geochemistry u pb geochronology and sm nd data from the paleoproterozoic western finland supersuite a key component in the coupled bothnian oroclines abstract the western finland wf supersuite characterized by supracrustal rocks and intruding plutonic rocks forms the core component of the coupled bothnian oroclines in the central part of the svecofennian orogen we present here geochemical isotope data detrital zircon age data on sedimentary rocks and age data on granitoid plutons within the wf supersuite in the pirkanmaa pb and pohjanmaa pob belt representing the southernmost limb of the southern and the hinge zone of the northern bothnian orocline respectively the tampere belt tb in the north and hame belt hb in the south bound the pb basaltic to picritic volcanic rocks in the wf supersuite with a proposed eruption age at ca 1 91 ga 1 92 1 90 ga show locally pillow structures and are typically associated with black shales cherts carbonate bearing rocks based on geochemistry and sm nd data a variable morb wpb type origin is evident and an evolution scenario from earlier wpb type rifting to more morb affinity ocean sea opening is proposed detrital zircon age distributions from sedimentary rocks with 1 91 ga maximum depositional age in the tb pb and hb have been compared and typically the age distributions of

multistage damming of the rhine river by tephra fallout during the 12 900 bp plinian laacher see eruption germany syn eruptive rhine damming i abstract the rhine the largest river in western europe was dammed during the plinian laacher see eruption lse 12 900 bp damming during the climactic plinian episode of lse occurred both upstream and downstream of the broad tectonic lower neuwied basin lnb that interrupts the narrow rhine canyon we here document details of the upstream damming at the bottleneck entrance to the lnb near the present city of koblenz our reconstruction is based on a high resolution analysis and correlation of the complex intercalation of primary fallout tephra relics with fluvially reworked laacher see tephra in the lnb tephra units representing complete eruptive cycles repeatedly fell on drained ground in between one minor and 4 major flooding events even at the base of side channels that had been active prior to the lse and that had been flooded by a preceding flooding event this demonstrates that flooding occurred generally during breaks and not during fallout events the repeated formation and breach of a dam at the upstream entrance of the lnb koblenz dam consisting of fallout components and driftwood washed together convincingly explains the multiple repetition of the drainage of the channels in the lnb followed by large magnitude flooding in rapid succession the strongly pulsating nature of the lse reflected in multiple interruptions of eruptive activity fundamentally controlled the damming and flooding dynamics the rhine became completely blocked during distinct fallout phases due to overloading with pumice that had fallen into the river and its major tributaries the temporary dam collapsed during eruptive breaks this is the first recorded example of tephra fallout damming a major watercourse the extremely low gradient of the rhine river allowed the repeated accumulation of large volumes of water in a long multi phase dammed up lake lake koblenz that extended along the upstream course of the river for up to c 30 km despite the low height damming of a major river while a large plinian eruption is in full progress represents an extraordinary challenge for hazard mitigation this is especially pertinent for an area close to and downwind from the vent and therefore simultaneously affected by massive plinian fall such as in the lnb

fluvial response to paleo equatorial climate fluctuations during the late paleozoic ice age the maritimes basin of atlantic canada preserves a 12 km thick stratigraphic succession deposited within a low latitude predominantly continental setting during the late paleozoic this paper examines the utility of using such successions for paleoclimate analysis with specific reference to changes in fluvial style modern rivers in the subhumid and semiarid tropics preserve a suite of features that are distinct from those in humid and arid environments these features include an abundance of sedimentary structures formed under froude transcritical to supercritical flow conditions and the presence of in situ vegetation growing within the channel environment this fluvial style is indicative of prolonged low flow conditions punctuated by intense precipitation events in a strongly seasonal climatic setting fluvial channel bodies within the maritimes basin preserve a remarkably similar style compared with these modern systems suggesting that a subhumid strongly seasonal paleoclimate was the dominant imprint on parts of the preserved successions strongly seasonal deposits are concentrated within four discrete stratigraphic intervals e1 e4 ranging in duration from 2 to 6 m y these intervals are correlated across the entire basin and between basins characterized by markedly different tectonic regimes which suggests that a coherent regional climate signal is recorded intervals are separated by deposits characteristic of humid and arid river systems and the alternation between these different fluvial styles implies that several pronounced long term changes in precipitation and runoff regimes occurred during the carboniferous within the region furthermore these intervals broadly coincide with major periods of southern hemisphere glaciation which suggests that glaciation had a profound and controlling effect on paleotropical climate

horizontal lamination and the sequence of bed phases and stratification under upper flow regime conditions two forms of horizontal laminae have been produced under upper flow regime conditions in a flume by deposition on a plane bed and on low in phase waves upper plane bed laminae consist of approximately equal numbers of fining upward fu and coarsening upward cu textural laminae heavy minerals when present are typically within or form cu laminae in horizontal laminae formed under low in phase waves fu laminae may predominate and are significantly thicker than cu laminae heavy minerals are associated with the tops of some fu laminae and are absent from cu laminae deposits aggraded on plane beds and low in phase waves both display low angle 13 14 upstream grain imbrication upper plane bed horizontal laminae are attributed to the effects of the burst sweep cycle on local bedload transport bursts form fu laminae and sweeps form cu laminae some fu laminae in in phase wave deposits are thought to form with downstream migration of bed waves on which sand size decreases from trough to crest heavy minerals are concentrated in the tops of these fu laminae due to their enrichment on the crests of the migrating bed waves other fu and cu laminae in in phase wave deposits probably formed by the action of bursts and sweeps as on upper plane beds a sequence of bed phases and stratification formed under upper flow regime conditions is described based on the observations here combined with descriptions from the literature with increasing flow strength the gradual sequence of bed phases is 1 plane bed 2 low relief downstream migrating inphase waves increasing in amplitude and wavelength with increasing flow strength 3 stationary in phase waves 4 upstream migrating including breaking wave antidunes the sequence of stratification formed under these bed phases is 1 upper plane bed horizontal lamination 2 in phase wave horizontal lamination to in phase wave foreset cross laminae 3 in phase wave drape laminae 4 antidune backset cross laminae

u pb and sm nd isotopic constraints on the evolution of the paleoproterozoic perapohja belt northern finland ten localities representing metasedimentary and igneous rocks from the northern part of the paleoproterozoic perapohja belt fennoscandian shield were sampled for zircon u pb dating by la mc icp ms and sm nd analysis by tims the results revealed two phases of granitic magmatism that took place at 1 99 ga and 1 79 1 77 ga the former age 1989 6 ma was given by the kierovaara granite pluton representing the first example of felsic plutonism of this age in northern finland though a type felsic volcanic rocks of the same age have previously been recognized the strongly negative initial nd values obtained for granitoids ranging from 3 9 to 11 4 are compatible with the previous sm nd data indicating the presence of archean basement in northern finland at the time of granite emplacement a garnet whole rock sm nd age of 1752 14 ma was measured for a mica gneiss constraining the time of metamorphic cooling down to the closure temperature of garnet detrital zircon grains from quartzite samples belonging to the jatulian palokivalo formation and a stratigraphically unassigned lithodemic unit the mellajoki suite yielded solely archean zircon populations in contrast our sample from the martimo formation representing kalevian sedimentation revealed a 66 paleoproterozoic detrital zircon component with the youngest zircon grains showing an age of ca 1 91 ga the new age data allow us to constrain the depositional time of this sedimentary unit between ca 1 91 and 1 88 ga the latter being the time of the synorogenic plutonism our geochronological data coupled with recent data from other karelian belts suggest that the depositional time period for rocks assigned to the kaleva spans ca 150 ma and the simple division of these rocks into two non coeval units the lower and upper kaleva has become problematic

early proterozoic nappe formation an example from sodankyla finland northern baltic shield the central lapland greenstone belt comprises rift related metavolcanic and metasedimentary rocks representing one of the largest supracrustal belts in the baltic shield the sodankyla area in the central part of the belt represents a complex thrust duplex within a nappe overlying belomorian archaean basement and autocthonous luirojoki calc silicate rocks here an early d 1 schistosity is axial planar to at least three coaxial generations of southward verging subhorizontal e w plunging d 1 folds associated with major southwards thrusting d 2 is represented by broad map scale upright ne trending folds in the south and crenulations in the north staurolite grade metamorphism represented by post tectonic andalusite staurolite kyanite assemblages occurred after d 2 folding later d 3 deformation was limited to local nw trending folds and sinistral faults the internal nappe like structure of the central lapland greenstone belt suggests that it represents the foreland of a large collisional complex cored by the lapland granulite belt

antidunes and traction carpet deposits in deep water channel sandstones cretaceous british columbia canada abstract sandstone beds intercalated with thick shale of the upper member of the haida formation albian turonian in the queen charlotte islands british columbia canada show thinly laminated graded bedding the sandstones generally are present as successions of more than a few tens of inversely to normally graded thin beds the beds at the base of each succession contain much coarser detritus than the beds at the top each succession of graded beds shows a lens shaped channel geometry wavy bed forms similar in shape to the hummock and swale topography that is thought to form hummocky cross stratification hcs characterize the uppermost part of the succession however grain fabric and spacing of the bed forms indicate that they are of antidune origin it is concluded that the successions of multiple laminated sandstone beds were produced by high density turbidity currents that were probably associated with a gravity transformation in which a successive freezing of traction carpets took place lack of shale in the succession together with the common presence of antidunes suggest that each succession was the product of one sedimentation event without decrease in stream power

early paleoproterozoic felsic and mafic magmatism in the karelian province of the fennoscandian shield abstract early paleoproterozoic 2 50 ga to 2 39 ga mafic and felsic intrusions are widespread in the fennoscandian shield the bimodal magmatic event reflects an extensional phase in the archean craton that had been tectonically quiescent for ca 200 ma after the amalgamation of the continent the early paleoproterozoic felsic intrusions are mostly granitic in composition only one syenitic pluton is known geochemically the felsic intrusions are a type the mafic layered intrusions were emplaced in the upper crust between the archean basement and overlying early paleoproterozoic volcanic rocks at least two different parental magma compositions were present in the mafic magmatic event partial melting of the archean lower crust due to heating effect of the hot mafic magmas most probably generated the felsic magmas which reflect the nd isotope composition of the source rocks the irregular distribution of coeval mafic and felsic intrusions within the karelian province is still an open question

quantification of tsunami induced flows on a mediterranean carbonate ramp reveals catastrophic evolution cool water carbonates are the dominant limestones in the mediterranean basin since the early pliocene their deposition typically resulted in ramp morphologies due to high rates of resedimentation several such fossil carbonate ramps are characterised by a bimodal facies stacking pattern where background deposition of subaqueous dune and or tempestite deposits is repeatedly interrupted by anomalously thick sedimentary units dominated by backset stratification formed by supercritical flows a multitude of exceptional triggers e g storms floods tsunamis have been invoked to explain the origin of these supercritical flows which in the absence of a quantitative analysis remains speculative as yet here for the first time the catastrophic evolution of one such mediterranean carbonate ramp on favignana island italy is quantified by combining 87sr 86sr dating outcrop based palaeoflow reconstructions and hydraulic calculations we demonstrate that rare tsunami induced flows occurring on average once every 14 to 35 kyr lasting a few hours only deposited the anomalously thick backset bedded units that form half of the sedimentary record in between such events cumulative two years of storm induced flows deposited the remaining half of the succession by the stacking of subaqueous dunes the two to four orders of magnitude difference in average recurrence period between the two flow types and their associated sedimentation rates emphasises the genetic differences between the two styles of deposition in terms of sediment transport the studied carbonate ramp was inactive for at least 99 of the time with gradual progradation during decennial to centennial storm activity carbonate ramp evolution attained a catastrophic signature by the contribution of rare tsunamis producing short lived high energy sediment gravity flows

