An outer-shelf cold-seep assemblage in forearc basin fill, Pliocene Takanabe Formation, Kyushu Island, Japan
An outer-shelf cold-seep assemblage in forearc basin fill, Pliocene Takanabe Formation, Kyushu Island, Japan
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日本九州岛上新世高锅组弧前盆地充填物中的外陆架冷泉组合
DOI：
10.2517/prpsj.7.297
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发表时间：
2003
期刊：
影响因子：
1.6
通讯作者：
Kazuhiro Kato
中科院分区：
文献类型：
--
作者：
R. Majima;Kazuhiro Ikeda;H. Wada;Kazuhiro Kato
摘要在日本西南部九州岛宫崎县新富町的久贺神社及其周边地区，发现了一个以大型双壳类Lucinoma sp.为主的冷泉无脊椎动物组合。该组合出露于宫崎群上新世高边组上部。出露于新富町的高边组是一个由上变粗和上变细的沉积物旋回序列，反映了海平面的变化：根据软体动物推断的古水深表明，水深从160 m ~ 200 m向上变浅至50 m，随后又变深至160 m ~ 200 m。冷泉组合发生在coarteries（泥质砂岩）和水深最浅（50米的水深）的水平，外大陆架环境附近的平均风暴波基地，在这个变浅和加深的周期。库格神社周边冷泉组合零星分布，南北向约100 m，东西向约60 m，地层厚度约12 m。许多有关节的和无关节的双壳类的接合面与层理平行，但也有一些有关节的双壳类的接合面与层理垂直，这是光泽壳类特有的生活位置。含该组合的泥质砂岩被自生碳酸盐胶结，胶结强度由强到弱，13 C含量严重亏损（δ 13 C = −30.69‰至−53.21‰，相对于PDB），这表明甲烷渗漏的影响。保存下来的亮壳类双壳类的生活位置和伴生碳酸盐的碳同位素比值表明该组合是原地形成的。许多贝壳的层理平行方向被推断为是由于在这样一个浅的环境中普遍存在的风暴引起的波浪和电流的原位改造。
Abstract A cold-seep invertebrate assemblage consisting mainly of the large bivalve Lucinoma sp. is exposed at the Kuge Shrine and surrounding area in Shintomi Town, Koyu County, Miyazaki Prefecture, Kyushu Island, southwestern Japan. The outcrops with this assemblage are in the upper part of the Upper Pliocene Takanabe Formation of the Miyazaki Group. The Takanabe Formation exposed in Shintomi Town is a cyclical sequence of upward-coarsening and upward-fining sediments, which reflects sea-level change: the paleobathymetry inferred from molluscs implies a shallowing upward from 160 m–200 m to 50 m in water depth, followed by a deepening back to 160 m–200 m. The cold-seep assemblage occurs in the coarsest (muddy sandstone) and bathymetrically shallowest (50 m in water depth) horizon, an outer shelf environment near the average storm-wave base, in this shallowing and deepening cycle. The cold-seep assemblage around the Kuge Shrine occurs sporadically in area of about 100 m in a north-south direction and about 60 m in an east-west direction, and is about 12 m in stratigraphic thickness. Many articulated and disarticulated bivalves are preserved with their commissure planes parallel to bedding, but some articulated bivalves are oriented normal to it, in the characteristic life position of lucinids. The muddy sandstone containing this assemblage is cemented strongly to weakly by authigenic carbonate depleted greatly in 13C (δ13C = −30.69‰ to −53.21‰ vs. PDB), which suggests the influence of methane seepage. The preserved life positions of the lucinid bivalves and the carbon isotope ratios of the associated carbonates suggest that this assemblage is autochthonous. The bedding-parallel orientation of many shells is inferred to have resulted from in-situ reworking by storm-induced waves and currents that are prevalent in such a shallow environment.