Carbonate Platform Margin Research Articles

Relationship between heterogeneity and seismic velocities of the Yudongzi Triassic oolitic reservoirs in the Erlangmiao area, northwest Sichuan Basin, China

In this paper, we use a range of parameters to quantitatively analyze the heterogeneity of oolitic reservoirs aiming to establish a predictive relationship between the reservoir heterogeneity and seismic velocities. The Yudongzi Oolitic shoals on the Triassic carbonate platform margin in the Erlangmiao area, NW Sichuan Basin, contain two facies elements: a stabilized sand flat and a mobile fringe. The stabilized sand flat consists mainly of oolitic limestones with moderately sorted grains of 0.1–6mm diameter that account for 40–65% of the rock components. The mobile fringe is characterized by residual oolitic dolostones in which the grains are extremely sorted with a diameter of 0.1–0.5mm and account for 70–80% of the rock components. These oolitic shoals occurred in the lower part of the Feixianguan Formation in each of the four sequence sets (i.e., Pss1–Pss4), and they were continuously progradational from Pss1 to Pss4. Diagenesis and reservoir properties of the oolitic shoals are closely related to their depositional environments, with the mobile fringe tending to contain potential reservoirs. Predictive relationships between the geological properties and seismic velocities of the oolitic reservoirs show that the content, mean diameter, mean radius ratio, and standard deviation of grains and dolomite content are linearly related to seismic velocities, while the porosity has a logistical correlation with seismic velocities. The seismic velocities decrease either with an increase of grain content or dolomite content, or with a decrease of mean grain diameter, mean radius ratio or standard deviation, and they decrease rapidly at first, and then slowly with an increase of porosity. With these correlations, impedance data from seismic exploration could be used to predict depositional environments, mineral composition, and porosity distribution of the oolitic reservoirs.

Diagenesis of a SE Asian Cenozoic carbonate platform margin and its adjacent basinal deposits

This study provides insights into the diagenesis of a SE Asian Tertiary carbonate platform and the variations in alteration that occur across platform top, margin, slope and adjacent basinal deposits. A petrographic, cathodoluminescent and stable isotopic study allowed evaluation of the diagenesis of the Kedango Limestone of Borneo. The Kedango Carbonate Platform developed as a predominantly low to moderate energy system in the semi-enclosed marine embayment of the Kutai Basin during the Eocene to Miocene. Early alteration through pervasive micritisation and minor isopachous fringing, syntaxial overgrowth and bladed to radiaxial cements was due to marine diagenesis on the basis of limited, relatively enriched stable-isotope compositions and non-luminescent cathodoluminescent (CL) characters. Minor early marine cementation formed just in the shallowest-platform top and margin deposits, and was likely promoted in high energy portions of the platform due to wave and/or current activity. Dissolution cavities, banded calcite cements with variable CL characteristics and depleted stable-isotope compositions, micritic sediment infiltration and alveolar textures are minor features, but together are indicative of localised subaerial emergence and meteoric diagenesis. The most prevalent and pervasive diagenetic feature of the deposits is the neomorphic alteration and replacement of metastable bioclasts and micritic matrix, together with contiguous calcitisation of pore spaces. Burial fluids with a marine character are inferred as the parent diagenetic fluid, since stable-isotope compositions for the neomorphic spar is largely consistent with precipitation from SE Asian Oligocene–Miocene seawater in a burial environment. Tectonic subsidence, the protected semi-enclosed marine embayment setting, and probably terrestrial runoff, together with component variability reflecting local environmental variations, are inferred to have been major controlling influences on the diagenesis of the Kedango Platform. Variations in diagenesis across bathyal, slope and shallow-platform deposits of the Kedango Limestone are not as marked as in other platforms from the region.

Comparison of basic features and origins of oolitic shoal reservoirs between carbonate platform interior and platform margin locations in the Lower Triassic Feixianguan Formation of the Sichuan Basin, southwest China

The oolitic shoal reservoirs of the Lower Triassic Feixianguan Formation carbonates in the Sichuan Basin of southwest China are an important target for gas exploration in the basin. Their occurrence, like other cases worldwide, can be divided into two locations in general, i.e., platform interior and platform margin locations. Their differences of reservoir features and origins, however, have not been investigated comprehensively due to different exploration degrees. This issue is addressed in this paper, to provide basic data and information for the basin’s hydrocarbon exploration and for the study of carbonate platform sedimentology and reservoir geology worldwide. We compared the features of these two types of reservoirs in detail, including the depositional and diagenetic features, pore types and petrophysical features. Based on the comparison, the origin of the reservoirs was further discussed. It is shown that the reservoirs in platform interior and platform margin locations differ significantly. The interior carbonates were deposited in moderate to high energy settings and the dominant lithologic type was limestone, which was weakly compacted and intensely cemented and has undergone meteoric dissolution. Pore types include intragranular dissolution and moldic pores, with low porosities ( 9% porosity and >0.1 mD permeability) and pore-throat structures than the interior reservoirs. These differences were influenced by both primary depositional features and secondary diagenesis. For the interior carbonate reservoirs, early meteoric dissolution, weak compaction and strong cementation are important controlling factors. By contrast, the factors controlling the formation of the margin carbonate reservoirs mainly include dolomitization, preservation of primary pores and burial dissolution.

