response of the inter tropical convergence zone to southern hemisphere cooling during upper ordovician glaciation stable isotope ratios of whole rock carbonates and faunas from three low latitude upper ordovician sections demonstrate a coherent pattern of shifting subtropical and tropical water masses and associated climate belts we suggest that tropical water beneath the inter tropical convergence zone 18o 10 to 15 was isotopically light reflecting high sea surface temperatures sst and reduced sea surface salinity sss subtropical water in both the northern and southern hemispheres is characterised by 18o values from 4 to 6 our interpretation is consistent with the published results from physical chemical modeling of ordovician seawater composition and climate models deriving absolute temperature and salinity values from ordovician 18ocarb data is problematic as they could reflect sst sss ice volume and potentially a variable diagenetic overprint and currently no independent constraints exist for any of these variables the itcz was positioned at 10 n of the equator during the early katian consistent with the likely position of the region of peak sea surface temperature sst during the late katian boda event the itcz moved across the equator to 10 s by analogy with the pliocene this resulted in a symmetrical distribution of sea surface currents about the equator this led to a net increase in heat flow towards the poles and lower latitudinal temperature gradients potentially explaining the migration of warm water faunas to higher southern latitudes during the boda event during the hirnantian the itcz shifted north of the equator resulting in a reduced poleward heat transport itcz position has been proposed as a likely forcing mechanism of neogene glaciation and the evidence from the ordovician suggests a discrete palaeo itcz whose position was controlled by solar insolation and latitudinal temperature gradients linked to ice volume changes and hadley cell dynamics we hypothesize that the position of the itcz was sensitive to the growing hirnantian ice sheet during the late ordovician we propose that latitudinal movement of the itcz during the late ordovician resulted in a major atmosphere ocean circulation re organization higher pco2 estimates for the ordovician caused more extreme changes in this circulation that were the likely primary cause of biotic mass extinction if the monsoon is considered an off equator itcz then we provide direct evidence of an ordovician monsoonal climate

channel formation and migration by mass flow processes in the lower carboniferous fluviatile fell sandstone group northeast england small steep sided narrow channels locally developed within the braided fluviatile lower carboniferous fell sandstone of northeast england are attributed to mass flow processes initiated by channel bank slumping the channels have sharp undisturbed margins and are filled with moderately well sorted medium to coarse sandstone identical to the sandstone which they cut most channel fills are structureless but a few show diffuse concentric marginal laminations laminations also occur towards the top of channel fills but they are less regular and show low amplitude trough like undulations channel margin inclinations range from 13 to 55 and channel depths vary between 1 7 m and 2 5 m the channels which are only seen in profile are orientated perpendicular to the local and regional palaeocurrent trend and cut through structureless sandstone and planar cross stratified sandstone characterized by compound and compound complex bar bedforms only one channel has both margins preserved and of the remainder seven only one has its northern margin preserved undercutting and retrogressive slumping of the sandy channel bank is thought to have initiated a subaqueous sediment gravity flow which moved across the main channel along a transverse scour at the base of the slip face of a large composite bar bedform such scours have strong asymmetric profiles with the steeper slope along the base of the active slip face northern side and the lower slope on the downstream southern side of the scour thus the northern channel margin was steeper less stable and more prone to collapse than the southern margin and is almost never preserved flow reconstruction indicates that on entering the channel a hydraulic jump developed at the change in slope and the flow became more turbulent and erosive scoured sand incorporated into the flow increased concentration and shearing along the boundaries reverse shear was also exerted by the overlying current on top of the flow but the central part remained relatively unaffected the presence or absence of marginal laminations is probably related to marginal shearing and flow dilution deposition occurred in response to deceleration and frictional freezing of the sand flow

visions of ice sheets in the early ordovician greenhouse world evidence from the peninsula formation cape peninsula south africa abstract the depositional architecture within the early ordovician peninsula formation south africa indicates a three tier stratal sequence hierarchy comprising likely 4th order autogenic 3rd order allogenic and 2nd order regressive transgressive sequence stacking pattern based on correlation with the global eustatic sea level curve we interpret the 3rd order sequences comprising predominantly lowstand braided fluvial deposits with thin early transgressive system tract marginally marine sandstones as orbitally induced glacioeustacy eustacy affected sand delivery base level changes and discharge patterns the cape peninsula lay at 30 s during the early ordovician yet the primary control on sedimentation was the waxing and waning of ice sheets in the source area we hypothesize the presence of an early ordovician ice sheet of 8 12 10 6 km 3 and a polar front at 40 s

