a seismic refraction study of the crustal structure associated with the adamawa plateau and garoua rift cameroon west africa summary recordings of quarry blasts along a 300 km profile across the adamawa plateau at the eastern end of the cameroon volcanic line permitted the determination of the crustal structure of the region to the north of the plateau in the area of the garoua rift an eastern arm of benue trough the crust is about 23 km thick and is underlain by upper mantle with a p wave velocity of 7 8 kms 1 the southern part of the plateau by comparison is associated with a more normal crustal thickness of about 33 km and underlain by upper mantle with a velocity of 8 0 kms 1 between these two areas the seismological experiment was not able to delineate how the crust changes however if the topography and gravity field reflect crustal thickness then there is a rapid change in crustal thickness at the northern edge of the adamawa plateau the study thus provides the first seismic evidence for major crustal thinning beneath the garoua arm of the west african rift system as well as providing evidence for an alternative model for the formation of the cameroon volcanic line

regional framework structural and petroleum aspects of rift basins in niger chad and the central african republic c a r abstract this paper overviews the regional framework tectonic structural and petroleum aspects of rifts in niger chad and the c a r the data base is from mainly proprietary exploration work consisting of some 50 000 kilometres of seismic profiles 50 exploration wells one million square kilometres of aeromagnetics coverage and extensive gravity surveys there have been 13 oil and two oil and gas discoveries a five phased tectonic history dating from the pan african orogeny 750 550 my b p to the present suggests that the western central african rift system wcas with its component west african rift subsystem was and central african subsystem cas formed mainly by the mechanical separation of african crustal blocks during the early cretaceous among the resulting rift basins in niger chad and the c a r seven are in the was grein kafra tenere tefidet termit bongor and n dgel edgi and three doba doseo and salamat are in the cas the was basins in niger and chad are all extensional and contain more than 14 000 m of continental to marine early cretaceous to recent clastic sediments and minor amounts of volcanics medium to light oil 20 api 46 api and gas have been discovered in the termit basin in reservoir source and seal beds of late cretaceous and palaeogene age the most common structural styles are extensional normal fault blocks and transtensional synthetic and antithetic normal fault blocks the cas doba doseo and salamat are extensional to transtensional rift basins containing up to 7500 m of terrestrial mainly early cretaceous clastics heavy to light oil 15 39 api and gas have been discovered in doba and doseo basins source rocks are early cretaceous lacustrine shales whereas reservoirs and seals are both early and late cretaceous dominant structural styles are extensional and transtensional fault blocks transpressional anticlines and flower structures the existence of a total rift basin sediment volume of more than one million cubic kilometres with structured reservoir source and seal rocks favours the generation migration and entrapment of additional significant volumes of hydrocarbons in many of these basins

thermal modelling of the melut basin sudan and south sudan implications for hydrocarbon generation and migration abstract the thermal history of the melut rift basin was studied in a number of wells and along a northeast southwest cross sectionacross the basin using 1 d and 2 d basin models modelling was conducted using petromod software to test subsidence thermal history and their implications for hydrocarbon generation migration and accumulations geotherms were found ranging between 24 and 44 c km and average 32 c km and present day heat flow was found to vary between 53 and 65 averaging 59 mw m 2 a geologicaly realistic paleoheat flow model with higher heat flow peaks of 75 70 and 65 mw m 2 during the first second and third rifting phases respectively calibrated against vitrinite reflectance data was employed the model was found sensitive to cenozoic erosions and a 600 m of exhumation did support the vitrinite data in providing a reasonable fit in most locations using the easy ro vitrinite kinetic model for maturity the average present day oil window was found between 1565 and 4050 m in the wells studied and between 1705 and 4200 m along the cross section using type ii kerogen for the aptian albian source rocks the hydrocarbon generation and expulsion were modelled and revealed that the generation started as early as 95 ma in some areas but significant generation rates were during the second rifting phase and expulsion started at around 85 ma calculations of the probable expelled amounts of hydrocarbon from the northern sub basin are high and up to 2 2 10 11 bbls of oil and 2 3 10 14 ft 3 of gas migration of oils from cretaceous source rocks kitchen to the cenozoic traps was probably through faults porous carrier beds and breaching of traps cap rocks the filling of these traps might be periodic during the rifting episodes and hydrocarbons remigrated from deeper traps to shallow ones it is possible that thinner and siltier cap rocks along the hydrocarbon migration path may have allowed gas to scape but holded the oil back it is most probable that part of the hydrocarbons might have been generated from shallower source rocks areas and a 2 d hybrid darcy flow path model have demonstrated this and predicted a number of accumulations which are similar to the known large discoveries such as palogue fal and adar yale oil fields

