shape sorting of flood transported synthetic clasts in a gravel bed river abstract an experiment was set up in a gravel bed river to test shape sorting of gravel size clasts synthetic clasts representing four shape types were used spheres were entrained in larger numbers than the other shapes and travelled furthest rods and blades were intermediate in mobility discs moved the shortest distance and in the least numbers the angle of repose and therefore the pivotability of clasts in combination with lift and drag forces exerted on the clast will determine if a clast will be entrained or not as well as the shear stress needed for the clast to maintain motion a sphere projects relatively higher than the other shape types of equal volume and will induce a higher degree of flow separation in its wake than other shape types therefore larger components of lift and drag forces exerted by water flow will be present to set this shape type in motion or to maintain motion the prolate clasts have a small angle of repose compared to discoid clasts and will roll much more readily the imbricated orientation assumed by discoid and sometimes blade shaped clasts is a very stable position very high applied shear stress will be needed to dislodge imbricated discs

two cryogenian glacial successions compared aspects of the sturt and elatina sediment records of south australia abstract two cryogenian glacial sequences the sturt and elatina occur within the adelaide rift complex s australia and are part of an 8 km thick succession recording rodinia fragmentation and subsequent passive margin development we compare and contrast these glacial successions presenting new data including detailed logged sections for both glacial sequences facies analysis reveals a total of six facies associations recording a range of glacial and glacially related processes these are a diamictite facies association ice proximal debris flows a sheet sandstone facies association fluvial deposits a flaser and rhythmically bedded facies association tidal flat a hummocky cross stratified sandstone facies association shoreface to offshore transition zone deposits and an underflow facies association recording offshore deposition a sixth facies association a ferruginous lonestone bearing siltstone is restricted to the top of the elatina formation and records rapid environmental amelioration and emergence from glaciation in the central flinders ranges the sturt succession commences with a glacial erosion surface ges which in agreement with previous workers was probably enhanced by rifting this ges is overlain in ascending order by the pualco tillite holowilena ironstone and wilyerpa formation with the warcowie dolomite member at the base of this latter formation a second ges is preserved within this latter unit and is overlain by a stratigraphic motif that records an alternation of offshore transition zone and offshore deposits significant base level falls within the wilyerpa formation are identified by fluvial deposits sitting disconformably upon offshore deposits in contrast within the elatina formation a single ges is proposed which defines the base of the formation the elatina formation is split into two informal members on the basis of analysis of sediment stacking patterns and the recognition of a sharp stratigraphic surface that divides the formation apart from a locally preserved basal diamictite the lower member consists of fluvial deposits whereas the upper member is dominated by a mixture of tidal flat sediments and debris flow deposits the boundary between these units is therefore interpreted as a hybrid surface recording tidal ravinement in some areas and a glacial advance in others deglaciation left contrasting deposits the sturt deglaciation culminated in deposition of black shale of the tindelpina shale member of the tapley hill formation seemingly as a sheet like deposit over the adelaide rift complex and beyond into the amadeus basin of central australia the elatina deglaciation meanwhile began first with deposition of the sixth facies association ferruginous dropstone bearing siltstone then proceeded with deposition of cap dolostone of the nuccaleena formation

unusual behavior in atmospheric angular momentum during the 1965 and 1972 el ninos abstract the global atmospheric angular momentum aam is known to increase with tropical eastern pacific sea surface temperature sst anomalies during el nino events using a reanalysis dataset the ratio of the monthly aam anomaly to el nino sst anomaly based on the nino 3 4 index is found to be approximately 1 angular momentum unit 1025 kg m2 s 1 per degree celsius for most post 1975 el ninos this ratio is much smaller however during the 1965 66 and 1972 73 el ninos raising the possibilities that either the early reanalysis data are in error due to sparse observations or the atmospheric response to the two early el ninos was unusual the possibility of a severe data problem in the reanalysis is ruled out by cross validating the aam time series with independent measurements of length of day the latitudinal structures of the zonal wind anomalies in 1965 66 and 1972 73 are examined for both the reanalysis and a set of general circulation model gcm simulations multiple gcm runs with specifie

late ordovician glacial record of the anti atlas morocco abstract late ordovician glaciogenic deposits are exposed intermittently along an 800 km long outcrop belt in the anti atlas mountains of southern morocco these deposits are of economic significance as potential oil bearing sandstones in the tindouf and boudenib basins and thus are here re examined as analogues to subsurface hydrocarbon reservoirs glaciogenic deposits of the upper second bani formation rest unconformably upon underlying shallow marine clastic deposits the unconformity is characterised by a series of palaeovalleys some 0 5 1 0 km wide and up to 100 m deep which may have been cut under elevated hydrostatic pressures as tunnel valleys beneath a late ordovician ice sheet the valleys and intervalley areas are filled with glaciogene sediments categorized into five facies associations namely 1 a tabular sandstone association shallow marine shoreface deposits 2 a massive sandstone and conglomerate ice contact debrites 3 meandriform sandstone deposits ice proximal sandur 4 stratified diamictites ice rafted debris and 5 sigmoidally bedded sandstones intertidal sandstones deformation in these sediments is ubiquitous and includes soft sediment striated pavements metre scale duplex systems thrust and fold belts of deformation affecting some tens of metres of sediment and pervasive lineations these features are interpreted to record the complex nature of deformation processes operating beneath a late ordovician ice sheet including sliding at the ice bed interface folding and deformation within the sediment column and a series of complex ramps detachments and shear zones within an unconsolidated pile of sediment beneath the ice sheet

the ordovician silurian boundary on cornwallis and truro islands arctic canada preliminary data four sections of the lower part of the cape phillips formation two outcrops on northeastern cornwallis island and one outcrop and one drill core from truro island northwest territories canada provide significant new data on the ordovician silurian boundary they show evidence of continuous sedimentation through the boundary interval and a continuous sequence of graptolite zones including the bohemicus and persculptus zones which have not been previously found in arctic canada strata yield graptolites including uncompressed specimens and conodonts through most of the sections the ordovicicus conodont zone occurs within the pacificus to lower persculptus graptolite zones the nathani conodont zone contains a transitional fauna a mixing of species typical of the preceeding ordovicicus zone and those generally regarded as silurian indicators this conodont zone ranges from the middle of the persculptus graptolite zone into the lower acuminatus graptolite zone and thus spans the ordovician silur

sequence stratigraphy of source rocks applied to the study of the kim meridgian tithonian in the north west european shelf dorset uk yorkshire uk and boulonnais france abstract in dorset and yorkshire england the sedimentation of kimmeridgianfrithonian age mutabilis to pallasioides zones shows ordered organic sequences the distribution of the organic matter content per cent total organic carbon petroleum potential hydrogen indices is not random but occurs as widespread primary organic rich cycles nested within each other these organic rich cycles with thicknesses of about 0 5 m show regular variations in organic carbon contents reaching a maximum value midway through the cycle they formed in response to widespread phenomena and can be correlated over long distances the cycles were deposited in marine environments mainly below the storm wave base they show little sedimentological evidence of sea level fluctuation either transgression or regression in the boulonnais france a more proximal setting the comparable strata show a greater lithological contrast and depositional environments corresponding to bathymetric changes are more easily determined a comparison of the distribution of organic matter through the sequence dated by ammonites in the three areas studied shows a good correlation between the distal dorset and yorkshire and the proximal boulonnais areas the main result of the biostratigraphic revision in the boulonnais france concerns the argiles de ch tillon which can be subdivided into two members these belong to the autissiodorensis zone autissiodorensis subzone and the gigas elegans zone respectively thus the kim meridgian tithonian boundary is located within this formation transgression does not appear to be the main factor determining the accumulation of autochthonous organic matter type ii not every major transgressive facies change seen in the kimmeridgian tithonian of the boulonnais is characterized by an enrichment in organic matter in the basinal area good source rocks with high petroleum potentials result from a conjunction between the physiography the transgression and the climate this last causes the variations in primary productivity which produce the organic rich cycles transgressions act mostly to reveal the climatic variations they are a necessary condition for the accumulation of the autochthonous organic matter but they are not sufficient by themselves as demonstrated by the transgressions in the autissiodorensis and elegans zones in the first instance the strata are depleted in organic matter in both the distal and proximal areas in the second instance type ii organic matter accumulated in both distal and proximal environments the vertical fluctuations in the total organic carbon contents the types of organic matter and the petroleum potentials of the organic rich cycles determine the oil potentials of transgressive sequences in sedimentary basins

high palaeolatitude hodh mauritania recovery of graptolite faunas after the hirnantian end ordovician extinction event marine shales directly overlying lower hirnantian uppermost ordovician glacially related sediments in mauritania northwest africa have produced a rich graptolite fauna spanning the ordovician silurian boundary in an area of high palaeolatitude the lowermost transgressive sandy shales are barren of graptolites but overlying shales show a sudden appearance of a diverse fauna indicative of the terminal ordovician persculptus zone suggesting that with improving conditions colonisation by a relatively cold tolerant fauna was possible this fauna is replaced by a low diversity assemblage dominated by long ranging taxa probably representing the basal silurian acuminatus and atavus zones with the extinction of the persculptus zone fauna conditions were still hostile to warm water silurian graptolites and a normalograptus fauna was again established a sudden influx of fairly diverse taxa marks the base of the acinaces zone and the establishment of a typical lower silurian fauna with the establishment of warmer water conditions

an oceanic model for lithological and faunal changes tested on the silurian record a model for oceanic cycles is presented the cycles are expressed as changes between more humid low latitude and cooler high latitude climates p episodes and dryer low latitude and warmer high latitude climates s episodes the cyclicity may be self regulating through changes in the co 2 storage capacity of the deep ocean due to temperature changes and through changes in the oceanic input and output of dissolved carbonate the model permits over 30 predictions regarding observable changes in both deep water and shelf lithologies and variations in the abundance diversity and extinction rate of planktic and benthic faunas sedimentary changes involve fluctuations in the rate of clay deposition and carbonate production the advance and retreat of carbonate deposition on the shelf edge the timing of oolite reef and black shale formation and changes between oxic and anoxic deep water faunal effects include both iterative evolution and intermittently widespread taxa lazarus taxa these predictions agree well with changes observed in well known sequence of wenlock and ludlow age as well as changes seen in selected older and younger sequences