association of gold with uraninite and pyrobitumen in the metavolcanic rock hosted hydrothermal au u mineralisation at rompas perapohja schist belt northern finland the perapohja schist belt comprises a supracrustal sequence of quartzites mafic volcanics and volcaniclastics carbonate rocks black shales mica schists and greywackes which were deposited from ca 2 44 to 1 91 ga during the rifting of the archaean basement in the eastern part of the fennoscandian shield metamorphism and multiple folding of the basin fill took place during the svecofennian orogeny 1 9 1 8 ga followed by intrusions of late orogenic 1 84 1 80 ga and post orogenic granitoids 1 79 1 76 ga the rompas au u mineralisation is hosted by deformed calcsilicate veins in mafic volcanic rocks and locally contains very high grade 10 000 g t au gold pockets with strict spatial association of gold minerals to uraninite and pyrobitumen chemical ages from the unaltered domains in the structure of uraninite indicate a 1 95 1 90 ga age for the deposition of the primary high temperature e g u th 100 in uraninite hydrothermal uranium mineralisation these data are in agreement with the results of previous u pb dating of uraninite by sims textural evidence suggests that metamorphic recrystallisation of the uraninite bearing quartz dolomite veins into calcsilicate mineral assemblages during the svecofennian orogeny 1 9 1 8 ga was followed by a hydrocarbon bearing fluid flow event and radiolytic polymerisation of hydrocarbons around grains of uraninite gold precipitated during a subsequent hydrothermal process in the fractures of uraninite as well as in the cracks and on the botryoidal surfaces of uraninite pyrobitumen nodules remobilisation and redeposition of uranium by these hydrothermal events produced secondary uraninite grains with chemical ages between 1 85 and 1 65 ga native gold is associated with galena altaite hunchunite nickeline and rare cobaltite pb bearing maldonite pyrite pyrrhotite chalcopyrite molybdenite and titanite raman spectra show disordered structure of undeformed pyrobitumen nodules in contrast with the well ordered graphite in calcsilicate veins mean random reflectance data for pyrobitumen indicate 270 340 c maximum temperature of thermal maturation this temperature range is also considered as the temperature of gold deposition results of multiple sulphur isotope analyses of organic material pyrite and acid volatile bound sulphur show distinct ranges of 34s values for sorg and scrs in uraninite pyrobitumen from 6 99 to 3 55 and from 10 02 to 4 41 respectively and uraninite pyrobitumen native gold mineral associations from 1 36 to 6 87 and from 0 42 to 9 7 respectively 33s data indicate local occurrence of nonmass dependent sulphur isotope fractionation owing to interaction of fluids with organic material concentration of lead in uraninite is depleted along the gold mineral filled fractures whereas the uranogenic lead isotope contents of galena altaite and hunchuite deposited in the same fractures are extremely high suggesting that the dominant source of lead for the crystallisation of these minerals was the radiogenic lead content of uraninite taking into account this source of radiogenic lead the calculated pb pb model ages for the lead minerals are between 1 75 and 1 70 ga sulphur and tellurium removal from the fluid by reaction with radiogenic lead released by uraninite appears to be an important mechanism in the strongly localised deposition of gold minerals scavenging of sulphur by pyrobitumen nodules from gold transporting fluids was an additional process triggering precipitation of gold carbon particles and organic functional groups in pyrobitumen probably acted as nucleation and adsorption centres for gold minerals

migratory tectonic switching western svecofennian orogen central sweden constraints from u pb zircon and titanite geochronology abstract the forsmark area in the western part of the svecofennian orogen central sweden is situated between two major palaeoproterozoic tectonic domains that show contrasting histories with respect to timing of igneous activity ductile deformation and metamorphism geological features common to both the adjacent domains are found at forsmark which consequently is a key area for understanding the tectonic evolution of the svecofennian orogen in this region new sims and tims geochronological data from several samples in a limited area at forsmark constrain the timing of igneous activity ductile deformation and metamorphism in this area u pb zircon dating in combination with field studies reveals the existence of two calk alkaline igneous suites at forsmark the older and most voluminous plutonic suite intruded at 1 89 1 87 ga and is affected by penetrative yet variably intense ductile deformation with the development of a ductile grain shape fabric under amphibolite facies metamorphic conditions and folding the younger and less voluminous hypabyssal suite intruded at 1 86 1 85 ga during the waning stages of this penetrative deformational phase furthermore u pb titanite dating shows that the area has been affected by one or more tectonothermal events after regional fabric development and possibly even after 1 83 ga the deformation during these events involved more restricted strain along discrete zones the forsmark data in combination with a compilation and evaluation of available geochronological data from the time interval 1 91 1 84 ga in central sweden points to the existence of at least two major tectonic cycles in this region each cycle is characterised by igneous activity associated at least partly with extension a short interval of compressional deformation and migration of the tectonic activity in this paper we discuss two highly contrasting tectonic models that may explain the cyclic tectonic evolution of the western part of the svecofennian orogen in central sweden the favoured model involves continuous subduction with a constant polarity beneath a single active continental margin combined with alternating subduction hinge retreat and advance the model involves migration of what has been described as tectonic switching in the younger accretionary orogenic systems of eastern australia palaeozoic lachlan orogen and new zealand

the sedimentary architecture of outburst flood eskers a comparison of ground penetrating radar data from bering glacier alaska and skeiðararjokull iceland we present ground penetrating radar gpr profiles that reveal the sedimentary architecture of an esker deposited during a surge associated outburst flood at the bering glacier alaska the wide up flow end of the esker contains a transition from large backset beds to large foreset beds interpreted to reflect composite macroform development in an enlarged part of the conduit by contrast the narrow down flow portion of the esker is dominated by plane beds interpreted to have been deposited where the conduit was constricted and the flow was faster a previously studied outburst esker at skeidararjokull iceland has a similar morphology and stratigraphic architecture this suggests that outburst floods generate distinct depositional signatures in eskers both in terms of morphology and sedimentary architecture identification of these distinct signatures in ancient eskers will help assess the paleohydraulic conditions under which ancient eskers formed and by extension the nature of meltwater drainage systems beneath the laurentide and eurasian ice sheets

deconvolving signals of tectonic and climatic controls from continental basins an example from the late paleozoic cumberland basin atlantic canada abstract the internal architecture and external form of fluvial channel bodies is governed by both intrinsic and extrinsic controls extrinsic controls such as climate eustasy and tectonism are believed to have modest effects on channel body types and geometries in high subsidence settings the effects of climate are particularly cryptic in high accommodation settings and have proven extremely difficult to separate from other extrinsic controls such as tectonics and eustasy the thick successions preserved in such basins have the potential to be relatively stratigraphically complete and are therefore valuable for interpreting basin evolution this study documents the external geometry and internal architectural styles of fluvial channel bodies in a spectacularly exposed 7 km thick mississippian permian section from the cumberland basin atlantic canada four fluvial styles are recognized perennial strongly seasonal ephemeral and fixed fluvial styles are not randomly distributed throughout the stratigraphic succession instead discrete stratigraphic intervals consist of predominantly one type of fluvial style four stratigraphic intervals e1 e4 in which strongly seasonal fluvial styles are predominant are recognized and alternate with intervals that are characterized by either perennial or ephemeral deposits this suggests that a coherent record of climate change as manifested by changes in precipitation and runoff regime is recorded in the internal architecture of sandbodies

sediment provenance in a tropical fluvial and marine context by magnetic fingerprinting of transportable sand fractions the sources and fluxes of sediment to the great barrier reef lagoon from northeastern australian rivers have been the subject of much concern and study with the large catchments of the burdekin and fitzroy rivers thought to be the key sources at present here the utility of newly developed magnetic fingerprinting methods for identifying sediment provenance both onshore and offshore and in association with individual large flood events is investigated within the burdekin catchment sediments are mobilized from different subcatchments by runoff generated by intense localized rainfall events magnetic measurements were made on untreated and acid treated samples of river channel sediments within the burdekin river subcatchments and from the estuarine and inner shelf depocentres of burdekin river sediments the acid treatment removes all discrete magnetic particles and coatings and leaves magnetic inclusions protected within host silicate grains as the basis of the measured magnetic signature of a sample the magnetic properties of the acid treated samples display statistically distinct sediment provenance groupings sand samples from the upper burdekin river appear magnetically distinct from samples from tributaries of the burdekin e g hann creek fanning river and also from nearby coastal rivers including the haughton suspended sand samples from a burdekin flood event in 2000 appear to have a different source compared with those from floods in 1998 and 1999 comparisons of the terrestrial acid treated sand fractions with the same acid treated sand size fractions from transects taken offshore suggest that the surface sediments in upstart bay and bowling green bay have different sources some of these sources are as yet unidentified but may represent the unsampled lower discharge south western burdekin subcatchments and or along shore drift of sand from the south perhaps even from the fitzroy river over millennial timescales of cyclone pumping the magnetic inclusion method precludes any obfuscation or confounding of sediment source which might arise from hydraulic sorting and or post depositional magnetic diagenesis or authigenesis

palaeoproterozoic accretionary processes in fennoscandia abstract accretionary processes contributed to major continental growth in fennoscandia during the palaeoproterozoic mainly from 2 1 to 1 8 ga the composite svecofennian orogen covers c 1 10 6 km 2 and comprises the lapland savo fennia svecobaltic and nordic orogens it is a collage of 2 1 2 0 ga microcontinents and 2 02 1 82 ga island arcs attached to the archaean karelian craton between 1 92 and 1 79 ga andean type vertical magmatic additions especially at c 1 89 and c 1 8 ga were also important in the continental growth the palaeoproterozoic crust is the end product of accretionary growth continental collision and orogenic collapse preserved accretional sections are found in areas where docking of rigid blocks has prevented further shortening the pirkanmaa belt represents a composite accretionary prism and other preserved palaeosubduction zones are identified in the gulf of bothnia and the baltic sea areas in the southern segment of the lapland savo orogen collision between the archaean continent lower plate and the palaeoproterozoic arc microcontinent assembly upper plate produced a special type of lateral crustal growth the archaean continental edge decoupled from its mantle during initial collision and overrode the arc and its mantle during continued collision

sedimentary architecture of large scale jokulhlaup generated ice block obstacle marks examples from skeiðararsandur se iceland during glacial outburst floods jokulhlaups large ice blocks are often released from the glacier margin ice blocks ground at points of flow deceleration generating distinctive patterns of scour and deposition scaled to the obstacle size the large scale ice block obstacle marks produced within the proglacial zone are diagnostic of jokulhlaups we use ground penetrating radar gpr to investigate the sedimentary architecture of ice block obstacle marks generated by the november 1996 jokulhlaup at skei ararjokull iceland the gpr data reveal a varied sedimentary architecture interpreted to suggest that ice block obstacle mark sedimentology is controlled by flow conditions sediment supply and whether ice blocks are deposited in isolation or as a cluster following ice block grounding deflection of flow down and away from the obstacle develops horseshoe vortices that maintain a non depositional hollow around the obstacle whilst continued outwash deposition away from the obstacle raises the level of the surrounding sandur where ice blocks ground early during the jokulhlaup hollows are infilled with foreset beds at the up flow end and backset beds antidune distally however where ice blocks ground on the waning stage when sediment flux is reduced antidunes are not preserved and the hollow is infilled with low angle foreset beds in both cases the lee side shadow ridge is composed of pseudo anticlinal beds proximal to the obstacle where shed vortices are strong and plane beds distally where flows converge and the shed vortices weaken where flow parallel ice block clusters develop the grounding of smaller ice blocks in the lee of the initial obstacle results in only limited shadow ridge aggradation these data provide improved insight into the controls on ice block obstacle mark sedimentary architecture and allow identification of high magnitude jokulhlaups in the sedimentary record

ordovician proglacial sediments in algeria insights into the controls on hydrocarbon reservoirs in the in amenas field illizi basin abstract ordovician proglacial deposits form gas reservoirs in the in amenas field illizi basin algeria depositional models were developed to understand the context and disposition of the main reservoirs through an evaluation of core and analogous outcrops from the tassili n9ajjer tunnel valleys initially accumulated sandstones with tractional structures subsequent failures of subaqueous grounding line sediment deposited proglacial debrites comprising poorly sorted argillaceous sandstone with granules these were interbedded with high density turbidity sandstones their fine grain size indicates they were dynamically disconnected from the lithologically varied debrites a lobate geometry has been defined for one subsurface composite turbidite periodic catastrophic outflows possibly evacuating subglacial lakes incised the network of subglacial tunnels and in the process delivered sand to the turbidite outwashes bedforms indicate high energy transcritical to supercritical outflows that were stable for extended periods during ice retreat a period of ice margin stability may have occurred due to grounding over the in amenas granitic palaeohighs outwash fan apices were located along this grounding line with feeder channels developing where the substrate was more easily eroded such as between the palaeohighs following further ice retreat deposition evolved to variably sinuous channels and thence to pelagic fines with dropstones

