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The current study presents results on depositional and diagenetic environments of the glauconite-bearing oolitic carbonates of the upper part of the basal third-order depositional sequence (DS6) of the Cambrian Fengshan Formation at Kelan in the North China Platform. The Fengshan Formation comprises calcareous mudstone and shale of shelf environment followed by calcareous mudstone, micritic limestone of deep ramp to oolitic limestones of shallow ramp environments. The glauconite-bearing oolitic carbonates are restricted in the upper part of the sequence which was deposited during the forced regressive event. Petrographic study revealed that the sediments comprised of both carbonate and non-carbonate framework constituents. These constituents are mostly comprised of rounded to elliptical, radial-concentric ooids, dolomitized ooids, bioclasts (trilobites, brachiopod, and echinoderms), glauconitic pellets with fibro-radiating rims, and pyrite crystals. The radial-concentric structure of ooids suggests high-energy shallow water deposition conditions. Also, the presence of glauconite along with oolitic grainstone in the restricted upper portion of the Fengshan Formation provides credible evidence of its deposition under high-energy conditions during relative sea-level fall. It is therefore inferred that the glauconite was precipitated on or in close proximity to the Cambrian calcite seafloor during the forced regressive event prior to calcite cementation. [ABSTRACT FROM AUTHOR]

According to the recent chronostratigraphic division of Cambrian, the Cambrian strata in the Kelan section can be subdivided into seven third-order sequences (DS1-DS7) based on cyclicity in sedimentary facies stacking patterns. The calcareous mudstone forming condensed section, micritic limestone comprising deep to middle ramp facies and the oolitic-grain bank facies in Series 3 and Furongian series represent the basic depositional fabric of Cambrian in northwestern part of Shanxi Province, North China Platform. These massive oolitic grainstones demonstrate that the oolitic-grain bank facies constitute the late-highstand systems tract or forced-regressive systems tract. The grains occupy upper parts of the third-order depositional sequences in response to relative sea-level fall. Furthermore, this forming pattern of oolitic-grain bank facies does not follow the standard model of sequence stratigraphy, in which deposition is believed to occur principally during sea-level rise, rather a continuous erosional unconformity develops during sea-level fall. Moreover, the microscopic analysis of oolitic grainstone shows the development of concentric and radial, rounded or elliptical, with or without nuclei, monocrystalline or polycrystalline, Girvanella or micritic ooids. The occurrence of diversified ooids in varying proportions provides a new dimension for studying evolution of the oolitic-grain bank in the North China Platform. The forming pattern of oolitic-grain bank controlled by their sequence-stratigraphic position in the Kelan section in the Shanxi province provides an important clue and a research direction for the regional correlation, as well as the paleogeographical reconstruction of the Cambrian Series 3 and Furongian series. [ABSTRACT FROM AUTHOR]