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

effect of eolian fe supply change on the phytoplankton productivity and community in central equatorial pacific ocean during the pleistocene a lipid biomarker approach abstract the effect of iron fe fertilization on biological productivity and community structure in the equatorial pacific a region with high nutrient and low chlorophyll concentrations has been debated for many years here we present lipid biomarker and geochemical proxy records in a sediment core from the central equatorial pacific spanning from 1100 to 600 ka to investigate the response of marine phytoplankton communities to eolian fe supply and to assess links to environmental fluctuations during the pleistocene we found a strong link between change in eolian fe input and phytoplankton community structure our data demonstrate that an increase in eolian fe supply across the 800 ka boundary resulted in a shift in the major primary producer from coccolithophores alkenones to diatoms epi brassicasterol and dinoflagellates dinosterol at the study site this strong link between changes in eolian fe supply and the community structure of plankton during 800 600 ka may have been associated with a change in atmospheric and ocean conditions due to southward migration of the intertropical convergence zone itcz the lipid biomarker results show that such a high supply of eolian fe might have led to a change in the dominant phytoplankton group however the opal record which is often used as a primary productivity indicator was not consistent with the biomarker results therefore the results indicate that multi proxy approaches should be used in paleoenvironmental studies because different proxies can reveal different aspects of environmental conditions thus contributing to a better understanding of paleoceanographic conditions

sequence stratigraphy and proveance on gondwana s margin the meguma zone cambrian to devonian of nova scotia canada the meguma zone is the second largest terrane in the canadian appalachians three thick sandstones cambrian upper ordovician and lower devonian alternate with two thick shales lower ordovician and silurian the succession is a marginal assemblage shoaling upward from deep sea fan complexes to coastal facies in terms of sequence stratigraphy the succession consists of a basal type 1 sequence the meguma supergroup and three overlying type 2 sequences collectively the annapolis supergroup interpretations of sedimentary environments and stratigraphic relations agree with those of classical systems tracts the meguma zone is a sedimentary sink for an enormous amount of well sorted fine grained sand and silt this quantity and its westward dispersal indicate a gondwanan derivation stratigraphic units of the west african craton mimic those of the meguma zone in lithology provenance dispersal succession and age during neoproterozoic time continental ice sheets rivers and wind moved sediment southeastward down a cratonic paleoslope from what is now morocco through mauritania and mali remnants of widespread sand sheets extend across southern mali this sand reservoir became the source rock for the meguma zone in the early cambrian the paleoslope reversed perhaps due to birth of iapetus these same agents eroded the malian sand sheets episodically during relative sea level lows in cambrian late ordovician and early devonian times remnants of the resulting northwestward moving cratonic sands and silts occur today as mesas and buttes in northern mali mauritania southern morocco and algeria the ultimate destination of this detritus was the continental margin of north gondwana this sediment now forms the meguma zone and other terranes of southern europe northern africa and the middle east

interpreting benthic oxygen levels in mudrocks a new approach quantified paleoecology and gamma ray spectrometry have been applied in the analysis of the kimmeridge clay a highly organic rich british jurassic mudrock decreasing benthic oxygen trends are reflected in decreasing species richness and dominance diversity values similarly the degree of fragmentation of the benthos reflects the benthic energy levels and covaries with benthic oxygen the calculation of authigenic uranium values from data gathered by gamma ray spectrometry shows enrichment in more oxygen deficient environments the good correlation between the independently derived paleoecological and authigenic u data indicates the importance of these techniques in environmental analysis of marine petroleum source rocks

source rock distribution and maturation in the zagros orogenic belt provenance of the asmari and bangestan reservoir oil accumulations abstract the zagros orogenic belt of iran is one of the most prolific petroliferous areas accounting for approx 12 of proven global reserves correlation techniques reveal that a majority of the oils variously reservoired in asmari oligo miocene and bangestan group upper cretaceous anticlinal structures share a common provenance oils of the n e khuzestan fields are an exception showing a conspicuously different origin candidate source potential for the crucial central zagros embayment and contiguous areas was assessed on field survey and from well materials collected during the decade from 1968 onwards although varying locally in vertical and lateral distribution attractive sources occur throughout the section included were silurian shales the sargelu middle jurassic garau gadvan lower cretaceous kazhdumi albian gurpi upper cretaceous and pabdeh paleogene fms maturation of these source units was essentially controlled by the foredeep subsidence related to the zagros orogeny being a neogene event hydrocarbon generation and migration are very recent and essentially synorogenic consolidation of this information with the real distribution of candidate sources application of carbon isotope based source oil assignments and biomarker results successfully differentiated the provenance of the generic oil groupings over much of khuzestan the kazhdumi was recognised as the major contributor elsewhere the pabdeh fm yields isotopically distinctive oils in n e khuzestan evaluation of this basin allowed identification of prolific source rocks in low energy euxinic argillaceous lime mudstones similarly it provided an early application of pyrolysis techniques carbon isotopic correlations and subsidence history maturation migration modelling approaches now routinely employed in basin evaluation

basin modeling of tadla basin morocco for hydrocarbon potential the tadla basin morocco is the easternmost major structural subdivision of the meseta south of the central massif after the basin remained relatively stable through the paleozoic movements during the variscan orogeny divided the basin into two subbasins a western subbasin where compressional forces formed large anticlines and an eastern subbasin where intense folding and faulting are associated with uplift and erosion the sedimentary basin fill includes numerous organic shales which provide the source beds also deposited were numerous shallow water to continental clastics and to a lesser extent carbonates providing several possible reservoir rocks for hydrocarbons the results from computer modeling show that the chief formations of hydrocarbon generation are carboniferous and silurian most of these formations may have generated hydrocarbons before variscan movements which probably destroyed preexisting oil and gas traps the only source rocks in the generation stage from mesozoic to the present are the upper visean shales the western part of the tadla basin was the most favorable area for the generation and maturation of hydrocarbons of the upper visean shales

early silurian sea level changes global sea level fluctuated markedly during the early silurian probably as a result of the waxing and waning of ice sheets in the south american portion of gondwana the highest sea levels of the silurian are recorded by the telychian upper crispus lower griestoniensis and spiralis lower lapworthi biozones other highstands occurred in the early aeronian during the convolutus zone mid aeronian guerichi zone and late turriculatus zone early telychian and early sheinwoodian low sea levels characterized much of the argenteus and sedgwickii zones aeronian the utilis subzone late guerichi early turriculatus zones early telychian the late telychian commencing in the mid lapworthi zone and after a period of apparently only small amplitude sea level fluctuations in the late sheinwoodian and earliest homerian the mid late homerian in particular the early nassa zone facies and faunal changes in the lower silurian do not support the p and s model of jeppsson and others but are consistent with the sea level changes proposed herein mid telychian marine red beds appear to have been deposited during a minor sea level fall immediately after a period of very high sea levels rather than during a transgressive episode as previously suggested comparison of the sea level curve presented herein with those constructed in the past is hampered by the lack of precision currently possible in the correlation of early silurian deep water graptolitic and shallow water shelly sequences improving the precision of this correlation should be a priority for future research

the paleozoic petroleum geology of central arabia exploratory drilling in central saudi arabia has established significant paleozoic petroleum potential in this region in recent discoveries the permian interval of the unayzah formation has been identified as the primary oil and gas reservoir the unayzah formation consists of alluvial and fluvial to shallow marine siliciclastic sandstones and siltstones that were deposited on a major pre unayzah unconformity saudi arabian paleozoic oil is a high gravity 43 53 degrees api crude with low sulfur content usually less than 0 07 that is classified as arabian super light oil gas with condensate has also been discovered in the unayzah formation the identified paleozoic traps are moderate relief fault generated structures with generally 30 100 m of closure unayzah structural traps developed primarily during the triassic to early jurassic however an earlier phase of significant pre unayzah structural development is also indicated between the devonian and late carboniferous and is related to a hercynian structural event post triassic structure in central arabia is mainly regional and results principally from late cretaceous to tertiary development of the central arabian arch the established source rock is the basal qusaiba shale a widespread organic rich shale that was deposited following regional deglaciation in the early silurian migration occurred regionally updip through unayzah formation reservoirs from the underlying qusaiba shale subcrop and or vertically along fault bounded zones on the flanks of structures from the underlying qusaiba shale interbedded upper permian shales and evaporites provide a top seal for the unayzah and form the basal sequence of the overlying transgressive khuff carbonate pre qusaiba reservoir targets include the cambrian ordovician saq sandstone the ordovician qasim formation and upper ordovician to lower silurian periglacial clastics of the zarqa and sarah formations high gravity low sulfur oil has also been discovered in this pre qusaiba clastic section the reservoirs of the pre qusaiba structures are fault bounded and sources laterally by the down faulted qusaiba shale

