influence of periatlantic climates and paleoclimates on the distribution and mineralogical composition of bauxites and ferricretes

paleomagnetism of the front range colorado morrison formation and an alternative model of late jurassic north american apparent polar wander a paleomagnetic study of the upper jurassic morrison formation in the front range of central colorado yields high unblocking temperature dual polarity magnetizations with respect to known paleohorizontal the inclinations absolute mean 57 3 degree 95 confidence interval 52 3 degree to 63 1 degree n 8 sites pass tilt and reversal tests whereas the dispersion in declinations can be attributed to apparent or real tectonic rotations and sedimentary processes the site centered colatitudinal locus of possible front range morrison poles partially overlaps the upper pole but it excludes the lower pole from the morrison formation on the colorado plateau as well as the 151 ma glance conglomerate pole from the basin and range province of southeastern arizona the authors offer various explanations for these disparities and suggest an alternative model of late jurassic north american apparent polar wander through approximately 70 degree n which is supported by late jurassic european poles with positive stability tests transferred to north american coordinates

geochemical behavior under tropical weathering of the barama mazaruni greenstone belt at omai gold mine guiana shield abstract mineralogical petrographical and geochemical studies of the weathering profile have been carried out at omai au mine guyana the area is underlain by felsic to mafic volcanic and sedimentary rocks of the barama mazaruni supergroup part of the paleoproterozoic greenstone belts of the guiana shield tropical rainy climate has favoured extensive lateritization processes and formation of a deeply weathered regolith the top of the weathering profile consists of lateritic gravel or is masked by the pleistocene continental deltaic berbice formation mineralogical composition of regolith consists mainly of kaolinite goethite and quartz and subordinately sericite feldspar hematite pyrite smectite heavy minerals and uncommon mineral phases nacrite ephesite corrensite guyanaite a specific feature of the weathering profile at omai is the preservation of fresh hydrothermal pyrite in the saprolith horizon chemical changes during the weathering processes depend on various physicochemical and structural parameters consequently the depth should not be the principal criterion for comparison purposes of the geochemical behavior within the weathering profile but rather an index that measures the degree of supergene alteration that has affected each analyzed sample independently of the depth of sampling thus the mineralogical index of alteration mia can provide more accurate information about the behavior of major and trace elements in regolith as opposed to unweathered bedrock it can also aid in establishing a quantitative relationship between intensity of weathering and mobility leaching or accumulation of each element in each analyzed sample at omai some major and trace elements that are commonly considered as immobile ex tio2 zr etc during weathering could become mobile in several rock types and cannot be used to calculate the mass and volume balance in addition due to higher immobile element ratios the weathered felsic volcanic rocks plotted in identification diagrams are shifted towards more mafic rock types and a negative adjustment of 20 units is necessary for correct classification in contrast these elements could aid in defining the material source in sedimentary rocks affected by weathering generally the rare earth element ree patterns of the bedrock are preserved in the saprolith horizon this can represent a potentially useful tool for geochemical exploration in tropical terrains strong negative ce and tb anomalies are displayed by weathered pillowed andesites which are explained by the influence of the water rock ratio

the elkhorn mountains revisited new data for the late cretaceous paleomagnetic field of north america the late cretaceous portion of north america s apparent polar wander apw path is poorly known due to a dearth of suitable post 85 ma reference poles a paleomagnetic re investigation of elkhorn mountains volcanics 80 ma was initiated to fill this void and to determine the degree to which the onset of an apparent rapid episode of apw during this time interval correlates with known late cretaceous tectonic events a total of 249 paleomagnetic samples 30 sites were collected from three separate areas within the elkhorn mountains using both alternating field and thermal demagnetization a characteristic direction of magnetization was isolate at 15 sites a95 15 which passes a fold test at the 95 confidence level the mean pole calculated from site virtual geomagnetic poles is located at 80 3 n 189 5 e n 15 a95 9 6 k 16 8 interestingly the elkhorn mountains pole is not concordant with the cretaceous stillstand poles but is nearly coincident with the paleocene reference pole and the recently published pole for the adel mountains dated at 76 ma these new data suggest the onset of apw which brought the pole from its cretaceous position to near its early tertiary position occurred prior to 80 ma and the transition was exceedingly rapid the onset of this rapid apw now appears to coincide more closely in time with many of the significant late cretaceous tectonic events e g the laramide orogeny however since the elkhorn mountains volcanics are floored by a major east directed thrust and have undergone crustal scale deformation the data should be viewed with caution until more results are available for this time interval

paleomagnetism magnetic anisotropy and mid cretaceous paleolatitude of the duke island alaska ultramafic complex we report paleomagnetic results from layered igneous rocks that imply substantial post mid cretaceous poleward motion of the insular superterrane western canadian cordillera and southeast alaska relative to north america the samples studied are from the stratiform zoned ultramafic body at duke island which intruded rocks of the alexander terrane at the south end of the southeastern alaska archipelago at about 110 ma thermal and alternating field demagnetization experiments show that the characteristic remanence of the ultramafic rocks has high coercivity and a narrow unblocking temperature range just below the curie temperature of magnetite this remanence is likely carried by low ti titanomagnetite exsolved within clinopyroxene and perhaps other silicate hosts the duke island intrusion exhibits a well developed gravitational layering that was deformed during initial cooling but below 540 c into folds that plunge moderately to the west southwest the characteristic remanence clearly predates this early folding and is therefore primary the fisher parameter describing the concentration of the overall mean remanence direction improves from 3 to 32 when the site mean directions are corrected by restoring the layering to estimated paleohorizontal all samples exhibit a magnetic anisotropy that is strong but nonuniform in orientation across the intrusion and we show that it has no significant or systematic effect on the site mean directions of remanence at least some of the anisotropy derives from secondary magnetite formed during partial serpentinization the mean paleomagnetic inclination 56 10 corroborates paleomagnetic results from five coeval silicic plutons of the canadian coast plutonic complex to the south and southeast and implies 3000 km 1300 km of poleward transport relative to the north american craton between mid cretaceous and middle eocene time the insular superterrane and coast plutonic complex shared a common paleolatitude history with more poleward transport than coeval inboard terranes

geochronology of the monteregian hills alkaline igneous province quebec three distinct groups of alkaline rocks can be recognized on the basis of silica content in the monteregian hills quebec strongly undersaturated slightly undersaturated to critically saturated and oversaturated the first two groups were derived from basanite and alkali picrite magmas respectively and yield distinct ages of intrusion clustered about 118 ma and 136 ma the age data are not readily explained by the translation of the north american plate across a fixed mantle hotspot but because the alkali picrite magmas were derived from greater mantle depths than the basanite magmas the data do suggest an upward migration of the zone of melting this change in the depth of melting may be a response to the upward transport of a metasomatic fluid perhaps with a coupled rise in mantle isotherms

