Peloidal Limestone Of Middle Permian In Northwestern Margin Of Yangtze Platform And Its Paleogeographic Implications

The basis for most paleoenvironmental interpretation of limestones is microfacies analysis, with abundances of individual grain types fundamental to the characterization of depositional environments, such as water energy, sedimentation rate, temperature, salinity. skeletal carbonates are typically produced within the environment of deposition. Different types of bioclasts indicate different sedimentary environments, such as algae and corals are major representatives of tropic shallow marine environments; abundant miliolid foraminifera represent restricted platform environments; a large number of brachiopods and bryozoans represent deep cold water environment; non-skeletal grains, such as oolid, oolitic grainstone usually represent high energy shallow water carbonate shoal environment. Numerous studies have examined surficial sediment compositions and textural characteristics within a range of modern carbonate environments, and these have traditionally been used as a basis for developing criteria for microfacies analysis.Peloids are small, rounded, spherical to ellipsoidal, cryptocrystalline carbonate aggregates commonly lacking internal structures. These aggregates generally vary in size from 30 to 100μm and rarely exceed 500μm. Peloids are major contributors to modern and ancient limestones, although their origins are not yet comprehensively understood. Macintyre (1985) grouped peloids into three categories according to their origin:fecal, detrital, and in situ precipitated. Based on the probable origin and diagnostic criteria, Flugel (2004) proposed nine subcategories:fecal pellets originating from organic excrements; algal peloids resulting from the abrasion of algae and calcimicrobes; bioerosional peloids resulting from boring and rasping by organisms; mud peloids as products of reworked cohesive mud; mold peloids resulting from internal molds of fossils; Bahamite peloids and pelletoids resulting from the alteration of grains; microbial and precipitated peloids, both formed in situ biochemically and chemically, respectively.Peloids occour in rock-building abundance forming peloid limestone or as only minor constituents of carboanate rocks together with other grain types. Peloids are common in shallow-marine tidal and subtidal shelf carbonate in reef and mud mounds. Many ancient peloidal limestones appear to have formed in shallow warm water environments. Fine-grained peloidal limestones are generally regarded as typical of shallow, low energy, resticted marine environments, while coarse peloid grainstone may be found in carbonate shoal environments. The geological records are so complex that we must figure out the formation of particles, based on a combination of evidence to interpret the original sedimentary environment.Shallow-water carbonate platform develops on the northwestern margin of the Yangtze platform during Middle Permian, with in which the middle member of the Chihsia Foamation and middle of Maokou Formation composed of carbonate shoals. More and more attentions are paid to the shoals, as which not only an important sedimentary facies, but also good oil and gas reservoirs. Most scholars defined it as bioclast shoal, due to the shallow water bioclasts and the spar filling in the pore space. However, there is some different opinion. Feng et al.(1997) consider it as biocrowd, which contains abundant bioclasts, pores between the grains filled with mud.Three outcrop sections on the northwestern margin of Yangtze platform were selected for detailed study of the shoals, adopting polarized light microscopy, cathodoluminescence microscopy, fluorescence microscopy and other methods. The grain types are mainly composed of peloids and cortoids, followed by shallow water bioclasts, the micrizition of which are common. Intergranular pores are filled by spar or lime mud. The peloids are classified into three types:fecal pellets, bahamite peloids and microbial peloids. The bahamite peloids are the most common grains observed. On the perspective of grain type, the depositional environment is the peloidal shoal, rather than bioclast shoal. The presence of both mud and spar in the pore space indicates the water energy was not strong enough to winnow off all the mud. Consequently the peloidal shoal develops in relatively low-medium energy environment.The abundance of bahamite peloids provide an opportunity for us to study the process of its formation. In light of detailed observation of the micriziting phenomenon of bivalves, calcareous algae and echinoderms, referring to previous similar researches, the process of micrizition is initially proposed, including:1. organisms disintegrate into fragments under the the actions of tidal currents and wind waves; 2. endolithic microborers attack the surface of bioclasts, producing little voids, which is filled with micritic aragonite. Mutiple repetition of boring and filling destroy the peripheral zone of the grains and results in the formation of circumgranular micritic envelope; 3. continued boring decreace the shell robustness, which further break into smaller fragments, then convert to cortoid under the followed abration and transport, eventually complete loss of the microstructure produce bahamite peloids.The peloid shoals are widespread in the west and north of the Yangtze platform. Tongkou section in Beichuan, Shuanghui section in Wangcang, Xidahe section in Baoxing, Dukou section in Xuanhan, and Qiaoting section in Nanjiang are selected for comparing with the three main sections, to obtain the spatial and temporal distribution of the peloidal shoal and spatial evolution of sedimentary facies. The peloidal shoals mainly develop in the midlle member of Chihsia Formation and middllle member of Maokou Formation. The distribution of the shoal in Chihsia Formation is lager, from the west magin to the southwest part of the platform, while in the Maokou Formation, the range of the shoal decreased, mainly developed in the south part of the Sichuan province.Combined with regional palaeogeographic and tectonic setting, the paleogeographic patterns of the northwestern margin of the Yangtze platform of middle Permian are reconstructed. From west to east, mainly develops tough, shelf, open platform with shoals, restricted platform facies belts. Compared with the carbonate bank on the edge of Nanpanjiang basin, the carbonate shoals on the western margin of Yangtze plarform are smaller in scale, mainly consist of peloids and cortoids, reflecting the characteristics of relatively low energy. As the thrusting of Longmen Mountain towards southeast since late Triassic, the middle Permian strata in the western margin of Yangtze platform are not exposed completely. This initially established sedimentary model should be confirmed in further study.