the morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae experimental studies of subcritical unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves flow depth was 0 11 m mean flow velocities were 0 86 1 0 m s 1 shear velocities were 0 058 0 71 m s 1 and dimensionless shear stresses were 0 56 0 87 bedwaves are asymmetrical in profile and range from 0 75 to 11 mm in height mainly 2 6 mm from 0 7 to 1 3 m in wavelength and have mean celerities of 10 mm s 1 flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs high resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave

syneresis cracks subaqueous shrinkage in argillaceous sediments caused by earthquake induced dewatering abstract syneresis cracks often confused with subaerial desiccation phenomena are traditionally ascribed to subaqueous shrinkage whereby salinity changes caused deflocculation of clay this and other previously proposed mechanisms fail to account for their occurrence in low energy typically non evaporitic facies stratigraphically sporadic distribution intrastratal formation under shallow burial depths variation in morphology degree of contraction generation of sedimentary dikelets as crack fills and deformation of dikelets and enclosing layers instead it is suggested that ground motion from strong synsedimentary earthquakes caused argillaceous sediments to dewater interbedded sands and silts to be almost simultaneously liquefied and injected into the resulting fissures and then these dikelets to be distorted comparative rarity of syneresis cracks in phanerozoic versus precambrian marine strata is considered to be primarily an evolutionary consequence of theological changes caused by increased organic binding of clay flocs in the water column greater input of organic matter into the sediment and the diversification of sediment dwelling bacteria and meiofauna in phanerozoic deposits

u pb rb sr and sm nd chronology of granitic basement hydrothermal albitites and uranium mineralization lagoa real south bahia brazil the granites orthogneisses hydrothermal albitities and rocks which have suffered uranium mineralization from the lagoa real district south state of bahia brazil have been investigated by u pb rb sr and sm nd techniques u pb values on zircons from the granitic protolith give an age of 1725 ma u pb on u mineralization dates the primary mineralization at 1395 ma and indicates a reworking at 480 ma which may represent the age of the thrusting of the lagoa real complex over espinhaco metasediments during the brazilian orogeny these two dates are given by rb sr on albitites but from sparse information and are not supported by unequivocal arguments the uranium deposition and sodium metasomatism however cannot be linked either with the thermal history of the granite or with brazilian thrusting sm nd gives scattered results which are suggestive of autochtonous reworkings of rees these results lead to the following suggestions 1 magmatic activity of subalkaline affinity existed within the sao francisco craton at about 1 7 ga 2 such subalkaline plutons are likely sources for u mineralization 3 at about 1 4 ga an unknown event caused hydrothermal activity leading to u deposition at lagoa real 4 brazilian overthrusting at about 480 ma did not play any genetic role in the hydrothermal activity

syndepositional tectonics recorded by soft sediment deformation and liquefaction structures continental lower permian sediments southern alps northern italy stratigraphic significance abstract the lower permian succession of the central southern alps lombardy northern italy was deposited in fault controlled continental basins probably related to transtensional tectonics we focussed our study on the stratigraphic record of the lower permian orobic basin which consists of a 1000 m thick succession of prevailing continental clastics with intercalations of ignimbritic flows and tuffs pizzo del diavolo formation pdv resting on the underlying prevailing pyroclastic flows of the cabianca volcanite the pdv consists of a lower part composed of conglomerates passing laterally to sandstones and distally to silt and shales a middle part pelitic with carbonates and an upper part alternating sandstone silt and volcanic flows syndepositional tectonics during the deposition of the pdv is recorded by facies distribution thickness changes and by the presence of deformation and liquefaction structures interpreted as seismites deformation is recorded by both ductile structures ball and pillow plastic intrusion disturbed lamination convolute stratification and slumps and brittle structures sand dykes and autoclastic breccias both the sedimentological features and the geodynamic setting of the depositional basin confidently support the interpretation of the described deformation features as related to seismic shocks the most significant seismically induced deformation is represented by a slumped horizon about 4 m thick on average which can be followed laterally for more than 5 km the slumped bed consists of playa lake deposits alternating pelites and microbial carbonates associated with mud cracks and vertebrate tracks the lateral continuity and the evidence of deposition on a very low angle surface along with the deformation liquefaction of the sediments suggest that the slump was triggered by a high magnitude earthquake the stratigraphic distribution of the seismites allows us to identify time intervals of intense seismic activity which correspond to rapid and basin wide changes in the stratigraphical architecture of the depositional basin and or to the reprise of the volcanic activity the nature of the structures and their distribution suggest that the magnitude of the earthquakes responsible for the observed structures was likely higher than 5 in order to produce sediment liquefaction and probably reached intensity as high as 7 or more the basin architecture suggests that the foci of these earthquakes were located close to the fault controlled borders of the basin or within the basin itself