Growth characteristics of Changhsingian (Late Permian) carbonate platform margin reef complexes in Yuanba Gas Field, northeastern Sichuan Basin, China

The Yuanba Gas Field is the second largest natural gas reservoir in the Sichuan Basin, SW China. The vast majority of natural gas reserve is embedded in the Changhsingian platform marginal reef complexes. Integration of facies analysis, well‐seismic calibration, well logs, seismic profiles, and RMS attribute allows reconstruction of the reef complex structure, and five growth cycles of overlapping accretionary reefs are recognized. The carbonate platform margin reef complex comprises reef core, fore reef and back reef facies associations. The retrograding–prograding growth pattern and northwesterly migrating trends of overlapping accretionary reefs were likely controlled by short‐term sea‐level changes superimposed by local currents prevailing in the eastern Palaeotethyan Ocean. Copyright © 2012 John Wiley & Sons, Ltd.

Stratigraphic architecture and computer modelling of carbonate platform margin, late ordovician Lianglitage formation, Central Tarim Basin

According to the different geometries and reflected characteristics in the seismic sections, the carbonate platform margin of the northern slope can be summarized as three basic depositional architectures in the Late Ordovician Lianglitage (良里塔格) Formation of the Tazhong (塔中) uplift. The type one mainly located in the west of the carbonate platform margin, and it showed obvious imbricate progradation from the interior to the margin of the platform. The type two was in the middle of the carbonate platform margin, which showed retrogradational stacking pattern in the same transgressive systems tract period, and the slope strata of the platform margin showed progradational sequence in the highstand systems tract period. The type three located in the east of the carbonate platform margin, and it showed the parallel aggradational architecture. The crossing well section along the northern slope of the Tazhong carbonate platform showed that the depositional thickness became thinner from the east to the west. The thickest belt located in the east of the platform margin, and became thinner rapidly towards the basin and the platform interior. These indicated that the paleogeomorphology of the Tazhong uplift was probably high in the west and low in the east during the period of the Late Ordovician Lianglitage Formation. According to the interpretation of seismic profiles and the computer modelling result, the depositional architectures of sequence O3l-2 showed aggradation, retrogradation and progradation from the east to the west of the carbonate platform margin during the transgression period. This meant that the accommodation became smaller gradually from the east to the west along the northern carbonate platform margin of the Tazhong uplift. The difference of the accommodation was probably caused by the difference of tectonic subsidence. Also, computer-aided modelling can be used to deeply understand the importance of various control parameters on the carbonate platform depositional architectures and processes.

Effects of diagenesis on the acoustic velocity of the Triassic oolitic shoals in the Yudongzi outcrop of Erlangmiao area, Northwest Sichuan Basin

The oolitic shoals of the Triassic carbonate platform margin in the Yudongzi (鱼洞子) outcrop of Erlangmiao (二郎庙) area in the northwestern Sichuan (四川) basin present a scarce oppor- tunity to quantitatively describe their diagenesis and its effects on the acoustic velocity. Using a detailed field geologic survey, profiles illustration of typical depositional system, and systematic testing, five types of diagenesis have been identified in the oolitic shoals: micritization, cementation, compaction and pressolution, dissolution, and dolomitisation. The cementation is composed of four subtypes (micrite cements, fibrous calcite cements, granular calcite cements, and blocky calcite cements). The dissolution is formed from three subtypes (freshwater selective dissolution, burial non-selective dissolution, and burial selective dissolution). The dolomitisation is composed of three subtypes (fine-crystalline dolo- mites, microcrystalline dolomites, and medium-crystalline dolomites). In order to quantitatively de- scribe the diagenetic fabric of oolitic shoals, the micritic grain content, calcite cement content, mean pore diameter, pore types, dolomite content, and dolomite types have been evaluated. Based on these data, the relationship between the acoustic velocity and diagenesis of oolitic shoals has

Paleoecology of the ordovician reef-shoal depositional system in the Yijianfang outcrop of the Bachu area, West Tarim Basin

The reef-shoal depositional system of the Ordovician carbonate platform margin is well exposed in the Yijianfang (一间房) outcrop of the Bachu (巴楚) uplift region, which offers an advantageous condition to study their paleoecology. Using a detailed field geologic survey and illustrated profiles of typical depositional systems, three types of genetic facies associations can be recognized in the reef-shoal depositional system: an organic reef, an organic shoal, and an upper slope. The organic reef is composed of three types of genetic facies (a reef base, a reef core, and fore-reef breccias); the organic shoal is formed from five types of genetic facies (tide channels, fore-reef inner shoals, fore-reef outer shoals, back-reef inner shoals, and back-reef outer shoals). The studies of the paleontological assemblage in each genetic facies of the depositional system indicate that the fauna preserved in each genetic facies are varied. The calathium, archaeoscyphia, bryozoan, and calcareous alga are well preserved in the organic reefs. The organisms preserved in the organic shoals are generally fragmented, while well-preserved girvanella and nuia siberica with a content of about 15% in the back-reef outer shoals are the most characteristic and different from others. The sinoceras, trilobites, and gastropods are well preserved in the upper slope deposits. The studies will demonstrate that the reef-shoal complexes developed above the base of the fair-weather wave base and that the original hydrodynamic conditions for the reef core forming is the stronger and become more and more low-energy from the inner part to outer part of the organic shoals.