the role of discharge variability in the formation and preservation of alluvial sediment bodies abstract extant planform based facies models for alluvial deposits are not fully fit for purpose because they over emphasise plan form whereas there is little in the alluvial rock record that is distinctive of any particular planform and because the planform of individual rivers vary in both time and space accordingly existing facies models have limited predictive capability in this paper we explore the role of inter annual peak discharge variability as a possible control on the character of the preserved alluvial record data from a suite of modern rivers for which long term gauging records are available and for which there are published descriptions of subsurface sedimentary architecture are analysed the selected rivers are categorized according to their variance in peak discharge or the coefficient of variation cvqp standard deviation of the annual peak flood discharge over the mean annual peak flood discharge this parameter ranges over the rivers studied between 0 18 and 1 22 allowing classification of rivers as having very low 0 90 annual peak discharge variance deposits of rivers with very low and low peak discharge variability are dominated by cross bedding on various scales and preserve macroform bedding structure allowing the interpretation of bar construction processes rivers with moderate values preserve mostly cross bedding but records of macroform processes are in places muted and considerably modified by reworking rivers with high and very high values of annual peak discharge variability show a wide range of bedding structures commonly including critical and supercritical flow structures abundant in situ trees and transported large woody debris and their deposits contain pedogenically modified mud partings and generally lack macroform structure such a facies assemblage is distinctively different from the conventional fluvial style recorded in published facies models but is widely developed both in modern and ancient alluvial deposits this high peak variance style is also distinctive of rivers that are undergoing contraction in discharge over time because of the gradual annexation of the channel belt by the establishment of woody vegetation we propose that discharge variability both inter annual peak variation and flashiness may be a more reliable basis for classifying the alluvial rock record than planform and we provide some examples of three classes of alluvial sediment bodies representing low intermediate and high very high discharge variability from the rock record that illustrate this point

architectural analysis of a volcaniclastic jokulhlaup deposit southern iceland sedimentary evidence for supercritical flow the 1918 eruption of the glacially capped katla volcano southern iceland generated a violent jokulhlaup or glacial outburst flood inundating a large area of m rdalssandur the proglacial outwash plain where it deposited ca 1 km3 of volcaniclastic sediment the character of the 1918 jokulhlaup is contentious having been variously categorized as a turbulent water flow a hyperconcentrated flow or as a debris flow based on localized outcrop analysis in this study outcrop based architectural analyses of the 1918 deposits reveal the presence of lenticular and tabular bedsets associated with deposition from quasi stationary antidunes and down current migrating antidunes and from regular based bedsets associated with transient chute and pool bedforms all of which are associated with turbulent transcritical to supercritical water flow conditions antidune wavelengths range from 24 to 96 m corresponding to flow velocities of 6 to 12 m sec 1 and average flow depths of 5 to 19 m this range of calculated flow velocities is in good agreement with estimates made from eyewitness accounts architectural analysis of the 1918 jokulhlaup deposits has led to an improved estimation of flow parameters and flow hydraulics associated with the 1918 jokulhlaup that could not have been achieved through localized outcrop analysis the observations presented here provide additional sedimentological and architectural criteria for the recognition of deposits associated with transcritical and supercritical water flow conditions the physical scale of sedimentary architectures associated with the migration of bedforms is largely dependent on the magnitude of the formative flow events or processes sedimentary analyses must therefore be undertaken at the appropriate physical scale if reliable interpretations regarding modes of deposition and formative flow hydraulics are to be made

bottomset architecture formed in the troughs of dunes and unit bars abstract bottomsets are formed in the troughs of migrating bedforms the flow and sediment dynamics downstream of bedforms differ and consequently the thickness grain size and internal structure of bottomsets vary greatly a classification of the controls on bottomset formation is proposed related to two and three dimensional processes flow and sediment unsteadiness and pre existing deposits structures and deposits created by flow and sediment unsteadiness as well as pre existing deposits are also considered in relation to changes they can induce in subsequent trough processes the classification is illustrated by observations from laboratory flumes the modern burdekin river australia and the rock record the roaches grit england and hawkesbury sandstone australia in bottomset examples described in this paper internal variability both vertically and laterally was much greater than that within the dominant foreset component complex compound structures were created by the interaction and predominance of different bottomset controls this combined with their high preservation potential makes them a highly useful paleoenvironmental indicator bottomset variation e g changes in the abundance of mud laterally will influence the permeability heterogeneity of fluvial reservoir rocks bottomsets formed under relatively steady conditions are likely to be laterally extensive and have similar characteristics over their entire length they may act as significant barriers to vertical flow conversely bottomsets generated in unsteady regimes tend to be highly variable potentially creating conduits between cross bed sets aiding inter set permeability

variations in character and preservation potential of vegetation induced obstacle marks in the variable discharge burdekin river of north queensland australia abstract vegetation induced obstacle marks are described from dalrymple bend in the burdekin river of queensland australia and their preservation potential is discussed the bend is divided into a sand and gravel covered lower bar and a vegetated upper bar obstacle marks comprising an erosional scour and a depositional sediment tail are recognised on the lower bar and the lower margin of the upper bar two types of obstacle marks are dominant in the lower part of the lower bar sediment tail dominated obstacle marks associated with inclined trees of melaleuca argentea silver leaved paperbark and comparatively small obstacle marks around stranded driftwood obstacle marks in the upper part of the lower bar are formed around grass clumps armoured with mud and gravel at the lower margin of the upper bar scours and gravelly sediment tails are recognised around mature tall trees in these cases the scour is 4 m in width and covered with alternating beds of fine sand and mud among the obstacle marks described sediment tail dominated obstacle marks around groves of m argentea have the highest preservation potential obstacle marks in the lower margin of the upper bar are also considered to have high preservation potential obstacle marks around stranded driftwood have the lowest preservation potential obstacle marks around grass clumps survive more than 1 year but their long term preservation potential is nonetheless low implications for the stratigraphic record are also discussed

slip and recrystallization of halite gouge in experimental shear zones abstract cylindrical specimens of tennessee sandstone with dry crushed halite between 35 precut surfaces are deformed at 70 mpa confining pressure at displacement rates from 10 2 to 10 6 cm s at 25 c and the shear strains of 16 0 under these conditions peak differential stress is attained at shear strains between 3 and 5 followed by quasi steady state behavior with subtle but possibly significant oscillations over the complete variation of conditions steady state stress levels range from 146 to 169 mpa at shear strains 6 0 the fabric of the halite gouge 0 2 to 1 0 mm thick changes with increasing shear strain from 1 a homogeneously elongated grains up to peak stress indicative of dislocation glide to 2 b a bimodal mosaic of recrystallized grains consisting of very fine grained neoblasts and relatively large euhedral porphyroblasts that is associated with a small stress drop to 3 c a mosaic of uniformly sized anhedral neoblasts produced from the coalescence of porphyroblasts grain size of the very fine grained neoblasts remain reasonably constant with increasing but that of the porphyroclasts and of the resulting mosaic decrease with increasing 100 of the porphyroblasts are strongly and conspicuously oriented at 45 to the shear zone boundary at 4 4 and progressively rotate with the finite strain axes so that at 9 4 100 are inclined at 70 80 and 10 20 to the shear zone it is argued that fabrics b and c are consistent with dynamic recrystallization by grain boundary migration from favorably oriented nuclei grains a ghost structure of aligned grain boundaries and impurities pervades all the shear zones and is oriented at progressively smaller angles to the shear zone boundary as shear strain increases there is a one to one correspondence between these angles and those calculated assuming simple shear this fact along with the constant thickness of the gouge with increasing shear strain indicates that the deformation is modeled by progressive homogeneous simple shear

interpreting flash flood palaeoflow parameters from antidunes and gravel lenses an example from montserrat west indies the wavelength of stationary water surface waves and their associated antidune bedforms are related to the mean velocity and depth of formative flow in sand bed flume experiments alexander et al 2001 found that lens structures were preserved during antidune growth and change and the dimension of the lenses were empirically related to antidune wavelength and thus could be used to estimate flow velocity and depth this study is the first to compare observations of formative flow conditions and resulting sedimentary structures in a natural setting testing the alexander et al 2001 relationship at a field scale trains of stationary and upstream migrating water surface waves were prevalent during the flash flood in october 2012 in the belham valley montserrat west indies wave positions and wavelengths were assessed at 900 s intervals through the daylight hours of the event within a monitored reach the wave data indicate flow depths up to 1 3 m and velocity up to 3 6 m s 1 sedimentary structures formed by antidune growth and change were preserved in the event deposit these structures include lenses of clast supported gravel and massive sand with varying internal architecture the lenses and associated low angle strata are comparable to sand bed structures formed from stationary and upstream migrating waves in flume experiments confirming the diagnostic value of these structures using mean lens length in the event deposit underestimated peak flow conditions during the flood and implied that the lenses were preserved during waning flow

the bothnian coupled oroclines of the svecofennian orogen a palaeoproterozoic terrane wreck the accretion of magmatic arcs gives rise to elongate linear orogens and is a key process in forming new continental crust many precambrian continents are however presently equidimensional or have large areas without any clear linearity such as the central part of the palaeoproterozoic svecofennian orogen 1 92 1 77 ga one way of forming an equidimensional continental domain is by buckling of a linear orogen about vertical axes of rotation into one or more coupled oroclines here we reinterpret existing data and demonstrate the occurrence of coupled bothnian oroclines in the svecofennian orogen palinspastic restoration of the southern and northern bothnian oroclines brings a 1000 km long segment of the svecofennian orogen into an originally linear nw striking geometry that restores the lithological belts metamorphic zones and structural vergences to a common direction and which indicates that the orogen consists of a sw facing arc which has been shortened along ne verging folds and thrust faults

variations in the architecture of hydraulic jump bar complexes on non eroding beds sediment accumulation downstream of hydraulic jumps can occur in many settings but the architectures of such deposits are poorly documented here three flume runs were used to examine the influence of sediment grain size and transport rate on the characteristics of hydraulic jump unit bars in one of these runs six hydraulic jump unit bars formed a hydraulic jump bar complex in another the same sediment was supplied more quickly and only two unit bars formed in the third run with the same sediment supply rate but different grain size only one large unit bar formed all unit bars developed in a similar way but their size and internal architecture differed they all resulted from a reduction in sediment transport capacity at the transition from supercritical flow to subcritical flow in the hydraulic jump after initial onset of sedimentation and unit bar formation generation of subsequent unit bars may be i related to small changes in sediment flux and ii independent of changes in the hydraulic jump continued sedimentation caused changes from oscillating to weak hydraulic jumps and hydraulic jump unit bars formed in both circumstances the flow of water and suspended sediment becomes shallower over the lee of the bar complex this leads to flow acceleration and a return to supercritical flow conditions in turn a chain of such features can form and generate a chute and pool bed morphology there is an inherent upper size limit to a hydraulic jump bar complex due to the changing flow conditions over the growing deposit as the water above it becomes shallower there is also an amplitude minimum for the development of foresets and subsequent unit bar growth hydraulic jump unit bars have architectures that should be recognizable in the rock record and because their size is constrained by the flow conditions their identification should be useful for interpreting palaeoenvironment

a succession of near orthogonal horizontal tectonic shortenings in the paleoproterozoic central lapland greenstone belt of fennoscandia constraints from the world class suurikuusikko gold deposit the relative and absolute timing of orogenic gold deposits in complex structural settings are active and challenging topics of research especially in precambrian greenstone belts the suurikuusikko gold deposit in central lapland greenstone belt is currently the largest primary gold producing deposit in europe located on a slight bend of the strike slip kiistala shear zone kisz gold is refractory and locked inside arsenopyrite and pyrite in this study different structural features were investigated along the kisz from the recently stripped etela pit which is the southern extension of the suurikuusikko ore body our data source ranges from aeromagnetic to high resolution aerial images x ray computed tomography scans of selected rock samples and regional geological and geophysical datasets the kisz has recorded five discrete deformation phases spanning between ca 1 92 and 1 76 ga the refractory gold at the suurikuusikko deposit formed during e w contraction related to the d1 thrusting phase this was followed by a n s shortening event d2 where most of the strain was taken up by the northern and southern thrusts tectonic vectors then switched from n s to ne sw and as a result dextral strike slip regime d3 commenced along the kisz this event exsolved invisible gold from the sulfides and remobilized it along with fractures a near orthogonal switch of the regional stress regime from ne sw to nw se flipped the kinematics of the kisz from dextral to sinistral d4 the last deformation phase d5 produced widespread veining under e w contraction and secured gold mineralization at the iso kuotko gold deposit within the kisz

