time scale and conditions of weathering under tropical climate study of the amazon basin with u series the rio solimoes amazonas amazon river and its major tributaries have been analyzed for u series nuclides 238 u 234 u 230 th 226 ra disequilibria have been measured in the dissolved 0 2 lm as well as bed sands u series disequilibria are closely related to major and trace element compositions and therefore reflect elemental fractionation during chemical weathering moreover while the dissolved load records present day weathering suspended particles integrate the erosion his tory over much longer time scales 100 ka lowland rivers are characterized by long time scales of chemical erosion p100 ka result ing in a high weathering intensity moreover exchange between suspended particles and the dissolved load may explain the u series signature for these rivers by combining u series and pb isotopes in suspended particles we show that erosion in the rio madeira basin occurred as a multi step process whereby the pristine continental crust was eroded several hundreds of ma ago to produce sediments that have then been integrated in the cordillera by crustal shortening and are currently eroded in contrast recent erosion of a pristine crust is more likely for the rio solimoes amazonas 10 ka the suspended particles of the rivers draining the andes solimoes ama zonas madeira suggest time scales of weathering ranging between 4 and 20 ka this indicates that suspended particles transported by those rivers are not stored for long periods in the andean foreland basin and the tropical plain the sediments delivered to the ocean have resided only a few ka in the amazon basin 6 3 1 ka for the rio amazonas at obidos nevertheless a large fraction of the sediments coming out from the andes are trapped in the foreland basin and may never reach the ocean erosion in the andes is not operating in steady state u series systematics shows unambiguously that rivers are exporting a lot more sediments than predicted by steady state erosion and that is a consequence of soil destruction greater than production by relating this observation to the short time scales of weathering inferred for the andes a few ka it appears that the erosion regime has been recently perturbed resulting in high denudation rates a possible explanation would be the increase in precipitation less than 5 ka proposed by recent paleoclimatic studies our results indicate that erosion responds rapidly to high frequency climatic fluctuations

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

meso archaean and palaeo proterozoic sedimentary sequence stratigraphy of the kaapvaal craton abstract the kaapvaal craton hosts a number of precambrian sedimentary successions which were deposited between 3105 ma dominion group and 1700 ma waterberg group although younger precambrian sedimentary sequences outcrop within southern africa they are restricted either to the margins of the kaapvaal craton or are underlain by orogenic belts off the edge of the craton the basins considered in this work are those which host the witwatersrand and pongola ventersdorp transvaal and waterberg strata many of these basins can be considered to have formed as a response to reactivation along lineaments which had initially formed by accretion processes during the amalgamation of the craton during the mid archaean faulting along these lineaments controlled sedimentation either directly by controlling the basin margins or indirectly by controlling the sediment source areas other basins are likely to be more controlled by thermal affects associated with mantle plumes accommodation in all these basins may have been generated primarily by flexural tectonics in the case of the witwatersrand or by a combination of extensional and thermal subsidence in the case of the ventersdorp transvaal and waterberg wheeler diagrams are constructed to demonstrate stratigraphic relationships within these basins at the first and second order levels of cyclicity and can be used to demonstrate the development of accommodation space on the craton through the precambrian

the volcano sedimentary succession of the archean sodium group ventersdorp supergroup south africa volcanology sedimentology and geochemistry abstract the volcano sedimentary succession of the sodium group in the south western corner of the kaapvaal craton rests unconformably on basement granites and is usually correlated with the middle ventersdorp supergroup this neoarchean 2700 ma large igneous province lip has been interpreted as deposited in a tectonic escape basin scenario following the collision of the zimbabwe and kaapvaal cratons or alternatively in a mantle plume induced cratonic rift basin the basaltic komatiitic to andesitic dacitic volcanic sequence and the accommodated sedimentary deposits developed in a graben to half graben setting within a cratonic failed rift system the sodium group sedimentary facies and geochemistry of volcanic rocks correlate well with the ventersdorp supergroup the major and trace element and ree geochemistry correspond with previously published results crow and condie 1988 grobler et al 1989 and the sr and nd isotopy confirms crustal contamination of the sodium group magmatic rocks a preliminary weighted mean 207 pb 206 pb zircon age of 2739 39 ma for a tuff in the upper ongers river formation is reported for the lower part of the sodium group the ongers river t kuip and omdraaivlei formations reflect sedimentation and volcanism in a rift induced graben setting with fast sedimentation from horst flanks and high burial rates in the graben valleys and subsequently a less pronounced topography post graben environment accompanying explosive phreatomagmatic and effusive mafic to acidic bimodal volcanism with abundant pyroclastics delivered ample material for graben fill deposits repeated horst reactivation has episodically exposed the basement to renewed erosion sedimentation along graben flanks comprised alluvial fans mass flow deposits braided river and related modes of deposition in distal shallow lakes volcaniclastic suspension and current sedimentation low density turbidites and stromatolitic reefs were accumulated

ignimbrites of basaltic andesite and andesite compositions from tanna new hebrides arc tanna island is part of a large volcanic complex mainly subsided below sea level on land two series of hydroclastic deposits and ignimbrites overlie the subaerial remains of a basal mainly effusive volcano the older tanna ignimbrite series oti late pliocene or pleistocene in age consists of ash flows and ash and scoria flow deposits associated with fallout tephra layers overlain by indurated pumice flow deposits phreatomagmatic features are a constant characteristic of these tuffs the younger late pleistocene pyroclastics the siwi sequence show basal phreatomagmatic deposits overlain by two successive flow units each comprising a densely welded layer and a nonwelded ash flow deposit whole rock analyses of 17 juvenile clasts from the two sequences vitric blocks from the phreatomagmatic deposits welded blocks scoriaceous bombs and pumices from the ignimbrites show basaltic andesite and andesite compositions sio2 53 60 in addition 296 microprobe analyses of glasses in these clasts show a wide compositional range from 51 to 69 sio2 dominant compositions at 54 56 58 5 and 61 62 sio2 characterize the glass from the oti glass compositions in the lower phreatomagmatic deposits from the siwi sequence also show multimodal distribution with peaks at sio2 55 57 5 61 62 and 64 whereas the upper ignimbrite has a predominant composition at 61 62 sio2 in both cases mineralogical data and crystal fractionation models suggest that these compositions represent the magmatic signature of a voluminous layered chamber the compositional gradient of which is the result of fractional crystallization during two major eruptive stages probably related to two caldera collapses the oti and siwi ignimbrites represent large outpourings from these magmatic reservoirs the successive eruptive dynamics from phreatomagmatic to plinian emphasize the role of water in initiating the eruptions without which the mafic and intermediate magmas probably would not have erupted

geochemistry and origin of late archean volcanics from the ventersdorp supergroup south africa abstract the ventersdorp supergroup 2700 2750 ma is comprised chiefly of mafic to intermediate subaerially erupted volcanics with smaller amounts of clastic sediments the lower unit the klipriviersberg group consists of basal basaltic komatiites overlain by basalts which show a progressive increase in mg number and ni content and a decrease in incompatible element contents with increasing stratigraphic height volcanics of the overlying platberg and pniel groups consist of basaltic andesites and andesites and some felsic volcanics all ventersdorp volcanics exhibit a subduction zone geochemical component and are similar in incompatible element distributions to volcanics from continental margin arc systems both tholeiitic and calc alkaline trends occur in the ventersdorp succession with the former dominating crustal contamination appears to have played a minor role in the evolution of ventersdorp magmas with the exception of basaltic komatiite the klipriviersberg lavas can be related by progressive melting of a garnet lherzolite source containing a subduction zone component followed by up to 30 of shallow fractional crystallization the basaltic komatiite can be produced from the same source at higher temperatures and the pniel lavas by shallow fractional crystallization of basaltic komatiite the platberg volcanics must come from a within plate enriched mantle source and mafic and felsic members of this group can be related by shallow fractional crystallization geochemical and geologic data are consistent with a model for ventersdorp magmas involving a subduction zone in which klipriviersberg and pniel magmas are produced from the mantle wedge and platberg magmas come from an enriched mantle lithosphere with a superimposed subduction zone component secular trends in klipriviersberg lavas can be explained by progressive adiabatic melting of an ascending plume rooted in the mantle wedge

tubular microfossils from 2 8 to 2 7 ga old lacustrine deposits of south africa a sign for early origin of eukaryotes supported by the national science centre poland grant 2011 01 b st10 06479 to j k and by the program of scientific and technological co operation between the governments of the republic of poland and the republic of south africa to j k and w a we appreciate the europlanet transnational access programme for funding access to nanosims facilities at the milton keynes open university uk cyprian kulicki and krzysztof owocki warsaw helped with sem eds and cameca microprobe analysis katarzyna janiszewska warsaw kindly prepared the micro tomograph movie and 2 d slices comments and suggestions by the editor and reviewer are greatly appreciated we thank the late edwin jackson owner of the omdraaivlei farm for his support hospitality and interest in stromatolites and early life the research was partially supported by the european union within the european regional development fund through the innovative economy operational programme poig 02 02 00 00 025 09 supported by nanofun poig 02 02 00 00 025 09

dating plio pleistocene glacial sediments using the cosmic ray produced radionuclides 10be and 26al we use the cosmic ray produced radionuclides 26al and 10be to date plio pleistocene glacial sediment sequences these two nuclides are produced in quartz at a fixed ratio but have different decay constants if a sample is exposed at the surface for a time and then buried by overburden and thus removed from the cosmic ray flux the 26al 10be ratio is related to the duration of burial we first attempted to date pre wisconsinan tills by measuring 26al and 10be in fluvial sediments beneath them and applying the method of burial dating which previous authors have used to date river sediment carried into caves this method however requires simplifying assumptions about the 26al and 10be concentrations in the sediment at the time of burial we show that these assumptions are not valid for river sediment in glaciated regions 26al and 10be analyses of such sediment do not provide accurate ages for these tills although they do yield limiting ages in some cases we overcome this difficulty by instead measuring 26al and 10be in quartz from paleosols that are buried by tills we use a more general mathematical approach to determine the initial nuclide concentrations in the paleosol at the time it was buried as well as the duration of burial this technique provides a widely applicable improvement on other means of dating plio pleistocene terrestrial glacial sediments as well as a framework for applying cosmogenic nuclide dating techniques in complicated stratigraphic settings we apply it to pre wisconsinan glacial sediment sequences in southwest minnesota and eastern south dakota pre wisconsinan tills underlying the minnesota river valley were deposited 0 5 to 1 5 ma and tills beneath the prairie coteau in eastern south dakota and adjacent minnesota were deposited 1 to 2 ma

