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

the geomorphology of the southwest guinea margin tectonic volcanic mass movement and bottom current influences abstract slope morphology controls were examined in two topographically different areas on the southern transform margin of the guinea plateau sectors g1 and g2 tectonic control is manifested by steep faulted slopes in both study areas and by basement warping in sector g1 and magmatic emplacements in sector g2 sediment deposition and the formation of bedforms such as sediment waves on the margin are influenced by topographically intensified bottom currents within the intermediate deep water mass the north atlantic deep water nadw as well as in the shallower equatorial surface water esw antarctic intermediate water aaiw at the uppermost slope plateau edge level thus unconsolidated sediments are eroded from steep slopes the upper and lower slopes canyon walls and the slopes of the volcanic cones and concentrated into slope parallel depocentres in sector g1 and ponded in moats between the volcanic cones in sector g2 surficial sediment waves which are apparently oriented at an angle to the slope range from 0 5 to 2 km in wavelength and up to 100 m in height mass movement of the sediment cover is significant on the lower slopes of both sectors and is the result of slope steepness and instability caused by the continuing tectonic activity

meso cenozoic long term landscape evolution at the southern moroccan passive continental margin tarfaya basin recorded by low temperature thermochronology abstract this paper presents the first regional study of low temperature thermochronology to be undertaken in the tarfaya basin at the southern moroccan passive continental margin the basin is characterised by vast subsidence since mid triassic times whereby up to 12 km of meso to cenozoic sedimentary rocks accumulated the study focused on the post rift vertical movements along a typical passive margin and besides dealt with the timing and maximum temperature reached by potential source rocks of the basin to unravel the t t development thermochronological analyses were performed on 50 outcrop and well samples from meso cenozoic rocks thermochronological data reveal a continuous subsidence phase in the offshore basin from mid triassic to recent times in contrast apatite u th sm he and apatite fission track data as well as thermal modelling point to an inversion of the northeastern onshore basin starting in the palaeogene at 65 55 ma the rock uplift and exhumation period resulted in the erosion of a 1 0 1 4 km thick cretaceous palaeogene sedimentary pile contemporaneously with peak atlas surface uplift in the cenozoic the exhumation stage could be an explanation for the increasing periodic influx of detrital material into the offshore and southern onshore tarfaya basin since palaeocene detrital apatite fission track ages from 92 16 to 237 35 ma of the upper cretaceous neogene succession indicate no heating above 60 c confirming immature to early mature cenomanian to campanian and eocene source rocks in the onshore tarfaya basin

oceanic plateau formation a cause of mass extinction and black shale deposition around the cenomanian turonian boundary the cenomanian turonian boundary 90 4 ma represents a major period of worldwide environmental disturbance the physical manifestations of this are elevated atmospheric and oceanic temperatures a significant sea level transgression and a period of widespread anoxia leading to the formation of oceanic black shales and the extinction of 26 of all genera elevated 13 c values and enrichment of trace elements in cenomanian turonian boundary sediments combined with a reduction in 87 sr 86 sr also imply a severe environmental perturbation at this time oceanic crustal production rates reached their highest level of the last 100 million years this was principally caused by extensive melting of hot mantle plumes at the base of the oceanic lithosphere and the development of vast areas up to 1 10 6 km 2 of thickened oceanic crust in the pacific and indian oceans the anomalous volcanism associated with the formation of these oceanic plateaux may have been responsible for the environmental disturbances c 90 ma these eruptions would also have resulted in the emission of large quantities of co 2 into the atmosphere leading to global warming additionally the emission of so 2 h 2 s co 2 and halogens into the oceans would have made seawater more acidic resulting in the dissolution of carbonate and further release of co 2 this run away greenhouse effect was probably put into reverse by the decline of the anomalous volcanic activity and by increased co 2 driven productivity in oceanic surface waters leading to increased organic carbon burial black shale deposition anoxia and mass extinction in the ocean basins

anoxic vs dysoxic events reflected in sediment geochemistry during the cenomanian turonian boundary event cretaceous in the umbria marche basin of central italy abstract one of the most spectacular expressions of global oceanic anoxic events oaes of the cretaceous is the livello bonarelli of central italy deposited during the cenomanian turonian boundary event ctbe or oae2 at around 93 5 ma at furlo in the umbria marche basin in italy the bonarelli consists of approximately 1 m of c org rich shales interbedded with radiolarian rich layers and contrasts sharply with the extensive over and underlying siliceous limestones several minor c org rich intervals herein termed black levels average thickness 4 5 cm are interbedded within the scaglia bianca beneath the bonarelli x ray fluorescence and icp ms analyses show that these sediments consist of different mixing systems with three end members 1 sio 2 caco 3 scaglia bianca 2 average shale caco 3 black levels and 3 average shale sio 2 bonarelli given the sharp lithologic contacts and the little geochemical mixing between the groups these are thought to represent rapid switching events between sedimentary modes major element concentrations are generally low except for si and p owing to dilution by silica up to 96 and carbonates as al normalised ratios for elements such as ti or zr taken as proxies for higher energy environments are close to average shale values excess sio 2 is most likely linked to biogenic processes rather than quartz element al ratios of ti mg k rb and zr are fairly constant pointing to a mostly homogeneous source area the black levels and bonarelli are characterised by lower mg al ratios compared to the scaglia bianca sediments suggesting variability in chlorite input which could be linked with short term sea level drops indicated by ti al ratios the bonarelli sediments are severely depleted in mn which is indicative of oxygen depleted bottom water conditions and an open marine environment while the black levels appear to have been deposited in a much more restricted setting a model involving partial physical isolation of the furlo site by a sill coupled to omz variations by extra basinal forcing mechanisms is proposed to account for these conflicting data based on re mo ratios the bonarelli sediments were deposited under varying oxygen depletion levels from suboxic to anoxic while re mo ratios in the black levels show deposition under less oxygen deficient conditions most minor elements many of them redox sensitive or sulphide residing are strongly enriched in the c org rich sediments further documenting the absence of oxygen and potential availability of hydrogen sulphide in the water column at times while high zn concentrations suggest elevated submarine hydrothermal activity during this time interval ba is also present in very high concentrations up to 1 4 with an average weighed concentration of 0 6 for the entire bonarelli and is probably indicative of high paleoproductivity in this area this is further supported by high p concentrations hinting at nutrient availability at this site

foredeeps versus subduction zones the geologic characteristics of foredeeps and accretionary wedges suggest that these features are distinguishable on the basis of the direction of the associated subduction east north east dipping subduction related accretionary wedges show high relief and broad outcrops of metamorphic rocks they are associated with shallow foredeeps with low subsidence rates in contrast west dipping subduction related accretionary wedges show low relief and involve mainly sedimentary cover the related foredeeps are deep and have high subsidence rates this differentiation is useful both for oceanic and continental subductions e g eastern vs western pacific subductions or east dipping alpine vs west dipping apenninec subductions in a cross section of the alps the ratio of the area of the orogen to the area of the foredeeps is at least 2 1 whereas this ratio is 0 22 1 for the apennines these ratios explain why foredeeps related to east or northeast dipping subduction are quickly filled and are bypassed by clastic rocks whereas foredeeps related to west dipping subduction maintain a deep water environment longer these differences support the presence of an eastward asthenospheric counterflow relative to the westward drift of the lithosphere detected in the hot spot reference frame even in the mediterranean where no hot spots are present in this interpretation the apennines foredeep was caused by the eastward push of the mantle acting on the subducted slab whereas the foredeeps in the alps were caused by the load of the thrust sheets and the downward component of movement of the upper adriatic plate these forces contrast with the upward component of the eastward mantle flow

forcing mechanisms for mid cretaceous black shale formation evidence from the upper aptian and lower albian of the vocontian basin se france calcareous nannoplankton palynomorph benthic foraminifera and oxygen isotope records from the supraregionally distributed niveau paquier early albian age oceanic anoxic event 1b and regionally distributed niveau kilian late aptian age black shales in the vocontian basin se france exhibit variations that reflect paleoclimatic and paleoceanographic changes in the mid cretaceous low latitudes to quantify surface water productivity and temperature changes nutrient and temperature indices based on calcareous nannofossils were developed the nutrient index strongly varies in the precessional band whereas variations of the temperature index reflect eccentricity since polar ice caps were not present during the mid cretaceous these variations probably result from feedback mechanisms within a monsoonal climate system of the mid cretaceous low latitudes involving warm humid and cool dry cycles a model is proposed that explains the formation of mid cretaceous black shales through monsoonally driven changes in temperature and evaporation precipitation patterns the lower albian niveau paquier which has a supraregional distribution formed under extremely warm and humid conditions when monsoonal intensity was strongest bottom water ventilation in the vocontian basin was diminished probably due to increased precipitation and reduced evaporation in regions of deep water formation at low latitudes surface water productivity in the vocontian basin was controlled by the strength of monsoonal winds the upper aptian niveau kilian which has a regional distribution only formed under a less warm and humid climate than the niveau paquier low latitude deep water formation was reduced to a lesser extent and or on regional scale only the threshold for the formation of a supraregional black shale was not reached the intensity of increases in temperature and humidity controlled whether black shales developed on a regional or supraregional scale at least in the vocontian basin the increased preservation of organic matter at the sea floor was more significant in black shale formation than the role of enhanced productivity

electromagnetic induction in new zealand analogue model and field results abstract the behaviour of electric and magnetic variations over north island new zealand is studied with the aid of a laboratory analogue model the source field frequencies used in the analogue modelling simulate naturally occurring geomagnetic variations of 5 120 min periods in phase and quadrature magnetic and electric fields for a selection of traverses for the modelled region of north island are presented since north island is of a relatively narrow cross section the field responses even for inland locations are expected to show strongly the effects of the surrounding ocean the irregular coastlines as well as the strait between north and south islands lead to coastal and inland field anomalies due to induced currents being deflected and channelled to produce localized current densities the comparison of model results with field station measurements obtained earlier individually by ingham and by midha for sites in the northeastern central and southern near cook strait regions of north island demonstrates the large role the ocean has in the observed field responses differences in the model and field results at some sites are expected and should reflect the effects of the local geology and the conductive substructure related to the complex tectonics of the region not simulated in the model

