Geology and stratigraphy of forearc basin sediments in Hokkaido, Japan: Cretaceous environmental events on the north-west Pacific margin

Abstract

Litho-, bio-, and chemostratigraphy of the Cretaceous forearc basin sediments exposed in Hokkaido, northern Japan allow a synthesis of the faunal, sedimentological, and environmental history of the north-west Pacific margin. Although the succession, named the Yezo Group, has yielded an abundant record of mid- to late Cretaceous invertebrates, monotonous lithologies of sandstone and mudstone, showing occasional lateral facies changes, have caused confusion regarding the lithostratigraphic nomenclature. Based on our wide areal mapping of the sequence, and analysis of litho- and biofacies, a new lithostratigraphic scheme for the Yezo Group is proposed. In ascending order, the scheme is as follows: the Soashibetsugawa Formation (Lower Aptian mudstone unit); the Shuparogawa Formation (Lower Aptian–lower Upper Albian sandstone-dominant turbidite unit); the Maruyama Formation (lower Upper Albian tuffaceous sandstone unit); the Hikagenosawa Formation (Upper Albian–Middle Cenomanian mudstone-dominant unit); the Saku Formation (Middle Cenomanian–Upper Turonian sandstone-common turbidite unit); the Kashima Formation (Upper Turonian–Lower Campanian mudstone-dominant unit); and the Hakobuchi Formation (Lower Campanian–Paleocene shallow-marine sandstone-conglomerate unit). In addition, we designate two further lithostratigraphic units, the Mikasa Formation (Upper Albian–Turonian shallow-marine sandstone-dominated unit) and the Haborogawa Formation (Middle Turonian–Campanian shelf mudstone/sandstone unit), which correspond in age to the shallower facies of the Saku and Kashima formations, respectively. Despite a lack of so-called “black shales”, because of siliciclastic dilution, our stratigraphic integration has revealed the horizons of oceanic anoxic events (OAEs) in the Yezo Group. The OAE1a horizon in the Soashibetsugawa Formation is characterized by a lack of foraminifers, macrofossils and bioturbation, and a prominent positive excursion of δ13Corg. A significant hiatus during the late Aptian and early Albian removed the OAE1b horizon. The OAE1c horizon in the Maruyama Formation shows a distinct negative excursion of δ13Corg with a concomitant high productivity of radiolarians. The OAE1d horizon in the middle part of the Hikagenosawa Formation consists of weakly laminated, pyrite-rich mudstone. Planktonic and calcareous benthic foraminifers are absent, whereas radiolarians are abundant above the OAE1d horizon. The mid-Cenomanian event (MCE) horizon is identified at the top of the Hikagenosawa Formation. Stepwise extinction of calcareous benthic foraminifers and a decrease in radiolarian diversity become apparent above the MCE horizon. In the study area, the OAE2 horizon has been well documented, and is placed in the middle part of the Saku Formation

Mesorbitolina (Cretaceous larger foraminifera) from the Yezo Group in Hokkaido, Japan and its stratigraphic and paleobiogeographic significance

“…In central Hokkaido, many limestone olistoliths (= "Orbitolina limestones": Yabe, 1901) 8) are intercalated in the Kirigishiyama Olistostrome Member of the Shuparogawa Formation, the lower part of the Yezo Group (Takashima et al, 2004) 5) ( Fig. 1-A).…”

“…3-A). The lower part of the Yezo Group in this area is composed of the Soashibetsugawa Formation (Lower Aptian mudstone unit) and the Shuparogawa Formartion (Lower Aptian-lower Upper Albian sandstone-dominant turbidite unit) in ascending order (Takashima et al, 2004) 5) (Fig. 3-A).…”

“…The Yezo Group comprises Aptian to Paleocene forearc basin deposits extending from southern Hokkaido to the Sakhalin Island (Far East Russia) (e.g., Okada, 1983;Takashima et al, 2004 Nagao (1962). 46) Note that the "Onodera Formation" of Nagao (1962) 46) is now considered as a locally-developed lithologic facies within the Kamiji Formation (e.g., Kawaguchi, 1997).…”

“…1) The olistostrome bed containing the limestone olistoliths in the Yezo Group in central Hokkaido has been assigned in age to the Late Aptian, based on planktonic foraminiferal biostratigraphy and chemostratigraphy (e.g., Nishi et al, 2003;Takashima et al, 2004 (Fig. 3-A).…”

“…An olistostrome contain-ing shallow-marine carbonates and correlative conglomeratic gravity flow deposits, has been considered as one of the important key units in the Yezo Group (Takashima et al, 2004;Iba et al, 2005). …”