rare earth elements of carbonate rocks from the bambui group southern sao francisco basin brazil and their significance as paleoenvironmental proxies abstract rare earth elements plus y rey data obtained for carbonate rocks of the bambui group in the southern portion of the sao francisco basin match the c and sr isotope variations that subdivide the unit in three chemostratigraphic intervals ci 1 to ci 3 the lower ci 1 samples exhibit flat rey shale normalised distributions that record a high freshwater input in the basin after a glacial period the subsequent marine transgression diminished the continental influence and initiated the development of la gd and y positive anomalies in the ci 2 samples interestingly a seawater like rey pattern is present in the ci 3 samples which correspond to the closure of the sao francisco basin rather than open marine conditions the ci 3 records a decrease in the silicate chemical weathering of sources located on the marginal belts of the sao francisco craton due to higher denudation rates and a change in the dissolved influx towards carbonate rocks the resultant high concentration of carbonate ions in the restricted sea induced a rey fractionation process similarly to that of the modern ocean constant positive ce ce values throughout the shallow and deep sections suggest anoxic early diagenetic conditions inefficient circulation and stratified water column which could have worked as a barrier for life diversification in the basin at the ediacaran cambrian limit finally our data show that the rey concentrations of ancient biochemical environments are not an unequivocal tool either to track open and restricted marine settings or to distinguish proximal and distal environments

lacustrine fan deltaic sedimentation in a proterozoic rift basin the sopa brumadinho tectonosequence southeastern brazil abstract the sopa brumadinho tectonosequence 100 to 200 m thick is the fourth unit in ascending order of the up to 2000 m thick mesoproterozoic espinhaco supergroup representing the depositional products of the third and main phase of the rift stage of the espinhaco basin the sopa brumadinho tectonosequence in the study area consists of diamond bearing lacustrine fan delta deposits bounded by prominent angular and erosional unconformities detailed lithofacies studies were carried out in the central part of the southern serra do espinhatco southeastern brazil to characterize the paleogeography and basin fill architecture of the sopa brumadinho tectonosequence thirteen lithofacies are present massive clast supported conglomerates lithofacies ug cs inversely graded clastsupported conglomerates lithofacies ig cs normally graded clast supported conglomerates lithofacies ng cs massive to crudely stratified clast supported conglomerates lithofacies cs cs sandy matrix supported conglomerates lithofacies s ms muddy matrix supported conglomerates lithofacies m ms talus breccia clast supported conglomerates lithofacies tb cs massive sandstones lithofacies sm horizontal stratified sandstones lithofacies sh trough cross stratified sandstones lithofacies st planar cross stratified sandstones lithofacies sp graded stratified sandstones lithofacies sg and pelites lithofacies p they represent the products of several types of subaerial and subaqueous sediment gravity flows ranging from turbidity currents to cohesive debris flows and ephemeral braided streams the deposits are usually arranged in coarsening to fining upward tectono depositional intervals each tectono depositional interval represents a progradational lobe which starts at the base with lacustrine pelites and evolves with time from distal subaqueous to proximal subaerial characteristics the fining upward part represents the abandonment phase of the depositional lobes the origin characteristics distribution and spatial arrangement of the various lithofacies the interbedding of gravityflow and stream flow deposits the predominance of debris flow conglomerates over aqueous current formed ones the semi radial fan like dispersion pattern of the paleocurrents and the recognition of associated subaerial and subaqueous depositional settings are all indicative of a lacustrine alluvial fan fan delta environment or a fan delta system an interplay between several intrabasinal and extrabasinal controls probably determined the fan evolution the sedimentation of the lacustrine fan delta systems of the sopa brumadinho tectonosequence probably took place in a n s elongated continental rift basin compartmentalized by n s trending normal faults and w e trending transfer faults under a tropical semi arid climate the basin fill architecture was probably determined by episodic subsidence clockwise block tilting and asymmetric graben evolution as well as by inherited characteristics of the source area additionally a new descriptive scheme for classification of conglomerate lithofacies is here proposed

the record of the varanger glaciation at the rio de la plata craton vendian cambrian of uruguay abstract a sedimentary succession included in the lower section of the playa hermosa formation from the playa verde basin uruguay is described two facies associations one mainly coarse to medium grained and other one fine grained have been defined fa i ii in the first one breccias conglomerates sandstones and minor mudstones were deposited in a subaqueous depositional setting proximal suggesting slope instability and resedimentation processes the second one contains diamictites rhythmites sandstones and mudstones and presents abundant evidence of soft deformation also interpreted to be deposited in a subaqueous environment distal dropstones clast layers diamictites rhythmites and varve like deposits are interpreted as ice rafting processes generated during a glacial episode this glacial related succession constitutes the first record from the varanger glaciation at the rio de la plata craton of the late neoproterozoic age and also represents one of the oldest sedimentary records after the collision of the rio de la plata and kalahari cratons a combined interaction of extensional faulting and glaciation in a tectonically active basin with locally high subsidence rates resulted in high rates of sedimentation and resedimentation processes as a whole the sedimentary succession sets a relevant datum to be used in future paleogeographic reconstructions of the vendian glacial record in southern south america

the formation of longitudinal fronts in a coastal plain estuary longitudinal estuarine fronts aligned parallel to the axis of the estuary are characteristic of the york river their time scale for genesis and decay is of the order of hours they are usually located at the inner edge of the shoals or over the main channel field measurements have shown marked intratidal differences in density and current speeds across this estuary these can be correlated with changes in depth and the strength of the advective processes differential advection between the channel and shoal regions when acting upon a constant longitudinal density gradient is shown to be of sufficient magnitude to generate strong lateral density gradients and thus fronts at particular times within the tidal cycle

aragonite crystal fans in neoproterozoic cap carbonates a case study from brazil and implications for the post snowball earth coastal environment abstract neoproterozoic cap carbonates are characterized by anomalous sedimentary features including sea floor precipitates represented by aragonite pseudomorph cement crusts and crystal fans these features are found in restricted periods of geological history and their generation in the context of the neoproterozoic has been attributed to very specific postglacial conditions leading to alkalinity oversaturation new sedimentary and geochemical results on well preserved neoproterozoic deposits of aragonite pseudomorph crystal fans at the base of the sete lagoas formation bambui group central brazil are reported the micrite settling facies shows the highest abundance of crystal fans arranged in centimeter scale layers interfingered with the micrite matrix this facies is interpreted as a result of postglacial calcium carbonate oversaturation in restricted areas characterized by marine dysoxic waters a numerical model constrains the kinetics of formation of aragonite crystals or micrite by calculating the induction times and precipitation rates of both carbonate species for postglacial conditions of elevated temperature and high atmospheric pco2 in these conditions the mg ca ratio of the neoproterozoic seawater is calculated at ca 1 2 alkalinity oversaturation is monitored by a variable evaporation degree aragonite formation is kinetically favored and the discrepancy between calcite and aragonite precipitation rate is greater as atmospheric pco2 hence alkalinity is higher these modeling conditions do not preclude the contribution of incomplete organic matter degradation to alkalinity as suggested by negative carbon isotope signatures in addition the large number of aragonite micrite pairs suggests a seasonal or paleoclimatic forcing other cases of abiotic aragonite precipitation through time are also briefly examined by comparison

residual circulation in shallow estuaries 1 highly stratified narrow estuaries long wave theory and simple turbulence closures have been used to show that three distinct types of circulation highly stratified weakly stratified and partially mixed arise in narrow shallow estuaries from the finite amplitude of the tide and the interaction of stratification with vertical mixing each type has a different dominant process causing the vertical exchange of salt and fresh water on the flood and each gives rise to a characteristic residual circulation the tidal circulation in highly stratified shallow estuaries described herein is the result of a finite amplitude internal motion driven by the barotropic tide shear instabilities at the interface are the major vertical exchange mechanism the residual circulation is caused primarily by ebb flood asymmetry in interface position and thickness a model based on our theoretical analysis shows that the interface thickness on flood is much less than the depth of flow up to a critical tidal amplitude at which the two layer flow is destroyed and correctly predicts the tidal amplitude of the neap spring transition in the columbia river estuary tidal frequency internal wave motion is not found in weakly stratified and partially mixed estuaries because of the reduced tidal shear shear instabilities are weak or absent and the residual circulation assumes a very different character as described in part 2

the neoproterozoic glacial record in the rio de la plata craton a critical reappraisal neoproterozoic glacial successions have been described in south america but the glacial deposits of the rio de la plata craton have been neglected in previous studies addressing the global distribution of glacially influenced successions the rio de la plata craton contains neoproterozoic glacial deposits in the sierra del volcan formation tandilia system argentina glacial influenced deposits in the playa hermosa and zanja del tigre formations dom feliciano belt uruguay and suspected glacially influenced deposits in passo da areia sao gabriel block brazil the tandilia system glacial record includesdiamictites dropstonesand rhythmitesdepos ited in glaciomarine conditions in a tectonically stable depositional setting the dom feliciano belt includes a thin section with ice rafted clasts in carbonates and a thicker section containing diamictites rhythmites outsized clasts and deformed beds in a volcano sedimentary succession the sao gabriel block occurrence deserves more attention to confirm any glacial influence in the fine grained part of the succession glaciation is considered to be contemporaneous with the gaskiers glaciation 580 ma with the exception of the carbonates with dropstones that may represent a previous event correlative with one of the glaciations described in the kalahari craton prior to kalahari rio de la plata assembly in the proto western gondwana margin

low frequency sea level variability in the vicinity of the east river tidal strait the east river is a narrow tidal strait connecting long island sound with the lower hudson estuary the sound and estuary both communicate with the new york bight subtidal fluctuations in sea level difference between the ends of the strait which are coherent with barotropic flow through the strait are produced mainly by direct setup in the sound and estuary and by sea level fluctuations primarily wind forced in the bight approximately 70 of the variance in subtidal sea level difference can be accounted for by the two components of wind stress the direction of dominant wind forcing wind stress direction producing maximum response varies considerably with frequency response functions relating sea level difference to wind stress components computed for 128 day series for sea level and winds allow us to interpret the direction of dominant forcing in terms of the relative contributions of direct setup and sea level fluctuations in the bight