age petrogenesis and tectonic setting of the permian bimodal volcanic rocks in the eastern jiamusi massif ne china abstract we present new in situ zircon u pb and hf isotope whole rock geochemical and sr nd isotopic data for volcanic rocks from the jiejinkou and baoqing areas in the eastern jiamusi massif these volcanic rocks are bimodal and consist of basalts basaltic andesites rhyolites and rhyolitic tuffs that can be subdivided into mafic and silicic groups zircon u pb dating by la icp ms indicates that these volcanic rocks were erupted between the early and middle permian 290 267 ma the mafic rocks in this area have positive e nd t 0 07 to 6 43 values and are enriched in light rare earth elements lrees and depleted in heavy ree nb and ta from these rocks the meta basalt of jinlu and basaltic andesite of taipinggou and haojiatun were derived from parental magmas generated by the partial melting of depleted mantle wedge material that was metasomatized by subduction related melts these magmas then underwent variable degrees of fractional crystallization and assimilated insignificant amounts of crustal material the meta basalt of liming likely originated from the metasomatized mantle derived melts hybridized by the convective asthenosphere during the evolution of the magmas in comparison the silicic rocks have negative e nd t and variable zircon e hf t values are enriched in the large ion lithophile elements liles and lree and are depleted in high field strength elements e g nb ta and ti yielding arc like geochemical signatures the geochemical and zircon e hf t characteristics of jiangfeng and longtouqiao rhyolites are indicative of formation from magmas generated by the partial melting of mafic lower crustal material whereas the liming meta rhyolite was probably produced from a source involving some depleted mantle components the bimodal volcanic rocks provide convincing evidence that the early middle permian volcanism in the jiamusi massif occurred in an extensional environment probably associated with slab break off during the westward subduction of the paleo pacific oceanic plate

alluvial fan to storm dominated shelf transition in the mesoproterozoic singhora group chattisgarh supergroup central india abstract mesoproterozoic siliciclastic strata at the basal part of the singhora group central india provide an example of a pre vegetation alluvial fan and braid delta depositional system detailed process based facies and paleoenvironmental analysis have led to the identification of 15 facies types grouped under four different facies associations which record paleoenvironmental settings ranging between continental alluvial fan and distal marine shelf two formations investigated are the rehtikhol formation formed in a conglomeratic or coarse arenaceous alluvial fan braid plain and delta setting and the overlying saraipalli formation which preserved the sedimentation record of a storm infested inner to distal shelf setting the preponderance of granular and sandy sediment gravity flow products with subordinate rock fall deposits within the alluvial fan system together with a pervasive west northwesterly paleocurrent is suggestive of a gently sloping beveled source area in the east southeast distal to the inferred fan a braid plain system developed with the decrease in the depositional slope streams within the braid plain had widths up to several hundred meters the braid plain system at the margin of the marine basin developed a delta with the deposition of wave reworked fluvial dominated sediments the crest trends of wave generated bedforms within the inferred delta sediments depict the northeast southwest shoreline interpreted for the singhora sea the prodeltaic basinal part of the singhora braid delta was storm infested the inner shelf acted as a zone of throughput with high velocity storm flows eroding deep narrow scours gutter casts and is interpreted as a zone of bypass operation of geostrophic currents on the singhora shelf has been inferred from the directional relationship of gutters with the inferred orientation of the shoreline a proximal distal relationship has been established within the shelf products based on the relative dominance of erosional and depositional features the singhora alluvial fan and braid delta facies are arranged in a fining upward sequence denudation of the source area increasing bathymetry with relative rise in sea level or a combination of both is inferred both extrabasinal and intrabasinal forcings possibly contributed to the deepening upward trend

