deformation temperatures vorticity of flow and strain symmetry in the loch eriboll mylonites nw scotland implications for the kinematic and structural evolution of the northernmost moine thrust zone

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

the enigma of oil and gas field growth growth in estimates of recovery in discovered fields is an important source of annual additions to united states proved reserves this paper examines historical field growth and presents estimates of future additions to proved reserves from fields discovered before 1992 field level data permitted the sample to be partitioned on the basis of recent field growth patterns into outlier and common field sets and analyzed separately the outlier field set accounted for less than 15 of resources yet grew proportionately six times as much as the common fields because the outlier field set contained large old heavy oil fields and old low permeability gas fields its future growth is expected to be particularly sensitive to prices a lower bound of a range of estimates of futu e growth was calculated by applying monotone growth functions computed from the common field set to all fields higher growth estimates were obtained by extrapolating growth of the common field set and assuming the outlier fields would maintain the same share of total growth that occurred from 1978 through 1991 by 2020 the two estimates for additions to reserves from pre 1992 fields are 23 and 32 billion bbl of oil in oil fields and 142 and 195 tcf of gas in gas fields

strain path partitioning within thrust sheets microstructural and petrofabric evidence from the moine thrust zone at loch eriboll northwest scotland abstract quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the loch eriboll area of the moine thrust zone and are used to interpret the detailed processes involved in fabric evolution the intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude a correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the moine thrust within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate mutually interfering shear bands globular low strain detrital quartz grains whose c axes are aligned sub parallel to the principal finite shortening direction z and quartz c axis fabrics which are symmetric both in terms of skeletal outline and intensity distribution with respect to mylonitic foliation and lineation non coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation tectonic models offering possible explanations for the presence of kinematic strain path domains within thrust sheets are considered

relationships between the loch ailsh and borralan alkaline intrusions and thrusting in the moine thrust zone southern assynt culmination nw scotland

crustal velocity structure associated with the eastern tennessee seismic zone vp and vs images based upon local earthquake tomography we present three dimensional p and s wave velocity models for the active eastern tennessee seismic zone etsz using arrival time data from more than 1000 local earthquakes a nonlinear tomography method is used that involves sequential inversion for model and hypocenter parameters we image several velocity anomalies that persist through most of the inversion volume some anomalies support the presence of known features such as an ancient rift zone in northern tennessee other anomalies reveal the presence of basement features that can be correlated with regional gravity and magnetic anomalies we image a narrow ne sw trending steeply dipping zone of low velocities that extends to a depth of at least 24 km and is associated with the vertical projection of the prominent new york alabama magnetic lineament the low velocity zone may have an apparent dip to the se at depths exceeding 15 km the low velocity zone is interpreted as a major basement fault juxtaposing granite rhyolite basement to the nw from grenville southern appalachian basement to the se relocated hypocenters align in near vertical segments suggesting reactivation of a distributed zone of deformation associated with a major strike slip fault we suggest that the etsz represents reactivation of an ancient shear zone established during formation of the super continent rodinia

comparative precambrian stratigraphy and structure along the mid continent rift the mid continent rift is a geophysically identified tectonic structure that has been traced from kansas northeastward into the lake superior district a related arm has been identified by gravity as extending from eastern lake superior southeastward into the lower peninsula of michigan this rift is of precambrian keweenawan age and began developing approximately 1 1 b y b p for most of its extent this continental class rift system is buried below phanerozoic strata outcrops are found only in east central minnesota northwestern wisconsin and the upper peninsula of michigan the most accepted tectonic model of the mid continent rift is that of a central horst partly covered by clastic rocks bounded by high angle faulting and flanked by basins filled with clastic rocks this model was developed in conjunction with field studies in the southwestern lake superior area and generally has been adopted for minnesota iowa nebraska and kansas a comparable model has yet to be presented for the related arm extending into southern michigan in the lake superior outcrop belt the oldest recognized rifted rocks are layered basalts of middle keweenawan age overlying these basalts is an upper keweenawan thick sedimentary sequence composed of the oronto older and bayfield younger groups these sedimentary rocks are almost entirely clastic with texture ranging from conglomerate to shale mudstone rock colors are principally reds and browns with secondary shades of green and white the oronto group is more immature mineralogically and texturally and more complex structurally than the bayfield group the oronto group also is differentiated by one of its units the nonesuch formation which contains organic shales that bleed liquid hydrocarbons from subsurface fractures and vugs in a michigan copper mine conditions of eposition for both groups are closely allied with an aquatic continental environment specific paleogeography ranged from alluvial fan and plain to deltaic flood plain fluvial and lacustrine southwest and southeast of the lake superior outcrop belt stratigraphic relationships are known principally from fewer than a dozen significant wells drilled into the precambrian within the mid continent rift trend analysis of the proposed upper keweenawan red clastic sedimentary rock column within these wells suggests that the lake superior stratigraphy can be provisionally correlated throughout the mid continent rift trend

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

lower tertiary and upper cretaceous source rocks in louisiana and mississippi implications to gulf of mexico crude oil the lower tertiary sparta formation wilcox group and the midway group in southern louisiana include organic rich source facies that generate crude oil at relatively high thermal maturities the wilcox group is an important source of wilcox crude oil and regional kerogen variations explain two crude oil subfamilies wilcox crude oils in downdip areas of southern louisiana migrated short distances but long range lateral migration about 150 km best explains wilcox crude oils far updip from mature source rocks crude oils in oligocene and younger reservoirs in southern louisiana migrated vertically from deep lower tertiary source rocks some crude oils in upper cretaceous tuscaloosa reservoirs were emplaced by long range lateral migration from tuscaloosa source rocks g ven little evidence of upper tertiary source rocks and the overmaturity problems of mesozoic source rocks most crude oils in upper tertiary and pleistocene reservoirs of the gulf continental shelf are best explained by vertical migration from deep lower tertiary source rocks even so it is simplistic to assume an exclusive lower tertiary origin many tertiary and pleistocene crude oils of this study probably include an overprint of high maturity hydrocarbons from mesozoic sources

new high pressure granulite event in the moine supergroup northern scotland implications for taconic early caledonian crustal evolution relict high pressure granulite facies assemblages have been discovered in mafic sheets within neoproterozoic moine supergroup psammites of northern scotland minerals and fabrics associated with the main 440 430 ma caledonian deformation and metamorphism d2 overprint these assemblages which preserve peak pressure temperature p t conditions of 11 12 kbar at 650 700 c indicating burial to 35 km the maximum age of the sheets is constrained by the post grenvillian ca 1000 ma age of the youngest detrital zircons in the host psammites the high p assemblage is constrained by the age of anatexis in the psammites at 467 10 ma assembling these data with p t and chronological data for the d2 d3 and later cooling events two partial clockwise p t t d time deformation paths emerge demonstrating that the high p event is not part of the evolution of the p granulite assemblages are interpreted to be a relict of an earlier taconic 470 460 ma history this new evidence suggests that the tectonic evolution of the caledonian nappes east of the moine thrust is more complex than previously thought

