Rhuddanian (Silurian) graptolite-bearing strata and their sedimentary environments in the Xike’er area of Jiashi, Xinjiang
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Abstract:
At the beginning of the Silurian, black shales were widely deposited across various latitudes, likely driven by large-scale global marine transgressions and extensive anoxic events. However, in contrast to the widespread black shales of the Lungmachi Formation in South China, the early Silurian Kalpintag Formation on the northwestern margin of the Tarim Basin consists predominantly of green-yellow silty mudstone, muddy siltstone, and siltstone. Although the formation is well developed and well exposed, it remains poorly documented. This study focuses on the basal part of the Kalpintag Formation at the Xike’er North section, approximately 1.5 km north of the Xike’erkule Town, Jiashi County, Kashi (Kashgar) Prefecture, Xinjiang. The basal part of the Kalpintag Formation at this section is characterized by greenish grey mudstone and yields a graptolite fauna with a total of eight species in four genera, including Korenograptus lungmaensis (Sun), K. jerini (Koren’ and Melchin), Metabolograptus parvulus (Lapworth), M. wangjiawanensis (Mu and Lin), Normalograptus skeliphrus Koren’ and Melchin, N. mirnyensis (Obut and Sobolevskaya), N. cf. ajjeri (Legrand), and Akidograptus? sp. This fauna indicates an early Rhuddanian age (Silurian), corresponding to the Akidograptus ascensus-Parakidograptus acuminatus Zone. In addition, we investigate the early Silurian sedimentary environments through integrated lithofacies, biofacies, and geochemical analyses, and further explore the dominant factors controlling the development of environmental phases based on stratigraphic correlation to the contemporaneous strata in South China.
