sedimentary and structural evolution of the northwest arabian red sea margin abstract an investigation of the northwestern margin of the arabian peninsula midyan region provides new insights into the two stage development of the red sea graben system the early red sea suez stage and the later red sea aqaba stage the early stage is characterized by the passive subsidence of a continental graben since the early oligocene after a period of enhanced subsidence of the basin in the early miocene this stage ended at the langhian serra valian boundary 14 ma the later stage was initiated by a release of stresses by strike slip movement along the newly formed aqaba levant transform creation of the new plate boundary was accompanied by stagnation of extension in the gulf of suez branch of the early rift stage strike slip motion on the aqaba levant structure is younger than 14 ma and is recorded by the post langhian compressional structures in the area of the midyan peninsula which developed by transpression along the curved path of a left lateral strike slip fault

evaporitic and biosiliceous cyclic sedimentation in the miocene of the gulf of suez depositional and diagenetic aspects abstract the middle to late miocene evaporite bearing formations which outcrop along the gebel zeit and gems highs in the southwestern gulf of suez egypt consist of thick calcium sulphate beds rhythmically interbedded with marlstones siltstones and claystones containing various amounts of biogenic silica as well as pure diatomites the calcium sulphate beds are composed mainly of gypsum and anhydrite subaqueous crustallisation in subpermanent brine ponds of marine origin predominated during deposition of the selenite and laminated gypsum early diagenetic interstitial growth of nodular anhydrite occurred episodically when the water level dropped in response to increasing salinity associated with the precipitation of halite in the deeper troughs nevertheless most of the nodular facies resulted from the burial conversion of gypsum to anhydrite which has been sometimes rehydrated into gypsum a last phase of gypsum dehydration has occurred at the surface related to the present hot and dry climatic conditions the biosiliceous deposits contain either well preserved opal a or diagenetically altered opal ct clinoptilolite quartz and even clays diatom frustules such diagenetic changes are commonly described in the oceanic biosiliceous deposits the fossil content of these diatomitic layers implies shallow marine conditions on the shelves of highs induced by block faulting the cyclic succession of evaporited and biosiliceous deposits recorded the alternation of high sea level stands associated with high organic productivity probably enhanced by seasonal input of nutrients and low sea level stands associated with hypersaline conditions and evaporite deposition locally the sharp contact between the diatomites and gypsum argues for a rapid evolution of salinity towards high concentration up to gypsum saturation and more the formation of finely laminated diatomitic sediments is known to require either high organic productivity or anoxic conditions in bottom intermediate waters and eventually both processes in the gebel zeit and gemsa areas a local association of the biosiliceous sediments with diagenetic carbonates and native sulphur resulting from processes of bacterial sulphate reduction confirms that anoxic conditions occurred in the deep water body or in the sediments large amounts of organic compounds were involved in these diagenetic processes immature organic matter was consumed during an early diagenetic stage of transformation it is assumed that this organic matter was supplied either by the biosilica rich interbeds or the organic rich laminated carbonates the initial organic content of the biogenic deposits was impoverished as early as the first stages of sedimentation and diagenesis although a large part of the organic matter could have been destroyed by these early diagenetic processes and further oxidation in outcrops these sediments can be considered as potential source rocks in nearby less exposed sequences

soil formation in coastal continental antarctica wilkes land recent antarctic soil studies suggest that in terrestrial ecosystems of coastal regions soil formation and chemical weathering occur to a greater extent than predicted by former models this paper summarizes pedogenic findings from the casey area on the coast of east antarctica and presents a proposal of soil formation sequences on a large scale data base soil organic matter accumulation and podzolization are important soil forming processes in regions outside the antarctic cold desert of the ross sea section the results suggest some correlations between the soil development and vegetation patterns nutrient supply in soil is affected by the high contents and availability of nitrogen phosphorus potassium and magnesium due to an input by seabirds the us soil taxonomy 8th edn gelisol order and the recently adopted world reference base for soil resources wrb cryosol soil group suggest a different understanding of impact of permafrost on soil formation according to the wrb and the us soil taxonomy organic permafrost affected soils are not cryosols but gelisols lithic haplorthels as well as haploturbels and to some extent aquaturbels and organic gelisols are suggested to be representative of the casey area in addition podzols occur to a great extent however the recent keys to soil taxonomy do not reflect the podzolization process in an adequate way and a classification is not possible according to a former version these soils would be lithic or pergelic haplocryods whereas according to the newly adopted wrb system these soils are spodic haplic cryosols

the sedimentary evolution of the red sea rift a comparison of the northwest egyptian and northeast saudi arabian margins abstract comparison of the sedimentary sequences outcropping on opposite sides of the red sea although based on relatively limited data reveals many similarities the cretaceous and paleogene pre rift series are very similar throughout the northwestern parts of the red sea and in the gulf of suez syn rift sediments both in saudi arabia and in egypt may be readily subdivided into a basal continental unit a transgressive marine series an evaporitic formation and a final pliocene mixed clastic carbonate phase these marked variations express a comparable geodynamic evolution on opposite sides of the rift however these similarities are modified by important differences which are partly chronological and concern the precocity of initial syn rift sedimentation in the midyan region of saudi arabia there are also important lithological differences within each of the major units the chronological and lithological differences may be due either to the diachronous evolution of the various segments of the basin or to different environments of sedimentation whose localisation differed with respect to a common rift axis

distribution of soil taxa in antarctica a preliminary analysis abstract only 0 35 49 500 km2 of antarctica is ice free these areas are scattered around the periphery of the continent and in interior mountain ranges making soil mapping difficult here we compile the results of mapping in five of the nine ice free areas that account for 79 of the ice free area on a reconnaissance scale and interpret the distribution of soil subgroups in soil taxonomy soils of antarctica are contained in four orders dominantly gelisols 84 13 suborders 27 great groups and 76 subgroups forty five percent of the soils of antarctica are orthels gelisols that show minimal cryoturbation and occur in dry landscape positions 38 of the soils are turbels showing cryoturbation and occurring in more moist landscape positions only 16 of the soils of antarctica lack permafrost in the control section and are classified as entisols gel great groups inceptisols gelepts suborder or gelaquepts or histosols gel great groups these soils occur almost exclusively along the western antarctic peninsula and at elevations below 30 m in the south shetland islands ssi and south orkney islands soi typic anhyorthels are the dominant soil subgroup comprising nearly 15 340 km2 or 31 of the soils in antarctica these soils occur primarily in central and southern victoria land but also occur in the thiel and pensacola mountains and shackleton range the prince charles mountains and the mountains of queen maud land typic haploturbels and typic anhyturbels occupy 14 and 13 of the soils of ice free regions of antarctica respectively most abundant in central victoria land they are common in most mountainous regions of antarctica the dominant soil processes of maritime antarctica are cryoturbation gleization melanization podzolization paludization and phosphatization in coastal east antarctica the major soil processes are rubification salinization calcification pervection and gleization the predominant soil forming processes in the transantarctic mountains include rubification salinization desert pavement formation and permafrost development measures of pedodiversity will be valuable in the selection of protected areas in antarctica

