Diagenetic Chert Research Articles

Conceptual model for early diagenetic chert and dolomite, Amuri Limestone Group, north‐eastern South Island, New Zealand

ABSTRACTThe Late Cretaceous to Early Eocene, dominantly micritic, Amuri Limestone Group (ALG) was deposited in an approximately NW trending trough, in eastern Marlborough, New Zealand. The ALG comprises: the Mead Hill Formation; the Teredo, Lower and Middle Limestone formations; and the Upper and Lower Marl formations. Chert and dolomite are concentrated in the Mead Hill Formation, which contains five of six recognized diagenetic zones: Zone I at the base of the ALG consists almost entirely of chert; Zone II consists solely of chert and dolomite; Zone III comprises chert and limestone; Zone IV is composed of chert plus dolomite; Zone V is a chertified mudstone; and the minor amounts of chert found in the Middle Limestone Formation comprise Zone VI. With the exception of Zones IV and V, chert decreases stratigraphically upwards and away from the basin centre. All the dolomites are composed of <1 mm diameter rhombohedra in discontinuous beds and lenses. Generally Ca‐rich, and non‐ to slightly ferroan, the dolomite contains approximately 500–900 ppm Mn and 200–400 ppm Sr. δ13C values average 1–2%PDB with δ18O ratios of about ‐4%PDB. Mass balance calculations indicate that the Mg2+ for dolomitization was derived from sea water. Sr, Fe and Mn concentrations are interpreted as indicating dolomite formation in the marine environment, with no influence from meteoric waters. The intimate association with pyrite implies dolomite formation in association with sulphate reduction, in the upper sediment column. δ18O data show that the bulk of the dolomite formed at temperatures below 50°C. All chert samples contain in excess of 90 wt% SiO2, about 1 wt% Al2O3 and 1 wt% from losses on ignition. Generally all other major elements total less than 2 wt% oxide. δ18O values range from 26·8 to 29·0%SMOW. Chert chemistry is consistent with the replacement of host carbonate and expulsion of carbonate‐bound components from the site of chertification, and the effective dilution by SiO2 of non‐carbonate‐bound insoluble residues. δ18O data indicate that chert formed in fluids of similar composition and temperature as the dolomite. The abundance of disseminated pyrite in cherts implies an association with sulphate reduction. Silica for chertification is thought to have initially come from dissolution of siliceous organisms. However, there is insufficient biogenic silica available to form the volumes of chert observed. It is suggested that the bulk of the silica came from SiO2‐rich pore waters generated by clay mineral reactions in the thick underlying mudstones. The ALG compacted down through these pore waters. Chert and dolomite nucleation are considered to have been penecontemporaneous. Dolomitization was initially probably the faster process, continuing as long as sulphate reduction prevailed and there was an adequate supply of Mg2+. The nucleation of chert, although initially slower (probably due to a relatively lower initial SiO2 supply), continued after cessation of dolomitization to the extent of completely chertifying the dolomite intercrystalline matrix. The amount of chertification decreased progressively as SiO2 supplies diminished, both stratigraphically upwards and away from the basin centre.

Sedimentology and petrography of early diagenetic chert and dolomite in the Late Cretaceous‐early Tertiary Amuri Limestone Group, eastern Marlborough, New Zealand

Abstract The dominantly micritic Amuri Limestone Group of eastern Marlborough, New Zealand, consists of six regionally extensive units: the Mead Hill Formation, Teredo Limestone Formation, Lower Limestone Formation, Middle Limestone Formation, and the Lower and Upper Marl Formations, which were deposited in a northwest‐trending trough. Six separate diagenetic zones (I‐VI) are recognised, based on visually estimated proportions of chert, dolomite, or limestone. Zones I‐V occur in the Mead Hill to Lower Limestone Formations, and Zone VI occurs in the Middle Limestone Formation. There is no chert or dolomite in the two Marl formations. The bulk of the chert and dolomite occurs in the basin centre (mainly in the Mead Hill Formation) where the limestone is thickest, but decreases in abundance stratigraphically upwards and towards the basin margins. Zone I consists almost entirely of chert which is either detrital rich or detrital poor. Zones II and IV consist of chert and dolomite in approximately equal propor...