major influences on the evolution of the 2 67 2 1 ga transvaal basin kaapvaal craton abstract the transvaal is one of three structural basins of the transvaal supergroup preserved on the kaapvaal craton the evolution of the neoarchaean palaeoproterozoic transvaal basin is ascribed predominantly to magmatism palaeoclimate and eustasy with plate tectonics playing a sporadic role the supergroup comprises basal protobasinal rocks followed by the black reef formation chuniespoort group and uppermost pretoria group immature siliciclastic and bimodal volcanic rocks of the protobasinal unit reflect a wide zone of rifting related to the c 2 7 ga ventersdorp supergroup mantle plume individual protobasinal successions were laid down in separate fault bounded basins controlled at least partially by greenstone belt orientations in the kaapvaal basement post magmatic post rifting thermal subsidence accommodated black reef fluvial sheet sandstones and the subsequent thick carbonate bif epeiric platform succession of the chuniespoort group subordinate mechanical subsidence accompanied this long lived thermal relaxation intense weathering due to neoarchaean atmosphere composition greatly reduced clastic sedimentation and the greenhouse palaeoclimate further encouraged carbonate sedimentation globally enhanced sea levels due to enhanced mid ocean ridge growth consequent upon either global magmatic events or supercontinent break up also played a pivotal role in chuniespoort epeiric basin evolution pretoria group sedimentation is ascribed to two cycles of rifting and subsequent thermal subsidence the first cycle appears to reflect plate tectonically induced rifting with an epeiric sea drowning the rift basin despite reduced sea levels due to the first major global glaciation the second pretoria cycle is most likely related to a major continental flood basalt event with thermal subsidence allowing a second and probably larger epeiric sea to advance onto the northern kaapvaal craton evaluation of the inferred relatively minor importance of plate tectonics in the evolution of the essentially intracratonic transvaal basin depends to a large degree on the age assigned to the limpopo orogeny during which the zimbabwe and kaapvaal cratons collided recent evidence for a c 2 0 ga collision supports the concept that catastrophic global magmatic events gradually gave way to plate tectonics during transvaal basin evolution

pahoehoe and aa in hawaii volumetric flow rate controls the lava structure the historical records of kilauea and mauna loa volcanoes reveal that the rough surfaced variety of basalt lava called aa forms when lava flows at a high volumetric rate 5 10 m3 s and the smooth surfaced variety called pahoehoe forms at a low volumetric rate 5 10 m3 s this relationship is well illustrated by the 1983 1990 and 1969 1974 eruptions of kilauea and the recent eruptions of mauna loa it is also illustrated by the eruptions that produced the remarkable paired flows of mauna loa in which aa formed during an initial short period of high discharge rate associated with high fountaining and was followed by the eruption of pahoehoe over a sustained period at a low discharge rate with little or no fountaining the finest examples of paired lava flows are those of 1859 and 1880 1881 we attribute aa formation to rapid and concentrated flow in open channels there rapid heat loss causes an increase in viscosity to a threshold value that varies depending on the actual flow velocity at which when surface crust is torn by differential flow the underlying lava is unable to move sufficiently fast to heal the tear we attribute pahoehoe formation to the flowage of lava at a low volumetric rate commonly in tubes that minimize heat loss flow units of pahoehoe are small usually 1 m thick move slowly develop a chilled skin and become virtually static before the viscosity has risen to the threshold value we infer that the high discharge rate eruptions that generate aa flows result from the rapid emptying of major or subsidiary magma chambers rapid near surface vesiculation of gas rich magma leads to eruptions with high discharge rates high lava fountains and fast moving channelized flows we also infer that long periods of sustained flow at a low discharge rate which favor pahoehoe result from the development of a free and unimpeded pathway from the deep plumbing system of the volcano and the separation of gases from the magma before eruption achievement of this condition requires one or more episodes of rapid magma excursion through the rift zone to establish a stable magma pathway

the archean proterozoic boundary in the kaapvaal province of southern africa abstract the kaapvaal province has long been considered unusual in that archean cover sequences developed over granite greenstone gneiss terranes 3060 25 ga whereas in other provinces the oldest cover sequences are generally 2500 ga however two unconformity bounded sequences 2 5 ga and of greater inter regional extent than the unconformably underlying archean sequences can now be recognized in the kaapvaal province the two basal unconformities of these sequences are candidates for the temporally defined 2 5 ga archean proterozoic boundary the older but geologically more discordant unconformity occurs at the base of the uppermost group of the ventersdorp supergroup the pniel group and in at least five other erosional remnants in central transvaal and adjacent botswana these other remnants include parts of the buffelsfontein and wolkberg groups which heretofore have been considered to be conformable protobasinal phases of the younger transvaal succession the pniel and the other remnants are truncated by the locally well known unconformity at the base of the transvaal succession on a world wide basis cratonization and the evolution of mineralogically mature cover sequences were and continue to be diachronous the kaapvaal province is unusual only in that it also contains archean cover sequences however with major conformities at about 2 5 ga and the presence of proterozoic unconformity bounded sequences the kaapvaal province seems little different than most other provinces that contain the archean proterozoic boundary

sequence stratigraphy and plate tectonic significance of the transvaal succession of southern africa and its equivalent in western australia the transvaal succession in the northeastern part of the kaapvaal province of southern africa is the erosional remnant of four 2 7 to 2 1 ga unconformity bounded stratigraphic units or sequences each of which is 102 to 103 m thick and spans 107 to 108 years three of these phanerozoic like cratonic sequences occur in the western part of the kaapvaal province and in the pilbara province of western australia thus the two provinces once were part of the same continent vaalbara the stratigraphically lowest sequence historically assigned to the transvaal succession is part of the ventersdorp supergroup in kaapvaal and the upper part of the fortesque group in pilbara it consists predominantly of arkosic sandstone and basaltic andesite it probably is 2687 2 ma the second sequence contains the quartz arenites dolomites and banded iron formations of the transvaal and griqualand west successions in the kaapvaal and the hamersley group in the pilbara the span of ages 2684 6 to 2432 31 ma lithologies and geographic extent of this sequence imply that the limpopo orogeny which welded the zimbabwe province to vaalbara is 2 47 ga not 2 67 0 05 ga the third sequence the pretoria and postmasburg groups in the kaapvaal and the lower part of the wyloo group in the pilbara probably is the clastic wedge generated by the limpopo orogeny the fourth sequence dominated by the felsites of the rooiberg group is restricted to northeastern kaapvaal sequence stratigraphy lithostratigraphy and lithofacies indicate that the pilbara rifted from the southern edge of western kaapvaal the penge iron formation in northeastern kaapvaal the asbesheuwels iron formation in southwestern kaapvaal and the brockman iron formation in the pilbara are the same formation in which cyclothemic units 0 6 to 15 0 m thick extended 1200 km

cavernous weathering in the capitol reef desert utah tafoni and honeycomb weathering are abundant in sandstones at capitol reef national park cavities are particularly common in talus blocks resting on alkali rich soil in vertical walls of desert washes and in sandstones containing calcareous cement chemical analyses x ray diffraction data and field observations suggest that salt weathering is the most important cause of disintegration but dissolution of calcite cement also occurs in calcareous sandstones cavernous weathering at capitol reef produces cavities similar in appearance to features found in arkose outcrops along the coast of northwest washington even though the environmental conditions at these two locations are very different

conditions for branching in depositional rivers it is often taken for granted that rivers organize transport into a single active channel in some net depositional environments however flow of water and sediment is distributed in several stable channels such branching rivers may be confined in valleys anabranching or anastomosed or unconfined on deltas distributaries and their existence confronts us with the very basic question of what governs the spatial organization of channel patterns in sedimentary landscapes current models for equilibrium channel morphology cannot predict the occurrence of branching rivers because they do not consider dynamical processes such as avulsion i e the rapid abandonment of a channel in favor of a new path at lower elevation the requisite conditions for avulsion have been the subject of ongoing debate here we resolve the conditions leading to channel avulsion and show that branching rivers occur when avulsion is the dominant mechanism of lateral channel motion a compilation of field and laboratory data demonstrates that avulsion frequency scales with the time required for sedimentation on channel beds to produce a deposit equal to one channel depth from the relative rates of bank erosion and channel sedimentation we derive a dimensionless mobility number that accurately predicts the conditions under which anabranching and distributary channels occur results may be directly applied to modeling landscape evolution over human and geologic time scales and for inverting formative environmental conditions from channel deposits on earth and other planetary surfaces

recycling of amazon floodplain sediment quantified by cosmogenic 26al and 10be the modern amazon trunk stream and tributaries contain substantial amounts of sediments that were formerly stored for 1 3 m y in floodplains by reworking of floodplain deposits this sediment is being incorporated into amazonian streams we conclude this from using the ratio of in situ produced cosmogenic 26 al to 10 be in detrital sediment from rivers draining the sediment producing andean guyana and brazilian shield areas as well as the largest amazonian lowland rivers samples from the modern amazon river yield 26 al 10 be ratios between 3 8 and 5 5 and thus represent mixing between a non buried sediment end member and a formerly buried sediment end member the non buried end member is sourced in the andes with a ratio of 6 5 reflecting recent erosion of hillslopes exposed to cosmic rays and the buried end member is potentially represented by deeply stored floodplain sediment as old as miocene age with a ratio of 3 that was partially shielded from cosmic rays this simple picture of binary mixing is complicated by grain size effects coarser grained sediment from non andean settings lowlands cratonic shields contains slightly more formerly buried material whereas finer grained lowland sediment mostly records non buried 26 al 10 be ratios and nuclide concentrations that are indicative of an andean source our assessment shows an incorporation of formerly buried old sediment of central amazonian floodplains to the fresh andean derived sediment mainly carried in the amazon trunk stream we interpret our results to mean that the amazon regularly avulses into these old deposits that crop out in more distal vicinities to the main river and temporarily leaves its current holocene channel

