Devonian to Triassic Successions of the Changning-Menglian Belt, Western Yunnan, China

Phanerozoic strata are distributed in several north-south trending zones in the central part of the Changning-Menglian Belt. Four types of Devonian to Triassic stratigraphic successions can be identified: (1) elastics with limestone lenses in the mid-section, changing up-section into alternations of fine elastics and cherts; (2) elastics with chert intercalations and limestone lenses, and topped by Permian basic volcanics; (3) elastics-basic volcanics-carbonates-clastics; and (4) limestones, dolomitic limestones-dark gray thin-bedded limestones, argillaceous limestones, mudstones and siliceous mudstones. Devonian to Triassic cherts occur in different horizons and different zones from east to west. These cherts are usually transitional to their neighboring elastics. There is no continuous Devonian to Middle Triassic chert sequence in the central zone of the Changning-Menglian Belt as Liu et al. (1991,1993) reported. Volcanics and the overlying carbonates described by some workers as "seamount" sequences     

Chertification in the Mississippian Lake Valley Formation, Sacramento Mountains, New Mexico

Chert distribution in the Lake Valley rocks is selective to mud-supported facies; it is not related to proximity to unconformities. The facies selectivity of the chertification is believed to be a function of the depositional distribution of indigenous silica as sponge spicules, an interpretation that is supported by high positive qualitative correlation of chert with spiculitic rocks. Petrography indicates that the spicules were all originally siliceous, and that they all went through a moldic stage during which many molds were compactively destroyed and distorted. Remaining molds were subsequently cemented by calcite or chalcedony. Chert distribution and spicule petrography argue for an intraformational source for much of the silica. Chert micro-fabrics are dominated by microquartz, a replacement of grains and lime mud; length-fast chalcedony, a pore-filling cement; and megaquartz, a post-chalcedony pore-filling cement. Petrography of compaction features within chert masses indicates that chertification occurred after some burial. Based on stratigraphic reconstruction this burial depth was a maximum of about 215 m. and was most likely a few metres to a few tens of metres. Petrography of chert-calcite cement relationships indicates that chertification occurred before and during first generation, pre-Pennsylvanian non-ferroan calcite cementation, and was completed before late-stage, post-Mississippian ferroan calcite precipitation. Petrography of chert clasts in basal Rancheria (Meramecian) and basal Pennsylvanian conglomerates proves these clasts derived from the Lake Valley Formation and were chertified before redeposition. Thus, some cherts in the Lake Valley are pre-Meramecian in age, but all are pre-Pennsylvanian in age. Furthermore, association of the cherts with the non-ferroan cements indicates the cherts were probably precipitated in meteoric phreatic lens established beneath the pre-Meramecian and pre-Pennsylvanian subaerial unconformities.     

渭北奥陶系的放射虫燧石岩

渭北奥陶系的放射虫燧石岩是在稳定的华北地台上形成的远洋沉积.它们产在赵老峪组深水碳酸盐地层的下部,并以层位稳定、纹层发育、富含放射虫骨骼、成岩交代组构清楚,以及在剖面上与薄层的硅质页岩呈韵律互层等为等征,表明是深水的放射虫软泥在成岩作用早期由生物蛋白石经溶解-再沉淀反应快速转变而成.这种产在地台上的远洋沉积虽不多见,却是海平面大规模上升及其伴生的重大地质事件的反映.渭北放射虫燧石岩的时代相当于中奥陶世卡拉道克期.当时正值全球性海平面上升,秦岭古海盆也在发生强烈扩张与俯冲,因此,华北地台南缘下沉,使渭北地区变成了弧后深水盆地的北部边缘.当时研究区的古地理位置也恰好处于低纬度的赤道附近.这些都为放射虫燧石岩的堆积提供了有利的条件.由于当时钙质浮游生物尚未大量兴起,所以其沉积作用不受CCD的控制,水深较之现代的类似物可能要浅得多.     

