sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

geochemistry of laterites stability of al goethite al hematite and fe super 3 kaolinite in bauxites and ferricretes an approach to the mechanism of concretion formation

high salinity intermediate water of the japan sea in the eastern japan basin conductivity temperature depth data from the midwinter in 1997 are examined for the presence of the intermediate water in the eastern japan basin in the northern part of the japan sea three different water masses can be detected with salinity extrema at intermediate depths between the main pycnocline and the upper portion of the japan sea proper water ujspw the shallower salinity maximum and the middle salinity minimum can be identified as the tsushima warm current water twcw and japan sea intermediate water respectively the deeper salinity maximum water with high temperature and high dissolved oxygen is identified as high salinity intermediate water hsiw and has density similar to ujspw 27 30 27 33 isopycnal analyses indicate that hsiw flows southeastward from the siberian coast where twcw outcrops and has an opportunity to become denser by direct cooling we conclude that twcw is the origin of hsiw which in turn is a source for ujspw

formation of 26 8 26 9 σθ water in the kuril basin of the sea of okhotsk as a possible origin of north pacific intermediate water by using all japanese hydrographic data obtained south of 55 n in latitude in the sea of okhotsk we compile a data set for this study our investigations with these data show that a large quantity of relatively cold fresh oxygen rich homogeneous intermediate water with a density range of 26 8 to 26 9 exists in the kuril basin we name this intermediate water the kuril basin intermediate water kbiw results of isosteric analyses show that a possible origin of kbiw is located in a region off the hokkaido coast in the southwestern sea of okhotsk we suggest that kbiw is produced by the isopycnal mixing of relatively dense soya warm water which appears only in early spring with the colder fresher water which originates from the northwestern okhotsk finally we demonstrate that kbiw may be a source water of the north pacific intermediate water npiw because it has the same density range and water properties

the effect of water on the 410 km discontinuity an experimental study 1 in order to evaluate the effect of h on the pressure and transition interval of the olivine wadsleyite transformation synthesis experiments were carried out in hydrous and anhydrous peridotite compositions at 1400 c under hydrous near saturated conditions the pressure of the transition is 1 0 0 5 gpa lower than under anhydrous conditions in addition the two phase interval appears to broaden from about 0 4 gpa 12 km in the anhydrous system to as much as 1 3 gpa 40 km in the hydrous system under hydrous conditions the spatial distribution of olivine and wadsleyite appears to be controlled by h diffusion extrapolating previously measured diffusion coefficients for h in olivine and in liquids it appears that h diffusion driven by gravitational equilibration may serve to narrow the transition interval between olivine and wadsleyite

continuing evolution of the pacific juan de fuca north america slab window system a trench ridge transform example from the pacific rim abstract many subduction margins that rim the pacific ocean contain complex records of cenozoic slab window volcanism combined with tectonic disruption of the continental margin the series of slab windows that opened beneath california and mexico starting about 28 5 ma resulted from the death of a series of spreading ridge segments and led to piecewise destruction of a subduction regime the timing and areal extent of the resultant slab window volcanism provide constraints on models that depict the subsequent fragmentation and dispersal of the overlying continental margin the initial pioneer slab window thermally weakened the overlying western transverse ranges and california borderlands region starting about 28 5 ma a second thermal pulse occurred in the same region starting about 19 ma during growth of the monterey slab window this additional heating combined with the capture of a partially subducted monterey plate fragment by the cocos plate initiated the pulling apart and rotation of the adjacent continental margin similarly the capture of guadalupe and magdalena plate fragments by the pacific plate and initiation of the guadalupe magdalena slab window about 12 5 ma are coeval with baja california pulling away from the mexico continental margin with the break along the comondu arc in crust already thermally weakened by about 10 my of volcanism in coastal california distributed crustal extension and subsidence accompanied the new transform plate boundary and continued until the slab windows cooled and plate motion coalesced along a through going system of strike slip faults the transform boundary continues to evolve and forward modeling predicts an instability with the current configuration as a result of convergence between the sierra nevada and peninsular ranges batholiths starting about 2 my in the future the instability may be resolved by a shift in the locus of transform motion from the san andreas fault to the eastern california shear zone or by breaking off another fragment of the mojave or southern sierra nevada crustal blocks and translating it northward

