ice sheet evolution in james ross basin weddell sea margin of the antarctic peninsula the seismic stratigraphic record the stratigraphic architecture of the james ross basin nw weddell sea continental shelf shows three major phases of deposition preglacial ice sheet growth and ice sheet dominated the preglacial phase shows stratigraphic sequences that are similar to low latitude margins during the ice sheet growth phase there is evidence for initial ice sheet grounding in the southern part of the basin which migrated in steps northward sediment supply was also high during the ice sheet growth phase during the ice sheet dominated phase both north and south parts of the basin inherited all the classic features indicative of glacial strata large glacial troughs hundreds of meters deep and tens of kilometers wide truncation of sequences overlain by chaotic seismic facies and topped with coherent laminated reflections and an oversteepened continental slope the james ross basin strata are divided into five units s5 s1 each unit is characterized based upon seismic facies and stratigraphic architecture and the age of each unit is inferred from a seismic stratigraphic age model end member strata s5 and s1 were drilled and dated during shaldril and help to constrain this age model in total 34 grounding events of the antarctic peninsula ice sheet apis are recorded on the continental shelf this is similar to the 31 grounding events imaged on the pacific side of the peninsula ice grounding on the continental shelf is shown to have occurred as early as late middle miocene to early late miocene in the southern part of the basin the ice sheet advanced northward to encompass the northern peninsula by late miocene time the seven oldest glacial unconformities are believed to predate all previously identified unconformities on the peninsula continental shelf an expanded section of late pliocene pleistocene deposits shows a minimum of 10 grounding events this record more closely matches the deep sea oxygen isotope record than previous seismic studies of antarctic margins have shown

combined flow processes and sedimentary structures on the shoreface of the wave dominated grande riviere de la baleine delta abstract shoreface sediments exposed in raised deltaic deposits along the hudson bay coast are characterized by a thick succession of predominantly parallel laminated and small scale cross laminated fine sand ripple forms where preserved include both combined flow and wave ripple geometries hummocky cross stratification is present in lower parts of the section similar facies have been found in cores from the present day shoreface measurements of waves currents and suspended sediment concentrations in 10 m water depth conducted over a 15 day period in the fall of 1999 captured four different meteorological periods these included an intense four day storm which sustained significant wave heights over 3 m for 15 hours and a storm surge of over 1 m height fair weather conditions produce weak alongshore tidal flows but oscillatory motions generated by small local waves predominate during storms orbital motions dominate the flow but a strong inertial flow generated by wind forcing is also present comparison with a published combined flow bedform stability diagram suggests that the predominant bedforms at 10 m depth would be small 2 d wave dominated ripples and small weakly asymmetrical 3 d ripples under fair weather conditions and large 3 d ripples and upper plane bed under storm wave conditions the predominantly interbedded parallel laminated and small scale cross laminated facies is therefore interpreted to be characteristic of the upper shoreface where the seabed experiences intense oscillatory motions under all storms and even some fair weather conditions combined flow ripples would form during storm events because of the stronger wind driven mean residual current hummocky cross stratification forms in deeper water during storms where the ratio of maximum orbital velocity to mean residual current velocity remains for longer periods in the large 3 d ripple field during the waning phase of storms a numerical bed predictor model using the 15 day data set as input also suggests bimodal conditions fluctuating between wave ripples and upper plane bed at 10 m water depth this model further indicates that combined flow ripples would form rarely but tends to underestimate the threshold of sediment suspension

discontinuity surfaces clinoforms and facies architecture in a wave dominated shoreface shelf parasequence abstract detailed outcrop analysis of a wave dominated shoreface shelf parasequence exposed in the book cliffs utah reveals minor stratigraphic discontinuities which define clinoforms two types of discontinuity are recognized each with a distinctive lithologic character and geometry nondepositional discontinuities are marked by an abrupt decrease in the thickness and amalgamation of storm generated event beds and are interpreted to record hiatuses in sedimentation these discontinuities define clinoforms with a concave upward geometry that dip gently 0 02 0 58 over distances of 800 6000 m down depositional dip erosional discontinuities are marked by an abrupt increase in event bed amalgamation grain size and sand content and are interpreted as enhanced storm wave scours these discontinuities define more steeply dipping 0 22 0 95 concave upward clinoforms that extend over 100 1600 m down depositional dip and 500 1500 m along depositional strike the distribution and amalgamation of minor stratigraphic discontinuities defines linear zones of distinctive facies architecture that are oriented parallel to the paleoshoreline trend using the simple assumption that the shoreface shelf equilibrium profile remained approximately constant for each type of discontinuity throughout shoreface migration the bruun rule intra parasequence facies architecture can be speculatively interpreted in terms of shoreline trajectory which reflects the balance between sediment supply and accommodation the resulting interpretations support the notion that shoreline trajectory exerts a strong control on intra parasequence facies architecture and preservation of the shoreface shelf profile

