erosional marks on consolidated banks and slump blocks in the rupen river north west india erosional marks including grooves current crescents and flutes are present on banks and slump blocks of the rupen river on the semi arid plains of north west india the channel has a width depth ratio of 6 and is incised up to 8 m through well consolidated fine grained aeolian sediment dated at 20 60 ka pedogenic carbonate nodules embedded in the channel walls have localized scour and reworked carbonate particles probably promoted corrasion during strong seasonal flow similar erosional marks are apparently scarce on modern river banks and a distinctive combination of conditions including a consolidated substrate and strong flow in a confined channel is probably required for their generation and preservation on channel walls and on slump blocks

sequence stratigraphy of the neoproterozoic infra krol formation and krol group lesser himalaya india a bstract a sequence stratigraphic study of terrigenous and carbonate rocks of the infra krol formation and krol group in the lesser himalaya fold and thrust belt of northern india was undertaken as part of a broader investigation of the significance of carbon isotope data in neoproterozoic successions eight regional stratigraphic discontinuities were traced over a distance of nearly 300 km and interpretations were anchored in a series of local studies involving the mapping of key beds and the measurement of closely spaced sections three of the regional surfaces are interpreted as sequence boundaries on the basis of 1 locally developed incised valleys 60 m deep 2 paleokarstic depressions with 50 m of mappable relief 3 subaerial dissolution and weathering products breccias and calcrete filling vertical fissures dikes cavities and shallow de pressions in underlying carbonate rocks and 4 small scale evidence for sub aerial exposure at an erosion surface the remaining five discontinuities are regional flooding surfaces identified on the basis of either facies changes with an abrupt upward deepening across the surface or transitions in facies stacking patterns typically from forestepping to backstepping a glacio eustatic origin is permitted although not required for the three sequence boundaries but no evidence has been found for marked lowering of sea level at other horizons a mismatch between the stratigraphic location of sequence boundaries and car bon isotope minima suggests that local diagenetic alteration or oceanographic phenomena unrelated to glaciation may be in part responsible for observed isotopic variation and that small ice sheets may have existed during apparently nonglacial times without producing either cap carbonates or negative carbon isotope excursions

half graben filling processes in the early phase of continental rifting the miocene namurungule formation of the kenya rift abstract the miocene namurungule formation crops out on the eastern flank of the kenya rift representing basin fill that developed in association with miocene rifting the formation is characterized by large volumes of volcaniclastics supplied passively by pyroclastic fall facies analysis reveals that this formation consists mostly of lacustrine delta deposits with minor alluvial fan deposits at its base the conspicuous occurrence of a flood plain facies with infrequent channel fill deposits in the lower part of this formation suggest that the drainage area was limited in the early stage of deposition pyroclastic fall would therefore have been an important source of sediment during the early stages of rift development the delta deposits are divided into two distinct successions based on lithological characteristics separated by a thick pyroclastic layer in the middle part of the formation the stacking pattern of the lower succession is retrogradational whereas the upper succession is characterized by a pile of prograding bodies both delta successions are interpreted to have accumulated in an underfilled basin the change in depositional mode from the lower to upper is considered to be due to a change in the balance between the sedimentation rate and the rate of lake level rise assuming constant sediment supply the apparent difference in flooding scale between the lower and upper successions is attributed to the topographical widening of the basin flat similar successions also occur in another basin in the east african rift ngorora formation central kenya in which pyroclastic sediments are dominant and upward decrease in sedimentation rate is recognized the differing in the stacking patterns between the namurungule formation and the ngorora formation is probably induced by basin width the narrow namurungule basin seems to have been sensitive to increased sediment supply owing to the expansion of the catchment area

