Argillaceous Carbonate Research Articles

Marine redox evolution in the early Cambrian Yangtze shelf margin area: evidence from trace elements, nitrogen and sulphur isotopes

AbstractNitrogen is an essential element for biological activity, and nitrogen isotopic compositions of geological samples record information about both marine biological processes and environmental evolution. However, only a few studies of N isotopes in the early Cambrian have been published. In this study, we analysed nitrogen isotopic compositions, as well as trace elements and sulphur isotopic compositions of cherts, black shales, carbonaceous shales and argillaceous carbonates from the Daotuo drill core in Songtao County, NE Guizhou Province, China, to reconstruct the marine redox environment of both deep and surface seawater in the study area of the Yangtze shelf margin in the early Cambrian. The Mo–U covariation pattern of the studied samples indicates that the Yangtze shelf margin area was weakly restricted and connected to the open ocean through shallow water flows. Mo and U concentrations, δ15Nbulk and δ34Spy values of the studied samples from the Yangtze shelf margin area suggest ferruginous but not sulphidic seawater and low marine sulphate concentration (relatively deep chemocline) in the Cambrian Fortunian and early Stage 2; sulphidic conditions (shallow chemocline and anoxic photic zone) in the upper Cambrian Stage 2 and lower Stage 3; and the depression of sulphidic seawater in the middle and upper Cambrian Stage 3. Furthermore, the decreasing δ15N values indicate shrinking of the marine nitrate reservoir during the middle and upper Stage 3, which reflects a falling oxygenation level in this period. The environmental evolution was probably controlled by the changing biological activity through its feedback on the local marine environment.

Aquifer Characterization and Groundwater Potential Using Integrated Geoelectric Sounding and Geoinformatics in West Maghagha Area, Minia Governorate, Egypt

The study area is located in the western extension of the Nile Valley near the boundary with the Western Desert, where the groundwater represents the potential water resource for future land development for both industrial purposes and agricultural reclamation. Historically, geoelectric methods proved prospective and practical in exploring for groundwater resources. In this study, 17 Vertical Electric Sounding (VES) were acquired and processed to reveal the subsurface distribution of the water bearing layers and identify the groundwater potential in West Maghagha area. After routine data analysis and calibration, the preliminary results are interpreted in light of the available geological data and indicated the presence of at least four geoelectric layers with model resistivity values up to 2000 Ω•m. The potential aquifer was encountered down to ~120 m depth with average thickness of 100 m and is made of argillaceous fractured carbonates. Despite the overall poor quality of this aquifer, the integrated geoelectric and hydrogeologic information indicated a possible potential occurrence of potable groundwater at the southern and northeastern parts of the study area. To improve understanding of the groundwater systems in the study area, detailed aquifer characterization is discussed through integration of the available geologic data, maps, and the geoelectric sections constructed from the VES.

Plicathyridine brachiopods (Athyridida) from the Frasnian (Late Devonian) of Western Europe and Middle East

Plicathyridine brachiopods (Athyridida) from the early–middle Frasnian of southern Belgium and northern France (Dinant Synclinorium) are systematically described for the first time. They include two species: Anathyris (Anathyris) calestiennensis nov. sp., and A. (A.) sp. indet. 1. They are uncommon in the mainly shally La Prée (Nismes Formation) and Ermitage (Moulin Liénaux Formation) members and are absent from the mixed argillaceous–carbonate late Frasnian succession in this area contrary to what is observed in Russia, notably in the East-European Platform and Siberia. In the Middle East, two species of Anathyris are recognized on the basis of a limited material from the Dascht-e-Nawar area in Afghanistan (A. (A.) sp. indet. 2) and the Kuh-e Kaftar mountains in Central Iran (A. (A.) sp. indet. 3). Anathyris (A.) calestiennensis nov. sp. is sometimes encrusted by epizoans (tabulate and rugose corals, and bryozoans) and rarely displays single, small circular drill holes. The past and current epizoan–brachiopod interactions are also discussed (Anathyris vs. Lingula, respectively).