Silicification and dolomitization of the Lower Eocene carbonates in the Eastern Desert between Sohag and Qena, Egypt

The Lower Eocene carbonates in the Eastern Desert between Sohag and Qena show remarkable diagenetic silicification and dolomitization beside other minor features of solution, cementation, syntaxial rim cement, compaction, recrystallisation and authigenic gypsum, anhydrite and halite formation. Two stages of silicification are recorded in the studied sequence, namely the syngenetic stage and the replacement stage. The first one is confined to the studied sequence, namely the syngenetic stage and the replacement stage. The in the open shelf lagoon facies as hard, dark silicified limestone concretions. The very early diagenetic chert formation was interrupted by the introduction of dolomite suggesting that dolomization postdates silification. The mode of occurrence of the syngenetic chert in the study area seems to be controlled by: the volume of the influx of hydrous silica in the interstitial water and the rate of lime mud deposition. Dolomites which are recorded here for the first time, represent about 40–60% of the carbonates and are confined mainly to the lime muds of the tidal flats and oyster bioclastic nearshore facies. The dolomitization phenomenon is considered to be of late diagenetic stage. Small elongate evaporite laths are scattered in the dolomitic lime mudstones. These authigenic evaporates associating the dolomitic facies of the tidal flats may suggest that hypersaline brines, rich in magnesium, were probably adjacent to these carbonates.

Secular Change in Chert Distribution: A Reflection of Evolving Biological Participation in the Silica Cycle

In the modern oceans, the removal of dissolved silica from sea water is principally a biological process carried out by diatoms, with lesser contributions from radiolaria, silicoflagellates, and sponges. Because such silica in sediments is often redistributed locally during diagenesis to from nodular or bedded chert, stratigraphic changes in the facies distribution of early diagenetic chert provide important insights into the development of biological participation in the silica cycle. The abundance of chert in upper Proterozoic peritidal carbonates suggests that at this time silica was removed from seawater principally by abiological processes operating in part of the margins of the oceans. With the evolution of demosponges near the beginning of the Cambrian Period, subtidal biogenetic cherts became increasingly common, and with the Ordovician rise of radiolaria to ecological and biogeochemical prominence, sedimented skeletons became a principal sink for oceanic silica. Cherts of Silurian to Cretaceous age share many features of facies distribution and petrography but they differ from Cenozoic siliceous deposits. These differences are interpreted to reflect the mid-Cretaceous radiation of diatoms and their subsequent rise to domination of the silica cycle. Biogeochemical cycles provide an important framework for the paleobiological interpretation of the organisms that participate in them.

The geological significance of manganese distribution in Jurassic-Cretaceous rocks of the Pindos Basin, Peloponnese, Greece

The Jurassic-mid-Cretaceous Radiolarite Series of the Pindos zone of the Peloponnese is a deep-water sediment sequence. It comprises Jurassic red mudstones, siliceous mudstones and ribbon-bedded cherts; and Cretaceous red mudstones, siliceous mudstones, cherts, calc-turbidites and terrigenous turbidites. Bedded manganese ores occur both in cherty calc-turbidites and ribbon-bedded cherts. In order to determine the origin of Mn enrichment, the transition element geochemistry and mineralogy has been determined for sediment samples from seven representative sections of the Radiolarite Series, representing an along and across strike section of the Pindos basin. Smectite and palygorskite are the principal clay minerals, with lesser mica, sepiolite and chlorite. Fe Al ratios in Jurassic rocks are close to values in continental detritus but increase upwards to the middle Cretaceous. Nevertheless, the association of volcanic rocks with Jurassic sediments is circumstantial evidence that the minor Mn enrichment is the result of hydrothermal processes. The hypothesis that high Fe Al in Cretaceous sediments is due to ophiolitic detritus is untenable in the Peloponnese, where sediment is derived from sources lacking ophiolites: the Fe enrichment must therefore result from hydrothermal processes, which may be related to formation of oceanic crust in the Pindos-Arvi zones. Mn concentrations are associated with diagenetic cherts at the top of calc-turbidites; in these settings, Mn correlates positively with Ca and negatively with Fe, suggesting that the concentration of beds of ore is diagenetic.

