西藏南羌塘坳陷昂达尔错地区侏罗系布曲组碳酸盐岩阴极发光特征与成岩阶段划分

通过岩石学特征研究和阴极发光分析,结合Fe、Mn、Sr元素特征及氧同位素资料,研究西藏南羌塘坳陷昂达尔错地区侏罗系布曲组碳酸盐岩阴极发光特征并进行成岩阶段划分。结果表明:(1)布曲组碳酸盐岩阴极发光强度总体较低,这与其较低的Fe、Mn含量有关,说明沉积期后非海源流体对碳酸盐岩的影响较小,海源流体在成岩过程中起主导作用;(2)伴随着成岩作用增强,不同岩石类型中的Mn含量逐渐增加、Sr含量逐渐降低,氧同位素值总体上向高负值方向偏移,对应的阴极发光强度逐步增加;(3)泥-微晶白云岩具有最强的阴极发光性和最高的Mn含量,在白云岩化过程中可能与近地表弱还原环境有关,蒸发泵和渗透回流作用可能是这些岩石白云石化作用的主要机理;而结晶白云岩则具有较弱的阴极发光性和较低的Mn含量,孔隙流体可能远离大气淡水,包裹体均一温度显示其白云岩化作用是在深埋藏环境中发生的;(4)布曲组碳酸盐岩经历了准同生-同生阶段的海水成岩环境→早成岩阶段的近地表-浅埋藏弱还原成岩环境→中-深成岩阶段的中-深埋藏深循环温压水和高盐度地层水成岩环境的连续改造→晚白垩世末期-古近系早期构造热事件热液改造阶段→表生成岩阶段的演化过程。     

Faulting and fracturing in the Jurassic carbonate ramp folds of southern Rif ridges (Northern Morocco)

In the South Rifian ridges (SRR), the dominated structures correspond to the faulted anticline characteristic of a foreland orogeny context, front of the Rif Alpine belt. These anticlines correspond to thrust propagation folds. Geometric model of these structures shows that the normal faults have controlled the Mesozoic sedimentation during extensive episodes and participated in determining areas of thrusting during Miocene compressional phases. However, the normal fault strike which is relative to the direction of the shortening determined the geometry of diverse folds whether into the frontal ramps, lateral, or oblique. In the meantime, the systematic fracturing study in the Jurassic limestone beds, in different parts of the folds with axes oriented E-W, NW-SE, and NE-SW, permits to propose a relative fracturing chronology and tries investigating the relationship between folding and fracturing. The three main fracture families, oblique, transversal, and axial, appear simultaneously during the amplification of the fold. The simple shear in the limb contributes the latest to the folding reactivation and the density of the intensification of these microfractures. Likewise, given the important downslope fold limb dip of the ramp propagation folds, theoretically the shear intensity is more important, and micro-fractures are more important in the downslope fold limb, thus the uphill one.     

A comparison of the early diagenetic environment in intertidal sands and muds of the Manukau Harbour,New Zealand

The early diagenetic environment of intertidal sandy sediments (sands) and muddy sediments (muds) is described and compared from two cores taken from an unpolluted part of the Manukau Harbour, New Zealand. Extraction techniques characterized the form of the trace elements (Fe, Mn, S, C, Pb, Zn, Cu) at different depths in the sediment. Dissolved forms of Fe, Mn, and S were measured in interstitial water. Nonresidual metal concentrations, humic acid, FeS, and FeS₂ are an order of magnitude higher in the muds than in the sands because of dilution by unreactive sand particles. Muds contain a larger proportion of metals in the mobile fractions; exchangeable (Mn), carbonate (Mn, Fe, Zn), and easily-reducible oxide (Fe, Mn, Zn, Pb). This is due to greater surface area (for Mn adsorption); the favorable conditions for MnCO₃, FeCO₃, and FeS precipitation; and higher concentrations of easily reducible iron oxide and humic acid. Therefore, compared to the sands, muds are more important as reservoirs for toxic metals, both in terms of quantity and availability. At either site there was very little difference between the forms of Zn, Pb or Cu identified by sequential extraction as sediments changed from oxic to anoxic conditions. One reason for this is that the amounts and proportions of some of the important components that bind metals, viz., amorphous iron hydrous oxides, humic acids, and FeS₂, do not change much. Other components that do change with redox conditions, for example, manganese phases and FeS, are only minor components of the sediment. Redox conditions, then, have relatively little effect on trace-metal partitioning in the sediment matrix of these unpolluted sediments.     

