Aragonite in Permian Reefs
Aragonite in Permian Reefs
复制标题
二叠纪珊瑚礁中的文石
作者：
J. Wendt
我们越深入地追溯地质记录，化石中文石的保存就越少。在古生代，文石遗迹仅在粘土质沥青矿床中发现，这些矿床在很大程度上抑制了不稳定矿物相的成岩蚀变（例如，美国上石炭统的巴克角沥青和肯德里克页岩1，以及苏格兰下石炭统的上油页岩群2）。然而，这些相中保存的古生物学信息仅限于那些生活在同等盆地环境中或被迁移到其中的有机遗骸。古代珊瑚礁生物的原始矿物学和微观结构通常只能通过与相当年轻的对应物的类比来推断。已知最古老的、保存有文石微结构的珊瑚礁生物是在南阿尔卑斯山的上三叠统（下卡尼期）发现的。在此，粘土-凝灰质沉积物早期埋藏了斑块礁石和礁石碎屑，以及随后的胶结作用导致碳酸盐孔隙度减小，可能抑制了文石微观结构向方解石的转变。在本报告中，我们描述了对钙质海绵的微观结构和成岩作用的研究，这些研究揭示了突尼斯南部杰贝勒特巴加上二叠统材料中的文石原生结构和胶结物。
PRESERVATION of aragonite in fossils decreases the further we go back in the geologic record. In the Palaeozoic, aragonitic remains have been found only in clayey–bituminous deposits which have largely inhibited diagenetic alterations of unstable mineral phases (for example, Buck-horn Asphalt and Kendrick Shale, both Upper Carboniferous of USA1, and Upper Oil Shale Group, Lower Carboniferous of Scotland2). Palaeontological information preserved in these facies is restricted, however, to those organic remains which lived in the equivalent basinal environments or were transported into them. Original mineralogy and microstructures of ancient reef organisms can generally only be deduced from analogies with equivalent younger counterparts. The oldest known reef-organisms with preserved aragonitic microstructures were found in the Upper Triassic (Lower Carnian) of the Southern Alps3. Here, early burial of patch reefs and reef detritus by clayey–tuffaceous sediments and porosity diminution of the carbonates by subsequent cementation have probably inhibited transformation of aragonitic micro-structures into calcite4. In this report we describe investigations of micrestructures and diagenesis in calcareous sponges which have revealed aragonitic primary structures and cements in material from the Upper Permian of Djebel Tebaga, Southern Tunisia.