depositional and tectonic setting of the lower silurian hydrocarbon source rock facies central saudi arabia the lower silurian qalibah formation consists of two members the upper sharawra member and the lower qusaiba member the qusaiba is the main hydrocarbon source rock for the known paleozoic hydrocarbon accumulations in central saudi arabia the qusaiba member is composed mostly of claystone and shale with interbeds of siltstone and sandstone depositionally the qusaiba member is interpreted to represent the delta toe clays whereas the sharawra member was deposited as pro delta sandstones of an immense fluviodeltaic system that dominated the silurian carboniferous of arabia the qalibah formation accumulated syndepositionally in at least two rapidly subsiding depocenters one depocenter was located in central and southern arabia and the other depocenter was located in northwestern saudi arabia the former trough began subsiding during the late ordovician following the taconic uplift the latter trough began subsiding during the early silurian at least 10 000 ft 3100 m of upper ordovician lower devonian strata accumulated in the trough located in central and southern saudi arabia about 3300 ft 1000 m of lower silurian sediments accumulated in the depocenter found in northwestern saudi arabia the development of these troughs documents a period of crustal extension probably related to the rifting of the lower paleozoic continental margin of saudi arabia the qusaiba member organic rich facies occurs near the base of the unit and has up to 8 total organic carbon content the development of favorable source rock facies in the qusaiba is observed along the flanks of the depositional thicks associated with the lower qusaiba member the basal qusaiba member organic rich facies was produced during periods of elevated primary productivity of organic matter within the water column in an open marine nonsilled basin setting deposition of the basal organic rich facies of the qusaiba member occurred as a condensed sequence along a sediment starved continental margin the basal qusaiba member organic rich facies accumulated in normally to partly oxygenated bottom water settings however the sediments below the sediment water interface were probably preserved in an anoxic setting geochemical data also demonstrate that the organic rich qusaiba member sediments found in the depositional thicks have characteristics consistent with a more oxidized sediment column geo chemical data also show that the basal qusaiba member organic rich facies deposited more distally to these depocenters are more dysoxic to anoxic in their characteristics the organic rich facies of the basal qusaiba member found in depositional thicks was produced under conditions that either reworked the organic rich sediments thereby oxidizing them or diluted the organic matter resulting in less preservation of potential source rock

bathymetric and isotopic evidence for a short lived late ordovician glaciation in a greenhouse period the end ordovician glaciation is distinct among phanerozoic glaciations in that co 2 levels were generally high yet major continental ice sheets accumulated on the gondwana supercontinent new oxygen isotopic data indicate substantial changes in sea water temperatures and ice volume coinciding with glacio eustatic changes in sea level reflecting the growth and decay of the gondwana ice cap major glaciation was apparently confined to the hirnantian and was 0 5 1 m y long rather than the 35 m y of earlier estimates carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep water circulation and then back again we believe that the changes in the carbon cycle effected a reduction in p co 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world

palaeoceanography bloom without fertilizer iron has been shown to stimulate productivity in certain areas of the modern ocean however it was not the primary driver of carbon burial in the equatorial pacific ocean for the past 10 million years

upwelling and productivity changes inferred from a temperature record in the central equatorial pacific a series of c37 39 alkenones synthesized by prymnesiophyte algae is commonly preserved in marine sediments and can be used for estimating past sea surface temperatures ssts 1 2 here we present an alkenone analysis of sediments taken from the central equatorial pacific ocean core w8402a 14gc 0 57 n 138 57 w which shows that sst varied slightly 2 c but coherently with milankovitch insolation cycles over the past 250 kyr the in phase response of the sst to the precessional component of insolation indicates that this part of the sst time series may be driven by changes in local trade wind strength and the longer 100 kyr eccentricity component indicates a response to basin wide south pacific winds using a simple heat balance model we use our palaeotemperature record to constrain the upwelling rates that have occurred in the past similar sst records from the eastern pacific could constrain the horizontal advection component of the sst record and allow for better estimates of upwelling upwelling and palaeoproductivity then could be compared to determine whether changes in equatorial nutrient levels actually cause equivalent changes in productivity

maximum extent of ice sheets in morocco during the late ordovician glaciation new field data demonstrate that during the late ordovician hirnantian glaciation an ice sheet expanding northwestwards over the anti atlas range reached into the southern meseta of northern morocco its growth to a glacial maximum position resulted in extensive subglacial erosion and deformation including the development of soft sediment striated surfaces and streamlined subglacial bedforms preserved between the high atlas of marrakech and rehamna these features imply that this ice mass extended 200 km further than previously thought and increase its size by at least ca 190 000 km2 comparable in area to the uk correlation between a measured section in the high atlas of marrakech and that of the southern meseta identifies sedimentary evolution within an ice contact system common to both these findings imply that the west african craton and northern morocco were in full glaciological communication during the latest ordovician palaeogeographic reconstruction shows that beyond the ice sheet south and southeastward palaeoslopes persisted on the shelf a palaeohigh beyond the main ice sheet was a major source for sand feeding delta systems that grew along the shelf as far as the shelf break this palaeohigh probably formed as a result of rift shoulder uplift and supported a satellite ice mass in the eastern meseta a thick 350 m underflow dominated deep marine fan was fed both from this shelf delta system and from glaciogenic debris derived from the main ice sheet the occurrence of this unexpected deep marine area in northern morocco implies that continued northward advance of the ice sheet was hampered by a dramatic break in bathymetry two depositional units are recognised across the meseta containing four distinct sedimentary cycles each recognised as a glacioeustatic response to the waxing and waning of ice masses elsewhere in west gondwana

new perspectives on graptolite distributions and their use as indicators of platform margin dynamics graptolite distributions in ordovician shelf slope and basinal facies in the great basin indicate that graptolites were scarce in open oceanic waters oceanward of the cordilleran shelf margin and that they thrived in waters above the margin this pattern is consistent with that of most zooplankton in modern oceans it follows from these observations that the depositional setting of typical graptolitic shale was the area of the sea floor under continental margin upwelling zones where graptolites flourished and within the oxygen minimum zone where their rhabdosomes were preserved with changes over time in relative sea level deep oceanic circulation and wind driven surface circulation the upwelling and oxygen minimum zones may have thickened or thinned migrated landward or oceanward and expanded laterally contracted or even disappeared the observed graptolite occurrences suggest that the primary graptolite biotope that is the habitat of diverse and abundant faunas was a relatively narrow belt of upwelling waters along and extending somewhat open oceanward from the continental margin provinces were maintained only to the extent that species could disperse along continental margins distribution of typical graptolitic strata may be used to interpret development of continental margins because such distribution incorporates a signal of sea level rise or fall oceanographic changes especially upwelling and tectonic events that led to creation and deterioration of upwelling conditions in which graptolites flourished

determination of organic matter content of appalachian devonian shales from gamma ray logs the organic matter content of the devonian shale of the appalachian basin is important for assessing the natural gas resources of these rocks and patterns of organic matter distribution convey information on sedimentary processes and depositional environment in most of the western part of the appalachian basin the organic matter content of the devonian shale can be estimated from gamma ray wire line logs using the equation phgro ggrb ggr 1 378a where phgro is the organic matter content of the shale fractional volume ggr the gamma ray intensity api units ggrb the gamma ray intensity if no organic matter is present api units and a the slope of the crossplot of gamma ray intensity and formation density api units g cm3 the quantities a and ggrb vary regionally and are mapped using data from gamma ray and formation density wire line logs organic matter contents estimated using this equation are compared with organic matter contents determined from direct laboratory analyses of organic carbon for 74 intervals of varying thickness from 12 widely separated wells the organic matter content of these intervals ranges from near zero to about 20 by volume excluding the cleveland member of the ohio shale and the lower part of the olentangy shale the distribution of the differences between volume percent organic matter content determined from core samples and estimated from gamma ray logs has a mean of 0 44 and a standard deviation of 1 98 which indicates that the accuracy of the gamma ray method is adequate for most geologic applications the gamma ray intensity of the cleveland member of the ohio shale and the lower pa t of the olentangy shale is anomalously low compared to other devonian shales of similar richness so that organic matter content computed for each of these units from gamma ray logs is likely to be too low wire line methods for estimating organic matter content have the advantages of economy readily available sources of data and continuous sampling of the vertically heterogenous shale section the gamma ray log in particular is commonly run in the devonian shale its response characteristics are well known and the cumulative pool of gamma ray logs forms a large and geographically broad data base the quantitative computation of organic matter content from gamma ray logs should be of practical value in studies of the appalachian devonian shale the general approach described here may also be applicable to other formations with physical and geochemical characteristics similar to the appalachian devonian shale

controls on reservoir quality in exhumed basins an example from the ordovician sandstone illizi basin algeria abstract petroleum exploration in many north african intracratonic basins targets early paleozoic sandstones as the primary reservoir objective these sandstones are often characterized by highly variable reservoir quality 0 0001 1000 md and the ability to predict and selectively target areas of enhanced porosity and permeability is crucial to unlock the hydrocarbon potential the objective of this study is to characterize the primary controls on reservoir quality in an ordovician field in the illizi basin of algeria through detailed core and petrographic analysis and establish if variations in thermal history across the field have a material impact on reservoir quality the best reservoir quality is observed in facies where primary intergranular porosity has been preserved in fine to coarse grained quartzarenites with less than 1 fibrous illite these lithologies are most commonly found within the high energy tidally reworked post glacial facies sandstones of the uppermost ordovician succession observed differences in quartz cement volume within compositionally and texturally similar samples from the southern and northern parts of the field are interpreted to reflect variations in thermal exposure due to deeper burial this interpretation is supported by field wide numerical modelling of sandstone diagenesis this study indicates that subtle variations in thermal history can have a material impact on the spatial trends in reservoir permeability thermal history therefore is an important consideration in reservoir quality studies in exhumed basins where variations in present day burial depth will be a poor guide for evaluating reservoir quality risk across a basin or play