description of a contourite depositional system on the demerara plateau results from geophysical data and sediment cores the demerara plateau belonging to the passive transform margin of french guiana was investigated during the iguanes cruise in 2013 the objectives of the cruise were to explore the poorly known surficial sedimentary column overlying thick mass transport deposits as well as to understand the factors controlling recent sedimentation the presence of numerous bedforms at the seafloor was observed thanks to newly acquired iguanes bathymetric high resolution seismic and chirp data while sediment cores allowed the characterization of the deposits covering the mass transport deposits modern oceanographic conditions were determined in situ using mooring monitoring over a 10 month period our data indicate the presence of a contourite depositional system along the demerara plateau most likely related to bottom current activity of the north atlantic deep water nadw however at the regional scale large longitudinal waves parallel the nadw flow their shape and orientation seem to be inherited from interactions between bottom currents and paleomorphologies expressed at the top of mass transport deposits their evolution is possibly driven by the intensity of bottom current activity that might have changed over time overall this work presents an integrated approach combining seismic and sedimentological evidence to study the processes at the origin of contourite formation in the demerara plateau other regional factors such as local slope values and slope instability also control sedimentation

differential opening of the central and south atlantic oceans and the opening of the west african rift system abstract plate tectonic studies of the development of the central and south atlantic oceans using seasat and geosat altimeter and magnetic anomaly isochron data now provide quantitative models of seafloor spreading through time such models enable an initial assessment of the differential opening between these two oceanic basins to be determined the equatorial atlantic is an integral part of this oceanic rifting process allowing stresses arising from the differential opening to be dissipated into both the caribbean and africa along its northern and southern boundaries respectively the tectonic model for the west african rift system based on geological and geophysical studies shows a series of strike slip fault zones diverging into africa from the gulf of guinea and dissipating their shear movement into the development of extensional basins orientated perpendicular to these faults zones the development of the west african rift system was contemporaneous with the early opening of the south atlantic continued to develop well after the final breakup of south america from africa and did not cease until the late cretaceous when there was a major phase of basin inversion and deformation santonian 80 ma deformation across the benue trough nigeria is broadly contemporaneous with dextral shear reactivation of the central african fracture system which in turn resulted in renewed extension in the sudan basins during the late cretaceous and early tertiary this paper illustrates the close linkage in both time and space between the history of the african rift basins and the opening of the atlantic both exhibit distinct phases of evolution with the rift basins developing in direct response to the differential opening between the central and south atlantic in order to dissipate stresses generated by this opening the mesozoic tectonic model proposed is therefore one of an intimate interaction between oceanic and continental tectonics

a synthesis of the sedimentary evolution of the demerara plateau central atlantic ocean from the late albian to the holocene abstract the demerara plateau off the french guiana and surinam margin corresponds to a prominent sub horizontal marginal plateau the interpretation of a large set of seismic data acquired along the demerara plateau and integration with previous seismic stratigraphy and drill studies enables a better understanding of the sedimentary evolution of the outer demerara marginal plateau since the end of the transform activity late albian three sedimentary evolutionary stages have been identified 1 the pre contourite stage late albian to early miocene is strongly controlled by the structure of the margin that influences the distribution pattern of post transform deposits through differential thermal subsidence between transform and divergent segments 2 the transitional stage middle miocene to early pliocene is expressed as a current controlled erosive surface resulting from the onset of bottom current interactions due to the establishment and strengthening of the modern thermohaline circulation that is coeval with the progressive closure of the isthmus of panama 3 the contourite stage middle pliocene to holocene is characterized by the development of a contourite depositional system that is linked to the activity of the north atlantic deep water circulation and by a complex interaction between alongslope and downslope processes our study highlights that marginal plateaus might be particularly prone to develop contourite features since they regionally influence ocean current dynamics in guiding and accelerating bottom contour currents marginal plateaus should thus be considered as a key target to study bottom current dynamics and paleocirculation patterns

changes in paleoenvironments in the atlantic ocean during cretaceous times results from black shales studies large parts of the cretaceous atlantic ocean are characterized by the wide spread deposition of organic carbon rich sediments black shales with a maximum occurrence during aptian albian and cenomanian turonian times in order to explain the origin of these black shales changes in the amount and type of organic matter in relation to changes in paleogeography paleoceanic circulation and paleoclimate have been investigated in detail the study is mainly based on calculation of accumulation rates of marine and terrigenous organic carbon organic carbon sedimentation rate relationships and estimates of paleoproductivity

paleomagnetic evidence for a neogene two phase counterclockwise tectonic rotation in the northern apennines italy paleomagnetic directions have been determined for a new collection of early oligocene and late miocene pliocene epiligurian clastic sediments from the frontal portions of the northern apennines these results are combined with cenozoic data from the literature to evaluate whether rotations of units in this region are related to the oligomiocene corsica sardinia rotation and or to younger phases of deformation of the apennine chain when corsica sardinia moved counterclockwise ov the coast of france the ligurian units located at its front were presumably pushed eastward and rotated counterclockwise above a main boundary thrust onto the adria africa margin we propose that about 24 out of a total of 52 of rotation observed in the epiligurian units can be related to the oligo miocene motion of the corsica sardinia block in partial agreement with previous conclusions and the remaining 28 to the pliocene tectonic phase at the apennine chain front which may have re activated thrust planes in the adria africa succession below the ligurian wedge 2000 elsevier science b v all rights reserved

organic rich sedimentation at the cenomanian turonian boundary in oceanic and coastal basins in the north atlantic and tethys one of the most striking results of the deep sea drilling project is the proof that organic rich sediments have a widespread geographical distribution during the period from upper cenomanian to middle turonian such sediments were drilled at north atlantic dsdp sites 105 135 137 138 367 398 551 603 they are also present from outcrops or oil wells on the shelf of the african continental margin senegal tarfaya and agadir basins in the tethys former alboran block southern spain algeria tunisia and umbrian apennines italy and in the north sea although these sediments have different lithologies and depositional environments from shelf areas to the deep sea their study mainly based on organic geochemistry with additional data on sedimentology biostratigraphy and palaeo bathymetry suggests that a unique pulse of organogenic accumulation characterizes the cenomanian turonian boundary event ctbe the content and type of organic matter are related to the depositional environment and organic preservation the organogenic accumulation is distributed according to various trends off the african continental margin the organic content increases from onshore areas to the shelf casamance area and moreover increases also in deep sea areas with a gradual transition from terrestrial type iii to marine type ii the best preservation of the organic matter being in the deepest areas i e site 367 off the american continental margin site 603 shows the same toc and type of organic matter as at site 105 the ctbe is also well recorded in the northern part of the atlantic celtic margin north sea by a drastic lithological change black shales within chalks but the type of organic matter is mainly terrestrial in the tethyan area the organic matter is of marine origin and well preserved results are compared with those of pratt 1984 from the western interior basin of the usa different hypotheses to explain this synchronous widespread accumulation of organic matter are discussed 1986 the geological society

the coast effect response in geomagnetic field measurements abstract two dimensional 2d numerical models of electromagnetic em induction for anomalous conductors in continental and island coastal regions are studied to determine constraints on ocean conductor separation distances that would permit to within an acceptable approximation a simple vector subtraction of the coast effect response the anomalous conductors are in the form of an upwelling or a depression in the asthenospheric conductive substratum at depth the coast effect response range yr is arbitrarily taken to be the distance from the ocean at which the b z b y n coast effect response is reduced to a value of 0 2 the response ranges are determined and presented empirically as a function of period for a range of conductive substratum depths island widths and ocean depths the response ranges are tested in 2d numerical models that include an anomalous conductor in the form of either an upwelling or a depression in the conductive substratum under the ocean and host earth the induction arrows for the anomalous conductor are shown to be in good agreement with the difference arrows obtained by subtracting the responses of a model without the anomalous conductor from the responses of the model that includes the anomalous conductor as an application difference arrows obtained by subtracting three dimensional laboratory analogue model coast effect responses from field site responses are employed in the interpretation of induction arrow responses available for a number of sites in the bohai bay region of china and the kii peninsula region of japan

occurrence of marine mid cretaceous sediments along the guinean slope equamarge ii cruise their significance for the evolution of the central atlantic african margin abstract during the equamarge ii cruise 1988 several coring attempts made along the slope edge of the southern guinean plateau led to the recovery of middle cretaceous sediments in two cores one contained a carbonate block dated as middle albian h rischi zone the micropaleontological and sedimentological data suggest an open marine environment and paleodepths of approximately 150 200 m another core included a dark shale level corresponding to the global anoxic event ctbe cenomanian turonian boundary event w archaeocretacea zone as shown by seismic reflection profiles these mid cretaceous beds lie very close to a major unconformity which distinctly divides the sedimentary cover into two units the lower one folded and faulted the upper one undisturbed geodynamic and paleoenvironmental implications of these new data better constrain models of evolution for this crucial area of the atlantic realm the compressive and extensional events which affected the equatorial atlantic during the progressive separation of africa from south america are thus more accurately dated these results also strengthen the hypothesis of an open seaway between the central and south proto atlantic as early as the middle albian

alkaline volcano of paleocene age on the southern guinean margin mapping petrology 40ar 39ar laser probe dating and implications for the evolution of the eastern equatorial atlantic abstract a segment of the southern guinea plateau margin s g p m was surveyed during the 1988 equamarge ii cruise of the r v jean charcot detailed seabeam mapping reveals a huge 3000 m high volcano which was called the nadir seamount culminating at 840 m below sea level and 6 adventive cones volcanics dredged between 2100 and 1300 m of water depth consist of hyaloclastite breccias and subaerial alkali basalts recovered as both massive blocks and eolian pebbles with a desert varnish the data are consistent with the growth of a volcano up to emergence and thereafter its dismembering and subsidence volcanological petrographic and geochemical features ree of the volcanics present similarities with those observed 1 in early basalts from fuerteventura canary islands and maio cape verde islands 2 in lamprophyric volcanism from the sierra leone rise and 3 in volcanics from the gorringe bank 40ar 39ar laser probe dating was performed on two biotite phenocrysts from a massive basalt using step heating and spot fusion procedures both experiments concordantly display a plateau age of 58 6 0 3 ma and an integrated age of 58 6 0 7 ma respectively the nadir seamount provides the third evidence of paleocene alkaline volcanism in the eastern atlantic after the gorringe bank and the northern sierra leone rise krause seamount these volcanoes are presumed to have been controlled by fractures zones the ages and compositions of the nadir and krause seamounts respectively suggest a connection between the volcanic chains of both the sierra leone rise and guinea f z which may have recorded the same hotspot track

