evolution of the eastern volcanic ridge of the canary islands based on new kar data the results of 64 new k ar age determinations together with 32 previously published ages show that after a period of erosion of the basal complex miocene volcanic activity started around 20 ma in fuerteventura and 15 ma in lanzarote forming a tabular succession of basaltic lavas and pyroclastics with a few salic dykes and plugs this series includes five separate volcanic edifices each one with its own eruptive history in fuerteventura several miocene eruptive cycles have been identified in the central edifice one around 20 17 ma followed by two others centred around 15 and 13 ma in the southern edifice the maximum of activity took place around 16 14 ma whereas in the northern one the main activity occurred between 14 and 12 ma in lanzarote a first cycle of activity took place in the southern edifice between 15 5 and 14 1 ma followed by another between 13 6 and 12 3 ma in the northern edifice three pulses occurred 10 2 8 3 6 6 5 3 and 3 9 3 8 ma an important temporal gap greater in fuerteventura than in lanzarote separates series i from the plio quaternary series ii iii and iv formed by multi vent basaltic emissions in fuerteventura the following eruptive cycles have been identified 5 2 9 2 4 1 8 1 7 0 8 0 4 and 0 1 ma in lanzarote the activity was fairly continuous from 2 7 ma to historic times with a maximum in the lower pleistocene eruptive rates in the series i edifices were on the average 0 1 0 01 km3 ka comparable but slightly smaller than in similar edifices in tenerife and la gomera but much lower than in gran canaria average post miocene eruptive rates were about 0 013 0 027 km3 ka in lanzarote and 0 003 0 007 km3 ka in fuerteventura all these volcanic edifices show a similar general sequence fissural eruptions erosion multi vent volcanism repeated at different periods in different parts of the eastern islands of the canaries the model of growth of the series i edifices is comparable to those in tenerife and la gomera long periods of activity sometimes greater than 6 m y with pulses separated by gaps however salic and intermediate differentiates frequent in tenerife and la gomera are very scarce in these islands the fuerteventura lanzarote ridge shows a decrease in volcanic activity with time and also a certain ssw nne polarity in the temporal development of volcanism

pedogenic carbonates and carbon pools in gypsiferous soils of a semiarid mediterranean environment in south italy abstract soil carbonates are key features in soils of arid and semiarid environment playing an important role from pedogenetic landscape history paleoclimatic and environmental points of view the objectives of this work were i to study pathways of pedogenic carbonate pc formation ii to distinguish between lithogenic and pedogenic inorganic c by using the natural c isotope abundance and iii to estimate the soil c pools in a gypsiferous semiarid mediterranean environment sicily italy five soil pedons developed on calcareous and non calcareous parent materials from holocene 10 000 years bp to upper tortonian 7 2 5 3 ma bp in age were surveyed during field soil description the highest stage of carbonate morphology was found in soils developed on non calcareous holocene colluvial deposits youngest deposits in age which also showed the highest amount of pc the great amount of pc in soils developed on youngest deposits was ascribed to a soil landscape relationships being located in a doline overhung by gypsum outcrops precipitation of ca 2 from gypsum dissolved by rainfall and biogenic co 2 is reliable the significant positive relationship between soil organic c and pedogenic carbonates 13 c values confirms that pc was formed from biogenic co 2 organic c pool in the first cubic meter of soil ranged from 17 to 42 kg whilst pedogenic inorganic c pool from 2 8 to 30 7 kg the estimated rate of inorganic c accumulation in soils developed on youngest deposits was 2 5 g m 3 y 1 whereas the rate was negligible on older parent material the hypothesized pathways of pc formation were ex novo precipitation of gypsum ca 2 and biogenic co 2 and dissolution of lithogenic caco 3 and re precipitation of ca 2 with biogenic co 2 from an environmental prospective investigated soils may act as a sink of c when ca 2 from gypsum is available for the formation of pedogenic carbonates

the tectonically controlled emplacement of a vertically sheeted gabbro pyroxenite intrusion feeder zone of an ocean island volcano fuerteventura canary islands abstract the miocene px1 gabbro pyroxenite pluton fuerteventura canary islands is a 3 5 5 5 km shallow level intrusion 0 15 0 2 gpa and 1100 1120 c interpreted as the feeder zone to an ocean island volcano it displays a vertical magmatic banding expressed in five 50 to 100 metre wide nne ssw trending alkaline gabbro sequences alternating with pyroxenites this emplacement geometry was controlled by brittle to ductile shear zones generated by a regional e w extensional tectonic setting that affected fuerteventura during the miocene at a smaller scale the px1 gabbro and pyroxenite bands consist of metre thick differentiation units which suggest emplacement by periodic injection of magma pulses as vertical dykes that amalgamated similarly to a sub volcanic sheeted dyke complex individual dykes underwent internal differentiation following a solidification front parallel to the dyke edges this solidification front may have been favoured by a significant lateral horizontal thermal gradient expressed by the vertical banding in the gabbros the fractionation asymmetry within individual dykes and the migmatisation of the wall rocks pyroxenitic layers result from the fractionation and accumulation of clinopyroxene olivine plagioclase crystals from a mildly alkaline basaltic liquid they are interpreted as truncated differentiation sequences from which residual melts were extracted at various stages of their chemical evolution by subsequent dyke intrusions either next to or within the crystallising unit compaction and squeezing of the crystal mush is ascribed to the incoming and inflating magma pulses the expelled interstitial liquid was likely collected and erupted along with the magma flowing through the newly injected dykes clinopyroxene mineral orientation as evidenced by ebsd and micro x ray tomography investigations displays a marked pure shear component supporting the interpretation of the role of compaction in the generation of the pyroxenites conversely gabbro sequences underwent minor melt extraction and are believed to represent crystallised coalesced magma batches emplaced at lower rates at the end of eruptive cycles clinopyroxene orientations in gabbros record a simple shear component suggesting syn magmatic deformation parallel to observed nne ssw trending shear zones induced by the regional tensional stress field this emplacement model implies a crystallisation time of 1 to 5 years for individual dykes consistent with px1 emplacement over less than 0 5 my a minimum amount of approximately 150 km3 of magma is needed to generate the pluton part of it having been erupted through the central volcanic centre of fuerteventura if the regional extensional tectonic regime controls the px1 feeder zone initiation and overall geometry rates and volumes of magma depend on other source related factors high injection rates are likely to induce intrusion growth rates larger than could be accommodated by the regional extension in this case dyke intrusion by propagation of a weak tip combined with the inability of magma to circulate through previously emplaced and crystallised dykes could result in an increase of non lithostatic pressure on previously emplaced mushy dyke walls thus generating strong pure shear compaction within the pluton feeder zone and interstitial melt expulsion these compaction dominated processes are recorded by the cumulitic pyroxenite bands

structural control of young basaltic fissure eruptions in the plateau basalt area of the arabian plate northeastern jordan abstract in the el hamadieh region part of the large intracontinental basalt plateau extending nw se from syria to jordan and saudi arabia in the northern part of the arabian plate young volcanic edifices formed by fissure eruptions have been investigated all geometrical dimensions and parameters of the ridges have been measured the field relationships documented the data evaluated and the structural patterns analysed using the attitude of the fissures as palaeostress indicators a stress strain model is introduced showing the superimposition of stress fields magma was extruded at the intersection of two regional structures the ne sw striking amman hallabat fold belt and the nw se trending azraq sirhan graben a first effusion with a relatively fluid magma took place in reactivated fold related conjugated hk 0 shear fractures with a compressional 1 stress trending nw se a second phase of fissure effusions produced a more viscous lava these fissures represent a hk 0 planes with an acute angle about the fold axis and b planes of normal faults along the southwestern shoulder of the azraq sirhan graben in the position of fold related reactivated conjugated 0 hl shear planes the geometrical relationships between the amman hallabat folds the azraq sirham faults and the volcanic fissures indicate a rotation of the local stress field with the originally nw se trending compressional 1 and the ne sw trending 3 stress changing into a ne sw acting tensional 1 stress

