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

deformational history of an archean fold belt eastern point lake area slave structural province n w t archean metasedimentary rocks in the eastern point lake area of the slave structural province preserve a sequence of archean structures consisting of two generations of folds f1 and f2 with littl

first cycle quartz arenites in the orinoco river basin venezuela and colombia modern first cycle quartz arenites are forming in the orinoco drainage basin by at least two distinct mechanisms common to both mechanisms is an environment of intense chemical weathering and extended time over which weathering can occur in the llanos andean foreland basin extended time is provided by temporary storage of orogenically derived sediments on extensive alluvial plains in lowland portions of the guayana shield long soil mineral residence times are produced by very low erosion and transport rates resulting from low relief and tectonic quiescence both weathering intensity and the duration of weathering are important in the modification of sand composition first cycle quartz arenites are produced in diverse tectonic settings and cannot be used by themselves to infer paleotectonic settings of ancient sequences furthermore although tectonic setting is of great importance in determining sand composition modifications during transport and deposition can overprint the effects of the tecto

loss of sand size feldspar and rock fragments along the south texas barrier island usa abstract to add to the limited quantitative data on the rate loss of sand size feldspar and rock fragments during transport in the beach zone the abundance of qfr grain types was determined for 31 samples of beach sand from padre island a barrier island 240 km long whose only source of river sand is from the rio grande at its southern end feldspar volcanic rock fragments carbonate rock fragments and unidentified rock fragments decrease irregularly and monocrystalline and total quartz increase irregularly northward the abundance of well rounded and spherical quartz grains show no trend which indicate that selective transport by roundness and shape is not taking place the decrease in abundance of feldspar and rock fragments northward in the southern half of padre island is the result of loss by wave action abrasion and grain breakage whereas the decrease of these grains in the northern half of padre island is a combination of abrasion breakage loss and dilution of grains from a more quartzose population to the north silicic volcanic rock fragments are as durable to beach abrasion as feldspar but carbonate rock fragments are not south of the mixing zone of rio grande sand with sand from a northern source feldspar and total rock fragments each decreased 1 absolute for each 19 km of beach transport this is 19 times faster than the rate loss of feldspar along the florida usa coast and 6 times slower than the loss of feldspar along the rio de janiero brazil coast additional case studies are needed to isolate the variables that control loss of unstable sand grains during beach transport

south america and a few grains of sand part 1 beach sands to study correlations between petrography and provenance of modern sands and thus contribute to improved interpretation of ancient sandstones this study reports on the petrography of 218 modern south american beach sands obtained from a shoreline of 25 300 km judged by quartz feldspar rock fragment ratios there are five major associations pacific 24 16 60 argentina 28 14 58 brazil 91 4 5 caribbean east 59 8 23 and caribbean west 54 17 29 study of potash feldspar volcanic rock fragments and the sio 2 al 2 o 3 ratio supplement q f rf ratios tectonics are the dominant control on the mineralogy both the pacific and brazilian associations which form 75 of south america s coastline identify correctly tectonic setting that is active margin composition for the pacific and passive margin for the brazilian however the argentine association would yield a serious error because sand with active margin composition overprints a passive margin variation of total quartz and feld

petrology chemistry and texture of modern river sands amazon river system key idea following the spirit of p d krynine the sands of the world s largest tropical river system are studied to reveal much about the general origin of sand and sandstone thin section and chemical study of 95 sand samples from the amazon river system in brazil peru ecuador and colombia a river system with great contrasts in climate and rock types reveals that sands derived from the andes are lithic arenites whereas sands derived from precambrian terrains are much richer in quartz as are sands in rivers draining tertiary molasse sand becomes more mature downstream along the main stem of the amazon solimoes maranon so that a sample analyzed at the mouth does not accurately reflect headwater composition dilution by quartz rich tributaries and quartz rich tertiary outcrops possible differential elimination of rock fragments and even possible weathering on the modern flood plain of the amazon contribute to enhanced maturity downstream we speculate that although data on modern large subarti

u pb zircon geochronology of the yellowknife volcanic belt nwt canada new constraints on the timing and duration of greenstone belt magmatism volcanic and sedimentary rocks in the slave province collectively termed the yellowknife supergroup have long been considered to represent laterally continuous and correlative stratigraphically coherent homoclinal sequences correlation based on the lithologic analysis of certain distinctive units has been most intensely applied to the yellowknife volcanic belt in the southern slave province u pb zircon data for seven samples collected from within the belt are reported here the distinctiveness of the townsite formation a term applied to three lenticles of rhyodacitic to dacitic tuffs breccias and pillowed flows in comparison to the overlying and underlying tholeiitic basalts kam group has led to its use as a key marker unit for stratigraphic correlations and palinspastic restorations of offsets on proterozoic faults in the area a sample from each of the three segments was collected for u pb zircon geochronology in addition numerous thinner tuffaceous beds from the kam group were also dated

detrital zircon age spectra for late archaean synorogenic basins of the eastern goldfields superterrane western australia abstract the kalgoorlie gindalbie kurnalpi duketon and burtville terranes were amalgamated by 2658 ma into the eastern goldfields superterrane which was later accreted cordilleran style onto the foreland of a pre existing continent late stage synorogenic basins developed within the eastern goldfields superterrane during the early stage of accretion the basins are similar to all late stage basins of archaean provinces developing late in the tectonic cycle in a strike slip regime they developed in two stages during the gap in granite emplacement between 2658 ma and 2648 ma basin analysis suggests that the bulk of sediments was derived from rocks readily identifiable today within the superterrane consistent with the provenance style of all late stage basins sediment derivation from the foreland occurred only during stage ii and is recorded only from basins proximal to the suture detrital zircon geochronology has corroborated those interpretations eleven new shrimp u pb detrital zircon datasets and 14 previously published datasets indicate that the bulk of zircon bearing sediment was derived from the 2715 ma to 2658 ma high ca ttg granites and volcanic rocks that define the terranes moreover primary modes in the detrital zircon age spectra of 2680 3 ma 2673 ma 2666 3 ma and 2660 3 ma coincide with the peaks of emplacement of high ca granites provenance from across the suture has also been corroborated with detrital zircon age spectra of the kurrawang basin a stage ii basin close to the suture having no match to rocks of the eastern goldfields superterrane defining modes of 2730 ma and 3000 ma some as old as 3750 ma and 3540 ma can be matched to an 2735 ma calc alkaline volcanic province and quartzites with 4350 3130 ma detrital zircon in the foreland the study shows that detrital zircon age spectra can be powerful supporters of tectonic analysis but for the study presented here their interpretation has been greatly assisted by prior basin analysis and a considerable geochronological database of source rock equivalents

composition of modern deep sea sands from arc related basins summary petrographic and chemical examination of modern deep sea sands shows a clear distinction between active and passive plate tectonic settings among active settings sands from forearc basins of island arcs and basins from strike slip continental margins can be distinguished but those from other arc related settings overlap considerably in composition in particular sands from continental margin subduction zones and those from the backarc of island arcs appear to be indistinguishable chemically arc related sands are very close in composition to ancient greywacke sandstones for instance sodium almost always exceeds potassium thus it is not necessary for sodium to be added to sands diagenetically to make greywackes as has been suggested the influence of tectonics on the composition of sandstones has long been a central feature of sandstone petrology the geosynclinal model of global tectonics stimulated much thought on this subject probably the best known ideas are those of krynine e g 1942 subsequently such research declined but the plate model for global tectonics has produced a new wave of interest e g crook 1974 schwab 1975 potter 1978 dickinson suczek 1979 much of this work has relied on data from ancient sediments where diagenetic changes and difficulty in de ciphering the tectonic setting limit the useful ness of the conclusions much more work is needed on recent sediments to establish a base line against which ancient sediments can be compared we have previously reported valloni maynard 1981 the results of our work on the petrography of a set of recent deep sea sands from piston cores in this paper we shall com pare this petrographic information with bulk chemistry and attempt to relate these to tecto nics in arc related settings

