FACIES MODELS FOR DEEPWATER GRAVITY- DIS-PLACED DEPOSITS IN ANCIENT MARINE BASINS, SOUTH CHINA

A spectrum of gravity-displaced deposits from isolated rock blocks through slides and slumps to various kinds of gravity flow sediments can be recognized through anatomizing deepwater deposits in many marine basins of South China.Their different association reflects the different types of sedimentary system to which they belong and their different distribution in each given system.Based on major sources those systems related lo gravity-displacement are divided into three categories: (1) gravity-displaced clastic system: (2) grivi-ty-displaced carbonate system; and (3) gravity displaced mixing sediment system.Category 1 includes two subdivisions of submerine fan and axial-transported system. In terms of the slope gradient of carbonate platform margin category 2 provides three kinds of the models or systems. 1) Ramp-type: besides some thin turbidites and small slumps, its sedimentary characteristics are the dominant fine deposits from sediment rain and the lack of gravity flow deposits containing the materials derived from shallow platform although reefs or mounds and shoals or ooids bands can exist in upper slope or outer shelf. 2) Middle slope gradient- type: it consists of four parts: a) a steeper bypass zone- upper slope which is indicated by periplatform fine sediments deposited in areas between numerous small channels that are perpendicular to the trend of the slope arid are filled with coarse gravity-displaced deposits; b) a gentle middle and lower siope which is the most important site for almost all kinds of gravity-displaced deposits; c) a slope toe in which channelized deposits disappear obviously but are replaced mainly by finer and thiner sheet beds of gravity flow depostits, and the combination of these beds constitutes a banded zone parallel to the trend of the slope along the slope toe; and d) a flat basin floor in which only fine deposits from sediment rain are developed and occasionally interlayered with thin turbidites. 3) Steep slope-type: it includes a very steep slope and a very adjacent fiat basin floor, and is characterized by the lack of channelized deposits and the development of gravity flow calcirudite sheets in large scale. In category 3 the framework of sedimentary systeem is the most complex and commonly is a combination of several subsystems, including: 1) carbonate slope system and clastic submarine fan; and 2) carbonate slope system and pyroclastic gravity flow axial-transported system.