a mid caradocian 453 ma drawdown in atmospheric pco2 without ice sheet development abstract previous workers have recognized a positive 13 c excursion during the mid caradocian in laurentian and baltic platform carbonates this study documents the mid caradocian guttenberg excursion gice in the kullsberg limestone in central sweden significantly we observed increases in both 13 c and 18 o unlike most previous studies of gice which only observed increased 13 c marine values brachiopods from the lower kullsberg limestone reflect pre excursion conditions having more negative 13 c 0 1 and 18 o 4 to 6 values compared with least altered marine cement from the upper kullsberg limestone which have more positive 13 c 2 1 3 6 and 18 o 1 8 to 3 6 values least altered marine cements from the kullsberg limestone include columnar calcite first generation equant calcite and translucent fibrous calcite from earliest to latest in timing of formation kullsberg seawater during the 13 c excursion event likely decreased in temperature by up to 15 c with an approximate 1 increase in 18 o h 2 o attributable to increased salinity during the mid caradocian the results of this study are consistent with multiple independent approaches to characterize the composition of the ordovician atmosphere hydrosphere systems specifically results of this study are consistent with formation of abiotic calcite at moderately low paleotemperatures during the gice excursion of 10 17 c and ordovician atmospheric pco 2 values of

post hunnebergian ordovician carbon isotope trend in baltoscandia its environmental implications and some similarities with that of nevada abstract carbon isotope changes during post hunnebergian ordovician time were investigated as the corresponding sequence of rocks is stratigraphically nearly complete in baltoscandia an attempt was made to use it to elaborate the general pattern of carbon isotope changes in most of the ordovician complications caused by several local hiatuses condensed sections and facies changes were mitigated by the study of overlapping sections a total of 326 new whole rock analyses and published data from 18 drill cores and 4 outcrops in estonia latvia and sweden were considered the following positive carbon isotope events were observed 1 the mid darriwilian excursion peak 13c value 1 9 in the aseri stage 2 the mid caradoc excursion 2 2 in the uppermost part of the keila stage 3 the first late caradoc excursion 2 3 in the lower part of the rakvere stage 4 the second late caradoc excursion 2 4 in the upper part of the nabala stage 5 the early ashgill excursion 2 5 in the lowermost part of the pirgu stage 6 the mid ashgill excursion 2 0 in the upper part of the pirgu stage 7 the widely known large hirnantian excursion in estonia the peak value reaches 6 7 in the porkuni stage the study interval comprises a long 19 ma period characterized by low magnitude carbon isotope changes and a following much briefer 12 ma interval with often medium scale shifts and a major event at the end the latter interval reflects onset in the mid caradoc of a period of variable climate alternation of arid and humid episodes which ended with a serious cooling and glaciation the baltoscandian and north american nevada 13c trends are in general lines very similar providing good prospects for global chemostratigraphical correlation and understanding of role of oceanic processes in carbon cycling

progradational sequences in miocene shoreline deposits southeastern caliente range california abstract an exceptionally well exposed marine nonmarine transition in middle miocene strata exists in the southeastern caliente range california about 50 individual progradational sequences form a succession that ranges in thickness from approximately 1000 m where predominantly nonmarine to more than 2500 m where predominantly marine paleogeographic evidence in basalt flows near the top of the succession and in overlying fluvial deposits indicates that these middle miocene strata were deposited across a north northwest trending shoreline a complete progradational sequence typically is several meters to a few tens of meters thick and includes strata that represent three intertonguing stratigraphic units individual sequences generally rest on a thin gravel deposit interpreted as a transgressive lag on an erosional surface the gravel is overlain by structureless siltstone or fine grained sandstone deposited at water depths where the rate of faunal mixing exceeded that of production of structures by physical processes these rocks grade upward into bedded fine sandstone deposited closer to shore where physical processes exceeded bioturbation crossbedded lenses of coarse sand or fine gravel in the upper part of this facies suggest the presence of fairly long period surface waves the bedded fine sandstone is sharply ove lain by a cross bedded coarse sandstone facies that is interpreted as a combined offshore bar rip channel surf zone assemblage cross strata dip dominantly offshore suggesting substantial deposition from rip currents a secondary shoreparallel mode of cross strata direction suggests longshore currents produced by surface waves from the northwest the crossbedded coarse grained sandstone grades upward into planar bedded medium grained sandstone that is interpreted as a beach foreshore this facies grades upward through structureless medium grained sandstone into nonmarine or lagoonal red and green mudstone of the caliente formation the middle miocene succession was deposited in a subsiding basin that was otherwise remarkably stable tectonically the position of the strand line differed no more than a few kilometers through a period of 1 to 3 m y the average duration of the transgressive regressive cycles a few tens of thousands of years together with their distribution in groups of three or four in the lower two thirds of the succession is consistent with the pattern of long term climatic cycles produced by periodicity of the earth s solar orbit and may be related to eustatic sea level changes attendant to the development of the antarctic ice cap changes in the pattern of progradation in the upper part of the succession and nearby basaltic eruptions may have been precursors to the onset of movement along th san andreas fault in this area 12 14 m y ago