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

palaeoclimatic interpretation of clay minerals in marine deposits an outlook from the continental origin abstract as distribution of clay minerals in modern oceans seems to be controlled by contemporary climates marine clays in ancient deposits have been widely used to reconstruct palaeoclimates in this review we examine this question mainly from the continental origin of the clay minerals and show the difficulties of retrospective analysis of the influence of climate on common marine deposits we examine successively how soils develop and behave are reworked and palaeoweatherings are preserved focusing the review on kaolinitic soils indicative of tropical wet climates several stages intervene between the time of clay mineral development in soils and their final deposition in the basin 1 the persistence over geological times of huge amounts of kaolinitic palaeoweathering materials in the landscapes may seriously alter the palaeoclimatic signal of kaolinite in the sedimentary record although the kaolinite deposited in recent sediments of the world oceans is consistent with wet climatic zones this is mainly because the major kaolinitic and bauxitic palaeosurfaces coincide with the tropical areas where kaolinite is still developing at present 2 possible development of deep kaolinitic profiles at high latitudes and cold conditions under co 2 enriched palaeoatmospheres or even by unusual acidic weathering linked with dry climates is misleading palaeoenvironments for palaeoclimatic reconstruction 3 erosion and transport processes always cloud the signal to some extent because frequently erosion products from the bedrock and different soil horizons will be mixed and major drainage systems may include soils of different climatic zones 4 the arrival of the soil clay assemblages in the basin inevitably lags against their formation on the continent and may occur several million years after formation of the soils on the continent and climate may have considerably changed at that time it is pointed out that sedimentologists and palaeoclimatologists should consider the complexity of the relations between soil and sedimentary clay minerals soil formation rates are slow and therefore the resolution of the palaeoclimatic record in marine clay may not be closer than 1 or 2 ma at present the palaeoclimatic interpretations of marine clay assemblages are yielding at best nothing more than rather broad palaeoclimatic information this approach cannot achieve the same degree of resolution as other techniques such as isotope or microfossil studies clay mineral assemblages may provide integrated records of overall climatic impacts whereas other techniques are more likely to reveal local or temporary climates

global change and long term geomorphology landscape evolution is on the same time scale as global tectonics biological evolution and major climatic change some features of global change reflected in landscape evolution result from the breakup of pangaea others relate to major climatic changes and yet others to a major change from a dominantly plains landscape of the mesozoic to an increasingly mountainous landscape in the tertiary worldwide deep weathering profiles of cretaceous and early tertiary age suggest widespread planation and warm moist climates erosion of deeply weathered regolith following the formation of new continental margins and tectonic uplift led to the deposition of unusual mature sediments many river patterns can be traced to early tertiary or older antecedents some climatic changes are due to orbital forcing but others may be related to changing positions of continents creation of new seaways or to mountain building the timing of mountain uplift climatic changes and biological changes must be resolved to test models of their mutual interaction geomorphic input on tectonics drainage evolution and deep weathering will be an essential component in dealing with these problems

neogene weathering and supergene manganese enrichment in subtropical south china an 40ar 39ar approach and paleoclimatic significance deep weathering profiles and associated supergene mn oxides deposits are widely distributed in south china however data on their ages are not available until most recently ar 40 ar 39 laser incremental heating analysis of potassium bearing supergene mn oxides collected from four weathering profiles in the qinzhou fangcheng mn belt guangxi province south china provides for the first time numerical constraints on timing of weathering and supergene mn enrichment thirty eight of 42 cryptomelane grains yield well defined plateau 36 grains or pseudo plateau 2 grains ages ranging from 17 6 0 5 ma to 4 71 0 08 ma 2 sigma the results when combined with data from mn oxides deposits from the adjacent guangdong and hunan provinces document a protracted history of weathering and supergene mn enrichment spanning from the earliest miocene to the latest pliocene in south china this indicates that warm and humid climatic conditions conducive to intense weathering and secondary mineralization prevailed across south china during the whole neogene the climatic conditions inferred from weathering geochronology are consistent with sedimentary records and fossil floral and fauna associations from cenozoic inland and marginal sea basins of south china the weathering geochronology data also provide temporal constraints on the initiation and intensification of the east asian monsoon that has been the cause for the south china s humid climate c 2007 elsevier b v all rights reserved

supergene alteration since the upper cretaceous on alkaline igneous and metasomatic rocks of the pocos de caldas ring complex minas gerais brazil abstract this contribution presents field evidence mineralogical observations and geochemical data of different modes of lateritic weathering which affected the alkaline magmatic rocks and their metasomatic alteration products since the upper cretaceous field relations demonstrate that ferralitic bauxitic weathering is confined to well drained hill crest and upper slope settings in the pocos de caldas area bauxite normally rests directly on parent rock stagnant drainage and reducing conditions beneath tropical peat deposits along valley floors and in lower slope situations favour sialitic saprolitic weathering the additional importance of high porosity and permeability is demonstrated by co occurring parent rocks with strongly contrasting response on weathering attack in normal nepheline syenites and phonolites nepheline breaks down first and nearly instantly upon weathering this results in high porosity giving water and atmospheric gases easy access to the more resistant minerals thus in well drained morphological positions bauxite is generated if the same original magmatic parent rocks are shielded against pervasive weathering attack by pre lateritic metasomatic alteration no comparable early permeability arises a fine grained and dense metasomatic assemblage of kaolinite illite microcline and smectite combined with the annealing of fractures and joints is able to prevent bauxite generation and saprolite is formed instead this metasomatically induced saprolite is significantly richer in illite than normal saprolite bauxite formation results in a strong depletion of mg ca na k si ni co cu zn rb ba sr y and ree sialitic saprolitic weathering leads to a generally less pronounced depletion of essentially the same elements silicon and ni are more efficiently retained than in bauxite mobile elements are mn ca na k mg co zn rb ba sr and ree cerium can be re precipitated under oxidizing local conditions closely associated positive and negative ce anomalies are due to inhomogenously concentrated precipitates in small scale spatial alternation with zones of nearly total depletion a general correlation between the type of laterite saprolite or bauxite and ce retainment or depletion does not exist host phases for most of the immobile trace elements are goethite and related plasmas in bauxites and saprolites immobility is exhibited by v pb ga nb zr th and u relict zircon is too rare to explain a seemingly retarded hree loss in saprolites

flow under standing waves part 2 scour and deposition in front of breakwaters abstract a 3 d general purpose navier stokes solver was used to calculate the 2 d flow in front of the breakwater the k sst shear stress transport model was selected as the turbulence model the morphologic model of the present code couples the flow solution with a sediment transport description and routines for updating the computational mesh based on the mass balance of sediment laboratory experiments of scour also were conducted in a wave flume to obtain data for model verification both in the numerical simulations and in the laboratory experiment two kinds of breakwaters were used a vertical wall breakwater and a sloping wall breakwater slope 1 1 5 numerically obtained scour deposition profiles were compared with the experiments the numerical results show that the equilibrium scour depth normalized by the wave height decreases with increasing water depth to wave length ratio although the numerical results obtained for vertical wall breakwaters are consistent with the existing experimental data including the present experiment the numerical results for the sloping wall case appear to be not very satisfactory