unravelling the depositional origins and diagenetic alteration of carbonate breccias carbonate breccias dissociated from their platform top counterparts are little studied despite their potential to reveal the nature of past shallow water carbonate systems and the sequential alteration of such systems a petrographic and stable isotopic study allowed evaluation of the sedimentological and diagenetic variability of the cenozoic batu gading limestone breccia of borneo sixteen lithofacies representing six facies groups have been identified mainly from the breccia clast on the basis of shared textural and compositional features clasts of the breccia are representative of shallow carbonate platform top and associated flank to basinal deposits dominant inputs are from rocky karstic shorelines or localised seagrass environments coral patch reef and larger foraminiferal rich deposits early pre brecciation alteration including micritisation rare dissolution of bioclasts minor syntaxial overgrowth cementation pervasive neomorphism and calcitisation of bioclasts and matrix was mainly associated with marine fluids in a near surface to shallow burial environment the final stages of pre brecciation diagenesis include mechanical compaction and cementation of open porosity in a shallow to moderate depth burial environment post brecciation diagenesis took place at increasingly moderate to deep burial depths under the influence of dominantly marine burial fluids extensive compaction circum clast dissolution seams and stylolites have resulted in a tightly fitted breccia fabric with some development of fractures and calcite cements a degree of facies specific controls are evident for the pre brecciation diagenesis pervasive mineralogical stabilisation and cementation have however led to a broad similarity of diagenetic features in the breccia clasts thereby effectively preserving depositional features of near original platform top and margin environments there is little intra clast alteration overprint associated with subsequent clast reworking and post brecciation diagenesis the diagenetic and to an extent depositional and clast characteristics of the batu gading deposits are diagnostic of breccia origins the predominance of early and pervasive stabilisation of calcitic components pervasive compaction resulting in a fitted texture and paucity of meteoric dissolution or cementation effects are collectively all indicators of slope deposition and lithification these features are comparable with other regional and global examples of submarine slope breccias and consistent with the prior interpretation that the carbonate platform at batu gading developed in a rotating wedge top basin wannier 2009 the results of this study along with regional analogues suggest the potential for reworked carbonates debris in slope settings to be a viable way of investigating carbonate platform variability and their subsequent alteration in the absence of preserved platform top or margin deposits

precambrian paleosols in canada although at least 38 precambrian paleosols have been reported in canada few have received the attention afforded most other geological phenomena the most intensively studied of these are widely distributed early proterozoic paleosols associated with the sub huronian and matonabbee new name unconformities criteria that have been most useful in identifying precambrian paleosols in canada include macrostructures ascending protolith disruption saprolith and corestones micromorphology sepic texture and peds mineralogical changes dissolution transformation geochemical signatures ascending depletion of some elements characteristic weathering indices and distinctive colour contrast diagenetic mechanical and geochemical overprinting of the paleosols evident in many canadian examples includes paleosol compaction veins cutting pedogenic features and widespread alkali and alkaline earth element metasomatism metamorphism and deformation also tend to mask pedogenic features as attested by th

searching for travertines calcretes and speleothems in deep time processes appearances predictions and the impact of plants common models for modern calcite precipitation in and around caves soils springs and streams involve co 2 supplied by thick high p co 2 biogenic soils which were probably thin or non existent before vascular plants indeed plant influenced chemical weathering might have caused accelerated terrestrial carbonate production from the devonian onwards however terrestrial carbonates have also been documented from the archaean proterozoic cambrian ordovician and silurian mechanisms which could have caused non marine carbonates to precipitate without organic rich soils are described and some geological events likely to have influenced non marine carbonate precipitation up to the origin of vascular plants are highlighted as organisms have evolved so have the petrographic characteristics of non marine carbonates some examples of this are also given here

internal anatomy of a mixed siliciclastic carbonate platform the late cenomanian mid turonian at the southern margin of the spanish central system the late cenomanian mid turonian succession in central spain is composed of siliciclastic and carbonate rocks deposited in a variety of coastal and marine shelf environments alluvial plain estuarine lagoon shoreface offshore hemipelagic and carbonate ramp three depositional sequences third order are recognized the atienza patones and el molar sequences the patones sequence contains five fourth order parasequence sets while a single parasequence set is recognized in the atienza and el molar sequences systems tracts can be recognized both in the sequences and parasequence sets the lowstand systems tracts only recognized for atienza and patones sequences are related to erosion and sequence boundary formation transgressive systems tracts are related to marine transgression and shoreface retreat the highstand systems tracts are related to shoreface extension and progradation and to carbonate production and ramp progradation sequences are bounded by erosion or emergence surfaces whose locations are supported by mineralogical analyses and suggest source area reactivation probably due to a fall in relative sea level transgressive surfaces are subordinate erosion and or omission surfaces with a landward facies shift interpreted as parasequence set boundaries the co existence of siliciclastic and carbonate sediments and environments occurred as facies mixing or as distinct facies belts along the basin mixed facies of coastal areas are composed of detrital quartz and clays derived from the hinterland and dolomite probably derived from bioclastic material siliciclastic flux to coastal areas is highly variable the maximum flux postdates relative sea level falls carbonate production in these areas may be constant but the final content is a function of changing inputs in terrigenous sediments and carbonate content diminishes through a dilution effect carbonate ramps were detached from the coastal system and separated by a fringe of offshore fine grained muds and silts as distinct facies belts the growth of carbonate ramp deposits was related to the highstand systems tracts of the fourth order parasequence sets during the growth of these ramps some sediment starvation occurred basinwards progradation and retrogradation of the different belts occur simultaneously suggesting a sea level control on sedimentation in the study area the co existence of carbonate and siliciclastic facies belts depended on the superimposition of different orders of relative sea level cycles and occurred mainly when the second order third order and fourth order cycles showed highstand conditions

