers and calcareous algae (Onoue & Sano 2007; Chablais et al. submitted). Sedimentological, micropaleontological and biostratigraphic results are presented with a particular attention to the foraminifers. Several microfacies types can be distinguished, probably reflecting the actual atoll morphology and its sub-depositional conditions.
The new discovered Upper Triassic benthic foraminifers can be grouped into four associations, each typical of specific depositional environment: (1) The lagoonal association is characterized by abundant Aulotortidae, Nodosariidae and Endotebidae; they are usually associated with Megalodonts bivalves. (2) The back- and fore-reef facies associations are dominated by Duostomina and Variostoma, often associated with cyanobacteria. (3) The reach and well diversified reefal association contains typical Norian-Rhaetian foraminifers such as Galeanella, Hoyenella, Ophthalmidium and Cucurbita. Cyanobacteria crusts, red algae, Carnian-Norian calcareous sponges and corals are also associated. (4) The shoal facies environment rich in ooids is distinguished by the almost monospecific association of Pilammina sulawesiana which, until today, has been only described in Sulawesi Island (Indonesia) (Martini et al. 1995).
The age of the foraminiferal associations indicates that the carbonate buildup sedimentation on top of Sambosan seamounts took place during Carnian to Rhaetian. This stratigraphic range of the Panthalassan seamount capping carbonates perfectly matches with that of carbonate platforms and reefs from Tethyan Realm such as the Dachstein Platform in the Alps (Enos & Samankassou 1998) and the Asinepe Limestone in Seram (Martini et al. 2004). The foraminiferal associations described here show a typical Tethyan affinity for tropical conditions, suggesting that the Sambosan seamount(s) very likely derive from low- to middle-latitudinal zone of Southern Hemisphere. This assumption is in agreement with the presence of endemic foraminifers and corals of Timor and Sulawesi, which confirm this palaeoposition (Stanley & Onoue, 2006). The Sambosan AC seamounts have to move more than 15,000 km to be accreted against Asian blocks. This distance, even if huge, seems to be in accordance with the velocity and the direction of tectonic plates related to the Neo-Tethyan ridge opening (G. Stampfli, pers. comm.).