Stratigraphy, Sedimentary Structures, and Textures of the Late Neoproterozoic Doushantuo Cap Carbonate in South China
Stratigraphy, Sedimentary Structures, and Textures of the Late Neoproterozoic Doushantuo Cap Carbonate in South China
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DOI：
10.2110/jsr.2006.086
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发表时间：
2006-07
影响因子：
2
通讯作者：
G. Jiang;M. Kennedy;N. Christie‐Blick;Huaichun Wu;Shihong Zhang
中科院分区：
文献类型：
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作者：
G. Jiang;M. Kennedy;N. Christie‐Blick;Huaichun Wu;Shihong Zhang
摘要中国南方陡山沱碳酸盐岩盖层覆盖于冰川作用下的南沱组之上。635 Ma），由横向持久、薄层状和正常级配的白云岩和石灰岩组成，表明相对深水沉积，最有可能在风暴浪基面以下。该碳酸盐岩的基部包含一套独特的密切相关的圆锥形结构，叠层石状洞穴，层平行片裂缝和胶结角砾岩。圆锥形火山岩的核是由堆积的洞穴组成的，洞穴由胶结物和角砾状的寄主微晶灰岩内衬。层状沉积物的上覆表明，层理的同沉积破坏导致了积极的海底表达。洞穴和席状裂缝包含内部沉积物，它们由原来文石等厚的葡萄状胶结物与针状辐射针内衬，现在被白云石和二氧化硅取代。黄铁矿和重晶石是常见的，方解石作为主要矿物被局部保留。这些特征与现代和古代甲烷渗漏具有相同的形态学和岩相学属性，其中甲烷气体和流体提供浮力的物理破坏力和显着胶结作用的碱性来源。在保存完好的石灰岩结壳和胶结物中，火山锥状结构内部和正上方的δ 13 C值低至−41‰，这为甲烷的影响提供了明确的证据，在整个盆地中，在同一基底帽碳酸盐岩水平上，广泛分布着相同的沉积构造和共生胶结物序列，这与天然气水合物失稳和甲烷渗漏的发展是一致的，冰后期海洋变暖考虑到广泛分布的类似功能，在同一地层水平在全球其他帽碳酸盐岩，我们认为，晚新元古代冰后期甲烷释放可能影响了海洋的氧气水平，以及有助于冰后期变暖通过温室效应的甲烷。
Abstract The 3- to 5-m-thick Doushantuo cap carbonate in south China overlies the glaciogenic Nantuo Formation (ca. 635 Ma) and consists of laterally persistent, thinly laminated and normally graded dolomite and limestone indicative of relatively deep-water deposition, most likely below storm wave base. The basal portion of this carbonate contains a distinctive suite of closely associated tepee-like structures, stromatactis-like cavities, layer-parallel sheet cracks, and cemented breccias. The cores of tepees are composed of stacked cavities lined by cements and brecciated host dolomicrite. Onlap by laminated sediment indicates synsedimentary disruption of bedding that resulted in a positive seafloor expression. Cavities and sheet cracks contain internal sediments, and they are lined by originally aragonitic isopachous botryoidal cements with acicular radiating needles, now replaced by dolomite and silica. Pyrite and barite are common, and calcite is locally retained as a primary mineral. These features share morphological and petrographic attributes with modern and ancient methane seeps in which methane gas and fluids provide both a force for physical disruption from buoyancy and a source of alkalinity for significant cementation. The presence of δ13C values as low as −41‰ in well preserved limestone crusts and cements within and immediately above the tepee-like structures provides unequivocal evidence for methane influence, and the widespread distribution of identical sedimentary structures and paragenetic cement sequences across the entire basin at the same basal cap carbonate level is consistent with gas hydrate destabilization and the development of methane seeps as a result of postglacial warming of the ocean. Considering the broad distribution of similar features at the same stratigraphic level in other cap carbonates globally, we suggest that the late Neoproterozoic postglacial methane release may have influenced the oceanic oxygen level as well as contributed to postglacial warming via the greenhouse effects of methane.