rates and processes of bedrock incision by the upper ukak river since the 1912 novarupta ash flow in the valley of ten thousand smokes alaska the rates and patterns of bedrock channel incision significantly influence landscape evolution and long term interactions among climate tectonics and erosion unfortunately only sparse field data are available to quantify the controls on river incision rates we exploit the diversion of the upper ukak river by an ash flow in 1912 to measure rates of incision along a newly formed bedrock channel minimum estimates of the rate of incision into intact rock vary from 0 01 to 0 10 m yr 1 this variation reflects differences in channel slope channel width lithologic facies and intensity of jointing as well as the effects of upstream knickpoint migration a streampower type incision model adequately explains the incision rate data provided 1 variations in channel width are prescribed on the basis of field measurements 2 the slope exponent is significantly less than unity n 0 4 0 2 and 3 observed downstream changes in lithologic facies and the intensity of jointing account for the apparent twofold downstream decrease in the coefficient of erosion despite the very rapid rate of incision calibrated stream power erosion coefficients for the ukak river k 2 4 10 4 m 0 2 yr 1 to 9 0 10 4 m 0 2 yr 1 are within the range of previously published estimates two plausible explanations for the low values of the slope exponent n are that incision rate is limited by either 1 a combination of physical weathering and hydrodynamic joint block extraction or 2 block fracture due to bedload impacts modulated on steeper channel segments by suspension of a significant fraction of the sediment load

geometry and evolution of salt structures in a marginal rift basin of the gulf of mexico east texas the geometry and distribution of salt structures in the east texas basin shown in an isometric block diagram reveal that a minimum of 1 500 m of mid jurassic louann salt accumulated in a north northeast trending trough floored by thinned continental crust above a threshold thickness of 600 m salt was mobilized by 1 loading beneath a carbonate wedge 2 differential loading by prograding terrigenous clastics and 3 basin edge tilting and erosion salt structures evolved from salt pillows to active diapirs with maximum gross growth rates of 400 to 530 m m y each stage lasted 10 to 30 m y three generations of diapirs were successively activated in different areas by different forces

seismic and sequence stratigraphy of the central western continental margin of india late quaternary evolution abstract seismic and sequence stratigraphic architecture of the central western continental margin of india between coondapur and south of mangalore has been investigated with shallow seismic data seismic stratigraphic analysis defined nine seismic units that are configured in a major type 1 depositional sequence possibly related to fourth order eustatic sea level changes comprising regressive lowstand transgressive and highstand systems tracts the late quaternary evolution of the continental margin took place under the influence of an asymmetric relative fourth order sea level cycle punctuated by higher frequency cycles these cycles of minor order were characterised by rapid sea level rises and gradual sea level falls that generated depositional sequences spanning different time scales during the regressive periods dipping strata were developed while erosional surfaces and incised valleys were formed during the lowstands of sea level terraces v shaped depressions lagoon like structures observed on the outer continental shelf are the result of the transgressive period in the study area we have recognised a complex erosional surface that records a long time span during the relative sea level fall regressive period and the following sea level lowstand and has been reworked during the last transgression we also infer that sedimentation processes changed from siliciclastic sedimentation to carbonate sedimentation and again to siliciclastic sedimentation marking an important phase in the late quaternary evolution of the western continental shelf of india we attribute this to an abrupt climate change at the end of the oxygen isotope stage 2 between the last glacial maximum and the bolling allerod event 14 000 yr bp this sensitive climate change warming favoured the formation of reefs at various depths on the shelf besides the development of fifty fathom flat a carbonate platform on the outer shelf off bombay developed prior to 8300 yr bp the highstand systems tracts were deposited after the sea level reached its present position

eaglebine play of the southwestern east texas basin stratigraphic and depositional framework of the upper cretaceous cenomanian turonian woodbine and eagle ford groups the woodbine and eagle ford groups of the southwestern east texas basin compose an emerging play which has generated considerable interest because of its potential for new hydrocarbon production from both sandstone and mudrock reservoirs however the play s stratigraphic and depositional relations are complex and directly relate to the play s exploration challenges productive woodbine and eagle ford sub clarksville sandstones intertongue with a poorly defined subregional mudrock dominated interval that thins southwestward toward the san marcos arch we propose dividing this succession into two intervals 1 the lower unit a high gamma ray unit at the base of this mudrock succession that is inferred to be equivalent to the maness shale of the washita group and to part of the lower eagle ford group on the san marcos arch and 2 an upper unit a basinward thickening zone of consistently lower gamma ray log facies inferred to be equivalent to the woodbine group pepper shale and the eagle ford group of the east texas basin because the cenomanian turonian boundary occurs within the eagle ford group of the east texas basin and the lower eagle ford section of the san marcos arch most of the maness through eagle ford succession exists as a much thinned section on the arch basinwide integration of the woodbine sequence stratigraphic framework shows that the number of fourth order sequences in the unit decreases westward from 14 in the basin axis to no more than 9 in the most active part of the eaglebine play because of their systematic depositional pinch out approaching the western basin margin the eagle ford group consists of three fourth order sequences capped by the sub clarksville sandstones that accumulated after the major late cenomanian early turonian flooding event recorded by a basinwide transgressive systems tract tst at the base of the unit depositional systems of the woodbine group vary within the study area even between stratigraphically adjacent systems on shelf siliciclastic systems include fluvial dominated delta incised valley fill fluvial and nearshore marine and wave dominated delta deposits

monsoon fluctuations in two 20 000 yr b p oxygen isotope pollen records off southwest india abstract two marine pollen diagrams for sediments off the southwest coast of india provide a 20 000 yr history of the nearby continental vegetation determined by the monsoon climate within the framework of the isotopic stratigraphy two important phases of the evolution of the monsoon climate are a very arid period about 22 000 18 000 yr b p and a very humid period culminating at 11 000 yr b p the very arid period corresponds to the lowest pollen representation of mangrove vegetation which is used here as an indirect indicator of monsoonal runoff this aridity is due to a very weak southwest airflow a great reduction of summer monsoonal rainfall and reduced runoff of the western ghats rivers the extension of the monsoon over india after the last glacial maximum has been a gradual process following the northward progression of the intertropical convergene zone the very humid period corresponding to the highest pollen representation of mangrove vegetation and the heaviest summer rainfall is a climatic response to the maximum summer insolation of the northern hemisphere at 11 000 yr b p

physically based modeling of bedrock incision by abrasion plucking and macroabrasion 1 many important insights into the dynamic coupling among climate erosion and tectonics in mountain areas have derived from several numerical models of the past few decades which include descriptions of bedrock incision however many questions regarding incision processes and morphology of bedrock streams still remain unanswered a more mechanistically based incision model is needed as a component to study landscape evolution major bedrock incision processes include among other mechanisms abrasion by bed load plucking and macroabrasion a process of fracturing of the bedrock into pluckable sizes mediated by particle impacts the purpose of this paper is to develop a physically based model of bedrock incision that includes all three processes mentioned above to build the model we start by developing a theory of abrasion plucking and macroabrasion mechanisms we then incorporate hydrology the evaluation of boundary shear stress capacity transport an entrainment relation for pluckable particles a routing model linking in stream sediment and hillslopes a formulation for alluvial channel coverage a channel width relation hack s law and exner equation into the model so that we can simulate the evolution of bedrock channels the model successfully simulates various features of bed elevation profiles of natural bedrock rivers under a variety of input or boundary conditions the results also illustrate that knickpoints found in bedrock rivers may be autogenic in addition to being driven by base level fall and lithologic changes this supports the concept that bedrock incision by knickpoint migration may be an integral part of normal incision processes the model is expected to improve the current understanding of the linkage among physically meaningful input parameters the physics of incision process and morphological changes in bedrock streams

oceanographic and eustatic control of carbonate platform evolution and sequence stratigraphy on the cretaceous valanginian campanian passive margin northern gulf of mexico an integrated sequence stratigraphic study based on outcrop core and wireline log data documents the combined impact of cretaceous eustacy and oceanic anoxic events on carbonate shelf morphology and facies distributions in the northern gulf of mexico the diverse facies and abundant data of the comanche platform serve as a nearly complete global reference section and provide a sensitive record of external processes affecting cretaceous platform development regional cross sections across the shoreline to shelf margin profile provide a detailed record of mixed carbonate siliciclastic strata for the hauterivian to lower campanian stages ca 136 to 80 ma the study window on the slowly subsiding passive margin allows the stratigraphic response to external forcing mechanisms to be isolated from regional structural processes three second order supersequences comprised of eight composite sequences are recognized in the valanginian barremian the aptian albian and the cenomanian campanian the valanginian barremian supersequence transitioned from a siliciclastic ramp to carbonate rimmed shelf and is a product of glacial ice accumulation and melting as well as variable rates of mid ocean ridge volcanism the aptian albian supersequence chronicles the drowning and recovery of the platform surrounding oceanic anoxic events 1a and 1b the cenomanian campanian supersequence similarly documents shelf drowning following oceanic anoxic event 1d after which the platform evolved to a deep subtidal system consisting of anoxic dysoxic shale and chalk in the time surrounding oceanic anoxic event 2 each period of oceanic anoxia is associated with composite sequence maximum flooding termination of carbonate shelf sedimentation and deposition of condensed shale units in distally steepened ramp profiles composite sequences unaffected by oceanic anoxic events consist of aggradational to progradational shelves with an abundance of grain dominated facies and shallow subtidal to intertidal environments because they are products of eustacy and global oceanographic processes the three supersequences and most composite sequences defined in the south texas passive margin are recognizable in other carbonate platforms and published eustatic sea level curves

uplift of the sierra nevada of california gravity and most seismic interpretations agree that the sierra nevada is at present isostatically compensated by a crustal root the most reasonable timing for emplacement of the root was during mesozoic batholithic intrusions this is difficult to reconcile with the evidence that major uplift of the mountains occurred in the past 10 m y a simple quantitative model for flexural isostasy of an elastic plate before and after breaking resolves this problem and explains the tilt of the sierra nevada block and variations in topography along the range we postulate that cooling and elastic strengthening of the mesozoic magmatic arc prevented its reaching local isostatic equilibrium during erosion in the cenozoic thus an overcompensated residual mountain range was held down elastically until basin and range extension broke the elastic plate along the owens valley and allowed rapid uplift the moho density contrast that best models the amplitude of both late cenozoic uplift and prior topography is slightly larger than seismic and gravity models suggest this implies that up to 20 of the uplift may be driven by density contrasts in the upper mantle the idea of overcompensated erosional remnants and their release upon lithospheric faulting has a more general application to anorogenic uplift of mountain blocks

potential distribution of subsurface methane in the sediments of the eastern arabian sea and its possible implications high resolution shallow seismic reflection data obtained in the continental shelf sediments off western india from 10 n to 22 n reveal the characteristic anomalous seismic signatures in the form of acoustic maskings of methane rich horizons a conservative estimate of the potential subsurface methane in these gas charged sediments is of the order of 2 6 tg and its contribution to the atmosphere with an annual flux of 0 039 tg ch4 appears to be quite significant from our limited results coupled with worldwide reports it is concluded that the seepage of methane 8 to 65 tg through the seabed of the continental shelves worldwide cannot be ignored for the estimation of global oceanic flux of methane and its budget