high resolution sequence stratigraphy of lower paleozoic sheet sandstones in central north america the role of special conditions of cratonic interiors in development of stratal architecture well known difficulties in applying sequence stratigraphic concepts to deposits that accumulated across slowly subsiding cratonic interior regions have limited our ability to interpret the history of continental scale tectonism oceanographic dynamics of epeiric seas and eustasy we used a multi disciplinary approach to construct a high resolution stratigraphic framework for lower paleozoic strata in the cratonic interior of north america within this framework these strata proved readily amenable to modern sequence stratigraphic techniques that were formulated based on successions along passive margins and in foreland basins settings markedly different from the cratonic interior parasequences parasequence stacking patterns systems tracts maximum flooding intervals and sequence bounding unconformities can be confidently recognized in the cratonic interior using mostly standard criteria for identification the similarity of cratonic interior and foreland basin successions in size geometry constituent facies and local stacking patterns of nearshore parasequences is especially striking this similarity indicates that the fundamental processes that establish shoreface morphology and determine the stratal expression of retreat and progradation were likewise generally the same despite marked differences in tectonism physiography and bathymetry between the two settings our results do not support the widespread perception that paleozoic cratonic interior successions are so anomalous in stratal geometries and constitute such a poor record of time that they are poorly suited for modern sequence stratigraphic analyses the particular arrangement of stratal elements in the cratonic interior succession we studied is no more anomalous or enigmatic than the variability in architecture that sets all sedimentary successions apart from one another thus paleozoic strata of the cratonic interior are most appropriately considered as a package that belongs in a continuum of variable stratigraphic packages reflecting variable controls such as subsidence and shelf physiography special conditions of exceptionally slow subsidence rate shallow bathymetry and nearly flat regional shelf gradient are manifest mostly by the presence of individual systems tracts of relatively long duration that extend for much greater distances across depositional strike than those that characterize successions deposited in more dynamic tectonic and physiographic settings these results suggest that if other cratonic interior successions are as anomalous as reported a low sediment supply may have played a primary role in development of their apparently condensed stratal architecture the results also lead us to suggest that a nonvegetated lower paleozoic landscape played a relatively insignificant role in the development of what are commonly perceived to be enigmatic stratigraphic features of sheet sandstones particularly their widespread yet thin geometry and a scarcity of shale and siltstone

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

stratigraphy and sedimentology of the middle ordovician hawaz formation murzuq basin libya the middle ordovician hawaz formation is a 200 m 660 ft thick succession made up of fine grained quartzarenites displaying a variable degree of bioturbation it records the deposition in a large scale low gradient estuary which was partially controlled by tectonic extension the upper boundary of the formation is marked by two erosion surfaces unconformities u1 and u2 related to the late ordovician glaciation the u1 and u2 erosion surfaces generated a pronounced paleotopography that controlled the deposition of the upper ordovician sequences tectonism influenced the paleogeography although faults were unimportant from the point of view of sedimentary thickness tectonic subsidence was moderate and accumulation rates were low physiography favored tidal power especially during transgressive episodes when the coastal embayment was flooded we defined 11 lithofacies forming 6 facies associations these associations are subtidal sandstones storm reworked shoreface sandstones shoreface to beach sandstones channel sandstone bodies nearshore to inner platform sandstones and k bentonites trace fossil assemblages match skolithos and cruziana archetypal ichnofacies on the basis of the dominant facies associations and ichnofacies we divided the formation into three informal units from base to top hw 1 hw 2 and hw 3 periodically volcanic ash was supplied to the basin from distal eruptive centers and was preserved as thin beds of k bentonite interstratified with the shoreface sandstones but not with the tidal dominated sandstones we divided the hawaz formation into five third order depositional sequences lowstand deposits were not identified the lower boundaries of transgressive systems tracts are tidal ravinement surfaces or sequence boundaries whereas the upper boundaries are flooding surfaces the transgressive systems tracts are constituted by early transgressive tidal deposits separated by a wave ravinement surface from the late transgressive storm dominated deposits highstand systems tracts consist of bioturbated shoreface to beach sandstones which record seaward shoreline progradation emilio obtained his ph d in geology from the universitat de barcelona in 1988 since then he has been a lecturer in basin analysis and petroleum geology he has been involved in several research projects on sedimentology and basin analysis in spain northern africa antarctica and south america his present day research interests include three dimensional modeling of sedimentary bodies and reservoirs a professor of stratigraphy at the universitat de barcelona m marzo s research interest focuses on the application of clastic sedimentology sequence stratigraphy reservoir modeling and basin analysis to the exploration and production of hydrocarbons he has been involved in several research projects funded by oil companies in southern europe the north sea south america and northern africa jordi m de gibert received his ph d from the universitat de barcelona in 1996 after a period at the university of utah he returned to barcelona in 1999 where he currently holds a position as a tenure track lecturer his interests and areas of expertise include trace fossils their paleobiological significance and their implications for understanding ancient depositional environments k tawengi received a b sc degree in geology from alfateh university libya in 1984 and an m sc degree in sedimentology and stratigraphy from durham university england in 1996 he worked as an explorationist with agip oil company in libya from 1985 to 2000 and since then has worked as a senior exploration member with repsol exploration in murzuq s a his main fields of interest are sedimentology stratigraphy and subsurface geology a khoja graduated in 1972 from the university of libya he received a diploma in petroleum geosciences from oxford polytechnic 1991 and an m sc degree from oxford brookes university 1993 he joined the national oil corporation of libya in 1972 and is presently the regional studies superintendent in the national oil exploration department nestor obtained his degree in geology from cordoba university argentina in 1982 and his postgraduate in petroleum geology in the university of cuyo argentina he joined ypf in 1984 and worked in their exploratory department in mendoza plaza huincul and neuquen until the year 2000 subsequently he became libya team leader for repsol ypf in madrid he is currently director of exploration and production in brazil

long period orbital climate forcing in the early palaeozoic facies indicators and geochemical proxies of early palaeozoic global climate cooling suggest episodes of fluctuating glacio eustasy and severe cold or glaciation from the mid late cambrian to silurian c 85 myr with a mean frequency of 2 6 myr long period orbital time series predicted through the phanerozoic are used to generate sine waves to test against these data the null hypothesis of no orbital influence is rejected with a high confidence level cooling episodes appear most frequent through the late ordovician leading up to the hirnantian glacial maximum but even greenhouse intervals of the early mid ordovician and early silurian provide evidence for periodic cooling episodes