testing models of the tertiary paleomagnetic field we examine selected paleomagnetic data of tertiary age in order to assess previously suggested models of the non dipole portion of the paleomagnetic field data from three localities on the african plate from dsdp leg 73 odp leg 115 and from gubbio indicate that the axial octupole contribution was near zero in the paleocene and was likely no greater than 5 6 of the axial dipole in the oligocene additional data from north america which can be linked to africa using well known relative plate motion parameters suggest that a positive axial quadrupole field has been present throughout the tertiary to investigate some of the practical consequences of these estimates we apply a non dipole field model 3 to 4 go go 0 g g l to paleomagnetic data from two odp sites on the african plate sites 710 711 and from the deccan traps we show that paleomagnetic directions from these deep sea sediments and from this flood basalt province appear consistent with a fixed hotspot model suggesting in particular that there has been little or no cenozoic drift of the rrunion hotspot 1 the paleomagnetic field configuration the application of paleomagnetic measurements to tectonic studies requires that the paleomagnetic reference field be accurately known usual practice in most tectonic studies invokes the dipole hypothesis to determine paleolatitude from the observed paleomagnetic directions observations from rocks that are only a few million years old generally support the dipole hypothesis the time averaged field configuration largely matches the field of a geocentric axial dipole gad however non dipole components can also be detected in the time averaged field 1 2 3 4 this long term non dipole field amounts to a few percent of the geocentric axial dipole and suggests that in detailed tectonic studies non dipole field effects cannot be neglected without consequence although these non dipole components of the mean field have been relatively small during the past few million years less than 5 of the gad larger non dipole fields may have existed during other intervals in earth history indeed two spherical harmonic studies of the time averaged field available for pre pliocene times 2 5 indicate significantly larger non dipole fields during parts of the tertiary and mesozoic amounting at times 0012 821x 90 03 50 1990 elsevier science publishers b v to 10 15 or more of the gad non dipole components of such magnitudes would lead to substantial systematic errors in paleomagnetic pole positions and thus could complicate tectonic interpretations and reconstructions considerably in this paper we attempt to test the reality of these prior indications of large non dipole contributions

longitudinal current induced by oblique waves along coastal structures the time and depth averaged equations in a porous medium are used to analyse how dissipation inside an infinitely long vertical breakwater generates an alongbreakwater ab current inside the structure this mean current is transferred seawards and leewards by turbulent diffusion based on a theoretical eddy viscosity law an analytical solution is obtained the mean current velocity profile depends on characteristics such as water depth wave height wave period and angle of incidence and on porous medium properties including porosity and friction in the neighbourhood of the structure the simultaneous action of waves and the ab current are taken into account to study the potential sediment transport patterns seawards of the structure the obtained results may help to explain the formation of bars almost parallel to the breakwater under certain circumstances the presence of the breakwater enhances the sediment transport in the zone closest to the seaward face which may induce scouring at the toe of the structure

test for bias in paleomagnetically determined paleolatitudes from pacific plate deep sea drilling project sediments paleolatitudes determined from paleomagnetic study of azimuthally unoriented sediment cores recovered by deep sea drilling on the pacific plate have been previously used in determining reference poles for the pacific plate apparent polar wander apw path from discordant paleoinclinations and corresponding paleolatitudes determined from these data some workers have inferred tectonic tilting or large horizontal motion of some deep sea drilling sites relative to other parts of the pacific plate here the reliability of these paleolatitudes is evaluated by comparing them with paleolatitudes determined from a pacific plate reference apw path determined from seamount poles skewness and amplitudes of magnetic profiles across magnetically lineated seafloor and paleocolatitudes from azimuthally unoriented basalt cores and from equatorial sediment facies the reference apw path includes a new determination of the early cretaceous pole from several types of paleomagnetic data which agree well with one another sediment paleolatitudes are found to be systematically less southerly than those predicted from the reference apw path a discrepancy that corresponds to inclinations shallower than expected thus most deep sea drilling sediment paleomagnetic paleolatitudes are unreliable and should be omitted from the data used in determining reference apw paths inclination and paleolatitude discrepancies noted in prior studies may be due not to tectonic causes but to shallowly biased inclinations

mid cretaceous paleomagnetic reference field for north america restudy of 100 ma intrusive rocks from arkansas we have obtained new paleomagnetic data from three middle cretaceous alkalic intrusive complexes in central arkansas magnet cove and potash sulfur springs intrusions about 100 ma 20 sites and granite mountain stock about 88 ma 6 sites out of 254 specimens measured 147 from 20 sites were accepted the remainders have scattered magnetization directions which experiments indicate were caused by lightning occurring during frequent electrical storms in this region the geologic evidence indicates that the intrusions have not been tilted since emplacement the directions of magnetization calculated with respect to present day horizontal yield a mean paleopole at 192 5 e 74 1 n n 20 k 34 a95 5 7 this does not differ at 95 confidence from that determined previously by scharon and hsu but the error is diminished by half the angular dispersion sd of site virtual geomagnetic poles vgps is 13 6 similar to the value of sd about 15 predicted by current paleosecular variation models the revised arkansas paleopole is combined with four other cratonic north american paleopoles of mid cretaceous age to yield a revised middle cretaceous reference pole at 196 e 71 n a95 4 9 the mean of nine paleopoles derived from overprints in rock units from the craton and craton margin which range in age from proterozoic to jura cretaceous is 202 e 68 n a95 8 2 this is not different from the reference paleopole indicating that overprinting was very widespread across north america in mid cretaceous time

phosphatized volcanic soils of fernando de noronha island brazil paleoclimates and landscape evolution abstract volcanic oceanic islands host soils that may hold important paleo environmental proxies based on vegetation and faunal evidences coupled with pedological and geomorphological indicators in this regard the main brazilian oceanic island of fernando de noronha fni still lacks a deeper and more complete analysis of its climatic history and soil evolution we select and collect representative fni soils to interpret their chemical physical and mineralogical properties and to explain their changing pedogenesis and geomorphic balance aiming at revealing the environmental and landscape evolutionary history of this island ten soil pedons on phonolites ankaratrites and the associated pyroclastic rocks were collected in june 2018 six profiles were classified as inceptisols one as entisol one as mollisol one as vertisol and another as the first oxisol ever described in a brazilian oceanic island the ph ranged between 5 0 and 6 0 in the more developed dystrophic and al saturated soils whereas alkaline phs were found in the eutrophic and na rich pedons for the latter group chemical and physical attributes are closely associated with the parent material composition whereas in the former present day soil features overprint paleoweathered regoliths and former pedogenetic processes high concentrations of available phosphorus p were observed in all soils and horizons even at depths greater than 150 cm and p forms were detected as amorphous or poorly crystalline phosphates al fe na ca and k high and low activity clay minerals coexist combined with gibbsite and unstable primary minerals indicating the polygenetic nature of fni soils a combined pedogeomorphological analysis indicated the existence of a high paleoweathered plateau with relict soils and erosional surfaces hosting younger soils results showed the succession of at least three distinct main paleoclimates since the pliocene and a post magmatic model for landscape evolution the unreported widespread and intense phosphatization of all soils and paleosurfaces is attributed to a past large population of birds nesting all over the island corroborating old historical records of the island s early occupation report the occurrence of brazil s first ornithogenic oxisol with more than 500 mg kg of bioavailable p

