the late devensian 22 000 bp irish sea basin the sedimentary record of a collapsed ice sheet margin the late devensian 20 ka bp glacial geology of the irish sea basin 4000 km2 is an event stratigraphy recording the entry of marine waters into a glacio isostatically depressed basin and the rapid retreat of the irish sea glacier as a tidewater ice margin marine limits occur up to 140 m o d across much of the central basin the ice margin was uncoupled from its bed exposing a subglacially scoured topography to glaciomarine processes the irish sea glacier was a major drainage conduit of the last british ice sheet calving of the marine ice margin resulted in fast flow surging of ice streams recorded by drumlin fields around the northern basin margin and tunnel valleys rapid evacuation of the basin may have stranded large areas of dead ice in peripheral zones e g cheshire shropshire lowlands and initiated the collapse of the ice sheet thick wedges of ice contact glaciomarine sediments were deposited during ice retreat as morainal bank complexes by successive tidewater ice margins stabilized at pinning points around the irish sea coast where morainal banks occur on the seaward side of drumlin swarms there is a clear sequential relationship between rapid ice loss from calving ice margins the development of fast flowing ice streams drumlinization and the pumping of subglacial sediment to tidewater raised delta complexes are locally associated with marine limits along the high relief coastal margins of wales east central ireland and the lake district associated valley infill complexes record downslope resedimentation of heterogenous sediments into the marine environment during ice retreat co eval offshore deposits are represented by well stratified glaciomarine complexes that infill a subglacially scoured topography that shows networks of tunnel valleys glaciomarine mud drapes occur well to the south of the maximum limit of grounded ice in the basin e g north devon scilly islands southern ireland the age of these distal sediments previously mapped as pre devensian tills is constrained by amino acid ratios basin rebound following deglaciation was rapid with over 100 m recovery in 3 ka and was followed by a low marine still stand peat accumulating in offshore areas now as much as 55 m below sea level has been drowned by the postglacial eustatic rise in sea level the glacio sedimentary model identified in this paper involving rapid ice retreat and related sedimentation triggered by rising relative sea level suggests that isotatic downwarping is an important mechanism for deglaciating continental shelves

the last british ice sheet a review of the evidence utilised in the compilation of the glacial map of britain this paper reviews the evidence presently available as at december 2003 for the compilation of the glacial map of britain see clark c d evans d j a khatwa a bradwell t jordan c j marsh s h mitchell w a bateman m d 2004 map and gis database of glacial landforms and features related to the last british ice sheet boreas 33 359 375 and http www shef ac uk geography staff clark chris britice html in an effort to stimulate further research on the last british ice sheet and promote a reconstruction of ice sheet behaviour based on glacial geology and geomorphology the wide range of evidence that has been scrutinized for inclusion on the glacial map is assessed with respect to the variability of its quality and quantity and the existing controversies in ice sheet reconstructions landforms interpreted as being of unequivocal ice marginal origin moraines ice contact glacifluvial landforms and lateral meltwater channels and till sheet margins are used in conjunction with available chronological control to locate former glacier and ice sheet margins throughout the last glacial cycle subglacial landforms drumlins flutings and eskers have been used to demarcate former flow patterns within the ice sheet the compilation of evidence in a regional map is crucial to any future reconstructions of palaeo ice sheet dynamics and will provide a clearer understanding of ice sheet configuration ice divide migration and ice thickness and coverage for the british ice sheet as it evolved through the last glacial cycle

hydrocarbon source rock potential and elemental composition of lower silurian subsurface shales of the eastern murzuq basin southern libya abstract a shallow borehole was drilled in lower silurian rhuddanian lower aeronian tanezzuft formation siliciclastic sediments at the eastern margin of the murzuq basin shale samples were retrieved from the borehole to study their source rock potential and trace element geochemical composition thirty shale samples analysed from borehole cdeg 2a reveal three geochemically distinct intervals shales from the top 23 38 m interval are weathered and have negligible organic content shales from the middle 38 42 m interval have moderate to good organic richness 0 6 1 7 wt toc with hydrogen index hi values up to 443 mg s2 g toc shales from the bottom 42 51 m interval have poor to moderate organic richness 0 07 0 8 wt toc with hi values up to 277 mg s2 g toc all samples have limited source rock potential and are immature to early mature for hydrocarbon generation it is unlikely that they have ever been buried deeper than 2 km low toc values inorganic geochemical redox proxies ce anomaly authigenic u th u v cr ni co and v sc indicate that the lower silurian tanezzuft formation shales from borehole cdeg 2a were deposited under oxic bottom waters explaining their relatively low toc values deposition took place in a shallow marine environment in a proximal position with regard to an early silurian palaeo shoreline this combined rock eval pyrolysis and whole rock geochemical study provides new insights into the silurian source rock distribution and hydrocarbon prospectivity in central north africa