a sequence stratigraphic model for an intensely bioturbated shallow marine sandstone the bridport sand formation wessex basin uk the bridport sand formation is an intensely bioturbated sandstone that represents part of a mixed siliciclastic carbonate shallow marine depositional system at outcrop and in subsurface cores conventional facies analysis was combined with ichnofabric analysis to identify facies successions bounded by a hierarchy of key stratigraphic surfaces the geometry of these surfaces and the lateral relationships between the facies successions that they bound have been constrained locally using 3d seismic data facies analysis suggests that the bridport sand formation represents progradation of a low energy siliciclastic shoreface dominated by storm event beds reworked by bioturbation the shoreface sandstones form the upper part of a thick up to 200 m steep 2 3 mud dominated slope that extends into the underlying down cliff clay clinoform surfaces representing the shoreface slope system are grouped into progradational sets each set contains clinoform surfaces arranged in a downstepping offlapping manner that indicates forced regressive progradation which was punctuated by flooding surfaces that are expressed in core and well log data in proximal locations progradational shoreface sandstones corresponding to a clinoform set are truncated by conglomerate lags containing clasts of bored reworked shoreface sandstones which are interpreted as marking sequence boundaries in medial locations progradational clinoform sets are overlain across an erosion surface by thin 10 m and overlie conglomerate lags at sequence boundaries sequence boundaries are thus interpreted as being amalgamated with overlying transgressive surfaces to form composite erosion surfaces in distal locations oolitic ironstones that formed under conditions of extended physical reworking overlie composite sequence boundaries and transgressive surfaces over most of the wessex basin clinoform sets corresponding to high frequency sequences are laterally offset thus defining a low frequency sequence architecture characterized by high net siliciclastic sediment input and low net accommodation aggradational stacking of high frequency sequences occurs in fault bounded depocentres which had higher rates of localized tectonic subsidence

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

the impact of paleogeography pco2 poleward ocean heat transport and sea level change on global cooling during the late ordovician abstract we performed sensitivity experiments with the global climate model genesis on two stages of the upper ordovician caradocian 454 ma ashgillian 446 ma under a range of atmospheric p co 2 values 8 18 pal pre industrial atmospheric level high and low sea level and two values of poleward ocean heat transport in order to determine the importance of these variables on global cooling we then coupled a three dimensional ice sheet model to the global climate model in order to investigate the necessary boundary conditions for ice sheet formation all simulations with a high sea level and normal heat transport remain free of ice sheets even with p co 2 levels as low as 8 pal in the caradocian simulations ice sheets form in three scenarios 1 with p co 2 of 8 pal and a low sea level and normal poleward ocean heat transport 2 with p co 2 of 8 pal and a high sea level and reduced 50 of normal poleward ocean heat transport and 3 with p co 2 of 15 pal and a low sea level and reduced poleward ocean heat transport in the ashgillian simulations ice sheets form in only two scenarios 1 with p co 2 of 8 pal and a low sea level and normal poleward ocean heat transport or 2 with p co 2 of 8 pal and a high sea level and reduced poleward ocean heat transport the ice sheets in the ashgillian experiments are larger and thicker than the ice sheets in the caradocian simulations because the southward movement of gondwana increased land area in the higher southern latitudes where ice sheets could grow the threshold for glaciation under ashgillian paleogeography is 8 pal and either a low sea level exposed shelves or a reduced poleward ocean heat transport while the paleogeographic evolution and a drop in p co 2 during the late ordovician cooled the global climate changes in additional factors were required to initiate ice sheet formation such as a drop in sea level a reduction in poleward ocean heat transport or a combination of both

on the origin of cenozoic and mesozoic third order eustatic sequences abstract the origin of third order eustatic sequences is reviewed by comparing recent sequence stratigraphic data to the latest best constrained astronomical model middle eocene to holocene icehouse sequences correspond to 1 2 myr obliquity cycles constraints from oxygen isotope records highlight the link between icehouse sea level lowerings sequence boundaries and 1 2 myr obliquity nodes mesozoic greenhouse sequences show some relation with the 2 4 myr eccentricity cycles suggesting that orbital forcing contribute to sea level change we suggest that during the icehouse large ice sheets associated with significant glacioeustatic changes 25 m up to 120 m changes were mainly governed by obliquity forcing during icehouse worlds obliquity forcing was the stongest control on global sea level and depositional sequences additionally during the middle eocene third order sequences were glacioeustatically driven in tune with 1 2 myr obliquity cycle suggesting that the presence of significant ice sheets is earlier than previously supposed i e early eocene in contrast during greenhouse worlds e g ephemeral small to medium sized or no ice sheets 0 25 m glacioeustatic changes the expression of obliquity in the sedimentary record is weak and intermittent instead the eccentricity signature which is the modulator of climatic precession is documented moreover we presume that greenhouse sequences on the myr scale are global and hence cannot be caused by regional tectonism e g intraplate stress or mantle hot blobs instead the eccentricity link implies a weaker glacioeustatic control because thermoeustasy is too small to explain the sea level changes stratigraphically well documented fourth order sequences may be linked to the astronomically stable strongest amplitude 405 kyr eccentricity cycle and possibly to 160 200 kyr obliquity modulation cycles fifth order sequences to the short 100 kyr eccentricity cycles and finally sixth order sequences to the fundamental obliquity 40 kyr and climatic precession 20 kyr cycles these astronomical cycles could be preserved in the sedimentary record and have been demonstrated to control sea level changes accordingly by placing depositional sequence orders into a high resolution temporal framework i e orbital periodicities standardization of eustatic sequence hierarchy may be possible