cerium anomalies in lateritic profiles the ree geochemistry and mineralogy have been studied in four lateritic profiles one derived from a syenite akongo sw cameroon the others being developed on a gneissic basement and located along a soil toposequence goyoum e cameroon there is a fractionation between lree and hree in the lateritic samples during weathering the weathered residual products being enriched in lree from la to eu and depleted in hree from gd to lu sampled waters are enriched in hree in relation to the syenite host rock a positive ce anomaly has been found systematically at the top of the saprolite beneath a zone of iron oxide accumulation in the laterite up to 2000 ppm ce may be present in the akongo profile cerianite ceo2 is present as very fine coatings in non ferruginous clayey domains primary ree bearing accessory minerals are weathered at the bottom of the profile specifically allanite and apatite are transformed into florencite and rhabdophane but these phases have no ce anomaly all the data are interpreted as the result of the following processes 1 1 ree leaching in a reducing environment 2 2 oxidation of ce3 to ce4 in an oxidizing environment 3 3 deposition of cerium as cerianite whereas the other ree remain in solution

paleomagnetism of middle late triassic plutons in southern maine fang wu and van der voo r 1988 paleomagnetism of middle late triassic plutons in southern maine tecionophysics 156 51 58 a paleomagnetic study of two small syenitic to alkali granitic plutons abbott and agamenticus southern maine yields paleopoles at 48o n 92 e and 48o n 99 o e with k ar ages of 221 f 8 and 228 5 ma respectively a third pluton the litchfield was much more weathered and did not contain stable magnetizations dual polarity magnetizations a dike contact test and excellent statistical parameters obtained after detailed thermal and alternating field demagnetization support our claim that the magnetizations and paleopoles are representative of the middle to late triassic paleomagnetic field however the two paleopoles fall about 10 to the southwest of previously published triassic apparent polar wander path segments for cratonic north america unless this small deviation can be ascribed to post triassic tilt of the plutons this requires a modification of the polar wander path to bring the poles into agreement a tilt correction of some 15 20 needs to be applied about a strike which is perpendicular to the area s geologic trends and the main coast line which appears to be highly unlikely given the stabilized tectonic setting of this area in post triassic time a small but significant revision of the north american cratonic apparent polar wander path may therefore be suggested by these results but this needs further corroboration the more southerly paleolatitudes indicated by these triassic results for north america would imply lesser displacements than previously proposed on the basis of triassic paleomagnetism for exotic terranes in the western cordillera

alkaline rocks from continental brazil abstract the present literature review on brazilian alkaline rocks stresses mainly the petrographic and geologic aspects at least eight different rock associations are represented agewise four groups are found among southern brazilian outcrops an older one about 240 m y only that is still poorly known the massifs clustered around jacupiranga about 130 m y old a few occurrences 100 110 m y old tunas itapirapua and the youngest and most important one with an age spread from about 80 m y to about 53 m y recent research shows a clear tectonic control both on local and on regional scales of alkaline occurrences in southern brazil alkaline magmatism was set off by pre drift and drift activity and is at its onset geologically related to basaltic activity emplacement of many massifs is also locally controlled by old lineaments e g in the minas gerais goias belt as is emphasized by geophysical data on a petrographic and chronologic basis several alkaline provinces are distinguished in southern brazil preliminary data on the cambrian precambrian rocks from southern bahia state a province with undersaturated syenites and litchfieldites and from cratonic areas in northern brazil are also presented

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

iron formations a global record of neoarchaean to palaeoproterozoic environmental history abstract iron formations if represent an iron rich rock type that typifies many archaean and proterozoic supracrustal successions and are chemical archives of precambrian seawater chemistry and post depositional iron cycling given that if accumulated on the seafloor for over two billion years of earth s early history changes in their chemical mineralogical and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving earth perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes the supply of nutrients to the oceans and ultimately diversification of the biosphere and complex life forms many of the key recent innovations in understanding if genesis are linked to geobiology since biologically assisted fe ii oxidation either directly through photoferrotrophy or indirectly through oxygenic photosynthesis provides a process for if deposition from mineral precursors the abundance and isotope composition of fe ii bearing minerals in if additionally suggests microbial fe iii reduction a metabolism that is deeply rooted in the archaea and bacteria linkages among geobiology hydrothermal systems and deposition of if have been traditionally overlooked but now form a coherent model for this unique rock type this paper reviews the defining features of if and their distribution through the neoarchaean and palaeoproterozoic this paper is an update of previous reviews by bekker et al 2010 2014 that will improve the quantitative framework we use to interpret if deposition in this work we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evolution of the oceans

two phases of cretaceous tertiary magmatism in the eastern desert of egypt paleomagnetic chemical and k ar evidence abstract paleomagnetic and radiometric data confirm the existence of two phases of igneous activity in the late mesozoic and cenozoic history of eastern egypt in the mid to late cretaceous the earlier and more extensive episode produced the wadi natash alkaline basalts and numerous small trachytic intrusions the ring complexes of abu khrug and gebel el kahfa were also emplaced at this time the mean paleopole position for the intrusive rocks is at 65 3 n 111 2 w a 95 11 4 31 sites chemical analyses of the trachytes show that they are enriched in nb zr and y and depleted in sr ba cr and ni suggesting either prolonged fractionation or limited partial fusion the second phase of activity during oligo miocene time is represented by small tholeiitic basalt flows on the red sea coast and along the nile valley the mean paleopole position for the flows is at 68 2 n 101 5 e a 95 27 8 3 sites the wide scatter is interpreted as reflecting secular variation the chronological and chemical data are used with studies of sedimentary formations coeval with the igneous rocks to apply a general model of continental rifting to the northern red sea the earlier alkaline phase of igneous activity is related to regional uplift while the second tholeiitic phase is associated with widening of the rift valley

detailed geochemical studies of the initial stages of weathering of alkaline rocks ilha de sao sebastiao brazil abstract detailed mineral and chemical studies of samples of syenite rocks with thick weathering rinds found in the alkaline massifs on ilha de sao sebastiao illustrate the complex interplay between variables controlling initial weathering processes an array of geochemical information was obtained by optical and electron microscopy x ray analyses mass spectroscopy and optical spectrometry sericite and metahalloysite are the principal phases nucleating and forming as the perthitic feldspar rock matrix disintegrates incipient feldspar dissolution and the formation of submicrometer x ray opaque secondary phases were observed in fresh rock material using transmission electron microscopy pore waters from which secondary phases nucleate and grow are under the constant influence as alteration proceeds of physical e g accessibility of mineral surfaces to percolating fluids and chemical e g chemistry of minerals providing the dominant solutes controls minor and trace element signatures do not vary significantly except for zn rb sr y ree and pb lathanide mobilities appear related to their host mineral stabilities with respect to ambient pore waters

