geometry of subducted plates and island arcs viewed as a buckling problem during subduction a plate9s leading edge is forced to squeeze into a smaller total width than it occupied at the surface yet the plate resists change of dimension in its own plane a possible outcome is that the plate buckles as it descends at a depth of 500 km the amplitude of buckling could be as much as one tenth the wavelength and dip angles for a plate descending at 45 on average could range from 35 to 55 observed plate geometries though not ideally regular are compatible with these suggestions before being subducted a lithosphere plate is convex upward at all points whereas to buckle it must become convex downward in a series of plunging synclines the pop through is most likely to occur after the leading edge has descended about 100 km at the moment of pop through the subduction hinge line changes from being smooth to being a series of arcs and cusps back arc spreading associated with the change can be 150 km or more in the arc embayments but should be zero opposite the cusps where a compressive regime is sustained in the pop through event elastic strain energy is released and the associated back arc spreading should be a comparatively sudden event on a geologic time scale

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

tracking slabs beneath northwestern pacific subduction zones article i nfo article history this study uses the amplitudes of bottom side reflected shear waves to constrain the morphology and dynam ics of subducted oceanic lithosphere beneath northwestern pacific subduction zones across honshu arc the 410 and 660 km seismic discontinuities are detected at the respective depths of 395 5 and 685 5 km within the wadati benioff zone their topographies are negatively correlated along slab dip showing the dominant effect of temperature on the olivine phase changes within the upper mantle transition zone the base of the upper mantle shows broad depressions as well as localized zones of shallow average depths be neath korea and northeast china the 15 km peak to peak topography west of the wadati benioff zones suggests that the stagnant part of the subducted pacific plate is not as flat as previously suggested eastward slab pile up is also possible at the base of the upper mantle across southern kuril arc the shear wave reflec tion coefficients of major olivine phase boundaries fall below 5 within the wadati benioff zone the appar ent reflection gaps and the spatial connection between a strong reflector at 900 km depth may imply 1 possible compositional variations at the top and bottom of the transition zone and 2 substantial mass heat flux across the 660 km seismic discontinuity we also identify strong reflectors within the subducted oceanic lithosphere at mid transition zone depths the depths and strengths of these reflectors are highly variable be tween honshu and southern kuril islands

origin and modification of early dolomites in cyclic shallow platform carbonates yeongheung formation middle ordovician korea abstract the middle ordovician yeongheung formation korea consists of numerous metre scale shallowing upward cycles which were deposited on a shallow marine carbonate platform depositional sequences developed on the inner platform were completely dolomitized regardless of constituting lithology whereas those developed on the outer platform were partially dolomitized the amount of replacement dolomite within a metre scale shallowing upward peritidal cycle decreases downward from the peritidal cycle cap in the partially dolomitized depositional sequences the cross platform distribution of dolomites and their stratigraphic position in the sequences suggest that the yeongheung dolomite formed syndepositionally the common existence of pseudomorphs of evaporite minerals and nodules in the yeongheung formation indicates that dolomitization of the yeongheung formation occurred in a sabkha type tidal flat environment it also occurred during each cycle of deposition influenced by high frequency sea level oscillations the present day yeongheung replacement dolomites are characterized by near stoichiometric to stoichiometric composition 49 9 to 52 1 caco3 mole and low strontium concentration 52 to 133 ppm the 18o value of the replacement dolomites ranges from 7 7 to 5 2 pdb this is much depleted relative to that of contemporary sabkha type dolomites these geochemical characteristics of yeongheung dolomite indicate significant post depositional modification the common existence of oomolds leached skeletal grains and dissolved evaporites in the yeongheung formation illustrates invasion of meteoric water into the sediments recrystallization of yeongheung dolomites in the infiltrated meteoric water at slightly elevated temperatures during shallow burial resulted in lighter 18o values and lower strontium concentration

continuity and facies heterogeneities of shallow carbonate ramp cycles sinemurian lower jurassic north east spain the lateral continuity and facies heterogeneities of metre scale cycles in a greenhouse lower jurassic sinemurian carbonate ramp from the northern iberian basin spain was evaluated from extensive field analysis carried out on a well exposed 12 km long outcrop eleven high frequency continuous cycles and their bounding surfaces are traceable laterally through the entire outcrop however three of these cycles are found to split laterally into discontinuous cycles of more limited distribution up to 3 to 5 km of lateral extent the continuous and discontinuous cycles have a similar field expression in one dimensional logs as a consequence the number of cycles that can be differentiated is variable along the logged sections i e from 11 to 16 cycles have variable facies heterogeneities and sedimentary trends depending on the environment of formation shallowing upward and symmetrical cycles occur in protected lagoon tidal flat areas and in the open marine high energy domain these cycles show significant facies heterogeneities which were controlled mainly by lateral migration of a mosaic of facies over an irregular topography deepening upward and aggradational cycles are generated in low energy sub wave base open marine areas facies are laterally homogeneous reflecting low potential for carbonate accumulation and inability to fill the created accommodation space in this low relief and relatively deep area cycle boundaries are generated by stages of rapid accommodation gain involving the flooding of the carbonate ramp they are more likely to originate from regional tectonic pulses related to the extensional tectonics operating in the northern iberian basin rather than greenhouse low amplitude eustacy discontinuous cycles tend to occur in thickened areas and are interpreted as originating from the infill of wedge shaped accommodation space resulting from differential subsidence i e local tectonic pulses in conclusion where thickness variations occur in extensional settings lateral continuity of cycles should not be expected in less well exposed or in one dimensional sections and in wells it would not be possible to distinguish continuous from discontinuous cycles or to understand such two dimensional heterogeneities identification of unique cycle forming mechanisms or attempting cyclostratigraphic long distance correlation of cycles is unrealistic without a detailed analysis of the architecture of cycles in laterally continuous outcrops

indosinian dextral ductile fault system and synkinematic plutonism in the southwest of the ogcheon belt south korea reply abstract the structure of the mylonite belt in the southwestern part of the ogcheon belt has been investigated and interpreted in the light of recent radiometric data the shear belt is composed of several en echelon dextral shear zones relayed by compressional or extensional tectonic bridges the dextral displacement along the fault system which may be extended to the hida zone of japan is of the order of 200 km it has brought into contact the ryeongnam block and its cambrian middle ordovician cover of north china affinity with the southwestern margin of the ogcheon belt characterized by cambrian middle silurian rift facies and shelf deposits of south china affinity synkinematic plutons are emplaced within the shear belt which is postdated by uppermost triassic unfoliated granitoids at the surface the shear belt generated elongated intermontane coal basins initiated as transtensional half grabens because of its post permian pre early jurassic age and its dextral kinematics we correlate the shear belt with the late stages of the akyioshi indosinian collisional orogeny of the east asian margin the synkinematic plutonism that accompanied the shear zone may be due to post collisional crustal stretching and subsequent anatexis at the mantle crust interface

magnesian calcite and chamositic ooids forming shoals peripheral to late ordovician ashgill muddy siliciclastic shores southern ontario in southern ontario ooids are associated with two distinct facies associations in the queenston formation the final stage of late ordovician ashgill taconic basin fill one facies consists of thin ooid and bioclastic grainstones interbedded with mudrock and lies near the base of the formation and in southwestern ontario also forms a local nw thickening wedge near the middle of the formation ooids have radial fibrous and radial concentric fabrics type a with chamosite illite and fe oxide laths at intercrystalline sites vertical lithologic and ooid abundance patterns indicate that thresholds to carbonate production were sensitive to changes in terrigenous sediment supply sea level circulation accommodation space and tectonism ooids in the second facies association are admixed with abraded fragments of open marine biota or occur burrow fills within a 30 cm thick interval of mudrock near the top of the preserved queenston succession a few metres below the ordovician silurian unconformity ooids have radial concentric and crosscutting patchy microcrystalline fabrics type b this unit may represent a transgressive or stillstand deposit modified by bioturbation the extent of preserved fabric suggests that both ooid types were originally magnesian calcite but type a ooids underwent greater burial alteration this is shown by crystalline mosaics that cross cut relict primary fabrics 13c values 1 82 to 0 67 and 18o values 4 46 to 10 57 more negative than marine calcite of similar age mn and fe concentrations more elevated above expected marine values and a luminescence similar to that of intergranular cements burial meteoric diagenesis was likely promoted by excellent permeability of the host sand we interpret authigenic chamosite and fe oxide to reflect diagenesis of iron bearing and clay detritus trapped during ooid growth type b ooids suffered less alteration 13c 1 1 to 6 64 and 18o 3 04 to 4 81 values overlap the expected marine range including 13c enrichment that occurs within the hirnantian latest ordovician excursion although mn and fe values are still higher than those of modern calcitic ooids negligible luminescence suggests that recrystallization occurred in the presence of marine derived pore fluids further burial alteration was inhibited due to low permeability of the host mud type a ooid facies in the queenston formation forms an ancient analogue for lesser known quaternary ooid shoals peripheral to tropical deltaic systems the facies of type b ooids while more enigmatic may preserve a geochemical herald of latest ordovician climate change the presence of minor chamosite in type a ooids defines a possible distal facies of the well known oolitic ironstones of similar age in the mid continental usa

first finding of eclogite facies metamorphic event in south korea and its correlation with the dabie sulu collision belt in china abstract metabasite lenses occur within granitic gneiss at bibong in the southwestern part of the gyeonggi massif south korea bibong metabasites experienced an initial eclogite facies metamorphism 17 0 20 9 kbar 835 860 c which was succeeded by a granulite facies metamorphism 11 5 14 6 kbar 830 850 c and finally overprinted by an amphibolite facies metamorphism 6 7 11 0 kbar 570 740 c the metabasites have sm nd internal isochron ages at 225 258 ma and the major elements of the metabasites display an island arc affinity the petrochemical and geochronological data suggest the southwestern part of the gyeonggi massif as an extension of the triassic dabie sulu collision belt in china