stratigraphic evolution from shoreface to shelf indenting channel depositional systems during transgression insights from the lower pliocene sua member of the basal upper onzole formation borbon basin northwest ecuador abstract shelf indenting canyons and their tributary systems are fairly common constituents of quaternary shelves of active continental margins but they have been rarely reported from older successions recognition of these prominent geomorphologic features in the ancient record has important implications not only for a proper understanding of shoreface to shelf depositional systems but also from a petroleum exploration standpoint as they represent efficient conduits for moving coarse grained river and nearshore borne sediments to the adjacent slope even during periods of relative rise in sea level coastal exposures of the lower pliocene sua member in the surroundings of sua northwest ecuador preserve the unusual juxtaposition of incising submarine channels onto nearshore deposits this succession accumulated along a narrow active continental margin during tectonically induced transgression and affords a rare opportunity to evaluate the stratigraphic evolution of such systems from an outcrop perspective a comprehensive facies characterization combined with application of sequence stratigraphic concepts has led to definition of the following physical surfaces and stratal units in ascending order i a polygenetic regionally extensive erosional surface resulting from the superposition of the wave ravinement surface onto the previous subaerial sequence boundary sb wrs ii a nearshore sand prone lithofacies succession comprising a condensed basal shellbed deepening upwards through lower shoreface bioturbated silty sandstones into inner shelf sandy mudstones iii two steep u shaped erosional features turbidite shelf entrenchment surfaces interpreted as shelf channels deeply incised into the subjacent nearshore sediments and marking an abrupt deepening of facies iv a thick fining upward sedimentary succession laid down within the confines of the channels by high and low density turbidity currents and including both bed load traction and suspended load deposits the overall fining and thinning upward character exhibited by the infill of these channels is thought to reflect decreasing flow energies and is consistent with the gradual cut off of clastic influx to their upper reaches in response to progressive detachment from an adjacent coastal source during relative rise in sea level based on detailed analysis of facies and a sequence stratigraphic interpretation of outcrop data this study contributes to extend the existing sequence stratigraphic schemes further attesting that shelf sediment bypass and deep water sedimentation can take place at sea levels other than lowstand

regressive and transgressive cycles in a rift basin depositional model and sedimentary partitioning of the middle jurassic hugin formation southern viking graben north sea abstract the jurassic hugin formation consists of shallow marine sandstones that belong to a significant hydrocarbon reservoir in the sleipner area in the norwegian north sea the formation encompasses coarsening upward units of mouth bar and shoreface facies interpreted to record delta outbuilding during regression and fining upward units with tidal channel dune and tidal flat facies interpreted as part of an estuary environment during transgression the correlations reveal that the studied part of the hugin formation consists of 8 sequences each with a transgressive and a regressive unit representing the transgressive systems tract and the highstand systems tract respectively the sequences are stacked retrogradationally landward as a result of rapid tectonic subsidence and rifting of the viking graben rifting led to the development of an elongate graben where tidal currents were amplified wave action damped and longshore drift as sediments supply reduced or absent lowstand and forced regressive systems tracts are not identified and their absence is interpreted to reflect suppression of relative sea level falls in a rapidly subsiding basin where the basin subsidence rate outpaced any potential fall in eustatic sea level through facies interpretation and sequence stratigraphic correlations between wells these regressive and transgressive units are shown to exhibit characteristic thickness trends in the form of sigmoidal shaped wedges stacked in an offset manner in a landward to basinward orientation these thickness trends illustrate sediment partitioning within the sequences and are explained by the relationship between accommodation versus sediment supply in terms of mass balance during regression the focus of sedimentation was pushed basinward and during transgression it was pushed landward as sediments were trapped there the mapping of these sequence stratigraphic units serves as input to reservoir models and to help increase recovery and identify new exploration targets

