marine geology of the bahama escarpment abstract a 2 km thick stratigraphic section of the bahama escarpment off cat island in the northwestern bahamas has been measured and sampled by submersible the escarpment from 3940 m upwards consists of lower and middle cretaceous peritidal lagoonal patch reef and backreef limestones unconformably overlain by an intermittent cover of maastrichtian and eocene pelagic limestone data from additional dive traverses and dredging elsewhere on the bahama escarpment indicate that shallow water platform limestones similar in age and lithofacies to the cat island sequence occur along the entire 800 km of the escarpment as far as navidad bank comparison of our section to stratigraphic data from coeval mesozoic carbonate platforms surrounding the gulf of mexico and extending up the east coast of north america as well as from other ancient and modern carbonate platforms reveals that the bahama sequences have the greatest similarity to deposits characteristic of the marginal zones 1 10 km wide of these platforms in particular the bahama sequence is most similar to the bank interior and backreef portions of the marginal zone and not to the reef portion it is clear from both modern and ancient examples that a purely constructive carbonate platform margin should consist of a reef and associated forereef deposits and not only of bank interior peritidal and lagoonal strata as exposed along the bahama escarpment therefore the reef and forereef debris which constituted the original seaward edge of the bahama must have been removed by erosion the amount of erosion is estimated as 1 5 km and is effected at present by spallation of joint blocks lithostatic pressure at 4 5 km depth may exceed the strength of the limestone comprising the platform resulting in the pervasive jointing observed in the lower portion of the escarpment subsidence of the cat island escarpment has been exponential decreasing from 60 m m y in the early cretaceous to 40 m m y in the middle cretaceous extrapolated values for the late cretaceous and tertiary are 20 m m y

re os geochronology of the lacustrine green river formation insights into direct depositional dating of lacustrine successions re os systematics and paleocontinental weathering abstract lacustrine sedimentary successions provide exceptionally high resolution records of continental geological processes responding to tectonic climatic and magmatic influences these successions are therefore essential for correlating geological and climatic phenomena across continents and furthermore the globe producing accurate geochronological frameworks within lacustrine strata is challenging because the stratigraphy is often bereft of biostratigraphy and directly dateable tuff horizons the rhenium osmium re os geochronometer is a well established tool for determining precise and accurate depositional ages of marine organic rich rocks lake systems with stratified water columns are predisposed to the preservation of organic rich rocks and thus should permit direct re os geochronology of lacustrine strata we present re os systematics from one of the world s best documented lacustrine systems the eocene green river formation providing accurate re os depositional dates that are supported by ar ar and u pb ages of intercalated tuff horizons precision of the green river formation re os dates is controlled by the variation in initial 187 os 188 os and the range of 187 re 188 os ratios as also documented in marine systems controls on uptake and fractionation of re and os are considered to relate mainly to depositional setting and the type of organic matter deposited with the need to further understand the chelating precursors of re and os in organic matter highlighted in addition to geochronology the re os data records the 187 os 188 os composition of lake water 1 41 1 54 at the time of deposition giving an insight into continental runoff derived from weathering of the geological hinterland of the green river formation such insights enable us to evaluate fluctuations in continental climatic tectonic and magmatic processes and provide the ability for chemostratigraphic correlation combined with direct depositional dates furthermore initial 187 os 188 os values can be used as a diagnostic tool to distinguish between lacustrine and marine depositional settings when compared to known oceanic 187 os 188 os values

carbonate periplatform sedimentation by density flows a mechanism for rapid off bank and vertical transport of shallow water fines existing theories of off bank sediment transport cannot account for rapid rates of sedimentation observed in bahama bank and florida shelf periplatform environments analysis of the physical processes operating during winter cold fronts suggests that accelerated off bank transport of shallow water mud may be achieved by sinking off bank flows of sediment charged hyperpycnal super dense platform waters

hydrocarbon evolution of the over mature sinian dengying reservoir of the neoproterozoic sichuan basin china insights from re os geochronology abstract rhenium osmium re os geochronology is a robust technique for constraining the accurate timing of hydrocarbon evolution but its effectiveness in over mature petroleum systems has not been well constrained here we conduct a case study in the neoproterozoic sichuan basin china i e the sinian dengying sequence 635 542 ma re os isotopic data from this highly mature hydrocarbon reservoir yield a re os isochron ages of 414 44 ma with an initial 187os 188os osi of 1 36 0 19 for bitumen and a 78 37 ma age with an osi of 3 21 0 17 for pyrobitumen coupled with thermal evolution of possible source rocks and fluid inclusion homogenization temperatures the 414 ma age marks the timing of oil generation and the 78 ma age indicates the timing of gas generation associated with the thermal cracking of oil and the formation of pyrobitumen in the reservoir a comparison of the osi of the 414 ma bitumen and that of possible source rocks at the time of bitumen formation reveal that it was primarily sourced from shale of the lower cambrian qiongzhusi formation the results of this study provide new insights to better understand the over mature oil gas system in the sinian sichuan basin and show that dengying formation reservoirs in the depocenter areas of the basin are potentially favorable for oil and gas exploration the results further highlight the utility of the re os geochronometer to constrain the accurate age and source of hydrocarbon evolution

oxygen isotope calibration of the onset of ice rafting and history of glaciation in the north atlantic region we report here that dsdp site 552a cored with the hydraulic piston corer on the west flank of rockall bank recovered an undisturbed sequence of alternating white deep sea carbonate oozes and dark coloured layers that are rich in glacial debris oxygen isotope analysis of the sequence together with detailed nannofossil and palaeomagnetic stratigraphy shows that the first major horizon of ice rafting occurred at about 2 4 myr and was preceded by a minor pulse of ice rafting at about 2 5 myr the carbon isotope record shows that the site has been bathed by a water mass of similar characteristics to present day north atlantic deep water at least since 3 5 myr

banktop responses to quaternary fluctuations in sea level recorded in periplatform sediments periplatform sediment from a 12 m core recovered from northwest providence channel bahamas contains a geochemical and paleontological record of sea surface temperatures ice volumes and the response of banktops to highstands of sea level a comparison of these data suggests that fluctuations of carbonate mineralogy in periplatform sediments result from fluctuations of sea level and from patterns of banktop sedimentation highstands of sea level that flood carbonate platforms are recorded in periplatfonn sediments as abrupt increases of exported sr rich aragonite derived from banktop orgiuiisms superposed on a background of sr poor aragonite pteropods and calcite foraminifera and coccoliths derived from planktonic sources the pulses of banktop sediment coincide with increases of water temperature determined from foraminiferal assemblages and with decreases in ratios of oxygen isotopes indicating decreased ice volume and rising sea level following these abrupt changes is a more gradual decline in bank derived sediment although paleotemperatures from foraminiferal assemblages and oxygen isotopic data clearly show that warm conditions and a highstand of sea level persist we suggest that this decrease of offbank transport is part of autocyclic sedimentation patterns of shallow water carbonate environments offbank transport is restricted as reefs sand shoals and islands reach sea level also green algal production may decline because progradation of tidal flats decreases lagoon area and the hydrologic and ecologic conditions change as lagoons are filled

petroleum source rock identification of united kingdom atlantic margin oil fields and the western canadian oil sands using platinum palladium osmium and rhenium implications for global petroleum systems this study demonstrates that petroleum and source rocks are enriched in pt and pd to the ppb level and that the 187os 188os composition coupled with the pt pd value permits the fingerprinting of petroleum to its source oils from the united kingdom atlantic margin sourced from the upper jurassic kimmeridge clay fm as well as source rock samples have been analysed for pt and pd when the pt pd value is compared with 187os 188os calculated at the time of oil generation osg the values from both the known source and the oils are similar demonstrating that they can be used as an oil to source fingerprinting tool this inorganic petroleum fingerprinting tool is particularly important in heavily biodegraded petroleum systems where traditional fingerprinting techniques e g biomarkers are severely hampered e g the world s largest oil sand deposit the west canadian oil sands wcos this has caused the source of the wcos to be hotly debated with no present day consensus between inputs from potential source units e g exshaw and gordondale fms 187os 188os and pt pd fingerprinting of the oil sands shows that the majority of the petroleum have similar 187os 188os and pt pd values supporting the hypothesis of one principal source analysis of the potential source rocks establishes that the principal source of the oil sands to be from the jurassic gordondale fm with a minor exshaw fm input thus the combination of previously pioneered re os petroleum geochronology with 187os 188os and pt pd values of petroleum permits both a temporal and spatial understanding of petroleum systems

controls on carbonate mineral accumulation in bahamian basins and adjacent atlantic ocean sediments abstract carbonate mineralogical studies of surface bahamian periplatform ooze containing pelagic calcite as well as bank derived aragonite and magnesian calcite were coupled with studies of the carbonate chemistry of the waters overlying these sediments the presence of metastable aragonite and magnesian calcite in the periplatform ooze presents a unique opportunity to assess the relation between the extent of carbonate saturation in the water column and the disappearance of metastable carbonate minerals in an area of the north atlantic ocean where intermediate and deep waters are saturated with respect to calcite aragonite is present in the fine fraction of the surface sediment to a water depth of 4 800 m but decreases sharply below 4 000 m this is also the depth limit of the occurrence of pteropod tests and fragments in the coarse fraction of the sediment the aragonite compensation depth should be by extrapolation between 5 000 and 5 200 m the saturation depth with respect to aragonite was estimated to be located between 3 800 and 4 500 m this corresponds fairly well with the depth of a sharp decrease of the fine aragonite as well as the depth of pteropod disappearance magnesian calcite in the fine fraction of the sediment decreases irregularly with depth and disappears between 3 700 and 4 000 m the calculated saturation depth with respect to magnesian calcite 13 mole mgco3 is between 900 and 1 500 m at this depth a significant drop in the concentration of magnesian calcite occurs this decrease of magnesian calcite from 25 to 15 percent could be caused by both dissolution and the increasing distance of deeper waters from the banks however the higher values of specific alkalinity of the waters ranging between 750 and 2 000 m when compared with the alkalinity values of the sargasso sea at the same latitude are interpreted to be indicative that some dissolution of magnesian calcite occurs in the bahamian basins at these water dept s our results show depressed levels by 500 m or more of the aragonite saturation depth in the water column as well as of the depth of aragonite disappearance in the surface sediment when compared with open ocean studies in the northwest atlantic in addition our results indicate that fine magnesian calcite 12 to 13 mole mgco3 disappears 1 200 to 1 500 m above the extrapolated disappearance depth of aragonite this observation is consistent with results for experimental studies showing that aragonite is more stable in seawater than a magnesian calcite of 12 to 13 mole mgco3

re os geochronology of organic rich sediments an evaluation of organic matter analysis methods rhenium and osmium in organic rich sedimentary rocks are dominantly hydrogenous but any nonhydrogenous component will influence the accuracy and precision of the re os date obtained to minimize the influence of any nonhydrogenous re and os we evaluate analysis of isolated organic matter from the whole rock together with whole rock analysis using a cro3 h2so4 digestion medium instead of inverse aqua regia for a black shale unit of the exshaw formation canada this unit previously returned a whole rock re os date of 358 10 ma model 3 geochim cosmochim acta 2002 using inverse aqua regia dissolution organic matter isolated from the whole rock matrix using the hf bf3 technique org geochem 20 1993 249 yields scattered data and a re os date of 449 220 ma model 3 mswd 616 the organic matter analyses show similar 187os 188os values but significantly lower 187re 188os values in comparison to the whole rock analyses we show that the re os systematics of organic matter are altered during chemical isolation and as such we suggest that the hf bf3 method should not be used for re os analysis of organic matter whole rock re os analysis using a cro3 h2so4 digestion medium yields significantly better regression analysis compared with the inverse aqua regia method and the re os data identify two distinct initial 187os 188os values for the sample set separate regressions of these data yield precise dates 366 1 9 6 mswd 2 2 and 363 4 5 6 ma mswd 1 6 model 3 which are indistinguishable from the age constraints for this formation 363 4 0 4 ma u pb monazite comparison of the re os dates obtained from aqua regia and cro3 h2so4 methods suggests that the former may contain nonhydrogenous re and os whereas the cro3 h2so4 method dominantly liberates hydrogenous re os from organic matter allowing for better stratigraphic age determinations and evaluation of the os isotope composition of seawater

segmentation and coalescence of cenozoic carbonate platforms northwestern great bahama bank seismic profiles over the northwestern great bahama bank reveal that it is formed by the coalescence of three smaller platforms and significant lateral progradation this conclusion is contrary to previous models which assumed that the bank is the result of continuous accumulation on one huge buildup in late cretaceous to early tertiary time a north south trending depression the straits of andros separated an eastern platform andros bank from a western platform bimini bank initially the straits of andros had dimensions similar to the modem tongue of the ocean but was progressively filled from east to west in early to middle tertiary time a second depression the bimini embayment formed within bimini bank by folding it had a maximum depth of about 470 m this depression was also filled from east to west in addition prograding systems built the western margin of bimini bank more than 25 km westward into the straits of florida

