湖南邵东马鞍山中泥盆统棋梓桥组沉积相

Scdimentary Facics in the Middle Devonian Qiziqiao Formation in Ma'anshan,Shaodong, Hunan Province

摘要:

湖南中部邵东县马鞍山中泥盆统棋梓桥组剖面,位于邵东廉桥北约8km的光坡乡,地质上处于猪婆大山背斜北西翼近转折端处(图1)。

Abstract:

The Middle Devonian Qiziqiao Formation section in Ma'anshan, Shaodong, central Hunan Province, an idealized section for studying sedimentary facies of ancient carbonates, is characterized by well-defined outcrops, complete sequences, simple structures and abundant typical indicators for sedimentary facies. The strata in the section may be divided into four parts: limestones in the Upper Devonian Shetianqiao Formation (D_3s); limestones and dolostones in the upper member of the Middle Devonian Qiziqiao Formation （D_2q²）; limestones, marls and shales in the lower member of the Middle Devon/an Qiziqiao Formation （D_2q¹）and quartzose sandstones in the Middle Devonian Tiaomajian Formarion (D_2t). In the Qiziqiao Formation with a total thickness of about 380 m, the following sedimentary focies types may be distinguished. the lower member of the Qiziqiao Formation is ascribed to lower-energy normal shelf environments where sea water became shallower and shallower upwards. The lower part of it is composed of dark grey green silty shales intercalated with calcareous nodules, marl lenticles, horizontal.be.dding and benthonic bioclasts the middle part is made up of the intercalations of biomicrite, argillaceous micrite and marl. mnterbedded with 2 to 3 layers of tufted coral beds with abundant well-preserved benthonic organisms and well-developed bioturbation structures, and the upper part of it is the rhythmic succession consisting of argillaceous banded micrite, biomicrite and nodular limestone with abundant benthomic organisms. The lower part of the upper member of the Qiziqiao Formation comprises organic reef facies. The reef body. a convex-upwards lenticle with a thickness of 24 m, consists of light grey massive reef limestone and dolostone. The reef-building organisms include massive, spherical and columnar Stromatopora and tabulates （Aheolues sp., Temnpor.a sp. ） etc. The organisms With the total content of 40 to 50％ are mostly autochthonous. The fillings contain bioclasts and lime mud. No beddings are present in the reef body.But there are two rhythmic successions caused by the variations in species and struc tures.The lower parts of them consist of Temnopora beds,with a thickness of 0.5 to 1.1 m, whereas the upper parts of them display the framework composed of massive Stromatoport and hermatypic corals. Binding and repeated wrapping between Stromnatopora and other organisms are common. The fillings in the upper part of the reef body are generally subjected to selective dolomitization. Temnopora tend to increase in quantity upwards and Strigocephalus beds and a great deal of reef gravels have also been observed. It follows that the evolution of the organic reefs was almost nearer to the late stage and that the reefs died finally because of the changes of the environments. The upur part of the uppc member of the Qiziqiao Formation belong to restricted tidal flat-lagoon environments in which rhythmic units consist of pelletal limestone, calcarenite, algal laminated lime stone and dolostone. A single rhythmic bed is 1 to 2 m in thickness; the lower part of it is composed of calcarenite or pelletal !imcstonc in which the small smale cross-beddings are observable,and the mi ddle part of it consists of algal laminated limestone in which fenestral structures are developed. Micritic limestone or dolostone occupy the top beds of it in which mud cracks and algal laminae are common and break plane is rare. The characteristic organisms in the study area include a great deal of Amphipora and blue green algae with small amounts of sponge spicules and benthomic organisms, indicating a shallowing-upwards tidal flat sequence. It can be seen from the above-mentioned features of the whole Ma'anshan section that the evolu tion of sedimentary facies underwent the open to restricted, i.e.shelf to tidal flat environments in the Qiziqiao stage.It is well-known that the features were attributed to a widespread regression during the late Middle Devonian.