a conceptual model for glaciogenic reservoirs from landsystems to reservoir architecture abstract glaciogenic sediments are present in many hydrocarbon producing basins across the globe but their complex nature makes it difficult to characterise the reservoir quality sedimentary units despite this ordovician glacial deposits in north africa and carboniferous permian glaciogenic sequences in the middle east have been proven to host significant economical hydrocarbon accumulations additionally discoveries have been made in the shallow

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

zarqa megafacies widespread subglacial deformation in the sarah formation of saudi arabia and implications for the sequence stratigraphy of the hirnantian glaciation

evidence for mechanically coupled lithosphere in central asia and resulting implications the recent dramatic increase in seismic anisotropy and surface global positioning system gps data for central asia permits a comprehensive examination of the mantle s role in mountain building a joint analysis of 178 shear wave splitting and 2000 gps observations using a new technique reveals that the crust and lithospheric mantle deform coherently arguing for crust mantle mechanical coupling during deformation the observed spatial variations in anisotropy refl ect the large scale pattern of lithospheric deformation as well as a change in deformational style from simple shear on the tibetan plateau transitioning to pure shear in surrounding regions

experimental modeling of pressurized subglacial water flow implications for tunnel valley formation tunnel valleys are elongated hollows commonly found in formerly glaciated areas and interpreted as resulting from subglacial meltwater erosion beneath ice sheets over the past two decades the number of studies of terrestrial tunnel valleys has continuously increased and their existence has been hypothesized also on mars but their formation mechanisms remain poorly understood we introduce here an innovative experimental approach to examine erosion by circulation of pressurized meltwater within the substratum and at the silicon substratum interface we used a permeable substratum sand partially covered by a viscous impermeable and transparent lid silicon putty below which we applied a central injection of pure water low water pressures led to groundwater circulation in the substratum only while water pressures exceeding the sum of the glaciostatic and lithostatic pressures led to additional water circulation and formation of drainage landforms at the cap substratum interface the formation of these drainage landforms was monitored through time and their shapes were analyzed from digital elevation model obtained by stereo photogrammetry the experimental landforms include valleys that are similar to natural tunnel valleys in their spatial organization and in a number of diagnostic morphological criteria such as undulating longitudinal profiles and tunnel shapes these results are consistent with the hypothesis that overpressurized subglacial water circulation controls the formation of tunnel valleys

ice keel scour marks in the geological record evidence from carboniferous soft sediment striated surfaces in the parana basin southern brazil abstract soft sediment glacially striated surfaces are common structures in ancient glacial successions such as the gondwanic paleozoic record and document ice erosion on non lithified beds in most cases they have been interpreted as the product of subglacial erosion beneath advancing marine or terrestrial glaciers although direct evidence for glacial sedimentation in associated strata is infrequent in this paper we document soft sediment striated surfaces of carboniferous age occurring in cross laminated sandstones of the lower itarare group southern parana basin brazil and interpret them as scour marks generated by keels of floating ice striated surfaces occur in closely spaced multiple stratigraphic horizons in the lower half of a deglacial sequence whose facies characteristics indicate sedimentation in subaqueous outwash and deltaic settings surfaces are laterally discontinuous commonly bordered by marginal berms and in some cases capped by mudstones with dropstones they have orientations that deviate from the regional paleo ice flow indicated by glacial grooves carved on the pre glacial basement the characteristics observed are consistent with modern and pleistocene ice keel scour marks in addition soft sediment striated surfaces from other gondwanic localities have similar attributes suggesting that ice keel scour marks may be more common in the geological record than previously considered and that vertically repeated striated surfaces do not necessarily indicate multiple ice sheet advances therefore paleo ice flow reconstructions based on soft sediment striated surfaces need to be revaluated considering that the movement of free floating ice masses is controlled by phenomena other than glacier advance

late ordovician glacial pavements revisited a reappraisal of the origin of striated surfaces late ordovician glacial pavements revisited

crustal fabric in the tibetan plateau based on waveform inversions for seismic anisotropy parameters 1 the tibetan plateau has the thickest continental crust on earth and fabrics within the crust that are anisotropic to seismic waves may provide clues to how it reached such extreme proportions and how it is currently deforming waveform modeling using a global minimization inversion technique applied to receiver functions computed from 11 stations spanning the north south length of the eastern plateau has yielded a suite of crustal models that include anisotropy these models suggest that the tibetan crust contains 4 14 anisotropy at different depths that is likely a result of both fossil fabrics and more recent deformation all models contain anisotropy in the surface layer and for most stations the alignment of the slow symmetry axis suggests a relationship with crustal fabrics associated with e w trending thrust faults or sutures middle to lower crustal anisotropy is present at most stations with a fast axis trending n s to nw se in the south nearly e w in the central plateau and n s to ne sw in the northern plateau this pattern appears consistent with recent ductile deformation due to both topographically induced flow and to boundary forces from subducting lithosphere at the northern and southern margins of the plateau the orientations of crustal anisotropy determined for most stations in this study are significantly different from shear wave splitting fast polarization directions implying distinct deformation in the crust and mantle