early holocene coastal environment change inferred from deposits at okinoshima archeological site boso peninsula central japan abstract a series of deposits recording the early holocene drastic changes in coastal environment was investigated in terms of sedimentology and paleontology at the okinoshima archeological site on the southwest coast of boso peninsula central japan the deposits ca 3 m thick exposed in trenches consist of structureless mud and overlying stratified sand divided into three units i ii and iii in ascending order unit i is poorly sorted sandy mud with abundant opal phytoliths unit ii gradually overlying unit i is poorly sorted sandy mud containing more organic material and pieces of wood and archeological remains unit iii overlies unit ii with a sharp erosional contact and is composed of well sorted medium grained sand intercalated by mud layers with abundant plant fossils unit i suggests a dry surface on a coastal plain near uplands unit ii was transported from the upland onto the coastal plain by a debris flow unit iii deposits resulted from high energy river and storm surge flows onto the coastal plain the succession records that the sedimentary environments of the study site changed successively from a slope near the uplands to a fluvial plain to a coastal plain in response to a sea level rise in the early holocene ams ages of the units are approximately 10 000 cal bp and deposition occurred within 1000 years

silica replaced oolites bedded shelf cherts and paleozoic changes in the silica cycle abstract oolites that were replaced by early diagenetic silica and were not reworked from their original depositional setting can serve as a useful tool to evaluate a previously suggested retreat of biosiliceous facies from peritidal environments to deeper water settings in the early paleozoic the silica replaced oolites suggest that the retreat was largely over by the middle ordovician which is consistent with emerging compilations of the spatial and temporal distribution ordovician cherts whether the retreat was a function of declining concentration of dissolved silica in shallow marine waters or of radiation of faunas from nearshore into offshore settings or some combination of both is an issue that may be clarified with improved resolution of the timing of the retreat a small upper carboniferous peak in silica replaced oolite abundance coincides with a minor rebound in shelf chert accumulation and the common presence of coastal flint deposits in upper carboniferous cyclothems suggesting that cherty facies returned to shallow water environments at this time

grain fabric of hummocky and swaley cross stratification abstract bed form and grain fabric of two types of hummocky and swaley cross stratification from the pleistocene formations in the kanto plain cental japan display clearly distinctive flow conditions of storm typhoon generated combined flow the grain alignment in the vertical profile of a domal structure of hummocky cross set together with variable strike directions of a lamina suggest that the cross stratification was produced under the waning stage of oscillatory dominant combined flow elsewhere the preferred grain orientation along the maximum dip direction of a large swaley structure shows a small angle of upcurrent imbrication both on the stoss and lee sides of the laminae the stratification within the swale is asymmetric but the strike directions of a lamina are persistently oriented in a preferred direction the fabric and the structure of the swale indicate that the cross stratification was formed as a very low angle mega ripple under unidirectional dominant combined flow the grain fabric of both cross sets clearly shows that in spite of a superficial resemblance to antidunes neither of the two structures was formed in flow conditions for antidunes

sedimentation in a subglacial lake enniskerry eastern ireland abstract field relationships between a subglacial lake fill from the enniskerry basin eastern ireland provide evidence for dynamic links between the sedimentary fill regional subglacial meltwater flow and regional ice dynamics the basin fill 6 7 km2 forms an integral part of an extensive 30 km subglacial meltwater system which developed parallel to north south ice flow along the western marginal sector of the late pleistocene irish sea glacier high magnitude sediment fluxes were directed into the subglacial lake southwards across the northern basin rim from two nye type channels and westwards by a rothlisberger type channel the resulting subaqueous fans typically consist of cones of coarse boulder gravels deposited by jet effluxes overlain by cross bedded sands and foreset gravels in places these are eroded and overlain by multistorey dish shaped channels cut and filled by subglacial meltwaters driven by a high hydrostatic head a variety of gravelly and sandy bedforms indicate that sediment migrated over efflux cores towards deeper parts of the basin facies variability within the ice proximal fans and spreads record deposition from a wide range of high and low density turbulent flows traction currents density underflows suspension and sediment gravity flows a surficial drape of winnowed diamict is related to areal resedimentation as jet effluxes closed down at one site the gravelly sequence shows a largely conformable transition upwards into glaciotectonised gravel and basal till in the lee of the basin rim elsewhere the upper part of the sediment pile is occasionally deformed by the drag of the ice roof a series of sequential events is postulated for the evolution of the subglacial drainage system and subglacial lake development initially a rock ridge transverse to ice flow resulted in the development of a relatively low pressure lee zone along the northern margin of the basin ice sheet decoupling within the basin was enhanced by water migration into this low pressure lee zone which also melted an ice roof cavity closure and maintenance of the ice roof was prevented by a fast ice flux generated by intense marine downdraw in the main ice lobe centred in the irish sea basin regional ice directed meltwater pressure gradients and erosion enhanced subglacial lake development in the lowest topographic point of the system major inlet and outlet nye type channels incised transverse to the rims of the basin testify to the intensity of ice directed drainage along the marginal sector of the irish sea glacier in a wider context the concepts of high magnitude subglacial meltwater flow coupled with high rates of debris flux invoked for the subglacial basin fill are also necessary to explain the thick tidewater sequences present in adjacent sectors of the irish sea basin

