hummocky cross stratification and megaripples a geological double standard abstract hummocky cross stratification in ancient sedimentary deposits is considered to be diagnostic of inner shelf storm deposits the geometry of the cross strata sets implies a low relief oval bedform of a few meters spacing orientation is poorly defined or random hummocky cross strata sets have been attributed to the action of strong storm wave surges megaripples of 2 5 m spacing are typical responses of the inner atlantic shelf of north america to storm flows side scan sonar records have shown that megaripples on the inner shelf are commonly hummocky oval to circular in plan view and lack asymmetry box cores and vibracores reveal gently inclined to horizontal lamination as the characteristic primary structure the megaripples am produced by combined flow storm currents these are alongcoast geostrophic flows with a mean flow component of 20 60 cm sec 1 a high frequency wave orbital current of similar or weaker intensity is present but its role in megaripple formation is subordinate to that of the mean flow the high velocity coastal margin of the flow is often jetlike and downwelling in nature and is characterize by a down current decrease in sand flux and rapid deposition of sand the bottom boundary layer of such flows is characterized by dense sediment suspensions 40 60 mgl 1 and is highly turbulent due to the interaction of the mean flow and wave orbital current components eddy diffusivity up to 200 cm2 sect at 40 cm above the bed sand transport in this coastal boundary zone is obliquely offshore but neither the density of the sediment suspension nor storm surge ebb appear to be important driving forces it is difficult to account for either modern hummocky megaripples or ancient hummocky cross stratification as responses to purely oscillatory flows combined flow currents are the typical response of mostmodern shelves and epicontinental seas to storms and presumably were so in the past hummocky cross stratified beds are generally too thick 20 80 cm to be accounted for by in situ resuspension they are more likely to have been despoited by combined flow currents in areas where these currents were experiencing a downstream velocity decrease and sediment deposition throughout much of the storm s duration evidence for strong scour in swales between hummocks and the smoothly rounded surface of the hummocks suggests a transitionlike bed regime measurements on the atlantic shelf likewi e suggest a transitionlike regime whose boundary layer is characterized by lower velocity than the transitional regime of unidirectional flow in flumes but more intense turbulence and a higher suspended sand concentration the lack of normal cross bedding in bothmodern and ancient hummocky bedforms indicates that these features did not migrate this characteristic may be a consequence of the complex trajectories followed by near bottom water particles in response to combined flow currents

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

discontinuity surfaces clinoforms and facies architecture in a wave dominated shoreface shelf parasequence abstract detailed outcrop analysis of a wave dominated shoreface shelf parasequence exposed in the book cliffs utah reveals minor stratigraphic discontinuities which define clinoforms two types of discontinuity are recognized each with a distinctive lithologic character and geometry nondepositional discontinuities are marked by an abrupt decrease in the thickness and amalgamation of storm generated event beds and are interpreted to record hiatuses in sedimentation these discontinuities define clinoforms with a concave upward geometry that dip gently 0 02 0 58 over distances of 800 6000 m down depositional dip erosional discontinuities are marked by an abrupt increase in event bed amalgamation grain size and sand content and are interpreted as enhanced storm wave scours these discontinuities define more steeply dipping 0 22 0 95 concave upward clinoforms that extend over 100 1600 m down depositional dip and 500 1500 m along depositional strike the distribution and amalgamation of minor stratigraphic discontinuities defines linear zones of distinctive facies architecture that are oriented parallel to the paleoshoreline trend using the simple assumption that the shoreface shelf equilibrium profile remained approximately constant for each type of discontinuity throughout shoreface migration the bruun rule intra parasequence facies architecture can be speculatively interpreted in terms of shoreline trajectory which reflects the balance between sediment supply and accommodation the resulting interpretations support the notion that shoreline trajectory exerts a strong control on intra parasequence facies architecture and preservation of the shoreface shelf profile

3 d characterization of a clastic reservoir analog from 3 d gpr data to a 3 d fluid permeability model a three dimensional 3 d 100 mhz ground penetrating radar gpr data volume is the basis of in situ characterization of a fluvial reservoir analog in the ferron sandstone of east central utah we use the gpr reflection times to image the bounding surfaces via 3 d velocity estimation and depth migration and we use the 3 d amplitude distribution to generate a geostatistical model of the dimensions orientations and geometries of the internal structures from the surface down to 12 m depth each sedimentological element is assigned a realistic fluid permeability distribution by kriging with the 3 d correlation structures derived from the gpr data and which are constrained by the permeabilities measured in cores and in plugs extracted from the adjacent cliff face the 3 d gpr image shows that gpr facies changes can be interpreted to locate sedimentological bounding surfaces even when the surfaces do not correspond to strong gpr reflections the site contains two main sedimentary regimes the upper 5 m co

synsedimentary formation of ooidal ironstone an example from the jurassic deposits of se central iran abstract the study of the upper lower to middle jurassic badamu formation in se of central iran represents the first report of jurassic ooidal ironstone in iran and exhibits a record of ferruginous radial ooid dominated ironstone the studies showed that this chamosite or ferric oxide chamosite type ooidal ironstone is similar to minette type ironstones a shoreface zone in an epeiric or epicontinental sea is proposed for formation and accumulation of the ferruginous ooids and the ooidal ironstone fe derived from lateritic weathering has been transported into the shoreface zone via a fluvial current during the synsedimentary formation of the ooidal ironstone during the periods with moderate to lower energy or weakly agitated and quiet environmental conditions suspended ferruginous small radial ooids and fe were increased in the lower shoreface subzone higher environmental energy resulted in the development of large concentric ooids and limited distribution of fe in the upper shoreface subzone higher environmental energy also prevented the development of radial ooids and restricted fe precipitation on carbonate ooids variations in environmental energy in the middle shoreface subzone were unfavorable for increasing ferruginous ooids and fe in the environment although short term periods with weakly agitated and quiet or moderate to lower energy conditions could had been existed in the middle shoreface subzone but precipitation of fe on a few small radial ooids was allowed environmental energy in the shoreface zone was the major factor controlling precipitation of fe especially suspended river fe loads on the ooids however precipitation of dissolved fe could have been related to oxidizing conditions in the environment also the probable presence of fe oxidizing bacteria in the cortexes of the ferruginous small radial ooids indicates that bacteria could have possibly contributed to precipitation of fe on the ooids in a calm environment