direct dating of mylonite evolution a multi disciplinary geochronological study from the moine thrust zone nw scotland rb sr dating of sub closure temperature syn deformationly crystallized white micas from the moine mylonites knockan and dundonnell has yielded ages which vary from 437 ma to 408 ma morphological and major element analyses of the micas indicate that all the micas within the analysed samples were re crystallized during the moine thrust shearing the ages yielded are therefore interpreted as marking the end of crystal plastic deformation associated with shearing on the moine thrust the variation in ages between samples is significantly greater than the individual analytical errors and may be a product of strain localization within the shear zone alternatively it may indicate flow of strontium rich fluids derived from outside the shear zone implying that feldspars may act as potentially unreliable initial isotopic reservoirs even in very high strain samples within the greenschist facies shear zones however the rb sr age are internally consistent and compatible broadly with existing geological and geochronological data for the region suggesting that even if the feldspars have not acted as perfect initial isotopic reservoirs the resulting errors were minimal the general cessation of ductile deformation at c 430 ma probably dates the general initiation of imbrication within footwall to the moine thrust the age of 430 ma is therefore likely to be a time of significant brittle displacement on the underlying ben more thrust k ar dating of the re crystallized moine mylonite micas yielded anomalously old ages in comparison to rb sr values as a result of incorporation of excess ar during shearing the degree of incorporated excess ar decreases away from the base of the shear zone into the hanging wall becoming indiscernible at c 1 5 km structurally above the base of the shear zone the distance c 1 5 km may represent the distance that fluid can have flowed out of the shear zone during deformation

the mesoproterozoic midcontinent rift system lake superior region usa abstract exposures in the lake superior region and associated geophysical evidence show that a 2000 km long rift system developed within the north american craton 1109 1087 ma the age span of most of the volcanic rocks this system is characterized by immense volumes of mafic igneous rocks mostly subaerial plateau basalts generated in two major pulses largely by a hot mantle plume a new ocean basin was nearly formed before rifting ceased perhaps due to the remote effect of the grenville continental collision to the east broad sagging subsidence combined with a system of axial half grabens separated along the length of the rift by accommodation zones provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post rift clastic sediments both along the rift axis and in basins flanking a central post volcanic horst pre rift mature quartzose sandstones imply little or no uplift prior to the onset of rift volcanism early post rift red bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings the exception is one gray to black carbon bearing lacustrine unit this early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red bed fluvial sediments with an extra basinal provenance

thermal effects and timing of thrusting in the moine thrust zone illite crystallinity studies in the foreland to the moine thrust belt show a widespread heating event which is interpreted as a thermal response to thrust sheet emplacement temperatures reached 275 c approximately 24 ma after the main thrusting events based on the relationships between syn thrusting intrusions and the thrust planes combined with radiometric dates from the moine sheet it is argued that movement on the moine thrust started between 440 and 430 ma ago contractional slip in the moine thrust belt ceased at about 425 ma ago and it is proposed that illite metamorphism attained temperatures of 275 c at about 408 ma ago

deep structure of the scottish caledonides revealed by the moist reflection profile the moine and outer isles seismic traverse moist is a deep crustal reflection profile shot at sea off the north coast of scotland in 1981 spectacular reflections are observed from the moho and from thrust zones within the caledonian fold belt and foreland deep reflection profiling of the continental crust of the united states under the direction of the cocorp group has amply demonstrated the value of this technique when applied to suitable geological problems see for example refs 1 2 a similar group the british institutes reflection profiling syndicate birps has now been set up in the uk to organize such projects funds are provided by the natural environment research council nerc through the core group based at cambridge university however a preliminary experiment of which moist is the result was organized through the institute of geological sciences igs

reviewing the configuration and extent of the midcontinent rift system abstract uncertainty exists in the configuration and extent of the midcontinent rift system mrs because of deficiencies in geophysical data and limited information from outcrops and basement drill holes additional ambiguity is caused by misunderstanding the definition of continental rifts six principal problematic regions in mapping the mrs are described gravity and magnetic data supported by drill hole and seismic reflection data show that the eastern lake superior rift segment of the mrs continues south from lake superior and connects to a much narrower rift in northern lake michigan the eastern margin of this transition is ill defined because of the lack of definitive anomalies and supporting seismic and drill hole data but is interpreted to occur near the u s canada border the rift segment in southeastern michigan intersects the grenville front and likely continues eastwards in modified form to near the boundary with canada cross cutting gravity and magnetic signatures may reflect grenvillian overthrusts near the terminus of the mrs in michigan the proposed southerly extensions of both branches of the rift system into oklahoma and ohio are based primarily on positive gravity anomalies but neither postulated extension appears to be associated with rifted troughs rather the gravity anomalies of the western branch are related to intrusive mafic rocks and those of the eastern branch are most likely related to deep crustal metamorphic rocks thrust into juxtaposition with less dense crust by grenville orogenesis recent paleomagnetic investigations in conjunction with high resolution radiometric dating imply that the mrs developed during the rapid southward movement of laurentia during a quiescent period along its eastern continental margin massive magmatic activity accompanying the rifting was likely due to rising mantle material that was displaced by subducted lithosphere along the southern margin the heated crust was made more ductile fostering rifting due to extensional stresses the nipigon embayment remains as a possible candidate for an early third branch of the mrs but current evidence is insufficient to include the fort wayne rift as part of the mrs future studies of the mrs would be well served by new age dating and high resolution seismic studies of the lithosphere

genetic stratigraphic sequences in basin analysis ii application to northwest gulf of mexico cenozoic basin the northwest gulf of mexico cenozoic sedimentary wedge illustrates the application of genetic stratigraphic sequence analysis and documents several general conclusions 1 sequences defined by regional marine flooding are the principal genetic stratigraphic units of the basin fill continental margins are characterized by repetitive episodes of basin margin offlap punctuated by periods of transgression and marine flooding of the depositional platform 2 continental margin outbuilding is concentrated at one or more shelf edge deltaic depocenters separated by interdeltaic bights depocenters remain fixed during a depositional episode but commonly relocate during transgression and flooding 3 a distinct syndepositional structural style in prograding continental margins results in sporadic uplift of a basin fringing peripheral bulge and accentuates preservation of shelf margin facies along zones of extensional normal faulting and enhanced subsidence 4 genetic stratigraphic sequences commonly reflect an evolving interplay among two or even three variables for example early cenozoic gulf sequences are most closely related to tectonic events of the intraplate source terrane which in turn affect rate and location of sediment supply and basin margin response to loading late cenozoic sequences more closely reflect proposed eustatic cycles

classification of offshore gulf of mexico oils and gas condensates five groups of petroleums can be recognized in the offshore gulf of mexico on the basis of carbon isotope and sulfur isotope ratios sulfur content and relative abundances of nickel and vanadium of 355 oils analyzed 341 96 1 are evidently closely related and cannot be subdivided on the basis of carbon or sulfur isotopes we suggest that these oils represent a single broad superfamily however two subgroups within this set are distinguishable on the basis of sulfur content and relative abundances of nickel and vanadium group 1a sigma 13 c 26 5 0 4 parts per mil sigma 34 s 5 6 2 4 parts per mil s 0 25 0 15 v ni v 0 27 0 10 and group 1b sigma 13 c 26 5 0 3 parts per mil sigma 34 s 5 9 1 2 parts per mil s 0 73 0 43 v ni v 0 67 0 07 group 1a oils are broadly distributed across the louisiana and adjacent texas continental shelf whereas group 1b oils are found along the louisiana shelf edge and continental slope group 1b oils have substantial genetic affinities to those from the campeche shelf offshore mexico of mesozoic origin the remaining 14 analyzed petroleums include supermature condensates from offshore texas and oils from offshore southern florida believed to be cogenetic with those from the jurassic smackover formation a three dimensional appraisal of geohistory suggests that petroleum generation began in the eocene in the region of the rio grande delta progressed coastwise to the mississippi delta area by the miocene and then occurred in regions of progres ively deeper water during the pliocene pleistocene