composition and architecture of braided and sheetflood dominated ephemeral fluvial strata in the cambrian ordovician potsdam group a case example of the morphodynamics of early phanerozoic fluvial systems and climate change abstract two fluvial facies associations braided and ephemeral fluvial are recognized in strata of the cambrian ordovician potsdam group in the ottawa embayment and quebec basin in northeastern north america braided fluvial sedimentation was dominated by the migration of coarse grained sandy dunes and rare unit bars and local gravels that built up low relief 0 4 5 m thick downstream accreting compound bars in very wide and shallow braided channels rare meter scale confluence scours incision and filling of decimeter thick bypass channels and thin 7 cm overbank deposition also occurred ephemeral fluvial sedimentation was dominated by the progradation of lobate terminal splays most of which were deflated and or reworked into thin 10 95 cm sheets by eolian processes during the long recurrence intervals between floods these strata were locally incised by rare 0 3 1 9 m thick scour filling high energy supercritical bedform strata and decimeter thick bypass channels both braided and ephemeral fluvial deposits consist of meter scale stacked high order stratal units separated by horizontal laterally extensive surfaces recording the recurring buildup avulsion and finally abandonment of channel belts in braided fluvial strata of the potsdam the dominance of thin 2 m elongated compound bars with low relief 5 bar margins and internal accretion surfaces 15 suggest that braided channels and in channel compound bars expanded laterally with negligible increase in flow depth during periods of increased discharge in the absence of bank stabilizing vegetation similar to other examples of the pre vegetated sheet braided style however in areas where bedrock topography inhibited the lateral expansion of channels thicker compound braid bars 1 2 3 5 m with higher relief accretion surfaces up to 35 like those observed in post silurian and modern systems preferentially formed in relatively deep channels ephemeral fluvial deposits here are similar to ephemeral examples throughout geologic history in that they mostly comprise poorly confined sheet like terminal splays that accumulated in topographic lows however compared to several post silurian examples strata in the potsdam exhibit considerably more eolian reworking of splays and preservation of supercritical bedform strata a manifestation of greater rates of surface water infiltration due to a lack of sheltering by rooted plants and propensity for unconfined flooding finally in the upper part of the potsdam two regional ephemeral fluvial units are interstratified with two braided fluvial units providing evidence for shifts in regional climate the sharp areally extensive surfaces that separate these fluvial systems are interpreted to approximate time lines and therein provide a means for relative and absolute chronostratigraphic correlation these changes are correlated with documented late cambrian to earliest ordovician global climate fluctuations with semiarid conditions and related ephemeral fluvial systems corresponding to global cooling events at ca 491 and 487 ma

