四川广元羊木-岩洞地区震旦纪-寒武纪界线地层序列及构造-沉积演化

绵阳-长宁拉张槽的形成机制及时间尚存在巨大争议,严重制约了灯影组油气勘探的部署。为更好地对拉张槽深水地层提供时代约束,深化拉张槽构造-沉积演化认识,基于露头描述、薄片观察、碳同位素测试,对广元地区4个露头剖面开展综合地层对比和沉积环境分析。岩石学及碳同位素特征表明,研究区实测4条野外露头剖面从下到上可划分为4个岩性段(Ⅰ~Ⅳ):岩性段Ⅰ为薄层泥质灰岩夹泥质白云岩条带,对应陡山沱组三段Shuram-Wonoka负偏段地层;岩性段Ⅱ以黑色页岩为代表,可对比陡山沱组四段;岩性段Ⅲ为块状白云岩-硅质岩序列,属于灯影组同期地层;岩性段Ⅳ以薄板状硅质岩为特征,相当于麦地坪组。其中,灯影组二段早期—麦地坪期,研究区从块状白云岩逐步过渡为薄板状硅质岩,见垮塌体及滑塌揉皱,指示台地转换为斜坡-盆地沉积环境。这种加深的沉积趋势与中上扬子台地相区变浅至暴露的过程完全相反,指示断裂活动影响下快速沉降的过程。

塔北埃迪卡拉系奇格布拉克组多类型共生鲕粒特征及其成因探讨

碳酸盐岩中的鲕粒是古沉积环境重建的重要载体之一。塔里木盆地北部露头剖面、星火1井区埃迪卡拉系奇格布拉克组潟湖相及浅滩相沉积中发现了多种类型鲕粒及其共生现象。文中通过剖面详勘、岩心观察以及薄铸片、阴极发光、环境扫描电镜等研究方法,阐明了不同类型鲕粒的基本特征及分布,探讨了鲕粒成核、生长的特殊沉积环境和成岩改造等过程。按显微结构、特殊形态、组合以及成岩改造,依次将研究区奇格布拉克组鲕粒划分出12种类型,其中大气淡水、快速拟晶白云岩化、重结晶、交代白云岩化等成岩作用主要发现于中等水动力条件形成的同心(圈)鲕、同心—放射状鲕等。近源风暴引起的底流和涡流产生的撕裂、磨损及搬运作用为泥晶(大)鲕、薄皮鲕、放射状泥晶(正常或大)鲕、脑状鲕以及部分球(或细菌)鲕、复鲕的成核和微生物作用提供了基本条件,而文石—白云石海中低能环境下的黏性软底质中广泛发育的文石—高镁方解石胶结促进鲕粒的形成;前者发育于潮道或浅滩环境,后者发育于潮坪—潟湖环境,短暂风暴潮或底流是两者共生的可能原因。微生物参与泥晶成核、一定水动力条件下台阶状生长、差异性成岩改造是前寒武纪多种类型鲕粒发育的主控因素。鲕模孔、粒间(内)溶孔、晶间孔、有机微孔等孔隙具有一定储集意义。对奇格布拉克组鲕粒的研究,有助于深化对前寒武纪古海洋和大气组分、黏性软底质中成核与生长中的水动力及微生物作用、早期成岩改造及孔隙形成与保存机理等的认识。

