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

criterion delineating the mode of headcut migration two modes of headcut migration are generally recognized 1 rotating headcuts that tend to flatten as they migrate and 2 stepped headcuts that tend to retain nearly vertical faces a mathematical description of the sediment detachment potential immediately upstream and downstream of the headcut is used to delineate these modes of migration the delineating parameter is the ratio of the time required to erode the headcut face from above to the time required to undermine the headcut face from below this erosional time scale ratio is a dimensionless function of flow sediment and geometry parameters for the limiting case of homogeneous cohesive soils the time scale ratio is a simple function of a froude number and the aspect ratio of drop height to normal flow depth this relationship is calibrated using original laboratory experiments of headcut migration in initially vertical headcuts and verified by independent field experiments of headcuts propagating in four different homogeneous cohesive soils

stratigraphic architecture palaeogeography and sea level changes of a third order depositional sequence the late turonian early coniacian in the northern iberian ranges and central system spain abstract this paper reports a high resolution stratal palaeogeographical and quantitative analysis of a late turonian early coniacian carbonate ramp the northern iberian ranges and its siliciclastic coastal margin the central system the analyzed succession comprises a complete 3rd order depositional sequence with a shelf margin wedge a transgressive systems tract and a highstand systems tract based on the existence of a subaerial exposure evidences and early diagenetic overprints b stratal relationships onlap offlap toplap regarding underlying and overlying sequences and c changes in the vertical facies evolution the depositional sequence was bounded by two major sequence boundaries these indicate that a major fall in relative sea level occurred in these boundaries the comparative study of the internal stratal surfaces and the vertical facies succession allowed the identification of superimposed higher order depositional parasequences sets and parasequences 4th to 5th order which record short term changes in the relative sea level thus when vertical facies trends in the parasequences are arranged in the framework of a parasequence set a clearer palaeogeographic picture is obtained and palaeogeographic maps can more clearly show the evolutionary trend of the basin each parasequence set was composed of a basal lowstand transgressive systems tract in turn composed of fringing siliciclastic shoreface deposits and offshore muds older sequences were partially exposed further landwards the sedimentation of these sands resulted in the progradation of the coastal systems during times of low relative sea level during which the basal parasequence set boundary was also generated subsequent transgression remobilised underlying lowstand sands the early highstand was composed of carbonate sediments in platform areas and mixed siliciclastic carbonate sediments landwards separated by a narrow fringe of muddy marl offshore sediments the late highstand showed a progressive shallowing trend with carbonate littoral and tidal sediments covering broad areas of the basin and lagoon facies were restricted to the northern outcrops during late highstand underlying transgressive systems tract and older highstand deposits were exposed landwards sea level and accommodation curves have been obtained from paleogeographical sedimentological and sequence stratigraphical data the mean sea level curve for the entire 3rd order sequence shows a rise of the sea level and then an overall stillstand but accommodation curve shows a positive trend along the sequence the 3rd order sea level cycle has a mean amplitude of 52 m and high frequency cycles have amplitudes of 20 40 m 4th order and about 1 5 4 m 5th order the mean final rise in the sea level 46 6 m is quite similar to the total accommodation deduced from fischer plot 45 75 m the sea level cycles are considered to be eustatic in origin and driven by orbital forcing in the milankovitch band based on the estimated 4th order 400 kyr and 5th order cycle lengths 80 kyr

microbial carbonates the geological record of the calcified bacterial algal mats and biofilms summary deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the archaean geologically best known in seas and lakes microbial carbonates are also important at the present day in fluviatile spring cave and soil environments the principal organisms involved are bacteria particularly cyanobacteria small algae and fungi that participate in the growth of microbial biofilms and mats grain trapping is locally important but the key process is precipitation producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation various metabolic processes such as photosynthetic uptake of co2 and or hco3 by cyanobacteria and ammonification denitrification and sulphate reduction by other bacteria can increase alkalinity and stimulate carbonate precipitation extracellular polymeric substances widely produced by microbes for attachment and protection are important in providing nucleation sites and facilitating sediment trapping microbial carbonate microfabrics are heterogeneous they commonly incorporate trapped particles and in situ algae and invertebrates and crystals form around bacterial cells but the main component is dense clotted or peloidal micrite resulting from calcification of bacterial cells sheaths and biofilm and from phytoplankton stimulated whiting nucleation interpretation of these texturally convergent and often inscrutable fabrics is a challenge conspicuous accumulations are large domes and columns with laminated stromatolite clotted thrombolite and other macrofabrics which may be either agglutinated or mainly composed of calcified or spar encrusted microbes stromatolite lamination appears to be primary but clotted thrombolite fabrics can be primary or secondary microbial precipitation also contributes to hot spring travertine cold spring mound calcrete cave crust and coated grain deposits as well as influencing carbonate cementation recrystallization and replacement microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water and thus relies uniquely on a combination of biotic and abiotic factors this overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas and in the past by temporal variation in abundance of marine microbial carbonates patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry stromatolites appeared at 3450 ma and were generally diverse and abundant from 2800 to 1000 ma inception of a proterozoic decline variously identified at 2000 1000 and 675 ma has been attributed to eukaryote competition and or reduced lithification thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the palaeozoic and reappeared in the late devonian microbial carbonates retained importance through much of the mesozoic became scarcer in marine environments in the cenozoic but locally re emerged as large agglutinated domes possibly reflecting increased algal involvement and thick micritic reef crusts in the late neogene famous modern examples at shark bay and lee stocking island are composite coarse agglutinated domes and columns with complex bacterial algal mats occurring in environments that are both stressed and current swept products of mat evolution ecological refugia sites of enhanced early lithification or all three

laboratory experiments on the influence of slope length on runoff percolation and rill development experiments to test relationships between slope length percolation and runoff were carried out in a laboratory flume under simulated rainfall at intensities from 24 2 26 mm h 1 a composite soil subject to sealing was mixed from a clay and a sand and tested on 2 39 m 7 18 m and 17 m slopes for a minimum of 200 min runoff discharge is not a simple function of rainfall excess and slope length but shows a complex pattern dominated by surface sealing rill development and headcut incision rill development by concentrated surface wash conformed to established threshold hydraulic conditions but subsequent headcut incision was necessary to breach the seal and significantly affect percolation runoff ratios headcut evolution is complex apparently reflecting hydraulic instability and possibly different stages in seal development headcut and rill incision shows a cyclic pattern interspersed with broad areas of sheetwash and colluvial deposition where percolation rates are very low

integrating facies and structural analyses with subsidence history in a jurassic cretaceous intraplatform basin outcome for paleogeography of the panormide southern tethyan margin nw sicily italy abstract we illustrate the tectono sedimentary evolution of a jurassic cretaceous intraplatform basin in a fold and thrust belt present setting cala rossa basin detailed stratigraphy and facies analysis of upper triassic eocene successions outcropping in the palermo mts nw sicily integrated with structural analysis restoration and basin analysis led to recognize and describe into the intraplatform basin the proximal and distal depositional areas respect to the bordered carbonate platform sectors carbonate platform was characterized by a rimmed reef growing with progradational trends towards the basin as suggested by the several reworked shallow water materials interlayered into the deep water succession more the occurrence of thick resedimented breccia levels into the deep water succession suggests the time and the characters of synsedimentary tectonics occurred during the late jurassic the study sections involved in the building processes of the sicilian fold and thrust belt were restored in order to obtain the original width of the cala rossa basin useful to reconstruct the original geometries and opening mechanisms of the basin basin analysis allowed reconstructing the subsidence history of three sectors with different paleobathymetry evidencing the role exerted by tectonics in the evolution of the narrow cala rossa basin in our interpretation a transtensional dextral lower jurassic fault system wnw ese present day oriented has activated a wedge shaped pull apart basin in the frame of the geodynamic evolution of the southern tethyan rifted continental margin the cala rossa basin could have been affected by jurassic transtensional faults related to the lateral westward motion of africa relative to europe

new material of the biomineralizing tubular fossil sinotubulites from the late ediacaran dengying formation south china abstract sinotubulites is a late ediacaran biomineralizing tubular fossil with a probable animal affinity it is characterized by millimeter to centimeter sized and multi layered tubes open at both ends the tube consists of two morphologically different walls a multi layered inner wall with weak ornamentations and a multi layered outer wall with transverse or oblique corrugations and sometimes longitudinal ridges the majority of previously published sinotubulites species are considered as synonymous with the type species s baimatuoensis three new species s triangularis n sp s pentacarinalis n sp and s hexagonus n sp are reported from the late ediacaran beiwan member of the dengying formation in southern shaanxi province south china the three new species are similar to the type species in having nested multilayered inner and outer tube walls however they are different in their polygonal cross sections and longitudinal ridges s baimatuoensis is more or less circular in cross section and lack longitudinal ridges on the outer tube wall whereas s triangularis s pentacarinalis and s hexagonus are respectively triangular pentagonal and hexagonal in cross section with three five and six longitudinal ridges on the exterior surface of the outer wall the new material adds to the diversity of late ediacaran biomineralizing animals the triradial pentaradial and hexaradial tubes of s triangularis s pentacarinalis and s hexagonus share some intriguing similarities in body symmetry with several early cambrian tubular fossils although these cambrian tubes are not open at both ends still it would be interesting to explore the tantalizing possibility of evolutionary continuity of triradial pentaradial and hexaradial tubular animals across the precambrian cambrian boundary

