Abstract:

The lower Trassic developed on the north-west slope of Shiwandashan basin in Guangxi is represented predominently by marine carbonate sediments,which can be subdivided into two opposite lithofacies zones according to their sedimentaty attributes. The north facies zone,occurred at carbonate platform,is featured by oolitic limestone,894-970m thick;while the southern one,developed in a basin environment,is represented by lamellar limestones,warty limestones and shales,as well as psephitic and calcarenites,the thickness of which varies from 307-521m. The lamellar limestones,warty limestones and the shales are autochthonous rocks,whereas psephitic and calcarenites are allochthonous sediments,i.e. carbonates of gravity flow origin. Gravity flow in present region can be divided into three types,notably debris,turbidity and grain. Of them,the most important one is debris flow which have givrn rise to more than a hundred meters of sediments. varying from bedded to massive coarse calcirudites and occupying 80-90% of the total thickness.Turbidity flow is composed of fine calcirudites and calcarenitos with positive gradation. This type of rocks is widely distributied and characterized by minor thickness for each individual bedding. Grain flow consists of medium calcirudites to calcarenites,with a rather unighe texture. On probability diagram showing gradation,all kinds of gravity flow sediments exhibit gentle and smooth curves covering a wide size range of particles,but each type of sediments have its own attributes. They are clearly different in their size range parameters and have distinction both on CM and scatering diaagrams. Gravity flow sediments were derived mainly from the simultaneous carbonate platform sediments(extraclasts),and also from micrites consolidated to some extent in the basin (intraclasts). A steeper slope primarily existed between the platform and the basin,the depth of which at the end of Trassic might be 800-1000m. The study on carbonate gravity flow not only can make direct contribution to dealing with sedimentary environment and compilation of lithofacies-paleoge-ographical map,but also give important guidance to petroleum prospecting and exploration.