||Radiolarian biostratigraphy of Late Cretaceous pelagic sediments in the Oman Ophiolite : Environmental change based on the radiolarian faunal transition across the Cenomanian/Turonian boundary
オマーンオフィオライトにおける後期白亜紀遠洋性堆積物の放散虫生層序 : 放散虫群集変遷に基づくCenomanian/Turonian境界の環境変化
Hara, Kousuke原, 康祐
114 , 2017-09-20 , 新潟大学
学位の種類: 博士（理学）. 報告番号: 甲第4373号. 学位記番号: 新大院博（理）甲第426号. 学位授与年月日: 平成29年9月20日
Lithostratigraphy and radiolarian biostratigraphy of Late Cretaceous pelagic sediments on basaltic extrusive rocks within the Oman Ophiolite were investigated in order to understand the nature of pelagic sedimentation, history of volcanic activity of the ophiolite, and radiolarian faunal transition and environmental change in this time period. Pelagic and metalliferous sediments commonly occur on lavas and at the boundaries between different volcanic units. The basaltic rocks have been subdivided into three volcanic units: the V1 lava with the N-MORB signature, the V2 lava formed by intra-oceanic volcanism, and the V3 lava generated by intra-plate seamount magmatism (Ernewein et al., 1998). The most thickly accumulated sediments, which rest directly on the V1 lava formed by ridge magmatism, were named the Suhaylah Formation (Fleet and Robertson, 1980). The metalliferous and pelagic sediments of this formation are subdivided into four lithologies in stratigraphic ascending order: metalliferous sediments, red mudstone with chert intercalations, micritic limestone, and red mudstone. The Suhaylah Formation is overlain by the Zabyat Formation (Robertson and Woodcock, 1983). This formation consists of ophiolite debris, redeposited sandstone- to siltstone-sized volcaniclastic rocks, and pelagic red mudstone. From the biohorizons of radiolarians, four radiolarian zones were defined. The Guttacapsa gutta Zone, obtained from metalliferous sediments and chert intercalations within red mudstone, could be correlated with the middle to lateset Cenomanian. The Rhopalosyringium scissum Zone, recovered from red mudstone and chert, is assigned to the lateset Cenomanian to early Turonian. The Foremanina schona Zone, recognized in micritic limestone, is correlated with the middle Turonian. The Eostichomitra perapedhia Zone, recovered from red mudstone of the uppermost part of the Suhaylah Formation and the top of the Zabyat Formation, is assigned to the late Turonian to Coniacian. The radiolarian age of the sediments overlying the V1 lava (middle–late Cenomanian: 96.5–93.9 Ma) is consistent with the high-precision U–Pb zircon age of crustal rocks formed by ridge magmatism (96.12–95.50 Ma: Rioux et al., 2016). In addition, the sediments on the V2 lava can be also correlated with the middle–late Cenomanian. These findings reveal that the change of the tectonic setting from mid-ocean ridge through subduction zone to oceanic thrusting occurred in a short period and the activities of the V1 to V2 lavas were terminated in late Cenomanian.
Based on the discrimination diagrams by Murray (1994), the rocks of the Suhaylah and Zabyat formations ranges from the ridge to pelagic environments. In particular, metalliferous sediments and chert intercalation plot within the ridge field, whereas the other sediments plots within the pelagic field. These findings suggest that the Suhaylah and Zabyat formations were deposited in the pelagic environment and were not influenced by inputs of terrigenous materials. Examination of radiolarian faunal composition shows that spherical Nassellaria such as the genera Hemicryptocapsa and Holocryptocanium increased dramatically in the OAE2 interval below the C/T boundary of the Suhaylah Formation. These features are also recognized in the European OAE2 horizons in which black shale was deposited (Bak, 2011). Therefore, the causes of extinction of the European Tethys and the Tethys of Oman are considered to be same, as follows: (1) the formation of LIPs (such as the Caribbean LIP) released CO_2, CH_4, and other greenhouse gases into the atmosphere, and then the temperature rose, (2) continental weathering was promoted, and a large amount of nutrient flowed into the ocean, (3) excessively supplied CO_2was absorbed by the ocean, and subsequent ocean acidification drastically damaged the primary producer, especially in calcareous nannoplankton (Du Vivier et al., 2015), (4) after that, cyanobacteria and dinoflagellates increased explosively on the ocean surface where nutrients were abundantly present (Ohkouchi, 2003; Ando et al., 2017), and (5) extinction of radiolarians occurred by gradually enlarging of the oxygen minimum zone mainly in the ocean middle layer due to excess supply of organic matter without decomposition (such as cyanobacteria and dinoflagellate remains). After the extinction of Cenomanian radiolarians, the Rhopalosyringium species occurs abundantly in the Suhaylah Formation, and their remarkable diversification can be recognized. On the other hand, this trend is not recognized in Europe. From this, it is considered that the radiolarian ecosystem was recovered quickly from environmental degradation in the Tethys of Oman that was open ocean, rather than closed European Tethys.