the terminal ediacaran tubular fossil cloudina in the yangtze gorges area of south china abstract the advent of biomineralizing metazoans in the terminal ediacaran age ca 550 539 ma represents a remarkable biological innovation in the history of life as a poster child of this evolutionary episode cloudina is widely regarded as a weakly biomineralizing tubular fossil with a global distribution therefore cloudina can both inform the evolution of animal biomineralization and facilitate terminal ediacaran stratigraphic correlation however this key taxon has not been fully described from the yangtze gorges area of south china where classical terminal ediacaran strata have been investigated extensively by paleontologists stratigraphers and geochemists here we document an assemblage of three dimensionally silicified tubular fossils from siliceous dolostone of the terminal ediacaran baimatuo member of the dengying formation in the yangtze gorges area western hubei province south china the baimatuo assemblage consists of sinotubulites which has been previously known in the baimatuo member as well as cloudina including c ningqiangensis c hartmannae c cf carinata and c sp indet and other unnamed tubular forms this discovery adds to the diversity of early biomineralizing metazoans in the yangtze gorges area and facilitates the biostratigraphic correlation of the cloudina sinotubulites co occurrence assemblage in terminal ediacaran strata

reconstructing sea level change from the internal architecture of stromatolite reefs an example from the mesoproterozoic sulky formation dismal lakes group arctic canada the mesoproterozoic dismal lakes group arctic canada contains a relatively thin yet regionally extensive stromatolitic reef complex that developed subtidally during a major transgression shoaled to sea level and was overlain by intertidal to supratidal carbonate and evaporite strata the september lake reef complex exhibits a complex internal architecture that records the interaction between stromatolite growth and changes in accommodation space derived from both higher order 4th or 5th order parasequence scale changes in sea level and the variable bathymetry of the sea floor reef growth which was initiated during three sea level cycles records progressive marine transgression over depositional lows that were formed during pre reef subaerial exposure and erosion of the underlying strata a fourth sea level cycle represented by spectacular coniform stromatolites with 10 m of synoptic relief marks a more dramatic rise in sea level and establishment of the main reef complex aggradation and ev

the origin of high frequency platform carbonate cycles and third order sequences lower ordovician el paso gp west texas constraints from outcrop data and stratigraphic modeling abstract the passive margin succession of the diablo platform lower ordovician west texas is represented by the second order sauk c supersequence set consisting of a basal transgressive clastic unit the bliss sandstone above the breakup unconformity marking the second order basal lowstand transgressive phase overlain by 750 m of drift related shallow marine platform carbonates the el paso group recording the second order highstand due to late paleozoic structuring of the gondwanan passive margin present exposures in texas are in an updip shelf position and lack internal stratal geometries across depositional strike so sequences and systems tracts are identified solely by the vertical stacking patterns of depositional subfacies and higher frequency fifth order cycles accommodation plots fischer plots of high frequency cycles gauge systematic shifts in third order accommodation of two complete third order sequences each about 2 m y long and 60 140 m thick within the el paso group this is expressed in the vertical succession of cycle types systematic changes in cycle thickness and variations in subfacies as revealed by histograms of subfacies types tied to fischer plots a complete el paso shelf sequence contains a thin lowstand systems tract of quartz arenite a thick transgressive systems tract dominated by upward thickening thrombolitic subtidal cycles and a highstand systems tract marked by dolomitic thinning upward peritidal cycles containing quartz sand the el paso third order depositional sequences have been correlated by biostrat graphy and cycle stacking pattern from the franklin mountains southeast to the diablo arch northeast into the ardmore basin and thence to the appalachian basin hence the third order sequences appear to be eustatic based on biostratigraphic correlation of accommodation plots around the periphery of gondwana time series analyses suggest that a strict allocyclic milankovitch driven glacio eustatic mechanism alone is insufficient to account for the higher frequency fifth order and lower frequency third order accommodation cycles we used a two dimensional forward computer simulation to compare allocyclic and autocyclic models for high frequency cycles the allocyclic simulation superimposes a deterministic milankovitch glacio eustatic driver on a lower frequency third order accommodation cycle and generates a two dimensional synthetic cross section composed of high frequency peritidal and subtidal cycles stacked into third order sequences the autocyclic simulation is generated by a ginsburg autocyclic mechanism for high frequency cyclicity triggered by third order accommodation changes the autocyclic simulation quantitatively replicates an autocyclic mechanism for fifth order cycle development linked to onshore sediment transport and seaward progradation of tidal flats the two dimensional internal facies architecture of a third order sequence and the high frequency cycle stacking patterns are simulated in both scenarios important differences in patterns of cycles and subfacies stacking in the simulations shed light on the interpretation of the diablo platform cycles and sequences as an alternative or a supplement to autocyclicity or milankovitch forced glacio eustasy we have studied stochastic models for driving high frequency changes in relative sea level these intriguing results maintain essentially all the features observed in the rock record our smoothed sto hastic simulations suggest that the cycle problem is much more complex than presently realized

cyclic ramp to basin carbonate deposits lower mississippian wyoming and montana a combined field and computer modeling study abstract the lower mississippian lodgepole lower madison formations 20 225 m thick developed along a broad 700 km storm dominated pericratonic ramp three types of fifth order upward shallowing cycles are recognized across the ramp to basin transition peritidal cycles consist of very shallow subtidal facies overlain by algal laminated tidal flat deposits which are rarely capped by paleosol breccia layers shallow subtidal cycles consist of stacked ooid grainstone shoal deposits or deeper subtidal facies overlain by ooid skeletal grainstone caps deep subtidal cycles located along the outer ramp consist of basal sub storm wave base limestone argillite overlain by storm deposited limestone which are capped by hummocky stratified to massive skeletal ooid grainstone deep subtidal c cles pass downslope into ramp slope facies composed to rhythmically interbedded limestone and argillite with local deep water mud mounds no upward shallowing cycles occur within the ramp slope facies average cycle durations calculated along the outer ramp are between 30 110 ky the fifth order cycles are stacked to form three third to fourth order depositional sequences which are recognized by regional transgressive regressive facies trends and cycles stacking patterns ramp margin wedges rmw developed during long term sea level fall and lowstand and consist of cyclic crinoidal bank and oolitic shoal facies which pass downdip into deep subtidal cycles transgressive systems tracts tst which onlapped the ramp during long term sea level rise include thick shallow and deep subtidal cycles peritidal cycles are restricted to the inner ramp highstand systems tracts hst developed during long term sea level highstand and fall and along the ramp are composed of early hst shallow subtidal cycles overlain by late hst peritidal cycles shallow through deep subtidal cycles characterize the hst along the ramp slope two dimensional computer modeling of the cyclic sequences suggests that for the assumed water depths of facies fifth order sea level oscillations of at least 20 25 m were required to generate deep subtidal cycles along the ramp slope synthetic sequences run with fifth order sea level amplitudes end page 1194 and tst deposition a feature not observed in the actual sequences other factors in addition to fifth order sea level oscillations likely played a role in generating synchronous peritidal and deep subtidal cycles during hst deposition these factors may include short term climatic changes which influenced the depths to storm wave reworking the moderate amplitude sea level oscillations suggested by the cyclic sequences may reflect the initial effects of carboniferous glaciation that was occurring in gondwana

