depositional facies of a mud shoreface in suriname south america a mud analogue to sandy shallow marine deposits abstract immense volumes of argillaceous muds are being deposited along the world s longest continuous mud coastline which faces the open equatorial atlantic not only are these muds deposited in an environment normally associated with sand deposits but just as in sandstones sedimentary structures within these muds record the dynamic processes that deposited them this study investigates a 100 km long portion of the suriname coast utilizing cores bathymetric surveys and field observations from nearshore and innershelf environments using terminology borrowed from sand coastlines the innershelf nearshore of suriname is divided into transition zone shoreface foreshore and backshore the transition zone consists of a mix of offshore sands and shoreface muds shoreface and foreshore are subdivided into bank and interbank zones these zones are transitory because mud banks which make up the bank zones migrate westwardly mud banks are up to 5 m high 50 to 60 km long 10 to 20 km wide and oriented at oblique angles to the coastline these banks generally consist of fluid muds which are so fluid that they interact with surface waves causing them to be altered and damped banks are attached to the shoreline and commonly produce progradation of the coastline substrate within the interbank zone is more consolidat d than within the mud banks because of lack of wave dampening wave energies are relatively high within the interbank zone producing erosion of the shoreface and shoreline sand beaches are generally formed within the erosional interbank shoreline the vertical sedimentary sequence within the shoreface muds is a record of bank migration and other sedimentary processes present wave and current energies are recorded by scour and fill features discontinuity features micro cross laminations lenticular laminations and parallel to subparallel laminations laminations appear to be created by boundary layer shearing within fluid muds which are above wave base and made mobile by wave generated shore parallel currents large scale evidence for migration of the mud banks is westward dipping acoustical reflectors within the banks these reflectors which are over a kilometer apart mark the boundaries of annual migration events biogenic structures are generally absent within the fluid bank muds but are present within the more consolid ted interbank zone and associated with discontinuity features which are buried interbank surfaces migration and vertical stacking of mud banks are recorded by vertical sequences of massive and laminated muds which were deposited by mud banks and discontinuity features which were formed by interbank zones the coastal deposits of suriname are a mud analogue to nearshore and innershelf sand deposits transition zones of both mud and sand systems reflect a transition from offshore to shoreface processes and sediments both mud and sand shoreface foreshore systems reflect sediment input and wave current and tidal processes that act on them mud banks of suriname resemble linear sand ridges on the continental shelf of the eastern u s a in shape oblique orientation to the coastline and orientation with respect to dominant direction of transport processes the sequence of massive and laminated muds with discontinuity features resembles laminated to burrowed sequences found in ancient and recent nearshore and shallow marine sands

hummocky cross stratification and megaripples a geological double standard abstract hummocky cross stratification in ancient sedimentary deposits is considered to be diagnostic of inner shelf storm deposits the geometry of the cross strata sets implies a low relief oval bedform of a few meters spacing orientation is poorly defined or random hummocky cross strata sets have been attributed to the action of strong storm wave surges megaripples of 2 5 m spacing are typical responses of the inner atlantic shelf of north america to storm flows side scan sonar records have shown that megaripples on the inner shelf are commonly hummocky oval to circular in plan view and lack asymmetry box cores and vibracores reveal gently inclined to horizontal lamination as the characteristic primary structure the megaripples am produced by combined flow storm currents these are alongcoast geostrophic flows with a mean flow component of 20 60 cm sec 1 a high frequency wave orbital current of similar or weaker intensity is present but its role in megaripple formation is subordinate to that of the mean flow the high velocity coastal margin of the flow is often jetlike and downwelling in nature and is characterize by a down current decrease in sand flux and rapid deposition of sand the bottom boundary layer of such flows is characterized by dense sediment suspensions 40 60 mgl 1 and is highly turbulent due to the interaction of the mean flow and wave orbital current components eddy diffusivity up to 200 cm2 sect at 40 cm above the bed sand transport in this coastal boundary zone is obliquely offshore but neither the density of the sediment suspension nor storm surge ebb appear to be important driving forces it is difficult to account for either modern hummocky megaripples or ancient hummocky cross stratification as responses to purely oscillatory flows combined flow currents are the typical response of mostmodern shelves and epicontinental seas to storms and presumably were so in the past hummocky cross stratified beds are generally too thick 20 80 cm to be accounted for by in situ resuspension they are more likely to have been despoited by combined flow currents in areas where these currents were experiencing a downstream velocity decrease and sediment deposition throughout much of the storm s duration evidence for strong scour in swales between hummocks and the smoothly rounded surface of the hummocks suggests a transitionlike bed regime measurements on the atlantic shelf likewi e suggest a transitionlike regime whose boundary layer is characterized by lower velocity than the transitional regime of unidirectional flow in flumes but more intense turbulence and a higher suspended sand concentration the lack of normal cross bedding in bothmodern and ancient hummocky bedforms indicates that these features did not migrate this characteristic may be a consequence of the complex trajectories followed by near bottom water particles in response to combined flow currents

comparing geology and well completions to production in the unconventional region of the cardium formation upper cretaceous northwest pembina field alberta canada drilling of horizontal wells in the cardium formation in the pembina field has abruptly increased since the successful completion of bonterra nexstar 4 25 47 03 w5 surface hole location 1 25 47 03 w5 bottom hole location in 2008 new cardium formation wells are targeting thinner lower quality reservoir intervals while implementing a variety of new completion techniques to improve hydrocarbon production the purpose of this study is to compare reservoir quality net pay mapping and completion techniques to production data to evaluate the successes and failures of geologic characterization and completion strategies well logs and routine core analyses data were used to evaluate reservoir properties and map net pay thicknesses subsequently production data from 125 horizontal wells were compared based on the following criteria presence of conglomerate sandstone density porosity net pay thickness wellbore orientation number of fractured stages fracture spacing lateral well length base fracturing fluid and average metric tons of proppant per stage in the unconventional parts of the pembina field well production based evaluation of net pay maps and reservoir characterization requires completion techniques to be considered conversely reservoir thickness and quality must be considered to accurately assess the success of different completion techniques the use of a 6 sandstone density porosity net pay cutoff can only be shown to be effective in identifying the most productive wells if well completions are considered our analyses indicate that wellbore orientation has a limited impact on well performance whereas decreasing fracture spacings and increasing lateral well length have the most significant impact on well performance