a regional gravity study of the west african rift system in nigeria and cameroon and its tectonic interpretation abstract this study brings together existing and new gravity data to investigate the nature and cause of the bouguer gravity field associated with the cretaceous west african rift system in nigeria and cameroon the new gravity measurements include data collected over the basement area between the benue trough and the cameroon border and fill an important gap in the gravity coverage after removal of the long wavelength negative anomaly from the observed gravity field the remaining positive gravity anomaly associated with the rift system is interpreted in terms of two and three dimensional crustal models these models are constrained by crustal thicknesses derived from a seismic refraction study carried out across and to the south of the yola rift the results of simple three dimensional gravity modelling indicate the crust beneath the lower and middle benue is approximately 20 km thick while beneath the gongola rift the crust is approximately 25 km thick relative to a normal crustal thickness of 34 km away from the rift assuming the thinned crust associated with the rift system is the result of simple lithospheric stretching then the maximum possible crustal extensions of 95 65 and 55 km have taken place perpendicular to the benue gongola and yola rifts respectively these crustal extensions do not necessarily reflect the total crustal movement affecting these rifts since the benue and yola rifts have been subjected to varying amounts of shear displacement during the cretaceous and early tertiary times crustal extension estimates across the west african rift system could be as much as four times greater than the published extension estimates across the east african rift system these rift systems have the same plate tectonic setting being the failed third arms of successful triple junctions and are the result of extensional and shear tectonics the differences in crustal extension and the resulting isostatic response of the lithosphere beneath these rifts can explain why the west african rift system has been associated with subsidence processes throughout its development whereas the east african rift system in northeast africa has been strongly affected by uplift

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

estimates of dinosaur speeds from a new trackway site in texas a method for estimating the speed of a dinosaurian trail maker from the size of its footprints and stride has been described by alexander1 however when applied to known trackways this method gave rather low speeds 1 0 3 6 m s 1 russell and beland2 estimated a speed of 1 77 m s 1 for a slowly moving ornithomimid and 7 54 m s 1 for an allegedly rapidly running ornithopod the latter estimate is questionable as there is some doubt concerning the number of prints and thus the stride length of the trail3 4 dinosaurs responsible for trackways in british columbia5 south wales6 and queensland7 all seem to have been moving slowly thus reliable estimates of dinosaur speeds are all rather low here i report dinosaur speeds based on trails at a new site from the lower cretaceous of texas some of which appear quite fast by alexander s method1

rift basins of interior sudan petroleum exploration and discovery in early 1975 chevron overseas petroleum inc commenced what became a major petroleum exploration effort in previously unexplored interior sudan with the cooperation of the sudanese government chevron has acquired a vast amount of geological and geophysical data during the past 12 years these data include extensive aeromagnetic and gravity surveys 36 040 mi 58 000 km of seismic data and 86 wells this information has defined several large rift basins now recognized as a major part of the central african rift system the sedimentary basins of interior sudan are characterized by thick nonmarine clastic sequences of jurassic cretaceous and tertiary age over 45 000 ft 13 716 m of sediment was deposited in the deepest trough and extensive basinal areas are underlain by more than 20 000 ft 6 096 m of sedimentary rocks the depositional sequences include thick lacustrine shales and claystones floodplain claystones and lacustrine fluvial and alluvial sandstones and conglomerates those lacustrine claystones deposited in a suboxic environment provide good oil prone source rocks reservoir sandstones have been found in a wide variety of nonmarine sandstone facies the extensional tectonism that formed these basins began in the jurassic early cretaceous movement along major fault trends continued intermittently into the miocene this deformation resulted in a complex structural history that led to the formation of several deep fault bounded troughs major interbasinal highs and complex basin flanks this tectonism has created a wide variety of structures many of which have become effective hydrocarbon traps during the past eight years several important oil discoveries have been made significant accumulations have been delineated in the heglig and unity areas where estimated recoverable reserves are 250 300 million bbl of oil

