Sedimentary facies and sea-level change across the Middle-Upper Devonian boundary at Caiziyan section， Guangxi: The mid-Givetian regression and its timing in South China
Li Hua1, Ma Xueping1, Wei Longming2
1 School of Earth and Space Sciences， Peking University， Beijing 100871 2 Department of Resource and Environment， Guilin University of Technology， Guilin 541004， Guangxi
Abstract：The Caiziyan section was located in a platform margin zone， with relatively abundant benthic and common-rare pelagic fossils， which may be regarded as a quite suitable reference section for stratigraphic correlation and division. On the basis of lithofacies and biofacies characteristics， sea-level change history is reconstructed for the interval from the Givetian to the Early Frasnian times. The Early Givetian Donggangling Formation of the study area may represent deposits under a persistent transgressive phase， probably with a slight sea-level fall at the end of the Lower varcus Zone. The lower part of the “Gubi” Formation is complex in lithology， probably resulting from extensive sea-level changes， including a significant transgressive event at the initial period of the Lower hermanni-cristatus Zone and the long-recognized mid-Givetian regression， whose timing has been precisely determined to be the Upper varcus Zone for the first time. The Late Givetian deposits host abundant tabulate corals and reef-associated bioclasts， including stromatoporoids， crinoid stems etc.， implying that the study area was probably located on the upper slope in front of a reef complex. The upper part of the “Gubi” Formation(conodont falsiovalis to lower transitans zones)is characterized by alternating medium-thin and thick bedded limestones(mainly with bioclasts of level-bottom organisms)， which indicates a change from fore-reef to fore-shoal environments， probably with little overall sea-level change across the Middle-Upper Devonian boundary. The Rongxian Formation(basal part)is characterized by grayish to gray-white thin-bedded to massive limestones， which may represent a significant sea-level rise in the Middle transitans Zone in South China. The finely-medium crystalline dolostone layer at Caiziyan is within the Upper varcus Zone， with normal marine fossils including corals and conodonts. The proposed interpretation for this dolostone layer is that a substantial sea-level fall event happened in the Upper varcus Zone in the Guangxi area， based on temporal and spatial correlation of the dolomitized layers and evidence of sea-level fall in other sections.
Li Hua,Ma Xueping,Wei Longming. Sedimentary facies and sea-level change across the Middle-Upper Devonian boundary at Caiziyan section， Guangxi: The mid-Givetian regression and its timing in South China[J]. JOPC, 2009, 11(5): 503-512.
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