thermal history and source rock characterization of a paleozoic section in the awbari trough murzuq basin sw libya abstract in some areas of the murzuq basin sw libya the lower silurian member comprises highly radioactive shales hot shale which are regarded as the most significant factor controlling petroleum generation in the basin in this context it was the goal of our project to study the distribution and maturity of the hot shale in the framework of basin evolution organic geochemical organic petrological and basin modelling methods were used to obtain a more thorough understanding four wells from the northern and central part of the awbari trough have been selected for this study the hot shale contains mainly type ii kerogen with high toc values ranging from 3 to 23 which is considered as an indication for excellent source rock potential a good petroleum potential is further supported by hydrogen indices varying between 220 and 380 mg hc g toc numerical 1d basin modelling was performed for four wells in order to assess the thermal maturity of the tanezzuft hot shale and its evolution through time mean random equivalent vitrinite reflectance results using vitrinite like particles for lower silurian hot shale pyrolysis t max values and fluorescence colours indicate that the hot shale is immature to early mature in the southern part and clearly more mature in the northern part of the basin other potential source rocks from silurian tanezzuft middle upper devonian awaynat wanin and lower carboniferous marar formations were analysed thermal history modelling indicates that the present day geothermal gradients and heat flows vary at different locations but that paleotemperature evolution is mostly related to deepest burial peak temperatures were reached either in the late carboniferous before hercynian uplift occurred or in the jurassic cretaceous the latter scenario being more probable because much oil has been preserved

origin of the infracambrian salt basins of the middle east abstract during the infra and lower cambrain the arabian gulf and zagros mountains hormuz formation south oman ara formation kerman in central iran ravar formation and the salt range province in pakistan salt range formation were all sites of salt precipitation in subsiding rift basins along the middle eastern edge of gondwanaland also from 600 to 540 ma a rift system extended westwards from the sinai peninsula and north egypt across ne africa while a second rift system about 600 to perhaps 510 ma extended northwards from the sinai peninsula across the jordan valley and dead sea into se turkey the development of these rifts was contemporaneous with the left lateral najd strike slip fault system which dislocated the arabian peninsula by 300 km the najd system may be interpreted as a transform fault system which accommodated rifting between ne egypt the jordan valley north pakistan kerman and south oman the najd system is parallel to the zagros line and the latter is also the eastern depositional limit of the infracambrian hormuz formation the infracambrian zagros line may be interpreted as a right lateral strike slip fault system which caused infracambrian rifting in the arabian gulf and zagros mountains palaeontological and radiometric age constraints together with stratigraphic considerations suggest that the syn rift evaporites were first precipitated during a transgression which commenced in the latest precambrian 600 ma and again during a lower cambrian regression the regression terminated a period dominated by marine carbonates and evaporites and initiated the deposition of progradational alluvial to marginal marine sediments finally in the late lower cambrian and middle cambrian transgressive marine deposits swept across the middle east

basement control in the development of the early cretaceous west and central african rift system abstract the structural framework of the precambrian basement of the west and central african rift system wcars is described in order to examine the role of ancient structures in the development of this early cretaceous rift system basement structures are represented in the region by large pan african mobile belts built at ca 600 ma surrounding the 2 ga west african congo and sao francisco cratons except for the small gao trough eastern mali located near the contact nappe of the pan african iforas suture zone along the edge of the west african craton the entire wcars is located within the internal domains of the pan african mobile belts within these domains two main structural features occur as the main basement control of the wcars 1 an extensive network of near vertical shear zones which trend north south through the congo brazil nigeria niger and algeria and roughly east west through northeastern brazil and central africa the shear zones correspond to intra continental strike slip faults which accompanied the oblique collision between the west african congo and sao francisco cratons during the late proterozoic 2 a steep metamorphic nw se trending belt which corresponds to a pre pan african ca 730 ma ophiolitic suture zone along the eastern edge of the trans saharian mobile belt the post pan african magmatic and tectonic evolution of the basement is also described in order to examine the state of the lithosphere prior to the break up which occurred in the earliest cretaceous after the pan african thermo tectonic event the basement of the wcars experienced a long period of intra plate magmatic activity this widespread magmatism in part relates to the activity of intra plate hotspots which have controlled relative uplift subsidence and occasionally block faulting during the paleozoic and the early mesozoic this tectonic activity was restricted to west of the hoggar west of air and northern cameroon during the late jurassic early cretaceous the st helena mantle plume may have had an important effect in weakening the lithosphere across a broad zone from brazil to sudan however causes of extension and stretching are likely to be found in a plate tectonic framework at that time the opening of both the indian and atlantic oceans were the dominant forces which favoured the development of the wcars and its propagation by strike slip movements and extensional displacements along pre existing pan african lithospheric zones of weakness