influence of tectonism and climate on lithofacies distribution and sandstone and conglomerate composition in the archean beaulieu rapids formation northwest territories canada abstract the 0 2 1 km thick archean beaulieu rapids formation is a late orogenic tectonically controlled sedimentary sequence overlying the beaulieu river volcanic belt unconformably on the west and bordering the major n trending beniah lake fault on the east local porphyry stocks characteristic of late orogenic successions were emplaced along strike of the fault four lithofacies comprise the beaulieu rapids formation 1 the 20 320 m thick conglomerate i 2 the 10 305 m thick siltstone sandstone 3 the 20 120 m thick conglomerate ii and 4 the 40 320 m thick quartz rich sandstone lithofacies stacking displays two distinct large scale fining upward sequences represented by the 300 375 m thick conglomerate i and sandstone siltstone sequence a and the 300 400 m thick conglomerate ii and quartz rich sandstone sequence b fining upward sequences a bounding unconformity and the beniah lake fault are compelling indicators of tectonic influence on sedimentation conglomerate i was deposited by debris flows traction currents and sheetfloods on the proximal medial and distal portions of alluvial fans or fan deltas respectively the overlying siltstone sandstone contains two distinct lithological units a sandstone dominated unit formed from fluvial dune migration and overbank deposition in a sandy braidplain setting with scattered ponds whereas sedimentary structures in the siltstone dominated unit are consistent with lacustrine deposition a second phase of tectonic uplift produced sequence b conglomerate ii was deposited in a proximal braided stream setting characterized by abundant longitudinal gravel bars the quartz rich sandstone developed from dune migration on a sandy braidplain mineralogy and textures reflect high sediment influx limited transport distance local sources and climatic influence clast populations in conglomerate indicate a prevalent mafic volcanic source whereas sandstones contain prominent quartz av 76 with subordinate feldspar 13 and lithic fragments 11 the disparity between clast and sandstone composition is a function of chemical weathering typical of a humid climate source areas are restricted to the mafic dominated beaulieu river volcanic belt the 2 8 ga gneisso plutonic sleepy dragon complex and fault related porphyry stocks the humid weathering aggressive archean climate was responsible for destroying labile minerals and lithic fragments in proximal source rocks rich in plagioclase accounting for the high quartz component in the sandstones the beaulieu rapids formation is similar to modern strike slip basins that form along major fault zones in terms of basin geometry faulted and unconformable contacts lithological units and sedimentary facies stacking and organization fault controlled basins of this nature commonly develop during the last stage of terrane accretion on archean cratons

evidence of tide storm and wave interaction on a precambrian siliciclastic shelf the 1 700 m y ortega group new mexico abstract the 1 700 m y ortega group in northern new mexico is a sequence of quartz arenites and subordinate mudstones in excess of 1 000 m sedimentation took place in diverse shallow shelf environments under the influence of tidal wave and storm processes time velocity asymmetry of tidal flow on the inner shelf produced large scale trough cross beds and tabular planar cross beds with multiple reactivation surfaces in contrast tabular cosets structured internally by small scale trough cross beds and herringbone cross beds resulted from symmetrical tidal flow on the inner shelf fair weather waves reworked the upper surface of tidal sand bodies on the inner shelf during quiet periods within the tidal rhythm during storms sand was entrained into the water column on the proximal inner she f leaving winnowed pebble lags at the top of tidal sand bodies sand transported by storm induced gradient currents was deposited on the outer shelf as discrete lobes progradation of sand lobes produced 1 11 m thick genetic packages which display an upward thickening of sandstone beds and are capped by cosets of tabular planar laminated sandstones storm sands were reworked by wave processes on the proximal outer shelf whereas storm deposited sands on the distal outer shelf remained unmodified no evidence of tidal processes exists on the outer shelf distribution of facies and paleocurrent patterns indicates that the shelf sloped to the south and experienced an overall transgression which culminated in the drowning of the outer shelf with onlap of black basinal muds absence of shallow water turbidites below storm wave base reflects low gradients on the shelf vertical transition from tide to storm dominated sedimentation in the ortega group does not represent a change in depositional style with time on the shelf rather all facies coexisted across the shelf with tidal processes prevailing on the shallow inner shelf and storm processes on the outer shelf the german bight in the north sea and parts of the bering sea off alaska are holocene counterparts for the depositional framework envisaged for the ortega group in both pro imal tide dominated environments pass distally into deeper water storm dominated environments

effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments with implications for provenance studies the bulk chemical compositions mineralogy and mineral proportions of sands and muds of the mallacoota basin in southeastern australia reflect the composition of weathering profiles mantling source rocks rather than bedrock muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles sands are strongly enriched in quartz relative to source rocks even in the headwaters of the fluvial system demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system k feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or k feldspar in weathering profiles subsequent erosion and sorting produce sands enriched in quartz with high k feldspar plagioclase ratios relative to source rocks the composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat

evolution of the western continental margin french italian alps sandstone mineralogy as an index of plate tectonic setting most plate tectonic developmental models for the western or french italian alps show that the western or french continental margin of the tethyan locally piedmont ocean exhibited several contrasting types of structural behavior from late paleozoic through tertiary time initially this margin acted as a rifted passive progressively subsiding atlantic type margin from late paleozoic through jurassic time during the cretaceous and tertiary active volcanism blueschist metamorphism and indications of rapid uplift and erosion suggest that active subduction of the piedmont and valais ocean basin s occurred producing a himalayan like sutured orogenic highland local island arc trench inland sea systems and possibly even transform faults all characteristics of orogenically active continental margins like those of the modern mediterranean and around the perimeter of the present day pacific ocean compiled analyses of modern sands from present day ocean basins and continental margins have been used

the duparquet formation sedimentation in a late archean successor basin abitibi greenstone belt quebec canada in the south central part of the abitibi greenstone belt a succession of archean epiclastic sediments at least 2 km thick was deposited in the duparquet basin the destor porcupine fault and a secondary fault that splays off the major fault define the margins of this basin which is at least 15 km long and up to 2 5 km wide sedimentary evidence of tectonic influence includes i rapid vertical and lateral facies changes over tens of metres ii cyclic repetition of facies associations iii mixed fining upward coarsening upward fining upward sequences in conglomerate units iv local derivation of most clasts and v asymmetric distribution of fan deltas and braid deltas along the strike of the fault bounded basin three basic facies associations are recognized i conglomerate sandstone facies association csfa ii sandstone argillite facies association safa and iii argillite sandstone facies association asfa the csfa which predominates at the faulted basin margins constitutes 60

evidence for the transition to an oxygen rich atmosphere during the evolution of red beds in the lower proterozoic sequences of southern africa abstract strata 2 5 to 1 8 ga in southern africa appear to span the transition from a non oxidizing atmosphere to one characterized by free oxygen red beds in 2 3 to 2 1 ga unconformity bounded sequences have matrices of stained clay minerals and are spatially associated with volcanic rocks ironstones or banded iron formations which evidently were sources of iron for the matrices however red beds in 1 9 ga unconformity bounded sequences not only matrix staining but also hematitic coatings on grains these beds also have far fewer interstratified ferruginous lithologies because red beds with stained clay matrices banded iron formations pyritic witwatersrand type conglomerates and iron depleted paleosols all occur in the 2 2 ga pretoria group formation of these lithologies may not have been time dependent until atmospheric oxygen approached 10 2 atm by 1 9 ga atmospheric oxygen was 10 2 atm causing lateritic weathering and red beds with hematitic coatings on grains

fan delta sequence in the archean fig tree group barberton greenstone belt south africa abstract the fig tree group of the barberton greenstone belt in south africa records the onset of tectonism and significant terrigenous and volcaniclastic sedimentation following a period of predominantly effusive volcanism in the southern part of the greenstone belt the lower fig tree conformably overlies the onverwacht group and can be grouped into either 1 volcaniclastic deposits consisting of pyroclastic and reworked volcanic detritus or 2 non volcanogenic terrigenous deposits of sandstone and conglomerate these petrologic groups are further divided into six lithofacies the lithofacies within the lower fig tree formation can be grouped into two facies associations 1 a subaqueous delta front association made up of fine grained airfall pyroclastic and slightly reworked pyroclastic debris along with resedimented land derived sandstone and conglomerate and 2 a subaerial association that is interpreted as forming from deposition of sandstone and conglomerate by flashy ephemeral stream processes on an alluvial fan surface the stratigraphic sequence coarsens upward from predominantly siltstone and fine grained sandstone at the base to pebble and cobble conglomerate at the top this sequence is interpreted as forming from the progradation of a fan delta into a body of relatively shallow quiet water composition of clasts in the conglomerates indicates that immediately underlying greenstone belt rocks or coeval volcanic rocks were the source for these coarse sedimentary accumulations erosion of uplifted greenstone belt terrain was apparently only to shallow levels both tectonic and volcanic activity played a role in producing several cycles of fan delta progradation