10be and 26al production rates deduced from an instantaneous event within the dendro calibration curve the landslide of kofels otz valley austria surface exposure dating requires the knowledge of cosmogenic nuclide production rates when determining time integrated production rates the exposure ages of the calibration samples need to be accurately known the landslide of kofels austria is very well suited for this purpose it is the largest landslide in the crystalline alps of austria dating back to 7800 100 years bc ams 14c dating of buried wood which is well within the 14c dendro calibration curve exposed quartz veins were sampled from the tops of large boulders from the toe of the landslide for analysis of 10be and 26al to calculate sea level high geomagnetic latitude 60 open sky radionuclide production rates corrections were applied for altitude and latitude for shielding by surrounding mountains for sample geometry vegetation and snow cover and for sample thickness the production rates for an exposure age of 10 000 years are 5 75 0 24 10be atoms yr g sio2 and 37 4 1 9 26al atoms yr g sio2 a 26al 10be ratio of 6 52 0 43 can be calculated the influence of the geomagnetic field on these production rates has been estimated using two different geomagnetic field records our production rates should be a good approximation for the use of surface exposure dating between about 5000 and 13 000 years bp

the origin of honeycomb weathering honeycomb weathering occurs throughout the world but the origin remains a matter of controversy wind erosion exfoliation frost shattering and salt weathering have been proposed as explanations although few attempts have been made to substantiate these hypotheses with chemical or mineralogical studies chemical analyses and field observations indicate that honeycomb weathering in coastal exposures of arkosic sandstone near bellingham washington results from evaporation of salt water deposited by wave splash microscopic examination of weathered surfaces show that erosion results from disaggregation of mineral grains rather than from chemical decomposition thin walls separating adjacent cavities seem to be due to protective effects of organic coatings produced by microscopic algae inhabiting the rock surface cavity walls are not reinforced by precipitation of elements released by weathering as has often been suggested at other locations honeycomb weathering develops rapidly and can be observed on surfaces that were planar less than a century ago

a review of tectonics and sedimentation in a forearc setting hellenic thrace basin north aegean sea and northern greece abstract exposure of the forearc region of the north aegean sea greece offers insight into evolving convergent margins the sedimentary fill of the thrace basin during the late eocene to oligocene time provides a record of subduction driven processes such as growth of magmatic arcs and construction of accretionary complexes this large sediment repository received sediment from two sources the southern outboard basin margin reflects the active influence of the exhumed accretionary prism e g pindic cordillera or biga peninsula while the northern inboard margin records the effect of the magmatic arc in the rhodope region the forearc basin sedimentary fills shoal upward into shallow marine strata but are dominated mainly by deep marine facies the depositional trend and stacking pattern are dominated by progradational patterns this trend which is observed in both basin margins is related to tectonic deformation rather than sea level fluctuations additional evidence for this tectonic uplift comes from the backstripping analysis the accretionary complex provided material into the forearc basin this material was transported northeast and formed a sand rich turbidity system that evolved upslope into shallow marine deposits stratigraphic data indicate that this turbidity system exhibits a successive landward inboard migration of the depocenter provenance data utilizing sandstone petrography conglomerate clast composition and bulk rock geochemistry suggest that this system reflects an increased influx of mafic material into the basin volcanic arc derived material was transported south and east and accumulated in deep marine settings both stratigraphic and provenance data indicate a seaward outboard migration of the basin depocenter and a significant increase in felsic detritus into the forearc

discovery of abundant pyroclasts in the namunagarh grit south andaman evidence for arc volcanism and active subduction during the palaeogene in the andaman area abstract the predominant clastic rocks of the late eocene namunagarh grit south andaman island are coarse to fine grained and pebbly immature greywacke sandstones these rocks have long been known and interpreted as epiclastic deposits formed from the weathering and erosion of accreted ophiolite in a trench slope setting this study documents abundant juvenile vesiculated fragments pumice clasts shards unbroken vesicles cognate lithics of andesite broken and whole euhedral fresh plagioclase and chlorite replaced glassy matrix in these sandstones the sandstones evidently have a pyroclastic origin based on structures and textures a coarse grained and massive and a finer grained bedded and graded sandstone facies are identified the coarse grained facies occurs in the lower part of the section while the finer grained one interstratified with mudstone occurs in the upper part major element chemistry indicates a predominantly andesitic composition for these rocks the facies characteristics indicate that the coarse grained facies was emplaced as debris flows while the finer grained facies was transported by turbidity currents preservation of intact glassy constituents and their abundance imply direct sedimentation or very rapid re sedimentation with limited traction and limited time for alteration during transport these pyroclast rich sandstones were laid down in a submarine forearc environment on an accretionary complex the petrology of the juvenile clasts indicates the subaerial or shallow subaqueous explosive eruption of crystal rich vesicular magma from andesitic arc volcanoes located on an arc massif inferred to have existed on the western margin of the burma malaya continent during eocene oligocene times this evidence of arc volcanism indicates active subduction accretion and emplacement of ophiolite and the associated sediments from the andaman java trench the pyroclast rich sandstone represents a distinct and hitherto unrecorded unit in the tertiary stratigraphy of the andaman islands

distinguishing strongly rheomorphic tuffs from extensive silicic lavas high temperature silicic volcanic rocks including strongly rheomorphic tuffs and extensive silicic lavas have recently been recognized to be abundant in the geologic record however their mechanisms of eruption and emplacement are still controversial and traditional criteria used to distinguish conventional ash flow tuffs from silicic lavas largely fail to distinguish the high temperature versions we suggest the following criteria ordered in decreasing ease of identification to distinguish strongly rheomorphic tuffs from extensive silicic lavas 1 the character of basal deposits 2 the nature of distal parts of flows 3 the relationship of units to pre existing topography and 4 the type of source as a result of quenching against the ground basal deposits best preserve primary features can be observed in single outcrops and do not require knowing the full extent of a unit lavas commonly develop basal breccias composed of a variety of textural types of the flow in a finer clastic matrix such deposits are unique to lavas because the chilled base of an ashflow tuff generally does not participate in secondary flow primary pyroclastic features are best preserved there massive flow banded bases are more consistent with a lava than a pyroclastic origin lavas are thick to their margins and have steep abrupt flow fronts ashflow tuffs thin to no more than a few meters at their distal ends where they generally do not show any secondary flow features lavas are stopped by topographic barriers unless the flow is much thicker than the barrier ash flow tuffs moving at even relatively slow velocities can climb over barriers much higher than the resulting deposit lavas dominantly erupt from fissures and maintain fairly uniform thicknesses throughout their extents tuffs commonly erupt from calderas where they can pond to thicknesses many times those of their outflow deposits these criteria may also prove effective in distinguishing extensive silicic lavas from a postulated rock type termed lava like ignimbrite the latter have characteristics of lavas except for great areal extents up to many tens of kilometers these rocks have been interpreted as ash flow tuffs that formed from low boiling over eruption columns based almost entirely on their great extents and the belief that silicic lavas could not flow such distances however we interpret the best known examples of lava like ignimbrites to be lavas this interpretation should be tested through additional documentation of their characteristics and research on the boiling over eruption mechanism and the kinds of deposits it can produce flow bands flow folds ramps elongate vesicles and probably upper breccias occur in both lavas and strongly rheomorphic tuffs and are therefore not diagnostic pumice and shards also occur in both tuffs and lavas although they occur throughout ash flow tuffs and generally only in marginal breccias of lavas dense welding secondary flow and intense alteration accompanying crystallization at high temperature commonly obliterate primary textures in both thick rheomorphic tuffs and thick lavas high temperature silicic volcanic rocks are dominantly associated with tholeiitic flood basalts extensive silicic lavas could be appropriately termed flood rhyolites

cosmogenic nuclide budgeting of floodplain sediment transfer abstract cosmogenic nuclides produced in quartz may either decay or accumulate while sediment is moved through a river basin a change in nuclide concentration resulting from storage in a floodplain is potentially important in large drainage basins in which sediment is prone to repeated burial and remobilization as a river migrates through its floodplain we have modeled depth and time dependent cosmogenic nuclide concentration changes for 10 be 26 al and 14 c during sediment storage and mixing in various active floodplain settings ranging from confined shallow rivers with small floodplains to foreland basin scale floodplains traversed by deep rivers floodplain storage time estimated from channel migration rates ranges from 0 4 kyr for the beni river basin bolivia to 7 kyr for the amazon river basin while floodplain storage depth estimated from channel depth ranges from 1 to 25 m for all modeled active floodplain settings the long lived nuclides 10 be and 26 al show neither significant increase in nuclide concentration from irradiation nor decrease from decay we predict a hypothetical response time after which changes in 10 be or 26 al concentrations become analytically resolvable this interval ranges from 0 07 to 2 myr and exceeds in all cases the typical residence time of sediment in a floodplain due to the much shorter half life of 14 c nuclide concentrations modeled for the in situ produced variety of this nuclide are however sensitive to floodplain storage on residence times of the cosmogenic nuclide composition of old deposits in currently inactive floodplains that have been isolated for periods of millions of years from the river that once deposited them is predicted to either increase or decrease in 10 be and 26 al concentration depending on the depositional depth these conditions can be evaluated using the 26 al 10 be ratio that readily discloses the depth and duration of storage we illustrate these models with examples from the amazon basin as predicted modern bedload collected from an amazon tributary the bolivian beni river shows no systematic change in nuclide concentration as sediment is moved through 500 km of floodplain by river meandering in contrast in the central amazon floodplain currently untouched by the modern river system low 26 al 10 be ratios account for minimum burial depths of 5 to 10 m for a duration of 5 myr the important result of this analysis is that in all likely cases of active floodplains cosmogenic 10 be and 26 al concentrations remain virtually unchanged over the interval sediment usually spends in the basin thus spatially averaged denudation rates of the sediment producing area can be inferred throughout the entire basin provided that nuclide production rates are scaled for the altitudes of the sediment producing area only because floodplain storage does not modify nuclide concentrations introduced from the sediment source area

