Silica diagenesis of Neogene diatomaceous and volcaniclastic sediments in northern Japan*

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.     

Petrography of Late Cambrian to Middle Ordovician radiolarian chert in Kazakhstan with special reference to the emergence of benthic animals in the pelagic realm

Upper Cambrian to Middle Ordovician radiolarian chert successions from Kazakhstan were studied to clarify the history of the emergence of benthic animals in ocean floor sediments. Radiolarian tests and clay were deposited for a period of 30 Myr without experiencing an influx of continent-derived coarse clastic materials. Red, grey and black cherts of the Upper Cambrian to the upper mid-Darriwilian are thinly laminated, and no trace of benthic animal activity is recognized in that time interval. Bioturbation structures and burrow traces in mid-Darriwilian stage red chert in Kazakhstan suggest that benthic animals colonized the location where radiolarian chert formed, but that there was a significant delay in colonization when compared with similar reported occurrences in Australia and Canada.     

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.     

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.     

Radiolarites et sédiments hypersiliceux océaniques: une comparaison

The mineralogical and chemical composition of Jurassic radiolarian cherts has been studied in Morocco (Rif), Italy (Lombardy basin and Apennines), Greece (Pindus zone and Vourinos Massif), some in close association with ophiolites. We have compared these samples with Cretaceous cherts from the NW Pacific (Leg 32) and with Cenozoic diatomaceous oozes from the Sea of Japan (Leg 31). The silica in the radiolarian cherts is quartz or chalcedony. Most of these rocks also contain feldspars and hematite while the clay fraction is composed of illite and/or chlorite generally associated with swelling clays and, locally, with kaolinite. In oceanic sediments all mineralogical species of silica have been detected (from opal to quartz), the clays generally being the same as those of the radiolarian cherts, the feldspars also being present. Based on the chemical composition of the radiolarian cherts, three facies can be distinguished: massive cherts, pelitic radiolarites and ferruginous radiolarites, the latter occurring only near the contact with volcanic basement. The chemical composition of cherts and diatom oozes from the Pacific is very close to the composition of radiolarian cherts. Although the mineralogy of radiolarian cherts can be related to several models (detrital, diagenesis of pelagic clays etc.) the detrital origin of part of the clay fraction seems certain. The origin of silica and its relation to the palaeolatitudes and the relatively confined nature of the Tethys oceans as well as the influence of volcanic inputs are evaluated, Chemical and mineralogical composition of radiolarian cherts shows that the diagenesis of the clay fraction is not a significant source of silica. Accumulation of diatom oozes in the Sea of Japan and in other areas, shows that the distance from continents and very deep seas are not essential to the development of siliceous sedimentation.     

An attempt to quantitatively reconstruct the paleo-primary productivity by counting the radiolarian fossils in cherts from the latest Permian Dalong Formation in southwestern China

A quantitative reconstruction method of paleo-primary productivity is present on the basis of the investigation on the faunal record in cherts. By obtaining the accumulation rate of radiolarian shells in the cherts and considering all aspects of silica loss before burial, such as dissolution in the eutrophic zone or within the sediments, it is possible to reconstruct the production rate of radiolarian in the surface water, which would be used to estimate the paleo-primary productivity. This method is applied to the end Permian chert sequence from Dalong Formation in Dongpan area, southwestern China. The resulting paleo-primary productivity of the ocean in end Permian south China is about 477.9 g C/m²/a during the sedimentation of cherts, which, to some extent, agrees with the primary productivity reported for some modern oceans.     

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.     

The radiolarian evidence for the accretion of the Fu-saki Formation with the inferred oceanic plate stratigraphy: A case of weakly-metamorphosed accretionary complex in Ishigaki Jima,southern Ryukyu Arc,Japan

The island of Ishigaki Jima, located in the western part of the southern Ryukyu Arc, Japan, is underlain by a basement comprising the Tumuru and Fu-saki formations. The former is a pelitic glaucophane schist with a metamorphic age of 220–190 Ma, and the latter is a weakly metamorphosed accretionary complex, composed mainly of chert, mudstone and sandstone with minor amounts of limestone and mafic rocks. The Fu-saki Formation was weakly metamorphosed at ∼140 Ma. Latest Carboniferous–Early Jurassic microfossils have been obtained from the limestones, cherts and siliceous mudstones of this formation, but no fossils have been collected from the phyllitic mudstones. The radiolarian fauna of the phyllitic mudstones described herein indicates a late Pliensbachian–early Toarcian (Early Jurassic) age. This result, when combined with existing data, enables the reconstruction of an oceanic plate stratigraphy, showing a succession of (in ascending order) Upper Carboniferous–Triassic cherts, Sinemurian–lower Pliensbachian siliceous mudstones and upper Pliensbachian–lower Toarcian phyllitic mudstones and sandstones. The radiolarians from the phyllitic mudstones are important in constraining the timing of the accretion of the Fu-saki Formation to the base of the Tumuru Formation.     