on the nature of transverse variability in a coastal plain estuary even though estuarine processes are inherently three dimensional most of the previous studies have focused primarily on the axial longitudinal and vertical variabilities recent surveys in the lower delaware bay indicate the presence of significant transverse variability in the salinity distribution and residual circulation pattern conceptual models are developed to examine the mechanisms which might explain the observed transverse structure results indicate that both the gravitational effect and wind forcing are capable of generating substantial transverse structure in drowned river valley estuaries with triangular cross sections

ages of protolith and neoproterozoic metamorphism of al p bearing quartzites of the veredas formation northern espinhaco brazil la icp ms age determinations on relict and recrystallized zircon and geodynamic consequences abstract the n s running espinhaco fold belt situated in the sao francisco craton in central eastern brazil separates this craton in a western and an eastern part along the paramirim corridor a brasiliano panafrican 650 500 ma deformation zone the meta sediments of the espinhaco basin consist of several synthems with dominantly siliciclastic deposits among them al and p rich lazulite bearing blue quartzite the amphibolite facies equivalent of a phosphatic sandstone of the veredas formation in the northern espinhaco form a conspicuous horizon zircon crystals from these quartzites have been dated using laser ablation inductively coupled plasma mass spectrometry the zircon crystals are both detrital and metamorphic in origin the latter exhibiting highly unusual textures produced by dissolution reprecipitation reaction and concomitant replacement of metamict areas ree y th u zircon fluid zircon xenotime detrital zircons reveal relict age groups of 3 1 3 3 ga 2 6 2 7 ga 2 0 2 3 ga and 1 5 ga the end of sedimentation and erosion of the magmatic and metamorphic rocks of the sao francisco craton can be estimated therefore as 1 5 ga the major metamorphic event recorded by reprecipitated zircon is neoproterozoic and documented by a lower intercept age of 634 19 ma which was calculated from analyses of individual spots on19 grains with a common relict orosirian rhyacian age of 2032 31 ma temporal equivalent to transamazonian the neoproterozoic age of the metamorphosed zircons of the vereda formation corresponds to the brasiliano panafrican remobilization of the sao francisco west congo craton along the eastern side of the espinhaco fold belt in the paramirim corridor

the effects of channels and shoals on exchange between the chesapeake bay and the adjacent ocean the role of bathymetric changes in determining the transport of water and salt in the lower chesapeake bay lcb was investigated using high resolution acoustic doppler current profiler adcp and conductivity temperature depth profiles a cross channel transect was repeated eight times during neap tides on october 6 7 1993 which illustrated the lateral structure of the longitudinal and transverse flow fields and the intratidal variations in the flow structure across the lcb amplitude and phase of the m2 tidal component as well as the mean flow velocity were calculated using least squares fitting at every point of a uniform grid obtained from the adcp data the results differ from the classical two layer pattern of estuarine circulation modified by coriolis effects but are consistent with recent hydrographic observations semidiurnal flow was highest over the navigational channels and lateral gradients were strongest in regions of sharp bathymetric changes the phase lag of the semidiurnal flow also showed lateral and vertical gradients that represented advances at the bottom with respect to the surface and over the shoals in relation to the channels the section of the water column measured indicated a mean outflow of 0 7 104 m3 s and a mean inflow of 1 3 104 m3 s the apparent gain of water by the estuary during the period of observation can be explained by meteorologically forced net barotropic inflow the depth averaged mean longitudinal flow consisted of inflow in the navigational channels and outflow over the shoals the mean transverse flow showed near surface convergence over the channels we propose a possible explanation for the observed flow and density structure as follows the barotropic and baroclinic forcing interact with the bathymetry to extend the inflow from the bottom to the surface thereby inducing a transverse circulation that yields near surface convergence over the channels

plate tectonics and the evolution of the british isles this paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics with illustrations particularly from the geological history of the british isles there is special emphasis on the mechanics of continental lithosphere in extension when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts an upper lithosphere stress concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid the very nature of plate tectonics as relative motion among a multi plate mosaic of plates provides a rational semi quantitative framework for large scale geological complexity in space and time the caledonian and hercynian history of the british isles may be explained semi quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions the rationalism of plate tectonics is exhibited on a smaller more direct field scale by many of the complexities of ophiolite complexes such as ballantrae scotland that can be explained as the effects of the evolution of ridge ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the southern uplands deformational and sedimentary patterns in nw europe from triassic times to the present day resulted from a complex interaction of extensional events related to diachronous atlantic opening and compressional events related to sequential alpine shortening phases those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences because huge areas much larger than the british isles must be analysed and synthesized before much sense can be extracted further difficulties arise from the palinspastic and preservation problems of convergent boundaries orogenic shortening telescopes and partially subducts facies belts most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened elongated and straightened to an apparent but deceiving simplicity therefore while plate tectonics provides a rational geological framework against which for example british basin evolution can be tested and explained we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively

neoproterozoic peritidal phosphorite from the sete lagoas formation brazil and the precambrian phosphorus cycle the neoproterozoic sete lagoas formation ca 610 ma of the sao francisco basin brazil is a succession of siltstone limestone and phosphorite phosphorite forms part of a previously unrecognized 150 to 200 m thick unconformity bounded depositional sequence lithofacies stacking patterns indicate that deposition was punctuated by higher order fluctuations in base level that produced aggradational parasequences these shallowing upward cycles record the progradation of phosphate rich intertidal flats over shallow subtidal deposits as accommodation filled the presence of mudcracks authigenic chert nodules lack of coarse terrigenous clastics and the abundance of silt with fine abraded quartz grains suggests accumulation along an arid coastline with significant aeolian input delivery of phosphorus adsorbed on aeolian fe oxyhydr oxide and clay is interpreted as having stimulated phosphogenesis in peritidal environments lithofacies associations indicate that windblown phosphorus promoted the establishment of cyanobacterial communities along the coast which produced photosynthetic oxygen and the suboxic conditions necessary for the precipitation of authigenic carbonate fluorapatite as in other precambrian phosphatic systems nearshore oxygen oases were a prerequisite for phosphorite accumulation because redox sensitive phosphogenic processes were pushed into the sediment to concentrate phosphorus in more distal anoxic environments phosphorite could not form because these biotic and abiotic processes were suspended in the water column which cycled phosphorus in sea water rather than at the sediment water interface such shallow water phosphorite is unlike larger younger neoproterozoic phanerozoic phosphatic deposits inferred to have formed in deeper upwelling related environments the increasing size of phosphatic deposits through the latest precambrian is interpreted as reflecting the progressive ventilation of the oceans during the neoproterozoic oxygenation event and resultant expansion of phosphogenic environments into distal settings the widespread cycling of bioavailable phosphorus at the sea floor not only produced the first true phosphorite giants but may have also been an important precondition for the evolution of multicellular animals

extensional collapse in the neoproterozoic aracuai orogen eastern brazil a setting for reactivation of asymmetric crenulation cleavage the aracuao orogen of eastern brazil is one of many brasiliano pan african orogens formed during the neoproterozoic assembly of gondwana its western edge bordering the sao francisco craton is the serra do espinhaco fold thrust belt in which top up to the west reverse sense faults west verging folds f1 and east dipping spaced to phyllitic cleavage s1 developed we have found that the kinematics of deformation changes markedly at the hinterland margin of this fold thrust belt here beneath a plateau known as the chapada acaua metadiamictite and fine grained pelitic schist comprise an east dipping belt that contains an assemblage of structures indicative of top down to the east normal sense shear this assemblage includes a cascade of f2 folds that refold f1 folds and verge down the dip of the belt s enveloping surfaces vertical tension gashes and top down to the east rotated clasts based on the presence of these structures we propose that the plateau exposes a regional scale normal sense shear zone here called the chapada acauashear zone casz because f2 folds refold f1 folds normal sense shear in the casz occurred subsequent to initial west verging thrusting considering this timing of motion in the casz we suggest that the zone accommodated displacement of the internal zone of the aracuao orogen down relative to its foreland fold thrust belt and thus played a role in extensional collapse of the orogen the casz trends parallel to preserved thrusts to the west and thus may represent an inverted thrust fault notably throughout the casz s1 schistosity has been overprinted by a pervasive west dipping asymmetric crenulation cleavage s2 the sigmoid shape of s1 surfaces in s2 microlithons require that slip on each s2 surface was top down to the west s2 cleavage is axial planar to the down dip verging f2 folds based on its geometry we suggest that s2 cleavage initiated either as an antithetic extensional crenulation cleavage during reverse sense shear or as a near vertical asymmetric crenulation cleavage formed during east west shortening of a preexisting east dipping schistosity subsequent normal sense shear in the casz reactivated this cleavage causing clockwise rotation of s2 domains as viewed looking along strike to the north in a manner similar to that of rotational bookshelf faults such movement could have accommodated concomitant vertical flattening of the casz during extensional collapse

sequence stratigraphic framework of proterozoic successions in eastern brazil abstract first order regional unconformities separate proterozoic supracrustal rocks of the gas bearing sao francisco craton and surrounding thrust and fold belts eastern brazil into four genetic stratigraphic units which constitute 1st order basin fill sequences espinhaco canastra paranoa vazante macaubas and bambui sequences the espinhaco 1st order sequence ca 1 73 1 50 ga records a stage of aborted lithospheric stretching of the sao francisco congo continental mass the espinhaco basin displays a steer s head geometry where four 2nd order sequences are recognized prerift rift transitional and flexural third order rift propagation unconformities allowed the definition of three 3rd order synrift sequences within the rift 2nd order sequence the 2nd order flexural sequence of the espinhaco basin can be subdivided into three 3rd order transgressive regressive sequences the canastra paranoa vazante and macaubas 1st order sequences ca 1200 630 ma comprise rift to drift successions deposited during the breakup of the supercontinent rodinia and the opening of the brazilide and adamastor oceans respectively to the west and east of the sao francisco paleocontinent according to seismic data the passive margin canastra paranoa vazante 1st order sequence can be divided into two 2nd order sequences rift and passive margin the passive margin 2nd order sequence can be further subdivided into three 3rd order sequences the outcropping canastra paranoa vazante sequence consists mainly of shallow marine deposits which contain black shales marls with intervals showing high total organic carbon toc contents comprising one of the highest potential hydrocarbon source rocks of the sao francisco basin the bambui foreland 1st order sequence ca 750 550 ma formed as a consequence of thrust loading related to shortening in the brasilia fold belt on the western flank of the sao francisco craton during the closing of the brazilide ocean and the assembly of the west gondwana supercontinent the bambui sequence shows two distinct successions which reflect the main basin paleogeographic settings foredeep and flexural ramp three transgressive progradational 2nd order sequences can be recognized in the ramp succession of the bambui basin