paired carbonate and organic carbon isotope variations of the ediacaran doushantuo formation from an upper slope section at siduping south china abstract intensive carbon isotope analyses from the doushantuo formation ca 635 551 ma in south china reveal decoupled carbonate and organic carbon isotopes 13ccarb and 13corg that have been interpreted as recording a large dissolved organic carbon doc reservoir in the ediacaran ocean or chemoautotrophic methanotrophic recycling modification of organic matter in a redox stratified ocean existing paired 13ccarb 13corg data however are mostly from sections of an intrashelf lagoon near the yangtze gorges area where basin restriction may have resulted in local isotope departure from the open ocean to better understand the spatial and temporal variations of the ediacaran 13c excursions we have conducted high resolution paired 13ccarb 13corg analyses of the doushantuo formation from an upper slope section at siduping hunan province south china the data reveal three negative 13ccarb excursions at the basal middle and upper doushantuo formation two prominent negative 13corg anomalies are also observed in black shales and shaly dolostones at the base and top of the doushantuo formation for the majority of the doushantuo formation however 13ccarb and 13corg values are decoupled similar to those observed in the yangtze gorges area both 13ccarb and 13corg show large up to 10 variations among shallow and deep water sections that are consistent with a large carbon isotope gradient in a stratified ediacaran ocean the large 13ccarb gradient between shallow and deep water sections implies the availability of sulfate in deep water environments where the addition of authigenic carbonate films cements into surface ocean derived carbonate particles in anoxic water column or pore water led to more negative 13ccarb values significant spatial 13corg variations suggest that the decoupled 13ccarb 13corg may not have been resulted from direct buffering of a large doc reservoir but from recycling of organic matter by chemoautotrophs and methanotrophs compilation of available paired 13ccarb 13corg data from the yangtze platform reveals alternating intervals of coupled and decoupled 13ccarb 13corg values up to the early cambrian 520 ma intervals with coupled 13ccarb and 13corg coincide with evidence for ocean oxygenation events this phenomenon may be explained by episodic growth and oxidation of a moderate doc reservoir 6 12 times of the modern ocean the upper doushantuo 13ccarb anomaly temporally crosses a major stratigraphic unconformity indicating that part of the upper doushantuo shuram 13ccarb excursion was formed during sea level regression large spatial and temporal variations of the upper doushantuo 13ccarb excursion may record a mixed signature of open ocean seawater overprinted by facies dependent isotope variation stratigraphic truncation and authigenic diagenetic modification along the unconformity the real primary 13ccarb excursion may be shorter lived than previously thought

the influence of bedrock geology on knickpoint development and channel bed degradation along downcutting streams in south central indiana geologic controls on knickpoint development and the mechanics of channel bed degradation were eluci dated by comparison of downcutting streams in south central indiana developed in lithologically homoge neous carbonate units and layered siliciclastic strata knickpoints formed in a single lithologic unit are stepped each step consists of a knickpoint face and tread the height of each step is equal to and controlled by bed thickness spatially knickpoints in homogeneous terrain are limited to jointed strata that dip 1 upstream or 2 downstream at angles less than the channel gradient knickpoints observed in lithologically layered strata are characterized by a single step in which resistant jointed rocks overlie and protect weaker units total knickpoint relief is influenced by thickness of the weaker strata knickpoints in layered siliciclastic strata are spatially limited to bedrock reaches produced by differential rates of channel incision knickpoint distribution is also influenced by joint

hydraulic conditions for rill incision under simulated rainfall a laboratory experiment a series of controlled laboratory experiments were conducted in order to obtain precise data on the hydraulic and sediment transport conditions during rill formation tests were carried out using a crusting prone binary mixed soil in a 15 m long flume at an average slope of 0 087 under simulated rainfall rainfall intensities varied from 30 35 mm h 1 and developed about 70 per cent of the kinetic energy of natural rainfall of similar intensity runoff and sediment discharge measured at the downstream weir were strongly influenced by rill forming processes essentially rill incision reduced runoff discharge as a result of increased percolation in rill channels but increased sediment discharge secondary entrainment processes such as bank collapse also increased sediment discharge at the weir knickpoint bifurcation and colluvial deposition however decreased sediment discharge rills always developed through the formation of a knickpoint the critical condition for knickpoint initiation was the development of supercritical flow and waves which mould and incise the bed prior smoothing of the soil surface by entrainment and redistribution of sediment facilitated supercritical flow statistical analysis showed that hydraulic and sediment transport conditions differed significantly in rilled and unrilled flows the relationship between sediment discharge rill erosion and flow hydraulics was found to be nonlinear conforming to a standard power function in the form y axb rills were also associated with significantly increased sediment transport capacities however rill initiation was not clearly defined by any specific hydraulic threshold instead rilled and unrilled flows were separated by zones of transition within which both types of flow occur

birth and demise of a middle jurassic isolated shallow marine carbonate platform on a tilted fault block example from the southern iberian continental palaeomargin abstract subbetic middle jurassic oolitic limestones of the jabalcuz formation crop out in san cristobal hill near jaen city andalucia spain between hemipelagic limestone and marl successions the jabalcuz limestones range in facies from calcareous breccias and micritic limestones to white cross bedded oolitic limestones recent erosion has exhumed a jurassic isolated shallow water carbonate platform on the san cristobal hill this shallow platform developed on a tilted fault block an almost continuous laterally extensive outcrop reveals tectono sedimentary features distinctive of block tilting in the different margins of the fault block the studied sections represent various palaeogeographic positions in the ancient shallow water carbonate platform and basin transition this exceptional outcrop allows to decipher the triggering mechanisms of the birth evolution and drowning of this jurassic isolated shallow water carbonate platform two shallowing upward depositional sequences separated by flooding surfaces can be distinguished on two different sides of the fault block in the southeastern part of the outcrop proximal sections grade vertically from distal talus fault breccias with bivalve and serpulid buildup intercalations to white cross bedded oolitic limestones defining the lowermost depositional sequence upwards overlying a flooding surface the second sequence with oolitic limestones prograding over micritic deposits is recorded in the southwest oolitic peloidal and more distal micritic facies alternate with notable southeastern progradation of oolitic facies in the upper part of the section which represents the upper depositional sequence the top of this second depositional sequence is another flooding surface recorded by the sedimentation of marls with radiolarians from the overlying formation in the northwestern outcrops the two depositional sequences are also almost completely preserved and can be differentiated a 100 m thick succession composed of a basal slumped unit of micritic and peloidal limestones overlain by cross bedded oolitic limestones represents the lowermost sequence the superposition of a newer slumped unit of micritic and peloidal limestones on the cross bedded oolitic limestones defines the lower part of the uppermost depositional sequence finally the central sections are less thick and comprise a facies succession consisting almost entirely of oolitic limestones locally oolitic limestones often show cross bedding and prograding clinoforms the cross bedding has centrifugal directions around the perimeter outcrops of san cristobal hill whereas in the centre palaeocurrents are highly variable the sense of the palaeoslopes as deduced from slumps also fits with a centrifugal pattern finally in the southwestern outcrops the demise of the carbonate platform is marked by backstepping oolitic facies the presence of lumachelle facies and onlapping hemipelagic facies all the data point to the birth evolution and drowning of an isolated shallow water carbonate platform that developed on a tilted fault block the fault block tilting controlled the development of talus breccias in the southeastern part and of slumps in the northwest dipping hangingwall ramp in the shallowest environments produced by the tilting uppermost part of the fault block a shallow marine carbonate platform developed carbonate production mainly oolitic kept up with relative sea level fluctuations and extended the platform at least to the northwest southeast and southwest based on observable data finally tectonics and sea level fluctuations drowned this carbonate platform

the origin of decoupled carbonate and organic carbon isotope signatures in the early cambrian ca 542 520 ma yangtze platform abstract the early cambrian ca 542 520 ma strata in south china record two prominent negative carbonate carbon isotope 13ccarb excursions of early nemakit daldynian n d and early tommotian ages across each of these excursions carbonate and organic carbon isotopes 13ccarb and 13corg are strongly decoupled regional correlation across a shelf to basin transect shows lateral heterogeneity of 13corg during the early middle n d but more homogenized 13corg values across the basin during the late n d and tommotian the temporal and lateral variations in 13corg suggest that decoupled 13ccarb and 13corg across the n d 13ccarb excursion were possibly caused by diagenetic alteration of organic matter and or amplification of detrital organic carbon isotope signature in low toc carbonates in contrast decoupled 13ccarb and 13corg of the upper n d and tommotian were likely resulted from chemoautotrophic methanotrophic biomass contribution to toc in organic rich black shale and carbonates the decoupled 13ccarb 13corg pattern from the lower n d strata ca 542 ma shows striking similarities with those from the basal ca 635 ma and upper ca 551 ma doushantuo formation in all three cases decoupled 13ccarb 13corg are seen in organic poor carbonates toc 0 1 and coupled 13ccarb 13corg occur in organic rich black shale and carbonates at the end of the negative 13ccarb excursion these similarities suggest that the shift from decoupled to coupled 13ccarb 13corg has no causal link with the terminal oxidation of a large oceanic doc reservoir given the pervasive anoxia euxinia in ediacaran early cambrian oceans local doc rich environments may have been common but a large oceanic doc reservoir capable of buffering the 13c of marine organic matter requires independent evidence