boundary layer cloud and drizzle variability in the southeast pacific stratocumulus regime abstract the southeast pacific stratocumulus regime is an important component of the earth s climate system because of its substantial impact on albedo observational studies of this cloud regime have been limited but during the past 5 yr a series of cruises with research vessels equipped with in situ and remote sensing systems have provided unprecedented observations of boundary layer cloud and drizzle structures these cruises started with the east pacific investigation of climate epic 2001 field experiment followed by cruises in a similar area in 2003 and 2004 pan american climate studies pacs stratus cruises the sampling from these three cruises provides a sufficient dataset to study the variability occurring over this region this study compares observations from the 2004 cruise with those obtained during the previous two cruises observations from the ship provide information about boundary layer structure fractional cloudiness cloud depth and drizzle characteristics this study indicat

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

sedimentology ichnology and paleoenvironments of the upper cretaceous wenonah and mt laurel formations new jersey abstract the wenonah and mt laurel formations of the new jersey coastal plain comprise a shallow subtidal coarsening upward regressive sequence that accumulated along a wave dominated coast in late cretaceous maestrichtian time the highly bioturbated muddy very fine to fine quartz sands of the wenonah formation were deposited mainly in the offshore zone of an inner shelf below storm wave base distinctive trace fossils include rosselia socialis and a planar form of zoophycos the mt laurel formation consists of two lithofacies in the northern new jersey coastal plain the formation is characterized by burrowed clean quartz sands that are thinly interbedded with muddy sand and mud characteristic trace fossils include ophiomorpha nodosa rosselia socialis two forms of skolithos and a delicate branching burrow deposition occurred mainly in the offshore shoreface transition zone between fair weather and storm wave base storm deposits include burrowed sands displaying trough cross stratification hummocky cross stratification lenticular and wavy ripple bedding and parallel lamination fair weather deposits consist of thoroughly bioturbated muddy sand and thin bedded mud at updip exposures shoreface deposits include trough cross strati ied burrowed fine to medium sands paleocurrents were bipolar with a predominant northwest onshore mode a massive sand lithofacies characterizes the mt laurel formation in the central and southern new jersey coastal plain and is characterized by highly bioturbated medium to coarse sand rare cross stratification indicates a polymodal paleocurrent pattern with predominantly northwest transport distinctive trace fossils include ophiomorpha nodosa and skolithos linearis this lithofacies accumulated mainly as shoals or bars in the transition and offshore zones of the inner shelf a shoreface ebb tidal delta facies was locally developed the marshalltown wenonah and mt laurel formations comprise a depositional sequence in the sense of haq et al 1987 that accumulated during a third order eustatic sea level cycle uza 4 4 the wenonah mt laurel regression occurred during a sea level high stand as sediments began to overflow estuaries created during the marshalltown transgression subsequent falling sea level intensified the regression and ultimately led to erosional truncation of the mt laurel sediments then eustatic rise in sea level during the late maestrichtian uza 4 5 led to ravinement during this ravinement event a fossiliferous and pebbly glauconitic quartz sand lithofacies was deposited as a transgressive lag in the transition and offshore zones of the inner shelf these transgressive sheet sand ediments comprise the lower part of the navesink formation the depositional model developed herein for the wenonah mt laurel regressive interval integrates both sedimentologic and ichnologic data this model is compared with the well known gallup and blackhawk cretaceous regressive sequences of the north american western interior it should be applicable in the evaluation of other regressive sequences in cretaceous strata of the u s atlantic coastal plain and western interior as well as epeiric sea cycles from other similar settings around the world

al2o3 tio2 ratio of the clay fraction of late ordovician silurian carbonate rocks as an indicator of paleoclimate of the fennoscandian shield abstract in the fennoscandian shield the fluctuations in arid humid climate conditions are inferred from the al2o3 tio2 ratio of the clay fraction of estonian carbonate rocks of the baltoscandian sedimentary basin the late ordovician climate of the fennoscandian shield was mainly semi humid with several humid pulses in the sandbian mid katian and hirnantian the post hirnantian climate was fluctuating between humid and arid until stable humid conditions were established in the middle and early late llandovery starting from the latest llandovery the climate turned semi arid and arid the ordovician silurian long term trend toward aridity was caused by the equator ward drift of the baltica plate shorter term fluctuations of al2o3 tio2 reflect climate changes of the global extent

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

sedimentology and high resolution sequence stratigraphy of the late middle to late miocene angostura formation western borbon basin northwestern ecuador an integrated analysis of facies and palaeontological content of the middle to late miocene c 12 4 9 1 ma angostura formation led to the identification of facies associations indicative of shoreface and inner shelf settings the angostura formation comprises eight high frequency sequences that are stacked to form a tectonically driven lowstand sequence set the most complete examples of sequences are bounded by transgressively modified unconformities and exhibit a threefold subdivision 1 a basal suite of deepening upward shoreface sediments transgressive systems tract including a base of cycle shell concentration 2 a mid cycle shell bed consisting of molluscan shells dispersed in a matrix of inner shelf muddy fine grained sandstones 3 a shallowing upward unit of inner shelf to lower shoreface sediments almost barren of mollusc fossils highstand systems tract biostratigraphic constraints allowed a reasonable correlation between sequence bounding unconformities and the late middle to late miocene high frequency glacio eustatic changes derived from recent 18 o studies this correlation has far reaching implications and leads to the following conclusions 1 glacio eustasy in tune with oxygen isotope changes at fourth order frequency 200 ka 1 ma duration may have been the principal factor regulating stratigraphic packaging in the angostura formation 2 these sequences provide an excellent shallow marine outcrop record of late middle to late miocene antarctic glaciations