a paleoproterozoic drowned carbonate platform on the southeastern margin of the wyoming craton a record of the kenorland breakup abstract the nash fork formation in the upper part of the early paleoproterozoic snowy pass supergroup medicine bow mountains of wyoming was deposited on a mature passive margin along the southern flank of the wyoming craton and straddles the end of the ca 2 2 2 1 ga carbon isotope excursion two drowning events marked by black shales subdivide the carbonate platform into three parts the lower nash fork formation consists of outer shelf to supratidal deposits represented by massive and stromatolitic dolomites heterolithic siliciclastics carbonates large silicified domal digitate stromatolites nodular dolomites and stromatolitic dolomites molds after evaporite crystals are pervasive in the heterolithic siliciclastics carbonates large silicified domal digitate stromatolites formed biostromes and bioherms following flooding events the middle nash fork formation comprises two intervals of black shale separated by inner shelf heterolithic siliciclastics nodular carbonates black shales are organic and pyrite rich contain turbidites and developed in response to drowning of the platform overlying massive dolomite of the upper nash fork formation was deposited in an unprotected intertidal setting and displays an upward shallowing trend terminated by a prominent karstic surface in the middle of the unit the nash fork formation is open marine with no evidence for restricted circulation on the carbonate platform the two drowning events on the carbonate platform are likely related to dissection of the mature passive margin associated with the breakup of kenorland the younger drowning event is associated with the end of the carbon isotope excursion the main building elements of the lower and middle nash fork carbonate platform are dolomitic mudstones and stromatolites macroscopic seafloor precipitates are volumetrically negligible with the exception of tufa deposits and domes in the massive and stromatolitic dolomites and possibly digitate stromatolites within domal digitate stromatolites the upper nash fork formation comprises dolomitic mudstones relatively rare stromatolites and inorganic precipitates that are more common than in the underlying carbonates styles of carbonate deposition on this early paleoproterozoic platform differ from those documented on late archean carbonate platforms there are fewer macroscopic seafloor precipitates and more dolomitic mudstones this pattern is considered to be related to a rise of the atmospheric oxygen level that led to a decrease in bicarbonate saturation in the ocean

evidence for the granulite granite connection penecontemporaneous high grade metamorphism granitic magmatism and core complex development in the liscomb complex nova scotia canada abstract upper amphibolite granulite facies gneisses and granites of the liscomb complex nova scotia canada which are exposed in a core complex within the cambro ordovician meguma group of southern nova scotia yielded concordant u pb zircon monazite ages of 377 2 and 374 3 ma respectively geochronological and geochemical data suggest a single devonian high grade metamorphic event which generated the granitic magma by partial melting of the fertile liscomb gneisses at a depth of 30 km the melting was also synchronous with an extensional event during which the gneisses were uplifted in a core complex associated with the intrusion of granitoids to a depth of 10 km subsequently the gneisses and granites underwent rapid exhumation before the deposition of unconformably overlying late fammenian rocks at 364 ma these events took place during terminal stages of the acadian orogeny and the onset of extensional tectonics in atlantic canada during the middle late devonian the close temporal and spatial association of liscomb gneisses granulites and granites their major and trace element compositions and their overlapping isotopic characteristics confirm the hypothesis that high grade metamorphism and generation of granitic melt are complementary processes as the liscomb granites are of similar age mineralogy and chemistry to the voluminous granitoid plutons found throughout the meguma terrane a similar process is indicated for the rest of the terrane

internal waves vs surface storm waves a review on the origin of hummocky cross stratification hummocky cross stratification hcs is considered a diagnos tic structure of surface storm activity at the shoreface offshore transition however the origin of hcs is still debated laboratory experiments have not yet reproduced it and direct observations on the continental shelves do not exist most hydrodynamic interpretations invoke pure oscilla tory flows unidirectional dominated combined flows and oscillatory dominated flows but they all share the assump tion of hcs to reflect the combined action of surface storm waves and related currents within this context of uncertain ties internal waves gravity waves propagating along the pycnocline provide an alternative mechanism to explain the origin of hcs internal waves breaking on the shelf create episodic high turbulence events and induce upslope and downslope currents as well as oscillatory flows at the depth where the pycnocline intersects the sea floor in this scenario both the oscillatory and the unidirectional components needed for hcs to form are not necessarily linked to surface storm waves but can occur at various depth as far and near there is a pycnocline where internal waves can form

macroforms large bedforms and rhythmic sedimentary sequences in subglacial eskers south central ontario implications for esker genesis and meltwater regime abstract eskers of south central ontario were deposited in closed subglacial conduits which were continuous main conduits this interpretation is supported by the intimate association of eskers with an anastomosing network of tunnel channels relatively continuous esker ridges minimal post formational disturbance of esker sediments intercalation of till and stratified sand and gravel diapiric folding at an esker core low variability in palaeocurrent direction and upslope flow paths down esker trends in clast lithology roundness and sphericity indicate continuous conduits with sediment supply by subglacial deformation of adjacent material into the conduits melt out of sediment from the conduit walls and fluvial resedimentation esker ridge morphology is attributed to synchronous erosion transportation and deposition along the main conduits such that ridge discontinuities are primarily explained as zones of non deposition during esker formation composite pseudoanticlinal and oblique accretion avalanche bed macroforms are identified gravel facies within these macroforms were deposited from fluidal flows or hyperconcentrated dispersions progradation of macroforms or migration of constituent large bedforms through the main conduits may have temporarily blocked constricted portions of those conduits and acted as possible internal autogenic controls on sediment availability and flow resistance the location of macroforms within conduits was primarily controlled by conduit geometry and sediment availability and later by feedback between macroform and conduit geometries sand and gravel units alternate rhythmically in vertical section rhythmicity is interpreted as a response to episodic flood flows controlled by seasonal changes in water supply but not necessarily representing annual events fans beads anabranched reaches and extended hummocky deposits are intimately associated with the main esker ridges they are interpreted in terms of subglacial cavities and localized flotation zones connected to the main conduits during flood events in phase wave structures are the products of hydraulic jumps in hyperconcentrated flood flows at flow expansions into swells within the main conduits into cavities connected to the main conduits or at a grounding line esker ridges record only the most powerful flood events whereas fans and beads record flow events in finer detail being primarily depositional reservoirs laterally fining deposits record the waning stages of conduit operation