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

sandy high energy flood sedimentation some criteria for recognition with an example from the devonian of s w england abstract many modern sand deposits resulting from episodic floods in semi arid zones are dominated by parallel laminations but parallel laminated sands are not a major feature in perennial streams single flood events may deposit over 1 5 m of parallel laminated sand hence ancient parallel laminated sand dominated alluvial deposits may be the product of ephemeral flows the trentishoe formation hangman sandstone group middle devonian of north devon consists in part of laterally extensive beds of parallel laminated fine grained red sandstones that build multistorey sand bodies individual beds are little more than 1 m thick but may extend laterally for hundreds of metres a vertical sequence of erosion surface sparse silt clasts parallel laminations silt drape is often found suggesting high flow stage deposition of plane bedded sand followed by a very rapid waning of flow with silt fallout in the final stages of the flood this sequence bears marked similarity to certain modern ephemeral flow sands

from alluvial fan to playa an upper jurassic ephemeral fluvial system neuquen basin argentina abstract the upper jurassic fill of the back arc neuquen basin includes a lowstand wedge known as the tordillo formation the studied deposits crop out along a n s oriented belt that runs parallel to the andean magmatic arc they are limited to the south by the east west oriented positive structure of the huincul arch formed as a result of upper jurassic tectonic inversion the tordillo deposits were formed in an arid fluvial dominated system characterised by systematic downstream changes in architectural style a gravelly and sandy bedload fluvial system is recognised in the southern upstream sector the reduced thickness and the coarse grain size suggest steep gradients excess of bedload supply and a low subsidence rate thicker and finer grained deposits prevail to the north and northeast they were formed under arid conditions in a wadi sand flat playa fluvial system this distal facies association indicates increased accommodation owing to high rates of subsidence relative to coarse siliciclastic sedimentation rates these low gradient deposits are characterised by cyclic alternations of mud dominated and sand dominated packages interpreted as high and low accommodation systems tracts the overall fining upward stacking pattern of the tordillo formation suggests a change towards higher accommodation rates this is accompanied by frequent development of soil horizons and darker primary and reworked pyroclastic deposits these attributes indicate a stronger explosive volcanic activity associated with increased precipitation and high water table emplacement towards the end of the tordillo lowstand wedge

the architecture of precambrian high energy tidal channel deposits an example from the lyell land group eleonore bay supergroup northeast greenland abstract within the 2 5 km thick late proterozoic lyell land group of the eleonore bay supergroup in east greenland numerous successions of stacked sheet sandstone bodies 10 70 m thick occur each succession consists of several 5 50 m thick multistorey sand sheets separated from each other by 0 2 2 m thick heterolithic mudstone or sandstone beds individual sand sheets are 0 2 3 0 m thick they have highly erosive lower contacts show a rough fining upward trend and are capped by a thin mudstone veneer containing desiccation cracks dominant sedimentary structures within the sand sheets are sets of planar cross bedding showing a range of characteristic tidal features such as successive bundles of foresets herringbone cross stratification ebb and flood caps and numerous reactivation surfaces horizontal lamination is abundant and may in some instances comprise more than half of the structures within a sand sheet the sand sheets represent high energy tidal channels with the multistorey sand sheets constituting larger tidal channel complexes the heterolithic beds represent mixed tidal flat sediment deposited during periods of tidal channel complex abandonment probably caused by channel aggradation and changing drainage patterns the tidal channels were wide shallow and very sandy with a very poorly developed channel morphology caused by lack of fine grained sediments and vegetation most likely they evolved within a meso to microtidal sandy back barrier setting where the channels migrated swiftly across tidal flats and reworked most interchannel sediment an architecture was developed consisting of stacked sand sheets interbedded with rare heterolithic mudstone beds the model presented incorporates sedimentary parameters and processes characteristic of a pre vegetative landscape and it has no direct modern analogue