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

an ichnological assemblage approach to reservoir heterogeneity assessment in bioturbated strata insights from the lower cretaceous viking formation alberta canada abstract facies scale trends in porosity and permeability are commonly mapped for reservoir models and flow simulation however these trends are too broad to capture bed and bed set heterogeneity and there is a need to up scale detailed bed scale observations especially in low permeability reservoir intervals here we utilize sedimentology and ichnology at the bed and bedset scale to constrain the range of porosity and permeability that can be expected within facies of the lower cretaceous viking formation of south central alberta canada three main facies were recognized representing deposition from the middle shoreface to the upper offshore amalgamated hummocky cross stratified sandstone facies facies s hcs consist of alternations between intensely bioturbated beds and sparsely bioturbated laminated beds trace fossil assemblages in bioturbated beds of facies s hcs are attributable to the archetypal skolithos ichnofacies and are morphologically characterized by vertical sand filled shafts vss bioturbated beds show poor reservoir properties max 10 porosity mean 85 1 md compared to laminated beds max 20 porosity mean 186 md bioturbated muddy sandstone facies facies s b represent trace fossil assemblages primarily attributable to the proximal expression of the cruziana ichnofacies four ichnological assemblages occur in varying proportions namely sediment churning assemblages sc horizontal sand filled tube assemblages hsf vss assemblages and mud filled lined or with spreiten mls assemblages ichnological assemblages containing horizontal max 30 porosity mean 1 28 md or vertical sand filled burrows max 10 porosity mean 2 2 md generally have better reservoir properties than laminated beds max 20 porosity mean 0 98 md conversely ichnological assemblages that consist of muddy trace fossils have lower porosity and permeability max 10 porosity mean 0 89 md highly bioturbated sediment churned fabrics have only slightly higher porosity and permeability overall max 15 porosity mean 1 29 md bioturbated sandy mudstone facies facies m b contain ichnofossils representing an archetypal expression of the cruziana ichnofacies four ichnological assemblages occur throughout facies m b that are similar to facies s b sc hsf vss and mls assemblages the sc max 15 porosity mean 21 67 md hsf max 20 porosity mean 3 79 md and vss max 25 porosity mean 7 35 md ichnological assemblages have similar or slightly lower values than the laminated beds max 20 porosity mean 10 7 md however mls assemblages have substantially lower reservoir quality max 10 porosity mean 0 66 md our results indicate that the most likely occurrence of good reservoir characteristics in bioturbated strata exists in sand filled ichnological assemblages this is especially true within the muddy upper offshore to lower shoreface where vertically oriented trace fossils can interconnect otherwise hydraulically isolated laminated sandstone beds this improves vertical fluid transmission the results of this work largely corroborate previous findings about ichnological impacts on reservoir properties unlike previous studies however we demonstrate that the characteristics of the ichnological assemblage such as burrow form and the nature of burrow fill also play an important role in determining reservoir characteristics it follows that not all bioturbated intervals attributed to the same facies should be treated equally when upscaling bed scale observations to the reservoir a range of possible permeability porosity values can be tested for model sensitivity and to help determine an appropriate representative elementary volume

mesozoic cenozoic tectonic relationships between the kuqa subbasin and tian shan northwest china constraints from depositional records abstract stratigraphic profiles in the kuqa subbasin located along the southern flank of the tian shan northwest china display similar mesozoic cenozoic evolutionary sequences of compositional maturity exist in conglomerate clasts sandstone framework grains and detrital heavy minerals the similarity of these time series profiles resulted from orogenesis and intracontinental deformation of the provenance area the ancestral tian shan mineralogical maturities of conglomerates and sandstones increase from triassic to middle jurassic and decrease from upper jurassic lower cretaceous to tertiary this trend parallels a similar decrease in maturity in detrital heavy minerals zircon rutile and tourmaline dominate in middle upper triassic to middle jurassic sandstone whereas unstable hornblende and pyroxene dominate the heavy mineral suites in the tertiary especially in miocene and pliocene strata these compositional trends are coeval with gradual southward migration of alluvial depositional fronts and depositional depocenters from cretaceous to miocene since the pliocene large scale southward migration of alluvial depositional systems occurred while depositional depocenters migrated northward based on the mesozoic cenozoic depositional records the tectonic evolution of the tian shan kuqa subbasin may be divided into five phases 1 in early triassic following compression and uplift since permian gravelly braided river alluvial plain depositional systems developing in the kuqa subbasin characterized by sands of mixed orogenic provenance type derived from the ancestral tian shan 2 from middle triassic to middle jurassic sediment composition of the kuqa subbasin reflects arc orogen and mixed orogenic sand types with extensive planation of the ancestral tian shan and the development of relatively stable coal bearing depositional environments 3 renewed compression uplift of the tian shan occurred in late mesozoic especially in cretaceous with most detrital modes of the kuqa subbasin indicating provenance attributes akin to the south tian shan 4 further intensive compression uplift and probably southward basinward propagation of the south tian shan occurred during miocene and 5 supracrustal gravity gliding and collapse occurred in the tian shan kuqa subbasin probably due to intensive deep compression with rejuvenated uplift and probable southward basinward structural propagation of the south tian shan developing since pliocene observed changes in sediment composition and depositional environment since pliocene time were also controlled by frequent and abrupt climate changes in addition to intensive uplift of the tian shan moreover all depositional records in the kuqa subbasin seem to reflect west to east differences in provenance along the tian shan probably indicating greater exhumation depth in the western tian shan than that in the eastern tian shan since early cretaceous

cenozoic crustal shortening between the pamir and tien shan and a reconstruction of the pamir tien shan transition zone for the cretaceous and palaeogene the magnitude of the late cenozoic crustal shortening during convergence of the pamir and tien shan was determined using a contemporary pattern consisting of facies zones palaeomagnetic data regarding the rotation of tectonic units and data on the structure of the tadjik depression by late cenozoic cretaceous and palaeogene facies zones were cut by the vakhsh trans alay overthrust and darvaz strike slip faults and a significant part of the cretaceous palaeogene tadjik basin was overthrust by the pamir massif the sediments of easternmost part of the basin are preserved in the tarim depression the facies zones of the southern slope of the afghan tadjik basin were deformed and moved northward a pattern of facies zones indicates a convergence between the pamir and tien shan over a distance of 300 400 km a number of cross sections through the tadjik depression were used to establish the structure before folding a rotation of tectonic units indicated by structural data conforms to the angles of rotation as determined in palaeomagnetic studies the data suggest 300 km of convergence between the pamir and tien shan stratigraphic lithological structural and palaeomagnetic data formed the basis for the construction of the palinspastically restored palaeogeographic and sedimentologic environments for the tadjik shallow sea basin which was situated between the pamir and tien shan before their convergence in the late cenozoic the maps were constructed for the eight stratigraphic levels of the cretaceous and palaeogene the tadjik sea was a bay in the enormous turan sea in the early aptian this bay was located in what is now the afghan tadjik basin in late cretaceous the eastern shore of the bay lay 600 700 km further eastward and in the eocene marine environments extended even further eastward but after the rupelian continental environments occupied all of this region 2000 elsevier science b v all rights reserved

control of reservoir geometry and stratigraphic trapping by erosion surface e5 in the pembina carrot creek area upper cretaceous cardium formation alberta canada 1 seven major erosion e surfaces are found within the 100 m thick cardium formation in the subsurface of alberta erosion surface e5 has up to 20 m of erosional relief in the carrot creek area and is cut into originally horizontal open marine sandstones and mudstones the e5 surface has been correlated into the pembina area by construction of a structure map based on 2470 resistivity logs the e5 surface is characterized by a series of sub parallel southeast trending steps with lower areas between the steps this topography is interpreted to have formed during erosional shoreface retreat in a period of alternating transgression and stillstand of relative sea level tectonic tilting of the basin floor is also suggested by the topography of the erosion surface the tilti g is related to the accretion of exotic terranes and loading the continental margin the erosion surface is overlain by a continuous veneer of conglomerate interpreted as a transgressive lag this conglomerate thickens to 20 m in the lower areas at carrot creek where it is interpreted as a shoreface deposit rather than an offshore bar these shoreface conglomerates form important reservoirs the veneer also thickens to about 9 m in parts of pembina but the thick conglomerates occur not in the lows but on top of the steps reconstruction of the geometry of the steps indicates that at times of unusually high sediment supply during transgression conglomerates were deposited as local bars in the lower shoreface the sandstones and mudstones below the erosion surface at pembina shale out northeastward partly forming a stratigraphic trap however these sediments are also truncated by the e5 erosion surface and buried by transgressive mudstones these mudstones contribute significantly to updip stratigraphic trapping the long narrow sand body trends mapped in pembina and adjacent fields are due to the erosive dissection of an originally extensive and continuous sheet of interbedded sandstones and mudstones and not as has been suggested to an offshore bar origin for the fields