understanding controls on rapid ice stream retreat during the last deglaciation of marguerite bay antarctica using a numerical model using a one dimensional numerical model of ice stream flow with robust grounding line dynamics we explore controls on paleo ice stream retreat in marguerite bay antarctica during the last deglaciation landforms on the continental shelf constrain the numerical model and suggest retreat was rapid but punctuated by a series of slowdowns we investigate the sensitivity of ice stream retreat to changes in subglacial and lateral topography and to forcing processes including sea level rise enhanced melting beneath an ice shelf atmospheric warming and ice shelf debuttressing our experiments consistently reproduce punctuated retreat on a bed that deepens inland with retreat rate slowdowns controlled by narrowings in the topography sensitivity experiments indicate that the magnitudes of change required for individual forcing mechanisms to initiate retreat are unrealistically high but that thresholds are reduced when processes act in combination the ice stream is however most sensitive to ocean warming and associated ice shelf melting and retreat was most likely in response to external forcing that endured throughout the period of retreat rather than to a single triggering event timescales of retreat are further controlled by the delivery of ice from upstream of the grounding line due to the influence of topography modeled retreat patterns are insensitive to the temporal pattern of forcing evolution we therefore suggest that despite regionally similar forcing mechanisms landscape controls significant contrasts in retreat behavior between adjacent but topographically distinct catchments patterns of ice stream retreat in the past present and future should therefore be expected to vary significantly

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

alluvial sandstone composition and paleoclimate i framework mineralogy abstract systematic variations in compositional maturity of first cycle fluvial sandstones are found in the cutler formation permian and fountain formation permian pennsylvanian in colorado and in the gondwana supergroup permian triassic of peninsular india these variations reflect changing climate during deposition climate is considered to be the critical factor affecting maturity because source rocks granite granite gneiss and tectono environmental setting alluvial systems associated with basement cored block uplifts were similar and remained relatively unchanged throughout deposition of all three of the units the roughly 3 200 m thick gondwana succession consisting of five sandstone petrofacies is characterized by the following sequence of change oldest to youngest in compositional maturity expressed in q f r percentages 54 42 4 88 10 2 62 34 4 82 16 2 99 1 0 this sequence is a function of the changing paleoclimate glacial arid temperate humid warm semiarid warm semihumid warm humid associated with the overall global limatic change and the changing latitudinal location of india as it moved from a location close to the south pole northward during the interval of time represented by gondwana deposition lithologic and fossil evidence indicates that during deposition of the fountain formation climate gradually changed from relatively wet and warm to at least semiarid and warm this climatic change is the main factor responsible for the difference in compositional maturity of the older fountain sandstone q f r 65 26 9 relative to the younger q f r 54 36 10 cutler sandstone was deposited in an arid climate and has maximum compositional immaturity q f r 49 44 7 a log log plot of the ratio of total quartz to total feldspar plus rock fragments against the ratio of total polycrystalline quartz to to al feldspar plus rock fragments is a sensitive discriminator of first cycle sandstones with differing climatic heritage bulk chemical composition data support interpretation of climate from framework mineralogy but by themselves are not sensitive enough to be unequivocal indicators the same is true for the ztr index and observations of degree and abundance of solution pits embayments on detrital quartz results of this study support the conclusion of earlier empirical and theoretical analyses which suggest that the optimum conditions for the production and preservation of a distinctive climatic signature on sand composition are met in extensional plate tectonic settings such settings are characterized by coarse grained crystalline parent rocks short transport distances in low order streams deposition in nonroutine environments and shallow burial diagenesis however even in such optimum settings only rarely will climate be a more important determinant of sandstone composition than the tectonic setting itself

physiographic controls on the composition of sediments derived from volcanic and sedimentary terrains on barro colorado island panama abstract the phenomena that control sand composition are complex dynamic and interlinked although source rock lithology is of critical importance in determining the allowable range of sand compositions chemical weathering may overprint the source rock signal substantially in the humid tropical environment of barro colorado island panama source rocks of similar lithological character produce sediments of differing compositions in different physiographic environments moreover sediments derived from source rocks of dissimilar mineralogical composition but produced on similar physiographic terrains may be nearly indistinguishable physiography influences the composition of sediments released from the source areas by controlling the duration of chemical weathering on relatively flat terrains rates at which loose solids are generated by weathering may exceed rates at which slope processes can remove this material soils are exposed to the weathering environment for long periods of time and are strongly leached consequently weathering products are cation poor and contain relatively immobile elements incompletely weathered detritus is not abundant and is greatly enriched in chemically resistant phases on steeper slopes potential solid erosion rates exceed weathering rates residence times of minerals within soils are short resulting in weathering products that are cation rich and contain an abundance of uns able phases on barro colorado island fluvial sediments are dominated by authigenic particles composed of secondary weathering products although the stability of many such particles in larger transportational and depositional systems and in the diagenetic environment has yet to be assessed they may be more important constituents of fluvial sediments in tropical environments than has been previously recognized

the influence of climate and topography on rock fragment abundance in modern fluival sands of the southern blue ridge mountains north carolina chemical weathering influences the detrital composition of sand size sediment derived from source areas subject to different amounts of precipitation in the coweeta basin north carolina of the grain types studied rock fragments are most sensitive to chemical degradation therefore their abundance is the best indicator of cumulative weathering effects destruction of sand size rock fragments by chemical weathering is a function of both the intensity and duration of chemical weathering experienced by grains in regoliths of the source area in the coweeta basin the intensity of chemical weathering is directly related to the climate via effective precipitation in individual subbasins whereas the duration of chemical weathering is inversely related to the relief ratio of the watershed therefore soils in watersheds with low relief ratios and high discharge per unit area experience the most extensive chemical weathering and sediments derived from these watersheds contain the lowest percentage of rock fragments the effects of climate alone cannot explain the systematic variation of rock fragment abundance in sediments from the coweeta basin the compositional imprint left on these sediments by chemical weathering is a function of both climate and topographic slope in the sediment source area

archean mafic dyke swarms near the cameron river and beaulieu river volcanic belts and their implications for tectonic modelling of the slave province northwest territories three dense swarms of archean mafic dykes which intrude the beaulieu river and cameron river volcanic belts and adjacent granitoid terranes of the sleepy dragon complex and meander lake complex in the southern slave province all contain clusters of multiple parallel sided sheeted dykes none of the swarms can be convincingly modelled as being part of an ophiolite assemblage the 3 km by 20 km step9nduck dyke complex comprising about 350 mainly metabasaltic to meta gabbroic dykes separated by screens of granitoid gneiss 2750 ma represents profuse mafic magmatism in the meander lake complex dense dyke swarms within the cameron river belt fed the lavas and are an integral part of the volcanic stratigraphy all basaltic magmas represented by the volcanic belts and the dyke sets are predominantly mantle derived fractionated tholeiites nd isotope geochemistry indicates that they were all contaminated by archean sialic material isotopic systematics are satisfied by mixing of depleted mantle and varying proportions of sleepy dragon type crust the dyke swarms in the sleepy dragon complex and the step9nduck swarm record extensional events in archean continental crust possibly they are subsurface equivalents of now eroded flood basalts that signalled the activity of mantle plumes during archean times this hypothesis is consistent with magmatic underplating of continental lithosphere and with extensive involvement of continental material