Composition and origin of North Atlantic deep sea cherts

Rocks described as chert from the North Atlantic, recovered during the Deep Sea Drilling Project, consist of quartz or disordered cristobalite. They occur in a wide variety of sediment types; only a few are associated with radiolarian oozes. Some of the cristobalitic cherts contain abundant clinoptilolite or palygorskite and sepiolite.The cherts have a variety of origins. They are most probably replacement cherts at sites 5, 6, and 7; direct alteration products of radiolarian ooze at site 8; and either alteration products of volanic glass or direct precipitates from hydrothermal solutions at sites 9 and 12. Evidence for these diverse origins is preserved as unusual mineral assemblages in these geologically young rocks.     

Displaced dolomites in radiolarian cherts of the Chichibu Belt on Shikoku Island,Southwest Japan

The northern tract of the Chichibu Belt on Shikoku Island, Southwest Japan, in places contains dolomites of Late Carboniferous age displaced into radiolarian cherts. The sections here examined are along the Niyodo gorge, central Shikoku. The sequence begins with thinly interbedded dolomitized radiolarian cherts and dolomites with a small amount of dolomitized radiolarian claystone and calcisiltite beds. These rocks, 5–10 m thick, are succeeded by a thick section of bedded and massive dolomites, commonly 40–50 m thick, which have thin intercalations of radiolarian claystone in the upper part. This dolomite sequence is depositionally overlain by a sequence, up to 50 m thick, comprising thinly interbedded radiolarian cherts and claystones, which, in turn, contain lenticular bodies of dolomite.Thin-section examination reveals that most of dolomites of the area have an arenitic or lithic texture, and should be termed doloarenite and dololithite. This means that dolomites are detrital. All lines of evidence suggest that the dolomites were not formed in the same depositional site in which the radiolarian cherts were being accumulated, but formed instead as allochthonous bodies which were displaced into a deep oceanic basin of chert deposition.The following sequence of events is postulated: (1) deposition of shallow-water calcareous sediments in a subtidal area; (2) dolomitization in a very shallow-water to supratidal environment; (3) displacement of dolomitized sediments, possibly mainly as debris flows into a deep-water, truely pelagic realm, in which siliceous radiolarian sediments were accumulating; (4) continued accumulation of siliceous sediments after the major influx of dolomitized sediments; and (5) minor influxes of dolomitized sediments during the continuous accumulation of siliceous sediments.     

Clinoptilolite, paragenesis and stratigraphy

Clinoptilolite, a zeolite of the heulandite group, occurs commonly in sediments as an authigenic mineral. In the Middle Eocene of southern Israel, it constitutes from a few per cent up to 80 per cent of the insoluble residue of the chalks and limestones. It is associated with opal C-T, montmorillonite and palygorskite. These chalks and limestones overlie the Danian-Palaeocene Taqiya marls which also contain a well-established clay mineral sequence consisting of opal C-T, montmorillonite, palygorskite, sepiolite, and clinoptilolite. This paragenesis of minerals is shown to be typical of the Upper Cretaceous to Eocene times. It is world-wide, occurs in deep-sea sediments as well as in shallow water sediments, and results from the abundance of silica which probably reflects a warmer climate during this time period. The concentration of magnesium in the sea-water and its ratio to the other cations seem to determine which authigenic silicate will be formed.     

Glauconitization of detrital silica substrates in the Barton Formation (upper Eocene) of the Hampshire Basin, southern England

Upper Eocene detrital silica grains (chert and quartz) of the Hampshire Basin display alteration and replacement fabrics by glauconite. Silica grains have etched surfaces due to glauconitization which appear green in reflected light and thin section. Quartz grains were glauconitized by surface nucleation and replacement, which spreads from the margin with progressive glauconitization, replacing the quartz grain interior. Chert grains were glauconitized by surface replacement and nucleation internally along cracks and in pores. Different forms of glauconite are associated with the two minerals; glauconite associated with quartz is generally highly-evolved whereas glauconite associated with chert is of the evolved variety. This is interpreted as being due to different surface-reaction control mechanisms associated with the two forms of silica. There is no evidence to suggest that glauconite evolved in stages from a nascent form. Two crystalline morphological forms of glauconite are found associated with both quartz and chert. Glauconite growing within a confined space has a laminated morphology whilst glauconite occurring on the surface has a rosette morphology.     

Boron in chert and Precambrian siliceous iron formations

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.     