deformation thermometry based on quartz c axis fabrics and recrystallization microstructures a review abstract for quartz rich tectonites two types of deformation thermometer are currently commonly employed 1 the quartz c axis fabric opening angle thermometer that provides an estimate of deformation temperatures when fabrics were locked in during dislocation creep and dynamic recrystallization 2 the quartz recrystallization thermometer that indicates a range of likely deformation temperatures based on observed microstructures and inferred mechanisms of dynamic recrystallization a critically important caveat in applying both thermometers is the assumption that deformation temperature is the primary controlling factor in recrystallization mechanisms and fabric development however fabric opening angles and recrystallization mechanisms are also sensitive to other variables such as strain rate and water weakening in this paper the development of these thermometers is reviewed and their potential sensitivities to competing factors such as temperature strain rate water weakening and in the case of opening angles 3d strain type are discussed examples of the application of these potential thermometers to naturally deformed quartz rich rocks are given and case studies of correlations between deformation temperatures estimated by these thermometers and temperatures of synkinematic metamorphism determined by petrology based thermobarometers are highlighted in the review attention is focused on problems associated with applying these thermometers to natural deformation and examples of such problems are discussed

tidal and storm influenced sedimentation from a cambrian transgressive passive margin sequence the basal cambrian clastic sediments in northwest scotland record a transgressive sequence of barrier island to tidal shelf quartz arenites eriboll sandstone lower member and pipe rock overlain by a storm dominated interval of mixed clastic carbonate sediments fucoid beds two shallowing events interrupt this sequence the lower near the middle of the pipe rock and the upper at the top of the fucoid beds which led to the deposition of the quartz arenites of the salterella grit variations in the proportion of storm deposited facies and in the thickness of tidally generated structures suggests that tidal currents progressively increased during deposition of the quartz arenites and then decreased until deposition of the mixed clastic carbonate deposits these features suggest amplification of tides as the shelf approached a resonant width during transgression further transgression produced a wide non resonant shelf where tides were frictionally damped and storm processes were dominant similar sequences of the same age in western newfoundland eastern greenland and spitsbergen may also have been produced by the shelf bordering the north american craton responding to tidal amplification followed by frictional damping

a possible southeastern extension of the midcontinent rift system located in ohio in 1988 a stratigraphic test core drilled in southwestern ohio penetrated a previously unknown structure and sedimentologic unit core analyses disclosed a sequence of lithic arenite and siltstone of probable pre phanerozoic age deposited within an alluvial fluvial environment a seismic reflection profile across the core site shows a sequence of strong horizontal paleozoic reflectors unconformably overlying eastward dipping layered units of poor reflectivity beyond the core hole total depth a regime of excellent reflectors is seen the pre mount simon upper cambrian sedimentary sequence encountered has been defined as the type section of the middle run formation point counter analyses of typical middle run formation samples indicate that this sequence is similar in composition to middle proterozoic units associated with the midcontinent rift system in the lake superior region the newly discovered structure in ohio is also similar to half graben structures composing the lake superior basin portion of the midcontinent rift on the basis of similarities in lithology stratigraphy structure and proximity to regional gravity anomalies the middle run formation and its apparent half graben basin is proposed as evidence for the probable extension of the midcontinent rift system southward from its normally accepted termination in southeastern michigan into southwestern ohio

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

modeling hydrocarbon generation from the paleogene source rocks in liaodong bay bohai sea a study on gas potential of oil prone source rocks abstract the exploration history of bohai sea reveals that liaodong bay is relatively rich in natural gas which is mainly gas condensate however the source rocks in liaodong bay share a medium maturity level with values of ro being generally three immature paleogene source rocks were selected to complete gold tube pyrolysis experiments the experiment results reveal that the secondary gas from oil cracking can be neglected until the cumulative yields of c6 c14 compounds reach the maximum corresponding to ro 1 3 above ro 1 3 oil starts to crack to wet gases substantially based on comparative analysis of pyrolysis results and source rock geochemical parameters we constructed a hydrocarbon generation model especially for hydrocarbon gas and validated the calculated ro values for pyrolysis conditions the c1 c5 yields for source rocks at 1 3 ro in gold tube pyrolysis approximate the maximum gas yield for source rock in open system pyrolysis indicating the majority of primary gas from kerogen cracking has been generated at 1 3 ro with its yields being 156 ml g toc 100 ml g toc and 117 ml g toc for ed3 es1 and es3 source rocks respectively this suggests that source rocks with ro

fast tectonometamorphism and exhumation in the type area of the barrovian and buchan zones the beginning of the grampian episode of tectonometamorphism in scotland is dated by collision and obduction of the ballantrae ophiolite complex at 478 8 ma for the first time we have used zircon and garnet to radiometrically date the peak of the caledonian buchan and barrovian tectonometamorphism in their type areas as contemporaneous at 467 2 5 ma detrital garnet from the neighboring southern uplands flysch terrane has the same radiometric age and occurs in 465 2 5 ma molasse in the midland valley we show that the entire grampian episode lasted 15 2 5 m y and that it took fewer than 7 6 m y to expose high grade rocks following the peak of metamorphism those who study precambrian orogeny should note the brevity of this scottish tectonometamorphic episode and the speed of exhumation

the framework of hydrocarbon generation and migration gulf of mexico continental slope abstract the occurrence of large volumes of crude oil and thermogenic gas in plio pleistocene reservoirs and holocene seeps of the gulf of mexico slope argues that the process of hydrocarbon generation and migration continues at the present time calculated thermal maturity models based on deep seismic stratigraphy indicate that cretaceous and possibly early tertiary sediments are presently within the thermal maturity range for generation and expulsion of crude oil the cretaceous and early tertiary sediments are suggested as possible source rocks because younger sediments are thermally immature for crude oil generation and older sediments are thermally overmature for crude oil preservation the calculated thermal maturity profile for the slope indicates that some generation of crude oil could occur as shallow as 6 km and that liquid hydrocarbons could be preserved as deep as 9 km thermogenic gas is stable at even greater depths migration with a strong vertical component must be invoked to explain thermogenic hydrocarbons in shallow reservoirs and seeps of the slope the great depth at which liquid hydrocarbons and dry gas can be preserved in reservoirs of the gulf of mexico salt basin indicates that huge volumes of prospective section remain essentially unexplored

