Long-distance correlation between tectonic-controlled,isolated carbonate platforms by cyclostratigraphy and sequence stratigraphy in the Devonian of South China

【摘要】：正During the early Middle Devonian in South China,an extensive carbonate platform was broken up through extension to create a complex pattern of platforms,and interplatform basins.In Givetian and Frasnian carbonate successions,five depositional facies,including peritidal,restricted shallow subtidal,semi-restricted subtidal,intermediate subtidal and deep subtidal facies,and 18 lithofacies units are recognized from measured sections on three isolated platforms.These deposits are arranged into metre-scale,upward-shallowing peritidal and subtidal cycles.Nine third-order sequences are identified from changes in cycle stacking patterns,vertical facies changes and the stratigraphic distribution of subaerial exposure indicators.These sequences mostly consist of a lower transgressive part and an upper regressive part. Transgressive packages are dominated by thicker-than-average subtidal cycles, and regressive packages by thinner-than-average peritidal cycles.Sequence boundaries are transitional zones composed of stacked,high-frequency, thinner-than-average cycles with upward-increasing intensity of subaerial exposure,rather than individual,laterally traceable surfaces.These sequences can be further grouped into catch-up and keep-up sequence sets from the long-term (second-order) changes in accommodation and vertical facies changes. Catch-up sequences are characterized by relatively thick cycle packages with a high percentage of intermediate to shallow subtidal facies,and even deep subtidal facies locally within some individual sequences,recording long-term accommodation gain.Keep-up sequences are characterized by relatively thin cycle packages with a high percentage of peritidal facies within sequences,recording long-term accommodation loss.Correlation of long-term accommodation changes expressed by Fischer plots reveals that during the late Givetian to early Frasnian increased accommodation loss on platforms coincided with increased accommodation gain in interplatform basins.This suggests that movement on faults resulted in the relative uplift of platforms and subsidence of interplatform basins.In the early Frasnian,extensive siliceous deposits in most interplatform basins and megabreccias at basin margins correspond to exposure disconformities on platforms.