concentrated and lean oolites examples from the nubia formation at aswan egypt and significance of the oolite types in ironstone genesis

pyrite ooids in devonian black shales record intermittent sea level drop and shallow water conditions upper devonian black shales of the eastern united states contain in places unusual beds of pyrite ooids textural and geochemical studies show that these originated as chamositic iron ooids that were replaced by pyrite during early diagenesis pyrite mimics the laminated texture of the precursor grain yet inclusions of silicate minerals and instances of partial replacement reveal the secondary nature of the pyrite pyrite ooids occur above erosion surfaces that are considered sequence boundaries because of large lateral extent chamositic precursor ooids indicate an oxygenated water column and wave interaction with seafloor sediments at the time of their formation this scenario agrees with earlier work that stipulates that erosion surfaces in devonian black shales reflect lowering of sea level that allowed wave reworking and erosion of earlier deposited black shales pyrite ooid beds thus furnish direct evidence of significant sea level drops during the accumulation of upper devonian black shales in eastern north america

de glauconiarum origine the glauconitic facies is widespread on present day continental shelves from 50 s to 65 n and at water depths between 50 and 500 m and is in particularly great abundance on the upper slope and outer shelf between 200 and 300 m it is also common in many ancient rocks of post late precambrian age it occurs as sand to pebble sized essentially green particles granular facies but also as a surface coating on particles and hardgrounds and as a diffuse impregnation film and diffuse facies we suggest the replacement of the term glauconite which has been interchangeably used to designate a morphological form and a specific mineral by glaucony facies and glauconitic smectite and glauconitic mica as end members of the glauconitic mineral family the widely accepted model of burst and hower for glauconitization requires a degraded micaceous 2 1 layer lattice structure parent clay mineral however detailed analysis of numerous samples of recent glaucony reveals that such a parent substrate is exceptional the model therefore requires modification generally the parent material is carbonate particles argillaceous kaolinitic faecal pellets infillings of foraminiferal tests various mineral grains and rock fragments that pass gradually into the commonly occurring green grains we show that the process of glauconitization is achieved by de novo authigenic growth of automorphous crystallites in the pores of the substrate accompanied by progressive alteration and replacement of the substrate it is this two fold evolution that causes the verdissement of granular substrates macrofossils and hardgrounds the authigenic mineral is an iron rich and potassium poor glauconitic smectite while new smectites are growing into the remaining pore space the earlier smectites are modified by incorporation of potassium producing decreasingly expandable minerals with a non expandable glauconitic mica as the end member this mineralogical diversity of the glauconitic mineral family explains the highly variable physical and chemical properties of glaucony four categories nascent little evolved evolved and highly evolved glaucony are distinguished glauconitization appears to be controlled by a delicate balance between degree of physical confinement of a particle and the amount of ionic exchange between the micro environment and ambient open marine sea water the optimum conditions for glauconitization are those of semi confinement as a result the interior of a grain is more glauconitized than its less confined periphery similarly for identical substrate types large grains 500 m provide more favourable substrates for glauconitization than lesser confined small grains on a larger scale the formation of glaucony is governed by the availability of iron and potassium and the balance between detrital influx and winnowing low accumulation rates expose grains to the open marine environment for sufficiently long times 105 106 years for highly evolved glaucony

search for milankovitch patterns among oolitic ironstones the stratigraphic record of about 175 oolitic ironstones is sparse yet it portrays several meaningful patterns repeated ironstone episodes occurred in only about 10 major sedimentary basins which were marked by relative tectonic stability or subdued orogeny intervals between episodes varied but were mostly a few to several million years long repetitions on this scale are commonly construed as global rise and fall of sea level the pattern persisted for a few tens of million years in fore deeps several tens of million years on unstable cratons and microcontinents and more than 100 m y in intracratonic basins within the congenial habitats an ironstone developed locally between each of several successive shallow marine upward shoaling detrital sequences these asymmetric units normally a few meters to a few tens of meters thick recurred on average every 250 to 500 kyr commonly fewer than 10 ironstone bearing sequences are involved during a very long lapse in normal sedimentation between such sequences stacked ironstones accumulated on the same short time scale speculation about short time scale patterns is poorly constrained but several other repetitive stratigraphic successions with a several hundred thousand year cadence have been attributed tentatively to the 400 000 year milankovitch climate cycle it is unclear how this would exert a eustatic control without the waxing and waning of continental glaciers

