late ordovician jaw bearing polychaetes from anticosti island eastern canada and their biogeographic significance the upper ordovician to lower silurian shallow marine succession of anticosti island eastern canada provides one of the most complete records across the hirnantian in the world this study reports a diverse assemblage of scolecodonts polychaete jaws from the upper katian and hirnantian vaureal ellis bay and basal becscie formations of western anticosti the collection of 10 samples includes ca 30 species representing 10 families the fauna is dominated by polychaetaspids mochtyellids paulinitids and polychaeturids the family xanioprionidae and genera pistoprion tetraprion and rakvereprion are documented for the first time from the ordovician of laurentia the anticosti polychaete fauna shows great similarity to the contemporaneous faunas of baltoscandia this is evidenced by a high relative abundance of mochtyellids and polychaeturids and a number of common species thus suggesting that the closing iapetus ocean at that time did not constitute a barrier for the dispersal of jaw bearing polych

stromatolitic framework of carbonate mud mounds abstract mud mounds are an important phanerozoic reef type generally formed in deeper water on carbonate shelf to basin slopes they commonly contain radiaxial calcite spar filled depositional cavities stromatactis but lack abundant metazoan framebuilders mud mounds are often flanked by steeply dipping bioclastic grainstones and sometimes attained widths and depositional reliefs of over a hundred meters they occasionally occur as basal cores on which developed shallow water metazoan constructed reefs stromatactoid mud mounds are composed of a framework of submarine cemented crudely reticulate masses or a succession of laminar crusts surrounded by cement filled cavities these cavities developed when unconsolidated sediment around the framework was removed by winnowing mud mound frameworks of all ages show strong geometric and petrographic similarities to shallow water cryptalgal structures especially thrombolites unlaminated stromatolites which occur in many phanerozoic shallow water reefs both thrombolites and mud mound frameworks are largely composed of lime mudstone cemented on the sea floor and contain fenestral fabric and diagnostic cryptalgal microstructure their depositional surfaces were irregular not bored or burrowed and rarely encrusted stromatolitic lamina ion although commonly not well developed in the subtidal zone anyway occasionally occurs in mud mounds it is proposed that mud mounds accumulated from organic probably blue green algal binding of locally derived lime mud and bioclasts deposited relatively slowly from suspension patchy distribution of algal mats resulted in areas of both bound and unbound sediment sporadic floodlike episodes of rapid sedimentation however temporarily smothered the living algal mat and also left layers of unbound sediment periods of winnowing washed out unconsolidated unbound sediment and promoted synsedimentary cementation of resulting stromatactoid cavities and algal bound framework

marine geology of sohm basin canadian atlantic margin multichannel seismic reflection seismic refraction and magnetic anomaly data are used to interpret the mesozoic sea floor spreading history and subsequent sedimentary evolution of the sohm basin southeast of nova scotia observed m series mesozoic magnetic anomalies show that oceanic crust in the sohm basin was generated at very slow rates of sea floor spreading 5 15 mm yr half rates the slow spreading created very rough crustal topography but the crustal thickness and seismic velocity structure are typical of normal oceanic basement basement depths corrected for sediment loading fall below the predicted age depth curve in the anomaly m25 m4 interval these depth anomalies and an associated regional negative geoid anomaly suggest that the sohm basin is underlai by asthenosphere denser and possibly colder than normal correlation of seismic reflection profiles in the basin to regional seismic stratigraphy together with age constraints from reflection pinch outs on dated oceanic crust provides a chronology for interpreting the seismic sequences the jurassic to lower cretaceous sequences are similar in thickness and acoustic character to sequences observed along the united states atlantic margin suggesting that the new england seamount chain had little effect on sediment distribution patterns within the early western north atlantic basin upper cretaceous sediments in the sohm basin are thin probably because seaward transport of sediment was restricted by eustatic sea level rise and by a sedimentary ridge uplifted because of salt tectonics along the central and upper continental rise progradati nal sedimentary wedges and erosional unconformities characterize the tertiary and quaternary sedimentary record along the lower continental rise the prograding sequences generally correlate to sea level lowstands repeated episodes of erosion by abyssal currents are marked by seismic unconformities horizon au lower oligocene lm lower miocene mm middle miocene and l upper pliocene the deep currents also controlled local patterns of deposition and modified cross slope sediment transport and accumulation patterns thick supra horizon l sediments cover the sohm basin and reflect rapid sediment influx caused by glacio eustatic fluctuations in sea level during the quaternary