Petrographic and geochemical characterization of microfacies in a lacustrine shale oil system in the Dongying Sag, Jiyang Depression, Bohai Bay Basin, eastern China

Petrology and Rock-Eval analyses have been conducted in the lower third and upper fourth members (lower Es3 and upper Es4) of the Shahejie Fm source rock intervals in well Fanye1 in the Dongying Sag, Jiyang Depression, Bohai Bay Basin, eastern China. The organic matter in rocks intersected by this well is dominated by lamalginite and bituminite, and most samples contain Type I or II kerogen. Microfacies A–H has been identified according to their organic-mineral composition and structure using polished thin sections under microscopy. The form and location of hydrocarbon residue occurrences (mainly as solid bitumen with fluorescence) are different in those microfacies: microfacies A, which is dominant in well Fanye 1, is calcareous mudstone without lamination, and the hydrocarbon residue is dispersed through the rock, with some being present in subtle microfractures. Microfacies B and F are mostly composed of shale, with adsorbed hydrocarbon indicated by fluorescence along mineral grains. Microfacies C is laminated argillaceous carbonate, with hydrocarbon residue mainly occurring in thin microfractures along the bedding plane. Microfacies D and E are mainly carbonate, and microfacies D is laminated with micrite and sparite, and hydrocarbon residue occurrence is not only in the microfractures along the bedding, but also in the micropores among the grains in the sparite layers, which is the main place for hydrocarbon residue occurrence in microfacies type E without lamination. Microfacies G and H are both micrite. Microfacies G shows bioturbation and hydrocarbon residue occurs within small, irregular microfractures along the bedding planes and pores. Only a little solid bitumen without fluorescence has been observed in microfacies H.The Rock-Eval oil saturation index (OSI) (S1×100/TOC) value is variable because of the different characteristics of these hydrocarbon residue occurrences. OSI values in microfacies A are normally high (74.8–242.8mgHC/g TOC, average of 117.2mgHC/g TOC), and likely much higher if microfractures develop (>200mgHC/g TOC). OSI values (averaging 43.7mgHC/g TOC) are lowest in microfacies F because of adsorption on the shales, and OSI values are the highest in microfacies D (371.3mgHC/g TOC) and E (199.9mgHC/g TOC) since the hydrocarbon residues are stored both in microfractures and pores between the grains. Both OSI values are high in microfacies G and H because of low TOC content. The combination of petrographic and OSI analysis appear to be a promising tool for characterizing shale oil systems.

CHARACTERIZATION OF THE LATE APTIAN TOP‐DARIYAN DISCONFORMITY SURFACE OFFSHORE SW IRAN: A MULTI‐PROXY APPROACH

The late Aptian disconformity at the top of the carbonate‐dominated Dariyan Formation is a regional‐scale surface recognised throughout SW Iran and adjacent areas to the south. This study investigates the disconformity using wireline logs, petrographic analyses, SEM observations, and CL and stable isotope data from an oilfield offshore SW Iran. The top‐Dariyan surface in the studied field is characterised by erosional incisions and clastic‐filled fissures which, together with secondary porosity in the underlying carbonates, uranium enrichment and meteoric infill cements, are interpreted as subaerial exposure‐related features. The clastic‐filled fissures were identified from gamma‐ray logs in horizontal wells, and infill material is composed of argillaceous carbonates, clays, quartz and iron‐bearing minerals. The correlation of GR logs in 19 vertical wells resulted in the recognition of erosional incisions (up to about 16 m deep) on the top‐Dariyan surface, together with thickening of siliciclastic sediments in the overlying basal part of the Kazhdumi Formation. Back‐filled sediments in erosional incisions may form potential exploration targets. Secondary porosity in the uppermost 20 m of the Dariyan Formation was created by meteoric processes during subaerial exposure. Cathodoluminescence microscopy of diagenetic cements, carbon and oxygen isotope data and elemental analyses were used to reconstruct the diagenetic history of the carbonates in the Upper Dariyan interval.

First direct evidence of sedimentary carbonate recycling in subduction-related xenoliths.