oolitic ironstones and contrasting ordovician and jurassic paleogeography distribution of abundant ordovician and jurassic oolitic ironstones provides a test for current ideas about factors controlling their origin these include warm climate and deep weathering dispersed continents and highstand of global sea level most of the jurassic ironstones developed in middle northern latitudes on the unstable european part of assembled laurasia at a time of low global sea level mild moist climate and abundant vegetation most of the ordovician ironstones accumulated around the northwestern margin of assembled gondwana and armorica during highstand of sea level and an absence of land plants and in high southern latitudes that supported an ice cap by the end of the period these contrasts demonstrate that neither dispersed continents nor major highstand of sea level was a necessary factor moreover requisite weathering in ordovician time may have been induced by adequate soil air co 2 pressure maintained by elevated atmospheric p co 2 in tne absence of land plants

tectonic evolution of nw iberia during the paleozoic inferred from the geochemical record of detrital rocks in the cantabrian zone the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges in age from ediacaran to early permian its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana ii an active continental margin or iii the margin of a drifting ribbon continent and iv a combination of the three aforementioned possibilities in this paper we present geochemical data from 21 samples taken in detrital rocks in the cantabrian zone from ediacaran to early permian age the results together with previously published detrital zircon ages from these strata allow a comprehensive analysis of changing tectonic scenarios and provenance through time collectively these data indicate that this portion of iberia was an active margin during the ediacaran that evolved to become part of the passive margin of gondwana at least from ordovician to late devonian times changes in geochemistry zircon age distribution and sm nd isotopes during the carboniferous are attributed to the far field effects of the onset of the collision between gondwana and laurussia and related processes such as recycling of older sedimentary sequences as well as the involvement of the rocks formed during the variscan orogeny and oroclinal buckling latest carboniferous and permian show a more juvenile sm nd and higher values of illite cristallinity that may support the hypothesis of lithospheric foundering after oroclinal buckling

extension of berner s new geochemical classification of sedimentary environments to ancient sediments using stable isotopes of carbon and oxygen in siderite nodules and lithology of host shales one can recognize geochemical environments proposed by berner in devonian carboniferous sediments of the appalachian basin his oxic post oxic and sulfidic diagenetic environments are well represented sediments of the methanic environment may be present but the evidence is equivocal the sulfidic sediments can be further subdivided into well laminated and bioturbated corresponding to the presence of h 2 s in the overlying water in the first case but confined to the sediment pore waters in the second journal abstract

distribution and characteristics of a middle ordovician oolitic ironstone in northeastern kansas based on petrographic and petrophysical properties a laurasian ironstone case study abstract the margins of gondwana are generally considered to be the major sites of oolitic ironstone production during the ordovician and appear to be linked with global eustatic sea level rise occurrences of oolitic ironstones within the north american craton are less well documented but provide important supplementary data the low latitude of laurasia contrasted with gondwana allows useful comparisons of climatic and temporal patterns of ordovician ironstone formation middle ordovician ironstones occur in siliciclastic sequences in the american mid continent and appear to become progressively younger as the epicontinental sea advanced from the southwest across a predominantly carbonate terrain in northeastern kansas the regional distribution pattern of primary syndiagenetic goethite iron oolites within the st peter sandstone indicate deposition peripheral to a north northeast trending chain of islands underlain by predominantly granitic rocks located along an ancestral nemaha uplift detailed compositional mapping in the subsurface was made possible by the distinctive petrophysical properties of the goethite zone and the extensive regional control of wireline logged exploration wells petrographic data from ironstone core and drill cuttings both validate log analysis and give insights on possible modes of genesis we propose that eustatic changes in sea level were the primary factor governing the formation and observed distribution patterns of the oolite bed s the relationship of the observed occurrence patterns to major rift related faults of the central north american rift system suggests that synsedimentary tectonism also influenced this process the most likely source of iron appears to be by derivation from intensive humid weathering of granite exposed extensively on the ancestral nemaha uplift archipelago