geodetic evidence for a low slip rate in the altyn tagh fault system the collision between india and asia has been simulated with a variety of computational models that describe or predict the motions of the main faults of east asia geological slip rate estimates of 20 30 mm yr 1 suggest that the largest of these faults the 2 000 km long altyn tagh fault system on the northern edge of the tibetan plateau absorbs as much of the indo asian convergence signal as do the himalayas partly by oblique slip and partly by contraction and mountain growth however the predictions of dynamic models for asian deformation and the lower bounds of some geological slip rates estimates 3 9 mm yr 1 refs 7 8 suggest that the altyn tagh system is less active here we report geodetic data from 89 91 degrees e that indicate left lateral shear of 9 5 mm yr 1 and contraction of 3 1 mm yr 1 across the altyn tagh system this result combined with our finding that at 90 degrees e tibet contracts north south at 9 1 mm yr 1 supports the predictions of dynamic models of asian deformation

440 ma ice stream in north africa landsat images aster digital elevation models aerial photographs and field investigations in the western murzuq basin libya and the adjacent tassili n ajjers algeria provide paleogeomorphological evidence for the existence of a late ordovician ice stream at least 200 km long and 80 km wide this includes mega scale glacial lineations an associated subglacial meltwater drainage system and ice front features this first comprehensive description of a pre cenozoic ice stream may help to identify other examples in the proterozoic to paleozoic glacial record reconstruction of the extent and behavior of former ice sheets and reservoir prospect analysis in glacially related successions have to take into account the potential occurrence of ice streams

anisotropy across the sorgenfrei tornquist zone from shear wave splitting abstract during the tor 1 passive seismic experiment in 1996 97 a maximum of 139 temporary seismograph stations were operating over the sorgenfrei tornquist zone stz in an area extending from northern germany through denmark to central sweden one of the objectives was to study horizontal anisotropy directions in the subcrustal lithosphere and asthenosphere across the trans european suture zone to achieve this goal broad band and intermediate period 5 s data of the tor 1 stations and additional stations of permanent networks grsn geofon were analysed for splitting of sks and skks phases as a result of the relatively dense station spacing the method offers good lateral resolution of anisotropy preliminary results suggest that the directions of the fast horizontal s wave velocity are affected by the stz in central europe and southern sweden far away from the stz fast s wave directions are approximately e w while they turn more northerly closer to the stz where they are approximately parallel to the trend of the stz no significant shear wave splitting was observed north of 57 n and east of 14 e small delay times between 0 2 and 0 5 s observed at the northernmost tor 1 station t40s and t60s may be controlled by anisotropy in a thickened crust the mantle contribution of horizontal anisotropy within the stz is probably constrained to an approximately 60 km thick zone in the depth range between 70 and 300 km the observations are consistent with a model where azimuthal anisotropy is not governed by present day mantle flow in the asthenosphere but rather is frozen into the subcrustal lithosphere during the last episode of tectonic activity

sedimentary imprints of former ice sheet margins insights from an end ordovician archive sw libya fromthe proterozoic to the quaternary the evolution of the earthwas characterised by recurrent periods of glaciation however the margins of many ancient ice sheets are poorly defined on palaeogeographic reconstructions the extent and outlines of ancient ice sheets can be better understood through careful documentation of sediments deposited at the ice sheet margin an outstanding example is provided herein based on an end ordovician archive in libya tihemboka area murzuq basin the four sets of structures include i subglacial glaciotectonic structures and soft sediment deformations from flowing glacier ice such as intraformational glacial striae intraformational deformation shear planes sheath folds normal microfaults and large scale glaciotectonic folds and thrusts ii structures resulting from overpressured subglacial meltwater flows such as clastic dykes and tunnel valleys iii proglacial depositional structures and facies related to highmagnitude meltwater floods such as sandstone intraclasts large scale bedforms resulting from supercritical flows climbing dune cross stratification and kettle holes and iv deformation structures resulting from free floating and nonglacier ice such as ice keel scours and ice crystal marks such a set of structures points to an ice marginal essentially continental depositional setting and provides an excellent suite of criteria to identify margins of ancient ice sheets in the stratigraphic record at a regional scale a reconstruction through time and space of the related depositional wedge is proposed this corresponded to a seismic scale n120 m in thickness 40 km in length ice marginal wedge in front of an essentially warm based ice sheet inducing concomitant large scale glaciotectonic deformation glacial basin and tunnel valley downcuttings the related ice front was associated with high energy meltwater flows feeding a network of deeply incised proglacial channels downstream and beyond them a fluvioglacial deltaic system shallow ice marginal permafrost most likely affected the depositional wedge at a larger scale the tihemboka ice marginal wedge is interpreted as related to a stillstand period over the gondwana platform developed over an estimated interval of a few thousands of years based on these data the conditions that arose in a particularly favourable context for the development the preservation and the identification of ice marginal wedges in the geological record are reviewed significant meltwater derived sediment deposition and aggradation in an accommodation space resulting either from preglacial inheritance glacial downcuttings and or glacio isostatic lithospheric flexure or active tectonic subsidence 1 myr are required for their formation and subsequent preservation