flat pebble conglomerate its multiple origins and relationship to metre scale depositional cycles carbonate flat pebble conglomerate is an important component of precambrian to lower palaeozoic strata but its origins remain enigmatic the upper cambrian to lower ordovician strata of the snowy range formation in northern wyoming and southern montana contain abundant flat pebble conglomerate beds in shallow water cyclic and non cyclic strata several origins of flat pebble conglomerate are inferred for these strata in one case all stages of development of flat pebble conglomerate are captured within stormdominated shoreface deposits of hummocky cross stratified hcs fine carbonate grainstone a variety of synsedimentary deformation structures records the transition from mildly deformed in situ stratification to buckled beds of partially disarticulated bedding to fully developed flat pebble conglomerate these features resulted from failure of a shoreface and subsequent brittle and ductile deformation of compacted to early cemented deposits failure was induced by either storm or seismic waves and many beds failed along discrete slide scar surfaces centimetre scale laminae within thick amalgamated hcs beds were planes of weakness that led to the development of platy clasts within partly disarticulated and rotated bedding of the buckled beds in some cases buckled masses accelerated downslope until they exceeded their internal friction completely disarticulated into clasts and transformed into a mass flow of individual cm to dm scale clasts this transition was accompanied by the addition of sand sized echinoderm rich debris from local sources which slightly lowered friction by reducing clast clast interactions the resulting dominantly horizontal clast orientations suggest transport by dense viscous flow dominated by laminar shear these flows generally came to rest on the lower shoreface although in some cases they continued a limited distance beyond fairweather wave base and were interbedded with shale and grainstone beds the clasts in these beds show no evidence of extensive reworking i e not well rounded or condensation i e no rinds or coatings a second type of flat pebble conglomerate bed occurs at the top of upward coarsening metre scale cycles the flat pebble conglomerate beds cap these shoaling cycles and represent either lowstand deposits or in some cases may represent transgressive lags the clasts are well rounded display borings and have iron rich coatings the matrix to these beds locally includes glauconite these beds were considerably reworked and represent condensed deposits thrombolites occur above the flat pebble beds and record microbial growth before initial transgression at the cycle boundaries a third

carbon isotope variability across the ediacaran yangtze platform in south china implications for a large surface to deep ocean δ13c gradient new isotope data obtained from relatively conformable carbonate rich strata of the ediacaran yangtze platform in south china reveal substantial 13 c variability in platform sections four negative 13 c anomalies with a nadir down to 8 pdb are present in the interval between the cap carbonate level 635 ma and the precambrian cambrian boundary 542 ma while in slope and basinal sections 13 c values are negative through the entire doushantuo formation 635 551 ma if these 13 c values are close to their primary seawater signature they imply a strong 10 surface to deep ocean 13 c gradient that is consistent with long term deep ocean anoxia and the presence of a large dissolved organic carbon doc reservoir the two prominent negative 13 c excursions within the doushantuo formation above the cap carbonate level are associated with shoaling and local exposure of the platform the anomalies may thus record remineralization of a large oceanic doc pool via sulfate reduction that transferred 13 c depleted carbon from the oceanic doc reservoir to the surface ocean during regression inconsistencies in ediacaran 13 c profiles globally and variations in south china in particular highlight the need for further evaluation of local departures in 13 c from an inferred average seawater signature

relation of eustasy to stacking patterns of meter scale carbonate cycles late cambrian u s a abstract an interbasinal study of late cambrian cyclic carbonate successions in the appalachian and cordilleran passive margins suggests that superimposed orders of eustasy controlled the development of large scale depositional sequences and the component peritidal to subtidal meter scale cycles that comprise them the focus of this paper is on the small scale cyclicity its probable control by milankovitch forced sea level oscillations and how stacking patterns of meter scale cycles can be used to define internal components of larger scale sequences and estimate variations in relative sea level fining upward peritidal cycles showing evidence of episodic emergence grade seaward into coarsening upward subtidal cycles which lack evidence of emergence and form a continuum across the cambrian carbonate platforms eustasy appears to exert the dominant control on the simultaneous development of peritidal and subtidal cycles on late cambrian carbonate platforms evidence for milankovitch forcing of glacio eustatic sea level oscillations is provided by a 4 1 bundling of fifth order meter scale cycles 96 ky within fourth order cycles spanning tens of meters 440 ky within the big horse member of the orr formation in the house range of utah the 4 1 bundling may manifest the short e centricity to long eccentricity ratio of the milankovitch astronomical rhythms systematic changes in the stacking patterns of meter scale cycles can be used in conjunction with fischer plots to define long term sea level cycles on fischer plots of peritidal cyclic successions long term relative sea level rises are characterized by thick subtidal dominated cycles with thin laminite caps long term relative sea level falls are defined by stacks of thin laminite dominated cycles that show brecciated cycle caps and quartz sands toward the relative sea level lowstand on fischer plots of dominantly subtidal cyclic successions long term sea level rise is characterized by storm dominated open marine carbonate cycles or thick deep ramp shale based cycles falling segments of the fischer plot are characterized by thin shallow subtidal cycles compos d of restricted lithofacies cycle stacking patterns parasequence sets provide the crucial link between the meter scale cycles parasequences and the larger scale sequences and their component systems tracts one and two dimensional models of peritidal and subtidal cycle development indicate that whereas peritidal cycle thickness is primarily controlled by accommodation space deeper subtidal cycle thickness is primarily controlled by sedimentation rate lithofacies within peritidal cycles reflect the sedimentologic response to fluctuations in sea level lithofacies within subtidal cycles may be related to fluctuations in the zones of fairweather and storm wave reworking that oscillated in harmony with sea level fluctuations and may have acted as a subtidal limit to upward aggradation the 2 d modelling illustrates how stacked peritidal to deep subtidal carbonate cycles with thicknesses compositions and stacking patterns similar to the late cambrian of north america can be generated by ilankovitch driven composite eustasy

thick skinned tectonics closing the rifian corridor abstract tectonic processes in the gibraltar region are associated with africa iberia convergence and the formation of the betic rif orogenic system the late miocene shortening recorded in the rif orogen resulted in gradual shallowing and eventual closure of the rifian corridor a narrow marine gateway connecting the atlantic ocean with the mediterranean sea this closure is associated with paleoenvironmental changes that ultimately led to the mediterranean messinian salinity crisis here we present a structural analysis based on a combination of field kinematic data and interpretation of reflection seismic lines acquired for petroleum exploration to understand the deformational phases associated with the closure of the rifian corridor we show the succession of three late miocene to present day events an initial thin skinned nappe thrusting followed by regional subsidence and continued by thick skinned contraction the transition from in sequence thin skinned tectonics during subduction to thick skinned contraction during continental collision resulted in significant acceleration of tectonic uplift and associated exhumation this is related to a change in the regional deformation linked to plate convergence but possibly also coupled with deep lithospheric or dynamic topography processes such a mechanism is also common for other mediterranean orogens during late stages of slab retreat where accelerated tectonics resulted in rapid sedimentation and associated basins evolution we conclude that the thick skinned contraction in the rif orogeny initiated in the late tortonian has created a cumulative uplift in the order of 1 km and provided high enough uplift rates to close the rifian corridor

tectono depositional pattern and evolution of the middle yangtze platform south china during the late ediacaran abstract the depositional environments and evolution of the yangtze block during the ediacaran cambrian transition remain uncertain preventing a thorough understanding of the coeval ocean stratification redox heterogeneity and organism distribution in this study detailed facies and sequence stratigraphic analyses were carried out on the dengying formation and its correlatives to better constrain the tectono depositional pattern and evolution of the middle yangtze area in the late ediacaran twenty five lithofacies were identified in nineteen sections and grouped into six depositional facies intrashelf basin intrashelf shoal restricted platform interior platform margin upper middle slope and lower siliceous slope basin the spatiotemporal distributions of these depositional facies reveal an isolated rimmed middle yangtze carbonate platform embayed with a westward opening intrashelf basin this intrashelf basin gradually became wider and deeper westwards linking to a nne trending basinal seaway that separated the middle yangtze platform from its upper yangtze counterpart the enclosure of the marginal shoal reef complex with an intrashelf shoal chain formed a large scale restricted lagoon and associated tidal flat in the platform interior based on key sequence boundaries and cycle stacking patterns two and a half transgressive regressive sequences were distinguished in the dengying formation and its correlatives in the twelve representative sections furthermore these sequences can be correlated well with those identified in other upper ediacaran successions elsewhere in upper yangtze namibia india and oman indicating a eustatic origin for these sequences however the complex platform basin configuration with significant facies and thickness variations reflects a transtensional tectonic regime in the middle yangtze area during the late ediacaran the abrupt changes in lithofacies and thickness across the southern principal platform margin and those from sandy shoals banks to storm influenced shelf along the northern portion of the intrashelf basin in the three gorges western hubei suggest synthetic extensional faulting across the margins during deposition the two synthetic extensional fault zones might have been induced by nne trending transtensional faulting along the eastern margin of the upper yangtze platform this structural pattern fundamentally controlled the depositional differentiation of the middle yangtze platform during the late ediacaran and also laid the foundation for the subsequent facies distribution and depositional evolution during the early cambrian terreneuvian epoch

