The problem on radiolarian shells reworking into the Lower Cretaceous molluscan facies in the Chichibu Terrane, eastern Shikoku
Ishida, Keisuke University of Tokushima Tokushima University Educator and Researcher Directory KAKEN Search Researchers
Hashimoto, Hisao Elementary School of Donari Town
Some Middle-Late Jurassic radiolarian shells were detected associated with Early Cretaceous autochthonous radiolarian and anmmonite assemblages from the Lower Cretaceous formations of molluscan facies in eastern Shikoku.
The modes of occurrence on these Middle-Late Jurassic radiolarian shells were summarized as follows: 1) Specific diversity of associate Middle-Late Jurassic radiolarians are restricted within several species of Tricolocapa and Stichocapsa genera. 2) Most of these Jurassic radiolarians are subspherical with closed distal end in shape. 3) Their sizes are limitted to 100-150μm in length and 80-100μm in diameter. 4) Lithologically, they are contained in laminated sandy mudstones and sandy siltstones. 5) Among these older radiolarians, Tricolocapsa plicarum, T conexa, T. fusiformis?, Stichocapsa convexa and S. naradaniensis are the index species of Middle to early Late Jurassic age. But the other species whose final appearances are known within Earliest Cretaceous such as Cinguloturris carpatica, Pseudodictyomitra primitiva and Eucyrtidiellum pyramis have possibility that their ranges reach into Barremian age. 6) All these Jurassic elements are yielded from the first transgressive sediments successively just above the Lower Cretaceous nonmarine formations in the Northern and the Middle Chichibu Terranes. 7) These ammonites and radiolarians bearing Lower Cretaceous formations are the continental shelf or upper submarine terrace sediments, because they construct cyclothem together with the coal-bearing and blackish sediments which unconformably overlie both the melangé type Jurassic formations in the Northern Chichibu Terrane and the molluscan facies Middle-Late Jurassic formations in the Middle Chichibu Terrane.
The above-mentioned evidences showed that these Jurassic radiolarian shells are the reworked fossils in the same manner as other detrital clastics in the Cretaceous sediments, probably derived from the Pre-Cretaceous basement similar to the Northern and the Middle Chichibu Terranes. Therefore, it is necessary to consider the problem of reworking and mixing by older materials when we deal with the microfossil biostratigraphy at nearshore sediments on such continental shelf and/or upper submarine terrace.
Journal of the Sedimentological Society of Japan
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