temporal and spatial correlations between changes in plate motions and the evolution of rifted basins in africa we present the results of a backstripping study incorporating over 200 wells in 14 different basins or groups of basins located on the african interior or continental margins these results presented as mean tectonic subsidence rates averaged over appropriate periods are compared with the opening histories and changes in plate motions of the atlantic and indian oceans and the tethys mediterranean area for seven time steps from the late jurassic until present the results indicate a temporal and spatial correlation between phases of rapid subsidence in african basins and changes in plate motions and therefore are promising in terms of applicability of large scale plate motions in the studies of paleo stresses in sedimentary basins especially at times of initial rifting and formation of subduction zones reactivation of the basins appears to be primarily controlled by the orientation of the basin and underlying basement structures with respect to the stress direction the basins are preferably formed within weaker zones mostly pan african shear zones and the distance from the active plate boundary to the weak zone plays a minor role in whether or not it will be reactivated the presence of three important hot spots which were active during the early stages of break up of gondwana may have been important in preweakening the lithosphere at their sites this may have played an additional role in the localization of the basin deformation

ferricretes as a source of continental oolitic ironstones in northern sudan abstract in northern sudan lateritic ferricretes and oolitic ironstones occur the latter in both fluvial and marine sediments oolites and ferricretes display a close similarity such as a similar geochemical enrichment spectrum including v which is a key element to distinguish ferricretes from simple ferribands formed in hydromorphic soils in addition elevated al content of goethite ferricrete derived nuclei of ooids and the existence of continental oolites as link between weathering crusts and marine deposits point to a lateritic source of sedimentary iron oolites

diagenetic alteration of chamositic clay minerals to ferric oxide in oolitic ironstone abstract in the clinton oolitic ironstone silurian of pennsylvania ooids that were initially composed of chamositic clay minerals were diagenetically altered to ferric oxide evidence for this alteration includes the preservation of intermediate stages in a variety of partly altered ooids finely laminated chamositic ooids in marginal zones of ferric oxide storm beds bed parallel transitions of ooid compositions in bioturbated oolite and spastolithic shapes of ferric oxide ooids electron microprobe analysis demonstrates that this oxic alteration did not leave significant abundances of residual silicon and aluminum based on textural characteristics and on a permian age of remagnetization the most likely time for the alteration was during early phases of the alleghany orogeny recognitio that the ooids were initially chamositic clay minerals resolves at least part of the oxidation reduction paradox maynard 1986 for ferrous phyllosilicate could presumably accumulate in the low oxygen conditions associated with the medial silurian oceanic anoxic event

a sporadic low velocity layer atop the western u s mantle transition zone and short wavelength variations in transition zone discontinuities teleseismic receiver function analysis of data from six dense arrays in the western u s is used to investigate mantle transition zone mtz discontinuities and the prevalence of a low velocity layer atop the 410 km discontinuity 410 lvl negative polarity ps arrivals indicative of a low velocity layer with a top 25 60 km above the 410 are identified in 8 11 out of 18 stacks of receiver functions from highly sampled back azimuth corridors the 410 lvl is interpreted as partial melt resulting from upwelling of hydrated mantle across a water solubility contrast at the 410 the 669 km mean depth of the 660 km discontinuity 660 and the magnitude of 660 topography suggest variable hydration locally approaching saturation in addition to 150 k lateral temperature variations beneath five arrays mean amplitudes of p410s and p660s increase monotonically with period from 2 to 10 s however greater variations are observed in the frequency dependence of p410s compared to p660s implying 410 thickness is more heterogeneous variable 410 thickness is attributed to changes in hydration modulating the width of the olivine to wadsleyite transition interval frequency dependence of p660s amplitudes suggests a broad velocity gradient consistent with multivariate phase changes in the olivine and garnet systems sporadic detection of the 410 lvl the magnitude and length scales of mtz discontinuity topography and inferred variations in hydration support the occurrence of vigorous small scale convection in the western u s mantle comparison of receiver functions with body wave tomography suggests small scale convection driven by sinking slab segments and lithospheric instabilities contributes to the intermittent nature of the 410 lvl

geochemistry and origin of the şaphane alunite deposit western anatolia turkey ore geology reviews 26 2005 39 50 reply

