Oolitic Ironstones Research Articles

Petrology and Genesis of the Bhainskati Iron Ore Deposit of Palpa District, Western Nepal

The Bhainskati Formation of the Tansen Group in Palpa area is known for hematite iron ore deposit for long time. A prominent band of hematite of about 1-2 km thickness extending >5 km was identified in the upper part of the Bhainskati Formation in the present study and the band is repeated three times in the area by folding and faulting. Petrographic study shows that it is oolitic ironstone of sedimentary shallow marine origin. Main minerals in the band are hematite, goethite, quartz, calcite, siderite and albite. Hematite content varies considerably among samples and occurs mainly as oolite and cement. The Bhainskati ironstone with its ferrous mineral assemblage and well-rounded texture of the ooids suggests prodeltaicto estuarine with shallow marine environment reduced clastic input.

Properties and available micronutrient status of some soils derived from Abeokuta formation in Nigeria

Available iron, zinc, copper and manganese were determined in six pedons located in upper slope, middle slope and valley bottom soils derived from Abeokuta geological materials in Nigeria. The soils had an average of 639.8 g kg−1 sand, 241.8 g kg−1 clay and 118.4 g kg−1 silt. The fertility status of the soils was low–medium with a strongly acid–neutral reaction, 1.3–15.1 g kg−1 organic carbon contents, moderate–high exchangeable bases and 1.38 mg kg−1 available phosphorus. Both Fe (122.50 mg kg−1) and Mn (111.40 mg kg−1) occurred at toxic levels, whereas the mean Cu (1.27 mg kg−1) and Zn (2.56 mg kg−1) contents were found to be adequate for most crops grown in the region. There were significant positive correlations among the micronutrients and also between soil pH, organic carbon, particle size fractions and micronutrients. The high levels of Fe and Mn were probably due to the presence of oolitic ironstone in the parent material.

Petrology and genesis of the Bhainskati iron ore deposit of Palpa District, western Nepal

The Bhainskati Formation of the Tansen Group in the Palpa area is known for hematite iron ore deposit for long time. A prominent band of hematite of about 1-2 m thickness and extending >5 km was identified in the upper part of the Bhainskati Formation in the present study. The band is repeated three times in the area by folding and faulting. Petrographic study shows that it is oolitic ironstone of sedimentary origin. Main minerals in the band are hematite, goethite, quartz, calcite, siderite and albite. Hematite content varies considerably among samples and occurs mainly as oolite and cement. The Bhainskati ironstone with its ferrous mineral assemblage and well-rounded texture of the ooids suggests shallow marine environment (prodeltaic to estuarine) with reduced clastic input. DOI: http://dx.doi.org/10.3126/bdg.v15i0.7418 Bulletin of the Department of Geology, Vol. 15, 2012, pp. 63-68

Iron-Coated Particles from Condensed Aalenian-Bajocian Deposits: Evolutionary Model (Iberian Basin, Spain)

Condensed intervals provide a framework to study evolution of iron-coated particles. This study examines the ironrich particles contained in the condensed carbonate deposits spanning from uppermost Toarcian to lower Bajocian in the northwestern Iberian Range (northern Spain), to unravel the controls on geochemical changes in the basin by determining variation in mineralogy and REY (rare earth elements and yttrium) as potential predictors of provenance. The articles are composed mostly of the ferrous phyllosilicate berthierine, and by Fe-oxyhydroxide goethite. Depositionally, iron-rich particles occur at specific horizons, and display changes in character, from iron cortoids (Stage 1) in lowest Aalenian sediments, passing to small-sized iron ooids (Stage 2) in early Aalenian strata and to more complex iron-grain aggregates and larger iron ooid–oncoid mixed particles (Stage 3) in the middle–upper Aalenian succession. In the lower Bajocian sediments that represent the most condensed interval, diagenetic processes strongly affected previously iron-coated particles. The genesis of the diverse suite of iron-rich particles is explained by petrographic and mineralogic studies revealing oscillation in redox conditions. Berthierine genesis is linked with suboxic conditions below the water–sediment interface. Subsequent periods of low rates of sediment accumulation allowed the exhumation of particles, and exposing them to oxic conditions that favored goethite formation. Definitive burial led to the development of isopachous circumgranular berthierinic cement rims and dissolution and replacement. Studies of REY and comparison with iron-rich samples of diverse provenance suggest a conceptual model for geochemical evolution of these deposits. The lack of both contemporaneous soil deposits and evidence of subaerial exposure in this arid paleoclimatic setting rule out weathering processes as the main source of iron. Instead, geochemical indexes (Y/Nb and Y/La ratios) suggest that coeval Iberian volcanism was the most plausible iron source. The sedimentological, mineralogical, and chemical attributes of these iron deposits provides proxies to interpret redox and geochemical fluctuations and integrate all the data in an evolutionary model. Variations in REY patterns offer a framework for correlation at both local and subregional scales. Likewise, as this study provides new geochemical data for the Aalenian period, including the Fuentelsaz GSSP worldwide reference section, it enhances knowledge of the evolution of the westernmost Tethyan basins and the significance of widely distributed oolitic ironstones. The data collectively reveal how the REY signature of condensed sediments can be used to obtain detailed information on the geochemical and paleoceanographic conditions of depositional basins.