ionospheric disturbances observed by dmsp at middle to low latitudes during the magnetic storm of june 4 6 1991 this paper extends a recent study of electric field penetration into the inner magnetosphere observed by the combined release and radiation effects crres satellite and the defense meteorological satellite program dmsp satellite f8 during the magnetic storm of june 4 6 1991 to consider its ionospheric consequences effects include the development of 1 km s subauroral ion drift said structures the formation of midlatitude density troughs and the vertical transport of equatorial plasma bubbles nearly simultaneous auroral electron and plasma drift measurements were acquired by three dmsp satellites with f8 and f9 in one hemisphere and flo in the other moderate to strong said structures were consistently detected for 10 hours during the early main phase of the storm weak saids were encountered during 8 hours of the early recovery phase dmsp data show that saids with similar characteristics developed at magnetically conjugate locations and extended for at least 3 hours in local time simultaneous measurements show that the saids spanned temporally grooving but latitudinally narrow plasma density troughs these observations suggest that the magnetospheric sources of saids act more like voltage than current generators energetic electron fluxes electric fields and plasma waves measured by crres indicate that during this storm the ring current shielding charges and said sources were located in regions of high plasma density characteristic of the plasmasphere the sequence in which dmsp detected equatorial plasma density irregularities is consistent with model predictions that stormtime electrodynamics at low latitudes operate on distinctive fast and slow timescales fejer and schcrliess 1997

apatite fission track and re os geochronology of the xuefeng uplift china temporal implications for dry gas associated hydrocarbon systems hydrocarbon evolution is extremely challenging to determine both temporally and spatially in complex tectonic settings here we investigate the western margin of the xuefeng uplift southern china which records multiple and protracted tectonic and hydrocarbon generation events this timing of initial oil generation is recorded by low maturity bitumen type a which yields an re os bitumen date of ca 430 ma consistent with basin models and a ca 405 ma bitumen rb sr date in contrast apatite fission track aft data yield considerably younger dates that reflect the timing and tectonic evolution of the yanshan orogeny from the northwest ca 150 ma to the southeast ca 70 ma the youngest aft date coincides with the western margin of xuefeng uplift where high maturity bitumen type b occurs that yields a ca 70 ma re os date the re os and aft dates imply that both the last stage of the yanshan orogeny and by inference the cessation of dry gas generation occurred ca 70 ma the re os data of this study imply that the re os chronometer can aid in constraining the timing of oil generation and secondary and or more mature hydrocarbon processes e g thermal cracking and or gas generation in hydrocarbon systems worldwide

timing and petroleum sources for the lower cretaceous mannville group oil sands of northern alberta based on 4 d modeling the lower cretaceous mannville group oil sands of northern alberta have an estimated 270 3 billion m3 bcm 1700 billion bbl of in place heavy oil and tar our study area includes oil sand accumulations and downdip areas that partially extend into the deformation zone in western alberta the oil sands are composed of highly biodegraded oil and tar collectively referred to as bitumen whose source remains controversial this is addressed in our study with a four dimensional 4 d petroleum system model the modeled primary trap for generated and migrated oil is subtle structures a probable seal for the oil sands was a gradual updip removal of the lighter hydrocarbon fractions as migrated oil was progressively biodegraded this is hypothetical because the modeling software did not include seals resulting from the biodegradation of oil although the 4 d model shows that source rocks ranging from the devonian mississippian exshaw formation to the lower cretaceous mannville group coals and ostracode zone contributed oil to mannville group reservoirs source rocks in the jurassic fernie group gordondale member and poker chip a shale were the initial and major contributors kinetics associated with the type iis kerogen in fernie group source rocks resulted in the early generation and expulsion of oil as early as 85 ma and prior to the generation from the type ii kerogen of deeper and older source rocks the modeled 50 peak transformation to oil was reached about 75 ma for the gordondale member and poker chip a shale near the west margin of the study area and prior to onset about 65 ma from other source rocks this early petroleum generation from the fernie group source rocks resulted in large volumes of generated oil and prior to the laramide uplift and onset of erosion 58 ma which curtailed oil generation from all source rocks oil generation from all source rocks ended by 40 ma although the modeled study area did not include possible western contributions of generated oil to the oil sands the amount generated by the jurassic source rocks within the study area was 475 bcm 2990 billion bbl

quantifying rates of syndepositional marine cementation in deeper platform environments new insight into a fundamental process abstract syndepositional marine cementation on carbonate platforms is a fundamental process that results in physical stabilization and lithification of sediments as well as reduction in primary porosity and permeability which may affect the migration of later diagenetic fluids rapid submarine cementation is commonly invoked to explain various aspects of carbonate platform geometry especially related to the preservation of steep slopes but the rates typically reported for cementation in deeper parts of the platform are mostly conjectural in an attempt to quantify the rates of syndepositional cementation in marginal platform environments substrates containing samples of carbonate sand ooids suspended above the sea floor were emplaced at various depths across a modern platform in the bahamas to evaluate the rates of cement growth through direct observation initial results indicate that cementation may lead to partial lithification of carbonate sands within a minimum of 8 months in water depths of up to 60 m these results provide some of the first direct evidence for rates at which cementation can occur in deeper marginal environments and confirms earlier hypotheses that very rapid syndepositional marine cementation may not be limited to very shallow subtidal and intertidal environments

re os geochronology for the nist rm 8505 crude oil the importance of analytical protocol and uncertainty abstract the national institute of science and technology nist research material rm 8505 is a venezuelan crude oil certified for its vanadium concentration it has recently been proposed as a matrix matched standard for interlaboratory comparison of re os isotopic analyses while serving as a benchmark for petroleum based re os method development here the nist rm 8505 oil is characterized using a multi solvent asphaltene precipitation permitting interlaboratory re os data comparison and evaluation of techniques used to derive re os isochron ages we present a detailed discussion of analytical blank sources critical evaluation of analytical uncertainties method improvements and data corrections additionally we provide a comprehensive discussion on the effect of data population size and analytical uncertainty on isochron statistics and the robustness of isochron ages in addition to nist rm 8505 2 43 0 15 ng g re 30 0 0 7 pg g os we provide the first re os data for nist srm 1634c 1 73 0 01 ng g re 32 5 0 8 pg g os and enviromat scp science 39 1 pg g re 1 01 0 02 pg g os two other commercially available hydrocarbon trace metal standards the application of several asphaltene asph precipitations using different n alkane solvents pentane heptane and decane for the nist rm 8505 significantly improved the spread of data in the 187re 188os 187os 188os space multi solvent asphaltene precipitations yield an asphaltene only model 1 isochron 75 24 ma and the maltene crude oil asphaltene triplets yield a more precise model 1 isochron age of 78 5 2 3 ma with a 187os 188osinitial of 0 877 0 021 our age is considerably older compared to a previously reported model 1 age of 62 7 5 7 ma based on c7 asph crude oil malt triplets liu and selby 2017 and considerably younger than the model 1 age of 98 4 9 5 ma based on asph separates liu et al 2019 the analytical reasons leading to these age differences are discussed in this paper the geological context for the nist 8505 oil is unknown therefore we use trace metal data ni v fe mo etc to derive plausible constraints for the origin of this oil the nist 8505 oil shows trace metal data similar to published data for oils from the urdaneta district the la paz mara and lagunillas oil fields and source rocks of the la luna formation in the maracaibo basin venezuela in this geological context our 79 ma age for the nist 8505 oil implies a late cretaceous early hydrocarbon generation event from older upper cretaceous source rocks and overlaps with the youngest age of potential source rocks in the maracaibo basin however the age of large scale oil generation in the maracaibo basin is significantly younger lasting from the oligocene to the pliocene

re os geochronology and os isotope fingerprinting of petroleum sourced from a type i lacustrine kerogen insights from the natural green river petroleum system in the uinta basin and hydrous pyrolysis experiments abstract rhenium osmium re os geochronology of marine petroleum systems has allowed the determination of the depositional age of source rocks as well as the timing of petroleum generation in addition os isotopes have been applied as a fingerprinting tool to correlate oil to its source unit to date only classic marine petroleum systems have been studied here we present re os geochronology and os isotope fingerprinting of different petroleum phases oils tar sands and gilsonite derived from the lacustrine green river petroleum system in the uinta basin usa in addition we use an experimental approach hydrous pyrolysis experiments to compare to the re os data of naturally generated petroleum in order to further understand the mechanisms of re and os transfer to petroleum the re os geochronology of petroleum from the lacustrine green river petroleum system 19 14 ma all petroleum phases broadly agrees with previous petroleum generation basin models 25 ma suggesting that re os geochronology of variable petroleum phases derived from lacustrine type i kerogen has similar systematics to type ii kerogen e g selby and creaser 2005a selby and creaser 2005b finlay et al 2010 however the large uncertainties over 100 in some cases produced for the petroleum re os geochronology are a result of multiple generation events occurring through a 3000 m thick source unit that creates a mixture of initial os isotope compositions in the produced petroleum phases the 187os 188os values for the petroleum and source rocks at the time of oil generation vary from 1 4 to 1 9 with the mode at 1 6 oil to source correlation using os isotopes is consistent with previous correlation studies in the green river petroleum system and illustrates the potential utility of os isotopes to characterize the spatial variations within a petroleum system hydrous pyrolysis experiments on the green river formation source rocks show that re and os transfer are mimicking the natural system this transfer from source to bitumen to oil does not affect source rock re os systematics or os isotopic compositions this confirms that os isotope compositions are transferred intact from source to petroleum during petroleum generation and can be used as a powerful correlation tool these experiments further confirm that re os systematics in source rocks are not adversely affected by petroleum maturation overall this study illustrates that the re os petroleum geochronometer and os isotope fingerprinting tools can be used on a wide range of petroleum types sourced from variable kerogen types

oxygen isotopes and sea level from the time that detailed oxygen isotope records derived from foraminifera living in the constant temperature environment of the abyssal ocean became available there has been a discrepancy between the ice volume record that these records imply and that derived from the altitude of dated coral terraces around the world here we re examine the data and conclude that the temperature of the abyssal ocean has been an actively varying component of the climate system

glacial versus interglacial sedimentation rates and turbidite frequency in the bahamas the southern tongue of the ocean is a 1300 m deep flat floored basin in the bahamas that receives large amounts of sediment from the carbonate platforms surrounding it on three sides we have examined five 8 13 m long piston cores and determined bulk sedimentation rates turbidite frequency and turbidite accumulation rates for the past two glacial and interglacial periods the mean of bulk sedimentation rates is four to six times higher in interglacial periods average accumulation rates of recognizable turbidites are higher by a factor of 21 to 45 and interglacial turbidite frequency is higher by a factor of 6 to 14 sediment composition indicates that increased interglacial rates are due to higher accumulation of platform derived material additional data from other bahamian basins as well as published material from the caribbean strongly suggest that highstand shedding is a general trend in pure carbonate depositional systems carbonate platforms without a siliciclastic component export more material during highstands of sea level when the platform tops are flooded and produce sediment the response of carbonate platforms to quaternary sea level cycles is opposed to that of siliciclastic ocean margins where sediment is stored on the inner shelf during highstands and passed on to continental rises and abyssal plains during lowstands of sea level

regional scale porosity and permeability variations in upper devonian leduc buildups implications for reservoir development and prediction in carbonates upper devonian carbonate rocks of the rimbey meadowbrook reef trend in the subsurface of central alberta are characterized by a wide range of porosity and permeability values dolostones show the highest values of absolute and average horizontal and vertical permeability for all rock types horizontal permeability averages tens of times to several hundred times the vertical permeability if considered irrespective of burial depth limestones and dolomitic limestones are more porous than dolostones there is an overall decrease of porosity and permeability in leduc formation carbonates with increasing burial depth and thermal maturity along the reef trend at relatively shallow burial depths 2000 m dolostones are significantly more porous and permeable than limestones dolostones appear to retain their porosity and permeability much better than limestones during burial indicating that dolostones are more resistant to porosity and permeability reducing processes particularly pressure solution thus reservoir quality in dolostone reservoirs decreases less severely with depth in addition porosity enhancement in dolostones by leaching in the burial environment during and after dolomitization is important although these significant relationships arenot new concepts they appear to be underappreciated the better porosity and permeability of dolostones relative to limestones at greater depths is of economic significance for exploration in upper devonian carbonates of the deep alberta basin and probably other carbonate areas where reservoirs are mostly found in dolostones and where limestones have a low reservoir potential and pose a considerable exploration risk the best reservoir potential in the leduc formation of the deep alberta basin occurs in dolomitized buildups that were connected to a regional conduit system