mesoproterozoic delta systems of the acurua formation chapada diamantina brazil abstract facies analysis distinct depositional paleoflow directions and grain size ranges suggest that different fluvial systems fed coeval distinct independent river dominated and tide dominated braid delta systems in the mesoproterozoic acurua formation the river dominated delta system is characterized by a vertical stacking of parasequences up to 30 m which are characterized by coarsening and thickening upward trends b gradationally based vertical succession of prodelta delta front distributary channels and floodplain c grain size of sandstone from delta front and distributary channels varies respectively from very fine to medium and medium to coarse d lacking of wave or tide reworking suggests delta development in a relatively low energy marine basin e paleocurrent readings indicate delta front sandstone bodies shifting laterally towards ne to se directions the tide dominated delta system is characterized by a vertical stacking of parasequences up to 25 m which are characterized by coarsening and thickening upward trends b gradationally based vertical succession of prodelta delta front and tide influenced distributary channels c grain size of sandstone from delta front and tide influenced distributary channels varies respectively from very fine to coarse and fine to granular d abundance of tide influenced sedimentary structures suggest tidal dominated delta development e consistent bidirectional paleocurrents with northwards ebb tide direction and no lobe shifting the studied braid delta systems belong to a highstand system tract and as such they exhibit classic parasequence sets with progradational stacking pattern the facies association variability and overall characteristics of these mesoproteozoic delta systems are compatible with delta systems of all ages however contrary to typically phanerozoic deltas which are characterized by progradational clinoforms the coalescence of the studied braid delta facies associations promoted fairly sheet like geometry limited by marine flooding surface that more likely resulted from the response of deposition on shallow and wide epeiric sea

a paleosol developed on hydrothermally altered granite from the hinterland of the witwatersrand basin characteristics of a source of basin fill near delmas in the transvaal a paleoweathering surface is developed beneath the black reef on hydrothermally altered granite hag au in the weathering profile averages 10 ppb compared with 5 ppb in the hag the hydrothermal alteration converted all plagioclase to muscovite and k feldspar removing na and ca from the rock and converting ferromagnesian minerals to chlorite discontinuous zones in the core were enriched in as sb co au and la superimposed on this hydrothermal alteration was a weathering event that penetrated to about 15 m k rb cs sr and ba were leached mg and fe were depleted in about equal proportion whereas ti and al were conserved consistent with weathering under a low o 2 atmosphere magnetite in the soil was destroyed and fe ti oxides converted to anatase sc and ta were also leached as a result of destruction of sphene and fe ti oxide without formation of secondary iron oxyhydroxides that re adsorb these elements in younger soils a later diagenetic event added

petrography and provenance of silicified early archaean volcaniclastic sandstones eastern pilbara block western australia intense post depositional alteration has profoundly affected sandstones in the volcanic portions of early archaean 3 5 3 3 ga greenstone belts the mineralogy and bulk compositions of most grains have been completely destroyed by pervasive metasomatism but grain textures are commonly well preserved consequently microtextural information coupled with present alteration compositions as determined petrographically can be used to estimate original framework modes silicified early archaean volcaniclastic sandstones assigned to the panorama formation and duffer formation warrawoona group eastern pilbara block western australia were originally composed of volcanic vrf and sedimentary srf rock fragments volcanic quartz feldspar traces of ferromagnesian minerals and pumice only volcanic megaquartz remained stable during alteration all other primary components were replaced by granular microcrystalline quartz gmc and sericite in most areas the sandstones were composed of dacitic to rhyolitic vrfs now totally replaced by sericite poor gmc and recognized by preserved microporphyritic textures in a few areas quartz poor dacitic to andesitic vrfs dominated the detrital assemblage minor srfs and mafic vrfs now replaced by gmc are recognized on the basis of colour internal structures and internal textures including skeletal possible spinifex textures detrital feldspar is represented by blocky sericite rich grain pseudomorphs a semi quantitative point count scheme developed for the analysis of heavily altered sandstones indicates the following primary detrital mode ranges for panorama arenites quartz 0 28 feldspar 0 28 vrfs 58 86 and srfs 0 25 in about half the point counted samples feldspar could not be distinguished from rock fragments in such cases both were counted as one grain type lv which makes up from 84 to 100 of the framework modes of these rocks these sands were derived from a terrane composed largely of fresh felsic volcanic rocks and sediments but locally including minor mafic ultramafic and sedimentary rocks much but not all of the felsic volcaniclastic sand represents reworked pyroclastic debris there is no evidence for contributions from plutonic or metamorphic sources the panorama modal assemblage represents a provenance that is lithologically more restricted than that of archaean greywackes and other siliciclastic units common in the sedimentary portions of these same early archaean greenstone belts and younger greenstone belts 3 0 2 7 ga

evolution of cretaceous cenozoic quartz pebble conglomerate gold placers during basin formation and inversion southern new zealand abstract numerous auriferous fluvial quartz pebble conglomerates qpcs are present within the late cretaceous recent sedimentary sequence in southern new zealand the qpcs formed in low relief settings before during and after regional marine transgression in alluvial fan and a variety of fluvial and near shore depositional settings in particular during slow thermal subsidence associated with late cretaceous early cenozoic rifting and during the early stages of orogenic uplift following mid cenozoic marine regression qpc maturity characteristics are complex and vary with sediment transport and recycling history stratigraphic proximity to the transgressive waipounamu erosion surface and the amount of first cycle detritus incorporated during recycling for pre marine qpcs the amount of first cycle detritus varies with tectonic intensity and proximity of the depositional setting to remnant cretaceous topography for post marine qpcs it varies with tectonic intensity and proximity to late cenozoic uplift of basement ranges qpcs do not form during a single bedrock erosion sediment deposition cycle non oxidised and or oxidized groundwater alteration kaolinisation of labile minerals in immature sediment and the upper part of underlying basement and repeated sedimentary recycling are fundamental processes of qpc formation regardless of the tectonic or sedimentary settings altered immature rock disaggregates easily upon erosion and alteration clays are winnowed to leave quartz rich residues containing resistant heavy minerals such as zircon and gold detrital sulfide survives recycling if deposition and burial in saturated sediments are rapid qpcs result only if sediment recycling is not accompanied by excessive erosion of fresh basement rock uplift of many parts of the otago schist belt since late miocene has raised rocks above the water table increased erosion rates and inhibited groundwater alteration and qpc formation qpc formation is still occurring in southland where the water table is high sediments are saturated and undergoing alteration and uplift and erosion rates topography and fluvial gradients are all low the qpcs accumulate as residual gravel on the valley floors of low competence streams that are slowly incising pervasively altered dominantly late miocene pliocene immature conglomerates qpcs formation essentially represents physical and chemical lagging of precursor strata accumulation of detrital gold and other heavy minerals is an inevitable consequence and most qpcs contain some gold three types of significant gold placer have developed in the qpcs type 1 placers are essentially eluvial and or colluvial in origin and form without significant fluvial transport by residual accumulation in low competence valleys during low rate uplift fluvial incision and qpc formation type 2 placers have formed during significant fluvial transport and subsequent fluvial incision mainly in higher energy proximal and medial reaches of larger pre marine eocene and post marine fluvial systems type 3 placers formed by wave base and marine current winnowing in the shallow shelf setting during low rate regional marine transgression especially in the eocene

late archaean synorogenic basins of the eastern goldfields superterrane yilgarn craton western australia abstract siliciclastic supersequences of the kalgoorlie and gindalbie terranes post date 2660 ma deformation uplift and erosion associated with the amalgamation of arc related terranes but were tectonically buried to greenschist or amphibolite facies during the orogenic shortening that accreted the eastern goldfields superterrane to the yilgarn craton the supersequences are synclinal remnants of late stage basins that were linear zones of subsidence adjacent to north trending strike slip faults fluvial facies merougil navajo jones creek scotty creek and yandal basins are dominated by sheet sandstones that represent the deposits of medial braid plains but proximal channel and bar complexes of cobble and boulder conglomerates are also preserved submarine facies kurrawang basin range from boulder to pebble conglomerates but are dominantly bouma turbidites with rare black shales and record the filling of a proximal canyon that fed a distal submarine fan axial depositional systems are preferentially preserved in all basins but transverse systems are preserved at the base of some fluvial deposits the gross vertical succession in all basins is from proximal up to distal the basins developed during two tectonic stages separated by an intervening period of uplift tilting and erosion with the southern kalgoorlie terrane recording change from a stage i fluvial to a stage ii deep marine basin stage ii fluvial deposits are preserved in the northern kalgoorlie terrane and in the gindalbie terrane sparse palaeocurrents establish sediment dispersal was to the south or southeast implying uplift to the north or northwest controlled palaeoslopes stacking of lithofacies is aggradational and is consistent with limited lateral migration of facies tracts in fault confined basins similar lithofacies and aggradational styles of facies stacking characterise syncollisional sequences of the cenozoic coastal range basin of taiwan and the present day syncollisional basins of southeastern taiwan are good analogues of late stage basins of the kalgoorlie and gindalbie terranes

