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

Luo Shunshe; Lyu Qiqi; Xi Mingli; He Youbin; Xiang Ji

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
Vol. 36, Issue 5, Pages: 745-755(2015)
Online First：28 October 2015，

Published：2015
DOI： 10.11743/ogg20150505
稿件说明：

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DOI：

Luo Shunshe, Lyu Qiqi, Xi Mingli, et al. Types,carbon/oxygen isotope characteristics and depositional environments of the Lower Ordovician contourites in Jiuxi and Yuanguping of North Hunan Province, China[J]. 2015, 36(5): 745-755. DOI： 10.11743/ogg20150505.

摘要

深水环境下等深流沉积研究已经开展50多年，在这50多年里虽然取得了一些成果，但对等深岩的形成机理和沉积环境研究较少。通过野外剖面实测、详细描述和碳氧同位素特征分析，对研究区形成等深岩的环流的特征及不同类型等深岩的形成环境进行了探索。根据其岩性特征，研究区下奥陶统盘家咀组发育有灰泥等深岩、粉屑等深岩、砂屑等深岩、砾屑等深岩和生物屑等深岩5种等深岩相，在垂向上主要发育由单一灰泥等深岩叠置组成的层序、灰泥等深岩与生物屑等深岩组成的层序和灰泥等深岩与砂屑等深岩组成的层序3种不完整等深岩层序和少量的完整等深岩层序。为了研究该等深流沉积形成时的古环境，分别对研究区不同类型等深岩与原地沉积进行系统采样做碳、氧同位素分析。研究表明，工区下奥陶统盘家咀组等深岩的碳同位素δ

C值分布于0.18‰～2.31‰，氧同位素δ

O值的范围为-10.93‰～-9.54‰，盐度指数Z值介于122.91～127.14，温度T值为16.98～24.81 ℃。与原地沉积相比，研究区等深岩的δ

C值、Z值、T值均较高，且随着等深岩的粒度增大，其δ

C值、Z值、T值均逐渐增高。这表明等深流沉积环境与原地沉积完全不同，等深岩主要形成于高盐度的较深水环境中，且盐度、水温越高，越有助于高能等深流沉积物砂屑等深岩和生物屑等深岩的形成。等深岩层序的形成周期均表现为古盐度、古水温、海平面先增大后减小的周期性变化。

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 δ

C values varying between 0.18‰ and 2.31‰

O 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 δ

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.