Factors Influencing the Composition of Shallow-Water Mixed Siliciclastic-Carbonate Sedimentation from the Cambrian Series 2:A case study on the Xiannüdong Formation of the Zhujiaba section (southern Shaanxi)
[Objective]The mixed siliciclastic-carbonate sediments (MSSs) contain both carbonate and detrital com-ponents that can concurrently carry aqueous and provenance signatures in their compositions. During the early Cam-brian,the MSSs were widely distributed in the Hannan-Micangshan area,western Yangtze Craton,but the factors in-fluencing their development have not been well known.[Methods]In this study,we performed a combined petrologi-cal and sedimentological analysis on the compositions of MSSs in the Xiannüdong Formation of the Zhujiaba section,southern Shaanxi.[Results and Conclusions]The results indicate that MSSs formed from the middle to the inner ramp settings during a regressive depositional cycle,and the highest level of admixture is generally distributed in the middle part of each upward-shoaling sequence compared to that of the lower and upper parts. Fine-grained detrital sediments and low-relief reefs developed in the middle ramp environment,and other carbonate fractions were mainly transported from shallow-water carbonate depositional settings. In contrast,ooid shoals and reefs developed in inner ramp environments with a small quantality of terrigenous sand-sized particles (quartz and lithic fragment). In addi-tion,the petrological evidence indicates that transported (dolomitized) carbonate grains and fine-grained detritus were concurrently mixed in the lagoon setting. The development of early Cambrian MSSs in the study area may be re-lated to the high carbonate productivity of shallow-water environments,as well as the active onshore-to-offshore trans-portation mainly induced by storm currents. This study may provide a case for understanding the factors influencing the development of deep-time MSSs and a reference for oil and gas exploration of Cambrian MSSs in the western Yang-tze area.
mixed siliciclastic-carbonate rocksXiannüdong FormationreefslithofaciesHannan-Micangshan area
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