sequence stratigraphy and chemostratigraphy of an ediacaran cambrian foreland related carbonate ramp bambui group brazil abstract in the terminal neoproterozoic drastic climate changes associated with biological innovations are coupled to isotope and elemental geochemical anomalies however lateral variability and local depositional controls may affect global geochemical signals which can only be tracked through a proper stratigraphic paleogeographic assessment here we investigate the sequence stratigraphy and chemostratigraphy of the basal units of the bambui group central east brazil this stratigraphic unit records a foreland basin system developed during the ediacaran cambrian west gondwana assembly and represents a 1st order sequence in which the two lowermost 2nd order sequences record major geochemical disturbances the first 2nd order sequence started with the deposition of a transgresive systems tract possibly in a postglacial scenario which accompanies a negative to positive 13ccarb excursion the early highstand systems tract represents the establishment of a marine carbonate ramp throughout the basin in terms of chemostratigraphy it corresponds to a 13ccarb plateau close to 0 and sr ca ratios around 0 001 the late highstand stage coincides with a remarkable increase in sr content and sr ca ratios at basinal scale occurrences of the cloudina sp late ediacaran index fossil were reported in this stage an erosional unconformity associated with a dolomitic interval locally including subaerial exposure features marks the top of the first 2nd order sequence this sequence boundary heralds an abrupt increase in 13ccarb values up to 14 these extremely high 13ccarb values and high sr ca ratios persist throughout the overlying sequence as a result of progressive and enhanced restriction of the foreland basin system basin restriction at this stage has implications for the paleontological and chemostratigraphic record of epicontinental basins of the west gondwana in the terminal ediacaran late ediacaran sr rich intervals in these basins show unusually non radiogenic 87sr 86sr ratios which may represent local depositional controls and deviations from the modern oceanographic models physiographic barriers and stressful conditions likely represented extreme environments for metazoan colonization

correlation of neoproterozoic terranes between the ribeira belt se brazil and its african counterpart comparative tectonic evolution and open questions

bathymetric flow rectification in a tropical micro tidal estuary abstract temperature salinity and current velocity data were recorded during the two principal atmospheric seasons affecting the tropical micro tidal estuaries in the western gulf of mexico tropical micro tidal estuaries in the western gulf of mexico share many characteristics among them a narrow connection between the estuary and the adjacent continental shelf therefore samples were taken during the dry april 2009 and the rainy july 2009 seasons in order to elucidate the effect of the narrow connection between the estuaries and the continental shelf on flow dynamics due to the similarities between the tropical micro tidal estuaries located in the western gulf of mexico the jamapa river estuary a shallow estuary with a narrow connection with the continental shelf and a 5 m wide navigational channel was used as a case study during the dry season april 15 17th 2009 there was a surface horizontal displacement of the salinity and the temperature front of 2 km while during the rainy season july 22 24th 2009 the salinity and temperature gradients were mainly observed in the vertical at 1 m depth in this particular case there was a marked difference between the northern and southern part of the estuary due to the presence of a second river discharge arroyo moreno which discharges always stays in the northern part of the estuary shallow area owing to the influence of the navigational channel on fluid dynamics finally a cyclonic recirculation was observed at the estuary mouth area according to model outputs the recirculation was observed when the ratio between the mouth and the estuary width were below 0 4 otherwise 0 4 the recirculation was not observed this should be a general behavior for all tropical micro tidal estuaries located in the western gulf of mexico

neoproterozoic glacial deposits from the aracuai orogen brazil age provenance and correlations with the sao francisco craton and west congo belt article i nfo glacigenic diamictite successions of the macaubas group are widespread in the western domain of the aracuai orogen east of the sao francisco craton brazil diamictites also occur on this craton and in the african counterpart of the aracuai orogen the west congo belt detrital zircon grains from the matrix of diamictites and sandstones from the macaubas group were dated by the u pb shrimp technique the geochronological study sets the maximum depositional age of the glacial diamictites at 900 ma and indicates multiple sources for the macaubas basin with ages ranging from 900 to 2800 ma sm nd tdm model ages determined on whole rock samples range from 1 8 ga to 2 5 ga and get older up section comparison of our data with those from the cratonic area suggest that these glacial deposits can be correlated to the jequitai and carrancas diamictites in the sao francisco craton and to the lower mixtite formation of the west congolian group exposed in africa the 900 1000 ma source is most probably represented by the zadinian mayumbian volcanic rocks and related granites from the west congo belt however one of the most voluminous sources with ages in the 1 1 1 3 ga interval has not been detected in the sao francisco congo craton possible sources for these grains could occur elsewhere in africa or possibly from within the brasilia belt in western central brazil

dynamics diffusion and geomorphological significance of tidal residual eddies many shallow tidal areas show the occurrence of ebb and flood surplusses organized in cell like structures or tidal residual eddies they seem to arise from a nonlinear transfer of vorticity from the oscillating tidal field to the mean field the irregular structure of the bottom topography or the coastline acting as a catalyst to produce the necessary vorticity gradients the dynamical characteristics of the eddies assist our understanding of the geomorphology of the sea bed

dynamics governing the response of tidal current along the mouth of jiaozhou bay to land reclamation 7 abstract data collected from the previous studies show that the tidal current along the mouth of jiaoz 8 hou bay jzb appears to have weakened whereas the spatial asymmetry stronger flooding in the north 9 region and stronger ebbing in the south region has remained nearly unchanged during the past aq4 several 10 decades of large scale land reclamation this study is conducted to explain the underlying dynamics for this 11 phenomenon the analytic evaluation of the tidal motion indicates that the tidal current in a small coastal 12 bay such as the jzb is linearly proportional to its length l if l is far less than one quarter of the tidal wave 13 length this relation suggests that the decrease in tidal current speed in jzb mouth results from the land 14 reclamation within the bay also the relationship between bay areas and tidal current along the jzb mouth 15 can be derived the results of this simple theoretical method for predicting the change in mean tidal current 16 amplitude after the land reclamation largely agree with previous three dimensional 3 d modeling studies 17 it is also found that the spatial asymmetry of the tidal current along the jzb mouth is controlled by the 18 sharp headland the local factor the unchanged shoreline around the headland leads to the stable spatial 19 asymmetry of the tidal current the weaker tidal current can explain the weaker residual current and the 20 unchanged asymmetry of the tidal current explains the unaltered pattern of inflow over the north region

tectono sedimentary evolution of sedimentary basins from late paleoproterozoic to late neoproterozoic in the sao francisco craton and aracuai fold belt eastern brazil abstract two first order regional unconformities separate proterozoic supracrustal rocks of the sao francisco craton and the aracuai fold belt eastern brazil into three genetic units these constitute three superimposed first order basin fill cycles the espinhaco macaubas salinas and bambui megasequences the paleo mesoproterozoic espinhaco megasequence ca 1730 1500 ma was deposited in an intracratonic rift sag basin in four stages pre rift rift transitional and flexural each of these stages is bounded by regional second order unconformities basin evolution was controlled primarily by tectonics although sea level variations were also important in the transitional and flexural stages after a hiatus of several hundred million years subsidence renewed during extension related to the early neoproterozoic breakup of the supercontinent rodinia with rift to passive margin sedimentation in the macaubas salinas basin ca 950 700 ma initial glaciogenic sedimentation possibly induced by uplift related to a mantle plume occurred in a rift setting subsequent a shelf slope deep sea passive margin deposits represent opening of a northern branch of the adamastor ocean tectonic magmatic climatic and eustatic processes all combined to control basin evolution the bambui megasequence ca 800 650 ma was deposited in a foreland basin during convergent and collisional tectonics related to closure of the brazilide ocean and generation of the brasilia fold belt west of the sao francisco paleocontinental region the evolution of the bambui basin whose fill is characterized by three transgressive progradational shallowing upward sequences was mainly controlled by tectonic processes

axial convergence in a well mixed estuary abstract transverse secondary circulations involving surface convergence observed in a well mixed estuary in north wales are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary the circulation and axial convergence however are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross section produced by non uniform advection of the longitudinal gradient through the channel a diagnostic model using measured mean distributions of cross sectional density indicates surface transverse velocities 0 1 ms 1 similar to those observed in the estuary the model further predicts appreciable transverse divergent currents at a fractional depth of 0 75 a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators

morphosedimentary analysis of the limoeiro stream center northern minas gerais state brazil clues to the eastward tilting of southern espinhaco range sites the morphosedimentary evolution of a sector of the southern espinhaco range northern minas gerais state brazil and its regional neotectonic setting are presented herein the studied site is a stream at base level flowing along the western border of the range its identification was accomplished by using remote sensing techniques including the analysis of 3d slope models generated with shuttle radar topography mission images the main geomorphological features of the identified site were surveyed in a 1 500 scale in addition seven stratigraphic logs are described a regional geologic compilation and facies diagrams were used as a basis for the integration of the geomorphological and stratigraphic data which produced some conclusions about the recent tectonic reactivations at the eastern border of sao francisco craton the results point to a local eastward tilting of the espinhaco range this fact is in accordance with previous findings in neighboring areas and results in the asymmetric evolution of the fluvial valley as well as the regional relief

observed and computed lateral circulation patterns in a partly mixed estuary abstract observations of lateral residual circulation patterns are presented for three sections in the upper reaches of the tamar estuary the data include measurements over spring and neap tidal cycles and are compared with predictions from a depth averaged numerical model of the lateral circulation the model shows features of the lateral structure which also occur in the observed data the computed mass transport stokes drift has a similar lateral structure at spring and neap tides for all sections and is up estuary in the deeper central part of a section and down estuary on the intertidal areas near the banks a distinctive feature of this drift is the reduction in lateral structure when axial density gradients are ignored the mass transport residual current has a characteristic lateral structure when the run off is not too high the current in the central part of a section is slow less than a few cm s 1 and may be up or down estuary depending on the magnitude of the run off the current over the intertidal areas is faster 10 cm s 1 and directed down estuary this distribution consists of three components a a run off induced current which has a river like form b density currents which are up estuary in the deeper part of a section and down estuary over the intertidal and shallow areas and c tidally driven currents which have a similar form to density currents and whose formation depends strongly on the existence of intertidal areas results from this paper are compared with published observations of lateral circulation patterns for several other estuaries

