Origin of cyclicity in Triassic-Jurassic radiolarian bedded cherts of the Mino accretionary complex from Japan

Abstract The abundance of magnetic microspherules in a Triassic-Jurassic continuous sequence of alternating chert and shale beds in the Mino accretionary complex, central Japan, was measured systematically. Depending on time, the magnetic microspherules extracted from shale beds change in abundance considerably from the minimum 0.9ppm/cm³ at latest Triassic ( ca 208Ma) and the maximum 75ppm/cm³ at late Early Jurassic ( ca 187Ma); however, the abundance is always higher approximately 10–100 (average 70) times than those from adjacent chert bed at any stratigraphic horizon. Such systematic difference reveals the origin of radiolarian bedded chert as cyclic-rapid accumulation of biogenic SiO₂ under extremely slow accumulative environments of shale with probable aeolian dust in origin. The accumulation data for individual shale and chert beds were obtained based on the microspherule abundance and radiolarian biostratigraphy, i.e., ca 0.018g/cm²Ka for lower Jurassic shale beds and ca 1.9g/cm²Ka for adjacent chert beds.
Duration time to make a chert-shale couplet corresponds to a dominantly 15–20Ka interval (average 23 Ka) in Upper Triassic bedded cherts with a low paleolatitude, whereas a 40–45 Ka interval (average 42 Ka) in Lower Jurassic ones which may been formed in higher latitude than Triassics before the final accretion to the Asian continental margin. Depending on paleolatitude, the cyclicity of 23 and 42 Ka may correspond to Milankovitch cycles which have been well documented in deep-sea sediments.     

藏南泽当卡多地区中生代放射虫动物群研究及其地质意义

雅鲁藏布江蛇绿岩带一直以来被认为是新特提斯洋岩石圈的残留,其与雅鲁藏布江缝合带内相关的海相地层共同记录了新特提斯洋的演化过程以及印度—欧亚板块的碰撞过程。位于藏南泽当西南卡多附近的硅质岩剖面中发现有保存较好的中生代放射虫化石,鉴定出放射虫31属39种,识别出4个放射虫组合,并将之与邻区及西特提斯地区放射虫化石带进行对比确定其时代:Alievium longispineum-Transhsuum maxwelli组合,晚巴通期—早卡洛夫期;Hsuum maxwelli-Triactoma mexicana组合,晚卡洛夫期—牛津期;Holocryptocapsa hindei-Hiscocapsa grutterinki组合,晚巴雷姆期—早阿普特期;Hemicryptocapsa polyhedra-Obeliscoites giganteus组合,晚阿尔布期—赛诺曼期。卡多硅质岩中所含放射虫组合可为泽当蛇绿岩的形成时限及新特提斯洋的演化提供重要的微体古生物依据。     

赣东北晚古生代放射虫化石综述

首次在赣东北蛇绿混杂岩带分布的弋阳县及德兴市境内被前人定为中元古界双桥山群与下震旦统志棠组中发现了一批晚古生代放射虫化石。对樟树墩和登山两处与含放射虫硅质岩共生的岩石组合、硅质岩的岩石学、地球化学及放射虫化石属种特点进行了综合分析 ,表明它们应是一套洋盆深水环境的沉积。这一新发现不仅为研究赣东北蛇绿混杂岩带的性质、形成时代 ,也为重新厘定“板溪群”的构造属性及华南大地构造格局问题提供了科学依据     

Polyphase accretionary tectonics in the Jurassic to Cretaceous accretionary belts of central Japan

Abstract Mesozoic accretionary complexes of the southern Chichibu and the northern Shimanto Belts, widely exposed in the Kanto Mountains, consist of 15 tectonostratigraphic units according to radiolarian biochronologic data. The units show a zonal arrangement of imbricate structure and the age of the terrigenous clastics of each unit indicates successive and systematic southwestward younging. Although rocks in these complexes range in age from Carboniferous to Cretaceous, the trench-fill deposits corresponding to the Hauterivian, the Aptian to Middle Albian and the Turonian are missing. A close relationship between the missing accretionary complexes and the development of strike-slip basins is recognizable. The tectonic nature of the continental margin might have resulted from a change from a convergent into a transform or oblique-slip condition, so that strike-slip basins were formed along the mobile zones on the ancient accretionary complexes. Most terrigenous materials were probably trapped by the strike-slip basins. Then, the accretion of the clastic rock sequence occurred, probably as a result of the small supply of terrigenous materials in the trench. However, in the case of right-angle subduction, terrigenous materials might have been transported to the trench through submarine canyons and deposited there. Thus, the accretionary complexes grew rapidly and thickened. Changes both in oceanic plate motion and in the fluctuation of terrigenous supply due to the sedimentary trap caused pulses of accretionary complex growth during Jurassic and Cretaceous times. In the Kanto Mountains, three tectonic phases are recognized, reflecting the changes of the consuming direction of the oceanic plates along the eastern margin of the Asian continent. These are the Early Jurassic to early Early Cretaceous right-angle subduction of the Izanagi Plate, the Early to early Late Cretaceous strike-slip movement of the Izanagi and Kula Plates, and the late Late Cretaceous right-angle subduction of the Kula Plate.     