geochemical relationships of petroleum in mesozoic reservoirs to carbonate source rocks of jurassic smackover formation southwestern alabama algal carbonate mudstones of the jurassic smackover formation are the main source rocks for oil and condensate in mesozoic reservoir rocks in southwestern alabama this interpretation is based on geochemical analyses of oils condensates and organic matter in selected samples of shale norphlet formation haynesville formation trinity group tuscaloosa group and carbonate smackover formation rocks potential and probable oil source rocks are present in the tuscaloosa group and smackover formation respectively extractable organic matter from smackover carbonates has molecular and isotopic similarities to jurassic oil although the jurassic oils and condensates in southwestern alabama have genetic similarities they show significant compositional variations due to differences in thermal maturity and organic facies lithofacies organic facies reflect different depositional conditions for source rocks in the various basins the mississippi interior salt basin was characterized by more continuous marine to hypersaline conditions whereas the manila and conecuh embayments periodically had lower salinity and greater input of clastic debris and terrestrial organic matter petroleum and organic matter in jurassic rocks of southwestern alabama show a range of thermal transformations the gas content of hydrocarbons in reservoirs increases with increasing depth and temperature in some reservoirs where the temperature is above 266 f 130 c gas condensate is enriched in isotopically heavy sulfur apparently derived from thermochemical reduction of jurassic evaporite sulfate this process also results in increased h2s and co2 in the gas and depletion of saturated hydrocarbons in the condensate liquids thermochemical sulfate reduction probably depends on the mineralogic composition of the reservoir rock as well as temperature because some deep 18 000 ft or 5 5 km and hot 320 f or 160 c smackover and norphlet reservoirs contain low sulfur petroleum

relationships between east texas field region and sabine uplift in texas the discoveries of spindletop field near beaumont texas on january 10 1901 and the east texas field located between the cities of longview and tyler texas on october 5 1930 are probably the two most important events in the exploration history of the american petroleum industry spindletop revolutionized both the american and world petroleum industry and created the need for petroleum geologists the east texas field had a profound and beneficial effect on depressed economic conditions of the time and it brought to the attention of geologists the great productive possibilities of stratigraphic type traps the east texas field has two outstanding features its tremendous size and the simplicity of its geologic trap it has produced 4 651 000 000 bbl through january 1 1980 and probably will produce another billion barrels the trap occurs where the truncated edge of the productive woodbine sand crosses regional nosing on the west flank of the sabine uplift although this discussion is based principally on a single 75 mi 120 7 km long east west dip electric log cross section conclusions were derived from studies that made use of several regional cross sections and maps these studies indicate that a major rise of the sabine uplift occurred after buda deposition and before woodbine deposition the woodbine apparently was deposited on a lower cretaceous peneplane which resulted from uplift and severe erosion of lower cretaceous strata woodbine sediments were deposited over the present crestal and flank areas of the sabine uplift on this eroded surface a later major rise of the sabine uplift occurred during upper woodbine and eagle ford deposition and the resulting erosion removed all woodbine strata from the higher reaches of the sabine uplift these eroded woodbine sediments were redeposited as reworked woodbine beds in texas and they were probably in part a source of eagle ford sands in texas and of tuscaloosa sands in louisiana at the end of early cretaceous deposition the east texas field region was structurally higher than the texas louisiana border area now occupied by the sabine uplift since that time the rise of the sabine uplift has changed that relationship so that the east texas field area now lies downdip from and on the west flank of the sabine uplift

a mechanistic model for river incision into bedrock by saltating bed load 1 abrasion by bed load is a ubiquitous and sometimes dominant erosional mechanism for fluvial incision into bedrock here we develop a model for bedrock abrasion by saltating bed load wherein the wear rate depends linearly on the flux of impact kinetic energy normal to the bed and on the fraction of the bed that is not armored by transient deposits of alluvium we assume that the extent of alluvial bed cover depends on the ratio of coarse sediment supply to bed load transport capacity particle impact velocity and impact frequency depend on saltation trajectories which can be predicted using empirical functions of excess shear stress the model predicts a nonlinear dependence of bedrock abrasion rate on both sediment supply and transport capacity maximum wear rates occur at moderate relative supply rates due to the tradeoff between the availability of abrasive tools and the partial alluviation of the bedrock bed maximum wear rates also occur at intermediate levels of excess shear stress due to the reduction in impact frequency as grain motion approaches the threshold of suspension measurements of bedrock wear in a laboratory abrasion mill agree well with model predictions and allow calibration of the one free model parameter which relates rock strength to rock resistance to abrasive wear the model results suggest that grain size and sediment supply are fundamental controls on bedrock incision rates not only by bed load abrasion but also by all other mechanisms that require bedrock to be exposed in the channel bed

some comments on two layer extensional models for the evolution of sedimentary basins simple extensional models that involve listric faulting in the brittle upper crust and plastic flow in the lower lithosphere have been shown to account for the subsidence history of various continental sedimentary basins and continental shelves in this paper we present structural evidence that extension in the continental lithosphere during rifting is not uniform we review evidence that the rifting process in major intracratonic rift basins is spasmodic rather than continuous that extension and subsidence within the rift during a tectonic phase is generally accompanied by uplift and erosion of the rift flanks that subcrustal heat input during rifting occurs over an area much broader than the rift proper and that there may be broad regional uplift before and or during rifting to explain the uplift of the flanks during rifting and their postrift subsidence we invoke two mechanisms subcrustal thinning by extension below the flanks and tilted major normal faults in the crust between graben and flanks the uplift and erosion of the flanks due to tilted faults probably do not significantly affect estimates of extension from subsidence analysis but uplift due to both sources can in certain cases be incorrectly attributed to regional uplift or doming we present a simplified analysis of the two layer extensional model for the elementary case in which extension is instantaneous and the crust is thinned by a different amount from the subcrustal lithosphere we provide a simple graphical method of data analysis to obtain the parameters that describe thinning of the crust and lithosphere during extension from the subsidence history application of this method to the central part of the pattani trough in the intracratonic gulf of thailand shows that extension and thinning of the lithosphere of a factor of 2 since late eocene can account for much of the subsidence history and the present day heat flow our analysis suggests that thinning of the crust in this area is 20 greater than thinning of the whole lithosphere assuming that none of the lithosphere is removed from the region of extension during rifting we show by simple geometric examples that a process of nonuniform extension implies that much greater thicknesses of sediment can be deposited in the center of a young basin than in the case of uniform extension of both crust and subcrustal lithosphere such an extensional process produces significant uplift of the flanks of the rift and as a result of erosion of the uplifted areas the effective cross sectional area of the basin can be increased by as much as 10 20 depending on the rate of erosion compared to the area that would have been created by a process of uniform extension finally we show that it may ultimately be possible to compute erosion rates from seismic cross sections of basins

recommendations for a uniform chronostratigraphic designation system for phanerozoic depositional sequences many published sequence stratigraphic frameworks lack a systematic and consistent designation for both depositional sequences and key surfaces despite the original goal to provide a fully integrated stratigraphic architecture including diagnostic age information based on a system in use for more than 10 yr at exxonmobil we recommend methodologies for a chronostratigraphic designation system cds using more uniform and robust sequence stratigraphic designations after objectively defining important physical stratigraphic surfaces biostratigraphic and other age constraining information is used to designate surfaces and the packages of rocks they bound this leads to the establishment of a sequence chronostratigraphic framework for a local area of investigation outcrop section field region or basin only after demonstrating clear well documented ties to phanerozoic global coastal onlaps or cycle charts are these sequences and associated surfaces considered as global entities and designated as such higher frequency sequences and surfaces are also accommodated in this cds alternative designations for areas with limited or poor quality chronostratigraphic information are also discussed the cds has proven to have great use in all phanerozoic strata different tectonic settings and depositional environments especially when chronostratigraphic age constraints are robust we have used this system at regional and basinal scales in many geographic locations to help reduce uncertainty in identifying and correlating reservoirs sources and seal rocks predicting the local distribution and quality of reservoirs as well as seals within a producing field and near field wildcats is also facilitated by this system this system has demonstrated use in the correlation of outcrop sections within a basin or between basins rigorous use of the cds proposed here will permit meaningful regional and or interbasinal correlation that is difficult to carry out with the diverse systems currently in use this uniform designation scheme will also facilitate communication within a company and between institutions as well as among academic investigators

uplift and extension at the gulf of suez indications of induced mantle convection reconstruction of tectonic movements since the initiation of the gulf of suez rift indicates that there has been 25 27 km of extension the uplift bordering rift indicates that lithosphere heating greatly exceeds that producible by uniform lithosphere extension i propose that small scale convection induced by the rifting augments the heating introduced by extensional rifting and produces the broad uplifts flanking the rift

thermally activated creep and flexure of the oceanic lithosphere thermal models of the oceanic lithosphere1 4 explain ocean floor bathymetry heat flux and gravity and geoid5 anomalies by the cooling and thickening of the lithosphere as it ages away from ocean ridges surface wave studies6 7 tend to confirm that thickness increases with thermal age thickness estimates based on the lithosphere s flexural response parameterized in terms of an elastic model floating on a inviscid substratum are however factors of two to three times less than those from the thermal and seismic models8 9 we show here that a unification of estimates can be achieved by a different yet simple parameterization of the bending response of the lithosphere the lithosphere is treated as a non convective layer that undergoes thermally activated creep according to its temperature at the time of loading the solutions to this problem for loads emplaced on lithosphere of increasing age are then used as a basis set to construct the solution to the problem of stress relaxation in a thermally ageing lithosphere we show that the predicted deformation passes through a series of apparently weaker quasi elastic states which can be equated with those of the elastic thin plate flexural models but the process is controlled by the thermal lithosphere and its associated thickness the controlling parameters are the basal viscosity of the lithosphere and the activation energy for creep in mantle rocks

evolution of atchafalaya lacustrine deltas south central louisiana abstract the entrenched pleistocene holocene mississippi alluvial valley was cut during the last low sea level stand and alluviation of the entrenched valley occurred during the holocene sea level rise regional gulf coast structures salt domes and faults have strongly influenced the morphology of the valley valley paleotopography has controlled depositional patterns and locations of major river courses as well as partially isolated large interdistributary basins within the alluvial deltaic plain interdistributary basin formation is further enhanced by distributary channel progradation switching and abandonment in conjunction with compactional subsidence of the alluvial plain surface geomorphology and sediment cores from the atchafalaya basin louisiana revealed that this basin has alternately accumulated fine grained organic rich sediments and terrigenous clastic dominated meander belt and lacustrine deposits basin subsidence and the presence of an active distributary channel in the basin are major determinants of the lithology of the sedimentary fill prograding lacustrine deltas of the atchafalaya river are the primary mechanism for filling the atchafalaya basin these upward coarsening deltas are relatively thin 3 m but are areally extensive tens to hundreds of kilometers and because they occur so frequently during the infilling process they account for a significant volume of the basin sequence the atchafalaya basin sequence documents lacustrine deltaic episodes dating from the early holocene maringouin and teche deltas to the present atchafalaya delta continued development of the atchafalaya river will rework portions of the backswamp and lacustrine deposits into meander belt deposits upon complete filling of the atchafalaya basin sediment will mostly bypass the basin and become deposited in the marine delta in atchafalaya bay