diversification and biogeography of scolecodont bearing polychaetes in the ordovician scolecodonts are common microfossils in palaeozoic rocks bearing witness to the extensive radiation of jawed polychacte worms during the ordovician period information on ordovician scolecodonts is increasing though it is still patchy and strongly skewed towards baltica and laurentia comparison of the faunas of these two palaeocontinents indicates that approximately 50 of the genera but only a few species had intercontinental distribution meagre data from other terranes also show that several genera were cosmopolitan in total more than 50 ordovician genera are known the first records are from the furongian latest cambrian but scolecodont diversity and abundance remained very low until the darriwilian the earliest jawed polychaetes had primitive usually symmetrical placognath ctenognath type jaw apparatuses xanioprionids tetraprionids and or conjungaspids seem to have played an important role in the early evolution and diversification of scolecodont beating polychactes the first more advanced taxa such as the polychaetaspids with labidognath type jaw apparatuses and mochtyellids with placognath type apparatuses and compound maxillae are first recorded in the earliest mid ordovician the most significant increase in global generic diversity but also regional species level diversity occurred in the late mid ordovician when many common taxa appeared accordingly the origination rate is highest in that interval since many ordovician polychaete genera are relatively long ranging extending into the silurian the extinction rate is low throughout the period increasing only slightly in the latest late ordovician the normalised diversity curve is characterised by an increasing trend in the mid ordovician followed by a plateau in the late ordovician c 2006 elsevier b v all rights reserved less

seismic evidence for anomalous crustal structure beneath mesozoic fracture zones in the gambia basin eastern equatorial atlantic abstract five sonobuoy refraction profiles shot in the eastern equatorial atlantic reveal the presence of thin and heterogenous crust in a zone at least 200 km wide near the southern margin of the gambia basin oceanic layer 2 v p 4 1 4 4 km s is detected on all profiles on some sections it is underlain by crustal material with apparent seismic velocities between 5 7 km s and 7 0 km s on others layer 2 directly overlies a zone with apparent velocities characteristic of normal upper mantle velocities 8 km s the formation of anomalous crust is attributed to a modification of accretion processes resulting from a close spacing of fracture zones during a period of slow spreading in the early cretaceous

saline tracer visualized with three dimensional electrical resistivity tomography field scale spatial moment analysis 1 cross well electrical resistivity tomography ert was used to monitor the migration of a saline tracer in a two well pumping injection experiment conducted at the massachusetts military reservation in cape cod massachusetts after injecting 2200 mg l of sodium chloride for 9 hours ert data sets were collected from four wells every 6 hours for 20 days more than 180 000 resistance measurements were collected during the tracer test each ert data set was inverted to produce a sequence of 3 d snapshot maps that track the plume in addition to the ert experiment a pumping test and an infiltration test were conducted to estimate horizontal and vertical hydraulic conductivity values using modified moment analysis of the electrical conductivity tomograms the mass center of mass and spatial variance of the imaged tracer plume were estimated although the tomograms provide valuable insights into field scale tracer migration behavior and aquifer heterogeneity standard tomographic inversion and application of archie s law to convert electrical conductivities to solute concentration results in underestimation of tracer mass such underestimation is attributed to 1 reduced measurement sensitivity to electrical conductivity values with distance from the electrodes and 2 spatial smoothing regularization from tomographic inversion the center of mass estimated from the ert inversions coincided with that given by migration of the tracer plume using 3 d advective dispersion simulation the 3 d plumes seen using ert exhibit greater apparent dispersion than the simulated plumes and greater temporal spreading than observed in field data of concentration breakthrough at the pumping well