quantitative subsidence analysis of the western canada foreland basin with implications for short term facies changes abstract this paper presents a quantitative study of jurassic lower tertiary basement subsidence in the western canada foreland basin corrected for local sediment loading and paleobathymetrical changes subsidence patterns have been investigated for the effects of erosion induced uplift by means of analytical estimations it is demonstrated that phases of rapid basin shallowing may be related to periods of dominant erosion of the cordilleran fold and thrust belt leading to 200 20 m flexural uplift of the lithosphere within distances of 300 km of the belt the amount of sediment that has been derived from the orogenic belt in such a period can be sufficient for a shelf edge progradation of at least 300 km myr 1 the history of the western canada foreland basin has been subdivided in five stages 200 154 7 ma si 154 7 124 5 ma sii 124 5 104 ma siii 104 77 7 ma siv and 77 7 60 ma sv recording polyphase lateral and temporal changes in the loading history the subdivision between stage i and stages ii v is based on a transition from significant vertical motions in the williston basin in the southeast and minor sedimentation to the west during si to continued vertical motions in the williston basin and major subsidence and sediment accumulation to the west during sii sv this major change forms the expression of the transition from passive to convergent margin conditions cf davies and poulton 1986 the sii siii transition is marked by the expansion of the area of tectonic activity towards the north due to a large scale plate reorganization cf chamberlain and lambert 1985 stage iv is characterized by generally continuous subsidence with deposition of shales interrupted by phases of rapid uplift with deposition of sandy shallow water units while the subsequent stage v is defined by similar tectonic processes but sedimentation conditions changed from mainly shale accumulation to mainly sand accumulation these latter two stages probably reflect lateral and temporal changes in the rate of progradation of the accretionary wedge

the low temperature thermo tectonic evolution of the western tian shan uzbekistan abstract the kyzylkum nurata region represents a key area in understanding the tectonic evolution of the western tian shan in this study we present new thermochronological data apatite fission track and apatite u th sm he and associated thermal history models for 45 igneous samples from the kyzylkum nurata segment of south tian shan on the territory of uzbekistan and tajikistan our data show that the kyzylkum nurata segment experienced a multi phase mesozoic thermal history that differs from previously studied segments of the tian shan a triassic 220 200 ma cooling signal is widespread throughout the tian shan and is interpreted as being associated with exhumation following the closure of the palaeo asian ocean following this period of fast cooling the kyzylkum nurata segment experienced a period of slow cooling and erosion in the early jurassic 190 160 ma however in contrast to other parts of the tian shan our study area preserves evidence for rapid cooling during the late jurassic early cretaceous 160 120 given that this rapid cooling signal was only recorded for samples in association with major structures e g relic suture shear zones we interpret this event as a period of fault reactivation related with tectonic processes at the eurasian margin of the tethys ocean subsequently the late cretaceous early palaeogene 100 50 ma is characterised by slow cooling and erosion since the late palaeogene the basement of the tian shan experienced cooling related to the india eurasia collision the thermal signal of this collision has been extensively recorded in the high altitude kyrgyz tian shan within the low relief kyzylkum nurata segment this cenozoic overprint is not recorded allowing for a detailed assessment of the mesozoic thermal and landscape evolution of the western tian shan our study demonstrates that the cretaceous uzbek tian shan was characterised by a series of parallel linear mountain belts that formed along suture zones during fault reactivation

indentation of the pamirs with respect to the northern margin of tibet constraints from the tarim basin sedimentary record the pamirs represent the indented westward continuation of the northern margin of the tibetan plateau dividing the tarim and tajik basins their evolution may be a key factor influencing aridification of the asian interior yet the tectonics of the pamir salient are poorly understood we present a provenance study of the aertashi section a paleogene to late neogene clastic succession deposited in the tarim basin to the north of the nw margin of tibet the west kunlun and to the east of the pamirs our detrital zircon u pb ages coupled with zircon fission track bulk rock sm nd and petrography data document changes in contributing source terranes during the oligocene to miocene which can be correlated to regional tectonics we propose a model for the evolution of the pamir and west kunlun wkl in which the wkl formed topography since at least 200 ma by 25 ma movement along the pamir bounding faults such as the kashgar yecheng transfer system had commenced marking the onset of pamir indentation into the tarim tajik basin this is coincident with basinward expansion of the northern wkl margin which changed the palaeodrainage pattern within the kunlun progressively cutting off the more southerly wkl sources from the tarim basin an abrupt change in the provenance and facies of sediments at aertashi has a maximum age of 14 ma this change records when the pamir indenter had propagated sufficiently far north that the north pamir was now located proximal to the aertashi region

cenozoic evolution of the eastern pamir implications for strain accommodation mechanisms at the western end of the himalayan tibetan orogen detailed field mapping geochronologic and thermochronologic analyses and petrologic investigations conducted along the southern segment of the late cenozoic kongur shan extensional system provide new information on the cenozoic tectonic evolution of the eastern pamir at the western end of the himalayan tibetan orogen field relations and cooling age patterns in the hanging wall and footwall of the active faults show a southward decrease in the magnitude of east west extension along the southern kongur shan extensional system from 20 km or less along the muztaghata massif in the north to l3 km along the tashkorgan fault in the south these results in conjunction with previously published work on the northern segment of the kongur shan extensional system show a general southward decrease in east west extension along the entire length of the extensional system consistent with models of extension primarily driven by oroclinal bending or radial thrusting of the pamir petrologic data 40 ar 39 ar cooling ages and monazite th pb ages from schists and gneisses in the footwall of the southern kongur shan normal fault along the muztaghata massif record two tectonic events that immediately preceded late miocene initiation of east west extension 1 high grade schists and gneisses experienced upper amphibolite facies metamorphic conditions 9 10 kbar 700 750 c dated as late oligocene to middle miocene by in situ ion microprobe analyses of monazite inclusions in garnet and 2 high grade schists and gneisses were subsequently rapidly exhumed to shallow crustal levels in the late miocene with 40 ar 39 ar biotite cooling ages of 7 5 9 ma rapid exhumation was accommodated in part by the east west striking south dipping shenti normal fault field relations and regional geologic correlations indicate that this exhumation event was related to the formation of the central pamir gneiss domes and the antiformal muztaghata massif is the eastward continuation of the sares dome of the central pamir these observations suggest that the antiformal gneiss domes of the central pamir have not been offset across the karakorum right slip fault from the qiangtang anticlinorium in tibet instead we propose that the development of the central pamir gneiss domes may have been related to oligocene miocene northward underthrusting and thickening of crust beneath the pamir