a structural transect across the coastal mobile belt in the brazilian highlands latitude 20 s the roots of a precambrian transpressional orogen abstract we present results of a detailed structural analysis from a 250 km long east west trending transect crossing the coastal mobile belt a part of the precambrian orogen which lies between the eastern edge of the sao francisco craton and the atlantic coast the region exposes amphibolite granulite grade metamorphic rocks and migmatites which formed at mid lower crustal depths during the brasiliano orogeny 0 63 0 52 ga this event marked the closure of the northernmost adamastor ocean and collision between the sao francisco and congo cratons during west gondwana assembly brasiliano deformation resulted in w vergent structures including thrust faults which accommodated kilometer scale transport of crystalline basement overturned kilometer scale folds sheath folds and penetratively developed gneissosity and schistosity isolated relics of an older folded fabric occur locally and may represent transamazonian 2 2 2 0 ga deformation the orogen is kinematically partitioned with the eastern 175 km dominated by moderately to steeply dipping north trending dextral strike slip and oblique slip faults and associated flower structures whereas the western 75 km is dominated by w vergent shallowly to moderately east dipping thrust faults the boundary between these two provinces may mark a brasiliano suture throughout the transect quartzite and metasedimentary belts form strongly deformed zones between massive crystalline basement thrust sheets the granulite cored serra do caparau massif the highest mountains in south america outside of the andes and guyana shield occupies a restraining bend between two brasiliano dextral shear zones the w vergent coastal mobile belt formed contiguously with the e vergent pan african west congo orogen now exposed along the conjugate margin of africa thus an important late precambrian boundary between structurally linked but kinematically opposed structural provinces must lie hidden in the extended offshore continental margins of either continent cretaceous opening of the south atlantic and separation of the west congo belt from the coastal mobile belt may have been structurally influenced by this boundary

transamazonian orogeny in the southern sao francisco craton region minas gerais brazil evidence for paleoproterozoic collision and collapse in the quadrilatero ferrifero abstract the paleoproterozoic transamazonian orogeny yielded a series of discrete orogens in brazil the present field study in the quadrilatero ferrifero qf indicates that the southern sao francisco craton region of the brazilian highlands preserves a portion of one of these orogens two sets of transamazonian structures occur in this region the first consists of northwest verging folds and thrusts affecting supracrustal sequences it is suggested that this set formed in a fold thrust belt setting shortly after 2 125 ga during the closure of a passive margin basin that had initiated along the margins of a preexisting continental mass at ca 2 5 ga the second set consists of structures defining the prominent dome and keel architecture of the qf this set a consequence of the emplacement of basement domes against supracrustal rocks at 2 095 ga may reflect the consequences of orogenic collapse narrow conglomerate filled intermontane basins may have formed coevally with dome emplacement formation of an ocean basin east of the present day sao francisco craton eventually occurred in late mesoproterozoic in effect the transamazonian orogen of the qf represents the collision and collapse stages of a paleoproterozoic wilson cycle the contractional phase of the transamazonian orogeny probably represents accretion of an offshore arc to the eastern and southeastern margin of the present day sao francisco craton region the arc and an associated suture may be traced in the brasiliano pan african orogen east of the sao francisco craton northwards into the northeastern lobe of the sao francisco craton clearly initial assembly of crustal blocks to form a larger continent involving south america occurred during the paleoproterozoic 2 1 ga post transamazonian rifting of this continent created the basins which were later inverted during the brasiliano assembly of gondwana

the lateral density distribution in a partially mixed estuary abstract the density distribution across the york river is characterized by distinct inhomogeneities especially in the upper 2 m of the water column the pattern of variability is repetitive and closely correlated with the semi diurnal tidal cycle and water depth in the shallow areas the water column remains vertically well mixed at all times the density differences are greatest at times of minimum currents at slack before flood the least dense water is located in a lens over the main channel towards the end of the flood cycle and at slack before ebb this is reversed with the least dense water situated over the shoals these density differences result in horizontal pressure gradients which at times may be of sufficient strength to generate localized lateral circulations such circulation patterns would form zones of convergence or divergence across the estuary depending on the particular density distribution

fluid inclusion microthermometry and the zr in rutile thermometer for hydrothermal rutile the zr in rutile thermometer is well established for the determination of metamorphic temperatures particularly in high grade metamorphic terrains and for sedimentary provenance studies the robustness of the rutile thermometry has not been tested on hydrothermal systems unlike quartz a common hydrothermal mineral with abundant fluid inclusions it is difficult to find fluid inclusions in rutile that are suitable for fluid inclusion microthermometry here we report fluid inclusion microthermometric measurements in rutile from the auriferous quartz kaolinite hematite vein that typifies the gold deposit of mil oitavas in the southern serra do espinhaco minas gerais brazil primary fluid inclusions in the rutile record moderately saline 10 12 wt nacl equivalent aqueous carbonic fluids with a total homogenization temperature of 250 c which were likely trapped at about 300 c and 2 0 kbar this temperature is approximately 200 c lower than that predicted by the zr in rutile thermometer for hydrothermal conditions of relatively low temperature direct measurements of homogenization temperatures in rutile hosted fluid inclusions should be preferred to the zr in rutile thermometer

numerical modelling of the sydney harbour estuary new south wales lateral circulation and asymmetric vertical mixing abstract a fully calibrated three dimensional hydrodynamic model of the sydney harbour estuary was used to determine the dominant forcing of regulating estuarine circulation in a sinuous channel under conditions of low river discharge during dry weather the system characterized by stratified flood and mixed ebb during spring tides and stratified flood and ebb during neap tides was analysed using model results of current profiles density field eddy viscosity and gradient richardson number at a mooring station and cross channel transect the magnitude and direction of lateral circulation was modulated in the flood ebb and spring neap cycles due to changes in the interactions between the tides and the complex geometry during spring tides a differential advection induced lateral baroclinic pressure gradient tends to drive the lateral circulation and stratifies the water column when the along estuary circulation is strong enough to break the stratification induced by the lateral baroclinic pressure gradient centrifugal force works in concert with nonlinear advection establishing a classical two layer helical flow across the channel channel bends also induce vertical mixing as a result of the overturning of the density fields the lateral circulation redistributes the differential advection induced lateral shear of the along estuary momentum and is transferred to either reinforce or cancel the along estuary circulation the lateral bathymetry variability in a channel is therefore the key factor causing the intratidal asymmetries in the along estuary circulations stratification suppresses lateral circulation during neap tides and generates a three layer lateral circulation structure in the water column during stratified ebb

marinoan glaciation in east central brazil abstract remnants of a neoproterozoic glaciation in east central brazil are represented by thin diamictite layers jequitai formation and correlative units locally overlying striated pavements on the sao francisco craton the diamictites are covered by the sete lagoas formation of the basal bambui group which is generally accepted to be a typical cap carbonate sequence although most authors have preferred a mid cryogenian post sturtian age for it based mainly on pb pb whole rock data the sete lagoas formation bears lithostratigraphic and isotopic characteristics that are identical to early ediacaran cap carbonates worldwide including a basal thin 0 10 m pale and flinty cap dolostone preserving a drop in 13c values from around 3 2 to 4 5 with associated 18o around 5 and crystal fan facies interpreted as aragonite pseudomorphs ediacaran zircons have been recovered from the middle of the sete lagoas formation constraining the deposition of its upper half to be younger than 610 ma rodrigues 2008 although there is an unconformity below the point where the zircons were collected it is short lived as suggested by the identical typically ediacaran 87sr 86sr values above and below 0 7074 0 7076 carbonate clasts from the jequitai formation and correlative diamictite bearing units in the fold belts that surround the sao francisco craton canabravinha and serra do catuni formations display similar ranges in 13c 6 7 to 2 6 suggesting the erosion of a pre glacial carbonate platform with negative 13c values i e the islay and or trezona anomalies the cratonic carrancas formation on the other hand yielded pale dolostone clasts with 13c in a small range between 4 2 and 3 4 and 18o values around 6 5 these clasts could be derived from the cap dolostone unit itself in which case the carrancas formation would represent resedimented basal sete lagoas formation and imply that sections of the sete lagoas formation sitting atop the carrancas formation are incomplete the base truncated sections have confused previous attempts to correlate the sete lagoas formation with other cap carbonate successions in light of the available lithostratigraphic isotopic and u pb zircon data we propose that the sete lagoas formation represents a basal ediacaran cap carbonate sequence 635 610 ma deposited after the marinoan glaciation in east central brazil

ice thrust features and a possible intertillite pavement in the proterozoic macaubas group jequitai area minas gerais brazil in north central minas gerais large bodies of quartzite up to 25 m long are found in tillite of the proterozoic macaubas group although one body is intact others are disrupted and intermixed with sheared siltstone and tillite two tillites separated by a thin conglomerate are found in the same area striations found on the top of the lower tillite display two directions of ice movement east west and northeast southeast the orientation of the quartzite bodies shear planes in the tillite and the striations on the lower tillite combine to suggest that the ice was moving in an east to northeast direction

tectonics and sedimentation in a paleo mesoproterozoic rift sag basin espinhaco basin southeastern brazil abstract sedimentologic paleogeographic stratigraphic structural and tectonic studies in a paleo mesoproterozoic metasedimentary succession espinhaco megasequence southeastern brazil indicates deposition in a rift sag basin four basin evolution stages are recognized prerift rift transitional and flexural the four stages can be represented by six unconformity bounded tectonosequences the unconformities are recognized in the field and mappable even on a regional scale the prerift and rift stages of the espinhaco basin were filled by products of continental depositional systems the prerift stage probably represents the first product of the rifting process before the development of the half grabens that characterize the rift stage during the rift stage mechanical subsidence due to lithospheric stretching was predominant and led to episodic rising of the depositional base level as a result the basin fill is characterized by coarsening upward intervals paleocurrent patterns indicate that block tilting and half graben subsidence uplift controlled sediment dispersion the first marine incursion within the espinhaco basin marks the change in the subsidence regime of the basin the evolution of the transitional and flexural stages was probably controlled by thermal subsidence due to thermal contraction of the lithosphere during cooling the transitional stage was characterized by relatively low subsidence rates higher subsidence rates and a consequent sea level rise characterize the flexural stage of the espinhaco basin in which three second order transgressive progradational sequences can be recognized