seafloor gas seeps and sediment failures triggered by the august 17 1999 earthquake in the eastern part of the gulf of izmit sea of marmara nw turkey abstract on august 17 1999 a major earthquake mw 7 4 occurred affecting a large area in northwestern turkey a 150 km long section of the north anatolian fault ruptured from the entrance of the gulf of izmit to golyaka in the east during the earthquake we utilized high resolution shallow seismic data acquired during a post earthquake cruise to find evidence of gas seeps gas charged sediments and sediment failure in the eastern izmit basin of the gulf of izmit the data showed anomalies that suggest the presence of increased gas in the gulf after the earthquake five major zones of offshore mass movement were found in the study area comparison of data collected before and after the earthquake indicates that the apparent amount of gas increased after the earthquake and the mass movement has a causative relationship to the august 17 1999 earthquake therefore we conclude that the seismic event presence of increased gas and the offshore mass movements in the study area are interrelated

quantitative testing of model of bedrock channel incision by plucking and macroabrasion river incision can play a role in societally relevant or human timescale which requires careful engineering planning and design physically based incision models can help us perform the engineering computations to prevent erosion damage to engineering structures such as bridges and channel bank protection works this paper presents a field application of our previously published work in which we developed a mechanistically based model of bedrock incision by plucking and macroabrasion a process of fracturing of the bedrock into pluckable sizes mediated by particle impacts from bedload the plucking macroabrasion incision model is tested quantitatively with unnamed drainage 1 in southern indiana which showed obvious channel incision over the past 19 years destroying a highway bridge foundation the field data of incision rate hydraulic conditions rainfall grain size characteristics and morphological conditions were collected in the field or obtained from previous studies and the national oceanic

dependence of the flexural rigidity of the continental lithosphere on rheology and temperature while the first order mechanical properties of oceanic and continental lithosphere are well defined and explain the observed increase in lithospheric flexural strength with thermal age the temporal variation of continental flexural rigidity following loading and its dependence on lithospheric composition and temperature structure is only just beginning to be appreciated we describe here the results of a quantitative theological model in which laboratory determined rock deformation data and transienttemperature distributions resulting from intra plate basin formation within continental interiors have been integrated continental flexural rigidity is shown to be controlled by crustal thickness lithospheric thickness through the temperature structure and the interaction of these factors during basin formation for thermally young lithosphere the flexural rigidity is dominated by the quartzo feldspathic theology of the crust while for the thermally older lithosphere it is dominated by the olivine rheology of the mantle flexural data through thermo mechanical models of lithospheric deformation provide a powerful constraint on both the transient and steady state temperature structure of the lithosphere

sequence stratigraphic controls on complex reservoir architecture of highstand fluvial dominated deltaic and lowstand valley fill deposits in the upper cretaceous cenomanian woodbine group east texas field regional and local perspectives an analysis of 31 whole cores 1600 ft 490 m and closely spaced wireline logs 500 wells penetrating the lower cretaceous cenomanian lower woodbine group in the mature east texas field and adjacent areas indicates that depositional origins and complexity of the sandstone body architecture in the field vary from those inferred from previous studies heterogeneity in the lower woodbine group is controlled by highstand fluvial dominated deltaic depositional architecture with dip elongate distributary channel sandstones pinching out over short distances typically 500 ft 150 m into delta plain and interdistributary bay siltstones and mudstones this highstand section is truncated in the north and west parts of the field by a thick maximum of 140 ft 43 m lowstand incised valley fill succession composed of multistoried coarse gravel conglomerate and coarse sandstone beds of bed load fluvial systems in some areas of the field this valley fill directly overlies distal delta front deposits recording a fall in relative sea level of at least 215 ft 65 m correlation with the woodbine succession in the east texas basin indicates that these highstand and lowstand deposits occur in the basal three fourth order sequences of the unit which comprises a maximum of 14 such cycles previous studies of the woodbine group have inferred meanderbelt sandstones flanked by coeval flood plain mudstones and well connected laterally continuous sheet sandstones of wave dominated deltaic and barrier strand plain settings this model is inappropriate and a full assessment of reservoir compartmentalization fluid flow and unswept mobile oil in east texas field should include the highstand fluvial dominated deltaic and lowstand valley fill sandstone body architecture

river incision into bedrock mechanics and relative efficacy of plucking abrasion and cavitation improved formulation of bedrock erosion laws requires knowledge of the actual processes operative at the bed we present qualitative field evidence from a wide range of settings that the relative efficacy of the various processes of fluvial erosion e g plucking abrasion cavitation solution is a strong function of substrate lithology and that joint spacing fractures and bedding planes exert the most direct control the relative importance of the various processes and the nature of the interplay between them are inferred from detailed observations of the morphology of erosional forms on channel bed and banks and their spatial distributions we find that plucking dominates wherever rocks are well jointed on a submeter scale hydraulic wedging of small clasts into cracks bashing and abrasion by bedload and chemical and physical weathering all contribute to the loosening and removal of joint blocks in more massive rocks abrasion by suspended sand appears to be rate limiting in the systems studied here concentration of erosion on downstream sides of obstacles and tight coupling between fluid flow patterns and fine scale morphology of erosion forms testify to the importance of abrasion by suspended load rather than bedload particles mechanical analyses indicate that erosion by suspended load abrasion is considerably more nonlinear in shear stress than erosion by plucking in addition a new analysis indicates that cavitation is more likely to occur in natural systems than previously argued cavitation must be considered a viable process in many actively incising bedrock channels and may contribute to the fluting and potholing of massive unjointed rocks that is otherwise attributed to suspended load abrasion direct field evidence of cavitation erosion is however lacking in terms of the well known shear stress or stream power erosion law erosion by plucking is consistent with a slope exponent n of 2 3 to 1 whereas erosion by suspended load abrasion is more consistent with a slope exponent of 5 3 given that substrate lithology appears to dictate the dominant erosion process this finding has important implications for long term landscape evolution and the models used to study it

petroleum related hydrocarbon seep age in a recent north sea sediment abstract in the summer of 1985 unocal carried out a 55 day surface geochemical survey in the norwegian sector of the north sea the program was set up to include a variety of geochemical techniques to aid evaluation of their effectiveness for detecting hydrocarbon seepages in the marine environment the ship was equipped with both bottom water and bottom sediment sampling devices and an analytical laboratory and computer network so that onboard measurements and real time data interpretation could be made during the survey 4100 line kilometers of bottom water data 11 000 sites and 320 sediment cores were obtained a major hydrocarbon gas anomaly was defined during part of the bottom water sampling program a set of sediment cores was taken in the vicinity of this anomaly the data from these cores are used to illustrate the fate of hydrocarbon migration through unconsolidated marine sediment migration by bubble ebullition transports hydrocarbons through the near surface sediment and produces gas plumes in the bottom water trapping of bubbles in the sediment will produce patchy temporal hydrocarbon anomalies in the sediment however extensive sediment anomalies such as those proposed to be developed by microseepage can only be formed at depth in the sediment below the level where sulfate is depleted from the pore water these anomalies result because vertical migration by molecular diffusion creates a steady state hydrocarbon oxidation zone sulfate reduction zone at depth in the sediment which efficiently eliminates hydrocarbon transport further to the surface

experimental study of abrasion and its role in producing downstream fining in gravel bed rivers abstract previous experimental studies on abrasion of gravel showed diminution rates much lower than those typical of gravel bed rivers this might be one reason many studies of downstream fining in rivers stress hydraulic sorting as the dominant fining mechanism previous experiments however did not replicate well the particle to particle impact between the bed load gravels and bed gravels in rivers on alluvial fans during floods the purpose of this study was to reexamine the effect of abrasion on downstream fining to determine if abrasion is responsible for longitudinal changes in gravel size in rivers on alluvial fans a rotating drum with three interior vanes was used to produce repeated collisions among test gravel particles in water the drum was only partially filled with water in order to simulate the impact velocity of particle collisions during a flood the test particles were slightly weathered andesite and chert taken from the bed of the watarase river japan experiments were conducted using three unisize grains l 7 0 128 mm to 6 5 91 mm m 5 5 45 mm to 5 0 32 mm s 4 0 16 mm to 3 5 11 mm and mixtures of two different sizes from the three size classes there are six principal results from the experiments 1 test gravel particles break frequently and decrease in weight rapidly 2 abrasion of chert produces mostly gravel while andesite produces mostly sand and silt 3 l size chert cobbles decrease in weight rapidly as a result of being broken into smaller pieces while andesite cobbles break so rarely that their weight decreases very slowly 4 s size andesite pebbles decrease in weight more rapidly than chert 5 abrasion depends strongly on the presence of different sizes because smaller fragments are crushed by larger gravel particles 6 diminution coefficients of andesite and chert obtained from the experiment are in the range of 10 3 10 1 km 1 diminution coefficients from this experiment are one to two orders of magnitude larger than those from previous experiments on abrasion this is mainly because of the larger impact velocities in this experiment diminution coefficients from the experiment are also consistent with those obtained from many japanese rivers on alluvial fans 10 2 10 1 km 1 this result suggests that downstream fining of river bed gravel on alluvial fans can be explained by abrasion

mechanism of subsidence of ancient cratonic rift basins cratonic basins commonly occur over ancient rift zones these inactive rift basins are recognizable by a positive linear bouguer gravity anomaly that may correspond to the axial gravity high found in modern rift valleys many of these basins undergo discrete periods of increased subsidence rates or reactivations long after the mass excess responsible for the linear gravity high was emplaced furthermore the reactivation of many cratonic basins occurs simultaneously with large scale compressional tectonics it is suggested that the driving force for subsidence is the isostatically uncompensated ancient mass excess the subsidence of these basins is modelled by a lithospheric flexure model with a nonlinear maxwell viscoelastic rheology solutions to this model indicate that basins may experience a low subsidence rate throughout geologic time the subsidence of a basin will stop only when isostatic compensation of the mass excess is achieved since ancient rift mass excesses may be uncompensated over long geologic time intervals the early thermal and structural evolution of rifts may not significantly influence later basin subsidence the models suggest that basins may be reactivated by any mechanism which lowers the effective viscosity of the lithospheric material allowing the uncompensated basin to settle toward an isostatic compensation depth faster than normal since viscosity is a strong function of temperature reactivation by a world wide increase in heat flow is suggested as a possible mechanism for the synchroneity of basin subsidence throughout a continent an increase of 15 in the geothermal gradient for example from 16 5 18 9 k km will cause about a 5 increase in subsidence this increase in heat flow however seems unlikely of producing by itself the magnitude of basin subsidence during a reactivation phase that is observed in the geologic record where up to 100 increase in subsidence might occur since the rheology of the lithosphere is nonlinear effective viscosity is also a strong nonlinear function of stress the presence of a regional compressive stress during periods of tectonism of 1 1 108 pa about 2 8 of the buckling strength of the lithosphere produces a short period of reactivated subsidence 105 yr during the reactivated subsidence the newly imposed regional stress relaxes sufficiently in the lower lithosphere to restore the effective viscosity to values similar to that before reactivation this suggests that reactivated subsidence caused by regional compression can be maintained as long as the stress level remains high in the lower lithosphere this may be accomplished by an intermittent application of the regional stress over time