tidal channel levee and crevasse splay deposits from a cambrian tidal channel system a new mechanism to produce shallowing upward sequences abstract two cliff exposures 1 km and 0 7 km long and 10 m high of the middle upper cambrian waterfowl formation were mapped in detail on the costigan thrust in southwestern alberta the formation here comprises 5 rock types intraformational conglomerate cross stratified grainstone wavy bedded grainstone mudstone massive dolomite mudstone and prism cracked cryptmicrobial laminated mudstone these rock types form a grainstone body in the upper 7 m of the outcrops and 4 fining upwards sequences in the lower 3 m the grainstone body is divided into 5 storeys each of which has a basal erosion surface overlain by an intraformational conglomerate with interbedded thrombolites storeys contain lateral accretion bedding that is composed of decimeter thick bedsets which are inclined up to 12 d grees relative to the basal erosion surfaces the fining upwards sequences comprise basal cross stratified grainstone overlain by wavy bedded grainstone mudstone that is capped by prism cracked cryptmicrobial laminated mudstones three fining upwards sequences are laterally continuous however one fining upward sequence pinches out along the outcrop face we interpret the grainstone body as the deposits of laterally migrating sinuous tidal channels it is not clear whether the stacking of channel deposits into storeys represents superimposed channel bar and channel fill deposits from a single channel belt or superimposed channel belts the fining upwards sequences may be shallowing upwards sequences in the sense of james 1984 and represent aggrading prograding tidal flat deposits a ternatively these sequences may be crevasse splay and levee deposits analogous to fluvial overbank flood sequences in this case these sequences may not be related to eustatic sea level changes although they resemble classic shallowing upwards sequences and punctuated aggradational cycles

a composite reference section for terminal proterozoic strata of southern namibia integrated sequence stratigraphic and chemostratigraphic data yield a framework for correlations of stratigraphic units in the terminal proterozoic to cambrian witvlei and nama groups of namibia coupled with precise u pb zircon age constraints these correlations make it possible to construct a composite reference section for use in calibrating terminal proterozoic chronostratigraphy the namibian reference section starts with two distinct glacial horizons and extends up to within 1 million years of the proterozoic cambrian boundary the two glacial horizons may represent each of two distinct varanger age glaciations better known from the north atlantic region from the higher of the two glacial horizons up the composite stratigraphy preserves one of the thickest and most complete available records of carbon isotope variability in post varanger terminal proterozoic seawater four carbon isotope chemostratigraphic intervals are recognized 1 a postglacial negative 13 c excursion npg interval 2 a rising interval pr interval of increasing positive delta 13 c values 3 a falling interval pf interval characterized by decreasing positive 13 c and culminating in near zero or negative values and 4 an interval of moderately positive relatively invariant 13 c values i interval that extends up to the unconformity that contains the proterozoic cambrian boundary each of these chemostratigraphic intervals can be recognized in widely separated correlative sections around the world by comparing sediment accumulation rate in the radiometrically calibrated namibian stratigraphy with sediment accumulation rates in correlative sections in arctic canada and oman a maximum age of 564 ma is estimated for the end of the younger varanger glaciation 25 m y younger than previous estimates

variations in architecture and cyclicity in fault bounded carbonate platforms early miocene red sea rift nw saudi arabia abstract the early miocene was a period of active rifting and carbonate platform development in the midyan peninsula nw saudi arabia however there is no published literature available dealing with the detailed characterization of the different carbonate platforms in this study area therefore this study aims to present new stratigraphic architectural models that illustrate the formation of different carbonate platforms in the region and the forcing mechanisms that likely drove their formation this study identified the following features formed during active rifting a a late aquitanian n4 fault block hangingwall dipslope carbonate ramp b a late burdigalian n7 n8 isolated normal fault controlled carbonate platform with associated slope deposits and c a late burdigalian n7 n8 attached fault bounded platform with reef buildups rimmed shelf developed on a footwall fault tip within a basin margin structural relay zone that formed coinciding with the second stage of rifting variations in cyclicity have been observed within the internal stratigraphic architecture of each platform and also between platforms high resolution sequence stratigraphic analysis shows to be parasequences the smallest depositional packages metre scale cycles within the platforms the hangingwall dipslope carbonate ramp and the attached platform demonstrate aggradational progradational parasequence stacking patterns these locations appear to have been more sensitive to eustatic cyclicities despite the active tectonic setting the isolated fault controlled carbonate platform reveals disorganized stratal geometries in both platform top and slope facies suggesting a more complex interplay of rates of tectonic uplift and subsidence variation in carbonate productivity and resedimentation of carbonates such that any sea level cyclicity is obscure this study explores the interplay between different forcing mechanisms in the evolution of carbonate platforms in active extensional tectonic regions characterization of detailed parasequence scale internal architecture allows the spatial variation in syn depositional relative base level changes to be inferred and is critical for understanding the development of rift basin carbonate platforms such concepts may be useful for the prediction of subsurface facies relationships beyond interwell areas in hydrocarbon exploration and reservoir modeling activities

sedimentary evolution of the ediacaran yangtze platform shelf hubei and hunan provinces central china abstract the development of the ediacaran doushantuo formation 635 551 ma on the yangtze platform is studied to analyse the environmental changes on this platform accompanying the ediacaran bioradiation in this study we will present a detailed facies analysis and propose a tentative stratigraphic analysis of seven selected sections that are located predominantly on the platform shelf situated in the hubei province the doushantuo yangtze platform displays two major depositional environments a rimmed carbonate platform and an open shelf the shift from one depositional model to another may be related to the paleotopography in horsts and grabens inherited from previous extensional movements the strata observed in the shelf deposits of the doushantuo formation are organised in three stacked facies successions these shallowing upward facies successions may be correlatable with other ediacaran sections worldwide

redox architecture of an ediacaran ocean margin integrated chemostratigraphic δ13c δ34s 87sr 86sr ce ce correlation of the doushantuo formation south china abstract early diagenetic silicification and phosphatization of the ediacaran doushantuo formation ca 635 to 551 ma in south china offer extraordinary taphonomic windows into the early evolution of multicellular eukaryotes including various algal groups and potentially animals in order to understand how the ecological and taphonomic distribution of these ediacaran eukaryotes was controlled by oceanic redox conditions it is critical to reconstruct the redox architecture of the sedimentary basin recently two alternative redox models have been proposed to account for the geochemical and sedimentary features of the doushantuo formation one argues that the unit was deposited on a continental margin where a metastable sulfidic wedge was dynamically maintained by a sulfate concentration gradient between shelf and basinal environments the other contends that the sulfidic water mass was largely restricted to the intra shelf basin behind a rimmed margin these two models make different predictions about the stratigraphic completeness and correlation of the doushantuo formation to test these predictions we generated high resolution time series trends of multiple isotopic and elemental tracers including 34s 87sr 86sr and ce ce to facilitate an integrated chemostratigraphic correlation between inner shelf xiaofenghe intra shelf jiulongwan and outer shelf yangjiaping and zhongling sections our correlations suggest that both the inner and outer shelf sections are stratigraphically incomplete relative to the intra shelf section the euxinic wedge model should be reconsidered insofar as it is based on a miscorrelation between sections viewed from our revised chemostratigraphic framework euxinic conditions on the platform appear to have been largely restricted to the intra shelf basin carbonates in the upper doushantuo formation at jiulongwan and their stratigraphic equivalents are characterized by a profound negative carbon isotope anomaly i e the shuram excursion coincident with a drop in pyrite sulfur isotope values and a significant rise in 87sr 86sr from 0 7080 to 0 7090 the integrated stratigraphic data from south china suggest that the onset of the shuram excursion is associated with enhanced oxidative continental weathering that delivered radiogenic strontium as well as sulfate to the ediacaran basin

source of silica and silicification of the lowermost cambrian yanjiahe formation in the three gorges area south china abstract the rise of siliceous organisms is generally considered to have driven the abrupt decrease of silica in the ocean during the ediacaran cambrian e c interval even if the exact timing and biological triggers of this event are still debated although ediacaran cambrian chert depositions are widely distributed multiple evidences converge to support their hydrothermal origin in this study we provide petrographic study supplemented by elemental concentration sem and raman spectral data on the siliceous rocks from the yanjiahe formation the three gorges area south china in order to ascertain the origin of included silica and discuss the involved processes of silica deposition contrary to most previous studies our analyses suggest that studied cherts mainly resulted from biogenic silica and subsidiary from abiotic precipitation influenced by surrounding hydrothermal activity and from detrital input the deposition model of silica includes direct deposition of siliceous bioclasts secondary replacement of carbonate by dissolved biosilica and microbial induced silicification from seawater the sink of silica recorded in the yanjiahe formation was characterized of a transition from typical precambrian to phanerozoic process in addition micropalaeontological and taphonomical analyses of the yanjiahe formation revealed two pulses of microfossil abundance and diversity in the fortunian and stage 2 respectively they can be correlated with a regression and an enrichment in dissolved phosphate that probably derived from enhanced detrital flux