Controls and predictability of carbonate facies architecture in a Lower Jurassic three-dimensional barrier-shoal complex (Djebel Bou Dahar, High Atlas, Morocco)

Spatial information on lithofacies from outcrops is paramount for understanding the internal dynamics, external controls and degree of predictability of the facies architecture of shallow‐water carbonate‐platform tops. To quantify the spatial distribution and vertical stacking of lithofacies within an outer‐platform shoal‐barrier complex, integrated facies analysis and digital field technologies have been applied to a high‐relief carbonate platform exposed in the Djebel Bou Dahar (Lower Jurassic, High Atlas, Morocco). The outer platform is characterized by subtidal, cross‐bedded, coarse grainstone to rudstone grading into supratidal, pisoidal packstone‐rudstone with tepees that together formed a 350 to 420 m wide shoal‐barrier belt parallel to the margin. This belt acted as a topographic high separating a restricted lagoon from the subtidal, open marine region. Low‐energy tidal flats developed on the protected flank of the barrier facing the lagoon. Lithofacies patterns were captured quantitatively from outcrop and integrated into a digital outcrop model. The outcrop model enabled rapid visualization of field data and efficient extraction of quantitative data such as widths of facies belts. In addition, the spatial heterogeneity was captured in multiple time slices, i.e. during different phases of cyclic base‐level fluctuations. In general, the lateral continuity of lithofacies is highest when relative water depth increased during flooding of the platform top, establishing low‐energy subtidal conditions across the whole platform, and when the accommodation space was filled with tidal flat facies. Heterogeneity increased during deposition of the relief‐building bar facies that promoted spatial diversification of depositional environments during the initial phases of accommodation space creation. Cycles commonly are composed of a thin transgressive tidal flat unit, followed by coated‐grain rudstone bar facies. Lateral to the bar facies, pisoidal‐grainstone beach deposits accumulated. These bar and beach deposits were overlain by subtidal lagoonal facies or would grow through the maximum flooding and highstand. There the bars either graded into supratidal pisoidal facies with tepees (when accommodation space was filled) or were capped by subaerial exposure (due to a sea‐level fall). Modified embedded Markov analysis was used to test the presence of common ordering in vertical lithofacies stacking in a stationary interval (constant depositional mode). Analysis of individual sections did not reveal any ordering, which may be related to the limited thickness of these sections. Composite sections, however, rejected the null hypothesis of randomness. The addition of stratigraphically significant information to the Markov analysis, such as exposure surfaces and lateral dimensions of facies bodies, strengthens the verdict of unambiguous preferential ordering. Through careful quantitative reconstruction of stratal geometry and facies relationships in fully integrated digital outcrop models, accurate depositional models could be established that enhanced the predictability of carbonate sediment accumulation.     

Biosedimentology of the Early Jurassic post-extinction carbonate depositional system, central High Atlas rift basin, Morocco