architectural elements from lower proterozoic braid delta and high energy tidal flat deposits in the magaliesberg formation transvaal supergroup south africa abstract three architectural elements are identified in the lower proterozoic magaliesberg formation pretoria group transvaal supergroup of the kaapvaal craton south africa 1 medium to coarse grained sandstone sheets 2 fine to medium grained sandstone sheets and 3 mudrock elements both sandstone sheet elements are characterised by horizontal lamination and planar cross bedding with lesser trough cross bedding channel fills and wave ripples as well as minor desiccated mudrock partings double crested and flat topped ripples due to the local unimodal palaeocurrent patterns in the medium to coarse grained sandstone sheets they are interpreted as ephemeral braid delta deposits which were subjected to minor marine reworking the predominantly bimodal to polymodal palaeocurrent trends in the fine to medium grained sandstone sheets are inferred to reflect high energy macrotidal processes and more complete reworking of braid delta sands the suspension deposits of mudrocks point to either braid delta channel abandonment or uppermost tidal flat sedimentation the depositional model comprises ephemeral braid delta systems which debouched into a high energy peritidal environment around the margins of a shallow epeiric sea on the kaapvaal craton braid delta and tidal channel dynamics are inferred to have been similar fine material in the magaliesberg formation peritidal complexes indicates that extensive aeolian removal of clay does not seem applicable to this example of the early proterozoic

climatic and tectonic influences on fan deltas and wave to tide controlled shoreface deposits evidence from the archaean keskarrah formation slave province canada the 2 6 ga keskarrah formation located in the central slave province northwest territories canada is a late orogenic tectonically controlled sedimentary sequence that developed under unusual climatic and depositional conditions the formation is adjacent to the crustal scale north trending beniah lake fault and overlies the 3 15 ga augustus granite the 2 69 2 7 ga mafic volcanic peltier formation and the turbiditic contwoyto formation unconformably principal lithofacies in the keskarrah formation include conglomerate sandstone and siltstone sandstone the conglomerate lithofacies represents coalescing gravelly streamflow dominated fan deltas adjacent to topographic highs up section quartz rich arenites and quartz arenites of the sandstone lithofacies are interpreted to be shallow water shoreface deposits influenced by wave action and tides the overlying feldspathic litharenites of the siltstone sandstone lithofacies are consistent with a lower shoreface to proximal offshore environment dominated by wave and tide interaction tidal influence in both sandstone dominated lithofacies is inferred from the presence of mudstone laminae between bedforms and on foresets of cross beds as well as from abundant reactivation surfaces with local mudstone drapes intense chemical weathering during the archaean resulting from elevated atmospheric co2 levels higher temperatures and moist climatic conditions played an important role in the development of quartz rich arenites that appear to be first cycle deposits few lithic fragments and feldspar grains are preserved due to in situ host rock weathering chemical weathering during transport and wave and tide action hydraulic sorting and abrasion in the shoreface environment contributed to the continued breakdown and transport of labile minerals increased proportions of lithic fragments in sandstone beds of the conglomerate lithofacies are the result of shorter transport distances from source areas to the depositional environment abundant conglomerate with up to 4 m large granitic boulders derived from the adjacent augustus granite and mafic clasts from the peltier formation indicate high relief and fault related uplift and subsidence the intimate association of fan deltas and wave and tide influenced shallow marine deposits in association with quartz rich sandstones forming in a high relief area make the keskarrah formation remarkable in the rock record

transgression of an estuarine channel and tidal flat complex the lower triassic of barles alpes de haute provence france the lower triassic succession of barles alpes de haute provence france comprises an unconformable quartz arenite sand body of 90m thickness the succession may be informally divided into i lower channellized cross bedded member overlain by ii an upper fining upward member the lower member comprises vertically stacked subtidal channel units separated into five major sand bodies by thin developments of fine grained channel margin and shoal deposits subtidal channel fill deposits are dominated by varying scales of cross bedding these scales vary systematically from the base to the top of the member with large scale planar sets dominating the lowest channel sand body sand body 1 medium scale planar and trough cross bedding characterizing sand bodies 2 4 the largest scale planar sets in the highest sand body sand body 5 this upward change in cross bedding scale is concomitant with a decrease in both the relief of major channel sand body erosion surfaces and the proportion of preserved interchannel shoal deposits the succeeding fining upward member comprises small scale tidal channel units overlain by channel shoal and tidal flat deposits tidal flat sequences are characterized by parallel laminated wave and current rippled sandstones separated by bioturbated fine grained siltstones and mudstones the vertical variation in facies of the lower triassic succession suggests two main periods of deposition the lower member is considered to preserve successively more seaward components of a transgressive estuarine complex the overlying upper member records the seaward progradation of tidal channel shoal and tidal flat environments the unconformity bounded nature of the lower member combined with its systematic variation in facies suggests it may represent an incised valley estuarine fill developed in response to an early triassic relative sea level fall and subsequent rise succeeding tidal channel and tidal flat deposits forming the upper fining upward member reflect a change in sediment supply and or rate of relative sea level rise comparable with a progradational shoreline it is unclear whether this final depositional episode represents a period of highstand progradation or a later lowstand shoreline system developed following a further period of relative sea level fall and rise

atmospheric deposition fluxes of 26 elements over the southern indian ocean time series on kerguelen and crozet islands 1 atmospheric deposition of dust is suspected to have a significant impact on biogeochemical processes in high nutrient low chlorophyll waters of the open ocean in this study we report time series of atmospheric deposition samples collected over two years at three different sites on kerguelen and crozet islands in the southern indian ocean total atmospheric deposition fluxes were measured for a large suite of elements al as ba ca ce co cr cu fe k la li mg mn na nd ni pb rb s si sr ti u v and zn most of them are identified as coming from sea salt or crustal sources but enrichment factor variabilities of pb as cr cu and v highlight an anthropogenic contribution during the austral winter for these five elements for al fe mn and si deposition fluxes are similar for both kerguelen and crozet islands fluxes for the other non sea salt elements exhibit differences below a factor of five with a decreasing gradient from crozet to kerguelen

alluvial fan sedimentation of the cutler formation permo pennsylvanian near gateway colorado the permo pennsylvanian cutler formation near gateway colorado is the most complete 1 334 m proximal section of alluvial fan sediments deposited along the western flank of the ancestral uncompahgre uplift cutler facies can be correlated with depositional processes observed on modern dry alluvial fans proximal cutler facies include matrix supported bouldery debris flow and channel form streamflood conglomerates midfan sedimentation in the cutler is represented by trough cross bedded granular braided stream sandstones laterally continuous streamflood conglomerates and sheetflood deposits laterally continuous streamflood conglomerate was deposited at the mouth of large channels near the intersection point and consists of a cross bed set as much as 2 m thick with a basal boulder cobble lag rippled and laminated siltstone with gravel channels represents distal sheetflood sedimentation pedogenic features include rhizocretions and calcareous nodules vertical changes in facies and maximum clast size delineate three megasequences on the scale of hundreds of metres thick each mega sequence is composed of a coarsening upward sequence of proximal facies overlain by a fining upward sequence of more distal facies coarsening upward sequences record periods of tectonic uplift and fan progradation whereas fining upward sequences result from tectonic quiescence and weathering back of the mountain front small scale cycles on the scale of ten metres occur within the larger megasequences and represent changes inherent to the alluvial fan system autocyclic sedimentologic data on cutler alluvial fan sediments at gateway support previous interpretations of semiarid or arid paleoclimate during permian time along the western flank of uncompahgria and may act as a standard of comparison for tests of the role of tectonism on sedimentation trends in the paradox basin

exceptions to the relationship between plate tectonics and sandstone composition abstract routine use of qfl qplsmlvm and lmlslv plate tectonic provenance diagrams has led to the recognition of error populations for these sandstones the interpretation of tectonic setting based on data exclusive of petrography does not coincide with the interpretation of tectonic setting based on their location on compositional diagrams the anomalous sandstones can be separated into at least four categories i sandstones deposited during the transition between tectonic regimes may be derived in part from relict source rocks ii the relative enrichment of sandstones in detrital quartz by weathering and or depositional reworking may lead to an inaccurate interpretation of tectonic setting from compositional data iii sandstones deposited in tectonic settings as yet unrepresented on he provenance diagrams may plot between the provenance fields or overlap existing fields iv the role of detrital carbonate rock fragments directly affects the location of data points on provenance diagrams and is an unresolved problem the first two categories represent permanent problems in the use of sandstone composition as an indicator of plate tectonic setting whereas the last two may only be temporary problems that can be solved by more data and a consensus on determining detrital modes recognition of anomalous sandstones in the rock record will reduce the probability of erroneous interpretations of tectonic setting and enhance the use of sandstone petrology as an indicator of plate tectonic setting