true polar wander an analysis of cenozoic and mesozoic paleomagnetic poles a true polar wander path is presented for the past 180 ma tracing the relative motion between the hotspot framework and the geomagnetic poles deduced from worldwide paleomagnetic data a relative displacement of 22 10 is observed the motion between the two reference frames has not been smooth episodes of rapid motion on the order of 80 100 mm yr include the present 65 50 ma 115 85 ma and 180 160 ma periods of slow or little polar motion less than 20 mm yr are observed at 50 5 ma and 160 115 ma the effect of plate motions is removed using morgan s plate motion model a19 this model is tied to a hotspot reference frame assumed to be fixed in the mantle the rotated paleopoles are grouped by ages and then averaged they are found to cluster about points distinct from the north pole as defined by the hotspots these points trace out a path of motion between the rotation axis and the hotspots or mantle this is interpreted as a shifting of the entire earth in response to a change in the principal axes of the moment of inertia of the mantle paleomagnetic poles of the past 7 m y are examined independently this eliminates a dependence on the fixed hotspot hypothesis since plate motions are essentially negligible for this time span there is a deviation of 5 2 between the present geographic north pole and the average of the paleopoles at 5 ma this is too great to attribute to plate motion interpreted as true polar wander the rate of this polar motion is about 1 m y astronomical measurements for the past 80 years have recorded actual motion of the earth s rotational axis up to 1 m y in the direction of eastern canada this agrees in rate with paleomagnetic polar motion of the past 7 m y and in both rate and direction for the past 2 m y

an energetics total load sediment transport model for a plane sloping beach bagnold s energetics based total load sediment transport model for streams is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed in both the bedload and suspended load the transport rate vectors are found to be composed of a velocity induced component directed parallel to the instantaneous velocity vector and a gravity induced component directed down slope the model is applied to idealized surfzone conditions leading to estimates of the local longshore and onshore offshore sediment transport rates as well as the equilibrium beach slope as a function of the local wave and current conditions the model is combined with a nonlinear longshore current model and spatially integrated to obtain predictions of the total longshore transport rate as a function of the incident wave conditions the results support the general form of the wave power equation except that the wave power coefficient is no longer constant but is instead a complex function of the incident wave and beach characteristics

hotspots in the southern oceans an absolute frame of reference for motion of the gondwana continents abstract the geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time that is motion of an order of magnitude less than the relative motion between plate pairs in most cases it is concluded that inter hotspot movement cannot be discerned for the period 100 m y to present and that widely distributed hotspots in the atlantic and indian oceans provide a frame of reference for plate motions following the disintegration of gondwanaland which is independent of paleomagnetism this frame of reference is absolute in that it gives the motion of the lithosphere with respect to the mantle hotspots the absolute motion model indicates that africa and antarctica are now moving only very slowly that there has been significant relative movement between east and west antarctica since the cretaceous and prescribes the relative motion between the somali and african plates

40ar 39ar ages for plutons of the monteregian hills quebec evidence for a single episode of cretaceous magmatism 40ar 39ar data for biotite and amphibole from six plutonic complexes of the monteregian petrographic province in quebec canada are presented generally uncomplicated and concordant incremental heating spectra are observed although there is evidence for minor excess 40ar at several localities biotites and amphiboles are concordant consistent with rapid cooling of the high level complexes ages for different phases of individual complexes are analytically indistinguishable and suggest that the various principal lithologies are cogenetic the 40ar 39ar ages of six monteregian complexes st hilaire rougemont johnson shefford brome megantic all fall within the very restricted range of 124 1 m y it is suggested that all of the monteregian plutons with the exception of oka were formed within a short interval 1 2 m y during the cretaceous consistent with paleomagmetic results the new ages along with geochemical characteristics support derivation of the alkaline magmas by a fixed mantle hot spot which also produced broadly similar complexes in new england and the chain of new england seamounts

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

paleomagnetism of the mount stuart batholith revisited again what has been learned since 1972 we have collected 20 new paleomagnetic sites from the mount stuart batholith and the adjacent beckler peak stock using thermal and low temperature demagnetization and rock magnetic tests we have found that the remanence in most of the batholith is carried by single domain magnetite the mean of the new mount stuart batholith sites is d 354 2 i 46 2 k 87 2 a95 4 6 n 11 and is similar to those of the beck and noson 1972 and beck and others 1981 studies examination of ar geochronology of hornblende and biotite from the mount stuart batholith finds that the southern part of the batholith cooled through the blocking temperature of magnetite at 91 ma the northern part cooled through the blocking temperature of magnetite at 86 ma and of pyrrhotite at 83 ma from these combined results we conclude that the paleomagnetic directions from the southern part of the batholith were acquired within 0 5 to 1 0 ma of the time at which the ah barometers cooled below their closure temperature use of the ah barometry to establish paleohorizontal for these rocks is thus well justified correcting the mount stuart direction for the tilt indicated by the aluminum in hornblende ah barometry the mean becomes d 355 9 i 50 6 this result places the mount stuart batholith at a latitude of 31 3 3 8 3 4 n at 91 ma by identifying the carrier of remanence in the mount stuart batholith as being single domain sd magnetite we have removed the major remaining uncertainty in interpreting the paleomagnetism of these rocks the characteristic magnetization repeatedly found in the mount stuart rocks clearly supports the microplate tectonic model of beck and noson 1972 presently manifest as the baja british columbia baja bc hypothesis

paleomagnetism of 122 ma plutons in new england and the mid cretaceous paleomagnetic field in north america true polar wander or large scale differential mantle motion a paleomagnetic study of cretaceous white mountains plutonic complexes in new hampshire and vermont yields high unblocking temperature dual polarity magnetizations in different types of igneous rocks the resulting pole position for three plutons 71 9 n 187 4 e a95 6 9 age 122 5 ma agrees with previously published mid cretaceous poles for north america which together give a mid cretaceous standstill reference pole slightly revised from globerman and irving 1988 at 71 2 n 194 1 e a95 3 7 n 5 studies we argue on the basis of the wide geographic distribution of these studies the variety in tectonic settings and rock types positive reversal tests and an overall reversal pattern consistent with geomagnetic polarity time scales that this mean pole represents the north american mid cretaceous reference field for nominally 36 m y 124 to 88 ma the standstill pole limits to within 4 the motion of the north american plate relative to the earth s spin axis during the same mid cretaceous interval the new england hotspot track 124 ma monteregian hills 122 5 ma cretaceous white mountains and 103 to 84 ma new england seamounts requires 11 4 of north poleward motion of north america in direct conflict with the paleomagnetic standstill a similar 13 discrepancy is independently demonstrated between the spin axis and the tristan da cunha hotspot track on the african plate during the mid cretaceous interval the hotspot spin axis discrepancies ended by 90 ma when it is shown that both atlantic hotspots agree with north american and african dipole paleolatitudes and present day locations nondipole fields are an unlikely explanation of the uniform motion of these two widely separated hotspots with respect to the spin axis leaving as possible interpretations true polar wander and large scale but differential mantle motion the southerly motion of the mid cretaceous louisville hotspot relative to the spin axis is ostensively at odds with what would be predicted under the true polar wander interpretation and points to differential mantle kinematics the motions of the three widely separated mid cretaceous hotspots with respect to the spin axis may be related to the recently proposed increase in global oceanic lithosphere production rates which gave rise to the mid cretaceous superplume