安徽巢湖地区中二叠统栖霞组灰岩中燧石成因

安徽省巢湖地区中二叠统栖霞组以发育一套滨海沼泽－浅海碳酸盐台地环境为主的细碎屑岩至碳酸盐岩沉积建造为特征,灰岩中常见结核状及条带状燧石。燧石多为椭球状和串珠状,部分燧石与灰岩间发育宽约0.5 cm的过渡带。镜下观察灰岩为微晶生物碎屑灰岩;过渡带也多由微晶方解石组成,多数钙质生物壳体被石英充填或半充填,扫描电镜下可见方解石微溶、石英充填溶孔的现象;燧石主要为隐晶及微晶石英,生物碎屑类型与灰岩中基本一致,且多被石英交代。岩石学特征表明死亡的生物在腐烂降解过程中形成的有机酸抑制了碳酸钙的沉淀,并使部分生物碎屑及灰泥发生溶蚀,胶质二氧化硅沉淀,形成燧石结核。燧石中w(Al)/w(Al+Fe+Mn)平均值为0.63,远大于热水沉积硅质岩的最大值0.35;Fe/Ti平均值为9.5,小于热水成因的最小值20;Al-Fe-Mn三角图投点位于非热液成因区域;燧石中Al₂O₃含量平均值为0.20%,远高于MgO、Na₂O、K₂O的含量。分析认为,该区燧石结核的二氧化硅可能来源于陆源物质。     

辽西下白垩统义县组湖相碳酸盐岩中的燧石成因

辽西义县鹰窝山下白垩统义县组砖城子层中部发育一套湖相碳酸盐岩,可识别出4种岩相类型,其中角砾化团块灰岩相中含有大量燧石,所含的燧石可据形态划分为结核状、透镜状和团块状3种类型,据颜色和透明度识别出两种类型。燧石以微晶石英和玉髓为主,含少量的白云石和方解石晶粒。该岩相系相对干旱气候背景下的洪积-漫湖沉积体系中较浅储水洼地沉积,火山活动也具一定的影响力。     

Silica diagenesis in Eocene shallow-water platform carbonates, southern Pyrenees

Spectacularly developed lower Eocene chert in the Corones platform carbonates of the Spanish Pyrenees is concentrated within a restricted, brackish-water, laminated ostracod-rich facies, which also contains abundant sponge spicules. The chert occurs as nodular, bedded and mottled varieties, and four petrographic types of quartz are developed: microquartz; length-fast (LF) chalcedony; megaquartz; and microspheres. δ¹⁸O values of chert range from 29·6‰ to 30·9‰ (SMOW), which correspond to a broad isotope rank common for biogenic and diagenetic replacement cherts. Calcian dolomite crystals with high Fe and Na are disseminated within the microquartz and LF-chalcedony, but are absent from the megaquartz and host carbonate. The chert is closely associated with desiccation cracks and with interstratal dewatering structures. Load casts are silicified, and laminae rich in sponge spicules are convoluted. Early cracks related to dewatering are filled by microquartz and quartz cements. Ostracod shells within chert are locally fractured; those in the host carbonate are commonly flattened. Late fractures are filled by LF-chalcedony and megaquartz. There is much evidence for the dissolution of sponge spicules and their calcitization in the carbonate host rock. Silica for the Corones cherts was derived from sponges during early diagenesis and shallow burial. Early mechanical compaction and sediment dewatering played a major role in sponge spicule dissolution, migration of silica-rich fluids and the consequent precipitation of chert. Quartz cements continued to be precipitated into the burial environment.     

