Oil & Gas Geology ›› 2015, Vol. 36 ›› Issue (5): 745-755.doi: 10.11743/ogg20150505

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Types,carbon/oxygen isotope characteristics and depositional environments of the Lower Ordovician contourites in Jiuxi and Yuanguping of North Hunan Province, China

Luo Shunshe1,2, Lyu Qiqi1,2, Xi Mingli1,3, He Youbin1, Xiang Ji1   

  1. 1. Laboratory of Exploration Technologies for Oil and Gas Resources, Yangtze University, Ministry of Education, Wuhan, Hubei 430100, China;
    2. School of Geosciences of Yangtze University, Wuhan, Hubei 430100, China;
    3. No.11 Oil Production Plant, PetroChina Changqing Oilfield Company, Qingyang, Gansu 745600, China
  • Received:2015-01-02 Revised:2015-08-20 Online:2015-10-28 Published:2015-10-23

Abstract: The literature on deep-water contourites is extensive after more than 50 years of research.However,many unknowns exist in terms of their forming mechanism and sedimentary settings.The article explores the characteristics of circulation and environment for forming various contourites in the Panjiazui Formation of the Lower Ordovician in the study area based on field measured section,detailed description and analyses of carbon and oxygen isotope characteristics.Lithologic analyses show that the formation contains 5 kinds of contourite lithofacies: calearenitie,calcilutite,caleilutitie,gravel and bioclastic contourities.Vertically,three types of incomplete vertical contourite sequences and some complete contourite sequences are recognized.The incomplete sequences include superimposed single calearenitie contourite sequence,interbedding calearenitie contourites and bioclastics contourite sequenceand interbedding calearenitie contourites and caleilutitie contourites sequencece.Systematically sampling and carbon/oxygen isotope analyses of the various contourites and autochthonous deposits indicate δ13C values varying between 0.18‰ and 2.31‰,δ18O values changing from -10.93‰ to -9.54‰,salinity index Z shifting from 122.91 to 127.14,and temperature(T) ranging between 16.98 and 24.81 ℃.Compared with the autochthonous deposits,the contourites in the formation have higher values of δ13C,Z and T.With the grain size of the rock getting bigger,these values continue to increase,showing that the sedimentary environment of contourites is totally different from that of the autochthonous deposits.The contourites were mainly formed in relative deep water environment with high salinity,and higher salinity and temperature were more favorable for the formation of certain high-energy contourites,such as caleilutitie and bioclastic contourities.The forming periods of contourite are all characterized by the periodical changes of first increasing and then decreasing palaeo-salinity,palaeo-temperature,and sea level.

Key words: carbon/oxygen isotope, contourite, Panjiaju Formation, Lower Ordovician, Mid-Yangtze platform

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