Upper Triassic shallow–water carbonates from the Naizawa Accretionary Complex, Hokkaido (Japan): New insights from Panthalassa

Abstract Due to their accretion on the Circum–Pacific area during the Jurassic and the Cretaceous, Upper Triassic carbonates from the Panthalassa occur very scarcely and with relatively poor preservation in accretionary complexes. However, they represent a unique opportunity to improve our knowledge of the depositional conditions in tropical regions of the Panthalassa. Recognized as a cradle of life, shallow–water carbonates are also of great importance to understand of how life evolved out of the Tethyan domain during the Triassic. Since 2007, several sedimentological and biostratigraphic studies, focused on the Upper Triassic shallow water limestone from the Circum–Pacific area, have been carried out as part of the REEFCADE project at the University of Geneva. Carbonates were thus reported in the southern part of Japan and in the Russian Far East. Hokkaido Island, in northern Japan, represents the missing link between those two areas. To fill this gap, five limestone outcrops, so far poorly described in the literature, were identified and sampled in the Pippu and Esashi areas (central and northern part of Hokkaido Island, respectively). Their related microfacies are presented in detail as well as their foraminiferal associations. The obtained age, based on foraminiferal biostratigraphy from both areas, is defined as Carnian and the facies similarities, associated with specific modes of occurrence, identify the outcrops from the two areas as part of the same depositional system. Based on microfacies interpretations, a hypothetical depositional model is presented. It corresponds to a intra–oceanic depositional system developed on the flanks of an emergent volcanic seamount. The strong similarity with synchronous systems from the Sambosan Accretionary Complex (southwestern Japan) is discussed.