paleozoic stromatactis and zebra carbonate mud mounds global abundance and paleogeographic distribution carbonate mud mounds with zebra and stromatactis structures are present in every paleozoic system and series but are more common in devonian and carboniferous deposits reaching their acme in mississippian system lower carboniferous rocks global distributions illustrate that mud mounds spanned the planet ranging from tropical to polar circles such a wide latitudinal span signifies that they not only grew in and occupied warm depositional environments but also in settings where oceanic waters were cold and seasonally light limited moreover their proliferation during the devonian and carboniferous was at a time when planet wide climatic ice house conditions are thought to have prevailed mud mounds therefore may also be products of cool and cold water carbonate sedimentation

hydrogeophysical tracking of three dimensional tracer migration the concept and application of apparent petrophysical relations 1 direct estimation of groundwater solute concentrations from geophysical tomograms has been only moderately successful because 1 reconstructed tomograms are often highly uncertain and subject to inversion artifacts 2 the range of subsurface conditions represented in data sets is incomplete because of the paucity of colocated well or core data and aquifer heterogeneity and 3 geophysical methods exhibit spatially variable sensitivity we show that electrical resistivity tomography ert can be used to estimate groundwater solute concentrations if a relation between concentration and inverted resistivity is used to deal quantitatively with these issues we use numerical simulation of solute transport and electrical current flow to develop these relations which we call apparent petrophysical relations they provide the connection between concentration or local resistivity and inverted resistivity which is measured at the field scale based on ert for media containing ionic solute the apparent petrophysical relations are applied to tomograms of electrical resistivity obtained from field measurements of resistance from cross well ert to create maps of tracer concentration on the basis of synthetic and field cases we demonstrate that tracer mass and concentration estimates obtained using these apparent petrophysical relations are far better than those obtained using direct application of archie s law applied to three dimensional tomograms from ert which gives severe underestimates

seismic and geologic evidence for early paleogene deepwater circulation in the western north atlantic previous studies of paleogeography global climate and foraminiferal stable isotopes have suggested that paleogene deepwater circulation was significantly different from that of today extracting details of deepwater history from the western north atlantic has been hampered by oligocene erosion and the lack of ideally located drill holes we report the discovery of a seismic disconformity on the sw bermuda rise that we have tied to a late paleocene erosional event in deep sea drilling project dsdp boreholes from this region distribution of this new seismic marker reflector ab indicates that it was formed by cyclonically circulating bottom water entering the western north atlantic from the south we attribute this vigorous deep circulation to a southern ocean source analogous to the effects of modern antarctic bottom water this interpretation is supported by isotopic comparisons that show the paleocene southern ocean was enriched in 13c relative to the pacific we distinguish reflector ab from the overlying reflector au on the basis of superposition and ages obtained at dsdp sites and piston cores using one of the cores we have estimated that the youngest age for reflector au is earliest oligocene 36 6 34 0 ma while its oldest age has been previously established as late eocene laminated sediments beneath reflector ab are enriched in organic carbon indicating that the early paleocene deep western north atlantic was occasionally anoxic these low oxygen levels may have been the result of sluggish deep circulation and or low latitude production of warm saline deep water wsdw sediments immediately above reflector ab show that deposition resumed on the western bermuda rise site 387 by the latest paleocene to earliest eocene biochron np10 57 2 0 6 ma while deposition resumed on the central bermuda rise site 386 during the late paleocene erosion nondeposition continued across much of the bermuda rise widespread resumption of pelagic deposition in the early eocene is consistent with 13c evidence for reduced southern ocean influence at this time however lower eocene sediments record well oxygenated conditions and 13c and 18o comparisons indicate that there were two early eocene deepwater sources the southern ocean and wsdw pak and miller this issue