This study documents a Liassic example of the long‐ranging effects of mass extinction on carbonate systems. Biohistoric constraints inherent in the Liassic carbonate depositional system are deciphered from normal‐marine, sub‐tidal deposits of the central High Atlas rift basin (Morocco) through ?Hettangian/Sinemurian to Early Toarcian times. The integration of results from the analysis of lithofacies, depositional geometries, microfacies, macrobenthos, carbonate build‐ups, carbon and oxygen stable isotopes, and rare earth element + yttrium distribution patterns allows the intrinsic (or biohistoric) control on the central High Atlas deposits to be separated from extrinsic factors, such as basin development and palaeoclimate. The survival interval in the aftermath of the end‐Triassic mass extinction persisted until the Early Sinemurian indicated by a severely depleted carbonate system impoverished in skeletal organisms. A tectonic pulse at the Early to Late Sinemurian boundary interval caused a basin widening with immigration of a marine fauna. However, until the latest Sinemurian (macdonelli Subzone of the raricostatum Zone) the deposits were dominated by filter‐feeding benthic heterotrophs (sponges, brachiopods, polychaetes and crinoids). During this stage, primary production within the enlarged basin must have been largely planktonic and there was a net‐flux of organic matter to the sea floor (oxygen minimum zone). A regional radiation of organic‐walled phytoplankton is inferred to explain the selective success of the filter‐feeding community and the occurrence of sponge mounds. Thus, significant effects of the end‐Triassic mass extinction are still present during the Late Sinemurian. Through almost the entire Pliensbachian a highly productive, shoal‐rimmed carbonate platform existed; it developed subsequent to tectonic reorganization and a marine recirculation event (radiolarian facies, Δδ¹³C ≈ ?1·1, strongly negative Ce‐anomaly). Photosymbiotic sediment producers (mainly large bivalves) now state the success of specialists and environmental equilibrium conditions. In the latest Pliensbachian the climax stage was reached with the development of a coralgal reef‐rimmed carbonate platform. The Liassic carbonate depositional system experienced a terminal, multicausal Early Toarcian drowning event during which most of the large bivalves became extinct.     

Concretions, isotopes, and the diagenetic history of the Oxford Clay (Jurassic) of central England

In interpreting the results of a petrographic and isotopic study of concretions, a range of subjects is discussed including the original texture of the Oxford Clay sediment, Jurassic palaeotemperatures, the diagenetic history of pore-waters and the palaeo-hydrology of central England. The concretions are all composed predominantly of calcite. They include precompactional, pyrite-rich concretions that later suffered an eposide of brecciation, and others that only commenced to form after compaction had crushed ammonite shells included in the bituminous clay sediment. Petrographic, chemical, and especially carbon isotope data demonstrate a dominantly organic source for the carbon in the early formed concretions. Oxygen isotopes indicate formation at the same temperatures (13-16°C) at which benthic molluscs were living. Concretion growth in pelleted, anaerobic mud proceeded concurrently with bacterial sulphate reduction and pyrite precipitation. Cracking of the concretions started at this stage: in a few concretions, the cracks were also partially filled with brown calcite. During post-compactional growth, δ¹³C increased and pyrite content decreased, showing waning organic influence; δ¹⁸O decreased. The brecciated concretions were intruded by clay in which baryte crystals grew; finally, most remaining voids were filled with strongly-ferroan calcite of δ¹⁸O about—7 PDB and δ¹³C about O PDB. This must indicate strong depletion of the pore waters in ¹⁸O. Mechanisms that might lead to this are reviewed. It is concluded that the sequence of mineralogical and chemical changes is most readily explained if originally marine porewaters, first modified by bacterial activity, were flushed from the compacting clays by water of ultimately meteoric origin. This had its source in palaeo-aquifers beneath the Oxford Clay. Speculative attempts are made to relate this history to the geology of the region.     

Stable isotope evidence for the origin of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel

The oxygen isotope compositions of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel, have been used to identify porewater types during diagenesis. Changes in porewater composition can be related to major geological events within southern Israel. In particular, saline brines played an important role in late (Pliocene-Pleistocene) dolomitization of these rocks. Diagenetic carbonates included early siderite (δ¹⁸O_SMOW=+24.4 to +26.5‰δ¹³C_PDB=?1.1 to +0.8‰), late dolomite, ferroan dolomite and ankerite (δ¹⁸O_SMOW=+18.4 to +25.8‰; δ¹³C_PDB=?2.1 to +0.2‰), and calcite (δ¹⁸O_SMOW=+21.3 to +32.6‰; δ¹³C_PDB=?4.2 to + 3.2‰). The petrographic and isotopic results suggest that siderite formed early in the diagenetic history at shallow depths. The dolomitic phases formed at greater depths late in diagenesis. Crystallization of secondary calcite spans early to late diagenesis, consistent with its large range in isotopic values. A strong negative correlation exists between burial depth (temperature) and the oxygen isotopic compositions of the dolomitic cements. In addition, the δ¹⁸O values of the dolomitic phases in the northern Negev and Judea Mountains are in isotopic equilibrium with present formation waters. This behaviour suggests that formation of secondary dolomite post-dates the tectonic activity responsible for the present relief of southern Israel (Upper Miocene to Pliocene) and that the dolomite crystallized from present formation waters. Such is not the case in the Central Negev. In that locality, present formation waters have much lower salinities and δ¹⁸O values, indicating invasion of freshwater, and are out of isotopic equilibrium with secondary dolomite. Recharge of the Inmar Formation by meteoric water in the Central Negev occurred in the Pleistocene, and halted formation of dolomite.     

Emplacement of the Jurassic Mirdita ophiolites (southern Albania): evidence from associated clastic and carbonate sediments

Sedimentology can shed light on the emplacement of oceanic lithosphere (i.e. ophiolites) onto continental crust and post-emplacement settings. An example chosen here is the well-exposed Jurassic Mirdita ophiolite in southern Albania. Successions studied in five different ophiolitic massifs (Voskopoja, Luniku, Shpati, Rehove and Morava) document variable depositional processes and palaeoenvironments in the light of evidence from comparable settings elsewhere (e.g. N Albania; N Greece). Ophiolitic extrusive rocks (pillow basalts and lava breccias) locally retain an intact cover of oceanic radiolarian chert (in the Shpati massif). Elsewhere, ophiolite-derived clastics typically overlie basaltic extrusives or ultramafic rocks directly. The oldest dated sediments are calpionellid- and ammonite-bearing pelagic carbonates of latest (?) Jurassic-Berrasian age. Similar calpionellid limestones elsewhere (N Albania; N Greece) post-date the regional ophiolite emplacement. At one locality in S Albania (Voskopoja), calpionellid limestones are gradationally underlain by thick ophiolite-derived breccias (containing both ultramafic and mafic clasts) that were derived by mass wasting of subaqueous fault scarps during or soon after the latest stages of ophiolite emplacement. An intercalation of serpentinite-rich debris flows at this locality is indicative of mobilisation of hydrated oceanic ultramafic rocks. Some of the ophiolite-derived conglomerates (e.g. Shpati massif) include well-rounded serpentinite and basalt clasts suggestive of a high-energy, shallow-water origin. The Berriasian pelagic limestones (at Voskopoja) experienced reworking and slumping probably related to shallowing and a switch to neritic deposition. Mixed ophiolite-derived clastic and neritic carbonate sediments accumulated later, during the Early Cretaceous (mainly Barremian-Aptian) in variable deltaic, lagoonal and shallow-marine settings. These sediments were influenced by local tectonics or eustatic sea-level change. Terrigenous sediment gradually encroached from neighbouring landmasses as the ophiolite was faulted or eroded. An Aptian transgression was followed by regression, creating a local unconformity (e.g. at Boboshtica). A Turonian marine transgression initiated widespread Upper Cretaceous shelf carbonate deposition. In the regional context, the southern Albania ophiolites appear to have been rapidly emplaced onto a continental margin in a subaqueous setting during the Late Jurassic (Late Oxfordian-Late Tithonian). This was followed by gradual emergence, probably in response to thinning of the ophiolite by erosion and/or exhumation. The sedimentary cover of the south Albanian ophiolites is consistent with rapid, relatively short-distance emplacement of a regional-scale ophiolite over a local Pelagonian-Korabi microcontinent.     

