Microfabric Properties of Lacustrine Sediments and Sedimentary Environments

Mircrofabrics from core sediments were observed in polarizing microscope and scanning microscope and used in interpreting the origin of three types of clay as well as the changes of sedimentary environments in four sites-the Honghu Lake,the Donghu Lake,the Wanghu Lake and the Dajiuhu Lake along the middle reach of the Yangtze River.Micro-beddings,orientation of particles and large sizes of micro-voids and particles with contacting patterns of face to face or edge to edge occurred in the light-colored clay formed since 0.4～0.5ka BP in the lakes.Their origin was ascirbed to clastic sediments from rivers surrounding the lakes.It was suggested that the lakes were connected with rivers and large amount of sedimens came from rivers into the lakes during that period.The typical microfabrics in the blue clay formed in 1～2.5ka BP were gelatinous texture,agglutinational texture,small sizes of particles and voids,low sphericity and diatoms of freshwater Class Centrium .Their origin was ascribed to the result of the interaction between organic colloids and clay colloids.It was suggested that the lakes in the Jianghan plain were of open and stable fresh water lake environments rich in planktons and with small amount of clay carried by rivers during this period.The main microfabrics in the black clay formed during 0.4～1ka Bp were gelatinous texture,large circular voids and plant fiber.The voids had a bimodal distribution.The sediment was formed mainly due to the accumulation of large amount of aquatic vascular bundle plants.It was suggested that the lakes were shallow and rich in aquatic vascular bundle plants,and some of them were swamping.The framework of fish bone in the sediments indicated that there was large number of fish in the Dajiuhu area during the Holocene warm period of 2.9～4ka BP,but the thermophilous fish did not migrate upwards apparently.