塔里木盆地西缘上寒武统下丘里塔格群热液白云岩改造时限:来自古地磁的约束

成岩流体活动时间的确定一直都是学界难以解决的问题。上寒武统下丘里塔格群广泛发育有热液白云石,但热液改造的时间缺乏可靠约束。基于热液改造形成的广泛重磁化对热液改造时间进行约束,在巴楚永安坝和柯坪水泥厂下丘里塔格群中共采集43个热液白云石脉定向古柱状样品,并制样测试了64件古地磁样品,再对其中的29件样品进行了岩石磁学分析。大多数样品磁性在580℃时也没有被完全清洗掉,一部分在高外部磁场上才得到饱和等温剩磁,指示高矫顽力、高阻挡温度载磁矿物,推测为赤铁矿;另一部分样品在低外部磁场上迅速达到饱和,指示低矫顽力、高阻挡温度载磁矿物,推测为磁铁矿。许多样品的退磁数据较离散,但少数样品退磁曲线具有典型的双分量特征。柯坪水泥厂2号采样点和永安坝2号采样点的高温特征剩磁分量对应的古地磁极位置能与前人在同地区测的早二叠世与晚二叠世古磁极位置良好对应。考虑到所测样品均为热液白云石脉,这部分高温特征剩磁分量显然是热液活动引起化学重磁化的结果,说明热液白云岩改造与塔里木板块二叠纪强烈的岩浆活动有关系。

塔里木盆地肖尔布拉克剖面肖尔布拉克组下段微生物碳酸盐岩沉积特征

塔里木盆地阿克苏地区肖尔布拉克剖面肖尔布拉克组下段除底部条带状泥晶白云岩外,主要为微生物碳酸盐岩沉积,但目前关于这套微生物岩的研究很少。通过详细的野外露头观察及室内镜下薄片鉴定,对研究剖面肖尔布拉克组下段的微生物碳酸盐岩类型进行了识别、分类,并重构了其沉积环境。结果显示:肖尔布拉克组下段共发育六种岩相类型,其中包括四种微生物碳酸盐岩类型:粗纹层状凝块岩,具窗孔凝块岩,厚层状凝块岩和块状凝块岩。粗纹层状凝块岩和具窗孔凝块岩主要分布于潮间带;厚层状凝块岩和块状凝块岩主要沉积于中深—浅水局限潮下带。不同的微生物岩类型发育于不同的位置,主要受控于海平面变化、古地理位置及微生物群落等因素。

Discovery of the Jiawengmen Stromatolite Assemblage in the Southern Belt of Eastern Kunlun, NW China and Its Significance

This paper reports a newly discovered Late Mesoproterozoic-Early Neoproterozoic stromatolite assemblage, named here the ＂Jiawengmen stromatolite assemblage＂, represented by a Conophyton-Baicalia association in the Jiawenmen area in the southern belt of the Eastern Kunlun. This stromatolite assemblage is dominated by large-scale conical stromatolites and related elements, i.e., Conophyton garganicus var. inkeni, C. cf. ressoti Menchikov, Jacutophyton cf., Conicodomenia f., which commonly co-exist with elements of the group of Baicalia. This assemblage can be correlated with that of the middle Jixian-middle Qingbaikou System in North and Northwest China, but is different from that in South China. Correlation can also be made with that in the upper horizon of the Middle Riphean-lower horizon of the Upper Riphean in the South Ural Mountains and Siberia of Russia, in North Africa, and in the Alaskan Peninsula of North America. These facts suggest that the Jiawengmen stromatolite assemblage probably colonized during 1300-850 Ma ago. Accordingly, the stromatolite-bearing carbonate rocks are then proposed to correspond to the middle Jixian System-middle Qingbaikou System or the upper Middle Riphean-lower Upper Riphean. Our stromatolite data further suggest that a Precambrian microblock, named the Xialawen microblock here, occurred in the southern belt of Eastern Kunlun, the western part of the Maqên microblock. Similar stromatolite assemblages in the Maqên microblock and those blocks that occurred in North China, Siberia and North Africa point to similar paleogeographic and paleoenvironmental conditions. These microblock and blocks were probably located at low latitudes and on the continental margins of the Rodinian supercontinent, where warm epicontinental seas were favorable to widespread colonization of stromatolites during the Late Mesoproterozoic-Eady Neoproterozoic. However, these stromatolite assemblages are quite different from those of the South China block, which is suggestive of different paleogeographic contexts, and probably also of a different tectonic affinity.