Silica diagenesis: origin of inorganic and replacement cherts

Silicification of originally non-siliceous sediments affects a wide variety of rock-types and materials and ranges from minor to pervasive. Partial and minor chertification occur mostly in Phanerozoic carbonates, carbonate-bearing sandstones, evaporites, and fossil wood. The source of the silica is predominantly biogenic. In petrified wood the silicification mechanism is a permeation or void-filling process, not a replacement. In this example, the sequence of silica-phase transformation is the same as that in deep-sea siliceous sediments. In many silicified rocks, particularly in certain carbonates, the transformation sequence is different from that in radiolarites or diatomites. The chemical environment and conditions of early diagenetic chert formation in shallow water carbonates are delineated by the general mixing model of Knauth (1979), but remain unknown for most other types. An exception are the flint nodules and bands of the English Chalk. A detailed geochemical study of the paramoudra flint structures by Clayton (1986) provided remarkable insight into the replacement process. Seven different recurring silica fabrics have been recognized in chert-replaced carbonates including equigranular (microcrystalline quartz or microquartz and megaquartz) and fibrous types (chalcedony, quartzine or length-slow chalcedony, lutecite, zebraic chalcedony and microflamboyant quartz). Among the latter, quartzine and microflamboyant quartz are common in, but by no means restricted to chert-replaced evaporites, for which Milliken (1979) recognized a sequence of seven quartz-fabrics. As a single criterion, only anhydrite inclusions in megaquartz, quartzine or microflamboyant quartz provide unequivocal evidence for an evaporite precursor. The relative timing between silicification and well-established diagenetic carbonate reactions shows that virtually all theoretically possible sequences occur. Chertification of carbonate host sediment thus may take place during early, intermediate or late diagenesis, and even during anchimetamorphism. Pervasive to complete silicification has been described in lacustrine, pedogenic and hydrothermal volcanogenic rocks and occurs on the scale of individual beds, members or entire formations. It may affect some of the aforementioned rock types too, for example carbonates. The source of the silica is predominantly inorganic. Chert formation in these environments includes direct chemical silica precipitation from solution through a gel stage. Magadi-type cherts result from the conversion of the hydrous sodium silicate magadiite into microcrystalline quartz which has occurred in East African rift valley lakes such as Lake Magadi in Late Pleistocene time. They are closely related to directly precipitated inorganic cherts which have been observed in alkaline environments of playa lakes. Silcretes originate from weathering and soil-forming processes under climatic and environmental conditions conducive to the formation of duricrusts and laterites. In more humid climates, however, non-weathering-profile silcretes occur which have been distinguished from weathering-profile silcretes. Four to five different silcrete fabric types have been established, including the (1) grain-supported (or ‘quartzitic’), (2) floating (terrazzo), (3) matrix (Albertinia and opaline), and (4) conglomeratic types. In volcanic edifices, silicification related to hydrothermal activity occurs (1) along the ascent-routes of the fluids (vents) within the volcanic complexes, (2) in isolated ponds and depressions on the sea-floor where the fluids discharge, mostly in the median rift valley of mid-ocean ridges, and (3) in geothermal areas on land associated with spreading lineaments, volcanic islands arcs, transform faults or continental hot spots. Attention has focussed on the associated base-metal concentrations and less on the silicification and accompanying iron enrichment processes. The Precambrian Banded Iron Formations may be indirectly or directly related to hydrothermal-volcanic processes. The origin of these cherty iron formations certainly requires more than one genetic model and at present remains a major unresolved problem despite massive efforts to tackle it.