on the recognition of areas with problematic sulphidic sediments using multi disciplinary data increased concentrations of several trace elements have been observed in water draining areas with sulphidic sediments along the baltic sea coast of sweden and finland some of these elements are harmful to the water environment the increased leaching of trace elements is often caused by antrophogenic activities such as ditching which cause oxidation of the sulphides and thereby acid soil conditions it is therefore important to recognise areas with sulphidic sediments and if possible avoid land use which causes sulphide oxidation the aim of this study was to find time effective methods which can be used to delineate areas with sulphidic sediments three areas in sweden with known occurrences of sulphidic sediments were studied data obtained from the mapping program at the geological survey of sweden sgu were first studied selected sites were thereafter visited and geoelectrical measurements were carried out by the use of several instruments the results imply that areas with sulphidic sediments are reflected as areas with high trace element contents on sgus biogeochemical maps which are based on analyses of trace elements in aquatic plants the same deposits are often shown as gyttja clay on the maps of quaternary deposits furthermore models derived from airborne electromagnetic measurements show that areas with sulphidic sediments often have a lower electric resistivity than surrounding sediments also the results from ground geoelectrical measurements suggest that the sulphidic sediments have a lower resistivity than surrounding fine grained deposits that interpretation was verified after stratigraphical studies and analyses of the sulphur contents of the sediments it is therefore suggested that data from ground geophysical measurements can be used for a detailed delineation of sulphidic sediments whereas data obtained from airborne geophysical and geological maps give an overview over potential areas with these sediments the absolute resistivity of the sulphidic sediments varies however between the three investigated areas complementary corings and stratigraphical descriptions are therefore always necessary to verify the occurrence of these sediments

paleoenvironmental conditions recorded by 87sr 86sr δ13c and δ18o in late pliensbachian toarcian jurassic belemnites from bulgaria the late pliensbachian toarcian jurassic sedimentological paleontological and geochemical belemnite 87sr 86sr 13c and 18o record is examined in two eastern tethyan bulgarian locations this interval contains the well known early toarcian ocean anoxic event t oae and its manifestation and temporal context is examined in bulgaria many of the features seen in south western europe are identified collapse of carbonate platform productivity at the pliensbachian toarcian boundary the t oae a short pulse of euxinic deposition in the falciferum zone an early toarcian rapid warming event seen in the belemnite 18o record that peaked around the falciferum bifrons zone boundary the long recognized positive 13c excursion in the late falciferum zone is also seen but a precursor sharp 13c negative excursion seen around the tenuicostatum falciferum zone boundary in most organic carbon records is not seen in the belemnite data a curious absence noted from other belemnite records subsequent perturbations in 87sr 86sr 13c and 18o suggest that there may be more global isotopic excursions in the early jurassic on the other hand belemnite sr isotope values from bulgaria are in accord with those recorded in western europe and hence demonstrating its value as a chronostratigraphic tool

reinterpretation of middle cambrian stratigraphy of the rifted western laurentian margin burgess shale formation and contiguous units sauk ii megasequence rocky mountains canada abstract until recently research on the renowned fossil animals of the burgess shale has advanced to a greater degree than an understanding of the rocks in which they are found studies addressing lithostratigraphy and hydrothermal petrography of the so called inner carbonate belt and adjacent chancellor group however have begun to re evaluate long standing hypotheses on the middle cambrian of western canada tectonic activity along the kicking horse rim buried remnants of neoproterozoic rifting during the cambrian period had more influence on local sedimentation than previously thought notably large scale collapse of the cathedral carbonate platform margin at 509 ma bp is evidence of reactivated basement faults these failures produced listric megatruncation surfaces having near vertical escarpments 150 m in height where they terminate against the platform the majority of what have been interpreted as shed olistoliths from the cathedral platform margin is herein shown instead to be carbonate mud mounds these grew in situ along the face of the cathedral escarpment and are associated with fossiliferous intervals in overlying basinal mudstones at three distinct stratigraphic horizons two within the burgess shale during the delamaran and marjuman stages the mounds nucleated where deep seated normal faults intersected the seafloor building primarily during periods of relative sea level transgression mound growth is associated with syndepositional exhalative activity inferred from stratigraphic relations sedimentology and geochemistry the new name monarch formation is proposed for the initial post collapse beds and mounds deposited against the cathedral escarpment early glossopleura zone the variably defined term takakkaw tongue is thereby confined to pre collapse slope deposits coeval to and correlative with the cathedral formation