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

origin of an esker like ridge erosion or channel fill sedimentology of the monington esker in southwest wales abstract a ridge of stratified gravel and sand near cardigan in southwest wales has previously been interpreted as an esker its sedimentology is exposed in sand pits and consists of cross bedded gravel and sand incised into a complex of climbing ripple cross laminated sand interbedded with silt and clay the succession records the progradation of fluvial channels across a former lake floor there is no clear sedimentological evidence that the sediments were deposited in an ice contact setting and therefore the feature should not be called an esker the interpretation favoured here is that the ridge is an erosional remnant either of a formerly more extensive outwash plain or of a kame terrace deposited when the adjacent valley was filled with dead ice recognition of an esker needs to be based on sedimentology and regional setting as well as on geomorphology

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

genesis of an esker like ridge over the southern fraser plateau british columbia implications for paleo ice sheet reconstruction based on geomorphic inversion abstract robust interpretations of meltwater systems operating during ice sheet decay are integral to reconstructing deglacial patterns and style yet over reliance on meltwater landform morphology with limited attention to morpho sedimentary relationships and basin scale geomorphic and stratigraphic context can lead to unreliable geomorphic inversion based paleo ice sheet reconstructions this problem is illustrated by the evolution of young lake esker like ridge yler formed in the young lake basin ylb on bc s southern fraser plateau during decay of the last cordilleran ice sheet cis we integrate data from digital elevation models aerial photographs sedimentary outcrops water wells and shallow geophysics ground penetrating radar electrical resistivity tomography previous interpretations of yler as both an esker and an ice contact poorly sorted stratified deposit emplaced by westerly flowing meltwater imply an eastward retreating ice margin geophysical data from a flat topped component of yler reveal slipface and planar bedded sand and gravel overlying lacustrine sediments characteristic of a hjulstrom delta eastward dipping foresets in a gilbert delta exist at the eastern terminus contextually our observations suggest despite esker like morphology yler was not deposited within a subglacial ice tunnel instead it formed through deposition of subaerial outwash between and or on dead ice in front of a regionally backwasting ice margin the complex deglacial evolution of ylb including a drainage reversal and formation of two glacial lakes supports northwestward backwasting of the cis and dead ice within ylb we conclude that accurate geomorphic inversion of meltwater landforms for deglacial paleo ice sheet reconstruction requires knowledge of landform scale morpho sedimentary relationships and basin scale geomorphic and stratigraphic context

endogenous and nonimpact origin of the arkenu circular structures al kufrah basin se libya the twin arkenu circular structures acs located in the al kufrah basin in southeastern libya were previously considered as double impact craters the arkenu craters the acs consist of a ne arkenu 1 and a sw structure arkenu 2 with approximate diameters of about 10 km they are characterized by two shallow depressions surrounded by concentric circular ridges and silica impregnated sedimentary dikes cut by local faults our field petrographic and textural observations exclude that the acs have an impact origin in fact we did not observe any evidence of shock metamorphism such as planar deformation features in the quartz grains of the collected samples and the previously reported shatter cones are wind erosion features in sandstones ventifacts conversely the acs should be regarded as a paired intrusion of porphyritic stocks of syenitic composition that inject the nubia formation and form a rather simple and eroded ring dike complex stock emplacement was followed by hydrothermal activity that involved the deposition of massive magnetite hematite horizons typical of iron oxide copper gold deposits their origin was nearly coeval with the development of silicified dikes in the surroundings plugs of tephritic phonolitic rocks and lamprophyres monchiquites inject the nubian sandstone along conjugate fracture zones trending nnw sse and ne sw that crosscut the structural axis of the basin