a dynamic interpretation of tristan da cunha volcano south atlantic ocean abstract new observations on the morphostructure eruptive characteristics and intrusive system of tristan da cunha an active intraplate volcano of predominantly trachybasaltic composition are combined with previously published data to arrive at a model of its internal structure volcanic activity on the island of tristan da cunha is characterized by a dual central and lateral feeding system the central activity occurred at the summit presumably on top of an axial magma column the eruptions produced small flows as well as a high percentage of pyroclastic deposits partly of hydrovolcanic origin the basaltic magma probably encountered a shallow water table or ephemeral crater lakes these pyroclastic products contributed in large part to the building up of a well developed central cone from the column a dense swarm of radial dykes were generated by hydraulic fracturing in contrast the lateral activity occurred on a much lower level on the flanks of the volcano the style of eruption was strombolian resulting in scoria cones and flows there is a concentration of strombolian vents around the base of the central cone this concentric pattern is ascribed to conical shear planes in depth created by uplift of the roof of the magma chamber the uplift may have contributed to the elevation of the central cone in addition a number of lateral vents occur along radial fractures that appear to be concentrated along two tectonic axes oriented n170 e and n80 e these directions are parallel to the compressive axes of the stress field in the ocean floor the most prominent axis n170 e connects the two most recent eruptions of rather unusual trachyandesitic entirely effusive character segmented en echelon dykes and fissures observed in plan and in vertical cliffs are related to slope instability when a dyke propagates to the surface it enters a zone of bimodal stress extension and shearing as a result of downslope movement under such conditions en echelon segmentation of the dyke takes place

geochronology of some canarian dike swarms contribution to the volcano tectonic evolution of the archipelago by g feraud g giannerini r campredon and c j stillman discussion

volcanic and morphological evolution of la gomera canary islands based on new k ar ages and magnetic stratigraphy implications for oceanic island evolution the spatial and chronological evolution of canary islands volcanism is due to eastward progression of the slow moving african plate over a mantle plume la gomera is located between the western shield stage islands 12 ma with 23 k ar ages and palaeomagnetic data we can constrain the timing of the main subaerial volcanic phases of la gomera the shield volcano c 9 4 8 0 ma the postshield peripheral lava flows c 7 4 5 0 ma the postshield horizontal lava flows c 5 4 4 3 ma and late intra canyon lava flows c 1 9 ma the horizontal lava flows fill a 10 km wide depression formed by a giant landslide garajonay the entire structure of la gomera is in relief inversion declining volcanic construction rates from the shield to the postshield stages are correlated with increasing erosion rates whereas the period of volcanic quiescence hiatus has low erosion rates the succession and duration of the hotspot volcanic stages in the canary islands are compared with those in the hawaiian islands thus underlining differences between slow moving and fast moving plate settings

40ar 39ar ages for the alkaline volcanism and the basement of gorringe bank north atlantic ocean abstract gorringe bank is situated on the europe africa plate boundary at the eastern end of the azores gibraltar fracture zone it has two summits gettysburg bank to the southwest and ormonde bank to the northeast we applied the 40 ar 39 ar stepwise heating method to date six samples of the alkaline volcanic rocks two gabbros from the ormonde bank and a dolerite from the gettysburg bank the results that the alkaline volcanism lasted probably for less than 6 ma 66 60 ma although the nature of this volcanism precludes any subduction feature during its setting the alkaline volcanism of ormonde is probably linked to upper cretaceous eocene compressive tectonic events the basement rocks of gorringe bank reveal distrubed 40 ar 39 ar age spectra one plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 ma and related to a variation in the direction of the relative movement between europe and africa the more probable age given by a plagioclase of another gabbro and by a dolerite 110 ma corresponds to tilting northeastward of the gorringe massif

reservoir compartmentalization get it before it gets you abstract this paper examines the impact of compartmentalization on oil recovery the importance of identifying it during field appraisal and methods to evaluate it using fluid data the impact on recovery factor is highlighted using a global database of oil field recovery factors as a function of reservoir complexity and compartmentalization and emphasized in two case studies the effect of compartmentalization on oil recovery demonstrates the benefit in characterizing compartmentalization correctly during appraisal so that the field can be developed in an optimal manner early characterization of field compartmentalization requires making maximum use of available fluid data during appraisal when interpreting fluid data to identify compartmentalization it is critical to take into account the different time scales for various fluid signals pressure contacts density composition to equilibrate and to be able to extrapolate to field production time scales this is essential to avoid false negatives compartments assumed absent due to homogeneous fluid properties when in fact fluids would have equilibrated even in the presence of compartments false positives where fluid differences are interpreted as evidence of compartments when in fact there has not been sufficient time for equilibration to occur and to resolve apparently conflicting data some fluid indicators are at equilibrium others are not rigorous simulation of fluid equilibration is a complex multiphase multidimensional process and is generally reserved for specialist in depth studies however order of magnitude evaluations can be made using analytical solutions in minutes allowing many what if scenarios to be considered and uncertainty to be assessed analytical solutions for estimating the time required for spatially varying fluid properties to revert to steady state distributions are reviewed all these mixing processes are shown to be diffusive in character an effective diffusion coefficient for each process can be calculated from the reservoir rock and fluid properties for an isothermal system the different time scales and distances for each fluid property variation to attain equilibrium can be compared on a single graph where the time elapsed since fluid perturbation is known analytical solutions can be used to estimate the degree of compartmentalization e g permeability of barriers these solutions lend themselves to the development of simple practical compartment assessment tools for industry practitioners

a new chronostratigraphical and evolutionary model for la gomera implications for the overall evolution of the canarian archipelago a review of the general volcano stratigraphy and geochronology of la gomera one of the lesser known canary islands has led to the establishment of a new evolutionary model the oldest edifice corresponds to the submarine stage built up between 20 and 15 ma the construction of the submarine edifice was followed by an important break in the activity about 4 ma and deep erosion of the edifice about 10 5 ma ago the main present day edifice the old edifice 10 5 6 4 ma emerged which was also submarine in its initial phases two different main stages are distinguishable the first stage was represented by a large some 22 km wide basaltic shield volcano the lower old edifice several lateral collapse events tazo and san marcos avalanches occurred during this time and were responsible for the removal of an important part of its northern flank in the second growth stage the upper old edifice the activity migrated southwards a 25 km wide composite volcano arose covering part of the remaining earlier shield volcano the felsic trachytic to phonolitic activity occurring in two separate episodes formed a significant component of this composite volcano finally one more recent large edifice the young edifice built up from 5 7 to 4 ma the lava flows of this younger edifice covered completely the centre and the south of the island and filled deep ravines in the north more evolved magmas including significant felsic magmas the third and last felsic episode occurred in this phase of activity the growth of la gomera was long lasting separated by an important gap in the activity in the middle miocene with no quaternary activity at all at the same time on tenerife the nearest island east of la gomera three large edifices grew separately roque del conde anaga and teno initially three separated islands from the available data it is inferred that the subaerial activity started earlier in the roque del conde edifice then on la gomera and later in teno in the nw and anaga in ne of tenerife which is the youngest of all these edifices these facts together with the irregular general progress of the volcanic activity support more complex views of the genesis for the canary islands than the simple hotspot model

magma flow in dikes from rift zones of the basaltic shield of tenerife canary islands implications for the emplacement of buoyant magma abstract dikes of rift zones in the miocene basaltic shield of tenerife have been studied along several profiles in order to record overall variations in lateral and vertical magma flow direction and sense dikes were sampled for anisotropy of magnetic susceptibility ams and other flow indicators and dike propagation indicators were also recorded at each sampling site bulk susceptibilities suggest a predominance of ferromagnetic minerals in the ams fabrics and most of the sampled dikes show normal fabrics magnetic fabrics have been classified according to clustering and imbrication shown by the maximum susceptibility axis on each dike margin a procedure that provides rapid information on the direction and sense of magma flow the flow of magma tends to be upward and steep beneath the volcanic centers of the basaltic shield massifs along the axes of rift zones and in those rift zones located across the flanks of the volcanic edifices roughly normal to the main rifts magma is mainly emplaced laterally and downward the flow of magma is mostly towards the external parts of the volcanic centers of the basaltic shield massifs and indicates that magma propagated toward regions of lower topography this interpretation agrees with magma being emplaced above the level of neutral buoyancy and with buoyancy as the driving pressure gradient the observed flow patterns can be explained in terms of density gradients within the volcanic edifices in which magma flow trajectories are toward less dense regions