an archean quartz arenite andesite association in the eastern baltic shield russia implications for assemblage types and shield history abstract shallow water sedimentary units are generally considered scarce in archean greenstone belts we describe an unusual quartz arenite subaerial andesite association within the archean hisovaara greenstone belt a fragment of the parandovo tikshozero belt within the karelian craton of the baltic shield the hisovaara greenstone belt consists of several lithotectonic assemblages 1 a komatiite tholeiite assemblage 2803 ma based on ages of cross cutting dikes 2 an andesite quartz arenite assemblage cut by similar dikes 3 an assemblage of coarse volcaniclastic rocks and 4 an upper mafic assemblage of tholeiitic basalts with minor pyroxene komatiite volcanic rocks the andesite quartz arenite assemblage 100 to ca 750 m thick has basal amygdaloidal fragmental andesites overlain by massive andesites then amygdaloidal and plagioclase phyric andesites unconformably overlying the andesite is a unit of quartz rich sandstones 6 40 m thick dominated by quartz arenite extending several km along strike at the north end the quartz arenite succession consists of basal andesite overlain by quartz arenite exhibiting hummocky cross stratification followed by aluminous coarse metasediments and sulfidic argillite and an unconformably overlying tholeiite komatiite unit at the south end the succession is basal andesite regolith cross bedded quartz arenite weathered andesites a second quartz arenite argillite and then subaerial rhyolite ree and hfse geochemistry has been obtained on the rocks of the andesite quartz arenite assemblage the quartz arenites contain low abundances and chondrite normalized patterns vary from relatively fractionated to flat with most of the variation related to grain size with pebbly units having higher abundance and more fractionated patterns combined major and trace element geochemistry indicates that a sodic felsic source with some admixture of mafic material will explain the geochemistry of the quartz arenites the andesites display moderately fractionated spidergrams with negative anomalies at ti ta and nb typical of arc volcanism the andesite quartz arenite assemblage represents accumulation of shallow water quartz rich sediments in a setting typical of the later stages of arc volcanism in which the volcanic edifice is subaerial at the southern end of the assemblage however at the north end our evidence is interpreted as indicating subaerial andesitic volcanism subsidence to a shallow marine basin which then deepens and rifts therefore the hisovaara andesite quartz arenite assemblage provides a linkage in archean greenstones between assemblages representing continental volcanism and a platform to rift setting the presence of an erosional interval in 2 8 ga greenstones suggests possible pre 2 7 ga orogeny in the baltic shield the pre 2 7 ga quartzrich sedimentation is similar in age to platformal assemblages in the pre 2 7 ga north caribou terrane of the superior province canada

numerical modeling of the evolution of arcades and rock pillars abstract arcades i e lenticular and other specifically shaped hollows controlled by discontinuities have recently been recognized as a weathering form typical for sandstones weathered quartzites granites or tuffs they are produced by accelerated weathering and erosion in stress shadows related to the redistribution of gravity induced stress along planar discontinuities in the rock these forms occur worldwide in various settings inland humid arid and coastal the origin of arcades has been demonstrated via physical experiments and supported by a relatively simplistic numerical modeling however details on their shaping and the evolution of related forms have not been explained we performed an advanced numerical modeling to produce various shapes of arcades and rock pillars during the erosion of rock masses dissected by discontinuities we demonstrate that the erosion model in which erosion takes place when the maximum principal stress is below a certain critical value can adequately describe the formation of arcades in the modeling we set higher critical values for stresses at discontinuities than in a homogeneous material representing a rock mass to represent the higher tendency for disintegration of the discontinuity material which was weakened by the discontinuity formation processes by applying various discontinuity geometries and values of critical stresses we were able to reproduce the formation of various arcade shapes and complex three dimensional clusters of arcade cavities with rock pillars discontinuities and stress controlled erosion weathering are the only necessary conditions for arcade formation