塔里木盆地西部地区中奥陶统一间房组沉积演化及其控制因素

塔里木盆地中奥陶统一间房组沉积演化和沉积层序的发育特征及其控制因素目前还存在争议。选择塔里木盆地西部良里塔格山地区一间房组为研究对象,基于沉积相和高频米级沉积旋回类型的识别及其垂向叠置样式的分析,明确一间房组的沉积演化和沉积层序的发育特征及其控制因素。研究表明:(1)一间房组沉积时期,良里塔格山地区整体为一个碳酸盐岩缓坡沉积体系,主要发育10种岩相类型和2个沉积相带(潮缘—局限潮下带和开阔潮下带);(2)一间房组发育两种高频米级沉积旋回,分别为潮缘—局限潮下沉积旋回和开阔潮下沉积旋回;(3)识别出2个半三级层序,层序界面主要为岩性转换面,其中一个完整的三级层序包括海侵体系域和海退体系域两部分;(4)沉积演化与层序发育的主要控制因素包括古地理格局、沉积微地貌及不同级次的相对海平面变化。其中古地理格局控制着整体的沉积相带分布,沉积微地貌决定着特定岩相(如瓶筐石礁)发育的优势部位,不同级次相对海平面变化则制约着沉积相的时空演化。

Characteristics and Dolomitization of Upper Cambrian to Lower Ordovician Dolomite from Outcrop in Keping Uplift, Western Tarim Basin, Northwest China

Late Cambrian to Early Ordovician sedimentary rocks in the western Tarim Basin, Northwest China, are composed of shallow-marine platform carbonates. The Keping Uplift is located in the northwest region of this basin. On the basis of petrographic and geochemical features, four matrix replacement dolomites and one type of cement dolomite are identified. Matrix replacement dolomites include （1） micritic dolomites （MD1）; （2） fine-coarse euhedral floating dolomites （MD2）; （3） fine-coarse euhedral dolomites （MD3）; and （4） medium-very coarse anhedral mosaic dolomites （MD4）. Dolomite cement occurs in minor amounts as coarse saddle dolomite cement （CD1） that mostly fills vugs and fractures in the matrix dolomites. These matrix dolomites have δ18O values of ？9.7‰ to ？3.0‰ VPDB （Vienna Pee Dee Belemnite）; δ13C values of ？0.8‰ to 3.5‰ VPDB; 87Sr/86Sr ratios of 0.708516 to 0.709643; Sr concentrations of 50 to 257 ppm; Fe contents of 425 to 16878 ppm; and Mn contents of 28 to 144 ppm. Petrographic and geochemical data suggest that the matrix replacement dolomites were likely formed by normal and evaporative seawater in early stages prior to chemical compaction at shallow burial depths. Compared with matrix dolomites, dolomite cement yields lower δ18O values （？12.9‰ to ？9.1‰ VPDB）; slightly lower δ13C values （？1.6‰-0.6‰ VPDB）; higher 87Sr/86Sr ratios （0.709165-0.709764）; and high homogenization temperature （Th） values （98°C-225°C） and salinities （6 wt%-24 wt% NaCl equivalent）. Limited data from dolomite cement shows a low Sr concentration （58.6 ppm） and high Fe and Mn contents （1233 and 1250 ppm, respectively）. These data imply that the dolomite cement precipitated from higher temperature hydrothermal salinity fluids. These fluids could be related to widespread igneous activities in the Tarim Basin occurring during Permian time when the host dolostones were deeply buried. Faults likely acted as important conduits that channeled dolomitizing fluids from the underlying strata into the basal carbonates, leading to intense dolomitization. Therefore, dolomitization, in the Keping Uplift area is likely related to evaporated seawater via seepage reflux in addition to burial processes and hydrothermal fluids.