sediment transport patterns in front of reflective structures under wind wave dominated conditions abstract this article presents a field study of the bed response near linear reflective structures under wind wave dominated conditions at the motril harbor spain on the mediterranean coast in 1999 the harbor breakwater was enlarged 600 m by adding vertical caissons before this enlargement took place no significant bed features had been observed in the area however at the end of 2004 high resolution bathymetry showed the existence of 3 4 bars parallel to the structure with lengths between crests of 0 70 m in the cross shore direction and heights of 0 25 cm toe erosion with an amplitude up to 25 cm was also observed along the structure the necessary conditions for the appearance of the bar system are 1 a standing wave pattern in front of the structure 2 local satisfaction of the wave height and wave period threshold limits for sediment mobility using an energy based approach we analyze the time series of sea states with sediment transport capacity after breakwater construction the cumulative distributions of the shear velocities related to wsw sw sea states show that all the bars have a greater probability of being n type features predominant bed load under generalized sediment transport conditions these conclusions were verified by matching nodal lines of standing wsw sw wave patterns with the bar locations the results suggest that the motril bar system of motril is possibly generated by the standing wave patterns induced by several sea states with peak periods of o 10 s and significant sediment transport capacity resulting from wsw sw severe storm events that occurred in motril after breakwater construction this article highlights the need to verify in field conditions the present threshold limits for sediment transport mobility

apparent polar wander of the mean lithosphere reference frame summary apparent polar wander in the mean lithosphere no net rotation no net torque uniform drag reference frame is compared with apparent polar wander in the hotspot reference frame over the past 100 myr palaeomagnetic poles and plate rotations previously used to determine an apparent polar wander path for the hotspot reference frame are here used to determine an apparent polar wander path in the mean lithosphere reference frame we find that the two paths are similar especially for late cretaceous time when a 10 20 shift of the pole occurred to first order the hotspots and lithosphere as a whole moved in unison relative to the palaeomagnetic axis during late cretaceous time a non dipole field explanation for the apparent shift can probably be excluded however either motion of the time averaged geomagnetic axis relative to the spin axis or polar wandering could have caused this shift the latter being the more likely explanation

palaeomagnetism and k ar ages of the south west african basalts and their bearing on the time of initial rifting of the south atlantic ocean summary palaeomagnetic and isotopic results from the kaoko lavas hoachanas basalts and dolerite sills of south west africa indicate that the upper triassic lower jurassic stormberg flows of south africa may have extended into sw africa and that younger igneous events of lower cretaceous age were simultaneous with the serra geral volcanism in brazil five analyses on three samples of the keetmanshoop sills gave k ar ages between 178 4 and 199 4 ma four analyses of two samples of the hoachanas basalts gave ages between 161 3 and 173 2 ma and eight analyses of five samples of kaoko basalt gave ages between 110 4 and 128 2 ma the components of remanent magnetization rm used to compute palaeomagnetic pole positions for the kaoko lavas 48 n 93 w a95 3 and for the hoachanas basalts 61 n 106 w a95 7 are stable to alternating field af and thermal demagnetization correlation on a pre drift map and on a map reconstructed for 112 ma bp before present between the palaeomagnetic poles from the kaoko and serra geral lavas suggests that the south atlantic had not opened appreciably by 112 ma bp cretaceous pole positions for s america and africa on a map reconstructed for 80 ma bp are also discussed

comments on the blag model factors affecting atmospheric co 2 and temperature over the past 100 million years since publication of the blag model several new items have appeared that bear on the question of paleo atmospheric co 2 levels first there is a problem between recent estimates in the literature of cretaceous sea floor spreading rates being similar to today and many reports of much more abundant cretaceous volcanism second the temperature dependence of weathering assumed in the blag model is in agreement with present data on the relation between temperature and average riverine silica concentrations third increased worldwide organic carbon burial rates and increased land volcanism during the cretaceous can be used to infer increased co 2 uptake via rock weathering at that time finally new model calculations show that the simple extrapolations of the blag model give a paleotemperature versus co 2 relation that is in agreement with the new model results within the probable error of paleotemperature estimates modified journal abstract

rare earth element mobility during conversion of nepheline syenite into lateritic bauxite at passa quatro minais gerais brazil abstract in a lateritic bauxite formed by weathering of nepheline syenite at passa quatro minas gerais state brazil bauxites on the hill tops directly develop from the syenite bed rock while downslope a kaolinitic layer occurs between bauxite and synetie a petrological investigation was performed on undisturbed weathered rock samples collected from a representative upslope pit the undisturbed weathered rocks were chemically analysed for major trace elements including ree and zr mass balance calculations were applied and the behaviour of the ree in the passa quatro weathering system was established compared to ree reference chondrite and to ree reference parent rock in the lateritic bauxite the results suggest that the first stages of weathering induce a volumetric change of 50 i e collapse with respect to the parent rock and remove ree with a slightly larger loss of the lree except ce compared to the hree in the upper layers where bauxite is more mature a net mass gain in ree is observed relative to the underlying layers this gain takes place during the reduction of the upper layer during the downward progression of the weathering front very significant ree losses occurs during the bauxitization processes throughout the upslope profile in addition the downslope kaolinitic system is demonstrated to be depleted in ree in the same proportions as the upslope bauxite we proposed that the ree exported in solution from the whole weathering mantle have enriched neighbouring watershed sediments

paleomagnetism of mesozoic kimberlites in southern africa and the cretaceous apparent polar wander curve for africa including those previously published good paleomagnetic data have been obtained from 48 of 51 sites in 28 separate kimberlites radiometric dates by one or more methods are available for 17 of these bodies and range from 81 to 145 ma with maxima around 90 and 120 ma these ages correlate with geochemical contrasts distinguishing what have been designated group i younger and ii older kimberlites smith et al 1985b stable magnetization appears to be associated with magnetite developed deuterically after emplacement but in most bodies secular variation has not been completely averaged in the remanence acquired the distribution of vgps calculated is distinctly but not sequentially streaked yet correlates broadly with the older and younger petrochemical types mean north poles calculated on this basis yield kp1 n 14 81 100 ma 64 1 n 226 1 e a95 5 2 and kp2 n 10 113 145 ma 47 6 n 269 9 e a95 9 7 with the exception of one undated pipe meltonwold which was not included in the averages and the melilititic male fissure at swartruggens in group ii average all the kimberlites sampled are normally magnetized including these new data 10 of 17 published african cretaceous poles meet acceptability criteria specified and define an apparent polar wander apw curve from 130 to 75 ma the curve appears to consist of two segments trending latitudinally before 100 ma and meridionally thereafter the latter trend is evident even in the distribution of the individual kp1 poles and may be related to the true polar wander speculated by gordon and livermore 1987 to have occurred in that interval

paleogeographic maps of the continents bordering the indian ocean since the early jurassic paleomagnetic data from the african plate from the early jurassic onward are critically re evaluated and updated resulting in a set of 30 paleopoles from which an apparent polar wander path is derived using 20 ma intervals this path is then used in conjunction with kinematic models of the indian and atlantic oceans to place other continents bordering the indian ocean eurasia india islands and plateaus madagascar seychelles mascarene and blocks involved in the collision of india with eurasia south tibet indochina in an absolute paleogeographic frame fixed with respect to africa paleogeographic maps are presented for the early jurassic pre breakup the jurassic cretaceous boundary anomaly ml5 the early cretaceous m2 the late cretaceous a 34 the cretaceous tertiary boundary a 29 the early eocene a 24 the middle eocene a 21 and the oligocene miocene boundary a 6 observed and predicted paleomagnetic directions for landmasses other than africa i e for western europe china korea india and australia which are not used in deriving the maps are found to be in very satisfactory agreement the quality of reconstructions as measured by rms discrepancies between observed and predicted paleolatitudes is of the order of 5 the main features of the paleogeographic maps are discussed with the aim of relating oceanic and on land geodynamic events the constraints provided by tibetan data on the india asia collision and the 20 ma reorganization of geology and plate boundaries in a band extending from the south china sea through tibet to the red sea system are discussed in more detail we emphasize the episodic nature of the breakup of gondwanaland and formation of the indian ocean and subsequent interaction with eurasia in which six major discrete events are identified separated by 30 ma phases of rather uniform plate motion