anatomy of turbidites and linked debrites based on long distance 120 x 30 km bed correlation marnoso arenacea formation northern apennines italy much of our understanding of submarine sediment laden density flows that transport very large volumes ca 1 to 100 km3 of sediment into the deep ocean comes from careful analysis of their deposits direct monitoring of these destructive and relatively inaccessible and infrequent flows is problematic in order to understand how submarine sediment laden density flows evolve in space and time lateral changes within individual flow deposits need to be documented the geometry of beds and lithofacies intervals can be used to test existing depositional models and to assess the validity of experimental and numerical modelling of submarine flow events this study of the miocene marnoso arenacea formation italy provides the most extensive correlation of individual turbidity current and submarine debris flow deposits yet achieved in any ancient sequence one hundred and nine sections were logged through a ca 30 m thick interval of time equivalent strata between the contessa mega bed and an overlying columbine marker bed correlations extend for 120 km along the axis of the foreland basin in a direction parallel to flow and for 30 km across the foredeep outcrop as a result of post depositional thrust faulting and shortening this represents an across flow distance of over 60 km at the time of deposition the correlation of beds containing thick 40 cm sandstone intervals are documented almost all thick beds extend across the entire outcrop area most becoming thinly bedded 40 cm in distal sections palaeocurrent directions for flow deposits are sub parallel and indicate confinement by the lateral margins of the elongate foredeep flows were able to traverse the basin in opposing directions suggesting a basin plain with a very low gradient small fractional changes in stratal thickness define several depocentres on either side of the verghereto high area the extensive bed continuity and limited evidence for flow defection suggest that intrabasinal bathymetric relief was subtle substantially less than the thickness of flows thick beds contain two distinct types of sandstone ungraded mud rich sandstone intervals record evidence of en masse debrite deposition graded mud poor sandstone intervals are inferred to result from progressive grain by grain turbidite deposition clast rich muddy sandstone intervals pinch out abruptly in downflow and crossflow directions in a fashion consistent with en masse debrite deposition the tapered shape of mud poor sandstone intervals is consistent with an origin through progressive grain by grain turbidite deposition most correlated beds comprise both turbidite and debrite sandstone intervals intrabed transitions from exclusive turbidite sandstone to turbidite sandstone overlain by debrite sandstone are common in the downflow and crossflow directions this spatial arrangement suggests either i bypass of an initial debris flow past proximal sections ii localized input of debris flows away from available sections or iii generation of debris flows by transformation of turbidity currents on the basin plain because of seafloor erosion and or abrupt flow deceleration a single submarine flow event can comprise multiple flow phases and deposit a bed with complex lateral changes between mud rich and mud poor sandstone

origin sequence stratigraphy and depositional environment of an upper ordovician hirnantian deglacial black shale jordan the upper ordovician succession of jordan was located 60 s less than 100 km from the hirnantian ice sheet margin new graptolite dates indicate glaciation ended in jordan in the late hirnantian persculptus biozone the succession records two glacial advances within the ammar formation and the subsequent deglaciations organic rich black shales batra formation form part of the final deglacial transgressive succession that in filled an existing low stand glacial continental shelf topography the base of the black shale is coincident with the maximum flooding surface during transgression interfluves and sub basin margins were breached and black shale deposition expanded rapidly across the region the top of the black shales coincides with peak highstand the expanding puddle model sensu wignall for black shale deposition adapted for the peri glacial setting provides the best explanation for this sequence of events we propose a hypothesis in which anoxic conditions were initiated beneath the halocline in a salinity stratified water column a fresher surface layer resulted from ice meltwater generated during early deglaciation during the initial stages of marine incursion nutrients in the monimolimnion were isolated from the euphotic zone by the halocline increasing total organic carbon toc and 13corg up section indicates the organic carbon content of the shales was controlled mainly by increasing bioproductivity in the mixolimnion the strakhov model mixolimnion nutrient levels were sustained by a continual and increasing supply of meltwater derived nutrients modulated by obliquity changes in high latitude insolation anoxia was sustained over tens to hundreds of thousands of years the formation of black shales on the north gondwana shelf was little different to those observed in modern black shale environments suggesting that it was the nature of the ordovician seas that pre disposed them to anoxia

interannual modes of variability in atmospheric angular momentum abstract the interannual variability of atmospheric angular momentum over a 26 yr period is studied regionally using monthly analyses of zonal winds derived from the global rawinsonde network variations in zonal mean momentum filtered to emphasize interannual timescales exhibit a coherent propagating signal emanating from low latitudes as identified in other studies using shorter records applying extended empirical orthogonal function eeof analyses to zonally varying data the authors isolate a dominant pair of eigenvectors whose principal component time series and spatial patterns are in quadrature with one another indicating oscillatory behavior the oscillation described by the two eeofs has a period of about 36 months and is linked a posteriori to the time evolution of the el nino southern oscillation phenomenon beginning as an anomaly over the tropics that extends from the indian ocean into the pacific the signal is observed to progress eastward and poleward into both hemispheres leading t

the origins of glacially related soft sediment deformation structures in upper ordovician glaciogenic rocks implication for ice sheet dynamics in the upper ordovician glaciogenic rocks of west gondwana many soft sediment deformation structures are attributed to glaciotectonic processes however little is known about the subglacial and ice marginal processes associated with this ice sheet a systematic description and interpretation of these soft sediment deformation structures is supplemented by a comparison with late quaternary and modern glacial settings to provide qualitative constraints on the character of ancient subglacial thermal regimes soft sediment striations were formed by discontinuous shear induced deformation beneath an ice sheet that oscillated between warm and cold based thermal regimes beneath warm based ice coupling across the ice bed interface was poor with sliding and loading occurring at this surface loading of soft waterlogged sediments allowed the development of fluted surfaces which are also associated with the formation of truncated folded zones during times of cold thermal regimes coupling across the ice bed interface was enhanced by freezing which caused changes in the position of the freezing front these discontinuous changes coupled with sustained subglacial shearing resulted in the development of stacked soft sediment striated surfaces in areas where the subglacial substrate had an irregular topography complex shear zones formed that comprised stacked duplex systems interlayered with highly strained units preserving pervasive folded lineations at the ice front large glaciotectonic deformation structures comparable with push moraines developed during sustained periods of re advance that created composite thrust and fold systems deformation was transmitted for up to 1 km from the ice front and reduced in intensity with distance other deformation structures include dome like folds that consist of central anticlines 30 to 70 m in width with tight rim synclines that formed by the load induced diapiric intrusion of muds an ice sheet surging mechanism is inferred for the development of composite thrust and fold systems and the coeval sediment diapirs these surges potentially caused the widespread reorganisation of a glacial load resulting in the pro to subglacial extrusion of soft sediments

an example of the relationship between rift and dome recent geodynamic evolution of the hoggar swell and of its nearby regions central sahara southern algeria and eastern niger abstract from essential gravimetrical and petrological data we propose a new model of geodynamic evolution for the volcanic hoggar swell and its nearby regions the eastern hoggar the tenere troughs and the north hoggar troughs correspond to a single distensive structure which developed during the late mesozoic by reactivation of n s and nw se trending pan african faults in response to the stress field probably associated with the central atlantic opening contemporaneously as suggested by the xenoliths entrained by the cenozoic alkali basaltic eruptions of the eastern hoggar an extensive lower crustal basic magmatism of tholeiitic affinity and probably associated with crustal stretching may have developed beneath the hoggarian part of this distensive structure on the other hand at the hoggar level a transverse lineament oriented ne sw oued amded cross cuts the distensive structure this lineament subsequently controlled the localization of the mio plio quaternary volcanic districts and possibly upper mantle modifications through reheating partial melting metasomatism and magmatic veining produced by astenospheric material transfer gas fluid kimberlitic to carbonatitic magmas resulting in a density reduction on the other hand the high deformation of the peridotitic inclusions of the cenozoic alkali basalts emplaced along the oued amded lineament may indicate that this lineament has also controlled the emplacement of small sized asthenospheric diapirs into the upper mantle thus in the hoggar it appears that doming was probably preceded by a rifting phase and that doming and cenozoic volcanism were possibly controlled by the intersection of this rift by a transverse wrench fault

burial history and kinetic modeling for hydrocarbon generation part ii applying the galo model to saharan basins the galo basin evolution model described in makhous et al 1997 is applied to evaluate hydrocarbon generation and migration histories in several saharan basins three basins the oued el mya ghadames and illizi are located in the central and eastern parts of the saharan platform and are investigated in detail the ahnet mouydir timimoun reggane and other basins located in the southern and western parts of the platform are also studied the modeling results combined with geochemical data are used in a synthesis of the regional framework the thermal gradients in the ghadames and illizi basins are greater than those in the oued el mya basin this difference is attributed to differences in sedimentation and subsidence rates to less hercynian erosion and to fewer occurrences of evaporates in the akfadou region of the ghadames basin and in the mereksen region of the illizi basin in the southern and western parts of the illizi province the major subsidence occurred before hercynian uplift very moderate hercynian uplift in the ghadames and illizi basins did not involve a significant decrease of temperatures and organic matter maturation continued but at slower rates as a result the realization of hydrocarbon potential appears to be higher than would be expected favorable traps are located near subsided areas where the source shales particularly the devonian were not subjected to uplift and erosion in this respect the eastern sahara including the ghadames and illizi basins is a favored province analysis of the distribution of present day temperatures and paleotemperatures in the paleozoic sediments of the triassic province oued el mya ghadames trias and north illizi basins combined with effective source rock occurrences shows that favorable conditions for hydrocarbon generation during the paleozoic occurred essentially in the southern and southwestern parts of the province hydrocarbon generation during the mesozoic took place mainly in the central and northern parts of this province however some of the hydrocarbon accumulations in the northern and northeastern parts of oued el mya basin and in the northern and western ghadames basin were subsequently destroyed by the hercynian orogeny in the illizi basin ordovician and devonian structures are the most likely traps for hydrocarbons generated during the paleozoic whereas in the ghadames and oued el mya basins structures formed prior to the late cretaceous are preferentially favorable for oil accumulation gas accumulations are found in traps formed in the late cretaceous or later when source shales were buried to 3 3 3 5 km and more

