Geology and Boreal bivalve assemblages of the Lower Jurassic Higuchi Group, western Shimane Prefecture, Japan

Abstract

The Lower Jurassic Higuchi Group, exposed in the western part of Shimane Prefecture, Japan, includes, in ascending order, the Orojidani and Higuchidani formations. The Orojidani Formation is composed of conglomerate, sandstone, and mudstone, the latter containing Oxytoma sp. and Pleuromya sp. The Higuchidani Formation consists mainly of dark gray mudstone, from which six genera and six species of bivalves were collected: Kolymonectes staeschei, Palmoxytoma cygnipes, Ryderia texturata, Pseudomytiloides matsumotoi, Oxytoma sp., and Pleuromya sp. This bivalve assemblage is characterized by Boreal elements such as K. staeschei and P. cygnipes, which are known exclusively from the Lower Jurassic of Russia and northern Canada; it contains no typical Tethyan species.

Palaeogeographic and tectonic setting of the Lower Jurassic (Pliensbachian‐Toarcian) Nishinakayama Formation, Toyora Group, SW Japan

“…For ammonoids, although Boreal‐type ammonoids (i.e., Amaltheus , Cleviceras ) were yielded both from the Higashinagano and Nishinakayama formations (Hirano, ; Nakada & Matsuoka, ), they are also characterized by the common occurrence of Tethyan fauna (i.e., Canavaria , Harpoceras , and Hildaites ; Hirano, ; Nakada & Matsuoka, ). Overall, these data suggest strong affinities with European Tethys (i.e., southern type) assemblages (Nagata et al, ). In addition, although the fossil‐plant assemblages from the Nishinakayama Formation are unique and not comparable with any other Early Jurassic floras from East Asia (Kimura, Naito, & Ohana, ), the Nishinakayama flora has been interpreted to have subtropical to tropical affinity (Kimura, Ohana, & Tsuji, ).…”

“…The bivalve assemblage from the Higashinagano Formation (underlying the Nishinakayama Formation; Figure ) is composed of mixed East Asian specific fauna, Boreal fauna (northern type), and Tethys fauna (southern type; Hayami, , , ). However, recent revision shows that it is characterized by mixed East Asian and Tethys fauna, with only rare Boreal fauna (Nagata, Komatsu, Shurygin, Ishida, & Sato, ). In addition, Hispaniastraea cf.…”

“…In addition, Hispaniastraea cf. ramosa , a coral species that is yielded typically from carbonate platforms of European Tethys and low latitude areas in the southern hemisphere (Rey, Andero, Garcia‐Hernandez, Martin Algarra, & Vera, ; Turnsek, Buser, & Debeljak, ; Turnsek, Seyfried, & Geyer, ), has recently been recorded from the Higashinagano Formation (Nagata et al, ). For ammonoids, although Boreal‐type ammonoids (i.e., Amaltheus , Cleviceras ) were yielded both from the Higashinagano and Nishinakayama formations (Hirano, ; Nakada & Matsuoka, ), they are also characterized by the common occurrence of Tethyan fauna (i.e., Canavaria , Harpoceras , and Hildaites ; Hirano, ; Nakada & Matsuoka, ).…”