Petrographic Approach Research Articles

Simple petrographic grain size analysis of siltstone reservoir rocks: An example from the Montney tight gas reservoir (Western Canada)

This paper presents a simple petrographic approach to measure grain size distributions of fine-grained tight reservoir rocks, simultaneous to other organic petrographic measurements. Application of reflected light microscopy on polished blocks is a routine methodology for organic petrographic analyses including maceral examination and measurement of vitrinite reflectance. While routine petrographic analysis is in progress, the presented method enables operators to provide simultaneous reporting of quantitative grain size distribution by random orientation measurement of grain diameters using a relatively quick and simple procedure. This method provides added value to the routine analytical information provided by organic petrography laboratories. Application of ultra violet (UV) incident light under both oil and water immersion objectives provides a far better visual distinction of grain boundaries compared to white incident light. The improved visual distinction is likely due to differences in the diffused UV energy between surface scattering of the crystalline grains and the subsurface scattering of the incident light from internal irregularities such as grain boundaries and cemented areas in the polycrystalline rock samples. As a result of this optical phenomenon, grain boundaries are highly visible as a light blue UV rim. We suggest a random orientation measurement of 50–250 grain diameters, depending on sample sorting in various parts of a polished block, to give a valid statistical dataset and a valid size distribution histogram.

Petrographic classification of unusual high-pressure metamorphic rocks

A great number of prehistoric stone implements from Western Europe are made of high-pressure (H P ) metamorphic lithotypes, such as Na-pyroxenites and fine-grained eclogites. These rocks represent minor “accessory” lithologies that occur as either small primary outcrops or secondary clastic deposits derived from erosion of the former. Due to their scarcity, these lithologies have been partly neglected by the Subcommission on the Systematics of Metamorphic Rocks (SCMR) of the International Union of Geological Sciences (IUGS). Their archaeometric relevance, however, is important. A new petrographic classification of these H P metamorphic lithotypes is presented here, which is consistent with the recommendations of the SCMR. Two simple diagrams are suggested, which are useful to classify “ jades ”, mainly consisting of Na-pyroxenes, and rocks mainly consisting of Na-pyroxenes and garnets. Other unusual H P rocks, sometimes used to make archaeological implements, are also considered and classified according to a serious petrographic approach.

Geochemical Siliceous and Silicified Facies of Phosphate Series of Ouled Abdoun Basin (Morocco)

Syn-sedimentary post-diagenetic phenomena are remarkable in Moroccan phosphates. Indeed they are reference deposits for geological study, where important interests are given of determination and identification of these geochemical phenomena. Silicification is one of the phenomena that we have studied to determine the diagenetic processes that control the new-formation of silica in phosphate series of Ouled Abdoun. This study was performed on deposit EL Halassa, which belongs to the Ouled Abdoun Basin; they spread out the Maastrichtian to Lutetian. At first, petrographic approach was applied on different terms composing the phosphate series. In a second step, a geochemical approach on the same silica and silicified facies of phosphate series Ouled Abdoun Basin for characterizing silicification deduced the probable links between this diagenetic phenomenon and facies and forms’ affect. Ten major trace elements were measured. According to the phases’ petrographic relationship, the results were presented and discussed: silicate phase, the carbonate phase and clay phase.

A radiometric and petrographic approach to risk assessment at Alte Madonie Mounts region (Sicily, Italy)