Mechanism of formation of Haddat Ash Sham ironstones (Oligo-Miocene), Makkah Al Mokaramah District, West Central Arabian Shield, Saudi Arabia

The present study aims to shed light on the mechanism of formation of the Oligo-Miocene oolitic ironstones of Haddat Ash Sham area, Saudi Arabia. These ironstones are enclosed within the middle part of the Oligo-Miocene siliciclastic succession of the western part of the Arabian Shield, western Saudi Arabia. The ironstone beds were formed during marine incursion and creation of short-lived starved time periods of high organic matter activities, ferrous iron, and low clastic input. The depositional and diagenetic processes involved in the formation of Haddat Ash Sham ironstones are summarized here as follows: (1) the deposition of detrital components (i.e., amorphous iron-bearing clays admixed with silt and sand-sized quartz grains) and their distribution by the waves and current actions in areas of different water depths (bars and inter-bar areas); (2) the deposition of the iron-bearing clays in slightly reducing transgressive conditions (dysaerobic zone) led to the authigenesis of green marine chamositic clays of variable mineralogical and chemical compositions according to the predominated depositional environments; and (3) in the upper parts of the depositional cycles, the iron-bearing clays become admixed with detrital quartz grains which resulted in the formation of silty and sandy ironstones of low iron content. The diagenetic processes led to the oxidation of the green chamositic clays and formation of amorphous Fe-oxyhydroxides, ferrihydrites, goethite, and hematite. These iron mineral phases are related to each other and show progressive steps of transformation during the diagenetic processes. The iron ooids represent in situ formed irregular domains formed during the diagenetic crystallization and dehydration of the amorphous iron oxyhdroxides resulted from the diagenetic oxidation of green chamositic clays. This is supported by the absence of detrital cores of the iron ooids, the gradational contact between the iron ooids and the enclosing matrix and also by the presence of many ooids of unclear and ill-defined internal structure.

Paleoenvironments and sequential stratigraphy of Ordovician marginal marine deposits of Sierra de Zapla (Sierras Subandinas, northwestern Argentina) and their relation with the Central Andean Basin

The best and most complete palynologically constrained Ordovician section across the Sierras Subandinas crops out in the southern part of the Sierra de Zapla along the Capillas River (Subandean Ranges, Jujuy Province). The dominantly marginal marine setting explains the scarcity of invertebrates throughout the section. Palynomorphs are the main tool in stratigraphic studies of the area and trace fossils are important aids in paleoenvironmental analysis. Based on a detailed facies analysis, geometry and paleocurrent analysis, and recognition of key surfaces, four depositional sequences (S1-S4) with different internal arrangements and compatible with 3rd order cycles were identified. The Zanjón Formation (upper part of Global Stage 2 and Global Stage 3) with unexposed base, initiates the stratigraphic section and it is characterized by a heterolithic facies association with subordinate storm layers, thin-bedded phosphate-rich micro-conglomerates and inarticulate-rich shell beds. Common subaerial exposure features like truncated ripple tops and mud cracks suggest tidal flat environments. This interval yields a mixed Skolithos-Cruziana ichnofacies. Towards the top, an increase in mottling and a gradual change into dominant purplish-red colors (Laja Morada Member of the Labrado Formation) indicate a protracted subaerial emergence during a Darriwilian (Global Stage 4) relative sea-level drop, and a fluctuating water table in interfluves. This unit is covered in sharp erosive boundary by the Lagunilla Member of the Labrado Formation, suggesting development of composite fluvial incisions. Sandy fluvial to tide-dominated estuarine fills and thoroughly-bioturbated subtidal hetherolithic beds overlie the sequence boundary representing a typical transgressive systems tract. Development of surfaces with dominant Skolithos forms are interpreted as representing Glossifungites ichnofacies typically indicating by-pass erosion and firm-ground development previous to reworking of interfluves by advancing ravinement surface. The Capillas Formation (upper Darriwilian and lower part of Global Stage 5), sharply levels the previous estuarine complexes and represents a fine-grained wedge that gradually coarsens up. This unit contains the only truly marine shelly fauna concentrated in few storm beds above the fine-grained interval that characterizes a maximum flooding stage. This is the only interval that can be positively correlated with sections in the Cordillera Oriental to the west and Bolivia. A gradual coarsening to silty sandstones and the replacement of Cruziana by Skolithos dominated ichnofacies suggest a shallowing-upward section, capped by the thick-bedded, high energy quartz sandstones with pipe-rock structures of the Centinela Formation (middle and upper part of Global Stage 5). A shallowing-upward trend indicates the progradation of deltaic complexes, which in turn were erosively truncated by the waxing stage of the Gondwanan Hirnantian (uppermost Ordovician) ice-cap represented by the Zapla Formation (Global Stage 6) along the Central Andes. Above the diamictite-rich Zapla Formation the Lipeón Formation (Silurian) is interpreted as related to repeated transgressive ravinement surfaces that truncated Fe+2 saturated estuaries after a waning glacial stage and isostatic rebound, favouring the deposition of oolitic ironstones and succeeded by the development of a Zoophycos dominated muddy shelf.

