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1.
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  相似文献   

2.
The Early Palaeoproterozoic Brockman Supersequence comprises banded iron formation (BIF), bedded chert, limestone, mudrock, sandstone, breccia, tuffaceous mudstone, ashfall tuff and, in sections not reported here, basalt and rhyolite. Density current rhythms are preserved in sandstones, mudrocks, tuffaceous mudstones and limestones. Relics of similar rhythms in BIF imply that its precursor sediments were also deposited by density currents. Hemipelagic deposits are siliciclastic or mixed siliciclastic–volcaniclastic mudstones. Bedded chert, chert nodules and the chert matrix of BIF preserve evidence for formation by diagenetic replacement. For bedded chert (and chert nodules), silica replacement occurred before compaction close to or at the sediment–water interface, indicating that it is siliceous hardground. The chert matrix of BIF formed during compaction but before burial metamorphism. Original sediments were resedimented from two sources: (1) limestone, mudrock, sandstone, breccia and tuffaceous mudstone from a shelf; and (2) BIF from within the basin realm. Shelf sediments were resedimented to basin-floor fans during third-order lowstands. The precursor sediments to BIF are interpreted to have been granular hydrothermal muds, composed of iron-rich smectite and particles of iron oxyhydroxide and siderite that were deposited on the flanks of submarine volcanoes and resedimented by density currents. Resedimentation occurred by either bottom currents or gravity-driven turbidity currents, and the resulting sediment bodies may have been contourite drifts. The concept that BIF records high-frequency alternating precipitation from ambient sea water of iron minerals and silica is negated by this study. Instead, it is postulated that the precursor sediments to BIF originated in much the same way as modern Red Sea hydrothermal iron oxide deposits, implying that at least the particles of iron oxyhydroxide originated from the oxidation of vent fluids by sea water. Several orders of cyclicity in basin filling establish a relationship between rising to high sea levels, episodic sea-floor hydrothermal activity and BIF that is reminiscent of the link between eustacy and spreading-ridge pulses.  相似文献   

3.
In order to assess the importance of siliceous sediments as a sink for oceanic B and to determine the effect of diagenesis on the mobilization of B, samples were analysed from chert nodules, bedded cherts, and siliceous banded iron formations from a variety of sedimentary environments and geologic ages. Boron analyses on bulk samples were made by prompt gamma neutron activation analysis. The distribution of B in rocks was mapped using α-track methods.Nodular Phanerozoic cherts typically contain 50–150 ppm B, and bedded cherts somewhat less. The B is initially concentrated in tests of silica-secreting organisms, but some is lost in early diagenesis as silica progressively recrystallises to quartz.Banded iron formation silica of Archean and Proterozoic age usually contains < 2 ppm B. This conforms with the view that such silica is not of biogenic origin but, since many iron formations are undoubtedly of marine origin, raises the question whether Precambrian oceans were impoverished in B. Analyses of Precambrian marine argillaceous sediments, averaging 70 ppm B, do not resolve this question.  相似文献   

4.
Nodular cherts can provide a window on the original sediment composition, diagenetic history and biota of their host rock because of their low susceptibility to further diagenetic alteration. The majority of Phanerozoic cherts formed by the intraformational redistribution of biogenic silica, particularly siliceous sponge spicules, radiolarian tests and diatom frustules. In the absence of a biogenic silica source, Precambrian cherts necessarily had to have had a different origin than Phanerozoic cherts. The Mesoproterozoic Belt Supergroup in Glacier National Park contains a variety of chert types, including silicified oolites and stromatolites, which have similar microtextures and paragenesis to Phanerozoic cherts, despite their different origins. Much of the silicification in the Belt Supergroup occurred after the onset of intergranular compaction, but before the main episode of dolomitization. The Belt Supergroup cherts probably had an opal-CT precursor, in the same manner as many Phanerozoic cherts. Although it is likely that Precambrian seas had higher silica concentrations than at present because of the absence of silica-secreting organisms, no evidence was observed that would suggest that high dissolved silica concentrations in the Belt sea had a significant widespread effect on silicification. The rarity of microfossils in Belt Supergroup cherts indicates that early silicification, if it occurred, was exceptional and restricted to localized environments. The similarity of microtextures in cherts of different ages is evidence that the silicification process is largely controlled by host carbonate composition and dissolved silica concentration during diagenesis, regardless of the source of silica.  相似文献   