a laboratory electromagnetic model study of the juan de fuca plate region the behaviour of the magnetic field variations over the juan de fuca plate region is studied using a scaled laboratory analogue model the model includes a simulation of the complex juan de fuca plate subducting the vancouver island region the subducting plate is modelled with a profile of increasing inclination from east to west horizontal offshore dipping at 10 under vancouver island and bending further under georgia strait to subduct the continent at 30 for the b c region and 45 for the washington oregon region the strike of the bending plate follows the general strike of the continental coastline with an abrupt change in direction 42 in the puget sound area the model substructure simulates a subducting plate overplated by a sediment layer several kilometres thick and underlain by a 30 km thick highly conducting upper asthenosphere the model source frequencies used simulate periods 5 120 min in the geophysical scale in phase and quadrature hx hy and hz magnetic field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for e and h polarizations electric field of the source approximately parallel and perpendicular respectively to the west coast of vancouver island the fields for three regions of the model over vancouver island over the olympic peninsula and over a linear portion of the u s coastline are examined in detail the general conclusion is that the effect of the dipping subducting plates is to significantly attenuate at short periods the maxima in the anomalies at the coastlines underlain by the 10 dipping plate while leading to anomalous vertical and horizontal fields over ranges as large as 500 km inland over a wide period range anomalous fields are observed over the offshore and inland knee bends of the subducting plates at all periods for both e and h polarizations for locations on land the in phase induction arrows point seaward and perpendicular to the strikes of the dipping plates for all periods while the quadrature arrows at short periods point landward and rotate to point seaward for periods greater than 20 min

a plate kinematic framework for models of caribbean evolution abstract we define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the caribbean region newly determined poles of rotation defining the approximate spreading histories of the central north and the south atlantic oceans are given for the late jurassic early cretaceous anomaly sequence of the central north atlantic we have used previously published definitions of fracture zone traces and magnetic anomaly picks redetermining the pole positions and angular rotations for various isochrons on an evans and sutherland interactive graphics system for magnetic anomalies younger than the cretaceous quiet period in both oceans we 1 used seasat altimeter data to help define fracture zone traces and 2 identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones by the finite difference method the relative paleopositions and the relative motion history between north and south america were computed this analysis defines the size and shape and the rate at which the size and shape changed of the interplate region between north and south america since the middle jurassic thus a plate kinematic framework is provided for the larger plates pertaining to the caribbean region in which can be derived more detailed scenarios for gulf of mexico and caribbean evolution north and south america diverged to approximately their present relative positions from late triassic to early campanian about 84 m y ago time this is the period during which the gulf of mexico and a proto caribbean seaway were formed since the campanian only minor relative motion has occurred from early campanian through to middle eocene times south america diverged only another 200 km and since the middle eocene minor n s convergence has occurred these very minor post early campanian motions have probably been accommodated by imperfect shear and compression along the atlantic fracture zones to the east of the lesser antilles and along the northern and southern borders of the caribbean plate accordingly it is suggested that from campanian time to the present the relative motions between the north and south american plates have had only minor effects on the structural development of the caribbean region primarily using the data of engebretson et al the convergence history of pacific plates with north america was calculated for two points near the western caribbean by completing finite difference solutions the convergence history of the pacific plates with the caribbean and south american plates can be approximated the direction and rate of convergence of the pacific plates with the americas may have controlled the style of subduction and possible microplate migration along the north american south american and western caribbean boundaries that define the eastern pacific plate margin

evolution of the southern guinea plateau implications on guinea demerara plateau formation using insights from seismic subsidence and gravity data abstract the guinea plateau offshore guinea and its conjugate the demerara plateau offshore french guiana comprise two of the most prominent passive continental margins in the atlantic ocean the conjugate plateaus formed as a result of two periods of rifting the jurassic opening of the central atlantic ocean and the northward propagating cretaceous opening of the southern atlantic ocean although several studies are published on the demerara plateau that explain the evolution of its multi rift history and the effect of rifting on its distinct geometry the guinea plateau and in particular its south eastern margin remain relatively unexplored in the literature here we present interpretations of the structure and evolution of the guinea plateau using recent 2 d and 3 d seismic reflection data collected at the intersection of the southern and eastern margins we substantiate our study with calculated subsidence curves at four locations along the southern margin as well as two 2 d gravity forward models along regional seismic reflection profiles to estimate stretching factors and crustal thicknesses we combine our results with previous studies concerning the south western guinea margin and compare them to published interpretations regarding the conjugate margins of the demerara plateau the resolved amounts of rift related volcanism listric style normal faults and moderate stretching factors suggest that a component of upper crustal asymmetry simple shear and depth dependent stretching may have persisted at the demerara guinea conjugate margins during cretaceous rifting of the equatorial segment of the southern atlantic ocean

late cretaceous to early quaternary organic sedimentation in the eastern equatorial atlantic abstract reconstructing the long term evolution of organic sedimentation in the eastern equatorial atlantic odp leg 159 provides information about the history of the climate ocean system sediment accumulation and deposition of hydrocarbon prone rocks the recovery of a continuous 1200 m long sequence at odp site 959 covering sediments from albian to the present day about 120 ma makes this position a key location to study these aspects in a tropical oceanic setting new high resolution carbon and pyrolysis records identify three main periods of enhanced organic carbon accumulation in the eastern tropical atlantic i e the late cretaceous the eocene oligocene and the pliocene pleistocene formation of upper cretaceous black shales off west africa was closely related to the tectono sedimentary evolution of the semi isolated deep ivorian basin north of the cote d ivoire ghana transform margin their deposition was confined to certain intervals of the last two cretaceous anoxic events the early turonian oae2 and the coniacian santonian oae3 organic geochemical characteristics of laminated coniacian santonian shales reveal peak organic carbon concentrations of up to 17 and kerogen type i ii organic matter which qualify them as excellent hydrocarbon source rocks similar to those reported from other marginal and deep sea basins a middle to late eocene high productivity period occurred off equatorial west africa porcellanites deposited during that interval show enhanced total organic carbon toc accumulation and a good hydrocarbon potential associated with oil prone kerogen deposition of these toc rich beds was likely related to a reversal in the deep water circulation in the adjacent sierra leone basin accordingly outflow of old deep waters of southern ocean origin from the sierra leone basin into the northern gulf of guinea favored upwelling of nutrient enriched waters and simultaneously enhanced the preservation potential of sedimentary organic matter along the west african continental margin a pronounced cyclicity in the carbon record of oligocene lower miocene diatomite chalk interbeds indicates orbital forcing of paleoceanographic conditions in the eastern equatorial atlantic since the oligocene miocene transition a similar control may date back to the early oligocene but has to be confirmed by further studies latest miocene early pliocene organic carbon deposition was closely linked to the evolution of the african trade winds continental upwelling in the eastern equatorial atlantic ocean chemistry and eustatic sea level fluctuations reduction in carbonate carbon preservation associated with enhanced carbon dissolution is recorded in the uppermost miocene 5 82 5 2 ma section and suggests that the latest miocene carbon record of site 959 documents the influence of corrosive deep waters which formed in response to the messinian salinity crisis furthermore sea level related displacement of higher productive areas towards the west african shelf edge is indicated at 5 65 5 6 5 55 5 2 4 8 ma in view of humid conditions in tropical africa and a strong west african monsoonal system around the miocene pliocene transition the onset of pronounced toc cycles at about 5 6 ma marks the first establishment of upwelling cycles in the northern gulf of guinea an amplification in organic carbon deposition at 3 3 ma and 2 45 ma links organic sedimentation in the tropical eastern equatorial atlantic to the main steps of northern hemisphere glaciation and testifies to the late pliocene transition from humid to arid conditions in central and western african climate aridification of central africa around 2 8 ma is not clearly recorded at site 959 however decreased and highly fluctuating carbonate carbon concentrations are observed from 2 85 ma on that may relate to enhanced terrigenous eolian dilution from africa

recurrent depletion of benthic oxygen with 4th order transgressive maxima in the cretaceous western interior seaway abstract a study was designed to investigate the relationship between depletion of benthic oxygen and 4th order sea level change within the cretaceous western interior seaway of north america three 4th order sea level change cycles were studied in strata belonging to the tropic shale and the tununk shale member of the mancos shale in utah these sedimentary deposits accumulated within prodeltaic environments in utah strata belonging to all three 4th order cycles exhibit evidence for depletion of benthic oxygen at times of peak 4th order transgression and development of condensed sections the evidence includes decreased intensities of bioturbation increased cr al ni al and zn al ratios and decreased mn fe ratios the data also suggest that depletion of benthic oxygen in the distal section preceeded that in the more proximal one and thus may reflect episodic expansion of an oxygen depleted water mass es from the pelagic to the prodeltaic environment of the seaway evidence for subsequent increases in benthic oxygen level with the onset of 4th order regressions is interpreted to reflect either aggradation of the seafloor above the oxygen poor water due to sediment accumulation or contraction of the oxygen depleted water mass from the prodeltaic part of the seaway or a combination of the two s organic c ratios of strata interpreted to have been deposited under conditions of depleted benthic oxygen are distinctly below 0 4 and thus were not useful for recognition of paleo oxygen levels in the prodeltaic environment this study highlights the importance of depletion of benthic oxygen as a mechanism for enhancing preservation of organic carbon in condensed sections on a hundred thousand year time scale our results underline the potential for source rock formation in similar prodeltaic deposits of other epicontinental seaways during periods of relatively high sea level and global depletion of benthic oxygen

mesozoic plate tectonic reconstructions of the central south atlantic ocean the role of the west and central african rift system abstract the mesozoic opening history of the central equatorial and south atlantic oceans is closely linked in time and space to the development of the cretaceous rift system in west and central africa geological and geophysical studies of the west and central african rift system indicate it to be dominated by a set of sinistral and dextral strike slip zones diverging from the gulf of guinea and terminating as extensional rift basins in niger and sudan plate tectonic studies of the opening history of the atlantic basins indicate four stages of development stage 1 the jurassic opening of the central atlantic stage 2 the early cretaceous 130 119 ma opening of the south atlantic with rifting propagating deep into africa via the benue trough stage 3 the opening of the equatorial atlantic 119 105 ma and stage 4 the linkage of these oceanic basins and development of a single opening mid oceanic rift system as seen today the west and central african rift system continued to develop until the end of the cretaceous i e into stage 4 when rifting was terminated by a compressional shear deformation event causing the folding of sediments in the benue trough and reactivation of the central african shear zone and extensional tectonics in the basins of southern sudan this event correlates with an important change in the relative opening of the atlantic basins during tertiary recent times the west african rift system has not been active however during this period domal uplift and volcanism have occurred in central cameroon and western sudan and a very active phase of volcanism has occurred within the last 10 ma along the present day cameroon volcanic line although these uplift and volcanic events indicate thermal processes in the upper mantle there is as yet no clear link between their development and plate tectonic processes