ground penetrating radar and its use in sedimentology principles problems and progress abstract ground penetrating radar gpr also referred to as ground probing radar surface penetrating radar subsurface radar georadar or impulse radar is a noninvasive geophysical technique that detects electrical discontinuities in the shallow subsurface

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

3d imaging of a reservoir analogue in point bar deposits in the ferron sandstone utah using ground penetrating radar most existing reservoir models are based on 2d outcrop studies 3d aspects are inferred from correlation between wells and so are inadequately constrained for reservoir simulations to overcome these deficiencies we have initiated a multidimensional characterization of reservoir analogues in the cretaceous ferron sandstone in utah detailed sedimentary facies maps of cliff faces define the geometry and distribution of reservoir flow units barriers and baffles at the outcrop high resolution 2d and 3d ground penetrating radar gpr images extend these reservoir characteristics into 3d to allow the development of realistic 3d reservoir models models use geometric information from mapping and the gpr data combined with petrophysical data from surface and cliff face outcrops and laboratory analyses of outcrop and core samples the site of the field work is corbula gulch on the western flank of the san rafael swell in east central utah the outcrop consists of an 8 17 m thick sandstone body which contains various sedimentary structures such as cross bedding inclined stratification and erosional surfaces which range in scale from less than a metre to hundreds of metres 3d depth migration of the common offset gpr data produces data volumes within which the inclined surfaces and erosional surfaces are visible correlation between fluid permeability clay content instantaneous frequency and instantaneous amplitude of the gpr data provides estimates of the 3d distribution of fluid permeability and clay content

origin of ooidal ironstones in relation to warming events cretaceous eocene bakchar deposit south east western siberia abstract an integrated study involving petrographical mineralogical and geochemical investigations proposes the origin of the bakchar ironstone deposit by upward migration of a mixture of fe rich brine and hydrothermal fluid through marine sediments trace element concentrations of the ironstone deposits rule out continental derivation of fe while high ree concentrations indicate mixed hydrogeneous and hydrothermal source three main types of ironstone within the bakchar deposit are characterized by distinct mineralogical assemblage of authigenic minerals corresponding to oxic methanic and weakly sulfidic dysoxic conditions changing redox conditions of the depositional environment methane seepage in warm seawater and availability of iron influx possibly determined the final mineralogy of three varieties of ironstones the association of siderite with ferrimagnetic sulfides like greigite and pyrrhotite in type 1 ore indicate anaerobic oxidation of methane with limited bacterial sulfate reduction the ironstone deposits of narym kolpashevo and bakchar are contemporaneous with geological events like ocean anoxic event 3 oae 3 cretaceous paleogene k pg boundary and palaeocene eocene thermal maximum petm respectively all corresponding to warm seawater conditions

hummocky cross stratification tropical hurricanes and intense winter storms most previous workers have inferred a storm origin for hummocky cross stratification which typically occurs in shallow marine deposits on the modern earth the only storms capable of profoundly affecting shallow marine depositional environments are severe tropical cyclones hurricanes and mid latitude winter wave cyclones intense winter storms this paper examines the palaeogeographic distribution including palaeolatitude and palaeogeographic setting of 107 occurrences of hummocky cross stratification ranging in age from the proterozoic to recent in each of these stratigraphic units both palaeolatitude and palaeogeography are consistent with a direct storm influence associated with the passage of hurricanes or winter storms directly over the site of deposition this palaeogeographic evidence lends support to the inferred storm origin for hummocky cross stratification further the distribution of the structure suggests that most occurrences 73 were generated by tropical hurricanes the remaining 27 being generated by intense mid latitude winter storms the preferential generation of hummocky cross stratification by hurricanes is consistent with 1 the known differences in the nature of the bottom flows generated by the two major storm types and 2 the inferred nature of the flows which form hummocky cross stratification hurricanes couple less effectively with the water column than do intense winter storms due to this ineffective coupling hurricane generated bottom flows tend to be oscillatory or multidirectional dominant with only minor unidirectional components of motion in contrast intense winter storms generally do couple effectively with the water column generating bottom flows which possess a dominant or significant unidirectional component most previous workers have suggested that hummocky cross stratification forms under oscillatory or multidirectional dominant flow thus it is conceptually reasonable that the vast majority of ancient occurrences of hummocky cross stratification were probably hurricane generated as suggested by the aforementioned palaeogeographic distribution the proterozoic palaeozoic neogene and quaternary were times when global climate was similar to that of today the distribution of hummocky cross stratification deposited during these times suggests that both hurricanes and intense winter storms occupied latitudinal belts during these times which were essentially identical to those occupied by their modern counterparts the mesozoic and palaeogene were non glacial times when global climate was much warmer than that of today the distribution of hummocky cross stratification deposited during this interval suggests that hurricanes occurred more frequently at higher latitudes during non glacial times than they do at present the possibility of a broadened hurricane belt during the mesozoic and palaeogene is consistent with climatic considerations a limited number of mesozoic and palaeogene rock units containing hummocky cross stratification were deposited in palaeogeographic settings that preclude a direct hurricane influence these examples were deposited in the middle latitudes suggesting that intense winter storms continued to form hummocky cross stratification in the middle latitudes during these much warmer times some previous workers have suggested that tsunamis may be capable of generating hummocky cross stratification the palaeogeographic distribution of the structure does not support an origin due to tsunamis lacustrine examples of hummocky cross stratification reported herein are the first known non marine occurrences they suggest that storm effects strongly influence the sedimentary record of some lakes

