Aragonite depositional facies in a Late Ordovician calcite sea,Eastern Laurentia

The Black River (Upper Ordovician – Sandbian) and Trenton (Upper Ordovician – Katian) groups are traditionally interpreted as a deepening-upward succession deposited in a progressively subsiding Appalachian Basin margin that contained warm-water, marine, photozoan deposits that pass upward into cool-water, marine, heterozoan carbonates. This succession is customarily interpreted to reflect an incursion of cold, high-latitude ocean waters into the area. This view is herein confirmed for coeval carbonates in the northern part of the basin, particularly the St. Lawrence Platform. They are now well explained in this study on the basis of recent studies of cool-water carbonates and calcite–aragonite seas. Overall the succession is one of Sandbian photozoan ramp deposits succeeded by Katian heterozoan ramp carbonates that changed back to photozoan ramp deposits prior to the Hirnantian glaciation. The current interpretation, that deposition took place throughout a calcite sea time, seems at odds with this series of strata. Instead it is herein proposed that deposition took place during an aragonite sea time wherein calcite sea-like sediments accumulated under cold ocean-water temperatures. Such an interpretation is supported by recent experimental data that supports the importance of seawater temperature on CaCO₃ polymorph precipitation. If correct, this means that some of the evidence for calcite sea deposition through time brought about by global tectonics, should be re-evaluated to make sure it was not simply cool-water carbonate production.     

The Rio Sassito sedimentary succession (Ordovician): a pinpoint in the geodynamic evolution of the Argentine Precordillera

In the Precordillera of western Argentina, an isolated outcrop of Llandeilian siliciclastics and Caradocian limestones (Rio Sassito succession) reveals a complex interplay between the tectonic and the sedimentary history of the Precordillera during Middle and Late Ordovician times. The succession is composed of a lower siliciclastic interval and an upper carbonate interval and is bounded below and above by erosional unconformities. Dating of these unconformities, which in many places merged to form one single surface, demonstrates that the most important erosional event took place prior to the deposition of the Rio Sassito succession. This erosional event is correlated to extensional tectonics during continental breakup and the separation of the Precordillera from Laurentia. Block faulting with the formation of horst and graben structures provided the topography for the establishment of a pelagic carbonate platform during the Caradoc. In our view, there are no indications that these phenomena are related to the accretion of the Precordillera to Gondwana or to the formation of an Ordovician supercontinent. The carbonate sediments are typical of temperate-water settings, characterized by the absence of ooids, oncoids, and algae, and by the presence of abundant abraded bioclasts, intraclasts, and peloids. The inference of a temperate-water environment does not, as previously supposed, indicate the accretion of the Precordillera to Gondwana, but is more likely related to global cooling prior to the Ashgillian glaciation.     

Storm-influenced siliciclastic and carbonate ramp deposits, the Lower Ordovician Dumugol Formation, South Korea

The Dumugol Formation (Lower Ordovician) in the southern part of the Baegunsan syncline, South Korea, contains mixed siliciclastic and carbonate ramp deposits. The ramp sediments were frequently influenced by storm events resulting in tempestites of sandstone-mudstone couplets, bioclastic grainstones to packstones, flat-pebble conglomerates, a skeletal lag layer and laminated calcisiltites. All tempestites are characterized by an erosive to sharp base, poor grading and a transitional upper boundary. The difference in lithology of tempestites appears to have been controlled by the nature of substrates and by proximality. For example, laminated calcisiltites have developed on the shallow carbonate ramp, flat-pebble conglomerates are closely associated with nodular limestones on shallow and deep ramps, and thin skeletal lag layers from fossiliferous argillaceous sediments formed in a basinal setting. The stratigraphic succession of the Dumugol Formation represents an initial transgression followed by a regression. The vertical facies change records the transition from a shallow siliciclastic ramp to a deep carbonate ramp, to a basin, shallowing to a deep carbonate ramp, and to a shallow carbonate ramp. Storm effects are mostly well preserved in shallow to deep ramp deposits.     