The main goal of this work was to assess the radiological hazard at Alte Madonie Mounts region (north-central Sicily, Italy) in response to rumours of an increase in the incidence of cancer in this area. A correlation between the natural radionuclide contents and the petrographic features of the soil and rock samples was also evaluated. A total of 41 samples of selected soils and rocks were collected, powdered, dried and sealed in 'Marinelli' beakers for 20 d prior to measurement to ensure that a radioactive equilibrium between (226)Ra and (214)Bi had been reached. A gamma-ray spectrometer was used to quantify the radioactivity concentrations. To determine (238)U and (232)Th activities, the 609.3-keV line from (214)Bi in secular equilibrium with (226)Ra and the 911-keV line from (228)Ac, with which (232)Th can be assumed to be in equilibrium, were used, respectively. The gamma transition of 1461 keV was used to determine (40)K activity. The average values of the concentrations of (214)Bi, (228)Ac and (40)K were 30, 17 and 227 Bq kg(-1), respectively, whereas the greatest values were 134, 59 and 748 Bq kg(-1), respectively. A linear relationship was found between the activity values of (214)Bi, (228)Ac and (40)K. An exception was found for a group of samples in which the (214)Bi activities were much higher than expected. The chemical compositions and mineralogical features of the samples permitted the justification of these anomalies. The results of the primordial radionuclide contents are reassuring from a radiation protection point of view because the activities of the uranium and thorium series products and of the (40)K do not present a significant radiological hazard.

Trojan pithoi: A petrographic approach to provenance of Bronze Age storage vessels from Troy

Large storage vessels from the Bronze Age periods at Troy were subjected to petrographic analysis in order to characterize and determine the provenance of the clay raw materials used for their production. The analyses reveal that the selected vessels were produced from different clay sources with different clay qualities. They can be assigned to distinct geological areas, between 10 and 20km south of Troy, in the Ezine region and surrounding areas. Different procurement patterns of raw clay can be also recognized.

Bronze Age pottery from the Aeolian Islands: definition of Temper Compositional Reference Units by an integrated mineralogical and microchemical approach

An integrated microchemical–petrographic approach is here proposed to discriminate the provenance of archaeological pottery artefacts from distinct production centres. Our study focuses on a statistically significant sampling (n=186) of volcanic temper-bearing potteries representative of the manufacturing and dispersion among the islands of the Aeolian Archipelago during the Bronze Age. The widespread establishment of new settlements and the abundant recovery of Aeolian-made ceramic in southern Italy attest for the increased vitality of the Archipelago during the Capo Graziano culture (Early Bronze Age–Middle Bronze Age 2; 2300–1430 BC). Potteries from three of the main known ancient communities (Lipari, Filicudi and Stromboli) have been studied integrating old collections and newly excavated material. Volcanic tempers have been first investigated through multivariate analyses of relative abundances of mineral and rock clasts along with petrographic characters. In addition, we performed in-situ mineral chemistry microanalyses by Electron Microprobe and Laser Ablation—Inductively Coupled Plasma Mass Spectrometry to assess major and trace element composition of the most common mineral phases.

PETROGRAPHIC ANALYSIS OF TRACE FOSSIL DESMOGRAPTON FUCHS, 1895 FROM THE MIOCENE OF NORTHERN APENNINES (ITALY): A METHODOLOGY TO DETECT HYPICHNIAL FEATURES AND IMPLICATIONS ON THE GRAPHOGLYPTID ETHOLOGY

The trace fossil Desmograpton Fuchs, 1895, as many other graphoglyptids, has been considered to be a typical pre-turbidite, three-dimensional burrow system formed in mud and preserved as a casting at the sole of turbidites. This classic interpretation was mostly based on hypichnial preservation, due to the lack of direct evidence (e.g. modern seafloor pictures) for pre-depositional or post-depositional origin. Due to the accidental series of processes invoked to explain the perfect hypichnial preservation of this delicate trace fossil, several doubts and questions have been raised. To try to clarify the sedimentary processed that lead this trace fossil to be preserved, 26 thin sections of Desmograpton dertonensis Sacco, 1888 and Desmograpton ichthyforme Macsotay, 1967 have been performed at sand/clay interface of thin-bedded sandstone turbidites in the Miocene marl deposits of Verghereto, Northern Apennines (Italy). Petrographic analysis of textures and grains of the burrow-filling sediments points out that most of the grains of burrows differ from those of overlying sand, being either more or less sorted and differ in composition and locally also in grain size. Burrows are occasionally characterised by an upper rim, more or less thick, enriched in glauconite, phosphates, pyrite or quartz. Axial cuts of the strings show that rim was involved in diagenetic compaction, squeezing and fluid migration modifying the original shape of the trace fossil. In pre-depositional conditions the oblique-oriented filling is the only explication to preserve the upper rim of original mud, while compaction produced asymmetry in hypichnial tunnels. This petrographic approach, performed for the first time on this graphoglyptid genus, also elucidates the complex interaction of several taphonomic processes that led to the preservation of Desmograpton . The application of a similar petrographic approach to other graphoglyptids could help to better understand their origin, ethology and their preservation processes.