Liquefaction, fluidization and erosional structures associated with bituminous sands of the Bracklesham Formation (Middle Eocene) of Dorset, England

ABSTRACT At Hengistbury Head, Dorset, the Boscombe Sands (Middle Eocene, Bracklesham Formation) are of estuarine channel facies. A mud-filled channel is exposed, the banks and eastern flank of which have a black carbonaceous stain, the degraded remains of a bitumen. At the time of deposition, the bitumen rendered the sediment firm and it was extensively burrowed by a Thalassinoides -forming organism (crustacean). The bituminous sand on the eastern channel bank suffered brecciation and dilation as a result of liquefaction and flowage of the underlying sediments. This is thought to have been due to rapid expulsion of pore water, possibly as a result of seismic shock. The layers of bituminous sand below the surface were ruptured during water-escape, resulting in localized zones of rapid flow causing fluidization and the development of dewatering pipes up to 1.2 m long. The estuarine sediments were subsequently transgressed during which the bituminous sand was exposed on the seafloor, when it was eroded into a hummocky topography and heavily burrowed. Blocks of bituminous sand were reworked into the marine basal conglomerate, composed mainly of flints, demonstrating the remarkable strength of the bituminous cement.     

Early diagenetic, Na-alunites in Miocene algal mat intertidal facies, Ras Sudar, Sinai

An early diagenetic non-hydrothermal deposit of Na-alunite [(Na,K)Al₃(SO₄)₂(OH)₆] is described from the Miocene Ras Maalab Group at Ras Sudar Sinai. The alunite is nodular and occurs along two horizons parallel to bedding. Displacive growth structures and disrupted bedding within the host rock, together with enterolithic growth structures within the nodules, indicate a period of growth prior to compaction. Linear arrays of fabric elements within the nodules, observed under the microscope, further support early diagenetic growth. The host sequence, believed to be the product of sedimentation within barred coastal lagoons has been subdivided into two lithofacies, one reflecting a subtidal open-lagoonal environment, the other an intertidal restricted-circulation environment. Anaerobic conditions of early diagenesis conducive to generation of H₂S and FeS₂ preceded an oxidation event produced either by contact with free-circulating oxygenated waters or by emergence. The sulphuric acid generated lowered the pH and clay minerals were converted to alunite.     

Multistage dolomitization and distribution of dolomitized bodies in Early Jurassic carbonate platforms (Southern Alps,Italy)

The Early Jurassic dolomitized carbonates are a hydrocarbon exploration target in Northern Italy. Of these carbonates, the Liassic Albenza Formation platform and the overlying Sedrina Formation shelf were studied to define a pervasive dolomitization model and to shed light on dolomite distribution in the sub‐surface. Field work, as well as analyses of well cores, stable isotopes, trace elements and fluid inclusions, was carried out on the outcropping thrust belt and sub‐surface deformed foreland of the Southern Alps. Petrographic analyses showed a first, pervasive, replacement dolomitization phase (D1) followed by volumetrically less important dolomite cement precipitation phases (D2, D3 and D4). The δ¹⁸O values fall between ?8·2‰ and 0·1‰ Vienna‐Pee Dee Belemnite with the more depleted samples belonging to dolomite cement‐rich dolostones; the δ¹³C ranges from 2·6‰ to 3·7‰ Vienna‐Pee Dee Belemnite. Analysis of trace elements showed different Fe and Mn contents in the sub‐surface and outcropping dolostones, and a higher Fe in the younger dolomite cements. An increase in the precipitation temperature (up to 130 °C from fluid inclusion data) and a decrease in diagenetic fluid salinity (from sea water to brackish) are observed from the first pervasive replacement dolomite to the dolomite cement phases. Field observations indicate that, in the Albenza Formation, dolomitization was limited to palaeohighs or faulted platform margins in the Early Jurassic carbonates. The pervasive replacement phase is interpreted based on a ‘compaction model’; the formation fluids expelled from compacting basinal carbonates could have funnelled along faults into permeable palaeohighs. The high homogenization temperature of the dolomite cements and decreased salinities indicate precipitation at great depth with an influx of meteoric water. These data, along with the thermal history, suggest that the dolomite cements precipitated according to the ‘tectonic squeegee’ dolomitization model. The dolomite precipitation temperature was set against the thermal history of the carbonate platform to interpret the timing of dolomite precipitation. The dolomite precipitation temperatures (90 to 100 °C) were reached in the studied formations first in the thrust fold belt (Early Tertiary, 60 Ma), and then in the foreland succession during the Late Tertiary (10 Ma). This observation suggests that the dolomite precipitation fronts moved southwards over time, recording a ‘diagenetic wave’ linked to the migration of the orogenic system. Observations suggest that the porosity increased during the first phase of replacement dolomitization while the dolomite cementation phases partially occluded the pores. The distribution of porous dolomitized bodies is therefore linked to the ‘compaction dolomitization’ model.     

