Zone Of Diagenesis Research Articles

Outcrop to petrography scale and new age constraint of the Jatisamit Mélange in the Luk Ulo area, Central Java, Indonesia

To resolve the formation of the Luk Ulo Mélange in the Karangsambung area of Central Java, the matrix-dominated Jatisamit Mélange, which is a member of the Luk Ulo Mélange, was examined. Outcrop and petrographic observations of sandstone boudins and their shale matrix from the Jatisamit Mélange were used to assess deformational mechanisms and formation. We suggest the Jatisamit Mélange underwent three deformation phases: (1) formation of a web structure in the sandstone boudins through a cataclasis mechanism; (2) formation of boudins and cleavage through a ductile deformation; and (3) formation of open fractures and Riedel shears through pressure increasing. These deformations occurred at relatively low to medium temperatures during progressively increasing differential stress. Using illite crystallinity, the matrix of the Jatisamit Mélange has a maximum metamorphic temperature of 150–200 °C (diagenesis zone) at a 4–5 km depth. In addition, calcareous nannofossil assemblages of the Jatisamit Mélange and other related rock units in Karangsambung suggest a depositional matrix age of middle Eocene–early Oligocene. Therefore, the mixing of the Luk Ulo Mélange occurred not earlier than the middle Eocene–early Oligocene at a shallow structural level in the suture zone during the collision of the southeastern margin of Sundaland with the Gondwana micro-continent.

Simulating meteoric and mixing zone carbonate diagenesis with a two-dimensional reactive transport model

Meteoric and mixing-zone diagenesis can dramatically alter the geochemical signatures of shallow marine carbonates. Most preserved pre-Cretaceous carbonates were deposited in shallow marine environments and thus may have been susceptible to meteoric and mixing-zone diagenesis. However, a quantitative understanding of how the geochemical composition of carbonates changes during diagenesis still requires further development. Here, we present a new two-dimensional (2D) reactive transport model coupled with a 2D coastal hydrology model to simulate carbonate diagenesis and provide insights into its impact on the isotopic and elemental compositions of carbonates in the geological record. Using this model, we have simulated the stratigraphic trends and relationship between isotopic records (for example, δ13C and δ18O values) observed in modern (Recent-Miocene) sections where the impact of meteoric diagenesis has been clearly characterized. Our model can also reproduce anomalous Neoproterozoic carbonate geochemical profiles where the effects of meteoric diagenesis have been debated. Further, our model indicates that linear carbonate C-O isotope co-variations can either be generated in the mixing zone between freshwater and marine pore waters, or in the freshwater phreatic zone with a downward decrease in recrystallization (with no net carbonate dissolution or precipitation) rate. In addition, numerous processes were observed to decouple δ18Ocarb values from other isotopic and elemental signatures during carbonate diagenesis, indicating that a lack of linear correlation between δ18Ocarb values and other geochemical variables does not necessarily suggest limited meteoric alteration. Sensitivity analyses show that the steady-state timescale is controlled by compositional differences between fluid endmembers, the calcite-water element distribution coefficient, the recrystallization rate, porosity, and the groundwater discharge rate. Given that reactive transport models have proven to be powerful theoretical tools in many disciplines of Earth sciences, our hope is that this model will promote a more quantitative understanding of meteoric and mixing zone diagenesis of marine carbonates.

Fluvial facies and petrography of Late Pleistocene Baneta sediments, Central Narmada Basin, Madhya Pradesh, India