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

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

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

coarse grained nonmarine volcaniclastic sediment terminology and depositional process modern and ancient volcaniclastic sedimentary sequences contain depositional units whose features cannot be attributed to fully turbulent dilute stream flow or viscous debris flow the characteristics of these poorly sorted sediments suggest rapid deposition from high concentration dispersions but not en masse sedimentation thus appears related to high discharge flows intermediate in sediment water ratio between stream flow and debris flow the term hyperconcentrated flood flow is proposed for describing this intermediate condition hyperconcentrated flood flow deposits are distinguished from debris flow deposits by lack of matrix support or reverse grading and instead exhibit distribution normal grading and horizontal stratification these deposits are distinguished from normal dilute stream flow deposits by lack of cross stratification in sand facies and by very poor sorting poor imbrication and numerous clasts with long axes oriented parallel to flow direction in gravel facies the horizontal bedding that dominates sandy hyperconcentrated flood flow deposits consists of sediment too coarse grained and strata too thick to have been produced in the boundary layer of the upper flow regime and should not be confused with the more familiar thin graded laminae of fine to medium grained sand often associated with parting lineation hyperconcentrated flood flow deposits are not unique to volcanic settings they also occur in arid alluvial fan sequences debris flow and hyperconcentrated flood flow deposits however are much thicker and more extensive in volcanic regions than on alluvial fans because explosive volcanism leads to rapid mobilization of large volumes of sediment and water on a scale unparalleled in nonvolcanic settings in volcanic regions therefore these deposits have greater preservation potential show greater lateral variability and are more voluminous transformation of channelized debris flow to hyperconcentrated flood flow by dilution with stream water recently observed at mount st helens is recorded in ancient volcaniclastic sequences and may serve as the primary mechanism for generating hyperconcentrated flood flow

depositional and tectonic setting of the archean moodies group barberton greenstone belt south africa the 3 22 3 10 ga old moodies group uppermost unit of the swaziland supergroup in the barberton greenstone belt bgb is the oldest exposed well preserved quartz rich sedimentary sequence on earth it is preserved in structurally separate blocks in a heavily deformed fold and thrust belt north of the inyoka fault moodies strata reach up to 3700 m in thickness detailed mapping correlation of measured sections and systematic analysis of paleocurrents show that the lower moodies group north of the inyoka fault forms a deepening and fining upward sequence from a basal alluvial conglomerate through braided fluvial tidal and deltaic sandstones to offshore sandy shelf deposits the basal conglomerate and overlying fluvial facies were derived from the north and include abundant detritus eroded from underlying fig tree group dacitic volcanic rocks shoreline parallel transport and extensive reworking dominate overlying deltaic tidal and marine facies the lithologies and arrangement of moodies group facies sandstone petrology the unconformable relationship between moodies strata and older deformed rocks presence of at least one syndepositional normal fault and presence of basaltic flow rocks and airfall fall tuffs interbedded with the terrestrial strata collectively suggest that the lower moodies group was deposited in one or more intramontane basins in an extensional setting thinner moodies sections south of the inyoka fault generally less than 1000 m thick may be correlative with the basal moodies group north of the inyoka fault and were probably deposited in separate basins a northerly derived southward thinning fan delta conglomerate in the upper part of the moodies group in the central bgb overlies lower strata with an angular unconformity this and associated upper moodies conglomerates mark the beginning of basin shortening by south to southeast directed thrust faulting along the northern margin of the bgb and suggest that the upper moodies group was deposited in a foreland basin timing orientation and style of shortening suggest that this deformation eventually incorporated most of the bgb into a major fold and thrust belt