the svecofennian orogen a tectonic model abstract recent geological studies have provided new concepts concerning the timing of the final break up of the archaean craton age data on detrital zircons from sedimentary rocks in sweden and finland show marked similarities and indicate pervasive homogenization of accumulated material geochemical and isotopic studies in sweden and finland suggest the existence of two volcanic belts separated by an area dominated by sedimentary rocks however the ages of metamorphism and plutonism differ in particular the late post 1 85 ga granitoid magmatism was more voluminous in sweden than in finland the svecofennian crust in finland is anomalously thick up to 65 km and is characterized by a high velocity lower crust of variable thickness the evolution of the svecofennian orogen is modelled by opening of the pre svecofennian ocean at 1 95 ga and progressive accretion of two arc complexes to the archaean craton between 1 91 1 87 ga initial accretion at 1 91 ga led to thrusting of the slices of an island arc complex and proterozoic marine strata onto the thickening craton margin subduction reversal with simultaneous anti clockwise rotation of the converging plate or convergence of another plate resulted in subduction below the newly accreted arc complex the tectonic style at the craton margin changed from thrusting to lateral displacement in the east in finland in the west in sweden the subduction zone retreated along a transform fault a large sedimentary basin closed in the east 1 89 ga ago when another arc complex collided against the accreted one collision resulted in lithospheric thickening subsequent delamination of the thickened lithosphere and compensation by a hot mantle underplate and large scale granitoid magmatism as a result of mixing between mantle magma and remelted arc complex material the thick dense layer that formed in the lower crust as the result of underplating kept the thickened crust in isostatic balance thus the exhumation rate in this area was similar to the rate in areas of thinner crust subduction and sediment accumulation continued in the west until at least 1 87 ga when large areas in the east were already stabilized an intracratonic transpressional zone developed as the result of continued convergence subsequent extensional collapse along the transpressional zone caused melting of svecofennian sedimentary rocks migmatization and generation of the 1 84 1 82 ga anatectic granites of southern finland and central sweden the voluminous post 1 85 ga granitoid magmatism in sweden is tentatively associated with an east west extension that is not related to the extensional collapse

structural setting of late svecofennian granites and pegmatites in uusimaa belt sw finland age constraints and implications for crustal evolution complex late svecofennian deformation patterns and voluminous granitic magmatism at 1 85 1 80 ga characterise the palaeoproterozoic uusimaa belt in sw finland detailed structural analysis and ion

timing of accretion intra orogenic sedimentation and basin inversion in the paleoproterozoic svecofennian orogen the pyhantaka area southern finland abstract the pyhantaka area in southern finland represents the accretionary svecofennian orogen presently exhumed to upper mid crustal level the post 1 87 ga volcanic sedimentary pyhantaka formation is separated from underlying cordierite paragneiss metagraywacke by an unconformity marked by an ancient weathering surface on top of the paragneiss and ultramature quartzite at the base of the pyhantaka formation structural studies and u pb datings carried out in the paragneisses metavolcanic rock interlayer and crosscutting granitoid rocks revealed four tectonic units separated from each other by shear zones and representing different depositional ages pre 1 87 ga and post 1 87 ga yet unidentified post 1 87 ga rock sequences are anticipated in the svecofennian orogen the model of tectonothermal evolution in the pyhantaka area includes contraction at 1 89 ga causing rapid burial of graywackes harkala graywackes to mid crustal levels where they were metamorphosed into cordierite gneisses and intruded by syntectonic granite at 1875 ma deposition of exhumation related graywackes himattu graywackes after 1 87 ga followed by development of a weathering surface and deposition of quartz sands indicate gradual stabilization of the upper crust a sequence of quartz sands immature sands volcaniclastic sands and basaltic rocks the pyhantaka formation and graywackes on top kesua graywackes were deposited in an intra orogenic rift basin during crustal extension that may have initiated 1 85 ga ago contraction at 1825 ma caused basin inversion with thrusting and folding of the post 1 87 ga sequence all rocks in the pyhantaka area were subject to a new high grade metamorphic event that culminated at 1 82 ga subsequent uplift of a pre 1 87 ga rock sequence relative to the post 1 87 ga sequence resulted in the present pattern of tectonic units the inferred evolution at pyhantaka conforms to several tectonic models presented for the svecofennian orogen however the occurrence of an ancient weathering surface and ultramature quartzite an evidence for a 10 20 ma stable period is more compatible with continental interior setting with outboard subduction than active continental margin setting during deposition of the pyhantaka formation

late palaeoproterozoic monazite and titanite u pb ages in the archaean suomujarvi complex n finland abstract the paper documents a late palaeoproterozoic episode of metamorphism and anatexis in the suomujarvi complex an archaean enclave in the palaeoproterozoic central lapland granitoid complex of northern finland u pb ages were obtained from monazite and titanite in four archaean granitoid rocks one quartzite and a massive granite the results show that growth of monazite and titanite in these rocks spanned a period of about 15 million years between 1780 and 1765 ma presumably reflecting complex processes of metamorphic re crystallisation invasion by granitic dykes veins and fluids and post peak metamorphic retrogression the timing of this activity corresponds to post orogenic granitoid magmatism throughout the baltic shield in this region between the lapland kola and that of svecofennian orogens it may represent a retarded response to their coeval ca 1 9 ga convergent collision the results support a proposed affinity of the suomujarvi complex with the belomorian rather than the archaean karelian province

flood behaviour of the burdekin river tropical north queensland australia strongly seasonal rainfall in north queensland australia causes extremely variable fluvial discharge and tropical cyclones commonly produce high magnitude short duration floods in the burdekin river catchment 129 500 km2 of eastern queensland rapid runoff and channel floodplain configuration lead to very fast flood wave speeds up to 4 34 m s 1 calculated the large catchment and varying tropical cyclone paths result in extreme variations in flood history at different sites changes in channel conditions bed elevation vegetation bedforms sediment surface character in successive events combined with complex rainfall patterns and highly unsteady flow make flood estimates extremely difficult because of the short duration of flow events and very rapid rate of change in discharge during individual events sediment transport rate bedforms and bed elevation change continuously for most of the year the channel is occupied by an underfit stream but this small perennial stream does not significantly rework the deposits or change channel form before the next major discharge event consequently there is little channel form recovery between major discharge events successive events have very variable size and consequently the bedforms and channel morphology are normally out of equilibrium with the flow and record the result of the most recent major events high magnitude floods transport coarse sediment onto the floodplain and the short duration of flooding and rapid runoff results in relatively low preservation of mud in the channel or overbank environment the holocene flood deposits include thick coarse sand and medium sand units and generally thin fine grained units

changes in the rates of floodplain and in channel bench accretion in response to catchment disturbance central queensland australia analysis of the changes in rates of catchment sediment storage can provide material evidence of the impact of landscape disturbance on catchment sediment flux a number of studies have suggested increased sediment yields from the rivers draining to the great barrier reef since european settlement in the mid nineteenth century many of these predictions which indicate increases between four to ten times the pre disturbance estimates are based on large scale catchment modelling that make some critical assumptions about pre disturbance erosion rates and or sediment delivery ratios in addition the majority have not been validated by empirical data this study uses single grain osl dating and 137cs depth profiles to determine pre and post floodplain accretion rates in theresa creek a subcatchment of the dry tropical fitzroy river basin we demonstrate that floodplain accretion rates have increased by three to four times since european settlement ca a d 1850 decreased rates of floodplain accretion since the mid twentieth century at sites contributed to by gullied terrain suggest a decrease in the supply of sediment derived from gully networks in contrast floodplain accretion rates from areas dominated by cultivation remain high widespread in channel benches deposited since european settlement are stable and appear to be important stores of large volumes of sediment low magnitude increases in post disturbance floodplain sedimentation rates 3 to 4 times in comparison to those reported from mainly temperate climates 10 to 100 times are attributed to the naturally high sediment loads typical of dry tropical catchments consequently previous predictions of large post disturbance increases in sediment yields from large dry tropical catchments draining to the great barrier reef are likely to be overestimates

bedrock benches and boulder bars floods in the burdekin gorge of australia the burdekin gorge of northeastern australia lies within the seasonal tropics and is characterized by high discharge variability slackwater sediments and paleostage indicators in the gorge record seven large floods that have occurred during the past 1 200 yrs these floods range in magnitude from 11 000 to 30 000 m 3 s 1 and are characterized by large downstream variations in hydraulics the floods generate high values of velocity boundary shear stress and stream power per unit area downstream fluctuations in these variables help to explain the location of boulder bars high flood levees small scale erosional features in the bedrock and the formation of inner channels boulder bars and flood levees form where shear stress and stream power decrease due to channel widening small scale erosional features such as potholes and troughs are best developed at sites of channel constriction and associated increases in shear stress and stream power the development of the inner channel appears to be controlled by complex interactions between bedrock lithology and structure and by flow hydraulics all of the channel features are related to the hydraulics of large floods which therefore are the dominant controls on many aspects of channel morphology in this bedrock gorge

depositional and erosional architectures of gravelly braid bar formed by a flood in the abe river central japan inferred from a three dimensional ground penetrating radar analysis abstract we conducted a ground penetrating radar gpr survey of gravelly braid bars in the abe river central japan to clarify the three dimensional 3d variations in their depositional facies under various geomorphologic conditions in september 2011 a ten year return period flood in the study area reworked and deposited braid bars after the flood we surveyed three bars with different geomorphologies using a gpr system with a 250 mhz antenna and identified seven fundamental radar depositional facies inclined reflections facies ia and ib horizontal to subhorizontal reflections facies iia and iib discontinuous reflections facies iiia and iiib and facies assemblage with a large scale channel shaped lower boundary facies iv combinations of these facies in dicate bar formation processes channel filling lateral aggradation and lateral and downstream accretion in the abe river aerial photographs and airborne laser scanning data were obtained before and after the flood the observed changes of the surface topography are consistent with the subsurface results seen in the gpr sections this study demonstrated that the erosional and depositional architecture observed among bars with different channel styles was related to river width and represented depositional processes for high sediment discharge the quantitative characterizations of the sedimentary architecture will be useful for interpreting gravelly fluvial deposits in the rock record

major palaeoproterozoic shear zones of the central fennoscandian shield abstract employing two recently studied crustal scale shear zones as type examples this paper summarizes the major palaeoproterozoic svecokarelian shear tectonics of the central fennoscandian shield and demonstrates that this part of the shield was not as stable during the svecokarelian orogeny as commonly assumed the collision of the svecofennian island arc with the karelian continent first created numerous nw se trending folds and thrusts of stages d1 and d2 which were then modified by successive shearing during stages d3 and d4 stage d3 built up a system of n s trending shear zones here named the savolappishearsystem the type example of which is the hirvaskoski shear zone this is a dextral strike slip shear zone at least 150 km long and 10 30 km wide characterized by blastomylonitic fault rocks and various structures such as hook folds z folds and sheath folds associated with the principal displacement zone synthetic riedel shears and pinnate shears the traces of the axial planes of f3 en echelon folds deviate 15 30 anticlockwise from the plane of the principal displacement zone other members of the savolappi shear system are the pajala shear zone in northern sweden and the russian north karelia shear zone in the east stage d4 created a conjugate shear system called the finlandia shear system the type example of which is the oulujarvi shear zone this is a ne sw trending sinistral strike slip shear zone more than 250 km long and 20 30 km wide across its southwestern end it is composed of a ne sw trending principal displacement zone synthetic riedel shears and pinnate shears with antithetic riedel shears in a nw se direction typical fault rocks within these shears are s c mylonites the axial plane traces of f4 folds of all scales diverge by 20 40 clockwise from the plane of the principal displacement zone the kuopio shear zone is a conjugate nw se trending counterpart of the oulujarvi shear zone as a whole the finlandia shear system forms a conjugate network of nw se and ne sw trending shear zones which occupies most of the northern and central fennoscandian shield

new geochronological and sm nd constraints across the pajala shear zone of northern fennoscandia reactivation of a paleoproterozoic suture abstract altogether 1130 u pb analyses on zircons from 25 samples across the pajala shear zone in northern fennoscandia were performed by la icpms methodology to study the origin of the zone the samples were also analyzed for their sm nd isotope systematics and geochemically the new data is tested with existing geological information to elaborate the provenance and maximum depositional ages the stratigraphic position of studied samples and finally to assess the tectonic evolution in the pajala shear zone area chromium enriched metasedimentary rocks are found within and to the east and west of the n s trending pajala shear zone no material originating from proterozoic felsic igneous zircons or mafic sm nd rocks was found in these rocks and thus only neoarchean or very early paleoproterozoic ages of deposition are possible our new results indicate that the bimodal 1 99 1 97 ga supracrustal rocks of the rovaniemi supersuite cover a large area east of the pajala shear zone based on the absence of archean zircons typical of the metasedimentary rocks covering the archean basement it seems that the basement and its sedimentary cover were not exposed at the time of deposition of the rovaniemi supersuite the metasedimentary rocks of the uusivirka supersuite located within the pajala shear zone vary from hornblende bearing metasedimentary rocks metapelite and meta arkose to orthoquartzite their characteristic features include a dominant proterozoic zircon population 65 87 with a high proportion of analyzed grains yielding ages between 1 96 ga and 1 91 ga and maximum depositional ages of 1 92 1 91 ga we record at least three metamorphic events at ca 1 92 1 90 ga ca 1 85 ga and at 1 82 1 78 ga of which the youngest can possibly be separated into two events at 1 83 1 82 ga and 1 79 1 77 ga respectively we propose a tectonic model in which continental breakup occurred at 2 1 2 05 ga followed by the development of a magmatic arc kittila arc at ca 2 0 ga above a subduction zone two archean continental units norrbotten and karelia collided at ca 1 92 1 91 ga the kittila arc was obducted upon the karelia continent as a foreland fold and thrust belt and the metasedimentary rocks of the uusivirka supersuite were emplaced as foreland deposits the proposed tectonic model is based on the absence of 2 44 ga layered intrusions and especially 2 06 ga rocks and 1 99 1 97 ga rocks of the rovaniemi supersuite that are found only east of the pajala shear zone in our model the pajala shear zone originated as a divergent plate boundary and was multiply reactivated after continental collision with both lateral and vertical movements