an alternative early opening scenario for the central atlantic ocean abstract the opening of the central atlantic ocean basin that separated north america from northwest africa is well documented and assumed to have started during the late jurassic however the early evolution and the initial breakup history of pangaea are still debated most of the existing models are based on one or multiple ridge jumps at the middle jurassic leaving the oldest crust on the american side between the east coast magnetic anomaly ecma and the blake spur magnetic anomaly bsma according to these hypotheses the bsma represents the limit of the initial basin and the footprint subsequent to the ridge jump consequently the evolution of the northwest african margin is widely different from the northeast american margin however this setting is in contradiction with the existing observations in this paper we propose an alternative scenario for the continental breakup and the mesozoic spreading history of the central atlantic ocean the new model is based on an analysis of geophysical data including new seismic lines an interpretation of the newly compiled magnetic data and satellite derived gravimetry and recently published results which demonstrate that the opening of the central atlantic ocean started already during the late sinemurian 190 ma based on a new identification of the african conjugate to the ecma and on the extent of salt provinces off morocco and nova scotia the identification of an african conjugate magnetic anomaly to bsma the african blake spur magnetic anomaly absma together with the significant change in basement topography are in good agreement with that initial reconstruction the early opening history for the central atlantic ocean is described in four distinct phases during the first 20 myr after the initial breakup 190 170 ma from late sinemurian to early bajocian oceanic accretion was extremely slow 0 8 cm y at the time of blake spur 170 ma early bajocian a drastic change occurred both in the relative plate motion direction from nnw sse to nw se and in the spreading rate an increase to 1 7 cm y after a small increase between chron m25 154 ma kimmeridgian and chron m22 150 ma tithonian the spreading rate slowed down to about 1 3 cm y and remained fairly constant until chron m0 125 ma barremian aptian boundary in addition kinematic reconstructions illustrate a significant spreading asymmetry during the early history of the central atlantic ocean the accretion rates were higher on the american side and led to the formation of more oceanic crust on this plate we infer that this asymmetry could be related to the fact that the thermal anomaly responsible for the significant magmatism of the central atlantic magmatic province camp was preferentially located below the african plate

ordovician reef and mound evolution the baltoscandian picture the widespread growth of reefs formed by a framework of biogenic constructors and frame lacking carbonate mounds began on baltica during ordovician time previously ordovician reef and mound development on baltica was considered to be sporadic and local a review of all known bioherm localities across the baltic basin reveals a more consistent pattern ordovician bioherms grew in a wide e w aligned belt across the baltic basin and occur in several places in norway substantial reef development began simultaneously across the region during the late sandbian early katian interval and climaxed during the late katian pirgu age the current spatiotemporal distribution of bioherms is a result of interdependent factors that involve original drivers of reef development such as relative sea level climate during the time of deposition and effects of post depositional erosion oceanographic conditions were likely more favourable during times of cooler global climates low sea level and glacial episodes at the same time the likelihood that bioherms are preserved from long term erosion is higher when deposited during low sea level in deeper parts of the basin a main factor controlling the timing of the reef and mound evolution was baltica s shift toward palaeotropical latitudes during late ordovician time the time equivalence between initial reef growth and the guttenberg isotope carbon excursion gice suggests that global climatic conditions were important