a new geochemical classification of sedimentary environments because of the relative constancy of ph in most subaqueous sediments and the general lack of measurability of eh these parameters are not practically useful for classifying sedimentary environments in their place a new classification is offered which derived from studies of modern sediments is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen anoxic environments in turn are divided into sulfidic and non sulfidic depending upon the presence of measurable dissolved sulfide anoxic nonsulfidic environments are further divided into postoxic or resulting from oxygen removal without sulfate reduction weakly reducing and methanic or resulting from complete sulfate reduction with consequent methane formation strongly reducing the environments are shown to succeed one another during early diagenesis in the order oxic post oxic sulfidic methanic iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record

a sedimentological approach to refining reservoir architecture in a mature hydrocarbon province the brent province uk north sea improved reservoir characterisation in the mature brent province of the north sea aimed at maximising both in field and near field hydrocarbon potential requires a clearer understanding of sub seismic stratigraphy and facies distributions in this context we present a regional high resolution sequence stratigraphic framework for the brent group uk north sea based on extensive sedimentological re interpretation of core and wireline log data combined with palynostratigraphy and published literature this framework is used to place individual reservoirs in an appropriate regional context thus resulting in the identification of subtle sedimentological and tectono stratigraphic features of reservoir architecture that have been previously overlooked we emphasise the following insights gained from our regional high resolution sequence stratigraphic synthesis 1 improved definition of temporal and spatial trends in deposition both within and between individual reservoirs 2 development of regionally consistent predictive sedimentological models for two enigmatic reservoir intervals the broom and tarbert formations and 3 recognition of subtle local tectono stratigraphic controls on reservoir architecture and their links to the regional middle jurassic structural evolution of the northern north sea we discuss the potential applications of these insights to the identification of additional exploration potential and to improved ultimate recovery

iron bacterial and fungal mats bajocian stratotype mid jurassic northern normandy france abstract the oolithe ferrugineuse de bayeux formation is located at the historical bajocian stratotype of sainte honorine des pertes north of bayeux normandy the condensed formation ranges from the base of the humphriesianum zone to the parkinsoni zone and is divided into four beds of decimetric scale three main microfacies are present 1 oncoid rudstones 2 ooid bioclastic packstones and 3 silty burrowed wackestones packstones sedimentation took place in a very quiet environment below the photic zone and below or near the storm wave base the general setting is a distal carbonate ramp its lower part characterized by hemipelagic sedimentation indicated by the presence of planktonic foraminifers the inferred depth is around 100 m free oxygen concentration was low dysaerobic conditions are indicated by a scarcity of benthic macrofauna ferruginous structures are numerous in the first two microfacies and absent in the last hematite staining is not uniform and follows many sedimentary patterns among the more widespread fe structures are perforation infillings with endolithic micro organisms microstromatolites oncoids ooids blisters coatings and hardgrounds these structures can be associated and none are mutually exclusive hematite coated filaments of different sizes and shapes are observed in the micrite matrix the walls of various organisms the calcite crystals associated with the fe cortical laminations the perforations and burrow the hardgrounds and microstromatolites petrographical and sem examinations suggest that the laminated crusts oncoids and hardgrounds are formed by microbial iron mats dominated by filamentous bacteria and fungi seven types of microbes are recognized filaments five morphotypes spheroidal bodies and stalked bodies filamentous microfossils of type 1 to 4 resemble the present day filamentous bacteria beggiatoales and cytophagaceae because of their large diameter and their branching nature filaments of type 5 are possibly filamentous fungi another argument in favor of fungi is the presence of stalked and spheroidal bodies that resemble zoosporangia and oogonia of some oomycota in deep calm and dysaerobic waters many interfaces e g between aerobic and dysaerobic waters are present in the sediments the stability of the soluble reduced state of iron fe 2 is higher at such interfaces and many ferric iron encrusted microbial fossils are observed iron could thus serve as an electron donor for microbial iron oxidation processes other microbial iron deposition pathways are also possible it appears that regardless of geological age paleozoic mesozoic and geographical location the same microbiological mechanisms are probably responsible for the red color in calcareous stratified or unstratified bodies the presence of fossilized iron encrusted bacteria and fungi at interfaces may therefore serve as an indicator of anoxic to dysaerobic conditions in various paleo micro environments