volcanic rocks of the witwatersrand triad south africa i description classification and geochemical stratigraphy abstract the witwatersrand triad contains thick volcanic sequences confined largely to the dominion group at the base and the ventersdorp supergroup at the top these volcanic sequences are of late archaean to early proterozoic age and are amongst the oldest supracrustal volcanic sequences erupted onto the archaean kaapvaal craton the volcanic rocks have suffered low grade greenschist facies metamorphism but primary textures and in some samples primary mineralogies are well preserved major and trace element analyses of a large number of samples of volcanic rocks from boreholes in the klerksdorp area western transvaal indicate that the chemical compositions of the lavas are largely unmodified by secondary processes and can be used for classification stratigraphic studies and petrogenetic interpretation the volcanics of the dominion group are a bimodal tholeiitic basalt rhyolite association with considerable compositional variation from basalt to tholeiitic andesite within the basaltic suite the ventersdorp supergroup is a weakly developed bimodal tholeiitic basalt dacite suite basic rocks in the klipriviersberg group at the base and in the overlying platberg group are largely basaltic whereas those of the allanridge formation at the top are largely tholeiitic andesites within the klipriviersberg group the more primitive samples occur at the top of the sequence phenocryst rich porphyries of the makwassie formation in the platberg group are largely of dacitic composition and have textural features suggestive of an ash flow origin a distinct geochemical stratigraphy is present within the volcanic sequences and application of discriminant function analysis and an empirical discrimination grid based on ratios of immobile elements p ti and zr allows for unknown samples to be placed within their correct stratigraphic unit

raindrop imprints in the late archaean early proterozoic ventersdorp supergroup south africa abstract well preserved raindrop imprints occur in ventersdorp rocks of randian age 2 7 ga on mudflakes at five different stratigraphic levels in a sequence which consists of alternating beds of arkose and tuff the raindrop imprints indicate that an atmosphere did exist and that the arkose was deposited in an environment periodically exposed to the atmosphere these are as far as is known the oldest raindrop craters ever to be described special conditions governing their formation and preservation are given

stratigraphy of the kos plateau tuff product of a major quaternary explosive rhyolitic eruption in the eastern aegean greece the kos plateau tuff kpt erupted during a moderate volume explosive rhyolitic event approximately 161 ka from a source south of kos in the eastern aegean sea six major stratigraphic units have been identified from a at the base to f uppermost unit a is a widespread vitric ash fall layer that is thickest 1 5 m and most extensive southeast of the source unit b is a 1 to 2 m thick low angle cross stratified armoured pumice lapilli and ash layer found on kos unit c resembles unit b but includes a greater abundance of lithic lapilli less fine ash is only diffusely stratified and is on kos and west of the source unit d includes a sequence of three non welded 1 to 20 m thick ignimbrites that extend radially 38 km from the source in areas of low topography unit e is a sequence of two non welded 3 to 8 m thick ignimbrites which occur radially from the vent regardless of topography 64 km from source unit f has a 6 m thick basal low angle cross stratified armoured pumice lapilli and ash part probably deposited radially from source the upper part of unit f is a widespread 1 m thick vitric ash fall layer found to at least 50 km from the source these six units represent a change in eruptive conditions from initial and final phreatomagmatic activity depositing fallout and internally stratified pyroclastic density current deposits to dry explosive during the more intense phases of the eruption which generated ignimbrites

evolution of an archaean intracratonic basin a review of the transvaal supergroup lithostratigraphy in botswana abstract several lithostratigraphic and radiometric dating studies over the last half century have generated a robust stratigraphic data set about the transvaal supergroup sedimentary deposits throughout southern africa the transvaal supergroup in southern africa represents one of the best examples of a precambrian intracratonic sequence including one of the best preserved and extensive archaean palaeoproterozoic carbonate platforms sediments of the transvaal supergroup are hosted on the kaapvaal craton in three areas of south africa and botswana critically the smallest of the three transvaal depocentres the kanye basin including the molepolole mochudi sub basin is the least known and the stratigraphic model is still incomplete the kanye basin in southern botswana exposes a mixed volcanic sedimentary succession spanning from the meso neoarchaean to the palaeoproterozoic including the complete sequence of the lower transvaal supergroup lts and the lower part of the upper transvaal supergroup uts although no consensus has been reached so far the deposition of the transvaal age sediments in botswana likely followed the same thermal and tectonic processes recorded in south africa albeit diachronously the initial stage of rifting was followed by sediments that formed in an epicontinental sea setting that recorded the deepening of the basin the younger sequence records the reactivation of the extensional tectonics alternating with post rift thermal subsidence the nomenclature of the lithostratigraphic units in botswana is often inconsistent and the correlations with the main transvaal basins in south africa are imprecise because of the importance of the transvaal supergroup for economic e g skarn deposits and placers and hydrologic e g the ramotswa trans boundary aquifer reasons the lack of a consistent nomenclature can introduce ambiguity in the scientific literature and lead to misinterpretations a thorough lithostratigraphic revision of the transvaal supergroup in botswana will also solve the puzzle of the transvaal age deposits in southern africa allowing reliable interpretations across the three basins here a comprehensive lithostratigraphic and chronostratigraphic description of the transvaal supergroup is provided based on literature data and new observations new lithostratigraphic charts are produced for the kanye basin revealing the stratigraphic relationships within the different transvaal basins and the effect of cyclicity in the evolution of this precambrian basin

zircon ion microprobe studies bearing on the age and evolution of the witwatersrand triad abstract the ventersdorp and witwatersrand supergroups and the dominion group collectively referred to as the witwatersrand triad represent some of the oldest and economically most important volcano sedimentary basins within the kaapvaal craton of southern africa precise age constraints which are vital to models aimed at understanding the formation mineralisation and evolution of these basins have been hampered by analytical and geological problems these problems have been overcome by this ion microprobe study on zircons from volcanic rocks within the three basins felsic and mafic volcanics from the ventersdorp supergroup yield ages of 2709 4 ma and 2714 8 ma respectively constraining the timing of the closure of the witwatersrand basin a maximum age for the witwatersrand basin is indicated by the age of 3074 6 ma for volcanics from the underlying dominion group thus a period of c 350 ma was available for the accumulation of the full witwatersrand basin succession unfortunately dating within the witwatersrand basin itself is not complete and future work could show that extended time breaks may exist within the basin these new data show that previous age determinations on the ventersdorp witwatersrand and dominion basins were inaccurate and that the economically important witwatersrand basin must now be recognised as archaean

ignimbrite depositional facies the anatomy of a pyroclastic flow a model is presented for the depositional regimes in a pyroclastic flow to explain the origin of facies in the taupo ignimbrite and to compare these with other published examples in this model a pyroclastic flow consists of a head a body and a tail the head which is where fluidization caused by air ingestion occurs generates layer 1 deposits on lithological grounds these are divided into layers 1 p which is fines depleted and rich in pumice and 1 h which is fines depleted and rich in lithics and crystals layer 1 p represents material thrown forwards from the flow head and is termed the jetted deposits layer 1 h represents material sedimented from within the flow head and is termed the ground layer the body represents the bulk of the flow and the tail is its trailing part which is slowed by ground friction these parts generate the layer 2 deposits which include the valley pond ignimbrite and its associated ignimbrite veneer deposit localized depositional modes within the body and tail generate distinctive coarse pumice concentration zones and pumiceous lee side lenses behind obstacles at its outer limits all of the flow on interfluves is affected by air ingestion fluidization producing the distant facies which combines features of both layer 1 and 2 deposits one important conclusion of this work is that the thicknesses and compositions of the various facies are not related in any simple way to the thickness and composition of the parent flow

steep rock lake sedimentology and geochemistry of an archean carbonate platform abstract the 2800 ma mosher carbonate formation at steep rock lake in south central canada is one of earth s oldest limestone deposits it is both thick up to 500 m and relatively well preserved and was one of the first precambrian successions to be noted for evidence of early life it continues to provide information on the antiquity of such processes as photosynthesis the carbonate contains a variety of fabrics and facies from apparently fine grained bedded limestones with domal and columnar stromatolites atikokania radial fans and sheet cracks to closely packed metric scale hybrid domes of originally aragonite seafloor crust interlayered with fenestral fabric here we review previous work provide new observations concerning these sediments and their geochemistry and discuss the inception overall development and ultimate demise of the steep rock platform we divide the mosher carbonate into the lower hogarth member and upper elbow point member these units constitute a shallow water carbonate platform succession the hogarth member overlies eroded 3000 ma tonalite and is at least 120 m thick thin horizons of domal pseudocolumnar columnar and fenestral stratiform stromatolites and occasional layers of atikokania radial crystal fans occur within relatively fine grained thin bedded limestone with iron carbonate horizons in the upper part meter scale domes occur locally with sheet cracks and stromatactis the irregular laminae that build the small domal and columnar stromatolites are interpreted to be lithified mats and biofilms presumably cyanobacteria dominated fine grained well bedded facies of the lower hogarth member and its varied association of small stromatolites is interpreted to have accumulated in a relatively protected inner platform environment numerous very thin horizons of iron carbonate in the lower and upper parts of the hogarth member suggest temporally limited influence of iron rich offshore waters vertical trends in fe mn ba sr ce and 13 c probably reflect progressive restriction of circulation with at least one abrupt but short lived change back to more open seawater conditions the upper hogarth member with large banded domes and fenestral crusts and stromatolites is interpreted as back margin facies the overlying elbow point member is at least 70 m thick and appears to be entirely composed of innumerable juxtaposed elongate smooth and thickly layered domes lacking intervening sediment each dome is typically several meters across and composed of centimetric alternations of cuspate fenestral fabric possibly microbial and crystal fan fabric that is probably abiotic seafloor aragonite precipitate adjacent cuspate fenestral and crystal fan fabric show significant differences in concentration of 13 c sr ba fe and mn this giant dome facies is interpreted as a platform margin deposit in which alternating calcite aragonite mineralogies within each dome reflect a laterally fluctuating offshore redox boundary negative ce anomalies positive gd anomalies and reduced positive eu anomalies relative to laterally correlative deep water iron formation occur in the majority of crystal fan samples and in some cuspate fenestral fabric samples ca carbonate precipitation at the margin of an anoxic iron rich sea would have been promoted by oxygenic removal of iron from seawater otherwise fe carbonate would have been favored over ca carbonate precipitation the mosher carbonate formation is interpreted as an early marine oxygen oasis a shallow platform that favored cyanobacterial productivity was sufficiently isolated from open marine circulation for the seawater to become relatively oxygenated and where carbonate sediment aggradation was a positive feedback that sustained these shallow water conditions limestone accumulation terminated as transgressive suboxic and anoxic seawater deposited first manganese oxide rich and then manganese oxide poor iron formation sediments