carbonate evaporite siliciclastic transitions in quaternary rift sediments of the northwestern red sea abstract the quaternary and recent coastal environments of the egyptian red sea and gulf of suez are characterized by rapid lateral variations carbonates evaporites and siliciclastic sediments exist in close proximity although the climate is exceptionally dry the numerous wadis draining high hinterland relief deliver important quantities of coarse clastics to coastal environments during the rare periods of flash flooding however because this efficient supply is sporadic coastal waters are contaminated for short catastrophic periods which are followed by a rapid return to clear water carbonate sedimentation the warm arid conditions on the other hand favour both carbonate reef production and evaporite mineral precipitation rapid lateral transitions between continental siliciclastic and marine carbonate facies are favoured by a particular morphology closely related to the active distensional tectonic setting within the red sea rift block faulting which began in the early miocene and continues today has resulted in numerous subparallel grabens and horsts situated between the onshore basement relief and the coast the structural depressions grabens tend to trap much of the episodic siliciclastic input favouring permanent carbonate production in the nearby littoral environments

lithospheric extension and the opening of the red sea sediment basalt relationships in yemen this paper analyses new field and radiometric age data on the timing of extension sedimentation and magmatism related to the opening of the red sea the first evidence of widespread extension is found in synsedimentary fissures in the fluviatile to shallow marine tawilah formation of cretaceous to early tertiary age the first indication of magmatism is the eruption of alkaline basalts 43 5 21 2 ma on to marine and non marine sandstones of lower to middle tertiary age the lack of an angular unconformity at the sediment basalt contact over a period of 25 myr corroborates a diachronous relationship between the tawilah sandstones and the yemen volcanics these data also indicate that the onset of volcanism in yemen was in the eocene as is the case in ethiopia and saudi arabia a passive rifting model is supported by early extension and sedimentation followed by syn and post tectonic magmatism

single borehole and cross borehole resistivity anomalies of thin ellipsoids and spheroids hole to hole resistivity measurement is a useful method of detecting deeply buried fractures and ore deposits in the subsurface with drilling costs continually rising there is a growing need for developing methods of borehole geophysics such as this in this study we present theoretical results relating to detection of thin oblate spheroids and ellipsoids with arbitrary attitude if we assume that individual fractures within a fracture zone are connected to each other and are of finite lateral and vertical extent then we can model the fracture zone as a thin conductive oblate ellipsoid or spheroid with arbitrary orientation of the major axis detection of such deeply buried fracture zones is the object of this study here the effects of the surface of the earth are neglected and the body is assumed to be enclosed within an infinite homogeneous mass the surface of the body is divided into a series of subsurfaces and a numerical solution of the fredholm integral equation is applied once a solution fo

volcano sedimentary characteristics in the abu treifiya basin cairo suez district egypt example of dynamics and fluidization over sedimentary and volcaniclastic beds by emplacement of syn volcanic basaltic rocks abstract this paper describes the neogene lava sediment mingling from the abu treifiya basin cairo suez district egypt the lava sediment interactions as peperites have been identified for the first time at the study area and can be used as paleoenvironmental indicators the identification of peperite reflects contemporaneous time relationship between volcanism and sedimentation and this finding is of primary importance to address the evolutional reconstruction of the abu treifiya basin characterization of the facies architecture and textural framework of peperites was carried out through detailed description and interpretation of their outcrops the peperites and sedimentary rocks are up to 350 m thick and form a distinct stratigraphic framework of diverse lithology that is widespread over several kilometers at the study area lateral and vertical facies of the peperites vary from sediment intercalated with the extrusive intrusive basaltic rocks forming peperitic breccias to lava sediment contacts at a large to small scales respectively peperites encompass five main facies types ascribed to i carbonate sediments hosted fluidal and blocky peperites ii lava flow hosted blocky peperites iii volcaniclastics hosted fluidal and blocky peperites iv sandstone siltstone rocks hosted blocky peperites and iv debris flows hosted blocky peperites soft sediment deformation structures vesiculated sediments sediments filled vesicles and fractures in lava flows indicate that lava flows mingled with unconsolidated wet sediments all the peperites in this study could be described as blocky or fluidal but mixtures of different clast shapes occur regardless of the host sediment the presence of fluidal and blocky juvenile clasts elucidates different eruptive styles reflecting a ductile and brittle fragmentation the gradual variation from fluidal to blocky peperite texture producing the vertical grading is affected by influencing factors e g the viscosity magma temperature confining pressure sediment fluidization and vapor film at the magma sediment interactions peperites in the study area record deposition within a shallow marine and fluvio lacustrine environment accumulated in a rift related basin developed during pre to syn rift phase respectively the facies transitions peperites in this area resulted from the explosive and sediment depositional processes which were mingled separately by volcanism under contrast geological conditions the development of such contrast in the depositional sequences reflects variation in the accommodation to sediment supply in the same accumulation space inside the depocenters during the rifting of the abu treifiya basin hydrothermal mineralizations comprising quartz and carbonate are restricted to peperites and lava flows

tectonic and sedimentary evolution of the gulf of suez and the northwestern red sea abstract the structural pattern of the gulf of suez and red sea rift is inherited from the basement tectonics n140 faults clysmic direction parallel to the regional elongation of the rift are associated with submeridian aqaba direction and sub e w faults duwi direction that also play an important role in the rifting processes the pre rift sedimentary cover includes cretaceous and eocene platform deposits it was slightly deformed prior to rifting the syn rift sedimentary sequence comprises four major units groups a to d delimited by unconformities group a oligocene to early burdigalian continental to restricted subaquatic deposits group b burdigalian to middle miocene open marine sediments group c middle to upper miocene major evaporitic episode and group d plio pleistocene which returns to an open marine environment four majors stages of structuration are recorded an initial stage of compressive deformation direction of shortening close to nw se characterized by strike slip movements and associated drag folds early group a a tilted block stage late group a corresponding to the period of highest extensional rate a horst and graben stage group b resulting in the cutting out of the tilted blocks by numerous synthetic faults group b corresponds to the period of highest subsidence rate when a flexuration stage of the margin expressed by a centripetal evolution of the subsidence is associated with major periferal uplift

the spacing of rift axis highs dependence upon diapiric processes in the underlying asthenosphere abstract detailed bathymetric mapping has revealed a near harmonic distribution of intermediate wavelength morphologic highs on slow to fast spreading centers one model suggests that the highs may result from the thermal instabilities generated by the interaction of cold fracture zone walls with the axial asthenosphere this may force a deepening of the isotherms near the fracture zone and an uplift of the isotherms midway between the fracture zones a preferred model based on newly accumulated data suggests that the highs may be a result of upwelling within the asthenosphere underlying the ridge axis where a linear region of instability develops at the base of a partial melting zone less dense material at depths of 40 80 km may penetrate the overlying material at discrete and ideally evenly spaced points harmonically spaced upwellings should generate excess magma at discrete points on the ridge axis creating volcanic highs the spacing between highs may depend upon the continuity and thickness of the deep axial instability layer the temperature and viscosity ratios within the magma and the distance between major transform faults instability of the diapirs may stimulate the migration of the highs through time

inherited discontinuities and neogene structure the gulf of suez and the northwestern edge of the red sea the gulf of suez and the northern red sea rifts are the result of intracontinental deformation during neogene times the initiation and the development of the rift are controlled by i four main trends offaults n1400 n1500 ns to n200 sub e w and n40 60 ii a zigzag faulting pattern iii two main tectonic events the first is characterized by strike slip displacements inducing the formation of antithetic tilted blocks the geometry of the blocks changes according to their orientation complex structures can result from the combination of several trends vertical movements are weak the second is characterized by synthetic normal movements forming a horst and graben pattern vertical displacements are important and induce the generation of the axial trough and uplift of the shoulders of the rift the zigzag pattern of the faults that govern the rift and the initial strike slip displacements cannot result from a simple extensional model but imply a reactivation of inherited discontinuities in the miocene stress system induced by the northern convergent boundaries of the african and arabic plates the major trends of faults that control the neogene structure have been active in this area since palaeozoic or even proterozoic times