crustal evolution of northeastern yeongnam massif korea revealed by shrimp u pb zircon geochronology and geochemistry abstract paleoproterozoic igneous activity and metamorphism are widespread in north china and korean peninsula as a part of a precambrian tectonic unit located in the southeastern margin of korea the northeastern yeongnam massif consists mainly of metasedimentary rocks and granitoids such as the buncheon granite gneiss hongjesa granite and leucogranite here we present their shrimp zircon u pb ages and geochemical compositions magmatic oscillatory zones of zircon grains from the buncheon granite gneiss yielded a concordia age of 1990 5 ma the detrital zircon analyses of metasedimentary rocks tightly constrain the depositional age between 2 02 ga and 2 00 ga subsequently the igneous and detrital zircons were overgrown during a metamorphic overprint at ca 1 95 ga dark cathodoluminescent metamorphic rims of zircon grains from the metasedimentary rocks and buncheon granite gneiss are dated at 1 89 to 1 84 ga which is contemporaneous with the emplacement age of leucogranite all granitoids are peraluminous and show distinct eu negative anomalies also the initial whole rock end values of the granitoids are consistently negative such geochemical features are indicative of recycled crustal source rocks with residual plagioclase the granitoids are enriched in large ion lithophile elements and depleted in high field strength elements in their normalized trace element patterns which is typical of subduction zone magmas our compilation of the literature shows that the yeongnam massif lacks late archean and neoproterozoic igneous or metamorphic phases which are well recorded in the other two precambrian massifs in korea namely the gyeonggi and nangrim massifs all three massifs however show a strong metamorphic imprint at ca 1 85 ga possibly associated with the formation of the supercontinent columbia there remain ambiguities on the correlation between korean and chinese precambrian tectonic units considering discrepancies in the geochronological framework of magmatism and metamorphism

the chemistry of river lake systems in the context of permafrost occurrence mongolia valley of the lakes part i analysis of ion and trace metal concentrations abstract this study provides a description of water chemistry in river lake systems located in central mongolia at the borderline of permafrost occurrence the analysis involved water samples collected from two river lake systems baydrag river boon tsagaan lake system and shargalyuut tuyn rivers orog lake system in the water samples ions and trace elements were detected and quantified additionally the parameters of ph electrical conductivity sec total dissolved solids tds and total organic carbon toc were determined principal component analysis pca was performed on the sample results water chemistry is mostly influenced by geochemical and hydrometeorological processes permafrost thawing could increase the concentration of nitrogen nh4 no3 as well as na and ca2 cl and so42 however it may also be an effect of other factors such as livestock farming seasonal drying out of lakes e g lake orog may also influence water chemistry by deflation of evaporites from exposed lake beds and at the same time with lower concentration of chemical compounds in water the pca shows that water samples can be divided into two groups the first group contains lake samples where water chemistry is shaped by prevailing evaporation processes whereas the second includes samples from rivers and springs water chemistry of the latter is predominantly influenced by geochemical and hydro meteorological processes

high resolution stratigraphic framework for mediterranean sapropel s5 defining temporal relationships between records of eemian climate variability a high resolution stratigraphic framework is presented for sapropel s5 which represents the low mid latitude climate optimum of the previous interglacial period eemian the framework is based on three sites along a transect from west to east through the eastern mediterranean and is further validated using a fourth site this method allows expression of s5 based proxy records of eemian climate variability along a standardised depth scale that offers unprecedented possibilities for assessment of spatial gradients and signal leads and lags in an interval where high resolution radiocarbon style dating cannot be performed our lateral comparison of s5 sapropels suggests that the onset of s5 in odp site 967c eratosthenes seamount was 1 6 centuries delayed relative to the onsets in more westerly sites

deltaic and shoreline sedimentation in saurashtra basin western india an example of infilling in an early cretaceous failed rift abstract the mesozoic rocks of the surendranagar district of saurashtra gujarat which overlie the early proterozoic basement and underlie the deccan basalt are interpreted to record the infilling of an early cretaceous failed riff the rift fill comprising four formations than surajdeval ranipat and wadhwan in ascending order consists predominantly of an interbedded terrigenous assemblage of seven major and three minor lithofacies the lowermost than formation is mainly composed of coarsening upward successions of carbonaceous shale including thin stringers of coal interbedded siltstone and shale and white and reddish crossbedded sandstone interpreted to be the deposits of delta distributary channels in the proximity of shoreline the succeeding surajdeval formation consists of lenticular bodies of crossbedded sandstone local lenses of conglomerate and laminated red shale and mudstone exhibiting recurring fining upward cycles minor lithofacies include flaser bedding and hummocky cross stratification these deposits are interpreted to have been laid down in the distal delta plain and nearshore tidal coastal environment the overlying ranipat sandstone is mainly white coarse medium to fine grained show ng profuse development of large and small scale planar and trough crossbedding including locally herringbone crossbedding probably deposited in subtidal tidal and beach environment the uppermost wadhwan formation contains lenses of fossiliferous limestone in the interbedded assemblage of pebbly coarse to medium sandstone and mudstone and is interpreted to have been deposited in estuaries and embayments overall the vertical lithic fill shows a progressive increase of coarse clastics from than to wadhwan with a general increase of fine clastics laterally along the length of the basin in the direction of paleoflow and paleoslope from northeast to southwest it is suggested that the development and infilling of the early cretaceous saurashtra failed rift including subsequent eruption of deccan basalt coincided with the pericratonic rifting and opening of the arabian sea to the west using the saurashtra basin as a model an idealized rift fill of the subsiding part should represent a succession of delta distributaries and tidal channels and interchannels in the lower part grading up to nearshore coastal plain embayments and estuaries as shoreline advanced landward through time the overall sequence tends to be transgressive through time and space

sequence stratigraphy and relative sea level history of the middle and upper ordovician of the nashville dome tennessee abstract eleven depositional sequences are present in the middle and upper ordovician of the nashville dome all but one lack lowstand systems tracts and many display evidence of subaerial exposure such as paleokarst or regional erosional truncation at their sequence boundaries which are combined with transgressive surfaces features formed during the transgression such as hardgrounds are more prominent at most sequence boundaries than are those features related to subaerial exposure all sequences have maximum flooding zones with aggradational stacking rather than discrete maximum flooding surfaces condensation features are much more common at transgressive surfaces than within the maximum flooding zone the expression of these parasequences and sequences changes with the onset of the taconic orogeny although parasequences from equivalent depositional environments can be recognized before and after the onset of the orogeny their facies composition changes completely sequences following the orogeny display fewer paleokarst features more pyritized and iron stained hardgrounds at flooding surfaces and transgressive surfaces and more phosphatic lags and mineralization at transgressive surfaces than those preceding the orogeny a relative sea level curve for the nashville dome has a long term 5 m y component presumably dominated by subsidence and a short term 1 3 m y component consisting of 10 50 m fluctuations that is primarily the result of eustasy

geochemical evolution of uraniferous soda lakes in eastern mongolia extremely high concentrations of uranium u were discovered in shallow groundwater fed hyperalkaline soda lakes in eastern mongolia a representative groundwater sample in this area is dilute and alkaline ph 7 9 with 10 mm tic and 5 mm cl in contrast a representative lake water sample is ph 10 with tic and cl each more than 1 000 mm groundwater concentrations of u range from 0 03 to 0 43 m l 1 lake water u ranges from 0 24 to 62 5 m possibly the highest naturally occurring u concentrations ever reported in surface water strontium isotopes 87sr 86sr varied in groundwaters from 0 706192 to 0 709776 and in lakes 87sr 86sr varied from 0 708702 to 0 709432 high concentrations of u na cl and k correlate to radiogenic sr in lake waters suggesting that u is sourced from local cretaceous alkaline rhyolites uranium rich groundwaters are concentrated by evaporation and u vi is chelated by co 3 2 to form the highly soluble uo2 co3 3 4 modeled evaporation of lakes suggests that a u mineral phase is likely to precipitate during evaporation

pretectonic and posttectonic emplacements of the granitoids in the south central okchon belt south korea implications for the timing of strike slip shearing and thrusting structural analyses of three granitic plutons in the south central okchon belt of the korean peninsula reveal that the baegnok and cheongsan plutons are pretectonic with respect to right lateral strike slip ductile shearing along the cheongsan shear zone and later top to the ese thrusting whereas the boeun pluton is posttectonic with respect to these two deformation events u pb sphene age data from the three plutons indicate that the intrusion ages of the baegnok cheongsan and boeun plutons are 222 7 2 1 ma 216 9 2 2 ma and 171 7 1 4 ma respectively microstructural evidence preserved in deformed rocks from the ne striking cheongsan shear zone and adjacent nne striking thrusts suggests that the right lateral ductile shearing event along the shear zone occurred before the ese directed thrusting since these thrusts crosscut the late triassic early jurassic taedong group we estimate that the thrusts developed at 180 ma during the daebo tectonic event just prior to the intrusion of the boeun pluton we also constrain the age of the cheongsan shear zone at middle late triassic time corresponding to the songrim orogeny so that the shear zone is not related to deformation along the middle jurassic honam shear zone 176 ma the honam shear zone is not a simple tectonic boundary between the okchon belt and the yongnam massif but an anastomosing ductile shear zone not only affecting the okchon belt and the yongnam massif but also crosscutting the boundary between them at a low angle we suggest that the right lateral cheongsan shear zone is a major boundary between the okchon zone and the yongnam massif with paleozoic cover rocks the kyonggi massif and the okchon zone south china block were juxtaposed against the yongnam massif and taebaeksan basin north china block along the cheongsan shear zone during a late stage of the middle triassic songrim orogeny probably related to the late paleozoic early mesozoic collision between the north and south china blocks

structural style of the okcheon fold thrust belt in the taebaeksan zone korea abstract structural interpretation of the distinct map pattern defined by highly connected thrust traces in the map view of the taebaeksan zone provides insight into the structural style of the northeastern okcheon belt the map pattern can generally be explained by either a folded imbricate fan or a hinterland dipping duplex the same geometry could also be formed by a complex combination of major imbricate thrusts and their connecting splays having the structural architecture of a typical fold thrust belt however a folded imbricate fan model is not adequate to explain the absence of late paleozoic to early mesozoic strata i e the pyeongan supergroup between two major thrusts viz the pyeongchang and machari thrusts in this area this result further suggests that the yeongwol area the western part of the taebaeksan zone is a duplex that corresponds to a more internal and deeper hinterland part of the fold thrust belt while the imbricate thrusts with low connectivity in the taebaek area the eastern part of the taebaeksan zone indicate a more external and shallower foreland portion of the belt in addition cross cutting relations and newly obtained sensitive high resolution ion microprobe shrimp u pb zircon ages from a cross cutting dike and syntectonic sedimentary rocks suggest limited thrust reactivation after the early jurassic and early tertiary respectively in spite of this the original geometry of the fold thrust wedge in the taebaeksan zone remained well preserved