abyssal current influence on the southwest bermuda rise and surrounding region abstract echograms 3 5 khz and bottom photographs reveal that the northward flowing antarctic bottom water aabw has strongly influenced the modern depositional regime on the southwest bermuda rise the spatial distribution of echo character types the orientation and nature of current controlled structures and limited current meter data show that aabw flows with varying intensities along three primary pathways around and over the southwest bermuda rise the main core of aabw flows clockwise around the eastern and western flanks of the southern bermuda rise roughly parallel to the 5400 m isobath this current bifurcates at 28 30 n 69 w where a portion flows northeast over the southwest bermuda rise and the remainder continues north along the physiographic boundary between the southwest bermuda rise and the hatteras abyssal plain secondary ribbons of aabw branch off the main core of aabw during its southerly journey along the southeastern bermuda rise and flow west through fracture zones finally a diffuse northward flowing aabw sweeps the entire southwest bermuda rise a progression of current controlled bedforms occurs beneath the main path of the aabw reflecting the spatially varying current velocities and sediment supply the main core of aabw flows west through the narrow vema gap creating erosional furrows along the border between the southwest bermuda rise and the vema gap current velocities greater than 20 cm s 1 are inferred from the bedforms in this region farther north along the southwestern edge of the bermuda rise sediment waves become more prevalent this transition from erosional to more depositional bedforms results from diminished current velocities 5 15 cm s 1 and increased sediment supply although some of these bedforms on the southwest bermuda rise appear to be relict their orientation is consistent with current meter data and abyssal current direction inferred from bottom photographs

polychaete palaeoecology in an early late ordovician marine astrobleme of sweden the post impact dalby limestone kulcruse upper ordovician of the tvaren crater southeastern sweden has been analysed with regards to polychaetes as represented by scolecodonts a palaeoecological succession is observed in the tvaren 2 drill core sequence as the vacant ecospace was successively filled by a range of benthonic nektonic and planktonic organisms scolecodonts belong to the first non planktonic groups to appear and constitute one of the most abundant fossil elements the polychacte assemblage recorded has an overall composition characteristic of that of the upper ordovician of baltoscandia oenonites vistulella mochtyella and the enigmatic xanioprion represent the most common genera whereas pteropelta protarabellites atraktoprion and xanioprion are considerably more rare the assemblage differs from coeval ones particularly in its poorly represented ramphoprionid fauna and the relatively high frequency of xanioprion a taxonomic succession and changes in abundance and relative frequency of different taxa is observed from the deepest part of the crater and upwards towards more shallow water environments the initial post impact assemblage does not however necessarily represent a benthonic colonization of the crater floor instead it seems to be a taphocoenosis as indicated by its taxonomic correspondence to the rim facies fauna recovered from dalby limestone erratics of the ringson island the tvaren succession has yielded considerably richer scolecodont assemblages than hitherto recorded from the approximately coeval lockne crater possibly as a consequence of shallower water settings in the former area

rough smooth basement boundary in the western north atlantic basin evidence for a seafloor spreading origin a distinctive change in the character of acoustic basement has been mapped within the mesozoic keathley magnetic anomaly sequence in the western north atlantic basin this boundary known as the rough smooth basement boundary separates relatively smooth acoustic basement on the older western side of the boundary from much rougher basement to the east south of bermuda the boundary is well defined and nearly isochronous corresponding to anomaly m 12 or m 13 128 m y b p north of bermuda the boundary is diachronous and more transitional with the change in basement relief occurring in a zone between anomaly m 21 and m 16 age crust 132 to 142 m y b p both the well defined and transitional changes in basement relief found in the western basin are also identifiable in seismic reflection records from the eastern canary basin and an analysis of magnetic anomaly patterns in the central north atlantic basin indicates that these changes occurred simultaneously on both sides of the ridge these results strongly suggest a primary seafloor spreading origin for the rough smooth basement boundary with the greater basement relief associated with slower spreading rates changing directions of relative plate motion and ridge jumps of isolated accretionary segments beginning in late jurassic early cretaceous time