tectonic processes in papua new guinea and past productivity in the eastern equatorial pacific ocean phytoplankton growth in the eastern equatorial pacific ocean today accounts for about half of the new production the fraction of primary production fuelled by externally supplied nutrients in the global ocean the recent demonstration that an inadequate supply of iron limits primary production in this region1 supports earlier speculation that in the past fluctuations in the atmospheric deposition of iron bearing dust may have driven large changes in productivity2 but we argue here that only small 2 nm increases in the iron concentration in source waters of the upwelling equatorial undercurrent are needed to fuel intense diatom production across the entire eastern equatorial pacific ocean episodic increases in iron concentrations of this magnitude or larger were probably frequent in the past because a large component of the undercurrent originates in the convergent island arc region of papua new guinea which has experienced intensive volcanic erosional and seismic activity over the past 16 million years cycles of plankton productivity recorded in eastern equatorial pacific sediments may therefore reflect the influence of tectonic processes in the papua new guinea region superimposed on the effects of global climate forcing

sequence stratigraphical interpretation of organic facies variations in marine siliciclastic systems general principles and application to the onshore kimmeridge clay formation uk

lower silurian hot shales in north africa and arabia regional distribution and depositional model abstract lowermost silurian organic rich hot shales are the origin of 80 90 of palaeozoic sourced hydrocarbons in north africa and also played a major role in petroleum generation on the arabian peninsula in most cases the shales were deposited directly above upper ordovician peri glacial sandstones during the initial early silurian transgression that was a result of the melting of the late ordovician icecap deposition of the main organic rich shale unit in the north african arabian region was restricted to the earliest silurian rhuddanian stage acuminatus atavus and probably early cyphus graptolite biozones during this short period 1 2 m y a favourable combination of factors existed which led to the development of exceptionally strong oxygen deficiency in the area in most countries of the study area the post rhuddanian silurian shales are organically lean and have not contributed to petroleum generation the distribution and thickness of the basal silurian hot shales have been mapped in detail for the whole north african region using logs from some 300 exploration wells in libya tunisia algeria and morocco in addition all relevant accessible published and unpublished surface and subsurface data of the lower silurian shales in north africa and arabia have been reviewed including sedimentological biostratigraphic and organic geochemical data the lowermost silurian hot shales of northern gondwana are laterally discontinuous and their distribution and thickness were controlled by the early silurian palaeorelief which was shaped mainly by glacial processes of the late ordovician ice age and by pan african and infracambrian compressional and extensional tectonism the thickest and areally most extensive basal silurian organic rich shales in north africa occur in algeria tunisia and western libya while on the arabian peninsula they are most prolific in saudi arabia oman jordan and iraq the hot shales were not deposited in egypt which was a large palaeohigh at that time the depositional model presented may help in better understanding the source potential of the basal silurian shales in less explored regions of north africa and arabia including morocco northern niger and the kufra basin in southeast libya

metacraton nature genesis and behavior article i nfo in this paper we show with examples that cratons involved in intercontinental collisions in a lower plate position are often affected by orogenic events leading to the transformation of their margins in some cases craton interiors can also be shaped by intense collisional processes leading to the generation of intra cratonic orogenic belts we propose to call these events metacratonization and the resulting lithospheric tract metacraton metacratons can appear similar to typical orogenic belts i e active margin transformed by collisional processes but are actually sharply different their main distinctive characteristics not all are present in each metacraton are 1 absence of pre collisional events 2 absence of lithospheric thickening high pressure metamorphism being generated by subduction leading to high gradient in strain and meta morphic intensity 3 preservation of allochthonous pre collisional oceanic terranes 4 abundant post collisional magmatism associated with shear zones but not with lithospheric thickening 5 presence of high temperature low pressure metamorphism associated with post collisional magmatism 6 intraconti nental orogenic belts unrelated to subduction and oceanic basin closures reactivation of the rigid but fractured metacratonic lithosphere will cause doming asthenospheric volcanism emplacement and mineral izations due to repetitive mineral enrichments this paper provides several geological cases exemplifying these different metacratonic features in scandinavia sahara central africa and elsewhere a special focus is given to the saharan metacraton because it is where the term metacraton originated and it is a vastly ex panded tract of continental crust 5 000 000 km 2

periodicity in the early palaeozoic marine realm evidence from the british isles suggests that during 3 intervals in the early palaeozoic conditions approximating to those of a non uniformitarian greenhouse g state earth were realized middle early upper cambrian st davids merioneth ages 30 ma mid ordovician llandeilo caradoc ages 32 ma and early silurian llandovery early wenlock ages 12 ma during these times sea levels were high and ocean waters below the zone of surface overturning poorly aerated and at times euxinic pelagic biotas graptoloids were diverse and in aerated portions of the highly productive shelf seas benthic biotas brachiopods flourished opposing conditions marking a well oxygenated o state earth occurred to varying degrees in the intervening intervals early cambrian comley age 20 ma late cambrian mid ordovician tremadoc late llandeilo ages 46 ma late ordovician ashgill age 4 ma and mid to late silurian post early wenlock age 15 ma within the constraints of the present data perturbations in the early palaeozoic marine realm do not appear to follow the same regular periodicity as in the mesozoic and caenozoic among a number of likely causes for this the greater tendency for the early palaeozoic seas to become anoxic seems the most important

palaeontological and geochemical characteristics of silurian black shale formations from the central brittany domain of the armorican massif northwest france abstract within the armorican massif as well as throughout the major part of the gondwanan northern margin llandovery to ludlow deposits are mainly represented by black shales with subordinate sandy beds in brittany these euxinic silurian facies lie more or less conformably on late ordovician deposits however based on graptolite evidence a stratigraphical break corresponding to part or whole of the llandovery is noticed the silurian black shales of brittany are devoid of benthic fauna while pelagic or epipelagic elements are very common these euxinic lithofacies are characterized by peculiar geochemical compositions which are discussed in terms of their stratigraphical and geographical distribution significant enrichment of sr y rb cr and ni higher concentrations of v up to 5600 ppm and greater volatile contents are particularly apparent in contrast with other terrigenous deposits from brittany a model for the sedimentary environment of the silurian black shales of the armorican massif is proposed the transgression induces the individualisation of basins partially separated from the open ocean in which the water column is stratified then the stratification disappears progressively during ludlow times

late ordovician glaciogenic reservoir heterogeneity an example from the murzuq basin libya in north africa late ordovician glaciogenic reservoirs contain large volume of recoverable hydrocarbon reserves but their heterogeneity and internal complexity means they are poorly understood to improve this understanding this paper presents the case study of the murzuq basin sw libya and a synthesis of the stratigraphic architecture of upper ordovician glaciogenic reservoirs the mamuniyat formation within it particular attention is paid to regionally extensive stratigraphic boundaries and the geometry of sandstone units of potential reservoir quality four disconformity bound units are recognised the bounding surfaces of which are flat or of high relief unit 1 the oldest and unit 3 are mud dominated and tend towards non reservoir in contrast units 2 and 4 the youngest are sand dominated have poor to excellent reservoir quality but distinctly different sedimentological architectures across the basin unit 2 comprises transitions from glaciofluvial intertidal sandstones to offshore turbidites and formed exclusively during glacial retreat the occurrence of a significant palaeotopography affected sandbody geometry with glaciofluvial intertidal deposits concentrating in the lee of these palaeohighs or infilling the bottom of tunnel valleys unit 4 is a compound stratigraphic unit recording the effects of glacial retreat and isostatic rebound in deep marine settings turbidite fans were deposited during retreat but in shallow water settings structural reactivation and sediment reworking had a more profound effect on sedimentation sedimentological architectures are variable at this level and include prograding coarse clastic wedges away from a palaeohigh or the infill of half graben basins during isostatic rebound

late early silurian wenlockian general circulation model generated upwelling graptolitic black shales and organic rich source rocks an accident of plate tectonics through an evolutionary process of plate tectonics the wenlockian physical world was composed of an oceanic northern hemisphere and a mainly continental southern hemisphere dominated by the giant continent of gondwana the high latitude position of gondwana placed much of its extensive margin in the middle latitudes laurentia and baltica occupied an equatorial position siberia and kazakh were to the north the global paleoclimate was created and driven by the diverse paleogeography of the two hemispheres in the model the northern hemisphere is dominated by strong zonality in all seasons in contrast the continental southern hemisphere reacts to wide temperature ranges of summer heating and winter cooling of gondwana the extensively flooded 28000 km margin of gondwana occupied the zone of the southern hemisphere westerlies these westerlies produced intense surface upwelling and by extension of present analogies high primary biologic productivity gondwana s northern margin extended far enough toward the equator that significant winter sea ice did not form in the southern hemisphere this allowed a vertically stratified sea to develop as well as probable dysaerobic and anoxic conditions a high organic flux of marine plankton accumulated on the sea floor where it was subsequently buried and preserved a strong positive correlation exists between upwelling generated by a general circulation model gcm on gondwana s northern margin and the distribution of southern hemisphere basins with source rocks containing types i and ii kerogens

fluvial processes and recumbently folded crossbeds in the pennsylvanian sharon conglomerate in summit county ohio u s a abstract the sharon conglomerate in northern ohio represents braided streams in incised paleovalleys it is distinguished by big compound bars huge scours apparently at braid channel confluences in front of the bars and abundant world class recumbently overturned crossbeds tangential and tabular foresets deformed into horizontally oriented v s and parabolic u s elsewhere these have usually been attributed to liquefaction by earthquakes followed by deformation under normal stream currents but earthquakes seem insufficiently frequent incapable of causing distinctive bar head upthrusting and unlikely to result in deformation that was not exactly parallel to paleoflow we suggest that sudden onslaught floods may provide a more attractive explanation in many cases