the early paleozoic carbon cycle a review of o c sr and s isotope trends for the entire phanerozoic shows that the present day values of isotope signals are similar to those at the proterozoic termination the sharp rise in 87sr 86sr since 65 ma has been attributed to an uplift and subsequent metamorphism and erosion associated with the himalayas and tibet this orogenic evolution has been postulated to have influenced the global organic and inorganic carbon cycles and climate as well a similar large scale orogeny the pan african event also dominated the neoproterozoic vendian times and the similarity of modern and neoproterozoic isotope values for seawater may therefore have had a comparable tectonic cause in this contribution we present the results of a numerical model of the coupled c alkalinity s sr cycles suggesting that the early paleozoic from early cambrian to late devonian evolution of sr o c and s seawater isotope signals could have been the consequence of progressive oxidation of a large reduced carbon reservoir exhumed during the pan african orogeny the 18o measured in brachiopod shells is used as a forcing of the model postulating that any change in the oxygen isotopic composition of seawater is the result of a disequilibrium in the organic carbon subcycle through the coupling of the oxygen isotopic and carbon cycles the calculated 13c 87sr 86sr and 34s are in good agreement with the data as is the reasonable calculated history for atmospheric pco2 and its relation to global climate

visions of ice sheets in a greenhouse world abstract backstripped eustatic estimates from new jersey and the russian platform show large 25 m and rapid 18 o record highlights the enigma this was the warmest interval of the past 200 my yet it was bracketed by two inferred eustatic falls of 25 m that were associated with two large 0 75 deep sea 18 o increases 92 93 ma mid turonian and 96 ma mid cenomanian we reconcile records of warm high latitudes with glacioeustasy by proposing that late cretaceous early eocene ice sheets generally reached maximum volumes of 8 12 10 6 km 3 20 30 m glacioeustatic equivalent but did not reach the antarctic coast hence coastal antarctica hence deep water remained relatively warm even though there were significant changes in sea level as the result of glaciation unlike the oligocene and younger icehouse world these ice sheets only existed during short intervals of peak milankovitch insolation leaving antarctica ice free during much of the greenhouse late cretaceous to middle eocene these results highlight the need to re evaluate the paradigm that polar ice sheets did not exist during times of warm high latitude climates

glaciation and deglaciation of the libyan desert the late ordovician record detailed outcrop studies at the flanks of al kufrah basin libya reveal the nature of glacially related sedimentation and post depositional deformation styles produced in association with the late ordovician glaciation during which ice sheets expanded northward over north africa to deposit the mamuniyat formation at the se basin flank jabal azbah the mamuniyat formation is sand dominated and incises interfingering braidplain and shallow marine deposits of the hawaz formation the glacially related sediments include intercalations of mud chip bearing tabular sandstones and intraformational conglomerates which are interpreted as turbidite and debrite facies respectively these record aggradation of an extensive sediment wedge in front of a stable former ice margin an increase in mudstone content northward is accompanied by the occurrence of more evolved turbidites a widespread surface bearing streamlined nw se striking ridges and grooves punctuates this succession the structures on the surface are interpreted to have formed during a regional north westward ice advance above siltstones bearing arthrophycus burrows and orthocone bearing sandstones beneath tidal bars testify to glaciomarine conditions for deposition of the underflow deposits beneath by contrast the northern basin margin jabal az zalmah is appreciably different in recording shallower water paralic sedimentation styles and major glaciotectonic deformation features although facies analysis also reveals northward deepening here a siltstone wedging from 8 to 50 m toward the north was deposited lower delta plain succeeded by climbing ripple cross laminated sandstones up to 60 m in thickness distal through proximal delta mouth bar deposits with occasional diamictite interbeds these rocks are deformed by thrusts and 50 m amplitude fault propagation folds the deformation locally sealed by a diamictite then overlain by conglomeratic lag during ultimate deglaciation integrating observations from both basin margins a model of fluvial dominated delta systems feeding a pulsed debrite and turbidite fan system in a shallow proglacial shelf is proposed

epipelagic chitinozoan biotopes map a steep latitudinal temperature gradient for earliest late ordovician seas implications for a cooling late ordovician climate abstract the early mid ordovician has long been considered a super greenhouse world based largely on high relative global sea levels and light stable oxygen isotope data from bulk carbonates an alternative and largely untested hypothesis has suggested that at least in equatorial palaeolatitudes there was a steady cooling trend through the early ordovician reaching the range of modern equatorial sea surface temperatures by the mid ordovician this hypothesis though controversial is supported by palaeobiogeographical studies of the early late ordovician sandbian zooplankton graptolite biotopes these are comparable to the modern planktonic foraminifera provinces and suggest a latitudinal temperature gradient that is similar to the modern one chitinozoans are also an important group of palaeozoic marine microfossils but with a poorly known biological affinity here we present an analysis of chitinozoan species palaeobiogeography for the early late ordovician sandbian c 460 ma which confirms that these microfossils and likely their parent organisms were epipelagic unlike the graptolites chitinozoans had their highest diversity and abundance south of 35 s during this time which strongly suggests adaptation to temperate to cold waters the distribution of chitinozoan biotopes during the gracilis time slice allows us to identify the austral subtropical subpolar and polar belts key oceanographic boundaries are identified as the subtropical subpolar transition at 35 s and tentatively the polar front at 55 70 s again these are in a similar position to those in the modern southern ocean chitinozoan biotopes also map a steep latitudinal faunal gradient that is comparable to that of the graptolites and that of modern plankton this likely indicates a steep latitudinal temperature gradient for early late ordovician seas and questions the notion that this interval of earth history had a greenhouse climate