re os geochronology and systematics in molybdenite from the endako porphyry molybdenum deposit british columbia canada precise re os molybdenite ages for the endako porphyry molybdenum deposit and the nithi mountain occurrence british columbia canada yield new information on the timing of molybdenite deposition three episodes of mineralization are indicated 1 154 ma 2 148 to 146 ma and 3 145 ma re os dates agree well with new u pb and 40 ar 39 ar data from granitoids of the nithi phase 155 2 1 5 ma the endako phase 148 4 1 5 ma and a preore dike 147 4 0 6 ma and the casey phase 145 1 0 2 ma plus hydrothermal biotite 144 5 0 75 ma villeneuve et al 2001 the excellent correlation between the re os dates from molybdenite and the ages of granitoids from the area indicates a direct genetic relationship between these plutons and molybdenite mineralization these re os data also provide further insight into the systematics and robustness of re os in molybdenite the endako molybdenum deposit is the product of two episodes of mineralization both associated with moderate salinity 5 15 wt nacl equiv and high temperature 440 c fluids a third low temperature 190 300 c hydrothermal event is associated with eocene faulting selby et al 2000 the clear distinction between two of the re os molybdenite dates in the deposit and the close agreement of these dates to the ages of granitoid intrusions suggest that the re os system in molybdenite is not reset by high temperature 440 c moderate salinity hydrothermal fluids in contrast magmatic biotite in a 147 ma preore dike has a 40 ar 39 ar age of 145 ma identical to the age of the hydrothermal biotite indicating that 440 c fluids reset the ar ar system in biotite hence we interpret that the re os isotope system in molybdenite is resistant to resetting by high temperature fluids 440 c in contrast to the 40 ar 39 ar system in biotite the re os system in molybdenite is also unaffected by postore hydrothermal fluids these data show that molybdenite can provide critical information in understanding sulfide mineral deposition in ore systems which is not recorded by other isotopic systems in alteration minerals

re os geochronology and fingerprinting of united kingdom atlantic margin oil temporal implications for regional petroleum systems we demonstrate that the re os system can be used to understand the temporal evolution of a petroleum system as well as aid in the identification of source units traditional geochemical methods high performance liquid chromatography carbon isotope gas chromatography and biomarker analysis indicate that the main source of 18 oils analyzed for re os geochronology from the united kingdom atlantic margin clair schiehallion cuillin and foinaven fields is late jurassic marine shales this is supported by the os isotopic composition 187 os 188 os of the late jurassic source 0 9 2 4 at the timing of oil generation being similar to that of the oil 68 ma 0 92 1 12 indicating that the re os system can potentially be used to fingerprint the source of an oil the re os data for the 18 oils yield an age 68 13 ma that agrees with both the relative basin models and absolute ar ar geochronology timing of oil generation indicating that that re os oil geochronology records oil generation events

brunhes matuyama boundary 790 k y date consistent with odp leg 130 oxygen isotope records based on fit to milankovitch template the age of the brunhes matuyama boundary until recently taken as 730 k y in standard time scales for the quaternary e g imbrie et al 1984 prell et al 1986 and in ocean drilling program results kroenke et al 1991 shackleton et al 1992 has been revised to between 780 k y and 790 k y based on both orbital tuning of oxygen isotope data e g shackleton et al 1990 and on new 40ar 39ar dates on volcanic rocks e g baksi et al 1992 we apply the revised chronology to an oxygen isotope record for ontong java plateau odp leg 130 western equatorial pacific using this time scale we find excellent agreement between the record and a template based on orbital forcing the equation for the template is where the change in ice mass is given as the difference between constant ice growth igr and variable amounts of melting melting depends on summer insolation at 65 n ins the current ice mass present ice and the average ice mass mem over a given time interval l the memory base l and the exponents are adjusted for best fit the fit to the relevant portion of the record yields an age of 790 6 k y for the brunhes matuyama boundary the age assignment is robust relative to substantial changes in parameter settings

the evolution of pleistocene climate a time series approach abstract in the later part of the pleistocene variations in global ice volume have been dominated by an approximate 100 000 year cycle analysis of 2 myr long oxygen isotope record from an equatorial pacific core indicates that this is true only for the last 900 000 years prior to this time the amplitude of the 100 000 year cycle is much reduced as is the variance of all oscillations with periods greater than 60 000 years based on results of time series analysis of this 2 myr long record the pleistocene glacial cycles can be divided into three sections 1 the late pleistocene 0 900 kyr b p where the variations in the isotope record are dominated by the 100 000 year cycle 2 the middle pleistocene 900 1450 kyr b p in which low frequency components are not as important as in the later period of the pleistocene and 3 the early pleistocene late pliocene 1450 2000 kyr b p where general reductions of importance at all frequencies is seen as compared to the later intervals recent modeling efforts which describe variations in global ice volume show that the dominant low frequency component observed in the late pleistocene can result from different time constants for the rate of glacial growth and decay in response to variations in the earth s orbital parameters it is hypothesized that during the early pleistocene the rate of growth and decay of glaciers were more similar and that continental erosion by successive glacial advances lowered the land surface in areas of ice cap formation to below sea level when the ice caps became marine based more rapid decay of the ice became possible

quaternary slope development of the western leeward margin of the great bahama bank abstract to date work on the great bahama bank s western leeward margin has centred chiefly on seismic scale expressions of carbonate sequences and systems tracts however periplatform slope sediments also exhibit very well developed cyclicity on scales of decimetres to several metres it is these small scale high frequency cycles within the larger scale facies successions of the quaternary which form the main topic of this paper previous studies have shown that the small scale cycles correlate to the orbitally forced high frequency sea level changes therefore these cycles should indicate how sea level has affected the slope development and thus platform margin evolution during this period through detailed high resolution sequence stratigraphy of the great bahama bank s leeward margin obtained via 18o isotope and mineralogical xrd analyses confined by u th dating and nannofossil bioevents a greater understanding of the bedding geometries within the pleistocene holocene seismic sequences and clues as to the nature of the slope development has been achieved the high resolution seismic profiles indicate that since the plio pleistocene change in geometry in which the great bahama bank developed into a rimmed platform continued steepening and subsequent progradation of the leeward margin has typified slope development during the quaternary which is described as an accretionary slope however on the basis of our observations we conclude that only the early to lower middle pleistocene section isotope stages 45 20 and the holocene isotope stage 1 of the leeward margin is accretionary this indicates that a degree of erosion and or by passing has occurred on the leeward margin since the lower middle pleistocene isotope stage 19 during the first part of this period isotope stages 19 12 erosion and or by passing occurred in the middle to lower slope regions and toe of slope by the end of the upper middle to late pleistocene phase isotope stages 11 2 erosion also occurred on the upper slope this erosion by currents at the toe of slope and oversteepening of the upper and middle slopes have led to back cutting upslope and resulted in the progressive retreat of the toe of slope towards the platform to the east however the rise in sea level since the last glacial maximum to its present day level has allowed high productivity on the platform top during the holocene and the deposition of a thick sediment wedge on the slope and sedimentation across the entire leeward flanks this has led to the redevelopment of an accretionary slope and continued westward progradation of the great bahama bank s western leeward margin

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

match between late pleistocene great bahama bank and deep sea oxygen isotope records of sea level throughout the pliocene and pleistocene a hierarchy of sea level fluctuations alternately flooded and exposed the great bahama bank controlling its sedimentary evolution the persistent rhythm of sea level is best shown in the upper pleistocene to holocene section in which 10 cycles of sea level rise and fall are identified in core borings on the lee side of the bank the same number of sea level oscillations is indicated within the brunhes chron of the deep sea oxygen isotope record in the bahamas sea level oscillations are inferred from meter scale parasequences of shallow marine carbonates interpreted largely as transgressive and highstand deposits sandwiched between discontinuity horizons interpreted as subaerial exposure horizons that developed during lower sea level subaerial exposure shifted from being largely restricted to the platform interior during the late pliocene olduvai chron to occurring across the entire lee side of the platform at the beginning of the brunhes chron this change on the platform is highly suggestive of the deep sea oxygen isotope proxy of sea level which indicates a fundamental paleoclimatic shift at approximately the same time despite dating limitations results from the bahamian cores can be used to push the calibration of the sea level record back to the late pliocene

direct radiometric dating of the devonian mississippian time scale boundary using the re os black shale geochronometer many phanerozoic time scale boundaries are characterized by oceanic anoxia and mass extinction events with the deposition of black shale the re os isotope system in black shale can be used to provide depositional ages for these rocks thus yielding direct radiometric ages for time scale boundaries we demonstrate that the re os black shale geochronometer can yield precise ages useful for time scale research through study of the devonian mississippian boundary within the exshaw formation canada the re os date determined places the devonian mississippian boundary at 361 3 2 4 ma 2 including uncertainty model 1 mean square of weighted deviates 1 2 in accord with recent calibration interpolated from u pb zircon dates 360 7 0 7 ma for the hasselbachtal section germany

geochemical constraints on models for secondary petroleum migration along the upper devonian rimbey meadowbrook reef trend in central alberta canada abstract the upper devonian rimbey meadowbrook reef trend of central alberta canada is one of the classical examples that was originally used to support gussow s model of differential hydrocarbon entrapment however the clear discrepancies existing between the ideal hydrocarbon distribution predicted by this model and the one actually observed led to several alternative geological models most of which have not been constrained by organic geochemical data the present study uses a wide range of bulk and molecular parameters to determine the oil source characteristics and thermal maturity along the reef trend the concentrations and isomeric distributions of alkylcarbazoles and non alkylated benzocarbazoles in oils are employed to study petroleum migration pathways and to constrain the existing secondary petroleum migration models the data indicates that gussow s principle is generally applicable as the oils in the up dip direction generally have longer implied migration distances although this is not always the case other factors not recognized previously such as the presence of two subfamilies of the duvernay formation sourced oils may also have contributed significantly to the discrepancies in the oil and gas distributions between the model predictions and the actual observational data

the growth potential of a bypass margin great bahama bank abstract observations and sampling to 350 m from a two person submersible off chub cay berry islands bahamas suggest that accretion is occurring to varying degrees in all subdivisions of the platform margin profile at the chub cay dive site a nearly vertical wall extends to 140 m subsea below and extending to the limit of the dives is a low relief fore reef slope ca 50 of limestone veneered with sediment this study supports the idea that the upper portion of the wall from 30 to 85 m the brow or deep reef is a principal source of talus now cemented that foots the windward margins of great bahama bank we infer that the brow grows outward so rapidly that it calves periodically the resulting debris largely bypasses the deeper wall but some is perched on the steep fore reef slope below where it is soon incorporated into the slope by submarine cementation this model further explains the observation that carbonate platform margins tend to steepen through time and shows how those steepened slopes can be maintained during their continued growth

quaternary fluctuations in the northern hemisphere trade winds and westerlies the mass accumulation rate and grain size of the total colian component isolated from pelagic sediment in two north pacific cores piston core kk75 02 under the prevailing westerlies and deep sea drilling project site 503 beneath the trade winds have been used to evaluate changes in the intensity of atmospheric circulation and source area aridity over the past 700 000 yr the eolian grain size a direct indicator of wind intensity fluctuates at periodicities similar to those calculated for the earth s orbital parameters of precession obliquity and eccentricity both sites display greater variability in wind intensity prior to 250 000 yr ago eolian accumulation rates an indicator of source area aridity fluctuate at periodicities similar to those of the glacial interglacial cycles lower eolian accumulation rates during colder glacial times most likely reflect increased glacial age humidity at central asian and central american source areas

organic maturity in the western canada sedimentary basin abstract organic maturity of strata in the western canada sedimentary basin has been investigated in numerous studies using petrological chemical and physical methods compilation of these data has facilitated a regional analysis of the maturity patterns and maturation history variation in maturity patterns can be divided into three levels basin wide 1st order regional 2nd order and local 3rd order first order variations are manifested by an overall increase in maturity of strata of the same age from east to west from the plains to the rocky mountain foothills and front ranges in response to progressively deeper burial and higher paleo geothermal gradients superimposed on this first order basinal scale variation in maturity are second and third order variations which reflect local differences in depth of burial conductive and advective heat transport or effects of thrust faulting maturity gradients in the axis of the basin are low averaging 0 10 log r omax km whereas in the front ranges and foothills the gradients are substantially greater average 0 25 log r omax km variations in maturity gradients reflect lower paleo geothermal gradients resulting from rapid sediment loading and subsequent unloading in the plains and higher conductive heat transport in the deformed belt the main component of maturity of most phanerozoic strata occurred during deep burial under upper cretaceous and or paleogene molasse in foredeeps developed in response to crustal loading during the easterly migration of the foreland fold and thrust belt as a result of the west to east propagation of deformation during the laramide orogeny deep burial maturation hydrocarbon generation and uplift occurred earlier in the foreland belt late cretaceous than in the plains to the east where the main component of maturity and hydrocarbon generation occurred as late as eocene in the plains because of the low maturity gradients a thick succession of strata currently is within the oil window in the deformed belt however higher maturity gradients have resulted in a thinner succession of strata within the oil window in response to higher paleo geothermal gradients strata in the deformed belt matured more quickly leading to more rapid hydrocarbon generation and migration than areas to the east in the southern part of the cordillera a significant component of maturation post dates structural deformation of the strata whereas in northeastern british columbia and adjacent parts of alberta maturity primarily pre dates structural deformation data from some deep bore holes and surface samples in the southern part of the cordillera provide evidence for maturity post dating or accompanying emplacement of major overthrust sheets tectonic burial in most areas however there is no clear evidence for the timing of maturity relative to faulting or evidence of frictional heating the maturity gradients and levels of maturity of surface and near surface strata in the western plains and rocky mountain foothills indicate that a succession several kilometres of paleogene strata were deposited in these regions foredeep and subsequently eroded in response to post laramide uplift