5.
根据产状和成因,本文将西秦岭中志留统含铀岩系中的硅质岩分为,(1) 成岩过程中硅化交代碳酸盐岩所形成的硅质岩,产于碳酸盐岩和泥质岩类所组成的透镜层的顶部或碳酸盐岩层内;(2) 早期成岩阶段形成的纹层状硅质岩,主要产于碳酸盐岩和泥质岩透镜层底部或呈薄层夹于粉砂岩、泥质岩层中。根据沉积环境分析及硅质岩的化学成分、REE分布模式和氧同位素组成,提出本区硅质岩形成于受大气淡水影响的局限浅海环境。  相似文献   

6.
The origin and diagenesis of cherts from Cyprus   总被引：1，自引：0，他引：1  
The Troodos Massif of Cyprus is overlain by a variety of cherts in pelagic chalks, volcanogenic sediments, radiolarites and radiolarian mudstones, all of Campanian to Upper Eocene age. There are two chert types, granular chert and vitreous chert. X-ray diffraction (XRD) reveals the silica polymorphs, disordered cristobalite and quartz. Silicification of the chalks varies from incipient, to bedded, granular cherts, all with disordered cristobalite as the main silica phase. Quartzitic cherts are restricted to the base of Upper Palaeocene and Lower Eocene calciturbidite beds. Disordered cristobalite predominates in the radiolarian mudstones at the foot of the sequence. The form of disordered cristobalite in cavities ranges from microspherules of radiating bladed crystals, the ‘lepispheres’ of the Deep Sea Drilling Project (DSDP) to bladed overgrowths, and fibrous silica. In contrast, within the fine grained matrix, the disordered cristobalite takes the form of partly coalescent crude microgranules and microspherules. Most of the chalcedonic quartz in Cyprus is derived by recrystallization of previously inorganically precipitated disordered cristobalite rather than by direct precipitation. According to the concept of impurity-controlled maturation the composition of host sediment controls the incorporation of exchangeable cations and other impurities into inorganically precipitated disordered cristobalite. With time (up to 100 million years) internal solid state reorganization of the disordered cristobalite is accompanied by gradual expulsion of impurities, until the cristobalite dissolves followed by quartz precipitation. Complete conversion to quartz takes place first in porous calcareous sediments free of impurities, as in the Cyprus calciturbidites; in fine grained clay-rich sediments, like Cyprus radiolarian mudstones, disordered cristobalite persists much longer. Impurity-controlled maturation also helps explain the diagenesis of Cyprus chert nodules.  相似文献   

7.
Marine diatomaceous siliceous sediments in Neogene sections of northern Japan contrast with the Monterey Shale of California in containing many intercalations of acidic volcaniclastic sediments. Diagenesis of these sediments from deep boreholes and surface sections was investigated. Three diagenetic zones—biogenic opal, opal-CT and quartz zones—are recognized in siliceous sediments, corresponding roughly to amorphous silica, low cristobalite and quartz zones in acidic vitric volcaniclastic sediments. Opal-CT consists almost exclusively of silica and water, while low cristobalite contains appreciable amounts of A1, Ca, Na and K. In subsurface sections, values of d(101) spacing of opal-CT decrease progressively with increasing burial depth. The progressive ordering is not associated with additional silica cementation. In surface sections, the behaviour of d(101) spacing is complicated owing to the modification of the progressive ordering developed during burial diagenesis by later silica cementation during uplift. The cementing opal-CT is probably precipitated from percolating groundwater which dissolves siliceous skeletons in porous diatomaceous mudstones overlying the opal-CT porcellanite. Opaline cherts that form during burial diagenesis are designated as early opaline chert, while those which form during uplift are later opaline chert. The later opaline chert contains two groups of opal-CT; one is progressively ordered opal-CT and the other is additionally cemented opal-CT with higher d(101) spacing than that in the host porcellanite. In diatomaceous siliceous sediments, early opaline chert is scarce. Most, if not all, opaline cherts in surface sections are of later origin.  相似文献   

8.
A Mesozoic continental margin sequence in the Othris Mountains, eastern Greece, contains a variety of both primary and replacement cherts occurring in several stratigraphic positions. Primary cherts, defined as these derived from siliceous sediment, show differences in bedform, sedimentary structures, mineralogy and chemistry. Examples described illustrate deposition by turbidity currents, traction currents and an in-situ pelagic “rain”. Chemical analyses show that major elements can be used to discriminate between different chert formations. Iron and manganese enrichment is a feature of cherts which have been deposited under steady sedimentation conditions.Stratigraphic relationships prove the older, current-deposited, Othris cherts to have been deposites on a volcanic ocean ridge where they probably accumulated in topographic depressions, and also between the ridge and the lower continental margin where they were deposited mainly by turbidites derived from an oceanic source. A younger suite which covers calcareous shelf, marginal and pelagic sediments is an in-situ deposit. These inferences illustrate how primary cherts can add to interpretations of ancient oceanic terrains by providing information on sea-floor topography and, potentially, circulation patterns. Some cherts can be used as facing indicators in sequences where no other wayup evidence is available.  相似文献   