Mineral Precipitation in the Quartz Reefs of the Bendigo Gold Deposit, Victoria, Australia

Gold precipitation at Bendigo postdates cleavage development and was initiated during folding, increased during a reverse faulting stage in bedding-parallel quartz veins, and culminated in quartz reefs associated with a strike-slip faulting event. Some gold was remobilized along carbonate-rich cataclasites associated with later brittle faults. Understanding the paragenesis of the sulfide and gangue minerals associated with the auriferous quartz reefs is critical to unraveling the cycle of ore genesis, from source to sink. In this study, a petrographic approach, closely tied to macroscopic structural observations, along with stable isotope and fluid inclusion analyses, places constraints on the relative timing of gold mineralization with respect to deformation. Early sulfide assemblages in bedding- and cleavage-parallel veins are dominated by pyrite-pyrrhotite-siderite, and are free of visible gold. Later assemblages in reactivated bedding-parallel veins and other fold-related veins are characterized by the presence of arsenopyrite-ankerite-gold. Fault-related veins and the massive quartz reefs are rich in sphalerite and galena with associated gold, but lack pyrrhotite. Small amounts of late-stage antimony-bearing minerals occur in many vein types and postdate the precipitation of gold. Decreasing temperature and increasing sulfur activity controlled successively younger sulfide assemblages in the quartz veins. Stable oxygen isotope data show that quartz from all vein types ( δ 18 O = 15.9–19.0) homogenized with the host rocks, whereas carbon ( δ 13 C =–14.0 to 4.1) and oxygen ( δ 18 O = 4.5–24.0) in carbonate have a wider range in values that is interpreted to be a function of decreasing temperature. This corresponds with early siderite, pyrrhotite, and anhedral pyrite, and with later ankerite and ferroan dolomite associated with arsenopyrite, galena, and euhedral pyrite that are crosscut by calcite veins. Fluid inclusions in quartz veins are predominantly composed of water with carbon dioxide, with smaller proportions of nitrogen and methane, particularly in the later strike-slip faults. The sporadic occurrence of methane suggests that there was an open fluid system with incoming fluids from an external source mixing with those in the host rocks. Although several recent studies have argued that synsedimentary preenrichment may be a significant factor determining the size and distribution of gold in the Bendigo orogenic gold deposits, we believe there is little direct or deposit-scale evidence for a relationship between original metal content in the host-rock metasedimentary rocks and the distribution of the gold mineralization in specific structural sites. It is suggested that deep-seated faults act as conduits for fluid flow and the source for the gold at Bendigo; the gold is externally derived from deeply sourced auriferous metamorphic fluids that have been focused into discrete structural sites.

The problem of provenancing English post-medieval slipwares: a chemical and petrographic approach

Trailed and combed slipwares are a common component of mid 17th- and 18th-century archaeological ceramic assemblages in England. Despite their widespread manufacture, pieces produced at different centres across England appear remarkably similar in terms of fabric and decoration. Inductively coupled plasma-atomic emission spectrometry and petrography are employed to determine if chemical, mineralogical or technological traits can be used to indicate manufacturing provenance. Sherds from Bristol, North Staffordshire and South Yorkshire were analysed. The results reveal that they form cohesive chemical and petrographic groups that correlate with their place of manufacture, indicating potential for future studies of slipware production and distribution.