Carbon in rocks and its rate of exchange with the exosphere is the least understood part of the carbon cycle. The amount of carbonate subducted as sediments and ocean crust is poorly known, but essential to mass balance the cycle. We describe carbonatite melt pockets in mantle peridotite xenoliths from Dalihu (northern China), which provide firsthand evidence for the recycling of carbonate sediments within the subduction system. These pockets retain the low trace element contents and δ18OSMOW = 21.1 ± 0.3 of argillaceous carbonate sediments, representing wholesale melting of carbonates instead of filtered recycling of carbon by redox freezing and melting. They also contain microscopic diamonds, partly transformed to graphite, indicating that depths >120 km were reached, as well as a bizarre mixture of carbides and metal alloys indicative of extremely reducing conditions. Subducted carbonates form diapirs that move rapidly upwards through the mantle wedge, reacting with peridotite, assimilating silicate minerals and releasing CO2, thus promoting their rapid emplacement. The assimilation process produces very local disequilibrium and divergent redox conditions that result in carbides and metal alloys, which help to interpret other occurrences of rock exhumed from ultra-deep conditions.

Sponge-microbial mound facies in Mississippian Tuscumbia Limestone, Walker County, Alabama

A new core from the Black Warrior Basin of Alabama contains a newly discovered mound lithofacies in the Tuscumbia Limestone (Meramecian). The Schlumberger-Alabama Power 1 Plant Gorgas well contains 37.2 m (122 ft) of carbonate rock assigned to the Tuscumbia. This formation overlies the Fort Payne Chert (Osagean), which was penetrated but not cored in this well. The Tuscumbia underlies calcareous shale and limestone of the Chesterian Pride Mountain Formation. Three lithologic units have been defined in the cored part of the Tuscumbia. The basal 2.8 m (9.25 ft) of the core consists of sponge-microbial boundstone (unit 1). This is overlain by 20.6 m (67.6 ft) of mixed carbonate strata dominated by mixed-particle grainstone, which increases in abundance upward (unit 2). Some of the grainstone is brecciated, suggesting exposure and paleokarst development following mound formation. Grainstone is interbedded with thin units of argillaceous cherty peloidal carbonate, sponge-microbial boundstone, and mixed-particle rudstone. Glauconite is common in the basal part of unit 2. Unit 2 is abruptly overlain by 13.8 m (45.2 ft) of bryozoan crinoid grainstone (unit 3) containing breccia beds, low-angle cross-strata, and immature carbonate paleosols. This unit is sharply overlain by the Lewis limestone of the Pride Mountain Formation, which here consists of 0.5 m (1.7 ft) of interbedded fossiliferous shale and mixed-particle packstone. The Lewis limestone is overlain disconformably by fenestrate-bryozoan-rich shale. The basal part of the core records growth of a sponge-microbial mound in relatively shallow water below normal wave base. The mound is not Waulsortian: stromatactis is absent, and matrix material is grainy. The upper contact is sharp, and borings are locally abundant. Abundant authigenic glauconite indicates reducing pore waters. The mound was buried by foreshoal grainstone, much of which is brecciated. Upper Tuscumbia bryozoan-crinoid grainstone formed in a mobile shoal that quickly aggraded to sea level. Although the upper Tuscumbia here is typical of the formation, the shoal buried a mound facies not previously reported from the Tuscumbia. Early diagenesis was dominated by marine cementation, syndepositional alteration, and fracturing. Burial diagenesis was dominated by calcite cementation, dissolution of siliceous spicules, stylolitization, chert formation, and later, emplacement of hydrocarbons and their subsequent transformation to pyrobitumen. Today, original interparticle voids are filled with a mixture of calcite cement, replacive chert, and solid hydrocarbons. Irregular nodules of chert replaced parts of the mound and the overlying heterogeneous unit. Dolomite partially replaced heterolithic strata of unit 2. Patches of once porous rock in units 1 and 2 contain abundant solid hydrocarbons, but porosity (0.7%–5.6%) and permeability (1–78 d) are low. In-place heating led to gas generation and to concomitant in-place solidification of liquid hydrocarbons.

Biostratigraphy and lithostratigraphy of the Mid-Carboniferous boundary beds in the Muradymovo section (South Urals, Russia)

