广西田林县下垌村石炭纪早期珊瑚礁基本特征及其沉积环境分析

The Basic Characteristics of Early Carboniferous Coral Reef at Xiadong Village in Tianlin, Guangxi, and Its Sedimentary Environment

广西田林地区发育有早石炭世全球罕见的后生动物格架礁,该珊瑚礁的主体部分由双型珊瑚(Diphyphyllum)、丛管珊瑚(Siphonodendron)、石柱珊瑚(Lithostrotion)以及笛管珊瑚(Syringopora)等建造而成。这些造架珊瑚个体一般以无性出芽繁殖生长和躲避障碍物弯曲生长的竞争模式占据更多的空间生态位,而群体之间则以越顶生长的策略取得竞争的胜利。礁体连续发育,无明显的间断性和旋回性,但造礁生物的组合在礁体纵向分布上存在一定差异,可识别出3个连续的生物组合单元,即Diphyphyllum组合、Siphonodendron-Lithostrotion组合和Diphyphyllum-Syringopora组合。对礁剖面岩石的微相特征研究,划分出了含少量生物碎屑粒泥灰岩、钙藻粒泥灰岩、生物碎屑泥粒灰岩、灰泥质颗粒灰岩、珊瑚格架岩和有孔虫生物碎屑亮晶颗粒岩等6种微相类型。礁体在发育过程中经历了由深水开阔台地相到浅水开阔台地相,珊瑚礁相到台内斜坡相再到台内浅滩相,标志着海平面逐渐下降、水体变浅、水动能不断增强的沉积环境。

A rarely Early Carboniferous metazon framework is unusually well developed in the Tianlin Region, Guangxi Province, China. This reef is constructed mostly by Diphyphyllurn, Siphonodendron, Lithostrotion and Syringopora. The individual of flame-building corals occupied more space niche by clonal budding and bending growth avoiding obstacle. Whereas, in order to achieving successful competition,the growth strategy of groups was overtopping. The reef grew continuously, and there was no significant discontinuity and cyclicity. But there are some differences among the biological assemblages of reef building coral during the frame-building period. Three continuous biological assemblage units can be identified, that are Diphyphyllurn assemblage units, Siphonodendron-Lithostrotio assemblage units and Diphyphyllum-Syringopora assemblage units. Six microfacies types may be distinguished, including a little-bioclast wackstone, calcareous algae wackstone, bioclast packstone, muddy grainstone, coral framestone and foraminifera bioelast grainstone. The sedimentary facies model transfers from the depth open carbonate platform facies, shallow open carbonate platform facies, coral reef facies, inner carbonate platform slope facies, to intraplatform shoal facies. The facies indicate that the sedimentary environment was a lowering of relative sea level, water-shallowing and enhanced hydrodynamic force.