Petrographic approach to the study of organic microfossils from the Irati Subgroup (Permian, Paraná Basin, Brazil)

Early diagenetic chert, infrequently exploited in Phanerozoic micropaleontology, was examined for organic-walled microfossils in petrographic thin sections of silicified dolostones from diverse levels and localities of the Assistência Formation (Permian, Paraná Basin) in the state of São Paulo, Brazil. In contrast to previous palynological studies of this formation, the use of thin sections allowed direct observation in three dimensions of common palynomorphs, as well as benthic microbial mats preserved in situ in various stages of their life cycles and degradation. As in palynological residues from the more well-known shale of this formation, the chert contains wind-dispersed pollen grains and phytoclasts derived from terrestrial sources and planktonic cryptarchs (unornamented coccoidal unicellular or colonial palynomorphs). However, only in the chert is it possible to see much more delicate microfossils, such as abundant cyanobacteria of the in situ benthic microbiota as well as chlorophycean microalgae of the microphytoplankton. Post-depositional processes affecting the formation have destroyed all but the most resistant organic remains in the other lithologies, such that only rare, degraded pollen grains are seen in the unsilicified dolostone of the formation, and in the shale the vast majority of microfossils have been compacted to flattened disks. On the other hand, early silicification not only preserved organic remains at an incipient stage of decomposition but also impeded significant further degradation due to compaction, recrystallization, and oxidation. Thus, the petrographic study of such chert can complement traditional palynological investigations in Phanerozoic rocks by furnishing hitherto unavailable information, especially with regard to benthic organic microfossils and fragile organic-walled phytoplankton normally absent from organic residues.

NMR relaxation of neritic carbonates: An integrated petrophysical and petrographical approach

Abstract A set of carbonate outcrop samples, covering a wide range of the sedimentary textures and depositional environments existing on carbonate systems, was studied through an integrated petrographical and petrophysical approach. With the aim of improving the understanding of the NMR (Nuclear Magnetic Resonance) signal of carbonates, this work is: 1) providing an atlas for various carbonate reservoir rock-types, 2) providing a workflow for integrating geological and petrophysical data and, 3) documenting common shortfalls in NMR/MICP analyses in carbonates. The petrographical investigation includes thin section and SEM (Secondary Electron Microscope) observations, whereas petrophysical investigation includes porosity (Φ), permeability (K), NMR, MICP (Mercury Injection Capillary Pressure), and specific surface area (BET) measurements. On the basis of NMR and MICP data, 4 groups of samples were identified: (1) microporous samples, (2) micro–mesoporous samples, (3) wide multimodal samples, and (4) atypical samples. The microporous samples allow us to define a maximum NMR threshold for microporosity at a T 2 of 200 ms. NMR and MICP response of the investigated carbonates are often comparable in terms of modal distribution (microporous, micro–mesoporous and wide multimodal samples). In particular, micritization, a well known but underestimated early diagenetic process, tends to homogenize the NMR signal of primarily different sedimentary facies. A grainstone with heavily micritized grains can display well sorted unimodal NMR and MICP signatures very similar, even identical, to a mudstone–wackestone. Their signatures are comparable to that of a simple sphere packing model. On the contrary, several samples (labeled atypical samples) show a discrepancy between NMR and MICP response. This discrepancy is explained by the fact that MICP can be affected by the physical connectivity of the pore network, in case of disseminated and isolated molds in a micrite matrix for instance. Similarly, NMR can differentiate pore classes but with less resolution. It does not rely on connectivity but can be affected by diffusional pore coupling, i.e. the diffusion of water molecules carrying the magnetization between micropores and macropores. The pore-coupling phenomenon, through its impact on the T 2 distribution, may disturb the permeability calculations from NMR data. For core plug characterization, NMR appears to be a complementary tool to MICP and should be used to complete rock-typing analysis in carbonates. For reservoir rock-typing, the obvious advantage of developing a NMR based approach is the use of NMR logging data providing continuous records of pore size distributions.