depositional and diagenetic c s fe signatures in early paleozoic normal marine shales the degree of pyritisation dop of cambrian silurian normal marine shales is always less than 0 70 similar to that found previously for sediments of devonian to cretaceous ages this indicates that dop can be used as a paleoenvironmental indicator of deposition from normal marine i e oxygenated bottom waters throughout the phanerozoic record although the range in dop is similar significant variations in normal marine cs ratios can occur relatively low cs ratios 1 5 result from lower reactive iron contents which are also independent of organic carbon variations reactive iron contents and hence normal marine cs ratios are able to vary at a regional or basinal level

deltaic coals of ferron sandstone member of mancos shale predictive model for cretaceous coal bearing strata of western interior the upper cretaceous ferron sandstone member of the mancos shale the coal bearing unit of the emery coalfield in central utah shows accumulation of clastic sediments in a lobate river dominated deltaic system that existed along the western shoreline of the interior cretaceous seaway during late turonian time five cycles of deltaic sedimentation each containing one major coal bed or coal zone are represented a clear genetic relationship exists between the geometries of the major coal beds of the emery field and the geometries of the delta front sandstone units with which they are associated the thicker part of each coal bed extends from the vicinity of the landward pinch out of the associated delta front sandstone landward to a distance of about 10 km this genetic relation forms the basis of a predictive model that can be used in designing more cost effective exploration programs in coal bearing strata of cretaceous age in the western interior

ironstones and green marine clays in the dongjeom formation early ordovician of korea abstract in the sino korean north china block the occurrence of ironstone is reported for the first time from the lower ordovician dongjeom formation korea the dongjeom formation comprises much quartzose sediment deposited on the extensive carbonate platform and is composed of three coarsening upward sequences ironstone beds are distributed on the tops of the lower two sequences suggesting that they mark approximately maximum flooding events during the relative lowstand of sea level when the dongjeom formation was deposited extensive weathering in the source areas coupled with higher order sea level rises resulted in the formation of the dongjeom ironstones the dongjeom ironstones include much green marine clays ironstones of the lowest sequence are characterized by glaucony facies and those of the middle sequence by chamosite facies chemical compositions of dongjeom glauconies reveal that they were originally nascent to slightly evolved 3 to 5 k 2 o wt and that they were affected by illitization after burial their immature compositions suggest that glauconies were formed in about 10 3 to 10 4 years or less on the seafloor the substrates for glauconitization are interpreted as detrital mineral grains regressions after glauconitization introduced iron oxides to the green grains eventually forming the iron oxide rich band under the most oxidizing conditions the distribution of glauconies in limited horizons suggests that the dongjeom glauconies are autochthonous in origin chamosites have granular shapes and are mixed with quartzose sand grains suggestive of reworking during storms the dongjeom chamosites were transformed from the precursor berthierines the occurrence of glauconies in correlative horizons in korea and north china may indicate that glauconitization in the host sediments which were deposited in far northern gondwana occurred under the influence of a coeval transgressive event

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

time specific aspects of facies state of the art examples and possible causes abstract the term time specific facies tsf was introduced to the scientific community by the late otto h walliser to refer to unique facies typical of particular narrow intervals some of which were related to bioevents in some senses however the concept was recognized much earlier and is even engrained in the very names of some geologic periods the concept of time specific facies is expanded slightly herein to include distinctive or unique regional to global characteristics of the sedimentary record that characterize particular intervals of geologic time the recognition of time specific and widespread processes in the sedimentary record is a critical step in unraveling the interplay between processes of differing scale these range from very short term but widespread facies that overlap with event deposits to general facies types that may persist for intervals up to 10s of millions of years in duration this paper briefly summarizes the history of development of the tsf concept provides examples to illustrate a few of the key aspects of time specific facies and offers a few tentative explanations for this phenomenon among the factors that control tsfs abrupt changes in redox conditions and early diagenetic mineralization sedimentary condensation often associated with abrupt sea level change altered climate and paleoceanography and biotic evolution and extinction seem to be most critical and permit a preliminary genetic classification of tsfs inevitably however many tsfs reflect multiple effects and some remain largely unexplained

a 3 d ground penetrating radar and wavelet transform analysis of the morphology of shoreface deposits in the upper cretaceous ferron sandstone member utah wavelengths of hummocky cross stratified hcs beds a common sedimentary feature of storm dominated shorefaces are documented for the first time using measurements in three dimensional 3 d ground penetrating radar gpr data for a well developed upper cretaceous lower shoreface succession at dry wash in the ferron sandstone member utah the shallow marine sequence consists of upward thickening hcs sand beds alternating with interstorm deposits the thickness variation of the storm beds indicates locally steadily growing storm intensity with at least four cycles weakly coarsening upward mud to very fine grained sand fair weather background deposits suggest a slow progradation of deposition with no significant change in environment the gpr interpretation mapped three conformable high continuity high amplitude reflections throughout the 3 d gpr data volume the interpreted radar surfaces rss are well correlated with tops of hcs sand beds and thus paleotopographic surfaces the associated radar units rus have a uniform thickness on average 0 8 m 2 6 ft the rus and the adjacent outcrop observations suggest that the shoreface sandstone at the dry wash site has a simple layered internal architecture the hummocky swaley surfaces generally dip westerly as a product of postdepositional structural alterations that are mostly in the shoreline direction and contain variable size structurally undulating rounded features a 2 d continuous wavelet transform analysis is applied to the detrended rss producing a multiresolution image decomposition of the gpr surfaces surface features with a wavelength range of 1 7 m 3 23 ft are in good agreement with the observations on modern hummocky shallow marine seabeds quantitative measurements indicate that the hummocky surfaces at the dry wash site are dominated by uniformly distributed circular to elongate bed forms with maximum correlation at 1 5 3 5 m 4 9 11 4 ft wavelength and that the deltaic sedimentary layers were simultaneously deformed by the middle campanian compressional stress of the sevier orogeny transmitted from the northwest quantitative information on the subseismic scale surface geometry of the hcs beds is expected to result in more refined reservoir models in addition the connectivity of units indicated by the scale of the morphology can be an indirect indicator of unit correlation and permeability paths

