SHELF-SLOPE SEDIMENTARY ENVIRONMENT OF MIDDLE TRIASSIC IN SOUTHWEST GUIZNOU

The sedimentary environment of Middle Triassic in Southwest and South Gui-zhou is controlled by the Kaiyuan-Pintang giant fault.The region on the west or northwest of this fault is a restricted carbonate platform or organic shoal facies on the margin of the platform, and the region on the east or southeast of the fault is a shelf-slope or trough basin facies, Banqi-Yata Region lies in the transitional zone between the platform area and trough basin area. In the section, the shelf facies (including the normal shelf deposit and tempestite ) , the slope facies ( including the normal carbonate slope deposit and contourite ) and the trough basin facies of the turbidite occur upwards correspondingly. The characteristics of the facies sequences indicate the transgressive sedimentary cycle of Middle Triassic. This paper deals with mainly the shelf-slope sedimentary characteristics of Middle Triassic in Banqi-Yata Region.The tempestite, contourite and turbidite are discussed in detail. There are three kinds of tempestite assemblages in the section, i.e. assmblage of shell-bed and calcisilitite, silty limestone, that of shell-bed and silty mudstone, and that of siltstone and mudstone. There exist typical structures, such as graded bedding, small hummocky crossbedding, wave-ripple cross-bedding, gutter cast.The characteristics of the tempestite, such as fine grain, thinner bed, very rare lag deposit and lack of amalgamtion indicate the tempestite belongs to the distal storm deposit on offshore shelf. The Banqi contourite is very different from the associated turbidite. Its beds are thin and grains well sorted but no graded bedding. Howere,the typical lenticular and wave-like cross-bedding develop. The turbidite in the study are mainly consists of the CDEF assemblage or DEF assemblage of the Bouma sequence, i.e.primarily fine grains. The channel-system deposit could not be found in the section and there is not a thinning or thickening upwards tendency either. The cycle curve in the corresponding diagram between bed numbers and bed thickness shows a sawtooth-like shape and the the characteristics of monotonous sequence. In the turbidite,"proxi-mal"and "distal" beds overlapped each other at random or thick-massive sandstones occur in "distal" beds.The above-mentioned characters indicate that the turbidite is not submarine-fan or typical basin-plain deposit but belongs to the trough longitudinal turbidite controlled by regional structures. Finally, the sedimentary pattern of Middle Triassic in the study region is presented. On the basis of the study on the Middle Triassic sedimentary facies, it is found that there's a close relationship between the sedimentary facies and the distribution of the middle-large scale Au deposits found out recently in the southwestern Guizhou, and that one kind of Au deposits lies in the shelf facies, another appears in the marginal subfacies of trough basin facies. In the light of the preliminary study, Au in the marginal subfacies mainly concentrates in the DE segments of Bouma sequence in the turbidite.