Sedimentology, biostratigraphy and event stratigraphy of the Early Aptian Oceanic Anoxic Event (OAE1A) in the Apulia Carbonate Platform Margin – Ionian Basin System (Gargano Promontory, southern Italy)

The stratigraphic and biotic signatures of severe environmental changes across the late Early Aptian Oceanic Anoxic Event (OAE1a) in central Tethyan settings have been investigated in the almost undeformed Apulia Carbonate Platform Margin-Ionian Basin system (ACPM-IB) exposed in the Gargano Promontory (southeastern Italy). The nature of the observed Lower Aptian biofacies shifts within the investigated carbonate system are analyzed using an integrated biostratigraphic approach, based on ammonite-calibrated orbitolinids and caprinids coupled with the published δ13C and δ18O record of coeval pelagic sections of the Ionian basin (Coppitella, eastern Gargano, and Paliambela, northwestern Greece). Detailed field analysis of facies and tracing out of key biosedimentary and chemostratigraphic markers of the Lower Aptian have elucidated the evolution of the platform margin and allowed integrated correlations across the Gargano margin to basin transition.It is shown here that the global-scale environmental changes leading to the OAE1a event (i.e., fluctuations of surface-ocean temperature, available trophic resources and, above all, pCO2) are matched consistently by significant shifts of the biotic associations inhabiting the ACPM. The onset of greenhouse, mesotrophic conditions in the surface ocean undergoing an increasing acidification by CO2 excess favored the rapid spread of calcite shelled, filter feeding, eurytopic opportunist organisms (chondrodontids, ostreids, chaetetids sponges, along with minor bryozoans) as well as echinoids and orbitolinids (foramol productivity mode); this change is recorded by the deposition of a 4–6 m Crisis Interval (CI) that sharply overlies the “Urgonian”-type, rudist-rich platform margin complex (Montagna degli Angeli Limestones) formed from predominantly aragonite shelled, stenotopic organisms (mostly caprinid rudists, with minor corals and Dasycladales and Bryopsidales green algae) (chlorozoan productivity mode). The CI heralded the incipient drowning of the ACPM which occurred immediately after the deposition of a hothouse “out of balance” brachiopod (Orbirhynchia nadiae)-cyanobacteria association (microbial productivity mode). The biostratigraphically constrained CI and the related drowning of the ACPM have been physically correlated with specific chemostratigraphic segments of the δ13Ccarb curve that is available for the facing, proximal Ionian Basin. The early (earliest ?) Late Bedoulian, greenhouse CI (early D. deshayesi – early P. cormyi zones) was deposited during the late C2 interval, whereas the early Late Bedoulian, hothouse “out-of-balance” guild and the ACPM drowning (early deshayesi zone) accompanied the subsequent negative excursion and culmination, respectively, at the very base of the Chemostratigraphic Selli Level (CSL) (latest C2 and C3 interval). These correlations permit regional to global interpretations of historical patterns and explanatory paleoeanographic and paleoecologic hypotheses; furthermore, they suggest that the Apulia shallow-water ecosystem reacted to the environmental disruptions linked with the OAE1a sooner than that of the facing open ocean. The onset of the drowning event was synchronous with the hothouse-induced bloom of the “out of balance” brachiopod-cyanobacteria association that occurred simultaneously with the pronounced negative excursion of the δ13Ccarb curve. This suggests a cause-and-effect relationship with the sudden environmental perturbations linked with the injection into the atmosphere-hydrosphere system of 13C-depleted CO2. The numerical age model of the major biotic and stratigraphic events at the Lower Aptian ACPM, along with their inferred genetic processes call for different timings and causal mechanisms associated with platform demise in northern and central Tethyan settings.