on sensitivity of surface to borehole resistivity measurements to the attitude and the depth to center of a three dimensional spheroid borehole to surface and surface to borehole resistivity measurements are versatile but not totally tested methods for detecting anomalies in the vicinity of a borehole the former method has been discussed by several authors alfano 1962 merkel 1971 merkel and alexander 1971 barnett 1972 snyder and merkel 1973 snyder 1976 daniels 1977 1978 and 1983 but the latter has not received much attention morrison 1971 and daniels 1977 are among the few who have addressed the problem each method has its own advantages surface to borehole resistivity measurements are made by placing a current source on the surface and measuring the apparent resistivity in a borehole in which the measuring electrodes are closer to the body than in the borehole to surface case pilot studies presented here suggest that the surface to borehole method can provide indicators of the attitude and the depth to the center of a body this paper illustrates a simple method for qualitatively determining the attitude and t

rock weathering processes and landform development in the sor rondane mountains antarctica abstract field observations of weathering processes and the related landforms combined with laboratory analyses of weathering products permit a synthetic evaluation of late cenozoic weathering environments in the sor rondane mountains antarctica an arid upland characterized by low temperatures and strong winds rates and character of weathering depend mainly on moisture availability and the bedrock geology under the humid weathering regime that occurs only locally around the margin of the present sheet frequent diurnal freeze thaw cycles in summer cause relatively rapid rock fragmentation most of the mountains are situated in the arid weathering regime under which rock breakdown is very slow unless the rock contains plenty of salts salt weathering becomes more intensive and extensive with exposure age as a result of salt accumulation in rock eventually producing soils as small as fine silt size lack of clay mineralization even in weathered rocks having been exposed above the ice sheet prior to 4 ma ago indicates that hydrolysis or carbonation of rock minerals has been insignificant during the past 4 ma the final products of weathering are due mainly to salt action and reflect the parent lithology resistant fine grained granite forms strongly oxidized tors carved with tafoni or fields of mushroom like boulders overlying the fractured bedrock less resistant rocks like biotite gneiss and amphibolite produce stone pavements underlain by saline silty soils up to 30 40 cm thick the thickness of which corresponds to the maximum thaw depth

three dimensional model results for an electrical hole to surface method application to the interpretation of a field survey an important objective of borehole geophysics is to maximize information about inhomogeneities in the vicinity of existing boreholes three dimensional models parallelepipedic inhomogeneities in a homogeneous half space are used here to characterize the responses of an electrical hole to surface method elecent electrode enterree where conductive or resistive inhomogeneities orebodies occur in the vicinity of a borehole two types of surface measurements are carried out simultaneously at each station electrical potential v as in mise a la masse and associated electrical field e these two measured parameters give rise to three other parameters two apparent resistivities one for each v and e case and another parameter quantifying the orientation of the e field the influence of various geometrical and electrical parameters such as the horizontal and vertical distances between the current source the stations and the inhomogeneity and the size and resistivity of the inhomogeneity on the five

miocene evaporites of the red sea rift yemen republic sedimentology of the salif halite abstract miocene evaporites are spectacularly exposed along the red sea coastal plain of yemen especially in the region of salif over one thousand metres of bedded halite containing regular millimetre thick laminae of anhydrite were precipitated below wave base which need not have been very deep in a relatively quiet and protected basin virtually cut off from the open sea of the mediterranean and indian ocean a metre thick unit of laminated and rippled anhydrite within the halite is the result of a major decrease in salinity within the basin gypsum overlying the halite is bedded and laminated but does contain vague vertical structures suggestive of an original selenitic mineralogy this sulphate was probably precipitated as bottom growth gypsum and decanted crystals more open circulation in the basin was established in the early pliocene when carbonate muds and siliciclastics were deposited these miocene evaporites were precipitated during the continental rifting and opening of the red sea gulf of aden when half graben structures were formed and fed by occasional influxes of marine water subsequent sedimentation and tectonic deformation led to major halokinesis the development of diapirs and exposure of the halite along the coastal plain

segmentation and along strike asymmetry of the passive margin in socotra eastern gulf of aden are they controlled by detachment faults on the island of socotra the southern passive margin of the gulf of aden displays along its strike two different types of asymmetric structures western socotra is made up of a series of southward tilted blocks bounded by consistently northward dipping normal faults eastern socotra consists of a broad asymmetric anticline with a steep northern limb and a gently dipping southern limb a zone of ne sw striking strike slip and normal faults separates the two areas the overall structure is interpreted as representing two rift segments separated by a transfer zone the along strike juxtaposition of crustal scale asymmetric structures on the southern margin of the gulf of aden is complemented by the asymmetry of the conjugate margins on either side of the gulf whereas the western socotra margin is narrow and characterized by oceanward dipping normal faults the conjugate oman margin is broader and dominated by horsts and graben considering that asymmetric structures in the upper crust are often associated with synthetic shear zones at deeper ductile levels we propose that the western and eastern socotra margin segments were controlled at depth by two detachment faults with opposite dips and senses of shear the normal faults of western socotra would sole out into a top to the north ductile shear zone whereas the asymmetric anticline of eastern socotra would be associated with a top to the south detachment fault

soil distribution in the mcmurdo dry valleys antarctica abstract the mcmurdo dry valleys mdvs are the largest ice free area ca 6692 km 2 in antarctica here we present a reconnaissance scale 1 2 million soil map of the mdvs the soil map units are subgroups as identified in the u s department of agriculture soil taxonomy the dominant soil subgroups in the mdvs are typic anhyorthels 43 typic haploturbels 36 and typic anhyturbels 14 soils of the mdvs represent an evolutionary sequence that include glacic haploturbels anhyturbels on holocene surfaces typic haploturbels anhyturbels on late quaternary surfaces typic anhyorthels on late to mid quaternary surfaces salic anhyorthels on mid to early quaternary surfaces and petrosalic petrogypsic petronitric anhyorthels on pliocene and older surfaces soils on silt rich tills of pliocene and older age generally are typic or salic anhyorthels they feature less weathering than younger soils because i they are derived from quartzose materials largely devoid of weatherable minerals and ii they have been subject to considerable wind erosion

gravity tectonics of topographic ridges halokinesis and gravitational spreading in the western ogaden ethiopia the cenozoic history of the western ogaden region of ethiopia between the ethiopian rift and the south afar margin is marked by uplift and incision of the ogaden plateau down to the gorrahei formation an upper cretaceous evaporite formation debuttressing of this and the overlying sedimentary formations resulted in widespread and spectacular gravitational spreading landforms over a minimum surface area of 15 000 km2 most of which remains unstudied after clearing up some misconceptions about the surface geology of the study area the kebenawa ridge in the audo range observations are reported that point to a tectonic style controlled by halokinesis and subsequently gravitational spreading the role of diapirism and karstification in the observed halokinesis is discussed as well as the influence of halokinesis on gravitational spreading spreading is in part akin to sackung in that ridge deformation features include a crestal graben and basal ridge topography extrusion and deformation was triggered by lateral ridge debuttressing ridge spreading also presents analogy with gravitational spreading of the canyonlands grabens in the needles district canyonlands national park utah the scale and the mechanisms are found to be basically similar but two differences are noted first incision by the drainage network in response to plateau uplift in ethiopia has debuttressed the topography along two parallel rivers instead of a single river the colorado river in utah secondly incision proceeded to the base of the evaporite layer in the ogaden whereas incision has not exceeded the top of the evaporite layer in utah these differences may have influenced the details of the spreading mechanisms in ways that remain to be investigated overall in ethiopia association of halokinesis and a transitional mode of gravitational spreading at the interface between narrow ridge spreading sackung and plateau spreading canyonlands type illustrates a fascinating and unusual ridge evolution style