Paleo-fluids characterisation and fluid flow modelling along a regional transect in Northern United Arab Emirates (UAE)

In the Northern Emirates, Jurassic and Lower Cretaceous platform carbonates of the Musandam parautochthonous units are tectonically overlain by siliciclastic units of the Hawasina–Sumeini allochthon, which derive from the former paleo-slope domain and a more distal basinal portion of the Arabian margin of the Tethys, respectively. All these tectonic units display numerous evidences of paleo-fluid circulations, accounting for dolomitisation and recrystallisation of the rock matrix (Musandam Platform units), as well as cementation of fractures. Polymict breccias of Upper Cretaceous Ausaq Formation which underlay the sole thrust of the Hawasina–Sumeini allochthon also record episodes of hydraulic fracturing, whereas fluid inclusion data indicate precipitation at high temperature in relation to paleo-fluid flow. Petrography of thin-sections (conventional and cathodoluminescence microscopic techniques) as well as fluid inclusion and stable isotopes analyses, were combined with micro-tectonic studies. These analytical data document (1) the paragenetic sequence of diagenetic products for the Musandam Platform (which constitutes a carbonate reservoir analogue) and Sumeini units of the Dibba Zone, as well as (2) the nature of the paleo-fluids circulating along fractures and the sole thrust of the Hawasina–Sumeini allochthon. The main results of this petrographic approach are qualitative, evidencing (1) the rapid and vertical transfer of hot fluids in the vicinity of the former slope to platform transition, accounting for episodes of hydrothermal dolomitisation, as well as (2) early (i.e. pre-orogenic) and late (i.e. post-orogenic) episodes of emersion of the carbonate units, accounting for additional interactions with meteoric fluids and karstification. In order to better link these diagenetic events with the overall burial, thermal and kinematic evolution of the Arabian margin, basin modelling with Ceres2D, including fluid flow and pore-fluid pressure modelling, was subsequently performed along a regional transect (D4) located in the vicinity of the samples localities and cross-cutting the Northern Oman Mountains from Dibba in the east up to the Arabian Gulf in the west. New subsurface constraints provided by deep seismic profiles were used to constrain the architecture of the cross-section, and to test various hypotheses on the lateral and vertical connection, timing and hydrodynamic behaviour of the faults. This Ceres basin modelling also provides new quantitative estimates of the paleo-fluid pathways, of the timing and velocities of the fluid transfers and of the evolution of pore-fluid pressures. Ultimately, this integration of petrographic studies on surface samples and coupled kinematic and fluid flow basin modelling provides an updated scenario for the succession of tectonically controlled episodes of fluid rock interactions, namely dolomitisation and karstification recorded in the Mesozoic platform carbonates of the Northern Emirates.

Late Matuyama climate forcing on sedimentation at the margin of the southern Alps (Italy)

Abstract The Pleistocene history of climate control on sedimentation in the Southern Alps–Po Plain system, northern Italy, was reconstructed using an integrated magnetostratigraphic, palynological, and petrographical approach on a 47-m-deep core. The core mainly consists of lacustrine sediments pertaining to the Bagaggera sequence, deposited at the foothills of the Southern Alps during the late Matuyama subchron (0.99–0.78 Ma). At that time, climate worsened globally and locally it caused the progradation of an alluvial fan unit onto the nearby Po Plain, triggering lake formation by damming of a tributary valley. These new data are used in conjunction with data from the literature to highlight and track the effects of climate forcing on sedimentation during the late Matuyama subchron in different orographic and geodynamic settings of the Southern Alps–Po Plain system as part of the greater Alpine area. We found that the episodes of alluvial fan and braidplain progradation observed in the southern foreland of the Alps during the late Matuyama global cooling seem broadly synchronous with the deposition of most of the so-called Gunz and Alterer Deckenschotter deposits in the northern forelands of the Alps as well as with the first major waxing of the Alpine valley glaciers, possibly around the Marine Isotope Stage 22 (∼0.87 Ma).