controls on geochemical expression of subaerial exposure in ordovician limestones from the nashville dome tennessee u s a abstract stable isotope and trace element data from 301 samples in six stratigraphic sections largely support previous inferences of subaerial exposure at third order sequence boundaries in mohawkian and cincinnatian limestones of the nashville dome they also reveal the presence of previously unrecognized surfaces of subaerial exposure for example evidence of subaerial exposure at the tops of parasequences suggests that at least some of the latter are better interpreted as high frequency sequences clusters of surfaces of subaerial exposure near some previously interpreted sequence boundaries suggest that the latter may represent sequence boundary zones some surfaces of subaerial exposure are at neither recognized sequence boundaries nor parasequence boundaries and may represent missed beats in which sea level fluctuations are preserved geochemically but not recognized on stratigraphic criteria recognition of these unexpected surfaces of subaerial exposure may in part depend on the close spacing of samples used in this study different sequence boundaries exhibit different degrees of geochemical alteration and more extensive alteration appears to characterize sequence boundaries of greater temporal significance overprinting of exposure surfaces is controlled by accommodation rate in that meteoric alteration as suggested by low c isotope composition and sr concentration is seemingly pervasive in an interval of slow accommodation but not in an interval of rapid accommodation 18o values show little relationship to exposure surfaces in these strata so that sr 13c plots are more useful than 18o 13c plots in recognizing meteoric diagenesis in aggregated data in this data set 13c excursions interpreted to result from pedogenic processes are at most 2 4 and extend at most only 1 5 meters below exposure surfaces less than the extent of such signatures in younger carbonates such minima are best developed in paleo lowland and paleo coastal settings and they are essentially unrecognizable in paleo inland settings where more section was removed by lowstand and transgressive erosion

tectonic and sedimentary evolution of the korean peninsula a review and new view abstract this review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts late proterozoic neogene in the korean peninsula the korean peninsula is part of the amuria plate and represents an important link between continental blocks of north and south china and the island arcs of japan the basement rocks exposed in the kyonggi and yongnam massifs consist of 2 7 to 1 1 ga high grade gneiss and schist these massifs are separated by the okchon fold belt which comprises metasedimentary rocks and bimodal meta volcanic rocks the stratigraphy of the okchon group is unclear at present the okchon basin was probably initiated as an intraplate rift prior to the late proterozoic the hwanggangri formation clast bearing phyllite most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin the stratigraphic relationship between the okchon group and the choson supergroup cambro ordovician of the taebaeksan basin is poorly constrained the choson supergroup unconformably overlies the yongnam massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments reflecting numerous sea level fluctuations the sequence is disconformably overlain by siliciclastic sequence of pyongan supergroup carboniferous triassic which formed most likely in shallow marine deltaic and fluvial environments the imjingang belt is an east trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics devonian carboniferous underlain unconformably by proterozoic basement rocks late proterozoic amphibolites of oceanic affinity were metamorphosed at about 8 13 kbar and 630 740 c during the late permian to the early triassic the south vergent contraction and top down to the north normal faulting are suggestive of a suture belt between the north china block sino korea craton and the south china block yangtze craton an extension of sulu belt across the yellow sea entire peninsula experienced strong deformation and metamorphism during the suturing event namely songrim orogeny during this orogenic event the kyonggi massif and the okchon basin accreted to the yongnam massif and taebaeksan basin along the south korean tectonic line running northeast southwest a series of northward trending thrust formed along the boundary zone to the east kaktong and kongsuwon thrusts and others piggyback basins locally developed along the thrust faults forming the taedong group the crustal deformation resumed in the early to late jurassic daebo event under contractional setting dextral ductile shearing associated with thrusting and folding continued in the mid southern part of the peninsula it was due to orthogonal northwestward subduction of the izanagi plate under the asian continent the yongnam massif experienced continuous dextral offset along the honam shear zone in the early cretaceous the izanagi plate began to subduct northward and caused formation of strike slip basins in retroarc setting i e kyongsang basin in the southeastern part and a number of small scale basins in the mid southwestern part of the peninsula small scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions extensive intrusion of granitoids occurred from triassic to early tertiary with a gap between 160 and 100 ma representing continental magmatic arc in the tertiary the southeastern margin of the korean peninsula experienced back arc opening pull apart basins formed in the miocene bounded by the yangsan and hupo faults the yonil group sedimentary fill of the pohang basin comprises more than 1 km thick siliciclastic sequence which represents deposition in fan delta systems on the hanging wall of the yangsan fault thick more than 10 km thick sediments in the ulleung basin margin were deformed in the late miocene due to the northward movement of kyushu block quarternary volcanic events in cheju island represent intraplate hot spots

spherical shell tectonics buckling of subducting lithosphere abstract the lithospheric plate is a spherical shell rather than a plane plate a spherical shell must buckle when it is bent inward we examined the possibility of lithospheric buckling upon subduction the lithosphere with its subducted portion is simulated by a hemispherical elastic shell bent inward at its circumferential edge by a uniform radial load the buoyancy force acting on the lithosphere seaward of the trench is simulated by clamping the shell along a parallel the deformable portion between the loaded and clamped edges corresponds to the subducting slab of lithosphere buckling analyses were made using the techniques of a linear stability analysis and a finite element method the wavelength of buckling depends on the thickness of the shell and the length of its deformable portion a longer wavelength is associated with a thicker shell and a longer deformable portion by scaling the shell thickness and the length of the deformable portion to the elastic thickness of lithosphere and the length of subducting slab respectively the wavelength of buckling can be compared favourably to the length of one unit of arcuate trench in a chain like continuation of island arcs the load to be applied for initiation of buckling is called the critical load a lower critical load is associated with a thinner shell with a longer deformable portion the excess weight of subducting slab provides a load large enough to initiate lithospheric buckling at a relatively early stage of subduction radial displacement of the circumferential edge at the critical state is an order of magnitude smaller than the depth of the leading edge of the wadati benioff zone indicating that most of the present subducting lithospheres are under a postbuckling state undulation of the shell in the postbuckling state is not purely sinusoidal but a successive continuation of arcs with cusps in between invoking the continuation of arcuate deep sea trenches with cusps at their junctions the cuspate feature of island arc chains is thus a natural consequence of lithospheric buckling and does not require any such irregularities as seamounts colliding with a trench collision of seamounts however can aid buckling greatly if they are aligned along a trench at an interval not very different from the inherent buckling wavelength

facies and depositional environment of the holocene evaporites in the ras shukeir area gulf of suez egypt the holocene evaporite sequence in the ras shukeir area conformably overlies marine shell banks and cross bedded to graded bedded beach sands and gravels the evaporite sequence is represented by gypsum anhydrite layers that are interbedded with mudstone layers field and petrographic investigations of the evaporite deposits revealed two facies types laminated evaporite facies primary and nodular to enterolithic anhydrite facies diagenetic the laminated evaporite facies is subdivided from the bottom to top into regular laminated evaporite chevron gypsum algal micrite laminations and wavy algal laminated evaporite based on their textures and fabrics the regular and wavy laminated evaporite facies are interpreted as primary deposits in a coastal lagoon and salina environment the chevron gypsum algal micrite facies formed by the growth of chevron gypsum at the sediment water interface within a shallow subtidal lagoonal environment that was characterized by extensive benthic algal mats the nodular to enterolithic anhydrite facies is secondary and formed diagenetically within a siliciclastic supratidal sediment some of the laminated evaporite facies have been diagenetically altered in a supratidal sabkha environment as evidenced by the following 1 the partial formation of nodular evaporite instead of laminated evaporite 2 disruption of gypsum laminations by plant roots and rootlets as well as by precipitation of lenticular gypsum on the root wall and 3 partial dissolution of halite laminae and the formation of wavy anhydrite laminae consequently the holocene evaporites in the ras shukeir area were deposited in a shallow semi closed to closed basin that was separated from the gulf of suez trough changing sea level led to progradation of the evaporite facies from subtidal to intertidal lagoon and salina to a supratidal sabkha

occurrence of changshania trilobita cambrian in the taebaeksan basin korea and its stratigraphic and paleogeographic significance in the cambrian the taebaeksan basin was a shallow marine siliciclastic carbonate system in which two contrasting types of lithofacies and biofacies occur in juxtaposition the shallow marine facies taebaek group comprises diverse trilobite taxa endemic to the sino korean block whereas the deep water oceanic facies yeongwol group is characterized by predominance of cosmopolitan and pelagic trilobites changshania is a representative furongian trilobite genus endemic to the sino korean block and has been employed as a zonal taxon for middle furongian in north china this article reports the occurrence of changshania from the deep water facies machari formation yeongwol group of the taebaeksan basin for the first time in korea this observation enables to correlate the eochuangia hana zone of korea with the changshania irvingella zone of north china with confidence the dominance of laminated dark gray to black shale and pandemic trilobites in the machari formation has been interpreted to indicate deposition in a dysaerobic deep water environment the occurrence of changshania along with some endemic polymerid trilobites though uncommon in the deeper water oceanic facies suggests that the machari formation was formed in the offshore region fringing the sino korean block during the cambrian

calcrete and other soils in quaternary limestones structures processes and applications quaternary limestones commonly include descriptions of calcrete caliche and soils which embrace both surficial deposits in which dissolution and cementing phenomena interact on a calcareous substrate and soils or other sediments where carbonates are precipitated as cements three genetic groups are recognized in which 1 a soil profile is developed by surface alteration of existing marine sediments 2 deposits accumulate superficially on a lithified surface and 3 soil like materials have been added to unconsolidated marine sediments by surface processes including infiltration sediments range from unaltered marine grainstones through poorly sorted wackestones to mudstones diagnostic structures include pisoliths and other glaebules laminae grain aggregates birdseyes and other vugs and rootlets as well as fractures and brecciation not found in other environments the most common precipitated carbonates are micrites but needle fibres whiskers and prismatic crystals occur crystal morphology is controlled in part by the level of supersaturation rapid evaporation leading to micrite precipitation and whiskers but also by the pressure of organic contaminants distribution of cements and structures is irregular relating to percolating groundwater streams cements may be isopachous but meniscus structures cement bridges and microstalactites occur these vadose textures may not be developed where evaporation rates are high irregular areas of replacive neomorphic spar and displacive calcite growths occur in some examples but criteria for recognition are not firmly established dated recent examples lie between 400 and 18 300 years b p but deposits on aldabra and in kenya are intercalated with marine sediments which extend to 200 000 years b p the recognition of these terrestrial intervals within otherwise marine sequences enables sea level changes to be identified and correlated with other climatic data to separate local patterns of uplift or subsidence

