Vestrogothia spinata(Phosphatocopina,Crustacea),Fossils of Orsten-type Preservation from the Upper Cambrian of Western Hunan,South China

Fossils of Orsten-type preservation represented by the crustacean Skaracarida and Phosphatocopida were found in western Hunan, South China in 2005, including the important phosphatocopid species Vestrogothia spinata based on exquisitely preserved soft-bodied specimens that allow the first growth stage to be reestablished. The taxonomy of Vestrogothia spinata is revised employing the character of a two-divided limb stem of the mandible. A new foundation for the phylogeny of the Phosphatocopina using mandible characters related to crustacean appendages is postulated. Vestrogothia spinata has only previously been found from Sweden.

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.

Porosity distribution in cyclic dolomites of the Lower Qiulitag Group(Upper Cambrian)in northwestern Tarim Basin,China

Increasing interests in hydrocarbon resources at depths have drawn greater attentions to the deeply-buried carbonate reservoirs in the Tarim Basin in China.In this study,the cyclic dolomite rocks of Upper Cambrian Lower Qiulitag Group from four outcrop sections in northwestern Tarim Basin were selected to investigate and evaluate the petrophysical properties in relation to depositional facies and cyclicity.The Lower Qiulitag Group includes ten lithofacies,which were deposited in intermediate to shallow subtidal,restricted shallow subtidal,intertidal,and supratidal environments on a carbonate ramp system.These lithofacies are vertically stacked into repeated shallowing-upward,meter-scale cycles which are further grouped into six third-order depositional sequences(Sq1 to Sq6).There are variable types of pore spaces in the Lower Qiulitag Group dolomite rocks,including interparticle,intraparticle,and fenestral pores of primary origin,inter crystal,and vuggy pores of late diagenetic modification.The porosity in the dolomites is generally facies-selective as that the microbially-originated thrombolites and stromatolites generally yield a relatively high porosity.In contrast,the high-energy ooidal grainstones generally have very low porosity.In this case,the microbialite-based peritidal cycles and peritidal cycle-dominated highstand(or regressive)successions have relatively high volumes of pore spaces,although highly fluctuating(or vertical inhomogeneous).Accordingly,the grainstone-based subtidal cycles and subtidal cycle-dominated transgressive successions generally yield extremely low porosity.This scenario indicates that porosity development and preservation in the thick dolomite successions are primarily controlled by depositional facies which were influenced by sea-level fluctuations of different orders and later diagenetic overprinting.

Conodonts from the Upper Cambrian Sesong and Hwajeol Formations in the Sabuk Area, South Korea

This paper presents a conodont biostratigraphic study on the Upper CambrianSesong and Hwajeol Formations in the Sabuk area, Korea. Two samples near the base of the SesongSlate at the Pukil section contain lower Upper Cambrian conodonts, comprising Furnishina furnishi,F. kranzae, F. pernica?, F. triangulata, Hertzina elongata, Laiwugnathus doidyxus?, Phakeloduselongatus, Muellerodus? obliquus, Westergaardodina matsushitai and W. moessebergensis. This is thefirst conodont record of the Upper Cambrian formations recovered yet in the northern limb of thePaekunsan syncline. The faunal assemblage is correlatable with the lower Upper Cambrian W.matsushitai Zone of North China. Four local biozones are recognized in the Hwajeol Formation, i.e.the Proconodontus, Eoconodontus notchpeakensis, cambrooistodus minutus and Cordylodus proavus Zonesin ascending order. This zonal scheme is essentially equivalent to that of the southern limb of thePaekunsan syncline and they are correlatable with zones in other parts of the world, including NorthAmerica, Australia, Iran and China.

Sequence stratigraphy and lithofacies paleogeography of the Middle–Upper Cambrian Xixiangchi Group in the Sichuan Basin and its adjacent area, SW China