羌塘盆地东缘中侏罗统布曲组碳酸盐岩成岩作用与成岩环境

羌塘盆地东缘布曲组碳酸盐岩的成岩作用类型主要有泥晶化作用、胶结作用、压实压溶作用、破裂作用、溶解作用等,其中以破裂作用、溶解作用和泥晶化作用最为发育。根据成岩组构可将碳酸盐岩的成岩环境分为海水、大气淡水和埋藏三种成岩环境。初步总结了布曲组剖面上成岩作用与沉积环境和海平面变化的关系。沉积旋回的下部以选择性的内溶孔和海底泥晶化作用为主要特征,中上部以发育压实、压溶的埋藏成岩作用为主。海平面到达最高点以后快速下降,以发育晚期的大汽淡水溶蚀作用为主要特征。初步总结了海进环境下成岩作用演化序列;探讨了羌塘盆地东缘的布曲组在油气普查与方面的重要意义。     

Capturing and modelling metre‐scale spatial facies heterogeneity in a Jurassic ramp setting (Central High Atlas,Morocco)

Each simulation algorithm, including Truncated Gaussian Simulation, Sequential Indicator Simulation and Indicator Kriging is characterized by different operating modes, which variably influence the facies proportion, distribution and association of digital outcrop models, as shown in clastic sediments. A detailed study of carbonate heterogeneity is then crucial to understanding these differences and providing rules for carbonate modelling. Through a continuous exposure of Bajocian carbonate strata, a study window (320 m long, 190 m wide and 30 m thick) was investigated and metre‐scale lithofacies heterogeneity was captured and modelled using closely‐spaced sections. Ten lithofacies, deposited in a shallow‐water carbonate‐dominated ramp, were recognized and their dimensions and associations were documented. Field data, including height sections, were georeferenced and input into the model. Four models were built in the present study. Model A used all sections and Truncated Gaussian Simulation during the stochastic simulation. For the three other models, Model B was generated using Truncated Gaussian Simulation as for Model A, Model C was generated using Sequential Indicator Simulation and Model D was generated using Indicator Kriging. These three additional models were built by removing two out of eight sections from data input. The removal of sections allows direct insights on geological uncertainties at inter‐well spacings by comparing modelled and described sections. Other quantitative and qualitative comparisons were carried out between models to understand the advantages/disadvantages of each algorithm. Model A is used as the base case. Indicator Kriging (Model D) simplifies the facies distribution by assigning continuous geological bodies of the most abundant lithofacies to each zone. Sequential Indicator Simulation (Model C) is confident to conserve facies proportion when geological heterogeneity is complex. The use of trend with Truncated Gaussian Simulation is a powerful tool for modelling well‐defined spatial facies relationships. However, in shallow‐water carbonate, facies can coexist and their association can change through time and space. The present study shows that the scale of modelling (depositional environment or lithofacies) involves specific simulation constraints on shallow‐water carbonate modelling methods.     