A Late Ordovician high-energy temperate-water carbonate ramp,southern Quebec,Canada: implications for Late Ordovician oceanography*

The Trenton Group (Late Ordovician), the youngest carbonate unit in the Taconic foreland basin of southern Quebec, is a tripartite unit with a distinctive coarse-grained middle part, the Deschambault Formation. Lithofacies of the Deschambault Formation are dominated by coarse-grained bioclastic/intraclastic limestones; finer-grained lithofacies are ubiquitous but subordinate. The complete spectrum of lithofacies indicates sedimentation ranging from above fairweather- to below storm-wave base. Skeletal components are indicative of the modern temperate-water bryomol association. Non-skeletal elements are represented by peloids and intraclasts. Accretion rates from areas of continuous sedimentation were low (<14 cm/10³ years). From sedimentological and faunal evidence, it is proposed that the Late Ordovician Deschambault ramp was bathed by temperate waters. The model compares favourably with modern cool-water shelves rimming the southern edge of the Australian continent. Palaeomagnetic data locate southern Quebec in a low latitudinal setting during the Late Ordovician. Upper Ordovician facies distribution in eastern Canada and progressive disappearance of some faunal provinces through Late Ordovician time are used to conclude that the initiation of the Late Ordovician glaciation that covered most of Gondwana was instrumental in easing northward movement of cold oceanic currents. This resulted in the rapid contraction of the southern hemisphere warm-water tropical belt from a 30° latitudinal-wide zone in the early Caradoc to a 15° zone in the late Caradoc.     

黔东北地区晚奥陶世岩相古地理

黔东北地区上奥陶统出露宝塔组、临湘组、五峰组和观音桥组.根据沉积物、沉积相特征变化,将黔东北地区上奥陶统划分为有障壁碳酸盐岩台地沉积、无障壁碳酸盐岩缓坡沉积和浅海陆棚沉积.板块构造运动在晚奥陶世开始变得剧烈,黔东北大部分地区因华夏板块挤压而隆起.宝塔期,黔东北地区发育碳酸盐岩缓坡模式,碳酸盐岩台地逐渐淹没,形成由古隆起...     

湖南奥陶纪沉积演化特征

提要：根据湖南地区奥陶纪沉积地层的野外考察和室内分析,并总结吸收前人对该区的研究成果,将湖南奥陶纪的构造沉积演化划分成了早奥陶世镶边型碳酸盐台地-陆棚-深水盆地、中奥陶世碳酸盐缓坡-陆棚-深水盆地、晚奥陶世早期碳酸盐缓坡-陆棚-深水盆地-陆棚边缘、晚奥陶世晚期局限浅海-深水盆地-陆棚边缘4个沉积阶段。处于扬子克拉通内的湘西北地区经历了镶边型台地-碳酸盐缓坡-局限浅海的演化阶段,沉积岩性也由碳酸盐岩逐渐被黑色泥页岩沉积所替代。位于克拉通边缘及华夏陆块之上的湘中、湘南地区则始终处于碎屑浅海沉积环境,盆地中心由南东向北西不断迁移。     

河北省平泉早奥陶世冶里组碳酸盐风暴沉积

河北平泉地区冶里组碳酸盐岩风暴沉积极为发育,含原地型风暴岩、异地型风暴岩、风暴浊积岩和生物截丘灰岩等4种类型.风暴沉积反映本区冶里组沉积时经历了一个完整的海进-海退旋回,当时区内古地理面貌为北高南低、海水北浅南深.此外,风暴沉积在露头层序地层划分中,对于海进体系域和高水位体系域的识别具有重要的指示作用.     

湘黔地区中奥陶世海平面上升的沉积响应

中奥陶世克拉道克阶(Caradocian)在全球范围内为海侵期,体现在全球范围广泛的黑色页岩沉积及笔石动物的大规模繁盛,湘黔地区这一时期地层发育完全,从碳酸盐台地-斜坡-外陆架盆地均有不同程度的响应。在台地区表现为碳酸盐台地的淹没,斜坡区表现为碳酸盐重力流及黑色页岩、粉砂岩沉积,形成碳酸盐与陆源碎屑的混合沉积,而在外陆架盆地区表现为黑色岩系大规模发育及笔石的大量繁盛。     