late miocene pleistocene aridification of asian inland revealed by geochemical records of lacustrine fan delta sediments from the western tarim basin nw china abstract continuous cenozoic deposits have accumulated in the tarim basin northwestern china and are crucial to investigating the aridification history of the asian interior which has long been thought to be linked with the growth of the tibetan plateau retreat of the para tethys sea and global cooling however less and poor climatic records from the region hinder our understanding of the questions above here we report continuous lithologic sedimentary color and geochemical caco3 and salt ions records from precisely dated late miocene early pleistocene sedimentary sequence from western tarim basin nw china they indicate that arid climate had prevailed within the basin at latest at ca 5 7 ma and gradually increased in degree until ca 3 7 ma between ca 3 7 and 2 1 ma a hydrologic event occurred which supplied more water from the tien shan after ca 2 1 ma the climate in the basin turned to hyper arid we attribute the stepwise drying of the asian inland since ca 5 7 ma to the late miocene pliocene episodic uplift of the tibetan plateau pamir and the tien shan and the connection of the pamir with the tien shan in remote response to the collision of india with asia

cretaceous paleomagnetism of the fergana basin and adjacent ranges central asia tectonic implications abstract paleomagnetic results have been obtained from cretaceous red beds sampled at fifteen localities in the fergana basin and adjacent ranges of the tien shan central asia the natural remanent magnetization of these rocks was accounted for by an late cretaceous overprint of normal polarity at fourteen localities and dual polarity presumably primary component at seven localities no definite correlation was found between the declinations of the two components and structural trends thus implying no oroclinal bending of the arcuate structures in the area studied inclination data are difficult to interpret due to probable tectonic movements inclination errors of various types and ambiguities in the cretaceous reference data all locality mean declinations were found to be deflected westward with respect to the eurasian paleomeridian the proposed counter clockwise rotation of the fergana block during the late cenozoic is shown to account for the main features of the alpine tectonics in particular the 200 km offset along the talas fergana fault is connected with rotation which contradicts the common idea that this displacement is of late paleozoic age the horizontal movements are most probably not older than 10 ma

arcuate pamir in the paleogene insights from a review of stratigraphy and sedimentology of the basin fills in the foreland of ne chinese pamir western tarim basin abstract the tectonic evolution of the pamir bears essential information for understanding intracontinental tectonic processes within the context of india asia convergence however it is unclear whether the arcuate shape of the pamir is entirely the result of the india asia collision or may have been in part an inherited pre cenozoic feature in this study we address the evolution of the ne chinese pamir during the paleogene by reviewing the stratigraphy of twelve sections in the western tarim basin and the establishment of sedimentary facies the results indicate three episodes of deposition of alluvial coarse sediments in the foreland regions of ne chinese pamir during the 1 early paleocene 2 late paleocene to early eocene and 3 middle eocene to oligocene respectively these are correspondingly attributed to 1 retroarc deformation in an andean type margin of the southern asia during subduction of the neo tethys ocean 2 the immediate response in the pamir to the india asia collision and 3 continued india asia convergence the alluvial conglomerates with grain size up to 5 10 cm imply a transport distance

oligocene clockwise rotations along the eastern pamir tectonic and paleogeographic implications despite the importance of the pamir range in controlling asian paleoenvironments and land sea paleogeography its tectonic evolution remains poorly constrained in time and space hindering its potential for understanding deep to surface processes we provide here new constraints on vertical axis tectonic rotations from the southwest tarim basin along the eastern flank of the pamir arcuate range based on paleomagnetic results two well dated eocene to oligocene sections previously analyzed using biostratigraphy and magnetostratigraphy yield consistently clockwise rotations of 21 6 4 2 in 41 to 36 ma strata then 17 1 6 5 in 33 to 28 ma strata at the aertashi section and 14 2 11 5 in 41 to 40 ma strata at the kezi section combined with a regional review of existing paleomagnetic studies these results indicate that most of the clockwise rotations along the eastern pamir occurred during oligocene times and did not extend systematically and regionally into the tarim basin in contrast on the western flank of the pamir tectonic rotations in cretaceous to neogene strata are regionally extensive and systematically counterclockwise throughout the afghan tajik basin this timing and pattern of rotations is consistent with paleogeographic reconstructions of the regional sea retreat out of central asia and supports a two stage kinematic model 1 symmetric rotations of either flanks of the pamir arcuate range until oligocene times followed by 2 continued rotations on its western flank associated with radial thrusting and along the eastern flank no further rotations due to decoupled transfer slip starting in the early miocene

paleomagnetism of cretaceous red beds from tadzhikistan and cenozoic deformation due to india eurasia collision abstract we have carried out structural and paleomagnetic studies in the tadzhik depression in order to evaluate the main features of the alpine tectonics of this area about 340 cores from 43 sites of lower cretaceous red beds were sampled from four different localities in the basin and adjacent ranges a well defined component of magnetization a of normal polarity with high unblocking temperatures up to 650 670 c was isolated from all the sites another component of magnetization b with unblocking temperatures between 650 and 680 c was isolated from only fifteen sites this component is bipolar the fold test is positive for both components we believe that component a was acquired during the cretaceous long interval of normal polarity comparison with eurasian reference data shows significant counterclockwise rotation of a locality close to the pamir wedge r 51 5 and another counterclockwise rotation from the inner part of the basin r 15 5 no significant rotations are observed at the two other localities on the periphery of the tadzhik basin

stress orientations from oil well fractures in alberta and texas many oil wells in alberta exhibit spalling of the walls known as breakouts which elongates the holes with the longer axis aligned northwest southeast this alignment is observed over an area in excess of 4 105 km2 in siltstones sandstones carbonate sediments and one shale formation through the stratigraphic column from devonian to cretaceous it is unrelated to dip of the beds we have elsewhere suggested that these breakouts are produced through stress concentration near the hole walls in a stress field having large unequal horizontal principal stresses and with the larger compression oriented northeast southwest it is probable that this northeast southwest principal stress is 1 this paper adds new data from oil wells in alberta and northern british columbia and shows that the breakouts and by inference the stress orientations are consistent through much of the western canadian sedimentary basin it also uses evidence from hydraulic fracturing in west central alberta and from steam injec