petroleum migration pathways and charge concentration a three dimensional model petroleum migration pathways through a basin are determined by the three dimensional distribution of discontinuous sealing surfaces which are usually parallel to bedding the petroleum migrates below the sealing surface taking the structurally most advantageous route the three dimensional distribution of migration pathways within the petroleum system can be modeled on a personal computer using a program based on the parameters discussed in this paper application of the model to the paris and williston basins demonstrates that a good correlation between predicted pathways and discovered accumulations can be made using simple models pathways form a dense network overlying generating areas in the central parts of basins toward the basin margins these routes commonly become increasingly focused into discrete pathways by the sealing surface morphologies eventually these pathways may reach the surface as seepages it is important to integrate surface outcrops of migration routes surface seepages into migration modeling deflection of the pathways from the structurally most advantageous route below the sealing surface may be caused by lateral sealing barriers due to facies variation in the carrier rock below the seal fault juxtaposition or cross formational seals such as salt intrusions deflection of pathways also occurs where there are hydrodynamic conditions in response to topography driven groundwater flow zones of vertical migration are associated with facies changes along the horizon of the sealing surface into a nonsealing facies or juxtaposition to nonsealing strata by faults vertical migration from either normally or abnormally pressured strata is most likely to occur into normally or lesser pressured strata at intrabasinal highs where hydrocarbons can be stored and transferred at times of temporary seal rupture reverse modeling of individual pathways from fields and seepages to potential sources is a powerful exploration tool it assists in predicting the boundaries of the leaking accumulations or areas of petroleum expulsion

volcanic complexes in the eastern ridge of the canary islands the miocene activity of the island of fuerteventura abstract fuerteventura has been since early stages of its growth the result of three different adjacent large volcanic complexes southern central and northern the definition of these volcanic complexes and their respective growing episodes is based on volcano stratigraphic morphological and structural criteria particularly radial dyke swarms each complex has its own prolonged history that might be longer than 10 m y during that time several periods of activity alternating with gaps accompanied by important erosion took place the evolution of each volcanic complex has been partially independent but all the three are affected by at least three miocene tectonic phases that controlled considerably their activity the volcanic complexes are deeply eroded and partially submerged in the core of the northern and the central volcanic complexes there is a set of submarine and plutonic rocks intensely traversed by a dyke swarm known as the basal complex the basal complex has been interpreted in different ways but all previous authors have considered it to be prior to the subaerial shield stage of the island here we advance the idea that the basal complex represent the submarine growing stage of the volcanic complexes and the hypabyssal roots plutons and dykes of their successive subaerial growing episodes two seamounts situated nearby southwest of the island might be interpreted as remains of two other major volcanoes these two volcanoes together with those forming the present emerged island of fuerteventura and finally those of famara and los ajaches situated further north on lanzarote constitute a chain of volcanoes located along a lineation which is subparallel to the northwestern african coastline and which may relate to early atlantic spreading trends in the area

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

the submarine volcanic succession of the basal complex of fuerteventura canary islands a model of submarine growth and emergence of tectonic volcanic islands three lithostratigraphic units have been distinguished in the volcanic succession of the basal complex of fuerteventura island these units are from bottom to top the submarine volcanic group the transitional volcanic group and the subaerial volcanic group these three groups record the submarine growth and emergence of the island the volcanism is represented by ultra alkaline and strongly alkaline igneous series the igneous activity was due to the presence of an anomalous zone in the sublithospheric mantle the low density of which also caused uplift of the mesozoic oceanic crust two extensional phases and an intervening contractional phase developed coeval to the generation of the volcanic succession the submarine volcanic group was deposited in the hanging wall basin of a large listric extensional detachment directed toward the ssw the transitional volcanic group was syntectonic with respect to a late inversion of the listric detachment finally the subaerial volcanic group resulted from a second episode of wnw extension this study of the evolution of the basal complex of fuerteventura serves as the basis for a tectonic model of submarine growth and emergence of volcanic islands

hotspot volcanism close to a passive continental margin the canary islands the canarian archipelago is a group of volcanic islands on a slow moving oceanic plate close to a continental margin the origins of the archipelago are controversial a hotspot or mantle plume a zone of lithospheric deformation a region of compressional block faulting or a rupture propagating westwards from the active atlas mountains fold belt have been proposed by different authors however comparison of the canarian archipelago with the prototypical hotspot related island group the hawaiian archipelago reveals that the differences between the two are not as great as had previously been supposed on the basis of older data quaternary igneous activity in the canaries is concentrated at the western end of the archipelago close to the present day location of the inferred hotspot this is the same relationship as seen in the hawaiian and cape verde islands the latter archipelago associated with a well defined but slow moving mantle plume shows anomalies in a plot of island age against distance which are comparable to those seen in the canary islands these anomalies cannot therefore be used to argue against a hotspot origin for the canaries individual islands in both archipelagoes are characterized by initial rapid growth the shield building stages of activity followed by a period of quiescence and deep erosion erosion gap which in turn is followed by a post erosional stage of activity the absence of post shield stage subsidence in the canaries is in marked contrast with the major subsidence experienced by the hawaiian islands but is comparable with the lack of subsidence evident in other island groups at slow moving hotspots such as the cape verdes comparison of the structure and structural evolution of the canary islands with other oceanic islands such as hawaii and reunion reveals many similarities these include the development of triple mercedes star rift zones and the occurrence of giant lateral collapses on the flanks of these rift zones the apparent absence of these features in the post erosional islands may in part be a result of their greater age and deeper erosion which has removed much of the evidence for their early volcanic architecture we conclude that the many similarities between the canary islands and island groups whose hotspot origins are undisputed show that the canaries have been produced in the same way

hydrodynamic effects on mission canyon mississippian oil accumulations billings nose area north dakota mission canyon oil production on the south flank of the williston basin provides an example of an area in the mature stage of exploration that shows significant hydrodynamic effects on oil accumulations related to stratigraphic traps the effects are illustrated by the billings nose fields and the elkhorn ranch field the reservoirs have low hydraulic gradients of about 2 m km 10 ft mi tilted oil water contacts with gradients of 5 m km 25 ft mi and variable formation water salinities that range from brackish to highly saline oil accumulations in some zones are displaced off structure and downdip to the northeast parallel to porosity pinch outs other zones are pure hydrodynamic traps lacking both structural and stratigraphic closure future success in exploration and development in the play will depend on recognizing the hydrodynamic effects and predicting oil displacement

hydrodynamic aquifer or reservoir compartmentalization changes in oil water contact owc depth across a field are often a signature of reservoir compartmentalization as a result of faulting or stratigraphic barriers such as shales but may equally well be caused by an underlying hydrodynamic aquifer in particular the pressure depth data obtained from an aquifer whose flow is changing over time can look very similar to that obtained from a compartmentalized reservoir misunderstanding which of these mechanisms causes the observed changes in owc across the field may result in poor estimates of oil in place and reduced recovery to address this problem an analytic expression is presented to estimate the time taken for a steady state tilted owc to be established once an aquifer starts flowing a comparison with simulations of hydrodynamic aquifers in homogeneous compartmentalized and heterogeneous reservoir models shows that this expression can be used in combination with the one derived by m k hubbert for the steady state tilt of the owc to clarify whether a reservoir contains barriers or baffles to flow or may simply have not yet reached equilibrium

miocene rifting of fuerteventura canary islands 1 the older geological units of the volcanic island of fuerteventura canary islands i e the so called basal complex and the lower part of the subaerial volcanic rocks show abundant structures indicative of a long lived period of miocene tectonic activity these structures include faults dike swarms kilometer scale folds tilted sequences and fissural and central volcanic edifices a detailed structural study based on geological mapping and the use of fault slip inversion techniques and statistical analysis of dike orientation has allowed the identification of three separated miocene deformation phases m d1 m d2 and m d3 the average extension directions determined for these phases are nw se nne ssw and ene wsw respectively a model of oceanic lithosphere rifting is proposed to account for this deformation history a buoyant anomalous sublithosphere mantle triggered the extension in the lithosphere beneath fuerteventura isolating it during the early and middle miocene from the plate scale collision regime predominant in the nw corner of the african plate

initiation and growth of gypsum piercement structures in the zechstein basin abstract the importance of tectonic processes in initiating halite diapirs has become much better understood in recent years less well understood is the development of diapiric structures involving rocks composed predominantly of gypsum below about 1000 m gypsum dehydrates to anhydrite which often obscures primary sedimentary textures if the strain associated with diapiric rise in the rock induces the transition to anhydrite obliteration of primary features in the gypsum can be expected in our study we infer that the diapiric movement in the werra anhydrite member of cycle 1 of the zechstein formation of europe occurred before the initial transition of gypsum to anhydrite based on the presence of pseudomorphs of bedded primary gypsum crystals the overburden lithologies and depositional environment and the mechanical properties of gypsum anhydrite and carbonate rocks faulting and differential loading of a shallow overburden were the key components in initiating the gypsum diapirism the transition to anhydrite occurred after burial and after cessation of diapirism in comparison the diapirism of calcium sulfate of the leine anhydrite into the leine halite members of cycle 3 of the zechstein formation probably occurred much later after burial and appears to have been triggered by halite diapirism which in turn triggered the dehydration reaction causing the calcium sulfate to become the incompetent phase relative to the halite