stratigraphic framework structural styles and seismic signatures of downdip yegua gas condensate fields central wharton county texas gulf coast abstract overpressured yegua gas condensate reservoirs in central wharton county occur within four cycles of sand deposition which are found far downdip of the major yegua sand depocenters gas condensate fields are found where these sandstones were deposited within at least two distinct yegua expansion fault systems possibly near or beyond the most seaward wilcox shelf edge four sand bearing intervals are present within sequences designated as 10 through 70 the 60 sandstones of the lower yegua formation anomalina umbonatus zone lie downdip of a pronounced flexure are associated with middle to outer neritic fauna and represent a complex of channeled turbidite deposits on the outer shelf or upper slope the 60 interval was deposited before movement on most yegua faults the sandstones are productive at phase four and gresham fields an upper lower yegua sand bearing interval 50 has not produced in the downdip fault trend but is the main cook mountain reservoir interval in mid dip fields the 40 sandstones at the base of the upper yegua formation form a small but thick gas productive birdsfoot delta in the phase four field area fed by channels which expand across several faults the 30 sandstones eponides yeguaensis zone occur primarily as thick strike elongate sand bodies interpreted to represent a barrier bar complex this sandstone sequence thickens markedly from 200 ft to over 800 ft over a fault of the phase four system at gresham field the sandstones produce at menefee black owl and gresham fields the 20 sand stones discorbis yeguaensis zone occur downthrown to the shanghai fault within the shanghai domino style glide fault system in central wharton county they represent proximal to distal delta front deposits of the shanghai delta system inferred to be fed by a channel the sandstones are productive at shanghai shanghai east and the el campo field complex the upper yegua intervals carry beach to middle neritic faunas and represent mostly shallow water high energy deposition the 20 30 and 40 sandstone intervals may represent drops in relative sea level in the area gas has been trapped in rollover anticlines downthrown to expansion faults of both the phase four and shanghai fault trends downthrown closures against sealing faults upthrown closures and stratigraphic traps most reservoirs are small in area 200 to 400 acres but prolific 1100 to 2100 mcf ac ft although traps of several thousand acres are present in the el campo complex stratigraphic complexities which form reservoir boundaries include channel boundaries and unconformities related to sandstone deposition structural complexities include antithetic faults closely spaced expansion faults and horsetail faults which die out upward into sand sequences seismic signatures of gas bearing yegua sandstones are of three classes 1 strong fluid contact reflectors with avo anomalies from thick sandstones as at black owl and shanghai fields 2 strong bedding parallel reflectors with avo anomalies derived from thinner gas filled sandstones as at phase four and 3 little reflection character from gas filled highly laminated sequences as at el campo avo anomalies can be severely affected by out of plane energy by skips in data acquisition and by masking in structurally complex areas the downdip yegua fields have been found to date by recognizing anomalous seismic signatures and delineating small closures future discoveries will probably result from exploration for specific reservoir intervals perhaps in more laminated zones which do not have a characteristic seismic expression from more sophisticated use of seismic signature analysis for direct hydrocarbon detection and from improved data quality in known trends

the role of rifting in the tectonic development of the midcontinent u s a abstract recent studies have proposed the existence of several major ancient rift zones in the midcontinent region of north america although the dating of some of these rifts and even the rift interpretations are subject to question an analysis of these paleo rifts reveals three major episodes of rifting keweenawan 1 1 b y b p eocambrian 600 m y b p and early mesozoic 200 m y b p the extent of these events documents that rifting has played a major role in the tectonic development of the midcontinent region this role goes well beyond the initial rifting event because these features display a strong correlation with paleozoic basins and a strong propensity for reactivation for example the eocambrian reelfoot rift was reactivated in the mesozoic to form the mississippi embayment and is the site of modern seismicity which suggests reactivation in a contemporary stress field of ene compression even though the importance of rifting can be established recognition of rifts and delineation of their complexities remain a major problem which requires more study

timing and heat sources for the barrovian metamorphism scotland abstract new shrimp u pb zircon ages of 472 2 5 8 ma and 471 2 5 9 ma are presented for the age of peak metamorphism of barrovian migmatites 40 ar 39 ar ages for white mica from the barrovian metamorphic series are presented and are recalculated using recently proposed revisions to the 40 k decay constants to allow more precise and accurate comparison with u pb ages the 40 ar 39 ar ages are found to vary systematically with increasing metamorphic grade between c 465 ma for the biotite zone and c 461 ma for the sillimanite zone there is no evidence for any significant metamorphic heating during the first 15 myr of the grampian orogeny before c 473 ma or the final 4 myr after c 465 ma the barrovian metamorphism occurred over a period of 8 myr within the 27 myr grampian orogeny the barrovian metamorphism records punctuated heating was temporally and spatially associated with large scale bimodal magmatism and developed within crust that was not overthickened the temporally distinct nature of the barrovian metamorphic episode within the grampian orogeny and its heating pattern and tectonic context are not consistent with significant heat contribution from thermal equilibration of overthickened crust rather the barrovian metamorphism records a transient phase of crustal thermal disequilibrium during the grampian orogeny temporal and spatial association with grampian bimodal magmatism is consistent with production of the barrovian metamorphic series within the middle crust as the result of advection of heat from the lower crust and or mantle the barrovian metamorphic series the classic example of orogenic regional metamorphism did not form in response to crustal thickening and thermal relaxation but appears to record large scale contact metamorphism

a reappraisal of the sibson scholz fault zone model the nature of the frictional to viscous brittle ductile transition along a long lived crustal scale fault outer hebrides scotland the widely cited sibson scholz conceptual fault zone model suggests that seismically active upper crustal brittle faults pass downward across a predominantly thermally controlled transition at 10 15 km depth into ductile shear zones in which deformation occurs by aseimic viscous creep the crustal scale outer hebrides fault zone ohfz in nw scotland has been described as the type example of such a continental fault zone it cuts precambrian basement gneisses and is deeply exhumed allowing direct study of the deformation products and processes that occur across a wide range of crustal depths a number of fault rock assemblages are recognized to have formed during a long lived displacement history lasting in excess of 1000 myr during caledonian movements that are recognized along much of the 190 km onshore fault trace brittle cataclasite bearing faults in the west of the ohfz are unequivocally overprinted to the east by a younger fabric related to a network of ductile shear zones field observations and regional geochronological data demonstrate that there is no evidence for reheating of the fault zone due to thrust related crustal thickening or shear heating microstructural observations show that the onset of viscous deformation was related to a major influx of hydrous fluids this led to retrogression with the widespread development of new finegrained phyllosilicate bearing fault rocks phyllonites and the onset of fluid assisted grain size sensitive diffusional creep in the most highly deformed and altered parts of the fault zone phyllonitic fault rocks also occur in older more deeply exhumed parts of the fault zone implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature depth range within crustal scale faults our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal scale faults containing interconnected networks of phyllosilicate bearing fault rocks will be characterized by long term relative weakness and shallow 5 km frictional viscous transition zones similar processes acting at depth may provide an explanation for the apparent weakness of presently active structures such as the san andreas fault