multiscale anatomy of a reservoir geological characterization of pembina cardium pool west central alberta canada geological reservoir elements that control a reservoir s conduit system are mineralogy grain size packing pore network lithohydraulic units lithologic continuity fractures and faults and rock fluid interactions all occur at different scales we have focused on four scales typical of a developed reservoir to characterize geological reservoir heterogeneity in the pembina cardium pool 1 megascale the field wide view 2 macroscale the interwell continuity 3 mesoscale the neighborhood of the well and 4 microscale the domain of grains and pores 1 megascale observations the reservoir top is characterized by a ridge and swale topography oriented northwest southeast a similar but more subdued topography is also characteristic of the unconformity between conglomerates and sandstones conglomerates are distributed irregularly and are generally shaly in beds thinner than 1 m thick conglomerates are common on the east and south flanks of underlying ridges and impart northwest southeast fluid production trends and yield zones of reduced pressures in thick conglomerates high flow capacities are detrimental to hydrocarbon recovery as they result in bypassing of injection fluids the conglomerate is a lithohydraulic unit as it is a body of strata distinct from neighboring strata by depositional diagenetic and tectonic imprintin s and by a characteristic set of petrophysical properties 2 macroscale characteristics reservoir lithofacies and lithohydraulic units are laterally discontinuous and form irregular patterns that can possess an irregular northwest southeast orientation fluid production trends oriented northeast southwest in areas of thin shaly conglomerates appear to be the product of fractures in these areas reorientation of injector wells to line drives results in higher oil recoveries 3 mesoscale properties in cores five lithofacies characterize the field they are 1 dark gray mudstone and siltstone 2 bioturbated thin and very thin bedded shale siltstone and very fine and fine grained sandstone 3 thin and very thin bedded shale siltstone and very fine and fine grained sandstone 4 medium to thick bedded very fine and fine grained sandstone and 5 conglomerate as each lithofacies possesses contrasting but partially overlapping petrophysical properties they are lithohydraulic units lithofacies 1 acts as the reservoir seal and restricts flow within the reservoir lithofacies 2 has high porosities low permeabilities and dominates the bottom half of the reservoir lithofacies 2 has a high storage capacity phgrh but its flow capacity kh i low lithofacies 3 reduces cross flow because its vertical permeabilities are very low lithofacies 4 has modest permeabilities and high porosities and is common to the top half of the reservoir this lithofacies flow capacity in t47 r7 w5m is 17 times greater than that of lithofacies 2 lithofacies 5 has modest porosities but very high permeabilities resulting in intervals with very high flow capacities 4 microscale investigations in the sandstones the mineral framework consists predominantly of quartz and chert whereas chert prevails in the conglomerates these mineralogic differences lead to reduced cementation by silica on cherts and result in the preservation of pore spaces resembling original pores in chert rich zones although quartz grains are modified by megaquartz overgrowths chert particles are altered by pressure solution fracturing and crushing in both sandstones and conglomerates framework grains are cemented by siderite calcite kaolinite and illite experiments with reservoir cores flooded with co2 enriched brines and data from waterflooding indicate that siderite and calcite are unstable under reservoir conditions this behavior can result in perm ability losses from release of insoluble fine materials and weakened responses during polymer floods mercury capillary pressure data from 28 measurements of lithofacies 4 sandstones indicate that mercury end page 1233 recovery efficiency increases with decreasing permeability and porosity this behavior if confirmed elsewhere in the field may explain the performance of areas that have reduced permeabilities and porosities but are responding well to waterflooding

timing of seawater retreat from proto paratethys sedimentary provenance and tectonic rotations in the late eocene early oligocene in the tajik basin central asia abstract the tajik basin is now located to the west of the pamirs but it formerly connected with the tarim basin occupied by a large epicontinental sea proto paratethys sea once extended from europe to central asia however the northward indentation and collision of the pamirs into the tian shan separated the tajik and tarim basins creating manifold implications and consequences consequently timing of the final seawater retreat from the tajik basin is important for understanding the evolution of the proto paratethys sea the effect of the northward indentation of the pamirs on the sea regression and the influence of seawater retreat on the cenozoic aridification in the tajik basin here we present a multidisciplinary study of upper paleogene strata from the central tajik basin our results demonstrate that the final seawater retreat from the center of the tajik basin occurred at 38 6 ma which is earlier than a previous age of 37 4 ma from the northeastern tajik basin such a late eocene sea retreat was quasi simultaneous with that in the tarim basin this final marine regression was mainly a consequence of the northward indentation and growth of the pamirs which was an intracontinental response to the india asia collision our new detrital zircon ages indicate that the provenance of upper eocene lower oligocene strata was from the pamirs rather than tian shan implying that the reactivation of the tian shan was later than the early oligocene our paleomagnetic data indicate a 30 counterclockwise rotation of the central tajik basin that is much bigger than the previous estimate of 17 and we propose that such a counterclockwise rotation mostly occurred after the late miocene finally different from the previous reported aridfication at 39 ma or 37 ma our evidence demonstrates that drought intensified since the start of the oligocene at 34 ma being related to reduced water vapor transport to central asia caused by global cooling induced sea level drop the retreating paratethys sea and weakened sea surface evaporation

using oxygen isotope ratios to quantitatively assess trapping mechanisms during co2 injection into geological reservoirs the pembina case study abstract carbon capture and storage ccs is considered a viable option for reducing co 2 emissions into the atmosphere from point sources such as coal fired power plants monitoring of co 2 storage sites is widely considered necessary for safety reasons and for verification of injected co 2 in the reservoir the latter is crucially dependent on the ability to determine co 2 trapping mechanisms and to assess pore space saturation of co 2 thus far attempts to determine co 2 pore space saturations at co 2 injection sites have had limited success here we present data from the pembina cardium co 2 monitoring project in alberta canada that demonstrate that changes in the oxygen isotope ratios 18 o of reservoir water can be indicative of the extent of pore space saturation with co 2 the 18 o value of injected co 2 at the injection site was 28 6 v smow and 18 o values of reservoir water at eight observation wells varied between 13 5 and 17 1 v smow before co 2 injection after commencement of co 2 injection the 18 o values of reservoir water at three observation wells increased between 1 1 and 3 9 due to the presence of large quantities of injected co 2 and equilibrium isotope exchange between water and co 2 our calculations revealed that reservoir water fully saturated with co 2 would only result in increases of 18 o h2o values of 0 4 hence the observed larger increases in 18 o values of reservoir water indicate free phase co 2 with estimated pore space saturations ranging from 12 corresponding to a 18 o increase of 1 1 to 64 18 o increase 3 9 of the non oil saturated pore space contributions to oxygen in the system from mineral dissolution were calculated to be less than 0 01 of total oxygen and therefore did not alter the 18 o value of the reservoir water significantly hence we conclude that changes in the 18 o values of reservoir water caused by the presence of injected co 2 can be used as a tracer for co 2 plume migration in the subsurface provided that the injected co 2 is isotopically distinct furthermore we submit that the extent of the changes in the 18 o values of the reservoir water provides a quantitative assessment of co 2 stored in dissolved form solubility trapping assuming no density driven convective overturn and as free phase co 2 structural stratigraphic and hydrodynamic trapping thereby elucidating the trapping mechanisms within the reservoir

thermochronological insights into the structural contact between the tian shan and pamirs tajikistan gilby jepson stijn glorie dmitry konopelko jack gillespie martin danisik noreen j evans yunus mamadjanov alan s collins

interaction of the indian and asian plates under the pamir and hindu kush regions insights from 3 d shear wave velocity and anisotropic structures

paleomagnetic evidence for cenozoic block rotations in the tadjik depression central asia this paper presents results of a paleomagnetic study of oligo miocene red beds of the tadjik depression in central asia we sampled about 530 cores at 69 sites and six localities across the depression and along the western border of the pamirs samples were thermally demagnetized and high temperature components appear to predate folding of upper tertiary age throughout the depression paleomagnetic inclinations are consistent with those observed on the stable turan platform at the western margin of the depression however they are shallower by about 30 than the inclination predicted from the reference apparent polar wander path this appears to indicate a 23 difference in latitude which is incompatible with paleogeographic reconstructions for the tertiary a sound interpretation of this anomaly would require a better constrained tertiary paleomagnetic reference for asia inside the tadjik depression paleomagnetic declinations are all significantly rotated counterclockwise with respect to those measured on the turan platform the eastern part of the depression is a domain of large rotation 52 13 to 46 15 whereas smaller amounts of rotation have occurred in the western part 27 14 to 14 15 the similarity between tertiary and cretaceous data available for the area shows that rotations have occurred since the miocene little or no paleomagnetic rotations are observed in the ranges bordering the northern and western parts of the depression paleomagnetic and structural data suggest that block rotations in the tadjik depression are associated with indentation of the pamirs into stable asia at a larger scale observed rotations are compatible with a model of regional sinistral wrenching along a strip running from the gulf of oman to lake baikal