the red sea gulf of aden re assessment of hydrocarbon potential abstract the results and conclusions of the undp world bank red sea gulf of aden regional hydrocarbons study project june 1989 january 1992 are summarized these derive from detailed examination of pooled public sector data provided to the project by the participating bordering states in the region these included samples and logs from about 56 wells drilled in the two basins out of a total of 71 and about 30 000 line km of reflection seismic data selected from 110 000 km available well logs were digitized and correlated and basinwide biostratigraphic lithostratigraphic and palaeoenvironmental studies source rock organic richness quality and maturity studies and sedimentological analyses studies were performed by contracted specialist staff in the uk and france these were integrated with basinwide seismic structural and isopach mapping for selected regional reflectors intervals by national task forces in cairo under project supervision academically acquired crustal seismic and other geophysical data and oil industry velocity and other borehole data were also integrated for the red sea modelling utilizing recently acquired geophysical data indicates crustal asymmetry for the red sea rift with oceanic crust of pull apart basin type flooring some portions of offshore egypt and sudan on the west and stretched continental crust underlying the eastern arabian flank these interpretations furthermore suggest that sinistral strike slip processes controlled the initial break up of the arabian plate shaping the african flank as a sharp plate boundary similar processes appear to have initially shaped the somalia flank of the gulf of aden rift only later did true seafloor spreading propagate in the eastern gulf of aden 10 12 ma about a regional lithostratigraphic nomenclature scheme for the miocene recent syn and post rift red sea successions keyed to detailed biostratigraphical distribution charts and microfaunal assemblage characteristics was erected tied to the well documented and understood petroliferous gulf of suez basin scheme this allows facies variations within the basin to be compared and allows consistent interpretation and correlations across the basin to which local informal nomenclature can be related pre rift pre oligocene marine sediments are present in most of the gulf of aden jurassic eocene but only incompletely on the basin margins in the red sea jurassic in the south upper cretaceous early palaeogene in the north significant pre rift oil gas prone upper jurassic cretaceous and palaeogene source rocks occur in gulf of aden sub basins initiated by late jurassic early cretaceous tensional tectonics with maturity levels ranging from early late to post mature whereas early synrift source rocks are generally immature in the red sea good quality pre rift source rocks outcrop in the north and south but are probably overmature for oil in the basin except in the extreme south if present good syn pre and post salt and post rift source levels occur throughout the basin but in the central southern sectors only the post salt remains within the oil window because of high geothermal gradients both basins are little explored attractive pre rift jurassic eocene exploration plays in rotated fault and horst blocks in predominantly carbonate but subordinate sandstone reservoirs are present in the gulf of aden confirmed by an important undeveloped eocene light oil discovery in offshore yemen with subordinate synrift potential where maturity is established relative continental shelf narrowness is economically limiting but wider sectors occur in the red sea the principal exploration plays are in miocene synrift pre and post salt successions rotated fault blocks occur in the pre salt but maturity for oil in the deeper levels is restricted to the northern sector these levels in the central and southern sectors are overmature except in the southernmost portion but excellent potential exists in the synrift post salt and early post rift sequences with a number of attractive play types reservoirs for synrift plays are mainly sandstones and potential is confirmed by several undeveloped gas and condensate discoveries and numerous oil seepages throughout the basin relatively wide continental shelves occur in the south and along much of the eastern flank

a new method for geoelectrical investigations underwater1 a new electrical method is proposed for determining the apparent resistivity of multi earth layers located underwater the method is based on direct current geoelectric sounding principles a layered earth model is used to simulate the stratigraphic target the measurement array is of pole pole type it is located underwater and is orientated vertically this particular electrode configuration is very useful when conventional electrical methods cannot be used especially if the water depth becomes very important the calculated apparent resistivity shows a substantial quality increase in the measured signal caused by the underwater targets from which little or no response is measured using conventional surface electrode methods in practice however different factors such as water stratification underwater streams or meteorological conditions complicate the interpretation of the field results a case study is presented where field surveys carried out on lake geneva were interpreted using the calculated apparent resistivity master curves

evidence for middle oligocene rifting of the gulf of aden and for late oligocene rifting of the southern red sea abstract recent biostratigraphic analysis of exploratory well samples drilled in the red sea and gulf of aden have revealed new dates for the earliest tertiary deep marine sediments in the region

three dimensional mise a la masse modeling applied to mapping fracture zones a numerical scheme applying the method of volume integral equations has been developed for borehole to borehole and borehole to surface modeling of the apparent resistivity response of a thin conductive body in a half space the inhomogeneity simulates a fracture zone in a geothermal system the algorithm is applicable for the direct current case when the buried electrode is either inside mise a la masse or outside near miss the body in implementing the scheme the integral equation is transformed into a matrix equation as a result of discretizing the inhomogeneity into rectangular cells all physical properties are assumed constant within each cell the rectangular cells are used through out execution of the algorithm the computed surface and subsurface apparent resistivity responses are examined for bodies of similar shape and size but with different orientations 1 vertical 2 horizontal 3 dipping at 60 degrees and 4 dipping at 30 degrees the four bodies produce apparent resistivity cr

soil characteristics on varying lithological substrates in the south shetland islands maritime antarctica abstract soils in ice free areas of livingston island have been forming since the last deglaciation in a maritime warmer climate that is more humid than in interior antarctica a soil survey was carried out on byers and hurd peninsulas to characterize the soils and investigate the processes soils were sampled on two different substrates mudstones and volcanic rocks in byers peninsula and greywackes in hurd peninsula sampling sites were located on raised beaches platforms and on volcanic outcrops across an altitudinal range from few m to 150 m a s l the ph electrical conductivity carbonates and organic matter showed similar patterns in each geomorphic unit but their values differed between bedrocks the sand silt and clay contents differed greatly in the soils on mudstones and volcanic rocks the elemental composition was closely related to mineralogy of parent materials mg k zn mn and fe were similar in the soils on all bedrocks the al content was highest on the volcanic rocks the largest difference in the ca content was between the soils on greywackes and mudstones the na pb and ba contents in soils on the mudstones differed greatly from the rest of bedrocks geochemical and mineralogical characteristics suggest that the main process involved in soil development was the mechanical disintegration of bedrock although there was also some leaching cryogenic processes play a key role in soil development but chemical weathering processes were also involved in soil evolution although limited in extent due to the restriction of water circulation to summer

termination of the arabian shelf sea stacked cyclic sedimentary patterns and timing oligocene miocene oman abstract in the janabah region of southeastern oman oligocene miocene limestones of the shuwayr warak and ghubbarrah formations are widely exposed they were deposited on an extensive shallow carbonate platform that was part of the arabian shelf and located on the gulf of aden s northeastern rift shoulder which emerged during the early miocene the uppermost part of the studied sedimentary succession developed immediately before the permanently subaerial exposure of the carbonate platform cyclic changes of intertidal and subtidal facies document a fluctuating relative sea level at different frequencies and a continuous decline of accommodation single erosive surfaces with palaeokarst cavities and caliche crusts separate larger depositional cycles these disconformities imply relatively long episodes of subaerial exposure and are interpreted to have been formed during lowstands of third order sea level cycles that denuded the platform taxonomic studies of the accompanying mollusc faunas and certain benthic foraminifers allow a correlation of the recognised subaerial disconformities with the ru4 ch1 to ch4 aq1 sequence boundaries of hardenbol et al hardenbol j thierry j farley m b jacquin t graciansky p c vail p r 1998 mesozoic and cenozoic sequence chronostratigraphic framework of european basins in graciansky c p hardenbol j jacquin t vail p r eds mesozoic and cenozoic sequence stratigraphy of european basins society for sedimentary geology special publication 60 13 603 this demonstrates that the termination of the arabian shelf sea must be back dated from the middle burdigalian to the early aquitanian