雅鲁藏布江缝合带及藏南地区硅质岩微组构特征及其地质意义

在雅鲁藏布江缝合带及藏南地区产出红、绿、黑、褐等多色调的中生代硅质岩.石英是它们的主要矿物,但不同样品的石英晶体微形貌各有特征,X衍射和红外吸收光谱特征表现出细微差别.夏鲁和彭错林分别作为生物成因和热水成因硅质岩的代表,它们在矿物演化路径、石英晶体微形貌和集合体微结构存在差异,为利用微组构区分不同成因硅质岩提供了参考依据.在夏鲁硅质岩中生物化石残骸内存在方英石和玉髓,为反演生物成因硅质岩的矿物演变序列提供了很好的证据.硅质岩成岩过程中微组构演变和元素迁移具有良好的对应关系.石英在重结晶过程中对杂质元素的剔除作用是促使某些微量元素产生富集的重要机制.从微组构角度切入硅质岩研究是一种新的研究思路,它综合运用微区微束手段对硅质岩的微观组构特征进行测试,进而探讨微观组构信息所指示的地质意义.结果对探讨硅质岩的成因、演化和藏南地区硅质岩容矿型Au-Sb-Cs矿床的成矿机理具有重要的指导作用.     

Chert spheroids of the Monterey Formation,California (USA): early‐diagenetic structures of bedded siliceous deposits

Chert spheroids are distinctive, early‐diagenetic features that occur in bedded siliceous deposits spanning the Phanerozoic. These features are distinct in structure and genesis from similar, concentrically banded ‘wood‐grain’ or ‘onion‐skin’ chert nodules from carbonate successions. In the Miocene Monterey Formation of California (USA), chert spheroids are irregular, concentrically banded nodules, which formed by a unique version of brittle differential compaction that results from the contrasting physical properties of chert and diatomite. During shortening, there is brittle fracture of diatomite around, and horizontally away from, the convex surface of strain‐resistant chert nodules. Unlike most older siliceous deposits, the Monterey Formation still preserves all stages of silica diagenesis, thus retaining textural, mineralogical and geochemical features key to unravelling the origin of chert spheroids and other enigmatic chert structures. Chert spheroids found in opal‐A diatomite form individual nodules composed of alternating bands of impure opal‐CT chert and pure opal‐CT or chalcedony. With increased burial diagenesis, surrounding diatomite transforms to bedded porcelanite or chert, and spheroids no longer form discrete nodules, yet still display characteristic concentric bands of pure and impure microcrystalline quartz and chalcedony. Petrographic observations show that the purer silica bands are composed of void‐filling cement that precipitated in curved dilational fractures, and do not reflect geochemical growth‐banding in the manner of Liesegang phenomena invoked to explain concentrically banded chert nodules in limestone. Chertification of bedded siliceous sediment can occur more shallowly (< 100 m) and rapidly (< 1 Myr) than the bulk silica phase transitions forming porcelanite or siliceous shale in the Monterey Formation and other hemipelagic/pelagic siliceous deposits. Early diagenesis is indicated by physical properties, deformational style and oxygen‐isotopic composition of chert spheroids. Early‐formed cherts formed by pore‐filling impregnation of the purest primary diatomaceous beds, along permeable fractures and in calcareous–siliceous strata.     

Chert occurrences in carbonate turbidites: examples from the Upper Jurassic of the Betic Mountains (southern Spain)

In Upper Jurassic carbonate turbidites of the Betic mountains (southern Spain), chert occurs in three morphologies: bedded chert, nodular chert and mottled chert. The last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock. The three types of chert are formed by replacement of limestones and are associated with different calcareous facies. Turbidite packstones of Saccocoma and peloids, and turbidite lime mudstones of pelagic material contain bedded and nodular cherts. The silicification textures are mainly micro- and cryptocrystalline quartz, with local chalcedonic quartz (both length-fast and length-slow) which is more common in the packstones. Only mottled chert is produced where calcareous breccia beds are silicified. Mottled chert consists of micro- and cryptocrystalline quartz, length-slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz. Beds and nodules are the result of early diagenetic silicification, with silica derived from the calcitization and dissolution of radiolarians and, subordinately, sponge spicules, whereas mottled chert is the consequence of later silicification in a probably Mg-rich environment. Early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones. Turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water. A silica-rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert.     

