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首页> 外文期刊>Facies >Sea-level changes as controlling factor of early diagenesis: the reefal limestones of Adnet (Late Triassic, Northern Calcareous Alps, Austria)
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Sea-level changes as controlling factor of early diagenesis: the reefal limestones of Adnet (Late Triassic, Northern Calcareous Alps, Austria)

机译：海平面变化是早期成岩作用的控制因素：阿德涅特的珊瑚礁灰岩（晚期三叠纪，北钙质阿尔卑斯山，奥地利）

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摘要

The uppermost Rhaetian Adnet reef is part of the Dachstein carbonate platform and is situated at the transition to the intrashelf Kössen Basin. Its diagenetic evolution is investigated focusing on dissolution cavities in the Tropfbruch quarry of Adnet (near Salzburg) stratigraphically situated immediately below the Triassic–Jurassic boundary. Sea-level changes due to global eustatic trends and regional tectonics are assumed to be the controlling factors in the development of a manifold diagenetic sequence characterized by phases of meteoric dissolution, marine and burial cementation, and internal sedimentation. Despite small-scale variations of the sequence, a superordinate pattern of diagenetic phases could be elaborated. Small-scale eustatic sea-level falls subordinate to a global regression trend caused subaerial exposures of the Adnet reef in the latest Rhaetian to earliest Hettangian. The result was karstification and meteoric dissolution of aragonitic coral skeletons (Retiophyllia) leading to the formation of biomoldic porosity. Coral septa which escaped dissolution were transformed into neomorphic calcite spar under meteoric–phreatic conditions. A first generation of dog-tooth cements precipitated sporadically on the altered coral skeletons. Eustatic sea-level rise in Early to Mid-Hettangian times caused a renewed flooding of the pore space of the Adnet reef by marine water and the influx of a first generation of internal sediments (IS I), derived from the karstified host rock of the Upper Rhaetian reef limestone. These internal sediments are overgrown by radiaxial-fibrous calcites (RFCs) whose oxygen-isotopic signature (δ18O = −1.3 (±0.7)‰) indicates precipitation in deeper (colder) water (18–21°C) due to a first phase of drowning. An intermediate phase of eustatic sea-level lowstand in the Late Hettangian is expressed by dissolution and corrosion of RFCs. Rapid drowning of the Dachstein carbonate platform due to eustatic sea-level rise and tectonic movements took place in the Early Sinemurian and a second generation of internal sediments (IS II) derived from the Lower Sinemurian Adnet Formation is washed into the dissolution cavities. Where IS II is absent, RFCs are overgrown by a second generation of dog-tooth cements with a bright-luminescent outer rim indicating the transition to negative redox conditions in the pore water during shallow burial. Burial diagenesis is represented by blocky calcite cements which occlude the remaining pore space. Depleted oxygen-isotope values and significant Fe contents indicate precipitation under reducing redox conditions and elevated temperatures of 30–50°C at burial depths of 420–870 m. Locally, replacive saddle dolomite is the latest diagenetic phase in the Adnet reef indicating crystallization under hydrothermal influences related to compressional subduction regimes of the Penninic Ocean.
机译：Rhaetian Adnet礁的最上层是Dachstein碳酸盐台地的一部分，位于向内陆架Kössen盆地的过渡处。研究了其成岩演化，重点是地层位于三叠纪-侏罗纪边界正下方的阿德特纳（萨尔茨堡附近）Tropfbruch采石场的溶蚀腔。假定由于全球成年趋势和区域构造而引起的海平面变化是形成多种成岩作用序列的控制因素，这些成岩作用序列以陨石溶解，海洋和埋葬胶结以及内部沉积阶段为特征。尽管该序列发生了小规模的变化，但仍可以详细说明成岩相的上级模式。小规模的向海平面下降是从属于全球回归趋势的，这导致了最近的Rhaetian到最早的Hettangian的Adnet礁的地下暴露。结果导致了古希腊珊瑚骨架（Retiophyllia）的岩溶化和陨石溶解，从而导致了生物分子孔隙的形成。逃逸溶解的珊瑚隔在流星-潜水条件下转变为新形方解石晶石。第一代狗牙水泥偶发地沉淀在改变后的珊瑚骨骼上。早至中唐奇时期的海平面上升引起海水对Adnet礁的孔隙空间的再次淹没，以及第一代内部沉积物（IS I）的涌入，这些沉积物是从喀斯特的岩溶化宿主岩中提取的。上拉第亚礁石灰石。这些内部沉积物被放射状纤维方解石（RFCs）长满，其氧同位素特征（δ 18 O = -1.3（±0.7）‰）表明在较深（较冷）的水中沉淀（18–21） °C）由于溺水的第一阶段。 Hettangian晚期的海平面低位活跃期的中间阶段通过RFC的溶解和腐蚀来表示。由于喜乐海平面上升和构造运动，Dachstein碳酸盐台地迅速淹没，并在Sinemurian早期发生，并将来自Sinemurian Adnet下部的第二代内部沉积物（IS II）冲入溶蚀腔。在没有IS II的情况下，RFC被第二代狗牙状胶合剂所覆盖，后者具有明亮的发光外缘，表明在浅埋时孔隙水向负氧化还原条件过渡。块状方解石胶结物掩埋了剩余的孔隙，代表了成岩成岩作用。氧同位素值的降低和大量铁的含量表明，在氧化还原条件降低和温度升高（30–50°C）下，埋深为420–870 m时会析出。在局部，替代性鞍状白云岩是阿德奈特礁最新的成岩相，表明在与Penninic海洋的压缩俯冲方式有关的热液作用下结晶。

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