geological constraints structural evolution and deep geology of the northwest scottish caledonides the front of the caledonian orogenic belt in nw scotland is marked by a zone of foreland thrusting which contains imbricates of lewisian basement proterozoic cover and cambrian shelf sediments faults within this moine thrust belt in most localities propagated in a foreland directed sequence therefore the earliest of these thrusts was the moine thrust which carried a basement assemblage of moine and lewisian rocks over the cambrian shelf the development of large duplex and imbricate structures in the footwall postadted movement on this thrust hence a restoration of these imbricates provides a minimum estimate of the eastern extent of the cambrian shelf no synsedimentary or prethrust extensional features are recognised within the cambrian succession which displays a remarkable stratigraphic and thickness consistency this implies a corresponding consistency in crustal structure beneath these cover rocks immediately prior to caledonian thrusting a balanced cross section constructed across the northern part of the moine thrust belt has given a restored width for the cambrian shelf of 54 km geophysical data suggest that the present foreland crustal thickness is 28 km if this is taken to be the crustal thickness beneath the cambrian shelf during caledonian thrusting a minimum cross sectional area of 1512 km of lewisian rocks must remain beneath the present outcrop of the moine thrust sheet this foreland basement wedge may have been imbricated subsequent to the development of the cambrian imbricates caledonian structures within the moine thrust sheet are interpreted as forming an imbricate stack with the moine thrust acting as a floor thrust the ductile imbricate thrusts possibly roofed into the naver slide late open folds which warp foliation slides and metamorphic isograds within the moine sheet may represent culminations in the foreland lewisian imbricates beneath the moine thrust these postulated culminations may have been sufficient to initiate gravity driven structures this may explain late movements on high level faults within the moine thrust belt

consortium for continental reflection profiling michigan surveys reprocessing and results reprocessing of consortium for continental reflection profiling data from central michigan with emphasis on deconvolution and velocity filtering before stack and more detailed velocity analysis has resulted in greatly improved imaging of a previously reported keweenawan rift buried beneath the paleozoic of the michigan basin the seismic stratigraphy of the sub paleozoic units can now be related to the basin filling middle keweenawan volcanic sequence and the upper keweenawan clastic assemblage in greater detail important structural and stratigraphic features can be seen for the first time for example reprocessing has imaged an unconformity within the upper keweenawan it is possible some of these newly imaged features are associated with a late keweenawan compressive episode furthermore reprocessing also reveals previously unrecognized sub keweenawan structures including a deeply penetrating reflection which may represent a major crustal fault and possible weak reflections from midcrustal 25 km and moho 43 km depths

basement cover relationships and structure within the moine rocks of central and southeast sutherland inliers of archaean to early proterozoic lewisian basement crop out within the proterozoic moine succession of central and se sutherland both the lewisian basement and the moine cover have been affected by two major tectonic events the earliest event probably occurred during the late proterozoic and involved minor folding and upper greenschist to upper amphibolite facies metamorphism intense deformation largely obliterated the unconformable relationship between the lewisian and moine rocks and imposed a planar banding on the basement lithologies subsequent reworking during the caledonian orogeny 500 400 ma resulted in ductile thrusting and folding and was associated with a widespread low to mid amphibolite facies metamorphic overprint within this context the lewisian inliers of central and se sutherland occur in two distinct structural settings i as allochthonous sheets which rest on major caledonian ductile thrusts ii in the cores of nw verging intensely curvilinear sheath folds of caledonian age

structure of the northern moine thrust zone loch eriboll scottish caledonides abstract this paper reviews the geology of the late ordovician silurian caledonian moine thrust zone in the loch eriboll region nw scotland we present new detailed mapping and balanced restored cross sections to examine its structural evolution this thrust zone comprises four major thrust slices carried by the moine lochan riabhach arnaboll and sole thrusts ductile mylonites were derived from lithologies in both the hangingwall moine schists and the footwall lewisian basement gneisses cambrian quartzites ordovician dolomites of the moine thrust the mylonites are bounded by the lochan riabhach thrust below and the moine thrust above the arnaboll thrust sheet comprises lewisian basement gneisses and cambrian ordovician sedimentary rocks over the sole thrust sheet which exposes a spectacular imbricate sequence of cambrian ordovician sedimentary rocks although thrusts sequentially propagated in sequence from higher moine to lower sole thrusts with time there is evidence for minor out of sequence thrusting and breakback thrusting thrusts evolve from deep ductile shears where strain is concentrated along mylonite zones up into the brittle field where they become discrete planar thrust faults the early silurian scandian metamorphism 435 415 ma is related to a himalayan style structure with sse directed subduction and wnw directed extrusion of a migmatitic core which we liken to channel flow we suggest that two major crustal scale thrusts that extend down into the upper mantle imaged on seismic profiles across the foreland the outer isles and flannan thrusts are unrelated spatially or temporally to the moine thrust sequence

defining the himalayan main central thrust in nepal an inverted metamorphic field gradient associated with a crustal scale south vergent thrust fault the main central thrust has been recognized along the himalaya for over 100 years a major problem in himalayan structural geology is that recent workers have mapped the main central thrust within the greater himalayan sequence high grade metamorphic sequence along several different structural levels some workers map the main central thrust as coinciding with a lithological contact others as coincident with the kyanite isograd up to 1 3 km structurally up section into the tertiary metamorphic sequence without supporting structural data some workers recognize a main central thrust zone of high ductile strain up to 2 3 km thick bounded by an upper thrust mct 2 vaikrita thrust and a lower thrust mct 1 munsiari thrust some workers define an upper lesser himalaya thrust sheet that shows similar p t conditions to the greater himalayan sequence others define the main central thrust either on isotopic nd sr differences differences in detrital zircon ages or as being coincident with a zone of young

fluorescent microscopy of particulate organic matter sparta formation and wilcox group south central louisiana abstract particulate organic matter pom isolated from core samples of mudstones in the mid eocene sparta formation and the late paleocene early eocene wilcox group in louisiana was characterized using transmitted and fluorescent light microscopy and rock eval pyrolysis observations of maceral fluorescence combined with programmed pyrolysis results better define source potential than visual examination using transmitted white light alone transmitted light microscopic characterization of pom indicates that amorphous macerals of mixed terrestrial and marine origin dominate samples from both the lagoonal and shelf depositional environments microscopic examination under fluorescent light indicates that some of the amorphous matter including terrestrially derived material fluoresces to some degree hydrogen index values show a positive correlation with the total number of fluorescing particles in the isolate as well as with the relative fluorescence levels comparison of sparta and wilcox samples indicates that significant changes occur in the assemblages at levels of thermal maturation associated with the onset of petroleum generation thermally mature samples contain a higher percentage of moderately fluorescent particles and lesser percentages of highly and weakly fluorescent particles this phenomenon is readily apparent in specific particle types namely amorphous nonstructured protistoclasts miospores and dinoflagellate cysts