quaternary reactivation of flexural slip folds by diapiric activity example from the western ebro basin spain in the western sector of the ebro basin two types of structures deform the quaternary terraces and pediments developed by the ebro river and its tributaries 1 folds up to 10 km long in the lower levels of fluvial terraces and 2 normal listric faults that produce tilting and rollover anticlines of the quaternary deposits both types of structures are linked to the geometrical and lithological features of the tertiary beds that underlie the quaternary deposits quaternary folds whose axes are parallel to the strike of the tertiary beds are the result of reactivation of tertiary large scale 60 km long folds due to diapirism of their gypsum cores where the gypsum units reach a maximum thickness of 3 000 m this reactivation produced flexural slip in some beds on the limbs of the folds bringing about the folding of the overlying quaternary terraces the mechanism of quaternary folding involves layer parallel shear in alternating tertiary units and folding linked to detachments and reverse layer parallel faults normal listric faults are widespread throughout the studied area they are partly parallel to low dipping tertiary beds and are the result of a nne ssw tectonic extension compatible with minor structures and focal mechanisms of recent earthquakes the relationship between the two kinds of quaternary structures indicates that diapirism of the gypsum cores of the anticlines was activated by extensional tectonics

gravity constraints on the structure and volcanic evolution of tenerife canary islands a detailed gravity study of tenerife constrains the internal structure of the island and its active magma system the maximum bouguer anomaly 2636 gu 1 gu 10 6 m s 2 0 1 mgal occurs over the sw part of the postshield las canadas edifice lce while smaller highs occur over the eroded shield volcanoes the main high extends ne and nw over the adjacent dorsal and santiago rift zones a deep low occurs over the northern lce and teide volcano the power spectrum of the anomalies suggests shallow causative structures 3 km inferred to belong to the island s dense core a model computed from the anomalies uses core and carapace densities of 2900 and 2360 kg m 3 respectively except beneath the main high where the core requires a higher density 3210 kg m 3 this part of the body is interpreted as a buried mafic volcano of substantial size the boca tauce volcano containing an old mafic plutonic complex the enigmatic las canadas caldera that truncates the lce shows strong asymmetry dense materials occupy the southern part whereas low density materials occupy the northern part to 2 3 km depth consistent with a vertical collapse origin high density materials in the south are related to the intracaldera extension of the dorsal rift zone the boundary between subcaldera domains is interpreted as the buried head of the giant icod landslide valley the icod landslide appears to have modified the caldera truncating its northern side and to have subsequently controlled the location of the active pico teide pico viejo vent systems voluminous chaotic deposits lying off northern tenerife previously associated with the formation of the caldera and icod valley are better interpreted as being derived from older mass wasting events the southern lce has been buttressed against lateral failure by the buried boca tauce volcano

petroleum hydrogeology of the cooper and eromanga basins australia some insights from mathematical modeling and fluid inclusion data mathematical modeling and fluid inclusion data analysis are used to reconstruct the petroleum hydrogeology of the australian cooper and eromanga basins our analysis focuses on the development of topography and compaction driven groundwater flow systems and their role in heat redistribution petroleum generation and oil and brine migration during basin evolution finite element models of basin transport processes are constructed along northeast southwest aa and northwest southeast bb cross sections that generally follow the present day groundwater flow patterns through these basins numerical results are presented in the copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received september 19 1995 revised manuscript received july 3 1996 final acceptance november 11 1996 2independent consultant 12 rinzee road dracut massachusetts 01826 3csiro division of petroleum exploration 51 delhi road north ryde nsw 2113 australia 4department of geology and geophysics university of minnesota 310 pillsbury drive se minneapolis minnesota 55455 e mail mperson darcy geo umn edu 5gza geoenvironmental inc 380 harvey road manchester new hampshire 03103 6santos ltd 101 grenfell street adelaide sa 7000 australia this research is supported by a grant from the petroleum research fund administered by the american chemical society prf 24184 ac8 we would also like to acknowledge the orpha and george gibson hydrogeology endowment and the mcknight land grant professorship program at the university of minnesota we thank the csiro division of exploration geosciences for providing support for travel costs within australia data provided by santos ltd and the department of mines and energy south australia are also gratefully acknowledged computational work performed on this project was done at the gibson computational hydrogeology laboratory at the university of minnesota which is supported by the national science foundation under ear 94 05807 special thanks go to m a habermehl of the australian geological survey organization david gravestock of the south australia bureau of mines and mineral resources and mike congreves of santos ltd for their assistance on interpreting the petroleum geology and hydrogeology of the cooper and eromanga basins the review comments of joseph toth stefan bachu and michelle walvoord as well as discussions with ward sanford greatly improved the quality of this manuscript three dimensional model results presented in this paper are available in animated form on vhs videocassette and can be obtained by contacting mark person at the address listed for him as the corresponding author

structure composition and age of the small islands of santa luzia branco and raso cape verde archipelago abstract the northern alignment of cape verde archipelago contains three small islands in its central sector santa luzia branco and raso on which there is very scarce geological petrological or geochronological information available the three islands along with the island of sao vicente arise on a continuous underwater relief less than 50 m deep and about 70 km long that formed from separate volcanic edifices the southernmost island raso is formed by accumulation of several tens of lava flows and some remaining strombolian cones an erosion surface covered by sediments allows the separation of two different episodes of growth the lower one older than 2 ma and the upper one between 0 9 and 1 2 ma the smaller island of branco is the remnant of a large stratovolcano about 6 km in radius 1000 m of altitude and between 5 and 6 ma in age santa luzia the northernmost and largest island of this particular group is essentially a remaining rift zone between 4 and 7 ma old at least 600 m high about 10 km long and 7 km wide the se extreme of the island is occupied by rocks of an older partially submarine stage of growth which also include gabbroic intrusions crossed by a dense network of dikes whose age may be estimated in 8 ma or more on the contrary the nw end shows the only remains of late post erosion activity that is of much less volumetric importance and it has been dated between 0 8 and 1 4 ma the three islands as a whole are formed by moderately or strongly alkaline basic and ultrabasic rocks similar to those found in the neighboring island of sao vicente ultraalkaline rocks only appear in raso and differentiate types as trachyandesites and phonolites are exclusively found in santa luzia the similar age found for the main stages of growth recorded in the mentioned four islands and even more in the other two islands santo antao and sao nicolau situated at the end of the barlovento alignment are not easily comprehensible with evolution models implying a simple spatial progression of the volcanic activity from east to west in this array of the archipelago on the other hand the temporal coincidence in periods of quiescence observed in the different islands seems to point to the existence of a cyclical activity for the ensemble

new tie points for the geomagnetic polarity time scale during the middle miocene from the mogan group on gran canaria and ocean drilling program leg 157 site 953 a thick sequence of volcaniclastic sediments drilled at site 953 during ocean drilling program odp leg 157 northeast of gran canaria canary islands contains an almost complete magneto stratigraphy back to the shield stage of the island 14 8 ma ago onshore a sequence of reversals has been identified and dated in 19 dominantly peralkaline rhyolitic ignimbrites one rhyolitic and one basaltic lava flow of the mogan group 13 35 13 95 ma which overlie basalt flows of the island s shield stage 14 ma the magneto stratigraphy of the ignimbrites onshore has been correlated with the marine magneto stratigraphy at site 953 determined in syn ignimbritic volcaniclastic turbidites which were deposited practically synchronously immediately following the entry of the parent pyroclastic flows into the sea around the circumference of the island the four polarity intervals recorded in the sequence of the mogan group ignimbrites correspond to c5acr c5acn c5adr and c5adn single crystal 40ar 39ar age determinations of the ignimbrites bracketing the polarity changes gave the following ages and uncertainties for the reversals c5ad t 13 95 0 07 ma c5ac o 13 89 0 08 ma and c5ac t 13 47 0 09 ma the newly dated polarity changes fit and refine the miocene age model proposed in the global polarity time scale