a review of the tectonics of the northern middle east region the structure of the northern part of the arabian platform is reviewed in the light of hitherto unpublished exploration data and the presently accepted kinematic model of plate motion in the region the palmyra and sinjar zones share a common history of development involving two stages of rifting one in the triassic jurassic and the other during late cretaceous to early tertiary times deformation of the palmyra zone during the mio pliocene is attributed to north south compression on the eastern block of the dead sea transcurrent system which occurred after continental collision in the north in southeast turkey the asymmetry of the palmyra zone is believed to result from northward underthrusting along the southern boundary facilitated by the presence of shallow triassic evaporites an important nw se cross plate shear zone has been identified which can be traced for 600 km and which controls the course of the river euphrates over long distances in syria and iraq transcurrent motion along this zone resulted in the formation of narrow grabens during the late cretaceous which were compressed during the mio pliocene to a large extent present day structures in the region result from compressional reactivation of old lineaments within the arabian plate by the transcurrent motion of the dead sea fault zone and subsequent continental collision

a new model for atmospheric oxygen over phanerozoic time a mathematical model has been constructed that enables calculation of the level of atmospheric o2 over the past 570 my from rates of burial and weathering of organic carbon c and pyrite sulfur s burial rates as a function of time are calculated from an assumed constant worldwide clastic sedimentation rate and the relative abundance and c and s contents of the three rock types marine sandstones and shales coal basin sediments and other non marine clastics red beds arkoses by our model values of o2 versus time using a constant total sedimentation rate agree with those for variable sedimentation derived from present day rock abundances and estimates of erosional losses since deposition this agreement is the result of our reliance on the idea that any increase in total worldwide sediment burial with consequently faster burial of c and s and greater o2 production must be accompanied by a corresponding increase in erosion and increased exposure of c and s on the continents to o2 consumption via weathering it is the redistribution of sediment between the three different rock types and not total sedimentation rate that is important in o2 control to add stability to the system negative feedback against excessive o2 fluctuation was provided in the modeling by the geologically reasonable assignment of higher weathering rates to younger rocks resulting in rapid recycling of c and s we did not use direct o2 negative feedback on either weathering of c and s or burial of c because weathering rates are assumed to be limited by uplift and erosion and the burial rate of c limited by the rate of sediment deposition the latter assumption is the result of modern sediment studies which show that marine organic matter burial occurs mainly in oxygenated shallow water and is limited by the rate of supply of nutrients to the oceans by rivers results of the modeling indicate that atmospheric o2 probably has varied appreciably over phanerozoic time during the late carboniferous and permian periods o2 was higher than previously because of the rise of vascular land plants and the widespread burial of organic matter in vast coal swamps a large decrease in o2 during the late permian was due probably to the drying up of the coal swamps and deposition of a large proportion of total sediment in c and s free continental red beds sensitivity study shows that major parameters affecting results are the mean c concentration in coal basins and the relative sizes of the reservoirs of young rapidly recycled versus old rocks less sensitivity was found for changes over time in total land area undergoing weathering and the use of direct o2 negative feedback on marine carbon burial good agreement for rates of c burial calculated via our model and via independent models which are based on the use of stable carbon isotopes indicates that the dominant factor that has brought about changes in atmospheric o2 level and the isotopic composition of dissolved inorganic carbon in seawater over phanerozoic time is sedimentation and not weathering or higher temperature phenomena such as basalt seawater reaction

terrestrial or waterlain glacial diamictites three case studies from the late precambrian and late ordovician glacial drifts in west africa abstract three examples of extensive diamictites within glacial sequences well exposed in the sedimentary cover of the west african cratonic platform are described they belong to the late precambrian and upper ordovician glacial periods and the paleogeography is summarized the late precambrian diamictites of the jbeliat group in adrar of mauritania displays characteristics of tillite deposited as lodgement or melt out till detail studies on structure clast shape nature and fabric texture in thin section clay mineralogy and carbonate content are given and discussed in order to delimitate useful criteria for tillite identification compared to this first example the two other diamictites from the late precambrian bakoye group in western mali and from the upper ordovician tichit group in southeastern mauritania have obviously been deposited in an aquatic environment waterlain glacial diamictites by the action of ice rafting bottom current and flow processes these examples show that for such ancient deposits sedimentological interpretations are partly solved by broad scale stratigraphical relationships however detailed sedimentological analyses remain essential for precise facies identification especially structural and petrographic studies must be encouraged in the lights of modern glaciodynamic processes

african oil past present and future eighty three sedimentary basins cover 50 of africa s 11 7 million square miles hydrocarbons are produced from 20 of these basins and a further 13 have non produced hydrocarbons yet of all the basins only 18 have been explored through a semi mature stage this paper presents a simplified classification of basins in africa africa s recoverable hydrocarbon reserves of 100 billion barrels of oil equivalent account for 7 of the world s total proven reserves this paper documents that reserve base s development through time together with the significant discoveries oil and gas occurrences and the play potential of each basin type are systematically described with examples demonstrating exploration problems regarding particular basins africa contains an abundance of unexplored sedimentary basins that should demonstrate relatively high success ratios but which must be evaluated with some caution because of political fiscal and logistical concerns end page 339

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

black shale grey shale fossils and glaciers anatomy of the upper ordovician silurian succession in the tazzeka massif of eastern morocco abstract a 400 m thick clastic succession of late ordovician through silurian age crops out in the tazzeka massif eastern morocco biostratigraphic data chitinozoa acritarchs constrain these rocks to the late katian through hirnantian coeval with glaciation in gondwana with a 8 myr hiatus at the ordovician silurian boundary sedimentological analysis reveals six facies associations including interbedded black and grey shale couplets poorly oxygenated shelf sediments a bioturbated shale with wave rippled sandstone inner shelf deposits several occurrences of diamictite of probable glaciogenic origin interbedded sandstone and mudstone storm and fair weather wave agitated shoreface rippled sandstone storm return flow deposits and amalgamated sandstone deposits delta front debris flows direct evidence for glaciation in the hirnantian deposits is poor third order sequence stratigraphic analysis reveals two transgressive systems tracts late katian and intra hirnantian two highstand systems tracts both intra hirnantian lowstand wedges probably corresponding to two principal glacial lowstands two maximum flooding surfaces late katian and intra hirnantian and a major ravinement surface hirnantian silurian contact remote from the centre of glaciation the tazzeka succession is suggested to be an excellent reference section with which to understand glacially moderated sea level changes in the late ordovician silurian late llandovery or younger shale geochemistry indicates organic enrichment toc 4 5 this organic enrichment greatly increases the known extent of the upper of two silurian organic shales across north africa potential hydrocarbon source rocks across the region

model for transgressive black shales many black shales in thick epicontinental basinal successions formed beneath an oxygen restricted puddle of deep water other black shales formed during rapid transgression when basinal deposition expanded to cover basin margin areas normally characterized by shallow water deposition an expanding puddle model is developed from the early jurassic of england wherein rapid sea level rise leads to marine sediment starvation due to sediment entrapment in flooded river valleys subsidence sea level rise and decreased sediment supply during early transgression are suggested to lead to rapid deepening and therefore the expansion of the are of the puddle such that in previously shallow water areas black shales rest on condensed basal transgressive lags or unconformities that in turn rest on shallow water facies reestablishment of sediment influx and aggradation in excess of any continued sea level rise cause shallowing throughout the basin and the cessation of black shale formation

flaser and wavy bedding in ephemeral streams a modern and an ancient example abstract flaser and wavy bedding are sedimentary structures characterized by alternating rippled sand and mud layers these structures often are considered to form mostly in tidally influenced environments published examples from fluvial environments are rare flaser and wavy bedding were found in two ephemeral stream deposits the jurassic kayenta formation and the modern wash in seven mile canyon both located in southeastern utah usa these examples demonstrate that flaser bedding can form and be preserved in ephemeral streams

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

effective petroleum source rocks of the world stratigraphic distribution and controlling depositional factors six stratigraphic intervals representing one third of phanerozoic time contain petroleum source rocks that have provided more than 90 of the world s discovered original reserves of oil and gas in barrels of oil equivalent the six intervals are 1 silurian generated 9 of the world s reserves 2 upper devonian tournaisian 8 of reserves 3 pennsylvanian lower permian 8 of reserves 4 upper jurassic 25 of reserves 5 middle cretaceous 29 of reserves and 6 oligocene miocene 12 5 of reserves this uneven distribution of source rocks in time displays no obvious cyclicity and the factors that controlled the formation of source rocks vary from interval to interval maps that show facies structural forms and petroleum source rocks were prepared for this study analysis of the maps indicates that several primary factors controlled the areal distribution of source rocks their geochemical type and their effectiveness i e the amounts of discovered original conventionally recoverable reserves of oil and gas generated by these rocks these factors are geologic age paleolatitude of the depositional areas structural forms in which the deposition of source rocks occurred and the evolution of biota the maturation time of these source rocks demonstrates that the majority of discovered oil and gas is very young almost 70 of the world s original reserves of oil and gas has been generated since the coniacian and nearly 50 of the world s petrole m has been generated and trapped since the oligocene the term petroleum as used in this study includes oil gas condensate and gas but does not include bitumen and other solid and semisolid oil derivatives