sedimentology of coarse grained alluvial fans in the markham valley papua new guinea abstract in alluvial sediment sequences recognition of a hierarchy of bounding surfaces and the shapes and associated lithofacies of the sediment bodies they define provide an appropriate framework for understanding associations among depositional forms the processes responsible for them and their controls on system development this methodology architectural element analysis integrates principles from geomorphology and sedimentology miall 1985 1988 it is used here to analyse the evolution of the modern umi fan and the alluvial fan part of the pleistocene leron formation in the markham valley papua new guinea a series of terraces has developed as the umi river has incised into its fan detailed stratigraphic analysis of the lowest terrace in exposures up to 25 m high and kilometres long reveals that the fan is dominated by sheetflood deposits with minimal preservation of either debris flow or hyperconcentrated flood flow sediments channel fill elements make up a larger proportion of exposures in the proximal fan than elsewhere while 95 of distal fan exposures are composed of sheetflood sequences these depositional features likely result from massive sediment dispersal associated with the rapidly uplifting upland terrain abundant sediment availability and flashy discharge regime channel fill units along with slope related deposits from debris and hyperconcentrated flood flows are only likely to be preserved in the trench backfilling phase following fan entrenchment reworking of deposits plays a dominant role in preservation of sheetflood deposits at the expense of slope related deposits an hierarchical framework of lithosomes is developed which characterizes various scales and bounding surfaces of depositional units which make up the umi fan cf miall 1988 decelles et al 1991 smaller elements observed in terrace exposures are described as first to fourth order lithosomes they reflect geomorphic processes which are evident in and adjacent to the modern umi river deposits which infill the trench are interpreted as a fifth order lithosome constrained by trench geometry proximal distal relationships are remarkably similar in the umi fan and leron formation fan sequences in both instances debris flow deposits are seldom observed the largest sixth and seventh order lithosomes which were deposited within a time frame of approximately one hundred thousand years are interpreted by analysis of the leron formation in relation to its tectonic setting the umi and pleistocene leron formation fans exemplify fan development in a post collisional molasse basin under a tropical monsoonal climate carbonate concretions observed in distal fan facies may provide a possible diagnostic feature of these tropical savanna fans

the middle devonian shoreline in north devon england the hollowbrook formation is an early middle devonian regressive succession of sandstones and heterolithic facies marking a passage from shallow marine shelf muds through a sandy shoreface to continental ephemeral stream sheet sands ten facies are recognized arranged in two sequence types the shelf sequence consists of alternations of bioturbated muddy sandstone fair weather sediment and shelly concentrates or parallel laminated fine sandstone storm deposits the nearshore and foreshore sequences coarsen upwards and pass from bioturbated muddy sandstone shelf to lenticular beds inner shelf then either wavy and flaser beds fair weather lower shoreface or hummocky beds and multistorey parallel laminated sands with bioturbated tops lower shoreface storm shoals upper shoreface wave ripple cross laminated sandstones follow then parallel and planar cross laminated sandstones foreshore and parallel laminated lithic sandstone continental sheet flood the lower parts of the shoreface sequence and inner shelf are burrowed by skolithos arenicolites and chondrites the sequence differs from the high energy 9oregon coast9 shore in the absence of a megarippled facies and presence of much bioturbation but closely matches the sequence found in modern low energy wave influenced sandy shores such as sapelo island georgia

cyclic sedimentation in tertiary lower rhine basin germany the liegendrucken of the brown coal open cast fortuna mine abstract in the lower rhine basin at the northwestern headlands of the schiefergebirge in western germany the oligocene miocene koln formation once was splendidly exposed in the former fortuna mine one of the open cast mines of the regional brown coal mining industry there a complex 60 m thick clastic marine shoreline succession was studied coastal marine sediments were formed along a high energy coast in the basal sediment fill of the basin due to repeated ingressions of the tertiary north sea a number of subtidal to supratidal coastal onlap cycles developed this study provides first knowledge about cyclic sedimentation in the tertiary of the lower rhine basin

a lower palaeozoic shallow water sequence in the eastern european variscides sw poland provenance and depositional history the gackowa formation of inferred cambrian ordovician age is part of the metamorphosed pre variscan basement of the southern kaczawa mountains of the sudetes region south west poland previously variously interpreted as lavas tuffs and sandstones it is shown to consist of about 200 m of originally well sorted siliclastic sedimentary rocks within a sequence of predominantly mafic volcanic rocks four facies have been distinguished based on relict sedimentary structures and textures and these suggest that deposition took place on a storm dominated shelf mostly above the wave base the petrography detrital zircon morphologies and geochemistry suggest affinity with and derivation in large part from acid volcanic rocks at a comparable stratigraphic position in a separate tectonostratigraphic unit to the south the gackowa formation and its acid volcanic source rocks have a clear continental geochemical signature in contrast with the mantle derived basic bimodal volcanic rocks below and above it is suggested that the parent acid magma originated by crustal melting during the ascent of these mantle derived magmas a continental crust setting is inferred for the deposition of the gackowa formation sandstones thus suggesting a continental setting for the associated within plate basalts the entire volcanic sedimentary succession resembles elements of the lower palaeozoic of germany and all may have formed in an initial rift setting during late precambrian to ordovician times

tidal sedimentation in part of the late silurian grampians basin southeastern australia abstract thick quartz arenite formations in the upper silurian grampians group of southeastern australia have been previously interpreted as fluvial deposits but detailed study of one of these formations the mount difficult sandstones shows that shallow marine conditions existed during their deposition in the southeastern part of the grampians basin the mount difficult sandstones are a succession of cross bedded quartz arenites with an overall fining upward trend four lithofacies associations are recognized the two lower associations record tidally dominated sedimentation whereas the two upper associations represent deposition in wave dominated environments boundaries are gradational and the associations arc interpreted as four spatially and genetically related environments within a t dal inlet and barrier beach system the basal tidal inlet channel fill complex overlies an erosion surface on tidally influenced coastal plain deposits of the silverband formation vertically stacked fining upward channel fills record lateral migration of tidal channels during transgression the overlying association was deposited in mixed tide wave conditions and tidal channel margin to ebb tidal delta environments seaward of the inlet are proposed the development of tidal inlet channels and a tidal delta suggests deposition of the mount difficult sandstones in a mesotidal setting tidal range 2 4 m with a time velocity asymmetry dominated by ebb tidal currents the increasing influence of waves is recorded by barrier beach deposits that migrated over the inlet delta system the barrier beach is represented by two lithofacies associations interpreted as upper shoreface and overlying foreshore deposits an arrangement that signals a change from transgressive to regressive progradational conditions thus the mount difficult sandstones represent a single transgressive regressive cycle most likely third order that was controlled mainly by the rate of subsidence in the tectonically active grampians basin under conditions of a relative highstand of sea level and high rates of sediment supply

archaean sedimentary sequences of the slave craton abstract the slave craton is composed of five archaean terranes that accreted during the neoarchean the central slave superterrane is the only tectonic block containing evidence of four major sedimentary successions including 1 a quartz arenite dominated succession deposited on an extensive tide influenced coastline along a rifted continental margin 2 a turbidite dominated succession that developed as submarine fans in a fore arc or intra arc basin 3 a second turbidite succession representing deep water deposition in a back arc basin and 4 late conglomerate dominated successions representing fluvial alluvial fan and shallow water depositional settings in strike slip basins deposition of the sedimentary rocks took place between ca 3 1 ga and 2 6 ga indicating a complex tectonic history an unconformity between the gneissic basement and the overlying quartz arenite succession marks a marine flooding surface consistent with a first order sea level change a major hiatus lies between the quartz arenite successions and arc backarc related volcanic deposits with interstratified and conformably overlying turbidite strata late conglomerate successions are exposed along north trending lineaments and overlie all sedimentary and volcanic rock units unconformably this depositional hiatus represents a fundamental change from compression and uplift to transcurrent motion during the final stages of craton stabilization between ca 2630 and 2580 ma the slave province experienced considerable pluton emplacement which disaggregated much of the once continuous stratigraphy a wheeler diagram illustrates the major tectonic events that occurred during formation of the central slave superterrane however remaining uncertainties concerning the evolution of the entire slave craton reflect the need for additional detailed field investigations