Baneta Formation, comprising of fining upward sequences of pebbly conglomerate, sandstone and siltstone, exhibits development of five distinct lithofacies, viz., massive pebbly conglomerate, large scale tabular cross bedded sandstone, horizontal parallel bedded coarse-grained sandstone, parallel laminated fine-grained yellowish sandstone and siltstone; representing channel lag, point bar and overbank flood plain deposits of mixed load meandering river. In these sediments, development of nodular, buckled bedded calcrete, rhizoliths and tepee is noticed. Granulometric studies of these sediments revealed presence of wide range of grain size classes, polymodal grain size distribution, moderate to very poor sorting, positive skewness and leptokurtic nature, supporting fluvial environment of deposition. Lithic arenitic nature, heavy mineral assemblage with dominance of augite and low ZTR index of these sediments indicate mineralogical immaturity and presence of illite, kaolinite and montmorllionite together with geochemical composition indicate their derivation from mixed provenance of Precambrian granite, metapelites, Vindhyan Supergroup, Gondwana Supergroup, Deccan trap basalt, and laterite. The thin sections studies reveal signatures of meteoric phreatic and vadose zone diagenesis related with semi-arid climate and subaerial exposure. The δ13C and δ18O content of calcretes indicate their pedogenic and/or shallow groundwater origin under semi-arid climatic conditions, and C3–C4 mixed vegetation with dominance of C4 vegetation. OSL and 14C dates of the samples from Baneta Formation suggest deposition of these sediments in Late Pleistocene.

Carbon and Oxygen Isotopes and Formation Conditions of the Upper Famennian Shale-Bearing Rocks in the Pripyat Trough, Southern Belarus

The behavior of stable carbon and oxygen isotopes in carbonates during the deposition and diagenesis of sediments in the bioproductive Upper Famennian Pripyat Trough (southern Belarus) is discussed. Limestones and clayey limestones (Corg 0.92 ± 0.11%) are characterized by very low δ13C values (–9.6 ± 0.3‰). Parental sediments of these rocks were deposited in the shallow-water zone during slow downwarping episodes of the seafloor. Lithification of the sediments took place in oxidative conditions of the diagenesis zone. Organic matter was actively oxidized by free oxygen. Carbon dioxide with isotopically light organic carbon formed in this process was used during the crystallization of diagenetic carbonates that are visually indiscernible from the sedimentational variety. Marls, clayey marls, and carbonate-bearing clays (Corg 6.02 ± 0.80%) are characterized by δ13C values as high as –3.5 ± 0.6‰. In combustible shales (Corg >10%), δ13C value is ‒1.2 ± 0.6‰). The clayey rocks mark the episodes of sedimentation in relatively deep-water conditions that appear during the uncompensated sagging of the basin floor. The diagenetic zone with free oxygen was significantly decreased or absent at all. Here, oxygen of marine sulfates was the main or single oxidizer of organic matter (sulfate reduction). The sulfate oxygen is a weaker oxidizing agent than free oxygen. Therefore, much more organic matter was retained and fossilized in clayey rocks than in carbonate rocks. Organic carbon released during the sulfate reduction and mobilized later for the diagenetic carbonate formation was insufficient for the significant decrease of δ13C values relative to the marine carbonate standard. Isotopic composition of carbonate oxygen in the studied rocks is invariable and does not depend on the content of clay and organic matter. In these rocks, δ18O values are at the level (approximately –5‰) shown for the Famennian in the Global Chemostratigraphic Chart. This fact is consistent with the supposition that oxygen isotope composition of atmospheric precipitates, which influenced the rock formation via continental flow, is close to that of sea water in low latitudes where the territory of Belarus was located in the Late Devonian.

Can Primary Ferroan Dolomite and Ankerite Be Precipitated? Its Implications for Formation of Submarine Methane-Derived Authigenic Carbonate (MDAC) Chimney

Microbes can mediate the precipitation of primary dolomite under surface conditions. Meanwhile, primary dolomite mediated by microbes often contains more Fe2+ than standard dolomite in modern microbial culture experiments. Ferroan dolomite and ankerite have been regarded as secondary products. This paper reviews the process and possible mechanisms of microbial mediated precipitation of primary ferroan dolomite and/or ankerite. In the microbial geochemical Fe cycle, many dissimilatory iron-reducing bacteria (DIRB), sulfate-reducing bacteria (SRB), and methanogens can reduce Fe3+ to Fe2+, while SRB and methanogens can also promote the precipitation of primary dolomite. There are an oxygen respiration zone (ORZ), an iron reduction zone (IRZ), a sulfate reduction zone (SRZ), and a methanogenesis zone (MZ) from top to bottom in the muddy sediment diagenesis zone. DIRB in IRZ provide the lower section with Fe2+, which composes many enzymes and proteins to participate in metabolic processes of SRB and methanogens. Lastly, heterogeneous nucleation of ferroan dolomite on extracellular polymeric substances (EPS) and cell surfaces is mediated by SRB and methanogens. Exploring the origin of microbial ferroan dolomite may help to solve the “dolomite problem”.

