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
Studies of OAE 2 sections beyond the Atlantic Ocean, Western Interior Seaway (WIS) and European pelagic shelf are limited. Here, we present initial osmium isotope stratigraphy ( 187 Os/ 188 Os-Os i ) from two proto-Pacific sites that span the Cenomanian-Turonian boundary interval (CTBI): the Yezo Group (YG) section, Hokkaido, Japan, and the Great Valley Sequence (GVS), California, USA; to evaluate the 187 Os/ 188 Os seawater chemistry of the proto-Pacific. Additionally we combine new 206 Pb/ 238 U zircon CA-ID-TIMS geochronology from five volcanic tuff horizons of the Yezo Group section to test and facilitate inter-basinal integration with the WIS using radio-isotopically constrained age-depth models for both sections, and quantitatively constrain the absolute timing and duration of events across the CTBI. The YG shows an almost identical Os i profile to that of the WIS, and very similar to that of other sites of the proto-Atlantic and European pelagic oceans (Turgeon and Creaser, 2008;Du Vivier et al., 2014). The characteristics of the Os i profile are radiogenic and heterogeneous (∼0.55-0.85) prior to the OAE 2, and synchronous with the inferred OAE 2 onset the Os i abruptly become unradiogenic and remain relatively homogeneous (∼0.20-0.30) before showing a gradual return to more radiogenic Os i (∼ 0.70) throughout the middle to late OAE 2. A 206 Pb/ 238 U zircon age of an interbedded tuff (HK017) in the adjacent horizon to the first unradiogenic Os i value constrains the age of the Os i inflection at 94.44 ± 0.14 Ma. This age, including uncertainty, agrees with the interpolated age of the same point in the Os i profile (94.28 ± 0.25 Ma) in the only other dated OAE 2 section, the WIS; indicating a coeval shift in seawater chemistry associated with volcanism at the OAE 2 onset at the levels of temporal resolution (ca. 0.1 Myr). Further, prior to the onset of OAE 2 an enhanced radiogenic inflection in the Os i profile of the YG is correlative, within uncertainty, with a similar trend in the WIS based on the U-Pb agedepth model. The interpolated ages, 94.78 ± 0.12 Ma and 94.66 ± 0.25 Ma for this Os i inflection in the YG and WIS, respectively, indicate that palaeocirculation was sufficient to simultaneously influence transbasinal seawater chemistry. In contrast, the pre-OAE 2 Os i profile for the GVS is disparate to that of the YG and those of the proto-Atlantic and European pelagic shelf locations. We interpret the pre OAE 2 heterogeneous Os i values (0.30-0.95) to record a palaeobasin that was regionally influenced interchangeably by both unradiogenic (hydrothermal flux) and radiogenic (continental flux) Os. We conclude that the Os i profiles from the proto-Pacific sections record both trends that are consistent globally (OAE 2 onset, syn and post OAE 2), but also show regional differences (pre OAE 2) between OAE 2 sections worldwide. As such the Os i profiles coupled with U-Pb geochronology facilitate the correlation of OAE 2 stratigraphy, and demonstrate both regional and global ocean dynamics.
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