variations of elastic plate thickness at continental thrust belts after examining 15 reliable estimates of the elastic thickness of the lithosphere where it underthrusts mountain belts in europe asia and the americas we conclude that the strength of continental lithosphere as a function of depth must be very different from that of oceanic lithosphere depending on the state of stress continental lithosphere may be either very much stronger or very much weaker than would be predicted for oceanic lithosphere of the same thermal age we explain this observation as the result of two competing effects a thicker thermal plate beneath the continents which allows a deeper elastic ductile transition for plates more than 100 million years old and a low activation energy for ductile creep in the continental crust which leads to massive failure of the upper elastic lithosphere at high bending stresses the 100 km values for elastic plate thickness of billion year old lithosphere require a thermal plate thickness of at least 250 km the magnitude of apparent thinning of the elastic plate at high stress is consistent with that calculated from a simple yield envelope which includes a ductile zone in the lower crust the low yield strength of continental materials leads to correlations between effective elastic plate thickness and all of the following dip of the underthrust plate radius of curvature of the thrust front in map view and total length of the mountain belt using insight derived from laboratory experiments on the buckling of spherical shells and drawing analogies with the geophysical characteristics of oceanic subduction zones we arrive at the following sequence of causes and effects the primary variable is the dip of the downgoing plate presumably prescribed by its own excess mass and pressure exerted by dynamic flow in the mantle where this dip is steep the rigidity of the plate is weakened due to inelastic yielding the weakened plate can then buckle at small wavelengths into short arcuate mountain belts thus we believe that the mantle ultimately controls the pattern of surface tectonics but that the nonelastic rheology of the lithosphere is the mechanism by which that control is imposed

state of stress within a bending spherical shell and its implications for subducting lithosphere summary the state of stress within a bending spherical shell has some special features that are caused by sphericity while most lithospheres are more like spherical shells than flat plates our ideas of the state of stress have been dominated by flat plate models as a consequence we might be missing some important aspects of the state of stress within subducting lithospheres in order to examine this problem we analyse spherical shell bending problems from basic equations we present two approaches to solve sphericalshell bending problems one by the variational approach which is suitable for globalscale problems and the other by the asymptotic equation which is valid to first order in h r where h is the thickness of the lithosphere and r is its curvature radius i e under the assumption of small curvature the form of the equation for displacement shows that wavelengths of deformation are determined by the spherical elastic evect and the gravitational buoyancy evect for which only the latter evect is included in the usual flat plate formulations in the case of the earth the buoyancy force is dominant and consequently spherical evects are suppressed to a large extent this explains why flat plate models have been successful for earth s lithospheric problems on the other hand the state of stress shows interesting spherical evects while bending fibre stress along the subduction zone is always important bending stress along the trench strike direction can also be important in particular when the subduction zone arc is small numerical results also indicate that compressive normal stress along the trench strike direction is important when a subduction zone arc is large these two stresses the bending stress and the compressive normal stress both along the trench strike direction may have important implications for intraplate earthquakes at subduction zones

paleoproterozoic stark formation athapuscow basin northwest canada record of cratonic scale salinity crisis abstract the paleoproterozoic stark formation a thick breccia unit formed during the transition from marine to non marine foreland basin fill in the athapuscow basin northwest territories canada four types of breccia occur in the stark formation 1 bedded breccias at the base of the unit overlying the basinal facies of the underlying pethei group are interpreted as debris flows 2 massive chaotic breccias occur in the lower part of the stark formation throughout the basin and these are interpreted to be evaporite collapse breccias 3 siltstone and mudstone breccia occurs in the upper part of the stark formation and is also interpreted as evaporite collapse breccia though much less evaporite was involved in its formation than the underlying chaotic breccia and 4 laminar accretionary breccia consisting of laminar carbonate clasts commonly with flat bottoms and convex up upper surface geometries these clasts occur in lenticular pods and are interpreted to form in caves following dissolution of evaporites or carbonates the widespread presence and abundance of evaporite pseudomorphs especially halite in all breccia types the stratigraphic restriction of the stark formation and irregular filling above this unit indicates that all but the bedded breccia formed by the dissolution and collapse of evaporites chaotic breccia directly above underlying pethei group shallow water and deep water carbonates indicates the evaporates formed across the platform and adjacent deep basin foundered megaclasts up to 1 5 km long and 40 m thick in basinal settings suggest cumulative evaporite thicknesses on the order of a few tens to a few hundreds of meters carbonate clasts in all the breccias contain abundant wave ripples planar laminations stromatolites and ooids these structures along with a paucity of subaerial exposure features indicates nearly all carbonate deposition in the stark was subaqueous thus by inference the evaporites in the stark formation were also partially subaqueous displacive halite pseudomorphs are common throughout all facies in the stark formation silicified and dolomite filled halite pseudomorphs are common along the contact between the evaporite collapse breccia and underlying carbonate platform pethei group and surrounding large clasts in basinal settings silicified hopper crystal casts within deep water rhythmites of the uppermost pethei group indicate precipitation of halite over deep waters gypsum pseudomorphs are exceedingly rare and anhydrite pseudomorphs were not observed the dominance of halite features in this basin and two penecontemporaneous basins kilihigok basin and wopmay orogen surrounding the slave craton suggests that relative to the present day seawater at 1 9 to 1 8 ga was oversaturated with respect to calcium carbonate had higher hco3 concentration and possibly was depleted in so42 the dearth of extensive thick bedded marine gypsum in the archean and early paleoproterozoic can be explained by low atmospheric oxygen content prior to 2 ga in conjunction with increased carbonate saturation before 1 6 ga

sequence stratigraphy and depositional facies of lower ordovician cyclic carbonate rocks southern missouri u s a abstract lower ordovician cyclic carbonate strata of southern missouri were deposited in a warm shallow epeiric sea on a fully aggraded carbonate platform sedimentological characteristics distinguish the jefferson city and cotter dolomites from the underlying gasconade and roubidoux formations mixed carbonate siliciclastic sedimentation characterizes the roubidoux formation with sandstones accounting for up to 60 of sedimentation the gasconade jefferson city and cotter dolomites exhibit an increased occurrence of chalcedonic chert nodules in very similar shape and texture to the gypsum and anhydrite nodules common on modern sabkha supratidal flats casts of halite and ghosts of gypsum laths also exist in the jefferson city and cotter strata but are rarely found in the underlying units facies analysis from drill cores and outcrop sections provides the basis for identifying two major meter scale cycle types type i cycles consist of algal stromatolites tidal flat laminites mechanical and algal ooid grainstones wavy peloidal wackestones and quartz sandstones interpreted as peritidal facies they are the dominant components of the roubidoux formation jefferson city dolomite and cotter dolomite type ii cycles consist mostly of subtidal facies such as strongly burrowed mudstone thrombolite boundstone and stromatolites type i cycles are thinner and represent highstand systems tracts whereas the thicker type ii cycles represent transgressive systems tracts and are dominant in the gasconade dolomite the cycle stacking patterns facies changes and the intrabasinal correlatability of fischer plots made from the widely spaced sections argue for a eustatic control on sea level fluctuation on the platform interbasinal correlation with other north american basins is possible using biostratigraphic information and comparison of fischer plots five missouri sequences correlate with those described for other regions the continent wide uniformity in cycle stacking patterns indicates a primarily eustatic control on lower ordovician meter scale cycle development regional tectonic and autocyclic controls probably account for general differences in sedimentation pattern among the correlated basins

the himalayan arc large scale continental subduction oroclinal bending and back arc spreading abstract palaeomagnetic results from the himalayan arc and southern tibet compared with simulated apparent polar wander paths for the indian plate show a consistent pattern of rotations of the himalayan arc relative to the indian shield varying gradually from 45 clockwise in the northwestern himalaya to slightly counterclockwise in the lhasa region this pattern is consistent with continental underthrusting of greater india beneath the tibetan plateau since the early miocene over at least 650 km at the longitude of western nepal and oroclinal bending since the latest miocene available palaeomagnetic observations support the steady state model for the formation of the himalayan arc with refinements as follows 1 collision between greater india s northern boundary and southern asia occurred at equatorial latitudes with progressive suturing from palaeocene in the northwestern himalaya until early eocene in the eastern himalaya 2 continuing convergence and indentation of greater india into southern asia over about 2000 km up to the early miocene resulted in southeastward extrusion of indochina and 3 neogene counterclockwise rotational underthrusting of greater india along the main central thrust with pliocene quaternary oroclinal bending of the himalayan arc

flexural response of the oceanic lithosphere at an arc arc junction implication for the subduction of aseismic ridges abstract the deformation of the oceanic lithosphere subducting at the junction of two trenches is studied by means of a three dimensional finite element analysis results show that the existence of a junction i e a change in trend of the trench axis yields a specific shape of the outer topographic rise in a convex junction area such as the japan and kuril trenches the topographic bulge presents a dome whereas in a concave junction area such as the java and sumatra trenches this bulge is less pronounced these theoretical results are confirmed by the bathymetry seaward of the junctions of the japan and kuril trenches and of the peru chile trench moreover the existence of the abnormal topographic dome in front of a convex junction contributes to the creation of normal faults which help the subduction of seamounts or of other bathymetric features in such areas

a middle proterozoic palaeokarst unconformity and associated sedimentary rocks elu basin northwest canada a major palaeokarst erosion surface is developed within the middle proterozoic elu basin northwestern canada this palaeokarst is named the sub kanuyak unconformity and truncates the parry bay formation a sequence of shallow marine dolostones that were deposited within a north facing carbonate platform under a semi arid climate the sub kanuyak unconformity exhibits up to 90 m of local relief and also formed under semi arid conditions when parry bay dolostones were subaerially exposed during a relative sea level drop of about 180 m caves and various karren developed within the meteoric vadose and phreatic zones their geometry size and orientation were largely controlled by northwest and northeast trending antecedent joints bedding and lithology near surface caves later collapsed forming valleys and intervening towers or walls and plains minor terra rossa formed on top of highs karstification was most pronounced in southern parts of bathurst inlet but decreased northward probably reflecting varying lengths of exposure time along a north dipping slope the kanuyak formation is up to 65 m thick and partially covers the underlying palaeokarst it consists of six lithofacies i breccia formed during collapse of caves as reworked collapse breccia and regolith ii conglomerate representing gravel dominated braided fluvial deposits iii sandstone deposited as braided fluvial and storm dominated lacustrine deposits iv interbedded sandstone siltstone and mudstone of sheet flood origin v dolostones formed from dolocretes and quiet water lacustrine deposits and vi red beds representing intertidal marine mudflat deposits rivers flowed toward the northwest and northeast within karst valleys and caves lakes were also situated within valleys marine mudflat sediments completely cover the palaeokarst to the north a regional correlation of the sub kanuyak unconformity with the intra greenhorn lakes disconformity within the coppermine homocline suggests that similar styles of karstification occurred over an extensive region the elu basin palaeokarst however was developed more landward and was exposed for a longer period of time than the coppermine homocline palaeokarst