diagenetic evolution of aptian evaporites in the namibe basin south west angola the widespread and dissected nature of the angolan gypsiferous salt residuals offers a uniquely detailed view of the lateral and vertical relations inherent to secondary evaporite textures which typify exhumed salt masses worldwide such secondary textures are sometimes misinterpreted as primary evaporite textures thin metre scale and patchy dome like gypsum accumulations are well exposed within strongly incised present day river valleys along the eastern margin of the namibe and benguela basins south west angola these sections are time equivalent to the main basinward subsurface evaporites aptian loeme formation which mostly consist of halite the gypsum here called the bambata formation is interpreted to represent the final residual product of fractional dissolution and recrystallization of the halite mass that occurred during late cretaceous margin uplift and continues today this halite underwent multiple episodes of diagenetic alteration between its deposition and its final exhumation leading to the formation of various secondary gypsum fabrics and solution related karst and breccia textures that typify the current evaporite outcrop four different diagenetic gypsum fabrics are defined thinly bedded alabastrine nodular alabastrine displacive selenite rosettes and fibrous satin spar gypsum current arid conditions are responsible for a thin weathered crust developed at the top of the outcropping gypsum but the fabrics in the main core of the current at surface evaporite unit mostly formed during the telogenetic stage of uplift prior to complete subaerial exposure alteration occurred as various dissolving and rehydrating saline minerals encountered shallow aquifers in the active phreatic and vadose zones geomorphological and petrographic analyses mostly based on the cross cutting relations and crystallographic patterns in the outcrop are used to propose a sequence of formation of these different fabrics

geochronological and structural study of tertiary and quaternary dikes in southern france and sardinia an example of the utilization of dike swarms as paleostress indicators ancient dikes swarms which are datable by geochronological methods can be used as paleostress indicators this study defines criteria by which suitable dike swarms can be selected this paper deals with tertiary and quaternary dike swarms of the french and sardinian peri alpine platform the results of geochronological and structural analysis show that the general trends of dikes ranging from nw se to n s intruded during the last 8 ma on the border of massif central and in languedoc parallel the general trajectories of the present stress field plane 1 2 as deduced from in situ measurements and from focal mechanism determination of earthquakes in detail dike swarms dated between 8 and 6 ma the region of coiron aubrac velay and causses in southern france possess a nw se orientation however those dated between 3 4 and 0 7 ma the region of logudoro montiferro and monte arci in sardinia and the region of escandorgue lodevois and agde in southern france are oriented roughly n s in southern france the stress orientations cannot be deduced from geophysical data with sufficient precision to determine whether the plane 1 2 orientation recorded by the dike propagation varies systematically in time or space nevertheless the fact that the oldest dike swarms 8 6 ma are oriented nw se indicate that the stress field may have been modified since that time

dike swarms key to the reconstruction of major volcanic edifices the basic dikes of la gomera canary islands the detailed study of the age and distribution of the basic dikes of la gomera canary islands has allowed us to reconstruct the evolution of its main volcanic edifice the dikes of the oldest unit the basal complex have a n70 80 dominant direction at regional scale on the other hand the dikes of the post basal complex subaerial units are the result of local stress fields the oldest swarm is composed of sills dated at 10 2 9 3 ma four younger radial dike swarms have been identified s1 s2 s3 and s4 with ages of 9 1 8 4 ma 8 2 6 7 ma 5 5 4 4 ma and 5 3 4 0 ma respectively the reconstruction of the magmatic focus location using these swarms shows a migration southwards with an average speed of 1 6 mm year this temporal sequence of parallel swarms sills and radial swarms is a pattern frequently repeated during the building of large insular volcanic edifices in the canary islands as well as in other archipelagos

morphological and geological aspects related to large slope failures on oceanic islands the huge la orotava landslides on tenerife canary islands huge volcanic landslides are one of the most hazardous geomorphological processes that can occur during the evolution of volcanic ocean islands the causes of these phenomena however are very complex and combine non volcanic and volcanic factors in the canary islands more than 20 events have been detected during the last decades a detailed analysis was carried out for la orotava amphitheatre on tenerife in order to understand the relationship between geomorphological and geological aspects and huge volcanic landslides the results indicated four major features that play a significant role in such mass movements deep erosive canyons high coastal cliffs widespread residual soils and structural axes high coastal cliffs and deep erosive canyons locally reduce the stability conditions and control both the seaward and the lateral boundary of the landslide weak residual soils formed above phonolitic pyroclastic deposits occur repeatedly in the stratigraphic column of la orotava and are characterised by their large extent thus one of these soils may have evolved into the slip surface of the failure part of the head scarp of the amphitheatre is defined by a volcanic rift zone as indicated by the measurement of dike orientation and a density map of eruptive vents the four features are not able to trigger a failure but to destabilise the volcano flank and determine the boundary of the slide therefore information on depth and orientation of canyons location and height of coastal cliffs stratigraphic repetition extension and thickness of residual soils and orientation and density of dikes and eruptive vents along structural axes should be incorporated into a hazard assessment on large landslides on volcanic islands

earlier history of the 70 ma old canary hotspot based on the temporal and geochemical evolution of the selvagen archipelago and neighboring seamounts in the eastern north atlantic major element trace element and sr nd pb isotope data combined with 40ar 39ar age determinations of volcanic rocks from the selvagen islands and neighboring seamounts in the eastern north atlantic reveal the earlier history of the 70 ma old canary hotspot a basanitic to phonolitic late shield stage intrusive complex 29 ma is exposed on selvagem pequena the evolution of selvagem grande can be divided into three magmatic phases a tephritic to phonolitic late shield stage intrusive complex 24 26 ma and two rejuvenated or post erosional stages 8 12 and 3 4 ma consisting of alkali basalt basanite and rare phonolite during the early to mid miocene volcanic hiatus 12 24 ma the top of the volcano was beneath sea level as evidenced by marine carbonate sediments 13 24 ma dated through correlation of 87sr 86sr with the seawater sr isotope curve the geochemistry of the shield stage lavas indicates that they derive from plume sources whereas the post erosional lavas are derived from metasomatized lithospheric sources five sampled seamounts to the east and northeast of the islands range in composition from alkali basalt and basanite to phonolite samples from dacia conception bank and lars were dated at 9 17 and 68 ma respectively geochemical data suggest that the dredged samples come from the post erosional stage of volcanism and therefore the dates represent minimum ages for the seamount volcanoes the elevation of erosional platforms formed at wave base decrease from selvagen grande 100 m above sea level to lars seamount 900 m below sea level suggesting a southwest to northwest age progression and that all of these seamounts are older than the selvagen islands trace element and sr nd pb isotopic composition of the selvagen islands and neighboring seamounts are consistent with their origin from the canary plume interaction of the weak canary mantle plume with a slow moving plate appears to be responsible for generating a 450 km wide irregular hotspot track extending 800 km from the youngest canary island of hierro in the southwest to lars seamount in the northeast

volcanic evolution of sao vicente cape verde islands the praia grande landslide abstract the island of sao vicente has undergone continuous volcanic activity from pliocene to pleistocene times the earliest evidence of activity corresponds to some 9 million years ago when a submarine edifice of alkaline basaltic affinity had already started to develop the island resulted from the growth of a single major edifice the sao vicente edifice built up in several distinguishable growth stages the early main stage occurred at 6 5 to 4 5 ma giving rise to an edifice of about 10 12 km in radius and more than 2500 m in height whose centre was located south of the present city of mindelo although the edifice in its earlier phase showed typical characteristics of a shield volcano the mindelo formation it adopted later on those of an ordinary composite volcano the madeiral monte cara formation a giant landslide event the praia grande landslide destroyed the ne sector of the edifice and left a 10 12 km large depression that was successively refilled by nephelinitic lava flows and some associated carbonatites the monte verde formation 4 5 3 1 ma the volcanic activity ceased about 3 2 ma ago when the edifice started to be deeply eroded until disappearing about 90 of its total volume only in recent times 0 3 ma very scarce and local strombolian activity has been developed aside the sao vicente edifice in the eastern sector of the island

origin of reservoir fractures in little knife field north dakota thin vertical planar fractures observed in the mission canyon formation at the little knife field are naturally occurring and appear to be extension fractures the predominant east west trend of the fractures measured in oriented core from six wells parallels the contemporary maximum horizontal compressive stress in the williston basin the fractures occur only in carbonate units but within the carbonates their occurrence is not lithology dependent fracture density measured in cores of the reservoir carbonates averages 1 ft 3 m of fracture per 2 3 ft 7 m of core the formation and mineralization of reservoir fractures were the most recent diagenetic events in the mission canyon formation at little knife study of aqueous and hydrocarbon fluid inclusions associated with the fractures reveals 1 fractures formed after the strata were buried to at least their present depth of 9 800 ft 2 987 m which indicates their age is post mesozoic 2 the pore fluid pressure gradient was normal hydrostatic immediately after if not during fracture system development 3 formation water salinity has remained fairly constant since fracture initiation 4 migration of hydrocarbons into the reservoir probably preceded fracture genesis and 5 methane concentration may have decreased since fracture initiation