Based on isotopic, lithologic and electrical data and logging cycle analysis technique, stratigraphic sequences in the Middle-Upper Cambrian Xixiangchi Group in the Sichuan Basin and its adjacent area are divided, and its sedimentary characteristics and evolution are analyzed. The Xixiangchi Group can be divided into 5 third-order sequences(Sql-Sq5), of which sequences Ⅰ to Ⅲ(Sql-Sq3) are relatively complete, sequences Ⅳ and Ⅴ are denuded in the Late Cambrian because of the Dian-Chuan paleo-uplifts.Third-order sequences of the Xixiangchi Group in this area have the characteristics of thin in the west and thick in the east, showing that the Caledonian paleo-uplift is a synsedimentary paleo-uplift and the paleogeomorphology in the platform is a gentle slope. Sequence Ⅰdevelops high stand systems tract and transgressive systems tract. The other third-order sequences are dominated by highstand systems tracts, and the transgressive systems tracts last shortly in time and are limited in area. The basic features of evaporative-restricted platform of gentle slope type developed continuously in the sedimentary period of the Xixiangchi Group, its sedimentary environment of "high in west and low in east" and the change of micro paleogeomorphology in the platform control the continuous development of sedimentary facies. Open platform is developed only in sequence Ⅰ and sequence Ⅱ, and the inner beach of the platform and the edge beach of the platform are mostly developed in sequence Ⅱ and sequence Ⅲ. It indicates that there are two platform margin zones in the study area,a relatively stable, large-scale platform marginal zone in NE Guizhou-Western Hunan and Hubei, and a moving and small-scale platform marginal zone in North Chongqing-Western Hubei.

From Basin Black Shales to Platform Carbonate Rocks:A Study on Sequence Stratigraphy for the Lower Cambrian of the Upper-Yangtze Region in South China

In the Upper-Yangtze region, especially in Guizhou Province and its adjacent areas, the Lower Cambrian is well developed and is marked by a succession from black shales of the basin facies to carbonate rocks of the platform facies. The drowning event of the platform occurring at the turn from Sinian to Cambrian resulted in a set of black shales, i.e. the Niutitang Formation, which makes up the bottom part of the Lower Cambrian. With the shoaling of the sedimentary environment, a set of carbonate rocks, i.e. the Qingxudong Formation, was formed in the top part of the Lower Cambrian. Thus, the Lower Cambrian in the study area makes up one second-order sequence that can be further subdivided into five third-order sequences, and forms a regularly cyclic succession of transgression-regression. There is a regularly vertical stacking pattern for the third-order sequences in the second-order sequence. From bottom to top, the succession of the ＂CS （condensed section） ＋HST （high-stand system tract）＂ of the third-order sequences is changed into the succession of the ＂TST （transgressive system tract）＋CS＋HST＂. Correspondingly, the drowning-type sequence boundary is changed into the exposure-type one. Therefore, both the second-order and the third-order sequences have similar sedimentary-facies architectures. A concomitant with these temporal changes, the Lower Cambrian with a thickness of 1000 m that contains five third-order sequences is changed into a condensed succession that cannot identify third-order sequences toward the southeast with the deepening of the sedimentary environment. According to the elementary features of the third-order sequences, i.e. the regularity o sedimentary-facies successions in space and the synchronism of sedimentary-environment changes in time, the detailed division of the third-order sequences at main logged sections in different paleogeographical background becomes the basis to establish the sequence-stratigraphic framework that can demonstrate two types of facies-changing surface and two types of diachronism in the stratigraphic records. This sequence-stratigraphic framework shows a growing process of the carbonate platform from the base of the condensed succession formed by black shales of basin facies. Resulting from the rapid transgression at the turn from Sinian to Cambrian the ecological space became open, which formed the antecedent condition of paleogeographical setting for ＂the Cambrian Biological Explosion＂. Ultimately, the genetic relationship between the depositional events and the biological-diversity events is very complex and there remain lots of problems that need further research in the future.

Controls on the organic carbon content of the lower Cambrian black shale in the southeastern margin of Upper Yangtze

Control of various factors, including mineral components, primary productivity and redox level, on the total organic carbon(TOC) in the lower Cambrian black shale from southeastern margin of Upper Yangtze(Taozichong, Longbizui and Yanbei areas) is discussed in detail in this article. Mineral components in the study strata are dominated by quartz and clay minerals. Quartz in the Niutitang Formation is mainly of biogenic origin, and the content is in positive correlation with TOC, while the content of clay minerals is negatively correlated with TOC. Primary productivity, represented by the content of Mobio(biogenic molybdenum), Babio(biogenic barium) and phosphorus, is positively correlated with TOC. The main alkanes in studied samples are nCC, and odd–even priority values are closed to 1(0.73–1.13), which suggest the organic matter source was marine plankton. Element content ratios of U/Th and Ni/Co and compound ratio Pr/Ph indicate dysoxic–anoxic bottom water, with weak positive relative with TOC. In total, three main points can be drawn to explain the relationship between data and the factors affecting organic accumulation:(1) quartz-rich and clay-mineral-poor deep shelf–slope–basin environment was favorable for living organisms;(2) high productivity provided the material foundation for organic generation;(3) the redox conditions impact slightly on the content of organic matter under high productivity and dysoxic–anoxic condition.