Geochemistry and mineralogy of the Al-rich shale from Baluti formation, Iraqi Kurdistan region: implications for weathering and provenance

The mineralogical and geochemical characteristics of the Upper Triassic Baluti shale from the Northern Thrust Zone (Sararu section) and High Folded Zone (Sarki section) Kurdistan Region, Iraq, have been investigated to constrain their paleoweathering, provenance, tectonic setting, and depositional redox conditions. The clay mineral assemblages are dominated by kaolinite, illite, mixed layers illite/smectite at Sararu section, and illite > smectite with traces of kaolinite at Sarki. Illite, to be noted, is within the zone of diagenesis. The non-clay minerals are dominated by calcite with minor amounts of quartz and muscovite in Sararu shale; and are dominated by dolomite with amounts of calcite and quartz in Sarki shale. Baluti shale is classified as Al-rich based on major and minor elements. The chemical index of alteration (CIA) is significantly higher in the Sararu than the Sarki shales, suggesting more intense weathering of the Sararu than the Sarki shales. The index of compositional variability (ICV) of the Sararu shale is less than 1 (suggesting it is compositionally mature and was deposited in a tectonically quiescent setting). More than 1 for Sarki shales (suggest it is less mature and deposited in a tectonically active setting). Most shale of the Baluti plot parallel and along the A-K line in A-CN-K plots suggest intense chemical weathering (high CIA) without any clear-cut evidence of K-metasomatism. Clay mineral data, Al enrichment, CIA values, and A-CN-K plot suggest that the source area experienced high degree of chemical weathering under warm and humid conditions, especially in Sararu. Elemental ratios critical of provenance (La/Sc, Th/Sc, Th/Cr, Th/Co, Ce/Ce*PN, Eu/Eu*PN, and Eu/Eu*CN) shows slight difference between the Sararu and Sarki shales; and the ratios are similar to fine fractions derived from the weathering of mostly felsic rocks. The Eu/Eu*CN, Th/Sc, and low K2O/Al2O3 ratios of most shales suggest weathering from mostly a granodiorite source rather than a granite source, consistent with a source from old upper continental crust. Discrimination diagrams based on major and trace element content point to a role of the felsic-intermediate sources for the deposition of Baluti Formation, and probably mixed with mafic source rocks at Sararu section. The chondrite-normalized rare earth elements (REE) patterns are similar to those of PAAS, with light REE enrichment, a negative Eu anomaly, and almost flat heavy REE pattern similar to those of a source rock with felsic components. The source of sediments for the Baluti Formation was likely the Rutba Uplift and/or the plutonic-metamorphic complexes of the Arabian Shield located to the southwest of the basin; whereas the Sararu shale was affected by the mafic rocks of the Bitlis-Avroman-Bisitoun Ridge to the northeast of Arabian Plate. The tectonic discrimination diagrams, as well as critical trace and REE characteristic parameters imply rift and active setting for the depositional basin of the shale of Baluti Formation. The geochemical parameters such as U/Th, V/Cr, V/Sc, and Cu/Zn ratios indicate that these shales were deposited under oxic environment and also show that Sarki shale was deposited under more oxic environment than Sararu.

