The sedimentary facies and tectono-stratigraphic successions of the Carboniferous–Lower Permian deposits in western South Qiangtang Block: Implication for a rifting process on the Gondwana margin

The South Qiangtang Block (SQB) was widely acknowledged as a part of the Cimmerian Continent, which rifted away from Gondwanan margin during the Early Permian. However, the sedimentary response to this rifting event has thus far not been unequivocally confirmed, which makes this event ambiguous. In this contribution, three stratigraphic successions are recognised, which are characterised by distinct facies and record different stages in the tectonic evolution of the basin and associated changes in the rates of basin subsidence and sediment accommodation. Succession 1 comprises glacio-marine deposits which are dominated by dark shales and diamictites (Cameng and Zhanjin formations) originated mainly from both gravity flow and downslope resedimentation. Succession 2 is predominated by shallow-water sandstones of delta setting (lower Qudi Formation). Succession 3 is composed of deposits of tidal flat and platform (upper Qudi and Tunlonggongba formations). The sedimentary facies changes fit well with marine rift-basin successions: (1) Succession 1 clearly records ‘sediment underfilled’ stage characterised by rapid tectonic subsidence, low sediment supply, and marks an early to climax stage of syn-rift; (2) deltaic sandstones facies of Succession 2 records ‘sediment filled and overfilled’ when sediment supply consumes up the accommodation, and marks a late stage of syn-rift during the tectonic quiescence period; (3) tidal-rhythmites of Succession 3 is likely a record of short-term autogenic cycles, indicating a possible post-rift stage. Therefore, these successions within the western SQB during the Early Permian was best explained by the tectonic subsidence resulted from the rifting of the SQB from the Gondwanan margin.