depositional environments of late cretaceous gongila and fika formations chad bornu basin northeast nigeria abstract organic geochemical and palynofacies studies of 172 ditch cuttings samples of possible source rock shales from the late cretaceous gongila and fika formations in the chad basin of ne nigeria were carried out to determine their paleoenvironments of deposition although dominated by amorphous organic matter c s ratios and molecular parameters suggest the mostly organic lean shales toc contents typically below 1 5 were deposited in a normal marine environment levels of oxygenation influenced by water depth in the depositional environment appear to control organic richness and quality of the dark grey shales the organic rich toc 2 0 upper part of the fika formation was deposited under anoxic conditions during the late cretaceous and could represent an oceanic anoxic event mature intervals where such conditions prevailed would have generated liquid hydrocarbon although none were sampled here a trend of increasing organic richness towards the central part of the larger chad basin observed in this and other studies supports the development of organic rich marine shales average toc contents of 2 3 of equivalent age in the termit basin where water depth would have been deeper and oxygen conditions at levels that permitted preservation of marine organic matter

onocerane and other triterpenoids in late cretaceous sediments from the upper benue trough nigeria tectonic and palaeoenvironmental implications abstract the aliphatic hydrocarbon bitumen fraction has been investigated in late cretaceous outcrop samples from the upper benue trough inundated by the sea only during the maximum mid cretaceous transgression thermal maturity in nearly all formations is pre oil window indicating only limited mainly tertiary burial the santonian deformation a significant thermal influence to the south west appears to have had little effect here onocerane i is the dominant triterpenoid in the pindiga formation but is absent elsewhere its origin is uncertain but it appears to be associated with deposition in a restricted basin possibly a brackish lagoon receiving dominantly terrestrial organic input a review of possible natural product precursor compounds suggests onoceradiene from a fern like plant was a likely source even carbon number predominance of n alkanes in the range c 16 to c 22 in some formations is tentatively interpreted to indicate hypersalinity

benue trough and benue chain it is suggested that the benue trough may have been initiated by the extension of the equatorial oceanic fracture zones onto the continent during the early stages of the separation of africa and south america a transcurrent movement along deep seated faults of the basement is responsible for the structure of the trough after a period of extension characterized by the sedimentary infilling of the trough and igneous activity the trough is turned into a chain through a short phase of compression a discrete pattern of en echelon folds and the strain pattern deduced from the analysis of large scale and microfracturing correspond to a n s to n160 e direction of compression coupled to a sinistral transcurrent movement along a ne sw direction the difference in age of the folding phase in the northern and southern parts of the chain indicates a diachronous nature for this tectonic event in relation to the wrench regime the tertiary evolution of the trough marked by a tensional regime resulting in an e w extension is to be related to the cameroon volcanic line

crustal structure of the khartoum basin sudan abstract the crustal structure of the northern part of the khartoum basin has been investigated using data from 3 permanent seismic stations within 40 km of khartoum and two modeling methods h k stacking of receiver functions and a joint inversion of receiver functions and rayleigh wave group velocities the khartoum basin is one of several mesozoic rift basins in sudan associated with the central african rift system results from the h k stacking indicate that crustal thickness beneath the khartoum basin ranges between 33 and 37 km with an average of 35 km and that the crustal vp vs ratio ranges from 1 74 to 1 81 with an average of 1 78 from the joint inversion of receiver functions and rayleigh wave group velocities we obtained similar results for moho depth as well as an average shear wave velocity of 3 7 km s for the crust these results provide the first seismic estimates of moho depth for a basin in sudan when compared to average crustal thickness for unrifted proterozoic crust in eastern africa our results indicate that at most only a few km of crustal thinning may have occurred beneath the khartoum basin this finding is consistent with estimates of effective elastic plate thickness which indicate little modification of the proterozoic lithosphere beneath the basin and suggests that there may be insufficient topography on the lithosphere asthenosphere boundary beneath the sudanese basins to channel plume material westward from ethiopia