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

high frequency eustatic sea level changes during the middle to early late ordovician of southern jordan indirect evidence for a darriwilian ice age in gondwana abstract the middle to early late ordovician hiswah and dubaydib formations in southern jordan provide a well constrained sedimentary record of 4th and 3rd order relative sea level change along the northern tectonically quiescent subpolar gondwana continental margin cyclic deposition occurred on a wave and storm dominated microtidal shelf dominated by shoreface and offshore transition zone facies cycles are commonly bounded by amalgamated bounding surfaces sb ts or sb mfs and are characterised by stacked transgressive systems tracts tsts the small facies dislocation across key sequence bounding surfaces indicates a 15 50 m change in relative sea level fourth order sequences are hypothesized to be paced by the long eccentricity 405 kyr cycle and consequently 3rd order sequences are calculated to be 1 2 m y and 2 4 m y long these correlate with the global eustatic sea level curve in comparison with mesozoic and cenozoic sequence stratigraphical data we conclude that the 1 2 m y cycles correspond with long obliquity cycles predominant in icehouse conditions and the 2 4 m y cycles with the long eccentricity cycle predominant during greenhouse conditions we propose a darriwilian ice age during which orbitally induced cold snaps caused the expansion and amalgamation of small medium scale ice sheets the reduced size of the correlative positive carbon isotope excursion compared to that in the hirnantian may indicate that the darriwilian ice sheets only reached the sea in a few places the absence of glacio terrestrial deposits of middle ordovician age is most likely due to subsequent glacial erosion during repeated ice advances up to and including those in the hirnantian the existence of darriwilian ice sheets is consistent with zooplankton province distributions that indicate a polar front at 40 s during the ice age it is likely that polar ice existed throughout the ordovician

ichnology of a late palaeozoic ice marginal shallow marine succession talchir formation satpura gondwana basin central india abstract the late palaeozoic glaciomarine talchir formation in the satpura gondwana basin central india hosts sparse to profuse trace fossils of various kinds although sporadically among the three principal depositional systems that gave rise to the talchir succession deposits of the ice contact fan delta system and the glacial outwash braidplain delta system lack trace fossils of any kind on the other hand within the deposits of the storm wave dominated shoreface shelf system a variety of animal traces is present including simple locomotion traces and non descript bioturbation three trace fossil assemblages could be recognized within these deposits assemblage i is dominated by cylindrichnus of the skolithos ichnofacies that occurs in storm laid medium to fine sandstones deposited in shoreface and transitional shoreface inner shelf environments in contrast assemblage ii comprising dominantly horizontal grazing cum deposit feeding traces cochlichnus helminthopsis phycosiphon planolites archaeonassa and psammichnites occurs on decimeter thick sandstone beds interlayered with mudstones deposited in lower shoreface to inner shelf environments trace fossil assemblage iii is known from a 6 9 m thick outer shelf to lower shoreface progradational succession showing a vertical transition from dark gray mudstone to fine sandstone the middle part of this particular section shows rhizocorallium and flat type zoophycos whereas the upper part hosts profuse cylindrichnus along with rosselia and teichichnus in lesser proportions this assemblage is comparable to the marine ichnofacies succession combining elements of both skolithos and cruziana ichnofacies the characteristic features of the talchir trace fossil assemblages are i low ichnodiversity ii presence of forms typically found the in marine environment iii co occurrences of vertical and horizontal traces of skolithos and cruziana ichnofacies iv presence of simple structures formed by trophic generalists v abundance of certain forms and vi presence of monospecific suites these characters are akin to those of brackish water ichnofaunas in the talchir succession the transition from an ice contact fan delta system to a relatively ice free open marine system indicates an enormous flux of fresh water into the sea reducing its overall salinity predominance of annelid and worm traces and the rarity of crustacean burrows in the talchir strata also suggest a stressful environment due to the prevailing cold climate and a lower than normal marine salinity it thus appears that mixing of fresh water during deglaciation might be a key controlling factor in shaping the characters of ichnofaunas in glacially influenced shallow marine environments