Accumulation and intense bioturbation of bioclastic muds along a carbonate-platform margin: Dry Tortugas, Florida

A sedimentological study was undertaken in the Dry Tortugas, lowermost Florida Keys, to examine processes influencing the accumulation and sedimentary-fabric characteristics of carbonate muds along the shallow margin of a carbonate platform. Sedimentological studies (physical properties, 234Th and 210Pb geochronology) were coordinated with observations of benthic biology and benthic-boundary-layer dynamics. Sampling was concentrated around a mud depocenter in Southeast Channel, a reentrant 20–30m deep in the southern edge of the Dry Tortugas platform.In the depocenter of Southeast Channel, accumulation rates of 0.10–0.32cmy−1 are estimated from 210Pb profiles. Likely sources of sediment are in shallower water, and include reefs and beds of calcareous green algae. Fine sediment (silt- and clay-sized) is advected to the seabed of Southeast Channel by a combination of tidal and storm-driven currents. Flows from shallow water decelerate upon entering deeper water in Southeast Channel, producing conditions favorable to long-term accumulation.Once deposited, sediment in Southeast Channel is subjected to bioturbation by a two-tiered benthic community. The surface zone (0–5cm) is mixed biodiffusively by a surface community of small bivalves and polychaetes (Db≈9cm2yr−1) over periods of weeks to months, and is characterized by mm-scale mottling. Other near-surface structures include biodeposited fecal mounds of deep‐deposit feeders and ephemeral physical stratification as thin (1–5cm) layers of fine sediment. Below ~5cm, the dominant post-depositional process is bioadvective mixing by a deep-deposit-feeding community of callianassid shrimp and notomastid polychaetes. The upper 10–15cm is mixed completely every 6–21years, and bioturbation below this depth is slower.Even though the study area has been hit by >10 major hurricanes in the last 120years (winds >190kmh−1), sediments preserved (i.e., below 5cm depth) in Southeast Channel during this period reveal wholly biogenic sedimentary fabric produced by the deep callianassid/notomastid community. Bioturbation is thus capable of erasing the stratigraphic signature of the most extreme sediment-transport events over decadal time scales.

Spatio-temporal evolution of a Tertiary carbonate platform margin and adjacent basinal deposits

The variability in low to moderate energy carbonate platform margins is poorly known from the geological record. Here, the spatial and temporal evolution of platform margin and adjacent basinal deposits is evaluated from the little known Tertiary Kedango Limestone that developed in a semi-enclosed marine embayment in SE Asia. The hypothesis here is that platform margin development will reflect regional and perhaps global influences, such as tectonics, eustasy or biotic change, rather than windward-leeward effects and storms that typically impact strongly upon open oceanic platforms.The development of the carbonate platform was determined through logging, petrography, facies evaluation, provenance and high-resolution dating studies. Eleven carbonate facies were identified from the 30km long western margin of the >600m thick platform and its adjacent slope and basinal deposits. Larger benthic foraminifera and coralline algal packstones and wackestones dominated in shallow waters. During the Oligo-Miocene, coral patch reef-related floatstones, rudstones and less commonly boundstones were also present on the platform top. Perhaps surprisingly for a low energy platform there was considerable variation along the platform margin and much reworking of material into slope and basinal deposits during the Oligo-Miocene. Reworked material includes shallow water bioclasts, clasts from older siliciclastics, fresh feldspars, lithified slope and platform top carbonate clasts, some of the latter showing evidence for karstification. The western platform margin varied laterally over a few kilometres from a gently sloping unrimmed platform, to a probable bank top, with in places coral-fringed, bypass and erosional faulted escarpment margins. Eustasy may have influenced shallowing and deepening trends on the platform top, but apparently had little impact on mass wasting. Instead platform margin development was strongly impacted by tectonics (including active faulting), terrestrial runoff, together with changes in carbonate producers and local variability on the platform top.

Compositional variations in calciturbidites and calcidebrites in response to sea-level fluctuations (Exuma Sound, Bahamas)

compositional variation of Pleistocene carbonate gravity deposits from the Exuma Sound Basin, Bahamas, was determined. Two types of gravity deposit were present in the cores of ODP Leg 101, Site 632A, i.e., calciturbidites and calcidebrites. In analogy with earlier studies, the compositional variations in the calciturbidites could be linked to different sources on the carbonate margin, i.e., platform interior, platform edge, and platform slope. Calciturbidites deposited during interglacial, sea-level highstands show a dominance of non-skeletal grains, largely derived from the platform interior, while calciturbidites of glacial, sea-level lowstands, show a dominance of skeletal platform-edge to platform-slope-derived grains. Thus, the calciturbidite composition can be used to reconstruct the position of absolute sea level. In addition, the mud content of the calciturbidites increased after Marine Isotope Stage 11. In contrast, the composition of the calcidebrites remained unaltered through time and showed a clear dominance of platform-edge-derived sediments during varying sea-level positions. The Bahamian carbonate platform is located in a tectonically stable passive-margin setting and the gravity-flow deposits were laid down in an environment exclusively controlled by eustatic sea-level fluctuations. This study shows that all types of gravity-induced carbonate deposits, calciturbidites, and calcidebrites, were deposited in response to global eustatic sea-level variations. The sediment composition could be linked directly to sediment input from specific facies realms along the carbonate platform margin. Hence, sediment composition analysis is a strong tool that may be used to discriminate between gravity-induced deposition triggered by eustatic sea-level changes and that related to tectonic events, when analyzing resedimentation processes in sedimentary basins.