Subdivision and Correlation of Middle and Upper Cambria in North China in Light of Sequence Stratigraphy

Correlation of Middle and Late Cambrian cyclic carbonates from the widely separated Zhangxia section, Qingxing section, Xishan section and Zhaogezhuang section has defined five major third-order depositional sequences. Correlation of biostratigraphy is used to establish the equivalency of stratigraphic sequences between individual sections. Even though the individ- ual sections are composed of different lithofacies and meter-scale cycles that reflect the varied environmental settings in the localities, the overall upward shallowing character of individ- ual sequences is evident. The sequences, named on their relation to the regional stages in range, are in ascending order: Zhangxia Sequence, Gushan Sequence, Guchang Sequence, Changshan Sequence and Fengshan Sequence, of which the upper four sequences were correlated with those defined by Osleger and Read in the United States, with the aid of biostratigraphic correlation. The Middle/Upper Cambrian boundary is tentatively suggested to be between the Guchang Sequence and the Changshan Sequence in North China. The optimization of the existing litho-and regional chronostratigraphic boundaries is discussed based on the sequence boundaries.

Conodont Biostratigraphy of the Middle Cambrian through Lowermost Ordovician in Hunan, South China

Since 1985, samples with a total weight of more than 14,000 kg, mainly from three key sections in western and northwestern Hunan, South China, have been processed for conodonts. In strata older than the late Late Cambrian paraconodonts have proved useful for stratigraphic subdivision and correlation. Thirteen conodont zones are proposed in the Middle Cambrian through lowermost Ordovician. The correlation between these zones and those of North China, western U. S.A., western Newfoundland, Canada, and Iran is discussed. In ascending order, these 13 zones are as follows: The Gapparodus bisulcatus-Westergaardodina brevidens Zone, Shandongodus priscus-Hunanognathus tricuspidatus Zone, Westergaardodina quadrata Zone, Westergaardodina matsushitai-W. grandidens Zone, Westergaardodina lui-W. am Zone, Westergaardodina cf. calix-Prooneotodus rotundatus Zone, Proconodontus tenuiserratus Zone, Proconodontus Zone, Eoconodontus Zone, Cordylodus proavus Zone, Cordylodus intermedius Zone, Cordylodus lindstromi Zone, and Cordylodus angulatus Zone (lower part). The Westergaardodina lui-W. ani and Westergaardodina cf. calix-Prooneotodus rotundatus Zones replace the Westergaardodina proligula and Westergaardodina cf. behrae-Prooneotodus rotundatus Zones, respectively, in the lowermost Upper Cambrian. Two new species (Westergaardodina Iui and Westergaardodina ani) and one conditionally identified species (Westergaardodina cf. calix) are described.

宿州符离集打鼓山白云岩矿地质特征及成因类型

安徽宿州市符离集地区赋存多个白云岩矿床,产于中上寒武系,风化面常呈黑褐色,新鲜面为灰、灰白色带肉红色,可与其它基岩区别。本文以该区打鼓山白云岩矿床为例,三个矿层对应矿石类型为鲕状白云岩、结晶质白云岩和泥质白云岩三种。白云石占95%以上,MgO平均含量20.72%,为深海相白云岩,物质来源于南部淮阳地盾长期剥蚀的Ca、Mg等以胶体方式搬运沉积而成。

Possible impact event in the Late Cambrian

The repeated investigation of the Batyrbay section has been done by the authors. Today we see in- creasing of interest to revising of volumes and boundaries of Stages and Series in the Cambrian and Ordovician, as well as to the events of this time. In this work the description of iron spherules and particles, found in the deep-water limestone layer of the conodont Cordylodus prirnitivus Zone in the middle Upper Cambrian of the Batyrbay section, Malyi Karatau of South Kazakhstan, and formed during the time of global sea-level changes known as a world-wide Lange Ranch Eustatic event, have been done. From our point of view, finds of iron spherules and particles may evidence about new unknown yet Event of cosmic origin, i.e. falling of a meteorite on Earth in the middle Late Cambrian.