Nodular chert from the Arbuckle Group, Slick Hills, SW Oklahoma: a combined field, petrographic and isotopic study

Nodular chert from the middle and upper Arbuckle Group (Early Ordovician) in the Slick Hills, SW Oklahoma, was formed by selective replacement of grainstones, burrow fillings, algal structures, and evaporite nodules. Chert nodules are dominantly microquartz with minor fibrous quartz (both quartzine and chalcedony), megaquartz, and microflamboyant quartz. Lepisphere textures of an opal-CT precursor are preserved in many (especially in finely-crystalline) chert nodules. The δ¹⁸O values of microquartz chert range from +23.4 to + 28.8^0/00 (SMOW), significantly lower than those of Cenozoic and Mesozoic microquartz chert formed both in the deep sea and from near-surface sea water. The δ¹⁸O values of chert decrease with increasing quartz crystal size. Silicification in the Arbuckle Group occurred during early diagenesis, with the timing constrained by the relative temporal relationships among silicification, burial compaction, and early dolomite stabilization. Silica for initial chert nucleation may have been derived from both dissolution of sponge spicules and silica-enriched sea water. Chert nucleation appears to have been controlled by the porosity, permeability, and organic matter content of precursor sediments. This conclusion is based on the fact that chert selectively replaced both porous grainstones and burrows and algal structures enriched in organic matter. Growth of chert probably occurred by a maturation process from opal-A(?), to opal-CT, to quartz, as indicated by the presence of opal-CT precursor textures in many chert nodules. Although field and petrographic evidence argues for an early marine origin for chert in the Arbuckle Group, the light δ¹⁸O values are inconsistent with this origin. Meteoric resetting of the δ¹⁸O values of the chert during exposure of the carbonate platform best explains the light δ¹⁸O values because: (i) the δ¹⁸O values of chert nodules decrease with decreasing δ¹⁸O values of host limestones, and (ii) chert nodules from early dolomite, which underwent more extensive meteoric modification than associated limestones, have lighter δ¹⁸O values than chert nodules from limestones. Increasing recrystallization of chert nodules by meteoric water resulted in progressive ¹⁸O depletion and (quartz) crystal enlargement.     

The significance of cherts as markers of Ocean Plate Stratigraphy and paleoenvironmental conditions: New insights from the Neoproterozoic–Cambrian Blovice accretionary wedge,Bohemian Massif

The Ediacaran to early Cambrian Blovice accretionary complex, Bohemian Massif, hosts abundant chert bodies that formed on an oceanic plate and were involved in subduction beneath the northern margin of Gondwana. Field relationships of cherts to their host, their microstructure and elemental as well as isotopic compositions revealed diverse processes of chert petrogenesis reflecting depositional environment and position on the oceanic plate. The deep-water cherts formed through a hydrothermal precipitation of silica-rich gels on outer trench swell of the subducted slab with none or only minor addition of terrigenous material. On the contrary, the shallow-water cherts formed in lagoons on seamount slopes, and at least some of them represent a product of hydrothermal replacement of former carbonate and/or evaporite precursors. For both chert types, the hydrothermal fluids were of low temperature and continuous pervasive hydrothermal alteration of oceanic crust, together with an elevated Si content in Neoproterozoic seawater, served as the major source of silica. On the other hand, minor carbon enrichment in chert is mostly linked to variable incorporation of organic matter that was deposited on the seafloor. Rare earth element (REE) systematics of the cherts indicate predominantly oxygenated environment for the shallow-water cherts whereas the deep-water cherts were deposited in diverse redox conditions, depending on their distance from hydrothermal vent. Using these data, we demonstrate that the cherts once formed a part of Ocean Plate Stratigraphy (OPS) now dismembered and mixed with terrigenous siliciclastic material to form OPS mélanges. Combining our data with those from the existing literature, we show that cherts can serve as significant markers of OPS since the Archean, recording a complex interplay between seafloor-related volcanic (production of MORB- and OIB-like magmas) and sedimentary processes, hydrothermal activity at mid-ocean ridges and seamount chains as well as at outer slopes of subducting slabs. However, the cherts also exhibit a secular change in composition and petrogenesis most profoundly affected by an overturn in seawater silica cycle across the Precambrian–Phanerozoic boundary.     