cyclostratigraphy of a carbonate platform as a tool for high precision correlation abstract mesozoic carbonate platforms of southern italy are thick 3000 m and well stratified bodies uplifted by the late tertiary quaternary deformation of the northern gondwanian continental margin they formed mostly in the shallow ramp open to restricted lagoon and peritidal supratidal settings and show a prevailingly cyclic organization consisting of m scale shallowing upward units we think that this cyclicity is demonstrably controlled by the earth s orbital perturbations individual cycle duration hierarchical organization numerical analysis of their sedimentological and petrophysical features the genetic mechanisms of the cyclicity have a climatic dictator allocyclicity that is coupled with eustatic oscillations evidenced by features related to subaerial exposure ranging from microkarst to more mature karst and paleosoils exposure features are widespread at the top of the cycles even when these units are formed by only subtidal deposits suggesting eustatic control this paper presents a study of three sections of the aptian in the campania region of southern italy here a total of 250 m has been studied at a cm scale sections include serra sbregavitelli central matese mountains 160 m upper aptian lower albian monte faito sorrento peninsula 55 m upper aptian lower albian and monte tobenna picentini mountains 35 m upper aptian the latter two outcropping 100 km sse of the former the three sections have a hierarchical organization composed of elementary cycles in turn grouped in bundles and superbundles that have been related respectively to the earth s short and long orbital eccentricity oscillation while the elementary cycles mostly appear to correspond to the precession or to a combination of precession and obliquity periodicities in these successions the stratal pattern shows a dominant aggradational configuration the thickness of the cycles and their groups appears environmentally controlled the thicker cycles being those formed in more open marine settings and vice versa if analysed in terms of sequence stratigraphy we may recognize in the superbundles a transgressive and a highstand system tract that together with maximum flooding surfaces and sequence boundaries was used to develop chronostratigraphic diagrams and to identify cycle omissions at the bundle and superbundle level the number and amplitude of omissions in the successions appear proportional to the environmental conditions the more restricted the water circulation in their original depositional sites the more prone they were to cycle omissions the sections of monte tobenna and monte faito were punctuated by omissions one of them at the superbundle level while in serra sbregavitelli we recognize a smaller number of gaps not reaching the superbundle level lower frequency oscillations 800 2000 k y are also evident and suggest a long term eustatic oscillation of the same periodicity during the lower cretaceous based on the hierarchical organization of the cyclic successions analysed and taking into account the recognized omissions we have correlated the serra sbregavitelli section sss with the two distant sections of monte tobenna corresponding to the lower sss and of monte faito corresponding to the upper sss using appropriate biostratigraphic markers a correlation appears possible at the scale of bundles of cycles 100 k y and sometimes also at the elementary cycle level 50 k y

integrated ediacaran sinian chronostratigraphy of south china abstract the ediacaran period was a critical time in the history of life and earth understanding the complex interactions between geological and biological events during this interval requires high resolution chronostratigraphy thanks to its uniquely favorable palaeogeographic setting south china is an ideal region for constructing a framework for global chronostratigraphic correlation of ediacaran strata here continuous ediacaran successions contain both siliciclastic and carbonate rocks that range from shallow to deep marine and contain rich fossil assemblages showing a variety of preservation styles as part of the present study 62 reference sections distributed throughout the yangtze platform were investigated analyses of the sedimentary facies and sequence stratigraphy of more than 40 sections revealed four major sequences in the shallow marine facies three major sequence boundaries occur respectively in the middle and upper parts of the doushantuo formation and in the middle part of the dengying formation subdivision and correlation of the ediacaran successions of the yangtze platform based on sequence stratigraphy is congruent with the carbon isotope chemostratigraphy of 12 reference sections 1075 samples the sequence boundary sb2 in the middle part of the doushantuo formation is suggested here as the lower xiadongian upper yangtzean series boundary of the ediacaran which coincides with several globally recognizable events namely 1 the end of the neoproterozoic glaciations 2 the first appearance of animals 3 the sudden increase in the diversity of acanthomorphic acritarchs and 4 a negative 13 c excursion the ediacaran system of south china may be subdivided into five stages furthermore the criteria for defining these stages may be applicable throughout the world also the composite carbon isotope profile of the ediacaran of south china can be used as a global reference for chronostratigraphic subdivision and correlation in addition to the globally recognizable negative 13 c excursion in the cap carbonate at the base of the ediacaran the negative 13 c excursion in the uppermost doushantuo wonoka shuram interval represents another globally correlatable major excursion the positive 13 c excursion immediately following the uppermost doushantuo excursion and ending at 548 ma is also useful for global correlation current ediacaran chronostratigraphy and global correlations indicate that the upper doushantuo wonoka shuram negative 13 c excursion ranges from ca 560 ma to 551 ma and therefore coincides with the evolutionary renovation of life feedback between evolutionary innovation and geochemical cycling was the key mechanism for this unique and most strongly negative 13 c excursion in earth history our chronostratigraphic correlations also imply that the earliest known metazoan biotas namely the weng an and characteristic ediacaran biotas do not constitute two temporally separate evolutionary lineages as proposed previously but represent two parallel evolutionary pathways preceding the cambrian explosion of animal life

numerical modeling of ooid size and the problem of neoproterozoic giant ooids temporal variation in ooid size reflects important changes in physical and chemical characteristics of depositional environments two numerical models are used to evaluate the effects of several processes influencing ooid size the first demonstrates that low supply of new ooid nuclei and high cortex growth rate each promote growth of large ooids the second model demonstrates that high average water velocity and velocity gradient also enhance ooid growth several neoproterozoic oolites contain unusually large ooids some reaching diameters of up to 16 mm while lower nuclei supply and higher ooid growth rate may have prevailed prior to the evolution of carbonate secreting organisms neither difference can explain the presence of giant ooids in neoproterozoic deposits because archean through mesoproterozoic ooids rarely exceed 5 mm in diameter in the presence of lower nuclei supply and higher growth rate high average water velocity may have allowed growth of such large ooids higher average water velocity could have been due to a prevalence of carbonate ramps over rimmed shelves during neoproterozoic time

longitudinal profile development into bedrock an analysis of hawaiian channels analysis of topographic maps of rivers incised into dated hawaiian lava flows shows that the long term average bedrock erosion rate along certain reaches is linearly related to stream power field observations suggest that two processes may control hawaiian channel downcutting 1 stream power dependent erosion including abrasion of the channel bed by transported particles and 2 step wise lowering caused by knickpoint propagation modeling results indicate that a simple stream power dependent erosion law predicts the straight to weakly convex longitudinal profiles characteristic of kauai channels but is insufficient to predict two other characteristic features the upslope propagation of knickpoints and the straight 5 8 channel slopes below the knickpoints thus more than this single transport law is apparently required to model bedrock channel incision field surveys also indicate that significant portions of the channel lengths below the knickpoints are mantled with large boulders we propose that

experiments on headcut growth and migration in concentrated flows typical of upland areas experiments were conducted to examine soil erosion by headcut development and migration in concentrated flows typical of upland areas in a laboratory channel packed sandy loam to sandy clay loam soil beds with preformed headcuts were subjected to simulated rain followed by overland flow the rainfall produced a well developed surface seal that minimized surface soil detachment during overland flow soil erosion occurred exclusively at the headcut and after a short period of time a steady state condition was reached where the headcut migrated at a constant rate the scour hole morphology remained unchanged and sediment yield remained constant a fourfold increase in flow discharge resulted in larger scour holes yet aspect ratio was conserved a sediment bed was deposited downstream of the migrating headcut and its slope depended weakly on flow discharge

stratigraphic position of the ediacaran miaohe biota and its constrains on the age of the upper doushantuo δ13c anomaly in the yangtze gorges area south china abstract the siliceous shale unit that hosts the ediacaran miaohe biota in the yangtze gorges area commonly referred to as the miaohe member has an age of 551 0 7 ma this unit is thought to be time equivalent with the doushantuo member iv which marks the top of the upper doushantuo negative 13 c excursion new bio sequence and chemostratigraphic studies presented here demonstrate that the miaohe member is significantly younger than the doushantuo member iv most likely time equivalent with the lower shibantan member of the dengying formation the new findings indicate that the top of the doushantuo formation i e the top of the upper doushantuo shuram 13 c excursion should be much older than 551 ma likely 560 ma the ocean oxygenation event documented from the doushantuo member iv black shales should also be older than 560 ma and predate the miaohe biota for more than 10 ma