a record of astronomically forced climate change in a late ordovician sandbian deep marine sequence ordos basin north china abstract the late ordovician pingliang formation on the southwestern margin of the ordos basin north china consists of rhythmic alternations of shale limestone and siliceous beds to explore the possible astronomical forcing preserved in this lithological record continuous lithological rank and magnetic susceptibility ms stratigraphic series were obtained from a 34 m thick section of the pingliang formation at guanzhuang power spectral analysis of the ms and rank series reveal 85 5 cm to 124 cm 23 cm to 38 cm and 15 cm to 27 cm thick sedimentary cycles that in ratio match that of late ordovician short eccentricity obliquity and precession astronomical cycles the power spectrum of the ms time series calibrated to interpreted short orbital eccentricity cycles aligns with spectral peaks to astronomical parameters including 95 kyr short orbital eccentricity 35 3 kyr and 30 6 kyr obliquity and 19 6 kyr and 16 3 kyr precession cycles the 15 cm to 27 cm thick limestone shale couplets mainly represent precession cycles and siliceous bed deposition may be related to both precession and obliquity forcing we propose that precession forced sea level fluctuations mainly controlled production of lime mud in a shallow marine environment and transport to the basin precession and obliquity controlled biogenic silica productivity and temperature dependent preservation of silica may have been influenced by obliquity forcing

middle ordovician darriwilian conodonts from southern tibet the indian passive margin implications for the age and correlation of the roof of the world

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

darriwilian shallow marine deposits from the sultanate of oman a poorly known portion of the arabian margin of gondwana the amdeh formation is a 3 4 km stack of sparsely fossiliferous quartzites and shales which crops out in the al hajar mountains near muscat here we describe the uppermost member am5 that can be dated biostratigraphically as darriwilian and which is the outcrop equivalent and probably the seaward continuation of the saih nihayda formation in the ghaba salt basin of northern oman the outcrops at wadi daiqa and hayl al quwasim consist of 690 m of quartzitic sandstones shales and bivalve rich shell beds trace fossils referable to the cruziana and skolithos ichnofacies abound the member comprises storm dominated shelf shoreface and delta deposits a number of new discoveries have been made in the outcrops fragments of the arandaspid fish sacabambaspis ossicles and moulds of the early disparid crinoid iocrinus two new genera of conodont an occurrence of the rare trinucleid trilobite yinpanolithus and palynological and sedimentological evidence of more continuous floian darriwilian deposition than is usual in the region sea levels during middle ordovician time are estimated to have been 50 200 m above present levels and a wide low gradient shelf covered much of arabia similar trace fossils and storm dominated micro tidal sedimentary rocks occur throughout the region small changes of sea level possibly caused by the growth and melting of polar ice sheets could lead to substantial seaward or landward shifts of facies belts the am5 deposits are thick compared to most equivalents in arabia implying active subsidence and a ready supply of sediment

unusual coarse clastic wave dominated shoreface deposits pliocene to middle pleistocene northern chile implications for coastal facies analysis abstract pliocene to mid pleistocene shallow marine strata exposed at gatico northern chile 22 30 s 70 14 w display unusual characteristics for coarse grained wave dominated shoreface deposits a 15 m2000 m strike section of pliocene to mid pleistocene strata exposed within small late pleistocene sea cliffs contain four facies beach foreshore channel upper shoreface and rip channel beach and upper shoreface facies predominate the latter consists of locally bioturbated coarse grained pebbly sandstone with 1 single sets of planar to tangential cross strata up to 3 5 m thick with reactivation surfaces and 2 sets of medium to large scale cross strata that were deposited on inclined set surfaces up to 15 some of the inclined set surfaces extend up dip of cross set terminations inclined parallel laminae and bioturbated beds lie subparallel to the inclined set surfaces interpretation of these unusual bedforms has been made through analogy with the present day microtidal wave dominated coastline at gatico by facies analysis and by comparison with modern day wave dominated shorelines the large scale sets of cross strata record the migration of simple two dimensional 2 d dunes and represent the slipface deposits of nearshore bars parallel laminae represent the bar surface medium scale dunes migrated up the inclined set bounding surfaces which lie subparallel to older preserved bar surfaces paleocurrent data indicate that bar migration was largely directed obliquely onshore towards the southeast the bar slipface deposits are considered to record the recovery of a nearshore bar immediately following storm conditions bar recovery involves re establishment of a slipface and rapid onshore migration erosion surfaces that appear as the extension of lower set boundaries beyond the termination of cross strata sets and reactivation surfaces are considered to be storm generated the episodic nature of bar migration is indicated by the development of heavily bioturbated horizons the small scale ascending cross sets may reflect fair weather conditions with northwestward dune migration up the modified bar slipface despite the abundance of barred shorelines around the world at present their deposits are not widely described from the rock record suggesting that they are either not preserved or not recognized the scale and internal characteristics of these nearshore bars closely resemble those of some tide generated bedforms e g compound cross strata in marine sediment with bimodal bipolar paleocurrents and evidence of episodic bedform migration the similarity of these north chilean deposits to documented tidal deposits suggests that an element of caution may be required in the identification of tidal deposits from the rock record