AbstractThe uninterrupted succession of the Mississippian–Pennsylvanian boundary beds in the Muradymovo section in the South Urals contains diverse fossils and has a high correlative potential. The Muradymovo section is located in the Zilair Megasynclinorium (ZM), which belongs to the West Uralian Subregion and displays carbonate-siliciclastic deep-water facies of the Bukharcha Formation, which is partly Serpukhovian (Kosogorian, Protvian and Yuldybaevian) and partly Bashkirian (Syuranian). In the southern ZM, the lower part of the formation contains argillaceous carbonates with beds of shale and siltstone, subordinate clastic limestones and limestone breccia, while the upper part is mostly limestone with cherty interbeds. In the north of the ZM, the formation mainly consists of limestone. The Muradymovo succession contains no identifiable gaps in the Mid-Carboniferous Boundary (MCB) portion and has a succession of foraminiferal, conodont, ammonoid and ostracod zones. The MCB in this section coincides with the base of the Bogdanovkian and is defined by the entry ofDeclinognathodus noduliferus.This level falls within the upper part of the foraminiferalMonotaxinoides transitoriusZone, is near the base of the ammonoidHomoceras–HudsonocerasGenozone and can be correlated worldwide.

Back-Reef Depositional Environments in a Lower Cretaceous (Sligo) Shelf-Margin Complex: Insights into Ultradeep Reservoir Preservation and Controls on Stacking Patterns in an Outer Platform Setting

The Early Cretaceous Sligo and Stuart City shelf-margin trends in Texas have been a subject of interest for over 60 years, attracting attention both because of their striking complex reef lithologies and their continued potential for oil and gas exploration. Numerous gas reservoirs have been discovered since the late 1950s, with a major push for revitalization and expansion occurring in the 2000s. The shelf-margin complexes originated as steep-walled reefs rimming the Comanche Platform. They evolved into a narrow (<6 mi) highly heterogeneous belt of banks, islands, shoals, and tidal flats with an associated highly productive shallow-water carbonate factory. Most characterization efforts for these systems have focused on known reservoirs in South Texas; very little work has investigated the margins outside of this region, particularly with respect to the Sligo shelf margin. This investigation presents the first well-documented long stratigraphic section for the Sligo margin in East Texas, which serves as an initial step in understanding this major shelf-margin complex. Facies are mainly associated with back-reef to back-reef lagoon environments and exhibit significant heterogeneity on the basis of variations in water depth and wave energy, with facies including argillaceous carbonate mudstones, oncolitic packstones, rudist biostromes, reef-derived grainstones, ooid grainstones, and algal laminate bindstones. Four depositional sequences are delineated based on broad changes in facies and stacking patterns related to water depth and energy regime, yielding new insight into controls on facies partitioning in a stable long-term shelf-margin complex. The pore network, which is limited to micropores, is largely facies-controlled and is related to the presence of originally Mg–calcite allochems such as miliolids and Lithocodium/Bacinella. As East Texas has significant structural complexity, the ultradeep buried Sligo Formation is an underexplored area with the potential for undiscovered reservoirs related to structural features. Geologic characterization of this core is an important step in understanding some of the porous lithofacies present within the Sligo shelf margin in East Texas.

Cambrian section in the east of the West Siberian geosyneclise (based on data from Vostok 4 well)

The study presents a description of a new type of Cambrian section penetrated in Vostok 4 well in the southeastern part of the West Siberian geosyneclise (Krasnoyarsk Territory). The drilled succession is well characterized by the fossil evidence and can be considered a key section for the east of the WSG. The section comprises all three divisions of the Cambrian and is subdivided into the Oksym, Tyya, Averin, Kolchum, and Evenki Formations, based on the composite log data for an interval of 2262–5105 m. The Oksym Formation (the age equivalent of the Usol’e Formation) and Kolchum Formation are first identified and described in this study. The section is mostly carbonate, argillaceous–carbonate, sulfate–carbonate, and terrigenous sediments deposited in a back-reef setting of the Yenisei salt-bearing basin.

Mineralogical and geochemical studies of boron-rich bauxite ore deposits in the Songqi region, SW Henan, China

The Songqi region, SW Henan, is an important bauxite province in China. The bauxite ore deposits occur unconformably on the top of Middle Ordovician argillaceous carbonates. The bauxite ores from the Songqi region are characterized by unusually high boron contents. In this paper, we report the mineralogical and chemical compositions of selected bauxite ores from the region. These new data, together with existing geochemical data of soils in the region, are used to evaluate the origin of boron enrichment in the bauxite ores. The compositional characteristics of tourmaline in the bauxite ores suggests that this mineral was likely derived from a meta-sedimentary protolith. Mass balance calculation reveals that tourmaline only accounts for a small portion of boron in the bauxite ores; the majority of boron in the ores must occur in other forms, possibly by ion absorption on the surfaces of diaspore and clay minerals. In the Songqi region, the Precambrian meta-sedimentary rocks are known to contain abundant tourmalines whereas the Cambrian–Ordovician argillaceous carbonates and the Upper Proterozoic shales are all enriched in boron in free ion state. We suggest that these rocks provided tourmaline and boron ion to the bauxite deposits in the region. Most known bauxite deposits in the Songqi region occur at the margins of boron-rich soil domains, suggesting that boron in soils is a potential exploration indicator for unknown bauxite deposits in this region.