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

archean and paleoproterozoic crust of the sao francisco craton bahia brazil geodynamic features abstract recent geological geochronological and isotopic studies allow the identification of four important crustal segments in the basement of the sao francisco craton in bahia the oldest is the gaviao block in the wsw part of the studied area and comprises granitic granodioritic and migmatitic continental crust including remnants of 3 4 ga ttgs which are amongst the oldest rocks in south america and are associated with archean greenstone belts the youngest segment is exposed in the itabuna salvador curaca belt which extends from se bahia along the atlantic coast to salvador then northwards into ne bahia it is mainly composed of a low k calc alkaline plutonic suite and also contains strips of intercalated metasediments and ocean floor back arc basin gabbro and basalt in the ssw part of the area the jequie block comprises granulitic migmatites with inclusions of supracrustal rocks intruded by many charnockite plutons in the ne the serrinha block is composed of orthogneisses and migmatites which form the basement for paleoproterozoic greenstone belts during the paleoproterozoic transamazonian cycle these four crustal segments collided resulting in the formation of an important mountain belt the regional metamorphism resulting from the crustal thickening associated with the collision occurred at around 2 0 ga major mineralizations were formed during the evolution of the four archean blocks and also during and after the paleoproterozoic collision

architectural elements and bounding surfaces in fluvial deposits anatomy of the kayenta formation lower jurassic southwest colorado abstract three well exposed outcrops in the kayenta formation lower jurassic near dove creek in southwestern colorado were studied using lateral profiles in order to test recent regarding architectural element analysis and the classification and interpretation of internal bounding surfaces examination of bounding surfaces within and between elements in the kayenta outcrops raises problems in applying the three fold classification of allen 1983 enlarging this classification to a six fold hierarchy permits the discrimination of surfaces intermediate between allen s second and third order types corresponding to the upper bounding surfaces of macroforms and internal erosional reactivation surfaces within the macroforms examples of the first five types of surface occur in the kayenta outcrops at dove creek the new classifications is offered as a general solution to the problem of description of complex three dimensional fluvial sandstone bodies the kayenta formation at dove creek consists of a multistorey sandstone body including the deposits of lateral and downstream accreted macroforms the storeys show no internal cyclicity neither within individual elements nor through the overall vertical thickness of the formation low paleocurrent variance indicates low sinuosity flow whereas macroform geometry and orientation suggest low to moderate sinuosity the many internal minor erosion surfaces draped with mud and followed by intraclast breccias imply frequent rapid stage fluctuation consistent with variable seasonal monsonal ephemmeral flow the results suggest a fluvial architecture similar to that of the south saskatchewan river through with a three dimensional geometry unlike that interpreted from surface studies of that river

medano creek colorado a model for upper flow regime fluvial deposition abstract medano creek in colorado provides a unique opportunity to study upper flow regime deposition it is a steep aggrading fluvial system in which antidunes are the dominant bedform medano creek is a straight single channel stream where flow is largely confined to a sequence of upstream migrating topographic steps pools and riffles a stable channel geometry antidunes and upperflow regime plane beds are arranged in an organized pattern on the surface of pool and riffle sequences as a response to spatially varied flow conditions in the creek antidunes form sedimentary structures consisting of low angle backset and foreset beds and subhorizontal laminae grouped into lenses laminae in a lens are often subparallel to the lens bounding surfaces on the downcurrent end and are truncated y the upper bounding surface on the upcurrent end upper flow regime plane bed and rhomboid ripples produce planar laminae through lateral shifting of the active channel and upstream migration of pools and riffles medano creek produces a facies sequence consisting of alternating 10 40 cm thick layers of planar laminae and low angle cross stratified lenticular bedding