middle miocene oceanographic event in the eastern gulf of mexico implications for seismic stratigraphic succession and loop current gulf stream circulation analysis of 1 500 km of high resolution single channel air gun seismic reflection profiles correlated with two drill cores demonstrates that the central west florida carbonate ramp slope experienced an oceanographic event in the middle miocene that resulted in a major stratigraphic break or succession of depositional sequences during the long term eustatic sea level rise in the early to middle miocene carbonate shelf margin deposition was characterized by seaward prograding sigmoidal clinoforms that built both laterally and vertically shelf margin sediments were relatively fine grained and rich in dolomite total organics and clay minerals approximately 12 15 m y ago tectonically controlled oceanographic closure of the isthmus of panama and or increased climatic gradients resulted in an intensification of oceanic circulation increased flow velocity of the loop current truncated antecedant clinoforms along the mid miocene shelf margin as the loop current for the first time became a dynamic oceanographic barrier to off shelf sediment transport carbonate deposition along the west florida slope changed dramatically from a series of prograding clinoforms to a pelagic slope front fill system bounded on its landward margin by contour current flow of an invigorated loop current shelf margin sediments presently represented as a condensed sequence became more carbonate rich coarser grained lower in organic content and depleted in dolomite our results suggest that oceanographic events can generate major seismic stratigraphic breaks that result in the succession of depositional sequences and systems along continental margins and that they may be as important as eustatic oscillations our data also suggest that modern loop current gulf stream circulation was initiated in the middle miocene controlled by tectono oceanographic and or climatic processes the challenge now is to identify seismic stratigraphic scale oceanographic events in the rock record differentiate them from eustatic responses and evaluate their paleoenvironmental significance

astronomic timescale for the pliocene atlantic δ18o and dust flux records of ocean drilling program site 659 high resolution benthic oxygen isotope and dust flux records from ocean drilling program site 659 have been analyzed to extend the astronomically calibrated isotope timescale for the atlantic from 2 85 ma back to 5 ma spectral analysis of the 18o record indicates that the 41 kyr period of earth s orbital obliquity dominates the pliocene record this is shown to be true regardless of fundamental changes in the earth s climate during the pliocene however the cycles of sahelian aridity fluctuations indicate a shift in spectral character near 3 ma from the early pliocene to 3 ma the periodicities were dominantly precessional 19 and 23 kyr and remained strong until 1 5 ma subsequent to 3 ma the variance at the obliquity period 41 kyr increased the timescale tuned to precession suggests that the pliocene was longer than previously estimated by more than 0 5 m y

new insights on the green river petroleum system in the uinta basin from hydrous pyrolysis experiments the tertiary green river petroleum system in the uinta basin generated about 500 million bbl of recoverable high pour point paraffinic crude oil from lacustrine source rocks a prolific complex of marginal and open lacustrine source rocks dominated by carbonate oil shales containing up to 60 wt type i kerogen occur within distinct stratigraphic units in the basin petroleum generation is interpreted to originate from source pods in the basal green river formation buried to depths greater than 3000 m along the steeply dipping northern margin of the basin producing fields in the altamont bluebell trend have elevated pore fluid pressures approaching 80 of lithostatic pressure and are completed in strata where open fractures provide permeability active hydrocarbon generation is one explanation for the origin of the overpressured reservoirs in this study experiments were undertaken to examine the mechanisms of hydrocarbon generation and accumulation in the uinta basin we combined analyses of representative source rocks from the entire green river stratigraphic section with detailed laboratory simulation experiments using both open and closed system pyrolysis this information provides new insights on lacustrine source rock lithofacies gas oil source rock correlations hydrocarbon generation kinetics and basin modeling the results show that the basal green river formation contains a unique type i source facies responsible for generation of paraffinic crude oils the classic type i oil shales in the upper green river formation correlate well with low maturity aromatic asphaltic samples we determined kinetic parameters for the source rocks and used them to develop basin models for hydrocarbon generation the models show that hydrous pyrolysis kinetic parameters are more consistent with the natural data in terms of predicted timing and extent of oil generation as compared to models using rock eval kinetics begin page 1334

highstand versus lowstand deposition on carbonate platform margins insight from quaternary foreslopes in the bahamas abstract examples of both highstand and lowstand deposition have been identified on the upper platform slopes of the tongue of the ocean bahamas highstand deposits consist of a thick wedge of fine grained skeletal and non skeletal sands and carbonate muds derived predominantly from the platform and deposited on slopes of 25 28 these sediments were deposited subsequent to platform flooding and may be more than 30 m thick along open leeward margins in contrast lowstand deposits consists of coarse skeletal sands gravels and boulders deposited on primary depositional slopes of 35 45 these sediments were derived from fringing reefs along the marginal escarpment and deposited during the initial rise of sea level following the last lowstand these variations in depositional character result in a distinctive signature of highstand versus lowstand deposition for steep sided platforms and further elucidate the response of carbonate platforms to high magnitude 5th order scale 20 000 yrs fluctuations in sea level

quaternary time scale for the ontong java plateau milankovitch template for ocean drilling program site 806 a 2 m y oxygen isotope record of globigerinoides sacculifer from the ontong java plateau based on cores from ocean drilling program leg 130 is dated by matching variations to an orbital template the procedure allows us to present the most complete quaternary record available for the western equatorial pacific the template generating algorithm describes a balance between growth and melting of ice following basic milankovitch theory ice growth is taken as constant while melting is taken to depend on summer insolation current ice mass and average past ice mass template settings must be changed once between 1 and 1 2 ma to reflect a major shift in climate template fits are strikingly good over much of the record and can be used to detect and fill gaps from core breaks and other disturbances one result of template dating is an exact age for the brunhes matuyama magnetic reversal boundary at 790 5 ka as well as several other precise dates 900 ka for the middle pleistocene climate shift 1070 1240 and 1450 ka for isotope stages 31 37 and 47 respectively sedi mentation rates fluctuate between 18 and 28 m m y a ca 400 ka cycle being the most prominent major anomalies arise within the transitional regime 1 2 to 1 ma the origin of the cycles is unknown we propose productivity variations in the western equatorial pacific

neoproterozoic cambrian petroleum system evolution of the micang shan uplift northern sichuan basin china insights from pyrobitumen rhenium osmium geochronology and apatite fission track analysis the neoproterozoic strata of the sichuan basin are a key target for oil and gas to evaluate the hydrocarbon evolution and its relationship with tectonic events in the micang shan uplift northernmost sichuan basin we apply solid bitumen geochemistry bitumen reflectance and fluorescence and re os geochronology the geochemistry of the solid bitumen indicates that it is highly mature pyrobitumen that formed contemporaneously with dry gas generation during oil thermal cracking the pyrobitumen is enriched in both re 106 191 ppb and os 3030 5670 parts per trillion the re os isotope data imply an early jurassic date for pyrobitumen formation which coincides well with age estimates from fluid inclusion data and basin modeling the re os date for pyrobitumen formation coupled with previously presented apatite fission track aft analysis show that exhumation of the neoproterozoic strata occurred during the cretaceous in the micang shan uplift this extensive uplift led to the erosion of any potential gas reservoirs and surface exposure of bitumen bearing neoproterozoic strata in contrast the more southern and central portions of the sichuan basin have experienced less severe exhumation and as a result neoproterozoic sourced gas systems are present this study shows that through the combined application of re os and aft methodologies the timing of gas generation and subsequent erosion of any potential gas reservoirs in the micang shan uplift northern sichuan basin can be quantified moreover the re os and aft data illustrate the potential to constrain the timing of gas generation in petroleum systems worldwide

bahama carbonate platforms the deep and the past abstract carbonate deposition prevailed for 150 ma in the bahama florida segment of the west atlantic margin and provides an interesting test for actualism present day facies patterns on the banks are controlled by the backward decrease of waves and tides and the concomitant increase of temperature and salinity variation in the troughs influx of sand and rubble from gravity flows varies with topography and distance from shallow water sources and allows one to define facies belts a rhythmic sequence of ooze and graded beds on the basin floors subdivided into a basin margin belt of coarse thick turbidites and basin interior with fine turbidites slope facies change with increase in height and declivity from accretionary to by pass to erosional regimes stratigraphic history of the bahamas is not simply a projection of the holo scene back in time both long term natural evolution decrease in subsidence upbuilding of the banks submarine erosion and outside factors climate eustacy have caused significant changes since the jurassic the bahamas seem to have evolved from a clastics evaporite province to a single carbonate evaporite platform and finally to an array of platforms and troughs during the platform trough stage the rate of upbuilding of the platforms decreased submarine canyon erosion increased platform flanks steepened as they grew higher and changed from accretionary to by pass to erosional slopes a change imposed by extraneous factors occurred in the pliocene when the great bahama bank changed from a giant reef rimmed atoll to a flat platform covered by oolites and peloid sands the bahamas share both long term trends as well as random changes by extraneous factors with other platforms in the geologic record compared to ancient platforms however they are unusually long lived and at a more advanced stage of growth they are deeply dissected by erosion their flanks are unusually high and steep and the troughs very narrow the neogene platform sequence is strongly controlled by eustatic sea level fluctuations

sources of periplatform carbonates northwest providence channel bahamas abstract sources and quantities of periplatform carbonate sediments have been determined in northwest providence channel bahamas by an end member method employing geochemical textural petrographic and sem data the holocene sediment over most of this open seaway 200 to 2 000 m deep between great bahama bank and little bahama bank is a drape of largely fine grained lime mud 50 cm thick shallow platform sources contribute 75 90 percent the remainder is planktonic foraminifera pteropods and coccoliths mud 62 m from each bank and the planktonic source are each geochemically distinct and can be quantified the two shallow platform sources dominate sedimentation in the deep basin trend surface maps of mud derived from each bank reveal no windward leeward asymmetry in cont ast to reported sand transport patterns this indicates that the fines are swept off both platforms by storm and tidal exchange concentric distribution of fines relative to source indicates no offset by currents suggesting that fecal pelleting must rapidly remove fine sediment from the water column canyon valleys and intercanyon highs have equal thicknesses of holocene sediment which indicates that pelagic processes of deposition presently dominate over gravity flow processes confirmation of this depositional process is that the deposition rate increased from 2 cm 1 000 years to 10 cm 1 000 years when postglacial sea level flooded the bank tops thus 80 percent of the present sedimentation rate results from bank top contribution furthermore the calculated mass of mud traceable to little bahama bank agrees excellently with the independent estimate of overproduction there in cores bank sediments resulting from the most recent sea level highstand sharply overlie planktonic sediments writing a sea level history in alternating stable and unstable mineral suites

petroleum evolution within the tarim basin northwestern china insights from organic geochemistry fluid inclusions and rhenium osmium geochronology of the halahatang oil field the newly discovered halahatang oil field in the northern tarim basin has a potential resource of more than 70 billion bbl of oil oil organic geochemical data from the halahatang oil field indicate that the oils are of moderate maturity biodegraded and represent one oil family derived from the same paleozoic marine source modeling of coeval aqueous and hydrocarbon bearing inclusion data provide fluid trapping temperatures and pressures of 100 c to 110 c and approximately 39 to 59 mpa 5656 8557 psi respectively the fluid inclusion data coupled with the previous basin model studies suggest a single prolonged oil migration event during the permian the re os isotope data of the oil yields an early permian re os age of 285 48 ma the age agrees with the timing of maturation of the paleozoic source via burial history modeling but is slightly older 5 55 m y than the oil migration and accumulation timing implied by the basin modeling coupled with fluid inclusion analysis and the published reservoir illite k ar dates thus the oil re os date suggests that oil generation in the halahatang depression of the tarim basin occurred during the early permian rather than the silurian as previously proposed with subsequent oil migration and accumulation occurring during the middle late permian as recorded by basin modeling coupled with fluid inclusion analysis and illite k ar dating in addition to promoting petroleum exploration in the tarim basin this study which combines crude oil re os isotope dating and traditional analytical methods organic geochemistry and fluid inclusion analysis to constrain petroleum evolution is applicable to hydrocarbon systems worldwide