evidence for middle amazonian catastrophic flooding and glaciation on mars abstract early geologic investigations of mars revealed some of the largest channels in the solar system outflow channels which appear to have mostly developed 3 byr ago these channels have been the subject of much scientific inquiry since the 1970s and proposed formative processes included surface erosion by catastrophic floods glaciers debris flows and lava flows based on the analysis of newly acquired mars reconnaissance orbiter mro context ctx 5 15 5 91 m pixel and high resolution imaging science experiment hirise 25 50 cm pixel image data we have identified a few locations contained within relatively narrow canyons of the southern circum chryse outflow channels that retain well preserved decameter hectometer scale landform assemblages these terrains include landforms consistent in shape dimension and overall assemblage to those produced by catastrophic floods and at one location to glacial morphologies impact crater statistics for four of these surfaces located within upstream midstream and downstream outflow channel surfaces yield an age estimate of 600 myr this suggests that the southern circum chryse outflow channels were locally resurfaced by some of the most recent catastrophic floods on the planet and that these floods coexisted within regional glacier environments as recently as during the middle amazonian

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

the mustajarvi orogenic gold occurrence central lapland greenstone belt finland a telluride dominant mineral system the mustajarvi gold occurrence lies in the southern part of the paleoproterozoic central lapland greenstone belt in proximity to the first order transcrustal venejoki thrust fault system the gold occurrence is structurally controlled by the second order mustajarvi shear zone which is located at the contact between siliciclastic metasedimentary and mafic to ultramafic metavolcanic rocks the main mineralization comprises a set of parallel veins and sulfidized rocks that are slightly oblique to the shear zone and are hosted by third order structures likely representing riedel r type shears the gold mineralized rock at mustajarvi is associated with pyrite that is present in 0 15 to 1 m wide quartz pyrite tourmaline veins and in zones of massive pyrite in the host rocks with thicknesses ranging from 1 15 to 2 m in unweathered rock hypogene gold is hosted by au and au bi telluride micro inclusions in pyrite whereas strong weathering at near surface levels has caused a remobilization of gold resulting in free gold deposited mainly in the cracks of oxidized pyrite the geochemistry of both mineralization styles is typical of orogenic gold systems with strong enrichments comprising au b bi co2 te and se and less consistent anomalous amounts of ag as sb and w unusual for orogenic gold deposits is the strong enrichment of ni and co which leads to the classification of mustajarvi as orogenic gold occurrence with atypical metal association

orthogonal switching of ams axes during type 2 fold interference insights from integrated x ray computed tomography ams and 3d petrography abstract we applied x ray computed microtomography ct in combination with anisotropy of magnetic susceptibility ams analysis to study metamorphic rock fabrics in an oriented drill core sample of pyrite pyrrhotite quartz mica schist the sample is extracted from the paleoproterozoic martimo metasedimentary belt of northern finland the ct resolves the spatial distribution shape and orientation of 25 920 pyrrhotite and 153 pyrite grains localized in mm thick metapelitic laminae together with microstructural analysis the ct allows us to interpret the prolate symmetry of the ams ellipsoid and its relationship to the deformation history ams of the sample is controlled by pyrrhotite porphyroblasts that grew syntectonically during d1 in subhorizontal microlithons the short and intermediate axes k3 and k2 of the ams ellipsoid interchanged positions during a subsequent deformation d2 that intensely crenulated s1 and deformed pyrrhotite while the long axes k1 maintained a constant position parallel to the maximum stretching direction however it is likely that all the three ams axes switched similar to the three principal axes of the shape ellipsoid of pyrite porphyroblasts from d1 to d2 the superposition of d1 and d2 produced a type 2 fold interference pattern

genesis of palaeoproterozoic iron skarns in the misi region northern finland sodic alteration is widespread in palaeoproterozoic greenstone and schist belts of the northern fennoscandian shield in the misi region that forms the easternmost part of the perapohja schist belt several small magnetite deposits show intimate spatial relationships with intensely albitised gabbros raising the possibility that regional sodic alteration released iron which was subsequently accumulated into deposits two of these magnetite deposits raajarvi and puro display a typical paragenesis as follows from oldest to youngest 1 diopside 2 actinolite tremolite magnetite chlorite biotite and 3 serpentine hematite chlorite mass balance calculations suggest that significant amounts of fe ca mg k cu v and ba were lost and na and si gained during the albitisation of the gabbro at near constant al ga ti and zr significant amounts of si ca fe and na were enriched in the formation of skarn related to magnetite deposits fe and v leached from country rocks deposited during the skarn alteration and formed the vanadium rich iron deposits while cu passed through the system without significant precipitation due to low sulphur fugasity variations in na ca mg k and ba contents reflect the composition of the infiltrating fluid during alteration conventional heating freezing measurements and proton induced x ray emission pixe analyses of the fluid inclusions related to actinolite tremolite magnetite stage alteration indicate that the fluids that caused the alteration and the fe mineralisation were complex oxidised highly saline h2o co2 fluids that contained high amounts of na ca k fe and ba as well as elevated concentrations of cu zn and pb the oxygen isotope thermometry suggest that temperature during the fe mineralisation stage was between 390 and 490 c calculated 18ofluid values of 6 1 9 8 smow and 13c values of calcites in the ores and skarns were between 7 7 and 10 9 pdb and most likely reflect admixture of 13c depleted possibly magmatic fluids with the marble wall rocks that show 13ccalcite values of 13 pdb the sims u pb data on the zircons in the albitised gabbro next to the raajarvi and puro deposits suggest that intrusion of the gabbro took place at 2123 7 ma and was accompanied by the formation of diopside skarn the tims data on the metasomatic titanites related to sodic alteration yielded ages of 2062 3 and 2017 3 ma iron was probably stripped from the mafic country rocks by sodic alteration between 2123 and 2017 ma driven by repeated brine influxes subsequently the metal rich brine was focused by a fault system and the iron was precipitated from this fluid by a combination of wall rock reaction fluid mixing and a drop in the temperature

crustal architecture of the inverted central lapland rift along the hukka 2007 profile we have studied the lateral velocity variations along a partly buried inverted paleo rift in central lapland northern europe with a 2d wide angle reflection and refraction experiment hukka 2007 the experiment was designed to use seven chemical explosions from commercial and military sites as sources of seismic energy the shots were recorded by 102 stations with an average spacing of 3 45 km two dimensional crustal models of variations in p wave velocity and vp vs ratio were calculated using the ray tracing forward modeling technique the hukka 2007 experiment comprises a 455 km long profile that runs nnw sse parallel to the kittila shear zone a major deformation zone hosting gold deposits in the area the profile crosses paleoproterozoic and reactivated archean terranes of central lapland the velocity model shows a significant difference in crustal velocity structure between the northern distances 0 120 km and southern parts of the profile the difference in p wave velocities and vp vs ratio can be followed through the whole crust down to the moho boundary indicating major tectonic boundaries upper crustal velocities seem to vary with the terranes compositional differences mapped at the surface the lower layer of the upper crust displays velocities of 6 0 6 1 km s both paleoproterozoic and archean terranes are associated with high velocity bodies 6 30 6 35 km s at 100 and 200 350 km distances the central lapland greenstone belt and central lapland granitoid complex are associated with a 4 km thick zone of unusually low velocities 6 0 km s at distances between 120 and 220 km we interpret the hukka 2007 profile to image an old partly buried inverted continental rift zone that has been closed and modified by younger tectonic events it has structural features typical of rifts inward dipping rift shoulders undulating thickness of the middle crust high velocity lower crust and a rather uniform crustal thickness of 48 km

near orthogonal deformation successions in the poly deformed paleoproterozoic martimo belt implications for the tectonic evolution of northern fennoscandia abstract structural mapping integrated with aeromagnetic and anisotropy of magnetic susceptibility ams data form complimentary and robust tools for analysis of regional structures and fabric overprinting relationships this multi scale approach is used to constrain the geometry and tectonic evolution of sparsely exposed and flat lying outcrops of the martimo belt mb of northern fennoscandia the mb records a history of near orthogonal deformations upper greenschist to lower amphibolite facies metamorphism between ca 1 92 1 79 ga the earliest deformation d1 involved east directed thin skin thrusting and the development of recumbent f1 folds and a n s trending s1 fabric d2 deformation is prominent in the central part of the mb and is characterized by n s shortening with the development of e w trending f2 folds an associated pervasive steeply dipping s2 foliation and type 2 fold interference pattern the mb and mellajoki suite exhibit regional scale d2 synformal and antiformal refolded structures respectively d3 is more heterogeneously developed and manifests bimodal orientations with nnw sse and wnw ese structural trends it is pronounced in the central and eastern parts of the mb but weakly developed in the western part the steep dip of s3 in the region suggests that it formed in response to ne sw bulk shortening d4 structures have a conspicuous nne ssw trend that is visible as lineaments on the aeromagnetic image and locally in the field the last deformation event d5 affected almost the entire mb and was characterized by broadly wsw ene oriented crustal shortening the n s oval shape geometry of the eastern part of the mb is attributed to the d5 event contrasting with well developed overprinting relationships between d1 and d3 structures the relative timing of d3 d5 could not be established in the study area but can be inferred from structures with similar trends in the central lapland granitoid complex which is located north of the mb the d1 d5 deformation scheme can be readily reconciled with the tectonic evolution of the lapland savo fennian svecobaltic and nordic orogens of the composite svecofennian orogen the age of d1 in the northern part of the lapland savo orogen is 1 91 ga based on crosscutting granitoids and maximum deposition ages of sedimentary rocks d2 1 90 1 89 ga has been linked to orogen parallel shortening during the late stage of southern part of the lapland savo orogen several orthogonal tectonic events in northern fennoscandia indicate amalgamation of crustal blocks around the core of the archean karelia continent at 1 92 1 77 ga and that fennoscandia was a key component of the columbia hudsonia nuna supercontinent

multiply deformed paleoproterozoic foreland fold and thrust belt in northern fennoscandia the peripheral kuusamo belt as a key example abstract the central lapland belt clb perapohja belt pb and kuusamo belt kb form an important part of northern fennoscandia and record ca 700 ma 2 5 1 8 ga of precambrian tectonic processes the 2 5 2 1 ga sedimentary volcanic successions in these belts form a basin wide surfaces with a wide lateral continuum tidal influenced shallow epeiric sea setting dominated at 2 4 2 1 ga followed by a continental breakup and subsequent formation of a passive margin at 2 1 ga the collisional evolution 1 93 1 76 ga started with an e w locally nw se and ne sw compressional phase d1 1 93 1 90 ga that resulted in at least 600 km long and in part over 300 km wide e vergent lapland foreland fold and thrust belt ftb the kb is the best preserved example of this ftb and shows e vergent imbricated structures in the east and e vergent out of sequence thrusting in the west the southern part of the kb has been affected by n s shortening d2 seen as upright to slightly inclined open to tight f2 folds and as oval shaped dome and basin structures the d3 overprinting causes folding with nw se trending spaced foliation in less competent rocks and parallel folds and faults in orthoquartzites the d4 ne sw trending overprinting deformation is characteristic for the high grade areas of the kb and is seen in outcrop to map scale tight folding in general the easternmost peripheral part of the e vergent lapland foreland ftb formed upon non stretched archean basement is only moderately affected by the later deformation events in the stretched part of the archean basement the earlier rift structures have facilitated the n s shortening and the d2 structures dominate the map pattern the d3 due to a sw ne shortening shows strong strain partitioning and localized deformation after prolonged heating the archean basement started to act ductile and long wavelength crustal scale ne sw trending d4 folds formed the metamorphic zonation in the kb from greenschist facies in the south to medium pressure amphibolite facies in the north is explained by a major d4 anticlinorium wavelength 70 100 km