Petrography And Mode Of Formation Of The Oolitic Ironstone Beds In The Campano-Maastrichtian Nkporo Formation In Leru, Southeastern Nigeria

The dark grey to black Nkporo Formation (Campanian – Maastrichtian) exposed in Leru area, southeastern Nigeria, contains several interbeds of oolitic ironstone in the upper parts. The ironstone consists of calcareous groundmass with pockets of unevenly distributed ferriferous ooids. The ooliths are “egg”- shaped and vary between 0.50 mm and 1.50 mm in diameter. From thin section studies, the oolitic ironstones are comprised of framework grains of chamosite ooliths in siderite cement and secondary limonite. The spheroidal shapes, absence of radial shrinkage fractures, lack of relict textures, and clean and clear, near-perfect cortices are indices of primary precipitation of the ooliths on the sea floor. The poor winnowing and presence of intact shell fragments indicate their deposition in a low energy environment. Concentric chamosite accretion on the nuclei developed while the nuclei were under low hydrodynamic turbulence on the sediment-water interface. Keywords: Petrography, Chamosite ooliths, Oolitic Ironstone, Primary precipitation, Diagenetic replacementGlobal Journal of Geological Sciences Vol. 6 (1) 2008: pp. 57-62

From diagenesis to hydrothermal recrystallization: polygenic Sr-rich fluorapatite from the oolitic ironstone of Saint-Aubin-des-Chateaux (Armorican Massif, France)

Four generations (I to IV) of Sr-rich fluorapatite were observed in the hydrothermally altered oolitic ironstone interbedded within the lower Ordovician sandstone of Saint-Aubin-des-Châteaux (Armorican Massif, France). Main type I is diagenetic to weak metamorphic; its remobilization along fractures gave type II, which just preceded the hydrothermal pyritization of the ironstone. Types I and II have a quite similar SrO content (~ 4.8 wt.%), with mean formulae (Ca4.73Sr0.24(Mn, Fe)0.02)∑=4.99(PO4)3.02[F0.68(OH)0.32] and (Ca4.75Sr0.24(Mn, Fe)0.01)∑=5.00(PO4)3.00[F0.63(OH)0.37], respectively. Types III and IV postdate the main hydrothermal process. Type III results from the breakdown of a Sr phosphate, lulzacite. It presents a patchy texture, where each constitutive sub-grain has a relatively homogeneous Sr content (from 1 to 7 wt.%), with an almost constant F ratio (0.83 apfu). The last type IV (geodic) shows a strong oscillatory growth zoning (Sr ↔Ca substitution) with SrO content ranging from 0 to 18 wt.%, and F close to 0.75 apfu. On the basis of trace analysis of REE and Y by LA-ICPMS, chondrite-normalized spectra of apatite types I, II and IV present dissymmetric convex shapes, with a significant deficit of LREE and a slight one of HREE relatively to MREE (Gd to Dy). Bond valence calculations indicate that this shape is determined mainly by the heterovalent substitution 2 Ca2+ → Na+ + ( REE 3+, Y), which controls the incorporation of REE and Y in the crystal structure of apatite. The best fit is for REE =Dy or Ho. This crystal chemical constraint favours at low temperature the fractionation of REE and Y between apatite and REE phosphates. This fractionation is stronger with monazite than with xenotime. The normalized spectrum of apatite type III has a symmetric convex shape, with a strong positive Eu anomaly inherited from its precursor lulzacite (close ionic radii of Eu2+ and Sr2+). Owing to its complex geochemistry and geological evolution, the Saint-Aubin ironstone appears as a basic example for the crystal-chemistry of apatite in low-temperature conditions, especially when phosphate-rich sediments and their metamorphic or hydrothermal derivatives are concerned.