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

late cryogenian neoproterozoic glacial and post glacial successions at the southern margin of the congo craton northern namibia facies palaeogeography and hydrocarbon perspective

the late ordovician early silurian glacial period abstract late ordovician to early silurian tillites and evidence of extensive glacial erosion were first discovered in the western sahara in the late 1950 s although the concept of a glacial period at this time had been around since the late 19th century further evidence of glaciation is now available from west africa south africa the arabian peninsula eastern north america west and central southe america and europe several glacial episodes can be distinguished the earliest evidence local mountain glaciation appears to be of caradoc age the major episode may have begun in caradoc time and lasted much of ashgill time when a major ice sheet covered at least north central and west africa which was then centered near the south pole however independent palaeontological and sea level evidence from areas far removed from the centre of glaciation suggests a time span for this episode of less than 2 ma this conflict has yet to be resolved the latest evidence of glaciation is wenlock in age but is confined to south america which by then was replacing africa over the south polar region however this latest episode may simply have been a rather intensive mountain glaciation rather than being of ice sheet dimensions the lack of land in the north polar region probably explains the absence of tillites from the then northern hemisphere land masses the total area affected by glaciation in early palaeozoic time in north africa alone is at least 6 106 km2 combined with other glaciated areas glaciation may have been on the scale of that of present day antarctica

sedimentology of an ordovician fan delta complex western libya abstract the ordovician haouaz formation in the western central gargaf arch of western libya provides a well exposed example of an ancient fan delta system the haouaz sequence consists of delta front deposits overlain by subaqueous distributary channels with intervening distributary mouth shoals and swash bars this delta front complex is abruptly and erosively overlain by braided fluvial and distributary channels of the lower deltaic plain lower deltaic plain deposits may show reworking by wave swash processes in the upper portion indicating marine destructive phases of delta building this process is attributed to diminished or absent detrital influx allowing wave reworking of the deltaic plain margin these reworked deposits may in turn be overlain by thin transgressive marine shelf sequences also formed during periods of minimal detrital influx the lower deltaic plain may also pass upward into an upper deltaic plain and alluvial plain environment

a stratigraphic investigation of the celtic sea megaridges based on seismic and core data from the irish uk sectors the celtic sea contains the world s largest continental shelf sediment ridges these megaridges were initially interpreted as tidal features formed during post glacial marine transgression but glacigenic sediments have been recovered from their flanks we examine the stratigraphy of the megaridges using new decimetric resolution geophysical data correlated to sediment cores to test hypothetical tidal vs glacial modes of formation the megaridges comprise three main units 1 a superficial fining upward drape that extends across the shelf above an unconformity underlying this drape is 2 the melville formation mfm which comprises the upper bulk of the megaridges sometimes displaying dipping internal acoustic reflections and consisting of medium to coarse sand and shell fragments characteristics consistent with either a tidal or glacifluvial origin the mfm unconformably overlies 3 the upper little sole formation ulsfm previously interpreted to be of late pliocene to early pleistocene age but here shown to correlate to late pleistocene glacigenic sediments forming a precursor topography the superficial drape is interpreted as a product of prolonged wave energy as tidal currents diminished during the final stages of post glacial marine transgression we argue that the stratigraphy constrains the age of the mfm to between 24 3 and 14 ka bp based on published dates coeval with deglaciation and a modelled period of megatidal conditions during post glacial marine transgression stratigraphically and sedimentologically the megaridges could represent preserved glacifluvial features but we suggest that they comprise post glacial tidal deposits mfm mantling a partially eroded glacial topography ulsfm the observed stratigraphy suggests that ice extended to the continental shelf edge

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

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

comparison of esker morphology and sedimentology with former ice surface topography burroughs glacier alaska topographic maps ofthe burroughs glacier ice surface and the surrounding land surface from 1948 to 1990 were used to generate subglacial hydraulic head maps and compare esker paths to hydraulic gradients hydraulic gradient at the glacier bed is controlled predominantly by ice surface slope but it is also influenced by the slope of the glacier bed eskers at burroughs glacier have been observed forming in subglacial and englacial tunnels most of the eskers formed in subglacial stream tunnels oriented parallel to the calculated hydraulic gradient where the former ice surface slope and slope of the bed do not coincide the relative influence of these factors on esker paths is analyzed one esker is oriented parallel to the land surface contours and perpendicular to hydraulic head contours implying an esker path controlled by the ice surface slope in another area a set of subglacially engorged eskers trends perpendicular to the land contours directly down the slope of the former glacier bed and parallels the calculated hydraulic head contours these eskers formed beneath thin ice in tunnels that were air filled much of the time and thus the slope of the glacier bed controlled the esker paths in a third area where the hydraulic gradient and glacier bed slope in the same direction a subglacially engorged esker trends down the land slope and the calculated hydraulic gradient most of the eskers at burroughs glacier are

