CHARACTERISTICS OF LACUSTRINE TURBIDITES AND THEIR TECTONIC SIGNIFICANCE

Turbidites are common in tectnically active lake basins. There are two kinds of turbidites, i. e. episodical and steady turbidites. Several examples are discussed in this paper based on the data from western Sichuan, China and Switzerland. Facts demonstrate that the recognition of the two is possible according to their sedimentary sequence, grain size distribution and their relations with the surrounding facies. The episodical turbidite might be triggered by catastrophic events, e. g. earthquake, volcanism and gravitational slumping. Owing to the small size of the basin in mountainous region,argillaceous breccia or brecciated mudstone occurs at the bottom of the sequence, indicating a slumping on great scale. The thickness of the breccia is rather variable. The content of the breccia is obviously related to the source rock nearby. The occurrence of the breccia is, therefore, a significant feature of the episodical turbidite of a lake basin. The breccia is found in the late Triassic Jiangzhou basin and in the lake Zurich where a catastrophic slumping took place 100 years ago and resulted in a turbidite sequence with argillaceous breccia at the bottom. The episodical lacustrine turbidites occur within the deep-water lacustrine muddy deposits as a result of catastrophic slumping and interrupt the normal facies sequence. The grain size distribution shows very typical characteristics on the log-probability plot coincident with the conclusion reached by Walker ( 1979 ) from the hydrodynamic analysis of the Bouma sequence. All the samples from the beds A and B of the Bouma sequence own a grain size distribution of suspended load, but the bed C is distinct with a traction component which is of log-normal distribution and appears as an independent population. The steady turbidites owe their origin to flood river. Although they have simi- lar profile structure except the lack of slumping-induced breccia, the position of the steady turbidite in the facies sequence is definite, usually they occur at the top part of a regradational sequence, indicating a largest flood stage, so that the existence of steady turbidite might indicate a climatic event prevailing in a wide area. The pattern of grain size distribution of the steady turbidite is basically the same as that of the episodical turbidite. But there are some deposited components in the bed A, which constitute an independent population with better sorting and reversed graded sequence. Lacustrine turbidites are typical members of the sequence of the incipient rift, and are of importance to understanding ancient rifts.