an ocean modeling study of the cenomanian atlantic equatorial paleo upwelling organic rich sediments and the consequences for a connection between the proto north and south atlantic abstract a reduced gravity ocean modelis used to deter mine the pattern of upwelling in the cenomanian atlantic the deposition of organic rich sediments on the margins ofthe equatorial region is found to be consistent with the pat terns of upwelling in simulations which incorporate only ashallow 300m equatorial connection between the northand south atlantic fracture zone ridges were probably re sponsible for the e ective shallowness of the gateway if adeep equatorial channel is assumed the modeling results areconsistent only with the morerecent turonian sedimentaryrecord we suggest therefore that reduced gravity modelsprovide a mean of predicting areas of the continental mar gins which may be underlain by hydrocarbon rich sedimen tary rocks introduction the equatorial region of the atlantic is a major route forwater transfer between the two hemispheres studies of itssedimentary cover have provided abundant evidence for ma jor environmental changeswithin this critical oceanographicgateway from late jurassic time among the important sed imentary consequences ofcirculation and sea level variationswas the development of petroleum rich sequences along thecontinental margins and in the adjacent deep water basins on the present continental shelves these occur over a widestratigraphic range from the early cretaceous to oligocene reyre 1966 nairn and stehli 1973 akpati 1978 kesse 1984 dumestre and carvalho 1985 herbin et al 1986 koutsoukos et al 1991 while in the deep sea organic richshales ranging in age from albian to late cretaceous havebeen found at several deep sea drilling project dsdp sites figure 1 the deep sea deposits have been attributedto oceanwide anoxic events oaes pedersen and calvert 1990 arthur and sageman 1994 the stratigraphic rangeof the continental margin black shales is much broader thanthe deep sequences however we have to look for othermechanisms apartfrom general oceananoxia to explain this these sequences could be ascribed to enhanced primary pro ductivity arising from upwelling of nutrients into the eu photic zone barron 1985 parrish 1995 parrish 1995 be

post rift stratigraphic architectures along the african margin of the equatorial atlantic part i the influence of extension obliquity abstract we investigated the variability of the first order crustal structure and early post rift stratigraphy of six segments of the african equatorial atlantic margin using sub surface data seismic and wells extension obliquity of the segments varies from 0 for the west ivory coast and ghana transform segments to 30 for the togo benin oblique segment and 75 for the east ivory coast normally divergent segment the sierra leone and liberia segments underwent probably deformation during both the early jurassic rifting of the central atlantic and the early cretaceous rifting of the equatorial atlantic with contrasted divergence obliquities for segments that underwent a single rifting we show that the higher the obliquity the wider the crustal thinning domain this has a major influence on the first order geometry of all the post rift horizons including the present day slope the lower the obliquity the larger the differential subsidence across the margin and the steeper the present day slopes of post rift horizons this also has a major influence on the flexural isostatic response of the lithosphere to thermal and erosion sedimentation driven un loads during the early post rift narrow transform segments underwent higher flexural and or thermal uplifts in the proximal domain than wider divergent segments along the same margin divergent segments therefore may preserve early post rift deposits in their proximal domains whereas they are not preserved on nearby transform segments

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

palaeogeography of radiolarite and organic rich deposits in mesozoic tethys siliceous and marine organic rich deposits are sometimes associated sometimes separate in space and time however both are generally accepted to be the result of high planktonic productivity among the siliceous marine deposits the phtanite family facies is distinguished from the radiolarite family facies by several characteristics they contain organic material and as a result are blackish vs red green for radiolarite facies their time of deposition corresponds with strong faunal modifications and they are deposited generally in shallower environments a palaeogeographic analysis of locations of tethyan biosiliceous and marine organic rich rocks both resulting from a high planktonic palaeoproductivity for three mesozoic high sea level intervals toarcian kimmeridgian and cenomanian show a during jurassic times these tethyan deposits were dissociated the siliceous deposits being closer to open ocean waters than the organic rich ones this is a common disposition in modern upwelling systems and suggests a common process b during cretaceous times these tethyan deposits were often associated i e both occur at the same site and are probably the result of a different process from that in the jurassic

formation and migration of hydrocarbons in deeply buried sediments of the gulf of cadiz convergent plate boundary insights from the hydrocarbon and helium isotope geochemistry of mud volcano fluids highlights clay dehydration water expelled from buried sediments drives mud volcanism rise of fluids mediated by crustal scale strike slip faults cross cutting wedge on active accretionary wedge petroleum accumulations were dismantled in neogene 4he enrichment and 13c ch4 50 in fluids reflect an open hydrocarbon system petroleum pools remain on shallow margin microbial gas vented out of active wedge abstract a geochemical study of the composition of hydrocarbon gases and helium isotopes 3he 4he in fluids from mud volcanoes mvs located on and out of the active accretionary wedge of the gulf of cadiz goc provides information on fluid sources and migrations in deeply buried sediments the goc is a tectonically active segment of the africa iberia plate boundary occluded beneath the thick sediments of an accretionary wedge dissected by crustal scale strike slip faults initially built during the miocene gibraltar arc subduction the wedge has since developed toward the w nw in an oblique convergent setting interstitial water expelled from clays undergoing diagenesis in buried sediments drives mud volcanism on the wedge with mvs located along strike slip faults mediating fluid ascent the large excess of radiogenic helium 4he in all goc fluids agrees with a clay mineral dehydration source of water hydrocarbon gases from all deepwater mvs bear methane having similar stable carbon isotope compositions of 50 vpdb whether fluids are highly enriched in methane relative to heavier homologues c2 or not methane ethane propane 10 to 10 000 we suggest that methane with 50 vpdb was largely diffused out of early generating source rocks and became dissolved in the water expelled by the buried sediments consistently low 3he 4he ratios suggest an open hydrocarbon system petroleum accumulations and 3he dissolved in the original sedimentary pore water have mostly escaped into the water column during the major late neogene compressional events at present some mvs vent ch4 rich fluids from dewatering sediments while other structures located on active thrusts additionally vent c2 rich gases generated by active cretaceous source intervals by contrast evaporitic seals preserved petroleum accumulations on the shallow moroccan margin while the westernmost mvs located out of the accretionary wedge vent microbial gas

shallow structures and evolution of the ivory coast and ghana transform margin abstract seismic profiles across the transform continental margin off the ivory coast and ghana western africa illustrate the structural style resulting from the early cretaceous phase of shear stress which leads to the final separation between the african and brazilian continental margins in this area most of the characteristic tectonic features observed along this portion of margin asymmetric grabens on the ghanean platform folds of the deep ivory coast basin the ivory coast ghana marginal ridge are believed to result from progressive transform contacts between the african and brazilian continents as their margins were created during early cretaceous time a major tectonic unconformity inferred to be of upper albian lower cenomanian age may be a direct consequence of the final separation of the continental margins the later evolution of the transform margin is chiefly explained by thermal subsidence

transform continental margins part 2 a worldwide review we present a global inventory of transform continental margins based on both a worldwide comparison of continent ocean boundary identifications with oceanic fracture zones traces and a compilation of published regional studies this inventory increases the number of identified transform margins from 29 to 78 these margins represent 16 of continental margins in cumulative length and 31 of non convergent margins we include morphological data published upper crustal sections continent to ocean transition locations and published moho shape data in the new database this review confirms that continent to ocean transitions are sharper at transform margins than at divergent margins it also emphasizes the structural diversity of transform margins associated with one third of transform margins we define marginal plateaus as a new type of relief that corresponds to a flat but deep surface inside the continental slope and that may be inherited from crustal thinning prior to transform faulting transform margin initiation appears to be favoured along propagating oceans and within cold and thick lithospheres

aeromagnetic anomalies and discordant lineations beneath the niger delta implication for new fracture zones and multiple sea floor spreading directions in the meso atlantic gulf of guinea cul de sac an aeromagnetic contour map compiled over shallow water and onshore portions of the nigerian continental margin shows several elongate long wavelength anomaly closures with some alternating polarity separated by steep gradient ne lineations the lineations are interpreted as new fracture zones or extensions of previously mapped ones the ne trend in the western delta region is concordant with ee fracture zone trends of the deeper gulf of guinea aeromagnetic lineations of the se niger delta basin however discordantly trend ene their termination against the former is interpreted as evidence of early sea floor spreading in a ene wsw direction in addition to the well documented ne sw spreading of the gulf of guinea and the rest of the meso atlantic sea floor the geophysical crustal structure indicate the existence of two early cretaceous triple junctions beneath the niger delta basin the two triple junctions further support the hypothesis that the african continent was a multi plate system in the niger delta region during the early opening of the atlantic

the cretaceous peri atlantic alkaline pulse paap deep mantle plume origin or shallow lithospheric break up abstract although the early jurassic initial phases of atlantic rifting were marked by the tholeiitic basalt flows and dikes of the central atlantic magmatic province camp the cretaceous era was characterised by widespread alkaline igneous activity on widely separated peri atlantic continental margins nearly half the peri atlantic alkaline rocks dated between 250 to 50 ma fall within the 125 80 ma range this period of intense alkaline activity during the mid cretaceous formed the peri atlantic alkaline pulse paap the mechanisms for the opening of the atlantic ocean as well as for the camp and later paap magmatic events have been attributed either to deep mantle plumes or to a combination of tensional forces lithospheric rifting and structural controls we describe the paap event for the first time and test the plume and structural models the paap displays evidence of strong structural control and appears to be related to reactivation of pre existing lithospheric structures throughout the peri atlantic zone early stages of continental separation and abrupt changes in plate motion were the most likely mechanisms for the reactivation of favourably oriented pre existing structures mid cretaceous pulses in alkaline magmatic activity were closely associated with major atlantic tectonic events the opening of the south atlantic the change in intensity and direction of both seafloor spreading and plate motion and the consequent fluctuation in intra plate stresses were responsible for two spikes in peri atlantic alkaline magmatism at 125 ma and 85 80 ma the general lack of evidence for a deep hotspot associated with the peri atlantic zone seems incompatible with a mantle plume origin on the other hand the spatial distribution of the paap along passive continental margins suggests an ideal setting for the development of edge driven convection we propose that the periodic reactivation of deep seated pre existing zones of weakness during the major stages of atlantic tectonic evolution combined with coeval asthenospheric upwelling due to edge driven convection and continental insulation flow enhanced the ascent of alkaline magmas this mechanism of shallow small scale upwelling during periodic structural reactivation provides a more likely explanation for the paap than the commonly proposed deep mantle plume scenario

the mesozoic to early cenozoic magmatism of the benue trough nigeria geochemical evidence for the involvement of the st helena plume c coulon p vidal c dupuy p baudin m popoff h maluski and d hermitte petrologie magmatique ura 1277 cerege bp 80 15545 aix en provence cedex 4 france centre de recherches volcanologiques ura 10 63038 clermont ferrand cedex france centre geologio ue et geophysio ue cnrs place bataillon j4095 montpellier cedex france institut de geodynamiq ue ura 1279 universite de nice sophia antipolis avenue einstein 06560 valbonne france laboratoire de geochronologie geochimie ura 176j place bataillon 34095 montpellier cedex france