esker characteristics in terms of glacier physics katahdin esker system maine the characteristics of large subglacially formed eskers such as the katahdin system are closely related to two special peculiarities of water filled tunnels along the beds of ice sheets 1 the water pressure approximates the weight of the overlying ice and 2 in tunnels that descend and those that ascend less steeply than 1 7 times the ice surface gradient the walls melt producing a sharply arched tunnel cross section whereas in those that ascend more steeply they freeze producing a wide low one the first peculiarity primarily governs the paths of these eskers it causes the tunnels to follow the paths ordinary rivers would follow were the land tipped downglacier 11 times the local ice surface gradient the paths therefore trend in the general direction of the former ice flow but tend to deviate so as to follow valleys and to cross divides at the lowest passes as observed ice surface gradients calculated from path deviations at two localities on the katahdin esker system indicate relatively thin sluggish ice the surface of which lay 200 m below the summit of mount katahdin in agreement with independent geologic evidence the second peculiarity primarily governs the form composition and structure of these eskers strong melting causes a large inflow of basal ice and entrained debris to the tunnel and produces sharp crested eskers of poorly sorted poorly bedded sand gravel and boulders with lithologies like the adjacent till whereas weaker melting produces multiple crested ones of similar composition freezing precludes inflow and produces broad crested eskers of fairly well sorted well bedded more water worn coarse sand with few large clasts ice surface gradients calculated from transitions from the multiple crested type to small areas of broad crested type on the katahdin system agree closely with those computed from the paths at nearby localities an anomalously low gradient calculated from a transition to an area of broad crested type approximately twice as wide and long as the probable ice depth apparently confirms that as expected the basal ice was supported by water pressure over most if not all of the width of the esker

macroforms large bedforms and rhythmic sedimentary sequences in subglacial eskers south central ontario implications for esker genesis and meltwater regime abstract eskers of south central ontario were deposited in closed subglacial conduits which were continuous main conduits this interpretation is supported by the intimate association of eskers with an anastomosing network of tunnel channels relatively continuous esker ridges minimal post formational disturbance of esker sediments intercalation of till and stratified sand and gravel diapiric folding at an esker core low variability in palaeocurrent direction and upslope flow paths down esker trends in clast lithology roundness and sphericity indicate continuous conduits with sediment supply by subglacial deformation of adjacent material into the conduits melt out of sediment from the conduit walls and fluvial resedimentation esker ridge morphology is attributed to synchronous erosion transportation and deposition along the main conduits such that ridge discontinuities are primarily explained as zones of non deposition during esker formation composite pseudoanticlinal and oblique accretion avalanche bed macroforms are identified gravel facies within these macroforms were deposited from fluidal flows or hyperconcentrated dispersions progradation of macroforms or migration of constituent large bedforms through the main conduits may have temporarily blocked constricted portions of those conduits and acted as possible internal autogenic controls on sediment availability and flow resistance the location of macroforms within conduits was primarily controlled by conduit geometry and sediment availability and later by feedback between macroform and conduit geometries sand and gravel units alternate rhythmically in vertical section rhythmicity is interpreted as a response to episodic flood flows controlled by seasonal changes in water supply but not necessarily representing annual events fans beads anabranched reaches and extended hummocky deposits are intimately associated with the main esker ridges they are interpreted in terms of subglacial cavities and localized flotation zones connected to the main conduits during flood events in phase wave structures are the products of hydraulic jumps in hyperconcentrated flood flows at flow expansions into swells within the main conduits into cavities connected to the main conduits or at a grounding line esker ridges record only the most powerful flood events whereas fans and beads record flow events in finer detail being primarily depositional reservoirs laterally fining deposits record the waning stages of conduit operation