the provenance of jurassic and lower cretaceous clastic sediments offshore southwestern nova scotia jurassic and cretaceous sandstones in the shelburne subbasin and fundy basin offshore nova scotia are poorly known but are of current interest for petroleum exploration the goal of this study is to determine the provenance of sandstones and shales which will contribute to a better understanding of regional tectonics and paleogeography in the study area mineral and lithic clast chemistry was determined from samples from conventional cores and cuttings from exploration wells using scanning electron microscopy and an electron microprobe whole rock geochemical composition of shales was used to test the hypotheses regarding provenance of mesozoic clastic sedimentary rocks in the sw scotian basin lower jurassic clastic sedimentary rocks in the fundy basin contain magnetite biotite and chlorite suggesting local supply from the north mountain basalt and meguma terrane whereas pyrope and anthophyllite suggest small supply from distant sources in the sw scotian basin detrital minerals lithic clasts and shale geochemistry from middle jurassic to early cretaceous indicate a predominant meguma terrane source and transport by local rivers rare spinel and garnet grains of meta ultramafic rocks only in the middle jurassic at the mohawk b 93 well suggest minor supply from the rising labrador rift via the same river that transported distant sediments to the fundy basin lower cretaceous sandstones from the mohican i 100 well contain minor garnet spinel and tourmaline from meta ultramafic rocks characteristic of sediment supplied to the central scotian basin at that time the dominant meguma terrane provenance precludes thick deep water sandstones in the eastern part of the shelburne subbasin but the evidence of middle jurassic distant river supply through the fundy basin is encouraging for deep water reservoir quality in the western part

breccia hosted manganese rich minerals of alpi apuane italy a marine redox generated deposit mn rich beds occur within the brecce di seravezza formation bsfm intercalated between norian metadolostone megalodontic marble and hettangian marble in the alpi apuane italy the bsfm is a rockfall talus deposit that accumulates at the base of the footwall scarp of normal faults bordering extensional marine coastal basins the type sequence of metabreccia ends with a bed of chloritoid schist interpreted as the metamorphic equivalent of lateritic soils the four main types of mn mineral assemblages are 1 braunite piemontite and rare hausmannite 2 piemontite braunite hollandite and minor rhodochrosite and kutnahorite 3 braunite hollandite and rare piemontite the other minerals are quartz calcite muscovite phlogopite baryte and minor hematite rutile and apatite during the alpine orogeny pressures of 4 6 kb and temperatures of 350 380 c were attained mn assemblages with hematite and phlogopite indicate o2 10 8 for t 350 c and o2 10 6 when braunite appears on the mn rich rocks ti fe nb sn ta hf th ga rb pr k na nd were identified as detrital in origin and are positively correlated with al u is positively correlated with co pb bi mo the rare earth element patterns normalized to seawater show a negative slope towards the heavy rare earth elements with positive ce anomalies in some samples positive ce and eu anomalies in other samples and negative ce and positive eu anomalies in a third group the eu concentrations are nearly constant and the eu anomalies reflect differences in the concentration of other rare earth elements for ce a positive correlation with al2o3 sio2 indicate that a detrital signature prevails on that produced by duration of seawater exposure but a significant good correlation of la tm lu with mg could indicate at least for these elements a seawater signature only in some samples ba eu sr show anomalously high concentrations likely of hydrothermal origin however generally in all the chemical variation diagrams the bsfm samples fall outside the field of hydrothermal and hydrogenous deposits as defined in the literature for other mn deposits in addition metabasites and metavolcaniclastic beds are absent from the bsfm sequence and bsfm coastal basins were for the most part separated from the open ocean and related streams that must be excluded as possible mn sources owing to the arid climate of late triassic the runoff was negligible and the groundwater oxidizing and alkaline were unable to mobilize mn the bsfm basins are then the most suitable depositional environment for mn the water column was stagnant and stratified only where bottom of the basin was deeper than the oxic anoxic boundary reducing marine bottom waters percolated within the fractured basement and leached mn from disseminated minerals of metamorphic rocks and then rising along fault and fractures supplied mn to the bottom waters afterwards when tectonic uplift and sealevel drop brought the bottom into the oxic field mn precipitated and formed the mn rich matrix of the bsfm deposits