华南地区二叠纪栖霞组燧石结核成因研究及其地质意义

燧石结核是华南地区二叠纪栖霞组的重要识别特征之一，其成因具重要的古地理、古海洋意义。通过对湖北黄石、江苏南京和广西来宾三地栖霞组燧石结核的岩石矿物学研究，确定了栖霞组燧石结核的矿物组成和成岩作用序列。研究区燧石结核主要由微石英、负延性玉髓、粗晶石英组成，并含少量白云石、方解石及生物碎屑。其中，微石英、负延性玉髓、正延性玉髓、白云石形成于早期成岩作用，方解石晶粒形成于晚期成岩作用，粗晶石英的形成则具有多期性。结合栖霞组菊花状天青石和海泡石成因研究结果，本文认为组成栖霞组燧石结核的硅质来源与当时全球硅质生物的繁盛有关。燧石结核内玉髓和白云石形成环境条件及形成时间的确定，为建立更加合理的燧石结核成因模式和白云岩化模式提供了重要资料，同时也对深入探讨本区二叠纪层状硅质岩的成因具启发意义。     

Some unusual alkali-expansive aggregates

The subgreywackes from the quarry at Alert, Ellesmere Island, N.W.T., Canada, have been found by length-change tests to have a definite potential for causing expansion of concrete by a type of alkali-aggregate reaction. The rocks are physically sound but have petrographic affinities with known alkali-expansive rocks in the maritime provinces. Rocks of this sort may be recognized by a petrographer familiar with this type of problem.     

Chronology of the last recession of the Greenland Ice Sheet

A new deglaciation chronology for the ice‐free parts of Greenland, the continental shelf and eastern Ellesmere Island (Canada) is proposed. The chronology is based on a new compilation of all published radiocarbon dates from Greenland, and includes crucial new material from southern, northeastern and northwestern Greenland. Although each date provides only a minimum age for the local deglaciation, some of the dates come from species that indicate ice‐proximal glaciomarine conditions, and thus may be connected with the actual ice recession. In addition to shell dates, dates from marine algae, lake sediments, peat, terrestrial plants and driftwood also are included. Only offshore and in the far south have secure late‐glacial sediments been found. Other previous reports of late‐glacial sediments (older than 11.5 cal. kyr BP) from onshore parts of Greenland need to be confirmed. Most of the present ice‐free parts of Greenland and Nares Strait between Greenland and Ellesmere Island were not deglaciated until the early Holocene. Copyright © 2002 John Wiley & Sons, Ltd.     

The Origin of Bedded Cherts of the Early Permian Gufeng Formation in the Lower Yangtze Area, Eastern China

Bedded cherts occurring in the Early Permian Gufeng Formation in the Lower Yangtze region, eastern China, are nearly 20-80 m in thickness and contain varying amounts of radiolarians and sponge spicules. There are three types of section for the Gufeng Formation: chert, carbonate and chert-carbonate mixing types. Bedded cherts mainly occur in the first and third types of section. The depocentres of silica are marked by some small (not larger than a few thousand square kilometres in area) rhombic or elliptic hollows and their formation was controlled by faults. Argillaceous volcanic rocks in cherts occur as numerous thin layers. Bedded cherts are characterized by higher Fe and lower Al contents, enrichment in such trace elements as As, Sb, Bi, Ga, Au, Ag and Cr, lower total REE abundance, negative Ce anomaly and varying degrees of HREE enrichment. These characteristics are important evidence for hydrothermal cherts. Minor amounts of substances of non-hydrothermal origin are mixed in the cherts. The tempera     

