Middle and Late Devonian brachiopod assemblages, sea level change and paleogeography of Hunan, China

Givetian through middle Famennian brachiopods of Hunan are used to define five biozones, comprising, in ascending order, the Stringocephalus Abundance Zone, the Ambocoeliid-Leiorhynchid Assemblage Zone, the Cyrtospiriferid Assemblage Zone, the Yunnanellina-Sinospirifer Assemblage Zone, and the Yunnanella-Hunanospirifer Assemblage Zone. They correspond approximately to the Po. hemiansatus Zone to Po. varcus Zone, S. hermanni Zone to Pa. punctata Zone, Pa. hassi Zone to Pa. linguiformis Zone, Pa. triangularis Zone to Upper Pa. crepida Zone, and Uppermost Pa. crepida Zone to Pa. marginifera Zone, respectively. Intensified rifting activity in South China during the Devonian began in the middle of the Lower Po. varcus Zone (a level marked by the first appearance of Po. rhenanus ), which is consistent with the Lower-Middle Givetian substage boundary suggested by the International Subcommission on Devonian Stratigraphy (SDS). This assumed that rifting activity in the Lower Po. varcus Zone was signified by the deposition of the Baqi Formation in Guangxi and was marked largely by the extensive deposition of the Qiziqiao Formation in central Hunan, which probably led to an opposite pattern of relative sea level change-rising in deeper water trough areas and lowering in carbonate platform areas. The previously recognized S. hermanni Zone sea level rise in platform margin areas of Guangxi can be identified not only in the trough areas but also in platform areas, which probably indicates an overall marine transgression at this time, rather than tectonically induced sea level change. Similarly, this level coincides with the Middle-Upper Givetian substage boundary suggested by the SDS. Although the sea level rise may have occurred in the deeper water trough areas of South China towards the end of the Givetian, platformal areas experienced uplifting at various degrees, especially in the central and northern regions of Hunan where Early Frasnian deposits were apparently absent in many places. Similarly, the end-Frasnian regression led to exposure of many places and absence of early Famennian deposits, which probably accounts for the restricted distribution of the Yunnanellina brachiopod fauna.