40ar 39ar chronology petrology and geodynamic setting of mesozoic to early cenozoic magmatism from the benue trough nigeria the benue trough is a continental scale intraplate tectonic megastructure which is part of mid african rift system this rift initiated in the latest jurassic was related to the opening of the central and south atlantic oceans mesozoic to early cenozoic magmatism accompanied this evolution two principal magmatic domains are evident the northern and southern benue in the northern domain magmatism is characterized by transitional alkaline basalts and transitional tholeiitic basalts acidic magmatism of peralkaline nature is also present in the southern benue several magmatic districts exhibit alkaline or tholeiitic affinities a detailed chronology of emplacement of this volcanism has been established using the 40 ar 39 ar radiometric method which lead to recognition of three periods of magmatic activity 1 147 106 ma well expressed in the northern benue represefited by transitional basaltic types 2 97 81 ma occurring only in the southern benue represented exclusively by alkaline intrusive rocks 3 68 49 ma restricted also to the southern benue with alkaline intrusions followed by tholeiitic subvolcanic rocks in the light of the general geodynamic evolution a scenario is proposed supported by the three chronological periods the late jurassic to albian magmatism occurred when the equatorial atlantic was still closed contemporaneous with the ne brazilian magmatism both magmatism could represent the forerunners of opening of the equatorial atlantic this activity occurred under a general extensional regime during which crustal strike slip faults gave rise to the emplacement of transitional alkaline basalts transitional tholeiitic basalts erupted along normal faults the second period of activity cenomanian to santonian restricted to the southern benue occurred after the opening of atlantic ocean during a period of decreasing extension this period ended with a santonian compressional episode the last period of activity from late maastrichtian to eocene is characterized by subsidence generated as an isostatic response to the early cretaceous crustal thinning and post rift thermal relaxation of the lithosphere expressed by tertiary e w extension

ocean wide stagnation episode in the late cretaceous in this paper we report that marine sediment locations spread over a wide area contain evidence of an anoxic interval during the mid cretaceous here called the cenomanian turonian black shale horizon ctbsh the occurrence of laminated carbonaceous shales with marine organic matter in an unoxidized state implies deposition and early diagenesis in anaerobic environments the stratigraphic record above and below the ctbsh in environments as different as the deep north atlantic ocean and marginal and epicontinental seas over enormous areas in north america north africa and northwestern europe is characterized by a stratigraphic gap a disconformity or a continuous but strongly condensed stratigraphic section the anoxic layer coincides in time with the maximum extension of a worldwide marine transgression of eustatic origin recorded on continental margins1 as well as on various cratons2 no simple model has yet been found to explain the occurrence of the ctbsh in such a wide range of depth and palaeoenvironmental conditions

the shallow structures of the guinea and ivory coast ghana transform margins their bearing on the equatorial atlantic mesozoic evolution abstract recent geological and geophysical data from a marine survey conducted off guinea and the ivory coast ghana region equatorial atlantic are discussed we interpret the southern guinean margin firstly as an area linked to the central jurassic atlantic and secondly as an area affected by the progressive transform opening of the equatorial atlantic in the early cretaceous off the ivory coast and ghana we have demonstrated distinct structural domains interpreted as consequences of the progressive transform contact between the west african and brazilian cratons also during the early cretaceous on both margins we observe a clear tectonic unconformity of probable late albian early cenomanian age we propose that this unconformity results from the final disruption between the two continental west african and brazilian crusts the geological structures generated along transform margins can be considered as good geological tools to constrain early phases of plate motions particularly around the equatorial atlantic

an analogue model study of electromagnetic induction in the tasmania region abstract laboratory analogue model magnetic measurements are carried out for a model of the region including tasmania bass strait with its highly conductive deep sedimentary basins and the south coast of mainland australia the model source frequencies used simulate naturally occurring geomagnetic variations of periods 5 120 min in phase and quadrature magnetic hx hy and hz field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for e polarization electric field of the source in the n s direction and for h polarization electric field of the source in the e w direction large anomalous in phase and quadrature model magnetic fields are observed over bass strait and the coastal regions at short periods for both e and h polarization but with increasing period the field anomalies decrease more rapidly for e polarization than for h polarization the difference in response with polarization for the bass strait region is attributed to current induced in the deep ocean for all periods being channelled through bass strait for h polarization but not for e polarization the persistent large coastal field anomalies elsewhere for h polarization can be accounted for by the coastal current concentrations due to currents induced in the deep ocean for all periods deflected to the south and to the north by the shelving sea floor and channelled through bass strait and around the southern coast of tasmania the phenomena of current deflection and channelling for h polarization for the geometry of the southern australia coastline and associated ocean bathymetry is particularly effective in producing field anomalies for a large period range the coastal horizontal hx and hy field anomalies present for e polarization at short periods and for h polarization at all periods do not extend far inland and thus for inland station sites somewhat removed from the coast should not present serious problems for magnetic soundings in field work the sharp vertical field hz gradient over tasmania at short periods which is predominantly in the e w direction for e polarization and the n s direction for h polarization is strongly frequency dependent becoming almost undetectable at 60 min the behaviour of the hz field gradients however are very similar from traverse to traverse over inland tasmania and thus the effects of the ocean should not present too serious a problem in the interpretation of field station studies the discrepancies between model and field station results should be useful in mapping geological boundaries in the region

analogue model study of electromagnetic induction in the newfoundland region abstract the behaviour of magnetic field variations over the newfoundland canada region is studied using a scaled laboratory analogue model the model source frequencies used simulate periods of 100 s 300 s 900 s and 1800 s in the geophysical scale contour views of in phase and quadrature phase parts of the three magnetic field components for two orthogonal source field polarizations for 100 s and 1800 s period variations are shown large anomalous in phase and quadrature model magnetic fields are observed over the strait of belle isle and cabot strait for all four periods studied with the anomalous fields reaching maxima when the skin depth is of the order of six times the depth of the strait the cape effect that is the large field enhancements due to deflection of induced current at capes is observed for all four periods enhanced fields are observed at the continental shelf edge for 1800 s variations but not for 100 s variations a general shift in the anomalous field from the in phase to the quadrature part for all three magnetic components is found for increasing period the magnitudes and directions of inter station induction arrows and single station induction arrows are studied the two types of induction arrows differ when an anomalous horizontal magnetic field is present this is especially evident near the coastline for a period of 100 s for all periods studied the in phase induction arrows point towards the nearest and largest current concentrations in traversing the coastline the single station quadrature arrows reverse direction for the 100 s period but not for the 1800 s period the model results indicate that there is no preference for using either single station or inter station induction arrows with respect to minimizing the effects due to coastal variations for inland locations further the model results show that for periods studied 100 s 1800 s the coastal effects would not produce serious perturbations to the field measured over crustal anomalies within the central regions of newfoundland

upper mantle structure beneath cameroon from body wave tomography and the origin of the cameroon volcanic line the origin of the cameroon volcanic line cvl a 1600 km long linear volcanic chain without age progression that crosses the ocean continent boundary in west central africa is investigated using body wave tomography relative arrival times from teleseismic p and s waves recorded on 32 temporary seismic stations over a 2 year period were obtained using a multichannel cross correlation technique and then inverted for mantle velocity perturbations the p and s wave models show a tabular low velocity anomaly directly beneath the cvl extending to at least 300 km depth with perturbations of 1 0 to 2 0 for p and 2 0 to 3 0 for s the s wave velocity variation can be attributed to a 280 k or possibly higher thermal perturbation if composition and other effects on seismic velocity are negligible the near vertical sides of the anomaly and its depth extent are not easily explained by models for the origin of the cvl that invoke plumes or decompression melting under reactivated shear zones but are possibly consistent with a model invoking edge flow convection along the northern boundary of the congo craton lithosphere if edge flow convection in the sublithospheric upper mantle is combined with lateral flow channeled along a fracture zone beneath the oceanic sector of the cvl then the oceanic sector can also be explained by flow in the upper mantle deriving from variations in lithospheric thickness

distribution of deep sea black shales of cretaceous age in the eastern equatorial atlantic from seismic profiling abstract the presence of thick 200 m carbonaceous shales of cretaceous age in the deep waters of the eastern equatorial atlantic was first revealed by drilling at dsdp sites 367 and 368 near the cape verde islands more recent seismic investigations show that black shales are widely distributed in this region in the gambia basin they are associated with a group of seismic reflectors lying at depths of 400 600 m below the sea floor these can be traced southwards from the drill sites into the sierra leone basin and eastwards beneath the african continental rise to depths at which ambient temperatures fall within the oil generation window in the gambia basin the reflectors continue westwards from the cape verde archipelago before terminating on turonian oceanic basement 92 ma south of the guinea fracture zone near 10 n extension of the same seismic sequence on to younger oceanic crust indicates that deep sea carbonaceous deposits formed close to the present equator as late as the early campanian 80 ma long after a series of oceanic anoxic events in the aptian albian oae1 and over the cenomanian turonian boundary oae2 led to widespread accumulation of black shales this late period of deposition may correspond to the more locally developed oae3 high surface productivity in equatorial upwelling zones during the late cretaceous probably played an important role in delivering large amounts of organic carbon to the sea floor when meridional movement of deep water between the south and north atlantic was restricted by closely spaced east west fracture zones in some areas seismic data show that the black shales level out rather than drape pre existing topography implying significant downslope transport of organic rich material from upwelling cells near the african margin

an analogue model study of electromagnetic induction for cape and bay coastlines abstract the electric and magnetic field variations across bay and cape coastlines are determined from scaled laboratory analogue models the effect on the field response of such features along an otherwise straight coastline is limited to a distance approximately equal to the dimensions diameter of the bay or cape of the irregularity for the e polarization of the source field the perturbations in hz and hy are more confined mainly to the immediate region of the bay or cape the coast effect hz hy for this polarization and variations with a 4 2 min period is approximately a factor of two larger for the cape than for the bay while the straight coastline hz hy is roughly mid way between the cape and bay values for h polarization the coast effect is again approximately a factor of two larger for the cape than for the bay but both values are larger than the value for a straight coastline the response of the various field components to the bay and cape features is decreased at longer periods and as a measure the coast effect is approximately a factor of 1 2 greater for the cape than for the bay for a 42 min period as compared with the factor of 2 for a 4 2 min period the bay and cape deflect the induced currents flowing in the ocean and modify the currents diffusing into the land this study demonstrates that data from coastal sites may be unreliable for correcting magnetic surveys or for analyses of short period geomagnetic source field variations