a new perspective on origin of the east sea intermediate water observations of argo floats abstract the east sea intermediate water esiw defined as the salinity minimum in the east sea hereafter es sea of japan is examined with respect to its overall characteristics and its low salinity origin using historical argo float data from 1999 to 2015 our findings suggest that the esiw is formed in the western japan basin 40 42 n 130 133 e especially west of the north korean front in north korean waters where strong negative surface wind stress curl resides in wintertime the core esiw near the formation site has temperatures of 3 4 c and less than 33 98 psu salinity warmer and fresher than that in the southern part of the es in order to trace the origin of the warmer and fresher water at the sea surface in winter we analyzed the data in three different ways 1 spatial distribution of surface water properties using monthly climatology from the argo float data 2 seasonal variation of heat and salt contents at the formation site and 3 backtracking of surface drifter trajectories based on these analyses it is likely that the warmer and fresher surface water properties found in the esiw formation site are attributed to the low salinity surface water advected from the southern part of the es in autumn

iron oxyhydroxide and sulphide mineralization in hydrocarbon seep related carbonate submarine chimneys gulf of cadiz sw iberian peninsula this paper aims to investigate the iron mineralization hosted in the submarine hydrocarbon seep related carbonate chimneys from the gulf of cadiz continental slope sw iberia chimneys are made of a general fine groundmass of major fe rich dolomite ankerite and calcite small grains of quartz and phyllosilicates and some foraminifer tests and ostracod shells primary porosity is frequently generated inside foraminifer chambers and ostracod shells and is filled with closely packed microcrystals of iron oxyhydroxide and sulphide minerals forming framboids some extremely peculiar multiframboidal textures are also detected corresponding to microcrystal groups spherical framboids and euhedral crystals without framboidal texture the mineralogy observed is mainly goethite pyrite pseudomorphs traces of tiny grains of pyrite were also observed cubic octahedral and pyritohedral are the habits observed in both minerals chemical analyses of framboids and euhedral crystals display high often erratic amounts of as co ni and mo in oxyhydroxides and mo pb v and co in sulphides versus almost always negligible concentrations of cu and zn a textural geochemical and mineralogical evolution is proposed to explain the coexistence of different morphologies in the multiframboidal texture a growth and aggregation of microcrystals as typical framboidal type mineralogical associations b development of euhedral habits c coalescence and homogenization of the microcrystal into large size euhedral to anhedral crystals and d formation of euhedral crystals or polycrystalline masses with complete loss of framboidal texture along this process an increase of the concentration of fe s mn and ti linked to a decrease in mg si al as p ca and v was observed the study of the iron mineralization in the gulf of cadiz is a first and can give clues to understand the complex geobiological interactions in this and other similar extreme hydrocarbon bearing submarine ecosystems

multiple seismic discontinuities near the base of the transition zone in the earth s mantle the seismologically defined boundary between the transition zone in the earth s mantle 410 660 km depth and the underlying lower mantle is generally interpreted to result from the breakdown of the gamma spinel phase of olivine to magnesium perovskite and magnesiowustite laboratory measurements of these transformations of olivine have determined that the phase boundary has a negative clapeyron slope and does indeed occur near pressures corresponding to the base of the transition zone but a computational study has indicated that because of the presence of garnet minerals multiple seismic discontinuities might exist near a depth of 660 km ref 4 which would alter the simple negative correlation of changes in temperature with changes in the depth of the phase boundary in particular garnet minerals undergo exothermic transformations near this depth acting to complicate the phase relations and possibly effecting mantle convection processes in some regions here we present seismic evidence that supports the existence of such multiple transitions near a depth of 660 km beneath southern california the observations are consistent with having been generated by garnet transformations coupling with the dissociation of the gamma spinel phase of olivine temperature anomalies calculated from the imaged discontinuity depths using clapeyron slopes determined for the various transformations generally match those predicted from an independent p wave velocity model of the region