sequence stratigraphy in the middle ordovician carbonate successions yeongheung formation mid east korea paleogeographic implications for a land detached ramp type platform abstract this study reveals the origin of the stratigraphic discrepancy between the yeongweol and taebaek groups and reconstructs paleogeographic model for the taebaeksan basin on the sino korean craton skc the global sea level fall in the earliest middle ordovician would have subaerially extensively exposed the lower ordovician peritidal successions forming the solution collapsed breccia zones on the sauk tippecanoe bounding surface the successive rise in sea level generated the fine grained siliciclastic successions in the taebaek group during early transgression since the carbonate production of the group was hampered by the strong input of siliciclastic sediments from terrestrial sources however such successions are absent in the yeongweol group because the yeongweol platform was distant from the siliciclastic sources and sustained carbonate production due to isolation from terrestrial influences the yeongweol group is interpreted as a land detached isolated ramp type platform as it responded to sea level fluctuations with restricted terrestrial influence whereas the taebaek group can be described as a land attached ramp type platform affected by a large input of siliciclastic sediments during lowstand and early transgression based on the comparison of stratigraphic patterns and configurations of land attached and detached ramp type successions this study concludes that the yeongweol sequence would have been formed on a land detached isolated carbonate ramp type platform located at the skc along the northeastern gondwana margin

cenozoic plate tectonic history of the northern venezuela trinidad area geological and geophysical data coupled with recent plate tectonic reconstructions suggest that the cenozoic geologic history of the northern venezuela trinidad area has been dominated by strike slip displacement of discrete crustal blocks allochthonous terranes within the area include metavolcanic rocks of the cretaceous villa de cura group and metamorphic rocks of the precambrian to cretaceous cordillera de la costa a relatively competent crustal block margarita block is defined by an outline around the metamorphic basement of margarita island the araya paria peninsula the northern range of trinidad and tobago island reconstruction of the margarita block to its original position requires at least partial closure of the falcon basin closure of the bonaire and cariaco basins and restoration of about 50 km of motion on both the oca and bocono faults post middle eocene eastward translation of the caribbean plate caused eastward motion of the margarita block a minor change in relative plate motion during the late oligocene or early miocene produced a right step in the moron fault forming the cariaco pull apart basin and el pilar fault zone maximum offset on el pilar fault is estimated to be no more than 125 km though displacement along the entire fault zone may have been greater transpressional stresses between the caribbean plate and northern south america caused folding of the serrania del interior of venezuela and the central range of trinidad eastward migration of transpressional stresses at the southeastern corner of the caribbean south american plate boundary is being accommodated by formation of oblique thrusts transpressive anticlines and downwarping of the crust bouguer gravity data suggest that jurassic aged atlantic oceanic crust is being depressed as the caribbean plate expands into the demerara plateau area this study suggests that the faults and transtensional transpressional compressional structures identified in this study are the result of stresses produced during the large eastward translation of the caribbean plate since the paleocene and are not the product of a shear couple

sequence sequence stratigraphy sea level changes and depositional systems in the cambro ordovician of the north china carbonate platform abstract the cambro ordovician strata in north china were deposited over a very extensive craton extending some 1500 km east west and 1000 km north south the dominantly shallow water carbonate succession reaches up to 2000 m in thickness and two megasequences transgressive regressive cycles can be distinguished lower cambrian through lower ordovician strata and middle through upper ordovician strata separated by a major palaeokarst the first megasequence consists of nine sequences which are generally 50 150 m in thickness the lower cambrian sediments consist of phosphorites and phosphatic sandstones deposited during the flooding of the craton carbonates mudrocks and evaporites were deposited in the early cambrian under an arid climate laying the foundation for the subsequent long period of shallow water carbonate deposition which lasted some 70 m y the middle and lower upper cambrian sequences consist predominantly of mudrocks and storm deposits tempestites in the lower part and oolitic grainstones and tidal flat lime mudstones in the upper part these represent outer mid and mid inner ramp depositional systems respectively of the transgressive and highstand systems tracts these sequences have strong similarities with the grand cycles in the cambro ordovician successions of north america in the upper cambrian there is a distinctive unit of glauconitic purple red mudstone several metres thick which is interpreted as the deposits of the maximum flooding of the first megasequence in the upper cambrian there was a phase of tilting of the north china carbonate platform to the north and storm deposits especially intraclastic conglomerates and hummocky cross stratified grainstones packstones were very common at this time also common in upper middle and lower upper cambrian strata are stromatolitic thrombolitic bioherms several metres in diameter the upper upper cambrian through lower ordovician strata are dominantly fine grained limestones and dolomites deposited in shallow subtidal and inter to supra tidal environments on a low energy epeiric sea platform this part of the succession is the regressive part of the first megasequence so that overall the platform shows an evolution from platform initiation to platform foundation to a ramp depositional system and then an extensive epeiric platform on a small scale the succession is composed of metre scale shallowing upward cycles parasequences arranged into cycle sets of 10 30 m thick the middle upper ordovician megasequence consists mostly of shallow water carbonates with several thick evaporite units minor palaeokarsts and palaeosoils separate the sequences which are composed of metre scale cycles regional uplift affected the north china platform in the ordovician and sedimentation did not resume until the carboniferous there is a good correlation of the two transgressive regressive megasequences described here with the global 2nd order relative sea level curve for the cambro ordovician

palaeokarst in the ordovician of the southern great basin usa implications for sea level history abundant and regionally unique dolostone lithoclast breccias occur throughout the shallow marine lower to middle ordovician pogonip group in the nopah range and adjacent ranges in eastern california and southern nevada breccia bodies display sharply cross cutting relationships with host dolostone bedrock stratigraphy they also show stratigraphic variability in size shape and dolostone clast composition but similarity in breccia matrix composition and framework texture and fabric these characteristics are consistent with a palaeokarst origin upsection changes in breccia clast lithology as well as multiple occurrences of associated quartz sand filled grikes solution widened fissures indicate multiple episodes of carbonate platform exposure and karstification repeated karstification is also indicated by stratiform bodies of quartz sand and thin terra rossa palaeosols that locally truncate breccias and grike systems thus bracketing karstified exposure surfaces facies successions and stacking patterns between recognized exposure surfaces are developed as transgressive regressive cycles and thus show depositional sequence architecture hence these breccias and other associated palaeokarst features are related to a succession of disconformities that provide a sequence stratigraphic framework for assessing ordovician relative sea level history of the south western cordilleran margin of laurentia

storm influenced siliciclastic and carbonate ramp deposits the lower ordovician dumugol formation south korea the dumugol formation lower ordovician in the southern part of the baegunsan syncline south korea contains mixed siliciclastic and carbonate ramp deposits the ramp sediments were frequently influenced by storm events resulting in tempestites of sandstone mudstone couplets bioclastic grainstones to packstones flat pebble conglomerates a skeletal lag layer and laminated calcisiltites all tempestites are characterized by an erosive to sharp base poor grading and a transitional upper boundary the difference in lithology of tempestites appears to have been controlled by the nature of substrates and by proximality for example laminated calcisiltites have developed on the shallow carbonate ramp flat pebble conglomerates are closely associated with nodular limestones on shallow and deep ramps and thin skeletal lag layers from fossiliferous argillaceous sediments formed in a basinal setting the stratigraphic succession of the dumugol formation represents an initial transgression followed by a regression the vertical facies change records the transition from a shallow siliciclastic ramp to a deep carbonate ramp to a basin shallowing to a deep carbonate ramp and to a shallow carbonate ramp storm effects are mostly well preserved in shallow to deep ramp deposits

sequence stratigraphy of the taebaek group cambrian ordovician mideast korea abstract this study focuses on the sequence stratigraphic implications of the taebaek group cambrian ordovician a mixed carbonate siliciclastic sequence ca 1200 m thick in the mideastern part of the korean peninsula the group contains 11 lithologic units formations that can be partitioned into four packages supersequences i iv each package comprises alternation of siliciclastic and carbonate successions corresponding to the second order sequence supersequence formed in a period of 10 100 m y supersequence i jangsan myeonsan myobong and daegi fms represents the initial inundation and subsequent drowning during the late early to middle middle cambrian it experienced the drowning between the middle and late middle cambrian forming type 3 sequence boundary supersequence ii sesong and hwajeol fms was formed by prolonged marine flooding during the late middle and late cambrian it was terminated by a large supply of siliciclastics due to the rejuvenation of the drainage basin and the subsequent shallowing at the cambrian ordovician boundary type 2 sequence boundary supersequence iii dongjeom dumugol and makgol fms comprises the lowstand coarse grained siliciclastics transgressive shale by marine flooding and shallow platform carbonates by highstand progradation in the early ordovician following the large scale eustatic fall of sea level in the earliest middle ordovician supersequence iv jigunsan and duwibong fms formed on the subaerial exposure surface type 1 sequence boundary by resumed marine flooding in the middle ordovician this study provides basis for basin scale correlation with the coeval sequences in northeastern china

huaiyuan epeirogeny shaping ordovician stratigraphy and sedimentation on the north china platform abstract ordovician conodont studies have revealed a depositional hiatus extending from the late floian to early darriwilian on the north china platform recognition of this widespread gap entails revision of the original concept of the huaiyuan epeirogeny with definition of two distinct regional tectonic events event 1 which initiated this hiatus and a subsequent event 2 that was responsible for terminating early palaeozoic sedimentation in the region from the late katian the timing of these two events partly coincides with widely recognized eustatic sea level falls and separates ordovician sedimentation into two episodes that can be broadly correlated with eustatic sea level rises in combination with these sea level changes the huaiyuan epeirogeny played a decisive role in shaping and controlling ordovician sedimentation and sequence stratigraphic architecture on the north china platform lower ordovician carbonates were deposited during an apparent regression decreasing accommodation space resulting from rapid sediment accumulation exceeding the overall rate of basement subsidence and eustatic sea level rise sedimentation ceased in the middle to late floian when basement uplift commenced in the south and extended northward to affect the entire platform the diachronous top surface of the lower ordovician succession reflects extensive erosion that is most pronounced in the south and southwest parts of the platform where the disconformity surface cut down into tremadocian or even upper cambrian strata deposition of the younger sequence darriwilian to katian was the result of the interplay between rejuvenated basement subsidence and the late middle ordovician eustatic sea level rise event 2 which was initially coupled with eustatic sea level fall induced by the end ordovician glaciations terminated ordovician deposition in the region with the top of the ordovician marked by an unconformity representing a hiatus of some 122 ma extending from latest ordovician to latest mississippian