Late Ordovician evolution of the intracratonic basins in north-west Gondwana

The Palaeozoic intracratonic basins in northwest Gondwana, i.e. the Amazonas, Parnaiba and Acera basins, probably opened during late Caradoc and Ashgill times. The fluviatile sedimentation later changed to littoral at the basinal margins. A transgression from the north-west region of Gondwana slowly overlapped the margins of the intracratonic basins. The transgression reached its maximum in the Rawtheyan (late Ashgill), as evidenced by fossiliferous shallow marine sediments in the Amazonas Basin. The Hirnantian glaciation in north Gondwana lowered the sea level, and in the Amazonas Basin a littoral sedimentation followed on shallow marine strata. From the opening of the basins onwards, a shallow sea probably existed close to the epicontinental basins in north-west Gondwana. The basins were connected via a narrow passage between the Guayana and Ivorian cratons.     

Global geoheritage significance of Ordovician stratigraphy and sedimentology in the Cliefden Caves area,central western New South Wales

Globally significant geoheritage features of the Cliefden Caves area, in the Belubula River Valley between Orange and Cowra in central western New South Wales, comprise a richly fossiliferous shallow-water limestone succession of Late Ordovician age (the Cliefden Caves Limestone Subgroup) overlain by deep-water laminites and allochthonous limestones of the Upper Ordovician Malongulli Formation. Key features of the Ordovician geology of the Cliefden Caves area that have been identified using the Geoheritage Toolkit as being of international significance are the abundance of unique and exceptionally diverse fossils in the Fossil Hill Limestone (forming the lower part of the Cliefden Caves Limestone Subgroup), which supplement detailed interpretation of carbonate-dominated deposition within an Ordovician volcanic island setting. The fossiliferous limestones preserve biostromes and local small bioherms of stromatoporoids and corals, and recurrent in situ and disarticulated/imbricated Eodinobolus shell beds formed in shallow, quiet-water, dominantly muddy carbonate sediments that passed up-sequence to clay-free carbonate environments. These mud-dominated carbonate sediments are interspersed with higher-energy conditions, represented by skeletal, lithoclastic and calcrete-ooid grainstones overlying disconformities, leading to the identification of subaerial disconformities and associated diagenesis in the Fossil Hill Limestone. The Fossil Hill Limestone is succeeded by massive limestones in the middle part of the Cliefden Caves Limestone Subgroup and then, in turn by the Vandon Limestone and the deeper-water graptolitic laminites of the Malongulli Formation—this completes a succession that is rarely preserved in the geological record, further enhancing the geoheritage significance of the Cliefden Caves area.     

塔里木盆地阿克苏-柯坪地区寒武系-奥陶系的沉积环境

本文讨论了塔里木西北缘厚一千余米的寒武系-奥陶系暗色碳酸盐岩夹细碎屑岩系的沉积环境。寒武系以粉-细晶白云岩、微晶叠层石白云岩夹细碎屑岩为主;奥陶系以含硅粉-细晶白云岩、内碎屑岩、细碎屑岩与灰岩的混合岩互层为主。根据矿物成分、生物化石、结构构造、岩石的元素、稳定同位素、矿物组合等判断,其沉积环境为半封闭的潮坪环境至开阔的陆棚沉积环境。     

冷水碳酸盐岩研究现状与展望

长期以来,海相碳酸盐沉积物被广泛认为是温暖浅海沉积环境中的产物,然而近年来国外研究表明,在冷水陆架环境中,也可以形成规模的碳酸盐沉积,即冷水碳酸盐岩。通过对大量文献的调研,综述了冷水碳酸盐岩的概念、地质特征、沉积模式等方面的研究进展,并展望了该研究领域未来的发展方向。研究表明,冷水碳酸盐沉积是指在(古)纬度约30°~35°以上的温带及寒带地区、温度约20℃以下的沉积水体中,或是在富营养的寒冷上升流水体中形成的碳酸盐沉积物或岩石,其沉积特征与暖水碳酸盐岩不同。冷水碳酸盐沉积物中的生物颗粒组合以底栖有孔虫、软体动物、苔藓虫等异养生物和钙质红藻为主,缺乏造礁珊瑚和钙质绿藻以及鲕粒、集合颗粒等非骨架颗粒,而且沉积物中灰泥基质含量较少,矿物成分以方解石为主,氧同位素较重,胶结作用弱,以破坏性成岩作用为主。冷水碳酸盐沉积形态以缓坡为主,波浪磨蚀和再沉积作用较强。古代冷水碳酸盐岩的沉积特征、识别标志及其作为储层的油气资源潜力尚处于探索阶段,仍需进一步深入研究。     