imaging and characterisation of subsurface solute transport using electrical resistivity tomography ert and equivalent transport models abstract we assess the usefulness of electrical resistivity tomography ert in imaging and characterising subsurface solute transport in heterogeneous unconfined aquifers a field tracer experiment was conducted at the krauthausen test site germany the spatial and temporal evolution of the injected nabr solute plume was monitored in a 2d ert image plane located downstream of the injection well for 90 days since ert maps changes in bulk electrical conductivity the reconstructed images at selected time intervals are first converted to solute concentration maps by postulating a linear relation the concentration maps are then analysed using an equivalent convection dispersion model cdm which conceptualises the aquifer as a homogeneous medium with a uniform mean flow velocity as demonstrated by associated synthetic model studies ert resolution in terms of recovered equivalent dispersivities is limited due to spatial smoothing inherent to the imaging algorithm since for heterogeneous media local concentrations within the plume deviate from those predicted by the equivalent cdm we also interpret the ert derived pixel breakthrough curves in terms of an equivalent stream tube model stm the stm represents transport in the aquifer by a set of 1d convection dispersion processes allowing the degree of mixing and the heterogeneity of transport within the plume to be quantified we believe that the observed tracer plume is satisfactorily described by the equivalent cdm probably because the tracer plume was small relative to the heterogeneity scale of the aquifer even though application of the stm revealed some deviation from the ideal homogeneous case the equivalent dispersivity in the stm matches the longitudinal dispersivity of the cdm closely consistent with predominantly homogeneous mixing however the stm analysis illustrates how ert results can be used to quantify the variability of parameters relevant to flow and transport in heterogeneous aquifers

crustal structure volcanism and opening mode of the ulleung basin east sea sea of japan abstract a study of the ulleung basin in the southwestern east sea sea of japan based on single and multi channel seismic reflection profiles and ocean bottom seismometer obs data suggests that the basin formed largely by pull apart opening and the deep northern basin is underlain by thicker than normal oceanic crust 10 km thick probably formed during the earliest stage of seafloor spreading volcanic activity in association with basin opening occurred over much of the basin but it ceased first in the south while it continued in the north until recently the principal evidence for pull apart opening includes 1 an overall rhomboidal shape of the basin consisting of two basement lows and a median high known to be characteristic of pull apart basins 2 the location and orientation of the major volcanic sources suggesting more or less n s opening and 3 the strike slip fault system along the western basin margin which appears to have guided the pull apart opening the lack of extensional deformation along the western basin margin which would have been prevalent if sw japan had rotated away from the korean peninsula as suggested by paleomagnetic data is also not in favor of the alternative rotational opening of the basin the absence of graben and rift topography in the acoustic basement in the deep northern ulleung basin is indicative of the non brittle or ductile nature of the crust tau p analysis of the obs data reveals that the igneous crust consists of upper and lower layers with velocities typical of oceanic layers the velocity depth profiles with a two gradient velocity structure is also characteristic of oceanic crust however the lower layer is much thicker than oceanic layer 3 and the change of velocity gradient which occurs at around 6 5 km s in typical oceanic crust occurs at about 6 km s suggesting that the crust is not a typical oceanic crust the meager basin size and short opening period may suggest that the basin opening did not lead to the formation of full fledged oceanic crust but instead resulted in embryonic or incipient oceanic crust thickened by upper mantle thermal perturbations evoked by rifting

vagrant benthos annelida polychaeta associated with upper ordovician carbonate mud mounds of subsurface gotland sweden micropalaeontological investigations of upper ordovician carbonate mud mounds and enclosing strata of subsurface gotland sweden demonstrate that jaw bearing polychaetes formed the most diverse faunal element associated with these build ups although not present within the mound cores intra mound facies scolecodonts or polychaete jaws occur abundantly immediately below and particularly above the mounds the supra mound facies also has the most diverse fossil assemblages by contrast to the scolecodont distribution the most diverse conodont faunas were recorded in the intra mound facies this reinforces the fact that scolecodont and conodont abundance and diversity numbers are commonly inverse to one another suggesting that these metazoans occupied different niches and responded differently to taphonomical processes the polychaete assemblage has no less than 27 species belonging to 12 genera of which oenonites mochtyella and pistoprion are the most abundant the assemblage has a characteristic baltic signature and is similar in taxonomic composition to coeval ones from other areas of the baltoscandian palaeobasin such as that of present day estonia a principal component analysis clusters the gotland assemblage most closely to those recorded from shallow to transitional shelf environments of estonia indicatino that the mud mounds were formed in such environments less

potential of electrical resistivity tomography to infer aquifer transport characteristics from tracer studies a synthetic case study 1 with time lapse electrical resistivity tomography ert transport processes in the subsurface can be imaged and monitored the information content of obtained spatiotemporal data sets opens new ways to characterize subsurface spatial variability difficulties regarding a quantitative interpretation of the imaged transport may arise from the fact that data inversion used in ert is generally underdetermined and that ert data acquisition is often limited to two dimensional 2 d image planes to address this problem we conducted a numerical tracer experiment in a synthetic heterogeneous aquifer with preset variability and spatial correlation of the hydraulic conductivity and monitored the tracer breakthrough in a 2 d image plane perpendicular to the mean flow direction using time lapse ert the tracer breakthrough patterns in the image plane were interpreted using equivalent transport models an equivalent convection dispersion equation to characterize the spatially averaged breakthrough and a stream tube model to characterize local breakthrough curves equivalent parameters derived from simulated and from ert inverted concentrations showed a good agreement which demonstrates the potential of ert to characterize subsurface transport using first order approximate solutions of stochastic flow and transport equations equivalent model parameters and their spatial variability were interpreted in terms of the local scale dispersion and the spatial variability of the hydraulic conductivity the spatial correlations of the stream tube velocity and of the hydraulic conductivity were found to be closely related consequently the hydraulic conductivity spatial correlation may be inferred from stream tube velocity distributions which can be observed with sufficiently high spatial resolution using ert

the crustal structure of the canary basin accretion processes at slow spreading centers multichannel seismic reflection and gravity data define the structure of mesozoic ocean crust of the canary basin formed at slow spreading rates single and multichannel seismics show a transition from smooth to rough basement topography from jurassic to cretaceous crust and a coeval change in crustal structure internal reflectivity of the rough basement area comprises upper upper middle or whole crust cutting discrete dipping reflections lower crustal reflectivity is almost absent and reflections from the crust mantle transition are short and discontinuous or absent for several kilometers in contrast crust in the smooth basement area is characterized by sparse lower crustal events and common reflections from the crust mantle boundary the crustal structure of fracture zones in the rough basement area is associated with depressions in the basement top and in most cases with thin crust in the smooth basement area fracture zones exhibit neither a clear topographic expression nor crustal thinning we interpret these characteristics as indicative of an increase in extensional tectonic activity and decrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from jurassic to cretaceous times in addition the crust imaged across the path of the cape verde hot spot in the canary basin exhibits a widespread lower crustal reflectivity very smooth topography and apparently thick crust our data document significant changes in the structure of crust formed at slow spreading rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spot beneath the mesozoic mid atlantic ridge

upper ordovician sequences of western estonia abstract the upper ordovician uppermost caradoc ashgill section of western estonia consists of a series of seven open shelf carbonate sequences depositional facies grade laterally through a series of shelf to basin facies belts grain supported facies shallow shelf mixed facies middle shelf mud supported facies deep shelf and slope and black shale facies basin locally a stromatactis mud mound occurs in a middle to deep shelf position shallow to deep shelf facies occur widely across the estonian shelf and grade laterally through a transitional slope belt into the basinal deposits of the livonian basin each sequence consists of a shallowing upward prograding facies succession sequences 1 upper nabala stage and 2 vormsi stage record step wise drowning of underlying shelf units lower nabala that culminated in the deposition of the most basinal facies fjacka shale in the livonian basin sequences 3 6 comprise the overlying pirgu stage and record the gradual expansion of shallow and middle shelf facies across the estonian shelf the porkuni stage sequence 7 is bracketed by erosional surfaces and contains the shallowest water facies of the preserved strata the uppermost part of the section normalograptus persculptus biozone is restricted to the livonian basin and includes redeposited carbonate and siliciclastic grains it is the lowstand systems tract of the lowest silurian sequence 8 sequence 7 and the overlying basinal redeposited material i e the lowstand of sequence 8 correspond to the latest ordovician hirnantian glacial interval and the bracketing unconformities are interpreted as the widely recognized early and late hirnantian glacial maximums the sequences appear correlative to upper ordovician sequences in laurentia graptolite biozones indicated that the estonian sequences are equivalent to carbonate ramp sequences in the western united states great basin and mixed carbonate siliciclastic sequences in the eastern united states appalachian basin cincinnati arch region these correlations indicate a strong eustatic control over sequence development despite the contrasting tectonic settings of these basins