major proterozoic basement features of the eastern midcontinent of north america revealed by recent cocorp profiling cocorp profiling in the eastern midcontinent of north america has 1 traced an extensive sequence of precambrian layered rocks beneath southern illinois indiana and western ohio 2 detected a broad zone of east dipping basement reflectors associated with the grenville front beneath western ohio and 3 discovered a wide region of west dipping reflectors penetrating most of the crust beneath eastern ohio the precambrian layered assemblage may be as much as 11 km thick beneath southern illinois extends at least 170 km in an east west direction and contains several strong reflectors that have a lateral continuity of 80 km or more industry seismic data indicate that the layering is extensive in a north south direction as well possible explanations for the layering include the silicic igneous rocks of the ca 1 48 ga eastern granite rhyolite province which are penetrated by basement drill holes throughout the region perhaps intermixed or underlain by mafic igneous or sedimentary rocks the 40 50 km wide zone of strong east dipping 25 30 reflectors beneath west central ohio corresponds to the position of the grenville front as determined from potential field and drill hole data these dipping reflectors in the upper and middle crust are interpreted to result from ductile deformation zones mylonites like those exposed at the grenville front in canada and imaged on the glimpce seismic reflection lines in lake huron both the cocorp and glimpce lines show a remarkably similar reflection geometry despite the more than 500 km separating the two profiles easternmost ohio appears to be underlain by pronounced west dipping 40 reflectors in the middle and lower crust which are also interpreted as marking a region of pervasive ductile deformation 80 km or more in width analogy with similar reflection packages elsewhere suggests that these reflections may mark a major collision zone the west dipping reflectors may be correlative with similar reflectors imaged on another cocorp survey in northern alabama the correlations suggested by these new results though tentative imply that the eastern midcontinent is composed of a relatively simple assemblage of crustal blocks

identification of organic rich lower tertiary shales as petroleum source rocks south louisiana abstract comprehensive organic geochemical evidence of organic rich marine shales in the lower part of the middle eocene claiborne group and the lower eocene paleocene wilcox group of south louisiana is now available that influences models for gulf coast petroleum origin the shales are the only post cretaceous age sediments in the northern gulf of mexico that meet recognized criteria for oil source rocks many organic rich paleogene shales contain terrestrially derived amorphous kerogen altered by microbial activity and display pyrolysis results consistent with type ii iii kerogen shelf edge depositional environments favor preservation of hydrogen rich kerogen seismic and sedimentologic interpretations suggest that marine character and thickness increase on the paleogene continental slope to the south the shales at burial depths in the 3050 4600 m depth range are at present thermally immature to late mature with respect to oil generation detailed geochemical analysis of extractable organic matter and kerogen isolates suggest an oil source correlation with tertiary reservoired oils in south louisiana and offshore in the adjacent gulf of mexico biomarkers of selected samples display high concentrations of c28 bisnorhopane and 18 oleanane that are also found in tertiary reservoired oils the key 18 oleanane biomarker is absent or not reported in gulf crude oils from cretaceous and jurassic source rocks burial and thermal history models suggest the timing of oil migration from paleogene source rocks is consistent with emplacement of oils in tertiary reservoirs the lower tertiary source rocks described here could offer new insight to understanding the origin of oil in other tertiary deltas

the nature of roof thrusts in the moine thrust belt nw scotland implications for the structural evolution of thrust belts buried thrust systems commonly analysed in terms of duplex structures are often interpreted as forming in a particular sequence this model considers that only one thrust is active at a time and the resulting structural geometry is well ordered however a reappraisal of structural geometries from the moine thrust belt in nw scotland has shown that many apparent duplex structures are more complex although the upper roof thrust of some duplexes may be folded and bulged by underlying imbricate slices in accordance with simple duplex models the footwall to these roof thrusts cut up and down stratigraphic section in conflict with the simple duplex models examples include the glencoul thrust with its underlying imbricate system and the foinaven duplex simultaneous slip on an array of imbricate thrusts can bulge duplex roofs relationships classically used to infer piggy back sequences of thrust propagation however activity on more hinterland ward imbricate thrusts can cause the roof onto which they branch to truncate more foreland ward structures in this fashion the duplex roof can cut up and down stratigraphic section mimicking an extensional fault by varying thrust activity in three dimensions within an imbricate system local parts of roof thrusts can generate domains of overstep behaviour that have only local significance existing interpretations of overstep relationships in the moine thrust belt as representing late orogenic low angle extensional faults may need revision the new interpretations of deformation activity where displacements in thrust arrays happen synchronously rather than sequentially are consistent with emergent thrust and fold belts with analogue experiments and with mechanical models of orogenic wedges

evidence for a major late precambrian tectonic event rifting in the eastern midcontinent region united states recently acquired gravity and aeromagnetic data delineate a large linear gravity anomaly which extends through eastern kentucky and tennessee and coincides with a zone of complex high amplitude magnetic anomalies basement lithologies in the area can be interpreted as a bimodal volcanic suite which is locally peralkaline in nature these volcanics appear to be metamorphosed where they lie east of the grenville front suggesting they predate the grenville metamorphic event the available gravity aeromagnetic seismic refraction and petrologic data along with regional correlations suggest that the best tectonic interpretation of these data is that a keweenawan rift zone extended through the area this rift can be roughly outlined by the gravity high which is locally offset suggesting the presence of transform faults the boundaries of this rift have been locally reactivated and in fact a recent earthquake was located along its western boundary in northern kentucky

mylonites lessons from eriboll eriboll has been an important area in understanding the geology and structure of the nw highlands of scotland it came to prominence during the highland controversy between nicol and murchison in the 1850s nicol recognized a major regional or grand dislocation the moine thrust zone at eriboll whereas murchison denied its existence an important element in the resolution of the controversy was the repeated observation of a distinctive schistose rock that often marked nicol s dislocation lapworth named it a mylonite and related its development to mechanical metamorphism resulting from tectonic milling movements along the dislocation peach horne and co workers whose contributions to early mylonite studies are often overlooked described the effects of increasing strain on mylonite development recorded the prominent lineation within mylonites and identified its kinematic significance via thrust geometries and a first use of shear sense indicators in this contribution those elements of the highland controversy that led to the identification of mylonites will be reviewed concentrating on the contributions of lapworth and of peach horne and co workers it will include an examination and discussion of the mylonites from lapworth s locality and of the mylonitized moine schists at eriboll it will conclude with comments on mylonite classification and nomenclature

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

thermal structure and tectonic evolution of the scandian orogenic wedge scottish caledonides integrating geothermometry deformation temperatures and conceptual kinematic thermal models in the caledonides of northwest scotland two independent geothermometers fe mg exchange and quartz c axis fabric opening angle are used to characterize the thermal structure of the lower part of the scandian 435 420 ma orogenic wedge within the moine ben hope and naver sgurr beag thrust sheets traced from west foreland to east hinterland fe mg exchange thermometry yields peak or near peak temperatures ranging from 484 50 c to 524 50 c in the immediate hangingwall of the moine thrust to 601 50 c in the immediate hangingwall of the ben hope thrust to 630 50 c in the naver thrust sheet preserved metamorphic facies and textural relationships are consistent with thermometric estimates deformation temperatures calculated from quartz c axis fabric opening angles across two similar orogen perpendicular transects also yield systematic increases glen golly ben klibreck 520 630 c ullapool contin 465 632 c traced towards the naver and sgurr beag thrusts in addition deformation temperatures show a pronounced increase along the leading edge of the moine thrust sheet moving south towards the assynt window which is interpreted to reflect deeper exhumation of the thrust plane above the assynt footwall imbricate stack because temperatures calculated from metamorphic assemblages are within error of the quartz fabric derived deformation temperatures that are of demonstrably scandian age the metamorphic sequence between the moine and naver sgurr beag thrusts is interpreted to have developed during the scandian orogeny integration of our results with previous 2d thermal mechanical studies allows development of new conceptual thermal kinematic models of scandian orogenesis that may be broadly applicable to other collisional systems furthermore it highlights the critical nature of coupling between orogen kinematic and thermal evolution