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

middle ordovician multi stage penecontemporaneous karstification in north china implications for reservoir genesis and sea level fluctuations abstract palaeokarsts contain important information about subaerial exposures due to sea level regression which has been considered as a dominant genesis of reservoirs herein we describe a typical sedimentary core section of the middle ordovician majiagou formation that records multi stage penecontemporaneous karstification in north china petrographic observations and stable isotopic data reveal high frequency penecontemporaneous subaerial exposure and resulting karstification evidence for this includes erosional surfaces gypsum breccias facies controlled dissolution vugs and vadose infillings fabric selective dissolution and negative carbon and oxygen isotopic excursions karstification generally occurred in the middle and upper parts of an upward shallowing sequence within a single depositional cycle leading to meteoric water dissolution of shoals and mounds this significantly enlarged the primary pores and generated new reservoir space as such high quality hydrocarbon reservoirs became well developed which are regularly and cyclically stacked at locations a few meters below the multiple exposure surfaces reservoir porosity also varies systematically with the high frequency sedimentary cycles and is generally at a maximum in the middle and upper parts of a cycle the multi stage penecontemporaneous subaerial exposure and resulting karstification reflect high frequency changes in relative sea level in north china at this time thereby providing a framework for identifying thin carbonate reservoirs in and around the ordos basin

spatial variation in late ordovician glacioeustatic sea level change abstract mass extinction of late ordovician marine fauna closely coincided with southern hemisphere glaciation the sequence stratigraphic architecture of shallow marine deposits informs estimates of glacioeustatic sea level change at sites both proximal and distal to the reconstructed ordovician ice sheet s and contemporaneous changes in ice volume a recent correlation framework for the stratigraphic architectures of one near and one far field late ordovician margin concluded that the late ordovician glaciation encompassed multiple long term cycles of ice volume growth and retreat with superimposed higher frequency cycles here we posit that similar to cenozoic glacial cycles glacial isostatic adjustment can preclude synchronous and similar magnitude or directional changes in late ordovician sea level between ice proximal and ice distal locations and hence distort a globally correlative sequence stratigraphy we explored whether long duration i e million year late ordovician glacial cycles should produce a globally coherent eustatic record of sea level change between ice proximal and ice distal margins using a gravitationally self consistent theory that accounts for the deformational gravitational and rotational perturbations to sea level on a viscoelastic earth model we adopted a late ordovician paleogeography and a synthetic continental ice sheet distribution and volume informed by the areal extent of glaciogenic deposits and geochemical records respectively we demonstrate that modeled million year late ordovician glacial cycles produce sea level histories on near and far field margins that differ from eustasy and from one another due primarily to elastic flexure and associated gravitational effects while predicted far field sea level histories faithfully preserve the temporal structure of modeled glacioeustasy their amplitude may differ from eustasy by as much as 30 40 the impact of glacial isostatic adjustment is largest at the margins of glaciated continents and these effects can be of the same order of magnitude as the eustatic and even induce a local sea level rise during an episode of ice growth and eustatic sea level fall and vice versa in this regard stratal surfaces of maximum regression and flooding expressed at near field margins need not reflect global eustatic trends in ice sheet growth and decay respectively and thus may not provide chronostratigraphic horizons for correlation with far field sequence stratigraphic architectures

co2 forced climate thresholds during the phanerozoic abstract the correspondence between atmospheric co2 concentrations and globally averaged surface temperatures in the recent past suggests that this coupling may be of great antiquity here i compare 490 published proxy records of co2 spanning the ordovician to neogene with records of global cool events to evaluate the strength of co2 temperature coupling over the phanerozoic last 542 my for periods with sufficient co2 coverage all cool events are associated with co2 levels below 1000 ppm a co2 threshold of below 500 ppm is suggested for the initiation of widespread continental glaciations although this threshold was likely higher during the paleozoic due to a lower solar luminosity at that time also based on data from the jurassic and cretaceous a co2 threshold of below 1000 ppm is proposed for the initiation of cool non glacial conditions a pervasive tight correlation between co2 and temperature is found both at coarse 10 my timescales and fine resolutions up to the temporal limits of the data set million year timescales indicating that co2 operating in combination with many other factors such as solar luminosity and paleogeography has imparted strong control over global temperatures for much of the phanerozoic

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

circum arctic petroleum potential more than 30 sedimentary basins surround the arctic ocean a reconstruction of the ancient structural depositional and climatic trends has been made of the more important basins to evaluate the relative hydrocarbon potential of this frontier area although tectonic movement has been active throughout the phanerozoic four periods of tectonism were of particular importance in forming and developing the arctic basins the caledonian orogeny during the devonian the formation of the ural mountains during the permian triassic collision of the siberian and baltic platforms the opening of the canada deep during the late jurassic early cretaceous and sea floor spreading along the nansen gakkel ridge during the early tertiary these events created a complex paleoenvironmental history for the arctic basins resulting in a variety of sources seals and reservoirs end page 101

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

shannon sandstone in wyoming a shelf ridge complex reinterpreted as lowstand shoreface deposits abstract the shannon sandstone campanian of wyoming was formerly interpreted as two stacked shelf ridge complexes sand was believed to have been transported from a time equivalent shoreline 110 150 km to the west and reworked or molded into ridges at the depositional site we show that this time equivalent shoreline section at lucerne wyoming consists of not one shoreface sandbody but two superimposed they are both storm dominated and the lower one terminates in beach facies capped by root traces there must have been a rise in relative sea level of at least 14 m to make accommodation space for the second shoreface sandbody in the salt creek area and the adjacent subsurface teapot dome there are two sandier upward facies successions in the lower succession there are abrupt vertical acies contacts between offshore bioturbated sandstones thicker hummocky cross stratified sandstones and coarser cross bedded sandstones there is also evidence that the cross bedded sandstones rest erosively on underlying facies the trace fauna in the sandstones includes macaronichnus segregatis rosselia socialis and ophiomorpha the m segregatis suggests a foreshore or upper shoreface depositional environment and r socialis indicates a lower to middle shoreface neither are characteristic of a shelf ridge complex the abrupt and probably erosive facies contacts along with the trace fauna suggest that the cross bedded sandstones in this succession represent a shoreface deposit that formed during a stage of actively falling relative sea level overlyi g muddy bioturbated sandstones indicate that shoreface deposition was terminated by a transgression the upper sandier upward succession contains facies and trace fauna similar to those of the lower succession and is also interpreted as a prograding shoreface rather than the sand moving 110 150 km from the shoreline and then being molded into shelf ridge complexes at salt creek we suggest that the shoreline moved into the salt creek area as the result of a fall in relative sea level it was transgressed and moved back westward a second fall of relative sea level moved the shoreline back to salt creek and a final transgression terminated shannon deposition