Sequence architecture and depositional evolution of theOrdovician carbonate platform margins in theTarimBasin and its response to tectonism and sea‐level change

AbstractThe sequence architecture and depositional evolution of theOrdovician carbonate platform margins in theTarimBasin,China, were formed in response to the interplay of tectonism and sea‐level change, their history being documented by the integrated analysis of many seismic lines, drilling and outcrop data. TheOrdovician carbonate system in the basin is divided into four composite sequences defined by major unconformities. Each sequence consists of a regional depositional cycle from transgression with an onlapping transgressive systems tract (TST) to regression with a prograding highstand systems tract (HST), and can be further subdivided into 10 third‐order sequences based on subordinate discontinuous boundaries at the carbonate platform marginal zones. Constrained by the marginal slope of the early‐riftedManjiaer aulacogen, the carbonate platform margins of the Lower and MiddleOrdovician that prograded eastward in an arcuate belt extending generally north‐south across the northern part of the basin. The development of theTazhong uplift due to compression resulted in an extensive paleokarst hiatus between the Middle and the UpperOrdovician in the south‐central basin, and subsequently constrained the formation of a peninsula‐shaped carbonate platform whose margins were controlled by marginal thrust‐fault belts of the paleo‐uplift during the LateOrdovician. In the northern basin, the LateOrdovician carbonate platform margin developed around the marginal slope of theTabei paleouplift. The transgressive–regressive cycles of the carbonate system are comparable and seem to have occurred simultaneously across the entire basin, suggesting that the cyclic sequence architecture was fundamentally controlled by eustatic fluctuations. Stacking patterns of the composite sequences varied due to the interplay between the accommodation produced by tectonism and sea‐level change, and the carbonate production rate. The reef–shoal facies complexes that developed along the platform margins, with paleokarst development at unconformities, constitute the major reservoir of large petroleum reserves in the basin.

Caratteri geologici e stratigrafici del settore esterno dell’Appennino sannitico-irpino (Campania, Italia)

Stratigraphic and geologic features of the external sector of Sannio-Irpinia Apennines (Campania, Italy).The very complex stratigraphic and tectonic features of the Sannio and Irpinia sectors of southern Apennines (Italy) are described. Facies analysis, stratigraphy, tectonics and geological mapping of the Late Cretaceous to Miocene units allowed us to identify six different tectonic units: the Sannio, Frigento, Fortore, Valle Tammaro, Daunia and Vallone del Toro units, which are strongly deformed and thrust eastward upon the buried Apulian thrust-system. All the basinal deposits are referred to an external basinal domain (Lagonegrese-Molisano Basin). The tectonic units are mainly formed by Upper Cretaceous to Early Miocene successions characterized by basinal and shelf-margin facies (Flysch Rosso Fm) and by basinal mainly calcareous-pelitic deposits of the Argille Variegate Group and Corleto Perticara fm, conformably followed by numidian flysch quartzarenites and arkosic arenites (Fragneto Monforte fm and San Giorgio fm). Inthe westernmost areas, the successions are characterized by frequent occurrence of coarse to fi ne calciclastic deposits settled by dense gravity flows (Flysch Rosso Fm) coming from a carbonate platform margin, as well as by slumps; they are followed by upper Burdigalian upper Langhian p.p. numidian flysch quartzarenites. The successions cropping out in the central-eastern zone are typically composed of pelagic sediments (varicoloured clays and shales) intercalated with fine carbonate turbidites and representing a more distal facies than the coarser proximal ones.The tectonic units are unconformably overlain by middle-upper Miocene terrigenous flysch deposits (Pietra Boiara unit, Reino-Morgia dei Rauli fm, Vallone Ponticello fm, Castelvetere flysch, Villanova del Battista fm, San Bartolomeo Flysch and Caiazzo sandstones), Messinian deposits (Altavilla group) and Pliocene unconformity-boundedmarine to alluvial units (Ariano Irpino supersynthem) referred to wedge-top depozone basins.The Lagonegro-Molise units of the Sannio-Irpinia Apennines show a very complicated structural style, that results from the Pliocene refolding of more ancient structures produced by middle-late Miocene deformation.

Geological exploration theory for large oil and gas provinces and its significance

