湖南泥盆纪棋子桥期层状礁的对比研究

COMPARATIVE STUDY ON THE MIDDLE DEVONIAN (QIZIQIAO STAGE) BIOSTROMES IN HUNAN

湖南泥盆纪棋子桥期的生物层状礁分布很广 ,类型也很复杂。根据它们不同的古生态特征 ,可分为 6种类型。不同类型分布在不同的古地理位置上 ,分别形成不同的沉积环境。某些类型的层状礁与其后形成的边缘礁关系密切 。

The late Middle Devonian(Qiziqiao stage) biostromes are well developed in Hunan. Six main types have been recognized.The biostromes of different types distributed respectively in different locations on the Devonian carbonate platforms or the upper parts of the platform foreslopes under different depositional environments. Biostromes of Type A are dominated by abundant irregular massive stromatoporoids as well as massive rugose corals forming organic skeletal framework.They are located at the narrow edges of carbonate platforms or the upper parts of platform foreslops and presumably developed under the clear turbulent shallow marine environments.In some places they formed parts of the flanks of marginal reefs. Biostromes of Type B are characterized by having nearly spherical massive corals and encrusting stromatoporoids.Their distribution and depositional environments are more or lass similar to that of Type A,except for the better endurance of terrigenous deposits.The biostromes of this type are found in many places below the biostromes of Type A,formed the bases of some marginal reefs. Biostromes of Type C are composed mainly of dendroid tabulate corals and encrusting stromatoporoids,and distributed in the wide area close to the margin of platforms.They might have been developed under weak hydrodynamic environments,occasionally suffered by storms.Somewhere,these biostromes can reach 20—30 m in thickness. Biostromes of Type D are dominated by tennis like stromatoporoids and dendroid tabulate corals.They might have been grown under restricted shallow marine environments and distributed in the areas distant from the margin of platforms. Biostromes of Type E are characterized by having thin lens like stromatoporoids.Their depositional environments and distribution are more or less similar to that of Type D.In some places these biostromes could be developing continuously and becoming small patch reefs. Biostromes of Type F are composed of blue green algae.They have been found in a narrow belt,close to the edge of Devonian carbonate platform.They might have been developed in the light penetrating zone,and growing upward close to the sea level . The biostromes of Type A and Type B are closely related to the marginal reefs,that they can be taken as the indicators of finding in situ marginal reefs.