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

characterization and discrimination of paleokarst breccias and pseudobreccias in carbonate rocks insight from ordovician strata in the northern tarim basin china abstract ordovician carbonates in the central and southern halahatang area of the tarim basin host highly productive hydrocarbon reservoirs previous studies suggest that the yingshan and yijianfang formations the primary producing intervals have been extensively karstified however the current author has not observed significant karst features in the formations based on the detailed study of 380 m long cores from 16 wells in the current study true breccias are only identified in two wells in the lianglitage formation including crackle and mosaic breccias as well as chaotic clast supported breccias the breccias are interpreted as karst related breccias that formed in near surface environments during pre silurian subaerial exposure and timing of karstification probably coincided with eustatic sea level fall caused by the middle katian glaciation widespread karst features of the yijianfang and yingshan formations identified as breccias or sediment filled cavities in many previous studies are actually only a mottled fabric pseudobreccia caused by selective diagenetic alteration and infiltration of bitumen in the deeper subsurface the color difference between pseudoclasts and pseudomatrix does not reflect a mixture of clasts and cave sediments but instead is characterized by distinct nevertheless commonly subtle variations in the limestone texture and whether bitumen is present one the basis of core and thin section observations sem scanning electron microscope and xrd x ray diffraction analyses the author presents criteria to distinguish karst breccias and pseudobreccias that can be applied to other similar carbonate deposits discrimination between karst breccias and nodular limestones is also presented differentiation of karst and pseudokarst features serves not only an academic interest but also has important applications for the exploration and development of carbonate reservoirs

closed form analytic solution of cloud dissipation for a mixed layer model abstractstratocumulus clouds play an important role in climate cooling and are hard to predict using global climate and weather forecast models thus previous studies in the literature use observations and numerical simulation tools such as large eddy simulation les to solve the governing equations for the evolution of stratocumulus clouds in contrast to the previous works this work provides an analytic closed form solution to the cloud thickness evolution of stratocumulus clouds in a mixed layer model framework with a focus on application over coastal lands the diurnal cycle of cloud thickness and whether or not clouds dissipate are of particular interest an analytic solution enables the sensitivity analysis of implicitly interdependent variables and extrema analysis of cloud variables that are hard to achieve using numerical solutions in this work the sensitivity of inversion height cloud base height and cloud thickness with respect to initial and boundary conditions such as bowen ratio

storm generated sedimentary structures in subtidal marine facies with examples from the middle and upper ordovician of southwestern virginia abstract the role of storms is of major importance in forming stratification in open shelf facies though storm processes on modern shelves are not fully understood enough is known to see how sediments are influenced storms transport sand away from coastal areas and suspend and transport sand and mud on the open shelf when wave base is lowered storms generate winnowed lag deposits of shells or gravel but coarser sediment and shells of megafauna are little transported except in very near shore environments the martinsburg formation middle and upper ordovician of southwestern virginia was deposited on a storm influenced open marine platform in contrast to the classic turbidite flysch basin of the martinsburg in the great valley of virginia to the northeast deposition of most martinsburg sediments was due to storms sediments deposited during single storm events occur in characteristic fining upward sequences 1 to 80 cm thick in both carbonate and terrigenous clastic dominated areas of the platform individual storm deposits consist of a sharp erosional base overlain by a couplet of whole fossil packstone and laminated calcarenite siltite or quartzose siltstone to very fine sandstone which is gradational to shale packstones are typically intraclastic have shells aligned parallel o bedding and convex up often with adhering or contained mud and have abundant infiltration textures such as cement filled shelter voids packstones grade abruptly to a laminated bed with horizontal plane lamination large and small scale in phase undulatory lamination and wave ripple cross lamination commonly occurring in ordered vertical sequences laminated beds contain escape burrows are weekly graded and have irregularly mounded upper bed surfaces laminated beds grade upward to shale containing horizontal branching burrows and marine fossils the faunal content of the fining upward sequences matches that of the interbedded shale storm generated beds vary greatly in thickness laterally and frequently pinch out completely fining upward sequences formed by scouring winnowing and suspension of the sand and mud during the height of the storm to form the erosional base and autochthonous lag of whole fossils storms preferentially preserve whole fossils rather than fragmenting them infiltration of finer sediment into the lag deposit and deposition of the laminated bed occurred as the storm waned laminated beds are molded into a distinctive suite of sedimentary structures by wave generated currents the shale represents deposition of finest storm suspended material and continued hemipelagic sedimentation similar sequences are widely distributed in the geologic record variability of storm generated sequences can be related to changing water depth thus storm deposits may be useful in basin analysis to estimate relative water depths and trace transgressive and regressive events