dykes faults and palaeostresses in the teno and anaga massifs of tenerife canary islands abstract a structural field study was made of 578 sheet intrusions mostly dykes and 153 mostly normal faults dissecting the anaga and teno massifs where a complex volcanic succession of tertiary age the old basaltic series representing the shield building stage of tenerife canary islands crops out many of the intrusions mostly sub vertical mafic dykes are emplaced by multiple magma injections with cumulative thicknesses mostly less than 2 m dyke tips are exposed and preserved for 12 of the dykes three differently oriented sets of dykes exist in the anaga massif nnw sse nne ssw e w whereas there is only one main set in teno trending nnw sse dyke swarms and other structural features having similar orientations also exist in other canary islands a minimum value of the horizontal component of extension induced by dykes is computed using a step of 5 of azimuth accounting also for the dip of dykes the cumulative crustal dilation is at least 300 m 4 in anaga and 270 m 6 in teno the maximum extension peaks at n75 in anaga and n60 in teno indicating a general prevailing extension in direction ene wsw most of the measured faults are normal and strike nnw sse computation of palaeostresses from inversion of fault slip data sets suggests the existence of a polyphase brittle deformation due to an extensional stress field with the minimum compressive principal axes trending ne sw and wnw ese

evaporite karst in three interior layered deposits in iani chaos mars abstract this paper describe the karst landforms observed in three interior layered deposits located in iani chaos a large depression located in the equatorial region of mars characterised by spectral signatures of monohydrated and polyhydrated sulfate such as kieserite and gypsum a morphological and morphometric survey of the ild surface morphologies through an integrated analysis of the available mars reconnaissance orbiter mro high resolution imaging science experiment hirise highlighted the presence of depressions of various shapes and sizes these martian landforms interpreted as doline of polygenetic origin resemble similarly karst landforms that can be observed both in different karst terrains on earth and in other regions of mars the karst landforms observed suggest a climatic change and the presence of liquid water probably due to ice melting in the late amazonian age

genesis and evolution of gypsum tumuli tumuli are hollow subcircular domes of the most superficial stratum of gypsum principally found in outcrops of macrocrystalline gypsum they vary from a few centimetres to several metres in diameter and reach maximum heights of a little more than 1 m the relationships between the morphostatistical parameters that define these formations are h r 3 and e r 9 where h is the elevation of the raised layer e its thickness and r the mean radius their genesis has caused some controversy over the involvement of phenomena such as hydration of anhydrite or tectonic processes capable of explaining this folding this paper shows their genesis linked to the dissolution of macrocrystalline gypsum and reprecipitation of microcrystalline gypsum within the same gypsiferous layer it has been calculated that to reach the theoretical saturation within the few centimetres thickness of the cap of the tumulus water infiltration velocities are required of between 0 002 cm s 1 for an uplifted stratum of 2 cm thickness and 0 03 cm s 1 for 30 cm thickness these velocities imply the existence of very slow rates of infiltration and or capillary movement of water within the gypsiferous layer the secondary microcrystalline gypsum is precipitated in the intercrystalline and intracrystalline voids of the gypsum crystals producing an increase in porosity and associated volume that causes the doming of the gypsiferous layer the development of tumuli is a cyclic process which is favoured by a sequence of short wet and dry intervals which in turn facilitate the almost simultaneous processes of dissolution and precipitation these conditions predominate in arid and semiarid climates where intense evaporation can occur suddenly following sporadic infiltration copyright 1999 john wiley sons ltd

graben structure in the las canadas edifice tenerife canary islands implications for active degassing and insights on the caldera formation abstract a graben structure has been identified at the western area of the las canadas caldera wall here referred as the los azulejos graben this graben is 1 km wide and is bounded by two major normal faults trending ne sw the los azulejos fault and the ucanca fault the graben was active for at least 0 5 ma from the end of the ucanca fm to the end of the guajara fm and before the collapse of the las canadas edifice that formed the western caldera a right lateral transtension regime operated in the graben as suggested by small fault orientations and kinematics the prolongation of the ne rift zone of tenerife to the canadas edifice is the most likely volcano tectonic scenario for the graben in this context inflation of phonolitic shallow magma chambers may have produced reverse faults and reactivation of normal faults an intense and widespread hydrothermal alteration here called azulejos type occurred mainly before the graben formation while a fault related hydrothermal alteration occurred during and after the graben diffuse carbon dioxide and hydrogen degassing in and around the las canadas caldera show relatively enriched values along a ne sw trend suggesting that faults in the los azulejos graben act as a pathway for deep seated gases to the surface diffuse degassing and hydrothermalism indicate that the graben area has been a zone of intense fluid circulation during the evolution of the las canadas edifice

intrinsic and scattering seismic wave attenuation in the canary islands the canary islands volcanic complex is studied in terms of coda wave attenuation a multiple lapse time window method based on the hypothesis of multiple isotropic scattering with uniform distribution of scatterers is applied to the available seismic data in order to obtain the intrinsic absorption qi 1 and the scattering attenuation qs 1 in the canarian lithosphere the analysis is performed for two hypocentral distance ranges from 0 to 80 km and from 0 to 140 km results show that in both cases and for all the studied frequency bands 1 2 2 4 6 8 and 8 10 hz intrinsic absorption dominates the low albedos found in the region indicate the low degree of heterogeneity in the canarian lithosphere at the scale length of the studied frequencies on the other hand the coda attenuation qc 1 calculated on the basis of the single scattering theory gives values near qi 1 for low frequencies and near the total attenuation qt 1 for high frequencies the degree of frequency dependence of the attenuation parameters is strong in all cases a correlation of the observed attenuation parameters with the geological evidence for a hotspot type archipelago and with other geophysical studies is suggested

stratigraphy structure and volcanic evolution of the pico teide pico viejo formation tenerife canary islands abstract the structure and volcanic stratigraphy of the pico teide pico viejo pt pv formation deriving from the basanite phonolite stratovolcanoes pt and pv and numerous flank vent systems are documented in detail based on new field and photogeologic mapping geomorphologic analysis borehole data and petrological and geochemical findings results provide insight into the structure and evolution of the pt pv magma system and the long term cyclic evolution of tenerife s post shield volcanic complex the pt pv formation comprises products of central volcanism mainly emplaced into the las canadas caldera lcc and contemporaneous products from adjacent rifts pt pv central volcanic products become more differentiated up section with felsic lavas dominating the recent output of the system this is attributed to the evolution of a shallow magma reservoir beneath pt that was emplaced early in the pt pv cycle on the intra caldera segment of tenerife s post shield rift system the rift axis has been the focus of pt pv intrusive and eruptive activity and has controlled the location of the stratocones the current geometry of the rifts reflects a major structural reorganisation defining the start of the pt pv cycle at 0 18 ma namely the truncation of the north side of the lcc lce by the giant icod landslide the internal stratigraphy of the pt pv formation suggests that pt developed early with pv developing as a satellite vent activity has since alternated between pt and pv due to episodes of vent blockage or chamber sealing these processes have allowed significant volumes of phonolitic magmas to develop and accumulate within the pt chamber which have vented through radial dike systems during tumescence episodes and from the rift system which has permitted lateral magma transport the pt pv magma system is a potentially hazardous source of future felsic eruptive activity on tenerife