sheet flow dominated gravel fans of the non marine middle cenozoic simmler formation central california abstract alluvial fan conglomerates of the middle cenozoic simmler formation in the cuyama gorge and southern la panza range of west central california were deposited in two half graben basins on the basis of paleocurrents and down fan fining of sediment both to the northeast the basins are thought to have been adjacent to the then active nacimiento and la panza faults three contiguous conglomerate lithofacies from most proximal to most distal are 1 clast supported crudely stratified coarse conglomerate with imbrication proximal talus and powerful sheet flow 2 clast supported well bedded internally unstratified conglomerate with common imbrication less common grading and thin interbedded coarse sands mid to upper fan deposits of highly sediment charged density modified grain flow sheet flows 3 stratified sandy conglomerate with imbrication distal alluvial sheet flows two additional lithofacies cut across the first three 4 parallel bedded graded coarse sands turbidity currents in floodbasin lakes and 5 matrix supported very poorly sorted conglomerates cohesive debris flows the alluvial fan deposits of the more northeasterly basin are overlain directly by the marine vaqueros formation the fans were dominated by sheet flow events the lack of cross bedding and minor occurrence of channels indicate that permanent streams were not established cohesive debris flows were unimportant the relationship of fan gravels both to the lake turbidites and to the conformably overlying marine sands indicates the existence of fan deltas as a result of late cenozoic transcurrent movement on the la panza fault a strand of the early san andreas system the two half graben basins have been moved so as to create a sedimentary mismatch

recycling and chemical weathering in tectonically controlled mesozoic cenozoic basins of new zealand sedimentation in molasse basins is controlled by tectonics however recycling and chemical weathering play a critical role in the compositional evolution of a sedimentary succession the cretaceous to pliocene molasse deposits of central otago new zealand are excellent examples of tectonically related deposits that were governed by the effects of chemical weathering and recycling preserved in fault controlled basins floored by flysch deposits of the otago schist the clastic successions contain ubiquitous unconformities and lithofacies consistent with alluvial fluvial and lacustrine depositional settings textural analysis of central otago sandstones establishes a general quartz enrichment and increased mixing of angular and well rounded quartz varieties up section consistent with a history of sediment recycling rare earth element ree patterns which reflect upper crustal compositions are similar for the flysch type otago schist permian early cretaceous a palaeo weathering profile and the overlying molasse deposits the development of quartz arenites is also consistent with high degrees of chemical weathering and erosion of the schist basement which contains numerous quartz veins although recycling has occurred sio2 and tio2 do not consistently show a negative correlation over time this reflects erosion of previously deposited quartz rich sediment and the otago weathering profile which produced an inverse stratigraphy cia values range from 52 for lithic rich coarse grained sandstones and polymictic conglomerate matrices to 93 for coarse grained to pebble rich quartz arenites individual samples were split into finer and coarser grained pairs 2 5 and 2 5 to 1 and were analysed separately the results show that finer grained samples contain higher ree abundances and less sio2 but the coarser grained miocene pliocene samples have higher cia values than their finer grained counterparts these coarse grained deposits are quartz rich and plot erratically on tectonic discrimination diagrams implying that using sio2 poor samples is more reliable for geochemical analysis overall the petrographic and geochemical results indicate that the main factors controlling the composition of the central otago molasse deposits were source composition chemical weathering and recycling studies of this nature can be conducted in archaean tectonically controlled molasse basins that are affected by similar allocyclic factors

composition of modern stream sand in a humid climate derived from a low grade metamorphic and sedimentary foreland fore thrust belt of north georgia abstract conditions of tectonics and drainage in north central georgia provide an excellent natural laboratory to study the composition of stream sand derived from a low grade metamorphic and sedimentary foreland fold thurst belt under humid climatic conditions the streams head in a high relief phyllite schist and metasandstone terrain and flow westward across a bounding fault into lower relief paleozoic shales cherty carbonates and quartz arenites the relative abundance of the source rock types in the drainage basin of each sample site was determined and a source rock ratio was calculated as the ratio of the area underlain by metamorphic rock to the total area of the drainage basin low grade metamorphic source rocks supply subequal amounts of monocrystalline quartz polycrystalline quartz and foliated quartz mica rock fragments to stream sediment whereas sand from the sedimentary source terrain consists primarily of monocrystalline quartz a surprisingly large number of pelitic rock fragments and chert mixing of sediment from both source rock types is recognizable on the quartz provenance diagram of young 1976 by a high instability index unstable polycrystalline quartz total polycrystalline quartz and a low polycrystallinity index polycrystalline total quartz this diagram is especially sensitive to the presence of sandstones as source rocks which further decreases the polycrystallinity index the ratio of metamorphic rock fragments to total rock frag ents also is directly related to the source rock ratio modern sand derived from the foreland fold thurst belt of north georgia can be distinguished from neogene subduction complex derived sand and collision orogen derived sand on qfl lmlslv and qplsmlvm provenance diagrams by containing less total feldspar more total quartz and no volcanic rock fragments the concepts developed from the recent sand are applied to the mississippian parkwood formation alabama and the cretaceous muddy formation montana the results suggest that both sandstone units were derived from foreland fold thrust belts but that the parkwood source area contained a greater proportion of low grade metamorphic source rocks than the source area of the muddy

sedimentology of a lacustrine fan delta system miocene horse camp formation nevada usa a 1600 m thick succession of the miocene horse camp formation member 2 exposed in east central nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan delta environments and nearshore and offshore lacustrine environments these four depositional environments are distinguished by particular associations of individual facies 14 defined facies subaerial and subaqueous fan delta facies associations include ungraded matrix and clast supported conglomerate normally graded matrix and clast supported conglomerate ungraded and normally graded sandstone and massive to poorly laminated mudstone subaqueous fan delta deposits typically have dewatering structures distorted bedding and interbedded mudstone the subaerial fan delta environment was characterized by debris flows hyperconcentrated flows and minor sheetfloods the subaqueous fan delta environment by debris flows high and low density turbidity currents and suspension fallout the nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments high energy oscillatory wave currents probably related to a large fetch reworked grains as large as 2 cm into horizontally stratified sand and gravel offshore directed currents produced uncommonly large typically 1 2 m thick trough cross stratified sandstone in addition stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high energy conditions alternating with periods of carbonate precipitation under reduced energy conditions massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout a depositional model constructed for the horse camp formation member 2 precludes the existence of all four depositional environments at any particular time rather phases characterized by deposition in subaerial fan nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan delta subaqueous fan delta and offshore lacustrine deposition this model suggests that high energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge thus preventing development of a subaqueous fan delta environment and promoting development of a well defined nearshore lacustrine environment low energy nearshore currents induced by shallow water along the

recognizing distinct portions of seamounts using volcanic facies analysis examples from the archean slave province nwt canada abstract archean volcanic rocks in the mafic dominated ca 2 66 2 69 ga point lake and beaulieu river belts slave province northwest territories are significant in demonstrating the facies that characterize specific portions of pillow volcanoes or seamounts irrespective of tectonic setting three distinct localities mapped in detail display facies consistent with 1 proximal deep water 2 medial to distal deep water and 3 medial shallow water seamount settings the proximal facies in the point lake belt include a 55 m thick non vesicular pillowed sequence cut by numerous mafic dykes and sills dykes contain multiple chilled margins indicating successive magma pulses which contributed to edifice construction abundant feeder conduits in addition to the absence of fragmental facies and vesicles are typical of the central deep water portion of seamounts where growth is initiated the medial to distal deep water facies in the point lake belt are represented by a 30 80 m thick assemblage of disorganized pillow breccia and pillowed and massive flows with 5 27 vesicularity massive non vesicular hyaloclastite intermingled with sedimentary material fluidal peperite in addition to thin shale units interstratified with pillow breccia and hyaloclastite indicate that sedimentation and volcanism were contemporaneous an increase in fragmental units and vesicularity relative to the proximal deep water facies is suggestive of the medial to distal part of a seamount in shallower water bedded tuffs laterally along strike with massive flows are the results of turbidity current deposition immediately following localized subaqueous eruptions a medial shallow water seamount setting is represented in the beaulieu river belt by a 5 85 m thick sequence of vesicular lobate pillowed and massive flows stratified pillow breccia and hyaloclastite and mafic dykes vesicularity ranges from 21 49 in pillowed flows 5 40 in massive flows and 20 35 in pillow breccia and hyaloclastite stratified pillow breccia developed along steep flow fronts in shallow water whereas bedded hyaloclastite formed during reworking and redeposition of autoclastic hyaloclastite on seamount flanks in shallow water the volcanic facies associations in the study areas are analogous to those of modern seamounts associated with the mid atlantic ridge and east pacific rise as well as mesozoic cenozoic seamounts in the canary islands fiji southwest japan the sea of japan and cyprus volcanological studies in the point lake and beaulieu river volcanic belts and subsequent comparisons with phanerozoic analogues demonstrate the manner in which distinct portions of ancient seamounts can be recognized in similar archean terranes