saltwater intrusion into the changjiang river a model guided mechanism study 1 the changjiang river cr is divided into a southern branch sb and a northern branche nb by chongming island as the river enters the east china sea observations reveal that during the dry season the saltwater in the inner shelf of the east china sea flows into the cr through the nb and forms an isolated mass of saltwater in the upstream area of the sb the physical mechanism causing this saltwater intrusion has been studied using the high resolution unstructured grid finite volume coastal ocean model fvcom the results suggest that the intrusion is caused by a complex nonlinear interaction process in relation to the freshwater flux upstream tidal currents mixing wind and the salt distribution in the inner shelf of the east china sea the tidal rectification resulting from the interaction of the convergence or divergence of tidal momentum flux and bottom friction over abrupt topography produces a net upstreamward volume flux from nb to sb with river discharge the net water transport in the nb is driven through a momentum balance of surface elevation gradient forcing horizontal advection and vertical diffusion in the dry season reducing the surface elevation gradient forcing makes tidal rectification a key process favorable for the saltwater intrusion a northerly wind tends to enhance the saltwater intrusion by reducing the seaward surface elevation gradient forcing rather than either the baroclinic pressure gradient forcing or the wind driven ekman transport a convergence experiment suggests that high grid resolution 100 m or less is required to correctly resolve the net water transport through the nb particularly in the narrow channel on the northern coast of chongming island

the age distributions of detrital zircons in metasedimentary sequences in eastern borborema province ne brazil evidence for intracontinental sedimentation and orogenesis detrital zircons in metasedimentary rocks attributed to three different complexes in the central and pernambuco alagoas domains of the borborema province northeastern brazil were dated by la icpms two samples from each of the following complexes were analyzed sert nia and surubim in the central domain and rio una in the pernambuco alagoas domain one sample of the sert nia complex yielded only paleoproterozoic zircons whereas neoproterozoic zircons predominate in the other with the youngest zircon indicating deposition after 642 ma one of the samples of the surubim complex yielded again only paleoproterozoic concordant zircon ages in the other sample although paleoproterozoic zircons predominate ages vary from the mesoarchean to the neoproterozoic the age of the youngest zircon in this latter sample indicates deposition after 850 ma and an overgrowth on an archean grain yielded an age of 623 6m a 2 interpreted as the age of peak metamorphism the two samples of the rio una complex yielded predominantly detrital zircons with ages in the ranges 2 2 2 1 ga and 1 7 1 6 ga in this complex the youngest detrital zircon suggests deposition after 917 ma and metamorphic zircons indicate peak metamorphic conditions between 600 ma and 617 ma the results of this study together with available data from the literature allow the following conclusions a the sert nia complex is a neoproterozoic sequence and not paleoproterozoic as previously thought b the similarity in lithological associations detrital zircon populations and c isotope signature between the sert nia and surubim complexes suggest that they belong to the same lithostratigraphic unit c although the rio una complex can be older than the sert nia surubim complex the lack of zircons younger than 900 ma can also result from insufficient sampling or absence of young rocks alongside the drainage system that collected detritus to this complex d the regional metamorphism related to the brasiliano orogeny attained its climax at 630 600 ma e the age spectra of detrital zircons require their derivation from erosion of rocks formed not only during known geological events in the borborema province and in the nearby sao francisco congo craton but also from the amazonian craton suggesting accumulation in an intracratonic setting within a preexisting large continent f the short timespan between deposition and orogenic deformation may explain the overall medium to high temperature metamorphism due to maintenance in the subsequent compressional event of the elevated geothermal gradients produced during broad scale lithosphere extension

residual circulation in shallow estuaries 2 weakly stratified and partially mixed narrow estuaries long wave theory is used herein to analyze circulation in weakly stratified and partially mixed estuaries unlike the highly stratified systems considered in part 1 the flows considered here have only a minimal tidal frequency internal wave component these estuaries may therefore be modeled as the sum of weakly interacting barotropic and baroclinic modes the dominant factors driving the residual flow are finite amplitude barotropic effects in weakly stratified estuaries and a combination of barotropic effects and steady horizontal density gradient forcing in partially mixed estuaries the dominant vertical exchange mechanism in the weakly stratified case is bottom boundary induced turbulence and that in partially mixed estuaries is believed to be random internal wave interactions a model of the columbia river estuary under weakly stratified conditions accurately predicts the observed residual velocity and salinity fields and the tidal amplitude at which a transition to a highly stratified state takes place the partially mixed state is unstable in the columbia river estuary because the tides and buoyancy input are both too strong

braidplain sedimentation in a proterozoic rift basin the sao joao da chapada formation southeastern brazil abstract the sao joao da chapada formation 100 200 m thick is the basal unit of the middle proterozoic espinhaco supergroup this up to 3000 m thick and slightly metamorphosed sedimentary succession is exposed in the central part of the southern serra do espinhaco southeastern brazil detailed lithofacies studies were carried out to characterize the sedimentation and paleogeography of the sao joao da chapada formation six lithofacies are present horizontal to low angle cross stratified sandstones lithofacies sh sl trough cross stratified sandstones lithofacies st planar cross stratified sandstones lithofacies sp ripple cross laminated sandstones lithofacies sr massive to horizontally stratified conglomerates lithofacies gm and fine grained rocks lithofacies f these are usually arranged in fining and thinning upward successions characterizing waning flood cycles which are always bounded by erosional surfaces no complete cycles have been found however and the cycles are normally composed only of lithofacies sh sl and st the other lithofacies are rare the observed sedimentary structures and their arrangement the geometry of the bodies mainly sheet like to slightly lenticular the low relief morphology of the erosion surfaces bounding the waning flood cycles the coarse and poorly sorted texture of the sediments and the measured paleocurrent pattern unidirectional with little dispersion throughout the study area point to deposition on a wide ephemeral braidplain characterized throughout by relative uniformity of facies and paleocurrent directions water depths on this subsiding braidplain were maintained by high discharge streams debouching onto it from the upland catchment areas as these rivers spread onto the depositional plain the changes in gradient and substratum transformed them into channels of remarkable width and very large width depth ratios these rivers ran in braided tracts and floods were temporarily confined within banks in these tracts channel switching events associated with tilting and subsidence of the basin floor resulted in the accumulation of extensive sandstone sheets interfingering laterally with one another and succeeding one another vertically which built up a braidplain succession extending up to 35 km transverse to the paleoslope the sao joao da chapada braidplain flanked the west margin of a n s elongated rift basin

the platiniferous au pd belt of minas gerais brazil and genesis of its botryoidal pt pd aggregates we delineate an approximately 240 km long north trending au pd pt belt in minas gerais brazil which hosts hydrothermal pd pt bearing gold mineralization in its southern part quadrilatero ferrifero and itabira district and alluvial pd pt au mineralization in its northern part southern serra do espinhaco the hydrothermal pd pt bearing gold mineralization locally known as jacutinga occurs as late orogenic quartz hematite talc kaolinite veins which truncate the brasiliano regional tectonic foliation of the itabiritic host rocks of the paleoproterozoic itabira iron formation the alluvial pd pt au mineralization occurs as botryoidal pd pt nuggets and delicate palladiferous gold aggregates in clastic and organic rich valley infill on quartzite and hematitic rocks of the paleo to mesoproterozoic espinhaco supergroup the au pd pt belt follows the regional trend of major thrust faults related to the 0 6 ga brasiliano orogeny and the hydrothermal jacutinga mineralization could be regarded as an oxidized variant of the orogenic gold deposit spectrum secondary botryoidal and arborescent pt pd aggegates and primary detrital au pd grains are peculiar to the alluvial platiniferous deposits of the northern part of the belt in the serra do espinhaco we suggest that their source was jacutinga style vein mineralization as deduced from abundant detrital hematite and rutile and the discovery of detrital hongshiite a pt cu alloy that forms bonanza pockets within jacutinga at itabira reconnaissance laser ablation inductively coupled plasma mass spectrometry analyses for trace metals in the botryoidal pt pd grains show that they have elevated tl and se contents of 0 9 to 354 and 54 to 2 700 ppm respectively these elements correlate positively with a tl se ratio of about 0 08 a value that contrasts with tl se 1 for hongshiite from the hydrothermal jacutinga system at itabira the low and roughly constant tl se ratios of the alluvial samples are close to the average value of 0 12 for fluvial waters we suggest that the botryoidal pt pd aggregates grew in situ by electrochemical metal accretion from dilute solutions carrying pt and pd from the dissolution of detrital pt pd minerals from jacutinga the connection between jacutinga and platiniferous alluvia indicates that there is a prospective trend of about 240 km for pd pt bearing gold mineralization reaching from ouro preto to diamantina the very high oxidation state required for efficient low temperature aqueous mobilization of pt pd and au was apparently controlled by the quartzitic and hematitic rocks in the serra do espinhaco but not in the southern belt where greenstone rocks buffered fluids to moderate ph and lower oxidation states

tracking connection and restriction of west gondwana sao francisco basin through isotope chemostratigraphy abstract c and sr isotope compositions of carbonate rocks from the intracontinental sao francisco basin can track ocean connections and restriction the lower three formations of the bambui group can be grouped into three chemostratigraphic intervals ci recording different evolution stages of the basin lowermost ci 1 comprises the basal cap carbonates of the sete lagoas formation displaying an initial c negative excursion followed by a coeval c and sr positive excursions 13 c values from 5 to 0 and 87 sr 86 sr ratios from 0 7074 to 0 7082 in 10 m of stratigraphic record it marks a change from a restricted shallow basin influenced by freshwater to a basin connected to external seawaters due to marine transgression ci 2 comprises carbonates of the middle portion of the sete lagoas formation with 13 c values around 0 and 87 sr 86 sr ratios around 0 7082 that matches those observed worldwide for the late ediacaran it records the onset of a gondwana sea pathway connecting several epicontinental basins allowing migration of index fossil cloudina sp uppermost ci 3 starts after a major positive excursion in the 13 c values reaching 16 and a steepened decrease of 87 sr 86 sr ratios to 0 7075 which are lower than those expected for the ediacaran cambrian boundary this interval comprises the upper sete lagoas serra de santa helena and lagoa do jacare formations and records the end of the connection of the sao francisco basin to the gondwana sea pathway setting a restricted epeiric sea restriction was probably caused by late ediacaran uplifting of orogenic belts surrounding the basin other west gondwana cloudina bearing units also display the same mismatch in the sr isotope ratios suggesting that the establishment of intracontinental basins inside large continental masses may challenge the use of isotope chemostratigraphy for interbasinal correlations