the structure of the guinean continental margin implications for the connection between the central and the south atlantic oceans the results of a recent geological geophysical survey conducted off guinea are combined with previous data to establish a preliminary stratigraphy and provide a structural sketch of this portion of the west african continental margin three sectors are distinguished a northwestern portion of the margin comprises a wide and deeply submerged plateau the guinean marginal plateau underlain by a thick sedimentary sequence and facing westward toward the gambia abyssal plain scismic stratigraphy and structures show clear analogies to the jurassic margins of the central atlantic including the presence of a cretaceous paleoslope covered by cenozoic deposits a southern area of the margin comprises a series of aligned w e trending acoustic basement features extending along the slope and bounding the guinean plateau to the south these features basement ridges and volcanic piles are related to a fracture zone system also documented by magnetic anomalies and gravity data the bordering deep sierra leone abyssal plain also dissected by e w trending oceanic fracture zones contains a sedimentary cover apparently not older than middle cretaceous between both sectors and between two nw se trending scarps lies an intermediate area the seismic profiles show that here the margin is dissected by faults creating a series of asymmetric horst and graben features progressively narrowing towards the s e and covered by untectonized but partly eroded upper cretaceous to cenozoic sediments

current controlled sedimentation in the equatorial atlantic examples from the southern margin of the guinea plateau and the romanche fracture zone abstract 3 5 khz and seismic reflection data were used in a study area of bottom current control of sedimentation in two areas of the equatorial atlantic a 100 km long segment of the romanche fracture zone rfz and a 30 km wide sector of the southern margin of the guinea plateau the rfz is the most important conduit for antarctic bottom water aabw into the eastern atlantic at mid depths ca 1500 4500 m north atlantic deep water nadw spreads southwards and eastwards whereas at the surface directly overlying the rfz is the equatorial divergence a zone of high biological production on the guinea margin a mixture of aabw nadw moving north washes the foot of the continental rise below 4250 m and the overlying nadw hugs the slope contours another divergence zone lies just to the north of this area on the southern margin of the guinea plateau sediment accumulation is controlled by contour currents within the nadw deposits are concentrated into a series of slope parallel sediment drift the surfaces of which are moulded into standing sediment waves in the slack region between areas of eroded or condensed sequences marking the loci of higher velocity current cores the outer edge and upper slope of the plateau under the influence of high velocities in the equatorial surface water are swept bare of unconsolidated sediment the lower rise sediments washed by the weak mixed aabw nadw water mass are generally flat surfaced in the rfz restricted sediment deposition and accumulation as well as the reworking of sediment surfaces document the influence of bottom currents flat surfaced sediments deposited on the floor of the axial valley below the ca 4500 m ccd are ponded into basins at the foot of the valley walls by a weak aabw on the northern flank of the north ridge pillsbury seamount and on the south ridge the overlying nadw has moulded the thin sediment cover into sediment waves a thicker accumulation of generally flat surfaced ponded sediments in the suspended valley were apparently deposited under a weaker current regime in the lee of the pillsbury seamount stronger bottom currents as well as steepness of slope appear to prevent sediment deposition on both the upper slope and crest of the north ridge

tectonic fabric of sierra leone west africa implications for mesozoic continental breakup a lineament map interpreted from landsat images has been integrated with available geological and geophysical data from both offshore and onshore regions of sierra leone the lineament patterns are related to intraplate and plate marginal reaction of pre existing structures during mesozoic rifting events that resulted in strong tectonic controls on magmatism of four main lineament trends the nnw sse to n s nne ssw and ene wsw trends are directly relatable to archaean fabric in the leo uplift a nw se trend reflects coast parallel late mesozoic dykes that follow the rokelide pan african fabric reactivated archaean nnw and n s trending structures nw se trending faults defining offshore basins are segmented and offset by ene wsw trending continental extensions of ocean fracture zones both the guinea and sierra leone fracture zones have nucleated on ene wsw trending sinistral archaean shear zones in the leo uplift the four lineament trends focused mesozoic magmatic events through protracted reactivation spatial and geometric relationships between the magmatic provinces and tectonic fabric indicate that intraplate deformation occurs far inland during rifting events the angular relationships between the permo triassic ne trending guinea belt the jurassic triassic nw se trending coast parallel dykes and the ene wsw trending ocean fracture zones centred on the eariy jurassic freetown basic igneous complex suggest that the sierra leone liberia continental margin evolved as an obliquely sheared rift rift transform passive margin during mesozoic continental breakup

a hierarchical tectonic model of the gulf of mexico and caribbean region abstract a quantitative computer graphic methodology hierarchical tectonic analysis has been used to model the tectonic evolution of the gulf of mexico and caribbean region this method employs a highly structured hierarchical description of the relative motions of pairs of tectonic elements in order to quantify the timing and style of plate interactions the main features described in this model are 1 1 a revised fit of pangea 2 2 the opening of the gulf of mexico in the middle late jurassic 3 3 the formation of a proto caribbean ocean due to the separation of north and south america during the early cretaceous 4 4 the formation of the greater antilles arc along the western margin of the proto caribbean ocean 5 5 the insertion of the farallon plate between north and south america during the middle cretaceous resulting in the northeastward advance of the greater antilles arc and the subduction of proto caribbean oceanic crust 6 6 the complete subduction of the proto caribbean ocean and the collision of the greater antilles arc with the bahamas platform in the late cretaceous 7 7 the truncation of the caribbean section of the farallon plate from its pacific counterpart and the formation of the panama arc latest cretaceous and 8 8 the westward movement of north and south america with respect to the caribbean plate resulting in the opening of the yucatan basin paleocene and subsequently in the evolution of the cayman trough and e w zone of strike slip

an analogue model study of em induction for an island near bay and cape coastlines abstract laboratory electric and magnetic model measurements were carried out to examine how the addition of a nearby elliptical island to an otherwise complex continental coastline involving bays and capes affects electromagnetic induction in coastal regions irregularities in a coastline in the form of bays and capes lead to significant spatial field variations in coastal regions due to variations in density of induced current flowing in the ocean caused by current deflection or channelling by the coastal irregularities the addition of a nearby island which for the appropriate field polarization can result in current channelling through the strait greatly influences these field spatial variations for e polarization the case of the model electric field of the inducing source parallel to the strait and for short period 4 2 min studied here variations the effect on the fields of the model elliptical island in the shallow coastal ocean at points on the nearby continental coastline was to increase by factors as large as two to three the h y amplitudes at the centres of the bay and the cape and the e x amplitude at each bay cape and straight coastline in the region of the strait as compared with the values for coastline models without an offshore island for h polarization the case of the electric field of the source perpendicular to the strait the effect of the island was to increase by a factor of two to three the h z coastline values at the edge of the bay and to decrease by a similar factor the value at the edge of the cape further the spatial variations in the fields for both polarizations over the island and over the strait were strongly influenced by current channelling through the strait thus any interpretation of field station measurements on such an island on the sea floor in the strait or on the continent in coastal areas near an island should take into account the effects due to current concentrations in the strait particularly at short periods

organic geochemistry of cretaceous black shales from the galicia margin ocean drilling program leg 103 organic carbon rich black shales from three different cretaceous episodes sampled during ocean drilling program leg 103 have been studied by organic geochemical methods rock eval analysis carbon isotope data and lipid biomarkers show organic matter to contain varying proportions of marine and continental materials in hauterivian barremian organic carbon rich turbiditic marlstones major amounts of land derived organic matter are found aptian albian black colored shales are interspersed within green claystones from which they differ by containing more marine organic matter an abbreviated layer of black shale from the cenomanian turonian boundary is dominated by well preserved marine organic matter downslope transport and rapid reburial within a predominantly oxygenated dcepwater setting created most of these examples of black shales except for the cenomanian turonian deposits in which decpwater anoxia may have been involved

abyssal circulation change in the equatorial atlantic evidence from cenozoic sedimentary drifts off west africa abstract seismic profiling in the equatorial atlantic reveals deep water 4500 m sediment bodies formed by current controlled deposition near the intersection of large offset fracture zones with the african margin a 600 km long drift accumulation the ivory coast rise lies north of the st paul transform near 3 n a smaller drift deposit has been identified along the northern side of the guinea transform at 10 n antarctic bottom water aabw which presently enters the eastern atlantic basins through the romanche and vema fracture zones probably played an important role in the development of these features proto aabw may have reached the equatorial region as early as mid eocene time before the establishment of permanent ice sheets in antarctica the ivory coast rise existed as a distinct sedimentary drift by the mid eocene as a result of deposition from bottom water moving southwards along the african margin and westwards parallel to the st paul fracture zone this early flow pattern is in the opposite sense to the present movement of deep water in the sierra leone basin a reversal in abyssal circulation may have been caused by the northward passage of the region across the paleoequator during the cenozoic

nearshore along strike variability is the concept of the systems tract unhinged sequence stratigraphic models assume that nearshore strata have relatively consistent and laterally persistent stacking at the systems tract scale and therefore may not fully describe the three dimensional stratigraphic architecture in areas displaying marked nearshore along strike variability a stratigraphic model of nearshore deposits is presented that corrects for this assumption by explaining variations in along strike stratal geometries in terms of a systematic change in the orientation of a shoreline trend or clinoform rollover a scenario comparable to deflection around a hinge the model defines hinge zones that are both fixed and moving with respect to time and was created from outcrop well log and seismic reflection data model end members predict contemporaneous progradational aggradational and retrogradational stacking bounded by surfaces displaying significant along strike changes in architecture implying that sequence stratigraphic surfaces can be diachronous we advocate examining the impact of stratigraphic variations parallel to depositional strike by testing for the presence of uniform versus differential progradation phenomena responsible for creating unhinged and hinged nearshore depositional systems respectively understanding these differences will improve subsurface predictions and provide a more complete understanding of the stratigraphic evolution of sedimentary basins

warm saline bottom water in the ancient ocean considerable isotopic evidence indicates that oceanic bottom water was much warmer in the geological past a simple convection model driven by multiple turbulent buoyant plumes and observations of present deep water formation have led to the development of a theory for the formation of warm saline bottom water it is suggested that changes in the size and configuration of marginal seas in net evaporation zones due to lithospheric plate motions and eustatic sea level change caused these seas to become sources of warm saline bottom water evidence from the palaeotemperature record and palaeogeography during the late cretaceous is used to reinforce the hypothesis climatic and chemical consequences of the formation of warm saline bottom water are discussed