controls and development of late quaternary periplatform carbonate stratigraphy in walton basin northeastern nicaragua rise caribbean sea the late quaternary sedimentary record in periplatform oozes and muds deposited on the northern nicaragua rise results from the interplay of four main controls 1 input of pelagic carbonates and bank derived fine aragonite and magnesian calcite 2 input of siliciclastic sediments 3 dispersal and removal of sediments by the caribbean current and 4 partial seafloor dissolution of metastable carbonates high accumulation rates of the calcite coarse sediment fraction throughout the study area demonstrate that planktonic foraminiferal productivity peaked during interglacial stages neritic carbonate productivity also peaked during interglacial stages when bank and shelf tops along the nicaragua rise were submerged within the photic zone because the bank and shelf tops remained mostly exposed during glacial stages the surface area available for the neritic carbonate productivity was drastically reduced to a narrow band along the margins of pedro bank and the southern shelf of jamaica a large volume of siliciclastic sediments transported from the coastal area of south america and the eastern and western regions of jamaica were deposited during glacial stages within the eastern and western deep extensions of walton basin this finding contrasts with the input pattern of siliciclastic sediments in walton basin itself where the largest input of siliciclastic sediments occurred at each of the interglacial climatic optima corresponding to intervals of maximum sea level transgression finally cores in water depths exceeding 1100 m display during interglacial stages aragonite accumulation rates systematically lower than rates in cores from water depths shallower than 1100 m this indicates that some bank derived aragonite and magnesian calcite has been partially removed by seafloor dissolution in areas from water depths exceeding 1100 m results from this study on the northeastern nicaragua rise indicate that not only do offbank transport and water column saturation state influence the late quaternary record but also that the variable strength of the caribbean current and the proximity of sources for siliciclastic sediments have played a major role in the development of the late quaternary periplatform high resolution stratigraphy

sedimentary patterns and geometries of the bahamian outer carbonate ramp miocene lower pliocene great bahama bank core logging and high resolution seismic data from odp leg 166 were used to analyse deposits of the neogene miocene lower pliocene bahamian outer carbonate ramp ramp sediments are cyclic alternations of light and dark grey wackestones packstones with interbedded calciturbidite packages and minor slumps cyclicity was driven by high frequency sea level changes light grey layers containing shallow water bioclasts were formed when the ramp exported material whereas the dark grey layers are dominantly pelagic calciturbidites are arranged into mounded lobes with feeder channels internal bedding of the lobes shows a north directed shingling as a result of the asymmetrical growth of these bodies calciturbidite packages occur below and above sequence boundaries indicating that turbidite shedding occurred during third order sea level highstands and lowstands highstand turbidites contain shallow water components such as green algal debris and epiphytic foraminifera whereas lowstand turbidites are dominated by abraded bioclastic detritus gravity flow depocentres shifted from an outer ramp position during the early miocene to a basin floor setting during the late miocene to early pliocene this change was triggered by an intensification of the strength of bottom currents during the tortonian which was also responsible for shaping the convex morphology of the outer ramp the miocene and lower pliocene of the leeward flank of great bahama bank provides an example of the poorly known depositional setting of the outer part of distally steepened carbonate ramps the contrast between its sedimentary patterns and the well known upper pliocene quaternary slope facies associations of the flat topped great bahama bank shows the strong control that the morphology of a carbonate platform exerts on the depositional architecture of the adjacent slope and base of slope successions

direct radiometric dating of hydrocarbon deposits using rhenium osmium isotopes rhenium osmium re os data from migrated hydrocarbons establish the timing of petroleum emplacement for the giant oil sand deposits of alberta canada at 112 5 3 million years ago this date does not support models that invoke oil generation and migration for these deposits in the late cretaceous most re os data from a variety of deposits within the giant hydrocarbon system show similar characteristics supporting the notion of a single source for these hydrocarbons the re os data disqualify cretaceous rocks as the primary hydrocarbon source but suggest an origin from older source rocks this approach should be applicable to dating oil deposits worldwide

late cenozoic eolian deposition in the north pacific asian drying tibetan uplift and cooling of the northern hemisphere a newly constructed record of eolian dust accumulation from the central north pacific shows that dust deposition increased by an order of magnitude quite rapidly at 3 6 ma we associate this sudden drying with the uplift of at least the northern portion of the tibetan plateau shutting off the indian ocean moisture source to central and western china this ten fold increase in atmospheric dust loading appears to be associated with the 1 m y long shift toward heavy 18o values that occurred at 3 6 2 6 ma the dust grain size record of wind intensity begins its late cenozoic coarsening a million years before the drying event at 4 5 ma the northern hemisphere cooling that results in intensification of the subpolar westerly winds may have as its ultimate cause the drawdown of atmospheric co2 in the latest miocene and the early pliocene closing of the panamanian seaway

evaluation of bitumen as a 187re 187os geochronometer for hydrocarbon maturation and migration a test case from the polaris mvt deposit canada abstract re os analyses of bitumen from the polaris mississippi valley type zn pb deposit demonstrate the potential utility of the re os system for establishing the absolute timing of hydrocarbon migration and generation the polaris deposit is hosted by carbonate rocks of the upper ordovician thumb mountain formation and sulphide mineralization during late devonian time is constrained by a rb sr sphalerite date of 366 15 ma and a paleomagnetic date bitumen occurs in open space fillings and coatings and is interpreted to have formed during the main and late stages of sulphide mineralization samples of bitumen were taken from a variety of locations from the main deposit at polaris re and os contents are 3 78 ppb and 55 1155 ppt respectively the 187re 188os ratios are very high ranging from 300 to 1800 and the os isotopic composition is moderately to highly radiogenic with 187os 188os ratios ranging from 3 3 to 13 7 calculated 187os 188os ratios at 366 ma are variable but an obvious grouping is defined by 11 of 16 analyses on a re os isochron diagram and these data yield a re os date of 374 9 ma mswd 12 model 3 in agreement with rb sr sphalerite and paleomagnetic dates for mineralization at polaris the combined dates suggest that the migration of fluids responsible for both pb zn mineralization and bitumen formation at polaris was broadly contemporaneous and was coeval with the late devonian ellesmerian orogeny though considerable further research is required to understand the re os isotopic systematics of migrated hydrocarbon systems bitumen re os geochronology may provide the possibility of establishing a more comprehensive understanding of mvt deposits sedimentary basin and petroleum systems

rapid growth rates of syndepositional marine aragonite cements in steep marginal slope deposits bahamas and belize abstract growth rates of marine botryoidal aragonite cements from steep 35 45 marginal slope deposits in the bahamas and belize have been determined by accelerator mass spectrometer radiocarbon dating of samples taken at the base and top of individual botryoids the pore filling cements which range from approximately 11 000 13 000 years old grew at average rates of 8 10 mm 100 yr with maximum rates 25 mm 100 yr radiocarbon dating of coexisting skeletal components indicates that cementation was syndepositional microsampling transects across individual botryoids for stable isotope analyses show little variation in 13c and 18o supporting the conclusion that cementation was extremely rapid although the cements show a progressive depletion in isotopic composition of approximately 1 13c and 2 18o from 13 ka to 11 ka the average variation 1 within individual por filling cements ranging in size from 2 mm to 32 mm bottom to top was 0 11 13c and 0 14 18o results of this study provide the first quantitative data on growth rates of marine carbonate cements in a marginal slope environment the data indicate that marginal slope deposits may lithify within several tens of years and suggest that geologically instantaneous cementation may be critical in stabilizing steep carbonate slope deposits at or above angles of repose

further evaluation of the re os geochronometer in organic rich sedimentary rocks a test of hydrocarbon maturation effects in the exshaw formation western canada sedimentary basin abstract re os isotopic analyses of a single organic rich sedimentary rock unit ors of known depositional age and at three levels of regional hydrocarbon maturity show that hydrocarbon maturation does not affect the ability the 187re 187os chronometer to yield a depositional age for such rocks we present re os isotope analyses from the late devonian exshaw formation in the subsurface of the western canada sedimentary basin alberta and obtain a re os isochron age of 358 10 ma 2 model 3 1 666 10 11 a 1 for samples ranging from hydrocarbon immature to overmature this age is within uncertainty of the established absolute age for the exshaw formation hydrocarbon immature and mature plus overmature samples show no significant age differences if regressed individually indicating that hydrocarbon maturation did not greatly disturb the re os isotope system in the exshaw formation as such we propose that the re os geochronometer may be used as a reliable tool for measuring the depositional ages of ors regardless of their level of hydrocarbon maturity we find that minimizing natural variation in the initial 187os 188os ratio is more important than avoiding hydrocarbon maturation in obtaining precise re os ages in particular the exshaw formation appears to contain a nonhydrogenous component of unradiogenic os in addition to the hydrogenous os load a subset of exshaw formation samples with 5 total organic carbon toc which should best reflect the hydrogenous os load alone yields a very well fitted isochron having a depositional age of 358 9 ma 2 1 666 10 11 a 1 with an initial 187os 188os ratio of 0 59 0 05 model 3 mean square of weighted deviates mswd 1 8 the initial 187os 188os ratio of this regression may provide an estimate of the os isotopic composition of local seawater at the time of deposition

the origin of pyrobitumens in upper devonian leduc formation gas reservoirs alberta canada an optical and eds study of oil to gas transformation pyrobitumens are common within deep basin devonian carbonate gas reservoirs of the western canada sedimentary basin incident light microscopy and sem eds have been used to study the origin of pyrobitumens from upper devonian isolated reefs of the leduc formation west central alberta reflectance properties romax and romin coke microtextures and relative s c ratios are used to classify the pyrobitumens in portions of reservoir units where nso enriched low aromaticity crudes may have been dominant oil to gas transformations produced mainly low reflecting isotropic to fine grained mosaic coke microtextures within the reservoir pyrobitumens type a in contrast where normal crudes were most likely dominant in the reservoir units prior to thermal cracking pyrobitumens with high bireflectance and very anisotropic coarse flow to domain cokes dominate type b oil to gas cracking within some of the reservoir units resulted in overpressuring and is represented by highly deformed pyrobitumens with fibrous coke textures and high bireflectance relative concentrations of type a and b pyrobitumens vary vertically and temporally in some of the reefal buildups the variations in the dominant pyrobitumen types are attributed to crude oil alteration which preceded crude oil to gas transformation e g tar mats in some zones whereas other zones may have been altered contemporaneously with crude oil cracking the later may have resulted from the oxidation and sulphur enrichment of the crude oil during thermochemical sulphate reduction this reaction is prevalent within devonian evaporite carbonate strata of the western canada sedimentary basin generating copious volumes of h2s the interaction between crude oil and sulphur enriched highly alkaline fluids is suggested by the presence of high s c very fine grained late stage calcined needle cokes within brecciated pyrobitumens

quaternary aragonite cycles and oxygen isotope record in bahamian carbonate ooze carbonate ooze in bahamian troughs displays cyclic variation in the content of aragonite a 10 m core devoid of turbidites shows 3 5 cycles each defined by a sharp increase followed by a gradual decrease of aragonite the aragonite curve matches closely the oxygen isotope curve of planktic foraminifera from the same core aragonite highs correspond to light oxygen isotope values and thus to interglacial stages content of terrigenous material is low during interglacials and higher during glacial periods intermittent flooding and exposure of the bahama banks is most probably not the cause of the cycles because the latest increase in aragonite precedes bank flooding by 8 000 yr and because the cycles are distinctly asymmetric with rapid increase and gradual decrease of aragonitic content carbonate dissolution cycles tied to the glacial rhythm of earth9s climate are a more likely explanation of the variations in aragonite content

further evaluation of the re os systematics of crude oil implications for re os geochronology of petroleum systems abstract here we further demonstrate asphaltene to be the main carrier of re and os in crude oil and additionally show that generally the less soluble fractions of asphaltene contain higher concentrations of re and os with the 187os 188os and 187re 188os values showing different trends between oils these observations are considered as evidence of the existence of different re and os carriers in crude oil the multiple heteroatomic ligands and porphyrins proposed as re and os hosts in crude oil are likely present as free molecules initially absorbed and occluded in the asphaltene aggregates and co precipitate with asphaltene in response to the addition of n alkanes the re os elemental and isotopic systematic behaviour revealed herein are interpreted to be the result of the involvement of re and os carriers in the aggregation and precipitation of asphaltene and the chemical make up of each particular crude oil asphaltene and maltene separated by a series of different alkanes show that asphaltene yields decrease from n pentane to n heptane and then tend to stay stable until n decane the 187os 188os values of the series of asphaltenes are extremely similar for each oil sample inhibiting the possibility of determining any re os dates with the asphaltenes of a single crude oil in contrast the maltene fractions exhibit re os isotopic variation nevertheless only the maltenes of one oil defined a reasonable isochron with an age similar to that of source rock as such this study highlights the need for a greater understanding in order to determine whether and in what cases fractions separated from a single crude oil by different alkanes can provide meaningful re os ages

rapidity of freshwater calcite cementation implications for carbonate diagenesis and sequence stratigraphy abstract in less than 10 years subaerially exposed holocene oolitic sand on eleuthera island bahamas was completely case hardened by freshwater vadose calcite cementation this phenomenon demonstrates the rapidity of freshwater carbonate diagenesis and explains in part why carbonate platforms preferentially shed unconsolidated sediment during highstands and not during lowstands when they are subaerially exposed it further proves contrary to what the exxon conceptual sequence stratigraphic models imply that carbonate deposits do in fact respond differently to subaerial exposure than siliciclastic deposits case hardening of unconsolidated carbonate sediment in response to subaerial exposure and early near surface freshwater diagenesis helps explain why shallow water platform sediments are preferentially shed into adjacent basins during highstands of sea level and not during lowstands during lowstands rapid subaerial lithification of carbonate sediments retards their reworking by wind or water once lithified these deposits are further stabilized by vegetation and karstification inhibiting their transport as unconsolidated sediment into adjacent deeper water basins observations from eleuthera island also document the reciprocal processes associated with freshwater carbonate diagenesis calcium carbonate released by partial dissolution of aragonitic ooids was locally reprecipitated as pore filling low magnesian calcite cement this reciprocity is confirmed by the confinement of freshwater dissolution and cementation to the upper 10 20 cm of the subaerially exposed body of oolitic sand below this case hardened surface oolitic sands remained unconsolidated only very minor amounts of calcite cement were required for effective casae hardening