field techniques for measuring bedrock erosion and denudation bedrock erosion rates in natural landscapes are usually slow of the order of millimeters per year or less and sophisticated techniques have been developed to measure them different techniques have proved to be valuable depending on the spatial and temporal scale on which information is needed on the environment and on the scientific question that is asked here we give an overview of the various methods that have been developed we introduce their working principles and outline their advantages and disadvantages further we provide comprehensive references to relevant literature both on the methods and on scientific examples of their application 2016 the authors earth surface processes and landforms published by john wiley sons ltd

the influence of bedrock geology on knickpoint development and channel bed degradation along downcutting streams in south central indiana geologic controls on knickpoint development and the mechanics of channel bed degradation were eluci dated by comparison of downcutting streams in south central indiana developed in lithologically homoge neous carbonate units and layered siliciclastic strata knickpoints formed in a single lithologic unit are stepped each step consists of a knickpoint face and tread the height of each step is equal to and controlled by bed thickness spatially knickpoints in homogeneous terrain are limited to jointed strata that dip 1 upstream or 2 downstream at angles less than the channel gradient knickpoints observed in lithologically layered strata are characterized by a single step in which resistant jointed rocks overlie and protect weaker units total knickpoint relief is influenced by thickness of the weaker strata knickpoints in layered siliciclastic strata are spatially limited to bedrock reaches produced by differential rates of channel incision knickpoint distribution is also influenced by joint

geometry and structure associated to gas charged sediments and recent growth faults in the ebro delta spain abstract in the ebro delta ne spain small to large scale synsedimentary deformation has been observed by means of high and low resolution seismics correlated with a network of wells lithoseismic units involved are middle upper pleistocene clays and gravels and holocene deltaic sequence gassy sediments and large river floor pockmarks are identified from seismic profiles along the ebro delta gas charged sediments are identified by zones of acoustic turbidity and other acoustic anomalies on seismic profiles most of the gas is believed to be biogenic in origin resulting from the decay of organic matter contained within rapidly developing shelf margin deltas a group of large symmetric river floor pockmarks also occurs along the inner delta plain both on the river floor surface and buried in the sediment column listric fault geometry is a typical scope shaped plane with a steep upper surface 40 60 that passes downward into a horizontal shear plane the shear plane appears seismically as chaotic and convoluted high amplitude reflectors and is correlated with well data showing layers that include gravel sand and clays associated structural features include reverse drag of reflectors antithetic faults and fluid escapes methane water the growth faulting along the ebro delta includes a break away and thin near surface layer created by overstepping of a sandy holocene delta slope the over steeping lies down dip from growth faults along the shelf break the overall growth faults province appears to represent a deeper seated general movement of the delta mass involving differential loading overpressure of fluids diapiric movements and extensional tectonics that cause listric faults and associated structures at different scales the recent growth faults are regularly spaced with the distance between successive faults controlled by the depth to the detachment layer three main fault systems have been identified a first or primary system spaced 5 2 10 5 km above a detachment layer at 55 95 m deep a second system spaced 3 5 6 km and a third system spaced 1 5 2 5 km above a detachment layer at 25 30 m the slope angle calculated by empirical equations ranges between 0 5 and 1 0 for the three systems

acoustic masking in sediments due to gases on the western continental shelf of india abstract surveys carried out on the western continental shelf of india indicate that the inner and middle shelf to a depth of 50 60 m is covered by acoustically transparent clays on the shelf off bombay the clays are thin near the shore where they overlie rocks or sands and increase to 20 35 m on the middle shelf beyond which they thin out at a depth of 50 60 m where relict oolitic sands are exposed the shallow seismic and echosounding records of these clays at places are marked by turbidity which may be categorised as 1 extending over a distance of 50 60 km and covering large areas of over 2500 km 2 and 2 resembling isolated inverted u shaped features the turbidity appears to rise from the underlying reflectors to within 5 10 m of the sea bed these features are very prominent in two areas viz on the middle shelf off bombay and on the inner shelf the shallow bore holes drilled and the cores collected in the area show bubbles or fissures in the clays of the turbid zone the gas predominantly consists of methane with terraces of higher hydrocarbons the composition of the gas and the occurrence of the gassy sediments on the middle shelf and in the vicinity of the estuaries and rivers favours their biogenic nature the common depth of occurrence of 5 10 m may represent the boundary between the sulfate reducing zone and the underlying methane producing zone perhaps the gases were formed by the degradation of large quantities of organic matter produced by increased productivity or because of higher influx of organic matter in the recent past however the location of some of the occurrences in the vicinity of an oil and gas field indicate that the gases in the sediments of the outer shelf off bombay may be petrogenic and those on the middle and inner shelf off bombay konkan karnataka and kerala may be biogenic

seismic and geochemical evidence for shallow gas in sediment on navarin continental margin bering sea marine seismic studies coupled with geochemical investigations demonstrate that hydrocarbon gases are ubiquitous in the near surface 250 m or 820 ft depth sediment of the navarin continental margin in the northern bering sea three types of acoustic anomalies appear to be related to the presence of gas in the sediment these anomalies are most prevalent in the northern half of the navarin basin acoustic anomalies attributed to gas hydrates and to diagenetic boundaries are present on seismic records of the lower slope between navarinsky and zhemchug canyons hydrocarbon gases methane through butanes are common in the surface 5 m or 17 ft depth sediment of the navarin continental margin methane the most abundant hydrocarbon gas is present in amounts ranging from 84 000 to 1 l l of wet sediment these concentrations are two to three orders of magnitude greater than the other hydrocarbon gases the highest concentrations of methane greater than 1 000 l l were measured in sediment of navarinsky canyon and over the central part of the navarin basin the source of methane is mainly biogenic but the hydrocarbon gas compositions in 17 of 141 cores suggest the presence of thermogenic gas most of these 17 cores are from the continental slope at water depths greater than 150 m 490 ft no direct correlation could be found between acoustic anomalies and gas concentrations in the sediment this lack of correlation is probably due to the limited penetration of the gravity corer and the spotty distribution of hydrocarbon concentrations

gas charged sediments on the inner continental shelf off western india abstract high resolution seismic reflection profiles on the inner continental shelf less than 60 m water depth off india from 10 n to 22 n show 5 35 m of weakly stratified acoustically transparent clays at water depths of 15 50 m the subbottom profiles are characterized by anomalous seismic signatures in the form of acoustic maskings these extend from the underlying late pleistocene unconformity often stretching along the bedding planes and occasionally surfacing on the sea bed these acoustic maskings could be due to shallow hydrocarbon gases mainly methane which might have been derived from biogenic sources or geothermal processes or both

lithospheric strength variations as a control on new plate boundaries examples from the northern red sea region abstract the complex plate boundary between arabia and africa at the northern end of the red sea includes the gulf of suez rift and the gulf of aqaba dead sea transform geologic evidence indicates that during the earliest phase of rifting the red sea propagated nnw towards the mediterranean sea creating the gulf of suez subsequently the majority of the relative movement between the plates shifted eastward to the dead sea transform we propose that an increase in the strength of the lithosphere across the mediterranean continental margin acted as a barrier to the propagation of the rift a new plate boundary the dead sea transform formed along a zone of minimum strength we present an analysis of lithospheric strength variations across the mediterranean continental margin the main factors controlling these variations are the geotherm crustal thickness and composition and sediment thickness the analysis predicts a characteristic strength profile at continental margins which consists of a marked increase in strength seaward of the hinge zone and a strength minimum landward of the hinge zone this strength profile also favors the creation of thin continental slivers such as the levant west of the dead sea transform and the continental promontory containing socotra island at the mouth of the gulf of aden calculations of strength variations based on changes of crustal thickness geotherm and sediment thickness can be extended to other geologic settings as well they can explain the location of rerifting events at intracratonic basins of backarc basins and of major continental strike slip zones

mesozoic hotspot epeirogeny in eastern north america geologic data especially conodont paleotemperatures imply that southeastern canada and new england have been elevated at least 4 km relative to the central appalachian region apatite fission track ages suggest that the uplift and consequent erosion occurred in cretaceous early tertiary time consistent with the formation of a broad swell as north america moved northwestward over the great meteor hotspot

the toarcian bajocian deltaic system in the north german basin subsurface mapping of ancient deltas morphology evolution and controls research focussing on the morphology and morphodynamics of modern river deltas has contributed much to the understanding of reservoir geometry and prediction of hydrocarbon splays and deep geothermal aquifers the interplay of allogenic and autogenic controls results in complex histories of modern river deltas comprising subsequent regressive and transgressive stages of delta formation the application to ancient examples is still ambiguous and in particular the detailed morphological description of ancient deltas is often hampered by limited subsurface data this basin scale study employs a data set of 15 cored wells and more than 450 logged wells in combination with lithofacies biofacies and architectural analysis to reconstruct high resolution subsurface facies and sand thickness maps of the toarcian bajocian deltaic system in the north german basin the results of this study show the evolution from smaller elongate river dominated delta types in the lower and upper toarcian 15 000 to 20 000 km and a larger lobate river dominated delta in the upper aalenian 40 000 km to a cuspate mixed river dominated and wave modified delta in the upper bajocian 25 000 km the river dominated parts are composed of subsequently formed of distributary deltaic channel belts and associated sheetsands of the delta plain and distributary mouth bar complexes with bar finger sand architectures of the delta front increasing marine processes in the bajocian resulted in modification of delta complexes forming a shoreface setting with foreshoals arc like chains of ooid bearing barrier shoals and back barrier lagoons the complex history of the toarcian bajocian deltaic system was mainly controlled by allogenic sea level fluctuations that triggered the delta progradation and subordinately by allogenic basin reorganisation autogenic process change of channel avulsion led to locally reduced sediment flux and culminated in lobe abandonment and transgressive shorelines differential compaction of underlying deltaic deposits result in strong differences in accommodation space and contributed to much to large scale delta shifts 300 km this article is protected by copyright all rights reserved

extension of structural and tectonic trends from the indian subcontinent into the eastern arabian sea abstract trends of basement highs faults and structural patterns on the india pakistan indus continental shelf and slope and in the deep waters extending to the lakshmi laxmi chagos laccadive ridge complex in the eastern arabian sea are parallel to the ancient precambrian structural and tectonic grain of the indian subcontinent these trends the occurrence of horst graben style features of the basement of the lakshmi ridge and of the region north and east of it both in deep waters and shelf areas and the oceanic nature of the basement west of the ridge suggest that the rifted transitional crust extends as far as the lakshmi chagos laccadive ridge complex the extension of rifted crust with horst graben style tectonic features into deep offshore areas of the arabian sea has implications for future hydrocarbon exploration in the region