Abstract In the period of “11 th Five-year Plan” (2006-2010), PetroChina proposed and developed a geological exploration theory for large oil and gas provinces, under which a group of major discoveries have been achieved. Large oil and gas provinces are large oil/gas-bearing areas consisting of several groups or belts of reservoirs (oil/gas fields) under the same large structural setting. These are determined by similar accumulation conditions, dominated by a certain type of hydrocarbon reservoir and overlaid vertically and connected horizontally. The combination of the large structural setting, favorable source rock and widely distributed heterogeneous reservoir is essential for development of large oil and gas provinces. Large oil and gas provinces are mainly developed in large structures such as continental depressions, foreland and marine craton basins, with large oil/gas-bearing areas and considerable reserves. Reservoirs are widely distributed, with low porosity and low permeability and high heterogeneity. Hydrocarbon distribution is not controlled by local structures and there is no uniform oil/gas/water contact but varying proportions of oil, gas and water. Based on the reservoir lithology, large oil and gas provinces are divided into clastics, carbonates and volcanic, which are subdivided into five sub-large oil and gas provinces, i.e. low porosity and permeability clastic; complex, steep and deep foreland structural; carbonate karst stratigraphic; carbonate platform margin reefal; and volcanic stratigraphic. Moreover, key technologies integrating seismic surveys, drilling, logging and formation tests have been developed. The exploration in large oil and gas provinces stresses the concept of “overall study, overall exploration and overall control”, and the evaluation method of “integrating exploration and development, and integrating production expansion and reserve growth”, to maximize the benefits of both exploration and development.

High-resolution stratigraphy and correlation of Cambrian strata using carbon isotopes: an example from the southern Appalachians, USA

This study presents an example of using well-constrained stratigraphic trends in carbon isotope composition of marine carbonate deposits as a tool for high-resolution correlation of successions that lack prominent biomarkers. A large, positive carbon isotope excursion is recorded within the Steptoean (Furongian, Upper Cambrian) strata of the southern Appalachians in Tennessee. A coeval excursion (also known as SPICE) has been reported for the western United States (the Great Basin area), China, Kazakhstan, and Australia, indicating that this is a global phenomenon. The anatomy of this excursion was determined through extensive sampling of homogenous micrite and dolomicrite from strata at the Thorn Hill locality in northeastern Tennessee. The δ13C values show an increase from the upper Nolichucky Shale into the overlying Maynardville Formation (Conasauga Group). The most positive δ13C values (+4 to +5 ‰ VPDB) correspond to the transition between the Maynardville and the Copper Ridge Dolomite (Knox Group). The δ13C values decline in the lower part of the Copper Ridge. This characteristic carbon isotope record is compared with the δ13C record from the Lee Valley locality, which is about 30 km away and contains a coeval succession of strata that represent similar carbonate platform depositional environments. A comparison of the δ13C records shows a remarkable similarity and provides the means for detailed correlation of these two stratigraphic successions. Next the results are compared with those from the Tazewell locality, which is in the northwesternmost outcrop belt of lower Paleozoic rocks in Tennessee. This section is separated from the Thorn Hill and Lee Valley sections by three thrust faults and is truncated at the base by a thrust fault. The strata at the Tazewell locality differ from those at Thorn Hill and Lee Valley in that they were deposited more closely to the carbonate platform margin. Despite these differences, carbon isotope records proved very useful for correlation among the three successions. The results of this study encourage application of similar approaches in correlation of Steptoean strata at more widely spaced outcrops within and beyond the Appalachian basin.

Geological and depositional setting of the Sirius Passet Lagerstätte (Early Cambrian), North Greenland

The Early Cambrian Sirius Passet Lagerstätte of North Greenland occurs in marine mudstones (Buen Formation) deposited in a slope environment along the eroded scarp of a pre-existing carbonate platform. The shallow-water platform is represented by dolostones of the Portfjeld Formation (Neoproterozoic – earliest Cambrian), which record a belt of tide-swept subtidal ooid shoals and microbial patch reefs at the outer edge of the platform. Solution features and meteoric cements attest to exposure of the platform, accompanied by fracturing, mass wastage and erosional retreat of the escarpment producing slope talus, and extensive debris sheets and olistoliths in basinal deposits. The marine mud-dominated siliciclastics of the Buen Formation, deposited in slope and shelf environments, record the transgression and onlap of the degraded platform in the Early Cambrian. The Sirius Passet Lagerstätte has yielded an arthropod-dominated fossil assemblage of over 40 species, many showing exceptional preservation of soft tissues; the finely laminated mudstones hosting this fauna accumulated from suspension in a poorly oxygenated slope sub-environment, such as an erosional embayment or abandoned slope gully. Although taphonomic features suggest that the fauna is mainly parautochthonous, some components (e.g., sponges, worms, the halkieriids and certain sightless arthropods) may be truly autochthonous. Comparison of the Sirius Passet locality with the renowned Middle Cambrian Burgess Shale of western Canada reveals similarities in overall depositional and tectonic setting: both accumulated in deep water adjacent to the steep, eroded margins of carbonate platforms — settings that subsequently sheltered the faunas from tectonic and metamorphic obliteration.

