Title
Characteristics, enviornment and geological dating of primary dolomite in the Neoproterozoic Pingwagou Formation at Hongliugou, Ruoqiang County, Xinjiang
摘要
新元古界白云岩是反映全球气候和生态环境等重要承载体之一，具有重要的理论意义及应用价值。本文报道了以原定为青白口系硝尔库里群中的平洼沟组含沥青微生物白云岩为代表的原生白云岩的特征、沉积环境及年代厘定等研究成果。通过岩相学、地球化学分析并结合锆石年代学等资料研究表明：平洼沟组是一套藻鲕粒-团粒云岩，绵层状、纹层状细粒、纹层-粗粒状胶结波纹状、锥状、穹窿、柱状叠层石和球粒黏结状微生物岩为主的原生白云岩，夹有风暴岩，属于潮间至潮下浅水沉积。平洼沟组发育了2个向上变浅-变深旋回的内-中缓坡微生物丘、滩相。广泛发育了葡萄状、纤状及纤柱状、放射-束状等海底胶结物，具有较高Mg/Ca、“帽型”为主的稀土配分模式，属于典型的文石海及原白云石沉积。综合叠层石圆锥-柱状的形态结构、正高漂移δC（PDB）（平均值7.80‰）、δO（PDB）（-1.82‰）和比当时海水稍高的Sr/Sr（0.708 07）以及下覆碎屑岩中锆石U-Pb年龄数据，将其厘定为Sturtian冰期后的成冰纪（685~635 Ma）的一套温暖气候下，富镁、富硅的海水以及海底热液参与的原生白云石沉积。平洼沟组虽历经漫长地质演化史，仍能保留一定的储集性，与下覆冰沟南组泥质烃源岩构成了一套生储组合，有一定油气勘探意义。
Abstract
The Neoproterozoic dolomite, one of the important carriers of ancient global climatic, biological, ecological and environmental information, is of great theoretical significance and application value.This study highlights the progress in characterization, environmental reconstruction and geological dating of a typical primary dolomite: the bitumen-bearing microbial Neoproterozoic dolomite from the Pingwagou Formation (previously believed to belong to the Nierkuli Group of Qingbaikou System) at Hongliugou section Ⅰ in Ruoqiang County.Petrographic and geochemical analyses as well as detrital zircon geochronology show that the Pingwagou Formation is mainly a set of intertidal to subtidal deposition of shallow-water primary dolomite mainly composed of algal oolite-granular dolomite, spongiostromate, laminated fine- and coarse-grained agglutinated stromatolites in wavy, conical, domal and columnar patterns, and microbolites in cemented spherulites intercalated with some tempestites.Two shallowing-up and deepening-down sub-cycles of bioherm and beach facies are developed in inner and mid ramps of the formation. Marine cements in grape-, fiber-, fibrous column-shaped or radial-fascicled patterns are broadly distributed with high ratios of Mg/Ca and a "cap-pattern" rare-earth distribution, indicating typical aragonite sea sediments and primary dolomites.Based on an overall consideration of stromatolite patterns, pronounced positive δC excursions (ave.7.80‰), δO (ave.-1.82‰) and elevated Sr/Sr values (ave.0.708 07) as well as zircon U-Pb dating of underlying clastics, we conclude that the formation is a set of primary dolomite deposition by Mg- and Si-rich seawater as well as seafloor hydrothermal inputs against a warm climate in the Neoproterozoic Cryogenian after the Sturtian Ice Age (685-635 Ma).Despite a long geological evolution, the dolomite in the formation is of certain reservoir properties and has formed an effective source-reservoir combination with underlying argillaceous source rocks, posing an interesting yet tough challenge to oil and gas explorers.
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