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

orebody vertical structure and implications for ore forming processes in the xinxu bauxite deposit western guangxi china abstract the xinxu bauxite orebodies which are contained in quaternary depressions overlying strata of permian to devonian age were formed through the break up oxidation and re sedimentation of a pre existing permian bauxite horizon with progressive karsitification the underlying strata of the quaternary orebodies were eroded out downwards in sequence gradually transiting from permian to devonian thus an older underlying stratum generally infers a higher degree of weathering a total of 47 exploration wells sampled each meter were chosen from several orebodies to evaluate the vertical distribution of four major components al 2 o 3 fe 2 o 3 sio 2 and loss on ignition in the ores the vertical evenness is quantified via a box gliding multifractal model the small width of the multifractal spectrum suggests that the vertical ore quality is even overall indicating an intertwining break up and mixing up processes during evolution of the quaternary orebody although the four components show complex spatial distributions topographic elevation displays a positive relationship with the mean fe 2 o 3 content and a negative correlation with the vertical unevenness of al 2 o 3 and fe 2 o 3 in the wells this is taken to reflect the more uniform oxidizing environment in the higher depressions which facilitates a homogeneous and greater increase of fe 2 o 3 content throughout the orebody in contrast the redox conditions in lower depressions are less oxidizing and varied in space inducing great variation in degree of fe 2 o 3 increase in the ores this study also demonstrates that in the xinxu deposit the ore forming redox environment has more impact on the ore quality than the degree of weathering as the contents show irregular vertical changes in the lower depressions it follows that it is the ancient redox environment rather than the modern conditions which affect the orebody vertical structure this clarifies the fact that the vertical structure of the orebody is jointly constrained by the physical melange accumulation of ores and the differences between the ancient redox environments in the higher and lower locations these factors should be the primary factors affecting the vertical structure of karst bauxite deposits

age progressive volcanism in the new england seamounts and the opening of the central atlantic ocean radiometric ages k ar and oar 39ar methods have been determined on dredged volcanic rocks from seven of the new england seamounts a prominent northwest southeast trend ing volcanic lineament in the northwestern atlantic ocean the oar 39ar total fusion and increnental heating ages show an increase in seamount construction age from southeast to northwest that is consistent with northwestward motion of the north american plate over a new england hot spot between 103 and 82 ma a linear volcano migration rate of 4 7 cm yr fits the seamount age distribution these ages fall within a longer age progression from the corner seamounts 70 to 75 ma at the eastern end of the new england seamounts to the youngest phase of volcanism in the white mountain igneous province new england 100 to 124 ma the new england hot spot estimated from the new radio metric ages and motion of north america in the hot spot reference frame to be near 28on 33ow probably generated a short line of mid to late tertiary age seamounts on the african plate but appears to be presently inactive the hot spot reference frame is used to calculate the motion of the north american plate away from the african plate from early cretaceous time to the present rominent magnetic anomalies recorded in the central atlantic seafloor give the positions of the spreading ridge for a bange of known ages hot spots that now underlie the african plate in the eastern central atlantic new england canary madeira cape verde azores could have produced cretaceous seamount and island chains on the north american plate during the early opening of the central atlantic each of these hot spots has been overridden by spreading ridges at pre dictable times some of these hot spot crossings are expressed as geochemical anomalies in the oceanic crust now far removed from spreading ridges

ripple and sandbar dynamics under mid reflecting conditions with a porous vertical breakwater abstract this research is an experimental study of ripple and sandbar dynamics under regular and random waves in partially reflective conditions as part of this study a series of small scale flume experiments were performed that reproduced the growth and migration of the bedforms starting from a flat bed or rippled bed with sediment transport in the bedload regime the results showed that the evolution and dynamics of sandbar geometry were slower processes than the evolution of ripples moreover they were governed by the wave field reflective conditions and sediment characteristics sandbar generation was controlled by the intensity of reflection whereas the location of the crests or deposition and erosion areas was constrained by the phase shift of the reflected waves significant differences were also found between sandbars under regular and random waves sandbars under regular waves showed flat or practically flat troughs in contrast sandbars under random waves were almost uniformly covered by ripples the experimental results showed that the concurrence of ripples and sandbars under partially reflected waves has a spatially modulating effect on ripple characteristics i e growth shape and migration celerity which could not be consistently interpreted by using the classical formulas valid for ripples under progressive waves and or without large scale bedforms this variability was more pronounced for regular waves than for random wave trains larger ripples develop in the nodes of the free surface envelope more or less corresponding to the sandbars crests whereas smaller ripples occurred in the antinodes or sandbars troughs the statistics of ripples geometry and celerity were computed with a sample stratification based on their position in reference to the sandbars in addition they were compared in two energetically equivalent tests with regular and random waves respectively although ripples under random waves had a larger wavelength and height than ripples under regular waves the celerity of migration was comparable our results showed that the sandbars modified the equilibrium geometry of ripples furthermore because of roughness streaming was induced by the highest and longest ripples in the sandbar crests the spatial modulation of the ripple celerity was found to be related to the local lagrangian mass transport velocity which was produced by the quasi standing wave inside the bottom boundary layer at the grain diameter scale

plate motions and the geomagnetic field ii jurassic to tertiary summary the analysis of the time averaged geomagnetic field is extended back to 200ma palaeomagnetic poles from the major plates have been carefully selected from recent compilations of reliable results for each region these were returned with their corresponding sampling sites to their locations at the estimated dates of magnetization in a fixed hotspots framework the corrected results were then grouped into 20ma windows at intervals of 10ma representing the past 100ma and 40ma windows at 30ma intervals for the more sparse 100 200ma data global means and fisher statistics were calculated for each window having included the axial quadrupole in the calculation the value of this coefficient which gave the maximum value for the fisher precision parameter tightest grouping of poles was taken as representative of each interval the results indicate that a small axial quadrupole of the same sign as the axial dipole may have persisted throughout the cenozoic this is equivalent to a northward offset axial dipole field far sided effect during the late cretaceous this component appears to have changed sign with respect to the dipole negative values seem to have obtained throughout the cretaceous long normal polarity interval corresponding to a southward offset dipole near sided effect the data distribution is inadequate for the resolution of the quadrupole at earlier times and zero values cannot be discounted little relative motion is implied between the hotspots and the geomagnetic axis for the past 90 ma the global mean polar path curving around the predicted fixed hotspots pole at a distance of typically 5 latitude with little sign of rapid tertiary polar wander as implied by studies of pacific data alone between 100 and 200 ma however there is a clear difference between the two reference frames amounting to 17 19 in the jurassic this may reflect motion of the mantle relative to the geomagnetic axis but may also include errors due to inaccurate determination of hotspot tracks and inter hotspot motion