cosmocalc an excel add in for cosmogenic nuclide calculations as dating methods using terrestrial cosmogenic nuclides tcn become more popular the need arises for a general purpose and easy to use data reduction software the cosmocalc excel add in calculates tcn production rate scaling factors using lal stone dunai and desilets methods topographic snow and self shielding factors and exposure ages erosion rates and burial ages and visualizes the results on banana style plots it uses an internally consistent tcn production equation that is based on the quadruple exponential approach of granger and smith 2000 cosmocalc was designed to be as user friendly as possible although the user interface is extremely simple the program is also very flexible and nearly all default parameter values can be changed to facilitate the comparison of different scaling factors a set of converter tools is provided allowing the user to easily convert cut off rigidities to magnetic inclinations elevations to atmospheric depths and so forth because it is important to use a consistent set of scaling factors for the sample measurements and the production rate calibration sites cosmocalc defines the production rates implicitly as a function of the original tcn concentrations of the calibration site the program is best suited for be 10 al 26 he 3 and ne 21 calculations although basic functionality for cl 36 and c 14 is also provided cosmocalc can be downloaded along with a set of test data from http cosmocalc googlepages com

eruption and emplacement of a basaltic welded ignimbrite during caldera formation on gran canaria the 14 1 ma old composite ignimbrite cooling unit p1 45 km3 on gran canaria comprises a lower mixed rhyolite trachyte tuff a central rhyolite basalt mixed tuff and a slightly rhyolite contaminated basaltic tuff at the top the basaltic tuff is compositionally zoned with a an upward change in basalt composition to higher mgo content 4 3 5 2 wt b variably admixed rhyolite or trachyte commonly 5 wt and c an upward increasing abundance of basaltic and plutonic lithic fragments and cognate cumulate fragments the basaltic tuff is divided into three structural units i the welded basaltic ignimbrite which forms the thickest part c 95 vol and is the main subject of the present paper ii poorly consolidated massive bomb and block rich beds interpreted as phreatomagmatic pyroclastic flow deposits and iii various facies of reworked basaltic tuff tuff unit i is a basaltic ignimbrite rather than a lava flow because of the absence of top and bottom breccias radial sheet like distribution around the central tejeda caldera thickening in valleys but also covering higher ground and local erosion of the underlying p1 ash a gradual transition from dense rock in the interior to ash at the top of the basaltic ignimbrite reflects a decrease in welding the shape of the welding profile is typical for emplacement temperatures well above the minimum welding temperature a similar transition occurs at the base where the ignimbrite was emplaced on cold ground in distal sections in proximal sections the base is dense where it was emplaced on hot felsic p1 tuff the intensity of welding especially at the base and the presence of spherical particles and of mantled and composite particles formed by accretion and coalescence in a viscous state imply that the flow was a suspension of hot magma droplets the flow most likely had to be density stratified and highly turbulent to prevent massive coalescence and collapse model calculations suggest eruption through low pyroclastic fountains 1000 m high with limited cooling during eruption and turbulent flow from an initial temperature of 1160 c the large volume of 26 km3 of erupted basalt compared with only 16 km3 of the evolved p1 magmas and the extremely high discharge rates inferred from model calculations are unusual for a basaltic eruption it is suggested that the basaltic magma was erupted and emplaced in a fashion commonly only attributed to felsic magmas because it utilized the felsic p1 magma chamber and its ring fissure conduits evolution of the entire p1 eruption was controlled by withdrawal dynamics involving magmas differing in viscosity by more than four orders of magnitude the basaltic eruption phase was initially driven by buoyancy of the basaltic magma at chamber depth and continued degassing of felsic magma but most of the large volume of basalt magma was driven out of the reservoir by subsidence of a c 10 km diameter roof block which followed a decrease in magma chamber pressure during low viscosity basaltic outflow

a 30 000 yr record of erosion rates from cosmogenic 10be in middle european river terraces abstract cosmogenic 10 be in river borne quartz sand records a time integrated erosion rate representative of an entire drainage basin when sequestered in a terrace of known age paleo erosion rates may be recovered from the nuclide content of the terrace material paleo erosion rates between 30 and 80 mm kyr are determined from terrace sediments 200 to 30 000 yr in age of the allier and dore rivers france and the meuse maas river the netherlands erosion rates determined from cosmogenic nuclides on terraces from the allier river are consistent with rates derived from the sedimentary fill of a lake in the allier catchment a strong decrease in cosmogenic nuclide derived erosion rates from terraces of the meuse river with late pleistocene to holocene age is observed the paleo erosion signal from cosmogenic nuclides records projection of the elevated late pleistocene erosion rate into the time integrated rates derived from middle european rivers

from source to sink preserving the cosmogenic 10be derived denudation rate signal of the bolivian andes in sediment of the beni and mamore foreland basins abstract the denudation rate signal of the bolivian andes as measured by cosmogenic 10 be in sediment is preserved in the floodplain of adjacent foreland basins even though these basins store the sediment for thousands of years this conclusion is drawn from comparing published andean source area denudation rates with new cosmogenic 10 be data as measured in the floodplains of the large beni and mamore basins for the entire beni basin including the sediment producing andes and the vast flooded plains of the foreland the cosmogenic nuclide derived denudation rate is 0 45 0 06 mm yr while that of the mamore basin is 0 55 0 19 mm yr by comparison the respective andean source areas erode at averaged rates of 0 37 0 06 mm yr upper beni and at 0 56 0 09 mm yr upper mamore we notice a decrease in variability of denudation rate with increasing spatial scale as small scale processes are averaged out sediment mixing within the floodplain damps scatter of nuclide derived denudation rates picked up in the source area on the temporal scale a remarkable agreement between cosmogenic nuclide derived rates averaging over a few kiloyears with those from fission track dating averaging over millions of years seems to suggest that cosmogenic nuclide derived denudation rates capture the long term erosional characteristics of the mountain belt the sum of these observations suggests that any sample collected along a river traversing a floodplain will yield the denudation rate of the source area this finding opens the unique possibility of constraining cosmogenic nuclide derived paleo sediment budgets for these large basins as the long term spatially averaged denudation rate signal of the sediment producing area is preserved in sedimentary archives

an early proterozoic three stage rift system kaapvaal craton south africa abstract a three stage model that constitutes a fully evolved rift system is proposed for a number of superimposed early proterozoic basins on the kaapvaal craton based on interpreted depositional systems and syndepositional tectonics this model consists of three superimposed basins 1 1 a pre graben stage that influenced the dominion group witwatersrand supergroup and klipriviersberg group of the ventersdorp supergroup 2 2 graben development that superimposed the platberg group bothaville and allanridge formations of the ventersdorp supergroup 3 3 post graben thermal subsidence that superimposed the black reef quartzite formation and the chuniespoort ghaap group of the transvaal supergroup oblique collision of the kaapvaal craton and zimbabwe craton is interpreted to have generated stresses that initiated the three stage system at different time intervals compressional deviatoric stress appears to have replaced tensional stress and kept the system in operation tectonically the system is analogous to the baikal rift system and the pannonian basin system while the general configuration of the superimposed basins is comparable to the north sea basin or the north china basin

channel floodplain geomorphology along the solimoes amazon river brazil across the cratonic landscape of brazil the solimoes amazon river transports to its delta plain 1240 mt of suspended sediment derived from andean erosion and reworks another 3200 mt of floodplain sediments distribution of these sediments has resulted in a variable along stream pattern of geomorphology the upstream reaches are characterized by sediment erosion in the main channel and deposition in floodplain channels that are an order of magnitude smaller in discharge than the main channel sediment deposition in and migration of the floodplain channels erases oxbow lakes of the main channel and yields an intricate scroll bar topography that forms the boundaries of hundreds of long narrow lakes in contrast downstream reaches are characterized by channels restricted by stabilizing long term levee building and floodplain construction dominated by overbank deposition overbank deposition buries the scroll bar topography resulting in a flat floodplain covered by a patchwork of large more equant shallow lakes on the basis of estimated rates of recycling of floodplain sediments the modern floodplain of the brazilian amazon could have been recycled in 5000 yr and is recycled more rapidly in the upstream than the downstream reaches the cratonic interior is interrupted by structural arches that bound intracratonic basins four of these arches cross the valley of the main river system at intervals of several hundred kilometres and impart a tectonic imprint on the channel floodplain geomorphology at this spatial scale structural arches appear to exert a primary influence by promoting entrenchment of the river as it passes through zones of deformation thus restricting channel movement for example as the river crosses the purus arch the valley narrows to 20 km compared to an average of 45 km the water surface gradient decreases sediment is deposited and yet the rate of channel migration is negligible hence the effect of the arches is to create a landscape where on the spatial scale of hundreds of kilometres the river is confined and entrenched in its valley is straight and is relatively immobile local valley tilting apparently unrelated to the arch structures also imprints the geomorphology in particular a tilted valley in the upstream reaches appears to have caused avulsions which have left behind the only large scale oxbow type features on the brazilian amazon river floodplain