塔里木盆地富满油田F_(Ⅰ)16断裂发育特征及其对油气分布的影响

利用三维地震、多层相干分析、振幅变化率、储层岩心分析及油气分布特征等资料,对塔里木盆地富满油田F_(Ⅰ)16断裂的空间展布特征、活动强度及期次进行了分析,并探讨了断裂的控储控藏作用。研究结果表明:①富满油田F_(Ⅰ)16断裂具有平面分段、垂向分层的特征。平面上由北往南可划分为3段,北段沿NE 10°展布,发育似马尾状构造;中段沿NE 20°展布,处于断裂走向转换部位,主要发育左阶斜列;南段沿NE 30°展布,具有强线性延伸特征,以右阶斜列为主;垂向上可划分为上寒武统底面以下的深部构造变形层和上寒武统底—奥陶系一间房组顶面浅部构造变形层,深层断裂活动较弱,分支发育较少,仅局部发育花状构造,浅层断裂活动强,花状构造普遍发育,且垂向上存在“多花叠置”及破碎带内地层同沉积的现象。②F_(Ⅰ)16断裂在富满油田的活动主要发生于加里东早期和加里东中期,加里东中期为主要活动期,强度较大且具有多幕次特征;活动强度在平面上表现为“北强南弱”,在垂向上表现出“浅层上拱、深层下降”的特征,浅层断裂活动更强烈。③研究区储层以断控缝洞型为主,受断裂破碎作用控制,平面上沿F_(Ⅰ)16断裂带呈条带状分布,垂向上具有深浅分层叠置的特征;受中寒武统膏盐岩的分布影响,F_(Ⅰ)16断裂带的油气分布具有“南油北气”的特征。

鄂西—渝东地区早寒武世克拉通内裂陷演化及对古地理格局的控制

克拉通内裂陷对四川盆地及周缘海相碳酸盐岩油气勘探有利区带具有重要的控制作用,大量露头证据揭示鄂西—渝东地区早寒武世克拉通内裂陷的存在。研究充分利用野外露头与钻井资料,结合相关构造背景与地球动力学成因,在前人研究基础上明确提出早寒武世鄂西渝东克拉通内裂陷发育的综合识别标志,并刻画了裂陷的沉积充填演化过程以及对古地理格局的控制。结果表明:1)裂陷呈南宽北窄、东缓西陡的不对称箕状形态;2)受裂陷西缘同沉积断裂控制,裂陷内地层沉积厚度与深—浅水沉积、斜坡相沉积均具明显分异性,分异程度与断裂活动性呈正相关关系;3)在早寒武世裂陷以补偿沉积充填为主,其内部由初期陆棚沉积向末期碳酸盐岩台地过渡,根据充填特性可将裂陷演化划分为裂陷(水井沱组沉积期)、坳陷(沧浪铺组沉积期)、填平补齐(石龙洞组沉积期)3个阶段;4)裂陷的“开启—闭合”是“罗迪尼亚大陆裂解—冈瓦纳大陆聚合”在中、上扬子之间的响应,该构造背景下的古地貌分异、同沉积断裂活动及海平面变化共同控制其古地理格局。鄂西—渝东地区早寒武世克拉通内裂陷的提出不仅为四川盆地克拉通内裂陷作用构造—沉积格局提供了新的认识,也对中上扬子地区油气勘探具有重要的指导意义。

鄂西宜昌地区寒武系页岩过剩钡成因及其对有机质富集的指示

通过对鄂西宜昌地区鄂阳页1井(EYY1)寒武系底部岩家河组和水井沱组下部的总有机碳(TOC)、总无机碳(TIC)、微量元素(Ba、U、Th)和稀土元素分析,并结合前人对中上扬子地区7个位于不同沉积环境区寒武系剖面的研究结果和已有数据报道,讨论了寒武系水井沱组富有机质页岩中过剩钡(Ba过剩)的成因、来源及其对海洋古生产力水平的指示。结果表明,中上扬子地区寒武系富有机质页岩普遍富集Ba过剩,而且从浅水内陆棚到深水外陆棚——大陆斜坡,再到热水沉积区,寒武系页岩的Ba过剩富集程度趋于增加,反映了与古陆物源区的距离远近以及古地理背景对Ba过剩富集的控制。寒武系富有机质页岩中所富集的Ba过剩有热液成因和生物成因两种来源,Ba过剩-Eu异常图版可用于推测其主要来源:黔北—湘西北地区所处的热水沉积区内Ba过剩含量极高(>10000μg/g),具有显著的正Eu异常,且Ba过剩与Eu异常值具有正相关性,指示Ba过剩主要为热液成因,其含量变化反映了热液活动的强度;鄂西地区和黔南地区Ba过剩含量相对较高(5000~10000μg/g),四川盆地西部Ba过剩含量相对较低(<1000μg/g),而且都不具备正Eu异常,指示Ba过剩为生物成因,其含量变化与古生产力水平有关。与中上扬子其他地区寒武系页岩的TOC含量、U/Th值及Ba过剩富集程度的综合对比表明,宜昌地区较高的古生产力水平和强还原条件为有机质的来源和保存提供了物质基础和有利条件,共同决定了寒武系水井沱组页岩有机质的富集。