CARADOC BRACHIOPODS FROM THE ARMORICAN MASSIF (NORTHWESTERN FRANCE)

Lower Caradoc rhynchonelliformean brachiopods have been recovered from the uppermost part of the Postolonnec Formation and the base of the Kermeur Formation (Upper Aurelucian-lowermost Burrelian) below and above a significant oolitic ironstone bed in the Veryarc'h section, Armorican Massif, France. A new species, Tenuiseptorthis loii n. sp. and four species, Howellites sp., Drabovia pentagonomya Havlíček, 1971, Tafilaltia destombesi Havlíček, 1970 and Triplesia cf. T. simplex Havlíček, 1971 are described. These brachiopod taxa confirm, at generic and specific level, the very close paleobiogeographic relationships existing within the North Gondwana Realm between the Armorican Massif, Spain, Portugal and North Africa during Late Ordovician time.

Palaeozoic oolitic ironstone of the French Armorican Massif: a chemical and structural trap for orogenic base metal–As–Sb–Au mineralisation during Hercynian strike-slip deformation

In the Saint-Aubin-des-Châteaux quarry (Armorican Hercynian belt, western France), an epigenetic hydrothermal alteration affects an oolitic ironstone layer intercalated within the Lower Ordovician Gres armoricain Formation. The hydrothermal overprint produced pervasive and massive sulphidation with stratoid pyritised lenticular bodies within the oolitic ironstone layer. These sulphide lenses are spatially associated with strike-slip faults and extend laterally from them. After the massive sulphidation stage (Fe–As, stage 1), subsequent fracturing allowed the deposition of base metals (stage 2) and Pb–Sb–Au (stage 3) parageneses in veins. The dominant brittle structures are vertical extension veins, conjugate shear veins and strike-slip faults of various orders. All these structures are filled with the same paragenetic sequence. Deformation analysis allows the identification of structures that developed incrementally via right-lateral simple shear compatible with bulk strain affecting the Central Armorican Domain. Each increment corresponds to a fracture set filled with specific parageneses. Successive hydrothermal pulses reflect clockwise rotation of the horizontal shortening direction. Geothermometry on chlorite and arsenopyrite shows an input of hot hydrothermal fluids (maximum of 390–350°C) during the main sulphide stage 1. The subsequent stages present a marked temperature drop (300–275°C). Lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds to the underlying Neo-Proterozoic basement. Lead isotope data, relative ages of deformation and comparison with neighbouring deposits suggest that large-scale fluid pulses occurred during the whole Hercynian orogeny rather than pulses restricted to the late Hercynian period. The vicinity of the Hercynian internal domain appears as a key control for deformation and fluid flow in the oolitic ironstones, which acted as a chemical and structural trap for the hydrothermal fluids. The epigenetic mineralisation of Saint-Aubin-des-Châteaux appears to be very similar to epigenetic sulphidation described in banded iron formation-hosted gold deposits.

Black smokers and density currents: A uniformitarian model for the genesis of banded iron-formations