estimating palaeo water depth from the physical rock record abstract palaeo water depth and its change with time is a key factor in environmental analysis and in sedimentology in general the literature published on this topic and on related subjects however is substantial albeit scattered within and between different communities geologists oceanographers engineers physicists etc here i focus on proxies for palaeo bathymetry as obtained from the physical rock record floral faunal and chemical evidence for palaeo water depth being equally important must be dealt with elsewhere an assessment of the main depth related parameters in actualistic settings is presented and their applicability to the fossil sedimentary record discussed the outcome reveals the full complexity of this topic there is for example no such thing as an average wave base depth applicable to a wide range of fossil case settings this because wave and current climates in general and particularly so in neritic and coastal settings are intriguingly complex furthermore observations from modern oceans are not a priori applicable to fossil ones acknowledging these problems and limitations critically evaluated quantitative proposals for depth indicative facies and features in the sedimentary record are presented and error bars discussed

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

stratigraphy petrography and dispersion of the lower permian syn eruptive deposits in the viar basin spain abstract this paper presents a stratigraphic petrographic and sedimentologic analysis of felsic syn eruptive deposits of the southernmost permian basin in europe viar basin an inner orogenic transtensional half graben located between two major geotectonic zones of the iberian variscan belt sw spain the syn eruptive deposits of the viar basin represent a mappable lithostratigraphic unit that is herein named the los canchales formation this unit forms a wedge shaped body bounded by alluvial fluvial deposits red layers that are in gradational contact the stratigraphic architecture of the los canchales formation is complex with frequent facies changes and interfingering among pyroclastic rocks resedimented volcaniclastic rocks and sedimentary rocks abundant fossil flora is also recorded a wax wane cycle of volcanic activity is recognized during the syn eruptive period two short lived volcanic episodes were recorded including ignimbrites and a possible phreatomagmatic event explosive volcanism generated pyroclastic flow surge and ash fall deposits in cases collapsing the drainage network they also modified the vegetation and created a new depositional setting with four successive sedimentary environments vegetated floodplain ephemeral sandy braidplain palustrine environment and again vegetated floodplain input of pyroclasts followed a nw to se longitudinal axial dispersal pattern

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

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

late ordovician climbing dune cross stratification a signature of outburst floods in proglacial outwash environments climbing dune scale cross statification is described from late ordovician paraglacial successions of the murzuq basin sw libya this depositional facies is comprised of medium grained to coarse grained sandstones that typically involve 0 3 to 1 m high 3 to 5 m in wavelength asymmetrical laminations most often stoss depositional structures have been generated with preservation of the topographies of formative bedforms climbing dune cross stratification related to the migration of lower flow regime dune trains is thus identified related architecture and facies sequences are described from two case studies i erosion based sandstone sheets and ii a deeply incised channel the former characterized the distal outwash plain and the fluvial subaqueous transition of related deltaic wedges while the latter formed in an ice proximal segment of the outwash plain in erosion based sand sheets climbing dune cross stratification results from unconfined mouth bar deposition related to expanding sediment laden flows entering a water body within incised channels climbing dune cross stratification formed over eddy related side bars reflecting deposition under recirculating flow conditions generated at channel bends associated facies sequences record glacier outburst floods that occurred during early stages of deglaciation and were temporally and spatially linked with subglacial drainage events involving tunnel valleys the primary control on the formation of climbing dune cross stratification is a combination between high magnitude flows and sediment supply limitations which lead to the generation of sediment charged stream flows characterized by a significant relatively coarse grained sand sized suspension load concentration with a virtual absence of very coarse to gravelly bedload the high rate of coarse grained sand fallout in sediment laden flows following flow expansion throughout mouth bars or in eddy related side bars resulted in high rates of transfer of sands from suspension to the bed net deposition on bedform stoss sides and generation of widespread climbing dune cross stratification the later structure has no equivalent in the glacial record either in the ancient or in the quaternary literature but analogues are recognized in some flood dominated depositional systems of foreland basins