elevated primary productivity of calcareous nannoplankton associated with ocean anoxic event 1b during the aptian albian transition early cretaceous 1 previous isotopic investigations of aptian albian oceanic anoxic event oae 1b from the western north atlantic blake nose posited that increased sea surface temperatures and decreased salinity led to stratification of the upper water column resulting in lowered dissolved oxygen and enhanced organic matter preservation we examined calcareous nannofossils from the same site in the western north atlantic blake nose to evaluate changes in surface water conditions prior to during and after the aptian albian oae1b the results of our analysis conflict somewhat with the previous interpretation that oae1b at blake nose was linked to water column stratification the signal from calcareous nannofossils indicates that in the late aptian prior to oae1b oligotrophic conditions in the surface waters were replaced by mesotrophic conditions that persisted throughout the oae event and into the early albian we speculate that increased surface water productivity facilitated to some degree the development of oae1b at blake nose

origin of the cretaceous bridge creek cycles in the western interior united states abstract alternating beds of burrowed limestone and laminated organic carbon rich marlstone in the bridge creek member of the greenhorn formation were deposited during maximum transgression of the cenomanian turonian sea in the western interior of the united states one view of the origin of the bridge creek limestone marlstone rhythms holds that they represent dilution cycles produced by alternating climates within the western interior the marlstones recording wet episodes when high runoff produced a sediment laden brackish lid on the seaway and the limestones recording dry episodes when runoff was low and the water column well mixed a second view holds that the rhythms represent productivity cycles the limestones representing times of high productivity of calcareous planktonic skeletons and the marlstones representing times of low productivity in this view the rhythms were produced by alternations in vertical circulation that characterized much of the world ocean at the time the productivity origin of the rhythms is supported by the presence of abundant calcispheres in the limestone beds the concentration of clay size quartz in many of the limestone beds the comparatively high 18 o and low 13 c in limestone beds the similarity in proportion of clay minerals in marlstone and limestone beds the pelagic depositional rate of the bridge creek member and by the discovery that bridge creek deposition began with a marlstone and not a limestone bed the dilution model is not consistent with these features and it has not been supported by modeling of paleoclimates and lithofacies that were predicted from it nor by macrofaunal distributions within the seaway the dilution model is not consistent with the occurrence of diverse and abundant planktonic foraminifera in both marlstone and limestone beds which indicate that near normal marine salinities prevailed during deposition of both bed by bed analysis of the bridge creek member may provide an analog for events in the tethys that are poorly recorded in deposits from the deep sea itself

induction arrows for a buried conducting plate abstract a scaled electromagnetic analogue model is used to study the behaviour of single station induction arrows over a conducting plate embedded in a homogeneous poorly conducting medium for a profile crossing the edge of the conducting plate the in phase arrow for all locations points towards the bulk of the good conductor while the quadrature arrow reverses direction directly over the edge consistently pointing towards the edge the results also show that both the in phase and quadrature arrow lengths are reduced with increasing conductor thickness and depth of burial the model results are applied to examples of a uniform depth ocean and a conducting sill embedded in a poorly conducting earth

the guinea continental margin an example of a structurally complex transform margin abstract the guinean continental margin corresponds to a wide hinge zone which developed between the jurassic central atlantic and the cretaceous equatorial atlantic the guinea marginal plateau that characterizes the southern border of this margin is a wide triangular morphological feature bounded to the west by a typical rifted margin slope segment while its southern edge corresponds to a narrow and morphologically complex continental slope probably resulting from intracontinental transcurrent motion during the early cretaceous on the basis of seismic profiles recorded in two selected areas located along the southern guinea margin a seismic stratigraphy is established at depth the plateau is underlain by a thick mesozoic and cenozoic sedimentary wedge made of two main ensembles a lower ensemble or sequence 1 consists of early cretaceous clastics an upper ensemble includes a series of four distinct sequences attributed to albian to tertiary deposits eastward the sedimentary cover of the southern plateau edge is locally pierced by volcanic bodies of paleocene age beneath the southern margin of the guinea plateau the sedimentary sequences appear deformed by faults and folds resulting from at least two main tectonic episodes a first event only affected sequence 1 which is cut by a set of extensional faults a second tectonic phase including folding reverse faulting and transcurrent faulting is responsible for the deformation of sequences 1 and 2 during this tectonic episode former normal faults have been reactivated as reverse faults resulting in a structural inversion a third tectonic phase was responsible for structures transverse to the general e w trend of the southern guinea margin and consisting of n s trending normal faults and associated volcanoes the spatial distribution of the tectonic trends clearly substantiates a polyphase tectonic activity that can be related to plate motion changes during early stages of the continental separation a first stage is characterized by a divergent rifting leading to n s trending synsedimentary normal faults during the cretaceous the tectonic activity increased as a general transform motion occurred between the guinea plateau and its american conjugate margin the demerara plateau local tensional stresses were responsible for a splay fault system active near the tip of the guinea plateau in late albian times a slight adjustment of the africa plate with respect to the south american one resulted in local squeezing of the whole sedimentary wedge after this short compressive event final continental separation occurred in an extensional regime characterized by a general collapse along two major scarps and volcanic activity

thinned and subsided continental crust on the rifted margin of eastern canada crustal structure thermal evolution and subsidence history summary seismic refraction measurements were carried out using ocean bottom seismometers over foundered continental crust in orphan basin and flemish pass on the continental margin north east of newfoundland tau p travel time inversion synthetic seismogram analysis and conventional layered model calculations were applied to the data the results show that these outer regions of the margin are underlain by thinned continental crust with a total depth to the m discontinuity of about 22 km there are two main crustal layers with p velocities of about 6 1 and 7 0 km s 1 which appear to be homogenous and separated by sharp interfaces these are overlain by a layer in which p velocities are about 5 5 km s 1 this is interpreted to be precambrian or palaeozoic basement mesozoic and cenozoic sediments cover the basement rocks and are over 4 km thick the results imply that crustal thinning to about 50 per cent of the original crustal thickness occurred the gravity anomaly data show that the thin crust has a maximum horizontal extent of about 450 km from the ocean continent boundary near orphan knoll landward to the outer continental shelf the subsidence history and thermal evolution of the region was computed assuming that the observed thinning is produced by horizontal extension of the lithosphere it is suggested that extension can only satisfy the observed crustal structure and elevation of the margin during the rift phase if more extension took place in the lower lithosphere than in the upper lithosphere the computed subsidence is compared to the observed subsidence and the total amounts of subsidence are similar however the shape of the observed subsidence curves measured in deep exploratory wells differs significantly from the predicted subsidence assuming cooling began when final continental breakup occurred in the late cretaceous the temperature distribution within the lithosphere due to extension may be related to the flexural rigidity of the plate as a function of both time and position across the margin therefore it is suggested that the response of the lithosphere to sediment loading and the large amplitude of the gravity shelf edge anomalies are directly related to the thermal history of the region

foraminiferal response to mid cenomanian upper cretaceous palaeoceanographic events in the anglo paris basin northwest europe we have analysed the sedimentology macrofossil event horizons including pulse faunas and foraminiferal succession 80 samples from an 8 m succession in the upper lower and lower middle cenomanian upper cretaceous at folkestone southeast england comparative sedimentological analysis of successions in southern and northeastern england indicates the presence of three hierarchies of cycle 20 kyr productivity chalk marl couplets cycles attributed to the milankovitch precession cycle 100 kyr sea level cycles attributed to the milankovitch eccentricity cycle and l myr cycles attributed to longer term sea level cycles previously recognised macrofossil pulse faunas occur preferentially at the bases of the 100 kyr eccentricity cycles we analysed the foraminiferal fauna across the lower middle cenomanian boundary at abbott s cliff folkestone in great detail 80 samples in 8 m the foraminiferal succession is also intricately related to the 1 myr sea level and 100 kyr eccentricity cycles changes related to the long term cycles include variations in the abundance of the benthic foraminifera tritaxia gyroidinoides lingulogavelinella and dorothia and an increase in benthic diversity in the transgressive deposits changes related to the 100 kyr eccentricity cycles include the presence of rotalipora ex gr reicheli in the lower part and rarely upper part of cycles the abundance of planktics both absolute and relative in the middle to upper part of cycles an increase in the proportion of gavelinella berthelini and g reussi in the upper part of cycles we present a stratified water column model to explain the changes related to the 100 kyr scale cycle 1998 elsevier science b v

accumulation of organic carbon rich sediments in the late jurassic and cretaceous atlantic ocean a synthesis abstract organic carbon rich sediments black shales were deposited at some 40 deep sea drilling project dsdp sites in the atlantic ocean during late jurassic and cretaceous times in order to reconstruct the depositional environments of black shale formation we have studied in detail changes in the occurrence of black shales as well as the type and amount of organic matter in relation to changes in paleogeography paleooceanic circulation and paleoclimate we have calculated accumulation rates of marine and terrigenous organic carbon and combined these with organic carbon content sedimentation rate relationships to obtain information about paleoenvironments main factors causing the preservation and accumulation of organic carbon are 1 the supply of terrestrial organic matter 2 the supply of marine organic matter 3 the oxygenation of the deep water and 4 the bulk sedimentation rate changes in the importance of these different factors and changes in the interrelationships among them determine the complex system of black shale formation and their cyclic variations increased supply of terrestrial organic matter due to increased river discharge and humid climate in northeast america southwest europe and south africa were important in the hauterivian albian northwest and northern northeast atlantic and in the late jurassic aptian south atlantic in the latter anoxic deep water conditions due to restricted circulation in the nascent largely isolated basin as well as higher oceanic productivity may have reinforced the high preservation rates of marine organic matter during that time off northwest africa higher flux rates of marine organic matter were recorded in areas of higher productivity which may have been caused by fluvial nutrient supply during the hauterivian and by intensified upwelling during albian turonian times in general the depositional environment was oxygenated and the content of marine organic matter was a function of the sedimentation rate furthermore at some places redeposition by turbidity currents or slumps may have been important for the accumulation of organic matter in the deep sea basins the only episode of widespread anoxia in the western and eastern basins of the north atlantic may have occurred in the cenomanian turonian when high amounts of marine organic matter were also preserved in areas of low productivity and low sedimentation rates