Occurrence of palygorskite and sepiolite in upper Paleocene–middle Eocene marine deep sediments of the Lagonegro Basin (Southern Apennines—Italy): Paleoenvironmental and provenance inferences

A mineralogical and geochemical study of clay lithologies and a biostratigraphic analysis of the carbonates from the deep-sea Lagonegro Basin (Southern Apennines—Italy) have been carried out to deduce in general the provenance of clay sediments and their paleoenvironmental conditions and particular to recognize the signature of the Paleocene–Eocene climatic global warming. The analysed succession comprising a wide stratigraphic interval of the Sannio Unit, spanning between Albian to the upper Oligocene–lower Miocene, is exposed near Accettura and Stigliano villages. Eighteen clay samples were analysed by XRD, XRF, SEM, TG-DTA. Their age was framed by biostratigraphic analyses carried out on carbonate sediments. Mineral assemblage of the clay sediments includes quartz, carbonates (calcite and dolomite), feldspars (plagioclase and k-feldspars), hematite, randomly illite/smectite mixed layers with a low illite percentage, kaolinite, discrete illite–muscovite, chlorite, palygorskite and sepiolite. The low illite percentage in randomly illite/smectite mixed layers indicates low diagenetic conditions for the studied successions. These features are unique for the Cretaceous–Tertiary successions of the Lagonegro domain and are particularly significant for the preservation of the native mineralogical assemblage useful to determine the provenance and paleoenvironmental conditions of the clayey sediments. Palygorskite and sepiolite are concentrated in the upper Paleocene–middle Eocene stratigraphic interval and particularly in the upper part of the early Eocene—lower part of the middle Eocene (biozone of Blow P 9–12). Clay sediments rich in palygorskite and sepiolite show a higher P 2O 5 amount and a lower kaolinite percentage, compatible with warm and arid climatic conditions typical of the global warming event well recorded in the southern tethyan margin. Likely palygorskite and sepiolite formed in lagoonal environment in nearby carbonate platform margins and then they were transported into the Lagonegro Basin as indicated by the well developed habitus of palygorskite. During the Paleogene the Lagonegro Basin and the nearby carbonate platforms represented a key sector the southern paleodomains of the Tethys. The discovery of these minerals gives a contribution to the reconstruction paleoenvironmental conditions of the Tethian paleo-margin during the early–middle Eocene.

Source and depositional processes of coarse-grained limestone event beds in Frasnian slope deposits (Kostomłoty-Mogiłki quarry, Holy Cross Mountains, Poland)

Source and depositional processes of coarse-grained limestone event beds in Frasnian slope deposits (Kostomłoty-Mogiłki quarry, Holy Cross Mountains, Poland)The Kostomłoty-Mogiłki succession is situated in the Kostomłoty transitional zone between the shallow-water Kielce stromatoporoid-coral platform and the deeper Łysogóry basin. In the Kostomłoty-Mogiłki quarry, the upper part of the Szydłówek Beds and Kostomłoty Beds are exposed. The Middle-Upper Frasnian Kostomłoty Beds are composed of shales, micritic and nodular limestones with abundant intercalations of detrital limestones. The dark shales and the micritic and nodular limestones record background sedimentation. The interbedded laminated and detrital limestones reflect high-energy deposition (= event beds). These event beds comprise laminated calcisiltites, fine-grained calcarenites, coarse-grained grain-supported calcirudites fabrics, and matrix-supported calcirudites. The material of these event beds was supplied by both erosion of the carbonate-platform margin and cannibalistic erosion of penecontemporaneous detrital limestones building the slope of this platform. Storms and the tectonic activity were likely the main causes of erosion. Combined and gravity flows were the transporting mechanisms involved in the reworking and redeposition.

Hydrothermal Zebra Dolomite in the Great Basin, Nevada—Attributes and Relation to Paleozoic Stratigraphy, Tectonics, and Ore Deposits