HYDROCARBON POTENTIAL OF THE UPPER CRETACEOUS SUCCESSION AT WELL 16/U‐1, ONSHORE QAMAR BASIN, EASTERN YEMEN

An Upper Cretaceous succession has been penetrated at onshore well 16/U‐1 in the Qamar Basin, eastern Republic of Yemen. The succession comprises the Mukalla and Dabut Formations which are composed of argillaceous carbonates and sandstones with coal layers, and TOC contents range up to 80%. The average TOC of the Mukalla Formation (24%) is higher than that of the Dabut Formation (1%). The Mukalla Formation has a Rock‐Eval Tmax of 439–454 °C and an HI of up to 374 mgHC/gTOC, pointing to kerogen Types II and III. The Dabut Formation mainly contains kerogen Type III with a Tmax of 427–456°C and HI of up to 152 mgHC/gTOC. Vitrinite reflectance values ranging between 0.3 and 1.0% and thermal alteration index values between 3 and 6 indicate thermal maturities sufficient for hydrocarbon generation.Three palynofacies types were identified representing marine, fluvial‐deltaic and marginal‐marine environments during the deposition of the Mukalla and Dabut Formations in the late Santonian — early Maastrichtian.

Burial Dissolution of Ordovician Granule Limestone in the Tahe Oilfield of the Tarim Basin, NW China, and Its Geological Significance

Abstract: With a comprehensive study on the petrology, geology and geochemistry of some Ordovician granule limestone samples in the Tahe Oilfield of the Tarim Basin, two stages of burial dissolution were put forward as an in‐source dissolution and out‐source dissolution based on macro‐microcosmic petrology and geochemistry features. The main differences in the two stages are in the origin and moving pass of acid fluids. Geochemical evidence indicates that burial dissolution fluids might be ingredients of organic acids, CO2 and H2S associated with organic matter maturation and hydrocarbon decomposition, and the in‐source fluid came from organic matter in the granule limestone itself, but the out‐source was mainly from other argillaceous carbonate rocks far away. So, the forming of a burial dissolution reservoir resulted from both in‐source and the out‐source dissolutions. The granule limestone firstly formed unattached pinholes under in‐source dissolution in situ, and afterwards suffered wider dissolution with out‐source fluids moving along unconformities, seams, faults and associate fissures. The second stage was much more important, and the mineral composition in the stratum and heat convection of the fluid were also important in forming favorable reservoirs.

Characteristics of the Lofer cyclicity in the type locality of the Dachstein Limestone (Dachstein Plateau, Austria)

In the present paper, the results of our studies in the type locality of the Dachstein Limestone are summarised in order to contribute to the correct interpretation of the Lofer cycles. In the sections studied on the Dachstein Plateau, the boundaries of the Lofer cycles are usually erosional disconformities showing karstification features. Penetration by karstic solution was not more than a few decimetres, since during the recurrent sea-level drops the platform only slightly emerged above sea level. The reddish or greenish argillaceous carbonate interlayers (facies A) cannot be interpreted as in situ palaeosol horizons. They are tidal flat deposits consisting predominantly of subtidal carbonate mud redeposited by storms that was mixed with reworked airborne fine carbonate particles and argillite and/or reworked lateritic soil, which were accumulated on the subaerially exposed platform. Rip-ups from consolidated sediment, blackened intraclasts and skeletons of tidal flat biota may have also contributed to the sediment of facies A. Erosional boundaries of most of the investigated cycles, and definite features of karstic solution beneath the disconformities, suggest periodical drops of sea level followed by a renewed transgression. This appears to confirm the allocyclic model for the explanation of the origin of the Lofer cycles.