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

tectonic stratigraphy and structural continuity of the northernmost moine thrust zone and moine nappe scottish caledonides the moine thrust zone and overlying moine nappe represent classic ground in scottish geology in which neoproterozoic moine metasedimentary rocks were translated towards the wnw over the laurentian foreland comprising lewisian basement gneisses and a cambrian cover succession during caledonian orogenesis at c 430 ma systematic mapping of both the moine nappe and underlying moine thrust zone in the loch hope to whiten head region of sutherland extends the tectonic stratigraphy developed in other parts of the thrust zone to the little studied north coast section an apparent continuum of deformation is recognized from the moine nappe down into the underlying thrust zone with a consistent gently ese dipping foliation associated with down dip mineral and extension lineations the polyphase fold history present within the moine nappe and the thrust zone results from progressive deformation during thrusting thrust related fold axes and bedding cleavage intersections are typically subparallel to the transport direction in high strain zones whereas gently north or south plunging attitudes are common in lower strain portions of the thrust zone thrusts are commonly deformed by folds developed within their footwalls consistent with a foreland propagating sequence of deformation the newly defined lochan riabhach thrust is interpreted as an out of sequence structure that sliced at low angles across the thrust zone and toward the wnw after initial thrust stacking comparison with the downfaulted segment of the moine nappe and thrust zone exposed on the foreland at faraid head demonstrates cross strike continuity of the thrust sheet template and structural history for at least 15 km to the nw

geochemistry and evolution of the early proterozoic post penokean rhyolites granites and related rocks of south central wisconsin u s a

moist and the continuity of crustal reflector geometry along the caledonian appalachian orogen new aspects of the deep structure of the scottish caledonides are revealed by the moine and outer isles deep seismic reflection traverse moist the caledonian foreland is underlain by an easterly dipping strongly reflecting surface cutting through the moho and traceable to more than 45 km depth thrusts within the foreland basement and caledonian orogen have been reactivated as normal faults bounding half grabens filled with sediments of late palaeozoic to mesozoic age the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the e on moist and is either a the westernmost of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or b a thrust lying further e which structurally overlies these easterly dipping reflectors by comparison with easterly dipping reflections on cocorp lines in the northern and southern appalachians the latter interpretation is preferred implying that many of the easterly dipping reflections on moist correspond to off shelf metasedimentary rocks with slivers of basement stacked and imbricated against the lewisian basement edge regional seismic refraction and conductivity data from northern scotland suggest that regions of the caledonian orogen have lower mid crustal velocity gradients and higher conductivities than the foreland which might be explained by this interpretation of the easterly dipping reflections despite similarities in crustal reflector geometries there are important differences between the moist and cocorp lines including 1 a remarkably continuous relatively horizontal moho seen on moist data at about 25 km depth and 2 the sedimentary basins offshore from northern scotland which have formed by reactivation under crustal extension of the easterly dipping thrusts furthermore it is probable that the easterly dipping reflectors in these areas of the moist and cocorp lines were formed at different times in the early palaeozoic

control of depositional environments eustacy and gravity and salt tectonics on sandstone distribution in an unstable shelf edge delta eocene yegua formation texas and louisiana abstract predicting the locations and geometry of downdip sandstone reservoirs in the yegua trend of texas and louisiana is a key challenge to explorationists in order to formulate and test depositional and stratigraphic models for this purpose well log seismic and biostratigraphic data were integrated over a distance of about 350 miles along strike the resulting correlation framework based on almost 4 500 well logs extends from updip on the stable shelf to the downdip limit of well control on the slope well logs provide the highest stratigraphic resolution in this data set on the stable shelf the entire yegua cook mountain sand bearing interval is about 1 000 1 500 ft thick shale markers which show tens to hundreds of miles of continuity along strike bracket progradational cycles 100 300 ft thick these log defined cycles comprise the basic mapping units used in this study seismic reflectors in this zone are essentially parallel downdip of the shelf complications were introduced by growth faulting erosion due to slope failure mass wasting of sediments overlying failure induced erosion surfaces and complex patterns of basin fill seaward of the shelf margin this complex zone thickens basinward to several thousand feet all of these zones were further complicated by contemporaneous and post depositional salt movement in the houston salt diapir province biostratigraphic data were used to determine general ages for downdip section to indicate prominent discontinuities and to show environmental trends large scale slope failures occurred several times during yegua deposition resulting in regional erosional unconformities it is not known if their origin is related to eustacy slope deposits that onlap extensive erosional surfaces are also well represented but their precise age relationship to updip systems is unclear paleoenvironments were interpreted from maps of log derived interval isopach net sand percent sand blocky sand and log facies each mapping unit is bounded by regionally correlatable shale zones interpreted to have formed during high stands strata bounded by the markers represent systems tracts comprising upper delta plain lower delta plain mouth bar distal mouth bar and shelf inferred local shelf bypassing and channel incision and basinward shifts in facies suggest small but significant falls in sea level during deposition each of these cycles resemble depositional sequences but in terms of their duration they are described as high frequency fourth order events small scale stratigraphic complexity within the mapped units suggests frequent autocyclic shifts of depocenters effective sand prediction downdip to the shelf edge and upper slope is accomplished using transgressive shale condensed section or flooding surface markers as a stratigraphic framework while recognizing the significant effects of eustatic sea level changes although widespread erosional unconformities possibly equivalent to sequence boundaries could be mapped on the slope neither unconformities nor correlative conformities could be identified sufficiently on the shelf to enable their detailed and regional mapping

a structural analysis of the moine thrust zone between loch eriboll and foinaven nw scotland abstract the northern part of the moine thrust zone as exposed around the valley of srath beag sutherland was developed by thrusts propagating in the tectonic transport direction deformation on any particular thrust surface evolved from dominantly ductile to dominantly brittle with time the foreland has been progressively accreted onto the overriding moine thrust sheet by duplex formation a process which has continuously folded the roof thrust and the rocks above its hanging wall fold culminations and depression can be related to lateral ramps which may give the rocks above the hanging wall a complex history of extensional and compressional strains normal to the transport direction folds within the thrust zone are laterally independent because they are controlled by short lived variations in deformation style on an evolving thrust footwall topography therefore there may be no correlation between structures across or along the thrust zone this variation limits the construction of balanced cross sections as structure cannot be projected onto particular section lines

origin of crude oil in the wilcox trend of louisiana and mississippi evidence of long range migration abstract geochemical characterization of crude oils from wilcox reservoirs in central louisiana and southwest mississippi suggests that they represent a single crude oil family that is distinct when compared to crude oils in deeper tuscaloosa and smackover reservoirs this observation is consistent with geologic constraints that suggest an origin of crude oil from within the wilcox group itself although shales of the shallow wilcox group in central louisiana and southwest mississippi contain gas prone kerogen and are thermally immature a more oil prone source facies is present in marine shales of the deep wilcox group in southcentral louisiana thermal maturity measurements based on pyrolysis suggest a broad area of effective wilcox source rock in southcentral louisiana migration distances from source to reservoir rocks of the downdip wilcox trend of southcentral louisiana appear to be relatively short however long range updip migration sometimes greater than 100 km from deeply buried wilcox source facies provides the best explanation for emplacement of crude oil in the shallow wilcox trend of central louisiana and southwest mississippi