the distribution of basaltic volcanism on tenerife canary islands implications on the origin and dynamics of the rift systems abstract one of the most characteristic features of volcanic islands is the existence of rift zones defined commonly as orientated eruptive fissures or parallel rows of elongate cinder cones and dyke swarms occasionally these rifts can appear at the birth of the volcanic island and persist until the last episodes of its constructions controlling the form and structure of the island e g azores islands in the case of tenerife canary islands it is possible to observe two rift zones santiago del teide and dorsal rifts running nw se and ene wsw marked by parallel rows of aligned cones and eruptive fissures additionally at the southern part of the island southern volcanic zone basaltic volcanism is characterized by scattered vents and apparently non coherently orientated eruptive fissures some authors relate the existence of the latter volcanism to a n s running rift zone that defines the third branch of a three armed rift system in the island in the present paper we first investigate the tectonic controls on the distribution of basaltic volcanism at the southern volcanic zone and their relation with the nw se and ene wsw rifts the numerical results obtained suggest that basaltic volcanism of the southern part of tenerife can be easily explained as the result of a extensional stress field derived from the combined effects of the nw se and ene wsw rifts as a second objective we have also investigated the origin of the santiago del teide and dorsal rift zones and their role on the formation of the original shield volcano and the subsequent evolution of the whole island our numerical results contrast with previously published explanations on the origin of the tenerife rifts that included fracturing due to volcano spreading or to deformation of the volcano due to magma intrusion we consider that volcanic activity in tenerife began throughout fissural volcanism along these structures that were already present in the oceanic basement progressively accumulating the basaltic series that gave rise to the construction of the composite shield volcano

giant miocene landslides and the evolution of fuerteventura canary islands abstract fuerteventura is the oldest island in the canaries archipelago following the growth of a pre miocene seamount its shield stage began 20 6 ma ago and ended in the mid miocene there followed a very extended period of quiescence then very minor quaternary post erosional volcanism the shield stage produced volcanoes up to 3000 m above sea level which were rapidly eroded by 17 5 ma both fuerteventura and lanzarote the older eastern most islands are in a post erosional stage and have subdued topography with elevations rarely rising above 400 m the western islands la palma and el hierro together with tenerife are the youngest in the archipelago and are still in their shield stage which began at most 7 5 ma ago they have a rugged topography with peaks rising to several thousand metres volcanic activity persists in these islands and in lanzarote the shield stage construction of the volcanically active western islands demonstrates that recent eruptive activity has concentrated in volcanic centres aligned in well defined rift zones in which major dyke emplacement has taken place and that volcano flank collapse has occurred with giant landslides initiated on the rifts in fuerteventura most of the shield stage volcanic rocks have been removed however evidence has now been advanced to show that major dyke swarms were emplaced in the oligocene and miocene these belonged to a line of intrusive complexes above which by the early to mid miocene volcanic peaks had been constructed three volcanic centres have been identified and in the central one a peak as high as the present mount teide on tenerife was rapidly denuded in less than 2 ma in the west of the island this erosion has exposed a window of more than 300 km 2 of submarine volcanics and sediments belonging to the pre shield phase seamount the principal mechanism of erosion seems to have been the generation of giant landslides analogous to those seen in the younger islands these appear to have removed some 3500 km 3 of lavas and volcaniclastics stripping the western sectors of the miocene volcanoes down to the pre shield phase basal complex and transporting the mass of volcanic material into the atlantic ocean as debris flows remnants of the shield volcanoes still exist in the eastern part of the island present day topographic variation indicates that the miocene base level of erosion is still visible in a central depression which runs through the axis of the island the eastern hills are composed of the miocene shield stage volcanics which still preserve a drainage pattern initiated on these peaks and the basal complex in the west has a juvenile landscape suggesting incision on a post miocene domal uplift of the volcano core structural links between intra island volcano tectonic events and the south atlas fault zone are not thought to be likely

impact of quarrying gypsum in a semidesert karstic area sorbas se spain the messinian gypsum of sorbas is intensively karstified with hundreds of dolines and numerous highly unusual exo and endokarstic forms amongst these the tumuli are especially spectacular as are the large caverns adorned with numerous speleothems some of which have not been described anywhere else in the world the extraction of the gypsum has impacted practically all the elements of the environment landscape water soil subsoil flora fauna etc and in particular all of the karstic forms in addition to endangering the caves and the natural treasures that they contain it affects the surface waters markedly modifying the hydrographic network groundwaters which now occur in the bottom of a quarry are also affected

regional source rock mapping using a source potential rating index a method has been developed to combine sediment thickness organic carbon content and thermal maturity data from potential source rock units into a single mappable parameter that can be used to indicate areas of potential hydrocarbon generation this parameter is computed by multiplying the average percent organic carbon by the effective source rock thickness of a formation this yields a richness factor that is then multiplied by maturity scaling factors to give source potential ratings for oil and or gas generation the rating values can be combined with kerogen type data if available to refine these assessments of generating potential the rating index can also be used with burial history curves and thermal maturation modeling to look at source potential during geol gic time an example is given of the rating index mapping applied to the bakken formation in the williston basin the source potential rating index provides objective semiquantitative measures by which the source potential of a single formation can be compared within an area or by which the source potential of two or more formations can be compared in the same or different basins in conjunction with structural analysis of a basin the resulting maps can help the explorer explain known hydrocarbon accumulations trace migration pathways and point toward new areas for exploration

geochronology of some canarian dike swarms contribution to the volcano tectonic evolution of the archipelago we present new conventional k ar and 39ar 40ar age data and geometric observations on dike swarms which intrude the islands of la gomera tenerife and fuerteventura canary archipelago the younger dated dikes of la gomera 5 4 5 2 ma were emplaced by a radial fissural structure independently superimposed on an older one dated between 10 5 and 7 0 ma the dated dike swarms of both the anaga and teno peninsula of tenerife are contemporaneous 5 8 3 6 ma while ages of 0 43 ma were measured on two dikes intruding the pedro gil ridge the dikes of the basal complex of fuerteventura give well defined 39ar 40ar plateaus when they are situated far away from basic and ultrabasic plutons and disturbed 39ar 40ar age spectra when they are situated in the vicinity of the plutons these disturbances are probably due to a thermal metamorphism the results of both the conventional k ar and 39ar 40ar stepwise heating methods show the existence of an important magmatic phase between 24 and 17 ma the dike swarms of the basal complex are partly contemporaneous with other dike systems intruding the basaltic series i of fuster et al 1968a this phase seems to correspond to the final stages of activity on the main nne ssw volcano tectonic axis following this a volcanic edifice was built on the nw part of jandia peninsula mainly between 15 4 and 14 2 ma the similarity of the ages obtained on dikes and lava flows shows the importance of fissural volcanism in these islands this study shows that dating of dike swarms reveals the existence of major eruptive phases an important part of the products of which have been eroded brief episodes of fissural volcanic activity sometimes probably as short as 200 000 years may be correlated with the migration of the lithosphere over a fixed hot spot the north south 3 5 ma old present and the nne ssw 24 17 ma old oriented dike swarms of la palma staudigel et al in prep and fuerteventura may represent the regional paleostress orientation plane 1 2 which affected the lithosphere during their intrusion although other phenomena may control the orientation of dikes these directions are concordant with the known contemporaneous nw african tectonics

submarine volcanism surrounding tenerife canary islands implications for tectonic controls and oceanic shield forming processes abstract using high resolution bathymetric data 97 seamounts within two large submarine areas n and se of tenerife have been identified and studied in detail main dimensions and associated parameters of these seamounts have been measured allowing a morphometric and morphologic classification on a shape basis in order to be certain of their volcanic origin several restrictive criteria were applied conical shape with maximum height 150 m basal ratio 1 aspect ratio 0 1 to the initial tally obtaining finally 31 regular shapes that can be identified without any reasonable doubt as volcanic seamounts these are very similar to submarine volcanoes described elsewhere on the sea floor most of these volcanic seamounts are aligned along two lines that follow the main volcano tectonic trends of the canary archipelago the nw se line crosses the island while the ne sw line coincides with a seismically active reverse fault that separates the tenerife and gran canaria insular blocks the distribution of the submarine volcanism to the north of tenerife permits a better knowledge of the island from its base on the sea floor and defines the tectonic control of the growth of the shield between 4 and 2 ma therefore the evolution of the submarine edifice of tenerife shows that the widespread parasite or satellite eruption centres are also very important in the overall growth of large shield edifices of volcanic islands

reconstruction of the differentiated long term exhumation history of fuerteventura canary islands spain through fission track and u th sm he data miocene intrusives and lower cretaceous siliciclastic sedimentary rocks from the basal complex in western fuerteventura were analyzed with low temperature thermochronometric methods such as fission track and u th sm he dating in order to reveal the evolution of the island s exhumation history the obtained thermochronometric data yields a very slow rate of cooling in the order of 1 5 3 c myr from 50 to 20 ma for the early cretaceous siliciclastic rocks these sedimentary units have never been heated significantly above 240 c after deposition and still record the submarine onset of the island s formation in the eocene intrusive bodies associated with the early miocene magmatic activity of the central volcanic complex of the island show rapid initial cooling rates of 50 70 c myr from 20 to 14 ma contemporaneous with the intrusions the cooling rate of the cretaceous sedimentary units increased to 25 35 c myr and it is inferred that this increase is associated with enhanced uplift and erosion of the central volcanic complex after 14 ma rates slowed down to 3 6 c myr palaeosols overlying the sedimentary units are themselves covered by pliocene basalt flows and reveal that the sedimentary rocks reached the surface before 5 ma the thermochronometric data obtained in this study for central fuerteventura is difficult to reconcile with the cooling history derived from previously obtained fission track and k ar data from the north western part of the island this inconsistency is likely to indicate that the exhumation history of fuerteventura is more complex and regionally subdivided than previously believed