Les Foraminifères planctoniques des brèches paléocènes du secteur de Tardets (Zone des Chaînons Béarnais, Pyrénées-Atlantiques)

The planktonic Foraminifera from the Palaeocene breccias of the Tardets area (‘Zone des Chaînons Béarnais’, Pyrenées-Atlantiques, France). Palaeocene marine breccias, the so-called ‘Etchebar/Bosmendiette breccias’, from the area of Tardets (Zone des Chaînons Béarnais, Pyrenées-Atlantiques, France) are interpreted as gravitational deposits (debris-flow) overlying a folded Jurassic and Early Cretaceous carbonate substrate and overlain, in turn, by the karstified rims of palaeocanyons, which, on the axis of the future range, were excavated during the “Laramide” orogenic phase at the end of the Cretaceous and the beginning of the Palaeocene. We clarify the cartographic, sedimentologic and petrographic characteristics of these breccias in this paper. Early/Middle Palaeocene (Late Danian–Early Selandian, P1c-P3 zonal interval) pelagic assemblages of ‘Globigerinacea’ are identified from new thin-section samples in their argillaceous carbonate matrix as well as within interbedded hemipelagites. These breccias are now well known and well dated along the Pyrenees from the ‘Corbières’ (to the east) to the Marble Thrust Sheet (to the west). The hypothesis of their diapiric origin founded on their supposed Aptian age must be consequently definitely rejected. To cite this article: M.-J. Fondecave-Wallez et al., C. R. Palevol 5 (2006) .

Review of the Ordovician rhynchonelliformean Brachiopoda of the East Baltic: their distribution and biofacies

Analyses of the distribution, in time and space, of approximately 300 Ordovician rhynchonelliformean brachiopods in the East Baltic allow the development of a faunal template for the Baltic Province (sensu stricto) within the context of the European Realm. Two different brachiopod magnafacies, the upper and lower ramp associations, are monitored through time. Changes in the brachiopod fauna through uppermost Hunneberg to the Porkuni stages are demonstrated from different drill core sections and some bedrock exposures located in facially contrasting areas across the region. The main developmental trends within the brachiopod biofacies of the shallower part of the palaeobasin (North Estonian facies belt) are characterized by relatively continuous changes in taxonomic composition including the evolution of endemics and the establishment of relatively persistent associations, especially during the later Ordovician. In the deeper parts of the palaeobasin (Central Baltoscandian confacies belt including the Livonian Tongue) the several different types, clearly determined by changes in environment, occur: Relatively low diversity associations in the red-coloured sediments, a well-defined assemblage associated with black shales and more diverse associations in the argillaceous carbonate deposits. The appearance and distribution of some shortlived associations including immigrants to the Baltic (Dactylogonia and Rhynchotrema during the Keila-Oandu event, the Holorhynchus association during the mid-Ashgill and the Hirnantia fauna during the late Ashgill) are probably associated with climatic and sea-level changes in the palaeobasin.

Use of spectral gamma-ray signature to interpret stratigraphic surfaces in carbonate strata: An example from the Finnmark carbonate platform (Carboniferous-Permian), Barents Sea

Spectral gamma-ray (GR) profiles were examined in well 7128/6-1, the stratigraphic reference section of the entirely subsurface Finnmark carbonate platform. Detailed bulk-chemical profiling of selected GR peaks shows that potassium (K) and thorium (Th) are mutually correlated and are a direct index of siliciclastic (aluminosilicate) content, whereas uranium (U) is uncorrelated with K, Th, and all other chemical components measured. Uranium tends to be enriched in thin shale and argillaceous carbonate layers within otherwise carbonate-dominated intervals. Uranium is thus associated with aluminosilicate minerals and is not particularly concentrated in dolomite. Two types of GR peaks are observed. Potassium-thorium-dominated peaks are suggested to indicate relatively major transgressions during which aluminosilicate detritus was derived from sources interior to the Fennoscandian shield. Uranium-dominated peaks correspond with relatively minor transgressions within intervals of cyclic shallow-water carbonate deposits. Uranium-enriched aluminosilicate detritus is suggested to be the product of extended subaerial exposure of the platform, during which U was concentrated by groundwater movement. These results can be useful as a basis for applying spectral GR signature as a tool for stratigraphic interpretation in uncored or incipiently understood carbonate sections.

