GRAVITY-DISPLACED DEPOSITS OF CAMBRIAN DEEP-WATER CARBONATES IN WEST HUNAN AND EAST GUIZHOU

The region of west Hunan and east Guizhou is at the northwestern border of the Cambrian marginal sea in south China.In this region a deep-water carbonate slope with tendency southwestward had developed since Qingxudong period of Early Cambrian. A huge carbonate platform was present at the northwestern side of the slope and a deep-water basin lay at the southeastern lateral. Many types of carbonate rocks resedimented by gravity were formed at the deep-water area of the basin margin. Based on the textures and sedimentary structures, they were identified as: 1 ) isolated rock-block 2 )sliding and slumping deposits and 3 ) gravity flow sediments including debris flow, turbidity current and grain flow deposits. They commonly occur in the Middle and Upper Cambrian and their maximum cumulative-thickness is above 400m. A particular association of the slope deposits consists of gravity-displaced sediments interbedding autochthonous sediments, the majority of which is laminated carbonate mudstone. Of all gravity-displaced carbonate rocks, debris flow and turbidity current sediments are of most significance in both thickness and distribution. Debris flow sediments being full of calcirudites and boulders, mostly belong to submarine channel-filled deposits on the slope, they stretch southeastward in banded form, namely perpendicularly to the trend of the slope. The layer of calcirudite, the thickness of individual bed and grain size of sediments decrease apparently toward basin margin; but sebimentary bodies by debris flow extend more like sheet at the end of channels. There are two types of turbidites; 1 ) one formed by low-density suspension and 2 ) the other formed by high-density suspension. The former is relatively thin ( 10 cm) in the thickness of each layer, and fine (at most 3mm in grain size), characterized by regular grading) it occurs in the complete or incomplete Bouma sequences, and can be found in various sub-environments in the deep-water basin. The later is relatively thick ( 20 cm), and coarse (max. grain size up to 8 cm)) characterized by coarse-tail grading, paralle-bedding and intermidiate or large-scale cross-bedding intervals, which are frequently combined with each other to comsti-tute the equivalent of a-b-c incomplete Bouma sepuence. One grain flow sedimentary unit always consists of three intervals one after another, gradually from the lower to the upper: 1 ) reverse grading, 2 ) massive, in which the mean grain size of sediments is the largest, and 3 ) normal grading; it has another important feature that there is almost no matrix but sparry cement of generation in the voids between grains. Isolated rock-blocks, from shallow-water environments of carbonate platform margin, are huge rock-fragments of shelf edge reefs built by Cyanophyta ( Epiphy-ion, Girvanella, etc. ) j enclosed by dark-coloured, fine and thin-laminated carbonate mudstonej the volume of fragments is over ten thousand m they are mainly deposited in the middle of the slope. The sequence of sliding and slumping deposits, four endmembers of which are sediments with curved-lamination, sedimentary folds, faults sedimentary in origin, and autobreecia, is a reliable indicator of the palaeoslope. The cumulative-frequency curves ( in probability scale ) of three types of gravity flow sediments have their own characteristics respectively) and on the scatter plots of grain-size parameters it is easy to distinguish debris flow from two other types) but on the main-factor loading plot in correspondence analysis by computer, three types can be differentiated more easily from one another, despite of some overlap) and on the C-M plot not only the distribution of the three types can be shown in the band-shaped area parallel to c = m line, but also their sorting degrees by Im values. Bathymetric-palaeowater maps by the thickness-stratum method for calculation of water depth indicate that, theslope is 1°- 4° gradient, the depth 20-600 nij the basinal-margin rise 20'-40', 600-800m) the basinplain 8 '-20', 800m, Palaeocurrent direction w