Crystallite Size of Diagenetic Chert in Carbonate Rocks--Probable Indicator of Prelithification Porosity: ABSTRACT

Silicification and dolomitization in seven constituent lithofacies of the Meramecian carbonate sequence in central eastern Missouri are genetically related and are facies dependent. In the arenite facies with variable lime-mud matrix (0-30%), both were probably caused by phreatic freshwater and seawater mixing. Silicification is characterized by microcrystalline quartz-replaced (MCQ-replaced) bioclasts and pore-occluding fibrous chalcedony (CHY); the latter is common as micronodules. In these facies-specific, silica-replaced bioclasts, MCQ has abnormally large crystallite size in parts of the profile characterized by blocky low-matrix cement. In contrast, the matrix-rich facies, characterized by extensively dolomitized matrix, have smaller crystalline size in the MCQ-replaced bioclasts. The conventional model of thermal annealing does not adequately explain the observed, intermittently anomalous, large crystallite size of the diagenetic chert in the Meramecian arenite facies, considering the regional thermal and burial history of the rocks. Also, the chert crystallite size inversely correlates with the matrix mud content in the facies. Therefore, the authors propose that the observed variation in the size of the chert crystallites reflects the degree of silica saturation in the pore water. The degree of silica saturation is a function of permeability controlled by the effective prelithification porosity in these carbonates. Preliminary data suggest that chertmore » crystallite size can be used to map the distribution of prelithification porosity in carbonate rocks that were substantially modified in texture during diagenesis. Oxygen isotopic composition of these cherts may help delineate the fluid-migration paths in these rocks. This information is useful in exploring for energy resources.« less

Exceptional preservation of photosynthetic organisms in silicified carbonates and silicified peats

Carbonaceous cherts in Proterozoic carbonate sequences provide an exceptionally clear record of early microbial life, but one that is significantly biased with respect to the range of environments inhabited by contemporary organisms. Many of the best preserved Proterozoic microfossil assemblages come from microbial mats and organicrich muds that accumulated in protected coastal areas where a combination of high productivity, limited water circulation, and, often, hypersalinity limitedpost mortemdegradation. The close distributional relationship between early diagenetic silica and organic matter can be explained in terms of a model developed by Leo and Barghoorn for the silicification of wood. Three factors appear to control the distribution of early diagenetic chert in Proterozoic sequences: sediment permeability, availability of silica in ground water solution, and locally high concentrations of organic matter in near-surface sediments. Of these, organic content appears to impose the major environmental bias. In terms of their excellent preservation, geochemistry of formation, and limited environmental coverage, Phanerozoic silicified peats bear comparison with their Proterozoic counterparts. Swamp dwellers may be the plants most likely to be preserved exceptionally well, but they may also be the plants least likely to give rise to new populations that will become ecologically widespread and evolutionarily important in subsequent periods. Allochthonous elements in permineralized peats may be unusually important to palaeobotany because they combine the exceptional preservation conferred by peat permineralization with ecological representation of floodplain and upland evolutionary cradles rather than swampland museums.

Early diagenetic bedded cherts of the Tuscan sequence (Miocene) near Città di Castello, Italy

Summary. A distinctive level of bedded, internally laminated, deep-sea cherts in the area west of Città di Castello, central Apennins, has been studied. These cherts form part of the Lower Miocene Monte Santa Maria Tiberina Formation, and belong to the Tuscan sequence, consisting of pelagic and turbidic sediments. The sediment is enriched in diatoms and Radiolaria. The diatoms provided silica as well as barium, now present as submicroscopical baryte concretions. Synsedimentary volcanism could have provided silica as well, through calcitization of volcanic fragments within the sediment. Since the laminations display early diagenetic microdeformations,. it can be concluded that the cherts themselves are an early diagenetic feature.

Siliceous Microfossils from the Lower Cambrian of Northwest Canada: Possible Source for Biogenic Chert

Round to oval, scalelike siliceous microstructures from cherts in Cambrian limestones of the western Yukon Territory suggest affinity with chrysophycean algae. At least six morphotypes present include porous forms with single branching processes and nonporous oval, ringlike forms. Partially dissolved specimens may indicate a contribution to contemporaneous or early diagenetic chert formation.