birdseyes fenestrae shrinkage pores and loferites a reevaluation abstract birdseyes birdseye limestone fenestrae fenestral fabric shrinkage pores and loferites are considered similar if not synonymous when occurring in lime mudstone or syndepositional dolomite especially in association with mudcracks and stromatolites compaction experiments indicate however that in the absence of early cementation these rugs can be completely obliterated whereas mudcracks and stromatolites remain relatively unchanged lumping of burrows and root tubes under the general terms birdseyes or fenestrae such as for example tubular fenestrae burrow fenestrae or root tube fenestrae is discouraged they should be called burrows or root tubes because the birdseyes fenestrae and shrinkage pores are so intimately associated with tidal flats in the minds of most geologists submarine cementation of pelletal and oolitic botryoidal grainstone under 5 to 6 m of water on the bahama banks has resulted in rugs provocatively similar to many birdseyes and fenestrae generally attributed to peritidal conditions in the literature birdseyes and fenestrae in ancient grainstones cannot therefore be reliably identified without consideration of the sedimentary sequence and assoc ated sedimentary structures

new u pb zircon ages of the ediacaran cambrian boundary strata in south china the ediacaran cambrian transition is a critical interval marking drastic biological oceanic and geochemical co evolutions in geological history but it is poorly constrained geochronologically in south china we here present two new sets of sims u pb zircon ages from ediacaran cambrian boundary strata dengying liuchapo and niutitang formations deposited in the slope basin environments of carbonate platforms two weighted mean u pb ages of 542 1 5 0 ma and 542 6 3 7 ma in the basal and mid upper liuchapo formation respectively in slope and basinal settings provide the first direct age set for the ediacaran cambrian boundary in south china another two u pb ages of 524 2 5 1 ma and 522 3 3 7 ma from the base of the overlying niutitang formation indicate that this widespread unit in south china was deposited about 20 ma after the onset of the cambrian

sedimentology diagenesis and stratigraphic occurrence of giant ooids in the ediacaran rainstorm member johnnie formation death valley region california the rainstorm member of the late neoproterozoic johnnie formation contains several distinctive features of possibly regional to global significance the johnnie oolite an incised valley with a carbonate breccia fill and an enigmatic bed of giant ooids the rainstorm giant ooids although the johnnie oolite and the incised valley occur in most johnnie formation outcrops scattered throughout the death valley region the giant ooids have only been observed in one area restricted to the southern nopah range if these giant ooids were deposited as part of the rainstorm member they would represent the youngest known occurrence of neoproterozoic giant ooids and the only such unit deposited after the last precambrian glaciation the gaskiers however since the giant ooid bed is associated with the incised valley fill it could simply be a large block of oolite derived from the stratigraphically lower beck spring dolomite known to contain giant ooids this study uses mapping petrography and isotope geochemistry to compare giant ooids from the rainstorm member and the beck spring dolomite with the goal of determining the provenance of the rainstorm giant ooids all lines of evidence suggest that the rainstorm giant ooids form a unit distinctly different from those of the beck spring dolomite carbon isotope values from the ooid bed indicate that it was probably deposited after the shuram 13 c excursion making this the youngest known precambrian giant ooid unit the most plausible explanation for this bed is that it is a raft derived from a so far unobserved pre incision rainstorm member giant ooid bed which would also have been the source of many of the giant ooid clasts in the incision breccia

new u pb age from the basal niutitang formation in south china implications for diachronous development and condensation of stratigraphic units across the yangtze platform at the ediacaran cambrian transition abstract the black shale dominated niutitang formation covers a large portion of the early cambrian yangtze platform in south china the base of this unit has been traditionally taken as the ediacaran cambrian boundary ca 542 ma recent radiometric ages from the basal niutitang formation and its correlative units however are significantly younger than 542 ma and raise questions about the diachrony of stratigraphic units across the yangtze platform at the ediacaran cambrian transition here we report a new u pb sensitive high resolution ion microprobe shrimp age of 522 7 4 9 ma from a tuffaceous bed of the basal niutitang formation in a deep water section in taoying guizhou province of south china this age in combination with existing ages and biostratigraphic data suggests that the base of the niutitang formation is of tommotian or upper meishucunian age and the ediacaran cambrian boundary is most likely within the liuchapo formation the depositional age of the unusual metal enriched ni mo pge sulfide deposits which are approximately 1 m above the ca 523 ma tuffaceous bed should be early tommotian upper meishucunian rather than early nemakit daldynian lower meishucunian regional correlation confirms the diachronous development and condensation of stratigraphic units across the early cambrian yangtze platform and provides important information for understanding the paleogeography and paleoceanographic events at the ediacaran cambrian transition in south china

geochronological constraints on stratigraphic correlation and oceanic oxygenation in ediacaran cambrian transition in south china the continuous late ediacaran early cambrian deep water successions of south china archive the complete evolution of seawater chemical conditions in the deep ocean during this critical time interval however the geochemical data from these poorly fossiliferous and condensed successions lack high resolution stratigraphic constraints hampering their interpretation for the spatio temporal evolution of the sweater chemistry in this time interval in this study we report a new sims and ca id tims zircon u pb age 545 76 0 66 ma total uncertainty for an ash bed at the lower liuchapo formation in the deep water longbizui section in western hunan province the new age suggests that the lower and the middle upper parts of the liuchapo formation in the deep water facies can be correlated with the lower dengying formation and the upper dengying lower zhujiaqing formations in the shallow water facies respectively this correlation implies that the correlative horizon of the ediacaran cambrian boundary in the deep water facies in south china is likely located near the base of a widespread negative 13corg excursion at the upper liuchapo formation based on our new geochronological framework the compilations of fe speciation mo and u data indicate that the deep ocean was characterized by widespread anoxic ferruginous water with intermittent euxinic water impinged on the middle lower slope in ediacaran cambrian transition and significant oxygenation events occurred in 533 520 ma the compilations do not support any significant oceanic oxygenation events in 551 535 ma

experimental study of knickpoint and longitudinal profile evolution in cohesive homogeneous material a series of experiments were conducted in homogeneous bedrock to study knickpoint and longitudinal profile evolution knickpoints are created by successive drops in base level which simulate intermittent uplift channel morphology and flow characteristics vary systematically along a knickpoint reach as the knickpoint lip is approached width decreases but depth velocity and bottom shear stress increase as a result knickpoints are rapidly destroyed through the process of 1 knickpoint replacement and 2 subsequent knickpoint inclination knickpoint replacement consists of two morphologically distinct erosional reaches the knickpoint face below the knickpoint lip which decreases in slope and an incising reach above the knickpoint lip which increases in slope knickpoint replacement is a significant process in the evolution of knickpoints in homogeneous bedrock within experimental limits this study supports the notion that knickpoints do not undergo parallel head ward retreat in homogeneous bedrock given intermittent uplift except in those possible cases of extensive and pervasive bedrock jointing only the erosional sediment transport discontinuity above the knickpoint is developed in cohesive sediments below the knickpoint a steep bedrock channel slope low sediment load and rapid decrease in grain size of sediment load preclude development of the aggradational sediment transport discontinuity knickpoint evolution can be characterized by three general models parallel retreat replacement and inclination these models are based on the following criteria within the knickpoint reach 1 relationship between bottom shear stress 0 and critical shear stress needed to initiate motion c 2 nature of the bed load transport discontinuities and 3 spatial variability of bedrock resistance to fluvial erosion

reappraisal of the ages of neoproterozoic strata in south china no connection with the grenvillian orogeny the jiangnan fold belt separates the yangtze and cathaysia blocks in south china and has long been considered grenvillian in age in order to place south china in central rodinia it consists of deformed early neoproterozoic strata that are unconformably overlain by undeformed late neoproterozoic strata and intruded by undeformed and unmetamorphosed granitic plutons zircons from the early neoproterozoic strata yield u pb ages as young as 830 ma and one granitic pluton has a zircon u pb age of ca 827 ma the 830 ma mafic rocks along the southeastern margin of the yangtze block have arc affinity geochemical characters whereas mafic rocks younger than 830 ma have typical ocean island basalt oib like compositions thus we suggest that the early neoproterozoic strata were deposited on an active continental margin prior to amalgamation of the yangtze and cathaysia blocks at ca 830 ma the overlying late neoproterozoic strata were deposited in the intracontinental rifted nanhua basin at 820 730 ma and probably reflect backarc spreading above the long lived 950 735 ma oceanic subduction zone along the northern and western margin of the yangtze block this model is consistent with the secular tectonic evolution of south china during the neoproterozoic the jiangnan fold belt is therefore not a grenvillian feature as previously suggested and there is no evidence to place south china in central rodinia instead we believe that south china was located in a marginal position relative to this supercontinent