facies analysis of the trypali carbonate unit upper triassic in central western crete greece an evaporite formation transformed into solution collapse breccias the trypali carbonate unit upper triassic which crops out mainly in central western crete occurs between the parautochthonous series plattenkalk or talea ori ida series e g metamorphic ionian series and the tripolis nappe comprising the tripolis carbonate series and including a basal phyllite quartzite unit it consists of interbedded dolomitic layers represented principally by algally laminated peloidal mudstones foraminiferal peloidal and ooidal grainstones as well as by fine grained detrital carbonate layers in which coarse baroque dolomite crystals and dolomite nodules are dispersed baroque dolomite is present as pseudomorphs after evaporite crystals nodules and rosettes which grew penecontemporaneously by displacement and or replacement of the host sediments sabkha diagenesis however portions of the evaporites show evidence of resedimentation pre existing evaporites predominantly consisted of skeletal halite crystals that formed from fragmentation of pyramidal shaped hoppers as well as of anhydrite nodules and rosettes salt crusts all microfacies are characteristic of peritidal depositional environments such as sabkhas tidal flats shallow hypersaline lagoons tidal bars and or tidal channels along most horizons the trypali unit is strongly brecciated these breccias are of solution collapse origin forming after the removal of evaporite beds evaporite related diagenetic fabrics show that there was extensive dissolution and replacement of pre existing evaporites which resulted in solution collapse of the carbonate beds evaporite replacement fabrics including calcitized and silicified evaporite crystals are present in cements in the carbonate breccias brecciation was a multistage process it started in the triassic but was most active in the tertiary in association with uplift and ground water flow telogenetic alteration during late diagenesis in zones of intense evaporite leaching and brecciation solution collapse breccias were transformed to rauhwackes the trypali carbonate breccias trypali unit are lithologically and texturally similar to the triassic solution collapse breccias of the ionian zone continental greece the evaporites probably represent a major diapiric injection along the base of the parautochthonous series metamorphic ionian series and also along the overthrust surface separating the parautochthonous series from the tripolis nappe phyllite quartzite and tripolis series the injected evaporites were subsequently transformed into solution collapse breccias

the origin of sulfate mineralization and the nature of the baso4 srso4 solid solution series in the ain allega and el aguiba ore deposits northern tunisia abstract ain allega and el aguiba are among the largest barite celestite deposits attributed to the triassic period in tunisia they are located in the flysch zone on the eastern edge of the triassic diapir of jebel hamra the ore body consists of dolomite intensely brecciated and surrounded by marls clay gypsum and dolomite which forms the hanging wall of the deposit and rimmed by paleocene marls the ore minerals show a cap rock type mineralization with various forms and types in particular impregnation in dolomite breccia cement replacement of carbonate cap rock and open space filling in the dissolution cavities and fractures ore minerals include sphalerite galena marcasite and pyrite principal gangue minerals are barite celestite calcite dolomite and quartz the ore minerals are hosted by the triassic carbonate rocks which show hydrothermal alteration dissolution and brecciation the study of barite celestite solid solution shows a bimodal distribution in which the composition is not continuous some intermediate compositions are missing 20 80 srso4 for the el aguiba ore deposit and 50 70 srso4 for the ain allega ore deposit x ray diffraction peaks in the baso4 srso4 series display considerable broadening at intermediate compositions the unit cell volume varies from 346 97 a3 in barite 100 baso4 to 308 29 a3 in celestite 100 srso4 this variation in cell parameters is attributed to the degree of substitution of barium by strontium since there is less variation between ionic radii of these elements consequently these substitutions caused variation in morphologies position and d spacing of peaks 200 011 113 312 122 112 and 111 some barite crystals have small scale strontium compositional banding which suggests that the crystals responded to regular fluctuations in physiochemical conditions during deposition the degree of supersaturation in ba2 and sr2 controls the growth mechanism and consequently the surface structure which in turn controls the mechanism of adsorption and incorporation of minor and trace elements into the growing crystal for a solid solution the transitional supersaturation for different growth mechanisms can be significantly different for the two end members one of the most interesting and intriguing phenomena observed in natural crystals of the ba sr so4 solid solution from this study area is the development of compositional oscillatory zoning consisting of alternating ba rich and sr rich layers the sulfur isotope analyses show significant variability for barite and celestite from 16 2 to 23 these values are interpreted as the result of the mixing of two sulfur end members in the mineralizing fluids corresponding to thermochemical sulfate reduction of messinian seawater together with triassic sulfate as sulfur sources fluid inclusion studies of celestite show that the baso4 srso4 solid solution in both deposits was precipitated from hot saline solution th 190 20 c 16 37 wt nacl equivalent in ain allega and 8 2 wt nacl equivalent in el aguiba consequently the precipitation could be from mixing of basinal brines with magmatic meteoric fluid

constraints on yield strength in the oceanic lithosphere derived from observations of flexure summary the wavelength and amplitude of outer rises seaward of sub duction zones and arches surrounding islands and seamounts are used to parameterize flexure profiles in terms of the moment and curvature at the first zero crossing the data show the clear age dependence in the mechanical thickness of the lithosphere up to 60 100myr saturation of moment at large curvature is interpreted in terms of a depth dependent yield strength for the lithosphere using relations adopted from laboratory experiments of rock deformation a comparison of theoretical curves with observed moments indicates that old oceanic lithosphere has no long term strength below about 40 km depth with no difference between 100 and 165 myr old crust moderate axial loading forces 200 mpa can explain most variations in the moment curvature observations except in the case of the kuril trench which appears anomalous given the age of the crust regional tension causes greater variability in moment as compared to regional compression because of the greater slope in the brittle failure envelope under tension the observations point to a lithosphere weaker than the prediction from experimental deformation of rocks of the possible weakening mechanisms elevated pore fluid pressure on faults does not predict the correct age dependence and is incompatible with earthquake focal mechanisms our favoured explanation is that the activation energy q appropriate for ductile flow at geological strain rates is lower than the values derived from laboratory extrapolations of dry olivine data taken at high temperatures if recent oceanic geotherms are reliable q in the lower lithosphere must be lower than 100kcal mol 1 the method used here is most appropriate for trench profiles with curvatures greater than 10 7m 1 for lower curvatures such as along seamount profiles small errors in the curvature estimate cause large changes in rheological parameters

sequence stratigraphy of a carbonate evaporite succession zechstein 1 hessian basin germany the evaporitic hessian zechstein basin is a sub basin of the southern zechstein basin situated at its southern margin twelve facies groups were identified in the zechstein limestone and lower werra anhydrite in order to better understand the sequence stratigraphic evolution of this sub basin which contains economically important potassium salts four different paleogeographic depositional areas were recognized based on the regional distribution of facies siliciclastic carbonate carbonate carbonate evaporite and evaporite shallowing upward successions are developed these allow the establishment of parasequences and sequences as well as correlation throughout the hessian basin and into the southern zechstein basin two depositional sequences are distinguished zechstein sequence 1 and zechstein sequence 2 the former comprises the succession from the variscan basement up to the lowermost part of the werra anhydrite including the kupferschiefer as part of the transgressive systems tract the highstand systems tract is defined by the zechstein limestone in which two parasequences are developed in large parts of the hessian basin zechstein sequence 1 is capped by a karstic subaerial exposure surface interpreted as recording a type 1 sequence boundary that formed during a distinct brine level fall low lying central areas central hessian sub basin werra sub basin however were not exposed and show a correlative conformity topography was minimal at the end of sequence 1 widely developed perilittoral sabkha and salina shallowing upward successions indicate a renewed rise of brine level interpreted as a transgressive systems tract because of inflow of preconcentrated brines from the southern zechstein basin to the north this marks the initiation of zechstein sequence 2 which comprises most of the lower werra anhydrite in the central hessian sub basin situated proximal to the brine inflow and on the ridges within the hessian basin physico chemical conditions were well suited for sulphate precipitation to form a thick cyclic succession it consists of four parasequences that completely filled the increased accommodation space in contrast only minor sulphate accumulation occurred in the werra sub basin situated further southwards and distal to the inflow as a result of substantially different sulphate precipitation rates during increased accommodation water depth in the region became more variable the werra sub basin characterized by very low sedimentation rates became increasingly deeper through time trapping dense halite brines and precipitating rock salt deposits werra halite this self organization model for an evaporitic basin in which depositional relief evolves with sedimentation and relief is filled by evaporite thereafter contradicts earlier interpretations that call upon the existence of a tectonic depression in the werra area which controlled sedimentation from the beginning of the zechstein

low and high frequency sea level changes control peritidal carbonate cycles facies and dolomitization in the rock of gibraltar early jurassic iberian peninsula the rock of gibraltar comprises two tectonically separated limbs of an isolated klippe of liassic gibraltar limestone formation both limbs have similar c 400 m thick sequences of inner carbonate platform facies arranged in high frequency metre scale shallowing upward peritidal cycles with emergent caliche caps four cycle types are recognized on the basis of vertically repeated successions of different sedimentary structures lithologies facies and biota when compared with other liassic cycles from fault bound platforms of the western mediterranean region all are found to be of similar scale facies and cycle type likely common origins are through milankovitch band allocyclicity or autocyclic tidal flat progradation superimposed on regional subsidence within the gibraltar limestone high frequency cycles are superimposed on a low frequency third order cyclicity that is revealed through the use of fischer plots to control the occurrrence of facies biota high frequency cycle types and dolomitization falling sea level and lowstand phases with reduced accommodation space are typified by restricted inner platform facies and cycles and by early reflux dolomitization transgressive and highstand phases with more accommodation space are characterized by the absence of early dolomites the incoming of inner platform microfossils i e foraminifera and calcareous algae and by less restricted marine facies i e oncoids shelly rudstones packstones and grainstones fischer plots have demonstrable value in the correlation and analysis of tectonically separated and geographically isolated cyclic sequences that lack prominent marker beds or stratigraphically useful biotas

