Recognition and Depositional Environment Interpretation of the Shallowing-Upward Sequence in the Cambrian Xixiangchi Formation, Northern Margin of the Upper Yangtze Block

Key words: 
• Northern Margin of the Upper Yangtze Block /
• Xixiangchi Formation /
• Microfacies of Carbonate Rocks /
• Shallowing-upward Sequence /
• Sedimentary Environment Interpretation

Abstract: [Objective] Shallowing-upward sequences are extensively developed in shallow-water carbonate environments, and the identification of shallowing-upward sequences in deep-water carbonate rocks holds a unique advantage for analyzing the evolution of shallow-water carbonate sedimentary environments. [Method] This study focuses on the Late Cambrian Xixiangchi Formation in Xiuqi Town, Chengkou County, on the northern margin of the Yangtze Platform. Through detailed field sedimentological analysis and laboratory microfacies analysis, shallowing-upward sequences in carbonate rocks were identified. The compositional characteristics and vertical stacking relationships were analyzed, thereby exploring the Late Cambrian sedimentary environment evolution in the upper Yangtze region. [Results and Discussions] In the Xixiangchi Formation, 14 sedimentary microfacies have been identified, including MF0 argillaceous shale, MF1 micrite, MF2 bright crystal gravel limestone, MF3 bright crystal sand limestone, MF4 muddy siltstone, MF5 dolomitic microcrystalline limestone, MF6 gravel dolomite, MF7 bright crystal sand dolomite, MF8 sandy gravel grain dolomite, MF9 sandy sandy lithic grain dolomite, MF10 sandy dolomite, MF11 laminated dolomite, MF12 bamboo leaf-like gravel dolomite, and MF13 grain dolomite with dissolution pores. Based on the microfacies associations, six decimeter-scale upward-shallowing tidal sub-environments (C1-1 to C1-6) have been recognized within the Xixiangchi Formation, representing subtidal deposition. Additionally, six decimeter-scale upward-shallowing tidal-subtidal transition sub-environments (C2-1 to C2-6) and two decimeter-scale upward-shallowing intertidal-subtidal transition sub-environments (C3-1 and C3-2) have been identified. These decimeter-scale upward-shallowing cycles constitute three secondary sea-level cycles. These three secondary cycles, from bottom to top, combine to form one upward-shallowing sedimentary cycle within the Xixiangchi Formation, representing a rapid transgression followed by gradual regression. [Conclusions] Based on the identification of shallowing-upward sedimentary sequences and the analysis of their compositional characteristics and vertical stacking relationships, it is believed that the Late Cambrian Xixiangchi Formation was significantly influenced by high-frequency sea-level changes. It as a whole underwent an early rapid marine transgression, followed by a late slow regression, and experienced multiple episodes of secondary sea-level fluctuations. This resulted in a restricted carbonate platform sedimentary model characterized by mixed intertidal subfacies, chalky lagoon subfacies, and inner-shelf grainy shoal subfacies. This study further confirms a global late Cambrian sea-level regression event.