framework of hydrocarbon generation and destruction in eastern smackover trend laminated lime mudstones of the lower member of the jurassic smackover formation are significant source rocks for crude oil across mississippi alabama and florida the source facies was deposited in an anoxic and perhaps hypersaline environment that preserved algal derived kerogen the distribution of kerogen along laminations of depositional origin and along stylolites of diagenetic origin resulted in efficient expulsion of crude oil with increasing thermal maturity crude oil initially emplaced in reservoirs was cracked to yield gas condensate and then methane rich in nonhydrocarbon gases such as hydrogen sulfide carbon dioxide and nitrogen early destruction of methane was driven by thermochemical sulfate reduction the thermal maturity framework of the smackover rend explains the distribution of hydrocarbon discoveries and suggests areas previously overlooked by exploration

migration of crude oil from the smackover source rock to jurassic and cretaceous reservoirs of the northern gulf rim abstract migration of distinctive crude oils from the upper jurassic smackover source rock across the northern gulf rim began at various times during the cretaceous regional source rock differences resulted in two compositional subfamilies of smackover crude oils type i crude oils in upper smackover reservoirs of east texas arkansas louisiana and mississippi reflect an origin from algal derived kerogen deposited in an anoxic environment type ii crude oils in upper smackover reservoirs of alabama and florida reflected an origin from kerogen preserved in a less anoxic environment with a terrestrial component comparison of molecular and carbon isotopic compositions of crude oils suggests that vertical migration of type i smackover crude oils charged many overlying upper jurassic lower cretaceous and upper cretaceous reservoirs some lateral migration to cretaceous reservoirs north of the updip limit of the smackover formation also occurred both type i smackover crude oils and geochemically dissimilar cretaceous sourced oils are found in upper cretaceous tuscaloosa and woodbine reservoirs because of impermeable barriers to migration especially in alabama vertical migration of type ii crude oils was less significant

geologic and infrastructure factors for delineating areas for clean coal examples in texas usa texas has a wide variety of areas that can be targeted for new clean coal facilities these areas are delineated by mapping spatial linkages between coal and lignite bearing formations groundwater and surface water resources and co2 sinks in brine formations for long term co2 storage or in mature oil fields with potential for enhanced oil recovery eor however a variety of infrastructure factors make it feasible to also target numerous areas outside coal and lignite basins in texas these infrastructure factors include pipelines for delivery of co2 to subsurface sinks and delivery of coal produced hydrogen to refineries ease of connection to existing transmission lines distribution of nonattainment areas where new clean coal facilities could be constructed and be compliant with strict air quality standards and railroads that can transport coal and other feedstock to new clean coal facilities primary regions in texas where favorably co located co2 source sink factors related to coal and lignite trends include the gulf coast the eastern shelf of the permian basin and the fort worth basin however areas outside coal and lignite basins particularly the permian basin where a new clean coal facility is being planned also have clean coal potential because of existing co2 pipelines and proximity to eor fields that can economically sustain new clean coal facilities

experimental study of bedrock channel alluviation under varied sediment supply and hydraulic conditions 1 channel incision into bedrock plays a critical role in mountain landscape evolution a bed that is completely alluviated cannot undergo incision as a result incision driven by the collision of bedload grains and bedrock requires transport at below capacity conditions for example where the bed is partially covered by alluvium in a mechanistic model of bedrock incision it is important to have a formula describing the relationship between the bedload transport rate and the degree to which the bed is covered by sediment the purpose of this work is to understand the mechanisms underlying the degree of bed exposure in bedrock rivers by means of an experimental study a number of experiments with various hydraulic and morphological bed conditions were performed in order to characterize this model here we focus on planar bedrock beds with some roughness the results suggest that the sediment supply channel slope hydraulic bed roughness the degree of areal coverage and thickness of antecedent gravel in the channel as well as the presence of boulders in the channel are major factors controlling bedrock exposure for sufficiently low ratios of sediment supply to transport capacity it was found that bedrock roughness can play a role in determining the degree of bedrock exposure for higher sediment ratios on the other hand the composite roughness associated with grain roughness bars and or antidunes dominates so that the underlying bedrock roughness no longer affects the degree of exposure of the bed for lower bed slopes i e less than 0 015 based on our experimental setting bedrock exposure decreases more or less linearly with increasing values of the ratio of sediment supply rate to capacity rate for higher bed slopes i e more than 0 015 there is a range of lower values of the ratio of sediment supply rate to capacity rate where a bedrock surface becomes fully exposed without any alluvial deposit for a given bedrock roughness this range expands solely with increasing slope regardless of shear stress within the upper range of values of the ratio of sediment supply to capacity rate a linear relationship between the degree of bedrock exposure and this supply to capacity ratio still prevails the addition of model boulders can be viewed as very high hydraulic bed roughness to the channel has been found to suppress the overexposure of bedrock and so restore a linear relation between the degree of exposure and the ratio of sediment supply rate to capacity rate formulations for estimating bedrock exposure as a function of sediment supply to capacity ratio under different river channel characteristics are proposed a linear relation between the degree of alluvial cover and the ratio of sediment supply rate to capacity rate is a previously proposed model the present study expands the result for other cases including runaway alluviation some landscape evolution models assume an abrupt shift between fully exposed bedrock and complete alluviation whereas others assume the linear relation the present study shows that both formulations are valid under different settings of river characteristics

relaxation of continental lithosphere an explanation for late cretaceous reactivation of the sabine uplift of louisiana texas with few unconstrained assumptions a simple quantitative model for flexural isostasy between two crustal blocks of different thickness that subsequently undergoes partial relaxation of accumulated stress can explain reactivation of the sabine uplift in the mid cretaceous thin salt over the sabine uplift indicates that it was a positive area in the middle jurassic but began to subside during the late jurassic during the late jurassic and early cretaceous the sabine uplift had no topographic expression and was the center of a large flat bottomed basin covering most of east texas and north louisiana reactivation of the sabine uplift in the mid cretaceous caused a minimum of 150 m of uplift and extensive erosion of lower cretaceous rocks during the late cretaceous and tertiary the sabine uplift resumed subsiding but at a slower rate than the adjacent basins since the mid cretaceous the sabine uplift has risen more than 1 km relative to the east texas basin we suggest that this structural history is consistent with the following simple quantitative model for flexural isostasy during formation of the gulf of mexico in the late triassic early jurassic by rifting and extension of the lithosphere brittle deformation of the crust created small scale wavelength of tens of kilometers lateral variations in crustal thickness initially the newly formed margin was in point wise airy isostatic equilibrium thus synrift and or early postrift subsidence was characterized by a series of rift valleys or half grabens separated by uplifts however as the margin cooled and contracted the lithosphere became stronger and subsequent loading was regionally compensated thus postrift subsidence was characterized by a broad regional downwarp if at some later point in time the rigid portion of the lithosphere was weakened or relaxed then lateral density variations would have been recompensated at shorter wavelengths thus areas of little or no crustal extension would have been uplifted and areas of high extension would have experienced additional subsidence gravity and total tectonic subsidence interpretations agree that the sabine uplift region is underlain by thicker crust than the adjacent east texas and north louisiana basins possible mechanisms for relaxation of continental lithosphere are thermal rejuvenation horizontal in plane stress viscous relaxation and extensional faulting

primary migration by oil generation microfracturing in low permeability source rocks application to the austin chalk texas fracturing of low permeability source rocks is induced by pore pressure changes caused by the conversion of organic matter to less dense fluids oil and gas these fractures increase the permeability and provide pathways for hydrocarbon migration an equation for the pressure change is derived using four major assumptions 1 the permeability of the source rock is negligibly small 0 01 d 10 20 m2 so that the pore pressure buildup by the conversion is much faster than its dissipation by pore fluid flow 2 the stress state is isotropic so that horizontal and vertical stresses are equal the source rock fails when the pore pressure equals the overburden pressure 3 the properties of the rock organic matter and fluids remain constant during oil generation this assumption is valid when the change in depth i e pressure and temperature is small 4 only two reaction rates are required for the conversions a low temperature reaction rate for the kerogen oil conversion e approx 24 kcal mol a approx 1014 m y and a high temperature reaction rate for oil gas conversion e approx 52 kcal mol a approx 5 5 1026 m y the equations for generation rate and pressure change are applied to the austin source rock by adjusting the several variables to fit geochemical data core saturations and observed levels of oil and gas production this application demonstrates that the equations are easily applied in calculating depths of primary migration for low permeability source rocks

shallow gas charged sediments off the indian west coast genesis and distribution abstract geophysical and geochemical surveys were carried out off goa central west coast of india to understand the genesis and distribution of shallow gases in marine sediments shallow gas charged sediments within the water depths of 15 to 40 m are reported all along the west coast and are characterized by gas masking high amplitude and reverse polarity reflections in high resolution seismic sparker data attributed to the presence of free gas these high amplitude reflections gas front are observed within a holocene fine grained wedge shaped sediment package overlying the maximum flooding surface the gas front lies between 1 2 and 5 m below the seabed and demarcates the transition from gas in the dissolved phase to bubble phase when the gas concentration exceeds the saturation level the observed and extrapolated sulfate methane transition zone smtz lies between 0 7 and 2 25 m below the seabed and it is related to the depth of the gas front methane concentration reaches saturation below the smtz and produces a bubble phase which lowers the p wave velocity and produces high amplitude reflection observed in the seismic data depleted carbon isotope ratios as low as 88 9 suggest a microbial origin for the methane in the study area high sulfate reduction rate high concentrations of pore water methane phosphate and ammonium in the shallowest site sasu123 1 at 14 7 m water depth suggest an enhanced availability of reactive organic matter required for microbially mediated biodegradation processes high gas flux in the shallow waters is also indicated by the shallowing of the gas front and the widening of the gas masked zone towards shore the present investigations do not testify the origin of holocene shallow gases either from buried channel deposits or the pleistocene sediments

