Conodont biostratigraphy and sedimentary facies of the Sanhedong Formation (Norian, Upper Triassic) in Weishan, western Yunnan Province
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Abstract:
The Norian (Upper Triassic) marine carbonate successions of the Sanhedong Formation were formed in the Lanping-Simao Block, western Yunnan Province. In this study, conodont fossils from the Sanhedong Formation were collected and identified. The conodont assemblage, dated as early Norian (Lacian), consists of Epigondolella vialovi,Epigondolella uniformis and Epigondolella aff. Triangularis. Facies and microfacies analyses of the Sanhedong Formation, in combination with field observation at the Sanhedong section, reveal three main facies, i.e., restricted-marine platform, open-marine platform and platform-margin reefs. They are further subdivided into nine microfacies. The presence of hydrothermal minerals, such as garnets, and the high conodont alteration index (CAI = 5) throughout the Sanhedong Formation indicate that hydrothermal actions may have affected the diagenetic process. The hydrothermal activities may be related to the upwelling of geothermal brine resulted from the subsidence of the continental rift margin and tectonic plate collision during the post-diagenetic process.
