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

Abstract

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 isotopes-sedimentary cycles) was proposed. Thus, the marine 3rd-order sequences were divided the global sea-level 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.