Abstract: The purpose of this paper is to present the results of my recent investigation of the Shetianqiaoian(Late Devonian)carbonate debris flow deposits in Hunan.In terms of rheology and grain-supported mechanism,the carbonate debris flows in the study area may be divided into cohesive and turbulent debris flows.The former,a kind of plastic and cohesive stratified flow,is characterized by high content of matrix and weak collision among grains,whereas the latter,a kind of cohesive-diluted turbulent flow by high content of grains and strong collision among grains,suggesting a complex grain-supported mechanism.The turbulent debris flows in this region are responsible for the formation of three types of calcirudites with:1)tabular structurc;2)graded structure,and 3) chaotic structure.
Four types of sequences have also been distinguished for the carbonate debris flow deposits in this region:1)cohesive debris flow deposits intercalated with thin-bedded micritic limestone;2)cohesive debris flow deposits graded into turbulent debris flow deposits;3)turbulent debris flow deposits passed into turbidity current deposits which is most common in the study area,and 4)debris flow deposits intercalated with siliceous rocks.
The development of the carbonate debris flows is closely related to the palaeogeography of the South Hunan“intraplatformal basin”,the eastern and western boundaries of which are controlled by the Shetianqiaoian synsedimentary faults.The steeper slopes of the basin facilitate the development of the carbonate gravity flow deposits.Generally speaking,the cohesive debris flow deposits are confined to the environments where the slopes are relatively smooth and gentle,and tectonic activity is relatively weak,whereas for the turbulent debris flow deposits,things are on the contrary. In the field,there is often a gradual change from the debris flow sequences to calcareous turbidite sequences.Due to greater depth and narrower width of the starved basin,the carbonate debris flows occurring on the steeper slopes tend to descend straight to the floor of the basin,resulting in the formation of the sequences of calcirudite intercalated with black thin-bedded siliceous rocks.