highly 13c enriched carbonate and organic matter in the neoproterozoic sediments of the bambuigroup brazil abstract paleontological and radiometric dating methods rb sr k ar and pb pb suggest neoproterozoic ages for sedimentary carbonate rocks of the bambuigroup in the southern part of the sa o francisco basin of central brazil the 13c values for carbonates fall within two ranges 2 to 2 and 10 to 16 for both groups coexisting organic carbon is depleted in 13c by 28 2 relative to the carbonates similar to numerous neoproterozoic examples worldwide secular variations in 13c in the bambuigroup show a significant positive excursion beginning with values near 0 in carbonates immediately above a diamictite and rising to values as high as 16 upsection the sr isotope data for carbonates from the bambuigroup agree with the age inferred from the secular variation in oceanic 13c values the striking feature in this study is that both the carbonate and total organic carbon are considerably more enriched in 13c than previously reported for neoproterozoic carbonates elsewhere it is not clear if these unusual 13c values are caused by primary or secondary processes

proterozoic first order sedimentary sequences of the sao francisco craton eastern brazil abstract the sao francisco craton in eastern brazil hosts sedimentary sequences deposited between the paleoarchean 3300 ma and late neoproterozoic 580 ma proterozoic successions occurring in this region comprise five 1st order sedimentary sequences which besides episodes of global significance record major basin forming events the ca 8000 m thick minas itacolomi 1st order sequence exposed in the brazilian mining district of the quadrilatero ferrifero and containing as marker bed the lake superior type caue banded iron formation tracks the operation of a wilson cycle in the paleoproterozoic era the quartz arenite dominated espinhaco i and ii sequences record at least two major rift sag basin forming events which affected the host continent of the sao francisco craton at around 1 75 ga and 1 57 ga the macaubas sequence and its correlatives in the extracratonic domains witness the individualization of a sao francisco congo plate in synchronicity with the break up of rodinia in the cryogenian period the sao francisco congo plate together with various fragments derived from rodinia reassembled to form gondwana in the ediacaran period in the course of the gondwana amalgamation orogenic belts developed along the margins of the craton its interior converted into foreland basins received the shallow water carbonates and pelites of the bambui 1st order sequence and its correlatives

provenance of the vazante group new u pb sm nd lu hf isotopic data and implications for the tectonic evolution of the neoproterozoic brasilia belt abstract the vazante group show varied u pb provenance patterns along the basin zircon ages range from 936 to 3409 ma but paleo and mesoproterozoic terrains constitute the main sources of the original sediments the youngest population 930 ma establishes the maximum depositional age of the group sm nd t dm data show the predominance of paleoproterozoic ages 1 90 2 08 ga and also indicate some input from younger sources in rocks of the lapa formation 1 67 to 2 0 ga in the upper part of the group whereas rocks of the serra do garrote formation present the oldest model ages 2 03 to 2 76 ga hf isotopic compositions of the detrital zircons indicate that they were derived mainly from recycled paleoproterozoic crust with a minor mesoproteroic juvenile component terranes within the sao francisco craton represent the main sources of detrital sediments of this group and reinforce the interpretation that it may be a passive margin sequence developed along the western margin of the original continent however the origin of mesoproterozoic grains remains uncertain slightly younger sm nd model ages in the lapa formation however are not entirely consistent with derivation solely from the craton and may indicate contribution from younger sources such as the neoproterozoic goias magmatic arc

neoproterozoic oceanic remnants in eastern brazil further evidence and refutation of an exclusively ensialic evolution for the aracuai west congo orogen the aracuai eastern brazil and west congo southwestern africa belts are counterparts of the same neoproterozoic orogen located between the sao francisco and congo cratons the macaubas group represents a major passive margin sequence and is a key unit for interpreting the evolution of that orogen the salinas formation is the distal rock assemblage of the macaubas group and consists of a deep sea sand mud sequence and a volcanic sedimentary unit called the ribeirao da folha facies the latter includes metamorphosed volcanic exhalative sediments associated with ocean floor basalts amphibolites the magmatic protoliths of these amphibolites crystallized at about 816 72 ma sm nd whole rock isochron nd t 3 8 0 2 regional metamorphism reached the amphibolite facies at about 630 ma rb sr whole rock isochron when slabs of ultramafic rocks were tectonically emplaced over the ribeirao da folha facies we consider this volcanic sedimentary facies and the coeval slabs of ultramafic rocks to be remnants of a branch of the adamastor brazilide ocean the extensive occurrence of syntectonic to late tectonic calc alkalic granitoids along the internal domain of the aracuai belt implies that a reasonably large amount of ocean crust was consumed via an east dipping subduction zone during formation of the aracuai west congo orogen

the effect of early alpine thrusting in late stage extensional tectonics evidence from the kulidzhik nappe and the pelevun extensional allochthon in the rhodope massif bulgaria abstract in the northeastern rhodope massif the kulidzhik nappe exposes a unique juxtaposition of a high grade basement allochthon onto a low grade mesozoic unit and the counterpart pelevun extensional allochthon belonging to the same unit the kulidzhik nappe tectonostratigraphy comprises structurally upward i a lower unit consisting of high grade basement orthogneisses ii a low grade greenschist phyllite unit consisting of jurassic extrusive rocks and metasedimentary rocks iii the nappe allochthon built by the lower high grade basement unit orthogneisses and iv eocene sedimentary rocks and oligocene volcanic cover rocks the pelevun extensional allochthon is heterogeneous and consists of mesozoic low grade unit marbles and greenschists and the upper high grade basement unit we have combined structure and kinematics with lithological information and 40 r 39 r geochronology to constrain the tectonic evolution and regional significance of the kulidzhik nappe and the pelevun extensional allochthon mineral chemistry reveals igneous phases of the granitic protolith of the allochthonous orthogneisses with textures related to ductile deformation and metamorphism higher than 500 c their trace element patterns are indistinguishable from the high grade basement orthogneisses in the eastern rhodope mineral chemistry of the metamorphic assemblage in the underlying greenschists is consistent with medium grade greenschist facies metamorphism at temperatures well below 450 c whose geochemistry defines transitional morb to iat affinities with a strong arc imprint the greenschist s composition is extremely similar to the composition of a supra subduction zone jurassic arc extrusive suite that occurs in the low grade unit of the eastern rhodope the structural elements in all of the tectono metamorphic units and the nappe surface indicate top to the nne tectonic transport deformation evolved from ductile to brittle conditions coevally with a progressive decrease from lower amphibolite to weak greenschist facies metamorphism towards the structural top two klippen gave plateau 40 r 39 r mica ages of 154 23 0 66 ma and 156 70 0 81 ma respectively which reflect cooling following the greenschist facies event below 350 c during shallow crustal level thrust emplacement and exhumation of the kulidzhik allochthon the hanging wall pelevun extensional allochthon preserves an internal nne directed ductile tectonic transport trend but is underlain by a ssw directed ductile brittle extensional detachment and has experienced ne sw brittle extension on high angle normal faults an amphibole 40 r 39 r inverse isochron age of 156 58 0 60 ma constrains a late jurassic upper greenschist facies tectono metamorphic event whereas a white mica plateau age of 39 66 0 47 ma provides evidence for middle eocene cooling and exhumation of the pelevun extensional allochthon in the hanging wall of the detachment we relate the kulidzhik nappe to late jurassic crustal deformation during arc continental margin collision that involved nne directed nappe staking and metamorphism of continental margin basement and island arc units the nappe shares a tectono metamorphic history with the nappes of the adjacent strandzha massif implying a region wide early alpine orogenic system our results reveal a record of early alpine thrust tectonics and show the significance of crustal accretion related assembly for the tectonic evolution of the rhodope massif both the late jurassic thrusting event and the subsequent cretaceous thrusting event thickened the rhodope crust creating crustal instability which influenced tertiary crustal extension

formation of the steer s head geometry of sedimentary basins by differential stretching of the crust and mantle if the lithospheric mantle is stretched over a wider region than the crust postrift stratigraphic onlap will occur at basin margins giving rise to a steer s head geometry space problems are avoided by making the total amount of extension in the lithospheric mantle equal to that in the crust the predicted pattern of onlap agrees well with that observed if the lithospheric mantle is stretched over a fractionally wider region than is the crust thus extensive stratigraphic onlap can be produced easily without recourse to sea level rise or flexural rigidity

late proterozoic glaciation in central eastern brazil synthesis and model abstract in the surroundings of the middle proterozoic sao francisco craton of central eastern brazil numerous traces of an ancient glaciation can be found almost all criterua necessary to prove an ancient glaciation occur including scratched and facetted pebbles a striated surface quarrying outwash plains eskers varvites dropstones decollement structures etc poorly sorted tillites with varying pebble composition are widely distributed continental tillites and glaciomarine sequences interfinger in the east and southeast of the craton traces of ice movement suggest flow directions of the ice towards the east and the southeast of the craton at the coast the wet base glaciers passed over to floating shelf ice in the northeast glaciers also came from the serrinha craton in northeastern bahia the glaciation covered an area of at least 1300 700 km the sequences containing traces of glaciation discordantly overlie archean to middle proterozoic rocks they in turn are overlain by late proterozoic calcareous and psammopelitic sediments of the bambui group present data indicate that glaciation took place around 1000 ma ago