A new interpretation of the structure of disordered α-cristobalite

Disordered -cristobalite which occurs extensively in bentonites, opals and particularly deep-sea cherts, has been previously interpreted in terms of a unidimensionally disordered structure in which cristobalite is interstratified with two-layer tridymite-like sheets. An alternative interpretation is that the structure is essentially tridymitic but that the sheets are stacked with random transverse displacement normal to the c axis, an arrangement similar to the translational, turbostratic stacking postulated for smectites. This interpretation was arrived at after a comparitive study of a silica phase in an Italian bentonite and a deep-sea chert, material which yielded an X-ray powder pattern almost identical with that of disordered -cristobalite, but electron diffraction patterns and infrared spectra more consistent with tridymite. It is suggested that this type of silica, which has been described almost universally as cristobalite, is more appropriately referred to as disordered -tridymite.Thanks are due to Mr R. H. S. Robertson, Pitlochry for the sample of bentonite and Mr D. S. Cassidy, Associate Curator, Antarctic Research Facility, Florida State University, U.S.A. for the sample of deep-sea chert.     

Archean cherts derived from chemical, biogenic and clastic sedimentation in a shallow restricted basin: examples from the Gorge Creek Group in the Pilbara Block

The >3·0 Ga chert sequence of the Gorge Creek Group is exposed at Ord Ranges about 50 km east of Port Hedland in the Pilbara Block. The chert sequence examined in this study is 15 m thick and consists of oxide-rich laminated chert, grey chert (silicified clastic rock), carbonaceous black chert and carbonate-rich laminated chert. Although the cherts have undergone postdepositional silica enrichment, such as cementation and metasomatic silicification, primary precipitation of silica at the site of deposition is indicated by abundant microstructures (mosaic and spherulitic structures). Other primary to early diagenetic components were carbonates, sulphates (gypsum and anhydrite) and organic matter. Although these mineral associations, on the whole, correspond to those of modern marine evaporites, they are different from modern equivalents with respect to abundant precipitation of amorphous silica and presumed primary precipitation of iron-carbonate (siderite). This feature is a possible manifestation of peculiar physicochemical conditions in the water mass from which the chemical sediments were precipitated; compared with modern ocean waters, the concentrations of Fe and Si were significantly higher and the pH value might have been lower. These conditions could be obtained by contributions of Fe- and Si-enriched hydrothermal solutions and continental run-off to the site of deposition. Grey cherts contain detrital quartz and altered Fe–Ti oxides and were formed in a period of input of terrigenous detrital materials. They are characterized by higher concentrations of TiO₂, Al₂O₃, Cr, Ni, Zn, Rb and Zr compared with the other types of chert and by very low (< 4) Al₂O₃/TiO₂ values. These features are attributed to the supply of terrigenous detrital materials that contain abundant Fe–Ti oxides (ilmenite and titanomagnetite) and fine TiO₂ particles. Such detrital materials might have been formed by extensive chemical alteration of source rocks and residual enrichment of Ti relative to Al.     

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。     

Stratigraphy and radiolarians of upper cretaceous sedimentary cover of the Arakapas ophiolite massif (Cyprus)

In the basal interval, sedimentary cover of the Arakapas ophiolite massif (southern Cyprus) is composed of metalliferous sediments of the Perapedhi Formation that is divided into three sequences based on diverse radiolarian assemblages. These are basal umbers of the Cenomanian age presumably (2–20 m), interlayering cherts and umbers of the Turonian-Coniacian (6–10 m), and opoka-like cherts of the Coniacian-Santonian. Higher in the succession, there are olistostrome deposits of the Moni Formation, which unconformably rest on the eroded underlying strata. In this formation also divisible into three sequences, the lower one 200 to 300 m thick is composed of variegated, presumably Campanian silty clays containing olistoliths of basic, presumably Upper Triassic volcanics, Lower Cretaceous sandstones, and opoka-like cherts and cherty limestones of the Albian-lower Cenomanian. Next sequence (100–200 m) is represented by alternation of variegated silty and green bentonitic clays of the Campaian, which enclose frequent olistoliths and horizons of fine-clastic olistostrome breccias. The upper sequence of upper Campanian-lower Maastrichtian bentonitic clays (50–100 m) contains interlayers of ash tuffs and clayey cherty sediments. Carbonate deposits of the upper Maastrichtian-Paleogene, conformably overlie the Moni Formation.     

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.     

ELECTRON-MICROSCOPIC EXAMINATION OF NAMURIAN BEDDED CHERTS, NORTH WALES (GREAT BRITAIN)

Electron microscopic examination of Namurian bedded cherts in North Wales (Great Britain) has revealed the existence of two dominant surface textures, granular and spongy, with a range of textures intermediate between the two. These textures can be used to differentiate between three principal lithologies within the cherts and between laminae and colour bands within the lithologies. A positive relationship is demonstrated between the presence of spongy texture and a high volume of impurities; there is a gradation, with increasing argillaceous/organic content, from granular through intermediate to spongy surfaces. It is suggested that this gradation reflects an increase in chalcedony over micro-crystalline quartz resulting from: (l) an increase in impurities; (2) an increase in the rate of precipitation of silica. The relationship between surface texture and type of lithology is considered to be consistent with the order of crystallisation for silica put forward by MILLOT (1960).     

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.