Diagenetic Alterations of the Upper Jurassic Scleractinian Corals, Hanifa Formation, Jabal Al-Abakkayn, Central Saudi Arabia

Abstract: During diagenetic stages, the aragonitic skeletons and the inter/intra-corallite cement of the upper Jurassic corals of Hanifa Formation either dissolved or subjected to diagenetic alterations including cementation, micritization, recrystallization, silicification, dolomitization and dedolomitization. The proposed sequence of diagenetic stages is as follows: early marine diagenesis, early meteoric and mixing zone diagenesis, late meteoric diagenesis, and shallow burial diagenesis. Each stage is characterized by certain diagenetic processes. The source of sulfate solutions for dedolomitization in the studied corals is the dissolved anhydrite deposits of the Arab–Hith Formations, sometime before their erosion. A possible source of silica, needed for the formation of chert and chalcedony, is the sponge spicules dispersed in many carbonates of the Hanifa Formation.

An experimental investigation of the role of microfracture surfaces in controlling quartz precipitation rate: Applications to fault zone diagenesis

We present the results of quartz growth experiments, which were designed to assess the role of microfracture surfaces in controlling quartz-precipitation rates during fault-zone diagenesis. Experiments were run in hydrothermal cold-seal vessels at 300–450 °C and 150 MPa confining pressure for up to 1344 h. Microfractures routinely form at grain contacts during these experiments. Microfracture kinematic-aperture distributions indicate that microfractures form within the first 48 h of each experiment. Regardless of experimental temperature or duration, microfracture-sealing cements account for approximately the same amount of new quartz cement in each experiment. With increasing experimental duration, sealed microfractures were progressively overgrown by grain-boundary overgrowth cements. Spatial and temporal trends in the distribution of overgrowth- and microfracture-sealing cements indicate that precipitation rates on newly formed microfractures greatly exceed those on detrital-grain boundaries. This effect persists regardless of natural iron-oxide grain coatings present in a subset of our experiments. While our results agree with previous research that demonstrated increased growth rates on fracture surfaces in faults in fully lithified rock, fundamental differences in the nature of deformation in our experiments provide insight into quartz cementation in cataclastic deformation bands in faults offsetting high-porosity sandstones.

Unusual polygenetic void and cave development in dolomitized Miocene chalks on Barbados, West Indies

Barbados provides an unusual case of polygenetic cave development within dolomitized chalks and marls of the Miocene Oceanics Group. These diagenetic processes are driven by a succession and interplay of tectonic uplift, fracturing, hypogene fluid injection, overprinting by mixing zone diagenesis, and mechanical and biological erosion in the current littoral zone. The significance of the voids and caves within the chalks on Barbados are: 1) these appear to be the first dissolution caves documented in dolomitized chalk, and 2) these features show a polygenetic origin documenting the diagenetic changes in lithology that allowed the development and preservation of these cave types.

Controls on regional variability in marine pore-water diagenesis below the seafloor in Upper Jurassic–Lower Cretaceous prodeltaic sandstone and shales, Scotian Basin, Eastern Canada

Diagenesis in the uppermost Jurassic to Lower Cretaceous deltaic sandstones and shales of the Scotian Basin is an important control on reservoir quality. Ferruginous zone (sub-oxic) marine pore-water diagenesis controls the initial formation of Fe 2+-silicates that are the precursors of grain-rimming chlorite that preserves porosity. This study assesses the regional controls on the type of marine pore-water diagenesis by studying the sedimentology, mineralogy, and geochemistry of the retrogradational units and underlying progradational units in parasequences from conventional cores in two wells in different parts of the basin. Coated grains preserve a record of whether marine pore-water diagenesis below the seafloor was dominantly in the ferruginous or sulphidic geochemical zone. Four types of coated grain were distinguished, each with a different mineral paragenesis. Mineralogical and chemical evidence of ferruginous zone diagenesis includes the presence of diagenetic chlorite and siderite, and the correlation of P with Fe or Ti. Pyrite and Fe-calcite are found where the sulphidic zone is more significant than the ferruginous zone. Ferruginous zone diagenesis was common in low-sedimentation rate retrogradational sediments with low organic carbon, and in delta-front turbidites and river-mouth sandstones. Estuarine, tidal flat and prodeltaic facies that are directly supplied by riverine sediments have a lower Fe:Ti ratio than do fully marine shoreface and open shelf facies as a result of input of detrital ilmenite and its alteration products. The relative contribution of colloidal iron (hydr)-oxides appears greater in distal low-sedimentation rate environments. Where large changes in sedimentation rate occurred at ravinement surfaces, the underlying progradational rocks have evidence of ferruginous zone diagenesis, whatever their facies. Rapid upward migration of the pore-water profile resulting from the change in sedimentation rate reduced the time available for mineral products to form in the deeper pore-water zones. This study has shown that the availability of Fe and organic carbon varying in a complex manner in marine deltaic sediments, but that the resulting diagenesis by marine pore-water can be predicted from facies and paleogeographic setting.