build up and depositional dynamics of an arc front volcaniclastic complex the miocene tepoztlan formation transmexican volcanic belt central mexico volcanic terrains such as magmatic arcs are thought to display the most complex surface environments on earth ancient volcaniclastics are notoriously difficult to interpret as they describe the interplay between a single or several volcanoes and the environment the early miocene tepoztlan formation at the southern edge of the transmexican volcanic belt belongs to the few remnants of this ancestral magmatic arc and therefore is thought to represent an example of the initial phase of evolution of the transmexican volcanic belt based on geological mapping detailed logging of lithostratigraphic sections palaeocurrent data of sedimentary features and anisotropy of magnetic susceptibility mapping of two dimensional panels from outcrop to field scale and geochronological data in an area of ca 1000 km 2 three periods in the evolution of the tepoztlan formation were distinguished which lasted around 4 myr and are representative of a volcanic cycle edifice growth phases followed by collapse in a magmatic arc setting the volcaniclastic sediments accumulated in proximal to medial distances on partly coalescing aprons similar to volcanic ring plains around at least three different stratovolcanoes these volcanoes resulted from various eruptions separated by repose periods during the first phase of the evolution of the tepoztlan formation 22ae 8t o 22 ae2 ma deposition was dominated by fluvial sediments in a braided river setting pyroclastic material from small andesitic dacitic composite volcanoes in the near vicinity was mostly eroded and reworked by fluvial processes resulting in sediments ranging from cross bedded sand to an aggradational series of river gravels the second phase 22ae 2t o 21 ae3 ma was characterized by periods of strong volcanic activity resulting in voluminous accumulations of lava and tuff which temporarily overloaded and buried the original fluvial system with its detritus continuous build up of at least three major volcanic centres further accentuated the topography and in the third phase 21ae 3t o 18 ae8 ma mass flow processes represented by an increase of debris flow deposits became dominant marking a period of edifice destruction and flank failures

sedimentology geochemistry and palaeogeographic implications of volcanic rocks in the upper archaean campbell group western kaapvaal craton south africa abstract tuffs and lava interbedded in campbell group carbonates and shales are investigated for their sedimentological and geochemical properties the most proximal tuffs occur at the southwestern margin of the kaapvaal craton in griqualand west they were deposited in a shallow marine to tidal carbonate environment from hydroclastic eruptions the tuffs along this margin are subdivided into two main facies types 1 graded fallout lapilli to ash tuffs containing accretionary lapilli and 2 current reworked laminated and rippled lapilli to ash tuffs calculation of water depth from settling velocities of tuff particles indicates deposition in depth of a maximum of some 30 m the tuff beds generally thin out towards the north and northwest but distal tuff layers are widespread in the campbell group and allow for some lateral correlation within regionally narrow limits single zircon age dates suggest that correlation over the entire basin is unreliable two geochemical groups of tuffs are tentatively distinguished one group includes andesites and basalts and tends to concentrate in higher stratigraphic levels than the other which consists exclusively of basalts volcanism was of a tholeiitic within plate character situated in a rifted cratonic setting that failed to develop an oceanic basin geochemical similarity to some karoo volcanic rocks is striking a cratonic rift basin model for the griqualand west depository in which carbonate sedimentation in shallow environments prevailed is discussed the sedimentation of carbonates was periodically interrupted by hydroclastic eruptions along a rifting graben or half graben system

clay mineral composition of river sediments in the amazon basin abstract clay minerals are important in evaluating the maturity of suspended sediments weathering intensity and source area however there are processes that can change the mineral assemblage such as river transportation deposition remobilization and tributary inputs in terms of water discharge and sediment yield the amazon is one of the largest rivers in the world most of the suspended sediments come from the andes crossing the lowlands before reaching the ocean this study measures the spatial distribution of clay mineral assemblages over the entire amazon basin the results obtained show the main features of the amazon river main stem and larger tributaries from their sources to their confluence clay mineral composition highlights the evolution of the madeira and maranon solimoes river which start in the andes with high illite chlorite content downstream smectite contents increase moreover all shield tributaries show high kaolinite content the lower amazon river is characterized by relative high smectite content different from the andean sources the clay mineral results show that suspended sediments of the amazon river have three main sources 1 the andes mountains 2 the amazon shields and 3 the piedmont basins especially the pastaza alluvial megafan and the fitzcarrald arch basin lateral bank erosion plays also a significant role by the introduction of more mature sediments into the river enriched in smectite

the reliability of anisotropy of magnetic susceptibility ams data as flow direction indicators in friable base surge and ignimbrite deposits italian examples abstract the base surge deposits in linosa and procida islands italy show minimum axes k3 of the anisotropy of magnetic susceptibility ams that are mostly parallel to the flow directions inferred from field evidence but sometimes perpendicular to them samples from different heights above the base of an ignimbrite bed vulsini area italy showing a coarse tail reverse grading and a fine grained basal layer have different magnetic fabrics at 10 and 40 cm from the base the declinations of maximum axes k1 of the ams are approximately 90 from the theoretical flow direction whereas at 60 and 75 cm from the base the k1 axes are approximately parallel to the flow direction this bed also shows a decrease upward of the magnetic foliation lineation and degree of anisotropy suggesting that the ams data could be used to obtain information about flow mechanisms the airm data in the case of the ignimbrite bed show that the long shape axes of the ferromagnetic grains are probably aligned with the maximum axes of the ams for this reason the change in the magnetic fabric at different distances from the base is probably due to differences in the flow behaviour rather than the domain state of magnetite grains this study shows that the ams lineations can indicate the flow direction in base surge and ignimbrite deposits but other geological and magnetic information is necessary to assess if these lineations are flow perpendicular or flow parallel

chronological correlations between the pilbara and kaapvaal cratons abstract the early geological development of the pilbara and kaapvaal cratons has many features in common attempts have been made to correlate geologically similar features of the two cratons and it has been postulated that they originated as contiguous components of a single continent vaalbara during this time the early geological histories of the pilbara and kaapvaal cratons are here compared in detail and the evidence that they were initially contiguous is assessed these comparisons indicate significant differences in the chronologies of magmatic events within the granite greenstone crusts of the pilbara and kaapvaal cratons in addition igneous correlatives emplaced during ca 2985 and 2782 ma magmatic events on the kaapvaal craton have not been identified on the pilbara craton and a well defined 2760 ma magmatic event manifest as widespread emplacement of granitic rocks into the pilbara granite greenstone basement and eruption of flood basalts of the lower part of the fortescue group is absent from the kaapvaal craton furthermore similarities in first and second order transgression regression cycles within the sedimentary supracrustal sequences may be attributable to global sea level fluctuations and thus may be irrevelant to the question of former contiguity however similarities in some aspects of the geological development of the pilbara and kaapvaal cratons imply that there were periods extending for between 60 and 200 ma of the archaean era during which the style of crust formation intensity of volcanism and subaerial erosion and magnitude of sea level fluctuations may have varied on a global scale such similarities include the overall duration of formation of the granite greenstone crusts from ca 3650 to 3100 ma the onset of craton wide erosion in the interval ca 3125 to 3000 ma the major episodes of flood basaltic volcanism between 2760 and 2680 ma the predominance of chemical carbonate and banded iron formation sedimentation between ca 2630 and 2440 ma and the transition to widespread clastic sedimentation within the interval 2440 to 2200 ma

biogenic origin of coastal honeycomb weathering honeycomb weathering occurs in two environments in late cretaceous and eocene sandstone outcrops along the coastlines of south west oregon and north west washington usa and south west british columbia canada at these sites honeycomb weathering is found on subhorizontal rock surfaces in the intertidal zone and on steep faces in the salt spray zone above the mean high tide level in both environments cavity development is initiated by salt weathering in the intertidal zone cavity shapes and sizes are primarily controlled by wetting drying cycles and the rate of development greatly diminishes when cavities reach a critical size where the amount of seawater left by receding tides is so great that evaporation no longer produces saturated solutions encrustations of algae or barnacles may also inhibit cavity enlargement in the supratidal spray zone honeycomb weathering results from a dynamic balance between the corrosive action of salt and the protective effects of endolithic microbes subtle environmental shifts may cause honeycomb cavity patterns to continue to develop to become stable or to coalesce to produce a barren surface cavity patterns produced by complex interactions between inorganic processes and biologic activity provide a geological model of self organization surface hardening is not a factor in honeycomb formation at these study sites salt weathering in coastal environments is an intermittently active process that requires particular wind and tidal conditions to provide a supply of salt water and temperature and humidity conditions that cause evaporation under these conditions salt residues may be detectable in honeycomb weathered rock but absent at other times honeycomb weathering can form in only a few decades but erosion rates are retarded in areas of the rock that contain cavity patterns relative to adjacent non honeycombed surfaces copyright 2010 john wiley sons ltd

neoarchean 2 7 ga lacustrine stromatolite deposits in the hartbeesfontein basin ventersdorp supergroup south africa implications for oxygen oases abstract the hartbeesfontein basin contains the most extensive deposits of archean lacustrine stromatolites on the kaapvaal craton with stromatolitic facies occurring over 100 km 2 in beds up to 7 m thick stromatolitic dolostones and cherts both preserve evidence of microbial processes dolomitic stromatolites have grumelous microspar textures between organic rich laminae suggestive of carbonate precipitation within microbial mats stromatolitic laminae within chert preserve detrital material beyond the angle of repose indicating the trapping and binding of grains by microbial mats stromatolitic cherts also preserve fenestral textures and filamentous microfossils many fenestrae have rounded shapes surrounded by filamentous laminae and appear to have formed in situ within stromatolite fabrics before lithification fenestrae within stromatolitic chert resemble hourglass associated fenestrae noted from recent silica stromatolites from yellowstone national park and are interpreted to originate from gas bubbles forming within stromatolite building mats the preservation of delicate structures in hartbeesfontein stromatolitic chert e g filamentous microfossils and gas related fenestrae implies rapid lithification of microbial mats while the mm to cm scale of fenestrae indicate equally rapid rates of microbial gas production textural and mineralogical evidence associated with gas related fenestrae support the presence of oxygenic photosynthesis which in turn strengthens previous hypotheses on archean lakes as potential oxygen oases before the great oxidation event

