Quantitative Evaluation of the Sea-level Drop at the End-Permian:Based on Reefs
【摘要】：正The amplitude of pre-Quaternary sea level drop,H,can be calculated by using the formula H = D + To. where To is the original thickness from the top of the tidal deposits on the reef core to the bottom of the tidal deposits on the reef front,or to the bottom of the ancient meteoric vadose zone,or to the edge of the mixed-water dolostone zone. The identity and similarity between the sea-level drop amplitudes calculated from different reefs far away from each other indicates that such sea-level changes are eustatic rather than relative changes.Evidence of an end-Permian sea-level drop has been found on the Changxingian(i.e.the end of the Palaeofusulina zone) reefs at Ziyun in South China,including algal laminated deposits,sabkha-related dolostone.desiccation cracks,dissolution collapse breccia. According to calculation based on the meteoric dissolution zone of the reef-core sequence at Ziyun,Guizhou province, the amplitude of the sea-level drop at the end-Permian is about 89.3 m.Calculation via the dolomitized upper part of the Changxingian reef in Lichuan,Hubei Province,yields an 88.9 m amplitude of the sea-level drop at the end-Permian. Comparison shows that the sea-level drop recorded in the two distantly located reefs may be of eustatic type.So the amplitude of the sea level drop of the Tethys Sea at the end-Permian might be at least 89.3 m.