Sedimentary geochemistry of chert from the Middle-Upper Ordovician in Shihuigou area, North Qilian orogenic belt and its tectonic implication

The North Qilian orogenic belt is an elongate tectonic unit that lies between the North China plate to the north and the Middle Qilian microplate to the south, and is formed by a collision of the two plates in the Caledonian. The Shihuigou Section from Yongdeng County, Gansu Province, is in the eastern sector of the North Qilian Mountains, spanning the Ordovician island-arc zones. The Zhongpu Group is distributed in the Shihuigou area and composed of medium-basic volcanic rocks and volcanic clastic rocks interspersed with cherts, limestones, slates, and metamorphic sandstones. The geochemistry of chert from the Zhongpu Group reveals that all cherts coexisting with island-arc volcanic rocks formed in a continental margin basin environment. Research results of the rare earth elements reveal that these cherts formed in a relatively deep-water basin with no significant terrestrial interference. Therefore, it is inferred that the North Qilian orogenic belt was previously an archipelagic ocean in the Ordovician. Translated from Geological Review, 2006, 52(2): 184–189 [译自: 地质论评]     

Biogenic chert in the Cow Head Group (Cambro-Ordovician), western Newfoundland

Chert in the Cow Head Group is mainly a replacement of limestone and shale and, to a lesser extent, an interparticle cement. Its field occurrences are distinct as: (1) silicified margins on coarse conglomerates and thinly bedded limestones; (2) nodules within limestone and shale; (3) pervasively silicified beds of limestone and shale; and (4) clasts or partial replacement of clasts within conglomerate. Radiolarians and sponge spicules are composed of microquartz or calcite and are particularly common in the Ordovician part of the succession where most chert occurs. In limestone spatially associated with chert, the use of cathodoluminescence demonstrates that calcite-replaced radiolarians and spicules are volumetrically more important than realized through transmitted-light petrography. Petrographic relations between siliceous and rare pyritized radiolarians further indicate that these particles may be dissolved prior to compaction. No trace of their former existence remains, other than indirectly through the presence of silicified limestone and shale. Crushed grains cemented by chalcedony indicate that chert was precipitated during or after compaction. The history of silicification and the replacement or dissolution of siliceous bioclasts is protracted, ranging from near the sediment-water interface, where it is concomitant with early limestone lithification, to deeper burial, postdating mechanical compaction.     

Stratigraphy and sedimentology of Jurassic bedded chert overlying ophiolites in the North Apennines, Italy