sediment production and delivery in the amazon river basin quantified by in situ produced cosmogenic nuclides and recent river loads we use cosmogenic nuclide derived denudation rates from in situ produced 10 be in river sediment to determine sediment production rates for the central amazon river and its major tributaries recent developments have shown that this method allows calculating denudation rates in large depositional basins despite intermediate sediment storage with the result that fluxes of the sediment producing hinterland can now be linked to those discharged at the basins outlet in rivers of the central amazonian plain sediment of finer grain sizes 125 500 m yields a weighted cosmogenic nuclide derived denudation rate of 0 24 0 02 mm yr that is comparable to the integrated rate of all main andean draining rivers 0 37 0 06 mm yr which are the beni napo mamore ucayali and maranon rivers coarser grained sediment 500 m of central amazonian rivers is indicative of a source from the tectonically stable cratonic headwaters of the guyana and brazilian shields for which the denudation rate is 0 01 0 02 mm yr respective sediment loads can be calculated by converting these cosmogenic nuclide derived rates using their sediment producing areas for the amazon river at obidos a sediment production rate of 610 mt yr results non andean source areas contribute only 45 mt yr a comparison with published modern sediment fluxes shows similarities within a factor of 2 with an average gauging derived sediment load of 1000 mt yr at obidos for example we attribute this similar trend in cosmogenic versus modern sediment loads first to the absence of long term deposition within the basin and second to the buffering capability of the large amazon floodplain the buffering capability dampens short term high amplitude fluctuations climatic variability in source areas and anthropogenic soil erosion by the time the denudation rate signal of the hinterland is transmitted to the outlet of the basin

evidence for 2 0 ga continental microbial mats in a paleodesert setting abstract early evolved microbial communities characterized the initial biological invasion of precambrian continental landscapes in modern arid settings microbial mats and biological soil crusts are well developed and stabilize sediment the paleoproterozoic makgabeng formation in south africa is one of the oldest and best preserved dryland systems on earth six types of microbial mat related structures are now recognized within these depositional systems this paper presents three newly discovered structures that include tufted microbial mat biological soil crusts and gas escape features in addition to three previously documented structures that include roll up features sand cracks and wrinkled features these discoveries demonstrate that microbial communities were well established and inhabited diverse continental settings by 2 0 ga approximately 200 million years after the onset of the great oxidation event

high temperature ash flow wet sediment interaction in the makwassie formation ventersdorp supergroup south africa abstract the late archaean ventersdorp supergroup in south africa constitutes a supracrustal succession consisting of mafic lava at the base that is overlain by a clastic wedge deposit known as the kameeldoorns formation which formed in grabens that were the result of extensional tectonics overlying these units is an intermediate to felsic ash flow deposit the goedgenoeg and makwassie formations that are now represented by feldspar and quartz feldspar porphyries at t kuip in the northern cape province spherical structures or clasts varying greatly in size occur at the contact of the makwassie formation with the underlying sedimentary kameeldoorns formation the larger clasts have cavities of different shapes and are regarded to have developed as a result of the deposition of a high temperature ash flow onto water saturated sediments

precise microbeam dating defines three archaean granitoid suites at the southwestern margin of the kaapvaal craton abstract precise microbeam u pb zircon dates have been determined for 17 granitoid samples which crop out along the southwestern margin of the kaapvaal craton in the marydale high two main types of granitoid are distinguished mainly by their normative quartz alkali feldspar plagioclase mineral proportions the draghoender granite type is generally tonalitic to trondhjemitic and some samples have very low heavy rare earth elements probably originating as melts of eclogitic protoliths their ages vary by 50 ma from 2946 9 ma to 2892 6 ma 2 reflecting a long period of magmatism possibly due to subduction the granodioritic to monzogranitic skalkseput granite type intrudes the draghoender granite in places and has a coherent age of 2901 14 ma these two granite types thus overlap in time and space the more evolved skalkseputs type could have been derived by melting of the draghoender type during assembly of the kaapvaal craton by collision of the kimberley and witwatersrand terranes a third granite type is exemplified by the 2721 6 ma monzogranitic steenkop granite gneiss which occurs south of prieskapoort and corresponds in age to a 2718 8 ma monzogranite dyke which intrudes a 2871 ma skalkseput monzogranite the previous age of 2718 8 ma for the skalkseput granite probably also dated a steenkop type dyke a basal conglomerate to the zeekoebaard formation of the ventersdorp supergroup has a single 2720 4 ma detrital zircon population south of the marydale high a narrow strip of draghoender type granite extends to franzenhof where it is exposed in a dwyka glacial outwash surface and is dated at 2931 9 ma the southernmost exposures of granite are windows in the dwyka group about 130 km southeast of marydale on both sides of the doornberg fault the 2905 7 ma maritzdam granite which is basement to ventersdorp supergroup volcanic rocks at kuip is monzogranitic and corresponds to the skalkseput type on the southwest side of the doornberg fault the 2907 4 ma welgevonden granite underlies marydale group volcanic rocks showing that the kaapvaal basement extends across the fault

sedimentation rates basin analysis and regional correlations of three neoarchaean and palaeoproterozoic sub basins of the kaapvaal craton as inferred from precise u pb zircon ages from volcaniclastic sediments abstract calculation of sedimentation rates of neoarchaean and palaeoproterozoic siliciclastic and chemical sediments covering the kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents zircons from tuff beds in carbonate facies of the campbellrand subgroup in the ghaap plateau region of the griqualand west basin transvaal supergroup south africa were dated using the perth consortium sensitive high resolution ion microprobe ii shrimp ii dates of 2588 6 ma and 2549 7 ma for the middle and the upper part of the nauga formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below wave base stratifera and clastic carbonate facies southwest of the ghaap plateau at prieska was of up to 10 m ma when not corrected for times of erosion and non deposition dates of 2516 4 ma for the upper gamohaan formation and 2555 19 for the upper monteville formation indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 ma to 62 ma on the ghaap plateau for these predominantly peritidal stromatolitic carbonates decompacted sedimentation rates were of 40 m ma to over 150 m ma bubnoff units the mixed siliciclastic and carbonate shelf facies of the schmidtsdrif subgroup and monteville formation accumulated with decompacted sedimentation rates of around 20 b for the kuruman banded iron formation a decompacted sedimentation rate of up to 60 b can be calculated thus for the entire examined deep shelf to tidal facies range archaean and phanerozoic chemical and clastic sedimentation rates are comparable four major transgressive phases over the kaapvaal craton followed by shallowing upward sedimentation can be recognized in the prieska and ghaap plateau sub basins in griqualand west and partly also in the transvaal basin and are attributed to second order cycles of crustal evolution first order cycles of duration longer than 50 ma can also be identified the calculated sedimentation rates reflect the rate of subsidence of a rift related basin and can be ascribed to tectonic and thermal subsidence comparison of the calculated sedimentation rates to published data from other archaean and proterozoic basins allows discussion of general precambrian basin development siliciclastic and carbonate sedimentation rates of archaean and palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical chemical and biological processes were active in the precambrian as found for the phanerozoic particularly for stromatolitic carbonates matching modern and neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite building microbiota the new data also allow for improved regional correlations across the griqualand west basin and with the malmani subgroup carbonates in the transvaal basin the nauga formation carbonates in the southwest of the griqualand west basin are significantly older than the gamohaan formation in the ghaap plateau region of this basin but are in part correlatives of the oaktree formation in the transvaal and of parts of the monteville formation on the ghaap plateau

dating sediment burial with in situ produced cosmogenic nuclides theory techniques and limitations abstract dating sediment burial over million year time scales is crucial in many areas of the earth sciences and archeology but is often difficult using traditional techniques sediment burial can be dated by the radioactive decay of cosmogenic nuclides provided that the sediment was exposed to cosmic rays prior to burial dating calculations are straightforward if sediment is buried deeply and rapidly enough to prevent cosmogenic nuclide production after burial however the analysis can be complicated by postburial production if sediment is insufficiently shielded from secondary cosmic ray nucleons and muons this paper discusses how buried sediments can be dated over timescales up to 5 myr using 26 al and 10 be in quartz

geochronology of a late archaean flood basalt province in the pilbara craton australia constraints on basin evolution volcanic and sedimentary accumulation and continental drift rates abstract eleven high precision 2 5 million years shrimp zircon u pb ages have been obtained from felsic rocks within a single stratigraphic section of late archaean volcanic and sedimentary rocks in the east pilbara of western australia the stratigraphic succession nullagine and mount jope supersequences in sequence stratigraphic terminology fortescue group in lithostratigraphic terminology is interpreted to be the rock record of three major geotectonic cycles that formed in an extensional rift related environment between about 2772 and 2715 ma the geochronology is constrained by a detailed stratigraphic framework based on unconformities and supported by a preliminary magnetostratigraphy field mapping geochemical and petrographic studies have shown that previously unrecognised thin felsic tuff bands are interbedded in subaerial flood basalt piles and mafic tuffs while flood basalts and proximal felsic volcanic rocks comprise by volume most of the volcanogenic components of the succession felsic volcanism is now known to have been active periodically through each geotectonic cycle the succession covers a time period of about 57 million years the lower 1400 m of a thick 1700 m clastic sedimentary succession from the oldest geotectonic cycle was deposited at a rate of about 100 m per million years over a mean time period of 14 million years in contrast a younger 150 m thick cogenetic tuff basalt unit accumulated in less than 3 million years and others probably accumulated at similar rates comparable to those of phanerozoic flood basalts unconformities in the succession are shown to be of variable duration and one unconformity marking the boundary between the first and second geotectonic cycles may represent a time gap of more than 10 million years the unconformity based stratigraphic framework the new geochronology and palaeomagnetic studies j geophys res 108 2003 b12 2551 emp 2 1 to 2 21 have been combined to determine a possible late archaean continental drift rate for one part of the succession implying a period of motion as fast as or up to five times faster than any known from the phanerozoic

