| The northwestern Sichuan Basin, which is located in the north margin of the Longmen Mountain Nappe Structural Belts, is one of the classic areas for the upper Carboniferous researches in Sichuan Basin and even in the upper Yangtze Plate. The upper Carboniferous was deposited in semi-enclosed gulfs of the western Yangtze Block, which composed of a set of semi-restricted platform and open platform carbonate rocks in this area. The strata are revised according to sedimentary types, sedimentary structures, lithological characters, and paleontological combinations. The upper Zongchanggou Formation, which was considered as one part of the lower Carboniferous, is amended to the upper Carboniferous Huanglong Formation in this paper. The upper Carboniferous is divided into four parts, as lower Huanglong Formation, middle Huanglong Formation, upper Huanglong Formation and lower Chuanshan Formation.Fifteen microfacies can be divided according to field observation and laboratory micro-analysises. The carbonate rocks are named following Dunham’s Classification of Limestones. Four sedimentary facies belts could be summarized as winnowed platform edge sands (FZ6), open platforms (FZ7), restricted platforms (FZ8) and platform evaporates (FZ9), according to vertical and horizontal microfacies combination. Depositional facies belts indicate that several transgressions and regressions have occurred during the late Carboniferous. The maximum transgression, characterized by widely outcropped carbonate rocks in the northwestern and eastern Sichuan Basin, is occurred in the upper Huanglong Stage.Many typical storm deposits, such as storm shell limestones, storm calcirudites, storm nodular limestones, storm calcarenites, scouring structures, graded beddings, knotty-like structures, scale beddings, horizontal beddings, current beddings, bioturbate structures and so on, are preserved in the Lower Huanglong Formation around Majiaoba area, Jiangyou County. Four storm layers are recognized, and five storm sedimentary combinations could be summarized. Combination Ⅰ, characterized by scouring structures, coarse gravel beddings, scale beddings, horizontal beddings, current beddings, bioturbate structures and dragonfly fossil concentration, is deposited nearby open platforms around normal wave base. Combination Ⅱ, represented by bioclastic limestones with similar knotty structures, graded beddings and massive beddings, is deposited nearby restricted platforms above normal wave base. Combination Ⅲ, characterized by coarse gravel beddings, graded beddings and massive beddings, is deposited nearby winnowed open platform edges above normal wave base. Combination Ⅳ is located above combination Ⅲ, and characterized by graded beddings, hummocky cross stratifications and massive beddings, as indicate that the combination Ⅳ have been deposited before the former storm flow completely disappeared nearby normal wave base. Combination Ⅴ, characterized by scouring structures, coarse gravel beddings and mudstones, is deposited nearby open platforms around fine weather wave base. Such tempestites are wholly proximal storms rocks.Oncoids are well preserved in the Chuanshan Formation in Majiaoba, Jiangyou County. Five core and lamina types and three structures types could be divided according to their microstructures. Six oncoid types are distinguished by their cores, laminas and shapes, such as big ball oncoids, small ball oncoids, long-shaped oncoids, gibbous oncoids, irregular oncoids and combined oncoids. They are deposited in winnowed platform edge sands, open platforms or restricted platforms separately. Both the distribution of the oncoids and their forming and depositional environments are discussed in this paper.(1) Forming environments and depositional environments are inconsistent. Oncoids contents have big sizes and are cemented by sparites in the strata, laminas are more complex and thicker than cores, constituents of core and matrix are different. The cores, which are deposited nearby open platforms with weak water power, suspended nearby winnowed platform edge sands with strong water power capturing and adhering carbonate particles. Additionally, biological clastics are seriously brokem in such samples.(2) Forming environments are not similar with depositional environments, at the same time the cores and the matrix are both composed by complete biological clasts. Medium spreaded oncoids contents have medium diameters and simple laminas and were cemented by sparites. Some biological clasts in their original places capture and adhere carbonate particles during suspended in open platforms state.(3) Forming environments are exactly same with depositional environments. The oncoids are deposited in restricted platforms, with low contents, anomalistic diameters, polytropical edges, simple laminas, low biological clastic and thin laminas.ICP-MS is used to determine REE contents of oncolite carbonate rocks of the Chuanshan Formatiom in Majiaoba, Jiangyou County. The analyses of the REEs are related to their depositional environments, especially ΣREE, LREE, HREE,δCe,8Eu and Y/Ho are displayed regularly. Total REE contents (ΣREE) vary from6.26ug/g to25.12ug/g. The average ΣREE value of all samples is15.29ug/g and under the average value of NASC (North American Shale Composite)(160.12ug/g). LREE are relatively rich and HREE are a little lack. Different LREE and HREE values of the samples indicate that REE are removed and LREE are fractionated more easily than HREE. NASC normalized REE patterns display low REE enrichments and relatively high REE loss. The LREE patterns are a little rightward heeling, and the HREE patterns are flat.δCe vary from0.43to0.74, and the average of all samples is0.59. They all have obviously negative δCe anomalies, as indicated strong Oxidation environments during the Chuanshan Stage. Some samples have negative8Eu anomalies and some have positive δEu anomalies. As indicate that REE characteristics are affected by ancient climate cycles-(glacial-interglacial cycle) and the periodical sea level changes. Y/Ho are closed to the lower limited of modern seawater, which imply that the ancient seawater is affected by the ice melt water. The REE are not only derived from the seawater, but also affected by the biodetritus and the genesis of oncolite. |
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