40ar 39ar dating of oil generation and migration at complex continental margins oil bearing fluid inclusions trapped within authigenic k feldspar in late jurassic reservoirs throughout the uk atlantic margin are evidence for an early oil migration event from late jurassic source rocks in the faeroe shetland basin previous compound specific isotope studies confirm the hydrocarbon provenance temperature composition time data show that the late jurassic source rocks were oil mature 113 3 5 ma 2 this new 40ar 39ar authigenic k feldspar age shows that the late jurassic source rocks were oil mature 30 40 m y prior to current estimates

dating of multistage fluid flow in sandstones through ultraviolet laser argon argon dating of potassium feldspar cements containing fluid inclusions we determined temperature composition time data for paleofluids in a sedimentary basin including data for an evolving episode of fluid flow recorded in distinct phases of cement the fluid evolved from mixed aqueous oil 83 million years ago to purely aqueous by 76 million years ago thus dating the time of oil charge in this reservoir

re os dating of maltenes and asphaltenes within single samples of crude oil abstract re os geochronology of oil may constrain the timing of oil formation and improve oil source and oil oil correlations typically asphaltene asph the heaviest and most re os rich oil fraction from multiple oils within an oil field or a larger petroleum system are analyzed to obtain sufficient spread in re os isotopic ratios a mathematical necessity for precise re os isochrons here we offer a new approach for re os geochronology of oil based on isotopic analyses of different fractions within a single sample of crude oil we studied three oils from the gela oil field southern sicily italy recovered from triassic jurassic stratigraphic intervals streppenosa noto and sciacca formations within the gela 1 well asph insoluble in n alkane and maltene malt soluble in n alkane fractions of oil were separated using n pentane n hexane n heptane and n decane solvents the asph contents of the sciacca and noto oils 26 33 wt are notably higher compared to the streppenosa oil 7 12 wt asph we present an optimized re os procedure with sample digestion in a high pressure asher followed by isotopic measurements using negative thermal ionization mass spectrometry very high metal contents of gela oils allowed acquisition of precise re os data systematic variations between the type of solvent used for asph precipitation and the asph content of the oil also known from the literature and the re os contents of the asph and malt fractions first observed in this study provide important practical applications for re os analyses of oil most re and os 96 98 in the noto oil are hosted in the asph fraction in contrast a significant portion of re and os 33 34 is stored in the malt fraction of the lighter but heavily biodegraded streppenosa oil collectively our new data on alkane distribution hopane and sterane biomarkers major and trace element contents and re os concentrations and isotopic ratios of the oils and their fractions support the presence of two oil families streppenosa oil a heavily biodegraded oil generated at an early stage of thermal maturation from a shaly source rock is clearly distinct from the noto and sciacca oils both generated at peak maturation from a carbonate source rock with no subsequent biodegradation two re os ages were obtained crude oil and asph from noto and sciacca oil yield a model 1 isochron age of 27 5 4 6 ma with an initial 187 os 188 os of 3 89 0 43 mswd 1 6 n 19 we interpret this oligocene age as the time of initial oil generation that was probably related to the onset of regional collisional tectonics in southern sicily asph and crude oil from the streppenosa oil yield scattered data perhaps related to effects of biodegradation streppenosa malt yield a model 1 age of 200 0 5 2 ma and initial 187 os 188 os of 1 39 0 11 mswd 0 52 n 4 this age and initial os isotopic composition are consistent with rapid oil generation shortly after late triassic source rock formation perhaps driven by magmatic related heating here we document a potentially powerful geochronological tool that can be applied to single samples of crude oil within each oil the 187 os 188 os and most of the 187 re 188 os ratios in the malt fractions are lower than in the corresponding asph fractions crude oils being physical mixtures of asph and malt have intermediate re os isotopic ratios hence the needed spread in data points for a re os isochron is obtained by analyzing components of a single crude oil this approach eliminates pitfalls in interpretation of re os data from multiple oils with possibly different generation migration or reservoir modification history

evaluation of the rhenium osmium geochronometer in the phosphoria petroleum system bighorn basin of wyoming and montana usa rhenium osmium re os geochronometry is applied to crude oils derived from the permian phosphoria formation of the bighorn basin in wyoming and montana to determine whether the radiogenic age reflects the timing of petroleum generation timing of migration age of the source rock or the timing of thermochemical sulfate reduction tsr the oils selected for this study are interpreted to be derived from the meade peak phosphatic shale and retort phosphatic shale members of the phosphoria formation based on oil oil and oil source rock correlations utilizing bulk properties elemental composition 13c and 34s values and biomarker distributions the 34s values of the oils range from 6 2 to 5 7 with oils heavier than 2 interpreted to be indicative of tsr the re and os isotope data of the phosphoria oils plot in two general trends 1 the main trend n 15 oils yielding a triassic age 239 43 ma with an initial 187os 188os value of 0 85 0 42 and a mean square weighted deviation mswd of 1596 and 2 the torchlight trend n 4 oils yielding a miocene age 9 24 0 39 ma with an initial 187os 188os value of 1 88 0 01 and a mswd of 0 05 the scatter high mswd in the main trend regression is due in part to tsr in reservoirs along the eastern margin of the basin excluding oils that have experienced tsr the regression is significantly improved yielding an age of 211 21 ma with a mswd of 148 this revised age is consistent with some studies that have proposed late triassic as the beginning of phosphoria oil generation and migration and does not seem to reflect the source rock age permian or the timing of re migration late cretaceous to eocene associated with the laramide orogeny the low precision of the revised regression 21 ma is not unexpected for this oil family given the long duration of generation from a large geographic area of mature phosphoria source rock and the possible range in the initial 187os 188os values of the meade peak and retort source units effects of re migration may have contributed to the scatter but thermal cracking and biodegradation likely have had minimal or no effect on the main trend regression the four phosphoria sourced oils from torchlight and lamb fields yield a precise miocene age re os isochron that may reflect the end of tsr in the reservoir due to cooling below a threshold temperature in the last 10 m y from uplift and erosion of overlying rocks the mechanism for the formation of a re os isotopic relationship in a family of crude oils may involve multiple steps in the petroleum generation process bitumen generation from the source rock kerogen may provide a reset of the isotopic chronometer and incremental expulsion of oil over the duration of the oil window may provide some of the variation seen in 187re 188os values from an oil family

seismic sedimentologic interpretation of a carbonate slope north margin of little bahama bank abstract analyses of seismic profiles and descriptions of sediments from odp leg 101 cores were combined to investigate the evolution of the northern margin of little bahama bank which has been prograding northward since the early mio cene modern depositional systems on the midslope and lower slope are found to be uncharacteristic of interpreted an cient sedimentary environments large slump masses covered much of the lower slope during the middle miocene pos sibly triggered by a regional tectonic event throughout the late miocene and most of the pliocene a channel and levee system meandered across the sediment apron of the lower slope the modern lower slope contains no meandering chan nels although gullies incised in the midslope funnel sediments to the base of slope apron actively promoting sediment bypass on an accretionary margin these changes in sedimentation pattern on the lower slope indicate increasing strength of ocean bottom contour following currents from the pliocene pliocene to holocene gravitational creep has produced large scale rotational movement of unlithified sediments and a major detachment surface along the base of the middle miocene slump masses these creep lobes extend far into the lower slope where sediments are contorted by propagation of and movement along multiple minor detachment surfaces

from the great bahama bank into the straits of florida a margin architecture controlled by sea level fluctuations and ocean currents high resolution multichannel seismic data collected across the great bahama bank margin and the adjacent straits of florida indicate that the deposition of neogene quaternary strata in this transect are controlled by two sedimentation mechanisms 1 west dipping layers of the platform margin which are a product of sea level controlled platform derived downslope sedimentation and 2 east or north dipping drift deposits in the basinal areas which are deposited by ocean currents these two sediment systems are active simultaneously and interfinger at the toe of slope the prograding system consists of sigmoidal clinoforms that advanced the margin some 25 km into the straits of florida the foresets of the clinoforms are approximately 600 m high with variable slope angles that steepen significantly in the pleistocene section the seismic facies of the prograding clinoforms on the slope is characterized by dominant partly chaotic cut and fill geometries caused by submarine canyons that are oriented downslope in the basin axis seismic geometries and facies document deposition from and by currents most impressive is an 800 m thick drift deposit at the confluence of the santaren channel and the straits of florida this santaren drift is slightly asymmetric thinning to the north the drift displays a highly coherent seismic facies characterized by a continuous succession of reflections indicating very regular sedimentation leg 166 of the ocean drilling program odp drilled a transect of five deep holes between 2 and 30 km from the modern platform margin and retrieved the sediments from both the slope and basin systems the neogene slope sediments consist of peri platform oozes intercalated with turbidites whereas the basinal drift deposits consist of more homogeneous fine grained carbonates that were deposited without major hiatuses by the florida current starting at approximately 12 4 ma sea level fluctuations which controlled the carbonate production on great bahama bank by repeated exposure of the platform top controlled lithologic alternations and hiatuses in sedimentation across the transect both sedimentary systems are contained in 17 seismic sequences that were identified in the neogene quaternary section seismic sequence boundaries were identified based on geometric unconformities beneath the great bahama bank all the sequence boundaries could be traced across the entire transect into the straits of florida biostratigraphic age determinations of seismic reflections indicate that the seismic reflections of sequence boundaries have chronostratigraphic significance across both depositional environments

bypass margins basin restricted wedges and platform to basin correlation upper devonian canadian rocky mountains implications for sequence stratigraphy of carbonate platform systems abstract carbonate platforms can commonly keep up with relative sea level rise because of high rates of sediment accumulation and platform aggradation surrounding basinal environments are commonly starved but can receive variable extrabasinal siliciclastic input and episodically deposited carbonate sediment if accumulation rates in basinal settings lag behind those of the platform a bypass or erosional margin can develop under these circumstances platform and basin depositional sequences become physically detached and direct correlation of basinal and platform sequences is hindered we report here the results of high resolution stratigraphic analyses of two upper devonian isolated carbonate platforms in western alberta that provide insight into the sequence stratigraphy of bypass margins and criteria for accurate correlation of platform and basinal sequences the slope and basin sequences surrounding the miette and ancient wall platforms consist of basin restricted onlapping wedges of fine grained background sediment deposited dominantly from suspension and coarse grained platform derived sediment redeposited by a variety of gravity flow mechanisms sequence boundaries are identified within the redeposited carbonate intervals identification of sequence boundaries and differentiation of highstand and lowstand slope and basinal facies was based on the geometry mineralogy and clast content of redeposited carbonate units highstand carbonates contain sheet like debris flows and turbidites with abundant slope derived clasts and background facies with high total carbonate content lowstand carbonates contain sheet like and channelized debris flows and turbidites with abundant platform derived clasts and background facies with low carbonate content and locally high amounts of organic carbon transgressive facies are dominated by initially carbonate poor and organic rich background sediments that display a progressive increase in carbonate content and decrease in organic carbon content these patterns are interpreted to record abundant background carbonate sedimentation during late transgression and highstand when the carbonate factory was robust highstand redeposited carbonates record slope erosion due to oversteepening and slope readjustment processes lowstand redeposited carbonates indicate platform and platform margin erosion and low background carbonate sedimentation when the platform was either exposed or under very shallow peritidal conditions high siliciclastic and organic contents during lowstand and early transgression may partly be the result of reciprocal sedimentation but alternatively may represent continuous siliciclastic supply during times with little dilution by fine grained carbonate sediment successive stages of platform development at miette and ancient wall were controlled by accommodation changes driven by relative sea level fluctuations backstripping analyses of strata from both platforms confirm that significant differential subsidence was a major control on variations in platform thickness and patterns of slope development greater subsidence at ancient wall fostered the development of a steeper bypass margin and different slope evolution compared to miette slope oversteepening also initiated a process of slope readjustment that eventually reduced the platform to basin gradient and facilitated regressive platform progradation in conventional siliciclastic sequence stratigraphy basin restricted wedges are interpreted as lowstand deposits on the basis of their geometry and position relative to an updip margin wedge shaped basinal units along the miette and ancient wall bypass margins contain both highstand and lowstand facies that straddle sequence boundaries the results of this study provide objective criteria for differentiating systems tracts in carbonate slope and basin environments through mineralogic and compositional analyses providing more accurate correlation of detached platform and basin sequences interpretation of carbonate basin restricted wedges as purely highstand or lowstand deposits may lead to erroneous conclusions regarding sequence stratigraphy platform to basin correlation and the volumetric partitioning of sediments deposited in different systems tracts

