Reconstruction of the Yangtze Ramp during Floian to Darriwilian(Ordovician)in South China:Its Morphology,Controlling Factors and Significances

A robust stratigraphic framework and a coherent depositional ramp model for the Zitai,Dawan,Meitan and Ningkuo formations of Floian–Darriwilian age(Early–Middle Ordovician)in the Yangtze(Daoba,Xiangshuidong,Daling,Gudongkou and Honghuayuan sections)and Jiangnan regions(Nanba section)was created based on lithofacies and major element analysis.Three siliciclastic(LF1–3)and six carbonate(LF4–9)lithofacies are recognized representing sediments that were deposited in mixed siliciclastic and carbonate ramp environment.The intensity of mixed sedimentation and terrigenous input were evaluated using the elemental proxies Intensity of Mixed sedimentation(IM)and Aluminum Accumulation Rate(Al AR),as well as their mean values during certain time intervals.Mixed sediments are most well-developed along the marginal Yangtze region,strongly impacted by recurrent influx of westerly derived terrigenous materials in response to global eustatic changes and regional tectonic movements,shaping the gently southeast-dipping morphology.Regular terrigenous influx resulted in periods of enhanced primary productivity on the Yangtze Ramp as evidenced by matching biodiversity peaks in planktonic organisms,i.e.,chitinozoans and acritarchs.Brachiopods and other shelly fauna were also able to proliferate as new niches developed along the gently dipping ramp floor with substrate changes.The biodiversification patterns suggest that terrigenous influx controlled in part by regional tectonics played a more important role than previously thought in the development of Great Ordovician Biodiversification Event in South China.