Environmental Redox Changes of the Ancient Sea in the Yangtze Area during the Ordo-Silurian Transition

Extensive organic-matter （OM） rich facies （black shales） occur in the Ordo-Silurian boundary successions in the Yangtze area, South China. To investigate the redox changes of the Yangtze Sea during the Ordo-Silurian transition, two OM sections （Wangjiawan in Yichang, Hubei Province, and Sanjiaguan in Zhangjiajie, Hunan Province） straddling the Ordo-Silurian boundary are studied. The measurements finished in this study include contents of the total organic carbon （TOC）, pyrite sulphur, and different species of Fe, including dithionite-extractable Fe （FED）, pyrite Fe （FeP）, HCl-extractable Fe （FeH）, and total Fe （FeT）, in black shales, as well as other redox proxies, such as the SIC ratio, the ratio between highly reactive Fe （FeHR = FeD ＋ FeP） and FeT, and the FeP/（FeP ＋ FeH） ratio, known as the degree of pyritization （DOP）. In the Wangjiawan section, the Middle Ashgill sediments have high FeHR/FeT ratios （0.20-0.77; avg. 0.45）, high DOP values （0.21-0.72; avg. 0.54）, and a relatively constant sulfur content independent of the organic carbon content. By the contrast, the mid-early Hirnantian deposits generally have low FeHR/FeT ratios （0.10-0.35; avg. 0.21）, low DOP values （0.11- 0.40; avg. 0.28）, and SIC values are clustering on the normal marine value （SIC = 0.36）. The late Hirnantian and early Rhuddanian deposits, similar to those of the Middle Ashgill deposits, are characterized by high FeHR/FeT ratios （0.32-0.49; avg. 0.41）, high DOP values （0.46-0.68; avg. 0.53） and fairly constant sulfur contents. These data suggest the occurrences of marine anoxia on the Yangtze Sea shelf during intervals of the Mid Ashgill, Late Hirnantian and Early Rhuddanian, and ventilated and oxygenated marine conditions during the mid-early Hirnantian time. The mid-early Hirnantian ventilated event was concomitant with the global glacial period, likely resulted from the glacio-eustatic sea-level fall and subsequent circulation of cold, dense oxygenated waters upon the shelf seabed. Accordingly, the abrupt change from oxygenated to anoxic marine waters from the late Hirnantian and the early Rhuddanian were resulted from the post-glacial rise of eustatic sea level. Combined with the data from Sanjiaguan section near the subemergent highland, salinity stratification of water columns are strongly evidenced by very low S/C ratios （ 0.00-0.08, avg. 0.02）, low FeHR/FeT ratios （0.10-0.37, avg. 0.27）, low DOP values （0.01-0.27, avg. 0.10）, and TOC contents （0.72%-4.27%, avg. 2.55%）. Under this circumstance, the anoxic water columns could have formed beneath the halocline, above which desalinized waters formed. In the Wangjiawan section, TOC contents are generally high （0.94- 9.32%, avg. 4.44%）, but low （0.35%-2.12%, avg. 1.29%） in the mid-early Hirnantian that is coincident with the oceanic oxic stage, together with relation of the organic contents to the stratal thickness, suggesting that the accumulation of the organic matter was mainly controlled by the oxygen levels of the water columns; on the other hand, productivity and depositional rate may also have played a role in the organic accumulation and preservation.

Types,Petrophysical Properties and Pore Evolution of Late Ediacaran Microbial Carbonates,Tarim Basin,NW China

