Deposition and Evolution of Carbonate Platform-Slope-Basin in Permian Guadalupian (Kungurian-Capitanian) in the Northern Margin of Yangtze Basin, China

The Yangtze Basin is located between the Qinling-Dabie orogenic belt and the Cathaysia landmass, and it was a part of the eastern Paleo-Tethys Ocean in the Permian. The study region is located in the north margin of the Yangtze Basin (the NMYB), and has a length of more than 1,000 km from east to west and a width of 70-450 km from north to south. The Yangsingian Series in the study region is equivalent to Kungurian Stage-Capitanian Stage. The Series, 200-500 m thick, is composed of mostly carbonate rocks such as megaconglomerates/megabreccias, calcarenites grainstones, packstones/wackestones etc., secondly cherts and clastic rocks. The Yangsingian Series has abundant fossils, such as algae, coral, brachiopod, foraminifera, echinoderm, fusulinid, ammonoid, radiolarian and sponge spicules etc.

The study region during the Middle Permian was a marginal sea. To the north, there was an open sea; to south and southwest, there were massive and chain carbonate platforms. The carbonate slope was situated between the platforms and the open sea, and it was north-northeast dipping, more than one thousand kilometers in length, and hundreds of kilometers in width.

The carbonate platform was an open-limited sea platform; it was mainly composed of bioclastic grainstone, packstone /wackestone, and micritic limestone, tens to hundreds of meters thick. On the platform, carbonate shoal sediments rarely developed; the shore-lagoon located on the edge of the platform mainly developed fine-grained clastic rocks and coal line or thin coal seams. The carbonate slope may belong to a new type of slope, a deposition-onlapping slope, which is also a gravelly to sandy slope. The slope can be divided into the upper, middle and lower slope. The upper slope, with a steep angle of 10-20°, was composed of grain-supported mud-free to mud-bearing megaconglomerates, which were mostly derived from the deposition of the debris flow. The middle slope, with a gentle slope angle of 5-10°, was composed of grain-supported or matrix-supported megaconglomerates, calcarenites, packstones/wackestones and layer cherts. There are diverse types of deposition processes in the middle slope, which include deposition of debris flow, contour current, upwelling flow, and in situ deposition. The lower slope, with a slope angle of less than 5°, developed siliceous shale, carbonaceous shale and calcarenites, with isolated limestone and chert nodules. The lower slope developed the deposition of contour current, upwelling flow, debris flow, suspension and avalanche. The sediments of the basin and margin were mostly composed of thin-bedded cherts, various kinds of shale, bearing manganese and phosphorus. They mainly formed under a layered stagnant anoxic depositional environment.

The study region in Middle Permian went through the carbonate platform-slope-basin depositional evolution, which can be divided into two major cycles and four secondary cycles. The major role controlling the cyclic sedimentation is sea-level changes, followed by tectonic activities. The activities of the Permian faults in the study region and the extension of the Paleo-Tethys Ocean to east were under the same tectonic setting. The development of the Middle Permian carbonate slope probably provides the evidence for the rapid expansion of the Paleo-Tethys Ocean eastward.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California