oxygen isotope records and salinity changes in the northeastern atlantic ocean during the last 18 000 years a comparison of the deglaciation 18o record of northeastern atlantic core su 81 18 with that calculated by adding the 18o changes due to sea surface temperature and ice volume variations shows that planktonic foraminifera have also recorded seawater 18o changes due to salinity local advection and evaporation precipitation e p changes using a simple ocean box model of meltwater dilution during the deglaciation we show that sea surface salinity covaried with temperature despite the injection of meltwater associated with the climate warming sea surface salinity was high when sea surface water was warm and low during cold events this implies that the combined effect of local e p changes and northward advection of saline subtropical water were the main factor responsible for the surface water salinity variations in the northeastern atlantic ocean during the last 18 000 years

three stages to form a large batholith after terrane accretion an example from the svecofennian orogen abstract the magmatic history of batholiths formed as the result of tectonic accretion mirrors the evolution of the underlying crust from island arcs to accretion and collapse chemical composition age and structure give information about the magma source and emplacement depths the target of this study is the central finland batholith cfgc that provides a magmatic record after terrane accretion during the svecofennian orogeny followed by orogenic collapse the crustal source of partial melting has changed during the crustal evolution and differentiation after the terrane accretion and when the stress regime has changed from contraction to extension here we present a three stage magmatic evolution model for accretional batholith in the svecofennian orogen in the first stage the lower crust of the accreted crust melts partially and produces calcic tonalitic melts that are emplaced above the upper middle crustal detachment zone group 1 1887 3 ma in the second stage the middle crust begins to melt producing calc alkalic granodioritic melts that are emplaced close to the surface group 2 1890 1886 ma and at both sides of the upper middle crustal detachment zone group 2 1884 1883 ma in the third stage the lower crust remelts and produces bimodal calc alkalic gabbroic and alkali calcic to alkalic granitic melts group 3 1881 1880 ma which are emplaced above the upper middle crustal detachment zone this kind of crustal differentiation produces a mafic lower an intermediate middle and a felsic upper crust the third stage represents the last magmatic event of the svecofennian accretional orogeny where the high velocity lowermost crust is formed during this stage crustal thickening and widespread mid crustal partial melting have inititated lateral flow after or concurrently with the emplacement of the group 2 granodiorite the lateral flow has reactivated large scale shear zones allowing the transportation of the group 3 rocks from the lower crust to the upper middle crustal detachment zone the change in the degree of deformation at the exposed parts from undeformed through local to pervasive is attributed to differences in the exhumation level

ground truthing late ordovician climate models using the paleobiogeography of graptolites 1 the 18o records obtained from conodonts suggest that during the mid ordovician equatorial temperatures stabilized at close to the present day a hypothesis broadly supported by published climate models however the degree to which equatorial temperatures represent global climate state and varied between different climatic modes greenhouse icehouse interglacial glacial and to what extent ordovician 18o and climate models truly reflect the global climate remain to be tested here we present paleobiogeographical data for the planktonic graptolites from the gracilis time slice i e early sandbian stage that just postdates the postulated onset of climate stabilization twinspan analysis and constrained seriation provide robust ecological groupings in paleobiogeographical presence absence data the highest level groups reflect tropical subtropical assemblages and a high southerly latitude assemblage constrained seriation defines latitude distinct biotopes that are considered equivalent to modern zooplanktonic provinces the distribution pattern of graptolite biotopes in the gracilis time slice show 1 that models explaining local graptolite ecological assemblages using lateral differentiation e g upwelling are to be favored above those using depth stratification and 2 a steep faunal gradient from equator to pole which is typical of a cool nongreenhouse world and comparable to the modern situation we therefore broadly support the climate stabilization hypothesis paradoxically the climate of the early late ordovician appears similar to that of the present day despite the higher atmospheric pco2 levels graptolite species were indicative of temperature controlled ocean water masses in much the same way as the modern zooplankton

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

the origin and recognition of laterally continuous carbonate cemented horizons in the upper lias sands of southern england abstract the upper lias sands of southern england contain numerous laterally extensive carbonate cemented horizons petrographical analyses of samples from outcrop sections and the marchwood no 1 borehole indicate that these horizons result from preferential cementation of bioclast rich clay poor sediments by comparison with interbedded clay rich bioclast poor sediments the alternation of the two sediment types is attributed to the effects of respectively fairweather and storm processes on a submerged marine shoal petrographical and ichnological data indicate an early distinction of the strongly and weakly cemented horizons the widespread extent of the cemented horizons as indicated by outcrop studies on the dorset coast is considered to be a direct consequence of episodic storm activity on the low relief shoal sedimentological palynological and petrophysical criteria are presented to assist in recognition of similar extensive cements in subsurface reservoir horizons that do not outcrop