transport of riverine sediment from different outlets in the pearl river estuary during the wet season abstract riverine sediment is important for the development of river deltas and for carrying both nutrients and contaminants into estuaries and coasts previous studies of sediment transport have focused on single sourced estuaries and paid less attention to multi sourced estuaries estuaries with multiple river discharge sources in this study we used the coupled ocean atmosphere wave sediment transport coawst modeling system to study the sediment transport in the pearl river estuary pre where riverine sediments come from four different outlets during a typical wet season 2007 about 6 8 million tons of riverine sediments were discharged into the pre most of which were trapped in shoals during neap tides while about 11 8 by mass was transported out of the estuary during spring tides the proportions of the multi sourced sediments trapped in the pre or escaping from the estuary depended on the morphology of the estuary and the location of the outlets in relation to the estuarine morphology the distributions of different riverine sediments were mostly independent in neap tides but overlapped more in spring tides the seaward sediment transport was dominated by river freshwater discharge in the upper estuary and by tidal forcing in the lower channels while tides and winds controlled the sediment transport in the lower shoals this study aids our understanding of sediment transport processes in multi sourced estuaries and has implications for coastal environmental management

climatic cycles recorded in glacially influenced rhythmites of the gowganda formation huronian supergroup abstract the gowganda formation of the 2 45 2 2 ga huronian supergroup contains glacially induced varve like rhythmites that potentially preserve a detailed record of climatic conditions during the paleoproterozoic era four rhythmic couplet thickness records were measured at two outcrops near wharncliffe ontario for the purpose of time series analysis the couplets which range from 1 to 32 mm thick are composed of alternating layers of siltstone and claystone time series analysis of the couplet thickness records using the mtm toolkit of mann and lees 1996 consistently revealed periodicities in the range of 2 2 2 9 couplets per cycle which is consistent with climatic cycles such as the quasi biennial oscillation qbo and the el nino southern oscillation enso observed in modern times this periodicity suggests that the rhythmic couplets represent annual deposits i e varves evidence for the presence of cycles at 3 0 4 9 couplets 6 6 6 9 couplets 8 8 9 2 couplets 22 8 couplets and 30 1 31 0 couplets were also observed in some couplet thickness records however the presence of these longer term cycles was inconsistent from site to site

kinematic evolution of the aracuai west congo orogen in brazil and africa nutcracker tectonics during the neoproterozoic assembly of gondwana abstract the neoproterozoic aracuai west congo a wc orogen is one of many brasiliano pan african orogens that developed during the assembly of west gondwana this orogen was split apart in mesozoic time due to opening of the south atlantic the aracuai orogen now underlies eastern brazil whereas the west congo belt fringes central africa s atlantic coast significantly at the time it formed the a wc orogen was bounded on the west north and east by the sao francisco congo craton a crustal block that had the shape of a lopsided upside down u thus the orogen was partially confined during tectonism in that it occupied an enclave surrounded on three sides by cratonic crust formation of the a wc orogen resulted in kinematically complex deformation substantial crustal shortening and production of a large volume of magma how such features could develop in this particular setting has long been a mystery our field studies in the aracuai orogen together with published data on the west congo belt characterize the kinematic picture of the a wc orogen and lead to a tectonic model that addresses its evolution in our model the a wc orogen formed in response to closure of the macaubas basin this basin was underlain by oceanic crust in the south but tapered northward into a continental rift which terminated against the cratonic bridge linking the eastern and western arms of the sao francisco congo craton closure occurred when the western arm now the sao francisco craton rotated counterclockwise towards the eastern arm now the congo craton this closure may have been driven by collision of the paranapanema amazonian and kalahari cratons against the external margins of the sao francisco congo craton rather than by slab pull associated with subduction of the macaubas basin s floor thus the process of forming the a wc orogen resembled the process of crushing of a nut between two arms of a nutcracker such nutcracker tectonics led to a series of kinematically distinct deformation stages initially internal portions of the orogen flowed northwards then substantial crustal thickening occurred in the orogen s interior and the deformation front migrated outwards producing thrust belts that overlapped the internal margins of the craton with continued closure space in the enclave became restricted and the orogen s interior underwent lateral escape to the south late stage extensional collapse triggered both production of late to post collisional granites and exhumation of high grade rocks from mid crustal levels

paleomagnetic evidence for the evolution of meso to neo proterozoic glaciogenic rocks in central eastern brazil abstract paleomagnetic studies on basic dikes in the eastern sao francisco craton which have isotopic ages of 1 0 1 1 ga define an apparent polar wander path for south america over this time interval the data indicate that the sao francisco craton was at paleolatitudes between 40 and 65 at the time of emplacement of these dikes neo proterozoic sedimentary glaciogenic rocks the macaubas group bebedouro formation ibia formation and carandai formation crop out in central eastern brazil an age of about 1 0 ga has been proposed for these glacial deposits paleogeographical reconstructions of south america show a continental movement coherent with paleoenvironmental models proposed for the macaubas group and suggest that the glacial period may have occurred between 1 01 and 1 08 ga

provenance and paleogeographic reconstruction of a mesoproterozoic intracratonic sag basin upper espinhaco basin brazil we would like to thank the capes cnpq grant no 304045 2010 1 and petrobras for their financial support for the field work and analysis

intensification of subtidal surface currents over a deep channel in the upper chesapeake bay a three dimensional hydrodynamic model of the upper chesapeake bay was used to examine the nature and cause of an intensification of subtidal southward surface current in the middle reaches of the basin the deep navigation channel along the eastern boundary was found to be ultimately responsible the deep channel allows the density and tidally induced subtidal currents to intensify over it producing the eastern intensification both mechanisms operate in the non rotating limit and consequently do not diminish with vanishing effect of the earth s rotation density induced forcing is predominantly baroclinic generating a northward undercurrent in the deep channel and a southward current aloft which attenuates westward tidal forcing is mostly barotropic producing southward mean current in the deep channel and return flow to the west historic data lend support to the model results

the determination and prediction of longitudinal dispersion coefficients in a narrow shallow estuary abstract measurements of velocity and salinity for parts of a flood and an ebb tide in the conwy estuary have been used to evaluate the time dependent longitudinal dispersion coefficients due to vertical and transverse oscillatory shear effects the values have been compared with semi analytical predictive formulae simple empirical expressions are suggested for relating the dispersion coefficients to hydraulic parameters for the conditions observed

anatomy of a basin controlled foreland fold thrust belt curve the tres marias salient sao francisco basin brazil abstract salients as common features of fold thrust belts correspond to convex to foreland map view curves hosting sedimentary sequences younger than 1 8 ga the intracratonic sao francisco basin of southeastern brazil encompasses two foreland fold thrust belts of opposite vergences the brasilia on the west and the aracuai on the east these belts culminate in the central portion of the basin in form of two almost touching antitaxial curves developed in response to diachronic orogenic events affecting the basin fill units during the ediacaran brasiliano pan african assembly of west gondwana located in the external sector of the thin skinned brasilia foreland fold thrust belt the ca 130 km long and 80 km wide tres marias salient involves the up to 2500 m thick ediacaran bambui sequence in order to characterize the overall architecture and processes involved in the generation of the tres marias salient we conducted a detailed structural investigation of the southern segment of the brasilia foreland fold thrust belt based on surface seismic well and aerogeophysical data interpretation the salient is frankly dominated by monotonous trains of upright chevron folds bounded by a small number west dipping thrust faults the structural traces of thrusts and fold hinges on maps trend preferentially ns in the central portion of the salient curving progressively toward ne and nw in its southern and northern limbs respectively the entire system is linked to a spoon shaped detachment zone located near the base of the ediacaran bambui strata whose maximum thickness roughly coincides with the salient culmination our analysis led to the conclusion that the tres marias salient corresponds to a basin controlled curve generated during a single and progressive deformational episode its fold dominated character and the high symmetry of its internal fabrics can be explained by the low friction of the associated detachment located along a lower bambui sequence black shale and the mechanical behavior of the pelite dominated deposits incorporated in the fold thrust wedge the occurrence of seepages and recent gas discoveries in the tres marias salient area point toward its potential for unconventional hydrocarbon resources our investigation suggests that the main basin controls that led to the development of the salient might have controlled the quality and distribution of proterozoic petroleum system elements

tidally driven residual circulation in shallow estuaries with lateral depth variation tidally driven residual circulation in shallow estuaries with lateral depth variation has been studied analytically using a two dimensional depth averaged model the solution is presented for a v shaped channel exchange flow is found to be correlated with the topography the magnitude of this exchange flow depends mainly on four parameters the ratio between the minimum depth on the shoal and the maximum depth in the channel the ratio between the tidal amplitude at the mouth and the mean depth the ratio between the length of the estuary and the tidal wave length and the ratio between the tidal timescale and the decay timescale due to friction generally a net landward flow occurs over the shoals and is balanced by a return flow in the channel the along channel residual velocity changes nearly linearly across the estuary the residual velocity decreases monotonically toward the head of the estuary the transverse residual velocity is convergent in the channel and divergent on the shoals the residual velocity is highly dependent on the length of the estuary when the length of the estuary is much smaller than a quarter of the wave length the residual velocity is relatively small when the length of the estuary is about a quarter of the wave length the magnitude of the residual velocity reaches its maximum when the length of the estuary is much larger than a quarter of the wave length the residual circulation approaches the pattern found in an infinite length estuary on the basis of the analysis of the solution we conclude that exchange flow generated by tides in a shallow estuary is the result of competition among several processes an inward flux is caused by local nonlinearity both in the bottom friction and from propagation of the tidal wave of finite amplitude this inward flux is larger on the shoals and smaller in the channel owing to larger nonlinearity in the shallow water the residual inward flow creates a setup residual surface elevation of water at the head of the estuary that produces a pressure gradient this residual pressure gradient is approximately uniform across the estuary and drives an outward flow that is larger in the channel than on the shoals in all of the parameter space the competition results in a net inflow on the shoals and a net outflow in the channel

neoproterozoic glacial dynamics revealed by provenance of diamictites of the bebedouro formation sao francisco craton central eastern brazil one of the main questions on neoproterozoic geology regards the extent and dynamics of the glacial systems that are recorded in all continents we present evidence for short transport distances and localized sediment sources for the bebedouro formation which records neoproterozoic glaciomarine sedimentation in the central eastern sao francisco craton sfc brazil new data are presented on clast composition based on point counting in thin section and shrimp dating of pebbles and detrital zircon cluster analysis of clast compositional data revealed a pronounced spatial variability of clast composition on diamictite indicating the presence of individual glaciers or ice streams feeding the basin detrital zircon ages reveal distinct populations of archean and palaeoproterozoic age the youngest detrital zircon dated at 874 9 ma constrains the maximum depositional age of these diamictites we interpret the provenance of the glacial diamictites to be restricted to sources inside the sfc suggesting deposition in an environment similar to ice streams from modern high latitude glaciers