In other parts of the world, previous workers have shown that sparry dolomite in carbonate rocks may be produced by the generation and movement of hot basinal brines in response to arid paleoclimates and tectonism, and that some of these brines served as the transport medium for metals fixed in Mississippi Valley-type (MVT) and sedimentary exhalative (Sedex) deposits of Zn, Pb, Ag, Au, or barite. Numerous occurrences of hydrothermal zebra dolomite (HZD), comprised of alternating layers of dark replacement and light void-filling sparry or saddle dolomite, are present in Paleozoic platform and slope carbonate rocks on the eastern side of the Great Basin physiographic province. Locally, it is associated with mineral deposits of barite, Ag-Pb-Zn, and Au. In this paper the spatial distribution of HZD occurrences, their stratigraphic position, morphological characteristics, textures and zoning, and chemical and stable isotopic compositions were determined to improve understanding of their age, origin, and relation to dolostone, ore deposits, and the tectonic evolution of the Great Basin. In northern and central Nevada, HZD is coeval and cogenetic with Late Devonian and Early Mississippian Sedex Au, Zn, and barite deposits and may be related to Late Ordovician Sedex barite deposits. In southern Nevada and southwest California, it is cogenetic with small MVT Ag-Pb-Zn deposits in rocks as young as Early Mississippian. Over Paleozoic time, the Great Basin was at equatorial paleolatitudes with episodes of arid paleoclimates. Several occurrences of HZD are crosscut by Mesozoic or Cenozoic intrusions, and some host younger pluton-related polymetallic replacement and Carlin-type gold deposits. The distribution of HZD in space (carbonate platform, margin, and slope) and stratigraphy (Late Neoproterozoic Ediacaran–Mississippian) roughly parallels that of dolostone and both are prevalent in Devonian strata. Stratabound HZD is best developed in Ediacaran and Cambrian units, whereas discordant HZD is proximal to high-angle structures at the carbonate platform margin, such as strike-slip and growth faults and dilational jogs. Fabric-selective replacement and dissolution features (e.g., collapse breccias, voids with geopetal textures) are common, with remaining void space lined with light-colored dolomite crystals that exhibit zoning under cathodoluminescence. Zoned crystals usually contain tiny ( ∼70 °C. The oxygen isotopic compositions of HZD are consistent with formation temperatures of 50–150 °C requiring brine circulation to depths of 2–5 km, or more. The few HZD occurrences with the highest concentrations of metals (especially Fe, Mn, and Zn) and the largest isotopic shifts are closely associated with Sedex or MVT deposits known to have formed from hotter brines (e.g., Th > 150–250 °C). These relationships permit that HZD formed at about the same time as dolostone, from brines produced by the evaporation of seawater during arid paleoclimates at equatorial paleolatitudes. Both dolostone and HZD may have formed as basinal brines, which migrated seaward from evaporative pans on the platform, with dolostone forming at low temperatures along shallow migration pathways through permeable limestones, and HZD forming at high temperatures along deeper migration pathways through basal aquifers and dilatant high-angle faults. The small MVT deposits were chemical traps where hot brines encountered rocks or fluids containing reduced sulfur. The abundant Sedex deposits mark sites where hot brine discharged at the seafloor in adjacent basins. Thus the distribution of HZD may map deep migration pathways and upflow zones between eastern shallow marine facies, where evaporative brine could have been generated, and western Sedex deposits, where heated brines discharged along faults into platform margin, slope, and basin facies. The small size and scarcity of Pb-Zn deposits and the abundance of barite deposits in the Great Basin suggests the brines were generally reduced, possibly due to reactions with carbonaceous rocks along deep migration pathways. While this scenario may have occurred at several times, the age and abundance of Sedex deposits suggest that such a hydrology was best developed in the Late Ordovician, Late Devonian, and Early Mississippian, possibly in response to episodes of extension and forebulge faults associated with the Antler orogeny. The improved understanding of HZD may aid future exploration for ore deposits in the Great Basin.

Depositional architecture of the late Ordovician drowned carbonate platform margin and its responses to sea-level fluctuation in the northern slope of the Tazhong region, Tarim Basin

The Tazhong Uplift of the late Ordovician is a drowned rimmed carbonate platform. the carbonate rock of the late Ordovician Lianglitage Formation in the northern slope of the Tazhong region is one of the significant petroliferous intervals. Based on petrofacies, depositional cycles, natural gammaray spectrometry and carbon/oxygen isotope data from the Lianglitage Formation, one 2nd-order, three 3rd-order and several 4th-order sequences have been recognized, and the late Ordovician relative sealevel fluctuation curve has been established. The sequences O3l-1 and O3l-2 on the platform are composed of highstand and transgressive systems tracts, but lack the lowstand systems tract. The sequence O3l-3 is a drowning sequence. The sequence O3l-1 overlapped the eroded slope and pinched out to the northwest and landward. The highstand systems tract in the sequence O3l-2 consists of low-angle sigmoid and high-angle shingled progradation configuration. Major sedimentary facies of the Lianglitage Formation include reef and shoal in the platform margin and lagoon, which can be subdivided into coral-sponge-stromatoporoid reef complex, sand shoal, lime mud mound, and intershoal sea. Reefs, sand shoals and their complex are potential reservoir facies. The reefs and sand shoals in the sequence O3l-1 developed in the upper of its highstand systems tract. In the sequence O3l-2, the highstand systems tract with an internal prograding configuration is a response to the lateral shifting of the complex of reef and sand shoal. The transgressive systems tract, in particular the sand shoals, developed widely on the slope of the platform margin and interior. The reefs in the sequence O3l-3 migrated towards high positions and formed retrograding reefs in the western platform and low relief in the platform interior. Basinward lateral migration of the reefs and pure carbonate rock both characterize highstand systems tract and show that the rise of the relative sea-level was very slow. Shingled prograding stacking pattern of the 4th-order sequences and reefs grow horizontally, which represents the late stage of highstand systems tract and implies relative sealevel stillstand. Reefs migrating towards high land and impure carbonate rock both indicate transgressive systems tract and suggest that the relative sea-level rose fast. Erosional truncation and epidiagenetic karstification represent a falling relative sea-level. The relative sea-level fluctuation and antecedent palaeotopography control the development and distribution of reef complexes and unconformity karst zones. Currently, the composite zone of epidiagenetic karstic intervals and high-energy complexes of reefs and sand shoals with prograding configuration is an important oil and gas reservoir in the northern slope of the Tazhong carbonate platform.