AGNOSTOID BIOSTRATIGRAPHY ACROSS THE MIDDLE–UPPER CAMBRIAN BOUNDARY IN HUNAN, CHINA

Abstract Many trilobite species of the order Agnostida have short stratigraphic ranges and global distribution in open-marine deposits. They are most abundant and most diverse from about the middle Middle Cambrian to the middle Upper Cambrian where they are proving to be excellent biostratigraphic indices. We describe unusually rich agnostoid faunas of that age from two thick, well-exposed sections, one near Paibi and one near Wangcun, in northwestern Hunan Province, China. All collections are from the Huaqiao Formation, the definition of which is revised and its stratigraphic content is expanded. Most of the fossils are well preserved in dark, laminated, argillaceous carbonates that were deposited in lower slope environments. These are assigned to 76 species and 33 genera. New species described are Agnostus? babcocki, Ammagnostus? cryptus, Ammagnostus histus, Ammagnostus hunanensis, Ammagnostus wangcunensis, Baltagnostus? ambonus, Linguagnostus stenus, Lisogoragnostus hybus, Lisogoragnostus mictus, and U...

The falling amplitude of carbon isotopic oscillations through the Lower to Middle Cambrian: northern Siberia data

Carbon and oxygen isotopes have been studied in Lower to Middle Cambrian fossiliferous carbonates from sections along the Malaya Kuonamka, Olenek, and Muna rivers of the Yudoma-Olenek facies belt, northern Siberian Platform. Marked temporal oscillations in delta 13C are recorded from shallow-water Nemakit-Daldynian to Atdabanian carbonates, as seen elsewhere on the platform. Rapid sea-level rise during the so-called "Sinsk event" led to starved-basin conditions from the middle Botomian (Bergeroniellus micmacciformis Zone) to late Amgan (Tomagnostus fissus Zone) and to deposition of organic-rich argillaceous carbonates of the Kuonamka Formation. Mayan carbonates were laid down during a phase of renewed and more rapid carbonate sedimentation, and relatively uniform delta 13C signatures fall from approx. +1.5omicron in the Anomocarioides limbataeformis Zone to near zero throughout the Lejopyge laevigata Zone. These results confirm that a long-term decline took place in maximal delta 13C values between the late Neoproterozoic and Middle Cambrian, accompanied by a dampening in the amplitude of shorter term oscillations. This was accompanied by a change from rift to drift patterns of crustal subsidence; from low to high sea levels; from an interior basin to outer platform locus for hydrocarbon source rocks; from rare to abundant metazoans in the biosphere; by the first major extinction in the Phanerozoic fossil record; and from "icehouse" to "greenhouse" conditions. We infer that factors leading to changes in the rate of carbon burial were amplified at times of low sea level, high climatic extremes, and a primitive biosphere (late Neoproterozoic), and dampened at times of high sea level, lower climatic variation, and a more advanced biosphere (Middle Cambrian).

TECTONIC CONTROLS ON THE EVOLUTION OF POROSITY IN THE CRETACEOUS MARDIN GROUP CARBONATES, ADIYAMAN OILFIELDS, SE TURKEY

The mid‐Cretaceous Mardin Group carbonates constitute the principal reservoir in a number of oilfields near the city of Adiyaman in SE Turkey (the Adiyaman, G. Adiyaman, Cemberlitas, Cukurtas, Bolukyayla and Karakus oilfields). Porosity development in these carbonates was controlled by two phases of brittle deformation. The first of these accompanied the emplacement of the allochthonous Koçali‐Karadut Complex in the Late Cretaceous (Maastrichtian), which principally influenced the development of porosity in the northern oilfields. Subsequently, movement during the Mio‐Pliocene on the transcurrent Adiyaman fault affected porosity development in the south.The topmost unit of the Mardin Group, the Karababa Formation, consists of argillaceous carbonates, whose permeability and porosity were increased as a result of abundant tectonically‐induced microfractures and stylolites. The underlying Derdere Formation includes porous limestones and dolomites, which are the principal reservoir units in the Adiyaman fields.Porosity versus depth and geothermal gradient versus depth curves indicate that porosity trends are controlled principally by the transcurrent Adiyaman fault and its antithetics. The Koçali‐Karadut thrusts have had less influence on the evolution of porosity in the Mardin Group carbonates in the study area.