塔里木盆地顺北地区中—晚奥陶世稳定碳同位素特征及意义

CHARACTERISTICS AND CORRELATION OF THE STABLE CARBON ISOTOPE RECORDS IN THE MIDDLE TO LATE ORDOVICIAN CARBONATES IN THE SHUNBEI AREA,TARIM BASIN,NW CHINA

本研究对塔里木盆地顺北地区3口典型钻井岩屑样品的稳定碳同位素组成进行分析,揭示盆地腹地中—晚奥陶世碳同位素变化特征,并与世界其他地区进行对比,探讨碳同位素作为井下奥陶系精细划分和对比标志的可行性。除少量样品因受后期成岩作用改造具有明显偏负的δ^13C值外,绝大多数样品保持较好的碳氧同位素值纪录。整体而言,在达瑞威尔期之前,δ^13C值分布较为稳定,略有偏负,大致在-1.0‰附近变化;此后,δ^13C值逐渐增大至+2‰并呈现出振荡性变化的特征。本研究识别出两次正漂移事件,分别对应国际上的达瑞威尔期中期碳同位素正漂移(MDICE)和凯迪早期的碳同位素正漂移(GICE)。其中,顺北地区井下的MDICE正漂移幅度较世界其他地区报道的略微偏低,GICE正漂移事件与其他地区发现的特征基本一致,可作为塔里木盆地井下中—晚奥陶世地层精细划分和对比的识别标志。

Middle to Late Ordovician stable carbon isotopes of three drill cores in the Shunbei area,Tarim Basin are analyzed.The results show that most of the samples retain the primary carbon and oxygen signatures of the Ordovician seawater,despite that rare samples display distinct negative offset of δ^13C and δ^18O values probably due to the alteration of later diagenesis.Prior to Darriwilian,the δ13C values are slightly more negative compared to the published carbon isotope curves elsewhere and vary around -1.0‰.Thereafter,the δ^13C values gradually increase to +2‰ and exhibit significant perturbation.Two positive excursion events are well identified for this interval,corresponding to the Middle Darriwilian carbon isotope excursion(MDICE)and the early Katian carbon isotope excursion(GICE),respectively.The correlation of the carbon isotope profile in the Ordovician of the Shunbei area,Tarim with those of other regions suggests that δ^13C can be used as a helpful tool for the subdivision and correlation of the Middle to Late Ordovician strata in the Tarim Basin.