geology of the early devonian oolitic iron ore of the gara djebilet field saharan platform algeria abstract the oolitic iron ore of the gara djebilet field occurs within the early devonian sediments of the tindouf basin algerian sahara particularly in the upper djebilet formation of pragian age three large lenses form three individual deposits extending e w for about 60 km namely gara west gara center and gara east the mineralization is interbedded with argillaceous to sandy sediments and it can be related to a barrier island palaeoenvironment bordered by an inner lagoon or shallow embayment and an epicontinental sea trapped by palaeozoic shoals the oolitic sediments show a mineralogy marked mainly by magnetite hematite goethite maghemite chamosite bavalite siderite apatite and quartz three paragenetic associations present a vertical distribution with a lower non magnetitic ore a magnetitic ore and an upper non magnetitic ore three petrographical facies types have been defined a cemented facies foc a detrital facies fod and a non detrital facies fond chemical data for the whole field show a difference between the lower non magnetitic ore fe 54 6 the magnetitic ore fe 57 8 and the upper non magnetitic ore fe 53 the magnetitic ore which corresponds mainly to the workable ore cutoff grade at 57 has the following composition sio2 4 9 al2o3 4 2 fe2o3 61 43 feo 19 2 and p2o5 1 8 the corresponding calculated economical ore reserves are 985 106t with 57 8 fe regarding the genesis of the oolitic iron ore a southern source is suggested for the iron with deposition taking place in a quiet environment there the ooids developed by an intrasedimentary accretion mechanism around detrital grain within an iron rich mud the gara djebilet field is an important occurrence of the north african palaeozoic ironstone belt extending from the zemmour to libya which also includes ironstones of ordovician silurian and devonian age

populations trends and cycles in combined flow bedforms abstract there are two size populations of unidirectional flow bedform spacings those of ripples and dunes in oscillatory flow by contrast asymmetrical bedforms might have a bimodal population but symmetrical ripples show a single log normal population of ripple spacings in combined flow data on ripple spacings are rare ripple spacings l and heights h of 182 combined flow bedforms with a range of l from 0 1 m to 2 8 m were measured in an upward shallowing storm dominated shoreline succession of sandstones of the fernie formation middle to upper jurassic in alberta canada statistical analysis of ripple spacings shows that the population of three dimensional 3 d bedforms belongs to a single log normal population that is distinct from population of two dimensional 2 d bedforms statistical tests also reveal an upward linear increase and a 16 bed cyclicity in l the linear trend is interpreted here as a proximality trend normally found in such successions and the 16 bed cycle as a shorter period climatic effect related to a milankovitch eccentricity cycle a complex interplay of wave and current parameters flow depth orbital diameter near bottom unidirectional velocity mixing length in the turbulence structure etc and climatic cycles of different periodicities from 0 4 m y to 0 1 m y seem to determine the size distribution of combined flow bedforms in a stratigraphic sequence

internal waves vs surface storm waves a review on the origin of hummocky cross stratification hummocky cross stratification hcs is considered a diagnos tic structure of surface storm activity at the shoreface offshore transition however the origin of hcs is still debated laboratory experiments have not yet reproduced it and direct observations on the continental shelves do not exist most hydrodynamic interpretations invoke pure oscilla tory flows unidirectional dominated combined flows and oscillatory dominated flows but they all share the assump tion of hcs to reflect the combined action of surface storm waves and related currents within this context of uncertain ties internal waves gravity waves propagating along the pycnocline provide an alternative mechanism to explain the origin of hcs internal waves breaking on the shelf create episodic high turbulence events and induce upslope and downslope currents as well as oscillatory flows at the depth where the pycnocline intersects the sea floor in this scenario both the oscillatory and the unidirectional components needed for hcs to form are not necessarily linked to surface storm waves but can occur at various depth as far and near there is a pycnocline where internal waves can form

observations of sand waves megaripples and hummocks in the dutch coastal area and their relation to currents and combined flow conditions 1 this paper aims to investigate the distribution and stability of large scale bed forms in response to storm and fair weather conditions in a shallow marine environment multibeam and side scan sonar data off the dutch coast median grain size 0 25 0 35 mm were collected to monitor sand waves 100 800 m and superimposed megaripples 1 40 m through multiple storm and fair weather events box cores were used to observe the vertical bed structure and grain size in the dutch coastal area two dimensional 2 d megaripples x 1 15 m are the dominant bed forms in current dominated 0 4 m s tidal flow regimes with oscillatory flows 0 4 m s undulating bed topography of mound like 3 d bed forms 20 40 m is observed immediately after storms these bed forms are covered by smaller 3 d megaripples which are related to sets of low angle converging laminae in box cores interpreted as hummocky cross stratification hcs the sand waves form compound bed forms of sets of 2 d and 3 d megaripples the morphology of the sand waves is a function of the general wind wave climate of the marine environmental setting with flat topped 3 d sand waves occurring in shallow wave dominated settings and 2 d sand waves occurring in the tide dominated environment farther offshore copyright 2005 by the american geophysical union

review of cenozoic ooidal ironstones abstract cenozoic tertiary ooidal ironstones coi in 20 districts 39 deposits developed between the equatorial zone and 60 n except for one eocene district in mid southern latitude stratigraphic distribution paleocene oi occur in northern pakistan western siberia southern germany northwestern venezuela and northeastern colombia eocene oi in western siberia southern germany northwestern romania central north africa central west saudi arabia southwestern central africa northwestern venezuela northeastern colombia south central usa and northwestern australia oligocene oi in northwest and west central kazakhstan central denmark and north central iran miocene oi in northwestern venezuela northeastern colombia and southeastern malaysia pliocene oi in southeastern ukraine geotectonic framework ten districts developed in an interior or fractured craton five along the south trending uralian seaway and one along the east trending northern european seaway the south trending trans saharan seaway in or near an early red sea embayment in southeastern malaysia and in northwestern australia ten districts lay near a eratonic margin one along divergent margin and nine along the broad east trending caribbean and tethyan seaways almost all coi accumulated during the paleogene relatively high stand of sea level especially in early and middle eocene time as sea level fell gradually in neogene time coi developed in only three marginal districts including the giant pliocene kerch taman deposit in southeastern ukraine sedimentary environment almost all of the coi developed in deltaic to shallow marine facies these are commonly associated with shoaling upward siliciclastic sequences a few are in mixed siliciclastic carbonate sequences a few coi apparently occurred in fluvial and lacustrine facies some of these may have been reworked from laterite or from marine ironstones sedimentary petrology many coi are less than a few tens of centimeters thick and sporadic the ooids are entirely or predominantly ferric oxide a few contain a minor amount of berthierine 7a only the oldest one northern pakistan contains chamosite 14a more than half of the coi are associated directly or more remotely with glauconite nearly all the coi need much more rigorous examination for both their individual composition and their comparison with older ooidal ironstones