The Upper Ediacaran microbial carbonates of the Tarim Basin are potential reservoir geobodies for future hydrocarbon exploration with rising interest in exploration for deeply-buried reserves.However,little knowledge has been acquired on the types of microbial carbonates that are present,the properties of the reservoir and the pore evolution,hampering predictions of high-quality reservoirs in these carbonates.Integrated with petrography and in-situ U-Pb dating geochronology analyses,this study aims to clarify the types of microbial carbonates present and to reconstruct the pore evolution processes of the potential reservoir rocks.The Upper Ediacaran microbial carbonates of the Tarim Basin can be divided into four types,based on their features in terms of different scales(macro-to micro-):microbial laminite,stromatolite,spongiomicrobialite and microbial-peloidal wackestone/mudstone.Petrophysical properties show that all these microbial carbonates have low porosity and very low permeability with poor connectivity.These carbonates were subject to long-term and complex diagenetic processes,mainly consisting of dissolution,compaction,pervasive dolomitization,cementation and fracturing.The most important reservoir spaces are contributed by vugs and dissolution-enlarged pores,which are likely to have been associated with the widespread uplift of the Aksu area in the terminal Ediacaran.In contrast,the cementation of the fine-to-medium crystalline dolomite greatly reduced the pre-existing pores.Pore types are closely related to different microbial fabrics,which played an important role in the pore evolution of the microbial carbonates.

川东北地区二叠纪晚期古海洋环境与有机质富集

【目的】二叠纪晚期是一个关键的地质历史时期,地球系统在构造、火山活动、气候、古海洋环境和生物群落等方面都发生了巨大变化。重建这一时期的古地理、古气候、古海洋环境有助于理解不同地质因素的相互作用机制,及其对该时期富有机质岩系及生物演化的重要影响。【方法】通过对四川盆地建峰剖面上二叠统深水序列的沉积学和地球化学(主微量元素含量、有机碳同位素、总有机碳含量)研究,明确了该时期的古地理环境,重建了海洋初级生产力和氧化还原环境的时空演化,探讨了多种因素对有机质富集的控制作用。【结果】建峰剖面的氧化还原环境依次经历了氧化—铁化—硫化—氧化四个阶段(Ⅰ~Ⅳ),初级生产力和陆源输入均在阶段Ⅱ_(2)和阶段Ⅲ较高,上升流在阶段Ⅱ_(2)、Ⅱ_(3)、Ⅲ和Ⅳ较为发育,火山活动仅在阶段IV较为频繁。因此,建峰剖面高初级生产力可能主要受陆源物质输入的影响,而上升流的影响次之,火山活动的贡献较小。【结论】结合西北乡剖面,推测该时期川东北地区硫化水团的形成受高生产力的控制,其上侵、下退则与海平面的波动有关;大隆组沉积期间盆地中部的初级生产力受上升流影响更大,而盆地边缘的初级生产力受陆源输入影响更大;初级生产力对大隆组的有机质富集起主导作用。

中上扬子地区震旦纪灯影组沉积期碳酸盐岩台地古地理格局及有利储集相带分布规律

【目的】中上扬子地区灯影组沉积期碳酸盐岩台地古地理格局及有利储集相带发育分布规律尚存争议,阻碍了灯影组油气勘探领域由绵阳—长宁拉张槽两侧向其他广大地区拓展。【方法】通过中上扬子地区灯影组30余条剖面(钻井)开展岩石类型划分及沉积相分析,【结果】进一步明确丘滩相以块状砂屑/鲕粒白云岩及柱状、穹窿状叠层石白云岩为特征,呈环带状分布在中上扬子台地(或内缓坡)边缘,向外过渡为中—外缓坡风暴岩或斜坡—盆地相区的滑塌角砾白云岩、泥质白云岩、硅质岩等。由于丘滩的阻挡作用,其内侧以低能的潮坪及潟湖沉积环境为主:其中潮坪相主要发育微生物纹层白云岩、穹窿—缓波状叠层石白云岩、凝块石白云岩夹砂屑白云岩、泥晶白云岩,纵向上常常形成米级—厘米级旋回;潟湖相主要由泥晶白云岩、含球粒/砂屑泥晶白云岩组成。相带时空分布表明灯影组在中上扬子地区总体呈现出进积的沉积趋势,内部包含2.5个层序,即三次进积(变浅)和两次退积(变深)旋回。【结论】由于灯影组在台地边缘—斜坡区域普遍遭到剥蚀,以及灯影组沉积期上扬子台地范围远超目前的板块边界,灯影组上部灯四段很少发现高能丘滩相带。在此背景下,局部地区(如松林—岩孔地区、中扬子台地周缘)灯影组中下部灯一段—灯二段(或蛤蟆井段、石板滩段)发育的丘滩相,是重要的有利储集相带。此外,处于海退体系域的灯二段中下部和灯四段上部(或相当层位)在中上扬子台地(或内缓坡)内部广泛发育微生物白云岩,也为储层发育有利相带。在中上扬子台地微生物白云岩广泛分布的背景下,进一步分析优质储层发育机理与分布规律才是找到油气接替区的关键。