Earliest Phanerozoic or latest Proterozoic fossils from the Arabian Shield

Cloud, P., Awramik, S.M., Morrison, K. and Hadley, D.G., 1979. Earliest Phanerozoic or latest Proterozoic fossils from the Arabian Shield. Precambrian Res., 10: 73--93. We report here the first biologically definable fossils from pre-Saq (pre-Middle Cambrian) rocks of the Arabian Shield. They include the distinctive helically coiled tubular filaments of the oscillatorialean blue-green alga Obruchevella parva as well as two size classes of spheroidal unicells of uncertain affinity. Also present is the conical stromatolite Conophyton and unidentified stromatolites. All occur in cherty limestones of the Jubaylah Group, northern Saudi Arabia, a nonmarine to locally marine taphrogeosynclinal sequence that fills depressions along the northwest-trending Najd faults. Conophyton has heretofore been found only in strata older than about 680 Ma (except for puzzling records in modern hot springs) while Obruchevella is so far known only from rocks between about 680 and 470 Ma old. Thus it appears that the Jubaylah Group is close to the Proterozoic-Phanerozoic transition. The simple spheroidal nannofossils are not diagnostic as to age. Their relationships within what appears to be early diagenetic chert suggest a classical algal-mat association. The brecciated and microchanneled appearance of much of the fossiliferous rock, its locally dolomitic nature, and the prevalence of cryptalgalaminate favors a very shallow, locally turbulent, and perhaps episodically exposed marine or marginal marine setting. The Jubaylah Group lies unconformably beneath the Siq Sandstone (basal member of the Saq Sandstone) of medial Cambrian age, rests nonconformably on crystalline basement, *The on Precambrian Stratigraphy of the International Union of Geological Sciences in July 1977 recommended the use of the time-honored term Proterozoic for time and events between Archean and Phanerozoic (James, 1978) and this usage has been adopted by the U.S. Geological Survey (Sohl and Wright, 1978). To date the Subcommission has not reached a decision on placement of the ProterozoicPhanerozoic boundary. In this paper, however, we follow the practice of including in the Phanerozoic, and thus regarding as post-Proterozoic, those sub-Cambrian rocks and events that are referable to the paleontologically defined Ediacarian System of Termier and Termier (1960) as discussed by Cloud (1976). For clarity in discussion, the term Precambrian with a capital P is here avoided and rocks older than Ediacarian (or Vendian are designated as pre-Phanerozoic.

Development of sedimentary basins on the lower trench slope : MOORE G. F. and D. E. KARIG, 1976. Geology, 4 (11): 693–697

We report here the first biologically definable fossils from pre-Saq (pre-Middle Cambrian) rocks of the Arabian Shield. They include the distinctive helically coiled tubular filaments of the oscillatorialean blue-green alga Obruchevella parva as well as two size classes of spheroidal unicells of uncertain affinity. Also present is the conical stromatolite Conophyton and unidentified stromatolites. All occur in cherty limestones of the Jubaylah Group, northern Saudi Arabia, a nonmarine to locally marine taphrogeosynclinal sequence that fills depressions along the northwest-trending Najd faults.Conophyton has heretofore been found only in strata older than about 680 Ma (except for puzzling records in modern hot springs) while Obruchevella is so far known only from rocks between about 680 and 470 Ma old. Thus it appears that the Jubaylah Group is close to the Proterozoic-Phanerozoic transition. The simple spheroidal nanno-fossils are not diagnostic as to age. Their relationships within what appears to be early diagenetic chert suggest a classical algal-mat association. The brecciated and microchanneled appearance of much of the fossiliferous rock, its locally dolomitic nature, and the prevalence of cryptalgalaminate favors a very shallow, locally turbulent, and perhaps episodically exposed marine or marginal marine setting.The Jubaylah Group lies unconformably beneath the Siq Sandstone (basal member of the Saq Sandstone) of medial Cambrian age, rests nonconformably on crystalline basement, and has yielded a K-Ar whole-rock age (on andesitic basalt) of ∼540 Ma. To judge from the fossils, however, that age may be as much as 100 Ma or more too young.