Banded iron-formations (BIFs) were comparatively abundant and widespread marine sedimentary rocks in the Archean and Lower Proterozoic eras, but thereafter they appear to be restricted to the Neoproterozoic and Paleozoic eras, although there are indications of similar rocks forming at present. BIFs are important as the major source of iron ore for industry and have also been used to support hypotheses regarding the evolution of life, oceans, and the atmosphere in the Archean and Proterozoic. They apparently formed in deep water and consisted of a semi-regular alternation of quartz (chert) and iron-rich minerals with little or no terrigenous sediment and low (< 1 wt.%) alumina content, in contrast to Proterozoic to Phanerozoic oolitic ironstones (Clinton–Minette style) that formed in shelf environments, were comparatively small and rare and contain abundant iron aluminosilicates and hydroxides but little or no chert. Although ooliths may be locally abundant in granular iron-formations (GIFs) they differ from the Clinton–Minette style oolitic ironstones that formed in shelf seas during periods of slow sedimentation. A common mode of origin for marine deep-water iron-formations is proposed, in which hot fluids, consisting of marine and connate water leaching iron, silica and other elements from mafic and ultramafic rocks associated with mantle plumes or mid-oceanic ridges and active spreading centres are released into the ocean at underwater hot springs (black smokers). On contact with cold marine water, the least soluble elements are precipitated in the form of colloidal hydrous silicates (clay minerals) and hydroxides close to the hydrothermal vent. The hydrothermal fluids are high in silica and low in alumina causing the precipitation of alumina-poor iron silicates (nontronite) that dissociate into iron hydroxide and amorphous silica during diagenesis. The amorphous silica is typically entrapped by iron oxide laminae to form bands of chert. Breaches of the iron oxide laminae permitted the escape of the gelatinous amorphous silica during compaction and dewatering leaving a chert-free residue as the protore of non-hydrothermal sedimentary high-grade iron ore. The rapid deposition and abundant included water formed unstable mounds and chimneys around the vents. Slumping of the mounds caused by compaction, dewatering, gravity sliding, and seismic events produced turbidity currents forming proximal fans of GIF, but colloidal particles remained suspended in longer-lived density flows to deposit the ultra fine-grained BIF over vast areas of the ocean floor. Episodes of density current deposition were separated by intervals of slow pelagic sedimentation and silicification of the sea floor. The density currents commonly caused minor erosion of the sea floor with rip-out clasts incorporated in the new layers. The iron-rich hydrothermal fluids triggered the precipitation of dissolved ferrous iron accumulated by anoxic weathering to produce the huge deposits of BIF in the Archean and Paleoproterozoic until the rise in atmospheric oxygen stopped the accumulation of ferrous iron in the oceans leaving only the hydrothermal source for later deposits.

A sequence stratigraphic model for an intensely bioturbated shallow‐marine sandstone: the Bridport Sand Formation, Wessex Basin, UK

AbstractThe Bridport Sand Formation is an intensely bioturbated sandstone that represents part of a mixed siliciclastic‐carbonate shallow‐marine depositional system. At outcrop and in subsurface cores, conventional facies analysis was combined with ichnofabric analysis to identify facies successions bounded by a hierarchy of key stratigraphic surfaces. The geometry of these surfaces and the lateral relationships between the facies successions that they bound have been constrained locally using 3D seismic data. Facies analysis suggests that the Bridport Sand Formation represents progradation of a low‐energy, siliciclastic shoreface dominated by storm‐event beds reworked by bioturbation. The shoreface sandstones form the upper part of a thick (up to 200 m), steep (2–3°), mud‐dominated slope that extends into the underlying Down Cliff Clay. Clinoform surfaces representing the shoreface‐slope system are grouped into progradational sets. Each set contains clinoform surfaces arranged in a downstepping, offlapping manner that indicates forced‐regressive progradation, which was punctuated by flooding surfaces that are expressed in core and well‐log data. In proximal locations, progradational shoreface sandstones (corresponding to a clinoform set) are truncated by conglomerate lags containing clasts of bored, reworked shoreface sandstones, which are interpreted as marking sequence boundaries. In medial locations, progradational clinoform sets are overlain across an erosion surface by thin (&lt;5 m) bioclastic limestones that record siliciclastic‐sediment starvation during transgression. Near the basin margins, these limestones are locally thick (&gt;10 m) and overlie conglomerate lags at sequence boundaries. Sequence boundaries are thus interpreted as being amalgamated with overlying transgressive surfaces, to form composite erosion surfaces. In distal locations, oolitic ironstones that formed under conditions of extended physical reworking overlie composite sequence boundaries and transgressive surfaces. Over most of the Wessex Basin, clinoform sets (corresponding to high‐frequency sequences) are laterally offset, thus defining a low‐frequency sequence architecture characterized by high net siliciclastic sediment input and low net accommodation. Aggradational stacking of high‐frequency sequences occurs in fault‐bounded depocentres which had higher rates of localized tectonic subsidence.