tunnel valleys current knowledge and future perspectives tunnel valleys are elongated incisions that are commonly interpreted as being the result of erosional processes by subglacial meltwater occurring under continental ice sheets the abundance size and the primarily coarse grained infill of these features have made tunnel valleys important hydrocarbon and groundwater reservoirs although numerous tunnel valleys have been described over the last century their formation and infill remain poorly understood this review summarizes and discusses the current knowledge of tunnel valleys providing an overview of the observations around the world morphological aspects that separate tunnel valleys from other landforms are discussed as well as the wide variety of sedimentary environments found to contribute to the infilling of these features the depth of the incision and the character of ice retreat significantly determine the final infill architecture the formational hypotheses proposed in the literature are assessed to test their wider applicability to all other tunnel valleys in order to find a generic model that helps in the prediction of the morphology and infilling of both pleistocene and pre pleistocene age a quasi steady state model with small meltwater outbursts that erode tunnel valleys near the ice margin seems compatible with most of the known valleys other proposed models require specific geographical or climatic conditions

push moraines and glacier contact fans in marine and terrestrial environments the local climatic regime and the mass balance state are important determinants of the dynamics of terrestrial and marine glacier fronts which in turn determine the sediments and landforms produced at the glacier front many modern glaciers undergoing overall retreat in areas of maritime climate produce winter push moraines during a late winter readvance followed by a summer retreat whilst in more continental regions no significant winter readvance occurs and annual push moraines are absent the frontal dynamics which lead to these changes are analysed and the form structure sequence and field relations of both terrestrial and marine push moraines are described from iceland spitsbergen and baffin island long term changes in mass balance leading to major glacier advances or readvances also generate large push moraines in terrestrial environments push moraine formation is accompanied by uplift rejuvenation and down cutting of outwash systems whose sediments become closely associated with glaciotectonic structures which permit pre syn and post tectonic sequences to be identified the development of ice marginal fan moraine complexes is modelled as a function of the relative magnitude of two parameters the velocity of ice movement and the calving rate a high ice velocity just exceeded by the calving rate gives closely spaced push moraines and confluent ice marginal fans a high velocity far exceeded by the calving rate produces closely spaced moraines but separate ice marginal fans a low ice velocity in combination with a high calving rate results in well separated and feebly developed push moraines while a low ice velocity and a low calving rate produces feeble push moraines and coalescent fans

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

observing plate motions in s e asia geodetic results of the geodyssea project this paper presents the final geodetic results of the geodyssea project the gps data from a 42 station network observed during two field campaigns 1994 1996 were analyzed by four groups using different software packages and analysis strategies the precision of both campaign coordinate solutions was found to be 4 7 mm for the horizontal and 1 cm for the vertical component the campaign solutions were merged into one unique solution which was accurately mapped into the itrf 96 reference frame the global accuracy of this solution with respect to itrf 96 is 1 cm while the resolution of the relative horizontal velocities is estimated to be at the level of 2 3 mm yr this solution was used as the basis for all scientific interpretations which are published in separate papers the velocity estimates of a part of the network provided the first direct measurement of a relative motion of the sundaland block with respect to eurasian plate

late ordovician glacial pavements revisited a reappraisal of the origin of striated surfaces the recognition of ice related unconformities is of prime importance when dealing with the sequential architecture of pre pleistocene glacial successions late ordovician striated surfaces strikingly preserved in north gondwana were long considered as abrasion surfaces at the sole of a grounded glacier overriding unlithified sediments or having resulted from the scouring action of drifting icebergs field observations from mauritania and libya show that these striated surfaces require an alternative interpretation which can be applied to similar surfaces described elsewhere such as in permo carboniferous or neoproterozoic glacial sequences a new model is proposed that involves superimposed concurrent decollement planes within a subglacial brittle shear zone in unlithified sand beds shear zones in subglacial sediments are well known in quaternary deposits but to date it has not been demonstrated that they may form striated surfaces occurring at continent scale with orientations consistent with palaeo ice flow reconstructions based on larger scale indicators