sequence stratigraphy of the archaean supracrustal belts of the pilbara block western australia abstract the stratigraphic succession of the 3580 2770 m y old granite greenstone terrain of the pilbara block is re assessed by the first and second order sequence stratigraphic techniques that are the standard for phanerozic terrains with the view to testing the conformity of stratigraphic cause and response a holistic tectonostratigraphic framework is presented that pairs the supracrustal and crustal rock records the resulting sequence scheme is tested against the cause of first order sequence cycles viz global sea level responses to supercontinent breakup and assembly three first order inter regional unconformities divide the stratigraphic succession of the supracrustal belts into four units of first order megasequence status in order of decreasing age these are named the warrawoona gorge creek roebourne and mount negri megasequences a total of 17 second order supersequences are also named although some may represent supersequence sets the megasequences and or their genetic crustal equivalents represent the preserved rock record of four megacycles of forearc arc and or back arc geotectonic evolution associated with the convergent margins of a pilbara continent individual supersequences are the rock record of separate second order basins or basin phases and reflect the different geotectonic components of each megasequence the four megacycles had respective longevities from oldest to youngest of 150 m y 230 m y 160 m y and 180 m y they are identical in their stratigraphic successions and predicted first order sea level cycles to phanerozoic first order tectono eustatic cycles and are interpreted as the same response to global tectonic change the warrawoona and gorge creek megacycles and the roebourne and mount negri megacycles are tectonogenetic pairs of the east pilbara and the west pilbara megacycle sets respectively the east pilbara megacycle set evolved over a period of 380 m y from 3490 ma to 3110 ma whereas the west pilbara megacycle set evolved over a period of 340 m y from 3110 ma to 2770 ma the two megacycle sets are identical in their longevity geotectonic components and predicted sea level cycles to convergent margin wilson and supercontinent cycles and are thereby interpreted individually as the oldest established records of the supercontinent cycle the implication of this interpretation is that a steady state global tectonic regime has been in operation from early in earth s history which has important consequences for theoretical modelling of the earth s early tectonic style and secular changes in its geochemical regime

precambrian sequence stratigraphy abstract sequence stratigraphy studies the change in depositional trends in response to the interplay of accommodation and sediment supply from the scale of individual depositional systems to entire sedimentary basin fills as accommodation is controlled by allogenic mechanisms that operate at basinal to global scales the change in depositional trends is commonly synchronized among all environments established within a basin thus providing the basis for the definition of systems tracts and the development of models of facies predictability all classical sequence stratigraphic models assume the presence of an interior seaway within the basin under analysis and are centered around the direction and types of shoreline shifts which control the timing of all systems tracts and sequence stratigraphic surfaces in overfilled basins dominated by nonmarine sedimentation the definition of systems tracts is based on changes in fluvial accommodation as inferred from the shifting balance between the various fluvial architectural elements the method of sequence stratigraphy requires the application of the same set of core principles irrespective of the age of strata under analysis from precambrian to phanerozoic the study of precambrian basins is often hampered by poorer stratal preservation and by a general lack of time control however where sedimentary facies are well preserved the lack of time control may be partially compensated by a good knowledge of facies architecture and relationships as well as by paleocurrent data the latter are particularly important to understand the stratigraphic record of tectonically active basins where abrupt shifts in paleoflow directions allow one to infer tectonic events and map the corresponding sequence bounding unconformities arguably the most important contribution of precambrian research to sequence stratigraphy is the better understanding of the mechanisms controlling stratigraphic cyclicity in the rock record and hence of the criteria that should be employed in a system of sequence stratigraphic hierarchy there is increasing evidence that the tectonic regimes which controlled the formation and evolution of sedimentary basins in the more distant geological past were much more erratic in terms of origin and rates than formerly inferred solely from the study of the phanerozoic record in this context time is largely irrelevant as a parameter in the classification of stratigraphic sequences and it is rather the stratigraphic record of changes in the tectonic setting that provides the key criteria for the basic subdivision of the rock record into basin fill successions separated by first order sequence boundaries these first order basin fill successions are in turn subdivided into second and lower order sequences that result from shifts in the balance between accommodation and sedimentation at various scales of observation irrespective of the time span between two same order consecutive events sequences identified in any particular basin are not expected to correlate to other first and lower order sequences of other basins that may be similar in age but may have different timing and duration

development of tectonic cyclothems in rift pull apart and foreland basins sedimentary response to episodic tectonism the thickest part of asymmetric rift pull apart and foreland basin fills commonly consists of large scale hundreds to thousands of metres thick tectonically generated cyclothems of fine grained marine lacustrine or longitudinal fluvial deposits and coarse grained transverse braid plain or alluvial fan deposits the appearance of coarse clastics in these basin fills is typically noted as the time of tectonically rejuvenated source area uplift based on the conceptual tie between relief and coarse grain size and on the application of the davis theory of landform development we propose the opposite interpretation that the commencement of fine grained sedimentation above coarse grained deposits in a tectonic cyclothem is the best indicator of renewed tectonic activity this reinterpretation is more consistent with 1 modern examples 2 the consideration of source area and sedimentary basin geomorphology 3 the disparity between the reaction rates of the various environments to subsidence 4 the disparity between the rates of tectonic uplift and erosion and 5 the controls on elastic wedge progradation in our model extensive coarse grained clastic wedge progradation is an indicator of tectonically quiescent phases

mixed carbonate siliciclastic sedimentation in forebulge grabens an example from the ediacaran bambui group sao francisco basin brazil abstract mixed carbonate siliciclastic successions have been described in multiple phanerozoic sedimentary settings recording the dynamic interplay of tectonics eustasy climate in situ carbonate production and variations in siliciclastic sediment supply the ediacaran bambui 1st order sequence i e bambui group covers most of the intracratonic sao francisco basin southeast brazil and encompasses thick packages of carbonate and fine to coarse grained siliciclastic strata recording a marine foreland basin stage that developed in the sao francisco plate during the neoproterozoic early paleozoic west gondwana assembly the ediacaran deposits unconformably overlie archean paleoproterozoic metamorphic assemblages of the sete lagoas basement high and fill a series of kilometer long grabens in the southern sao francisco basin seismic data reveal that these troughs developed through the extensional reactivation of ancient basement structures along with the tectonically driven forebulge uplift of the sete lagoas high in the early evolutionary stages of the bambui basin cycle based on the detailed description of continuous drill cores of a well recently drilled during hydrocarbon exploration campaigns we recognized two transgressive regressive 2nd order sequences preserved within one of the focused grabens i sequence 1 includes the glaciogenic deposits of the basal carrancas formation that grade upward into the carbonate ramp successions of the sete lagoas formation ii sequence 2 contains the siliciclastic dominated and deep water to deltaic strata of the serra de santa helena formation and passes upward into peritidal carbonates of the lagoa do jacare formation these sedimentary successions encompass suites of retrogradational aggradational and progradational lower rank cycles and are bounded by erosional surfaces regional seismic interpretation well data and the available literature indicate that most of these deposits and their correlatives are continuous over most of the sete lagoas basement high showing remarkable variations within the studied forebulge grabens our analysis demonstrates that the depositional architecture of sequence 1 and sequence 2 marks the interplay of successive episodes of tectonically driven forebulge uplift and regional eustatic and climatic changes the latter are mainly recorded by the lower second order sequence sequence 1 which contains typical sedimentary features deposited in the aftermath of an important glaciation affecting lower latitudes during the late neoproterozoic an additional local accommodation control is restricted to the forebulge grabens and is associated with episodic and local subsidence pulses as indicated by lower to higher rank cycles it has been responsible for the deposition of thicker transgressive to early regressive successions a few gravitational flow deposits and the development of the deltaic to shallow marine carbonate cycles of sequence 2 the overall stratigraphic architecture of the studied successions suggests that several basin mechanisms operating during the phanerozoic were also present in the late neoproterozoic the close association between organic rich post glacial shales potential reservoir facies and rift related migration paths and hydrocarbon traps indicate that forebulge grabens represent important elements to consider in hydrocarbon exploration campaigns

patterns of sedimentation in the precambrian abstract the principle of uniformitarianism may be applied to precambrian basin evolution and to the sedimentary record as a whole the major difference in the precambrian eon lay in variability of rates and intensities of processes controlling weathering erosion transport deposition lithification and diagenesis this paper examines precambrian sedimentation patterns within the larger framework of earth evolution pre rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension all reworked by marine current systems and localised turbidites from c 4 to 3 2 ga greenstone belts provided the predominant settings for the thin passive margin carbonates bif stromatolitic evaporites pelites and quartzites and lesser synorogenic turbidites conglomerates and sandstones that accompanied the volcanic and volcaniclastic rocks typical of these settings common palaeoenvironments were high gradient alluvial fans low sinuosity braided rivers and relatively shallow marine settings subject to wave and tidal action and turbidity currents although continental crustal growth continued largely through greenstone belts until c 2 7 ga the witwatersrand basin c 3 0 2 7 ga kaapvaal craton south africa reflects initial stabilisation of the oldest craton with an epeiric sea accumulating largely fluvial detritus subject to tidal inland and storm wave craton marginal reworking within a retroarc foreland basin setting neoarchaean palaeoproterozoic sedimentation is discussed within a framework of two global superevents at c 2 7 ga and 2 2 1 8 ga each encompassing major changes in earth s evolution related to the supercontinent cycle mantle superplumes peaks in crustal growth rates and significant biochemical changes within the atmosphere hydrosphere system concomitant globally raised sea levels led to chemical and clastic epeiric seas within which the first giant carbonate platforms developed and deposition of iron formation peaked globally at c 2 5 ga the first global glaciation at c 2 4 2 2 ga provides little support for the snowball earth theory but does suggest a negative feedback loop model whereby intraglacial co 2 related warming and synglacial decreases in weathering alternated up to three times models for palaeo atmospheric and oceanic evolution are mutually exclusive but by c 1 8 ga the existence of large landmasses and free oxygen in earth s atmosphere enabled erg development and red bed sedimentation globally the full spectrum of phanerozoic modern sedimentary environments was thus present on earth a third postulated superevent at c 0 8 0 6 ga essentially recreated conditions experienced at c 2 2 1 8 ga with additionally at least three global refrigeration events