eruption transport and deposition of ignimbrite a case study from mexico abstract the acatlan ignimbrite covers 150 km 2 around a vent situated 40 km southwest of guadalajara mexico the upper and main ignimbrite flow unit is compositionally zoned and has white or pink rhyolitic pumice in its lower part and black andesite in its upper these parts are believed to have been erupted continuously from the same magma chamber this zonation has been important in correlating an extremely coarse lithic rich deposit a co ignimbrite lag fall deposit with this ignimbrite the co ignimbrite lag fall deposit indicates that the flow unit formed by continuous collapse of an explosive eruption column and enables the source vent to be located the vertical compositional grading of the flow unit also provides a means of testing the various proposed mechanisms of ignimbrite emplacement turbulent vs high concentration flow the preservation of compositional zonation and other features in the deposit indicate that flow was not turbulent details of the grain size distribution indicate that laminar flow was important early in the flow history when collapsing material from the eruption column was superimposed in stratigraphical sequence initial high gas flow rates associated with flow segregation and formation at the site of column collapse rapidly decayed away from vent and later in its history the flow degassed to a semi rigid plug overriding a sheared basal layer

scaling factors for production rates of in situ produced cosmogenic nuclides a critical reevaluation abstract new scaling factors are presented describing the altitude and latitudinal dependence of production rates for in situ produced cosmogenic nuclides the new factors incorporate the influence of the non dipole contributions to the geomagnetic field on the cosmic ray flux the currently used scaling factors of lal earth planet sci lett 104 1991 424 439 are based on the assumption of a dipole field the overall strategy used in deriving the new scaling factors is however very similar to that of lal and relies on the same and or similar nuclear data in this reevaluation data are only used if the effective geomagnetic parameters inclination horizontal field strength can be reconstructed for the time of measurement the absorption free pathlengths for cosmic rays selected for this study are based on observational data at altitudes relevant for exposure age dating sea level to 7000 m at sea level and latitudes between 20 and 40 the new factors are up to 18 lower and at high altitudes more than 30 higher than those of lal in addition to accounting for the influence of the effective geomagnetic field and providing more applicable estimates of the new factors also allow for correction for the significant deviations from the standard pressure altitude relationship that exist in the atmosphere 10

geodynamics of trench advance insights from a philippine sea style geometry abstract for terrestrial parameter sets trench retreat is found to be nearly ubiquitous and trench advance quite rare largely due to rheological and ridge push effects recently updated analyses of global plate motions indicate that significant trench advance is also rare on earth being largely restricted to the marianas izu bonin arc thus we explore conditions necessary for terrestrial trench advance through dynamical models involving the unusual geometry of the philippine sea region in this subduction system a slab pull force from distal subduction is transmitted to the overriding plate at the pacific trench our 2d modeling demonstrates that trench advance can occur for terrestrial rheologies in such special geometries we observe persistent trench advance punctuated by two episodes of back arc extension characteristic features of the model such as time interval between extensional episodes high back arc heat flow and stress state of philippine plate correspond to processes recorded in the region

new geological framework for western amazonia brazil and implications for biogeography and evolution abstract although many of the current hypotheses to explain the origin and distribution of the amazon biodiversity has been based directly or indirectly on geological data the reconstruction of the geological history of the amazon region is still inadequate to analyze its relationship with the biodiversity this work has the main goal to characterize the sedimentary successions formed in the brazilian amazon in the neogene quaternary discussing the evolution of the depositional systems through time and analyzing their main controlling mechanisms in order to fill up this gap radar image interpretation sedimentological studies and radiocarbon dating allowed the mapping of plio pleistocene to holocene units along the solimoes amazonas river brazil this integrated work led to the characterization of five sedimentary successions overlying miocene deposits of the solimoes pebas formation which include the following ica formation plio pleistocene deposits q1 37 400 43 700 14c yr b p deposits q2 27 200 14c yr b p deposits q3 6730 2480 14c yr b p and deposits q4 280 130 14c yr b p these deposits occur mostly to the west of manaus forming nw se elongated belts that are progressively younger from sw to ne indicating a subsiding basin with a depocenter that migrated to the ne the reconstruction of the depositional history is consistent with significant changes in the landscapes hence the closure of a large lake system at the end of the miocene gave rise to the development of a plio pleistocene fluvial system this was yet very distinct from the modern drainage with shallow energetic highly migrating braided to anastomosed channels having an overall northeast outlet this fluvial system formed probably under climatic conditions relatively drier than today s during the early pleistocene there was pronounced erosion followed by a renewed depositional phase ca 40 000 14c yr b p with the development of prograding lobes and or crevasse splays associated with a lake system i e fan delta and or fluvial flood plain areas after a period of erosion a fluvial system with eastward draining channels started to develop at around 27 000 14c yr b p the fluvial channels were overflooded in mid holocene time this flooding is attributed to an increased period of humidity with a peak between 5000 and 2500 14c yr b p the data presented herein support that rather than being a monotonous area the amazonia was a place with frequent changes in landscape throughout the neogene quaternary probably as a result of climatic and tectonic factors we hypothesize that these changes in the physical environment stressed the biota resulting in speciation and thus had a great impact on modern biodiversity

composition and trace element geochemistry of detrital clay and heavy mineral suites of the lowermost amazon river a provenance study abstract compositions of clays and heavy minerals as well as trace element geochemistry of the lowermost amazon sediments reflect source regions and the intense chemical weathering conditions illite and montmorillonite indicate mountainous sources under physical weathering conditions whereas kaolinite is related to the soil composition under humid tropical conditions where chemical weathering predominates the suite of unstable heavy minerals on the other hand reflects some sediment transport from the andes more supply from tributaries draining the basic and metamorphic rocks from the southeastern guiana shield and strongest influence of the upstream amazon and solimoes areas these source signals are damped by transitional storage and intense lateral sediment exchange between channel and floodplain the brazilian shield contribution via the xingu river seems to be of minor importance although the alter do chao and the barreiras formations which are products of erosion of the shields supply suites of stable heavy minerals that reflect recycling rare earth and trace element data from the clay and heavy mineral fractions provide additional clues to the source areas in the downstream direction superimposed upon provenance of sediments from an undifferentiated arc the andes is provenance from strongly weathered continental crust the precambrian shields as well as sedimentary recycling small scale provenance can be delineated by the most immobile elements heavy rare earth elements in the heavy mineral fraction increase downstream toward estreitos to the para river

benthic perspective on earth s oldest evidence for oxygenic photosynthesis the great oxidation event goe is currently viewed as a protracted process during which atmospheric oxygen increased above 10 5 times the present atmospheric level pal this threshold represents an estimated upper limit for sulfur isotope mass independent fractionation s mif an archean signature of atmospheric anoxia that begins to disappear from the rock record at 2 45 ga however an increasing number of papers have suggested that the timing for oxidative continental weathering and by conventional thinking the onset of atmospheric oxygenation was hundreds of million years earlier than previously thought despite the presence of s mif we suggest that this apparent discrepancy can be resolved by the earliest oxidative weathering reactions occurring in benthic and soil environments at profound redox disequilibrium with the atmosphere such as biological soil crusts and freshwater microbial mats covering riverbed lacustrine and estuarine sediments we calculate that oxygenic photosynthesis in these millimeter thick ecosystems provides sufficient oxidizing equivalents to mobilize sulfate and redox sensitive trace metals from land to the oceans while the atmosphere itself remained anoxic with its attendant s mif signature as continental freeboard increased significantly between 3 0 and 2 5 ga the chemical and isotopic signatures of benthic oxidative weathering would have become more globally significant from a mass balance perspective these observations help reconcile evidence for pre goe oxidative weathering with the history of atmospheric chemistry and support the plausible antiquity of a terrestrial biosphere populated by cyanobacteria well before the goe

a comparative study of the geochemical and isotopic systematics of late archaean flood basalts from the pilbara and kaapvaal cratons geochemical and sm nd isotopic data are reported for igneous rocks of the 2 76 to 2 69 ga fortescue group of the pilbara craton western australia and the 2 70 ga ventersdorp supergroup of the kaapvaal craton southern africa the metamorphic history of the fortescue group has also been investigated using whole rock rb sr and pb pb isochron techniques igneous of both sequences have mixed tholeiitic and calc alkaline affinities and show immobile element correlations indicating derivation from chemically heterogeneous lree enriched sources both sequences have similar negative nd values between 1 5 and 4 4 for the fortescue group and 0 and 3 4 for the ventersdorp supergroup which are not correlated with mg mg fe2 the negative nd values of the fortescue group rocks probably do not result from crustal assimilation within crustal level magma chambers during or following differentiation as the fortescue group felsic differentiates have nd values within the ranges of values found in associated mafic rocks and zircon xenocrysts are absent in the felsic differentiates of the fortescue group instead these results are consistent with either extensive crustal contamination of primitive komatiitic or picritic fortescue group parent magmas prior to their differentiation or derivation from negative nd or enriched mantle sources the presence of zircon xenocrysts in some ventersdorp samples provides compelling evidence of some crustal contamination although correlations between immobile element ratios such as la yb ti zr ti sc and v zr and nd indicate derivation from chemically heterogeneous enriched mantle sources a whole rock sm nd isochron date of 3308 138 ma 600 ma older than the time of eruption indicated by u pb zircon data and within 300 ma of the formation of the kaapvaal granite greenstone terrane may date an sm nd fractionation event within the subcontinental lithospheric mantle sources of the ventersdorp rocks rb sr and pb pb isochrons obtained for several flows from the fortescue group register hydrothermal events occurring between 2 45 and 2 0 ga and probably associated with burial metamorphism similarities in the field geology geochemistry and isotopic characteristics of the fortescue and ventersdorp sequences and phanerozoic examples of continental flood basalt volcanism suggest a common mode of origin possibly involving the interaction of asthenospheric mantle plumes with subcontinental lithospheric mantle sources which have been modified by subduction processes