Primary depositional and diagenetic features in the banded iron formation and associated iron-ore deposits of Noamundi,Singhbhum district,Bihar, India

The Banded Hematite Jasper (BHJ) Formation of Noamundi region in Bihar, belonging to the lower part of the Iron Ore Group of early Precambrian age (c. 2900–3200 Ma), exhibits numerous primary depositional and diagenetic features, both in BHJ as well as the associated iron ore deposits. Observed primary depositional features include banding and bedding of different geometric-types, surface-markings including interference ripple-marks, current crescents, linear markings, scour-and-fill structures, etc. and within-mass microstructures such as spherulites, granules, discs and maculose cylindrical bodies. Diagenetic features, such as fabric changes, micro-dessication structures, gravity-density features, etc. and motion-and-disruption features of various kinds are also seen. The significance of various features in terms of probable mode and environments of deposition of BHJ and the iron ore beds has been considered. In general, shallow water environment of deposition in a region proximal to the shoreline with a rather steep paleoslope of the shelf has been surmised.     

Southernmost outcrops of marine lower Tertiary carbonate rocks in NE-Africa (Gebel Abyad,Sudan)

Early Tertiary carbonates (Gebel Abyad Formation) are described from an isolated limestone plateau from the northwestern Sudan. They represent the southernmost outskirts of marine, Tertiary sediments in northeast Africa, which can be connected with transgressive/regressive cycles of the Tethys onto the African craton. Microfacies studies of the carbonates yielded exclusively shallow marine sediments which dried up temporarily, and with changing rates of terrigenous input. They contain a low diverse fauna and flora, poor in individuals. The paleogeographic reconstruction of this isolated occurrence was possible by comparing sediments from southern Egypt.

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Zusammenfassung Aus dem nordwestlichen Sudan werden alttertiäre Karbonate (Gebel Abyad Formation) beschrieben, die ein isoliertes Vorkommen darstellen. Diese repräsentieren die südlichsten marinen tertiären Ablagerungen in Nordost-Afrika, die mit transgressiv/regressiven Zyklen der Tethys auf den afrikanischen Kraton in Verbindung gebracht werden können. Die mikrofaziell bearbeiteten Kalke stellen ausschlie\lich flachmarine Ablagerungen dar, mit Anzeichen für zeitweiliges Trockenfallen, und wechselnden Anteilen terrigener Zufuhr. Sie enthalten eine gering-diverse, individuenarme Fauna und Flora. Faziell vergleichbare Ablagerungen in Südägypten lassen eine Rekonstruktion dieses isolierten Kalkvorkommens zu einem paläogeographischen Modell zu.

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Résumé Cette note décrit des roches carbonatées d'âge tertiaire inférieur (formation du Jebel Abyad) qui font partie d'un plateau isolé dans le nord-ouest du Soudan. Elles représentent l'extension méridionale maximale de la sédimentation marine tertiaire dans le nord-est de l'Afrique, sédimentation qui peut Être reliée aux cycles de transgressionsrégressions de la Téthys sur le bloc continental africain. L'étude des microfacies amène à les identifier comme des dépÔts marins de faible profondeur, temporairement asséchés et mÊlés d'apports terrigènes en quantité variable. Ils renferment une flore et une faune pauvres en espèces et en individus. La comparaison avec des sédiments du Sud égyptien permet de les intégrer dans un modèle paléogéographique.

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Syndepositional and diagenetic features in the pyrite ores of Amjhore,Bihar, India

The sedimentary pyrite deposit of Amjhore, Bihar associated with the Bijaigarh Shale of the Upper Vindhyan Group (Proterozoic age) exhibits characteristic syndepositional features suggestive of both calm and disturbed conditions of deposition. The pyrite ores also show typical diagenetic features.     