late ordovician early silurian glaciofluvial deposits preserved in palaeovalleys in south jordan abstract periglacial fluvial strata of late ordovician age infilling incised palaeovalleys in the lower part of the shallow marine siliciclastic khreim group late ordovician to silurian are described from outcrops in south jordan the palaeovalley fill predominantly comprises poorly sorted medium to coarse grained sub arkose with subordinate beds of pebble to cobble size diamictite clasts include subangular rip up sandstone and sparse shelly fossils derived from the local bedrock together with exotic rounded pebbles and cobbles derived from the cambro ordovician sandstones and granitoid precambrian rocks in the palaeohinterland situated to the south the palaeovalley fill is locally characterised by synsedimentary slump folds micro faulting and contorted bedding these are attributed either to gravitational slumping at palaeovalley margins or collapse due to melting of stagnant supporting ice at some localities the sandstones are typically trough cross bedded and exhibit upward fining trends terminating in current ripples suggesting episodic infill of the palaeovalleys the few palaeocurrent measurements available from these sedimentary structures indicate a general palaeoflow to the north coincident with the general orientation of the exposed palaeovalleys the palaeovalleys may have been formed by glacial and or fluvial processes their predominantly sandstone fill is interpreted as fluvial and or glaciofluvial in origin and was probably deposited as proximal proglacial outwash at some distance from the ice sheet which lay to the south in present day saudi arabia the batra mudstone which overlies coeval non channelised glaciofluvial sandstones yields an early llandovery fauna thus constraining the upper age of the palaeovalley infilling by analogy with similar strata in saudi arabia erosion of the palaeovalleys and infilling is likely to have taken place during ashgill to early llandovery times erosion of the palaeovalleys and their subsequent infilling is associated with the late ordovician early silurian glaciation of gondwanaland widely reported from central saudi arabia and in the subsurface from jordan

lower palaeozoic unconformities in an intracratonic platform setting glacial erosion versus tectonics in the eastern murzuq basin southern libya the stratigraphic record of the eastern murzuq basin has been importantly influenced by deformation resulting in angular and or deeply erosional unconformities though the overall context is intracratonic major transgressive events and the ordovician glaciation are nevertheless documented allowing the delineation of tectonic eustasy or climate driven unconformities lower palaeozoic key events and related unconformities that characterize the north gondwana platform have therefore a signature in the eastern murzuq basin the basement cover unconformity also known as the infra tassilian surface truncates all the deformed and metamorphosed lower cambrian and older rocks above is a middle cambrian to lower ordovician megasequence murizidie and hasawnah fms which is in turn truncated by an intra ordovician angular unconformity this megasequence is unconformably overlain by a middle ordovician hawaz fm to silurian tanzzuft and akakus fms megasequence which includes the upper ordovician glaciogenic unit mamuniyat fm bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations tillites and glaciotectonic structures the middle ordovician to silurian megasequence is finally truncated by a base devonian angular unconformity overlain by fluvial sandstones regarding the possibility that those fluvial deposits may be as younger as late devonian in the eastern murzuq basin based on palaeoflora the so called caledonian unconformity might be here a much younger mid eifelian surface and the occurrence of the lower devonian tadrart fm is questioned the upper ordovician glacial erosion surface which is sometimes referred to as the taconic unconformity usually truncates middle ordovician strata in the murzuq basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra ordovician deformation event in the murizidie southeastern murzuq basin the infra tassilian surface the intra ordovician unconformity and the upper ordovician glacial erosion surface amalgamate together here an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision the main part of which relate to the intra ordovician tectonic event the upper ordovician climate related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the lower palaeozoic although it presents most of the physical attributes of tectonic driven unconformities

links between iron input and opal deposition in the pleistocene equatorial pacific ocean iron is often a limiting nutrient in ocean regions that have a constant supply of other macro nutrients and changes in iron supply over time have been linked to fluctuations in primary productivity marine sediments from the equatorial pacific ocean show that over the past million years iron input was linked to the export and burial of biogenic silica

a practical model for organic richness from porosity and resistivity logs a practical method the delta log r technique for identifying and calculating total organic carbon in organic rich rocks has been developed using well logs the method employs the overlaying of a properly scaled porosity log generally the sonic transit time curve on a resistivity curve preferably from a deep reading tool in water saturated organic lean rocks the two curves parallel each other and can be overlain since both curves respond to variations in formation porosity however in either hydrocarbon reservoir rocks or organic rich non reservoir rocks a separation between the curves occurs using the gamma ray curve reservoir intervals can be identified and eliminated from the analysis the separation in organic rich intervals results from two effects the orosity curve responds to the presence of low density low velocity kerogen and the resistivity curve responds to the formation fluid in an immature organic rich rock where no hydrocarbons have been generated the observed curve separation is due solely to the porosity curve response in mature source rocks in addition to the porosity curve response the resistivity increases because of the presence of generated hydrocarbons the magnitude of the curve separation in non reservoirs is calibrated to total organic carbon and maturity and allows for depth profiling of organic richness in the absence of sample data this method allows organic richness to be accurately assessed in a wide variety of lithologies and maturities using common well logs

coherent interannual and decadal variations in the atmosphere ocean system 1 we investigate the source of poleward propagating atmospheric zonal wind anomalies originating at the equator and penetrating to high latitudes in both hemispheres in conjunction with enso dickey et al 1992 and report the discovery of similar variability on decadal and longer timescales since atmospheric dissipation times are generally on the order of a month or less we examine the ocean as a memory source for these globally coherent anomalies this hypothesis is substantiated by the observation of complementary oscillation in the sea surface temperature sst field further we detect a robust decadal variability 10 12 yrs in both the sst and contemporaneous atmospheric angular momentum aam series analyzing gisst sst data beginning in 1902 we confirm this decadal mode and find signatures of longer multidecadal sst variability centered in the equatorial and north pacific

large scale channel fill structures in late ordovician glacial deposits in mauritania western sahara abstract late ordovician siliciclastic glacial and related deposits on the north gondwana continental shelf comprise striking incised features referred to as large scale channel fill structures these are described from exposures and aerial photographs in two areas adrar and hodh in mauritania the channel fill structures are up to several kilometres in length and several hundred metres wide they are slightly sinuous narrow sandstone bodies deeply incised into the late ordovician glacial drift and the cambro ordovician bedrock palaeogeographical reconstructions indicate that these structures were located in the ice marginal zone they are preferentially oriented parallel to palaeo ice flow directions sedimentary facies analysis reveals three vertically stacked architectural units infilling a u shaped erosional basal unconformity unit 1 is thin and made up of conglomeratic sandstones of various origins comprising debris flows deltaic like progradational foresets and trough cross stratified sands in adrar architectural unit 2 shows sheet like evenly laminated fine to medium grained well sorted sandstones characteristic of a high energy marine environment in contrast in the hodh area unit 2 displays vertically stacked successions of coarse grained sandstones that indicate high discharge sediment laden flows these are interpreted as meltwater sediments in both adrar and hodh architectural unit 3 is made up of gravelly coarse grained trough cross stratified sandstone emplaced in a braided low sinuosity fluvial environment the three stage infilling history and the palaeogeographic location combined with size and shape criteria make it possible to compare the channel fill structures with pleistocene tunnel valleys found in areas of low relief in cool temperate and low arctic zones such features are incised by high pressure subglacial meltwater and later infilled by proglacial to postglacial deposits based on this comparison architectural unit 1 represents subglacial or proglacial outwash high sediment discharge in unit 2 took place in a proglacial environment in the hodh area while an isostatically downwarped shelf was responsible for a marine incursion in the adrar channels and deposits in unit 3 are fluvial postglacial sediments identification of preserved channel fill structures may be useful in reconstructing late ordovician ice sheet dynamics

recurring patterns of total organic carbon and source rock quality within a sequence stratigraphic framework sequence stratigraphy has been combined with total organic carbon toc analysis from well logs core and cuttings to develop a model of toc accumulation in marine source rocks routine well profiling of toc in marine source rocks has revealed that recurrent patterns in the vertical distribution of toc occur that can be explained with sequence stratigraphic concepts the model assumes that the principle controls on organic carbon accumulation are 1 the presence of low oxygen conditions at the sediment water interface and 2 sedimentation rate under these circumstances toc will vary inversely with terrigenous clastic influx if accommodation is high clastic influx is low in distal locations and the resulting weight percent toc in basinal sediments is increased th maximum toc in a vertical profile through a single marine sequence probably correlates to the maximum flooding surface toc decreases above this surface due to increasing sediment dilution during highstand progradation and decreases below it with the higher sedimentation rate of the older transgressive systems tract commensurate with toc increase towards the maximum flooding surface the organic matter type becomes more marine and therefore increasingly oil prone several marine source rock examples are presented as well as a lacustrine example which also conforms to the marine model

continental margin evolution of the northern arabian platform in syria copyright aapg 1993 reprinted by permission of the aapg whose permission is required for further use see also http atlas geo cornell edu syria best 1993 html

the hydrocarbon systems of north africa abstract north africa s hydrocarbon reserves are spread over a wide range of reservoirs and play types but derive from only a few key source rock intervals particularly those in the silurian late cretacous and eocene source rock maturity together with the timing of the generation relative to trap forming and destructive events seem to be the key controlling factors on success or failure at a basin level in most cases generation clearly post dates trap formation although a more complex relationship is evident in various parts of algeria where some traps did not form until gas was being generated source rock deposition and maturation can be tied to two main extensional tectonic cycles which ended in the compressional and destructive events of the hercynian and alpine orogenies remaining exploration potential in mature provinces may be tied largely to subtle and stratigraphic traps in addition significant frontier potential can be identified in two main fairways corresponding to regions where silurian devonian and cretaceous eocene sources respectively are likely to have generated petroleum critical factors in these areas are timing of generation and preservation the gas discoveries in the ahnet basin provide analogues for palaeozoic generation preservation models in the interior silurian devonian source fairway similarly the key to success for the mesozoic basins of the atlas region lies in demonstrating a favourable relationship between trap formation destruction and generation from late cretaceous and eocene source rocks