hummocky cross stratification is not produced purely under progressive gravity waves hummocky cross stratification is a type of bedding in sedimentary rocks caused by the draping of sand around low steepness three dimensional bedforms this distinctive form of stratification has been thought to be diagnostic of the former action of storm waves and hence its occurrence has been used to reconstruct ancient palaeogeographies and marine processes here i examine theoretical and empirical aspects of hummocky cross stratification pertaining to its origin and argue that the notion that it was produced purely under progressive storm waves is incompatible with observed hummock spacings and possible values of the ratio a under waves a is the amplitude of oscillation near the bed and is the bedform spacing my view is supported by recent observations on naturally occurring hummocky megaripples from the sea floor of the inner atlantic shelf of north america

structural and seismic evidence for intracontinental subduction in the peter the first range central asia this paper examines the stratigraphy structure and seismicity of the peter the first range an actively deforming foreland basin in soviet tadjikistan the range represents a highly deformed segment of a large intracontinental basin the tadjik depression which is being shortened in response to the cenozoic convergence between two orogenic massifs the pamir and the tien shan ranges the late jurassic to paleogene stratigraphy of the peter the first range includes more than 2 000 m of shallow marine to nonmarine sediments deposited under relatively quiescent conditions close to sea level in contrast the correlative units of the northernmost pamir directly south of the peter the first range show evidence of proximity to a major uplifted source area to the south these mesozoic early cenozoic strata are overlain by thick sequences of alluvial sandstone and conglomeratic units that coarsen upward through the neogene and record the progressive uplift of the pamir shortening within the peter the first range is recorded by kilometer scale isoclinal upright to over turned folds and imbricate north verging thrusts that emerge near the northern edge of the range and mark the active tectonic boundary with the tien shan most of the deformation appears to postdate the accumulation of the mio pliocene molasse the stratigraphy and structure of the region are best explained by a model of the mesozoic tadjik depression as an intracontinental foreland basin developed in a back arc position with respect to the active andean or collisional orogens located in northern afghanistan and the pamir seismicity in the region is dominated by earthquakes extending from the surface to 12 km depth apparently occurring within the deformed sediments of the peter the first range a subhorizontal zone of relatively low activity at this depth may mark the location of a basal detachment that underlies a structurally thickened sedimentary section this aseismic zone is in turn underlain by a south dipping belt of seismicity that extends from about 17 to 35 km depth and which can be traced updip to the seismogenic crystalline basement of the tien shan range to the north we interpret this south dipping structure as a zone of intracontinental subduction that may be an updip continuation of the enigmatic intermediate depth wadati benioff zone beneath the pamir range to the south

timing cause and impact of the late eocene stepwise sea retreat from the tarim basin west china abstract a vast shallow epicontinental sea extended across eurasia and was well connected to the western tethys before it retreated westward and became isolated as the paratethys sea however the palaeogeography and the timing of this westward retreat are too poorly constrained to determine potential wider environmental impacts let alone understanding underlying mechanisms of the retreat such as global eustasy and tectonism associated with the indo asia collision here an improved chronostratigraphic and palaeogeographic framework is provided for the onset of the proto paratethys sea retreat at its easternmost extent in the tarim basin in western china is provided five different third order sea level cycles can be recognised from the cretaceous palaeogene sedimentary record in the tarim basin of which the last two stepped successively westwards as the sea retreated after the maximum third incursion new biostratigraphic data from the fourth and fifth incursions at the westernmost margin of the tarim basin are compared to our recent integrated bio magneto stratigraphic results on the fourth incursion near the palaeodepocentre in the south western part of the basin while the fourth incursion extended throughout the basin and retreated at 41 ma base c18r the last and fifth incursion is restricted to the westernmost margin and its marine deposits are assigned a latest bartonian early priabonian age from 38 0 to 36 7 ma near top c17n 2n to base c16n 2n similar to the fourth the fossil assemblages of the fifth incursion are indicative of shallow marine near shore conditions and their widespread distribution across eurasia suggests that the marine connection to the western tethys was maintained the lack of diachronicity of the fourth incursion between the studied sections across the southwest tarim basin suggests that the sea entered and withdrew relatively rapidly as can be expected in the case of eustatic control on a shallow epicontinental basin however the westward palaeogeographic step between the fourth and fifth incursions separated by several millions of years rather suggests the combined long term effect of tectonism possibly associated with early uplift of the pamir kunlun shan thrust belt the fourth and fifth regressions are time equivalent with significant aridification steps recorded in the asian interior thus supporting climate modelling results showing that the stepwise sea retreat from central asia amplified the aridification of the asian interior

chondrites a trace fossil indicator of anoxia in sediments the trace fossil chondrites a highly branched burrow system of unknown endobenthic deposit feeders occurs in all types of sediment including those deposited under anaerobic conditions in some cases such as the jurassic posidonienschiefer formation of germany chondrites occurs in black laminated carbonaceous sediment that was deposited in chemically reducing conditions in other cases such as numerous oxic clastic and carbonate units throughout the geologic column chondrites typically represents the last trace fossil in a biotutbation sequence this indicates that the burrow system was produced deep within the sediment in the anaerobic zone below the surficial oxidized zone that was characterized by freely circulating and oxidizing pore waters

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

paleomagnetism in the tajikistan continental shortening of european margin in the pamirs during indian eurasian collision abstract a stable and well defined magnetization withd 350 i 55 5 and 95 4 carried by hematite is isolated in the red beds of the tajik basin the fold test is positive 99 probability and this magnetization predates the late eocene early oligocene folding phase in the tajik basin the coordinates of the vgp are 82 n 323 e dp 4 dm 6 with a paleolatitude of dushanbe 38 38n 68 51e of35 6 5 a high blocking temperature component with weak intensity and reverse polarity has been demonstrated in the site of ragoon but it has not been isolated this component predates the long cretaceous normal period and is compatible with the older stratigraphic age attributed to the formation valanginian these arguments indicate that this component is likely of primary origin the other component of higher intensity and clearly isolated in all the sites is a pre folding remagnetization the process of remagnetization is attributed to the tectonic events such as fluids migration and slight heating during a few million years which took place just before the first late eocene early oligocene phase of folding an anticlockwise rotation of 26 encountered by the region of dushanbe is in good agreement with the indentation model of collision leading to anticlockwise rotation west of the indian wedge and clockwise rotation to the east a comparison of the paleolatitude of dushanbe computed from the direction of magnetization and the paleolatitude expected from the apparent polar wander path for eurasia gives an estimate of the post collision continental shortening north of the tajik basin this shortening did not exceed a few hundred kilometers and was probably between 100 and 300 km an estimate of the roles of the different classical mechanisms of crustal shortening is discussed and a paleogeographic reconstruction giving the location of the southern margin of eurasia at 45 ma is proposed the paleolatitude deduced from the prefolding remagnetization of the red beds of the tajik basin is then compared with the results obtained by soviet workers in tertiary sediments of the same area revealing strong contradictions in interpretation this contradiction emphasizes the problem of the rigidity of european plate