hydrogeology sequence stratigraphy and diagenesis in the paleoproterozoic western thelon basin influences on unconformity related uranium mineralization abstract the paleoproterozoic statherian thelon formation consists of unmetamorphosed sandstone and conglomerate that was deposited in the thelon basin a continental basin in northern canada this study presents the first detailed sedimentological and sequence stratigraphic analysis of the expansive and under studied western thelon basin and its relation to uranium mineralizing processes in the vicinity of the boomerang lake unconformity related u prospect four lithofacies comprise the thelon formation in the western thelon basin consisting of breccia conglomerate sandstone and minor mudstone these are grouped into four facies associations that represent deposition in alluvial fans and braided fluvial streams three stratigraphic sequences were identified based on the recognition of widespread surfaces that mark significant periods of subaerial exposure or changes in depositional environment these unconformity bounded sequences are composite in nature and can be subdivided into six high frequency sequences hfs low accommodation and high accommodation systems tracts are delineated instead of the traditional systems tracts that are linked to changes in relative sea level as no marine deposits are recognized in the thelon formation two significant strataform paleohydrogeological units were distinguished in the thelon formation based on the relative timing of key diagenetic events such as compaction grain dissolution and cementation diagenetic aquicludes became isolated from basinal fluid flow due to stylolite catalyzed early diagenetic quartz cementation and occlusion of secondary intergranular macroporosity by kaolinite and muscovite and minor dickite during later peak diagenesis diagenetic aquifers conducted u bearing basinal fluids throughout diagenesis and are marked by an abundance of peak diagenetic dickite sequence stratigraphic correlation indicates that diagenetic aquicludes which were not effective at conducting u bearing basinal fluids overlie the unconformity at the boomerang lake prospect and extend along the entire strike length of a related exploration trend in contrast thick diagenetic aquifers lie approximately 10 km to the north of the boomerang lake prospect along a different exploration trend the western and eastern portions of the thelon basin experienced depositional environments and diagenetic conditions distinct from one another suggesting that they were separate sub basins the western thelon basin contains fewer of the well cemented facies that compartmentalized basinal fluid flow than the eastern thelon basin and contains a kaolin dominated diagenetic mineral assemblage rather than a muscovite dominated assemblage that is common in the eastern thelon basin in these regards the western thelon basin evolved more similarly to the eastern portion of the athabasca basin where thick regional kaolin rich diagenetic aquifers were fluid conduits for the movement of the u rich fluids that formed high grade unconformity related u deposits

new insights into the mechanics of fluvial bedrock erosion through flume experiments and theory river incision into bedrock drives the topographic evolution of mountainous terrain and may link climate tectonics and topography over geologic time scales despite its importance the mechanics of bedrock erosion are not well understood because channel form river hydraulics sediment transport and erosion mechanics coevolve over relatively long time scales that prevent direct observations and because erosive events occur intermittently and are difficult and dangerous to measure herein we synthesize how flume experiments using erodible bedrock simulants are filling these knowledge gaps by effectively accelerating the pace of landscape evolution under reduced scale in the laboratory we also build on this work by providing new theory for rock resistance to abrasion thresholds for plucking by vertical entrainment sliding and toppling and by assessing bedrock analog materials bedrock erosion experiments in the last 15 years reveal that the efficiency of rock abrasion scales inversely with the square of rock tensile strength sediment supply has a dominant control over bed roughness and abrasion rates suspended sediment is an efficient agent of erosion and feedbacks with channel form and roughness strongly influence erosion rates erodibility comparisons across rock concrete ice and foam indicate that for a given tensile strength abrasion rates are insensitive to elasticity the few experiments that have been conducted on erosion by plucking highlight the importance of block protrusion height above the river bed and the dominance of block sliding and toppling at knickpoints these observations are consistent with new theory for the threshold shields stress to initiate plucking which also suggests that erosion rates in sliding and toppling dominated rivers are likely transport limited major knowledge gaps remain in the processes of erosion via plucking of bedrock blocks where joints are not river bed parallel waterfall erosion by toppling and plunge pool erosion feedbacks between weathering and physical erosion erosional bedforms and morphodynamic feedbacks between channel form and erosion rates despite scaling challenges flume experiments continue to provide much needed tests of existing bedrock erosion theory force development of new theory and yield insight into the mechanics of landscapes

the northern gulf of mexico during oae2 and the relationship between water depth and black shale development despite their name oceanic anoxic events oaes are not periods of uniform anoxia and black shale deposition in ancient oceans shelf environments account for the majority of productivity and organic carbon burial in the modern ocean and this was likely true in the cretaceous as well however it is unlikely that the mechanisms for such an increase were uniform across all shelf environments some like the northwest margin of africa were characterized by strong upwelling but what might drive enhanced productivity on shelves not geographically suited for upwelling to address this we use micropaleontology carbon isotopes and sedimentology to present the first record of oceanic anoxic event 2 oae2 from the northern gulf of mexico shelf here oae2 occurred during the deposition of the well oxygenated inner neritic lower estuarine lower tuscaloosa sandstone the overlying organic rich oxygen poor marine tuscaloosa shale is entirely turonian in age we trace organic matter enrichment from the spinks core into the deepwater gulf of mexico where wireline log calculations and public geochemical data indicate organic enrichment and anoxia throughout the cenomanian turonian boundary interval redox change and organic matter preservation across the gulf of mexico shelf were driven by sea level rise prior to the early turonian highstand which caused the advection of nutrient rich oxygen poor waters onto the shelf this results in organic matter mass accumulation rates 1 2 orders of magnitude lower than upwelling sites like the nw african margin but it likely occurred over a much larger geographic area suggesting that sea level rise was an important component of the overall increase in carbon burial during oae2

some northern sources of atmospheric methane production history and future implications northern sources including wetlands and perhaps gas hydrates contribute significantly to the ch4 content of the atmosphere methane production from northern wetlands including bogs swamps and ponds is probably very seasonal being most important in late summer with significant evasion in autumn as lakes overturn the strong recovery of beaver populations in canada from near extinction 50 years ago to present abundance may also be important both in creating new wetlands and in the alteration of them wetlands that have been altered by beaver activity produce orders of magnitude more methane than beaver free wetlands in the arctic methane gas hydrates represent a significant source of methane which may become more important if arctic warming occurs as part of global climate change the danger of a thermal runway caused by ch4 release from permafrost is minor but real other high latitude sources of ch4 include arctic peat bogs and losses from natural gas production especially in the soviet u

heat transfer and intraplate deformation in the central indian ocean nineteen new heat flow measurements made across deformed oceanic lithosphere in the central indian ocean and previously published data show that heat flow is significantly higher than predicted by models for cooling oceanic lithosphere over much of the region many of the temperature depth profiles are nonlinear upward convection of water is the most likely explanation for the curvature of the temperature profiles since other possible causes including variations in bottom water temperatures conductivity changes with depth in the sediments and experimental error can be eliminated this interpretation requires water velocities of the order of 7 x 10 sup 8 m s which is unusual because the lithosphere is relatively old 72 82 m y and a thick sedimentary cover 1 2 5 km is present these observations suggest that the processes causing deformation of the plate have increased the heat flux through the sediment water interface we infer that extra heat is being generated at shallow depths perhaps less than 35 km in the plate although the specific mechanism by which deformational energy is converted into heat is difficult to determine

selective sorting and abrasion of river gravel i theory most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers it has been surmised that this downstream fining is due to some combination of selective sorting by which finer grains are preferentially transported downstream and abrasion by which individual particles are reduced in size here a framework for the simultaneous treatment of both phenomena is developed the analysis is restricted to bed load transport of gravel only abrasion due to binary collisions between moving bed load and stationary bed particles is considered the formulation includes the possibility of differential basin uplift and subsidence due to tectonism and mixtures of rock types with differing coefficients of abrasion the formulation of selective sorting is based on the concept of hiding as applied to a surface based bed load transport relation abrasion is first treated in terms of a lagrangian formulation for individual grains and t

steady propagation of delamination events delamination of the lithospheric thermal boundary from overlying continental crust propagates laterally from the line of initiation accelerating as the sinking slab of detached lithosphere grows longer this propagation has been numerically modeled with steady state equations in a moving reference frame by matching an interior finite element solution to flexible boundary conditions which represent the mechanical and thermal response of the surroundings the form of the solution depends on the shear coupling of intruding asthenosphere to the top of the sinking slab across a thin layer of crustal material without coupling the tip of the intrusion cools and stiffens to form a wedge dividing the crust cold mode with coupling the intrusion is forced to convect and remains ductile hot mode the cold mode can propagate at all velocities the hot mode has a lower limiting velocity of 1 2 cm year but offers less resistance at higher speeds resistance to delamination includes a constant term from the buoyant crustal downwarp plus a velocity proportional term representing viscous deformation however the proportionality constant of the latter term is only weakly dependent on crust and lithosphere viscosities matching this resistance to loading lines of 100 to 800 km slabs sinking in a mantle of 1022 p velocities of 0 3 8 0 cm year are obtained changes in viscosity affect this rate but cold mode delamination is unstoppable except at continental margins or by failure in the sinking slab the surface expression of delamination is a leading outer rise followed by a submarine trough with a large negative free air anomaly which finally evolves into a 1 km plateau if crustal viscosity and velocity are both low however there is a montonic crustal uplift with no trough thus the present lack of linear supracontinental oceans does not preclude delamination at up to 4 cm year driven by slabs up to 400 km in length

a multidisciplinary approach in determining the maceral kerogen type and mineralogical composition of upper cretaceous eagle ford formation impact on pore development and pore size distribution abstract maceral kerogen or organic matter type and mineralogical composition with the associated porosity of the upper cretaceous eagle ford formation karnes county texas was investigated in karnes county the calcareous mudstone of the eagle ford formation was deposited in an open marine outer shelf to upper slope environment to adequately characterise all components of this mudrock a multidisciplinary approach was implemented to analyse 26 core samples from a single producing well that completed within the eagle ford formation analyses included organic petrology organic geochemistry light and electron microscopy and spectroscopy pore size distribution analyses and x ray diffraction the bitumen reflectance ranges between 1 94 and 2 08 calculated vr o 1 60 1 69 indicating the mudrock is in the condensate rich gas window the toc content ranges between 1 7 and 7 2 with an average of 3 9 mineral composition is dominated in descending order by calcite clay quartz and plagioclase calcite varies between 32 and 87 with an average of 57 porosity measured by helium pycnometry ranges between 3 5 and 10 3 with an average of 7 5 organic petrology coupled with backscattered and field emission scanning electron microscopy indicates matrix bitumen as the most significant contributor to the toc content and porosity matrix bitumen contains significant meso and microporosity and is enhanced by the secondary cracking of the bitumen an increase in bioclastic mostly foraminifera and siliciclastic components correlates with lower toc content due to the lower volume of organic rich matrix peloids fecal pellets are calcareous organic rich and lack terrigenous sediment indicating a zooplankton source pelagic deposition of peloids is interpreted as a primary mechanism of amorphinite deposition which is converted to hydrocarbons and matrix bitumen the mixed siliciclastic calcareous matrix that surrounds the peloids is comprised of matrix bitumen and dispersed organics from land plants vitrinite fusinite and inertodetrinite the siliciclastic component increases in the middle of the eagle ford and marks the boundary maximum flooding surface between the lower tst and upper hst section of the formation the inorganic and organic components suggest a mixed terrigenous and marine source with both inorganic and organic sediments being derived from the proximal ouachita thrust belt and from pelagic organisms within the overlying water column the lower tst eagle ford formation contains more micro and mesopores than the upper hst eagle ford formation due to higher toc content in the lower eagle formation these results illustrate that a multidisciplinary approach is necessary to identify how pore network are distributed within mudrocks and how this distribution is controlled by organic inorganic deposition and hydrocarbon generation processes