fault charged mantle fluid contamination of united kingdom north sea oils insights from re os isotopes 187 os 188 os as expected from a kimmer idge clay formation source however oils from the viking graben with which the brent fi eld is associated and main east shetland basin display unradiogenic 187 os 188 os in this study we seek to demonstrate that these anomalous results are due to interaction with a mantle like fl uid that propagated through the main fault zone of the viking graben and east shetland basin this study enhances our understanding of the re os oil systematics and supports cur rent ideas regarding fault architecture crustal thinning and associated fl uid fl ow and interac tion in the north sea oil system abstract united kingdom north sea oils sourced from the kimmeridge clay formation contain unradiogenic 0 17 to 0 48 and radiogenic 1 04 to 3 34 187 os 188 os values the unra diogenic 187 os 188 os values are spatially associated with the main basin bounding faults of the viking graben and east shetland basin in contrast the radiogenic 187 os 188 os values are associated with north sea basins located farther away from the basin boundary faults we suggest that crustal thinning and strain localization within the viking graben and east shetland basin are suffi cient to have allowed basin bounding faults to propagate to suffi cient depth to act as conduits for mantle derived fl uids to interact with oil this hypothesis is sup ported by previous geochemical data for north sea oil fi elds furthermore we demonstrate the use of os isotopes as an important tracker of crustal scale fl uid dynamics and petroleum migration pathways in extensional basins

sedimentation on a lower jurassic carbonate platform flank geometry sediment fabric and related depositional structures djebel bou dahar high atlas morocco abstract along the southern margin of the high atlas morocco carbonate platforms of lower jurassic age developed in a transtensional basin the south atlas trough complete exposure and preservation of the platform margin flank and basin of the djebel bou dahar allowed the subdivision of the slope into an upper mid and lower slope based on geometry sediment fabric and depositional structures at ca 50 m below the platform margin coral algal patch reefs and ooid sand shoals grade into the depositional slope the straight upper slope showing discontinuous bedding at angles up to 29 extends down to ca 250 m and consists mainly of poorly sorted mixtures of sand and gravel almost devoid of mud avalanching rockfall and shearing of the coarse gravelly sediment are the dominant processes readjusting the upper slope the middle and lower part of the slope profile extending down to ca 500 m below the margin is separated from the upper slope by a distinct slope break slope angles range from 19 near the slope break down to 4 8 where the slope merges with the basin plain the irregularly bedded mid slope is characterized by scouring and deposition of muddy sands and non cohesive gravelly debris flows sedimentation on the dominantly muddy concave upwards lower slope can be separated into evenly bedded shales muddy sands and mud free gravel sand mixtures deposited as debris sheets the results of this study emphasize the relationship between sediment fabric and the slope geometry of a carbonate platform flank the overall geometry internal heterogeneity and depositional structures are suggested to be strongly related to the original character of the sediment supplied to the djebel bou dahar platform flank

comment on behavior of re and os during contact between an aqueous solution and oil consequences for the application of the re os geochronometer to petroleum geochim cosmochim acta 158 2015 1 21 abstract in a recent study mahdaoui et al 2015 simulated the contact of oil with re os bearing aqueous fluids in petroleum reservoirs and concluded that both metals could be rapidly and substantially enriched in the oil fraction these findings could have significant geological ramifications for the use of rhenium osmium re os geochronology in the age dating of oil deposits however the lack of data reproducibility between parallel experiments and misused parameter of recovery rate has cast doubt on the main conclusions of the paper re analyses of the raw data provided sufficient evidence to suggest that in petroleum basins with very low abundances of re and os the extraction of these metals to oil is unlikely to be a geologically instantaneous process as the authors implied in their study in addition the possibility of reactor leakage in the contacting experiments cannot be completely ruled out

magnetostratigraphic dating of shallow water carbonates from san salvador bahamas magnetostratigraphic results are reported here from a sequence of late neogene quaternary shallow water carbonate sediments from a continuous core drilled on the island of san salvador bahamas on the basis of the remanent magnetism of 136 samples from a 91 m measured section of core the polarity sequence can be correlated with the magnetic polarity time scale from the gilbert chron early pliocene through the late brunhes chron late pleistocene holocene magnetic polarities were determined on the basis of relative up down direction in the unoriented core extraction studies of the magnetic particles reveal the presence of single domain crystals of magnetite resembling those produced by the magnetotactic bacteria and algae the sequence of reversals provides a minimum of six new major chronostratigraphic markers for the pliocene pleistocene of the bahamas it confirms and refines the local timing of both the lithologic change from skeletal to nonskeletal sediments and the disappearance of coral and molluscan species from the bahamas as upper late pliocene between 2 6 and 2 7 ma that the primary magnetic remanence is preserved in shallow water carbonates including replacement dolomites suggests that this technique could be used to date similar tertiary and possibly even older carbonate sequences the establishment of a reliable magnetostratigraphy provides refined dating of shallow water carbonates and regional faunal appearances or disappearances sediment accumulation rates subsidence and depositional events

anatomy of a modern open ocean carbonate slope northern little bahama bank the open ocean carbonate slope north of little bahama bank consists of a relatively steep 4 upper slope between water depths of 200 and 900 m and a more gentle 1 2 lower slope between depths of 900 and 1300 m the upper slope is dissected by numerous small submarine canyons 50 150 m in relief that act as a line source for the downslope transport of coarse grained carbonate debris the lower slope is devoid of any well defined canyons but does contain numerous small 1 5 m hummocks of uncertain origin and numerous larger 5 40 m patchily distributed ahermatypic coral mounds sediments along the upper slope have prograded seaward during the cenozoic as a slope front fill seismic facies of fine grained peri platform ooze surface sediments show lateral gradation of both grain size and carbonate mineralogy with the fine fraction derived largely from the adjacent shallow water platform near surface sedimentary facies along the upper slope display a gradual downslope decrease in the degree of submarine cementation from well lithified hardgrounds to patchily cemented nodular ooze to unlithified peri platform ooze controlled by lateral variations in diagenetic potential and or winnowing by bottom currents submarine cementation stabilizes the upper part of the slope allowing upbuilding of the platform margin and controls the distribution of submarine slides as well as the headward extent of submarine canyons where unlithified sediments are heavily bioturbated and are locally undergoing dolomitization upper slope sediments are also conditioned eustatically resulting in vertical cyclic sequences of diagenetically unstable aragonite and magnesian calcite rich and stable calcite rich carbonates that may explain the well bedded nature of ancient peri platform ooze sequences lower slope sediments have prograded seaward during the cenozoic as a chaotic fill seismic facies of coarse grained carbonate turbidites and debris flow deposits with subordinate amounts of peri platform ooze coarse clasts are internally derived from fine grained upper slope sediments via incipient cementation submarine sliding and the generation of sediment gravity flows gravity flows bypass the upper slope via a multitude of canyons and are deposited along the lower slope as a wedge shaped apron of debris parallel to the adjacent shelf edge consisting of a complex spatial arrangement of localized turbidites and debris flow deposits a proximal apron facies of thick mud supported debris flow deposits plus thick coarse grained ta turbidites grades seaward into a distal apron facies of thinner grain supported debris flow deposits and thinner finer grained ta b turbidites with increasing proportions of peri platform ooze both the geomorphology and sedimentary facies relationships of the carbonate apron north of little bahama bank differ significantly from the classic submarine fan model as such a carbonate apron model offers an alternative to the fan model for palaeoenvironmental analysis of ancient open ocean carbonate slope sequences

density cascading off shelf sediment transport evidence and implications bahama banks abstract suspended sediment concentrations in shelf waters are often appreciable but sediments on their own rarely contribute enough density to cause suspension currents to transport them offshelf however sediment charged hyperpycnal high density waters are often generated over continental shelves because of the same set of air sea interactions intense heat and moisture flux responsible for thermohaline production and circulation of deep ocean water masses we propose the term density cascading to describe the processes by which hyperpycnal waters are released from shelf areas and together with entrained sediments sink to their compensation level in the adjacent ocean density cascading probably involves a range of physical processes from pelagic draping of slopes and basins with shelf muds to initiating turbidity currents capable of scouring canyons and plunge pools sediment bypassing of slopes and deposition of sands in deep basins thermodynamic air sea interactions can generate hyperpycnal water at all latitudes therefore density cascading has important implications not only to the sedimentology of modern carbonate platforms such as the florida shelf and bahama banks and stratigraphy in ancient limestones but also to offshelf sediment flux in siliciclastic environments

evaluating re os systematics in organic rich sedimentary rocks in response to petroleum generation using hydrous pyrolysis experiments abstract successful application of the 187re 187os geochronometer has enabled the determination of accurate and precise depositional ages for organic rich sedimentary rocks ors as well as establishing timing constraints of petroleum generation however we do not fully understand the systematics and transfer behaviour of re and os between ors and petroleum products e g bitumen and oil to more fully understand the behaviour of re os systematics in both source rocks and petroleum products we apply hydrous pyrolysis to two immature hydrocarbon source rocks the permian phosphoria formation toc 17 4 type ii s kerogen and the jurassic staffin formation toc 2 5 type iii kerogen the laboratory based hydrous pyrolysis experiments were carried out for 72 h at 250 300 325 and 350 c these experiments provided us with whole rock extracted rock and bitumen and in some cases expelled oil and asphaltene for evaluation of re os isotopic and elemental abundance the data from these experiments demonstrate that the majority 95 of re and os are housed within extracted rock and that thermal maturation does not result in significant transfer of re or os from the extracted rock into organic phases based on existing thermodynamic data our findings suggest that organic chelating sites have a greater affinity for the quadravalent states of re and os than sulphides across the temperature range of the hydrous pyrolysis experiments both whole rock and extracted rock 187re 188os ratios show small variations 3 3 and 4 7 for staffin respectively and 6 3 and 4 9 for phosphoria respectively similarly the 187os 188os ratios show only minor variations for the staffin and phosphoria whole rock and extracted rock samples 0 6 and 1 4 and 1 3 and 2 2 these isotopic data strongly suggest that crude oil generation through hydrous pyrolysis experiments does not disturb the re os systematics in ors as supported by various studies on natural systems the elemental abundance data reveal limited transfer of re and os into the bitumen from a type iii kerogen in comparison to type ii s kerogen 0 02 vs 3 7 suggesting that these metals are very tightly bound in type iii kerogen structure the 187os 188os data from the pyrolysis generated phosphoria bitumens display minor variation 4 across the experimental temperatures with values similar to that of the source rock this indicates that the isotopic composition of the bitumen reflects the isotopic composition of the source rock at the time of petroleum generation these data further support the premise that the os isotopic composition of oils and bitumens can be used to fingerprint petroleum deposits to specific source rocks oil generated through the hydrous pyrolysis experiments does not contain appreciable quantities of re or os 120 and 3 ppt respectively in contrast to natural oils 2 50 ppb and 34 288 ppt for re and os respectively which may suggest that kinetic parameters are fundamental to the transfer of re and os from source rocks to oils from this we hypothesise that at the temperatures employed in hydrous pyrolysis re and os are assimilated into the extracted rock as a result of cross linking reactions

a second type of highly asphaltic crude oil seepage stranded on the south australian coastline abstract strandings of semi solid to solid asphaltic bitumen along the coastline of south australia have been reported as far back as the late 1800s hitherto only a single variety now referred to as asphaltite has been attributed to seepage from the nearby bight basin the geochemistry of the asphaltites suggest they were derived from a marine source rock deposited under anoxic or euxinic conditions most likely a cretaceous ocean anoxic event and were generated within the early main oil window here we identify a new type of semi solid asphaltic bitumen collected following a severe storm event in 2016 termed asphaltic tars these viscous oils bear a strong geochemical resemblance to the asphaltites both oil types have high asphaltene contents identical n alkane carbon isotopic profiles and near identical source specific sterane distributions however several notable geochemical variations can distinguish these new strandings from the asphaltites the most notable of these differences include heavier bulk sulphur isotopic composition extremely high abundances of re and os with distinct 187re 188os and 187os 188os values and thermal maturity parameters consistent with generation in the late oil window the differences in sulphur isotopic composition and re os systematics could be considered evidence that despite their other source specific similarities the asphaltic tars originated from a different source rock however alteration of these two parameters can occur due to thermochemical sulphate reduction conclusive identification of this alteration process typically relies on further diagnostic parameters which are unfortunately not available in the case of coastal oil strandings this introduces uncertainty to the correlation of these two types of asphaltic oil in either scenario the similarities between these two types of oil suggest their source rock s contained highly comparable organic matter inputs we therefore attribute the origin of these new asphaltic tar strandings to natural seepage from the offshore bight basin

