The Cambrian-Ordovician boundary in the Tarim Basin, NW China: Geochemistry and geophysics data anomalies

Autor: Fan Tai-liang, Wei Duan, Hans G. Machel, Wang Hongyu, Gao Zhiqian
Rok vydání: 2017
Předmět:
Zdroj: Journal of Petroleum Science and Engineering. 156:497-512
ISSN: 0920-4105
DOI: 10.1016/j.petrol.2017.06.034
Popis: This paper presents outcrop, petrographic, geochemical, well log and seismic data which together characterise the T80 unconformity that occurs between the carbonate-dominated Ordovician and Cambrian successions in the Tarim Basin, NW China. In the region the T80 unconformity represents the Cambrian-Ordovician transition surface controlled by both large-scale tectonic movements and sea level changes. The unconformity had a major influence on platform-margin architectures and depositional environments in the Palaeozoic carbonates below and above the unconformity. Seismic profiles show that a progradational ramp platform margin developed in the Upper Cambrian below the T80 unconformity, while a retrogradational rimmed platform margin developed in the Lower Ordovician above the T80 unconformity. The Upper Cambrian carbonates are dolomitized whereas limestones dominate in the Lower Ordovician strata. At four measured outcrop sections near the northwestern margin of the basin (Yingshan, Yonganba, Penglaiba and Shuinichang), extensive dissolution porosity from karstification is developed below the T80 unconformity surface. The unconformity is associated with various geochemical anomalies. δ13C and δ18O ratios in the dolomitized Upper Cambrian strata immediately below the unconformity surface display negative excursions, paired with significantly reduced concentrations of major and trace elements such as, K, Ti, Sr, Fe, Ca, Si, Cl and S, while Mg and Ba are enriched relative to the Ordovician strata above. Furthermore, it is proposed that changes in the natural gamma radiation and Sr element concentrations can be used as a new tool for the identification and correlation of the Cambrian-Ordovician boundary, especially where diagnostic fossil markers are missing.
Databáze: OpenAIRE
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