Chronicles of vadose zone diagenesis: cone-shaped iron oxide concretions, Triassic Trujillo Formation, Palo Duro Canyon, Texas

Unusual cone-shaped iron oxide concretions occur in the Late Triassic, lower fluvial sandstone member of the Trujillo Formation at Palo Duro Canyon in the Texas panhandle. In situ concretions are significant because they record both historical information about past processes that occurred within the geologic unit and present-day information about the ability of the unit to conduct fluids. The dominant orientation of the concretions is cone-apex up, body radiating down and out, with long axis perpendicular to bedding. Concretion morphologies are associated with the sedimentary texture and primary bedding structure of the host rock and the corresponding hydrologic regime (i.e. advection versus dispersion for iron-transport behavior). Three lithofacies in the lower Trujillo member exhibit different cone forms. Field observations of cone orientation and morphology suggest vadose conditions for diagenetic precipitation of iron oxide cements, with timing potentially represented by the major pre-Miocene unconformity.

Modeling the Radiocarbon Reservoir Effect in Lacustrine Systems

The modern water (both pre- and post-atmospheric nuclear testing) of most lakes has an anomalously old apparent radiocarbon age due to what is commonly referred to as the “reservoir effect.” In contrast to marine settings, this14C-offset phenomenon is primarily caused by pre-aged carbon discharged to lakes by rivers and/or groundwater. In this paper, a 2-component box model based on the principle of14C mass balance in lake water and in the early diagenesis zone was formulated to address the relative importance of terrestrial inputs, autochthonous production, and biogeochemical processes in the14C reservoir of a lacustrine system. The model was tested using observed data from Lake Qinghai, the largest inland water body in China. Our inverse modeling using Markov chain Monte Carlo (MCMC) techniques yields best estimates of the δ14C of DIC in river (∼118% modern) and groundwater (∼76% modern), as well as the δ14C of DOC in river water (∼70% modern) during the post-bomb era. Assuming that these parameters remain constant over time, our modeling indicates that both the DIC and DOC pool of this lake have reservoir ages of about 1500 yr for the pre-bomb era, generally consistent with estimates obtained by extrapolation of the age-depth models of 2 sediment cores to the sediment surface.

Sedimentary facies and climate control on formation of vivianite and siderite microconcretions in sediments of Lake Baikal, Siberia

Authigenic vivianite and siderite microconcretions were found, respectively, in hemipelagic and deltaic facies of 600-m-long BDP-98 sediment section from Lake Baikal. Textural investigations of these microconcretions show that they are typically <1 mm in size, irregular in shape and composed of aggregated crystallites. Dissimilar orientation of vivianite and siderite crystallites suggests formation at different depths in the sediment; up to tens of centimeters for vivianite and tens of meters for siderite. Chemical analyses of both the vivianite and the siderite indicate cation composition characterized by minor amounts of Mn, Ca and Mn apart from the dominating Fe. Rather limited and distinctive carbon isotopic composition of the siderite, with d 13 CVPDB values between about +13 and þ16‰, implies formation of the mineral in the metha- nogenic zone of diagenesis. Isotopic composition of oxygen in the siderite (d 18 OVPDB values be- tween about )10 and � 11‰) is consistent with crystallization temperature at about 10-30� C and water d 18 OSMOW values between about )10 and � 16‰. The distribution of the authigenic miner- als in the section suggests changes in both sedi- mentary facies and climate, where vivianite formation was controlled by hemipelagic deposi- tional conditions during the Pliocene and Qua- ternary, whereas siderite reflects impact of deltaic conditions during the Miocene.