sequence stratigraphic context of extensive basin margin evaporites middle jurassic gypsum spring formation wyoming u s a abstract ancient marine evaporites are commonly far more extensive than any known modern evaporite system this is true not only for basin central saline giants but also for evaporites that occur in basin margin settings and the origin of these basin margin systems is unclear extensive basin margin evaporites of the middle jurassic gypsum spring formation are well exposed along the eastern flank of the bighorn basin in wyoming usa and a facies and sequence stratigraphic analysis was undertaken to understand the origin of these evaporites and associated deposits the gypsum spring formation was deposited on a northwestward dipping mixed evaporate carbonate siliciclastic ramp with facies and depositional environments that include distal ooid shoal grainstone open marine shallow subtidal peloidal skeletal wackestone to packstone restricted shallow subtidal laminated lime mudstone peritidal dolostone and microbialite as well as red mudstone and gypsum deposited on mud flats sabkhas and salinas three depositional sequences are present with the j 1 j 1a and j 1b sequence boundaries characterized locally by chert pebble lags silicified horizons and iron rich paleosols evaporites accumulated in a basin margin position with 1 6 m beds of gypsum forming in salinas and sabkhas and nodular anhydrite and gypsum forming in red mudstones deposited on sabkhas and mud flats the limited lateral extent of salina type gypsum beds and the variable number of these beds among the exposures indicates substantial autocyclic control on the formation of salinas salinas are well developed in the tst of the j 1 sequence and sabkhas and mud flats are well developed in the j 1 hst the j 1a hst and the j 1b tst the great lateral extent of the gypsum spring evaporites reflects the original expanse of the depositional environment in which they formed and is not the result of diachroneity of a narrow evaporite precipitating belt such as the modern day coastal sabkhas of the trucial coast the great lateral extent of gypsum spring evaporites suggests that they were precipitated mainly from continental brines on an expansive desert mudflat

biostratigraphy and biofacies of the middle darriwilian ordovician conodonts from the laoshidan section in the western margin of the north china craton abstract middle ordovician conodonts from the sandaokan and zhuozishan formations of the laoshidan section at the western margin of the north china craton consist of 39 identified and six indeterminate species belonging to 21 genera and three taxa left in open nomenclature at both genus and species levels of particular biostratigraphic importance is the presence of morphologically advanced species of histiodella associated with several age diagnostic endemic taxa making it possible to recognize international conodont biozones in the main part of the sequence three histiodella based zones are recognized in the studied section including the histiodella cf holodentata histiodella kristinae and histiodella bellburnensis zones in ascending order they can be directly correlated with the conodont zonations established in western newfoundland canada and tarim western china and compared well with the conodont biozones of baltoscandia and yangtze central china however the same stratigraphic interval on the north china platform is barren of conodonts multivariate statistical studies on these conodonts allow recognition of four conodont biofacies scalpellodus biofacies panderodus biofacies drepanoistodus biofacies and phragmodus biofacies turnovers of the conodont biofacies are related to either sea level changes or mixing of shallow water conodonts due to down slope transportation excluding the effect of the allochthonous conodonts the transgressive regressive patterns demonstrated by the conodont biofacies compare closely to the published sea level curves for the western margin of the north china craton

continental underplating model for the rise of the tibetan plateau abstract subduction of continental lithosphere entrained in an oceanic plate for distances of several hundred kilometers can lead to continental underplating whereby the continental crust with or without its lithospheric mantle is detached at depth from the main part of the sinking slab and rises buoyantly beneath the overriding plate the rising continental material causes first extension in the overriding plate as the asthenosphere flows laterally and later rapid isostatic uplift when it reaches the base of the overriding lithosphere this continental underplating model is applied to the geology of the tibetan region and the following stages are identified 100 ma to 50 ma oceanic subduction leading to continental collision 50 ma to 20 ma pause in convergence with possible delamination of indian crust 20 ma to 5 ma continental subduction initially at an angle of 30 of indian lithosphere along the main central thrust 5 ma to 2 ma rise of the continental lithosphere displacing the asthenosphere beneath tibet and causing widespread graben formation 2 ma to present contact between the underplating indian continental lithosphere and the overlying asian crust leading to rapid rise of the surface altitude by 3 4 km present day convergence between india and tibet albeit at a rate much less than the total india eurasia convergence continues as continental underthrusting

three dimensional model of the cascadia subduction zone using earthquake hypocenters western washington abstract the identification of seismically active fault planes is important for assessment of earthquake potential as well as development of neotectonic models and analysis of stress systems for a region an automated set of computer programs seispln has been developed for the identification of three dimensional 3 d alignments of seismic hypocenters the seispln technique is based on rotating a 3 d elongate grid system through the hypocenter distribution the number of events in each cell is tabulated and those cells which contain statistically significant numbers of events are considered to be potential fault surfaces a multiple linear regression is applied to the set of hypocenters in each delineated cell to model the 3 d geometry of each of the inferred fault surfaces we applied the technique to earthquakes located by the washington regional seismic network to separate the seismic events associated with the subducted juan de fuca jdf plate from events in the overlying crust a total of 259 jdf associated events were delineated and the resulting 3 d models of the jdf slab beneath washington reveal a complex combination of arching and flattening also a steep orientation for the slab is observed beneath the southern portion of the washington cascades while a much shallower orientation is inferred beneath the northern cascades

spherical shell tectonics on the buckling of the lithosphere at subduction zones abstract the buckling phenomena of the subducting lithosphere due to the sphericity of the earth are studied by a nonlinear finite element method fem and the effects of geometrical nonlinearity are examined the subduction of the lithosphere is modelled by a hemispherical shell squeezed at its circumferential edge in scaling the geometry of the subducting lithosphere two parameters are employed one is the thickness of the hemispherical shell i e the thickness of the lithosphere and the other is the length of its deformable portion i e the length of the descending slab to simulate the bending of the lithosphere at the trench uniform inward load is applied to the free circumferential edge of the deformable portion which corresponds to the leading edge of the subducting lithosphere at the opposite edge of the deformable portion i e at the trench a built in boundary condition is imposed no displacements and no rotations are allowed with a load greater than some critical value the deformable portion of the shell buckles and undulates azimuthally the investigation of this buckling phenomenon can be summarized as follows 1 1 buckling with a shorter wavelength is associated with a thinner and shorter shell mechanical irregularities simulating lithospheric tearing or seamount collision do not alter the buckling wavelength very much after appropriate scaling the lengths of subduction zones coincide with the buckling wavelength calculated by f e m 2 2 sharp cusps pointing outward from the shell appear during the postbuckling undulation without any irregularities in shell property and in the applied load the shape of this cuspidal undulation is quite similar to that actually observed at an arc arc junction 3 3 buckling can take place in the stress environment likely to exist in the earth these results strongly suggest that the arcuate shapes of subduction zones are in fact generated by buckling of the subducting lithosphere

unraveling the origin of carbonate platform cyclothems in the upper triassic durrenstein formation dolomites italy abstract facies analysis of the durrenstein formation central eastern dolomites northern italy indicates that this unit was deposited on a carbonate ramp as evidenced by the lack of a shelf break slope facies or a reef margin together with the occurrence of a molechfor biological association its deposition following the accumulation of rimmed carbonate platforms during the ladinian and early carnian marks a major shift in growth mode of the triassic shallow marine carbonates in the dolomites the durrenstein formation is characterized by a hierarchical cyclicity with elements strongly suggestive of an allocyclic origin including a subaerial exposure features directly above subtidal facies within meter scale cyclothems b purely subtidal carbonate cyclothems c symmetric peritidal carbonate cyclothems and d continuity of cyclothems of different orders through facies boundaries the durrenstein cyclothems are usually defined by transgressive and regressive successions and so most of them probably originated from sea level oscillations their allocyclic origin allows their use for high resolution correlations over distances up to 30 km a stratigraphic section in the tre cime di lavaredo area encompassing the upper part of the durrenstein formation and the lower part of the overlying raibl formation upper carnian was studied using time frequency analysis a strong milankovitch signal appeared when interference arising from a variable sedimentation rate was estimated and removed by tuning the short precession line in a spectrogram all of the principal periodicities related to the precession index and eccentricity calculated for 220 ma are present p1 21 9 ky p2 17 8 ky e1 400 ky e2 95 ky and e3 125 ky along with a peak at a frequency double that of the precession which is a predicted feature of orbitally forced insolation at the equator components possibly related to earth s obliquity at ca 35 ky and ca 46 ky are present as well the recovery of milankovitch periodicities allows reconstruction of a high resolution timescale that is in good agreement with published durations of the carnian based on radiometric ages the recognition of a milankovitch signal in the durrenstein and lower raibl formations as well as in other mesozoic carbonate platforms strongly supports a deterministic and predictable rather than stochastic control on the formation of carbonate platforms carbonate platforms might thus be used in the future for the construction of an astronomical time scale for the mesozoic

carbonates and relative changes in sea level abstract in the geologic record some of the most accurate gauges of changes in sea level are the sediment type geometry and diagenesis of carbonate shelves and platforms this is because carbonates frequently occur at or very near sea level and are usually less compacted than siliciclastics world wide changes in relative sea level the sum of eustatic sea level changes sedimentation and crustal movements have occurred repeatedly and cyclicly through geologic time producing characteristic responses in carbonates 1 1 relative rises in sea level usually caused by the cumulative effect of tectonic subsidence and eustatic rise may result in the following 1 1 a drowned carbonate reefs or platforms here carbonate growth potential is exceeded by relative sea level rise and is characterized by shallow water sediments overlain by hardgrounds and or deep water sediments some of which may be condensed sequences 1 2 b platforms where only the fast growing rim and patches of the interior are able to match sea level rise while the remainder of the platform is drowned temporarily 1 3 c platforms which keep up and maintain a flat top at sea level and contain shallow water sediments whose thickness at the least matches the height of the sea level rise if terrigenous supply is limited prograding sheets of shelf carbonate occur frequently capped by supratidal evaporites with prograding shelf margin carbonate clinoforms and turbidites if terrigenous supply is high the shelf carbonates encroach on deltaics 2 2 relative drops in sea level caused by crustal uplift or by subsidence being outpaced by a eustatic drop in sea level cause karst and soil development over shelves and platforms deposition of deep water evaporites in adjacent semi enclosed basins and in open marine basins the deposition of deltaic and aeolian clastics that bypassed the shelf falls are accompanied by platform wide fresh water diagenesis during relative sea level rises marine diagenesis is common in the subtidal portions of the shoaling upward carbonates and fresh water diagenesis and dolomitization and sulphate deposition is common in their intertidal and supratidal portions the stratigraphic significance of these reponses to relative sea level change is that many are tied to eustatic events and so are predictable within a basin of deposition