three dimensionality of slab detachment due to ridge trench collision laterally simultaneous boudinage versus tear propagation the detachment of subducted tectonic plates is a process that has been increasingly associated with collisional scenarios and the end of subduction in various locations worldwide in particular the propagation of slab detachment tearing of a subducting plate has been described in conceptual models as a cause for spatially and temporally progressing surface effects such as slab gap volcanism and uplift however there is little understanding of the causes and dynamics associated with three dimensional 3 d slab tearing especially in the case of ridge trench collision here we show using fully dynamic 3 d numerical models that the process of detachment due to ridge trench collision depends on the geometry of the ridge segments approaching the trench for a finite laterally symmetric slab the 3 d detachment process occurs nearly simultaneously along strike by way of boudinage type necking and opening of holes central to the slab for a case involving the approach of two offset ridge segments to the trench slab tearing occurs in the form of 1 a vertical propagating separation along the age offset boundary within the slab that was previously weakened by a transform weak zone and 2 horizontal propagating detachment controlled by lateral transfer of slab pull to adjacent surface plate segments however lateral decoupling between offset adjacent plate segments and the propagating nature of the vertical and horizontal tearing are dependent upon fracture zones remaining weak through the subduction zone whether detachment occurs simultaneously along strike or propagates laterally the process is controlled by plastic yielding of the slab interior when young lithosphere entering the trench can no longer support slab pull

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

the nature of the 660 kilometer discontinuity in earth s mantle from global seismic observations of pp precursors the 660 kilometer discontinuity which separates earth s upper and lower mantle has been detected routinely on a global scale in underside reflections of precursors to ss shear waves here we report observations of this discontinuity in many different regions using precursors to compressional pp waves the apparent absence of such precursors in previous studies had posed major problems for models of mantle composition we find a complicated structure showing single and double reflections ranging in depth from 640 to 720 kilometers that requires the existence of multiple phase transitions at the base of the transition zone the results are consistent with a pyrolite mantle composition

mode of extension and rifting history of upper tiburon and upper delfin basins northern gulf of california 1 the crustal structure of the northern gulf of california transtensional margin has been investigated by a 280 km long nw se profile including deep multichannel seismic reflection and densely sampled refraction wide angle reflection seismic information combined with gravity modeling the seismic and gravity modeling constrains two thinned crustal areas corresponding to the upper delfin and the upper tiburon basins on both sides of the profile toward the baja california peninsula and the mexico mainland a progressive thickening of the continental crust is observed our results indicate that the crustal thickness is 19 km below the coastline and it decreases to 14 and 17 km below the upper delfin and upper tiburon basins respectively in the area between both basins the crust thickens to 19 5 km there are significant lateral thickness variations for the different levels of the crust the interpreted structure is consistent with the existence of an aborted rift below the upper tiburon basin prominent dipping reflections in the multichannel data under upper tiburon basin and the ridge between upper tiburon and upper delfin basins can be explained as a mylonite like zone related to a detachment fault this interpretation suggests that the structural evolution of upper tiburon basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust the mode and locus of extension have evolved from a core complex in upper tiburon to a narrow rift mode in upper delfin basin

evidence of upper mantle processes related to continental rifting versus oceanic crust in the gulf of california summary receiver functions from teleseismic events recorded by stations around the gulf of california are used to map the upper mantle seismic discontinuities we observe a mean transition zone thickness comparable to the global average for most of the region a low velocity layer is detected above the 410 discontinuity that varies in thickness along the gulf of california the 660 discontinuity shows complex waveforms south of latitude 30 n as a result of the phase change of garnet to perovskite within the transition zone a complex behaviour of the receiver functions is observed mainly at the southern end of the gulf the north south variations of this zone are likely associated with a slab window at the northern gulf resulting from the cessation of subduction of the farallon plate 12 ma and the subduction of the guadalupe and magdalena microplates at the southern end resulting in a hydrated upper mantle our results suggest that change in rifting styles occurring along the gulf of california mirrors deeper processes in the upper mantle

ignimbrite flare up and deformation in the southern sierra madre occidental western mexico implications for the late subduction history of the farallon plate geologic mapping and structural data for the southern part of the smo demonstrating that most of this volcanic province was built in two episodes of ignimbrite flare up in oligocene 31 5 28 ma and early miocene 23 5 20 ma time and that extensional deformation occurred mostly before the transfer of baja california to the pacific plate extensive ignimbrite successions with 40 ar 39 ar ages clustering at 23 and 21 ma cover most of the southern smo thus correlating in age with ignimbrites exposed in southern baja california and central mexico grabens with a 020 to n s orientation developed in the east almost concurrently with this volcanic episode half grabens and nnw striking listric normal fault systems formed at the end of middle miocene as far as 150 km from the present coast a belt of left lateral transpressional structures formed along the southern boundary of the smo during the same period we link these magmatic and tectonic events to the evolution and dynamics of the farallon and north america plates during the miocene particularly we propose that a first detachment of the lower part of the farallon plate in early miocene time produced a transient thermal event and partial melting of the crust via mafic underplating middle miocene extension would be related to a second detachment event resulting from the slowing subduction that preceded the final capture of the magdalena microplate by the pacific plate at 12 5 ma transpression at the southernmost end of the smo occurred along the inland projection of the magdalenacocos plate boundary and may be explained by a difference in subduction rate and by a temporal convergence between the two plates in the eve of the end of subduction of the magdalena plate index terms 5480 planetology solid surface planets volcanism 8450 8150 tectonophysics evolution of the earth plate boundary general 3040 8109 tectonophysics continental tectonics extensional 0905 keywords ignimbrite flare up sierra madre occidental western mexico extensional tectonics slab detachment