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

flow units connectivity and reservoir characterization in a wave dominated deltaic reservoir meren reservoir nigeria the meren e 01 agbada formation middle miocene reservoir offshore nigeria consists of a lower progradational shoreface succession terminated by a minor sequence boundary overlain by a progradational and retrogradational shoreface succession deposition occurred in a wave dominated delta front as indicated by the presence in core of hummocky cross beds slumped units and turbidites eight flooding surfaces were correlated and isopach maps sandstone quality trend maps and mudstone quality trend maps were constructed for each parasequence this work revealed a complex reservoir architecture characterized by shoreface clinoforms and a history of progradation and retrogradation cycles three different three dimensional geological characterizations of the e 01 reservoir were built a geostatistical model that used only well data a more geologically complex facies based model that used the sandstone quality trend maps in addition to well data and the most geologically complex sequence stratigraphic model that used mudstone quality trend maps in addition to the above data the three models were analyzed in terms of sandstone continuity and connectivity to hypothetical injector and producer wells only the sequence stratigraphic model predicted significant vertical compartmentalization through tortuosity generated by flooding surface mudstones waterflood fluid flow simulation of a downdip sector of the geologic models predicts similar recovery for the three models but a significantly different distribution of unswept oil only the sequence stratigraphic model identified parasequences with abundant unswept oil that are large enough to be economic infill prospects history matched full field fluid flow simulations verify both the reservoir compartments predicted by the sequence stratigraphic model and infill targets

platform collapse and pelagic seamount facies jurassic development of central tunisia abstract in central tunisia the lower liassic facies consists of peritidal carbonates deposited on a wide platform which has been broken into irregular tilted blocks in the early pliensbachian black shale was deposited in some of the down faulted blocks during the early toarcian oceanic anoxic event the bajocian bathonian deposits are represented by ironstones and hemipelagic ammonite and zoophycos rich marl limestones the upper jurassic is composed by pelagic limestones and debris flow deposits the middle jurassic deep marine sedimentation was frequently associated with condensed sections submarine erosion leading to stratigraphic hiatuses the latter results from the interaction between global sea level fluctuations and regional tectonic activity generating an uplifted block in central tunisia in relation with early mesozoic rifting and associated distensional drifting of the north african tethyan margin this horst block has acted as a starved pelagic shoal on which sedimentation has been recorded especially during retrogradational and the aggradational phases

laminaset geometries in fine grained shallow marine sequences core data from the rannoch formation north sea and outcrop data from the kennilworth member utah usa and the bencliff grit dorset uk sandbodies from storm dominated marine and marginal marine environments commonly contain intervals of laminated fine sandstones a characteristic of such lamination is the presence of low angle cross lamination in order to model correctly the effects of such lamination on a waterflood of an oil bearing shoreface sequence it was necessary to quantify the geometry of the laminaset elements this challenge has been greatly complicated by the lack of outcrop of the formation of interest the middle jurassic rannoch formation of the north sea only occurs in the subsurface where it is not possible in core to measure the aspect ratio of laminasets directly in this study the laminaset geometry data that can be obtained from core e g apparent set thicknesses were collected for the rannoch formation these data were compared with similar data from potential outcrop analogues in 1 the cretaceous kennilworth member of the blackhawk formation in utah usa and 2 the upper jurassic bencliff grit from the dorset coast uk a quantitative analysis of laminaset geometries has been used to compare subsurface core with potential outcrop analogues the rannoch formation core is characterized by numerous low angle truncations we have measured these features in two wells means of 7 2 and 12 1 mean apparent set thicknesses were 0 24 and 0 19 m in the outcrop sections studied truncation angles ranged from 9 6 to 13 4 and mean set thicknesses from 0 24 to 0 34 m mean bounding surface dips of 5 8 and 8 6 and mean laminaset lengths of 2 3 and 4 1 m were also measured directly in the field and by using photomosaics on the basis of this comparison the kennilworth member in utah was found to be the most suitable and the geometries i e aspect ratios measured there were used to generate an appropriate geometry of rannoch laminaset geometries for use in engineering studies laminaset length 2 0 m laminaset thickness 0 2 m

a depositional model for the liassic minette ironstones luxemburg and france in comparison with other phanerozoic oolitic ironstones

detailed internal architecture of a fluvial channel sandstone determined from outcrop cores and 3 d ground penetrating radar example from the middle cretaceous ferron sandstone east central utah ideally characterization of hydrocarbon reservoirs requires information about heterogeneity at a submeter scale in three dimensions detailed geologic information and permeability data from surface and cliff face outcrops and boreholes in the alluvial part of the ferron sandstone are integrated here with three dimensional 3 d ground penetrating radar gpr data for analysis of a near surface sandstone reservoir analog in fluvial channel deposits the gpr survey covers a volume with a surface area of 40 x 16 5 m and a depth of 12 m five architectural elements are identified and described in outcrop and well cores using a sixfold hierarchy of bounding surfaces internally the lower four units consist of fine grained parallel laminated sandstone and the upper unit consists of medium grained trough cross bedded sandstone the same sedimentary architectural elements and associated bounding surfaces are distinguished in the gpr data by making use of principles developed in seismic stratigraphic analysis to facilitate comparison of geologic features in the depth domain and radar reflectors in the time domain the radar data are depth migrated the gpr interpretation is carried out mainly on migrated 100 mhz data with a vertical resolution of about 0 5 m measures of the spatial continuity and variation of the first and second order bounding surfaces are obtained by computing 3 d experimental variograms for each architectural element each radar begin page 1584 facies the maximum correlation length of the dominant internal features ranges between 4 and 6 m and the anisotropy factor ranges between 0 6 and 0 95