tectonic and climatic controls on rift escarpments erosion and flexural rebound of the dhofar passive margin gulf of aden oman we investigate the respective roles of climatic parameters and the flexural rigidity of the lithosphere in the erosion history and behavior of two adjacent rift escarpments along the northern coast of the gulf of aden in oman at this 25 myr old passive margin we define a type 1 scarp which is high sharp crested and has retreated 25 30 km inland from its master fault and a type 2 scarp which exhibits a more rounded profile lower relief and still coincides with its mapped normal fault trace since about 15 ma the margin has been seasonally affected by monsoon precipitation but with contrasting effects at the type 1 and type 2 escarpments depending on the position of the intertropical convergence zone in the geologic past during peak monsoon conditions both scarps experienced heavy rainfall and runoff whereas during monsoon starved conditions such as today the type 2 scarp experienced a foggy moist climate while the type 1 scarp remained much drier in order to assess the relative effects of climate and flexural parameters on the present day morphology of the dhofar margin we present onedimensional numerical models of erosion and flexure along two profiles representative of the type 1 and type 2 scarps unlike most surface process models previously published where present day topography is the only criterion by which to evaluate the quality of model outputs model behavior here is additionally constrained by independent estimates of denudation provided by geological cross sections well defined fault traces and other stratigraphic markers the best fitting models indicate that the type 1 escarpment formed under relatively arid climatic conditions and was affected by significant erosion recession and flexural uplift due to a low 7 km effective elastic thickness in contrast the morphology of the type 2 fault scarp was smoothed by a more humid climate but a high effective elastic thickness 15 km prevented it from uplifting or receding in addition we show that the sedimentary load acting at the foot of the escarpments exerts significant influence on their morphological evolution though this parameter is often neglected in other scarp evolution models

structural influence on hydrocarbon entrapment in the northwestern red sea egypt the northwestern part of the egyptian red sea has attracted the attention of many geologists because it lies at the triple junction of the main rifts between the red sea the gulf of aqaba and gulf of suez the geometry of the fault system in this area of the basin clearly indicates an extensional setting the area has a southwestward regional dip and it has experienced more extension than the rest of the gulf of suez six tectonic stages different in their stress history sedimentary fill and depositional setting are recognized for the northwestern red sea 1 cambrian to early cretaceous stage 2 late cretaceous to oligocene stage 3 early miocene stage 4 early to middle miocene stage 5 middle to late miocene stage and 6 post miocene stage magnetic gravity seismic surface and subsurface data from the northwestern red sea delineate several elongate structural highs separated by elongate troughs both highs and troughs have the same northwest southeast direction as the clysmic trend some of the highs are dissected by cross elements that trend northeast southwest and east northeast west southwest and laterally offset these highs the tectonic framework of the northwestern sector of the red sea had a significant influence on hydrocarbon generation migration and accumulation the evidence for this influence includes the following 1 the troughs form the main source kitchens where the precambrian basement rocks are present at depths exceeding 4877 m e g gemsa trough 2 the magnitude of throw on the clysmic fault is critical in the entrapment mechanisms 3 the cross elements play a major role in the hydrocarbon migration and accumulation because they truncate the extension of most of the oil fields and form their bounding faults and 4 all the discovered oil fields in the study area are structural and or combination traps and show good matches to the magnetic anomalies

mixed carbonate siliciclastic sedimentation on a miocene fault block gulf of suez egypt abstract the miocene sediments at abu shaar red sea egypt include both carbonates and detrital silicates whose distribution is closely related to a pre miocene basement fault block as such they demonstrate the relationships between local distensional tectonics and sedimentation because of the vertical bathymetric relief of the basement block the carbonates include both a spectacular talus and massive platform facies in spite of the presence of scattered corals sedimentation is not truly reefal as deposits consist essentially of muddy bioclastic and silicate sands the most typical ecological elements are numerous spectacular stromatolites rigidity of the sedimentary mass was mainly induced by early marine cementation carbonate siliciclastic transitions on and around the abu shaar platform were determined mainly by basement morphology and related pre miocene tectonics

the red sea 50 years of geological and geophysical research abstract the red sea displays lithosphere that is in transition from rifting to drifting and therefore provides enormous opportunity to understand incipient seafloor spreading development despite more than 50 years of extensive geological and geophysical research there remains significant conjecture concerning the timing of sea floor spreading initiation the extent of spreading along the axis of the red sea and the geodynamic processes responsible for the onset of crustal extension red sea tectonic models based on geological data are dominated by single stage rift models involving protracted stretching of continental crust followed by sea floor spreading at ca 5 ma and include both asymmetric and symmetrical extension models geophysical data suggest that an oceanic crust occurs beneath the red sea shelves although exactly how this crust has evolved remains undecided equally arguments have been made in favor of the sedimentary shelves forming on extended continental crust and that oceanic crust is restricted to the present day red sea axial valley however our synthesis shows that none of the models proposed so far is applicable to the entire red sea basin the distribution and timing of the red sea crustal extension and sea floor spreading may have been influenced by subduction related far field forces due to complex plate interactions along the northern edge of the arabian plate during early miocene our reassessment suggests that following the arrival of the afar plume the southern red sea underwent rapid extension culminating in the transition from rifting to seafloor spreading which stalled during the early miocene in the central red sea due to either slowing of the convergence between arabia and eurasia or the onset of the arabian plate passive margin collision with eurasia off axis magmatism is focused on the arabian margin and afar depression and has been episodically active since the miocene we consider a formation of the red sea as a hybrid process involving both active and passive rifting

geomorphology and soils distribution under paraglacial conditions in an ice free area of admiralty bay king george island antarctica abstract the main pedological geomorphological and cryogenic features of keller peninsula part of admiralty bay king george island maritime antarctica were mapped and quantified with emphasis on the relationship between the ice retreat process melt out landform development and soil distribution moraines protalus scree slopes inactive glacial cirques uplift marine terraces biogenic landforms aretes and felsenmeer were mapped scree slope is the main landform covering approximately 25 of the peninsula indicating prominent paraglacial features inherited glacial landforms such as lateral moraines highland plateau and exhumed u shaped valleys are now being exposed in north keller by ice shrinkage of former ice protecting cover landforms influenced soil formation and stability cryosols and leptosols wrb roughly corresponding to gelisols and entisols sss respectively are the most common soil classes with an overall tendency of absent permafrost in the coastal areas changing to sporadic permafrost at mid slope and discontinuous permafrost with greater altitude and substrate stability

punctiform initiation of seafloor spreading in the red sea during transition from a continental to an oceanic rift transition from a continental to an oceanic rift occurs in the red sea by initial emplacement of oceanic crust in regularly spaced hot points which serve as nuclei for axial propagation into segments of oceanic crust accretion and for initiation of seafloor spreading hot points develop above upwelling mantle diapirs caused by density viscosity inversion beneath continental and oceanic rifts