sandstones glaciers burrows and transgressions the lower palaeozoic of jabel az zalmah al kufrah basin libya abstract field studies in the jabel az zalmah region at the northern flank of al kufrah basin provide new insights into the stratigraphy and evolving depositional environments of cambro ordovician through devonian sediments at the base of the succession the hawaz formation is about 300 m thick and is characterised by frequently skolithos burrowed fine to medium grained sandstones with occasional silty intervals and rare conglomerates deposited in a generally high energy tidal shelf setting bioturbation is pervasive and includes spectacular cruziana traces produced by early to mid ordovician trilobites at the bottom and the top of the hawaz formation respectively these strata were incised during the late ordovician hirnantian glaciation ice sheets flowing over jabel az zalmah cut a palaeo topography of up to 100 m relief this relief is overlain by a coarsening upward succession of clastic sedimentary rocks recording re advance of an ice sheet which overrode the uppermost sandstones ice sheet retreat was accompanied by marine transgression and erosion which left a thin transgressive lag conglomerate capped sharply by siltstones of the tannezuft formation progradation resumed after post glacial maximum flooding depositing tide storm and river influenced parasequences of a major delta complex 200 m thick acacus formation fluvial incision and the development of a disconformity preceded the deposition of the tadrart formation 50 m thick a succession of trough cross bedded braided river deposits transgression and the re establishment of shallow or marginal marine systems is represented by the extensively bioturbated binem formation in which zoophycos burrows offer a tentative middle devonian stratigraphic fix an understanding of the lithological character of the lower palaeozoic strata in jabel az zalmah together with the character of unconformities that punctuate the succession will be vital for successful hydrocarbon exploration in this frontier basin which has until now been unsuccessful

re evaluation of the petroleum potential of the kufra basin se libya ne chad does the source rock barrier fall abstract the kufra basin is a large underexplored palaeozoic intracratonic sag basin in se libya and ne chad with extensions into nw sudan and sw egypt the basin fill consists of shallow marine to fluvial deposits ranging in age from infracambrian to cretaceous geologically the basin is very similar to the murzuq basin in sw libya which recently presented libya with its largest oil discovery for over a decade most of the hydrocarbon play elements known from the murzuq basin also occur in the kufra basin thick porous cambro ordovician sandstones are present and would form good reservoirs lower silurian shales may act as effective seals and there are potential structural traps in seismically defined fault blocks however the source rock availability in the kufra basin is currently unclear one of the two main source rock candidates in the basin is a lower silurian shale unit tanezzuft formation the tanezzuft shales have been described as being up to 130 m thick in outcrops at the basin margins but the shales were found to be replaced by siltstones and sandstones in two dry exploration wells drilled in the northern part of the basin by agip between 1978 and 1981 hot shales developed at the base of this widespread silurian shale unit form important source rocks in many areas of north africa and arabia these hot shales are interpreted to have been deposited in palaeodepressions such as incised valleys of the preceding lowstand or intrashelf basins during the initial transgression after the melting of the late ordovician ice cap the areal distribution of the organic rich unit is therefore discontinuous fieldwork in the kufra basin has shown that the basal tanezzuft horizon is not exposed on the northern and eastern margins of the basin deep infracambrian rift grabens have been interpreted on seismic lines from the kufra basin and in analogy to oman and algeria could contain organic rich infracambrian deposits the infracambrian succession in the kufra basin may contain a second major potential source rock and warrants further investigation

identification of source rocks on wireline logs by density resistivity and sonic transit time resistivity crossplots source rock formations generally show a lower density a lower sonic transit time and a higher resistivity than other sediments of equal compaction and comparable mineralogy this phenomenon can be used to identify source rocks on wireline logs provided the source rocks have a minimum thickness within the resolution of the sondes used and are sufficiently rich in organic matter classification rules have been established to assist in the recognition of source rocks on a combination of logs because of the low density contrast between water and organic matter the method becomes inaccurate at high water saturations i e low compaction within limits the amount of organic matter contained in a sediment can be estimated from log anomalies when source rocks become mature free oil is present in addition to kerogen and the resistivity increases by a factor of 10 or more

early paleozoic paleogeography of the northern gondwana margin new evidence for ordovician silurian glaciation during the early paleozoic transgressions and the distribution of sedimentary facies on the northern gondwana margin were controlled by a regional nnw sse to almost north south striking structural relief in early silurian times a eustatic highstand enabled the sea to reach its maximum southward extent

a diatom record of co 2 decline since the late miocene abstract extratropical sea surface temperature records from alkenones record a dramatic cooling of up to 17 c over the last 14 ma but the relationship between this cooling and greenhouse gas forcing has been elusive due to sparse and contrasting reconstructions of atmospheric co2 for the time period alkenone carbon isotopic fractionation during photosynthesis has previously been used to estimate changes in pco2 over this interval but is complicated by significant changes in cell size of the alkenone producing coccolithophorids over this time period in this study we reconstruct carbon isotopic fractionation during photosynthesis e p using organic compounds trapped within the frustules of pennate diatoms in sediments from the eastern equatorial pacific ocean at ocean drilling program site 846 over the last 13 ma physical separation of pennate diatoms prior to measuring carbon isotopic fractionation enables us to obtain a record with constant cell geometry eliminating this factor of uncertainty in our pco2 reconstruction in the past 11 ma e p declines from 15 5 to 10 3 using the classic diffusive model and taking into account variations in opal content alkenone concentration and coccolith sr ca as indicators of past productivity and growth rate and sea surface temperature records from the site we estimate a decline in pco2 from 454 41 to 250 15 ppmv between 11 and 6 ma models accounting for changing the significance of active carbon uptake for photosynthesis which likely produce more accurate co2 estimates suggest a significant larger pco2 decline of up to twice that shown by the classic diffusive model in average from 794 233 ppmv at 11 ma to 288 25 ppmv at 6 ma considering growth rates varying between 0 5 and 1 7 day 1 large uncertainties in the pco2 estimated between 8 and 11 ma using the active uptake model are related to the growth rate used for calculations together these results suggest co2 forcing for this period of steep decline in temperatures

the lower silurian qalibah formation of saudi arabia an important hydrocarbon source rock 1 the lower silurian qalibah formation represents a distinct lithostratigraphic time transgressive sequence it is disconformably bounded below by the upper ordovician to possibly lower silurian sarah formation and above by the continental to marginal marine lower devonian and possibly upper silurian tawil sandstone formation this paper redefines the qalibah formation within these boundaries in its reference section near the town of al qalibah in northwest saudi arabia the qalibah is 499 m thick whereas in a subsurface composite section it reaches 955 m the qalibah formation is a coarsening upward progradational sequence consisting of a lower qusaiba shale member and an upper sharawra sandstone member the basal hot shale of the qusaiba is a black euxinic bottomset shale up to 70 m thick which was deposited during the early silurian sea level rise following the deglaciation of gondwana this paleozoic shale is organic rich and the most likely source rock of the low sulfur high gravity oil condensate and gas discovered in recent years in the paleozoic rocks of central and eastern saudi arabia lithologically similar silurian sequences were deposited over most of the broad stable shelf of gondwana from the middle east to the african sahara

estimates of the earth s spin geographic axis relative to gondwana from glacial sediments and paleomagnetism abstract the ice centres of the permo carboniferous and late ordovician glaciations of gondwana have been quantitatively estimated their positions relative to the inferred mean paleomagnetic poles are not significantly different from those of the quaternary ice centres relative to the present day geographic poles these data suggest it is unnecessary to postulate that the paleozoic earth had marked non dipole components in the geomagnetic field or that the average of the dipole axis was significantly different from the geographic axis or that there was a significantly different tilt to the geograpic axis the short lived late ordovician ice age is tentatively attributed to equally short episodes of faulting within gondwana these are believed to have created transient highlands which converted a previously snow free summer climate into one in which snow could accumulate and build ice caps once the faulting ceased the topography decayed and the climate reverted to its previous snow free summer slate

sources of fe to the equatorial pacific ocean from the holocene to miocene abstract biological productivity in the modern equatorial pacific ocean a region with high nutrients and low chlorophyll is currently limited by the micronutrient fe in order to test whether fe was limiting in the past and to identify potential pathways of fe delivery that could drive fe fertilization i e dust delivery from eolian inputs vs fe supplied by the equatorial undercurrent we chemically isolated the terrigenous material from sediment along a cross equatorial transect in the central equatorial pacific at 140 w and at ocean drilling program site 850 in the eastern equatorial pacific we quantified the contribution from each potential fe bearing terrigenous source using a suite of chemical and isotopic discrimination strategies as well as multivariate statistical techniques we find that the distribution of the terrigenous sources i e asian loess south american ash papua new guinea and ocean island basalt varies through time latitude and climate regardless of which method is used to determine accumulation rate there also is no relationship between flux of any particular fe source and climate moreover there is no connection between a particular fe source or pathway eolian vs undercurrent to total productivity during the last glacial maximum pleistocene glacial episodes and the miocene biogenic bloom this would suggest an alternative process such as an interoceanic reorganization of nutrient inventories may be responsible for past changes in total export in the open ocean rather than simply fe supply from dust and or equatorial undercurrent processes additionally perhaps a change in fe source or flux is related to a change in a particular component of the total productivity e g the production of organic matter calcium carbonate or biogenic opal

glacial to interglacial fluctuations in productivity in the equatorial pacific as indicated by marine barite an empirical correlation between marine barite baso4 accumulation rate in core top sediment samples from two equatorial pacific transects at 140 w and 110 w and the estimated primary productivity of the overlying water column were used to evaluate glacial to interglacial changes in productivity fluctuations in barite accumulation rates down core indicate that during glacial periods of the past 450 000 years the productivity in the central and eastern equatorial pacific was about two times that during intervening interglacial periods this result is consistent with other evidence that productivity was high in the eastern and central equatorial pacific during the last glacial