Evolution of Isotopic Carbon Dioxide–Water Systems in Lithogenesis: Communication 1. Sedimentogenesis and Diagenesis

Isotope-geochemical features of the formation of carbon and oxygen isotopic compositions in sedimentary and diagenetic carbonates are considered. Isotopic criteria for the identification of early diagenesis zone are proposed. The transition from sedimentogenesis to diagenesis (upper boundary of the early diagenesis zone) is accompanied by the alteration of carbon isotopic composition in the HCO– 3(hydrosphere)–HCO– 3(pore water of sediment)system. The lower boundary of early diagenesis zone is registered by the alteration of oxygen isotopic composition in the pore water of sediments and authigenic carbonates.

Isotopic Composition (δ13C, δ18O) of Carbonate Concretions from Terrigenous Deposits in the Northern Caucasus

The isotopic composition of carbon and oxygen was studied in carbonate concentrations with different compositions (calcite, dolomite, and siderite) from Jurassic, Cretaceous, and Tertiary terrigenous deposits of the Northern Caucasus. Wide variations in the isotopic composition (from 41.4 to 18.1‰ for δ13C and from 11.7 to 33.5‰ for δ18O) point to different formation conditions in the early diagenesis zone and the later catagenesis zone.

Low-grade metamorphism in the Scottish Southern Uplands terrane: deciphering the patterns of accretionary burial, shearing and cryptic aureoles

ABSTRACTSystematic studies of metapelitic grade linked with the geological re-survey of the Southern Uplands have been used to generate a contoured metamorphic map currently covering nearly two-thirds of the terrane. These studies, based on approximately one pelite sample per 2·5 km2, have used XRD measurements of clay mineral reaction progress, particularly illite crystallinity, to delineate zones of diagenesis and low-grade metamorphism in the imbricated Ordovician and Silurian strata. The regional pattern revealed by metapelitic zonal sequences does not agree with earlier observations that grade increases across the strike, from SE to NW. Instead, the map shows considerable variations in metamorphic trends, including patterns of grade increasing from older into younger strata, indicative of accretionary burial. Other patterns identified include those generated by high strain rates in the Moniaive Shear Zone (MSZ), and by extensive low-temperature cryptic aureoles associated with late granitic intrusions. The present pattern is the result of uplift that generated normal movement on reactivated thrust faults and differential block movement on NW-trending faults.Regional metamorphic patterns were generated by burial and underplating in an accretionary thrust stack. Subduction was initiated in the early Caradoc and probably ceased in the early Wenlock. Metapelitic patterns suggest that two levels of accretion are exposed in the terrane. Strata accreted to the toe of the prism and stacked above the décollement zone are typically at late diagenetic grade. Underplated strata below the décollement are typically at anchizonal grades with moderate to well-developed slaty cleavage. Coherent thrust-bounded tracts of strata at both levels were rotated and buried to produce a syntectonic depth-controlled pattern of meta-morphism. Shear zone metamorphism at depths of 12 km or more was probably confined to the underplated lower level of the thrust stack, and Devonian granitic intrusions were also emplaced mainly within the underplated strata.

Oxygen index as an indicator of early organic maturity

Most maturity indices can be used during catagenesis and metagenesis but there are only a few indicators to show the degree of organic maturation within the diagenesis zone. A previous study has already demonstrated that the oxygen index (OI) measured by Rock-Eval analysis provides reliable information about the early evolution stages of coals and type III kerogens. The main purpose of this study was to investigate the conditions of OI application in both natural and artificial evolution. The results of experiments presented here revealed that the accuracy of OI in estimating the evolution level depended on the amount and chemical composition of the organic matter. Natural series of low-rank coals showed OI to be a valuable rank parameter not only during diagenesis but in early catagenesis also. OI appeared to be a sensitive indicator during both simulated diagenesis and the entire oil window for gas-prone coal having a substantial humic contribution to its precursor material. In contrast, the OI of oil-prone coal and coal with mixed oil and gas potential proved to be a valuable rank parameter only up to the beginning of the oil window. Though it could be utilized successfully for describing depth profiles in organic-rich intervals of early maturity in wells, its use in determining natural maturity was restricted by the organic-richness of rocks, as well as by the effect of the mineral matrix. Providing humic material is present, OI can be used to follow the artificial thermal maturation of type II kerogen in the absence of mineral matrix.