palaeomagnetism of the mesozoic serra geral formation southern brazil abstract palaeomagnetic results from 20 volcanic sequences and 11 intrusive bodies sills and dykes of the serra geral formation parana basin are reported in this paper the sequences are widespread all over the basin while sills and dykes ponta grossa arch come from the northeastern portion three mean palaeomagnetic poles were computed for the serra geral formation which account for a time interval of 15 ma pole sg1 is located at 85 s 108 e 95 1 1 n 18 and represents the main phase of the magmatic activity in the basin with a mean age of 135 ma pole sg2 is located at 82 s 38 e 95 7 8 n 2 and represents a younger magmatic phase with an associated age of 130 ma pole sg3 72 s 37 e 95 6 8 n 10 is the youngest pole it is computed from the intrusive rocks and its age is assumed to be not younger than 118 ma the lower limit of the cretaceous normal magnetic interval these three poles describe a shifting path which suggest that the south american platform moved 5 southwards and rotated 10 clockwise during the lower cretaceous preceding the south atlantic opening

discrimination between different water bodies from a multilayered aquifer kaluvelly watershed india trace element signature in the multilayered aquifer of kaluvelly india comprising various sedimentary layers overlying a charnockitic basement concentrations of trace elements were measured in aquifer formations and in groundwaters to identify geochemical tracers for water bodies the two main sandstones cuddalore and vanur originate from the charnockites and the cuddalore sandstone has experienced lateritization in the studied area two charnockite end members were identified a dioritic and a granitic one mineralogical composition and whole rock ti concentrations confirmed the charnockite which displayed the granitic composition as the parent rock of the two sandstones titanium distribution indicates that the cuddalore sandstone originates from a more intense weathering of the parent material than the vanur sandstone despite extensive differences in trace element contents recorded in aquifer formations only a few trace elements were suitable to distinguish the water bodies among soluble elements li in the vanur aquifer and ba in the charnockite and carbonaceous aquifers can be used as tracers as the input of these elements in solution is mainly regulated by the available stock for a given mineralogical origin there is a direct link between the aquifer formation composition and water signature with the exception of as concentrations of redox sensitive elements were not preserved during pumping because of oxidation preventing their use as tracers low mobility elements such as la ce th zr nb hf or ta were too insoluble to be detected in waters and or to record the aquifer formation signature their input in solution was not regulated by the available stock but by the dissolution rate of rock forming minerals only ti can be used to distinguish between two out of the three aquifers charnockite and vanur the specific behavior of ti recorded in these waters may be linked to rutile inclusions within plagioclases and to the influence of climate on ti solubility

bars bumps and holes models for the generation of complex beach topography in shallow water any two waves of the same frequency are shown to produce complex patterns of drift velocity above the sea bed if the longshore components hi h of the wave numbers of the two waves are different these steady flow patterns exhibit a longshore periodicity of wave number hi h irrespective of whether the waves propagate in the same direction say hi he positive or in opposite directions he negative the interaction of two edge wave modes is examined in detail the drift velocities are calculated and a simple sediment transport model is used to predict the beach topography that would be in equilibrium with these flow patterns as expected crescentic sand bars are produced by the special case of standing edge waves hi h intriguingly for all other cases a combination of complex transverse bars plus meandering or straight offshore bars result patterns that are surprisingly reminiscent of many published descriptions of complex rhythmic topography the extension of the model to three or more waves produces topography that appears to be very irregular although the pattern should repeat over sufficiently long distances along the beach even with only three waves these distances may be very large compared to the scale of the bars

eogenetic diagenesis of chinle sandstones petrified forest national park arizona usa a record of late triassic climate change fluvially derived tuffaceous chinle sandstones from petrified forest national park provide a well preserved late triassic archive of climate information petrographic analysis of 38 chinle sandstones provides new insight into the depositional history and evolution of palaeoclimate during chinle deposition this study focuses on the relationship between climate and meteoric diagenesis as a guide for constraining climate change in western equatorial pangea during the late triassic petrographic analysis of chinle sandstones reveals their wide range of textural attributes as well as pedogenic and shallow burial diagenetic features that occurred during the late triassic these diverse petrological characteristics are indicative of the evolving late triassic climate when placed into a well constrained stratigraphic and geochronological framework the stratigraphic succession is characterized by variations in the abundance of framework grains detrital matrix weathering intensity of feldspar and volcanic rock fragments and the mineralogy of clay cements climate records from chinle palaeosol geochemistry indicate a progression from wet to dry conditions this trend is also reflected in the meteoric diagenetic features of chinle sandstones during deposition of the lower chinle elevated rainfall promoted the weathering of labile volcanic detritus to kaolinite whereas feldspars especially plagioclase were partially or completely dissolved in the upper chinle a trend towards drier conditions favoured the formation of smectite and less feldspar dissolution resulting in a higher abundance of well preserved plagioclase grains shallow burial meteoric weathering reactions in chinle sandstones reflect the evolving climate during the late triassic

a plate kinematic framework for models of caribbean evolution abstract we define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the caribbean region newly determined poles of rotation defining the approximate spreading histories of the central north and the south atlantic oceans are given for the late jurassic early cretaceous anomaly sequence of the central north atlantic we have used previously published definitions of fracture zone traces and magnetic anomaly picks redetermining the pole positions and angular rotations for various isochrons on an evans and sutherland interactive graphics system for magnetic anomalies younger than the cretaceous quiet period in both oceans we 1 used seasat altimeter data to help define fracture zone traces and 2 identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones by the finite difference method the relative paleopositions and the relative motion history between north and south america were computed this analysis defines the size and shape and the rate at which the size and shape changed of the interplate region between north and south america since the middle jurassic thus a plate kinematic framework is provided for the larger plates pertaining to the caribbean region in which can be derived more detailed scenarios for gulf of mexico and caribbean evolution north and south america diverged to approximately their present relative positions from late triassic to early campanian about 84 m y ago time this is the period during which the gulf of mexico and a proto caribbean seaway were formed since the campanian only minor relative motion has occurred from early campanian through to middle eocene times south america diverged only another 200 km and since the middle eocene minor n s convergence has occurred these very minor post early campanian motions have probably been accommodated by imperfect shear and compression along the atlantic fracture zones to the east of the lesser antilles and along the northern and southern borders of the caribbean plate accordingly it is suggested that from campanian time to the present the relative motions between the north and south american plates have had only minor effects on the structural development of the caribbean region primarily using the data of engebretson et al the convergence history of pacific plates with north america was calculated for two points near the western caribbean by completing finite difference solutions the convergence history of the pacific plates with the caribbean and south american plates can be approximated the direction and rate of convergence of the pacific plates with the americas may have controlled the style of subduction and possible microplate migration along the north american south american and western caribbean boundaries that define the eastern pacific plate margin