the end ordovician mass extinction a single pulse event abstract the end ordovician mass extinction eome is widely interpreted as consisting of two pulses associated with the onset and demise of the gondwana glaciation respectively with the second pulse eradicating the distinctive glacially related hirnantian benthic biota hbb a global review of occurrence data of latest ordovician benthic marine organisms reveals that virtually all warm water benthic assemblages previously assigned to the hbb comprise two distinct and clearly postglacial faunas both younger middle and late hirnantian respectively than the cool water hirnantia fauna latest katian to early hirnantian the newly recognised three transitional benthic faunas i e tbfs 1 3 can be closely tied to graptolite conodont and chitinozoan biozonations the hirnantian isotope carbon excursion hice and the glaciation thereby providing an integrated much higher resolution timescale for understanding the tempo and nature of the eome at this finer resolution we postulate a more profound impact of the first pulse of the eome than hitherto envisaged as evidenced by opportunistic expansion of the hirnantia fauna globally and the complete absence of metazoan reefs in its immediate aftermath we also argue based on high quality data from well documented benthic groups in south china i e brachiopods tabulate and rugose corals trilobites and sponges that the magnitude of the second pulse of the eome caused by the deglaciation has been overestimated because the two postglacial faunas i e tbfs 2 3 were part of a subsequent recovery phase of marine ecosystems rather than contributing to biodiversity decline thus it is more plausible to reinterpret the eome as a single pulse rapid event that was followed by a prolonged initial recovery intermittently impeded by climatic shocks through the hirnantian prior to the onset of a progressive reestablishment of marine ecosystems during the early silurian rhuddanian and aeronian associated with an overall amelioration of climatic conditions

ground penetrating radar for high resolution mapping of soil and rock stratigraphy ground penetrating radar is a technique which offers a new way of viewing shallow soil and rock conditions the need to better understanding overburden conditions for activities such as geochemical sampling geotechnical investigations and placer exploration as well as the factors controlling groundwater flow has generated an increasing demand for techniques which can image the subsurface with higher resolution than previously possible the areas of application for ground penetrating radar are diverse the method has been used successfully to map ice thickness water depth in lakes bedrock depth soil stratigraphy and water table depth it is also used to delineate rock fabric detect voids and identify karst features the effective application of the radar for the high resolution definition of soil stratigraphy and fractures in bedrock is highlighted the basic principles and practices involved in acquiring high quality radar data in the field are illustrated by selected case histories one example demonstrates how radar has been used to map the bedrock and delineate soil horizons to a depth of more than 20 m two case histories show how radar has been used to map fractures and changes of rock type to 40 m range from inside a mine another case history demonstrates how radar has also been used to detect and map the extent of groundwater contamination the corroboration of the radar results by borehole investigations demonstrates the power and utility of the high resolution radar method as an aid for interpolation and extrapolation of the information obtained with conventional coring programmes with the advent of new instrumentation and field procedures the routine application of the radar method is becoming economically viable and the method will see expanded use in the future

density structure of io and the migration of magma through its lithosphere we model the density structure of the outer several tens of kilometers of io on the basis of the assumption that condensed volatiles mainly so2 exist at shallow depths in a mafic to ultramafic lithosphere which has a finite porosity due to its accumulation as a mixture of pyroclastic and effusive volcanic deposits which include condensed volatiles the porosity decreases with increasing depth due to pressure driven compaction and temperature driven melting of the volatiles and this controls the bulk density structure of the lithosphere we also model the densities of ascending magmas as a function the types and amounts of volatiles that are available in their source regions the relative densities of the accumulated volcanic deposits and of the magmas rising through them control whether the magmas will erupt at the surface or will stall to form intrusions which can grow into magma reservoirs we find that the depths at which magma reservoirs may exist are strongly influenced by the void space fraction in surface deposits and only weakly controlled by the mass fraction of so2 in the deposits and the mass fraction of so2 in the magmas once nucleated a reservoir has the potential to grow to a very large vertical extent 15 km total height before the magma immediately beneath its roof is again buoyant relative to the surrounding rocks the presence of large volume reservoirs may explain the great extent of the larger lava flow fields seen on io and the longevity of some eruptions the presence of very soluble volatiles such as water in magmas would have virtually no effect on these findings because these volatiles would influence the densities of magmas only very near the surface though they would even in very small amounts then have a dramatic effect on the explosivities of eruptions

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

a ground penetrating radar survey of a delta front reservoir analog in the wall creek member frontier formation wyoming ground penetrating radar gpr measurements in conjunction with outcrop sedimentology were carried out at murphy creek reservoir in the upper cretaceous turonian wall creek member of the frontier formation in wyoming the objectives were to apply gpr to map geometrical details of a top truncated lowstand delta front and to estimate the volumes of the prograding bar deposits of the delta lobe eleven gpr profiles totaling about 4400 m 14 435 ft were acquired using 50 mhz antennas on a coarsely spaced two dimensional grid of lines lying parallel and perpendicular to the average depositional dip ground penetrating radar reflections were detected from within the outcrop to a depth of about 10 15 m 33 49 ft four southerly dipping major surfaces identified in the gpr data are correlated with the boundaries of progradational delta front facies stacked as distal mouth bar deposits in the outcrop the major boundaries correspond to lithological changes between relatively clean sandstones that are interpreted to have been deposited during floods with high sediment supply alternating with bioturbated sandstones and mudstones deposited during interflood periods with correspondingly low sedimentation rates these two lithological units which also correspond to the two main gpr facies repeat at least three times with no change in dominant average sand grain size subsequent erosion by transgressive ravinement caused the significantly truncated lowstand delta long after the sandstones were deposited the bar assemblage volume at successive stages of growth is estimated using measurements from the outcrop and the gpr data the migrating bars have an estimated average half length of 650 m 2132 ft a lower bound on the average volume of the bar is 3 9 106 m3 1 37 108 ft3 as the volume of the bars increases the bar deposits appear to have a landward as well as a basinward component of accretion