Magnetostratigraphy of the uplifted former atoll of Niue, South Pacific: implications for accretion history and carbonate diagenesis

Cenozoic carbonate platforms are a potential rich source of information concerning eustasy but their prerequisite dating is fraught with difficulties. Paleomagnetism has long been established as a highly successful tool for dating deep-sea sediments but its applicability to shallow-water reef carbonates has only recently been explored. Here we derive a detailed magnetostratigraphy of the carbonate platform at Niue on the basis of cores drilled into lagoonal facies to a maximum depth of 303 m below surface. The carbonates consist of limestone which has been partially dolomitized and contain 16 zones of meteoric diagenesis caused by a succession of sea-level falls. The paleomagnetic record, spanning Chron C4r (Tortonian) to Chron C2n (late Piacenzian) and covering about 7.1 m.y. of deposition and erosion, was tied to the Geomagnetic Polarity Time Scale (GPTS) using verified control points. Coincidence of high accretion rates (up to 62.1 m/m.y.) and high sea levels caused the Tortonian-early Messinian section to mirror the contemporaneous GPTS polarity reversal intervals. In contrast, decelerated subsidence coupled with frequent eustatic fluctuations yielded compressed and truncated polarity intervals in the younger section. During late Messinian, the accretion rate was anomalously high (114.1 m/m.y.) probably due to a short-lived endogenic heating event. Isothermal remanent magnetization data suggest that magnetite is the dominant magnetic carrier in the Niue carbonates. NRM intensities of limestones (mean 0.126 ± 0.95 mA/m for n = 76) and dolomites (mean 0.121 ± 0.097 mA/m for n = 73) are indistinguishable and their NRM/SIRM ratios are similar (close to 10 −2) suggesting they underwent the same process of remanence acquisition. The NRM at Niue are comparable in their intensities to those reported from San Salvador and Mururoa but much weaker than those of deep-sea sediments. In general, meteoric diagenesis and dolomitization did not overprint the primary magnetization record at Niue. One exceptional case of magnetic resetting, depicted within a well-developed meteoric diagenesis zone, suggests that magnetic overprinting cannot be ruled out in carbonate platforms possessing weak primary magnetizations and therefore their polarity records must be evaluated on a case by case basis.

Geochemical zones of diagenesis in siliciclastic sediments

Geochemical zones of diagenesis in siliciclastic sediments

Illite crystallinity study of the Cretaceous Shimanto Belt in the Akaishi Mountains, eastern Southwest Japan

Abstract Illite crystallinity (IC) analyses in the Upper Cretaceous Shimanto accretionary complex of the southern Akaishi Mountains, eastern Southwest Japan confirm the applicability of this technique for evaluating the grade of diagenesis/metamorphism in a sediment‐dominated accretionary complex. Reproducibility analyses of IC values show a variance of about 15% from the mean. Data from three traverses, which transect across‐strike sections of ∼25 km, clearly demonstrate that the IC distributions have specific overall trends. The IC values belong to the lQwer anchizone and the zone of diagenesis. The IC distributions may be controlled locally by structural features, but there are no distinct relationships with regional‐scale geological structures. This may indicate that the heterogeneous geothermal rise affected the pre‐existing structural and diagenetic/metamorphic framework of the accretionary sequence. Along‐strike variations of grade tend to increase toward the northeast where a Middle Miocene granitoid occurs. Hence, the original diagenetic/metamorphic framework of this part of the Shimanto Belt was presumably overprinted during the Middle Miocene.