弗拉—法门转折期气候—海洋环境变化及生物危机成因探讨

【目的】晚泥盆世弗拉—法门(Frasnian-Famennian,F-F)转折期是地质历史的一个关键时期,海洋生态系统发生了重大变化并造成了低纬度地区浅海底栖生物的大量灭绝。关于该时期的古气候、古海洋环境变化以及生物灭绝的成因机制研究目前已取得一定的进展,但仍然存在争议且各环境要素之间的相互作用尚不明确。【方法】在系统梳理F-F转折期古气候、古海洋环境变化研究现状的基础上,结合华南的研究实例探讨了F-F转折期的古海洋缺氧模式。【结果与结论】F-F转折期,气候在变冷的趋势下出现多次快速的暖—冷交替;缺氧范围和程度各地表现不一,但主要集中在低纬度浅海地区上Kellwasser层附近。因此,F-F生物危机并不是单一因素造成的。频繁、短暂的火山活动引起大陆风化作用增强,一方面导致了气候快速的暖—冷交替,另一方面促进了陆地向海洋中输入营养物质造成浅海的富营养化和缺氧。各个环境因素相互制约/影响对低纬度浅海地区生命造成极大的环境压力,最终导致了F-F生物危机。

Geochemical changes across the Ordovician-Silurian transition on the Yangtze Platform,South China

The trace element and rare earth element(REE) variations across the Ordovician-Silurian succession are presented from two outcrop sections on the Yangtze Platform:the Nanbazi section,Guizhou Province,deposited in a shallow platform interior setting,and the Wangjiawan section,Hubei Province,deposited in a deeper basinal environment.Geochemical analysis of closely spaced samples through three intervals,the Wufeng,Guanyinqiao and Longmaxi,revealed vast palaeoceanographic changes.Some geochemical proxies,including Th/U,Ni/Co,V/Cr,and V/(V+Ni) ratios,together with sedimentary facies and biotic data,indicate that an anoxic condition on the most of the Yangtze Platform during the Wufeng and Longmaxi intervals,but an oxic condition during the Guanyinqiao time.The shift of the anoxic to the oxic environment during the Guanyinqiao time coincided with a global sea-level lowstand,in parallel with the global glaciation.The Longmaxian anoxic environment was a result of a global sea-level rise,which may be synchronized with a mainly catastrophic event in the latest Ordovician.Although the two sections generally show similar variation patterns of trace and REE concentrations and some element ratios,a minor difference occurs between the Wangjiawan and Nanbazi sections,likely reflecting a difference in depositional setting during the accumulation.Such an oceanic oxygen-level variation may add a useful constraint to the current arguments on the cause and consequence of the latest Ordovician mass extinction.