on the use of global potential field models for regional interpretation of the west and central african rift system abstract the use in regional interpretations of the earth gravity model egm08 and earth magnetic model emag2 is evaluated by comparison to ground gravity and aeromagnetic data in the central sector of the west and central african rift system wcars the comparison includes upward continuation spectral analysis and pseudogravity calculation and statistical evaluation a correlation between emag2 which contains roughly 25 km resolution aeromagnetic data in the region and near surface aeromagnetic data over wcars is only true for the very low wavelength part but a strong similarity between egm08 and ground gravity data can be confirmed interpretation of the egm08 data allows identifying and confirming the position of major structural trends and provides new information on the crustal architecture the lineaments limiting different grabens forming the logone birni basin and in the southern chad basin are identified the results presented here show the use of egm08 data for regional interpretations over cameroon and adjacent countries which can overcome the absence and sparseness of data in developing countries and remote areas

the walking speed of dinosaurs from the peace river canyon british columbia canada the walking speed of seven dinosaur ichnospecies from the lower cretaceous peace river canyon british columbia canada have been determined the speeds range from approximately 1 to 3 m s 1 4 10 km h 1

speeds and gaits of dinosaurs abstract the known relationships of speed gait and body size derived mainly from mammals are used to determine the gaits and theoretical maximum speeds of dinosaurs speed estimates are made for 62 dinosaurs representing 51 genera and are supplemented with information from comparative anatomy and from dinosaur trackways it is concluded that smaller bipedal dinosaurs were capable of running at speeds up to 35 or 40 km h the so called ostrich dinosaurs are credited with maximum speeds less than 60 km h and possibly as low as 35 or 40 km h larger bipedal dinosaurs were probably restricted to walking or slow trotting gaits with maximum speeds in the range 15 20 km h most quadrupedal dinosaurs seem to have been restricted to a walking gait stegosaurs and ankylosaurs may have had maximum speeds as low as 6 8 km h sauropods may have attained 12 17 km h and some ceratopsians may have been capable of trotting at speeds up to 25 km h on a weight for weight basis the speeds of dinosaurs are generally lower than those of mammals

organic geochemical characteristics and depositional models of upper cretaceous marine source rocks in the termit basin niger abstract upper cretaceous marine mudstones are widely known as the most important source rocks in the western african rift basins however geochemical studies on their occurrence and formation mechanism are scarce in this study a detailed geochemical investigation within a sequence stratigraphic framework was carried out to reconstruct the palaeoenvironmental conditions and to establish models on the depositional history of upper cretaceous marine mudstones in the termit basin niger western african a total of six third order sequences dsq1 dsq2 ysq1 ysq2 ysq3 and msq1 from bottom to top were identified in the upper cretaceous the distribution of biomarkers from 33 mudstone samples in different sequences indicate that mudstones from ysq3 were mainly deposited in a suboxic to oxic environment with significant contributions from mixed terrigenous higher plants and lower aquatic organisms in fresh brackish water column in contrast the other sequences ysq2 ysq1 dsq2 and dsq1 mainly represent a suboxic to anoxic marine environment with a stratified water column 0 59

ice complex formation in arctic east siberia during the mis3 interstadial abstract a continuous 15 m long sequence of ice complex permafrost yedoma exposed in a thermo cirque at the southern coast of bol shoy lyakhovsky island new siberian archipelago dmitry laptev strait was studied to reconstruct past landscape and environmental dynamics the sequence accumulated during the marine isotope stage 3 mis3 interstadial between 49 and 29 ka bp in an ice wedge polygon the frozen deposits were cryolithologically described and sampled on a vertical bluff between two ice wedges according to sedimentological and geochronological data the section is subdivided into three units which correlate with environmental conditions of the early middle and late mis3 period palynological data support this stratification the stable isotope signature of texture ice in the polygon structure reflects fractionation due to local freeze thaw processes while the signature of an approximately 5 m wide and more than 17 m high ice wedge fits very well into the regional stable water isotope record regional climate dynamics during the mis3 interstadial and local landscape conditions of the polygonal patterned ground controlled the ice complex formation the sequence presented here completes previously published mis3 permafrost records in northeast siberia late quaternary stadial interstadial climate variability in arctic west beringia is preserved at millennial resolution in the ice complex a mis3 climate optimum was revealed between 48 and 38 ka bp from the ice complex on bol shoy lyakhovsky island