Properties of Ironstone Lateritic Gravels in Relation to Gravel Road Pavement Construction

A laboratory investigation was conducted to determine properties of five samples of ‘oolitic ironstone’ lateritic gravel aggregates which are relevant to their use as road construction materials. Measured values of physico-chemical, physico-mechanical as well as index properties and compaction characteristics are similar to those of other lateritic, gravels occurring in West Africa which are used in road pavement applications. These lateritic clayey gravels have good workability as engineering construction materials and are rated fair to good as road aggregates in terms of probable in-situ behaviour based on water absorption values. Relationships between maximum dry unit weight and optimum moisture content as well as those between California bearing ratio and some derived soil parameters such as grading modulus, plasticity product or plasticity modulus can be described using a third-order polynomial function. The maximum dry unit weight has a bimodal frequency distribution and can be predicted from logarithm to base 10 of compaction energy with fairly good accuracy.

Diamictite and oolitic ironstones, a sedimentary association at Ordovician–Silurian transition in the north Gondwana margin: New evidence from the inner nappe of Sardinia Variscides (Italy)

The contact between the Silurian black phyllite and the Cambro–Ordovician underlying rocks has been investigated over different tectonic units, affected by green-schist facies metamorphism, in the inner nappe zone of the Sardinia Variscides. In spite of strain and metamorphism, the field work highlighted the occurrence of diamictic sediments. In the Canaglia Tectonic Unit the diamictite consists of dark, massive metamorphic claystone bearing chamositic ooliths, chamositic nodules and millimetre to centimetre sized clasts, dispersed, or gathered in clusters, within the muddy matrix. In the Argentiera Tectonic Unit the diamictite consists of angular clasts, ranging in size from few millimetres to several decimetres, scattered within a finely laminated black sericitic meta-argillite. Field data, textural and compositional analyses suggest a glacio-marine environment for the formation of the diamictites. The Canaglia diamictite deposited in a protected, glacial-influenced, shore. Compositionally it can be defined as ironstone; in the Upper part it hosts a horizon of clast-supported conglomeratic hard ironstone, mostly made of magnetite, which testifies for sub-aerial reworking. The source of the iron is to be related to local, glacio-eustatic driven, emergence of Upper Ordovician alkaline mafic volcanics. These are widespread in the uppermost Ordovician of the Canaglia Unit, possibly linked to the rifting stage that invested the north Gondwana margin, before the uppermost Ordovician–early Silurian sea level rise. The Argentiera diamictite deposited beyond the iron-rich diamictite in the outer euxinic shelf that was reached by rain out of rafted debris.

Iron–phosphorus relationship in the iron and phosphorite ores of Egypt

Iron and phosphorite ores are very common in the geological record of Egypt and exploitable for economic purposes. In some cases these deposits belong together to the same geographic and geologic setting. The most common deposits include phosphorites, glauconites, and iron ores. Phosphorites are widely distributed as a belt in the central and southern part of Egypt. Sedimentary iron ores include oolitic ironstone of Aswan area and karstified iron ore of Bahria Oasis. Glauconites occur in the Western Desert associated with phosphorites and iron ores. As these ores are exploitable and phosphorus in iron ores and iron in phosphorites are considered as gangue elements, the iron–phosphorus relationship is examined in these deposits to clarify their modes of occurrences and genetic relationship based on previously published results. Phosphorus occurs mainly as carbonate fluorapatite (francolite). Iron, on the other hand, occurs in different mineralogical forms such as glauconites, hematite, limonite and goethite. In P-rich rocks (phosphorites) no relationship is observed between iron and phosphorus, which in turn indicates that the FeP model is unlikely to interpret the origin of the late Cretaceous phosphorites and the association of phosphorites and glauconites in Egypt. In Fe-rich rocks (iron ores and glauconites) also no relationship between iron and phosphorus is observed. The present work, therefore, does not support the hypothesis that there is a genetic relationship between phosphorus and iron in sedimentary rocks.