in situ x ray diffraction study of post spinel transformation in a peridotite mantle implication for the 660 km discontinuity abstract we present the phase relations in anhydrous cao mgo feo al2o3 sio2 pyrolite to examine the influence of compositional difference between pyrolite and mg2sio4 on the post spinel phase transformation it is shown that in the pyrolite system the transformation occurs at about 0 5 gpa lower pressure relative to mg2sio4 we have carried out several in situ x ray diffraction experiments on ringwoodite to mg perovskite ferropericlase and backward transformations and found that post spinel transformation boundary can be expressed as p gpa 0 0005 t k 23 54 using the gold equation of state by tsuchiya t tsuchiya first principles prediction of the p v t equation of state of gold and the 660 km discontinuity in earth s mantle j geophys res 108 2003 doi 10 1029 2003jb002446 the interval for coexisting ringwoodite and mg perovskite was found to be 0 1 0 5 gpa the discrepancy between our data and the depth of the seismic discontinuity global average 654 km is about 20 km at 1850 k based on the results of in situ measurements we confirmed that the difference in chemical composition between pyrolite and mg2sio4 cannot modify the clapeyron slope of the post spinel transformation using experimental data and assuming the average mantle temperature 1850 k at 660 km we can account for only a half of variations in the depth of the 660 km discontinuity in subduction zones and at hot spots an additional explanation for the observed seismological variations at the 660 km discontinuity is required and may reflect influence of other minor components or volatiles

a change from warm humid to cold dry climate conditions recorded in lower barremian clay dominated continental successions from the se iberian chain ne spain abstract a combined facies and clay mineralogy analysis of the continental succession recorded in the lower part of the blesa formation early barremian oliete subbasin ne spain allowed us to specify the palaeoclimatic and palaeoenvironmental conditions under which this sedimentary record originated six outcrops including clays clays marls and dm thick interbedded palustrine lacustrine limestone levels were logged and sampled for x ray diffraction and optical and electron microscopy analysis the lower levels consist of red clays with abundant ferruginous pisoids 1 5 mm in size and macropisoids 5 mm whereas the intermediate and upper levels are ochre and violet clays marls with few or no pisoids the pisoids normally consist of a nucleus and a cortex made up of several layers some pisoids have a fragment of a previous pisoid acting as a nucleus indicating local re working processes both the nucleus and the cortex are formed by hematite goethite variable contents of kaolinite and minor proportions of ilmenite their texture and mineralogy show an in situ growth interbedded limestone beds within the clay and clay marl dominated levels are formed by micrite microsparite and sparite these limestones are cemented by kaolinite and they also have pisoids indicating the occurrence of edaphic processes the red clays have the lowest calcite content and the highest hematite anatase diaspore goethite and kaolinite content by contrast the ochre and violet clays marls have higher illite content smectite is only detected in the ochre clays marls this mineral association is characteristic of lateritic palaeosols formed under warm and humid conditions kaolinite and smectite textures indicate that they are authigenic whereas illite may be of detrital origin the upward decrease in kaolinite hematite anatase diaspore and goethite content along with the increase in quartz content and the presence of smectite and illite reflect a change from warm and humid conditions to drier and colder conditions during the early barremian in the studied area