湘西—黔东地区寒武系清虚洞组地层特征与铅锌成矿关系

湘西—黔东地区寒武系发育完整,是一套从黑色岩系到碳酸盐岩的沉积序列。下寒武统清虚洞组由灰岩段和白云岩段组成,纵向上构成总体向上海水变浅的沉积相序列,同时反映了缓坡型碳酸盐岩台地的发育过程。该组空间上从北西至南东可识别出潮坪相、局限台地相粉细晶云岩、台地边缘浅滩相粉细晶灰岩和台地边缘(滩)丘相微晶灰岩、浅-深缓坡相、陆棚相泥灰岩、台地前缘盆地等沉积。研究表明,地层岩性、岩相古地理对铅锌矿具有明显的控制作用,微晶丘是主要容矿层,与微生物和海底热水关系密切。铅锌成矿明显与浊流沉积和风暴沉积等事件沉积相关,浊积岩、微晶丘、砾(粒)屑灰岩构成一完整的铅锌控矿序列。     

晚奥陶世五峰期扬子板块沉积模式

本文从沉积相分析及稀土元素地球化学特征来探讨扬子板块在晚奥陶世五峰期的沉积模式,认为五峰期扬子板块南部为古陆,东西北为断续分布的碳酸盐岩台地、生物礁和隆起区所环绕,中部为半封闭的浅水滞流海盆,稀土元素分析表明扬子扳块中部广泛分布的五峰组形成于陆浅海而非深海小洋盆。     

Structure,Age, and Settings of Formation of Ordovician Complexes of the Northwestern Frame of the Kokchetav Massif,Northern Kazakhstan

This work presents the data on the structure, geochronology, and formation settings of the Ordovician sedimentary and volcanogenic-sedimentary complexes of the Sterlitamak, Mariev, and Imanburluk structural and formational zones located in the western and northwestern frames of the Kokchetav massif (Northern Kazakhstan). In addition, the results of detailed stratigraphic, geochemical, and geochronological studies of the reference section of the Ordovician deposits of the Mariev Zone are given. The studied section is composed of carbonate, terrigenous, and less commonly volcanogenic-sedimentary deposits, confined to a wide stratigraphic interval from Tremadocian Stage of the Lower Ordovician to the lower Sandbian Stage of the Upper Ordovician. For the first time, the study of conodont assemblages made it possible to establish the Early to Middle Ordovician age of the most ancient limestone–dolomite sequence, which was previously conventionally attributed to the Cambrian. The above-lying tuffaceous–terrigenous Kupriyanovka Formation is now attributed to the Middle Ordovician. On the basis of compositional features of the lithoclastic tuffs composing the middle part of the formation, we assume that it was formed within the island arc zone. Limestones from the base of the youngest terrigenous–carbonate Kreshchenovka Formation are attributed to the lower part of the Sandbian Stage of the Upper Ordovician. The study of the geochronology of detrital zircons from terrigenous rocks of the limestone–dolomite sequence has shown that the Early Neoproterozoic quartzite–schist sequences of the Kokchetav massif were the most probable provenance area during its deposition. It was established that there was the change of sedimentation environments from closed lagoons to a relatively deep sea basin with normal salinity and intense circulation of water masses in the northwestern frame of the Kokchetav massif during the Ordovician. During this period of time, there was a sufficiently high level of erosion of provenance areas that resulted in the deposition of thick strata of terrigenous material. A general tendency of the deepening of sedimentation environments from the Early to Late Ordovician was interrupted by sea level rises in the Dapingian and early Darriwilian ages.     