pleistocene sea levels and tectonic uplift based on dating of corals from sumba island indonesia we collected and dated fossil corals from the uplifted reef tracts of sumba island mass spectrometry tims and ams was used to obtain reliable and precise radiochronological results it is possible to identify outcrops corresponding to major sea level high stands isotope stage 1 starting at 7 1 kyr bp 5a c a 86 kyr bp 5c c a 105 kyr bp 5e between 119 and 132 kyr bp and 9 c a 305 kyr bp the radiochronological data are thus broadly consistent with the timing of sea level fluctuations predicted by the astronomical theory of paleoclimates comparison with similar data from other terrace sequences and from benthic 18o records indicates that the average tectonic uplift rate at cape laundi sumba is on the order of 0 2 0 5m kyr the lowest values 0 3m kyr lead to the build up of polycyclic topographic terraces in which it is possible to find corals grown during different interstadial or even interglacial times

precise re os ages of organic rich mudrocks and the os isotope composition of jurassic seawater rhenium and osmium isotope and abundance data have been obtained on precisely located samples from three suites of immature organic rich mudrocks from jurassic coastal outcrops in england the data provide accurate whole rock ages of 207 12 ma 181 13 ma and 155 4 3 ma for suites of hettangian toarcian exaratum subzone and kimmeridgian sensu anglico wheatleyensis subzone samples these new re os ages are indistinguishable within the assigned analytical uncertainties from interpolated depositional ages estimated from published geological timescales and establish the importance of the re os dating technique for chronostratigraphic studies early diagenetic pyrite nodules possess levels of re and os which are 1 2 orders of magnitude lower than in the enclosing organic rich mudrocks indicating that these elements had already been removed from sediment pore waters at the time of nodule formation thus the re os isotope system in these organic rich mudrocks has been closed since or from very soon after the time of sediment deposition because most of the re 98 and os 95 99 8 in the mudrocks is shown to be hydrogenous the 187os 188os i of the samples is interpreted to be that of contemporaneous seawater the data thereby provide the first estimates of the os isotope composition of jurassic seawater during the earliest jurassic hettangian the seawater 187os 188os ratio was extremely unradiogenic 0 15 it had increased to 0 8 at the end of the early jurassic toarcian 20 ma later while in the late jurassic kimmeridgian the seawater 187os 188os ratio was 0 59 the most likely explanation for the unradiogenic os isotope composition of hettangian seawater is that the contribution of unradiogenic os to the oceans from the hydrothermal alteration of oceanic crust greatly exceeded the input of radiogenic os from the continents at that time this interpretation is in line with observations suggesting that global weathering rates were low in the hettangian and that increased hydrothermal and volcanic activity preceded the break up of pangea the re os ratios of hettangian mudrocks and by inference of contemporaneous seawater are similar to those of mudrocks deposited at later times during the jurassic and argues against the unradiogenic os in hettangian seawater being derived from extraterrestrial meteoritic sources

rhenium osmium geochronology and oil source correlation of the duvernay petroleum system western canada sedimentary basin implications for the application of the rhenium osmium geochronometer to petroleum systems the re os geochronometer has been applied to many petroleum systems worldwide however it is debated if the re os systematics in petroleum actually record the timing of oil generation here we investigate the re os isotope systematics of the duvernay petroleum system in the western canada sedimentary basin which has been shown to be a relatively simple petroleum system that is associated with oil generated during the late cretaceous early eocene laramide orogeny from a single source the organic geochemistry of the duvernay oils pristane phytane ratios of 1 5 smooth homohopane profile c29 c27 c28 regular sterane distribution and predominance of diasteranes over regular steranes strongly suggests the oil source to be that of type i ii marine organic matter of the upper devonian duvernay formation the asphaltene fraction re os isotope data of the duvernay oil yield an age of 66 31 ma which is in excellent agreement with the timing of the main stage hydrocarbon generation of the duvernay formation based on basin models further the similarity between the 187os 188os compositions of the duvernay formation source rock 0 46 1 48 and the oil 0 55 1 06 at the time of oil generation supports the hypothesis that the 187os 188os composition of an oil is inherited from the source unit at the time of oil generation and therefore shows no or limited influence from interaction with basin fluids this study supports that the re os isotope systematics of an oil can yield the timing of oil generation and be used to trace its source

the geology of eleuthera island bahamas a rosetta stone of quaternary stratigraphy and sea level history abstract a 5 km stretch of coastline in north eleuthera reveals a long and detailed stratigraphy that includes all known surficial limestone units in the bahamas and supplements the record with several previously unrecognized ones eight paleosol bounded limestone parasequences comprise at least six interglacial periods the lithostratigraphy demonstrates cyclicity at several frequencies 10 5 10 4 20 40 ka and 10 3 years and displays a variety of distal to proximal shoreline facies indicative of shifting depocenters associated with changing sea levels stratigraphy petrology pedology and whole rock aminostratigraphy are used to correlate units and subunits among the 12 described sections amino acid ratios are also converted to absolute age estimates which support the lithostratigraphy the parasequences are correlated with marine oxygen isotope stages 1 to 13 or older evidence of middle pleistocene highstands are abundant in the eleutheran stratigraphy including paleo sea levels of decreasing age at 2 m 7 m 20 m associated with stages 11 and or 9 and two near present highstands during stage 7 a complex sea level history is associated with substage 5e while substage 5a is represented by near shore aggradation of coastal dune complexes in eleuthera and throughout the bahamas concordance of sea level deposits between bermuda and the bahamas reinforce their tectonic stability while the abundance of highstand evidence during the middle pleistocene contradicts suggestions that platform subsidence has obscured all evidence of these events below present sea level the high resolution late quaternary stratigraphy of eleuthera is unrivaled among geologic records from stable carbonate coastlines and thereby offers a rosetta stone for interpretation of the quaternary evolution of the bahamas and sea level history over the past 500 ka

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

source rock contributions to the lower cretaceous heavy oil accumulations in alberta a basin modeling study the origin of the immense oil sand deposits in lower cretaceous reservoirs of the western canada sedimentary basin is still a matter of debate specifically with respect to the original in place volumes and contributing source rocks in this study the contributions from the main source rocks were addressed using a three dimensional petroleum system model calibrated to well data a sensitivity analysis of source rock definition was performed in the case of the two main contributors which are the lower jurassic gordondale member of the fernie group and the upper devonian lower mississippian exshaw formation this sensitivity analysis included variations of assigned total organic carbon and hydrogen index for both source intervals and in the case of the exshaw formation variations of thickness in areas beneath the rocky mountains were also considered all of the modeled source rocks reached the early or main oil generation stages by 60 ma before the onset of the laramide orogeny reconstructed oil accumulations were initially modest because of limited trapping efficiency this was improved by defining lateral stratigraphic seals within the carrier system an additional sealing effect by biodegraded oil may have hindered the migration of petroleum in the northern areas but not to the east of athabasca in the latter case the main trapping controls are dominantly stratigraphic and structural our model based on available data identifies the gordondale source rock as the contributor of more than 54 of the oil in the athabasca and peace river accumulations followed by minor amounts from exshaw 15 and other devonian to lower jurassic source rocks the proposed strong contribution of petroleum from the exshaw formation source rock to the athabasca oil sands is only reproduced by assuming 25 m 82 ft of mature exshaw in the kitchen areas with original total organic carbon of 9 or more

stratigraphy and structure of southern blake plateau northern florida straits and northern bahama platform from multichannel seismic reflection data approximately 2 100 km of 24 fold multichannel seismic reflection data reveal much about the subsurface geology for a large part of the continental margin east of florida discordance between the westward dipping pre breakup sediments and the eastward sloping basement along the edge of the blake plateau is interpreted as an effect of a splinter of continental margin derived from the african plate by a spreading center jump in the middle jurassic early rifting centered under the main part of the blake plateau became inactive as a spreading center jump shifted the active rift to east of the present blake escarpment along the blake spur magnetic anomaly in the northern florida straits the data reveal that the breakup unconformity underlain by triassic lower jurassic arkosic volcaniclastics extends from southern florida to the western bahama banks these volcaniclastics are associated with the rift crust of intermediate nature formed just prior to and during breakup of the north american and african continental plates back reef platform deposits of limestones dolomites and evaporites of late jurassic to albian age extend from the blake bahama escarpment westward beneath florida these deposits formed what once was a megabank extending over a wider area than the present smaller isolated bahama banks the formation of the florida straits and bahama channels occurred during the cenomanian transgression only on the present bahama banks and florida platform did shallow water carbonate deposition persist to maintain a shallow bank environment evidence of recurring scour by current erosion is found in the florida straits erosional events apparently occurred in the middle cenomanian middle paleocene early middle eocene and eocene oligocene which coincidentally are times of lower eustatic sea level according to vail et al 1977a this evidence of florida current scour indicates that the current was present as far back as the cenomanian major faulting appears to have dropped the northeast providence channel relative to the western bahamas after the albian submarine erosion and bank buildup created the channels and smaller relief features like great abaco knoll beginning in about the cenomanian a carbonate bank margin and reef complex was present along the bahama escarpment since the middle jurassic apparently these organic buildups seeded on originally shallow structural relief on oceanic basement created during the spreading center jump to the position of the blake spur magnetic anomaly the bank margin apparently has retreated at least 15 km from a late jurassic earliest cretaceous position now marked by a bench below the au unconformity active faulting occurred along the great abaco fracture zone at least through the late cretaceous and perhaps into the tertiary these relatively young tectonic events together with the post albian faults in providence channel indicate interactions between the atlantic and caribbean plates and that extensions of faulting have taken place far to the northeast of cuba and the greater antilles

k ar and ar ar dating the aim of this chapter is to present the k ar and ar ar dating techniques in the context of noble gas studies since there are already several recent texts on k ar and ar ar dating dickin 1995 mcdougall and harrison 1999 the focus of this section will be aspects of argon transport and storage in the crust which affect k ar and ar ar dating including ar loss from minerals by diffusion and ar gain by minerals or excess argon the k ar dating technique was one of the earliest isotope dating techniques developed soon after the discovery of radioactive potassium and provided an important adjunct to u pb and u he dating techniques the ease of measurement and ideal half life 1250 million years see table 2 below for dating geological events has made this the most popular of isotopic dating techniques aldrich and nier 1948 first demonstrated that 40ar was the product of the decay of 40k and soon after k ar ages were being measured in several laboratories most often using an absolute method such as a mcleod gauge to measure argon concentrations the first published k ar results by such a technique were those of smits and gentner 1950 who analyzed sylvite from the buggingen oligocene evaporite deposits obtaining an age of 20 million years mass spectrometers which simultaneously measured very small amounts of gas and the isotope ratios necessary to make corrections for atmospheric contamination quickly replaced manometric techniques crucially the use of static vacuum techniques pioneered by john reynolds at the university of california berkeley meant that mass spectrometers were sufficiently sensitive to analyse the small amounts of gas released from common rocks and minerals although the earliest mass spectrometers were built in house the introduction of the commercially available ms10 farrar et al 1964 a small 180 metal mass spectrometer built for

pliocene pleistocene platform facies transition recorded in calciturbidites exuma sound bahamas abstract the composition of pliocene pleistocene calciturbidites of odp hole 632a exuma sound bahamas has been determined by point count analysis of thin sections in order to correlate basin platform events which influenced sedimentation strontium isotope chronostratigraphy has been used to correlate basin turbidite compositional variations to the evolution of the platform as documented by magnetostratigraphy magnetostratigraphy of great bahama bank cores shows that between 3 4 and 2 5 ma late pliocene ooids and peloids became the dominant sediments across the platform after this period the platform has been subjected to a series of subaerial exposure events this led to slow accumulation on the platform and starvation of the basin around 1 ma the platform was reflooded the transgression being followed by an increased production of ooids there is a change in calciturbidite composition in the earliest pleistocene coinciding with a regional unconformity pleistocene turbidites containing abundant skeletal material and ooids overlie pliocene early pleistocene turbidites rich in mud and skeletal material the duration of the hiatus represented by the unconformity based on 87 sr 86 sr dating is 1 0 0 7 ma prior to this hiatus sedimentation rates in the basin are an order of magnitude lower than present day accumulation rates we conclude that reflooding of the platform around 0 8 ma led to shedding of both non skeletal and skeletal sediments the change of great bahama bank from a pliocene reef rimmed skeletal platform to the present day flat topped bank dominated by non skeletal sediments can be correlated with the observed change in turbidite composition

catastrophic collapse of the west florida carbonate platform margin a large truncation surface has been discovered on seismic reflection profiles along the distally steepened ramp of the west florida carbonate platform margin this structure interpreted as a submarine slide scar is at least 120 km across and up to 30 km wide the slide has removed as much as 300 350 m of late paleogene and early neogene strata collapse occurred in the middle miocene within an episode of seaward progradation during a relative sea level highstand postcollapse depositional sequences thicken abruptly across the structure this suggests catastrophic slope failure gravitational instability created by high rates of sediment accumulation is believed to have been the triggering mechanism such large scale platform margin collapse is likely to generate megabreccia debris flows as well as aid in the landward retreat of precipitous high relief carbonate escarpments