graptoloid evolutionary rates track ordovician silurian global climate change graptoloid evolutionary dynamics show a marked contrast from the ordovician to the silurian subdued extinction and origination rates during the ordovician give way during the late katian to rates that were highly volatile and of higher mean value through the silurian reflecting the significantly shorter lifespan of silurian species these patterns are revealed in high resolution rate curves derived from the conop constrained optimization scaled and calibrated global composite sequence of 2094 graptoloid species the end ordovician mass depletion was driven primarily by an elevated extinction rate which lasted for c 1 2 ma with two main spikes during the hirnantian the early silurian recovery although initiated by a peak in origination rate was maintained by a complex interplay of origination and extinction rates with both rates rising and falling sharply the global 13 c curve echoes the graptoloid evolutionary rates pattern the prominent and well known positive isotope excursions during the late ordovician and silurian lie on or close to times of sharp decline in graptoloid species richness commonly associated with extinction rate spikes the graptoloid and isotope data point to a relatively steady marine environment in the ordovician with mainly background extinction rates changing during the katian to a more volatile climatic regime that prevailed through the silurian with several sharp extinction episodes triggered by environmental crises the correlation of graptoloid species diversity with isotopic ratios was positive in the ordovician and negative in the silurian suggesting different causal linkages throughout the history of the graptoloid clade all major depletions in species richness except for one were caused by elevated extinction rate rather than decreased origination rate

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

ecostratigraphical interpretation of lower middle ordovician east baltic sections based on brachiopods a detailed ecostratigraphical framework is established for the lower middle ordovician kundan regional stage of the east baltic area corresponding to the asaphus expansus a raniceps and a eichwaldi trilobite zones lower darriwilian the study is based on approximately 6200 brachiopods collected bed by bed from limestone sections in northern estonia harku trench and saka and western russia putilovo quarry lava river canyon and lynna river valley with in addition the first detailed systematic assessment of the kundan brachiopods of the east baltic these sections represent an oblique depth transect some 400 kilometres long deepening eastwards five biofacies associations have been recognized using detrended correspondence and cluster analyses a shallow water lycophoria association a transitional gonambonites association and two deeper water associations the soft substrate orthis callactis and the hard substrate orthambonites associations a separate fifth soft substrate association is present in the marl beds at the main locality of putilovo quarry the associations reflect a combination of palaeo water depth and substrate the biofacies facilitate an ecostratigraphical correlation along the transect and third and fourth order sea level curves are reconstructed reflecting mainly eustasy the sea level was relatively low early in the kundan but then rose significantly into the a raniceps biozone this corroborates the recent discovery of possible small early darriwilian ice caps on gondwana

geomorphological and sequence stratigraphic variability in wave dominated shoreface shelf parasequences physical stratigraphy within shoreface shelf parasequences contains a detailed but virtually unstudied record of shallow marine processes over a range of historical and geological timescales using high quality outcrop data sets it is possible to reconstruct ancient shoreface shelf morphology from clinoform surfaces and to track the evolving morphology of the ancient shoreface shelf our results suggest that shoreface shelf morphology varied considerably in response to processes that operate over a range of timescales 1 individual clinoform surfaces form as a result of enhanced wave scour and or sediment starvation which may be driven by minor fluctuations in relative sea level sediment supply and or wave climate over short timescales 10 1 10 3 years these external controls cannot be distinguished in vertical facies successions but may potentially be differentiated by the resulting clinoform geometries 2 clinoform geometry and distribution changes systematically within a single parasequence reflecting the cycle in sea level and or sediment supply that produced the parasequence 10 2 10 5 years these changes record steepening of the shoreface shelf profile during early progradation and maintenance of a relatively uniform profile during late progradation modern shorefaces are not representative of this stratigraphic variability 3 clinoform geometries vary greatly between different parasequences as a result of variations in parasequence stacking pattern and relict shelf morphology during shore face progradation 10 5 10 8 years these controls determine the external dimensions of the parasequence

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

the turbulence structure in a continental stratocumulus cloud from millimeter wavelength radar observations abstract the turbulent scale vertical velocity structure in a continental stratocumulus cloud is studied using a 3 mm wavelength doppler radar operating in a vertically pointing mode the radar observations provided 30 m sampling in the vertical with 2 s averages of 10 000 samples vertical velocity measurements were made continuously for an 8 h period and were further supported by measurements of cloud base height from a laser ceilometer and liquid water path from a microwave radiometer during the beginning of the observational period the cloud layer extended between 200 and 800 m the vertical velocity variance profiles evolved systematically over the period from a well defined peak in the upper part of the cloud layer of 0 7 m2 s 2 to a peak in the lower part of the cloud layer of 0 2 m2 s 2 as the layer became decoupled later in the observing period the vertical velocity skewness during the well coupled conditions was negative through most of the cloud consistent with the presence of relatively n

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

fluvial shallow marine glaciofluvial depositional environments of the ordovician system in jordan abstract the ordovician system cropping out in southern and west central jordan consists entirely of a 750 m thick clastic sequence that can be subdivided into six formations the lower disi formation starts conformably above the late cambrian umm ishrin formation according to cruziana furcifera occurring in the upper third of the disi formation an early ordovician age is confirmed the disi formation consisting mainly of downstream accretion da fluvial architectural element was deposited in a proximal braidplain flowing n ne from the southerly located arabian nubian shield towards the tethys seaway the braidplain depositional environment evolved into a braidplain dominated delta through the middle and upper parts of the disi formation and the lower part of the overlying um saham formation the delta was replaced by siliciclastic tidal flats that in turn evolved into an upper to lower shoreface environment through the upper part of the um saham formation the depositional environment attained the maximum bathymetric depth during the deposition of the lower and central parts of the third unit the hiswa formation where offshore graptolite rich mudstone with intercalated hummocky cross stratified tempestites were deposited the tethys seaway regressed back through the upper part of the hiswa formation promoting a resumption of the lower upper shoreface sedimentation oscillation between the lower to upper shoreface depositional environment characterized the entire fourth unit the dubaydib formation as well as the tubeiylliat sandstone member of the fifth unit the mudawwara formation the depositional history of the ordovician sequence was terminated by a glaciofluvial regime that finally was gradually replaced by a shoreface depositional environment throughout the last unit the ammar formation