破译结核中的沉积环境和同沉积构造活动信息——以黔湘边界地区黑色岩系为例

黑色岩系赋存油气、煤、页岩气等能源资源和铁、锰、磷、钡等多种固体资源,沉积成藏和沉积成矿条件的恢复是资源预测的基石,古沉积环境和同沉积构造的恢复因而受到额外重视。在黔湘边界地区的黑色岩系中有大量结核,而结核对原始记录有封存特性,因此,在对野外露头剖面观察和室内显微分析的基础上,通过沉积作用—构造作用的一体化分析,发现不同层位的结核在矿物成分和结构类型上差别很大:(1)上震旦统—寒武系纽芬兰统留茶坡组(Z-C l)的结核是重晶石、钙质、磷质和硅质等多成分的集合体,砾屑或砂屑结构;两层结核的胶结温度不同,直接覆盖重晶石矿的结核层温度较高;(2)寒武系第二统杷榔组(-C_2p)的结核以黄铁矿为主要成分,发育三期生长构造:核心层呈草莓状结构,外面两层呈肾状结构;(3)寒武系第二统乌训组(-C_2w)结核的矿物成分为钙质和黄铁矿,并以钙质为主,黄铁矿呈星点状散布其中,有四期生长构造,第一和第三期黄铁矿含量较高。从不同层位的结核中分别破译出不同的古沉积环境条件和同沉积构造的活动信息:(1)留茶坡组含重晶石结核属低温热液成因,还原环境局限在断裂带附近;(2)杷榔组黄铁矿结核形成于氧化还原界面附近,环境具备高浓度活性铁的介质条件;(3)乌训组钙质—黄铁矿结核形成于低浓度铁的周期性还原环境,推测为基底的幕式陷落引发频繁的相对海平面变化,从而导致水介质氧化还原性的频繁变化。因此,结核研究有助于重建黑色岩系的介质条件,对资源预测有重要意义。

塔里木盆地中央隆起区上寒武统—下奥陶统白云岩储层中两类白云石充填物:特征与成因

在塔里木盆地中央隆起区,下古生界白云岩储层中的白云石充填物从形态上可分为菱形和鞍状两类,它们具有不同的岩石学特征,菱形白云石内部结构简单均一,而鞍状白云石内部结构比较复杂,由内核、环带和外缘三部分组成。地球化学组成分析表明,菱形白云石可能形成于成岩作用的早期,成岩物质可能来源于改造的海水,其沉淀是一个相对稳定的过程,后期仅少量受到富铁流体的改造。而鞍状白云石的形成则可能与断裂诱发的深部热流体活动密切相关,其物质可能来源于深部热液在运移过程中对围岩白云石的溶解,鞍状白云石内部复杂的结构可能反映了断裂的幕式活动。因此,两类白云石充填物反映了不同的成岩流体环境,特别是鞍状白云石很可能是热液溶蚀作用发生的重要标志。在白云岩储层研究中,今后需要进一步深入详细分析充填物的产状、充填作用发生的时间和机制,对于研究分析白云岩储层形成机制和储层预测都有重要意义。

四川盆地及周缘地区中上寒武统洗象池群层序地层与岩相古地理演化特征

以同位素组成地球化学资料为基础,结合岩性与电性特征,利用测井旋回分析技术,对四川盆地及周缘地区中上寒武统洗象池群进行地层层序划分并分析沉积特征与演化规律。洗象池群可以划分为Sq1—Sq5共5个三级层序,其中层序Sq1—Sq3保存相对较完整,层序Sq4与Sq5在寒武纪末期大面积剥蚀,展现出滇—川古隆起的特征。各三级层序均呈西薄东厚,显示出加里东古隆起是同沉积古隆起且台内古地貌为缓坡特征。层序Sq1同时发育高位体系域和海侵体系域,其他三级层序主要发育高位体系域,海侵体系域发育短暂且局限。洗象池群沉积期在台内以缓坡型蒸发—局限台地的基本面貌发育,西高东低的沉积环境与台内微古地貌变化控制着沉积相的持续发育,其中开阔台地只发育在层序Sq1和Sq2,台内滩与台缘滩在层序Sq2和Sq3最发育。洗象池群在四川盆地周缘地区存在相对稳定且规模较大的黔东北—湘鄂西台缘带和具有迁移性且规模较小的渝北—鄂西台缘带。