long periodic variations in the earth s obliquity and their relation to third order eustatic cycles and late neogene glaciations abstract evolutive spectra of four high resolution climatic proxy records reveal that the spectral power in the obliquity frequency band increases during five well defined intervals in the last 5 3 million years these intervals correspond to periods of high amplitude variations in the tilt obliquity of the earth s axis connected with a 1 2 myr cycle and to sea level lowstands of third order eustatic cycles in the haq et al 1987 curve this implies that the development of major plio pleistocene glaciations proceeded episodically and that third order cycles are glacio eustatically controlled in at least two of these intervals sinistrally coiled neogloboquadrinids entered the mediterranean at times of glacial periods one interval starts around 2 8 ma and is characterized by the first occurrence of n atlantica s another interval begins at the plio pleistocene boundary around 1 8 ma and is characterized by the common occurrence of sinistrally coiled n pachyderma also during the late miocene third order cycles correlate well with the 1 2 myr cycle of obliquity a significant drop in the third order eustatic curve starting at the tortonian messinian boundary for instance coincides with a period of high amplitude variations in the obliquity time series connected with the 1 2 myr cycle

transgressive wave ravinement on an epicontinental shelf as recorded by an upper pennsylvanian soil nodule conglomerate sandstone unit kansas and oklahoma u s a abstract a thin 3 25 cm persistent conglomerate sandstone unit occurs at the base of the transgressive systems tract of an upper pennsylvanian cyclothem below a transgressive limestone in outcrop and subsurface in se kansas and ne oklahoma it has an erosional base and a sharp top and is composed of calcitic clasts some shell fragments and quartz sand and rare coal fragments clasts are rounded equant to elongate coarse sand to pebble sized and moderately to well sorted they are micritic to microsparitic or radial fibrous calcite grains and pisoids micritic and microsparitic clasts are heterogeneous and contain rounded to elongate moulds and radiating or concentric spar or micrite filled cracks pisoids have micrite cores and superficial calcite or clay rims the clasts have similar texture and composition to the calcitic nodules and rhizoconcretions in underlying paleosols and thus were probably derived from these soil nodules upper shoreface erosion during shoreline transgression excavated the underlying paleosols and eroded coeval transgressive deposits landwards of the shoreline the eroded sediments were then reworked and transported to the lower shoreface and inner shelf by storm return flows to be deposited as a soil nodule conglomerate sandstone unit its base is an amalgamated surface of wave ravinement and initial transgression as well as the lower sequence boundary the widespread conglomerate sandstone unit suggests extensive transgressive wave ravinement on an epicontinental shelf the transgressive record composed of upward deepening dominantly fine grained lithofacies defines a new type t c4 of transgressive record which is composed of mixed carbonate and siliciclastic lithofacies with a simple transgressive lag on a low relief shelf

a coupled climate ice sheet modeling approach to the early cenozoic history of the antarctic ice sheet abstract the sudden widespread glaciation of antarctica and the associated shift toward colder temperatures near the eocene oligocene boundary 34 ma represents one of the most fundamental reorganizations of the global climate system recognized in the geologic record this glacial inception and the subsequent evolution of the early east antarctic ice sheet eais are simulated using a new coupled global climate dynamical ice sheet model accounting for the paleogeography greenhouse gas concentrations changing orbital parameters and varying ocean heat transport suites of long 105 yr climate ice sheet simulations are used to investigate the effects of declining atmospheric co2 compared to those of the tectonic opening of southern ocean gateways and the timing of mountain uplift in the antarctic interior in contrast to the established paradigm for the glaciation of antarctica which centers on the opening of the southern ocean gateways and the thermal isolation of the continent our results show that declining cenozoic pco2 may have played the dominant role first small isolated ice caps formed on the highest antarctic plateaus then as a co2 threshold between 3 and 2 pre industrial level pal was crossed height mass balance feedbacks were initiated during orbital periods with cold austral summers triggering much larger highly dynamic terrestrial ice sheets as co2 continued to decline these isolated ice caps eventually merged into a permanent continental scale eais in our model neither the opening of the southern ocean gateways nor mountain uplift significantly affected the timing of the major ice sheet transition given a scenario of gradually declining co2 from 4 to 2 pal over 10 million years around the eocene oligocene boundary

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

rip currents as a geological tool this paper considers the nature and sedimentary significance of rip currents rip currents are the main factor responsible for the transport of coarse sediments from the littoral zone to greater depths such sediments deposited outside the zone of wave deformation may be identified as storm rip current increments within sediments deposited during fair weather composite beds deposited during a given wave cycle by storm rip currents are closely similar to turbidites and many so called fluxo turbidites described from flysch deposits using data gathered in studies conducted on the modern baltic coast supplemented by experimental work and theoretical considerations a sedimentary model is proposed the model may be used to interpret possible rip current deposits among shallow water turbidites and both modern storm sediments and ancient tempestites language en