A slope to outer-shelf cold-seep assemblage in the Plio-Pleistocene Kazusa Group, Pacific side of central Japan
A slope to outer-shelf cold-seep assemblage in the Plio-Pleistocene Kazusa Group, Pacific side of central Japan
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日本中部太平洋一侧上里奥-更新世 Kazusa 群的外陆架冷泉组合斜坡
DOI：
10.2517/prpsj.7.279
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发表时间：
2003
影响因子：
2.8
通讯作者：
R. Majima
中科院分区：
文献类型：
--
作者：
T. Kitazaki;R. Majima
摘要日本中部太平洋一侧横滨市上上新统至下更新统Ofuna组和下更新统Kazusa群Koshiba组斜坡至外陆架地层中存在冷渗组合。为了了解组合的三维范围，取了四个岩心：一个岩心平行于层理，三个岩心垂直于层理。岩心和露头研究表明：(1)该组合的地层厚度至少为37m，横跨Koshiba组和下伏的Ofuna组之间的界线；(2)与层理平行，南北方向宽至少16m，东西方向长30m；(3)该组合主要由大型(最大直径可达10 cm)的Lucinoma属、Conchocele属和Acharax属的关节双壳类组成，呈集合体或零星贝壳；(4)包裹体的基质中的自生碳酸盐胶结物由密到稀，亏损13C(δ13C=−47.99‰~−55.06‰)，表明碳酸盐是甲烷微生物氧化作用的产物；(5)存在厚约10m的次表层，其特征是：(A)基质全部由纯白色碳酸盐组成的角砾岩层，(B)寄主沉积物的角砾状碎屑与极其丰富的双壳类碎屑一起出现，其中许多是非常小的，以及(C)岩性边界与研究区整体倾角极不一致。观察结果5a-c共同表明地下物质非常活跃的渗漏和/或爆炸性渗出，并在研究区域的地下存在“麻点”。有20个凝灰岩层可观察到，并可在露头之间准确对比，没有出现冷渗组合。然而，这些凝灰岩层不能与岩心中的凝灰岩床相关联。这可能是由于高度活跃的生物扰动和渗漏材料的严重渗漏和/或爆炸性渗漏。我们推测，这些活动和事件分散、干扰、角砾化和/或重新浓缩了原始沉积的凝灰岩床。
Abstract A cold-seep assemblage is present in slope to outer-shelf strata of the Upper Pliocene to Lower Pleistocene Ofuna Formation and the Lower Pleistocene Koshiba Formation of the Kazusa Group in Yokohama City, on the Pacific side of central Japan. Four cores were taken in order to learn the three-dimensional extent of the assemblage: one core was parallel to bedding and three were normal to bedding. Study of the cores and outcrops shows that: (1) the assemblage is at least 37 m thick stratigraphically and extends across the boundary between the Koshiba Formation and the underlying Ofuna Formation; (2) parallel to bedding, the assemblage is at least 16 m wide in a north-south direction and 30 m long in an east-west direction; (3) the assemblage consists mostly of large (up to 10 cm in maximum diameter), articulated bivalves of the genera Lucinoma, Conchocele, and Acharax, in aggregations or as sporadic shells; (4) authigenic carbonate cement in the matrix enclosing the assemblage ranges from dense to sparse and is greatly depleted in 13C (δ13C = −47.99‰ to −55.06‰), indicating that the carbonate resulted from the microbial oxidation of methane; (5) there is a subsurface horizon about 10 m thick that is characterized by: (a) scoria beds in which the matrix consists entirely of pure, white carbonate, (b) abundant brecciated clasts of the host sediment occurring along with extremely abundant bivalve fragments, many of which are very small, and (c) lithologic boundaries greatly discordant with the overall dip within the study area. Observations 5a–c together suggest very active seepage and/or an explosive effusion of subsurface material, and the presence of “pockmarks” in the subsurface of the study area. Twenty tuff beds are observable and accurately correlatable among outcrops where no cold-seep assemblage occurs. However, these tuff beds cannot be correlated with those in the cores. This is probably due to highly active bioturbation and by severe seepage and/or the explosive effusion of seep materials. We infer that these activities and events dispersed, disturbed, brecciated and/or reconcentrated the originally deposited tuff beds.