palaeozoic oolitic ironstones on the north american craton abstract more than 40 mostly minor palaeozoic oolitic ironstones oi accumulated on low latitude cratonic north america almost entirely in usa a few middle cambrian oi among the oldest anywhere were deposited on the western cratonic shelf of usa widely scattered late cambrian ones developed on the southwestern southeastern and northeastern flanks of the transcontinental arch middle ordovician oi were deposited on the cratonic interior and on the southern and southeastern cratonic margins in latest ordovician time neda oi spread across north central usa east of the arch and south of the laurentian upland minor ones developed in the southern part of the taconian foreland basin after major orogeny early and middle silurian clinton oi flourished throughout the foreland basin in eastern usa producing the largest oi deposit on the north american craton minor early late silurian oi accumulated in the central part of the basin middle and late devonian oi in the acadian foreland basin in northeastern cratonic usa developed progressively westward of the encroaching catskill delta an isolated middle devonian one accumulated in southwestern cratonic usa and latest devonian ones in north central usa east of the arch during this palaeozoic episode sites of oi deposition shifted eastward across the craton most of the oi were deposited during a hiatus in normal sedimentation accumulating in the upper part of shoaling upward sequences some early palaeozoic ones occur in glauconitic siliciclastic carbonate facies and contain mostly spherical hematitic ooids these suggest derivation from iron rich soils developed on glauconitic deposits oi in wholly siliciclastic facies containing distorted chamositic ooids with cores of mud peloids were common in later palaeozoic time these oi like many other phanerozoic ones suggest a synsedimentary to early diagenetic origin

sedimentologic character of ancient muddy subaqueous deltaic clinoforms down cliff clay member bridport sand formation wessex basin u k abstract muddy subaqueous clinoforms are a common feature of many modern deltas particularly those developed in basins with strong waves tides or oceanographic currents ancient examples have only rarely been reported implying that they are under recognized herein the sedimentological characteristics of muddy subaqueous deltaic clinoforms from the lower jurassic down cliff clay member of the bridport sand formation wessex basin uk are described and interpreted as a reference to re evaluate other ancient shallow marine mudstone successions deposits below the down cliff clay clinoforms consist of erosionally based bioclastic sandy limestone beds intercalated with upward thickening fossiliferous claystones and siltstones facies association a which record upward increasing water depth under conditions of minor input of clastic sediment and reworking by storm waves these deposits are downlapped by foreset to toeset deposits of the subaqueous deltaic clinoforms which comprise claystones and siltstones that contain calcareous nodules and are variably bioturbated by a low diversity trace fossil assemblages dominated by chondrites facies association b these foreset to toeset deposits record slow deposition undecompacted sediment accumulation rate of c 4 4 m myr from suspension fall out and distal sediment gravity flows under conditions of intermittent and or poor oxygenation of bottom waters clinoform foreset deposits dip paleoseaward at 2 and consist of siltstones and sandy siltstones that are moderately to completely bioturbated by a high diversity trace fossil assemblage facies association c thin