Daly Basin,Northern Territory: lithostratigraphic revision resolves context of incongruous Ordovician fossils

A local succession of interbedded dolostone, limestone and glauconitic sandstone in the central Daly Basin of the Northern Territory, dated as Early Ordovician, has for many years appeared incongruous in terms of lithology and age relative to mapped formations of the Cambrian Daly River Group. Geological mapping and stratigraphic drilling have now shown that this interval, recently named the Florina Formation and described here, unconformably overlies the karstified surface of the uppermost formation of the Daly River Group, the Oolloo Dolostone. It is the youngest formation of the Daly Basin succession, but due to the lengthy hiatus between it and the underlying units, it is not included within the Daly River Group. It comprises three intervals of carbonate rocks alternating with thicker intervals of siliciclastic rocks. The latter are viewed as forming under dominantly moderate energy, shallow marine conditions with sediments derived from a distant terrigenous source. The carbonate rocks were dominantly subtidal, too far offshore to receive significant amounts of terrigenous material but shallow enough to be above storm wave base. The Oolloo Dolostone is formally divided here into two members, the lower Briggs Member and the upper King Member. Both consist largely of dolostone, but the Briggs Member is typically well bedded, contains ooids and has a minor component of quartz sand. It accumulated mainly as ooid shoals seaward of tidal flats. The overlying King Member is massive to coarsely bedded with only traces of terrigenous sediment and was deposited in deeper water seaward of the Briggs Member.     

Evolution of an early Proterozoic foreland basin carbonate platform, lower Pethei Group, Great Slave Lake, north-west Canada

The Taltheilei, Utsingi, McLean and Blanchet formations form a 175–390 m thick carbonate platform-to-basin succession in the lower part of the PaleoProterozoic Pethei Group, preserved in the eastern arm of Great Slave Lake. Carbonates accumulated along the south-east margin of the Slave Craton within a foredeep formed during the collision of the Slave and Churchill Cratons. The rocks include eight, predominantly microbial, carbonate facies that comprise five facies associations representing (1) shallow-water rimmed shelf, (2) shallow-water open shelf, (3) shallow-water ramp, (4) upper slope and deep ramp, and (5) lower slope and basin plain environments. Microbialite facies grew by organically mediated precipitation of spar and micritic cement and trapping and binding of lime mud. These wholly subtidal facies typically reflect progressive shallowing and changing geometry of the lower Pethei sea floor, from ramp, to open shelf, to shallow rimmed shelf, with associated slope and basin plain deposition. Repeated relative sea-level changes influenced platform growth. This resulted in five shallowing upward packages; each separated by an incipient drowning event of varying magnitude. Antecedent topography and the size of the preceding drowning event strongly influenced the initial growth of each interval. This repeated pattern is attributed to interaction between (a) the inherent tendency of microbial carbonates to aggrade vertically, (b) changing sedimentation rates and (c) readjustments of relative base level. The lower Pethei succession is one of few PaleoProterozoic examples of carbonate platform growth within a foreland basin. It has (1) a low gradient profile, (2) extensive slope and basin plain carbonate production and sedimentation, (3) no ooids, (4) minor terrigenous clastic sediments, and (4) a mobile, submergent shelf rim lacking substantial carbonate sand shoals.     

A Middle Ordovician Drowning Unconformity on the Northeastern Flank of the Okcheon (Ogcheon) Belt, South Korea