fracture network of the ferron sandstone member of the mancos shale east central utah usa abstract the fracture network at the outcrop of the ferron sandstone member of the mancos shale was studied to gain an understanding of the tectonic history of the region and to contribute data to studies of gas and water transmissivity related to the occurrence and production of coal bed methane about 1900 fracture readings were made at 40 coal outcrops and 62 sandstone outcrops in the area from willow springs wash in the south to farnham dome in the north of the study area in east central utah two sets of regional vertical to nearly vertical systematic face cleats were identified in ferron coals a northwest striking set trends at a mean azimuth of 321 and a northeast striking set has a mean azimuth of 55 cleats were observed in all coal outcrops examined and are closely spaced and commonly coated with thin films of iron oxide two sets of regional systematic joint sets in sandstone were also identified and have mean azimuths of 321 and 34 the joints of each set are planar long and extend vertically to nearly vertically through multiple beds the northeast striking set is more prevalent than the northwest striking set in some places joints of the northeast striking set occur in closely spaced clusters or joint zones flanked by unjointed rock both sets are mineralized with iron oxide and calcite and the northwest striking set is commonly tightly cemented which allowed the northeast striking set to propagate across it all cleats and joints of these sets are interpreted as opening mode mode i fractures abutting relations indicate that the northwest striking cleats and joints formed first and were later overprinted by the northeast striking cleats and joints burial curves constructed for the ferron indicate rapid initial burial after deposition the ferron reached a depth of 3000 ft 1000 m within 5 2 million years m y and this is considered a minimum depth and time for development of cleats and joints the sevier orogeny produced southeast directed compressional stress at this time and is thought to be the likely mechanism for the northwest striking systematic cleats and joints the onset of the laramide orogeny occurred at about 75 ma within 13 7 m y of burial and is thought to be the probable mechanism for development of the northeast striking systematic cleats and joints uplift of the ferron in the late tertiary contributed to development of butt cleats and secondary cross joints and probably enhanced previously formed fracture sets using a study of the younger blackhawk formation as an analogy the fracture pattern of the ferron in the subsurface is probably similar to that at the surface at least as far west as the paradise fault and joe s valley graben farther to the west on the wasatch plateau the orientations of ferron fractures may diverge from those measured at the outcrop

quartz arenites review and interpretation abstract quartz arenites orthoquartzites are exceptional sands pettijohn et al 1972 that in the past have formed extensive and commonly thick formations they are somewhat enigmatic in that they do not appear to be forming at the present time this paper examines the origin of quartz rich sands documents the range of environments in which they are found and suggests a model for their genesis quartz sand is most commonly formed by tropical weathering of coarse grained quartz bearing rocks such as granite it may be deposited as texturally immature fluvial sandstones generally under a humid climate with drying of the climate quartz rich bauxitic soils and sandy alluvium can be a substantial source of poorly rounded dune sands for major deserts quartz sand when transported into shallow marine environments becomes very well rounded through reworking by tidal currents such sand may spill into deep oceanic environments and by sedimentary or tectonic transport may come to rest in both stable and active tectonic settings quartz rich sandstones can also be formed by diagenetic destruction of labile framework grains of immature sandstones precambrian orthoquartzites are commonly thick and widespread when compared to younger examples the thinness of post silurian quartz arenites may be largely due to the trapping effect of land vegetation and solution of quartz by organic acids in weathering profiles there is an abundance of shallow marine and fluvial quartz arenites in the early paleozoic and the cretaceous this is due to widespread continental flooding and a humid world climate following the disintegration of supercontinents the shallow marine examples are commonly associated with bituminous sometimes phosphatic shale glauconite and possibly chamositic ironstone there is an abundance of eolian quartz arenites in the permo triassic this is related to a widespread dry continental climate during assembly of the pangean supercontinent

origin and depositional environment of ordovician stratiform iron mineralization from zamora nw iberian peninsula the ordovician stratiform iron deposits at zamora nw iberia are arranged in several levels ranging between 0 2 and 1 5 m in thickness which are interstratified in the upper member of the pielgo quartzites formation arenig the sandy nature and trace fossils corresponding to the ichnogenus cruziana and daedalus together with major and trace element contents of this formation suggest an inter and subtidal shallow marine depositional environment which on a global scale formed part of a broad shelf situated in the northern margin of the gondwana continent the iron mineralization displays a foliated and banded structure due to the alternation of quartzitic phosphatic apatite chamositic chamositic biotitic and ferriferous magnetite and hematite beds they have high tio2 ta sc v nb co zn and y contents the magnetite contains unusually large amounts of tio2 v cr and ni there is also a clear depletion in eu and the eu sm cn ratio is 1 the chamosite contains high concentrations of cr and v these results suggest that iron was supplied from the weathering of a continental source in combination with volcanic activity such as within basin basic volcanism or the presence of basic volcanic rocks in the exposed land the physicochemical conditions of iron mineral crystallization calculated from chamosite compositions are the following log f o2 38 8 to 30 7 log f s2 13 2 to 9 5 and t 200 to 330 c these results together with the 18o value 2 of the magnetite suggest that chamosite and magnetite were crystallized during later diagenesis and early low grade metamorphism under redox conditions below the magnetite hematite buffer