the sequence stratigraphic concept and the precambrian rock record an example from the 2 7 2 1 ga transvaal supergroup kaapvaal craton abstract sequence stratigraphic concepts are applied to the 2 7 2 1 ga transvaal supergroup which constitutes the sedimentary floor to the bushveld igneous complex the transvaal succeeds the foreland fill of the witwatersrand supergroup and in its lowermost portions is partly synchronous with the ventersdorp supergroup lavas that followed upon the witwatersrand sediments in the stratigraphic record the unconformable contacts at the base and the top of the transvaal supergroup mark significant changes in the overall tectonic setting which qualifies them as first order sequence boundaries sedimentation within the transvaal basin was controlled by cycles of extensional and or thermal subsidence resulting in the accumulation of sequences separated by stages of base level fall generating sequence bounding subaerial unconformities the transvaal first order sequence is subdivided into five second order unconformity bounded depositional sequences i e the protobasinal black reef chuniespoort rooihoogte timeball hill and boshoek houtenbek sequences the span of time of these second order sequences averaging 130 ma is much longer than the duration of the second order phanerozoic cycles which is explained by the less evolved and thus slower plate tectonic processes that dominated the late archaean early proterozoic times each transvaal second order sequence preserves a variable number of systems tracts mainly as a function of the strength of the erosional processes associated with the ravinement surfaces and subaerial unconformities in a complete succession a sequence would include a basal lowstand systems tract lst followed by transgressive tst highstand hst and falling stage fsst systems tracts the protobasinal and rooihoogte timeball hill sequences only preserve the lst and tst due to the strong post depositional erosion the black reef sequence is composed of lst tst and hst the chuniespoort sequence preserves the tst hst and fsst but lacks the lst due to ravinement erosion the boshoek houtenbek sequence develops the complete succession of systems tracts common features may be emphasized between the same types of systems tract developed within different sequences the lst includes high energy coarse prograding facies such as alluvial fans and fan delta systems that fill the irregularities of the pre existing topography and lead to the peneplanation of the depositional areas the tst may be recognized from retrogradational stacking patterns that are associated with the transgression of marine environments the hst marks the normal regression of the previous marine environments which assumes coeval aggradation of fluvial and basinal facies within an overall progradational framework the fsst would normally include only basinal facies age equivalent with the correlative subaerial unconformities in our case study the stages of base level fall resulted invariably in fully nonmarine environments within the transvaal area generally leading to the development of subaerial unconformities in particular circumstances the nonmarine environment may still accumulate and preserve fluvial alluvial fan or lacustrine sediments as a function of the relative position between topography and local equilibrium profiles and base levels the interpretations of base level rise and fall resulting from the application of the sequence stratigraphic concept to the 2 7 2 1 ga transvaal supergroup enable estimation of sea level changes during deposition of this succession with an understanding of local tectonic conditions on the kaapvaal craton and of coeval sedimentation elsewhere in africa eustatic and relative sea level changes and variation in continental freeboard conditions may be suggested for the transvaal supergroup

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

evolution of the slave craton greenstone belts in the slave craton represent the episodic accumulation of volcanic and sedimentary rocks over as much as 200 m y in the western craton volcanic and sedimentary rocks were deposited between 2 81 and 2 60 ga on 2 9 3 1 ga sialic basement not tectonically juxtaposed with it deposition of 2 6 ga turbidite and conglomerate sequences across the entire slave craton was synchronous with or followed by the main phase of regional deformation and metamorphism at 2 6 2 58 ga and final stabilization of the craton

structural development of an archean orogen western point lake northwest territories the point lake orogen in the central archean slave province of northwestern canada preserves more than 10 km of structural relief through an eroded antiformal thrust stack and deeper anastomosing midcrustal mylonites fault restoration along a 25 km long transect requires a minimum of 69 km slip and 53 km horizontal shortening in the western part of the orogen the basal decollement places mafic plutonic volcanic rocks over an ancient tonalitic gneiss complex ten kilometers to the east in the keskarrah bay area slices of gneiss unroofed on brittle thrusts shed molasse into several submerged basins conglomerates and associated thinly bedded sedimentary rocks are interpreted as channel levee and overbank facies of this thrust related sedimentary fan system the synorogenic erosion surface at the base of the conglomerate truncates premetamorphic or early metamorphic thrust faults formed during foreland propagation while other thrusts related to hinterland progressing imbrication displace this unconformity tightening of synorogenic depositional troughs resulted in the conglomerates present localization in synclines to the west of associated thrust faults and steepening of structural dips eastern parts of the orogen consist of isoclinally folded graywackes composed largely of mutti and ricci lucchi turbidite facies b c and d interpreted as submarine fan deposits eroded from a distant volcanic arc thrust faults in the metasedimentary terrane include highly disrupted slate horizons with meter scale duplex structures and recrystallized calcmylonites exhibiting sheath folds and boudin trains with very large interboudin distances the sequence of fabric development and the overall geometry of this metasedimentary terrane strongly resembles younger forearc accretionary prisms conditions of deformation along the thrusts parallel the regional metamorphic zonation amphibolite facies in the basal decollement through greenschist facies shear zones to cataclastic crush zones in the region of emergent thrusts in keskarrah bay depth differences can account for only half of the metamorphic gradient thermal profiles which increased downwards in obducted greenstone belts and synthrusting plutonism explains other high metamorphic gradients a tectonic model involving the collision of an accretionary prism with a continental margin best explains the structural and sedimentological evolution of the orogen

a terrestrial shallow marine transition in the archean opemisca group east of chapais quebec abstract the stella and hauy formations of the archean opemisca group in the eastern chapais syncline feature numerous fluvial to shallow marine transitions throughout the stratigraphic sequence during the first depositional phase stella formation relatively high gradient alluvial fans prograde into a shallow marine environment the coarse clastic sediments reflect a scott type depositional environment the initial depositional environment is envisaged to be a high energy wave and storm flood dominated coastline bordered by coastal alluvial fans shallow marine deposits consist of coarse to very coarse grained sandstones and intraclast and pebble to cobble conglomerates the prevalent shallow marine sediments are related to storm and or flood deposition terrigenous sediments of the donjek type depositional environment prevail during the second depositional phase hauy formation the shallow marine sediments although still wave and storm flood dominated are finergrained and when correlated with the terrigenous sediments imply a lower gradient and a maturing of the coastline furthermore contemporaneous volcanism of andesitic shoshonitic to basaltic composition occurs during this evolutionary stage the sedimentary basin gradually evolved during the two depositional phases the repetition of terrigenous and shallow marine sediments throughout the stratigraphic sequence coarsening and fining sequences in the conglomerates and the alluvial fan setting strongly suggest a fault bounded basin margin tectonic uplift of the hinterland initiates alluvial fan progradation onto the shelf whereas quiescent periods are documented by marine transgression the clast composition of the conglomerates and the alluvial fan environment suggest local derivation of clasts the synvolcanic chibougamau pluton and the lake dore complex acted as a stable basement during deposition of the opemisca group the small landmass served as a stable platform for shallow marine deposition even when tectonically influenced a late phase intraplutonic cratonic development of an initial backarc basin system internal zone of the abitibi greenstone belt is proposed as the overall depositional setting of the chapais basin