ediacaran chemostratigraphy and paleoceanography at a shallow marine setting in northwestern hunan province south china abstract the geochemical and lithological features of the ediacaran strata at yangjiaping in northwestern hunan province south china record information about the depositional environments redox conditions and carbon cycling in a shallow water setting in this region the chemostratigraphic profile of the inorganic carbon isotope reveals three negative anomaly intervals in the ediacaran section that comprises the thick 360 m doushantuo and the thin 110 m dengying formations two of the three intervals at the base of the doushantuo formation and the pc c boundary correspond to the global trend another anomaly interval in the uppermost 120 m of the doushantuo formation appears large amplitude fluctuations of the inorganic carbon isotope as well of the oxygen isotope this interval can be correlated to one of the anomaly intervals of the three george section either at the upper or at the middle doushantuo formation the latter correlation scheme implies association with the gaskiers glaciation and demands a substantial hiatus at the doushantuo dengying boundary decoupling between organic and inorganic carbon isotopes was recognized in two intervals of the lower doushantuo formation and the upper doushantuo to the lower dengying formations the lower and upper intervals were likely attribute to unusual carbon cycling reflecting from development and diminishing of the dissolved organic carbon doc reservoir respectively shallow water sedimentary facies and high organic carbon isotopic values are characteristic of the yangjiaping section especially in the oolitic facies of the middle doushantuo formation and in the supratidal facies of the lower dengying formation in these intervals yangjiaping was in an isolated lagoonal setting where the carbon cycle was independent of that of the open ocean in the south china region

petrology and geochemistry of chert on the marginal zone of yangtze platform western hunan south china during the ediacaran cambrian transition the ediacaran cambrian transition was one of the most critical intervals in earth history during this interval widespread chert was precipitated commonly as a stratal wedge in carbonates along the southern margin of the yangtze platform south china the chert wedge passes into a full chert succession further basinward to the south east four lithotypes of chert are identified across the marginal zone in western hunan mounded vein brecciated and bedded chert the mounded chert is characterized by irregular to digitiform internal fabrics generally with abundant original vesicles and or channels that mostly are lined by botryoidal chalcedony cements with minor quartz and barite crystals the host chert or matrix of these mounds is dominated by amorphous cryptocrystalline silica commonly disseminated with pyrite the vein chert with minor quartz locally generally cross cuts the overlying dolostone and chert horizons and terminates under the mounded and or bedded chert bodies the brecciated chert commonly occurs as splayed intrusions or funnel shaped wedges and cross cuts the topmost dolostones the bedded chert the most common type generally is thin to medium bedded and laminated locally it is composed of amorphous silica with minor amounts of black lumps microthermometry of fluid inclusions from vein and void lining minerals mainly quartz and barite revealed homogenization temperatures from 120 to 180 c for the trapped primary fluids compositionally these chert deposits generally are pure with sio2 92 wt and only minor fe2o3 and al2o3 contents most of which show positive europium anomalies in rare earth element patterns especially for the mounded chert all these data suggest that the marginal zone chert deposits resulted from a low temperature silica rich hydrothermal system in which the mounded chert was precipitated around the releasing vents i e as silica chimneys the vein and splayed brecciated chert however was formed along the syndepositional fault fracture conduits that linked downward while the bedded chert was precipitated in the quieter water column from the fallout of hydrothermal plumes onto the sea floor these petrological and geochemical data provide compelling evidence and a new clue to the understanding of the extensive silica precipitation rapid tectonodepositional and oceanic changes during the ediacaran cambrian transition in south china

dynamic redox conditions control late ediacaran metazoan ecosystems in the nama group namibia the first appearance of skeletal metazoans in the late ediacaran 550 million years ago ma has been linked to the widespread development of oxygenated oceanic conditions but a precise spatial and temporal reconstruction of their evolution has not been resolved here we consider the evolution of ocean chemistry from 550 to 541 ma across shelf to basin transects in the zaris and witputs sub basins of the nama group namibia new carbon isotope data capture the final stages of the shuram wonoka deep negative c isotope excursion and these are complemented with a reconstruction of water column redox dynamics utilising fe s c systematics and the distribution of skeletal and soft bodied metazoans combined these inter basinal datasets provide insight into the potential role of ocean redox chemistry during this pivotal interval of major biological innovation the strongly negative 13c values in the lower parts of the sections reflect both a secular global change in the c isotopic composition of ediacaran seawater as well as the influence of local basinal effects as shown by the most negative 13c values occurring in the transition from distal to proximal ramp settings critical though is that the transition to positive 13c values postdates the appearance of calcified metazoans indicating that the onset of biomineralization did not occur under post excursion conditions significantly we find that anoxic and ferruginous deeper water column conditions were prevalent during and after the transition to positive 13c that marks the end of the shuram wonoka excursion thus if the c isotope trend reflects the transition to global scale oxygenation in the aftermath of the oxidation of a large scale isotopically light organic carbon pool it was not sufficient to fully oxygenate the deep ocean both sub basins reveal highly dynamic redox structures where shallow inner ramp settings experienced transient oxygenation anoxic conditions were caused either by episodic upwelling of deeper anoxic waters or higher rates of productivity these settings supported short lived and monospecific skeletal metazoan communities by contrast microbial thrombolite reefs found in deeper inner and mid ramp settings supported more biodiverse communities with complex ecologies and large skeletal metazoans these long lived reef communities as well as ediacaran soft bodied biotas are found particularly within transgressive systems where oxygenation was persistent we suggest that a mid ramp position enabled physical ventilation mechanisms for shallow water column oxygenation to operate during flooding and transgressive sea level rise our data support a prominent role for oxygen and for stable oxygenated conditions in particular in controlling both the distribution and ecology of ediacaran skeletal metazoan communities

digital characterization of thrombolite stromatolite reef distribution in a carbonate ramp system terminal proterozoic nama group namibia the stratigraphic architecture of a terminal proterozoic carbonate ramp system ca 550 ma nama group namibia was mapped quantitatively with digital surveying technologies the carbonate ramp consists of a shoaling upward ramp sequence in which thrombolite stromatolite reefs developed at several stratigraphic levels the reefs are associated with grainstone and heterolithic facies and exhibit diverse geometries and dimensions related to the position in the sequence stratigraphic framework laterally extensive reefs with a tabular geometry developed when accommodation was relatively low whereas discontinuous oblate dome shaped reefs developed during times when accommodation space was relatively high collecting sedimentological and stratigraphic data digitally in an extensive canyon system allowed a comprehensive documentation of the three dimensional 3 d architecture and dimensions of the reefal buildups both deterministic and stochastic methods were used to extend outcrop observations to construct 3 d models that honor the observed stratigraphy in particular the accuracy with which dimensions of reefal buildups can be measured is critically important in the statistical modeling of the dome shaped buildups calculations and corrections can be applied directly to the digital data set and serve as input during model building the final 3 d model faithfully reproduces the outcrop distribution of facies and geological objects and has a high spatial resolution compared with petroleum industry reservoir models the organization of the reefal buildups in the stratigraphic framework has direct implications for reservoir continuity and connectivity in analogous settings the digital characterization and 3 d outcrop models presented in this article can be subsequently used to condition dynamic reservoir simulation modeling of geologically similar areas

the stratigraphic complexity of the middle ediacaran carbon isotopic record in the yangtze gorges area south china and its implications for the age and chemostratigraphic significance of the shuram excursion abstract the middle ediacaran shuram excursion represents the most pronounced negative carbon isotopic shift in earth history and has been considered as evidence for a profound disturbance to the global carbon cycle and proposed as a key chemostratigraphic marker for ediacaran stratigraphic subdivision and global correlation previous study has revealed a pronounced negative 13c shift en3 in the upper doushantuo formation of south china which has been interpreted as an equivalent of the shuram excursion detailed 13c investigation of multiple sections of the ediacaran doushantuo formation around the huangling anticline western hubei province south china indicates that the 13c variation in the upper doushantuo formation is more complex than previously reported in the western region of the huangling anticline a moderately positive 13c excursion here termed the diaoyapo positive excursion bisects two strongly negative 13c excursions here termed the jiuqunao negative excursion and the younger miaohe negative excursion with a 551 ma ash bed capping the miaohe negative excursion in the central region there is only one negative 13c excursion whereas in the eastern region the negative 13c excursion is absent the variable stratigraphic expression of these 13c excursions is most parsimoniously interpreted as reflecting local facies variation diagenesis and or a cryptic unconformity the more complete but more complex 13c chemostratigraphic record in the western region of huangling anticline implies that depending on how this record is correlated with the shuram excursion the latter may be significantly older than 551 ma if it does represent a global chemostratigraphic marker