organic geochemistry of the silurian tanezzuft formation and crude oils nc115 concession murzuq basin southwest libya abstract thirty six silurian core and cuttings samples and 10 crude oil samples from ordovician reservoirs in the nc115 concession murzuq basin southwest libya were studied by organic geochemical methods to determine source rock organic facies conditions of deposition thermal maturity and genetic relationships the lower silurian hot shale at the base of the tanezzuft formation is a high quality oil gas prone source rock that is currently within the early oil maturity window the overall average toc content of the hot shale is 7 2 wt with a maximum recorded value of 20 9 wt by contrast the overlying deposits of the tanezzuft formation have an average toc of 0 6 wt and a maximum value of 1 1 wt the organic matter in the hot shale consists predominantly of mixed algal and terrigenous type ii iii kerogen whereas the rest of the formation is dominated by terrigenous type iii organic matter with some type ii iii kerogen oils from the a b and h oil fields in the nc115 concession were almost certainly derived from marine shale source rocks that contained mixed algal and terrigenous organic input reflecting deposition under suboxic to anoxic conditions the oils are light and sweet and despite being similar were almost certainly derived from different facies and maturation levels within mature source rocks the b oils were generated from slightly less mature source rocks than the others based on hierarchical cluster analysis hca principal component analysis pca selected source related biomarkers and stable carbon isotope ratios the nc115 oils can be divided into two genetic families family i oils from ordovician mamuniyat reservoirs were probably derived from older palaeozoic source rocks whereas family ii oils from ordovician mamuniyat hawaz reservoirs were probably charged from a younger palaeozoic source of relatively high maturity a third family appears to be a mixture of the two but is most similar to family ii oils these oil families were derived from one proven mature source rock the early silurian rhuddanian hot shale there is a good correlation between the family ii and iii oils and the hot shale based on carbon isotope compositions saturated and aromatic maturity parameters indicate that these oils were generated from a source rock of considerably higher maturity than the examined rock samples the results imply that the oils originated from more mature source rocks outside the nc115 concession and migrated to their current positions after generation

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

density driven deformation structures in glacigenic consolidated diamicts examples from traeth y mwnt cardiganshire wales u k abstract density driven deformation may occur when a sediment of higher bulk wet density is deposited on top of one of a lower bulk wet density and occurs commonly in glacigenic sediments by recognizing density driven deformation of gravels and sands embedded in diamicts it is possible to reconstruct relative preconsolidation states of the diamicts density driven structures indicate that during deformation the diamict behaved as a viscous fluid thus providing important evidence for the viscosity and physical state of diamicts after deposition which has implications for the depositional environment the density driven process and its implications for reconstructing depositional environments is discussed criteria are given for the recognition of density driven deformation stages of gravels and sands deposited on top of a viscous mud

did lingering ice sheets moderate anoxia in the early palaeozoic of libya the hirnantian glaciation of west gondwana produced a glacially sculpted topography which is draped by organic rich latest ordovician and early silurian hot shales although these are the most important early palaeozoic source rock in north africa organic enrichment is distributed unevenly for example in al kufrah basin libya hot shales are elusive but outcrop analysis at the western basin demonstrates why this is the case the topmost mamuniyat formation of hirnantian age comprises glaciogenic sandstones passing upward into mixed facies of the tanezzuft formation which has a latest ordovician early silurian age the basal tanezzuft formation contains a shelly carbonate cool water deposits accumulated under oxygenating conditions and bioturbated sandstone succession above hummocky cross bedded and graded sandstone intervals are intercalated with shale and siltstone storm influx onto a muddy shelf these are interrupted by several lonestone bearing intervals ice rafted debris a striated pavement of subglacial origin and manganese oxide crusts and concretions the concretions and bioturbation imply oxygenation of the sea floor during transgression these putative glacial deposits were deposited following the main phase of the hirnantian glaciation at the same stratigraphic level as hot shales elsewhere in northern gondwana lingering ice caps may have produced well oxygenated marine waters precluding hot shale deposition

devensian glacigenic sedimentation and landscape evolution in the cardigan area of southwest wales hambrey michael davies j r glasser n f waters r a 2001 devensian glacigenic sedimentation and landscape evolution in the cardigan area of southwest wales journal of quaternary science 16 5 pp 455 482 rae2008