geology and hydrocarbon accumulations columbus basin offshore trinidad the columbus basin situated on the eastern shelf of trinidad lies at the eastern extremity of a belt of severe deformation along the northern boundary of south america that has been affected by compressional and wrench tectonics in pliocene pleistocene time two major structural trends are present in the columbus basin a series of east northeast trending anticlines and north northwest oriented normal faults the basin was filled during late miocene to holocene time with sediments deposited by an ancestral orinoco river draining a hinterland to the southwest the pliocene pleistocene section which contains the hydrocarbon accumulations in the columbus basin was deposited in three coarsening upward sedimentary sequences followed by a late pleistocene transgressive sequ nce traps for hydrocarbon accumulation were formed by an easterly trending pliocene pleistocene wrench system with associated east northeast oriented anticlines combined with north northwest oriented normal faults oil was sourced in the late miocene lower cruse formation whereas gas was derived both from pliocene pleistocene pro delta shales and as a late high temperature phase of lower cruse hydrocarbon generation the north northwest faults formed migration conduits from the oil source rock to pliocene pleistocene reservoirs the temporal relationship of faulting to oil generation is a major factor affecting the distribution of oil and gas the size of hydrocarbon accumulations is limited to some extent by a lack of an effective hydrocarbon seal particularly in the western half of th basin

geology and hydrocarbon accumulations columbus basin offshore trinidad abstract the columbus basin on the eastern shelf of trinidad lies at the eastern extremity of a belt of severe deformation along the northern boundary of south america that has been affected by compressional and wrench tectonics in the pliocene pleistocene two major structural trends are present in the columbus basin a series of ene trending anticlines and nnw oriented normal faults the basin was filled during the late miocene to holocene with sediments deposited by an ancestral orinoco river draining a hinterland to the southwest the pliocene pleistocene section which contains the hydrocarbon accumulations in the columbus basin was laid down in three coarsening upward sedimentary sequences followed by a late pleistocene transgressive sequence traps for hydrocarbon accumulation were formed by an easterly trending pliocene pleistocene wrench system with associated ene oriented anticlines combined with nnw oriented normal faults oil was sourced in the late miocene lower cruse formation whereas gas was derived both from pliocene pleistocene pro delta shales and as a late high temperature phase of lower cruse hydrocarbon generation the nnw faults formed migration conduits from the oil source rock to pliocene pleistocene reservoirs the temporal relationship of faulting to oil generation is a major factor affecting the distribution of oil and gas the size of hydrocarbon accumulations is limited to some extent by a lack of an effective hydrocarbon seal particularly in the western half of the basin end of article last page 500

hydrocarbon generation and migration routes in the east texas basin the east texas basin is a prolific mature hydrocarbon province producing oil and gas from several reservoirs and a variety of trap types many of the liquid hydrocarbons discovered in the basin are trapped in structures related to movement of the underlying louann salt by determining the structural evolution of the basin we constructed a framework to model the generation of hydrocarbons in the basin geochemical data indicate three major oil types jurassic oil lower cretaceous oil and upper cretaceous oil the jurassic source is mature throughout the basin and began to expel oil at approximately 88 ma the distribution of jurassic oil in cretaceous reservoirs shows that vertical migration routes predominated prospective lower cretaceous source rocks are only matur in the deep central portion of the basin where expulsion began about 47 ma distribution of this oil type suggests that lower cretaceous source rocks occur only in localized areas of the east texas basin organic rich upper cretaceous shales are immature in the main part of the basin but are mature south of the angelina caldwell flexure where they reached peak generation at approximately 20 ma long distance lateral migration routes are necessary to explain the distribution of this oil type migration routes to the giant east texas field may be 100 km or more modeling of this basin suggests an exploration approach in mature basins of defining migration pathways and seeking traps astride them traps in this position have a better probability of being filled and all else being equal are likely to be better fields than traps located away from the major migration routes

sulfidization processes in seasonally hypoxic shelf sediments a study off the west coast of india abstract continental shelves are the major sites for organic carbon burial and sulfate reduction organic carbon degradation and sulfidization along continental shelves are expected to play a significant role in global carbon sulfur iron c s fe cycles here we report sediment pore water chemistry and iron sulfur speciation in two short cores ssk 42 9 and 10 collected off malvan from the seasonally hypoxic shelf region off the west coast of india wci at water depths of 30 and 13 m respectively concentration profiles of pore water so42 nh4 dic hs and depth integrated sulfate reduction rates jso4 suggest a variable influence of sedimentation rates the composition of organic matter marine and terrestrial and anaerobic oxidation of methane aom on the pore fluid chemistry chromium reducible sulfur crs and organic bound sulfur obs are the detectable solid phase sulfur species in the studied sediment cores sulfur content and isotope ratios of chromium reducible sulfur crs and organic bound sulfur obs phases produced via hs sx2 pathways sulfidization and sulfurization show contrasting profiles in ssk42 9 and 10 apparently influenced by the availability of reactive iron the relative significance of early and late diagenetic processes source of obs detritus of marine origin and sulfurized organic molecules and diffusion of hs produced via aom across the smtz the marked influence of aom driven sulfate reduction and the diffusion of isotopically enriched hydrogen sulfide across the sulfate methane transition zone smtz is apparent from the 34scrs profiles in ssk 42 10 sediment toc ts ratios recorded in this study are significantly less than that of average modern marine surface sediments 2 8 1 underlying oxygenated water and this is attributed to the enhanced sulfidization and burial of iron sulfur minerals in the seasonally hypoxic regions since the inner shelf of wci experiences seasonal alternation from normoxia to hypoxia which may have a profound influence on the benthic community structure nature and depth of bioturbation bioirrigation we propose that the impact of these processes need to be studied at a significantly higher resolution for better understanding c s fe biogeochemical cycle

nearshore sediment characteristics and formation of mudbanks along the kerala coast southwest india in order to gain insight into the formation dynamics of mudbanks off the kerala coast of india extensive surveying of the nearshore bathymetry along with sediment characterization was undertaken the textural and geotechnical properties of the surficial sediments of a mudbank were determined during pre monsoon monsoon and post monsoon periods the mudbank sediments were clayey silts with high water and organic carbon contents high atterberg limits and low bulk density and therefore very susceptible to entrainment during the monsoon the mudbank regime was characterised by enhanced turbidity and a benthic fluff layer triggered by the increasing swell of the early monsoon period re suspension exposed a more consolidated previously sub bottom layer which exhibited lower water content and greater shear strength than the pre monsoon seabed texturally the monsoon seabed was similar to the pre monsoon seabed with the same modal grain size but the proportions of sand and coarse silt increased nearshore while the proportions of fine and very fine silt increased offshore there was a seaward fining textural gradient at all times but this became pronounced during the monsoon period paradoxically the monsoonal seabed displayed greatly reduced wet bulk density it is hypothesized that this was due to the presence of gas probably methane in the sediments while the pre monsoon sediments were fully saturated the monsoon sediments were only 83 saturated we speculate that the gas was forced into the surficial sediments either by wave pumping at the onset of the monsoon or by seaward flowing subbottom freshwater derived from monsoonal rains with the waning of the monsoon the benthic fluid mud layer rapidly disappeared and the seabed returned to its pre monsoon state as suspended sediments were redeposited the mudbank regime is therefore essentially an in situ phenomenon it is suggested that the mudbanks are palimpsest marshy lagoonal deposits rich in organic matter and derived gas that were submerged after a marine transgression the surficial sediment is annually entrained during the monsoon but erosion is limited by the formation of the benthic fluid mud layer which attenuates wave generated turbulence although some fine sediment disperses alongshore and offshore most is returned to the seabed as the monsoon declines

seafloor gas seeps triggered by a northern california earthquake abstract a series of marine surveys using high resolution seismic reflection and side scan sonar profiling was conducted on the klamath river delta from 1977 to 1985 in november 1980 near the middle of this 8 year time period a major earthquake magnitude 7 occurred in the area the earthquake caused liquefaction of shelf deposits off the klamath river and while mapping the sediment failures we observed an inordinate number of gas vents and acoustic anomalies in the sediment gas charging and water column gas clouds our analyses indicate that a change in gas seepage occurred after the earthquake there was a marked increase in gas abundance immediately after the earthquake a gradual decrease over the next year and stabilization at pre earthquake gas levels within five years or less the acoustic anomalies were observed in pre quaternary rocks as well as in surficial holocene deposits and gas seeps overlay truncated anticlines faults and syncline exposures thus the gas released may have been in part thermogenic in origin a large area 20 km 2 on the delta failed during the earthquake and the abundance of gas seeps and craters in the area suggests that sediment degassing may have been an important process contributing to failure

convective thinning of the lithosphere a mechanism for the initiation of continental rifting thinning of the lithosphere by heat carried convectively to its base is a possible mechanism for the initiation of continental rifting the lithosphere asthenosphere boundary marks the transition from a quasi rigid behavior of the mantle on geological time scales to a viscous one the boundary lies at or close to the solidus of the mantle material and its position is a function of temperature and pressure additional heat carried to the boundary will cause it to rise and the lithosphere to thin a one dimensional model is used to study the upward movement of the boundary from an initial equilibrium position as a consequence of an increase in the heat flux from the convecting mantle the ratio of the amount of thinning to the initial thickness is directly proportional to the increase in heat flux from the asthenosphere and inversely proportional to the difference between the final subcrustal geothermal gradient and the slope of the transition temperature profile the rate of thinning is determined by the initial and final equilibrium thicknesses the thermal diffusivity the transition temperature profile and the plume temperature profile it depends only slightly on any latent heat associated with the lithosphere asthenosphere transition during a large fraction of the time between the initial and final equilibrium states the lithosphere thins at a rate which is inversely proportional to the square root of time t even 300 km thick lithospheres may be thinned to 50 km or crustal thicknesses in tens of million years by the heat advected by mantle plumes this heat contributes to an enhanced surface heat flow because of the constant level of radioactive heat production in the crust the ratio of the final to the initial surface heat flow increases much less than the ratio of the final to the initial heat flow from the asthenosphere for large increases in asthenospheric heat flow the lithosphere is almost thinned to the crust before any significant change of surface heat flow occurs uplift due to thermal expansion upon lithospheric thinning is of the order of 102 103 m a rayleigh taylor instability acting on the thinning lithosphere may further increase thinning rates and cause magmatic intrusions of the lithosphere and surface volcanism

a seeping sea floor in a ria environment ria de vigo nw spain the occurrence of gas accumulations in the ria de vigo nw spain have been characterized by the authors in previous research pockmarks frequently appear on seismic and sonar records covering ca 45 of the sea floor of the study area which indicates that gas expulsion is not an uncommon phenomena in the coastal ria environment here we report the occurrence of gas seepage for the first time along the coast of nw spain side scan sonar echo sounder and high resolution seismic techniques were used for mapping gas expulsion features some expulsion pockmarks sit over elongated features that represent bottom marks created by anthropogenic activity thus these anthropogenic sites may act as preferential venting zones for gas as well as being potential hazards on a muddy sea bed such as that of the ria where gas accumulates just below the surface of the sea floor