miocene biofacies development and geological history of the gulf of suez egypt abstract miocene sediments of the gulf of suez were deposited in a rapidly subsiding rift which became active during late oligocene and early pliocene tectonic episodes complex lithological variations associated with the variable submarine structure commonly preclude detailed correlation of drilled subsurface successions thick middle to late miocene evaporites and their geometry obscure seismic interpretation of prospective sub evaporitic early and middle miocene sequences the thick miocene sediments are believed to have significantly furnished the source as well as the reservoirs and seals for the oil discovered in the gulf of suez over four billion barrels of oil have been established as recoverable reserves in miocene reservoirs a new gulf wide detailed correlation scheme is established based upon a series of newly defined microfaunal assemblage zones these zones are based on holistic analysis of microfauna utilizing planktonic and benthonic foraminifera diatoms otoliths and pteropods dating by planktonic foraminifera and calcareous nannofossils indicates the zones are isochronous vertical and lateral variations in assemblage characteristics have been identified mapped and are concluded to be the results of vertical and lateral differences in salinity water depth water clarity lithological variation and oxygen availability the correlative applicability of this scheme within the gulf of suez provides potential for it to serve as a reference for drilled sequences in the red sea where thicker evaporite sequences make sub evaporitic stratigraphic correlation even more difficult owing to the greater seismic attenuation isochronous palaeoenvironmental variations are detected utilizing the zonation scheme both within the gulf of suez and the red sea region which will assist in the correlation and mapping of stratigraphic sequences of hydrocabon potential detection confirmation and the degree of displacement of faults is also improved by the application of the scheme

thermochronological data from sudan in the frame of the denudational history of the nubian red sea margin low temperature thermochronology provides information on the timing of rifting and denudation of passive margins and the red sea with its well exposed young rift margins is a suitable setting for its application here we present new apatite fission track aft data from sudan northern hinterland and red sea coastal areas from the former region we obtained ages between 270 2 ma ad 253 53 ma and from the coastal belt between 83 8 ma and 39 7 ma these data prompted a review and comparison with low temperature thermochronological data from the whole nubian red sea margin and a discussion on their implication in assessing the margin evolutionary style aft data are available for egypt and eritrea as well as apatite u th he ahe ages for two transects transversal to the margin in eritrea both in egypt and eritrea aft data record a cooling event at about 20 25 ma early miocene and an earlier more local cooling event in egypt at about 34 ma early oligocene the thermal modeling of the sudan samples provides an indication of a rapid cooling in miocene times but does not support nor rules out an early oligocene cooling phase the re assessment of new and existing thermochronological data within the known geological framework of the nubian and conjugate arabian margins favours the hypothesis that early rifting stages were affecting the whole gulf of suez red sea gulf of aden system since the oligocene these precocious more attenuated phases were followed by major extension in miocene times as to the mode of margin evolution aft age patterns both in egypt and eritrea are incompatible with a downwarp model the distribution of ahe ages across the eritrean coastal plain suggests that there the escarpment was evolving predominantly by plateau degradation copyright 2009 john wiley sons ltd

cenozoic epeirogeny of the arabian peninsula from drainage modeling it is generally accepted that the arabian peninsula has been uplifted by subcrustal processes positive residual depth anomalies from oceanic crust in the red sea and in the gulf of aden suggest that a region surrounding this peninsula is dynamically supported admittance calculations surface wave tomography studies and receiver function analyses all imply that regional topography is generated and maintained by some combination of mantle convective circulation and lithospheric thickness changes despite these significant advances the spatial and temporal uplift rate history of the arabian peninsula is not well known here we show that a regional uplift rate history can be obtained by jointly inverting 225 longitudinal river profiles that drain this peninsula our strategy assumes that shapes of individual river profiles are controlled by uplift rate history and moderated by erosional processes we used local measurements of incision rate to calibrate the relevant erosional parameters in our inverse algorithm uplift rate is permitted to vary smoothly as a function of space and time but upstream drainage area remains invariant we also assume that knickzone migration is not lithologically controlled implications of these important assumptions have been investigated our results suggest that the arabian peninsula underwent two phases of asymmetric uplift during the last 20 30 ma at rates of 0 05 0 1 mm a 1 the southwestern flank of the peninsula has been uplifted by 1 5 2 5 km regional stratigraphic constraints the age and composition of volcanism paleosol formation incised peneplains emergent marine terraces and thermochronometric measurements corroborate our calculated patterns of uplift progressive development of three domal swells along the western margin of the peninsula is consistent with localized upwelling of hot asthenospheric mantle

effects of geologic noise on cross borehole electrical surveys using a finite element algorithm which allows for subsurface current and potential electrodes in dc resistivity we have analyzed the detection of a thin 2 d conductive inhomogeneity in the presence of several sources of geologic noise the pole pole array with the current electrode fixed in one borehole and the potential electrode movable in adjacent boreholes is the main array of concern the sources of noise are surface topography buried topography random geologic noise quasi random geologic noise nontarget inhomogeneities layering and a vertical contact for several positions of a downhole source electrode normalized apparent resistivities have been computed these resistivities have been contoured in section view as appropriate to cross borehole investigations for the models studied here surface topography buried topography random and quasi random geologic noise do not obscure the anomaly due to the thin conductive inhomogeneity in a vertically or horizontally layered earth the anomal

the memory of the accreting plate boundary and the continuity of fracture zones a detailed aeromagnetic anomaly map of the mesozoic seafloor spreading lineations southwest of bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation the magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone flowline trends the linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones during the period of mesozoic geomagnetic polarity reversals when 1200 km of central north atlantic seafloor formed the atlantic accreting boundary consisted of stationary elongate spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells normal oceanic crustal structure was formed in the spreading center cells but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries which parallel flowlines of mesozoic relative plate motion in the central north atlantic we suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest here each spreading center cell independently goes through its cylce of stress buildup stress release and crustal accretion after which its memory is refreshed the temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists

propagation of an oblique spreading centre the western gulf of aden abstract the east west trending gulf of aden rift between arabia and somalia is spreading along a n35 e direction it propagates westward inside the african continent toward the afar triple junction oblique to the spreading direction these combined propagation and oblique rifting generate a special faulting pattern associated with an irregular distribution of volcanism we analyse this pattern using a structural map derived from the analysis of multibeam bathymetric data and backscattering images from the 1995 tadjouraden cruise in the western gulf of aden the rift valley is bounded by faults trending from n110 e to n130 e in the eastern part of the surveyed area where the valley trends n90 e and by large faults in the central western part where the valley is oriented n70 e the direction of extension deduced from the fault histogram is n37 e 10 over the whole area whatever the rift trend therefore this bend of the plate boundary does not result from a change in the stretching direction this bending increases the rifting obliquity westward and therefore modifies the faulting pattern faults display two distinct patterns either en echelon on the walls indicating a right lateral component of shear or purely normal and parallel to the rift trend inside the valley volcanoes are mostly located in the eastern part of the rift where they are aligned into swarms trending roughly perpendicular to the extension direction deduced from fault directions backscattering images show that recent lava flows fill the axial valley of the eastern part whereas in the central and western parts volcanoes are scattered in the middle part of the valley this distribution is interpreted as a westward propagation of the volcanism and as the direction of deformation migration the thickness of the upper brittle layer of the lithosphere is estimated from the width if the inner graben is inside the rift it indicates a deepening of the brittle ductile transition from east 6 km to west 16 km these values are typical of oceanic and continental systems respectively and consistent with the depth of earthquakes in the western part this points out the existence of a transitional area between 44 e and 45 20 e characterised by the lack of recent volcanism the nature of this mechanical transition is discussed in terms of propagation of an spreading centre into a continental or transitional afar type lithosphere

chemical and biological properties formation occurrence and classification of spodic cryosols in a terrestrial ecosystem of east antarctica wilkes land abstract soils with the morphological features of podzols are widespread in coastal east antarctica neither the world reference base of soil resources wrb nor soil taxonomy st adequately classify these soils so we recommend the addition of spodic cryosol at the second hierarchical level in wrb and spodorthel at the great group level of st we divide the spodic cryosols into ornithogenic and non ornithogenic spodic cryosols based on inorganic chemistry soil organic matter composition and microbiology the occurrence of spodic cryosols depends on parent materials soil texture surface pattern solar energy input and vegetation patterns antarctic spodic cryosols form on solid rocks outwash sediments and abandoned penguin rookeries and represent 20 of the soil landscape of wilkes land with one third ornithogenic and two thirds non ornithogenic ornithogenic spodic cryosols are related to tectonically uplifted abandoned penguin rookeries all spodic cryosols contain large amounts of carbon and nitrogen during pedogenesis iron complexed with organic matter in the topsoils has been altered to oxides and crystalline iron minerals but aluminium remains almost entirely 100 organically bound throughout the profile meltwater and a moss cover are essential for podzolization on the non ornithogenic sites and at the ornithogenic sites the chemically aggressive solution from dissolved guano has bleached the uppermost ae horizon and translocated iron and aluminium into the subsoil the low salinity suggests that downward leaching exceeds upward transport by evaporation intensity of podzolization is determined by microclimate soil microbial effects and parent materials the permafrost table may cause precipitation of organic matter in the subsoil mg and p levels are high the p level is very high in the ornithogenic soils k p ph base saturation bs and bacterial colonization reflect the influence of guano in the ae horizon of ornithogenic soils colonization by lichens and or mosses depresses bacterial activity the mean c n ratio of approximately 11 and the abundance of carboxylic carbon compounds probably derive from organic acids and oxidized carbohydrates migration of organic acids non humified carbohydrates and n containing compounds is suggested as the main mechanism of podzolization

subsidence history crustal structure and evolution of the somaliland yemen conjugate margin 1 we have used biostratigraphic data from deep exploration wells to determine the tectonic subsidence history of the somaliland northwestern somalia yemen conjugate margin a poorly known margin in the central part of the gulf of aden bathymetry and magnetic anomaly data suggest the gulf of aden is a young feature that formed following the rifting apart and breakup of the african and arabian plates 32 ma our tectonic subsidence data suggest however that the present day gulf of aden developed on an earlier mesozoic rift system the oldest episode of rifting initiated at 156 ma and lasted for 10 ma and had a nw se trend we interpret the rift as a late stage event associated with the breakup of gondwana and the separation of africa and madagascar at 80 ma there is evidence of an intermediate rift event which correlates with a rapid increase in spreading rate on the ridges separating the african and indian and african and antarctica plates and a contemporaneous slowing down of africa s plate motion the combined effect of all three rifting events has been to thin the crust and upper mantle by up to a factor of 2 the amount of thinning deduced from the wells is in accord with the crustal structure inferred from available seismic refraction data and process oriented gravity and flexure modeling the margin is asymmetric with a steeper gradient in the moho on the yemen side than the somaliland side the main discrepancy is on the yemen side where the gravity derived moho is 10 km deeper than the well derived moho we attribute the discrepancy to the addition of material at the base of the crust since rifting possibly magma sourced from the afar plume

upwarp of anomalous asthenosphere beneath the rio grande rift continental rifts are possible analogues of mid ocean ridges although major plate tectonic features are less clearly observed1 current thermal models of mid ocean ridges2 4 consist of solid lithospheric plates overlying the hotter less viscous asthenosphere with plate thickness increasing away from the ridge axis the lithospheric lower boundary lies at or near the melting point isotherm so that at greater depths higher temperatures account for lower viscosity lower seismic velocities and possibly partial melting upwarp of this boundary at the ridge axis concentrates heat there thus lowering densities by expansion and raising the sea floor to the level of thermal isostatic equilibrium at slow spreading ridges a major central graben forms owing to the mechanics of magma injection into the crust5 topography heat flow gravity and seismic studies support these models on the continents a low velocity channel has been observed although it is poorly developed beneath ancient cratons6 9 plate tectonic models have been applied to continental basins and margins10 12 but further similarities to the oceanic models remain elusive topographic uplift is often ascribed to airy type isostatic compensation caused by crustal thickening rather than thermal compensation in the asthenosphere here we discuss the rio grande rift in southwestern united states teleseismic p wave residuals show that regional uplift is explained by asthenosphere uplift rather than crustal thickening

genesis properties and classification of cryosols from admiralty bay maritime antarctica abstract soils from maritime antarctica are different than those from other antarctic climatic zones and are among the least studied soils in antarctica we here present analytical and morphological data for the main soils found in admiralty bay king george island in the south shetland group fifty six 56 pedons were described sampled and analyzed according to standard international methods and classified according to both u s soil taxonomy and the wrb the soils are generally weakly developed and have chemical physical and morphological characteristics strongly influenced by the parent material we identify four main soil groups i basaltic andesitic ii acid sulfate iii weakly ornithogenic and iv ornithogenic soils small adjustments are proposed for both classification systems for allowing a more adequate classification for maritime antarctica greyish colored eutric lithic and typic haploturbels turbi leptic cryosols with neutral to alkaline reaction are the most common soils in admiralty bay and are usually developed from basaltic andesitic tills yellowish acid dystric sulphuric haploturbels turbi thionic cryosols are restricted to areas affected by pyritized andesites umbriturbels turbi umbric cryosols only occur at sites under bird influence ornithogenic soils are the most developed soils in admiralty bay and show pronounced chemical physical and morphological alteration of parent materials with formation of umbric epipedons and phosphatic b horizons fibristels organo turbic cryosols are restricted to hydromorphic areas under thick moss carpets

genesis mineralogy and ecological significance of ornithogenic soils from a semi desert polar landscape at hope bay antarctic peninsula abstract large amounts of organic matter of marine origin are seasonally deposited on ice free soils by birds and mammals especially penguins in some restricted antarctic terrestrial ecosystems the incorporation of this material into the mineral soil matrix becomes the main pathway for the widespread formation of phosphate minerals and ornithogenic soils enhancing local biodiversity and complexity these soils have been well studied in maritime antarctica and comparatively neglected in the cold dry polar antarctica hence we studied the influence of penguins on soil and landscape formation at hope bay a transition climatic zone in antarctic peninsula where no previous pedological study has been undertaken with regards to soil phosphatization ten pedons nine of which are ornithogenic were described sampled and analyzed for physical chemical and mineralogical properties results indicate that the slow mineralization of bird guano coupled with high levels of organic matter and reduced leaching notably ca contribute to reduced soil acidification and low exchangeable al3 content unlike ornithogenic soils from maritime antarctica phosphate minerals typical of the phosphatization process such as taranakite minyulite leucophosphite struvite and fluorapatite were detected in hope bay they are subjected to very slow rates of dissolution neoformation of secondary phosphatic minerals and mineralogical transformation and leaching processes ornithogenesis is viewed as a mechanism of sea land nutrient transfer enriching the latter since the parent rocks are chemically poor sedimentary materials on the other hand the presence of kaolinite indicates wetter climate conditions in the past with periods with greater moisture and weathering compared with the current semi polar desert environment we conclude that the phosphatization process overprints previously weathered sedimentary materials