In situ stable isotopic constraints on dolomitizing fluids for the hydrothermally-originated saddle dolomites at Keping, Tarim Basin

Detailed characterization of diagenetic fluid behaviour and evolution is commonly hindered by shortage of appropriate and economic methods to carry out in situ analysis in the tiny diagenetic mineral crystals. Using micromill sampling method, this study presents in situ stable isotopic data for the growth zones of saddle dolomite crystals in the hydrothermally-altered dolostones of Upper Cambrian (Furongian) at Keping, Tarim Basin, NW China. These data show minor and large variations in δ13C ( 0.7‰ to 1.5‰ VPDB) and δ18O ( 8.3‰ to 13.8‰ VPDB), respectively, from the cores to outer rims of the dolomite crystals and suggest that saddle dolomites precipitated from dolomitizing fluids with similar carbon sources but oscillatory temperatures during different formation stages. This scenario is confirmed by microthermometry of fluid inclusions within the growth zones of dolomite crystals. This study indicates that in situ isotope analysis could provide more detailed information related to the source and pathway of dolomitizing fluids, facilitating better characterization of dolomitizing fluids and processes.

End-Guadalupian mass extinction and negative carbon isotope excursion at Xiaojiaba,Guangyuan,Sichuan

The end-Paleozoic biotic crisis is characterized by two-phase mass extinctions;the first strike,resulting in a large decline of sessile benthos in shallow marine environments,occurred at the end-Guadalupian time.In order to explore the mechanism of organisms' demise,detailed analyses of depositional facies,fossil record,and carbonate carbon isotopic variations were carried out on a Maokou-Wujiaping boundary succession in northwestern Sichuan,SW China.Our data reveal a negative carbon isotopic excursion across the boundary;the gradual excursion with relatively low amplitude(2.15‰) favors a long-term influx of isotopically light 12 C sourced by the Emeishan basalt trap,rather than by rapid releasing of gas hydrate.The temporal coincidence of the beginning of accelerated negative carbon isotopic excursion with onsets of sea-level fall and massive biotic demise suggests a cause-effect link between them.Intensive volcanic activity of the Emeishan trap and sea-level fall could have resulted in detrimental environmental stresses and habitat loss for organisms,particularly for those benthic dwellers,leading to their subsequent massive extinction.

Early dolomitisation of the Lower-Middle Ordovician cyclic carbonates in northern Tarim Basin, NW China

High-frequency metre-scale cycles are present within the Lower-Middle Ordovician carbonate successions in northern Tarim Basin, NW China. These metre-scale cycles were variably dolomitised from top to bottom. Three types of replacive dolomites were recognised, including dololaminite(very finely to finely crystalline, planar-s to nonplanar-a dolomite;type-1), patterned dolomite(finely crystalline, planar-s dolomite; type-2), and mottled dolomite(finely to medium crystalline,nonplanar-a(s) dolomite; type-3). Petrographic evidence indicate these dolomites were primarily deposited in supratidal to restricted subtidal environments, and formed in near-surface to shallow burial realms. Geochemically, all types of dolomites have similar δ13C and 87Sr/86 Sr ratios comparable to calcite precipitated in equilibrium with the Early-Middle Ordovician seawater. These geochemical attributes indicate that these dolomites were genetically associated and likely formed from connate seawater-derived brines. Of these, type-1 dolomite has δ18O values(.4.97‰ to.4.04‰ VPDB) slightly higher than those of normal seawater dolomite of the Early-Middle Ordovician age. Considering the absence of associated evaporites within type-1 dolomite, its parental fluids were likely represented by slightly evaporated(i.e., mesosaline to penesaline) seawater with salinity below that of gypsum precipitation. More depleted δ18O values(.7.74‰ to.5.20‰ VPDB) of type-2 dolomite and its stratigraphic position below type-1 dolomite indicate the generation of this dolomite from mesosaline to penesaline brines at higher temperatures in near-surface to shallow burial domains. Type-3 dolomite yields the most depleted δ18O values(–9.30‰to –7.28‰ VPDB), pointing to that it was most likely formed from coeval seawater-derived brines at highest temperatures in a shallow burial setting. There is a downward decreasing trend in δ18O values from type-1 through type-2 to type-3 dolomites, and in abundance of dolomites, indicating that the dolomitising fluids probably migrated downward from above and persisted into shallow burial conditions.