Effects of Middle–Late Permian sea-level changes and mass extinction on the formation of the Tieqiao skeletal mound in the Laibin area, South China

摘要

A skeletal mound is described for first time from a Middle–Late Permian succession that contains the Guadalupian–Lopingian (G/L) boundary at the Tieqiao section in the Laibin area, Guangxi Province, South China. The Tieqiao mound grew on a deep-water, south-facing carbonate ramp during the late Middle Permian. The mound features a range of grainstone–packstone to rudstone–floatstone facies together with sponge boundstone facies. Both sponges and algae acted as frame builders which colonised skeletal debris on the ramp and initiated mound growth. Diverse benthic biota comprising mound dwellers and constructors proliferated in the Tieqiao mound complex. The water depth changes recorded in the Tieqiao mound successions reflect the refined trajectory of global sea-level changes during the great regression at the end of the Middle Permian. The timing of sea-level lowstand coincides with disappearance of the Jinogondella granti conodont zone at Tieqiao. The change to sea-level rise pre-dated the G/L boundary. Growth and demise of the mound is interpreted to have been controlled by changes in sea-level. Abrupt disappearance of both body fossils and fossil fragments of leading fossil groups at Tieqiao is interpreted as the result of the G/L mass extinction rather than environmental change. The potential extinction horizon is *30 cm above the G/L boundary at Tieqiao and also records a rise in sea-level. The regional fall in sea-level destroyed the Tieqiao mound, but the G/L crisis halted redevelopment of the mound in the earliest Late Permian. Most mound builders suffered severely the Lazarus effect of the G/L mass extinction.