lower palaeozoic iron oolites and volcanism from a baltoscandian perspective the ordovician iron oolites in baltoscandia are generally geographically extensive but lean 30 ooids sensu bhattacharyya d p 1989 concentrated and lean oolites examples from nubia formation at aswan egypt and significance of the oolite types in ironstone genesis in young t p taylor w e g eds phaneorozoic ironstones geological society special publication 46 93 103 and have undergone minimal reworking and diagenetic alteration the ooids were formed in a normal shallow marine environment around a nucleus of a calcite skeletal grain the oolites were formed during the period of the highest global iron oolite production ever most of the major oolites from this period have been altered in various ways which has obscured their origin the presence of volcanic matter in the baltoscandian and some other european oolites points to a volcanic origin a conclusion further supported by the ree distributions in iron ooids which are close to those in associated volcanic tuffs the global 87sr 86sr curve for phanerozoic seawater in general and for the palaeozoic in particular shows that negative excursions in the sr isotope ratio often coincide with periods of high global iron oolite productivity this temporal coincidence points to a possible causal link iron ooids volcanic ashes and bentonite beds occur in the ordovician of baltoscandia during a negative excursion in the 87sr 86sr curve the variation in the 87sr 86sr ratio in seawater mirrors the balance between the two major sources of sr mid ocean ridge volcanism and continental weathering and indicate that volcanism could be the general cause of most palaeozoic iron oolites two cambrian iron oolites occur in baltoscandia and at least one of them is suggested to be of volcanic origin

lower ordovician iron ooids and associated oolitic clays in russia and estonia a clue to the origin of iron oolites abstract on the baltic platform a lower llanvirn ordovician iron oolite can be traced for a distance of 1200 km from norway to the east of lake ladoga in russia this oolite is usually thin seldom exceeding 0 5 m and is dominated by goethite limonite type ooids the easternmost part of the oolite from tallinn to ladoga is examined here the oolitic limestone is intercalated with oolitic clay beds the mineralogical chemical and isotopic composition and other indicators point to volcanic ash being the source for the clay similarities in ree distribution patterns and immobile element contents between ooids and the oolitic clay suggest that the ooids were also formed from volcanic ash

phanerozoic oolitic ironstones geologic record and facies model oolitic ironstonesl composed predominantly of ooidsl of ferric oxide and chamosite accumulated throughout much of phanerozoic time figure 1 and some of them have been important ore deposits they are built of two very common products of weathering iron oxide and clay but they constitute a distinctive and subordinate sedimentary facies for which there is no simple modem analogue as a result considerable disagreement has attended more than 125 years of discussion about their origin early analyses of these rocks for example the minette are of france focused mainly on their chemical and mineral composition and on processes of diagenesis more recent work has described their sedimentary textures and structures as well as their general facies and depositional environments ad eleye 1973 brookfield 1973 corbin 1980 hallam 1975 karpov et a11967

early to middle devonian ironstone and phosphorite in the northwestern gondwana parnaiba basin brazil a record of an epeiric margin paleoceanographic changes abstract pragian to givetian successions in the caninde group parnaiba basin brazil present phosphorites ironstone and organic rich sediments that globally coincide with greenhouse conditions and an increase in continental weathering ocean oxygen levels and black shale deposition events these facies are associated with the evolution of the northern gondwana epeiric sea margin of the rheic ocean and can tell us how ocean chemistry changed and the paleoceanographic factors involved in this particular transition of the ocean record we conducted an integrated study based on sedimentology sequence stratigraphy petrography scanning electron microscopy and x ray diffraction to understand the paleoenvironmental stratigraphic and diagenetic controls lithofacies stacking patterns indicate that deposition occurred during marine transgressions resulting in four depositional sequences late eifelian and givetian maximum flooding surfaces are represented by phosphogenesis and francolite precipitation associated with condensed zones pragian emsian to givetian ironstones were represented by authigenic coated phosphatic iron silicate odinite chamosite clinochlore francolite and siderite precipitation these are related to normal regressions under lowstand and highstand systems tract oxic waters prevailed and episodic upwelling is interpreted as the source of nutrients that maintained fe si and p supply on a storm influenced to storm dominated shelf with an additional source of iron also associated with river discharges phosphorite was formed in a stratified ocean near storm wave base under anoxic sulfide reducing bacteria microbial communities and suboxic conditions productivity fueled by recycled p shallower anoxic conditions near the fair weather wave base existed possible related to a shift in the locus of upwelling and then resumption to oxic conditions indicate that water column stratification was not a stable feature due to variations in base level ventilation terrigenous or marine contribution and progressive microbial degradation at the beginning of late eifelian transgression an event of increased delivery of continental flux and eutrophication can also be proposed the link between organic rich intervals and third order black shale events daleje chot c and kacak suggests that extinction events can be tracked by analyzing sections in south america global correlation with ironstone precipitation and organic rich sediments shows that stepped periods of c sink conditions probably contributed to the ocean oxygenation