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

characterization of pebble fabrics in modern terrestrial glacigenic sediments pebble fabric data are available from several facies of glacigenic sediments deposited by modern glaciers where sedimentary processes can be observed or inferred with relatively little ambiguity over 100 samples from contemporary environments illustrate fabrics characterizing melt out till deformed and undeformed lodgement till sediment flow deposits and ice slope colluvium lodgement till fabric variability is related to the two layer structure of these sediments a structureless friable upper layer with low shear strength and high consolidation coefficient overlying a very compact material of horizontal platy structure fabric strength assessed by eigenvalue analysis is weaker and pebble dip is more dispersed in the upper structureless horizon stronger fabrics in the lower platy horizon may be primary depositional fabrics which are destroyed by subglacial shearing to give weaker fabrics in the upper horizon alternatively upper horizon fabrics may be characteristic of all recently deposited lodgement tills with stronger fabrics developing at depth by dewatering and consolidation there is a general reduction in fabric strength and an increase in particle dip associated with the transition from melt out tills through undeformed and deformed lodgement tills to sediment flow deposits and ice slope colluvium there is however considerable overlap in the fabric strengths characteristic of sediment flow deposits and deformed lodgement tills fabric data from modern glacial sedimentary facies are used to assist in interpreting the mode of deposition of some quaternary glacial sediments relatively strong fabrics characteristic of melt out tills and undeformed lodgement tills are more likely to be diagnostic of genesis than weaker fabrics associated with deformed sediments

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

depositional environments and sequence stratigraphy of paralic glacial paraglacial and postglacial upper ordovician siliciclastic deposits in the murzuq basin sw libya abstract the application of sequence stratigraphy methods to glaciogenic deposits is more problematic than in normal paralic deposits because changes in the relative sea level are strongly influenced by an interplay between glacial advance and retreat and concomitant isostatic loading and rebound of the continental shelf the study of outcrop and subsurface sections of the upper ordovician glaciogenic melaz shuqran and the mamuniyat formations murzuq basin sw libya yielded important information on understanding changes in the relative sea level related to glacier movements three depositional sequences were recognized depositional sequence one ds 1 corresponds to the entire melaz shuqran formation that was bounded below by subglacial erosion surface i e sequence boundary sb formed during glacier advance into shallow water areas this surface correlates with a transgressive surface ts in deep water areas and records an initial rise in the relative sea level owing to glacier advance and loading of the continental shelf transgression occurred due to the slower rate of eustatic sea level fall than the isostatic loading the transgressive systems tract tst comprises shoreface sandstones and offshore mudstones diamictites with ice rafted debris further rise in relative sea level was associated with glacial retreat which resulted in sediment starvation in deep water areas and the formation of a condensed section these deposits represent glacial depositional systems the highstand systems tract hst comprises prograding deltaic deposits which was formed when the rate of sediment supply exceeds the rate of relative sea level rise the hst deposits represent paraglacial depositional systems depositional sequence two ds 2 corresponds to the lower and middle part of the mamuniyat formation and is bounded below by deep erosional surface sb that cuts into ds 1 as an incised valley which was formed during glacial advance across the continental shelf incision occurred due to the faster rate of eustatic sea level fall than the rate of isostatic loading glacier retreat and the concomitant rise in relative sea level resulted in the deposition of braided fluvial sediments in a lowstand systems tract lst and tide dominated estuarine sediments in a tst these deposits represent glacial depositional systems the hst sediments represent prograding foreshore to shoreface deposits which were formed when the rate of sediment supply exceeded the rate of relative sea level rise these deposits represent paraglacial depositional systems depositional sequence three ds 3 corresponds to the upper part of the mamuniyat formation and is bounded below by an erosional surface sb that was formed as a result of isostatic rebound associated with relative sea level fall ds 3 lst comprises gilbert type deltaic conglomeratic sandstones grading upwards into sandstones subsequent relative sea level rise during the late ordovician resulted in deposition of shoreface tst sandstone further rise in the relative sea level resulted in sediment starvation in deep water areas and the formation of thin hardground layer i e condensed section that marks the upper boundary of late ordovician deposits this study provides predictive model for the spatial and temporal distribution of ancient glaciogenic depositional facies that may have important implications in hydrocarbon explorations