Magnesian calcite and chamositic ooids forming shoals peripheral to Late Ordovician (Ashgill) muddy siliciclastic shores: southern Ontario

In southern Ontario, ooids are associated with two distinct facies associations in the Queenston Formation, the final stage of Late Ordovician (Ashgill) Taconic basin fill. One facies consists of thin ooid and bioclastic grainstones interbedded with mudrock, and lies near the base of the formation, and, in southwestern Ontario, also forms a local NW-thickening wedge near the middle of the formation. Ooids have radial-fibrous and radial-concentric fabrics (Type A), with chamosite, illite, and Fe-oxide laths at intercrystalline sites. Vertical lithologic and ooid abundance patterns indicate that thresholds to carbonate production were sensitive to changes in terrigenous sediment supply, sea level, circulation, accommodation space, and tectonism. Ooids in the second facies association are admixed with abraded fragments of open-marine biota, or occur burrow fills, within a <30-cm-thick interval of mudrock near the top of the preserved Queenston succession, a few metres below the Ordovician–Silurian unconformity. Ooids have radial concentric and crosscutting patchy microcrystalline fabrics (Type B). This unit may represent a transgressive or stillstand deposit modified by bioturbation. The extent of preserved fabric suggests that both ooid types were originally magnesian calcite, but Type A ooids underwent greater burial alteration. This is shown by crystalline mosaics that cross-cut relict primary fabrics; δ 13C values (−1.82‰ to +0.67‰) and δ 18O values (−4.46‰ to −10.57‰) more negative than marine calcite of similar age; Mn and Fe concentrations more elevated above expected marine values; and a luminescence similar to that of intergranular cements. Burial meteoric diagenesis was likely promoted by excellent permeability of the host sand. We interpret authigenic chamosite and Fe-oxide to reflect diagenesis of iron-bearing and clay detritus trapped during ooid growth. Type B ooids suffered less alteration: δ 13C (+1.1‰ to +6.64‰) and δ 18O (−3.04‰ to −4.81‰) values overlap the expected marine range, including 13C enrichment that occurs within the Hirnantian (latest Ordovician) excursion. Although Mn and Fe values are still higher than those of modern calcitic ooids, negligible luminescence suggests that recrystallization occurred in the presence of marine-derived pore fluids. Further burial alteration was inhibited due to low permeability of the host mud. Type A ooid facies in the Queenston Formation forms an ancient analogue for lesser known Quaternary ooid shoals peripheral to tropical deltaic systems. The facies of Type B ooids, while more enigmatic, may preserve a geochemical herald of latest Ordovician climate change. The presence of minor chamosite in Type A ooids defines a possible distal facies of the well-known oolitic ironstones of similar age in the mid-continental USA.

New lithostratigraphical, sedimentological, mineralogical and palaeontological data on the Mesozoic of Belgian Lorraine: a progress report

A multidisciplinary study of some extensively cored boreholes together with the new 1/25,000 geological mapping of Wallonia led us to propose a new lithostratigraphic canvas for Belgian Lorraine. This area is located on the N-E border of the Paris Basin, south of the Ardennes; the studied stratigraphic interval covers the Keuper to the Toarcian. Each of the new lithostratigraphic units is interpreted by macroscopic and microscopic sedimentological observations. Detailed palynological and claystratigraphical analyses were also performed, providing additional stratigraphical, palaeoecological and sedimentological data. The Habay Formation (conglomerates and red mudstones) is a fluvial unit, with immature channel conglomerates and paleosoils. The Attert Formation (dolomitic marls with gypsum and pseudomorphs) exhibits an evaporitic trend. The Mortinsart Formation (sands and marls) corresponds to a restricted marine unit, evolving towards an alluvial plain (Levallois Member). The second cycle begins with the Jamoigne Formation (bioturbated marls and limestones), a marine subtidal restricted unit, evolving towards a more sandy series (Metzert Member). The base of the third cycle corresponds to the rest of the Luxembourg Formation, composed by a superposition of sand waves. The Ethe Formation (laminar mudstones and marls) marks a deepening of the basin and the outset of marine dysaerobic conditions. The Aubange Formation (bioturbated marls with sandstones and limestones) is characterized by the reappearance of a normal benthic fauna. The Grandcourt Formation (laminar mudstones and marls) marks a return to open marine dysaerobic conditions. The Mont-Saint-Martin Formation (marls, sandy marls and oolitic ironstone) is a highly regressive unit, while the Longwy Formation (limestones) marks the initiation of a carbonate platform.

