CAMPANIAN-MAASTRICHTIAN CLAY-RICH SEQUENCES ALONG THE NORTH PACIFIC MARGIN: KEY SEQUENCES FOR EARLY COOLING HISTORY OF CRETACEOUS GREENHOUSE EARTH

Cretaceous shelf and fore-arc basin sandstone and mudstone are preserved in the coastal regions of Japan, Far East Russia, and the Pacific coast of Canada and USA. Several of these sequences have been variously assessed in terms of their biostratigraphy and chemostratigraphy, and correlated to the Aptian through Maastrichtian. In addition to macro- and microfossil biostratigraphy, carbon isotope (δ¹³C) stratigraphy has also identified some important event horizons within these successions, such as OAE2. Owing to the clay-rich nature of the strata, parts of the sequences yield excellently preserved calcareous fossils available for oxygen isotope thermometry (Moriya et al., 2003).

This study examines the Campanian-Maastrichtian interval. Its chronostratigraphy, including detailed δ¹³C stratigraphy, has been summarized recently (Voigt et al., 2012) and it records the initial phase of global cooling of the Cretaceous greenhouse Earth (Moriya, 2011; Friedrich et al., 2012). As the northern paleo-Pacific Ocean had a large heat capacity, its paleoceanography should provide insights for understanding the subsequent environmental transition from greenhouse to ice house Earth.

The Yezo Group and its equivalent in Hokkaido (Japan) and Sakhalin (Russian Far East), as well as the Nanaimo Group of the Canadian Pacific coast (British Columbia), are examined in this study. From the Yezo Group, a clear negative δ¹³C excursion as large as 1.4–2 per mil has been identified. On Sakhalin, its Campanian-Maastrichtian boundary age is constrained by local bio- and magnetostratigraphy, and the excursion is thus identified as the Campanian Maastrichtian Boundary Event (CMBE), associated with some subevents.

δ¹³C stratigraphy of the Nanaimo Group is under analysis. On the basis of magneto- and biostratigraphy (Haggart et al., 2011; Ward et al., 2012), the CMBE interval is expected to be located in the mudstone-dominated intervals of the overlying Northumberland or Spray formations.

Friedrich, et al., 2012, Geology, 40, 107-110; Haggart et al., 2011, Can. Paleont. Conf., Field Trip Guidebook No. 16, 31-62; Hasegawa et al., 2003, Palaeo-3, 189, 97-115; Moriya, 2011, Paleont. Res, 15, 77-88; Moriya et al., 2003, Geology, 31,167–170; Voigt et al., 2012, Newsl. Str., 45, 25-53; Ward et al., 2012, GSA Bull., 124, 957-974.