timescales for the evolution of seismic anisotropy in mantle flow 1 we study systematically the relationship between olivine lattice preferred orientation and the mantle flow field that produces it using the plastic flow recrystallization model of kaminski and ribe 2001 in this model a polycrystal responds to an imposed deformation rate tensor by simultaneous intracrystalline slip and dynamic recrystallization by nucleation and grain boundary migration numerical solutions for the mean orientation of the a axes of an initially isotropic aggregate deformed uniformly with a characteristic strain rate show that the lattice preferred orientation evolves in three stages 1 for small times recrystallization is not yet active and the average a axis follows the long axis of the finite strain ellipsoid 2 for intermediate times the fabric is controlled by grain boundary migration and the average a axis rotates toward the orientation corresponding to the maximum resolved shear stress on the softest slip system 3 for the fabric is controlled by plastic deformation and average a axis rotates toward the orientation of the long axis of the finite strain ellipsoid corresponding to an infinite deformation the infinite strain axis in more realistic nonuniform flows lattice preferred orientation evolution depends on a dimensionless grain orientation lag parameter x defined locally as the ratio of the intrinsic lattice preferred orientation adjustment timescale to the timescale for changes of the infinite strain axis along path lines in the flow explicit numerical calculation of the lattice preferred orientation evolution in simple fluid dynamical models for ridges and for plume ridge interaction shows that the average a axis aligns with the flow direction only in those parts of the flow field where 1 calculation of provides a simple way to evaluate the likely distribution of lattice preferred orientation in a candidate flow field at low numerical cost

circular cryogenic structures from the hirnantian deglaciation sequence anti atlas morocco abstract more than a hundred circular structures are observed at the topmost part of the late ordovician glaciogenic record in the central anti atlas southern morocco the structures were formed in the outer glaciated platform and characterize the hirnantian deglaciation sequence they are distributed within a 20 meter thick stratal interval and at present relicts of them still occupy several square kilometers in map view the circular to ovoidal structures which comprise fluvial sandstone beds and are sealed by shallow marine strata occur in closely spaced clusters they are 13 320 m in diameter with half of them in the 40 90 m range stratal wedging and bending as well as extensional and reverse centimeter to meter scale soft sediment fractures demonstrate that formation processes initially involved synsedimentary uplifts subsequent subsidence and collapse resulted in the formation of circular structures which essentially correspond to localized depocenters in the form of small scale sag basins regarding the overall deglaciation context the circular structures are interpreted as periglacial in origin with vertical movements ascribed to cryogenic processes involving the growth and then decay of bodies of segregation ice the size and diameter distributions which are comparable to the recent younger dryas and current lithalsa structures suggest that hirnantian circular features relate to similar genetic processes and can be referred to as lithalsa like structures a conceptual emplacement model is proposed which considers that a long term sedimentary aggradation over the fluvial plain prevented any significant landforms either a bump or a depression from being expressed at the depositional surface at the time of the circular structure formation the hirnantian lithalsa like structures constitute firm evidence for a late ordovician discontinuous permafrost during the deglaciation stage assuming that the late quaternary meridional zonation is valid it suggests the anti atlas was at latitudes in the 55 65 interval if combined with the rare pingo structures described in the literature which attest that coeval continuous permafrost conditions occurred at higher paleo latitudes the moroccan circular structures provide evidence for an alternate south pole position during the hirnantian 444 ma

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

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

late ordovician tunnel valleys in southern jordan theupperordovicianglacialrecordofsouthernjordan ammarfm essentiallycon sists of palaeovalley infills and of a subordinate time transgressive fluvial to shallow marine suc cession overstepping both the palaeovalleys and interfluvial areas valley size depth 60 160 m width 1 3 km steep 20 508 margins internal organization and depositional facies point to an origin as tunnel valleys the tunnel valleys are infilled by either fluvioglacial sandstones or fluvio deltaic coarsening upward successions including fine grained clayey sediments re occupation of previous valleys is evident in places at least three generations of tunnel valleys are inferred from cross cutting relationships although they most probably only reflect temporary standstills and minor re advances related to the overall recession following the main glacial advance recorded in saudi arabia petrophysical measurements indicate that higher permeabilities are located in the glacially related strata 1 5 3 darcy in fluvioglacial infills with a somewhat reduced porosity 22 28 relative to the preglacial sandstones owing to a higher clay content probably of diagenetic origin sandstone amalgamation however gives the fluvioglacial sandstones a high reservoir quality