Chemical and petrographic evidence for thermal cracking and thermochemical sulfate reduction of paleo-oil accumulations in the NE Sichuan Basin, China

Significant extents of heterogeneity are observed in the optical properties, reflectance values and S/C atomic ratio values of solid bitumens in the carbonate reservoirs of the Lower Triassic Feixianguan and Changxing formations in the Puguang gas field, the largest in marine carbonates in China. The chemical and stable carbon isotope compositions of the gaseous alkanes in the produced gases also depend on field location, with no clear correlation with the hydrogen sulfate content. These data indicate that the gas reservoirs are highly compartmentalized. This manuscript describes an integrated chemical and petrographic approach for distinguishing solid bitumens formed by thermal cracking and by thermochemical sulfate reduction (TSR). The fact that most reservoir bitumens have only been partially affected by TSR suggests that sulfate supply appears to be a major limiting factor for the TSR reactions involving gaseous alkanes.

Eocene-Oligocene sedimentation in the external areas of the Moldavide Basin (Marginal Folds Nappe, Eastern Carpathians, Romania): sedimentological, paleontological and petrographic approaches

Eocene-Oligocene sedimentation in the external areas of the Moldavide Basin (Marginal Folds Nappe, Eastern Carpathians, Romania): sedimentological, paleontological and petrographic approachesThe Marginal Folds Nappe is one of the most external tectonic units of the Moldavide Nappe System (Eastern Carpathians), formed by Cretaceous to Tertiary flysch and molasse deposits, piled up during the Miocene closure of the East Carpathian Flysch basin, cropping out in several tectonic half-windows, the Bistriţa half-window being one of them. The deposits of this tectonic unit were accumulated in anoxic-oxic-anoxic conditions, in a forebulge depozone (sensuDeCelles & Giles 1996), and consist of a pelitic background sporadically interrupted by coarse-grained events. During the Late Eocene the sedimentation registered a transition from calcareous (Doamna Limestones) to pelitic (Bisericani Beds) grading to Globigerina Marls at the Eocene-Oligocene boundary, and upward during the Oligocene in deposits rich in organic matter (Lower Menilites, Bituminous Marls, Lower and Upper Dysodilic Shales) with coarsegrained interlayers. Seven facies associations were recognized, and interpreted as depositional systems of shallow to deeper water on a ramp-type margin. Two mixed depositional systems of turbidite-like facies association separated by a thick pelitic interval (Bituminous Marls) have been recognized. They were supplied by a "green schists" source area of Central Dobrogea type. The petrography of the sandstone beds shows an excellent compositional uniformity (quartzarenite-like rocks), probably representing a first cycle detritus derived from low rank metamorphic sources, connected with the forebulge relief developed on such a basement. The sedimentation was controlled mainly by different subsidence of blocks created by extensional tectonic affecting the ramp-type margin of the forebulge depozone.

Redox conditions during sedimentation of the Middle Jurassic (Upper Bajocian–Bathonian) clays of the Polish Jura (south-central Poland)

Depositional redox conditions of the uppermost Bajocian–Bathonian (Middle Jurassic) ore-bearing clays of the Gnaszyn/Kawodrza area in the Polish Jura have been determined using an integrated geochemical (Th/U and U/Th ratios, degree of pyritisation (DOP), sulphur stable isotopes, biomarker analysis) and petrographic approach (measurements of pyrite framboid diameters, and microfacies analysis). The Th/U and U/Th ratios indicate that oxic conditions prevailed on the sea-floor during this interval, and 34S isotopes suggest open-system conditions. DOP values, however, are rather scattered, and may reflect oxic, dysoxic, or anoxic conditions. We consider that the DOP values result from reducing conditions within the sediment and the chemistry of the pore-waters, rather than true sea-floor redox conditions. Pyrite framboid populations also indicate that dysoxic conditions prevailed within the sediment, beneath an oxygenated water column. Biomarker data did not provide any evidence of water column stratification or anoxia during sedimentation of the Middle Jurassic clays.