flexural modeling of the midcontinent rift a basement profile obtained from seismic reflection data has been used to constrain a two dimensional flexural model of basin formation for the midcontinent rift at a latitude of 45 sup 0 25 n model parameters included the thickness of the elastic plate the basin width and the maximum basin thickness modeling suggests that flexure produced a deep narrow basin along the rift axis and that the crust was thinned at the time of basin formation to an elastic thickness of 9 6 km for a plate ruptured by rifting or 5 7 km for an unbroken plate with corresponding maximum basin thicknesses of 14 km and 16 km respectively the plate thickness depends most strongly on the basin width and is well constrained by the seismic data although erosion may have narrowed the basin the maximum basin thickness is poorly constrained because of the lack of seismic data for depths greater than about 10 km and because the strata at the center of the rift have been disturbed by a postrift compressional event which produced the st croix horst despite uncertainty about the basin thickness the load required to flex the crust to produce the midcontinent rift basin is too large to be attributed tomore the weight of the central flood basalts unless the basin subsided into a fluid less dense than the solidified basalts on the basis of seismic refraction data and by analogy with other rifts we hypothesize that a magnetic rift pillow intruded in the lower crust the basaltic pillow subsequently solidified to produce a large high velocity region in the lower crust centered under the rift axis as determined from deep seismic refraction this crystallization and cooling may be responsible for the sag phase of rift evolution as evidenced by laterally widespread occurrence of postvolcanic sediments copyright american geophysical union 1988 less

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

the 1 1 ga midcontinent rift system central north america sedimentology of two deep boreholes lake superior region abstract the midcontinent rift system mrs of central north america is a 1 1 ga 2500 km long structural feature that has been interpreted as a triple junction rift developed over a mantle plume as much as 20 km of subaerial lava flows mainly flood basalts are overlain by as much as 10 km of sedimentary rocks that are mostly continental fluvial red beds this rock sequence known as the keweenawan supergroup has been penetrated by a few deep boreholes in the search for petroleum in this paper two deep boreholes in the upper peninsula of michigan are described in detail for the first time both the amoco production 1 29r test herein referred to as the st amour well and the nearby hickey creek well drilled by cleveland cliffs mining services were 100 cored the former is 7238 ft 2410 m deep and the latter is 5345 ft 1780 m deep the entirety of the stratigraphic succession of the hickey creek core correlates very well with the upper portion of the st amour core as determined by core description and point counting of 43 thin sections selected out of 100 studied thin sections two lower paleozoic units and two keweenawan red bed units the jacobsville sandstone and the underlying freda sandstone are described the jacobsville is largely a feldspatholithic sandstone and the freda is largely a lithofeldspathic sandstone below the freda the remaining footage of the st amour core consists of a thick quartzose sandstone unit that overlies a heterogenous unit of intercalated red bed units of conglomerate sandstone siltstone and shale black shale individual basalt flows and a basal ignimbritic rhyolite this lower portion of the st amour core presents an enigma as it correlates very poorly with other key boreholes located to the west and southwest while a black shale sequence is similar to the petroleum bearing nonesuch formation farther west there is no conglomerate unit to correlate with the copper harbor conglomerate other key boreholes are distributed over a 1300 km distance along the better known southwest arm of the triple junction mrs and can be correlated rather well with the units that are exposed in the lake superior region however a definitive explanation of the anomalous deeper st amour stratigraphy is elusive and any explanation is tenuous a possible explanation for this anomalous stratigraphy may be the geographic proximity of the st amour borehole to the keweenawan hot spot mantle plume the suggested thermal force behind the development of the mrs similarly a drastic change in structural architecture may be explained by this geographic relationship thus within the locale of this rifting center complexities of expansion tectonics may well be responsible for igneous and sedimentary sequences that differ considerably from those found farther west along the rift arm

a geophysical profile of the southern margin of the midcontinent rift system in western lake superior a 70 km long seismic reflection profile in western michigan provides new insight into the nature distribution and structure of the keweenawan supergroup volcanic and overlying sedimentary rocks and the controversial keweenaw fault along the southern boundary of the midcontinent rift system in the lake superior basin interpretation of the 5 s reflection data constrained by surface geology magnetics and gravity modeling shows that volcanic rocks which cropout north of the keweenaw fault dip northerly to depths of the order of 17 km south of the fault volcanic rocks overlain by 2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the south range the volcanic pile within the basin thickens rapidly to the north of the keweenaw fault suggesting that the volcanics were deposted in an extensional fault bounded basin clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event the thickness of the volcanic filled basin implies that the upper crust was almost completely broken during the rifting event the available evidence is interpreted to show the keweenaw fault as a moderate to high angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence there is no evidence from the seismic profiling for major faulting except for the keweenaw fault intrusions or folding of the keweenawan supergroup in this region

thrust sequences and the significance of low angle out of sequence faults in the northernmost moine nappe and moine thrust zone nw scotland in common with most thrust belts the moine thrust zone scotland preserves a foreland propagating sequence of thrusting however significant examples of local to regional overstep sequences are also recognized and their interpretation remains controversial remapping of the northernmost moine thrust zone has defined a thick belt of mylonites in the centre of which lies the ductile wnw directed moine thrust the base of the mylonite belt however is marked by a regionally significant low angle west directed brittle fault here termed the lochan riabhach thrust that truncates thrust related structures in its footwall comparison of the tectonic stratigraphy of the mylonite belt with that preserved in a downfaulted outlier to the west at durness indicates that the lochan riabhach thrust cuts down section in its footwall and therefore appears to be extensional locally it is unequivocally breached by underlying imbricate thrusts we prefer to interpret the lochan riabhach thrust as an extensional fault that formed during a hiatus in thrust stacking we relate this faulting episode to the changes in the stability of the evolving orogenic wedge and reiterate that such extensional faults may be common although possibly unrecognized features in many other thrust belts

aeromagnetic and gravity anomaly constraints on the crustal geology of ohio the u s geological survey and the ohio geological survey recently compiled aero magnetic anomaly maps and revised gravity anomaly data for crustal studies of ohio updated structural and lithologic maps of the precambrian basement surface provide additional constraints to limit interpretational ambiguities of the geopotential anomaly data the combined modeling of five east west gravity and magnetic anomaly profiles supports a generalized crustal model for ohio involving a lower zone of intermediate composition a more felsic and relatively homogeneous middle zone and an upper zone consisting of intrusive and extrusive igneous rock in the western third of the state and medium grade grenville metamorphic rocks to the east modeling of the geopotential field anomalies indicates the presence of large dense lower crustal bodies in western and south central ohio that may be related to a paleo rift system large possibly anorthositic masses in southern ohio post tectonic granitization in northwestern ohio and a volcanic source for igneous rocks in southwestern ohio may also be interpreted from the combined anomaly analysis

the midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting