drivers of holocene sea level change in the caribbean abstract we present a holocene relative sea level rsl database for the caribbean region 5 n to 25 n and 55 w to 90 w that consists of 499 sea level index points and 238 limiting dates the database was compiled from multiple sea level indicators mangrove peat microbial mats beach rock and acroporid and massive corals we subdivided the database into 20 regions to investigate the influence of tectonics and glacial isostatic adjustment on rsl we account for the local scale processes of sediment compaction and tidal range change using the stratigraphic position overburden thickness of index points and paleotidal modeling respectively we use a spatio temporal empirical hierarchical model to estimate rsl position and its rates of change in the caribbean over 1 ka time slices because of meltwater input the rates of rsl change were highest during the early holocene with a maximum of 10 9 0 6 m ka in suriname and guyana and minimum of 7 4 0 7 m ka in south florida from 12 to 8 ka following complete deglaciation of the laurentide ice sheet lis by 7 ka mid to late holocene rates slowed to

carbonate recrystallization in shallow marine environments a widespread diagenetic process forming micritized grains abstract integrated morphologic petrographic and mineralogic analyses of sediment grains of the miliolid foraminifer archaias and the green alga halimeda from shallow marine environments in florida the bahamas and belize document pervasive syndepositional recrystallization in these grains scanning electron microscopy shows that original skeletal rods and needles recrystallize on the sea floor to a variety of equant micritic fabrics in thin section this textural alteration corresponds to progressive micritization with loss of the familiar golden brown color exhibited by fresh miliolids and halimeda and gradual development of gray cryptocrystalline carbonate x ray diffraction and electron microprobe studies show that this textural alteration may be accompanied by mineralogical changes from mg calcite to aragonite or from aragonite to mg calcite our findings support petrographic studies that reported syndepositional recrystallization in a wide variety of carbonate grains about thirty years ago but which have been largely ignored together these studies indicate that carbonate recrystallization is a widespread process of early diagenesis extensive recrystallization in shallow tropical seas challenges basic principles regarding the setting and timing of textural and mineralogic alteration of metastable carbonate grains in addition recognition of recrystallization as an important process of micritization contradicts prevailing theories that micritization results solely by infilling of microborings finally our findings raise fundamental questions concerning the nature of carbonate crystal structure mechanisms of crystal growth and driving forces for recrystallization answering these questions will require sedimentologists to step beyond traditional bounds into fields such as biomineralization and materials science

an immense shield volcano within the shatsky rise oceanic plateau northwest pacific ocean most oceanic plateaux are massive basaltic volcanoes however the structure of these volcanoes and how they erupt and evolve is unclear because they are remote and submerged beneath the oceans here we use multichannel seismic profiles and rock samples taken from integrated ocean drilling program core sites to analyse the structure of the tamu massif the oldest and largest edifice of the shatsky rise oceanic plateau in the north western pacific ocean we show that the tamu massif is a single immense volcano constructed from massive lava flows that emanated from the volcano centre to form a broad shield like shape the volcano has anomalously low slopes probably due to the high effusion rates of the erupting lavas we suggest that the tamu massif could be the largest single volcano on earth and that it is comparable in size to the largest volcano in the solar system olympus mons on mars our data document a class of oceanic volcanoes that is distinguished by its size and morphology from the thousands of seamounts found throughout the oceans

last interglacial reef growth beneath belize barrier and isolated platform reefs we report the first radiometric dates thermal ionization mass spectrometry from late pleistocene reef deposits from offshore belize the location of the largest modern reef complex in the atlantic ocean the results presented here can be used to explain significant differences in bathymetry sedimentary facies and reef development of this major reef area and the results are significant because they contribute to the knowledge of the regional geology of the eastern yucatan the previously held concept of a neotectonically stable eastern yucatan is challenged the dates indicate that pleistocene reefs and shallow water limestones which form the basement of modern reefs in the area accumulated ca 125 130 ka significant differences in elevation of the samples relative to present sea level 10 m have several possible causes differential subsidence along a series of continental margin fault blocks in combination with variation in karstification are probably the prime causes differential subsidence is presumably related to initial extension and later left lateral movements along the adjacent active boundary between the north american and caribbean plates increasing dissolution toward the south during pleistocene sea level lowstands is probably a consequence of higher precipitation rates in mountainous southern belize

deep crustal structure and magmatic processes the inception of seafloor spreading in the norwegian greenland sea summary an extensive investigation into the deep crustal structure of the conjugate norwegian and e greenland margins utilizing two ship expanded spread and wide aperture cdp profiling has led to the identification of a unique suite of crustal structures developed during the onset of seafloor spreading on both margins a region of low velocity crust was found proximal to marginal escarpments immediately seaward of the low velocity crustal block seaward dipping reflectors occur within thick crust that is interpreted to be wholly igneous a product of voluminous melt production enhanced by convective partial melting processes this earliest formed oceanic crust is actually several kilometres thicker than the low velocity crust or the adjacent continental crust normal oceanic crustal thicknesses are not produced until about 5 my after the initiation of spreading when convective partial melting has abated and the magma budget has reduced the extrusive unit reduces to normal thicknesses long before the intrusive and plutonic section indicating that the thickening or thinning of oceanic crust is accomplished primarily by variations in the latter

long submarine lava flows observations and results from numerical modeling long 100 km lava flows are relatively common on mars and venus and have been identified on the moon but they are rarely documented on earth however although 75 of the earth s surface is covered by water only a small percentage of the ocean floor has been investigated at a resolution sufficient to unequivocally identify the boundaries of long submarine lava flows even so basaltic lava flows as long as 110 km have been identified on the deep 1500 m seafloor near hawaii and the east pacific rise ambient conditions on the deep ocean floor may favor the development of long lava flows for the following reasons first high pressures 15 mpa keep volatiles dissolved in basaltic lavas preventing viscosity increases associated with exsolution and vesiculation second seawater rapidly quenches the surface of submarine basalt flows so that an insulating glass layer 1 5 cm thick encases submarine flows within seconds after their emplacement this glass rind effectively insulates the molten flow interior from additional heat loss making submarine basalt flows behave as well insulated subaerial tube fed flows thus for identical basalt flows emplaced on the deep seafloor and subaerially a submarine flow could advance farther before stopping results of numerical modeling indicate that thin 1 m submarine basalt flows behave similarly to identical subaerial flows but thicker submarine flows may advance significantly farther than their subaerial counterparts

crustal structure off norway 62 to 70 north abstract extensive geophysical surveys have been undertaken on the volcanic passive continental margin offshore norway between 62 and 70 n during the last 25 years three main margin segments have been identified the lofoten vesteralen margin the voring margin and the more margin the main features of the margins are prominent marginal highs including seaward dipping reflector sequences and an up to 22 km thick volcanic and transitional crust prominent escarpments the voring plateau escarpment and the faeroe shetland escarpment and up to 12 km deep post jurassic sedimentary basins east of the escarpments velocity depth solutions from about 250 sonobuoys expanding spread profiles and refraction profiles have been compiled and contoured isovelocity horizon contour maps and velocity transects outline a crust which broadly thickens from an oceanic crust with a normal oceanic type velocity structure to a ca 35 km thick continental crust with a continental velocity structure beneath the norwegian coast anomalous features include local crustal thickening below the more and vering marginal highs and high velocity bodies in the lower crust in the extension of the precambrian lofoten vesteralen archipelago the free air seasat derived gravity anomalies show a good correlation with the high velocity bodies and show prominent ne trending highs from the rockall plateau porcupine plateau region over the more voring and lofoten vesteralen margins to the southwestern barents sea

evolution of florida bay islands from a supratidal precursor evidence from westernmost bob allen key and sid key cores from the interior portions of westernmost bob allen key and sid key document island nucleation from a supratidal precursor developed on a paralic peat deposit whereas cores from exterior portions of these islands document development of marine mudbanks progradation or colonization by mangroves and supratidal sedimentation the supratidal precursor beneath these islands consists of eroded remnants of coastal tidal flats or local topographic highs that remained supratidal throughout the holocene sea level rise sedimentologic and biostratigraphic evidence suggest erosion of mangroves by storms or inundation of mangroves by storm deposits is a common precursor to subsequent sediment aggradation on both islands if other florida bay islands develop from mangrove colonization of marine mudbanks then data from westernmost bob allen key and sid key indicate that nucleation from a supratidal precursor and mangrove colonization of marine mudbanks are both viable mechanisms for island initiation the abse

the first antarctic geomagnetic reference model arm 1 spherical cap harmonic analysis has been applied to obtain a reference model of geomagnetic secular change for antarctica valid for the last forty years in this paper we use the latest available observatory data to update this model and to compare it with the 8th generation igrf in addition the selected set of total field values used for the generation of the oersted initial field model have been employed together with observatory data to develop the first complete antarctic reference model arm this model improves the fit to the secular variation deduced from observatory data by about 60 relative to igrf and the fit to observatory and satellite field data by 8 the model allows merging data sets taken at different altitudes and epochs in antarctica where significant temporal geomagnetic variations occur

the composition and structure of volcanic rifted continental margins in the north atlantic further insight from shear waves abstract imaging challenges caused by highly attenuative flood basalt sequences have resulted in the understanding of volcanic rifted continental margins lagging behind that of non volcanic rifted and convergent margins massive volcanism occurred during break up at 70 of the passive margins bordering the atlantic ocean the causes and dynamics of which are still debated this paper shows results from traveltime tomography of compressional and converted shear wave arrivals recorded on 170 four component ocean bottom seismometers along two north atlantic continental margin profiles this traveltime tomography was performed using two different approaches the first a flexible layer based parameterisation enables the quality control of traveltime picks and investigation of the crustal structure the second with a regularised grid based parameterisation requires correction of converted shear wave traveltimes to effective symmetric raypaths and allows exploration of the model space via monte carlo analyses the velocity models indicate high lower crustal velocities and sharp transitions in both velocity and vp vs ratios across the continent ocean transition the velocities are consistent with established mixing trends between felsic continental crust and high magnesium mafic rock on both margins interpretation of the high quality seismic reflection profile on the faroes margin confirms that this mixing is through crustal intrusion converted shear wave data also provide constraints on the sub basalt lithology on the faroes margin which is interpreted as a pre break up mesozoic to paleocene sedimentary system intruded by sills

crustal architecture and deep structure of the ninetyeast ridge hotspot trail from active source ocean bottom seismology a 550 km long transect across the ninetyeast ridge a major indian ocean hotspot trail provided seismic refraction and wide angle reflection data recorded on 60 ocean bottom instruments about 24 000 crustal and 15 000 upper mantle arrivals have been picked and used to derive an image of the hotspot track two approaches have been chosen i a first arrival tomographic inversion yielding crustal properties and ii forward modelling of mantle phases revealing the structure at the crust mantle boundary region and of the uppermost mantle away from the volcanic edifice seismic recordings show the typical phases from oceanic crust that is two crustal refraction branches pg a wide angle reflection from the crust mantle boundary pmp and a wave group turning within the upper mantle pn approaching the edifice three additional phases have been detected we interpret these arrivals as a wide angle reflection from the base of material trapped under the pre hotspot crust pm2p and as a wide angle reflection pnp and its associated refraction branch pn from a layered upper mantle the resulting models indicate normal oceanic crust to the west and east of the edifice crustal thickness averages 6 5 7 km wide angle reflections from both the pre hotspot and the post hotspot crust mantle boundary suggest that the crust under the ridge has been bent downwards by loading the lithosphere and hotspot volcanism has underplated the pre existing crust with material characterized by seismic velocities intermediate between those of mafic lower crustal and ultramafic upper mantle rocks 7 5 7 6 km s 1 in total the crust is up to 24 km thick the ratio between the volume of subcrustal plutonism forming the underplate and extrusive and intrusive volcanism forming the edifice is about 0 7 an important observation is that underplating continued to the east under the wharton basin during the shield building phase however ninetyeast ridge was located adjacent to the broken ridge and was subsequently pulled apart along a transform fault boundary therefore underplating eastwards of the fracture zone separating the edifice from the wharton basin suggests that prolonged crustal growth by subcrustal plutonism occurred over millions of years after the major shield building stage this fact however requires mantle flow along the fossil hotspot trail the occurrence of pnp and pn arrivals is probably associated with a layered and anisotropic upper mantle due to the preferential alignment of olivine crystals and may have formed by rising plume material which spread away under the base of the lithosphere

separation of lithospheric external and core components of the south polar geomagnetic field at satellite altitudes we present a new approach to producing scalar magsat magnetic anomaly maps based on correlation coefficient filtering and the use of almost all of the available orbits our method differs from earlier techniques with respect to the following 1 passes are selected based on their variance properties rather than planetary indices such as kp 2 the core field model is least squares fit to individual passes and subsequently removed instead of subtracting the model directly this technique replaces band pass filtering or polynomial trend removal methods 3 each selected pass is sorted geographically and by local time placed into one of four different altitude bands and correlation coefficient filtered with the two adjacent passes the filtering is the second step toward isolating the static lithospheric signal from the more dynamic external field signals 4 least squares collocation is used to grid the correlated passes subsequently the dawn and dusk maps are also correlation filtered providing another step toward removal of external fields 5 the four resultant total field maps are continued to a common altitude and again correlation filtered for the static lithospheric anomalies 6 the filtered results are then averaged together to provide a new total field map of the lithosphere south of 40 s latitude our total field map differs from previous efforts over the crustal blocks of west antarctica we obtained a positive anomaly over edward vii peninsula extending into the byrd subglacial basin and obtained a negative anomaly over the ellsworth mountains and parts of the byrd subglacial basin also a positive anomaly extending from the ross sea to offshore wilkes land is present in our map however this feature is absent in other maps positive anomalies marking the weddell sea in previous efforts are not present in our map prominent external field anomalies in the quadrant offshore of wilkes land are present in all previous efforts however these signatures are reduced in our total field map this map significantly advances efforts for the identification and interpretation of magnetic anomalies of the south polar lithosphere

early cenozoic crust at the norwegian continental margin and the conjugate jan mayen ridge multichannel seismic profiles at the voring plateau margin off norway and the northern jan mayen ridge have provided a framework for the early tertiary evolutionary history of these areas which were adjacent prior to the opening of the norwegian sea we propose that their paleogene evolution is quite similar and that the two acoustic basement reflectors ee and jo were formed in the early eocene by extrusion of flow basalts from the time of opening to about anomaly 23 time oceanic seafloor was generated by icelandic type spreading during that time the central jan mayen fracture was the main transform the seaward dipping reflector sequences are flow basalts generated by the icelandic spreading the existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation during the early eocene subaerial spreading episode which is related to the north atlantic volcanic province basalt flows also covered the adjacent thinned and heavily intruded continental crust it is proposed that the continent ocean boundary is located just landward of anomaly 24b a short distance seaward of the voring plateau escarpment reflector k is only recognized landward of this boundary in a region of transitional crust the escarpment experienced syndepositional faulting and the most elevated parts of the voring marginal high did not subside below shallow water depths before oligocene miocene time we suggest that the voring margin and the jan mayen ridge represent volcanic passive margins the evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust

deep crustal reflectors under reydarfjordur eastern iceland crustal accretion above the iceland mantle plume summary deep seismic reflection profiling on the faroe iceland ridge experiment fire imaged a sequence of dipping events in the upper to crust beneath the tertiary lava pile in reydarfjordur eastern iceland these events dip towards the spreading centre at which the crust was formed and continue down to 6 s two way time corresponding to depths down to 15 18 km similar crustal reflections have been observed previously within the crust in other areas of iceland although the reflective events located by previous surveys have not been interpreted rigorously in this study we calculate synthetic seismograms from modelled sequences of lava flows to examine the reflective character of a lava le the synthesized lava pile sections are based on field measurements made in the reydarfjordur area the 1 km vertical exposures on the sides of the fjord are composed of lava flows within stratigraphic units which have a range of compositions and some intercalated tuffaceous clastic horizons we compare the synthetic seismograms with our seismic reflection data to test the interpretation that the deep dipping reflections are caused by a gently dipping sequence of subaerially erupted lavas we conclude that the upper and mid crustal reflections that we observe can be interpreted as lava flow sequences similar to those seen at the surface with the main reflections resulting from packets of flows of similar thicknesses the reflectors that we have imaged beneath iceland are similar to those defining the seaward dipping reflectors on the volcanic continental margins and we therefore suggest that the seaward dipping reflections commonly result from packets of lavas with similar distributions of thicknesses the occurrence of lavas down to 15 18 km beneath reydarfjordur can be explained partly by multiple episodes of accretion of the upper crust although the volcanic processes forming the crust in iceland generally lead to advection of extrusive material to mid crustal levels the production of an average extrusive layer totalling 15 km thickness is within reasonable estimates of the current volcanic productivity the advection of frozen lava flows to mid crustal levels contributes to the maintenance of the subsolidus crust that is observed on iceland and the zeolite minerals within the extrusive sequence may be responsible for the crustal low resistivity zone that has been observed using magnetotelluric techniques

a model of the secular change of the geomagnetic field for antarctica abstract an improved model of geomagnetic secular change for the antarctic was developed using observatory annual mean values measured in antarctica during the last 40 years spherical cap harmonic analysis scha with a power series time dependence was used to model spatial and temporal variations of main field differences at spatial wavelengths from 3000 to 13 000 km the model was designed to facilitate merging satellite airborne marine and terrestrial magnetic data sets recorded at very different epochs in the antarctic where significant annual geomagnetic changes have occurred it improves the fit to observatory data by up to about 50 relative to the international geomagnetic reference field

carbonate cycle stacking patterns and hierarchies of orbitally forced eustatic sealevel change ordered stacking of meter scale upward shallowing cycles manifested as repeated thickness hierarchies within cratonic carbonate sequences has been considered primary evidence for causal relations between orbitally forced climate change and eustatic sealevel variation explicit in this argument is the assumption that each individual cycle represents a single sealevel rise sensitivity testing of one dimensional forward models incorporating a depth threshold of carbonate accumulation indicates that multiple upward shallowing cycles may originate during any single rise in sealevel variation in resultant cycle stacking pattern is sensitive to changes in sealevel amplitude sedimentation rate and subsidence rate with highest stacking ratios ratio of number of cyclic units to number of eustatic periods produced by low sealevel amplitudes and high sedimentation and subsidence rates these relations indicate that direct correlation between multiple frequency eustatic sealevel variations and meter scale cycle stacking hierarchies is unwarranted thus prevalent interpretations regarding the stacking record of orbital forcing in ancient carbonate rock sequences should be reevaluated

geophysical response of flood basalts from analysis of wire line logs ocean drilling program site 642 voring volcanic margin analysis of wire line logs at ocean drilling program site 642 the deepest scientific hole yet drilled in an offshore large igneous province lip reveals the main features of the extrusive complex constructed during formation of a volcanic margin a characteristic cyclic log response images tholeiitic lavas and thin tuffs corresponding to the seaward dipping reflectors in the seismic record flows are 0 6 18 5 m thick while most sediments are 2000 m focused resistivity kicks the large flow scale variations in physical properties show that representative core measurements must be densely sampled both on flow and sequence scales site 642 log responses appears typical for the north atlantic volcanic province and other lips but differs from logs in deep ocean crustal holes principal differences in log responses reflect mode of crustal emplacement and alteration hence site 642 forms a reference hole for extrusive constructions on oceanic lips

volcanism on the rockall continental margin continental break up in the northern north atlantic was accompanied by massive volcanism this produced sequences of basalt flows erupted close to sea level which are imaged as seaward dipping reflectors on seismic reflection profiles they comprise the upper 4 5 km of transitional crust in both the edoras bank and hatton bank areas of the rockall continental margin detailed surveys show that the reflectors are convex upward on dip lines but are sub horizontal on strike lines they are probably produced by aerially extensive basalt flows extruded from linear fissure vents aligned parallel to the rift axis along the rockall margin two sequences of seaward dipping reflectors occur separated by an 9outer high9 near the foot of the continental margin the landward set of basalt flows are emplaced above stretched and intruded continental crust while the seaward set span the region from highly distended continental crust to fully oceanic crust the outer high is capped by basalt flows but exhibits little coherent internal reflectivity we discuss possible origins of the outer high as a residual block of thinned and intruded continental crust or as a central volcano cut by fissure vents similar to those found in the neovolcanic zones of iceland

factors influencing magmatism during continental breakup new insights from a wide angle seismic experiment across the conjugate seychelles indian margins we present a model of the northern seychelles continental margin derived from controlled source wide angle seismic traveltime inversion and teleseismic receiver functions this margin has been widely cited as a classic example of rifting in association with a continental flood basalt province the deccan traps however we do not find the typical set of geophysical characteristics reported at other margins linked to continental flood basalts such as those of the north atlantic the oceanic crust formed immediately after breakup and throughout the first 3 ma of seafloor spreading is just 5 2 km thick less than half that typically seen at other volcanic margins the continent ocean transition zone is narrow and while two packages of seaward dipping reflectors are imaged within this transition they are weakly developed beneath the thinned continental crust there is an approximately 4 km thick layer of high velocity material 7 5 7 8 km s that we interpret as mafic material intruded and underplating the lower crust however we believe that this underplating most likely happened prior to the breakup overall the observations show that the rifting of india from the seychelles was characterized by modest magmatism the spatial extent of the deccan flood basalt province is therefore smaller than previously thought we speculate that either the lateral flow of deccan related hot material beneath the breakup region was hampered perhaps as the rifted margins did not intersect the center of the deccan source or there was incomplete melt extraction from the wide melting region that formed between the rapidly diverging plates if the latter explanation is correct then the rate of plate separation as indicated by the initial seafloor spreading rate is more important in controlling the volume of magmatism generated during continental rifting than has been previously recognized

antarctic geomagnetic reference model updated to 2010 and provisionally to 2012 abstract the antarctic reference model arm has been here updated using recent geomagnetic data measured over the antarctic continent from both ground observatories and satellites this regional geomagnetic model is based on a spherical cap harmonic analysis scha of geomagnetic field measurements over a polar cap of 30 half angle centred at the geographic south pole fixing the maximum spatial expansion index k max 8 and the maximum temporal order of polynomials q max 4 the importance of updating arm model lies for instance in its usefulness for the reduction of magnetic surveys performed during the period of model validity over the antarctica or for geomagnetic anomaly field estimations moreover so far arm still remains the only regional reference magnetic model specifically constructed for the antarctic continent the present updated version can be considered valid from 1955 5 to 2010 0 with predictive coefficients up to 2012 0 the model includes the most recent available data but in contrast to previous versions not only does it take advantage of a stricter selection of satellite data in order to consider even quieter periods of external magnetic activity but it also includes ground observatory data previous to 1960 going back to 1955 8 like the previous versions the new updated model has been tested and compared with major global models to show its reliability over the region under investigation

the mount wright arc a cambrian subduction system developed on the continental margin of east gondwana koonenberry belt eastern australia abstract the mount wright arc in the koonenberry belt in eastern australia is associated with two early to middle cambrian lithostratigraphic groups developed onto the late neoproterozoic volcanic passive margin of east gondwana the gnalta group includes a calc alkaline basalt andesite dacite suite mount wright volcanics interpreted to represent the volcanic component of the arc volcaniclastic gnalta group rocks now buried in the bancannia trough represent the continental back arc developed immediately behind the arc in a manner analogous to the modern taupo volcanic zone of new zealand east of the gnalta group is the ponto group a deep marine sedimentary package that includes tholeiitic lavas bittles tank volcanics and felsic tuffs interpreted as part of a fore arc sequence the configuration of these units suggests the mount wright arc developed on continental crust in response to west dipping subduction along the east gondwana margin in contrast with some models for cambrian convergence on other sections of the delamerian orogen which invoke east dipping subduction and arc accretion by arc continent collision this convergent margin was deformed by the middle cambrian delamerian orogeny which involved initial co axial shortening followed by sinistral transpression and oroclinal folding around the edge of the curnamona province

late pliocene to holocene platform evolution in northern belize and comparison with coeval deposits in southern belize and the bahamas the full text is not available on soar access this article via the commercial databases licensed by university libraries http libcat wichita edu vwebv holdingsinfo searchid 817 reccount 30 recpointer 1 bibid 1334578

divergent wrench faulting in the belize southern lagoon implications for tertiary caribbean plate movements and quaternary reef distribution analysis of 500 km of multichannel seismic reflection data in the southern lagoon of belize reveals that its tertiary structural evolution has been punctuated by two major transtensional episodes first left lateral strike slip motion parallel to the coast together with a major component of extension took place during the paleocene producing a series of southwest northeast trending en echelon normal faults which created a topographic relief of ridges and troughs both the types of structures and their timing of formation are consistent with models that propose that cuba swept past the eastern coast of belize and the adjacent yucatan peninsula moving along a left lateral transform boundary until it collided with the bahamas platform the second tectonic event occurred during the pliocene and or later revealing very recent activity along the north american caribbean left lateral transform boundary high angle normal faults with a southwest northeast orientation dominate the style a comparison between the deep structure and the present bathymetry of the lagoon reveals that portions of the barrier platform and elongate to rhombohedral platform atolls such as laughing bird cay are located over ancient structural highs whereas bathymetric lows such as the victoria channel are long lived troughs this suggests that quaternary reef foundations are deeper than previously thought and that structure exercised a major control on sedimentation throughout the tertiary evolution of the southern lagoon

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

vertical resolution of thick beds thin beds and thin bed cyclothems resolution has two common definitions the first of these relates to determination of the position of a reflector in time or space resolution by this definition is inversely related to pulse width or sharpness of the wavelet and potentially directly related to bandwidth of the frequency spectrum the second definition of resolution relates to determination of the spacing of close features such as thin beds this type of resolution is determined by the rayleigh criterion which inversely relates resolution to wavelength resolution of thin beds improves with shorter wavelength or higher frequency in comparison to the rayleigh criterion the ricker and widess criteria improve the resolution potential of thin beds but they rely on models of the reflectors and require that the beds be isolated it is a long established and widely accepted axiom that the best wavelets contain bandwidths of at least a couple of octaves this results in wavelets with reduced side lobes and ringiness a more easily interpret

stratigraphy and correlation of the region surrounding the irdp drill hole 1978 reydarfjordur eastern iceland the 1 9 km thick sequence intersected by the irdp 1978 drill hole at reydarfjordur eastern iceland is correlated with the westward dipping exposed volcanic pile on the basis of extensive field mapping and paleomagnetic studies the base of the landward extension of sea floor magnetic anomaly 5 epoch 9 can be located at the top of the grjota olivine tholeiite formation intersected at about 1 5 km in the core and exposed at sea level about 9 km east of the drill site other exposed marker formations can also be identified in the core the drill site is located within a relatively dense dyke swarm 10 crustal dilation previously interpreted as the feeder of the overlying volcanic formations field observations including vertically discontinuous dykes and the lack of lava feeder relationships suggest that the dykes were intruded by lateral injection from the south and are considerably younger than and thus could not have fed the exposed lava pile in the vicinity of the drill site it is proposed that about 10 3 m y ago roughly coinciding with the lower boundary of anomaly 5 the reydarfjordur volcanic center ceased to be active and volcanic activity shifted about 20 km to the west where the breiddalur and thingmuli volcanic centers later developed the exposed stratigraphic succession between thingmuli and reydarfjordur volcanic centers is composed of lavas deposited in flank areas relatively removed from a paleovolcanic zone which now probably lies buried to the west of the irdp drill site the lowest 450 m of the core may represent lavas related to activity in the reydarfjordur volcanic center 13 5 km to the east

dolomitization of holocene mg calcite supratidal deposits ambergris cay belize dolomitized crusts and associated sediments 845 2925 yr old on ambergris cay belize central america compose a significant portion of the holocene sediment section on many supratidal flats the dolomitic sections are as thick as 0 7 m and contain an average of 70 calcic dolomite dolomite occurs as a replacement of high mg calcite micrite matrix and allochems and as passively precipitated cements isotopic analyses suggest that dolomitization appears to be promoted primarily by reactions of the magnesian calcite supratidal sediments with essentially near normal marine waters the effects of seasonal influxes of meteoric and hybrid fluids however are also clearly evident in the crusts as indicated by isotopes dolomite crystal etching the selective leaching of aragonite and the local loss of mg 2 from the host micrite and included skeletal fragments the amounts rapid rates of dolomitization and initial mg calcite mineralogy of the sediments make the ambergris cay supratidal flats exceptional in the holocene and this area may be a model for the genesis of some ancient peritidal dolomites

a compilation of igneous rock volumes at volcanic passive continental margins from interpreted seismic profiles abstract the rifting and breakup of continents and subsequent seafloor spreading give rise to passive continental margins many of which are underlain by enormous volumes of igneous rocks and termed volcanic passive margins the relationships between the igneous rocks rifting and mantle plumes remain unresolved despite extensive studies and proposed alternative models to support such studies we use published previously interpreted seismic reflection and refraction data to compile a dataset of igneous rock volumes and geometries at volcanic passive continental margins the volmir volcanic passive margin igneous rocks dataset is based on margin crossing profiles on which the volumes and geometries of both shallow seaward dipping reflector sdr and deeper high velocity lower crustal hvlc units can be measured it includes information about the ages of continental breakup and profile distances from the associated euler pole and from hotspots that may have played roles in the breakup process despite local variations the dataset shows general patterns the average ratio of sdr to hvlc volumes 0 3 0 4 is relatively consistent between profiles implying that formation of these units is related during continental breakup the volumes of igneous rocks display a moderate positive correlation with distances from the euler pole but only a weak negative correlation with distances from the nearest hotspot at the time of margin formation suggesting that in continental breakup lithospheric processes passive rifting have greater effects than hotspots active rifting these results suggest that this dataset will be useful for exploring aspects of the rifting process relationships between volcanic rifted margins and potential explanations for their similarities and differences

symmetric conjugation of continent ocean boundary structures along the norwegian and east greenland margins abstract a characteristic structural feature of the norwegian continental margin that has been described from multichannel seismic data as well as dsdp and odp drilling is a major buildup of layered extrusive basaltic rocks forming wedges that dip seaward along the entire margin models for their origin describe them as either products of a late stage intra continental rift phenomenon with the volcanic rocks overlying attenuated continental crust or as an integral component of the oceanic crust formed during the first few million years of seafloor spreading in the norwegian greenland sea from studies of the norwegian margin alone it was not possible to unequivocally distinguish between the two proposed models so an investigation of the conjugate portions of the east greenland margin was undertaken which used two ship multichannel seismic acquisition as its focus we found that the wedges were not only present on the east greenland margin but that details of their along strike variability are mirrored on both margins together with their spatial relationship to the oldest magnetic lineation pattern the wedges are relatively small in the northern part of the margin where they occur almost entirely landward of lineation 24 moving south we see that the wedges broaden encroach into the pattern of lineation 24 then develop into two separate wedges the seaward of which is associated with lineation 23 lineations 24 and 23 on both margins are distorted when associated with wedges we believe that these data lend strength to a tectonic association of the wedges with nascent plate boundary processes their observation in many margins throughout the world suggest that a prolific volcanic phase may be commonly associated with the inception of a divergent plate boundary and we believe that the most likely mechanism to generate the excess volcanism is the convective partial melting scheme recently proposed by mutter and buck 1986 and mutter et al in press

seismic structure of basalt flows from surface seismic data borehole measurements and synthetic seismogram modeling we use the wide angle wavefield to constrain estimates of the seismic velocity and thickness of basalt flows overlying sediments wide angle means the seismic wavefield recorded at offsets beyond the emergence of the direct wave this wide angle wavefield contains arrivals that are returned from within and below the basalt flows including the diving wave through the basalts as the first arrival and p wave reflections from the base of the basalts and from subbasalt structures the velocity structure of basalt flows can be determined to first order from traveltime information by ray tracing the basalt turning rays and the wide angle base basalt reflection this can be refined by using the amplitude variation with offset avo of the basalt diving wave synthetic seismogram models with varying flow thicknesses and velocity gradients demonstrate the sensitivity to the velocity structure of the basalt diving wave and of reflections from the base of the basalt layer and below the diving wave amplitudes of the

evidence for a thick oceanic crust adjacent to the norwegian margin the oceanic crust created during this first few million years of accretion in the norwegian greenland sea lies at an unusually shallow depth for its age has a smooth upper surface and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate seaward dipping reflectors these have been attributed to lava flows generated during a brief period of subaerial seafloor spreading we describe the results of inversions of digitally recorded sonobuoy measurements and two ship expanded spread profiles collected over the oceanic crust adjacent to the norwegian passive margin we find that the crust of the deep lofoten basin is indistinguishable from normal oceanic crust in thickness and structure closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2 while the layer 3 region retains approximately the same thickness the area over which the seaward dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in layer 2 thickness in the very oldest crust immediately adjacent to an escarpment that probably marks the continent ocean boundary we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke lava interface in the thick crust comparing the structure of the thick crust to that of eastern iceland we find a strong resemblance especially in the expansion in thickness of material with layer 2 velocities these results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous building a thick pile of lavas erupted from a subaerial spreading center

seismic response and construction of seaward dipping wedges of flood basalts voring volcanic margin geological and geophysical data from the 900 m of volcanic basement drilled at ocean drilling program site 642 provide the framework for studying seismic properties of huge extrusive constructions on volcanic margins the main part of the drilled section corresponding to a prominent seaward dipping reflector sequence consists of subaerially emplaced tholeiitic basalt flows and thin interbedded sediments the basalts exhibit a characteristic velocity and density lava flow distribution reflecting changing porosity pore aspect ratio distribution and alteration stacks of laterally continuous basalt flows appear to have thin layer transverse isotropic properties for typical wavelengths in multichannel seismic data vertical seismic profiling and average sonic log velocities are similar 3 77 km s and 3 88 km s respectively while comparable refraction velocities are 10 20 higher synthetic seismogram modeling based on downhole logs shows that basement reflectors originate from interference and tuning effects of numerous basalt flow and interbedded sediment interfaces though the most continuous reflectors are related to thick flows seismic models based on the characteristic velocity and density basalt flow distributions and site 642 stratigraphy show that reflector truncation and onlap may be caused by seismic interference phenomena in a sequence of landward thinning flows the base of the dipping reflector sequence reflector k correlates with flows in the lower part of upper series basalts while the transition from basaltic to underlying dacitic andesitic lavas correlates with a locally defined reflector a model for the breakup related volcanism includes 1 prebreakup dacitic andesitic volcanism 2 early breakup basaltic volcanism infilling the prebreakup relief 3 main breakup stage with intense focused volcanism large subsidence and lava pounding and 4 late breakup volcanism during a period of decreased subsidence and local off axis activity

sediments facies tracts and variations in sedimentation rates of holocene platform carbonate sediments and associated deposits northern belize implications for representative sedimentation rates full text is not available on soar publisher s website http jsedres geoscienceworld org cgi content abstract 74 4 498 doi 10 1306 012004740498

micritized skeletal grains in northern belize lagoon a major source of mg calcite mud abstract lime mud in northern belize lagoon is composed predominantly of silt size cryptocrystalline grains and clay size clumps both of which consist of anhedral equant crystals less than 0 5 m in size this mud is 70 90 mg calcite containing 9 10 mole mgco3 detailed petrographic morphologic and mineralogic analyses suggest that the cryptocrystalline grains are micritized skeletal grains and that the mud formed by abrasion and bioerosion of these altered grains micritization of skeletal grains occurred primarily by recrystallization of original skeletal carbonate laths prisms and blades of mg calcite aragonite and calcite to equant crystals of micritic mg calcite with additional alteration by micritic infilling of microborings and skeletal cavities because of d fferences between crystal morphologies and mineralogies of original skeletal components and their micritized counterparts mud produced by diminution of micritized grains is distinct from that produced by skeletal breakdown the abundance of cryptocrystalline grains in the bahamas and other shallow carbonate environments suggests that micritized grains are potentially an important source of lime mud that has not previously been identified

dolomitization of holocene shallow marine deposits mediated by sulfate reduction and methanogenesis in normal salinity seawater northern belize abstract dolomite constitutes an average of 12 of the holocene organic rich sediments over a 15 km2 area of the cangrejo shoals mudbank in northern belize although it defines a laterally persistent stratiform body that averages 3 m thick it is present throughout the 7 6 m thick sediment section these transgressive sediments are less than 6400 years old and were deposited in shallow marine environments of normal salinity the dolomite is dominantly cement and average crystal size is 7 m there are no significant correlations among amount of dolomite vs sediment texture mineralogy porosity or mole mgco3 in associated particulate high mg calcite depth or location on the shoals the dolomites are poorly ordered and calcic 39 5 44 5 mole mgco3 with low mean mn 210 ppm and relatively high mean sr 1034 ppm concentrations there is no evidence of recrystallization or geochemical alteration of the dolomite 18o values of the dolomites range from 0 5 to 2 8o oopdb and the mean value 2 1o oo suggests that the dolomite precipitated from normal salinity pore water dolomite 13c values range from 5 2o oo to 11 6o oopdb mean seawater 13c 0 5o oo which suggests dolomitization promoted by both bacterial sulfate reduction and methanogenesis in environments with anoxic pore water dolomitization attending these organodiagenetic reactions apparently was reversible over time and episodic rather than continual precipitation is indicated requisite mg and ca were provided by seawater and by some dissolution of host sediments the most rapid period of dolomitization may have been during early transgression when relatively high sedimentation rates sustained high levels of organodiagenesis and pore water alkalinities

interpretation of a vertical seismic profile conducted in the columbia plateau basalts seismic reflection data are often of poor quality when recorded in areas where volcanic rocks are present at or near the surface in order to investigate this phenomenon a vertical seismic profiling vsp experiment was conducted in the columbia plateau basalts so that the behavior of seismic energy in subsurface volcanic rocks could be observed directly thus giving insight into data acquisition in volcanic terrains the lithologic section at the vsp site consists of low velocity 400 m s to 900 m s alluvium in the uppermost 50 m beneath which are layers of high velocity about 5800 m s high density basalts interbedded with clay layers with much lower velocities about 1700 m s and densities these large velocity and density contrasts dramatically influence wave generation and propagation in spite of the small source borehole offset 61 m large amplitude s waves are generated by the downgoing p waves when they reach a shallow 250 m clay basalt boundary these s waves in turn generate strong

sedimentary architecture and genesis of holocene shallow water mud mounds northern belize cangrejo and bulkhead shoals are areally extensive holocene biodetrital mud mounds in northern belize they encompass areas of 20 km2 and 35 km2 in distal and proximal positions respectively on a wide and shallow water microtidal carbonate shelf where storms are the major process affecting sediment dynamics sediments at each mound are primarily biodetrital and comprise part of a eustatically forced dominantly subtidal cycle with a recognizable deepening upward transgressive systems tract condensed section and shallowing upward highstand systems tract antecedent topographic relief on pleistocene limestone bedrock also provided marine accommodation space for deposition of sediments that are a maximum of 7 6 m thick at cangrejo and 4 5 m thick at bulkhead despite differences in energy levels and location facies and internal sedimentological architectures of the mud mounds are similar on top of pleistocene limestone or buried soil developed on it are mangrove peat and overlying to laterally correlative shelly gravels deposition of these basal transgressive premound facies tracked the rapid rate of sea level rise from about 6400 6500 years bp to 4500 years bp and the thin basal sedimentation unit of the overlying mound core appears to be a condensed section following this the thick and complex facies mosaic comprising mound cores represents highstand systems tract sediments deposited in the last 4500 years during slow and decelerating sea level rise within these sections there is an early phase of progradationally offlapping catch up deposition and a later and current phase of aggradational keep up deposition the mound cores comprise stacked storm deposited autogenic sedimentation units the upper bounding surfaces of which are mostly eroded former sediment water interfaces below which depositional textures have largely been overprinted by biogenic processes associated with thalassia colonized surfaces vertical stacking of these units imparts a quasi cyclic architecture to the section that superficially mimics metre scale parasequences in ancient rocks the locations of the mud mounds and the tidal channels transecting them have apparently been stable over the last 50 years characteristics that might distinguish these mud mounds and those mudbanks deposited in more restricted settings such as florida bay are their broad areal extent high proportion of sand size sediment fractions and relatively abundant biotic particles derived from adjoining open shelf areas

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

eruption and emplacement of flood basalt an example from the large volume teepee butte member columbia river basalt group flows of the teepee butte member grande ronde basalt issued from a vent system in southeastern washington northeastern oregon and western idaho three distinct basalt flows were erupted the limekiln rapids flow the joseph creek flow and the pruitt draw flow together these mappable flows cover more than 52 000 km sup 2 and have a volume exceeding 5 000 km sup 3 a portion of the vent system for the joseph creek flow is exposed in cross section in joseph canyon washington it is one of the best preserved columbia river basalt group vent complexes known the vent complex is about 1 km in cross section 30 m high and composed of deposits characteristic of hawaiian type volcanism the vent is asymmetrical the eastern rampart consists of intercalated pyroclastic deposits and thin pahoehoe flows the western rampart is composed wholly of pahoehoe flows flows of the teepee butte member are compositionally homogeneous and were emplaced as sheet flows each having several local flow units our study supports the importance of linear vent systems and the westward palouse slope along with the large volume lava flows in controlling the distribution of columbia river basalt group flows other factors including the number of active fissure segments andmore topography modified the shape of the flows and the number of flow units 45 refs 19 figs 2 tabs less

tuning effect and interference reflections from thin beds and coal seams the ricker and widess criteria of resolving thin beds are established and widely accepted axioms analysis of tuning effects from an isolated thinning bed is commonly used in estimating the thickness of a thin bed however frequency and amplitude tuning effects may also result from a sequence of thin beds depending on the thicknesses acoustic properties and the relative location of thin beds in space tuning effects from interference reflections can sometimes deform the seismic wavelet and cause complex waveforms coal seams as thin as 1 20 of the predominant wavelength can produce reflections and these reflections may be a composite of primary and multiple reflections interference reflections from two closely spaced coal seams were recorded and used to interpret a seam anomaly roll that may create adverse conditions for longwall mining the seismic data and computer model results correlated with findings of several boreholes

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

age of seychelles india break up many continental flood basalt provinces are spatially and temporally linked with continental break up establishing the relative timing of the two events is a key step in determining their causal relationship here we investigate the example of the deccan traps and the separation of india and the seychelles whilst there has been a growing consensus as to the age of the main phase of the deccan emplacement 65 5 1 ma chron 29r the age of the rifting has remained unclear we resolve this issue through detailed seafloor magnetic anomaly modeling supported by wide angle and reflection seismic results of the north seychelles and conjugate laxmi ridge gop rift margins and geochemistry and 40ar 39ar geochronology of rocks from the north seychelles margin we show that syn rift volcanics offshore the seychelles islands in the form of seaward dipping reflectors were most likely erupted during chron 28n and the first organized seafloor spreading at the carlsberg ridge also initiated during this chron at 63 4 ma the severing of the seychelles occurred by a south eastward ridge propagation that was completed by the start of chron 27n 62 ma a brief pre 28r phase of seafloor spreading occurred in the gop rift possibly as early as 31r 32n 71 ma initial extension at the margin therefore preceded or was contemporaneous with the deccan emplacement and separation of the seychelles was achieved less than 3 5 ma afterwards this is the shortest time interval between flood basalt emplacement and break up yet reported for any continental flood basalt rifted margin pair a contributing factor to the apparently short interval in the deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere however we conclude that external plate boundary forces rather than the impact of a mantle plume were largely responsible for the rifting of the seychelles from india

sedimentology and budget of a recent carbonate mound florida keys the sedimentology of a recent carbonate mound is investigated to further our understanding of mound building communities surface and subsurface mound sediments and the overall sediment budget of mounds nine sedimentary facies of the surface of tavernier mound florida keys are described these sediments are composed of neogoniolithon halimeda porites mollusc and foraminiferal grains and lime mud muds rich in aragonite and high magnesian calcite show little mineralogical variation over the mound surface geochemical evidence suggests that the mud is mainly formed from breakdown of codiacean algae and thalassia blade epibionts production rates of the facies are established from in situ growth rate experiments and standing crop surveys annual calcium carbonate production is c 500gm 2 intermediate between reef and other bay and lagoonal environment production rates in the caribbean the internal structure of the mound studied from piston cores and sediment probes indicates that seven facies can be identified five of these can be related to the present day facies and occur in the upper part of the mound gravel mound stage the remaining two facies characterized by molluscs and aragonite rich muds occur in the lower part of the mound mud mound stage and are most similar to facies from typical florida bay mud mounds mangrove peats within the mound indicate former intertidal areas and c14 dates from these peats provide a time framework for mound sedimentation the mound appears to have formed because of an initial valley in the pleistocene surface which accumulated mud in a shallow embayment during the holocene transgression a sediment budget for the mound is presented which compares production rates from present day facies with subsurface sediment masses during the mud mound stage production rates were similar to accumulation rates and the mound was similar to the present day mounds of florida bay during the gravel mound stage 3400 yr bp present day conditions were more normal marine and the establishment of porites and neogoniolithon on the mound increased production rates 10 over accumulation rates this excess sediment is thought to be transported off the mound to the surrounding seabed models are proposed which divide carbonate mounds on the basis of internal versus external sediment supply comparisons are made with other recent and ancient mounds similarities exist between the roles of the biotic components of late palaeozoic mounds but major differences are found when structures and early diagenesis are compared

magnitude and timing of episodic sea level rise during the last deglaciation a succession of elevated ridge deposits on the south florida margin was mapped using high resolution seismic and side scan sonar imaging in water depths ranging from 50 to 124 m the ridges are interpreted to be subtidal shoal complexes and paleoshorelines eolian dune or beach formed during the last sea level transgression oolitic and skeletal grainstones and mixed skeletal peloidal ooid packstones were recovered using a research submersible all of the grains are of shallow water or intertidal origin and both marine and nonmarine cements were identified formation and preservation of these features are attributed to episodic and rapid changes in the rate of the deglacial sea level rise at the onset of the termination 1a 18o excursion this high resolution record of sea level change appears to be related to deglacial processes operating on submillennial time scales and supports increasing evidence of rapid episodic fluctuations in ice volume climate and ocean circulation patterns during glacial interglacial transitions