Errata: Early Diagenetic Chert Nodules in Bhander Limestone, Maihar, Satna District, Madhya Pradesh, India

Errata: Early Diagenetic Chert Nodules in Bhander Limestone, Maihar, Satna District, Madhya Pradesh, India

Early Diagenetic Chert Nodules in Bhander Limestone, Maihar, Satna District, Madhya Pradesh, India

Differential compaction of oolites in the limestones around and lack of compression of the same within most of the chert nodules in the upper part of the Bhander Limestone point to emplacement of silica at early stages of dewatering-compaction. As precipitated, silica was possibly a plastic material and its time of hardening varied within and between the nodules. The degree of deformation of the oolites varied according to the stage of hardening of the silica within the nodules at the time of compaction. Replacement of carbonate sediments took place due to addition of carbon dioxide derived from decomposition of organic matter, preferentially concentrated in the burrows formed by deposit feeders. Silica, on the other hand, was immobilized initially in the burrows by adsorption on organic matter. In all probability, silica was derived by post-mortem dissolution of siliceous organisms trapped in the sediment. Contemporaneous with silicification, dissolved carbonates released from areas enriched in organic matter were reprecipitated in zones lacking organic matter as calcito veinlets, that were subsequently deformed during compaction. Circumstantial evidences are, therefore, consistent with the hypothesis postulated by Siever for the origin of early diagenetic chert nodules in limestones.

Preliminary report of microstructures of probable biologic origin from the Mescal Formation (Proterozoic) of central Arizona

Abstract Microstructures of probable biologic origin have been described within early diagenetic chert from near the top of the Mescal Formation of Proterozo c (1.2–1.4 b.y.) age, from exposures in the vicinity of Horse Camp, McFadden Peak Quadrangle, Arizona. The structures, nearly all hematite in composition, are interpreted as replacements of microorganisms and have been divided into three categories: Type A, filamentous chains; Type B, discrete and grouped spherules; and Type C, globular aggregates. The Type A structures are interpreted as replacements of filamentous blue-green algae. Type B structures are thought to be replacements of blue-green algae, although the possibility exists that some may represent chlorophycean algae. Type C structures are tentatively interpreted as pyrite framboids subsequently altered to hematite.

Early Diagenetic Chert in the Marble Falls Group (Pennsylvanian) of Central Texas

ABSTRACT Chemically-precipitated quartz, confined to a thin interval at an unconformity within the Marble Falls Limestone, originated during an episode of Early Pennsylvanian subaerial exposure. The chert is principally early diagenetic chalcedony and microcrystalline quartz that formed as pore-filling cement and as replacement products for carbonate allochems. Preservation of incipient carbonate cement, stabilized at the margins of pore spaces when microquartz was precipitated in pore centers, and reworked siliceous allochems provide the principal evidence for early diagenetic silica emplacement. Corrosion surfaces in carbonates at the base of siliceous intervals indicate silica was precipitated from pore fluids undersaturated with respect to CaCO3. Organic matter preserved in microquartz probably played an important role in precipitation of silica by locally reducing silica solubility and promoting precipitation.

Nature and Origin of Early and Late Cherts in the Leadville Limestone, Colorado

Two generations of chert have been observed in the Mississippian Leadville Limestone of west-central Colorado: (1) an early chert inferred to have precipitated from hypersaline marine waters of high pH after those waters seeped into carbonate muds prior to final burial and lithification of the carbonate; and (2) a late chert that appears to have precipitated from ground waters as amorphous silica after initial lithification but prior to or during karst erosion of the formation in Late Mississippian (?) and Early Pennsylvanian time. A third type of microcrystalline quartz, hydrothermal jasperoid, is associated with ore deposits of Laramide age in the Leadville, but it is not discussed here. Conditions in west-central Colorado at the time that the early cherts formed in the Leadville were probably similar to those associated with the formation of early diagenetic chert in modern sediments, and the early chert in the Leadville is believed to have formed in a manner similar to the modern early cherts. The silica of the late chert appears to have come from Paleozoic sandstones below the Leadville. It was carried by slightly acid artesian waters into the formation, probably in concentrations of less than 60 ppm SiO 2 Super-saturation of amorphous silica occurred within the Leadville and probably was obtained by evaporative concentration at the water-air interface during dry seasons. Dissolution of the limestone by the slightly acid waters may account for the removal of the calcite that is replaced by chert.