generation migration and entrapment of hydrocarbons on southern norwegian shelf the southern norwegian shelf 56 degree 58 degree 30 n has proved to be one of the most prolific hydrocarbon provinces in europe recoverable reserves of about 4 8 billion bbl of oil plus oil equivalent are found predominantly in upper cretaceous and danian age chalk and jurassic sandstones the excellent quality of seismic data moderate to dense well control and the detailed geochemical evaluations of the kimmeridge clay the principal source rock make the southern norwegian shelf an ideal area for hydrocarbon generation studies a geochemical analysis of the area contained five basic steps 1 construction of a nine layered three dimensional grid summarizing the burial history of the sediments by using well control and seismic data 2 calculation of geothermal gradients 3 source rock analysis to investigate variations in thickness and richness of the kimmerdige clay study area 4 maturation study based on the results of the three initial steps and 5 volumetric analysis and prospect evaluation major factors affecting hydrocarbon accumulations in the structural and stratigraphic closures of the study area include the amount of oil generated within the catchment areas of the various closures and particularly in the case of the chalk fields the level of hydrocarbon generation of the kimmeridge clay directly under themore crest of the structure and presence of faults to act as conduits for vertical migration of hydrocarbons from the upper jurassic kimmeridge clay to the upper cretaceous danian chalk the expulsion and migration efficiency hydrocarbons in place hydrocarbons generated within the catchment area of individual closures for tested closures average approximately 8 for the study area with individual culminations having values as high as 40 or as low as 0 based in part on the previously mentioned factors less

geology of the campanian m1 sandstone oil reservoir of eastern ecuador a delta system sourced from the amazon craton abstract the campanian m1 sandstone member of the napo formation is one of the main reservoirs of the capiron tiputini oil play system of the eastern part of the oriente basin of ecuador this study presents a geological model for this reservoir at the apaika and nenke oil fields included within the block 31 and the eden yuturi oil field operated by petroamazonas ep the geological characterization of the m1 sandstone is based on an integrated study which includes core description electric log analyses and seismic data interpretation the sedimentological descriptions from available cored sections show that m1 sandstone records a variety of fluvial to shallow marine processes including mass flow and delta front turbidity currents not previously documented the stratal stacking pattern interpreted from electric logs indicates that the succession reflects deltaic progradation rather than the backstepping pattern typically displayed by the underlying albian to santonian sediments of the napo formation seismic rms amplitude defines clinoform geometries with sigmoidal sand body architecture prograding from east to west correlation with neighboring regions in colombia suggest that the m1 sandstone was probably part of a large river dominated delta system sourced from the east during major campanian progradation the location of the source area of sediment was determined using u pb detrital zircon dating provenance ages are uniform within the m1 sandstone cluster in a population that ranges from 1 4 to 1 6 billion years these ages indicate that sands were derived mostly from erosion of the lithotectonic rio negro jurena province located to the northeast of the studied area detrital ages from the overlying maastrichtian tena formation include ages between 80 and 200 ma pointing to an important change in the source areas located in the eastern cordillera of ecuador the change in sediment supply from east to west recorded in the tena formation coincide with the initial episodes of the andean orogeny and may be driven by the collision of fragments of the caribbean large igneous province clip against the continental margin of northwestern south america

late pleistocene holocene sediments on the spanish continental shelves model for very high resolution sequence stratigraphy abstract sequence stratigraphy analysis of high resolution seismic profiles geopulse uniboom and 3 5 khz of late pleistocene holocene sediments has been carried out on five sectors of the spanish continental margin four types of depositional settings are distinguished in these sectors 1 low subsident ramps alboran margin cadiz gulf 2 high subsident 2m kyr ramps alicante valencia 3 ria type morphology on the atlantic passive margin ria de muros and 4 fault scarp morphology systems with subsidence balearic margin a type 1 sequence is interpreted in all these sectors being composed of lowstand systems tract transgressive systems tract and highstand systems tract this conforms to the basic concepts of sequence stratigraphy and each systems tract correlates with a particular part of the last eustatic hemicycle characteristic shelf features such as terraces terraces with beach deposits and progradational sediment wedges evidence a complex stacking of lesser sub sequences in all the systems tracts which must be related to very short period sea level stillstands and fall we propose a very high resolution sequence stratigraphy model in which the last sea level hemicycle is punctuated by p cycles 4500 years which give rise to the neo glacial events h cycles 2200 950 years and c cycles 500 50 years these cycles interact with each other thus establishing the placing of the high and low sea levels this attention to detail 1 explains sedimentary evolution on both the shelf and upper slope during late pleistocene holocene time 2 illustrates some departures from the classical sequence stratigraphy model and 3 also demonstrates that the late pleistocene holocene eustatic curve is not one simple transgression but is modulated by three differing period cycle groups below the milankovitch band our model is delimited by fluctuating sea level during pleistocene holocene times such features should be identifiable on any continental margin however localized features occur due to subsidence and continental shelf morphology which determ ine the location and depth of sedimentary bodies generated in each eustatic cycle

evolution of salt structures east texas diapir province part 2 patterns and rates of halokinesis the effects of salt mobilization on aptian and younger post 112 ma strata in the east texas basin can be used to illustrate patterns of dome growth through time and to estimate rates and amounts of salt movement pre aptian domes grew in three areas around the margin of the diapir province apparently in pre aptian depocenters maximum dome growth along the basin axis coincided with maximum regional sedimentation there during the middle cretaceous aptian albian and cenomanian in the late cretaceous the sites of maximum diapirism migrated to the periphery of the diapir province diapirism began after pillows were erosionally breached leading to salt extrusion and formation of peripheral sinks the duration of pillow and diapir stages of growth was subequal ranging from 10 to 30 ma post diapiric stage of growth continued for more than 112 ma in some cases diapirs grew fastest in the early cretaceous when peak growth rates ranged from 150 to 530 m ma 490 to 1 740 ft ma declining in the early tertiary to 10 to 60 m ma 30 to 200 ft ma assuming steady state conditions over periods of 1 to 17 ma strain rates for the rise of the east texas diapirs averaged 6 7 10 16 sec peak gross rate of growth averaged 2 3 10 15 sec similar to slow orogenic rates the evolution of east texas salt domes essentially ended in the early tertiary with uplift rates less than 30 m ma 100 ft ma

field instrumentation for high resolution parallel monitoring of bedrock erosion and bedload transport river incision is fundamental in shaping the earth s surface in mountainous regions with steep river beds fluvial bedrock erosion by bedload transport is an important mechanism forming channels however there are only a few complete field datasets that can be used to improve process understanding and evaluate erosion models especially at the process scale to provide a simultaneous dataset of hydraulics bedload transport and bedrock erosion at high temporal and spatial resolution a new measuring device has been installed in the erlenbach a gauged stream in the swiss pre alps in this stream bedload transport rates can be calculated from surveying deposits and from geophone plate sensors and bedload transport samples can be taken directly by an automated moving basket system to measure bedrock erosion rates simultaneously two natural stone slabs were mounted flush with the channel bed in a steel frame hosting various measurement devices force sensors below the slabs record normal stress and shear stress at a point erosion rates on the slab surfaces are continuously measured at sub millimetre precision at three locations on each slab in addition the slab topography is monitored following erosive flood events in this article i the erosion scale device is described ii data resolution and data quality is assessed by means of tests and event data and iii the first transport event is discussed the erosion scales are confirmed to provide data at high spatio temporal resolution for process analysis the preliminary data show evidence for the tools effect in bedrock erosion the bedrock slabs can be exchanged to obtain measurements for catchments with different lithologies for comparison copyright 2014 john wiley sons ltd

assessment of undiscovered conventional oil and gas resources onshore claiborne group united states part of the northern gulf of mexico basin the middle eocene claiborne group was assessed for undiscovered conventional hydrocarbon resources using established u s geological survey assessment methodology this work was conducted as part of a 2007 assessment of paleogene neogene strata of the northern gulf of mexico basin including the united states onshore and state waters dubiel et al 2007 the assessed area is within the upper jurassic cretaceous tertiary composite total petroleum system which was defined for the assessment source rocks for claiborne oil accumulations are interpreted to be organic rich downdip shaley facies of the wilcox group and the sparta sand of the claiborne group gas accumulations may have originated from multiple sources including the jurassic smackover formation and the haynesville and bossier shales the cretaceous eagle ford and pearsall formations and the paleogene wilcox group and sparta sand hydrocarbon generation in the basin started prior to deposition of claiborne sediments and is currently ongoing primary reservoir sandstones in the claiborne group include from oldest to youngest the queen city sand cook mountain formation sparta sand yegua formation and the laterally equivalent cockfield formation a geologic model supported by spatial analysis of petroleum geology data including discovered reservoir depths thicknesses temperatures porosities permeabilities and pressures was used to divide the claiborne group into seven assessment units aus with three distinctive structural and depositional settings the three structural and depositional settings are 1 stable shelf 2 expanded fault zone and 3 slope and basin floor the seven aus are 1 lower claiborne stable shelf gas and oil 2 lower claiborne expanded fault zone gas 3 lower claiborne slope and basin floor gas 4 lower claiborne cane river 5 upper claiborne stable shelf gas and oil 6 upper claiborne expanded fault zone gas and 7 upper claiborne slope and basin floor gas based on monte carlo simulation of justified input parameters the total estimated mean undiscovered conventional hydrocarbon resources in the seven aus combined are 52 million bbl of oil 19 145 tcf of natural gas and 1 205 billion bbl of natural gas liquids this article describes the conceptual geologic model used to define the seven claiborne aus the characteristics of each au and the justification behind the input parameters used to estimate undiscovered resources for each au the great bulk of undiscovered hydrocarbon resources are predicted to be nonassociated gas and natural gas liquids contained in deep mostly 12 000 ft 3658 m present day drilling depths overpressured structurally complex outer shelf or slope and basin floor claiborne reservoirs the continuing development of these downdip objectives is expected to be the primary focus of exploration activity for the onshore middle eocene gulf coast in the coming decades

biomarkers and carbon isotopes of oils in the jurassic smackover trend of the gulf coast states u s a abstract the geochemistry of 25 oils from 23 fields in alabama and the mississippi and louisiana arkansas jurassic salt basins was evaluated results show that the oils were generated by a carbonate source rock which was deposited under highly anoxic conditions and which contains mainly marine derived organic matter the mississippi and louisiana arkansas oils are geochemically similar indicating similar depositional environments for the source although the alabama oils were also derived from a carbonate source rock they are dissimilar to the mississippi and louisiana arkansas oils terpane biomarkers suggest that in addition to marine derived organic matter the source for the alabama oils had an organic input from a more near shore paralic environment i e with a component of terrestrially derived kerogen within each area the oils are similar therefore the norphlet and upper smackover oils in alabama share a common source and the upper smackover cotton valley and some of the lower tuscaloosa oils where production is from faulted structures in mississippi also share a common source maturities of the oils in the three areas vary from low in the updip mississippi salt basin high in the louisiana arkansas salt basin to very high in portions of alabama based on the maturity of oils in mississippi oil generation and migration commenced during the cretaceous when the source was at modest levels of thermal maturity oils migrated relatively short distances into nearby reservoir rocks some oils reached high maturities in the reservoirs resulting in abundant late forming bitumen and pyrobitumen deposition in pore spaces