Chert and obsidian procurement of three Corsican sites during the 6th and 5th millenniums BC

Provenance studies of Corsican siliceous raw materials used during, the Neolithic focused mainly on obsidian. They showed an almost systematic use of Monte Arci sources (Sardinia). Chert studies have long been dodged, whereas the multiplicity of potential origins, in Sardinia and in continental areas, may provide complementary spatial information about diffusion patterns and interaction phenomenon in the Middle Tyrrhenian during the Neolithic. We studied obsidian and chert industries of three Corsican sites: Renaghju (Early Neolithic), Monte Revincu and Vasculacciu (both Middle Neolithic). In order to assign a provenance, 2241 chert archaeological samples were characterized by petrographic approaches and 100 obsidian artefacts were submitted to geochemical analyses. Each site provides a specific trend regarding relative abundances of raw materials, provenance and consumption patterns. Considered in the wider perspective of the Neolithic Corsican context, results reveal procurement variations from a chronological as well as a geographical point of view. Those variations may echo economic and social evolutions undergone in Neolithic societies of the Tyrrhenian area.

Accumulation and Preservation of Reworked Marine Pyrite Beneath an Oxygen-Rich Devonian Atmosphere: Constraints from Sulfur Isotopes and Framboid Textures

Abstract The Devonian Appalachian Basin provides a unique window to the unusual geochemical conditions required to form and preserve detrital metal sulfide deposits during the oxygen-rich Phanerozoic Eon. The Devonian Leicester Pyrite Member, a marine transgressive lag, occurs in the northern Appalachian Basin of western New York and is a reworked and redeposited acid-resistant residuum dominated by a wide range of pyritic clasts in the absence of carbonate grains. These iron sulfide grains include 10−2 m nodules, framboids, and pyritized fossil material (e.g., cephalopods, burrows, and steinkerns). A previous model for the formation of the Leicester Pyrite Member requires establishment of a strong and persistent density and chemical stratification in the Appalachian Basin. Although this model permits erosion under the low-oxygen conditions required for pyrite preservation, little geochemical evidence has been provided in corroboration, and the chemical conditions responsible for the formation, deposition, and preservation of the pyrite lag have remained speculative. Using a combined geochemical and petrographic approach focusing on the pyritic fraction and emphasizing sulfur isotopes and pyrite framboid size distributions, the specific redox conditions responsible for the formation of the Leicester Pyrite Member are addressed. The wide ranges of textural and isotopic data observed for the Leicester Pyrite Member, with δ34S values that range from −22 to +54% at a single location, are consistent with the general idea of lag concentration of pyritic debris formed under diverse diagenetic and possibly water-column (syngenetic) conditions. Energy propagation, possibly storm related, along the redox transition of the pycnocline and chemocline would have favored pyrite preservation, while still permitting enough oxidation and concomitant acid production to dissolve associated hydrodynamically equivalent carbonate debris. Intermittence in the stability of water-column stratification would have enhanced this oxidation, although an association with the overall oxygen deficiency that characterized the deposition of the overlying Geneseo Shale Member is demonstrated by the occurrence of microlaminated, Geneseo Shale Member-like clasts within the Leicester Pyrite Member. Our geochemical results confirm that during times of atmospheric oxygenation, pyrite reworking and lag formation require high-energy but low-oxygen conditions at pycnocline depths in a subaqueous marine setting. While not unique to the Leicester Pyrite Member, the specific sets of environmental conditions required for pyrite preservation differ from those of the fluvial settings in which Archean detrital pyrite and uraninite deposits formed. We suggest, therefore, that the presence of Phanerozoic pyrite lags such as the Leicester Pyrite Member does not compromise the utility of Archean detrital pyrite as a paleoredox proxy delineating early deficiency of atmospheric oxygen.

Intérêt de l'analyse pétrographique des spéléothèmes pour les reconstitutions paléoenvironnementales