on the roughness of mesozoic oceanic crust in the western north atlantic summary seismic reflection profiles from mesozoic oceanic crust around the blake spur fracture zone bsfz in the western north atlantic have been widely used in constraining tectonic models of slow spreading mid ocean ridges these profiles have anomalously low basement relief compared to crust formed more recently at the mid atlantic ridge at the same spreading rate profiles from other regions of mesozoic oceanic crust also have greater relief the anomalous basement relief and slightly increased crustal thickness in the bsfz survey area may be due to the presence of a mantle thermal anomaly close to the ridge axis at the time of crustal formation if so the intracrustal structures observed may be representative of an atypical tectonic regime

seafloor spreading history of the western north atlantic basin derived from the keathley sequence and computer graphics abstract an analysis of sea surface magnetic profiles and an aeromagnetic contour chart west of bermuda yields a new tectonic framework for the keathley m series seafloor spreading magnetic anomaly sequence changes in spreading direction derived from the western keathley sequence occur at times corresponding to m24 m21 and m11 of which m11 is a time of continental breakup the models facilitate new identifications of the anomalies between m16 and m4 obviating the need for anomalously slow spreading between m11 and m4 previously suggested by other workers and by us and placing the location of the well defined rough smooth basement boundary at m11 the models show that spreading rates contemporaneously vary among profiles collected between flowlines as determined by a recently presented kinematic history this strongly suggests that individual ridge axis segments act independently of another in detail our reconstruction technique records in map form where the ridge crest has made significant departures from perfect symmetry and a predictable spreading rate during its evolutionary history among eight profiles collected parallel to seafloor spreading flowlines in the western north atlantic encompassing only 2700 km of normal oceanic crust we can identify more than fourteen ridge jump events there is evidence for a small 30 km ridge jump to the east of many ridge axis segments at m24 this is supported in the eastern north atlantic conjugate crust of the canary basin where m25 and m24 are poorly represented a series of four ridge jumps to the east along one paleoridge axis segment now located at 36 n 65 w is related to the passage of the mesozoic mid atlantic ridge crest over the verde hotspot at 137 m y b p the project magnet magnetic anomaly contour chart for the area between 29 n and 40 n has been used to determine stage poles of motion for processes occurring at the ridge crest related to its segmentation which have a component oblique to spreading between chrons m21 and m11 these trends may relate to the motion of the ridge crest over the mantle

large scale natural gradient tracer test in sand and gravel cape cod massachusetts 1 experimental design and observed tracer movement a large scale natural gradient tracer experiment was conducted on cape cod massachusetts to examine the transport and dispersion of solutes in a sand and gravel aquifer the nonreactive tracer bromide and the reactive tracers lithium and molybdate were injected as a pulse in july 1985 and monitored in three dimensions as they moved as far as 280 m down gradient through an array of multilevel samplers the bromide cloud moved horizontally at a rate of 0 42 m per day it also moved downward about 4 m because of density induced sinking early in the test and accretion of areal recharge from precipitation after 200 m of transport the bromide cloud had spread more than 80 m in the direction of flow but was only 14 m wide and 4 6 m thick the lithium and molybdate clouds followed the same path as the bromide cloud but their rates of movement were retarded about 50 relative to bromide movement because of sorption onto the sediments