Type division and controlling factor analysis of 3rd-order sequences in marine carbonate rocks

Type division and controlling factor analysis of 3rd-order sequence are of practical significance to tec-tonic analysis, sedimentary environment identification, and other geological researches. Based on the comprehensive analysis of carbon and oxygen isotope trends, paleobathymetry and spectral-frequency of representative well logs, 3rd-order sequences can be divided into 3 types： （a） global sea level （GSL） sequence mainly controlled by GSL change;（b） tectonic sequence mainly controlled by regional tectonic activity;and （c） composite sequence jointly controlled by GSL change and regional tectonic activity. This study aims to identify the controlling factors of 3rd-order sequences and to illustrate a new method for classification of 3rd-order sequences of the middle Permian strata in the Sichuan Basin, China. The middle Permian strata in the Sichuan Basin consist of 3 basin-contrastive 3rd-order sequences, i.e., PSQ1, PSQ2 and PSQ3. Of these, PSQ1 is a GSL sequence while PSQ2 and PSQ3 are composite sequences. The results suggest that the depositional environment was stable during the deposition of PSQ1, but was activated by tectonic activity during the deposition of the middle Permian Maokou Formation.

Carbon and oxygen isotope variation and its implication for marine sequence: A case study of Ordovician in Tarim Basin

Carbon and oxygen isotopes of marine carbonate rocks or fossil shells could indicate global sea-level relative change.Generally,heavier carbon isotopes or lighter oxygen isotopes reflected rise of global sea-level relative changes,and vice versa.For carbonate rocks deposited before Tertiary,carbon isotope was more stable than oxygen isotope,and thus it was more suitable to indicate global sea-level relative fluctuation.Within time limit of the 3rd-order sequence(1-10Ma),large scale migration of carbonate platforms would not occur and carbonate productivity would not change greatly,therefore,a research method of marine tectonic sequence(i.e,a contrastive analysis method of carbon and oxygen isotopessedimentary cycles)was proposed.Thus,the marine 3rd-order sequences were divided the global sealevel sequence which was controlled by global sea-level change and the tectonic sequence which was controlled by regional tectonic subsidence and uplift.The global sea-level fluctuation indicated by carbon and oxygen isotopes were consistent with variation of sedimentary cycles and water depth of the global sea-level sequence,but were not consistent with variation of sedimentary cycles and water depth of the tectonic sequence.As a case study of Ordovician in Tarim Basin,two 3rd-order global sea-level sequences were identified in the Lower Ordovician,while six 3rd-order tectonic sequences controlled by collision and compression between Arkin island arc,Kudi active continental margin uplift and Tarim Plate were identified in the Middle and Upper Ordovician.Particularly,OSQ6 in the Upper Ordovician(Lianglitage Formation)was a typical tectonic sequence.