the formation of fluvio lacustrine ferruginous pisoliths in the extensive palaeochannels of the yilgarn craton western australia abstract iron pisoliths and nodules are common within the fluvio lacustrine kaolinitic clays of tertiary paleochannels developed in the yilgarn craton of southern western australia deposition within these channels began in the late eocene with sediments comprised a fining upward sequence from sands at the base to lacustrine clay deposits above iron pisoliths are found randomly distributed in the clays in deposits more than 300 km apart the pisoliths are composed of goethite and hematite and minor quartz and clays they are 2 to 30 mm in diameter and formed of an inner nucleus surrounded by up to 200 iron oxide laminations there is no evidence for transportation of pisoliths into the channels but the internal fragmented nature of some of the pisoliths suggests some movement has occurred and multiple generations of pisoliths have developed the interface between river water and sediment was a redox boundary between reduced slightly acidic sediments and oxidised river water pisoliths formed on or just beneath the surface of the sediment by redox reactions likely mediated by microorganisms pisoliths have undergone cycles of exposure growth and movement linked to the height and course of the palaeoriver system

the effect of severe storms on the ice cover of the northern tatarskiy strait the tatarskiy strait is the northernmost region of the japan sea with an ice covered area in winter of about 4 104 km2 this study uses the daily passive microwave images of the region from the special sensor microwave imager ssm i which was launched in 1987 to estimate the production of ice and japan sea bottom water in winter the prevailing winds in the strait are northerly and cold these conditions create a region of reduced ice concentration in the northern strait which leads to an enhanced ice production throughout the season in addition to these prevailing winds each winter one or two severe storms generate very strong northerly winds and cold temperatures in the strait these storms create a large transient polynya in the strait at a time simultaneous with the greatest heat flux out of the open water calculation of the ice growth from open water in the northern strait yields about 25 km3 of ice per season of which severe storms provide about 25 the total ice production is sufficient to form about 5 12 102 km3 of the japan sea bottom water which is about 50 100 of the renewal rate required from 14c data because the oxygen bottom layer thickness decreased between 1969 and 1984 the study also investigates the frequency of the severe storms in the northern strait during 1966 1990 and finds that these storms are about 3 times as frequent at the beginning of this period than at the end this suggests the storms may play a role in the generation of the japan sea bottom water

petrographic microchemical studies and origin of the agbaja phanerozoic ironstone formation nupe basin nigeria a product of a ferruginized ooidal kaolin precursor not identical to the minette type the campanian maastrichtian agbaja ironstone formation of the nupe basin nigeria forms a major part of the about 2 billion tons of iron ore reserves of the middle niger embayment the ironstone deposits were previously reported to be similar to the minette type ironstones because of their depositional patterns composition and inferred origin four rock types are recognized within the agbaja ironstone formation ooidal pack ironstone pisoidal pack ironstone mud ironstone and bog iron ore in the ironstones kaolinite of both the groundmass and the ooids pisoids is of lateritic origin whereas the associated quartz mica and heavy minerals are of detrital origin ooids and pisoids were formed by mechanical accretion of platy kaolinite crystals by rolling on the sea floor in a near shore environment and were subsequently transported and deposited together with a fine grained kaolinitic groundmass pyrite mainly framboidal and siderite both exclusively occurring as pseudomorphs of goethite and or hematite are diagenetic whereas goethite is post diagenetic in origin resulting from the ferruginization of the kaolinitic precursor crandallite gorxeicite goyazite bolivarite and boehmite are also post diagenetic in origin hematite was formed from the dehydration of goethite whereas gibbsite restricted to the upper part of the deposit is of recent and in situ lateritic origin the presence of newly formed authigenic pyrite and siderite now replaced by hematite and goethite are indicators of a reducing environment during diagenesis the absence of diagenetic chamositic clay minerals evidently caused by a low mg concentration suggests that fully marine conditions were not established during sedimentation this is supported by the lack of fossils brecciated shell materials and bioturbation features in the deposit reworking and redeposition of the primary constituents are inferred from broken pisoids nuclei of pisoidal ooidal fragments in pisoids and high iron concentrations present in the pisoids and ooids compared to that of the groundmass these observations indicate that the agbaja ironstone deposits of the lokoja study area exhibit some environmental and mineralogical characteristics that are markedly different from other known deposits of minette type where primary chamositic clay minerals generally form the protore for the ironstones the recognition of kaolinite as the precursor constituent and the occurrence of similar deposits of the same age late cretaceous in nigeria sudan and egypt have implications for the paleoenvironmental interpretations of phanerozoic ironstone deposits