2 d and 3 d resistivity image reconstruction using crosshole data theoretical changes in the distribution of electrical potential near subsurface resistivity anomalies have been studied using two resistivity models the results suggest that the greatest response from such anomalies can be observed with buried electrodes and that the resistivity model of a volume between boreholes can be accurately reconstructed by using crosshole data the distributive properties of crosshole electrical potential data obtained by the pole pole array method have also been examined using the calculated partial derivative of the observed apparent resistivity with respect to a small cell within a given volume the results show that for optimum two dimensional 2 d and three dimensional 3 d target imaging in line data and crossline data should be combined and an area outside the zone of exploration should be included in the analysis in this paper the 2 d and 3 d resistivity images presented are reconstructed from crosshole data by the combination of two inversion algorithms the first algorithm uses the alpha center method for forward modeling and reconstructs a resistivity model by a nonlinear least squares inversion alpha centers ex press a continuously varying resistivity model and the distribution of the electrical potential from the model can be calculated quickly an initial general model is determined by the resistivity backprojection technique rbpt prior to the first inversion step the second process uses finite elements and a linear inversion algorithm to improve the resolution of the resistivity model created by the first step simple 2 d and 3 d numerical models are discussed to illustrate the inversion method used in processing data from several field studies are also presented to demonstrate the capabilities of using crosshole resistivity exploration techniques the numerical experiments show that by using the combined reconstruction algorithm thin conductive layers can be imaged with good resolution for 2 d and 3 d cases the integration of finite element computations is shown to improve the image obtained by the alpha center inversion process for 3 d applications the first field test uses horizontal galleries to evaluate complex 2 d features of a zinc mine the second field test illustrates the use of three boreholes at a dam site to investigate base rock features and define the distribution of an altered zone in three dimensions

diversity of neogene seismites of the northwest red sea egypt a characteristic sedimentary expression of rifting abstract the syn rift neogene sediments outcropping along the northwest red sea coast exhibit a series of early remobilization structures formed close to the paleo sediment water interface developed both in inclined and in subhorizontal sedimentary substrates their origins cannot be explained only by simple gravity movement they are formed in a variety of sediments including floodplain mudstones shallow marine sandstones and micritic limestones as well as in nonlithified conglomerates although the type of sediment undoubtedly determines in part their preservation they are not limited to any particular sedimentary environment among the twelve types of deformational structures described the most frequent are various types of breccia which generally have a clearly defined base brecciation is most intense at the base and often tends to grade up into undisturbed beds there being no sharp break at the top these breccias have not been transported and adjacent elements may be fitted together giving asymmetric brecciated folds nonbrecciated fold structures are also common especially in continental floodplain mudstones which have been deposited on subhorizontal surfaces other remobilisation structures include load casts and other hydroplastic phenomena affecting individual beds certain sandstone horizons have been completely homogenized due to fluidification finally sedimentary dykes and other fracture fillings up to 10 m in height are also frequent certain structures especially the cm scale reverse faults exhibit both compressional and dilatory movements their stratiform disposition and independance with respect to paleoslopes suggest that these synsedimentary remobilization structures express periodic instability which is best explained by repeated earthquake shocks their similitude with respect to structures resulting from recent earthquakes at el asnam algeria and near the san andreas fault california tend to confirm this interpretation although they have not yet been related to particular structural elements their frequence indicates that these seismite structures are a characteristic expression of red sea rifting

magnetostratigraphy and sedimentary evolution of the late miocene to early pleistocene sediments quseir region egyptian red sea an integrated sedimentological and magnetostratigraphic study has allowed a detailed understanding of the late miocene to early pleistocene evolution of the sediments in the quseir region of the egyptian red sea coast palaeomagnetic samples were collected from sections in six wadis covering the shagara formation the gabir and samh members of the wardan formation and the abu dabbab formation evaporites remanence properties are carried by magnetite haematite and goethite the characteristic remanence is typically carried by detrital magnetite and haematite with more recent overprints predominantly associated with haematite and goethite produced by the weathering of diagenetic pyrite the magnetostratigraphy has allowed the following detailed age assignments for the lithostratigraphic units the shagara formation ranges in age from late pliocene late piacenzian to middle pleistocene 0 6 2 5 ma the gabir member is latest messinian to earliest piacenzian in age 3 5 5 5 ma and the samh member late tortonian to mid messinian 6 0 7 5 ma the age of the top of the abu dabbab formation is probably mid tortonian 8 ma disconformities occur between all the lithostratigraphic units with a local angular unconformity between the shagara and wardan formations lowstands in global sea level appear to have a strong influence on the timing of these disconformities characteristic mixed alluvial and reef facies of the shagara formation are a response to the ephemeral wetter climate following the initiation of northern hemisphere glaciation at 2 4 ma enhanced by rift margin uplift of basement complexes to the west this tectonic activity was concentrated in the early piacenzian the marine gabir member was deposited during the early pliocene and latest messinian high sea level stands the late tortonian early messinian age and sedimentological character of the samh member indicates this unit was affected by marine flooding events which ultimately produced during drawdown phases the off shore zeit formation evaporites in half graben depocentres

analysis of geological hiatus surfaces across africa in the cenozoic and implications for the timescales of convectively maintained topography geological maps are a powerful but underutilized tool for constraining geodynamic processes and models unraveling the cenozoic elevation history of africa and distinguishing between competing upli

physical characterization of rock masses using borehole methods a convenient view of geophysical phenomena is based on the categorization of techniques according to the governing partial differential equation the laplace diffusion or wave equation each category is examined nonmathematically in the context of those borehole methods used to investigate large volumes of a rock mass away from the borehole in which the instrumentation is deployed the published literature reflects the variety of applications some of which occur more frequently than others our discussion of the physics focuses on electric potential electromagnetic induction and seismic methods respectively as representatives of each category choice of a single technique for characterization of rock masses involves a compromise which depends upon the geophysical problem and the type of information desired the complementary nature of the information means however that integrated studies using two or more types of methods may be worthwhile

mantle flow at oceanic spreading centers models derived from ophiolites abstract the internal structures of eleven ophiolite complexes have been structurally investigated the attitude of the foliation and the lineation in the peridotites is related to asthenospheric flow in the uppermost mantle at an oceanic spreading center from the orientation of these structures with respect to the paleohorizontal plane two oceanic situations are distinguished the horizontally spreading asthenosphere illustrated by the table mountain massif newfoundland is characterized by 1 a homogeneous regular and sub horizontal plastic flow pattern over large distances 2 b flat paleo mono and absence of large scale folding 3 c rare economical chromite deposits and depleted mantle the diapirically spreading asthenosphere illustrated by the acoje type philippines displays 1 a inhomogeneous and sub vertical plastic flow pattern 2 b deformed paleo moho and large scale folds 3 c important economical chromite deposits and less depleted mantle presumably due to secondary magmatic impregnation this leads to vogt s model of asthenospheric flow beneath an oceanic ridge the direction of which being equated with the dike swarm in this model the horizontally spreading asthenosphere is derived from the diapirically spreading one in the following way the vertical mantle flow at the diapir center diverges horizontally in every direction including that parallel to the ridge at about 50 km from the diapir center the mantle flow becomes progressively perpendicular to the ridge due to the damming effect of transform faults an important economical consequence of this model is that large chromite deposits are expected preferentially in the diapiric type of ophiolite because chromite is preferentially precipitated at the diapir center from pristine magma