Characteristics And Controlling Factors Of The Thrombolites From Cambrian Series 3, Western Henan

Abundant thrombolites were well exposed in the Third Member of Mantou Formation and Zhangxia Formation(Cambrian Series 3) of Dengfeng and Mianchi Areas of Western Henan. They can be recognizable easily in the wild because of its obvious macroscopic character. The analysis of the macro-morphology, micro-characteristics, growth patterns, sedimentary environment and control factors of the different types of thrombolites can provide evidences for the study on the spatial distribution and evolution of Cambrian microbial carbonate.7 beds of Cambrian thrombolites from Dengfeng Area are mainly formed by microbial absorbing, capturing and bonding miccrite and bioclastic. They are divided into three typies including small-columnar, massive and dendritic thrombolites based on the sedimentary environment and growth patterns, The evolution from dendritic via massive to small columar-thrombolites reflects environmental changes from the subtidal platform zone with low energy and deep water to the intertidal oolitic shoal zone with high energy and shallow water.5 beds of Cambrian thrombolites from Mianchi Area can be divided into two typies including small-columnar and patch, according to the macroscopic growth morphology and the different characteristics between clots and fillings. Abundant different types of cyanobacteria fossils are found in the thrombolites in Mianchi Area. The Girvanella fossils are filamentous, with well-developed bent pipe structure, which are divided into around the debris, banded, and cloddy distribution of three types; The Renalcis fossils are spherical, which are formed by color black lime mud or micritic calcite under poor light, appered as a dense clusters or bead-like pattern distribution; The Epiphyton fossils take the form of forked branches while cross section is circular. The morphology of bacterial communities can be divided into two categories including dendritic and clostered.The development of thrombolites begin with the storm-induced flat-pebble limestones and end with the occurrence of thickbedded oolitic limestones in sustained high-energy environment. The internal structures and macro morphology of thrombolites and their origin and evolution are controled by water depth and hydrodynamic conditions, attached substrates, influx of terrigenous clays and barren of metazoan.