SEDIMENTARY CHARACTERISTICS AND ENVIRONMENTAL SIGNIFICANCE OF THE MUDDY TURBIDITES IN THE INDOSINIAN NANPANJIANG FORELAND BASIN

The Middle and Late Triassic terrigenous turbidite fan as the stereo deposits in the Indosinian Nanpanjiang foreland basin consists of the Banna （T₂¹） and Lanmu （T₂₂） turbidite fans. The muddy turbidites formed on the channel leveed banks, between the channels, on the lobate margins and the basinal plains comprise the following complete or incomplete Piper （1978）sequence associations: （1） E₁+E₂+E₃+（F）; （2）E₁+E₂+（F）; （3）E₂+E₃+（F）; （4）E₂+（F）, and （5）E₃+（F）. The main mineral compositions include clay minerals and small amount of quartz, feldspar and calcite fragments. The clay minerals are composed of illite, chlorite and kaolinite; the latter is developed only in the Lanmuian muddy turbidites, suggesting that the Lanmu turbidite fan is closer to the littoral zone than the Banna turbidite fan. The F member of the Lanmuian muddy turbidites differs essentially from that of the Bannaian muddy turbidites. The former contain the horizontal laminae assembled by complete thin-crusted planktonic bivalves, whereas the latter either absent or poorly-developed are composed dominantly of argillaceous compositions. In addition, illite from the F member of the Lanmuian muddy turbidites displays a freely dispersion pattern and is non-turbidity in origin.
The study of calcium carbonate contents in the muddy turbidites shows that the Bannaian turbidite basin beneath the carbonate compensation depth （CCD） and the Lanmuian turbidite basin above the depth may be ascribed respectively to Types Ⅵ and Ⅱ by Hesse（1975）.
During the deposition of the low-density turbidity currents, the decrease in submarine shear stress caused by the progressive decrease in sandy sediment grains was responsible for the formation of the complete sequence of the muddy turbidites.