petrological and geochemical differentiation of lateritic iron crust profiles abstract lateritic iron crust profiles developed from metabasic rocks in west africa burkina faso were studied from a petrographical and geochemical point of view at the base of profiles a thick gray purple kaolinitic horizon occurs with frequent macroscopic preservation of original pre existing structures in such an horizon chemical balances clearly show that part of kaolinite crystals could be explained by an absolute accumulation from circulating solutions higher up in the profiles fe accumulation leads to two main facies purplered hematitic spots and nodules and yellow kaolinitic and goethitic spots mottled clay horizon hematite and goethite distribution in that horizon is regulated by water activity in microenvironments hematitic nodules form as kaolinite crystals are completely dissolved in goethitic spots fe accumulation is much less abundant and the first generation of well crystallized kaolinite is replaced by a poorly crystallized and smaller kaolinite crystals showing a greater substitution of al 3 for fe 3 in the upper part of profiles all the kaolinite is gradually dissolved and the maximum of ferruginous concretion formation leads to a hard and nodular iron crust

the fate of upwelled waters in the great whirl august 1995 abstract the great whirl is a large anticyclonic gyre that develops off the northern somali coast during the southwest monsoon in august 1995 the noaa ship malcolm baldrige surveyed the seaward edge of the upwelling zone associated with this gyre the fate of recently upwelled water was followed by mapping surface property distributions along a cool surface feature that extended seaward along the northern edge of the great whirl surface properties t s and chlorophyll a surface velocity adcp and xbt and ctd casts were interpreted in relation to the trajectories of three instrumented surface drifters deployed in the feature cool surface waters correspond in space to the shoaling of the upper thermocline and offshore advection from the coast surface chlorophyll a concentrations decreased from 2 to 3 g l 1 in the upwelling zone to 0 5 1 5 g l 1 in the surface feature and contiguous waters maximum surface velocities in the great whirl were 250 cm s 1 with velocities 100 cm s 1 along the northern perimeter of the gyre decorrelation time scales for u and v velocity components and chlorophyll a fluorescence from the drifters were on the order of 4 to 7 days these times are comparable to those over which the drifters were ejected from the great whirl into the socotra gyre decorrelation times for sea surface temperature were somewhat longer 10 days all three platforms passed between the somali coast and socotra within a week of their deployment and then traveled east into the northern arabian sea

phanerozoic ironstones an introduction and review s u m m a r y ironstones particularly ooidal ironstones have long fascinated sedimentary geologists and have generated an enormous variety of interpretations but have remained poorly understood a recent upsurge in interest in this group of rocks has generated much new information derived from many different disciplines within the earth sciences the first part of this introduction examines recent advances and attempts to produce a synthesis of them with particular reference to the genesis of marine ooidal ironstones current models for the formation of marine oolitic ironstone formations are discussed and a working model based on various lines of recent research is proposed this model invokes the intrasedimentary formation of berthierine ooids in marine environments during postoxic diagenesis the ooids may be mechanically modified and mineralogically transformed by reworking as well as by later diagenesis goethite ooids may be oxidized berthierine ooids reworked lateritic ooids or primary grains the formation of oolitic ironstoneformations is favoured by a break in clastic sediment supply continued supply of iron and physical reworking many sedimentary environments may supply these parameters but changes in sea level leading to the flooding of land masses and the reworking of suitable terrestrial soils may be both an important source for the iron and provide a break in clastic sediment supply in the second part of this introduction the terminology and classification are discussed and conventions proposed the term berthieroid is introduced as a non specific term for material of undetermined berthierine or chamosite composition the term ironstone is proposed as a petrological term and as an informal lithostratigraphic term a deposit composed of ironstones may be termed an ironstone formation it is recommended that a dunham style of classification is employed petrographically with ironstone replacing the stone of the original carbonate classification the nomenclature of allochems in ironstones is also discussed a glossary of ironstone terminology is provided