heavy mineral shadows a new sedimentary structure formed under upper flow regime conditions its directional and hydraulic significance abstract distinct heavy mineral accumulations named here as heavy mineral shadows were observed on a current lineated bedding plane in the quartzose whirlpool sandstone silurian near georgetown ontario the shadows are irregular in form but show one consistent feature in plan along lines parallel to the current lineation one side of a shadow is defined by an abrupt boundary between the heavy mineral rich dark area and adjacent quartz rich light area while the other side is characterized by a gradual transition into the light area comparison with paleocurrent directions determined from nearby sedimentary structures and with grain imbrication a few millimeters below the bedding plane on which they occur indicates that the paleoflow was from the quartz rich areas towards th sharply defined side of the heavy mineral shadows a set of flume experiments was undertaken with a mixture of 97 by weight quartz sand and 3 chromite sand as the bed material stream power during each run was within the range of upper flow regime plane bed stability with increasing stream power the chromite sand behaved as follows 1 heavy mineral shadows were stable identical in form to those described above moving intermittently beneath a streaky faster moving quartz sand bed load 2 shadows were completely destroyed and the chromite sand formed a diffuse layer beneath the quartz sand in transport and 3 chromite sand was concentrated across the crests of very low standing wave bed forms with wavelengths of 0 4 m and amplitudes as small as 1 5 min although the data available is too limited to allow anything more than speculation on their origin the recognition of heavy mineral shadows provides a new tool for paleocurrent studies flume experiments also suggest that its presence on bedding planes may have paleohydraulic implications

magnetotelluric array in the central finnish lapland ii 3 d inversion and tectonic implications abstract the northern part of the fennoscandian shield records several palaeoproterozoic phases of rifting crustal shortening metamorphism and structural reactivation the geologic history has left behind a complex crustal geoelectric structure as evidenced by magnetotelluric mt data from the central finnish lapland acquired in the context of the masca project in 2014 the data are characterized by strong 3 d effects such as high phase tensor skew values and anomalous induction vectors interestingly however at the same time a dominant e w principal direction from the phase tensor data can be inferred 3 d conductivity models derived using the modem code display high crustal conductance 10 000 s in the vicinity of the central lapland greenstone belt the perapohja belt and the kuusamo belt conductors in the northern and southern part of the study area are separated by a resistor coinciding with the central lapland granitoid complex a remarkable feature is an arc shaped conductor inside the northern part of the central lapland granitoid complex which continues into the central lapland greenstone belt in the north the conductor is associated with a major induction vector anomaly and also coincides with extreme responses negative phase tensor principal values which are discussed in an accompanying paper the conductive structures in the models are interpreted as deeply buried graphite and sulphide bearing metasedimentary rocks or as reactivated archaean shear zones the revealed geoelectric structures also partly correlate with seismic reflection and other geophysical data from the area a possible explanation for the observed pervasive e w principal direction of the phase tensor data could be the aulacogen failed rift suggested in recent tectonic evolution models

complexity in the behavior and recrystallization of monazite during high t metamorphism and fluid infiltration abstract a detailed study of monazite grains in silica undersaturated and quartz bearing mg al metapelite from the oygarden group of islands east antarctica reveals a complex history of growth and recrystallization during two separate events in the neoproterozoic and earliest cambrian monazite grains from garnet poor and garnet rich metapelite preserve core domains that have ages corresponding to growth and or recrystallization at granulite facies p 9 1 0 gpa t 850 900 c conditions during the rayner structural episode between 930 and 890 ma high th rims 22 wt tho 2 that are in textural equilibrium with sapphirine orthopyroxene symplectites formed after garnet during late rayner decompression occur on monazite grains in garnet rich assemblages and give electron microprobe ages 883 18 ma within error of core domains 903 14 ma these high th rim domains are interpreted to have formed through recrystallization of liberated inclusions via a process dominated by coupled dissolution reprecipitation reactions facilitated by transient fluid films on the large surface areas in the symplectite and not by new growth in garnet poor metapelite monazite grains that have grain boundaries in textural equilibrium with the granulite facies assemblage also show alteration to higher th compositions on rims and along vein like fractures this compositional shift is accompanied by partial to complete resetting of ages to 500 ma along with minor modification of core domain ages textures and patterns of chemical age resetting are interpreted to also be the result of a coupled dissolution reprecipitation reaction process but in contrast to garnet bearing assemblages this alteration occurred during a fluid influx at lower temperatures during regional pan african tectonism this study highlights the susceptibility of monazite to resetting with fluid present conditions dramatically increasing the potential for recrystallization

plant material deposition in the tropical burdekin river australia implications for ancient fluvial sediments abstract in the deposits of the burdekin river north queensland australia plant material character and directional fabric together with the sedimentary facies character are generally diagnostic of depositional environment plant material is selectively entrained transported and deposited in all flow conditions ranging from dry season minimal flow

lateral and vertical facies relationships of bedforms deposited by aggrading supercritical flows from cyclic steps to humpback dunes abstract the preservation of bedforms related to supercritical flows and hydraulic jumps is commonly considered to be rare in the geologic record although these bedforms are known from a variety of depositional environments this field based study presents a detailed analysis of the sedimentary facies and stacking pattern of deposits of cyclic steps chutes and pools antidunes and humpback dunes from three dimensional outcrops the well exposed middle pleistocene successions from northern germany comprise glacilacustrine ice contact subaqueous fan and glacial lake outburst flood deposits the studied successions give new insights into the depositional architecture of bedforms related to supercritical flows and may serve as an analogue for other high energy depositional environments such as fluvial settings coarse grained deltas or turbidite systems deposits of cyclic steps occur within the glacial lake outburst flood succession and are characterised by lenticular scours infilled by gently to steeply dipping backsets cyclic steps formed due to acceleration and flow thinning when the glacial lake outburst flood spilled over a push moraine ridge these bedforms are commonly laterally and vertically truncated and alternate with deposits of chutes and pools and antidunes the subaqueous fan successions are dominated by laterally extensive sinusoidal waveforms which are interpreted as deposits of aggrading stationary antidunes which require quasi steady flows at the lower limit of the supercritical flow stage and high rates of sedimentation humpback dunes are characterised by downflow divergent cross stratification displaying differentiation into topsets foresets and bottomsets and are interpreted as deposited at the transition from subcritical to supercritical flow conditions or vice versa gradual lateral and vertical transitions between humpback dunes and antidune deposits are very common the absence of planar parallel stratification in all studied successions suggests that the formation of these bedforms is suppressed in flows characterised by hydraulic jumps under highly aggradational conditions the large scale lateral and vertical successions of bedforms are interpreted as representing the temporal and spatial evolution of the initial supercritical flows which was strongly affected by the occurrence of hydraulic jumps small scale facies changes and the formation of individual bedforms are interpreted as controlled by fluctuating discharge bed topography and pulsating unstable flows

coarse grained floodplain deposits in the seasonal tropics towards a better facies model abstract floodplain deposits in semiarid and subhumid settings with erratic rainfall may contain relatively high proportions of sand because of climatically controlled patterns of discharge and vegetation cover ancient formations with abundant floodplain sands may record fluvial systems with extreme discharge variability of this type but are difficult to interpret because of the paucity of published modern analogues the lower burdekin and haughton river alluvial and delta plains of north queensland australia are used here to illustrate this type of fluvial architecture the modern burdekin and haughton channels have gravelly sand beds and transport a mixed load in the wet season the channels have low relief levees that decrease in amplitude downstream across the low gradient floodplains tropical cyclone activity results in high magnitude short duration sediment charged floods that inundate large areas of the coastal lowland bankfull discharge is infrequently exceeded return interval 25 years because the channels are enlarged by major cyclone induced runoff events and between these events flows are too small to modify the channel form significantly suspended sediment loads and the proportions of these that are sand are particularly high when intense rainfall occurs after prolonged drought when vegetation cover is reduced as a consequence of these factors most of the deposition on the levees and floodbasins occurs in very large discharge events with significant suspended sand load these floods carry a lot of sand out of the channel and efficiently in suspension over the floodplains because of deep inundation boreholes drilled in these floodplains encountered sharply bounded and gradationally bounded sand beds that fine upward or show no vertical grain size trend these are interbedded with thin silt clay and minor gravel beds holocene paleochannels make up about 30 of the floodplain area and consist predominantly of coarse sand that is similar to modern channel bed deposits the overbank sand in contrast is predominantly fine medium grained and beds have distinctly different granulometry from each other and from channel floor deposits efficient sediment partitioning in the runoff and extreme variation in flood magnitude explain variations in sand bed granulometry in the alluvial valley and upper delta plain individual sand beds are difficult to correlate between adjacent boreholes even over very short distances tens of meters in contrast on the lower delta plain overbank sands have a more sheet like geometry this is explained by the lower lying landscape negligible levee development greater inundation depths and reduced surface roughness in addition the channel bodies are thinner and wider and may be connected to overbank sand wings

gravel antidunes in the tropical burdekin river queensland australia the geological record is punctuated by the deposits of extreme event phenomena the identification and interpretation of which are hindered by a lack of data on contemporary examples it is impossible to directly observe sedimentary bedforms and grain fabrics forming under natural particle transporting high velocity currents and therefore their characteristics are poorly documented the deposits of such flows are exposed however in the dry bed of the burdekin river queensland australia following tropical cyclone induced floods long wave length up to 19 m gravel antidunes develop during short days high discharge flows in the upper burdekin river maximum recorded discharge near the study reach over 25 600 m s in february 1927 flood water levels fall quickly metres in a day and flow is diverted away from raised areas of the river bed into subchannels exposing many of the high stage bedforms with little reworking by falling stage currents gravel bedforms were observed on the dry river bed after the moderate flows of february 1994 max 7700 m s and january 1996 max 3200 m s the bedforms had wave lengths in the range 8 19 m amplitudes of up to 1 m with steeper stoss than lee faces and crest lines generally transverse to local peak discharge flow direction the gravel fabric and size sorting change systematically up the stoss and down the lee faces the antidune deposits form erosive based lenses of sandy gravel with low angle downstream dipping lamination and generally steep upstream dipping a b planes the internal form and fabric of the antidune gravel lenses are distinctly different from those of dune lee gravel lenses the erosive based lenses of low angle cross bedded gravel with steep upstream dipping a b planes are relatively easy to recognize and may be diagnostic of downstream migrating antidunes the antidune gravel lenses are associated with thick to 1 m high angle cross bed sets ancient antidune gravel lenses may be diagnostic of episodic high discharge conditions and particularly when they are associated with high angle cross bedded gravelly sand they may be useful for palaeoenvironmental interpretation

age and tectonic framework of structurally controlled palaeoproterozoic gold mineralization in the hame belt of southern finland abstract a number of gold occurrences in the palaeoproterozoic svecofennian domain of southern finland show a clear structural control on mineralization and are classified as orogenic gold samples of pre syn and late to post mineralization rocks taken from two gold prospects in the somero tammela gold zone of the hame belt have been studied to gain absolute ages of mineralization by using sims u pb of zircon and titanite and ar ar single grain step heating of hornblende and biotite gold mineralization in the somero tammela gold zone is closely related to quartz veins and controlled by wsw ene to sw ne trending shear zones and faults formed during prolonged dextral oblique contraction in the late stages of svecofennian orogenic evolution zircons from a felsic volcanic host rock give an age of 1881 3 ma interpreted as the crystallization age of the rock and providing a maximum age for mineralization inherited zircons in a gold bearing quartz vein with the youngest zircon rim of 1832 15 ma give a maximum age of quartz vein emplacement a 1786 7 ma mean 207pb 206pb age of a pegmatitic quartz feldspar dyke providing a minimum age for mineralization is interpreted as reflecting lead loss whereas an 1815 4 ma zircon age is probably closer to the crystallization age of the dyke this age overlaps with 1805 4 5 4 ma 39ar 40ar age of hornblende from a quartz vein 1 78 and 1 76 ga 39ar 40ar biotite cooling ages mark the minimum age for mineralization in summary the age of mineralization can be estimated at ca 1 82 1 79 ga which is in good agreement with the age of dextral shearing in southern finland the controlling structures wsw ene to sw ne dextral shear zones and nw se oriented faults formed during approximately wnw ese oriented sub horizontal shortening during crossing ductile brittle boundary conditions the steep shear zones and faults and the intersection lines of the two sets impart a directional permeability allowing channelling of the mineralizing fluids in these structures this was supported by prolonged dextral oblique contraction enabling continuous or episodic slip along the faults and shear zones the late svecofennian tectonic episode could be described as an extension contraction cycle in a convergent plate margin setting above a retreating subduction zone oblique extension 1 86 1 83 ga caused by slab rollback was followed by a rapid switch to oblique contraction at ca 1 82 ga induced by arrival of positively buoyant crustal fragments e g oceanic ridge oceanic plateau or continental sliver at the subduction zone the contractional period characterized by dextral transpression provided a favourable setting for orogenic gold mineralization the termination of the svecofennian orogenic evolution in finland after 1 79 ga might be caused by lock up of the subduction zone and slab break off the striking nw se alignment of orogenic type gold occurrences in the svecofennian domain of southern finland suggests that the late svecofennian 1 82 1 79 ga gold mineralizing event may be regionally more widespread and of greater importance than hitherto expected