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

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

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

normalograptus kufraensis a new species of graptolite from the western margin of the kufra basin libya normalograptus kufraensis sp nov occurs as monospecific assemblages in the tanezzuft formation at the western margin of the kufra basin jabal eghei southern libya these graptolites have parallel sided rhabdosomes with long straight virgellae climacograptid thecae and a full straight median septum n kufraensis is intermediate between ordovician graptolites from the n angustus perner lineage and the younger sister species n ajjeri legrand and n arrikini legrand n kufraensis differs from these taxa as follows it is broader than n angustus it has greater thecal spacing than n ajjeri or n arrikini a table comparing measurements of n kufraensis with 44 other normalograptus taxa differentiates it from other members of this morphologically conservative group even though n angustus and n ajjeri are very long ranging graptolites a stratophenetic approach suggests that the specimens from jabal eghei may be of late hirnantian or younger age the faunal composition and preservation suggests these graptolites occupied the cratonic invader biotope the stratigraphic succession records deglacial flooding and fluctuating of redox in the tanezzuft formation with the graptolites indicating a short lived interval of anoxia

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

reservoir heterogeneity of ordovician sandstone reservoir bedinan formation se turkey diagenetic and sedimentological approachs abstract the sandstone bearing reservoir levels bedinan formation in the diyarbakir basin se turkey show strong heterogeneity in porosity permeability and capillary pressure which influences oil production rates of the studied 6 wells and beclouds the prediction of hydrocarbon reserves therefore comprehensive understanding of the sandstone reservoir characteristics has been necessitated to optimize its ultimate hydrocarbon recovery maximize its lifetime and to make more accurate performance predictions for the use in economic risk assessment in this study integrated analysis of paleogeography depositional characteristics petrographic features provenance pore structure reservoir heterogeneity rating diagenetic stage and its possible impacts on reservoir quality were examined by employing thin section analysis sedimentary structure analysis petrophysical interpretations geostatistic methods x ray diffraction xrd and non destructive x ray computed tomography it is concluded that the upper ordovician sands were laid down under shallow marine conditions lower shoreface to upper shoreface in an ice distal glaciomarine environment and have been subjected to a complex history of diagenesis the sandstone succession was sourced from plutonic rocks and derived from a stable craton the presence of detrital and diagenetic rooted clay coating minerals illite kaolinite illite smectite mixed clay layer together with cementation by quartz overgrowth and carbonate concretions in the matrix of the sandstone layers are the main factors deteriorating the reservoir quality by plugging pore throats and decrease production rates and oil recovery however the type of clay coating minerals and the amount of those are the key factors modifying reservoir architecture in comparison with the cementing materials besides that the integration of illitization of kaolinite and smectite in mesodiagenetic stage the presence of minerals such as quartz feldspar and illite altering the pore spaces from oil wet to water wet and compaction induced micropores are culprits in causing high water saturation in the sandstones even in the zone above oil water contact

sedimentology and origin of deposits from a small ice dammed lake leirbreen norway this paper describes recent proglacial lacustrine sediments exposed by the drainage of a small probably never more than 0 03 km2 ice dammed lake basin at leirbreen central jotunheimen norway the dominant facies include ripple laminated massive and horizontally stratified sands massive and horizontally laminated silts and irregularly laminated fine sands and silts the major control on lake circulation and the nature and distribution of these facies was an underflow driven by a subglacial meltwater stream which formed the major lake input although much of the sedimentary sequence indicates a pulsatory input the proximal character of this small lake prevented the development of classic varved silts compressional deformation of shoreline sediments was due to winter lake ice push other deformational processes included the grounding of icebergs water escape and syn sedimentary downslope collapse observations from an adjacent small ice marginal lake at leirbreen provide support for several of the inferences drawn from the sediments of the former ice dammed lake

the origin and glaciodynamic significance of sandstone ridge networks from the hirnantian glaciation of the djado basin niger the djado basin niger was located beneath the inner part of the late ordovician ice sheet the felar felar formation consists mainly of glaciomarine deposits associated with the major ice sheet recession within the glaciation and is bounded by two glacial unconformities structures corresponding to sandstone ridges are found within the felar felar formation sandstone ridges are several metres high about 10 m wide and hundreds of metres long these structures are organized in extensive anastomosed to subpolygonal networks the association of sandstone ridge networks with the later glacial unconformity and with other glacial evidence suggests sub glacial conditions for their origin sandstone ridge sedimentological characteristics indicate that sandstone ridges result from the scouring of the felar felar formation by sub glacial turbulent and pressurized meltwater then subglacial cavities were infilled with sand derived from glacial abrasion sandstone ridge networks are comparable with tunnel channels and document unusual drainage structures of the inner part of the palaeo ice sheet