subglacial to proglacial depositional environments in an ordovician glacial tunnel valley alnif morocco abstract this paper presents the sedimentary analysis of an exceptional ordovician glacial tunnel valley in the eastern part of the anti atlas the valley infill comprises two major glacial erosion surfaces striated pavements each overlain by a fining upward glacial unit these units are composed of five distinct facies associations recording the evolution from subglacial to proglacial environments and an additional sixth facies association overtopping the tunnel valley infill and associated with post glacial environments the tunnel valley infill also records a transitional environment between the subglacial and proglacial settings which is compared with the antarctic ice sheet margin these three environments are defined by the position of the grounding line and the coupling line the new proposed depositional model also differs from usual ordovician depositional models in which the main tunnel valley infill is interpreted as essentially proglacial outwash deposits in a range of glaciomarine to glaciofluvial environments overall a substantial part of the valley infill 50 of volume was deposited in a subglacial setting the sedimentary bodies could form potentially thick and laterally extended although these were limited by the shape and extent of the subglacial accommodation space finally the sedimentary record when compared with regional analogues also provides information for the palaeogeographic reconstruction of the ordovician ice sheet in this region

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

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

trace fossils on a late ordovician glacially striated pavement in algeria abstract an exceptional exposure of late ordovician glaciogenic sediments crops out in dider se algeria within the tassili n ajjer region the sediments consist of sandstones and diamictites sandwiched between a mid ordovician fluvial and tidal sandstone in tahouite formation below and early silurian shale oued imirhou formation above stratigraphic discontinuities within the late ordovician glaciogene succession include palaeovalley incisions and glacially striated pavements striation and fluting of a soft bedded sediment beneath an ancient ice sheet is supported by abundant dewatering structures and soft sediment gouges interpreted to have been produced by the action of stone ploughing in dider two types of previously undescribed circular structures sit in negative relief on this glacial pavement namely 1 paired thumb shaped impressions 2 cm in diameter and 3 mm in depth and 2 a 5 cm wide impression with 3 segmented nested cycles a framboid or aggregate origin may be appropriate for the smaller of the features but the larger impression is interpreted as biogenic internal complexity is characteristic discounting concretion moulds and water escape structures a biogenic origin as a coelenterate resting trace is proposed with speculation on conditions of exceptional preservation in an ancient periglacial environment

hirnantian sea level changes in the baltoscandian basin a review abstract correlation of hirnantian sections of the baltoscandian basin reveals four major transgressive regressive sea level cycles related to waxing and waning of ice sheets at high southern hemisphere paleolatitudes the correlation between stratigraphic members in estonia and latvia shows transgressions regressions in the first half of the hirnantian when sea level fluctuated between shallow and deep shelves during the second half of the hirnantian the sea regressed into the former deep shelf leaving north estonia as a mainland after that the sea retreated to the central baltoscandian basin and the whole baltic shelf stayed subaerial this final regression brought the coastline to the vicinity of scania in the west and of lublin basin in the east in the latest hirnantian during ice cap melting sea level began to ascend the baltic shelf submerged again in the early silurian this transgression of latest hirnantian earliest silurian indicates fast melting of ice in gondwanan ice sheets we are proposing that the destabilisation of great amount of gas hydrates due to diminishing of water column pressure during the last deepest regression acted as a trigger for rapid warming of climate the ascertainment of sea level changes presumes correlations between different parts of the baltoscandian basin comparison of published data on stratigraphy lithology faunal finds chemical elements and stable carbon isotopes was the ground for tracking hirnantian changes

subglacial emplacement of tills and meltwater deposits at the base of overdeepened bedrock troughs the sedimentary infill of subglacially eroded bedrock troughs in the alps are underexplored archives for the timing extent and character of pleistocene glaciations but may contain excellent records of the quaternary landscape evolution over several glacial interglacial cycles the onset of sedimentation in these bedrock troughs is often reflected by diamicts and gravels directly overlying bedrock in the deepest basin segments subglacial or proglacial depositional environments have been proposed for these coarse grained basal units but their characteristics and origin remain controversial this article presents results from drill cores that recovered a coarse grained basal unit in a major buried bedrock trough system in the lower glatt valley northern switzerland the excellent core recovery allowed a detailed study combining macroscopic microscopic and geochemical methods and gives unprecedented insights into the transition from erosion to deposition in overdeepened bedrock troughs these results show that the basal infill comprises diamicts interpreted as subglacial tills separated by thin sorted interbeds originating from subglacial cavity deposition the stacking may thus represent repeated switching between a coupled and decoupled ice bed interface indicating an ever transforming mosaic of subglacial bed conditions decoupling in response to high basal water pressures is probably promoted by the confined subglacial hydraulic conditions resulting from the bedrock acting as aquitard the narrow reverse sloping outlet and a large catchment area while stratigraphic and lithological evidence suggests that erosion and the onset of basal sedimentation occurred during the same glaciation different scenarios for the relative timing of infilling in relation to formation and glaciation of the bedrock trough are discussed overlying deltaic and glaciolacustrine sediments suggest deposition during subsequent deglaciation of the bedrock trough the basal sediment characteristics are in agreement with previous reports in hydrogeological and seismic exploration and suggest the occurrence of similar basal successions in other subglacially overdeepened basin in the alps and elsewhere