microbial structures in oolitic iron formations ferriferous ooids and microoncoids occur in many sedimentary iron formations these ferriferous coated grains are found in fine grained carbonate rich groundmasses iron mineral encrusted microbiota are observed in both the coated grains and the groundmass the branching nature and other morphological features of the microorganisms suggest fungal origins for the oolitic iron ores particularly of the lower jurassic lorraine minette the similarity of the microbial structures in coated grains and their groundmasses suggests that both had developed within microbial mats growing under calm environmental conditions the contribution of stromatolitic marine fungal mats to the fast extraction and immobilization of iron and thus to the genesis of iron ores is demonstrated observations on well preserved freshwater derived fungal stromatolites of the tertiary and laboratory experiments with organotrophic fungal mats confirm the findings on the jurassic and imply a general role of microorganisms in the formation of such deposits

examples of biogenic support of mineralization in two oolitic iron ores lorraine france and gara djebilet algeria abstract after inspection it appears that oolites in the aelenian of lorraine and devonian of gara djebilet are very often nubecularidea the mineralization of these oolitic shaped elements results from a metasomatic process this observation supports the metasomatic theory and leads to the need for more attention being paid to the shaped elements of the microfacies

petrography mineralogy and geochemistry of the late eocene oolitic ironstones of the jebel ank southern tunisian atlas abstract the oolitic ironstones ore deposit of jebel ank central tunisia is a simply folded stratiform ore body of about 2 5 8 m thickness located in the upper part of the epicontinental souar formation late eocene and is covered by the continental segui formation mio pliocene the deposit contains about 20 mt of ore with an average grade of 50 fe generally oolitic iron deposition occurs in shallow water lagoonal environments the jebel ank deposit lies between two regional disconformities late eocene and miocene and is evidence of a transitional stage at the end of regional regression before renewed transgression the footwall of the oolitic iron ore bearing bed consists of a fine grained sandstone bed 10 20 cm thick pinching out laterally westward into green clays the hanging wall is composed of thin bedded limestone and clay alternations 2 3 5 m thick iron occurs in the form of cryptocrystalline goethite with limited al fe substitution the goethite contains around 48 fe 5 al and up to 1 5 p jarosite alunite and manganese minerals cryptomelane psilomelane and manjiorite are supergene secondary minerals probably related to descending surface fluids these manganese minerals occur as accessory minerals with the goethite and are most abundant at the lowermost part of the succession showing varied morphologies local cement space filling and free centimeter sized nodules fe oolites in the deposit are similar to those documented in many other oolitic ironstone deposits the dominant fe oolite type 90 has a concentrically laminated cortex with no nucleus the nuclei of the oolites that do have a nucleus are most commonly detrital quartz grains major elements in high grade samples fe 2 o 3 65 vary within a limited range and show higher concentrations of sio 2 average 7 85 and al 2 o 3 average 5 1 with minor tio 2 mno mgo na 2 o k 2 o and so 3 less than 1 paas normalized trace elements of bulk samples and fe oolite generally show similar behavior both are enriched in v co ni mo as zn and y and are depleted in cu rb zr nb ba and hf anomalous v cr ni zn and ree y are correlated with goethite paas normalized ree y patterns of both bulk samples and fe oolite show slight hree enrichment positive ce with negative y anomalies the mineralogy goethite and cryptomelane along with the geochemistry si vs al as cu mo pb v zn vs ni co binary plots zn ni co triangular diagram ree y content and patterns and ce ce vs nd and ce ce vs y n ho n binary plots of the studied oolitic ironstone are congruent with a hydrogenetic type while two possible sources of iron for jebel ank ironstone can be proposed i submarine weathering of glauconite rich sandstone and ii detrital iron from adjacent continental hinterland the later is the more plausible source of iron based on paleogeographic setting the occurrence of fine sandstone underlying the iron level occurrence of mn ores in the lower part of the fe ores succession high phosphorous zinc ree y concentrations and y ho ratios and low la ce ratios