Palaeontological and facial features of the Upper Jurassic Hochstegen Marble (Tauern Window, Eastern Alps)

Micropalaeontological, microscopic and mineralogical investigations of the ductily deformed and greenschist-facies metamorphic Hochstegen Marble in the Tauern Window shed new light on its stratigraphy and fades.
New radiolarian and sponge spicule discoveries have been made in cherty limestone marbles. They confirm previous age assignments and permit for the first time a more exact micropalaeontological age determination of early Tithonian for the upper parts of the marble. Forty morphotypes of radiolarians could be distinguished; in one sample a Fisher diversity index of 6 is reached indicating deeper marine conditions. The spicule fauna is also diverse and shows affinity to the S-German Malm. In respect to all the data it can be presumed that carbonate sedimentation of the Hochstegen Marble took place in a deeper marine environment at the southern margin of the European continent (Helvetic realm) during the whole Late Jurassic.     

Continuity and facies heterogeneities of shallow carbonate ramp cycles (Sinemurian,Lower Jurassic,North‐east Spain)

The lateral continuity and facies heterogeneities of metre‐scale cycles in a greenhouse Lower Jurassic (Sinemurian) carbonate ramp from the northern Iberian Basin (Spain) was evaluated from extensive field analysis carried out on a well‐exposed 12 km long outcrop. Eleven high‐frequency continuous cycles and their bounding surfaces are traceable laterally through the entire outcrop. However, three of these cycles are found to split laterally into discontinuous cycles of more limited distribution (up to 3 to 5 km of lateral extent). The continuous and discontinuous cycles have a similar field expression in one‐dimensional logs. As a consequence, the number of cycles that can be differentiated is variable along the logged sections (i.e. from 11 to 16). Cycles have variable facies heterogeneities and sedimentary trends depending on the environment of formation: shallowing‐upward and symmetrical cycles occur in protected lagoon–tidal flat areas and in the open‐marine, high‐energy domain. These cycles show significant facies heterogeneities, which were controlled mainly by lateral migration of a mosaic of facies over an irregular topography. Deepening‐upward and aggradational cycles are generated in low‐energy, sub wave‐base, open‐marine areas. Facies are laterally homogeneous, reflecting low potential for carbonate accumulation and inability to fill the created accommodation space in this low‐relief and relatively deep area. Cycle boundaries are generated by stages of rapid accommodation gain, involving the flooding of the carbonate ramp; they are more likely to originate from regional tectonic pulses (related to the extensional tectonics operating in the northern Iberian Basin) rather than greenhouse low‐amplitude eustacy. Discontinuous cycles tend to occur in thickened areas and are interpreted as originating from the infill of wedge‐shaped accommodation space resulting from differential subsidence (i.e. local tectonic pulses). In conclusion, where thickness variations occur in extensional settings lateral continuity of cycles should not be expected. In less well‐exposed, or in one‐dimensional sections and in wells, it would not be possible to distinguish continuous from discontinuous cycles, or to understand such two‐dimensional heterogeneities. Identification of unique cycle‐forming mechanisms or attempting cyclostratigraphic long‐distance correlation of cycles is unrealistic without a detailed analysis of the architecture of cycles in laterally continuous outcrops.     

Clastic sediments associated with radiolarites (Tauglboden—Schichten, Upper Jurassic, Eastern Alps)

During the early Upper Jurassic, widespread deep-sea radiolarites were deposited in most parts of the Northern Limestone Alps. In the formation described (Tauglboden-Schichten), these pelagic sediments interfinger with local-source clastic material. Depending on the topography and the kind of material, either slides and slumps, mudflows, grain flows or turbidity currents operated and formed slump-folded beds, mud-flow breccias, fluxoturbidites or turbidites. A breccia had been traced over an area of 20 km². Its variation is described in terms of lithological columns, bed thicknesses, maximum grain sizes and grain orientations. It forms a tongue-shaped body, which was probably a part of a submarine fan. The fluxoturbidites of the proximal area grade distally on three sides into turbidites within 3–5 km. The clastic material consists of marls and limestones of Rhaetian and Jurassic age. It was probably derived from a tectonically uplifted palaeo-high by an interplay of tectonics and gravity. The clastics were deposited on submarine fans bordering this high. In its lithology the formation closely resembles certain marginal facies of flysch troughs.