paleokarst breccias calcretes silcretes and fault talus breccias at the base of upper proterozoic windermere strata northern canadian cordillera in the ogilvie mountains west central yukon the upper proterozoic lower mount harper group informal name is stratigraphically equivalent to basal windermere supergroup rocks that occur in other areas of the north american cordillera breccias at the base of the lower mount harper group are of two different but in part coeval origins a normal fault that was active during late proterozoic time forms the southern boundary of the lower mount harper group and is genetically related to a wedge of paleotalus breccia that thins rapidly northward from the fault a different breccia layer is preserved on a disconformity surface that separates lower harper group strata from a thick succession of underlying dolostones this disconformity breccia contains silcretes and calcretes that provide evidence of several episodes of subaerial exposure rare interbedded debris flows display features that suggest coincidence of the last exposure event with initiation of deposition of the mount harper group sediments silcretes and calcretes in this succession suggest that at the onset of windermere deposition in this area a temperate to equatorial probably semi arid to arid climate regime existed basal breccias in windermere equivalent strata of other areas in the cordillera generally have been interpreted as fault related but some of these breccias also contain features compatible with paleokarst origin

structure and composition of the precambrian crust along the polar profile in the northern baltic shield abstract along the 500 km long polar profile the continental crust of the northern baltic shield consists of several archaean and early proterozoic crustal segments in the south in the karelian province the upper crust consists of a 3 1 2 6 ga old archaean substratum with a 0 6 km thick low to medium grade volcanosedimentary cover here shallow basin facies rocks of the central lapland complex have been thrust northwards onto the karasjok kittila greenstone belt the repeatedly folded cover rocks form linear troughs between domes and culminations of granitoids the 47 48 km thick crust of the karelian province is of a shield type it is characterised by a low velocity layer interpreted as a horizon of decollement below the supracrustal and granitoid rocks in the lapland granulite belt early proterozoic high grade metapelites and metapsammites with minor metaigneous intercalations have been thrust to the southwest they are separated from the underlying karelian province by the tectonic mixture zone of the tanaelv belt in the interpretations two alternative models are given for the shape of the lapland granulite belt it is either a synformal nappe or a ne dipping body pinching out at depth and overthrust from the north by the archaean inari terrain both the lapland granulite belt and the inari terrain have a 42 44 km thick crust with a relatively thin lower crust marked by lower seismic velocities the early proterozoic polmak pasvik pechenga belt separates the archaean inari terrain from the archaean sorvaranger terrain in the north this belt is interpreted as a suture zone which continues to depth it may either dip steeply towards irregularities in the moho or form a sw dipping low angle plane below the lapland granulite belt the northernmost part of the baltic shield is concealed beneath a 0 7 km thick very low grade metamorphic late proterozoic cover this cover is divided into two parts by the trollfjord komagelv strike slip fault here the crust is again of a normal shield type with a thickness of 42 46 km

placer formation in gravel bedded rivers a review the literature on placer formation processes within fluvial systems is widespread and ranges between detailed laboratory studies of the hydrodynamic segregation processes through to the intuitive interpretation of the distribution of minerals within geological sections however there are few if any comprehensive reviews of the literature surprisingly given the economic importance of placers the theoretical framework relating to the hydrodynamics of physical grain sorting is not well developed and there are relatively few detailed laboratory hydraulic investigations to inform theory in this wide ranging review the history of the development of principles of placer formation is explored as far as possible in a non technical fashion a consideration is given to the hydrodynamics of physical grain sorting above lower stage and upper stage bedforms and the typical internal sedimentary structures associated with placer concentrations are detailed finally examples of the depositional environment of diamond tin and gold placers are considered

petrology of nattanen type granite complexes northern finland abstract the nattanen type granites form a group of post tectonic and post metamorphic 1 77 ga old epizonal multiple granite intrusions in the granulite belt granitic gneiss surroundings in northern finland the granite complexes the tepasto pomovaara and nattanen stocks and the riestovaara batholith in finland and the juvoaivi stock in the u s s r are composed of various slightly peraluminous microcline plagioclasebiotite granites monzogranites that contain sphene and magnetite as typical minor constituents geochemical evolution within the complexes is demonstrated by the increase of sio2 and rb and decrease of tio2 feo mgo cao sr ba and zr from the oldest to the youngest granite varieties molybdenum mineralization is associated with the latest intrusive phases of the tepasto and juvoaivi stocks the isotope geochemical studies indicate a major archaean crustal component in the source of the nattanen stock that is situated within the early proterozoic granulite belt

sedimentology of the upper burdekin river of north queensland australia an example of a tropical variable discharge river the burdekin river is an example of a class of tropical streams which experience two to four orders of magnitude variation in discharge in response to seasonal but erratic monsoonal rainfall floods of the burdekin rise abruptly reaching peak discharges of up to 40 000 m s in less than 24 h maintain peak flow for up to a few days and recede exponentially the geomorphology and deposits of these rivers reflect the extreme discharge fluctuations and have not previously been described a stretch of the upper burdekin river comprising four bends and one straight reach was examined near the town of charters towers the river bed is largely exposed for most of any year with a small misfit perennial channel carrying low stage flow major geomorphic elements of bends include point bars with ridge and swale topography three distinct types of chute channels avalanche slipfaces up to 5 m or more high around the downstream edges of bars and on the outer part of one point bar an elevated vegetated ridge straight reaches are flat or gently inclined sand and gravel covered surfaces much of the river bed is covered by well sorted in places gravelly coarse to very coarse grained sand with local accumulations of pebble to boulder gravel lower parts of the river bed are periodically draped by mud which is desiccated on exposure dunes and plane beds are the most commonly occurring bedforms with local development of gravelly antidunes most bank tops and upper vegetated bars are covered by silt and fine grained sand the river bed also hosts a low diversity but locally high abundance flood tolerant flora dominated by the paperbark tree melaleuca argentea which plays an important role in controlling the distribution of sediment the gross geomorphology of the river bed and most of the sedimentary features are interpreted as having formed during major bankfull or near bankfull flows which have a recurrence of about 18 years based on 65 years hydrographic data the initial rapid drop in discharge following flood peaks appears to preserve flood peak features on upper bars more or less intact whereas lower areas are subjected to variable degrees of modification during falling stage and by more frequent non bankfull discharge events

a revised geodynamic model for the lapland kola orogen abstract the paleoproterozoic lapland kola orogen in fennoscandia has been studied for decades and several plate tectonic models have been proposed including one sided subduction zone either towards sw or nw or two opposite verging subduction zones before the collision based on new structural and isotope data from finland and recently published data from russia we propose a revised tectonic model for the paleoproterozoic lapland kola orogen the main components are foreland in the ne followed by cryptic suture inari arc retro arc basin and retro arc foreland in the sw the latter three constitute the inari orocline subduction towards present sw and subsequent arc magmatism inari arc started at ca 1 98 ga followed by voluminous sedimentation in the deepening retro arc basin underplating of a mid ocean ridge caused flat subduction and magmatic flare at 1 92 ga over a broad distance in the retro arc basin rapid heating led to melting of the retro arc basin sediments and voluminous amounts of granulite facies diatexites formed during collision d1 at 1915 1910 ma large thrust nappes formed on the foreland deformation in the retro arc basin is seen as recumbent folding and shearing of diatexites in the lower parts of the basin and thrusting of metatextite diatexite packages in the upper parts a post collisional stage is seen as 1904 ma appinites and decompression derived granites at 1 90 1 89 ga renewed shortening d2 due to far field effects in sw at 1 88 1 87 ga led to thick skin shortening of the archean middle crust large scale crustal duplexing of already cooled granulites towards the retro arc foreland and inclined upright folding of granulites in the opposite direction towards the inari arc a switch in the stress field from ne sw to nw se led to orogen parallel contraction and buckling started along a dextral strike slip fault zone to form the inari orocline buckling is seen in the bending of pre orocline fabrics and formation of syn orocline fabrics radial conical folds d3 radial fractures a strike slip fault zone and thrusting at the hinge zone the end result is a mega scale parallel multi layer fold composed of the inari arc retro arc basin and possibly also the heated retro arc foreland

a review of recent research in fluvial sedimentology this volume has been compiled to represent the current status of research into fluvial sedimentology as part of the proceedings for the 5th international conference on fluvial sedimentology 5icfs brisbane july 1993 the volume contains papers on a wide variety of topics representing the diversity of interests covered by the icfs this review serves to introduce these papers within the context of recent research post1988 in fluvial sedimentology icfs papers may be somewhat arbitrarily divided into those which report fundamental studies of sediment transport studies of modern alluvial and coastal systems studies of environmental management of fluvial systems facies analyses of ancient fluvial sequences studies of controls on alluvial stratigraphy and basin fill and applied sedimentological studies of fluvialhosted mineral resources it is interesting to compare the balance of papers in this volume with that of previous icfs volumes miall 1978 collinson and lewin 1983 ethridge et al 1987 marzo and puigdefabregas 1993 as a guide to changing emphasis in fluvial sedimentology figure 1 illustrates the distribution of papers over the five icfs meetings categorised as above among various trends two are worthy of comment the first being the marked peak in interest in controls on fluvial sequence and basin fill at the 4icfs and the second the influx of papers on environmental management in this volume although there has been a steady level of activity in fundamental aspects of sediment transport and bedform generation the rela

1 9 1 8 ga old strike slip megashears in the baltic shield and their plate tectonic implications abstract the tectonic evolution of the ca 2 0 1 75 ga old svecokarelian fold belt is reviewed and evidence is presented for large scale intraplate strike slip movements along ductile megashears after the formation of the kola collision suture and the neighbouring granulite tanaelv thrust belt around 1 9 ga ago dextral shearing was initiated along n s trending megashears subsequent anticlockwise rotation of the initially nne ssw oriented principal compressive stress caused dextral shearing along a nw se trending megashear and reversal in the sense of shearing in the n s trending ones further anticlockwise stress rotation to a total of about 120 brought an end to sinistral shearing along the n s megashears around 1 8 ga ago and caused reversal to sinistral slip along the nw se megashear both the older 1 9 1 85 ga and younger 1 84 1 8 ga parts of this evolution are recorded within the karelian province and its southwestern margin where consolidation of the lithosphere took place shortly after 1 9 ga ago in the svecofennian province where crustal accretion did not start until around 1 9 ga ago the older movements may have caused synaccretional crustal folding but with increasing consolidation the deformation was concentrated along megashears although it is still not possible to interrelate the function of active subduction zones and intraplate megashears the evolution traced so far provides support for plate tectonic interpretations of the early proterozoic geodynamics of the baltic shield

an abandoned channel fill with exquisitely preserved plants in redbeds of the clear fork formation texas usa an early permian water dependent habitat on the arid plains of pangea abstract a well preserved plant assemblage at the colwell creek pond locality of leonardian kungurian age provides an opportunity to evaluate taphonomic conditions in a dryland alluvial setting a narrow channel body incised to 5 m depth through red paleo vertisols contains 2 m of varicolored laminated mudstone with graded layers and plant material x ray diffraction analysis of individual laminae indicates the presence of chlorite illite kaolinite and mixed layer clay with hematite in red and gray layers and goethite in yellow brown laminae no carbonate was identified and the total organic carbon content is minimal the fine sediment accumulated in a shallow abandoned channel from suspension and gentle underflows probably linked to seasonal inflow and analysis of lamina thickness suggests that standing water may have persisted for up to a few millennia the preservation of lamination is attributed to a lack of bioturbation possibly linked to a paucity of subsurface oxygen low productivity elevated salinity rapid deposition or a combination of these factors minimal bioturbation may also reflect the limited use of freshwater ecospace during the early permian clay rich paleo vertisols complete the fill with drab root traces that indicate growth of vegetation in a strongly seasonal setting abundant plant material in the laminated beds includes branches of walchian conifers the possible cycadophyte taeniopteris spp and the comioid possible peltasperm auritifolia waggoneri they were derived from an adjacent riparian zone and preserved as 3d goethite petrifactions much of the foliage shows evidence of arthropod herbivory although a humid climatic episode cannot be ruled out the exceptional abundance and preservation of the plants probably reflects the persistence of an oxbow lake on a relatively arid alluvial plain where riparian plants experienced periodic moisture stress but had access to groundwater nearly year round rapid burial in standing water the lack of bioturbation in the laminated sediments and early biomineralization probably explain the exceptional preservation of the plant remains