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

the sedimentary record of carboniferous rivers continuing influence of land plant evolution on alluvial processes and palaeozoic ecosystems abstract evidence from modern rivers and the deep time geological record attests to the fundamental importance of plant life for the construction of physical habitats within fluvial environments data from an extensive literature review and original fieldwork demonstrates that many landforms and geomorphic features of modern river systems appear in the palaeozoic stratigraphic record once terrestrial vegetation had adopted certain evolutionary advances for example stable point bars are associated with the onset of rooted plants in the siluro devonian and avulsive and anabranching fluvial systems become common at the same time as extensive arborescent vegetation in the carboniferous in this paper we demonstrate a correlation between the diversification of physical fluvial environments and the expansion of terrestrial fauna and flora with an emphasis on the culmination of these trends within carboniferous alluvial systems many extrinsic factors have been considered as possible controls on the evolutionary timelines of terrestrialization for organisms however a fundamental prerequisite for achieving terrestrial biodiversity was the variety of physical habitats especially riparian systems available for newly evolved organisms in association with abundant lowland meandering systems the widespread appearance across carboniferous alluvial plains of fixed channel and anabranching reaches created further physical landforms for colonization and would have promoted increasingly complex hyporheic flow regimes furthermore the associated increase in arborescent vegetation and supply of large woody debris to inland and coastal rivers would have created a wealth of microhabitats for continental organisms we argue that the expanding extent and diversity of physical alluvial niches during the palaeozoic is an underappreciated driver of the terrestrialization of early continental life the study of the deep time fossil and stratigraphic record also illustrates that vegetation is a fundamental prerequisite for the creation of biogeomorphic alluvial landforms and physical habitats and microhabitats

sedimentary talc in neoproterozoic carbonate successions abstract mineralogical petrographic and sedimentological observations document early diagenetic talc in carbonate dominated successions deposited on two early neoproterozoic 800 700 million years old platform margins in the akademikerbreen group svalbard talc occurs as nodules that pre date microspar cements that fill molar tooth structures and primary porosity in stromatolitic carbonates in the upper fifteenmile group of the ogilvie mountains nw canada the talc is present as nodules coated grains rip up clasts and massive beds that are several meters thick to gain insight into the chemistry required to form early diagenetic talc we conducted precipitation experiments at 25 c with low so4 synthetic seawater solutions at varying ph mg2 and sio2 aq our experiments reveal a sharp and reproducible ph boundary at 8 7 only above which does poorly crystalline mg silicate precipitate increasing mg2 and or sio2 aq alone is insufficient to produce the material the strong ph control can be explained by mg silica complexing activated by the deprotonation of silicic acid above 8 6 8 7 ft ir tem and xrd of the synthetic precipitates reveal a talc like 2 1 trioctahedral structure with short range stacking order hydrothermal experiments simulating burial diagenesis show that dehydration of the precipitate drives a transition to kerolite hydrated talc and eventually to talc this formation pathway imparts extensive layer stacking disorder to the synthetic talc end product that is identical to neoproterozoic occurrences early diagenetic talc in neoproterozoic carbonate platform successions appears to reflect a unique combination of low al concentrations and by inference low siliciclastic input near modern marine salinity and mg2 elevated sio2 aq and ph 8 7 because the talc occurs in close association with microbially influenced sediments we suggest that soluble species requirements were most easily met through microbial influences on pore water chemistry specifically ph and alkalinity increases driven by anaerobic fe respiration

carbonate platform growth and cyclicity at a terminal proterozoic passive margin infra krol formation and krol group lesser himalaya india the infra krol formation and overlying krol group constitute a thick 2 km carbonate rich succession of terminal proterozoic age that crops out in a series of doubly plunging synclines in the lesser himalaya of northern india the rocks include 18 carbonate and siliciclastic facies which are grouped into eight facies associations 1 deep subtidal 2 shallow subtidal 3 sand shoal 4 peritidal carbonate complex 5 lagoonal 6 peritidal siliciclastic carbonate 7 incised valley fill and 8 karstic fill the stromatolite rich peritidal complex appears to have occupied a location seaward of a broad lagoon an arrangement reminiscent of many phanerozoic and proterozoic platforms growth of this complex was accretionary to progradational in response to changes in siliciclastic influx from the south eastern side of the lagoon metre scale cycles tend to be laterally discontinuous and are interpreted as mainly autogenic variations in the number of both sets of cycles and component metre scale cycles across the platform may result from differential subsidence of the interpreted passive margin apparently non cyclic intervals with shallow water features may indicate facies migration that was limited compared with the dimensions of facies belts correlation of these facies associations in a sequence stratigraphic framework suggests that the infra krol formation and krol group represent a north to north westfacing platform with a morphology that evolved from a siliciclastic ramp to carbonate ramp to peritidal rimmed shelf and finally to open shelf this interpretation differs significantly from the published scheme of a basin centred on the lesser himalaya with virtually the entire infra krol krol succession representing sedimentation in a persistent tidal flat environment this study provides a detailed neoproterozoic depositional history of northern india from rift basin to passive margin and predicts that genetically related neoproterozoic deposits if they are present in the high himalaya are composed mainly of slope basinal facies characterized by fine grained siliciclastic and detrital carbonate rocks lithologically different from those of the lesser himalaya