9.
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.  相似文献   

10.
The Middle Marker is a thin (3–6 m) sedimentary unit at the base of the Hooggenoeg Formation in the 3.4 Ga old Onverwacht Group, Barberton Mountain Land, South Africa. The original sediments consisted largely of current-deposited volcaniclastic detritus now represented by green to buff-colored silicified volcaniclastic rock and fine-grained gray chert. Black chert, possibly formed by the silicification of a non-volcaniclastic precursor, makes up a significant part of the unit. The Middle Marker is underlain and overlain by mafic and commonly pillowed volcanic flowrock. Although the original sediment has been replaced by and/or recrystallized to a microquartz, chlorite, sericite, carbonate and iron oxide mosaic under lower greenschist-grade metamorphism, sedimentary textures and structures are remarkably well preserved. Textural pseudomorphs indicate the primary volcaniclastic sediment consisted of a mixture of crystal, vitric and lithic debris. Middle Marker sediments were deposited as a prograding, cone-flanking volcaniclastic sedimentary platform in a relatively-shallow and locally current/wave-influenced subaqueous sedimentary environment. Available paleocurrent data indicate a largely bimodal, orthogonal distribution pattern which is quite similar to both ancient and modern shallow marine/shelf systems. Diagnostic evidence for tidal activity is lacking. As felsic volcanic activity waned, an extensive airfall blanket of fine-grained volcanic ash and dust was deposited in a low-energy subaqueous environment. The sedimentary cycle was terminated with a renewal of submarine mafic volcanism. Middle Marker volcaniclastic sediments accumulated in an anorogenic basin removed or isolated from the influence of continental igneous and metamorphic terranes. Although compositionally dominated by a volcanic source, Middle Marker sediments owe their final texture and sedimentary structures to subaqueous sedimentary rather than volcanogenic processes.  相似文献   

11.
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.  相似文献   

12.
Research Status and Advances in Chert and Permian Chert Event   总被引：1，自引：0，他引：1  
Yao Xu  Zhou Yaoqi  Li Su  Li Dou 《地球科学进展》2013,28(11):1189-1200
Cherts are concret research objects for petro-tectonic framework and are widespreadly distributed in special sedimentary environments along orogenic belts and depositional basins. Cherts hold abundant origin related information, which plays important role in palogeographic and paleooceanograpic study. This paper reviewed the history and the advancement of research on chert, especially paying more attention to the research progress about both bedded chert and nodular chert in domestic and international research areas. Combined with Permian cherts distribution in China, we discuss research achivements of silica sources, sedimentary environments and original mechanism from cherts deposited in different sedimentary environments such as oceanic rift, oceanic basin and continent shelf. “Permian Chert Event” (PCE) has been used to discribe the widespread biogenic Permian cherts that occurred around the eastern margins of Panthalassa and Paleotethys Ocean. Research results on Permian cherts in eastern Paleotethys Ocean, particularly the Yangtze areas, are examined here, which suggest Permian cherts are not typically biogenic in origin but severely influenced by hydrothermal sedimentation. The seasonal melting of sea ice is assumed engine for PCE in northwest edge of Pangea Supercontinent, but that dose not work in China. Therefore, a hypothesis different form thermohaline circulation and upwelling is proposed here: there would be a periodical hydrothermal fluid system in the eastern Paleotethys Ocean, providing the source of silica for the development of Permian Chert Event on both south and north continent shelves. What is more, some intensively studied Permian outcrops in the Yangtze areas have the potential to be the typical outcrops for PCE research in China.  相似文献   