high energy flood events recorded in the mesoproterozoic meall dearg formation nw scotland their recognition and implications for the study of pre vegetation alluvium the mesoproterozoic neoproterozoic torridonian sandstone of nw scotland has renewed sedimentological significance following recent advances in the understanding of pre vegetation alluvium as it is one of the most extensive and easily accessible successions of such strata worldwide this paper presents the first modern sedimentological analysis of the unit9s constituent meall dearg formation late mesoproterozoic recognizing a dominance of alluvial facies with subordinate aeolian facies alluvial strata within the meall dearg formation contain direct evidence for event deposition by high energy floods including the following 1 widespread upper and transitional upper flow regime elements 2 frequent stacking of successively lower flow regime elements 3 common subcritical subaqueous dune fields with superimposed ripple marks 4 occasional thin desiccated mudstones 5 evidence that microbial mats colonized substrates during intervals of sedimentary stasis together these strands of primary sedimentary geological evidence indicate that the alluvial deposition of the meall dearg formation was typified by supercritical flows during high energy floods punctuated by prolonged intervals of sedimentary stasis the preservation potential of all of the features was boosted by highly aggradational sedimentary conditions these primary observations have implications for the norm in pre vegetation alluvium and confirm that despite increasing recognition of diversity within precambrian fluvial systems classical pre vegetation motifs of high energy alluvial flood deposits preserved as sheet braided alluvium are still an archetypal sedimentary signature in some instances using supportive evidence from the meall dearg formation we recommend that the term sheet braided be used in inverted commas in future studies this emphasizes the polygenetic depositional nature of the 20 1 width thickness fluvial style while maintaining the value of the term in the isolation of a key characteristic of pre vegetation sedimentary architecture

grain fabric of a laboratory antidune the grain fabric of a laboratory antidune composed of coarse to medium grained sand shows a high angle upcurrent imbrication both on the upstream and downstream sides such a grain fabric differs from a normal dune or a megaripple where grain alignment displays an upcurrent imbrication relative to the depositional slope in the foreset lamina and or near parallelism with the slope surface in the toe set lamina recognition of antidunes in ancient rocks is difficult but detailed examination of grain fabric of cross stratification makes it possible to distinguish antidunes or backset bedding from normal dunes or megaripples

formative conditions and sedimentary structures of sandy 3d antidunes an application of the gravel step pool model to fine grained sand in an experimental flume antidunes and their sedimentary structures can be useful in reconstructing paleo hydraulic conditions especially for large discharge events however three dimensional 3d antidunes in sand sized sediments have not yet been studied extensively as compared to either two dimensional 2d antidunes or antidunes in gravel sized sediments in this study we estimated formative conditions of gravel step pool morphologies and applied them to the formation of 3d antidunes over a sand bed formative conditions are expressed in terms of a relationship between the water discharge per unit width and the bed slope flume experiments demonstrated that 3d mound like antidune configurations and their associated internal sedimentary structures could be preserved internal sedimentary structures were characterized by shallow lens like structures whose bases were erosional although gently dipping concave upward lamination was dominant convex upward lamination was occasionally observed the dimensions of lenticular lamina sets can be used to estimate antidune geometry thus if 3d antidunes can be interpreted in the stratigraphic record it is possible to estimate the paleo hydraulic parameters such as water discharge and bed slope more precisely than previously copyright 2010 john wiley sons ltd

sequential porphyroblast growth and crenulation cleavage development during progressive deformation abstract six stages of crenulation cleavage development during a single deformation event d2 can be recognised in both matrix and porphyroblast inclusion trails in the robertson river formation n e australia these stages progress from undeformed to crenulation and differentiated crenulation of s1 through development of differentiated crenulation cleavage s2 and differentiated schistosity s2 to homogeneous foliation s2 these rocks underwent prograde metamorphism during d2 and chloritoid garnet staurolite andalusite and sillimanite isograds have been mapped most rocks are rich in porphyroblasts which commonly contain well defined inclusion trails the geometry of these trails varies from one mineral type to the next depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the schistose matrix the deformation history involved progressive bulk inhomogeneous shortening and the strain is very heterogeneous on all scales partly as a consequence of this hence each of the six stages of crenulation cleavage development formed locally at various times during d2 therefore if a porphyroblast grew late in the deformation it could overgrow all stages of crenulation cleavage development including the very early ones the temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others that were unstable in the new conditions preserving them from destruction hence the precise timing of mineralogical reactions deduced from isograds and mineral chemistry relative to the stages in schistosity development can also be determined in the andalusite zone the sequence of porphyroblasts in decreasing order of age is biotite garnet biotite staurolite andalusite dissolution of garnet occurred during growth of staurolite and especially andalusite and dissolution of staurolite accompanied andalusite growth

environmental response to a large explosive rhyolite eruption sedimentology of post 1 8 ka pumice rich taupo volcaniclastics in the hawke s bay region new zealand abstract the 1 8 ka eruption of the taupo ignimbrite from the taupo volcanic centre in the central north island of new zealand produced a 30 km3 ignimbrite covering an area ca 160 km in diameter centred on lake taupo remobilisation of its pyroclastic detritus triggered major alluviation in the years to decades after its emplacement in the hawke s bay region in the eastern north island three large rivers rise in the ignimbrite impacted mountainous headwaters in areas that were buried by up to 40 m of loose unconsolidated pumiceous material analysis of preserved volcaniclastic sediments along valleys shows that resedimentation occurred in two main stages characterised by contrasting sediment water ratios the first comprised erosion and redeposition of ignimbrite debris by lahars e g debris flows and hyperconcentrated flows the second was marked by decreasing sediment water ratios and the re establishment of braided fluvial systems in the former river valleys as recorded by such deposits as trough planar low angle and antidune cross bedded planar bedded and ripple cross laminated sands and gravels however there is no systematic chronological trend towards lower pumice contents in these fluvial facies four characteristic sets of co occurring lithofacies were produced during the sedimentary response 1 laharic remobilisation association 2 upper flow regime dominated stream flow association 3 lower flow regime dominated stream flow association and 4 lacustrine association individual lithofacies were strongly controlled by the unusual hydrodynamic properties of pumice particularly its low and variable density automated settling tube analysis of water saturated pumiceous sediment confirms that sieve analysis is misleading with respect to the hydrodynamic behaviour of such sediments recasting granulometric data in terms of effective hydraulic grain size shows better sorting of mixed component sediments but statistical sorting and mean grain size parameters do not discriminate between the deposits of tractional and overcapacity parent flows therefore particle behaviour is addressed by using both lithofacies and petrofacies in the description and classification of the remobilisation deposits

reactive monazite and robust zircon growth in diatexites and leucogranites from a hot slowly cooled orogen implications for the palaeoproterozoic tectonic evolution of the central fennoscandian shield sweden monazite in melt producing poly metamorphic terranes can grow dissolve or reprecipitate at different stages during orogenic evolution particularly in hot slowly cooling orogens such as the svecofennian owing to the high heat flow in such orogens small variations in pressure temperature or deformation intensity may promote a mineral reaction monazite in diatexites and leucogranites from two svecofennian domains yields older coeval and younger u pb sims and emp ages than zircon from the same rock as zircon precipitated during the melt bearing stage its u pb ages reflect the timing of peak metamorphism which is associated with partial melting and leucogranite formation in one of the domains the granite and diatexite belt zircon ages range between 1 87 and 1 86 ga whereas monazite yields two distinct double peaks at 1 87 1 86 and 1 82 1 80 ga the younger double peak is related to monazite growth or reprecipitation during subsolidus conditions associated with deformation along late orogenic shear zones magmatic monazite in leucogranite records systematic variations in composition and age during growth that can be directly linked to th u ratios and preferential growth sites of zircon reflecting the transition from melt to melt crystallisation of the magma in the adjacent ljusdal domain peak metamorphism in amphibolite facies occurred at 1 83 1 82 ga as given by both zircon and monazite chronology pre partial melting 1 85 ga contact metamorphic monazite is preserved in spite of the high grade overprint by combining structural analysis petrography and monazite and zircon geochronology a metamorphic terrane boundary has been identified it is concluded that the boundary formed by crustal shortening accommodated by major thrusting

age and structural constraints on the tectonic evolution of the paleoproterozoic central lapland granitoid complex in the fennoscandian shield abstract the paleoproterozoic central lapland granitoid complex clgc forms the northern core of the fennoscandian shield we present here new u pb age data and review the existing ones from plutonic and metasedimentary rocks of the clgc and integrate the results with the structural field observations to decipher the tectonic history of this region granitoids vary in age from 2 43 ga to 1 78 ga whereas metamorphic zircon overgrowths both in granites and metasedimentary rocks vary between 1 90 ga and 1 76 ga with several age peaks indicating prominent metamorphic pulses the almost continuous growth of metamorphic zircon domains favors that the central part of the clgc was near solidus for 100 120 ma a succession of five deformations and associated foliation producing events which were near orthogonal to each other have been identified east vergent thrusting and stacking d1 commenced at ca 1 92 ga and led to the onset of barrovian type metamorphism followed by a radical tectonic shift that yielded e w trending d2 structures exhumation at 1 88 1 87 ga caused decompression melting of thermally evolved thickened crust and led to the emplacement of large volumes of granites and triggering of buchan type metamorphic conditions ne sw collision d3 at the southwestern margin of fennoscandia heterogeneously overprinted the s3 fabric on d2 structures orogen parallel nnw sse buckling at 1 87 1 85 ga formed oroclines in the hot linear arc segments renewed nnw sse shortening d4 at 1 84 1 81 ga produced long wavelength folds and caused the formation of domal structure in the clgc the final 1 78 1 76 ga far field collision d5 in the west overprinted all the pre existing structures this complex multi directional tectonic scenario provoked the intermittent exhumation phases between different contraction events of the clgc

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

a consideration of the dune antidune transition in fine gravel hydraulic data defining the dune antidune transition in fine gravel are compared with potential flow theory and information is drawn from published experiments and field based studies attention is given to both transitional bedforms and the development of downstream migrating antidunes in the latter case most data pertain to sand beds and not to gravel empirical data provide some weak support for the theoretical notion that the transition occurs at progressively lower froude numbers at greater relative depths although a critical froude number of 0 84 may reasonably be applied for the beginning of the dune to antidune transformation lag effects and a possible depth limitation ensure that transitional bedforms may persist across a broad range of froude numbers from 0 5 to 1 8 this latter observation has great relevance for palaeohydraulic estimates derived from outcrop data whereas the application of theoretical bedform existence fields based upon potential flow theory to fine gravel was previously purely speculative the addition of experimental and field data to these plots provides a degree of confidence in applying stability theory to practical geological problems for the first time laboratory data pertaining to downstream migrating gravel antidunes are compared with theory these bedforms have been reported from certain experimental near critical flows above sand or gravel beds but have been observed infrequently in natural streams however there are no detailed studies from natural rivers and only a few contentious identifications from outcrops nevertheless the limited hydraulic data conform to theoretical expectations

unit bar architecture in a highly variable fluvial discharge regime examples from the burdekin river australia unit bars are relatively large bedforms that develop in rivers over a wide range of climatic regimes unit bars formed within the highly variable discharge burdekin river in queensland australia were examined over three field campaigns between 2015 and 2017 these bars had complex internal structures dominated by co sets of cross stratified and planar stratified sets the cross stratified sets tended to down climb the development of complex internal structures was primarily a result of three processes i superimposed bedforms reworking the unit bar avalanche face ii variable discharge triggering reactivation surfaces and iii changes in bar growth direction induced by stage change internal structures varied along the length and across the width of unit bars for the former down climbing cross stratified sets tended to pass into single planar cross stratified deposits at the downstream end of emergent bars such variation related to changes in fluvial conditions whilst bars were active a hierarchy of six categories of fluvial unsteadiness is proposed with these discussed in relation to their effects on unit bar and dune internal structure across deposit variation was caused by changes in superimposed bedform and bar character along bar crests such changes related to the three dimensionality of the channel and bar geometry when bars were active variation in internal structure is likely to be more pronounced in unit bar deposits than in smaller bedform for example dune deposits formed in the same river this is because smaller bedforms are more easily washed out or modified by changing discharge conditions and their smaller dimensions restrict the variation in flow conditions that occur over their width in regimes where unit bar deposits are well preserved their architectural variability is a potential aid to their identification this complex architecture also allows greater resolution in interpreting the conditions before and during bar initiation and development