菲律宾海及其邻近海域表层沉积物中放射虫的分布对不同区域环境的响应

为了解菲律宾海放射虫的区域分布特色,利用同样的样品处理方法,对菲律宾海及其邻近海域的44个表层沉积样中的放射虫进行对比分析,鉴定统计了500个属种,物种多样性较高.菲律宾海表层沉积物中放射虫的群落结构和丰度变化幅度较大,反映了菲律宾海更为复杂的区域生态环境或沉积环境;南海北部放射虫丰度非常高且罩笼虫目占据较大优势,表明...     

Geochronology and Geochemistry of the Radiolarian Cherts of the Mada’er Area, Southwestern Tianshan: Implications for Depositional Environment

In the southwestern Tianshan,the geologic ages of many strata and their depositional environments are still poorly constrained because of complex structures.The Mada'er area is located in the Kuokesaleling belt,Southwestern Tianshan.The cherts from the former Wupata'erkan Group contain abundant radiolarian fossils,including 10 species which are identified as late Devonian to early Carboniferous in age.Eleven chert samples have SiO_2 contents ranging from 88.80 wt%to 93.28 wt%, and 2.02 wt%to 3.72 wt%for ...     

下扬子区早三叠世孤峰组层状硅质岩成因

下扬子区早三叠世孤峰组中产出的层状硅质岩厚度为近20—80m,含有数量不等的放射虫和海绵骨针。孤峰组剖面有硅质岩型、碳酸盐岩型和硅质岩-碳酸盐岩混合型3种类型。层状硅质岩主要产在第一、第三2种类型剖面中。硅质沉积中心是一些菱形或椭圆形小型(面积在数千平方公里以内)凹地,其形成受断裂控制。硅质岩中产出泥化火山岩,层薄,且层数很多。硅质岩富Fe贫Al,富集As,Sb,Bi,Ga,Au,Ag,Cr等微量元素。稀土元素总量低,Ce负异常,重稀土有不同程度的富集,具有热水硅质岩的特征。硅质岩中混有少量非热水成因的物质。硅质沉积时海水温度约为几十度至160余度。     

Preliminary report on Lower Jurassic radiolaria of Gondwana origin from the Kawhia coast, New Zealand

Abstract Well-preserved radiolarians from the Newcastle Group in southwest Kawhia, New Zealand, constitute the first record of Lower Jurassic radiolarians from in situ deposits in high latitudes of the Southern Hemisphere on the margin of Gondwana. The radiolarians were extracted from carbonate nodules from five horizons in the Rewarewa Formation and the lower part of the Arawhero Formation, in the Murihiku Terrane. The radiolarian-bearing sequence, which lies within the upper part of the type section of the local Aratauran Stage, is roughly datable as Hettangian-Sinemurian from rare ammonite occurrences. The radiolarian assemblages consist, on average, of 80–90% spumellarians and 10–20% nassellarians. Spumellarians include species of the following genera: Archaeotriastrum, Crucella, Emiluvia (?) Homeoparo-riaella, Orbictilifomaa, Pantanellium, Paronaella (?), Pseudocrucella, PseIIdoheliodiscus, Spon-gostaurus and Spongotrochus. Nassellarians are composed of species of Ragotum, Bipedis, Droltus, Jams (?) Perispyridium (?) Raoultius, Riedelius, Saitoum and Thetis. From data of Lower Jurassic radiolarian faunas of Europe, North America and Japan, the New Zealand fauna shows stronger affinity with those of the European Tethys such as Turkey (e.g. De Wever 1982) and the Northern Alps (Kozur & Mostler 1990) than with faunas from other areas of the circum-Pacific. This connection between the European Tethyan and New Zealand faunas is not well explained by presently accepted continental reconstructions (Smith et al. 1994) for the Early Jurassic.     

南海南部1百万年以来的放射虫动物群特征

定量分析了ODP184航次1143站岩心中的117个放射虫样品，得出南海南部1MaB．P．以来地层中的放射虫动物群特征如下：自1MaB．P．以来放射虫的数量分布丰度值呈现由低到高的变化趋势，并具有规律性的旋回特征，体现了1MaB．P．以来该海区放射虫动物群由贫乏向繁盛的逐步演化过程，以及海洋环境的几个阶段性变化。放射虫属种组成呈现明显的热带暖水动物群的组合面貌，优势种的组成与南海中北部海区及其它低纬度大洋相比存在一定差异，体现了该海区独特的放射虫组成特征。此外，指示温度的特征种在1MaB．P．来地层中的分布也显示了热带、亚热带海区放射虫的特殊指示作用。     

Characteristics of Albaillella （ Albaillellarian ,Radiolarian） Fauna from Guadalupian to Lopingian Series in Permian, South China

On the basis of establishment of radiolarian biostratigraphy and conodont biostratigraphy,a radiolarian Albaillella fauna in the transitional environment from Guadalupian to Lopingian Series in Permian was found at a pelagic chert section in southeast Guangxi, South China. Radiolarian Albaillella is one of the most sensitive biology to the transitional environment. The Albaillella fauna shows an ecological evolutionary process from Guadalupian to Lopingian: declined stage-recovery stage-flourishing stage. The study of characteristics of the Albaillella fauna in the transitional environment may provide more information, not only for the subdivision and correlation of a high-resolution biostratigraphy, but also for influence of radiolarian Albaillella fauna on the pre-Lopingian mass extinction.