pore geometry control on reservoir properties walker creek field columbia and lafayette counties arkansas walker creek field in southern arkansas produces hydrocarbons from oolitic packstones and grainstones of the jurassic smackover formation the relationships between pore geometry and reservoir quality in these rocks were evaluated using petrographic methods and mercury injection capillary pressure analyses results indicate that reservoir quality is controlled by pore geometry which in turn is determined by depositional and diagenetic processes reservoir rocks at walker creek were deposited as prograding grainstone shoals in a shallow water high energy environment diagenetic processes including early marine cementation compaction and deeper burial pressure solution and calcite cementation modified the original pore system primary interparticle porosity is the dominant effective pore type and is most important in terms of reservoir performance secondary microporosity is also abundant comprising a significant percentage of total porosity locally up to 100 but is generally ineffective the rocks were subdivided into lithofacies based on their depositional and diagenetic origin within each lithofacies reservoir marginal reservoir and nonreservoir rock types are identified based on their pore geometry and characteristic capillary pressure curve the reservoir rock types are less cemented oolitic and oolitic skeletal grainstones with steeply sloping capillary pressure curves and flat plateaus indicating relatively homogeneous pore sizes that are well interconnected by large pore throats the marginal to nonreservoir rock types include mudstones wackestones muddy packstones and cemented grainstones with generally small interparticle and moldic pores gently sloping capillary pressure curves with poorly defined or no plateaus indicate variable pore and pore throat sizes a close correlation is found among parts of the capillary pressure curve shape such as slopes plateaus and displacement pressure and aspects of pore geometry such as pore throat size pore throat size distribution and pore interconnectivity in addition capillary pressure analyses provide guidance in evaluating overall reservoir quality by rock type as well as answer specific questions regarding net pay criteria as a function of height above free water level

the deltaic nature of amazon shelf sedimentation despite the annual discharge of more than a billion tons of sediment by the amazon river the sedimentary environment near the river mouth has little subaerial expression and thus does not meet the classic definition of a delta the river mouth however is not an estuary either these observations raise a major question as to what type of sedimentary environment the amazon river mouth represents seismic stratigraphy has been examined on the continental shelf at the mouth of the amazon river using high frequency 3 5 khz seismic records from about 6 000 km of ship track these records demonstrate three regions 1 the amazon river has built a subaqueous feature which stretches for hundreds of kilometres offshore and alongshore from its mouth the feature is prograding seaward and accreting upward and it contains fine scale stratification typical of classic deltas the feature forming at the mouth of the amazon is a subaqueous delta it differs from classic deltas primarily in its lack of subaerial expression subaqueous deltas such as the amazon represent the general case of a major river entering an energetic oceanic regime

physical processes and fine grained sediment dynamics coast of surinam south america abstract the prograding holocene mud wedge between the amazon and orinoco rivers in the trade wind belt of northeastern south america provides a modern day example of muds accumulating under moderate wave energy conditions gigantic shore attached mudbanks 10 km 20 km composed partly of thixotropic fluid mud gel front this coast every 30 60 km to form a buffer to wave attack and a temporary storage for fine grained sediments this mesotidal coast tide range 2 0 m with gentle offshore slope 0 0006 allows the exposure twice a day of extensive tidal flat deposits which are backed by mangrove swamps on a well developed chenier plain complex field experiments were conducted in surinam du ing 1975 and 1977 to provide new information on process form relationships in this interesting but unusual muddy environment simultaneous measurements of waves currents tide elevation suspended sediment concentration and variations in mud density show that soft intertidal and subtidal muds are suspended at both tide and wave frequency suspended sediment concentrations typically exceed 1 000 mg l at the surface as incoming solitary like waves partially disperse fluid mud into overlying water on a falling or rising tide redeposition of mud may occur near time of high tide the strong attenuation of shallow water waves by these muds provides conditions that are favorable for further sedimentation high concentrations of suspended fluid mud together with solitary like waves from the northeast throughout the year can lead to extraordinarily high net sediment transport rates in the nearshore zone calculations based on solitary wave theory and on data obtained from this study indicate that 15 65 106 m3 of mud can move along shore each year without involving breaking waves the concept of radiation stress and a nearshore circulation cell or bedload transport farther offshore outside the zone of wave dominance wind driven currents and the guiana current combine to transport muds to the northwest consistent with the observed direction of mudflat migration

lithology dependent diagenetic control of reservoir properties of conglomerates falher member elmworth field alberta conglomerates in the falher member of the lower cretaceous spirit river formation of alberta are the reservoir rocks for the giant 2 3 tcf or 5 8 1010 m3 elmworth gas field three types of conglomerates are present a unimodal granules or pebbles lacking any matrix b bimodal grain supported a framework of pebbles with fine to medium sand in the interstices and c bimodal sand supported pebbles floating in fine to medium sandstone the distribution of these three types is controlled by the original depositional environment the granules and pebbles are composed of chert and silicified sedimentary and volcanic rock fragments the sand grains are dominantly quartz during diagenesis very small 01 0 1 mm drusy quartz crystals form on the cherts and rock fragments whereas quartz grains undergo heavy quartz overgrowth formation because of differences in nucleation and growth of quartz crystals as a consequence most unimodal conglomerates low in quartz have not experienced major reduction of porosity and permeability bimodal conglomerates both sand and pebble supported have lost significant amounts because of cementation of the quartz rich matrix streaks and patches of kaolinite and calcite cements have also reduced porosity and permeability locally gas production from falher wells therefore de ends not only on the amount of conglomerate but also on the proportion of unimodal and bimodal types

controls on reservoir quality in fan deltaic conglomerates insight from the lower triassic baikouquan formation junggar basin china abstract fan delta sandy conglomerates form important oil and gas reservoirs recently a new giant oilfield was discovered in the lower triassic baikouquan formation t1b in the mahu sag of the junggar basin northwestern china however the fan delta sandy conglomerate reservoirs are strongly heterogeneous most have poor permeability and host little hydrocarbons while a subset at the delta front exhibit high porosity and permeability forming high quality reservoirs to explain this disparity the sedimentary facies diagenesis and reservoir quality of the sandy conglomerate strata and their interactions were studied detailed logging of 253m cores indicates that the t1b was deposited in a lacustrine fan delta complex comprising ten lithofacies from coarse matrix supported pebbly conglomerate gm to fine massive mudstone fm in four facies associations interpretation of optical microscope backscattered and scanning electron microscope imaging show that these ten lithofacies underwent differential diagenesis especially dissolution of minerals such as orthoclase dissolution was particularly prevalent in the subaqueous distributary channel deposits of sandstones st hyperpycnal flow massive sandstones sm and grain flow deposits of sandy gravelly conglomerates gcs at the delta front forming abundant secondary pores these pores enhanced the reservoir performance and led to high porosity and permeability within these three lithofacies for these reservoirs continuous winnowing at the delta front led to relatively high primary porosities and subsequent mineral dissolution during burial diagenesis closely related to acidic mn rich hydrocarbon bearing fluids significantly improved the porosities further migration of hydrocarbon bearing fluids was facilitated by faults cutting through the reservoir and underlying source rocks this led to the development of belts of high quality reservoir at the delta front near the fault conduits this model for reservoir development and charge suggests that further reservoirs can be expected within the t1b of the mahu sag