四川盆地南川地区娄山关群碳酸盐岩成岩作用研究

四川盆地南川地区中—上寒武统娄山关群储层主要为台地边缘滩沉积,以白云岩为主。主要成岩作用类型有:白云岩化、溶蚀、破裂、胶结充填及压实压溶作用,对储层形成有重要贡献的成岩作用是白云岩化作用、溶蚀作用及破裂作用。根据白云石标型特征、有序度、碳氧同位素的分析,认为娄山关群白云岩成因有3种:准同生、混合水以及埋藏白云岩化作用,以准同生白云岩化作用为主。储层发育3期溶蚀作用:大气淡水溶蚀、埋藏溶蚀和表生溶蚀,以表生溶蚀作用为主。白云岩化作用和表生溶蚀作用是形成优质储层的关键,而胶结、充填作用破坏了大量的原生和次生孔隙,不利于储层的发育。

上扬子区中、上寒武统的层序地层划分和层序地层格架的建立

上扬子区特别是贵州及邻区的中、上寒武统发育完整,是一套从少量灰岩到大套白云岩所组成的地层序列;以娄山关群白云岩为代表的自北西而南东的进积作用穿时,代表了与二级海退作用相对应的碳酸盐台地进积作用的基本特征,从而形成一个复杂而有序的层序地层格架。在研究区域,特别是浅水台地背景之中,从厚度较小的含化石的陡坡寺组到厚度近千米的贫乏化石的娄山关群白云岩,给层序地层研究带来了许多困难;而在较深水背景之中的中、上寒武统,沉积相序列的有序性正好弥补浅水背景中的不足。中、上寒武统组成一个二级层序,其中可以进一步划分为七个三级层序,形成一个有序的海侵—海退旋回序列。伴随着上述时间变化的同时,从北西向南东,古地理背景由浅变深,中、上寒武统由大套白云岩地层相变为泥页岩夹灰岩地层。在不同古地理背景的典型剖面的层序划分的基础上,根据三级层序的两大属性———“空间上相序的有序性和时间上环境变化的同步性”,可以建立研究区域能反映出地层记录中“两种相变面和两种穿时性”的层序地层格架。研究区域的中、上寒武统层序地层格架的建立表明:虽然地层的沉积记录较为复杂,但是建立在以“沉积物的时间演化序列与沉积相的空间展布形式所代表的规律性”为基础的层序地层研究,可以从复杂的地层记录之中寻找出更多的规律性。

湘西中上寒武统界线层型侯选剖面沉积特征

湘西中、上寒武统花桥组和车夫组主要由斜坡相和盆地边缘相的碳酸盐岩、泥质碳酸盐岩及钙质泥岩组成。重力流沉积十分发育，其中花垣排碧剖面以碎屑流砾屑灰岩和近源浊积岩为主；永顺王村剖面以浊流沉积为主；桃源瓦尔岗剖面则以远源浊积岩为主。成岩作用主要为海底埋藏胶结作用、新生变形作用和白云石化作用。排碧剖面大多数岩样酸不溶物重量含量在１０％以下，王村和瓦尔岗剖面大多数岩样在１０％以上。全岩δ１８Ｏ值介于－１１．２‰至－５．６‰（ＰＤＢ）之间；δ１３Ｃ值为－１．９‰至３．４‰（ＰＤＢ）。从Ｌｉｎｇｕａｇｎｏｓｔｕｓｒｅｃｏｎｄｉｔｕｓ带开始，δ１３Ｃ有较大的正漂移，漂移值达３‰（ＰＤＢ）以上。沉积花桥组和车夫组时，湘西过渡区经历了４次海平面升降事件，台缘斜坡逐渐变陡，由缓坡型逐渐发展为镶边型和沟槽型。在湘西的诸多剖面中以王村剖面最适合于作为中、上寒武统界线层型候选剖面，界线选择在花桥组顶部。