seismic model of a progradational carbonate platform picco di vallandro the dolomites northern italy we present a seismic model of a well exposed carbonate platform to basin transition from the triassic succession at picco di vallandro the dolomites northern italy the core of this model is a detailed lithologic cross section that represents 1500 m of stratigraphic section over a distance of 3 5 km the cross section is based on detailed measured sections photo interpretation and careful projection of surface mapping into the plane of section samples from the measured sections provide control on velocity and density distribution and these data were used to generate an impedance model from the cross section the impedance model was convolved with a zero phase 25 hz peak frequency wavelet to produce the seismic model the results illustrate how the platform to basin transition might be imaged by conventional seismic reflection data how the reflections relate to the distribution of lithology and through going geological surfaces and how constructive and destructive interference and seismic resolution combine to produce individual reflections the picco di vallandro section displays several scales of progradation and retrogradation ranging from stacked buildups that are several hundreds of meters thick to the interfingering of individual beds the seismic model displays this complexity with varying degrees of success the individual buildups are well imaged but as expected smaller features are not as clearly reproduced by the model intermediate scale sections that represent platform retrogradation are shown as surfaces of onlap in the seismic model but field work shows that the platform was still shedding sediment while retreating and that no simple onlap surface exists in the outcropping section a similar lack of stratigraphic resolution is shown by the model at the downlapping clinoforms which do not correspond to iscrete stratal surfaces but to the toe of slope position where major bedding units thin below seismic resolution smaller features such as the position of downslope terminations of carbonate breccia wedges are not resolved by the model toe of slope packages of limestone turbidites are represented only by reflection segments of higher amplitude the decreasing seismic resolution is caused by limited seismic bandwidths and by the effects of constructive and destructive interference interference has the largest effects at the complexly interfingering buildup to basin transition or at the most detailed part of our model an understanding of these types of seismic limitations and their controls is important as we try to extract more and more stratigraphic information from seismic data p

seismic wave attenuation in volcanic rocks from vsp experiments seismic wave attenuation in the columbia plateau basalts and snake river plain volcanics was analyzed using vertical seismic profiling vsp data the computation of attenuation coefficients is based on fitting a straight line to the logarithm of amplitude ratios computed for fixed values of frequency and variable depth this approach does not require any assumptions on the dependence of q on frequency for the columbia plateau basalts the attenuation coefficients obtained from the field data are smaller than those computed from the synthetic vsp generated using the sonic and density logs indicating that the observed attenuation is related to scattering effects and is substantially larger than the intrinsic attenuation of basalt therefore it is concluded that only a lower bound for q can be established in agreement with recent findings by other authors the effective attenuation of seismic energy in basalts about 1 1 10 4dbs m for the peak frequency is comparable to the effective attenuation observ

three term taylor series for t2 x2 curves of p and s waves over layered transversely isotropic ground the arrival time curve of a reflection from a horizontal interface beneath a homogeneous isotropic layer is a hyperbola in the x t domain if the subsurface is one dimensionally inhomogeneous horizontally layered or if some or all of the layers are transversely isotropic with vertical axis of symmetry the statement is no longer strictly true though the arrival time curves are still hyperbola like in the case of transverse isotropy however classical interpretation of these curves fails interval velocities calculated from t 2 x 2 curves do not always approximate vertical velocities and therefore cannot be used to calculate depths of reflectors to study the relationship between velocities calculated from t 2 x 2 curves and the true velocities of a transversely isotropic layer we approximate t 2 x 2 curves over a vertically inhomogeneous transversely isotropic medium by a three term taylor series and calculate expressions for these terms as a function of the elastic parameters it is shown that both inhomogeneity and transverse isotropy affect slope and curvature of t 2 x 2 curves for p waves the effect of transverse isotropy is that the t 2 x 2 curves are convex upwards for sv waves the curves are convex downwards for sh waves transverse isotropy has no effect on curvature

holocene shallow subtidal dolomitization by near normal seawater northern belize calcic dolomite cements compose an average of 5 of the upper 4 3 m of subtidal deposits 18 o 2 compositions of the high sr dolomites mean october 1900 ppm together with near normal salinity and inherently normal mg ca ratio of pore fluids suggest precipitation from near normal seawater tidal and wind driven circulation of seawater through the sediments supplies most of the mg for dolomitization which appears to be promoted by elevated pore water alkalinity resulting from bacterially mediated oxidation of organic matter and locally early stages of methanogenesis rapid dolomitization here supports the idea that significant quantities of dolomite can form syndepositionally from normal seawater in shallow subtidal deposits

modeling studies of interference reflections in thin layered media bounded by coal seams high resolution seismic data collected over a major u s coal basin indicated potential complex problems associated with interference reflections these problems differed from those normally encountered in the exploration of oil and gas because of differences in the geologic boundary conditions modeling studies were conducted to investigate the effects of overlapping primary reflections and the composite reflection that result from stacking individual wavelets a modified empirical formula of lindseth s linear relationship between acoustic impedance and velocity is used to extrapolate velocity information from density logs to provide appropriate geophysical properties for modeling the synthetic seismograms generated from density and synthetic sonic logs correlated well with the processed seismic data a 150 hz ricker wavelet is used to convolve with the computer models and the models showed that certain anomalous composite reflections result from the superposition of overlapping primary reflections de

dip moveout error in transversely isotropic media with linear velocity variation in depth levin 1990 modeled the moveout within common midpoint cmp gathers of reflections from plane dipping reflectors beneath homogeneous transversely isotropic media for some media when the axis of symmetry for the anisotropy was vertical he found departures in stacking velocity from predictions based upon the familiar cosine of dip correction for isotropic media here i do similar tests again with transversely isotropic models with vertical axis of symmetry but now allowing the medium velocity to vary linearly with depth results for the same four anisotropic media studied by levin show behavior of dip corrected stacking velocity with reflector dip that for all velocity gradients considered differs little from that for the counterpart homogeneous media as with isotropic media traveltimes in an inhomogeneous transversely isotropic medium can be modeled adequately with a homogeneous model with vertical velocity equal to the vertical rms velocity of the inhomogeneous medium in practice dip moveout dmo is based on the assumption that either the medium is homogeneous or its velocity varies with depth but in both cases isotropy is assumed it turns out that for only one of the transversely isotropic media considered here shale limestone would v z dmo fail to give an adequate correction within cmp gathers for the shale limestone more fortuitously the constant velocity dmo gives a better moveout correction than does the v z dmo less