on the architecture of high latitude continental margins the influence of ice sheet and sea ice processes in the polar north atlantic abstract the presence of ice during the late cenozoic distinguishes the nature and rates of processes on high latitude margins from those elsewhere ice sheets terminating in marine waters deliver icebergs meltwater and debris to high latitude seas sea ice influences ocean salinity structure and downslope water and sediment transfer and also transports fine grained sediments over long distances these cryospheric processes have led to the development of a distinctive sedimentary architecture on modern high latitude continental margins large submarine fans made up almost entirely of stacked debris flows are present around the norwegian greenland sea large slides are located in a variety of settings relative to rates of sediment delivery from quaternary ice sheet margins but no large slides have been mapped on the east greenland margin however extensive channel systems and sediment wave fields are present in the greenland basin probably related to intermittent downslope flow of dense water and turbidity currents the extensive ne greenland shelf was not innundated by ice sheet advance during recent full glacial conditions allowing sea ice and deep water production during both interglacials and full glacials changes in the nature and rate of sedimentation within the greenland basin should provide clues on the rate of dense water production with implications for thermohaline circulation in the north atlantic other erosional and depositional features on the norwegian greenland sea margins include canyons and contourite drifts high relief tectonic features influence sediment reworking by turbidity currents at abyssal depths a simple conceptual model for glacier influenced marine sedimentation summarizes the role of cryospheric processes in high latitude margin sedimentary environments

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

the palaeo valley infilling glaciogenic sarah formation an example from rahal dhab palaeo valley saudi arabia the sarah formation is a glaciogenic sedimentary unit deposited along the gondwana margin during the latest ordovician ice age and represents a major hydrocarbon reservoir in northern saudi arabia large scale glacial palaeo valleys cut into the qasim formation and were infilled by the sarah formation post glacial transgression in the earliest silurian resulted in the deposition of the qusaiba shale member and associated organic rich basal source rocks which cap the sarah formation infilled palaeo valleys this unique setting makes the sarah formation an important emerging exploration target in saudi arabia this paper focuses on the facies and depositional architecture in seismic scale outcrops of the sarah formation in north western saudi arabia the rahal dhab palaeo valley provides a 100 km long dip oriented cross section which has been covered by 24 vertical sections sedimentary architectural analyses at metre to kilometre scale and by three cored shallow boreholes in the rahal dhab palaeo valley the sarah formation was deposited in a proglacial setting that ranged from marginal marine to offshore pro delta and is made up of three units i sarah sandstone ii sarah shale and iii uqlah the present study shows the relationships between these three units and architectural controls on reservoir quality in this system this paper contributes to the regional understanding of the sarah formation and the new depositional model of the rahal dhab palaeo valley provides an outcrop reservoir analogue for hydrocarbon exploration in adjacent areas this article is protected by copyright all rights reserved

late ordovician ice marginal processes and sea level change from the north gondwana platform evidence from the valongo anticline northern portugal abstract in the valongo anticline northern portugal the upper ordovician age sobrido formation records glacioeustatic sea level changes and glacially influenced depositional environments on the north gondwana platform associated with the hirnantian glaciation new stratigraphic data from the sobrido formation taking into account lateral facies variations not exposed in the type section allow for a better understanding of hirnantian glacial sedimentary processes sea level changes and palaeogeography the lower member of the sobrido formation is composed of quartzites alternating with siltstones and mudstones the transition to the upper member is most commonly an erosional contact marked by a ferruginous horizon the lowermost part of the upper member contains a thinly laminated and rhythmically bedded mudstone that is capped by a thin mm scale shelly bed and overlain by massive diamictites laterally conglomerate beds that have erosion bases are rhythmically interbedded with slates and diamictites that sometimes contain outsized mudstone clasts the uppermost part of the upper member is very variable in thickness laterally and contains massive to laminated diamictite beds intercalated with quartzites conglomerates and slates massive diamictites commonly exhibit thin horizons with fe oxides and occasionally manganese phosphate and oolitic chamosite the lower member of the sobrido formation is interpreted as ice proximal outwash that is transitional laterally to subaqueous ice contact fans formed by debris and mass flows erosion followed by chemical weathering of the lower member and variably within the lowermost part of the upper member is interpreted as a response to sea level fluctuations and development of spatially variable anoxic conditions and groundwater migration within isolated marine basins facies within the upper member of the sobrido formation are interpreted as ice distal laminated diamictites and ice proximal deposits massive diamictites quartzites and conglomerates groove casts and hummocky cross stratification within quartzite beds suggest deposition in a shallow shelf setting the upper ordovician lower silurian transition is locally marked by a ferruginous horizon black basal quartzites are interpreted to record development of an erosional unconformity during silurian sea level rise in other areas a chloritoid layer indicates proximity to dry land these palaeoenvironmental interpretations suggest dynamic changes in sedimentary and geochemical processes in response to sea level and related palaeogeographic changes during the upper ordovician and transitional to the lower silurian on the north gondwana platform