In the North Apennines of Italy, Upper Jurassic bedded chert stratigraphically overlies ophiolitic rocks and is overlain by Lower to Middle Cretaceous pelagic limestone and shale, and Upper Cretaceous flysch. The bedded chert, best exposed in East Liguria and on Elba, is typically 30–80 m thick, but occasionally reaches 150–200 m thickness. It consists of two main alternating lithologïes: siliceous mudstone (SM) and radiolarite (R). Chert sections commonly show characteristic stratigraphic changes. Lower cherts display a striking rhythmic alternation of R and ferruginous SM beds. In middle cherts, SM beds are much less ferruginous and shalier intercalations are locally present. In upper cherts, R beds are less frequent and SM beds are essentially non-ferruginous. R beds are generally 1–4 cm thick, and consist of 80–90% quartz, 5–15% clays and usually < 1% hematite. They are commonly parallel-laminated, and rarely size-graded. In size-graded beds, large radiolaria are more abundant near the bed base (commonly together with ophiolitic or SM clasts) and small radiolaria more abundant near the bed top. Sorting is poor throughout most R beds. R beds are interpreted as turbidites (cf. Nisbet & Price, 1974). Model calculations suggest that typical settling velocities of radiolaria during redeposition are < 1 cm sec^?1, which is low and of restricted range relative to the 1–10 cm sec^?1 settling velocities of clastic grains of comparable size range. Radiolaria therefore should have only a limited tendency to grade and sort during deposition from a turbulent current. SM beds are commonly 1–7 cm thick, although much thicker ones occur near the base of sections, and consist mainly of 50–70% quartz, 15–35% clays and 0–15% hematite. Microscopic clay-silica aggregates and highly corroded remnants of radiolaria are common. SM beds are interpreted as mainly ambient pelagic sediment which accumulated slowly in topographic lows, and which was modified by near-surface dissolution of biogenic silica. In SM beds which contain two texturally different layers, the lower one is interpreted as the top of the underlying radiolarian turbidite. North Apennine cherts represent the first sediment deposited on oceanic crust formed during the opening of the North Apennine part of the Tethys. The ophiolitic basement had a rugged topography which favoured the redeposition of siliceous sediment. Hematite and local Mn enrichments in SM beds in the lower chert sections represent hydrothermal precipitates inferred to have originated at a spreading axis. During seafloor spreading, accumulation of siliceous sediments progressively reduced the topography. Deposition of ophiolitic detritus within the sediments phased out during early chert sedimentation, and the hydrothermal contribution during early-middle chert sedimentation. As local basins filled, during late chert sedimentation, radiolarian turbidites became less frequent. The first limestones at the top of chert sections are calcareous ooze turbidites derived from above the CCD and deposited slightly below it. Gradual descent of the CCD to ocean floor depths at the end of the Jurassic (Bosellini & Winterer, 1975) led to the replacement of siliceous by carbonate sedimentation.     

广西晚古生代构造沉积背景的初步研究

广西西部存在晚古生代连续深海沉积,其中各个时代玄武岩广泛分布,具大洋板内地化特征。桂西和柳州地区晚古生代硅质岩有明显的铈负异常,指示本区可能属远洋沉积环境。狭窄的深水地层条带具强烈的构造形变,所环绕的碳酸盐台地却产状平缓。区域地质分析表明,广西在晚古生代可能代表扬子和印支地块之间的古特提斯水道,碳酸盐台地可能为广海中的水下高地。     

Sedimentology of cherts in the Early Proterozoic Wishart Formation, Quebec-Newfoundland, Canada

The siliciclastic Wishart Formation of the Early Proterozoic Labrador trough is a high-energy shelf deposit. Wishart sandstones contain both interstitial chert with textures of void-filling cement and thin chert intercalations contaminated with siliciclastic mud. Although volumetrically minor, these cherts occur in several thin, areally extensive stratigraphic intervals. The Wishart contains intraclasts of both the chert-cemented sandstone and the impure chert layers (as well as several other types of chert sand and gravel). This suggests the cherts formed penecontemporaneously, which is consistent with the absence of any signs of replacement in all but one of the chert types and the clear-cut distinctions between chert types, even where they are side by side in a single thin section. The origin which appears to be most compatible with available evidence is that the cherts represent silica precipitated from thermal waters that rose through the sediments of the Wishart shelf and discharged into suprajacent seawater. A biogenic origin is unlikely in view of the lack of appropriate organisms during the Early Proterozoic and the rapidity with which the cements formed. A volcanogenic origin is unlikely because volcaniclastic textures are plentiful in associated formations but absent from the Wishart. Precipitation induced by evaporative concentration is unlikely in view of the widespread evidence of tidal currents and the lack of evidence of desiccation in the Wishart. Finally, the cherts are not restricted to the lowest-energy facies, and therefore they presumably did not accumulate as a background sediment. Deposition of silica above the sediment/water interface was probably made possible by ambient concentrations of silica that were significantly higher than those of Phanerozoic seawater. Cherts with similar textures occur in other Early Proterozoic sediments, most notably arenitic or granular iron-formations.