sedimentation patterns during the precambrian a unique record abstract although the similarities between depositional processes and products as well as the analogous controls on basin filling and evolution appear to have enjoyed great uniformity throughout the sedimentary rock record a noticeable distinction exists in the rates and intensities of a broad range of geological processes in the precambrian epoch this paper searches for distinctiveness in the precambrian sedimentary record both siliciclastic and carbonate through an extensive though not exhaustive review of the relevant literature augmented by new observations while differences in precambrian deltaic aeolian glacial and possibly also lacustrine deposits and settings appear to have been small their large scale development was controlled largely by a combination of temporal and geodynamic influences essentially of global compass in this regard the onset of the supercontinent cycle and major perturbations in palaeo atmospheric composition appear to have been significant marine environments provide a poor platform for precambrian phanerozoic comparisons of sedimentation patterns as those from the former period are preserved almost exclusively in epeiric settings an environment essentially lacking on modern earth for the shallow marine carbonates biological mediation of chemical sediment deposition changed radically from dominance by microbial biota in the precambrian to a combination of metazons protozoans and algae for the skeletal carbonates of the phanerozoic despite it being widely recognized that precambrian channel systems were braided in all environments deltaic tidal alluvial fluvial as a consequence of the lack of vegetation and poor development of soils the fluvial setting has some enigmatic aspects amongst these is evidence for ponding of muddy detritus in apparently sandstone bed load dominated braided systems with effects on local palaeoslopes which have resulted in unusual palaeohydraulic parameters for precambrian fluvial systems this is perhaps a field of research which holds greater promise when investigating sedimentation patterns prior to the phanerozoic

forced regressive wedges on a neoproterozoic siliciclastic shelf chandarpur group central india abstract outcrop based sequence analysis across the transition from the chaporadih to the kansapathar formation of chandarpur group chattisgarh supergroup central india resolved a long standing uncertainty on occurrence of depositional discontinuity through undoubted identification of a forced regressive wedge and associated intraformational unconformity process based facies analysis identified eleven different facies types that are grouped into five different facies associations paleoenvironments range from shelf shoreface foreshore and beach tide influenced estuary and delta bounded between the surface of marine erosion at its base and subaerial unconformity marked by paleosol development at its top the coarse grained wedges of shoreface sandstone is interpreted as forced regressive in origin the basinward downstepping of two such regressive wedges bears evidence for lowering of wavebase in connection with relative fall in sea level the valley created during the falling sea level was filled by sediments in lowstand transgressive and highstand depositional motif the aggradational to weakly progradational bedsets of foreshore beach sediments record the history of early slow rise in base level with steepening in the rate of relative sea level rise the coastline transformed from wave dominated to tide dominated wide variability in the balance between sediment supply and accommodation is recorded in the facies types and stacking motif of transgressive estuarine sections at different studied locations the highstand tidal delta is a supply driven normal regressive product at stable or marginally rising base level

sequence concepts at seismic and outcrop scale the distinction between physical and conceptual stratigraphic surfaces abstract the sequence stratigraphic terms maximum flooding surface and downlap surface as currently applied are ambiguous examples of these intra sequence surfaces are summarised from high frequency mid pleistocene sequences and from a cretaceous recent seismic megasequence both from new zealand at any one locality a mid pleistocene sequence contains up to four stratal discontinuities in ascending order the sequence boundary ravinement surface local flooding surface and downlap surface these physical surfaces which occur in outcrop are regionally diachronous and should be differentiated from theoretical isochronous horizons such as the time of maximum flooding horizon corresponding to maximum shoreline transgression and the time of peak eustatic or local relative sea level horizons corresponding to the highpoint of the eustatic and relative sea level cycles respectively in seismic studies the boundary between the transgressive and highstand systems tracts is usually located at the downlap surface on the basis of a major thermo tectonic sea level cycle in the canterbury basin it is shown that the downlap surface is not a single regional surface and that the change in slope associated with toes of successive prograding clinoforms rises in stratigraphic height basinwards the downlap surface therefore does not usually coincide with the maximum flooding horizon in plio pleistocene cyclothems a discrete unit the mid cycle shellbed straddles the contact between the transgressive and highstand systems tracts this unit might be classified within its own systems tract the condensed section systems tract csst alternatively the position of the boundary between the transgressive and highstand systems tracts can remain unspecified or unknowable