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

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

forced regressive shoreface sandstone from himalayan foreland implications to early himalayan tectonic evolution abstract sedimentology and sequence stratigraphic analysis of the 31 ma old marker white sandstone unit from the subathu sub basin nw himalayan foreland suggest it to be a forced regressive wedge frw formed during the transition from the marine subathu formation to the continental dagshai formation the frw is bounded between the surf diastem below and type 1 unconformity at the top and differs from rsme regressive surface of marine erosion occurring below bounded frws described from other classical coastal foreland settings correct identification of bounding surfaces of a frw has an important implication to the estimation of rate of relative sea level rsl fall a faster rate of rsl fall higher than the sedimentation rate has been postulated for the erosion of the lower shoreface and rsme using the logged thickness of the subathu dagshai transition zone including the white sandstone bounded between the surf diastem and unconformity available chronology and eustatic sea level fall 0 023 mm year at 31 ma a higher rsl fall than the sedimentation rate 0 07 mm year has been inferred during the deposition of the white sandstone petrography of sandstones and their sr and nd isotopic compositions indicate a major provenance switch over from dominant mafic ultramafic to metamorphic source from white sandstone 31 ma onwards attesting the link between hinterland tectonics provenance and forced regression the provenance switch over at 31 ma was earlier inferred to be driven by proto himalayan thrust propagation in the foreland using a simple isostatic model on the contrary a mechanism of accelerated surface uplift at a rate of 0 10 0 15 mm year is suggested for both provenance change and forced regression

long lived glaciation in the late ordovician isotopic and sequence stratigraphic evidence from western laurentia the timing and causes of the transition to an icehouse climate in the late ordovician are controversial results of an integrated 13c and sequence stratigraphic analysis in nevada show that in the late ordovician chatfieldian stage mid caradoc a positive 13c excursion in the upper part of the copenhagen formation was closely followed by a regressive event evidenced within the prominent eureka quartzite the chatfieldian 13c excursion is known globally and interpreted to record enhanced organic carbon burial which lowered atmospheric p co2 to levels near the threshold for ice buildup in the ordovician greenhouse climate the subsequent regressive event in central nevada previously interpreted as part of a regional tectonic adjustment is here attributed in part to sea level drawdown from the initiation of continental glaciation on gondwana this drop in sea level which may have contributed to further cooling through a reduction in poleward heat transport and a lowering of p co2 by suppressing shelf carbonate production signals the transition to a late ordovician icehouse climate 10 m y before the widespread hirnantian glacial maximum at the end of the ordovician

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

modeling the early paleozoic long term climatic trend the early paleozoic climate has been described as warm and equable however recent data based on conodont oxygen isotopic composition reveal a large long cooling trend through the ordovician followed by an abrupt cooling during the late ordovician glaciation this long term climate change is associated with a major radiation in the earth life history nonetheless the driving mechanisms for this cooling trend remain unknown carbon dioxide consumption by the weathering of fresh rocks from volcanic arcs has recently been suggested as a possible driver for this climate change however the impact of the plate motion context has not been explored yet although it might have a major impact on atmospheric co 2 levels simulations with a climate model coupled to a biogeochemical model geoclim show that the atmospheric co 2 decreased from more than 20 pal 5600 ppmv in the furongian down to approximately 10 pal 2800 ppmv in the llandovery before rising again in the early devonian we suggest that changes in geography and exposure of fresh volcanic rocks on continents are required to explain the large co 2 drawdown that led to the onset of cooler to glacial conditions from the middle ordovician to the llandovery the weathering of fresh volcanic rocks is itself responsible for 33 of the late ordovician atmospheric co 2 decrease the rest being related to the continent motion through the intertropical convergence zone itcz mean annual continental temperature falls by 3 c in the early ordovician reaching 13 5 c during the glacial interval and rises to 16 c in the early devonian

glacial onset predated late ordovician climate cooling the ordovician glaciation represents the acme of one of only three major icehouse periods in earth s phanerozoic history and is notorious for setting the scene for one of the big five mass extinction events nevertheless the mechanisms that drove ice sheet growth remain poorly understood and the final extent of the ice sheet crudely constrained here using an earth system model with an innovative coupling method between ocean atmosphere and land ice accounting for climate and ice sheet feedback processes we report simulations portraying for the first time the detailed evolution of the ordovician ice sheet we show that the emergence of the ice sheet happened in two discrete phases in a counterintuitive sequence of events the continental ice sheet appeared suddenly in a warm climate only during the second act and set against a background of decreasing atmospheric co2 followed steeply dropping temperatures and extending sea ice the comparison with abundant sedimentological geochemical and micropaleontological data suggests that glacial onset may have occurred as early as the middle ordovician darriwilian in agreement with recent studies reporting third order glacioeustatic cycles during the same period the second step in ice sheet growth typified by a sudden drop in tropical sea surface temperatures by approximate to 8 degrees c and the further extension of a single continental scale ice sheet over gondwana marked the onset of the hirnantian glacial maximum by suggesting the presence of an ice sheet over gondwana throughout most of the middle and late ordovician our models embrace the emerging paradigm of an early paleozoic ice age

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