restoration of cenozoic deformation in asia and the size of greater india a long standing problem in the geological evolution of the india asia collision zone is how and where convergence between india and asia was accommodated since collision proposed collision ages vary from 65 to 35 ma although most data sets are consistent with collision being underway by 50 ma plate reconstructions show that since 50 ma 2400 3200 km west to east of india asia convergence occurred much more than the 450 900 km of documented himalayan shortening current models therefore suggest that most post 50 ma convergence was accommodated north of the indus yarlung suture zone we review kinematic data and construct an updated restoration of cenozoic asian deformation to test this assumption we show that geologic studies have documented 600 750 km of n s cenozoic shortening across and north of the tibetan plateau the pamir hindu kush region accommodated 1050 km of n s convergence geological evidence from tibet is inconsistent with models that propose 750 1250 km of eastward extrusion of indochina approximately 250 km of indochinese extrusion from 30 to 20 ma of indochina suggested by se asia reconstructions can be reconciled by dextral transpression in eastern tibet we use our reconstruction to calculate the required size of greater india as a function of collision age even with a 35 ma collision age the size of greater india is 2 3 times larger than himalayan shortening for a 50 ma collision the size of greater india from west to east is 1350 2600 km consistent with robust paleomagnetic data from upper cretaceous paleocene tethyan himalayan strata these estimates for the size of greater india far exceed documented shortening in the himalaya we conclude that most of greater india was consumed by subduction or underthrusting without leaving a geological record that has been recognized at the surface

anomalous low palaeomagnetic inclinations from oligocene lower miocene red beds of the south west tien shan central asia summary we studied oligocene to lower miocene red beds from the western margins of the south west ghissar range the westernmost part of the tien shan fold belt central asia a total of 19 sites were sampled on both limbs of a gentle syncline after removal of an unstable component at 200 to 300 c the natural remanent magnetization in these red beds from 18 sites is accounted for by one characteristic component chrm which persists up to 680 c intermediate directions of the geomagnetic field were recorded at five sites after tilt correction the overall mean direction for the remaining 13 sites is d 3 i 30 k 19 95 8 9 both the reversal and the fold tests are positive haematite is found to be the main remanence carrier in these red beds the calculated mean declination fits well with the reference data besse courtillot 1991 and no rotation with respect to eurasia is detected in accord with geological data in contrast the mean inclination is about 30 shallower than expected we argue that this result is not due to large scale northward transport of the area mean inclinations showing a similar amount of shallowing were also reported from the tadzhik and fergana basins and kirghiz tien shan from various rock types thomas et al 1993 1994 and we are of the opinion that neither inclination error nor compaction could have led to such a consistent pattern the analysis of all reliable early tertiary palaeomagnetic results extracted from the global paleomagnetic data base for the northern part of the eurasian plate reveals that inclination anomalies are very common in this data set whatever the rock type the general tendency is a progressive increase in the anomaly amplitude from the atlantic ocean to central asia the size and localization of this tertiary field anomaly is comparable with the present day non dipole field anomaly this suggests that there may have been a long term standing non dipole anomaly in the tertiary earth magnetic field perhaps correlated to major features of the contemporaneous plate tectonics for example the india asia collision

causes of permeability variation in a lower cretaceous reservoir unit in the west siberian basin russia abstract samples of the lower cretaceous bs 10 reservoir taken from 11 oil fields in the central part of the west siberian basin show a permeability variation of more than 4 orders of magnitude at the scale of the core plug much of this variation occurs in samples that fall within the fine grained wentworth class however application of 2 d fourier analysis and other methods of petrographic image analysis demonstrates the existence of a real but subtle variation in grain size that provides an important control on permeability reservoir quality will be best in the facies that represent the highest depositional energy

new cretaceous paleomagnetic poles from the tarim basin northwestern china most paleomagnetic studies of the tarim have been focused on the paleozoic with little data published for mesozoic rocks however the ancient configuration of this part of asia has been affected by the indian eurasian collision during the second french chinese karakorum expedition from kashkar to kunjerab in the autumn of 1990 we sampled about 500 cores from 53 sites of upper and lower cretaceous red sandstones from three areas near uytak yingjisha and wuqia 38 5 39 5 n 75 76 5 8 isothermal remanent magnetization acquisition indicates the presence of minerals with high magnetic coercivity stepwise thermal demagnetization confirms that hematite is the dominant carrier and allowed the separation of low temperature components ltc 600 c in most samples the mean direction of ltcs is aligned with that of the present earth field normal and reversed polarities have been found for the htcs of each age group htcs show positive fold tests for at least one sub study where the attitudes of bedding were sufficiently different to allow the test to be performed more important the four sub studies from yingjisha and wuqia give fully compatible results results from the uytak area show a wide dispersion in declinations as could be expected from the rather severe deformation and proximity to major thrust and strike slip faults data obtained in the kuche area from northern tarim by li and co workers and in southern junggar by chen and co workers are statistically identical to the new data from yingjisha and wuqia data from paicheng in northern tarim appear to have been subject to local rotation excluding those sites which appear from paleomagnetic study to have been significantly rotated we calculated a mean cretaceous pole for the tarim basin at 69 1 n 223 88 ass 3 5 n 5 this result indicates that the total post cretaceous rotation of tarim relative to junggar and to eurasia has been negligible 2 6 6 1 and 0 4 4 6 respectively continental shortening and rotation between tarim and junggar within the tien shan are not resolved by paleomagnetism although the values obtained 400 t 600 km 3 6 are compatible with tectonic models of the range a major discrepancy warranting further geological investigation is that between the tarim and north china block poles which would imply some 1400 1060 km of north south relative displacement

far field contractional polarity changes in models and nature abstract a change in contractional polarity occurs when the direction of tectonic transport switches along strike this switch is conditioned by lateral variations in rheology or inherited asymmetries such as contrasts in structure or changes in the polarity of subduction zones the parameters controlling contractional polarity changes are less understood in situations when the strain is transferred at large distances from indenters analysing this type of strain transfer is critical for understanding the mechanics of thrusting in fore or back arc settings of orogenic areas comparison of crustal scale analogue modelling with the inversion of the black sea back arc and the formation of the new guinea new britain fore arc suggest that far field changes in contractional polarity are related to rheological contrasts across inherited normal faults the initial extension creates rheological weak zones that localize the subsequent far field contractional deformation along groups of thrusts with opposite vergence along the strike of the system the largest amount of far field contractional deformation is recorded in the transfer zone located between the two indenters moving in opposite directions and is particularly high when inverting oblique extensional systems

morphology and origin of erosion surfaces in the cardium formation upper cretaceous western interior seaway alberta and their implications for rapid sea level fluctuations the morphology of three basin wide erosion surfaces has been defined using 7869 resistivity well logs over an area of about 50 000 km2 the lowest of these designated e6 is topographically smooth and has no incised steps or low areas it cuts into marine mudstones and indicates either an uninterrupted erosive transgression following a major lowering of relative sea level or wave scouring of the substrate following a very minor lowering of sea level erosion surface e6 5 also cuts into marine mudstones but its overall morphology is less well understood owing to subsequent dissection by surface e7 this surface e7 is characterized by three strike parallel linear low areas that die out northwestward these linear lows are up to 40 km wide 36 m deep and have irregular to symmetrical cross sectional profiles they cut into and are filled with marine mudstones and their origin is enigmatic the differences in the morphologies of the surfaces suggest the interplay of several controlling parameters