first evidence for cambrian glaciation provided by sections in avalonian new brunswick and ireland additional data for avalon gondwana separation by the earliest palaeozoic abstract the first evidence for cambrian glaciation is provided by two successions on the avalon microcontinent the middle lowest cambrian middle terreneuvian series and fortunian stage stage 2 boundary interval has an incised sequence boundary overlain by a fluvial lowstand facies and higher olive green marine mudstone on hanford brook southern new brunswick this succession in the lower mystery lake member of the chapel island formation may be related to melting of an ice sheet in avalon the evidence for this interpretation is a muddy diamictite with outsized up to 10 cm in diameter proterozoic marble and basalt clasts that penetrated overlying laminae in the marine mudstone that eustatic rise was associated with the mudstone deposition is suggested by an approximately coeval rise that deposited sediments with watsonella crosbyi zone fossils 650 km away in avalonian eastern newfoundland a sea level rise within the watsonella crosbyi chron at ca 535 ma may correspond to a unnamed negative 13 c excursion younger than the basal cambrian excursion bace and the zhuce excursion in stage 2 of the upper terreneuvian series cambrian dropstones are now also recognized on the northern gander margin of avalon in continental slope rise sedimentary rocks in southeast ireland although their age early middle cambrian is poorly constrained dropstones in the booley bay formation provide additional evidence for cambrian glaciation on the avalon microcontinent besides providing the first evidence of cambrian glaciation these dropstone deposits emphasize that avalon was not part of or even latitudinally close to the terminal ediacaran cambrian tropical carbonate platform successions of west gondwana

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

ordovician and silurian sea water chemistry sea level and climate a synopsis following the cambrian explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans the ordovician and silurian periods witnessed three subsequent major biotic events the great ordovician biodiversification event the end ordovician extinction the first animal extinction and second largest of the five mass extinctions of the phanerozoic and the early silurian postextinction recovery there are currently no simple explanations for these three major events combined extrinsic geological and intrinsic biological factors probably drove the biodiversifications and radiations and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography palaeoclimate and sea water chemistry the present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the ordovician and silurian including sea level changes geochemical proxies 13c 18o 87sr 86sr of the ocean waters and the evolution of the atmosphere oxygen and carbon dioxide content less

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

does porewater or meltwater control tunnel valley genesis case studies from the hirnantian of morocco several ordovician tunnel valleys are exposed in the moroccan anti atlas mountains including the alnif and the foum larjamme tunnel valleys located 150 km away from each other sedimentological and deformational analyses of these two glacial troughs reveal that differing processes lead to their formations the alnif tunnel valley contains numerous deformation structures within sediments both below and above the main glacial erosion contact surface ball structures and clastic dykes occur within preglacial sediments down to 35 m below glacial incisions while overlying glacial sediments contain fluted surfaces clastic dykes dewatering structures folds and radial step normal faults the characteristics of the alnif tunnel valley can be explained by a porewater pressure driven model of formation where the localized increase of basal shear stress and porewater pressure underneath subglacial deforming zones lead to the development of a dense hydrofracture network in the preglacial bed these processes of hydraulic brecciation promoted subglacial remobilization of the preglacial material and contributed to the formation of the tunnel valley the foum larjamme tunnel valley displays undisturbed preglacial sediments and few dewatering structures at the base of the glacial sedimentary infill which suggests relatively low porewater pressures within the tunnel valley during formation this second type of tunnel valley where porewater pressure remained relatively low appears to have been formed by meltwater erosion the undulating base of the foum larjamme tunnel valley implies progressive erosion by a stable subglacial braided network of nye channels or alternatively by channels migrating laterally during episodic minor subglacial outbursts these two tunnel valleys highlight the regional variability of processes involved in the formation of tunnel valleys the distribution of palaeo ice streams in north africa illustrate that morphologies and processes involved in the formation of tunnel valleys vary between ice stream and inter ice stream zones due to variations in meltwater availability the topography and bed lithological properties