The significance of speleothem petrography in paleoenvironmental reconstruction Speleothem petrography, like oxygen and carbon isotopic ratios, is influenced by various hydrological, chemical, biological and physical factors at the time of growth. While speleothem-based paleoenvironmental studies using stable isotope records are increasingly numerous, interpretations remain problematic. This paper aims to highlight the significance of a petrographical approach to the study of speleothems as a complement to isotopic studies. First, a synthesis of the main calcite crystalline fabrics encountered in speleothems is presented and a terminology suggested. A summary of the current state of knowledge about the relationships between fabrics and (i) the conditions of speleothem precipitation and (ii) speleothem carbon and oxygen isotopic ratios is then provided. It appears that some fabrics are essentially controlled by two main parameters : 1) the chemical efficiency of the solution ; and 2) the drip rate and variability. As a function of the respective importance of these two parameters, fabrics evolve within a continuum from elongated columnar fabric (typically, high intensity and low variability - low chemical efficiency) to dendritic fabric (low intensity and high variability - high chemical efficiency) via two intermediate forms, compact and open columnar fabrics. Other fabrics are associated with specific conditions of precipitation. For example, a high concentration of impurities in the source water can produce a microcrystalline fabric ; precipitation in a tunnel environment can give rise to squat columnar fabric ; and recrystallisation can produce a mosaic fabric. Combined with known conditions of precipitation, fabrics can be used to evaluate the quality of the stable isotope signals recorded by speleothems and can help in the interpretation of paleoenvironmental records.

Microstratigraphy of calcite cements in Pliocene cool-water limestones, New Zealand: relationship to sea-level, burial and exhumation events

Abstract The diagenetic evolution of thick, cool-water Pliocene limestones that formed within a forearc basin to accretionary wedge setting in eastern North Island, New Zealand, can be usefully tracked by applying at the thin-section scale the concepts of stratal patterns (onlap, offlap, discontinuity surfaces) in sedimentary sequences. The petrographic approach, supported by geochemical data, involves recognizing genetically related packages of zoned cements under cathodoluminescent (CL) light, named cement suites, which are bounded in thin section by (correlative) diagenetic discontinuities, including dissolution surfaces, renucleation surfaces and/or fractures. Based initially on detailed petrographic study of the early Pliocene Kairakau Limestone formation, a bryozoan-epifaunal bivalve-barnacle grainstone to rudstone, this procedure identifies five distinctive cement suites labelled K1–K5 separated, respectively, by discontinuity surfaces d1 to d4, and referred to collectively as the Kairakau diagenetic motif. Suites K1 and K2 have a pre-compaction origin and are inferred to have formed in a sedimentary system paced by high-frequency glacio-eustasy cycles, and reflecting deposition in transgressive (TST), highstand (HST) and regressive (RST) systems tracts, followed by initial shallow burial. Cement suite K1 is developed only locally, typically immediately above (early TST) and sometimes below (late RST) sequence boundaries. It consists of neomorphosed marine turbid cements growing upon the abraded surface of skeletons, and is bounded above by a dissolution surface (d1). Pre-compaction cement suite K2 has this dissolution surface at its base and a fracture surface (d2) at its top. K2 cements formed from oxidizing waters, either under shallow marine burial or, more likely, mixed marine-meteoric influences; they are inferred to relate mainly to the HST-RST portion of a depositional cycle. Post-compaction cement suites K3–K5 comprise pore-filling and fracture-hosted cements that formed during the burial of depositional sequences by overlying sequences, and during subsequent uplift. Suite K3 comprises ferroan calcite cements precipitated from compaction-driven reduced fluids that are terminated against fracture event d3, whereas suites K4 and K5 are interpreted as telogenetic cement phases that formed from meteoric, dominantly oxidizing, waters during uplift and exhumation of the whole succession and are separated by fracture and dissolution surface d4. Significantly, the same Kairakau diagenetic motif is developed in all the other Pliocene limestone occurrences in the study area. In an attempt to explain the emplacement of the successive cementing aquifers within limestones of different ages and separated by thick siliciclastic deposits, a cement stratigraphic model for the Pliocene succession concludes the paper, utilizing the concept of onlap and downlap cementational trends within the pre- and post-compaction cement suites of the eastern North Island carbonates. Ideal pre-compaction onlap-downlap diagenetic suites K1 and K2 mimic the evolution of the depositional environment from marine to subaerial forced mainly by short-term high-frequency (10 4 – 10 5 a) relative sea-level changes, whereas their post-compaction counterparts (suites K3–K5) record burial followed by exhumation of the sediment pile forced by subsidence and tectonic mechanisms of longer duration (10 5 –10 7 a).