paleosols in shallow marine carbonate sequences abstract shallow water carbonate sequences in the geological record commonly exhibit numerous exposure surfaces reflecting emersion of sediment due to progradation processes and sealevel oscillations some ancient platform sequences contain hundreds of such surfaces associated with paleosols fossil soils typically calcretes a wide range of macro and microstructures are associated with these calcretes which are reviewed profile types and stages in profile development can be defined the nature of the exposure surface and associated paleosols can be used to identify the nature of the prevailing climate although hydromorphic effects related to marine flooding of the platforms can modify the paleosols a potentially important use of paleosols is to define the nature of sequence boundaries several studies have identified sets of paleosols with varying maturities reflecting changes in rates of accommodation space creation during sealevel rises and falls

sequence stratigraphy in the middle ordovician shale successions mid east korea stratigraphic variations and preservation potential of organic matter within a sequence stratigraphic framework abstract the jigunsan formation is the middle ordovician shale dominated transgressive succession in the taebaeksan basin located in the eastern margin of the north china platform the total organic carbon toc content and some geochemical properties of the succession exhibit a stratigraphically distinct distribution pattern the pattern was closely associated with the redox conditions related to decomposition bulk sedimentation rate dilution and productivity to explain the distinct distribution pattern this study attempted to construct a high resolution sequence stratigraphic framework for the jigunsan formation the shale dominated jigunsan formation comprises a lower layer of dark gray shale deposited during transgression and an upper layer of greenish gray siltstone deposited during highstand and falling stage systems tracts the concept of a back stepped carbonate platform is adopted to distinguish early and late transgressive systems tracts early and late tst in this study whereas the highstand systems tracts and falling stage systems tracts can be divided by changes in stacking patterns from aggradation to progradation the late tst would be initiated on a rapidly back stepping surface of sediments and just above the surface exhibits a high peak in toc content followed by a gradually upward decrease this trend of toc distribution in the late tst continues to the maximum flooding surface mfs the perplexing toc distribution pattern within the late tst most likely resulted from both a gradual reduction in productivity during the late tst and a gradual increase in dilution effect near the mfs interval the reduced production of organic matter primarily incurred decreasing toc content toward the mfs when the productivity was mainly governed by benthic biota because planktonic organisms were not widespread in the ordovician results of this study will help improve the understanding of the source rock distribution in mixed carbonate siliciclastic successions within a stratigraphic framework particularly for unconventional shale reservoirs

a middle ordovician drowning unconformity on the northeastern flank of the okcheon ogcheon belt south korea abstract the maggol limestone of ordovician age was deposited in the taebaeksan taebacksan basin which occupies the northeastern flank of the okcheon ogcheon belt of south korea carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the maggol limestone probably culminated in subaerial exposure of platform carbonates in the early middle ordovician earliest darriwilian elsewhere this subaerial exposure event is manifested as a major paleokarst unconformity at the sauk tippecanoe sequence boundary beneath the middle ordovician succession and its equivalents most in notably north america and north china due to its global extent this paleokarst unconformity has been viewed as a product of second or third order eustatic sea level fall during the early middle ordovician the sauk tippecanoe sequence boundary in south korea however appears to be a discrete marine flooding surface in the upper maggol limestone strata beneath this surface represent by a thinning upward stack of exposure capped tidal flat dominated cycles that are closely associated with multiple occurrences of paleokarst related solution collapse breccias this marine flooding surface is onlapped by a thick succession of thin bedded micritic limestone that is eventually overlain by a middle ordovician condensed section this physical stratigraphic relationship suggest that second and third order eustatic sea level fall may have been significantly tempered by regional tectonic subsidence near the end of maggol deposition the tectonic subsidence is also evidenced by the occurrence of coeval off platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias i e the yemi breccia in the basin with continued tectonic subsidence a subsequent rise in the eustatic cycle caused drowning and deep flooding of the carbonate platform forming a discrete marine flooding surface that may be referred to as a drowning unconformity this tectonic interpretation contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously suggested thus it is proposed that the taebaeksan basin in the northeastern flank on the okcheon belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early middle ordovician

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

the relationship between plate curvature and elastic plate thickness a study of the peru chile trench the age of the nazca plate where it enters the peru and northern chile trenches varies from 30 ma in the north to 45 ma in the south as its dip beneath the south american continent steepens from 13 to 30 if the elastic thickness te of oceanic lithosphere depends only on its age and therefore thermal state we would expect that te determined from fitting the flexure of the lithosphere over the outer rise as revealed in the depth geoid and gravity anomalies would increase from the peru trench in the north to the northern chile trench further south we find that the opposite is true the lithosphere appears suffer outboard of the peru trench than it does offshore chile to explain deflections of the lithosphere seaward of peru the isotherm controlling the elastic ductile transition must be near 800 c providing the thermal structure of the plate is that predicted by the standard thermal plate model because the decrease in plate stiffness to the south is correlated with an increase of plate curvature over the outer rise and outer trench wall we interpret our result in terms of inelastic yielding of the oceanic lithosphere when bent to high strains the magnitude of the reduction in strength however suggests that the standard oceanic yield strength envelope underestimates the amount of failure for lithosphere bent to high strains unless the stress field off northern chile is characterized by 100 mpa level tension the more highly bent segment of subducting lithosphere at this trench also dips at a steeper angle at greater depth beneath the continent but detailed analysis of plate geometry does not support a relationship between slab dip as determined by earthquake hypocenters and elastic behavior over the outer rise

vertical distribution of physical chemical features of water and bottom sediments in four saline lakes of the khangai mountain region western mongolia characterizing vertical profiles of water columns is important to understanding the biochemical cycles and nutrient dynamics of an aquatic ecosystem physical and chemical properties of surface water in mongolian saline lakes have been studied over the past decade but chemical composition along a vertical profile of the water column has not been well documented four major saline lakes lakes telmen oigon tsegeen and khag in northwestern mongolia were studied to determine geochemical behavior of microelements and to assess major ionic components and trace elements associated with vertical profile of various water samples and sediments water samples were vertically collected from each sampling depth with a vacuum deep water sampler in july 2013 physical chemical parameters major ionic components and trace elements were analyzed by conventional methods and inductively coupled plasma mass spectrometry concentrations of microelements and mineralogical composition of sediment samples from near shore were characterized by x ray fluorescence spectrometry and x ray diffraction this study was apparently the first to characterize chemical composition along vertical profiles of these lakes sodium chloride and sulfate ions were largely conserved in all water samples except lake oigon depending on salinity type these lakes were divided into sodium sulfate lake lake khag or chloride lakes due to geomorphological characteristics strontium 3 18 mg l was the only ion in the highest concentration across all depths of water and sediment samples among the lakes sediments were comprised mainly of quartz anorthite albite and orthoclase in conclusion physical and chemical properties in lake oigon were highly variable and dissimilar among lakes furthermore we inferred that vertical profiles of water columns were intimately linked to variations in chemical composition of salt lakes in the western mongolia

bending of spherical lithosphere axisymmetric case summary effects of sphericity are commonly ignored in the lithospheric bending problem in order to examine its effects i solve a simple axisymmetric spherical shell model the full solution and the asymptotic solution are derived from the basic equations and their relationship to the flat plate solution is examined for displacement effects of sphericity are small and use of the flat plate solution produces results that are numerically indistinguishable from those of the spherical solution the most significant effect of sphericity appears in the stress in particular the normal stress along the strike direction of the trench this stress is approximately given by eu r where e is young s modulus u is the vertical deformation of the shell and r is its radius of curvature if the shell lithosphere is bent downwards and reaches 30 km this stress can become about 5 kbar in the earth while plastic behaviour may set in under such high pressure conditions and analysis beyond elasticity theory may be required sphericity may be a cause of large compressive stress in the trench strike direction this stress may play an important role in forming the overall shape of the earth s subduction zones

the frasnian famennian mass extinction insights from high resolution sequence stratigraphy and cyclostratigraphy in south china two sequences sfr sfa each 1 1 2 myr in duration are recognised in the strata across the frasnian famennian f f transition both in carbonate platform and interplatform basinal successions in south china the sequence boundary between the two sequences is placed a little below the top of the frasnian the sequences are basically composed of coarsening upward bed thickness increasing upward cycles and shallowing upward cycles parasequences in basinal and platform deposits respectively which stack into cycle sets typically six to eight cycles 10 and 12 cycle sets are identified in sequences sfr and sfa respectively these cycle sets can be further grouped into larger scale composite cycle sets herein termed mesocycle and megacycle sets with two and four cycle sets respectively this vertical cycle stacking pattern and the hierarchy of cyclicity suggest a milankovitch style of forcing such that the cycles and cycle sets were formed in response to the orbital perturbations of precession 16 18 kyr and eccentricity v100 kyr in duration respectively in the basinal cycles smaller scale rhythmic stratification beds typically six to eight beds in a cycle are extensive and were likely caused by millennial scale climatic forcing in the lower sequence sfr the latest highstand deposits consist of calciturbidites and debrites in deep water strata and fenestral limestones in shallow water strata representing a major third order sea level fall within these deposits four cycle sets are further identified in both coeval deep water and platform successions succeeding deeper water organic rich facies within which three cycles occur are the transgressive deposits of the overlying famennian sequence sfa these cycles represent three higher frequency 16 18 kyr sea level fluctuations and accompanying anoxia superimposed on a major third order sea level rise the f f boundary is placed at the top of the first cycle based on conodont data thus a major sea level fall and then a rise occurred in the f f transitional period faunal and sedimentological data reveal a massive biotic decline in concert with the major sea level fall and a further biotic demise coinciding with the major sea level rise and its three superimposed higher frequency sea level fluctuations and accompanying anoxia the f f biotic crisis was therefore characterised by two episodes of step down extinction on the basis of milankovitch orbital rhythms the first major biotic extinction took place over v400 kyr and the subsequent event was v50 kyr in duration i e v450 kyr for the entire event at the same time as the massive decline of normal marine fossils during the latest frasnian sea level fall there was widespread cyanobacterial growth and a thriving of planktonic calcispheres suggesting eutrophic conditions this situation could have caused a severe biotic loss as a result of the deterioration of surface water clarity and formation of anoxic bottom waters due to over