13.
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  相似文献   

14.
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.  相似文献   

15.
The Precambrian–Cambrian Athel Silicilyte is a 400 m thick, salt‐encased siliceous succession in the South Oman Salt Basin. It is a self‐sourcing hydrocarbon reservoir and comprises up to 95% microcrystalline quartz and exhibits wavy discontinuous lamination, comprising thin, alternating organic‐rich and silica‐rich layers. Textures and geochemical fingerprinting indicate that it is a primary precipitate formed by microbially mediated precipitation of silica from sea water, within the water column at the sulphidic/oxic interface. The unique occurrence of the Athel Silicilyte in the terminal Proterozoic implies that optimal conditions for this style of silica precipitation occurred only briefly. Basin anoxia, coupled with the growth of microbial mats, low pH and high silica pore water saturations, created optimal chemical conditions for silica precipitation. Volumes of microcrystalline quartz are highest within the transgressive and early highstand systems tract and towards the centre of the Athel Basin. At the basin margins, and within the late highstand systems tract, volumes of microcrystalline quartz decreased as the volume of detrital sediment increased. Mass‐balance calculations indicate that silica‐enriched sea water would have been supplied to the basin by infrequent marine incursions that replenished ambient sea water in the upper part of the water column. In conclusion, precipitation of the Athel Silicilyte was driven by the coincidence of basin restriction, limited clastic input, availability of organic matter and water column anoxia. The observation that there are few documented examples of chert deposits younger than ca 700 Ma, prior to the Cambrian explosion, suggests that although silica budgets within marine basins probably remained high prior to the evolution of silica‐secreting organisms, direct precipitation from sea water was restricted. This is tentatively related to the gradual increase in alkalinity of sea water through the Palaeo‐Proterozoic and Meso‐Proterozoic, such that silica precipitation could only occur through the coincidence of basin anoxia and low siliciclastic input.  相似文献   

16.
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.  相似文献   

17.
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18.
The broad range of time over which ribbon bedded cherts were deposited does not extend into the present marine environment, and no ribbon cherts have been recovered from the sea floor by the Deep Sea Drilling Project. The depositional environment of bedded cherts is difficult to determine, but extra-silicic impurities in the rock may offer clues about the provenance of the non-biogenic component. To test the usefulness of relative abundances of the extra silicic components in extracting information on the depositional environment of the chert, I analyzed the major element chemistry of chert samples from a broad range of environments including ophiolite-associated chert from the Franciscan Formation of California, deep-sea chert and porcellanite from the northwest Pacific (DSDP Leg 32), shallow pelagic shelf chert nodules from the Chalk of Britain, continental marginal basin chert from the Monterey Formation of California, and continental marginal basin chert from the Pindos Zone of Greece. The ratios FeO/A12O3, TiO2/A12O3 and A1/A1+Fe+Mn were considered in detail. The interpretative logic is simple but empirically supported by observations of these ratio values at different depositional environments in the Pacific: A1 is concentrated most highly in continental material while Fe and Mn are more concentrated in pelagic sediments. FeO/A12O3 can be used to differentiate between ophiolite associated chert and chert associated only with other sediments. TiO2/A12O3 is not a useful indicator, possibly because of the equalizing effect of widespread eolian transport. The A1/A1+Fe+Mn ratio was measured in detail in one stratigraphic section in central continental Greece. This ratio varied with the type of sediment admixture, decreasing in value after the influx of ophiolite debris-bearing sediments, even when their presence was undetectable in hand sample or under petrographic microscope.To help clarify the paleogeography of the main study area, the Pindos Zone, and to identify sources and dispersal patterns of extra-basinal materials, isopach maps of sedimentary facies of the Pindos were constructed. Superimposed directly upon the series of imbricated thrust slices that comprise the Pindos Zone, the maps are at best compressed pictures of the Pindos Sea Floor. Persistent regional variation of facies thicknesses over time suggests the existence of several smaller depressions surrounded by submarine highs in the Pindos Basin.  相似文献   

19.
邱振  王清晨  严德天 《岩石学报》2011,27(10):3141-3155
广西来宾地区中上二叠统广泛发育硅质岩,蓬莱滩剖面和铁桥剖面是本地区中上二叠统出露最好的剖面.本文对蓬莱滩剖面28件硅质岩(分别为中二叠统茅口组3件与上二叠统合山组25件)的主量和稀土元素进行了分析研究,并结合岩石学特征,认为:蓬莱滩剖面合山组硅质岩沉积于受到陆源物质影响的边缘海盆环境,它们的硅质主要来源于硅质生物,为生物成因;而其茅口组硅质岩则沉积于远离陆源物质影响的边缘海盆环境,它们的硅质主要来源于热液,为热液成因.通过与铁桥剖面的热液成因硅质岩的沉积背景对比研究,结果表明:中晚二叠世时期,来宾地区为富硅的边缘海盆环境,而陆源物质输入程度的差异是造成本地区形成不同成因硅质岩的主要因素.  相似文献   

20.
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