The Maggol Limestone of Ordovician age was deposited in the Taebaeksan (Taebacksan) Basin which occupies the northeastern flank of the Okcheon (Ogcheon) Belt of South Korea. Carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the Maggol Limestone probably culminated in subaerial exposure of platform carbonates in the early Middle Ordovician (earliest Darriwilian). Elsewhere this subaerial exposure event is manifested as a major paleokarst unconformity at the Sauk-Tippecanoe sequence boundary beneath the Middle Ordovician succession and its equivalents, most in notably North America and North China. Due to its global extent, this paleokarst unconformity has been viewed as a product of second- or third-order eustatic sea level fall during the early Middle Ordovician. The Sauk-Tippecanoe sequence boundary in South Korea, however, appears to be a discrete marine-flooding surface in the upper Maggol Limestone. Strata beneath this surface represent by a thinning-upward stack of exposure-capped tidal flat-dominated cycles that are closely associated with multiple occurrences of paleokarst-related solution-collapse breccias. This marine-flooding surface is onlapped by a thick succession of thin-bedded micritic limestone that is eventually overlain by a Middle Ordovician condensed section. This physical stratigraphic relationship suggest that second- and third-order eustatic sea level fall may have been significantly tempered by regional tectonic subsidence near the end of Maggol deposition. The tectonic subsidence is also evidenced by the occurrence of coeval off-platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias (i.e., the Yemi Breccia) in the basin. With continued tectonic subsidence, a subsequent rise in the eustatic cycle caused drowning and deep flooding of the carbonate platform, forming a discrete marine-flooding surface that may be referred to as a drowning unconformity. This tectonic interpretation contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously suggested. Thus, it is proposed that the Taebaeksan Basin in the northeastern flank on the Okcheon Belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early Middle Ordovician.     

湘西-黔东地区寒武系都匀阶清虚洞期 岩相古地理与铅锌成矿关系研究

依据地层记录中的两种相变面和两种穿时性,结合上扬子地区寒武系十余条代表性剖面的沉积特征及其横向展布规律,应用等时面优势相成图方法编制了寒武系都匀阶清虚洞期岩相古地理图.区内古地理格局具有西高东低、北浅南深(水深)的特点,沉积自北西向南东超覆,层位逐渐抬高穿时;岩性、厚度、颜色和沉积相类型上的差异是统一沉积背景下沉积物相变、穿时的结果.清虚洞组由灰岩段和白云岩段组成,以发育缓坡型碳酸盐台地为特征,纵向上构成总体向上变浅的沉积相序列,同时反映了缓坡型碳酸盐台地的生长发育过程;横向上从北西至南东发育从混积潮坪相、浅缓坡相、中缓坡相、深缓坡相和陆棚-盆地等沉积.岩相古地理对层控铅锌矿具有明显的控制作用,主要分布于中缓坡相藻灰岩微晶丘中,浊积岩、微晶丘、砾(粒)屑灰岩构成一完整的铅锌控矿序列.     

A Middle Ordovician Drowning Unconformity on the Northeastern Flank of the Okcheon (Ogcheon) Belt,South Korea

The Maggol Limestone of Ordovician age was deposited in the Taebaeksan (Taebacksan) Basin which occupies the northeastern flank of the Okcheon (Ogcheon) Belt of South Korea. Carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the Maggol Limestone probably culminated in subaerial exposure of platform carbonates in the early Middle Ordovician (earliest Darriwilian). Elsewhere this subaerial exposure event is manifested as a major paleokarst unconformity at the Sauk-Tippecanoe sequence boundary beneath the Middle Ordovician succession and its equivalents, most in notably North America and North China. Due to its global extent, this paleokarst unconformity has been viewed as a product of second- or third-order eustatic sea level fall during the early Middle Ordovician. The Sauk-Tippecanoe sequence boundary in South Korea, however, appears to be a discrete marine-flooding surface in the upper Maggol Limestone. Strata beneath this surface represent by a thinning-upward stack of exposure-capped tidal flat-dominated cycles that are closely associated with multiple occurrences of paleokarst-related solution-collapse breccias. This marine-flooding surface is onlapped by a thick succession of thin-bedded micritic limestone that is eventually overlain by a Middle Ordovician condensed section. This physical stratigraphic relationship suggest that second- and third-order eustatic sea level fall may have been significantly tempered by regional tectonic subsidence near the end of Maggol deposition. The tectonic subsidence is also evidenced by the occurrence of coeval off-platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias (i.e., the Yemi Breccia) in the basin. With continued tectonic subsidence, a subsequent rise in the eustatic cycle caused drowning and deep flooding of the carbonate platform, forming a discrete marine-flooding surface that may be referred to as a drowning unconformity. This tectonic interpretation contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously suggested. Thus, it is proposed that the Taebaeksan Basin in the northeastern flank on the Okcheon Belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early Middle Ordovician.