Evolution Of The Cambrian Carbonate Platform Margin And Slope In East Guizhou And West Hunan,-Relationship To Tectonics Of Yangtze Continental Margin

Carbonate platforms tend to grow near to sea level. Thus they record the combined history of subsidence and sealevel fluctuation on passive margins. Sedimentology research on the Cambrian carbonate platform margin and slope in east Guizhou and west Hunan provides detailed information on subsidence history of the south-east passive margin of the Yangtze Plate as well as on sedimentary models and evolution of the margin. Main results are:1. The Cambrian and lower Ordovlcian carbonate platform developed on an underlying siliclas- tic shelf. The evolution of carbonate platform can be divided into four stages. (1)ramp evolved into steepened ramp. (2)initially drowned platform rapidly evolved Into ramp,followed by accretionary platform. (3)plalform covered by terrigenous influx rapdly evolved into ramp. followed by accre- tionary platform and finally by-pass margin platform. (4)by-pass margin platform evolved into a ccretionary platform.2. The Cambrian to lower Ordovician carbonate platform margin aggraded and prograded throughout this time, reflecting steady tectonic subsidence and suggesting that the south-east margin evolved into mature passive margin through crust thinning and subsidence.3. The favourable tectonic setting and climate resulted in a shallowing-upward megacycle that first coarsens upward, then fines upward. A deeper water ramp or slope shallowing upward into a marginal shoal, and eventually lagoon and tidal flats formed on platform. A coarsing and thicken- ing upward megacycle formed on slope.4. The progradation of platform margin shifted the shelf break eastward and seaward. As a result, calcirudites of the slope cycle rise eastward.5. Episodic and uneven progradation of the platform margin produced a variety of secondary cycles of different duration, position, and extent.6. The geometry of slope deposits indicates development of a slope apron as well as a base-of-slope apron in the early late Cambrian and a debris,sheet at the end of the Cambrian.7.Various mud and clast-supported fabrics of debris flow sediments, and calculations of debris static sterngth and paleoslope gradients show a variety of support mechanism. Pulsating movement and flow transformation to turbidity currents as welllasslide transformation to flows were common in the submarine carbonate debris flows.