13:45 〜 15:15
[BCG05-P01] Late Early Cretaceous radiolarian assemblages from limestone gravels washed out from the basal conglomerate bed of the Pliocene Naarai Formation in Choshi area of central Japan
キーワード：放散虫、名洗層、銚子層群、白亜紀、Aptian、Albian
The Lower Cretaceous Choshi Group (Barremian–Lower Albian) of shallow marine strata is well exposed along a sea cliff at the easternmost point of the Choshi Peninsula in Chiba Prefecture, Central Japan. The Pliocene Naarai Formation unconformably overlies the Choshi Group with a basal conglomerate at Nagasaki-hana Cape, which lies at the southern end of the sea cliff trending north-south. Various types of beach gravel present at the Nagasaki-hana Cape include those washed out from the basal conglomerate of the Naarai Formation. Kashiwagi et al. (2013) reported Late Tithonian (late Late Jurassic) to Cenomanian (early Late Cretaceous) radiolarians from the cobble of vertebrate bone-bearing calcareous rocks from Nagasaki-hana Cape. However, wide-ranging radiolarian age intervals have prevented accurate provenance analyses which represent denudation-redeposition sedimentation after the Late Cretaceous in the Choshi area.
In this study, radiolarian assemblages were extracted from four grey, white-coloured limestone gravels collected from Nagasaki-hana Cape. In thin sections, abundant radiolarian tests and minor amounts of foraminiferal tests were scattered within the micritic limestone. Majority of the radiolarian tests were pyritised internal moulds with smooth surfaces and/or euhedral crystal planes, or composed of framboidal aggregates. Radiolarians remaining in the biogenic silica test could be identified at a specific level and used for age determination. The radiolarians extracted in this study are Archaeodictyomitra vulgaris Pessagno, Cryptamphorella spp., Diacanthocapsa betica ODogherty, Diacanthocapsa ovoidea Dumitrica, Diacanthocapsa spp., Holocryptocanium barbui Dumitrica, Dictyomitra montisserei (Squinabol), Pseudodictyomitra lodogaensis Pessagno, Rhopalosyringium spp., Siphocampe sp., Stichomitra communis Squinabol, Stichomitra japonica (Nakaseko and Nishimura), Stichomitra mediocris (Tan), Thanarla brouweri (Tan), Thanarla conica (Squinabol), Turbocapsula spp. and Xitus spicularius (Aliev) in nassellarians, and Acaeniotyle spp., cortical shells with two polar spines, forms with three rays radially, and circular disc-shaped forms with peripheral rod-like spines in spumellarians. These radiolarians co-occur within the UAZone 9-10 defined by ODogherty (1994), indicating a late Aptian–early Albian age (ODogherty, 1994; Danelian et al., 2007; Danelian, 2008).
The late Aptian radiolarian age for the limestone gravel is concordant with the ammonoid age of the Toriakeura Formation of the Choshi Group, indicating a late Aptian deposition (Obata and Matsukawa, 2009). The Toriakeura Formation is primarily composed of alternating sandstone and mudstone beds and thick-bedded massive mudstone with abundant calcareous nodules. Similarities in both lithofacies and ages suggest that limestone gravel in the basal conglomerate of the Naarai Formation may have been derived from calcareous nodules of the Toriakeura Formation. The early Albian radiolarian age is consistent with those of the Nagasakihana Formation, which consist primarily of thick-bedded coarse-grained sandstones with mudstone intercalations. Regarding the possible origin of limestone gravels in the Naarai Formation, they also could have originated from an eroded limestone facies that was contemporaneously heterogeneous with the currently exposed Choshi Group. Some ammonoid fossils comprising Tithonian (Late Jurassic) and late Albian (Early Cretaceous) were reported as solid and/or worn specimens from the basal conglomerate of the Naarai Formation, and they could have been derived fossils from unexposed strata in this area (Obata et al., 1975; Obata and Matsukawa, 2005, 2009). As revealed by the ammonite fossils, radiolarian fossils also suggest that the basal conglomerate of the Naarai Formation contains gravel derived from the shallow marine strata of late Early Cretaceous age.
In this study, radiolarian assemblages were extracted from four grey, white-coloured limestone gravels collected from Nagasaki-hana Cape. In thin sections, abundant radiolarian tests and minor amounts of foraminiferal tests were scattered within the micritic limestone. Majority of the radiolarian tests were pyritised internal moulds with smooth surfaces and/or euhedral crystal planes, or composed of framboidal aggregates. Radiolarians remaining in the biogenic silica test could be identified at a specific level and used for age determination. The radiolarians extracted in this study are Archaeodictyomitra vulgaris Pessagno, Cryptamphorella spp., Diacanthocapsa betica ODogherty, Diacanthocapsa ovoidea Dumitrica, Diacanthocapsa spp., Holocryptocanium barbui Dumitrica, Dictyomitra montisserei (Squinabol), Pseudodictyomitra lodogaensis Pessagno, Rhopalosyringium spp., Siphocampe sp., Stichomitra communis Squinabol, Stichomitra japonica (Nakaseko and Nishimura), Stichomitra mediocris (Tan), Thanarla brouweri (Tan), Thanarla conica (Squinabol), Turbocapsula spp. and Xitus spicularius (Aliev) in nassellarians, and Acaeniotyle spp., cortical shells with two polar spines, forms with three rays radially, and circular disc-shaped forms with peripheral rod-like spines in spumellarians. These radiolarians co-occur within the UAZone 9-10 defined by ODogherty (1994), indicating a late Aptian–early Albian age (ODogherty, 1994; Danelian et al., 2007; Danelian, 2008).
The late Aptian radiolarian age for the limestone gravel is concordant with the ammonoid age of the Toriakeura Formation of the Choshi Group, indicating a late Aptian deposition (Obata and Matsukawa, 2009). The Toriakeura Formation is primarily composed of alternating sandstone and mudstone beds and thick-bedded massive mudstone with abundant calcareous nodules. Similarities in both lithofacies and ages suggest that limestone gravel in the basal conglomerate of the Naarai Formation may have been derived from calcareous nodules of the Toriakeura Formation. The early Albian radiolarian age is consistent with those of the Nagasakihana Formation, which consist primarily of thick-bedded coarse-grained sandstones with mudstone intercalations. Regarding the possible origin of limestone gravels in the Naarai Formation, they also could have originated from an eroded limestone facies that was contemporaneously heterogeneous with the currently exposed Choshi Group. Some ammonoid fossils comprising Tithonian (Late Jurassic) and late Albian (Early Cretaceous) were reported as solid and/or worn specimens from the basal conglomerate of the Naarai Formation, and they could have been derived fossils from unexposed strata in this area (Obata et al., 1975; Obata and Matsukawa, 2005, 2009). As revealed by the ammonite fossils, radiolarian fossils also suggest that the basal conglomerate of the Naarai Formation contains gravel derived from the shallow marine strata of late Early Cretaceous age.