the cenomanian turonian boundary event in central poland abstract stable isotopes 13c 18o caco3 content carbon content insoluble in hcl corg and in the carbonates ccarb and foraminifers have been investigated in the cenomanian turonian boundary interval rotalipora cushmani zone and lower whiteinella archaeocretacea zone in southeast central poland a higher content of corg in the uppermost rotalipora cushmani zone and lowermost whiteinella archaeocretacea zone a 13c and 18o stable isotope anomaly in the uppermost cenomanian and substantial changes in the foraminiferal assemblages have been recorded a major carbon stable isotope excursion with a shift of up to 1 6 pdb recorded in the lower part of the whiteinella archaeocretacea zone within the marly limestone in the uppermost cenomanian a late cenomanian expanding oxygen minimum zone is thought to be responsible for changes in foraminiferal assemblages

a plate tectonic setting for mesozoic rifts of west and central africa abstract africa by virtue of its central position within gondwana has recorded much of the complex history of plate interactions which have progressively fragmented this supercontinent since the early mesozoic continental reconstructions reveal both a temporal and spatial relationship between the development of the continental margins of africa and the formation of rifted sedimentary basins deep within the african continent the multi stage opening of the atlantic ocean and associated rifting in west and central africa provide one of the most impressive examples of ocean continent tectonic interactions during the early cretaceous the onset of rifting along the future margins of the south atlantic is contemporaneous with intra continental rifting generating both strike slip and extensional basins within west and central africa the syn rift phase of intra continental basin development continued until the santonian by which time africa and south america had been physically separated for approximately 10 ma except for minor rifting during the senonian and tertiary the short lived phase of deformation at about 80 ma marks the transition into the post rift or sag phase of basin development this deformational event can be correlated with a period of plate motion changes recognised from fracture zone geometries seen in both the central and south atlantic oceans using present day stress analogies cretaceous rifting in africa can provide a means of indicating the regional palaeostress directions within africa at this time

the cenomanian turonian oceanic anoxic event i stratigraphy and distribution of organic carbon rich beds and the marine δ13c excursion marine strata deposited during late cenomanian and early turonian time display lithological faunal and geochemical characteristics which indicate that significant parts of the world ocean were periodically oxygen deficient at or very close to the cenomanian turonian boundary between 90 5 and 91 5ma ago oxygen deficiencies were particularly marked over a period of 1ma this short lived episode of oceanic oxygen deficiency has been termed the cenomanian turonian oceanic anoxic event oae the widespread distribution of anoxic sediments deposited synchronously during such a short lived event indicates that such sediments are not simply the product of coincidental local climatic or basinal water mass characteristics but are the result of a global expansion and intensification of the cenomanian turonian oxygen minimum zone related to feedback between sea level rise and regional palaeoceanography the palaeoceanography of the cenomanian turonian oae is discussed in detail in a companion paper by arthur et al 1987 from authors

the toarcian black shale event in northern italy abstract during the early toarcian jurassic c 192 ma organic rich sediments were deposited in many areas of europe this paper reports the major findings of the first part of an organic geochemical investigation of the toarcian oceanic anoxic event oae thirty two black shales and associated lithologies were studied from seven localities in the southern alps of northern italy toarcian organic rich sediments up to 4 3 toc were deposited in various palaeoceanographic settings within the lombardy belluno and julian basins and on the trento plateau results of biomarker analyses indicates that in all sections mixed contributions from algae and bacteria dominate a variable but generally minor terrigenous input the preservation of significant quantities of marine derived organic matter and the typically well developed sedimentary lamination suggest that the black shales accumulated under oxygen deficient conditions in all palaeoceanographic settings molecular evidence suggests that the algal derived components of the sedimentary organic matter is partially degraded in many of the basinal samples this observation is here interpreted as being the result of aerobic decomposition of algal debris during transport through the water column other lines of evidence indicate that oxygen depletion intensified during the later stages of the toarcian oae with enhanced preservation of authochthonous organic matter in the light of these results the preferred model for the accumulation of black shales on the tethyan southern margin incorporates anoxic basin bottom waters overlain by a relatively well oxygenated water column

the cenomanian turonian boundary event on the moroccan atlantic margin agadir basin stable isotope and sequence stratigraphy abstract correlations of four sections constrained by stable isotope stratigraphy and micropalaeontology along a 80 km long e w oriented transect from the western high atlas to the present day coast taghazoute show that the cenomanian turonian boundary event ctbe occurred within shallow water oyster bearing limestones and marls and not in the overlying black shale the age of the laminated shale is early turonian as the planktonic marker helvetoglobotruncana helvetica is found from its first centimetres upward the underlying deposits hosting the ctbe event are bounded by two unconformities interpreted as emersion surfaces and represent the strongest seawardshifts of facies in the local cenomanian sedimentary wedge the 13 c anomaly is only recorded in the coastal taghazoute section not in other sections landward meaning that the ctbe occurred here during a short transgressive phase within an overall regressive trend the lower turonian black shale of the taghazoute section is transgressive onto the second unconformity and pass landward to white evenly laminated chert bearing mudstones then to bioturbated mudstones in the easternmost sections of the transect western high atlas

active concomitant counterclockwise rotation and northwards translation of africa during the albian campanian time a paleomagnetic study on the wadi natash alkaline volcanic province 104 78ma south eastern desert egypt abstract africa was a key tectonic unit in the geologic evolution of the tethyan realm in the middle late cretaceous during the rifting of the south equatorial atlantic ocean this research sheds light on the tectonic evolution of africa during the albian campanian time the two end members of the wadi natash alkaline volcanic province wnavp 104 78 ma in the south eastern desert of egypt were studied paleomagnetically the wnavp 24 5 n 34 25 e is made up of a thick succession of differentiated lava flows grading from alkali olivine basalt aob followed by hawaiite mugearite and benmoreite flows with abundant sandstone interbeds all intruded by a large number of trachyte phonolite tr ph circular lava domes and ring dykes forming conspicuous landmarks in the province the oldest flows of the aob 104 7 ma and the youngest tr ph plugs and ring dykes 86 78 ma as well as the interbedded turonian taref sandstone formation previously known as nubian sandstone were sampled all over the province which covers more than 500 km 2 the isothermal remanent magnetization irm study pointed to magnetite family as the major remanence in wadi natash volcanics with little contribution from goethite hematite minerals on the other hand goethite and or hematite are the dominant remanence carriers in the interbedded taref sandstone after the progressive stepwise thermal demagnetization of all samples the characteristic remanence chrm direction for each rock type was visually isolated and its best fit line was calculated followed by the site means rock unit means and the paleomagnetic poles this paleomagnetic study revealed that 1 in the tilt corrected coordinates the mean chrm of the oldest aob flows n 12 sites was dec inc 340 10 k 54 95 6o yielding a north pole at 55 n 250 e the chrm of the youngest tr ph ring dykes and plugs n 10 sites was dec inc 354 3 5 k 60 95 6 3o yielding a north pole at 66 5 n 229 e 2 on the other hand the site mean chrms of the taref sandstone interbeds n 8 sites in the present day in situ coordinates are clustered around the present day geomagnetic field with a mean dec inc 3 33 k 37 95 9 3 yielding a north pole at 83 n 191 e the two paleomagnetic poles of the aob and the tr ph are in accordance with albian campanian poles from africa including madagascar and those rotated from the main cratons such as the north american craton europe including greenland and iberia south america and india using the published plate kinematics and paleomagnetic euler pole pep rotation parameters paleogeographically the two obtained poles of wadi natash volcanics place the african plate at two different azimuths and paleolatitudes during albian cenomaniam and santonian middle campanian tectonically these two paleomagnetic poles point to an active synchronous northwards translation associated with a counterclockwise rotational convergence of africa towards eurasia during the albian campanian time between the time of the aob eruption 104 7 ma and the intrusion of the tr ph 86 78 ma africa concomitantly rotated counterclockwise about 14 and translated northwards towards eurasia about 6 of latitude

thick turbidite successions from supply dominated shelves during sea level highstand emphasis on the association between relative sea level lowstand and the formation of sandy deep water fans has tended to downplay the significance of high sediment supply and its potential to create deep water fans even during sea level highstands the lance fox hills lewis shelf margin in southern wyoming suggests that high supply was critical in causing the accretion of this moderately wide maastrichtian shelf margin at a minimum rate of 47 8 km m y and the generation of large sand rich fans during every shoreline regression across the shelf it is surprising that fans developed from shelf margin clinoforms that show systematically rising shelf edge trajectories proxy for rising relative sea level as well as from those that show flat trajectories stable to falling relative sea level however the latter producing more sediment bypass resulted in bigger and thicker fans whereas the former produced somewhat smaller and thinner fans we term the former highstand fans and suggest caution in using the lowstand model for high supply systems

biogeography and diversity of south atlantic cretaceous echinoids implications for circulation patterns abstract this survey of echinoid species living on the african and south american margins of the south atlantic ocean in cretaceous times is based on a review of published species and abundant unpublished material the synthetic approach is used to ascertain how the south atlantic was colonised by echinoids from the time it first opened in the early cretaceous until communications were established with the north atlantic and mediterranean tethys in the late cretaceous the survey focuses on the biogeographical origin of immigrant species and their relations with indo madagascan north atlantic and mediterranean tethyan faunas qualitative variations in species diversity are also studied where available fossil material permits together with the in situ evolution of certain lineages and their endemism finally echinoid colonisation of the south atlantic during the cretaceous is viewed against palaeocurrent patterns the survey shows within the limits of material available that no echinoid fauna attests to the existence of the narrow marine trough separating south america and africa in the barremian in the aptian west african echinoids display a mediterranean tethyan influence indicating a probable link between south east and west of africa via the strait opened between africa and madagascar echinoid fauna on the brazilian margin includes species found in what is now continental north america and on the pacific seaboard of south america during the albian south america separated completely from africa and normal marine communications were established with the western tethys to the north and the southern ocean domain to the south the echinoid fauna of the south atlantic during the albian and early cenomanian was particularly rich and diversified with the earliest ubiquitous species on both margins of the ocean in the late cenomanian early turonian a tethys south atlantic ocean link probably formed in west africa through the sahara and nigeria favouring the influx of mediterranean tethyan species to the south atlantic during the mid turonian coniacian the trans saharan channel closed slowing down colonisation of the south atlantic by mediterranean tethyan echinoids as the only connection was via the central atlantic this period was marked by the stability of a number of lineages that survived without major morphological changes in areas colonised during cenomanian times in the late senonian the south atlantic was largely open to the neighbouring ocean domains and a new trans saharan channel linked the mediterranean tethys and south atlantic directly this allowed substantial faunal exchanges and most senonian echinoids found on the south atlantic rim are common to both its eastern and western margins