Mineralogy and geochemistry of Late Ordovician phosphate-bearing oolitic ironstones from NW Sardinia, Italy

¶In the Nurra region, NW Sardinia, oolitic ironstones are interlayered within coarsening upward metasedimentary sequences of siltstone, sandstone, breccia, and conglomerate. A Caradoc-Early Ashgill age is suggested by the analogies with metasediments of Central-Southern Sardinia following the Sardinian tectonic phase. The sequences including oolitic ironstones are overlain by black metapelite of inferred Hirnantian to early Silurian age. The ooids consist of chamosite, siderite or magnetite or, rarely, stilpnomelane. Chamosite ooids consist of up to 30 lamellae and sporadically show clustering of magnetite grains in core and rim, and widespread late replacement by siderite. The alternation of chamosite and Fe-oxide observed in a few samples points to an ooid transport over the crests and hollows of megaripples in a continental shelf at 0–60 m depth, and/or a random displacement of the littoral environments in a rapidly evolving continent-sea transition zone producing an alternation of oxidizing and reducing conditions. Black phosphate clasts, including older Fe-ooids, frequently occur. The oolitic ironstones of Nurra are variable mixtures of an Al-Si-Ti rich- detrital component and a Fe-rich chemical component.

Stratigraphy and sedimentary history of the Nepal Tethys Himalaya passive margin

The sedimentary history of the Nepal Tethys Himalaya began with deposition of thick carbonates in the Cambro?–Ordovician, followed by a mixed siliciclastic–carbonate epicontinental succession recording two major deepening events in the Early Silurian and Late Devonian. Fossiliferous carbonate ramp deposits in the Tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins (“rift stage”). Break-up in the mid-Early Permian, locally associated with effusion of tholeiitic lava flows, was followed by a transgressive sandy to shaly, locally coal-bearing or bioclastic unit capped by condensed pelagic carbonates in the Middle to Late Permian (“juvenile ocean stage”). Subsidence of the cooling stretched crust led close to bathyal water depths in the Olenekian, but then slowed down in the Middle Triassic to increase again sharply in the Late Triassic owing to renewed extensional tectonic activity and sediment loading during up- and out-building of the Indian continental terrace. Deposition of tropical platform carbonates finally became widespread in the middle Liassic (“mature passive margin stage”). The initial fragmentation of Gondwana in the Middle Jurassic led to rejuvenation of the Indian craton and deposition of quartzo-feldspathic hybrid arenites, capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late Bathonian/middle Callovian. Next, a discontinuous pelagic grey marly limestone unit was followed by the ammonoid-rich offshore Spiti Shale in the Late Jurassic. The final disintegration of Gondwana began in the Berriasian, when quartzose siliciclastics derived again from the rejuvenated Indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the Valanginian? followed in the Hauterivian to Albian by more felsic differentiates such as the trachyandesites exposed in the Lesser Himalaya 120 km to the south. A widespread drowning episode, fostered by waning volcaniclastic supply during a global eustatic rise, is documented by a major glauconitic horizon deposited at middle southern latitudes in the late Albian, overlain by “Scaglia-like” pelagic limestones in the latest Albian. The final part of sedimentary history, during the rapid northward flight of India and its collision with Eurasia, is not documented anywhere in Nepal due to later erosion of Upper Cretaceous to Lower Tertiary strata.

Depositional environments of the Kurnub group (Early Cretaceous) in northern Jordan

Seven facies associations comprise the Early Cretaceous Kurnub Group (KG) in northern Jordan. They are assigned to the following five depositional environments: proximal to distal alluvial braidplain; tidal flat; tidal marsh-coastal swamp; inner shelf; and meandering river. According to the evolution of these depositional facies associations, the KG can be split into three regressive-transgressive depositional sequences. This development of the depositional environment of the KG through the Early Cretaceous Epoch is portrayed as a six-block depositional model. The depositional history of the KG began with the first regression, when a conglomerate and sandstone facies association of a proximal to distal alluvial braidplain system was deposited above a regional angular unconformity, truncating Triassic and Jurassic strata. Afterwards, the Tethys Seaway inundated northern Jordan, giving rise to the first transgressive sequence, consisting of a tidal flat-mixed carbonate-siliciclastic and coastal swamp-carbonaceous shalelignite facies association. Due to the second regression of the sea, a medial to distal unconfined braided river system resumed in northern Jordan. The second transgression of the Tethys covered most of the region during the middle interval of the KG, and deposited a mixed carbonate-siliciclastic sequence with interbedded inner shelf carbonates, subtidal oolitic ironstones and tidal marsh-coastal swamp black shales and lignites. During the next interval of the KG, the Tethys regressed from the area and was replaced by a moderate sinuosity meandering river system that deposited the third regressive fluvial sequence. The third transgression of the sea developed a marginal to shallow marine depositional system, which affected northernmost Jordan, giving rise to a mixed carbonate-siliciclastic tidal flat and an inner shelf carbonate sequence. The climate in Jordan during the Early Cretaceous Epoch was generally humid and temperate.