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

geophysical evidence of cretaceous volcanics in logone birni basin northern cameroon central africa and consequences for the west and central african rift system abstract detailed analyses and interpretation realized by combining existing 2d reflection seismic and gravity magnetic data of the logone birni basin lbb in the west and central african rift system wcas have revealed the distribution of the main buried volcanic bodies as well as their relationships with the structural and tectonic evolution of this basin the volcanic activity in the lbb is restricted to the cretaceous period three main volcanic episodes are identified and are associated to the neocomian late albian and cenomanian turonian rifting phases respectively the volcanic bodies within the lower cretaceous are either lying directly on basement or are mainly interbedded with the contemporaneous sediments whereas the upper cretaceous bodies are morphologically expressed in the forms of dykes and sills the volcanic activity was more intense in the western region of the central lbb zina sub basin along the cameroon nigeria border whereas it was scanty and scattered in the other parts of the basin the main volcanic dykes are found on the flanks of the major faults bounding basement horsts or in crestal positions in association with syndepositional faults although wcas is associated with large amount of crustal extension and minor volcanism the intense volcanic activity observed in lbb during the cretaceous suggests that the intrusive zone during this period was confined to the basement beneath the study area flanked respectively to the north south and southwest by the lake chad poli and chum triple junctions tensional stresses generated by this localized domal uplift accounts for most of the observed tectonic structures where major faults transected the entire lithosphere thus providing conduits for magma migration

chronology and geodynamic setting of cretaceous cenozoic rifting in west and central africa abstract the development of the early cretaceous to palaeogene west and central african rift system which extends from nigeria benue trough to kenya anza trough can be related to the build up of intraplate tensional stresses during the break up of gondwana which caused reactivation of pre existing zones of lithospheric weakness repeated changes in the intraplate stress regime of africa are reflected by phases of crustal extension alternating with episodes of compression many of these events can be correlated with changes in rates of seafloor spreading in the central and south atlantic oceans as reflected in flowline patterns the west and central african rifts can be considered as typical passive rifts which evolved in response to the build up of intraplate stresses however the st helena hot spot appears to have been located beneath the equatorial plate boundary at approximately 120 ma and may have played an important role in weakening the lithosphere during extension

gravity study of the central african rift system a model of continental disruption 1 the ngaoundere and abu gabra rifts abstract a regional compilation of published and unpublished gravity data for central africa is presented and reveals the presence of a major rift system called here the central african rift system it is proposed that the junction area between the ngaoundere and abu gabra rift arms in western sudan forms an incipient intraplate triple junction with the as yet unfractured but domally uplifted and volcanically active darfur swell it is only the darfur swell that shows any similarities to the uplift and rift history of east africa the other two rifts arms are considered to be structurally similar to the early stages of passive margin development and thus reflect more closely the initial processes of continental fragmentation than the structures associated with rifting in east africa

petroleum geology of the melut basin and the great palogue field sudan abstract this paper overviews the framework structural and petroleum geology aspects of the melut rift basin in sudan the data base is from mainly proprietary exploration work consisting of more than 20 000 km of seismic profiles 2000 km2 of 3d seismic and more than 50 exploration wells there have been more than 15 oil and oil and gas discoveries the melut basin is a meso cenozoic rift basin accompanied by the formation and development of the central african shear zone casz on the pre cambrian crystalline and metomorphic basement of lower relief three stages of rift development and fracturing have been identified stronger in the early cretaceous and paleogene and weaker in the late cretaceous source rocks are the lower cretaceous lacustrine shales whereas reservoirs and seals are both paleogene and upper cretaceous dominant structural styles are large scale anticlines in the paleogene sequences and antithetic normal fault blocks in the upper cretaceous and paleogene heavy to light oils 8 87 61 5 api and gas have been discovered in the paleogene and only light oils and gas in the upper cretaceous pay zones the primary pay zone in the melut basin is the paleogene the great palogue field with ultimate recoverable oil reserves of more than 900 million barrels was discovered in the northern melut basin the gentle slopes of each subbasin are favorable areas for petroleum accumulation and enrichment the faulted anticlines within the paleogene syn rift sequences are the main trap types whereas the antithetic fault blocks of the upper cretaceous sequences are secondary