the dyke swarms of the amanay massif fuerteventura canary islands spain abstract the sheeted dyke swarms of the amanay massif in fuerteventura exhibit variable trends but generally strike n45 e main and n110 subsidiary on the basis of petrographic studies the dykes are classified as ankaramitic basalts trachybasalts hawaiites and lamprophyres camptonites this compositional variability is attributed to different extents of partial melting of the mantle and to differences in the degree of fractional crystallization we present a detailed account of field characteristics mineralogy and petrography of the sheeted dyke complex that intruded the sedimentary sequence plutonic and submarine volcanic formations

rifting recurrent landsliding and miocene structural reorganization on nw tenerife canary islands we studied mechanisms of structural destabilization of ocean island flanks by considering the linkage between volcano construction and volcano destruction exemplified by the composite teno shield volcano on tenerife canary islands during growth tenerife episodically experienced giant landslides genetically associated with rifting and preferentially located between two arms of a three armed rift system the deeply eroded late miocene teno massif allows insights into the rifting processes the failure mechanisms and related structures the semicircular geometry of palaeo scarps and fracture systems breccia deposits and the local dike swarm reconfigurations delineate two clear landslide scarp regions following an earlier collapse of the older los gigantes formation to the north the rocks around the scarp became fractured and intruded by dikes substantial lava infill and enduring dike emplacement increased the load on the weak scarp and forced the flank to creep again finally resulting in the collapse of the younger carrizales formation once more the changing stress field caused deformation of the nearby rocks a fracture belt formed around the scarp and dikes intruded into new concentric directions the outline size and direction of the second failed flank of teno very much resembles the first collapse we suggest structural clues concerning mechanisms of recurrent volcano flank failure verifying the concept that volcano flanks that have failed are prone to collapse again with similar dimensions

crustal extension from exposed sheet intrusions review and method proposal abstract sheet intrusions are probably the most widespread magmatic features on earth they create a space for themselves by dilating the host rock the exposed sheet intrusions allow an estimate of a minimum value of the extension to which the crustal layers and the volcanic edifices are subject the correct evaluation of the contribution of sheet intrusions to the crustal extension is of great importance in various geologic settings the current methods used to evaluate the extension due to exposed sheet intrusions are reviewed in this paper the effect of the dip of sheet intrusions on the evaluation of horizontal crustal extension is analysed if the dip effect is neglected and the intrusions are sub vertical the error due to the dip effect in the extension evaluation is small if the intrusions are shallow dipping the error may be great and varies as a function of the dyke dip for dykes with dip range 45 65 the error due to neglecting the dip effect is 30 to 15 increasing discrepancies of more than 25 are produced on the computed dilation when the intrusions progressively dip less than 50 a simple method based on structural field measurements of exposed sheet intrusions and on trigonometric considerations is presented as an improvement of pre existing methods to estimate the horizontal component of the crustal extension using this method field data of 1154 dykes from different geodynamic contexts are analysed the horizontal component of extension is computed using dyke data from the tertiary massifs of lanzarote and tenerife and from the active stratovolcanoes of monte somma vesuvius and etna since the analysed dykes are steeply dipping the cumulative difference between the extension computed neglecting and taking into account the dip effect remains less than 10 for all the data sets the percentage difference is nearly constant and cumulatively higher for monte somma where the mean dip of the intrusions is smaller in lanzarote 237 dykes emplaced between 15 5 and 3 7 ma give a peak extension of 276 m in direction n135 and a time averaged minimum extension rate of 1 9 10 2 to 2 3 10 2 mm a 1 in tenerife an extension of 562 m is computed in the direction n65 from 563 dykes emplaced from 7 4 to 3 3 ma the time averaged minimum rate of extension for the tertiary massifs of tenerife ranges from 0 14 to 0 25 mm a 1 the peak extension computed for monte somma is 81 7 m in the direction n90 based on 96 exposed sheets very likely most of monte somma sheets intruded within 12 ka giving a time averaged minimum extension rate of 7 mm a 1 from 184 exposed dykes of etna a minimum extension rate of 6 2 mm a 1 is computed peak extension is 212 m in direction n35 the poor resolution of age constraints may affect deeply the reliability of the extension rates however from the examples presented it appears that the extension rates of pre erosional successions from intraplate volcanic islands can be smaller than those of active stratovolcanoes this may suggest a more active dynamics in the stratovolcanoes

the history of intrusive activity on the island of la palma canary islands abstract the island of la palma located in the northwest of the canary archipelago consists of two main geological units 1 the subaerial extrusive coberta series covers most of the island and consists of alkali basaltic lavas phonolitic plugs and pyroclastic rocks erupted between 1 61 ma and the recent 2 the basement complex is exposed in the barranco de las angustias and the caldera de taburiente and consists of the submarine intrusive and extrusive seamount series and a basal plutonic complex alkali basaltic dikes of different ages and orientations intruded in both units three groups of dikes and sills can be distinguished based on their field relationships petrography geochemistry and ages the oldest group i represent the feeder dikes to the seamount series intruded between 2 9 and 4 0 ma b p they were originally steep with a n05 w strike and perpendicular to the originally nearly flat bedding of the layered seamount series subsequent to emplacement group i dikes and the extrusives of the seamount series were rotated into their present orientation group ii sills intruded parallel to the bedding of the seamount series 45 230 are coeval with or younger than group i but older than the erosional unconformity separating the coberta and the basement complex i e 1 6 ma 50 vol of the volcanic edifice of the la palma seamount and highlight the importance of intrusive processes for island growth intrusions into central oceanic volcanoes like la palma may result in an uplift and rotation of more than 30 km 3 by up to 45

stress orientations during oligocene volcanism in trans pecos texas timing the transition from laramide compression to basin and range tension orientations of dikes and hydrothermal veins indicate that east northeast compression occurred during the main pulse of volcanism in west texas 39 to 32 m y ago direction of compression was essentially the same as that during eocene laramide deformation the observations support a continental arc tectonic setting the beginning of extension between 32 and 30 m y ago coincides with a regional change in style of magmatism from intrusions and eruptions spanning a wide range in chemical composition to basaltic volcanism in texas and to bimodal but dominantly rhyolitic volcanism in northern mexico

the felsic dikes of la gomera canary islands identification of cone sheet and radial dike swarms on the northern part of la gomera there exists a great abundance of trachytic phonolitic dikes showing a broad diversity in dip and strike several methods have been applied in order to separate these dikes in different sets localise the area from where they derive and reconstruct the geometry of the swarms the oldest dikes correspond to a radial swarm dated at 8 ma the felsic activity migrated then southwestwards and a second radial swarm and a cone sheet complex were developed between 7 5 and 6 4 ma ago the cone sheet complex is 10 km in diameter and shared its centre with that of the second radial structure the cone sheets exhibit an outward decrease of dip angle whilst every individual sheet maintains a constant inclination this geometry reflects the existence of an ancient single dome shaped shallow magma chamber situated some 1650 m below present sea level the eastern radial swarm represents a felsic episode that could mark the ending of the lower old basalts the earlier subaerial activity of la gomera the two other dike swarms represent a younger episode coeval with the upper old basalts

evolution of the north flank of tenerife by recurrent giant landslides geomorphologic analysis of submarine and subaerial surface features using a combined topographic bathymetric digital elevation model coupled with onshore geological and geophysical data constrain the age and geometry of giant landslides affecting the north flank of tenerife shaded relief and contour maps and topographic profiles of the submarine north flank permit the identification of two generations of post shield landslides older landslide materials accumulated near the shore 40 km and comprise 700 km3 of debris thickening towards a prominent axis suggests one major landslide deposit younger landslide materials accumulated 40 70 km offshore and comprise the products of three major landslides the la orotava landslide complex the icod landslide and the east dorsal landslide complex each with an onshore scar a proximal submarine trough and a distal deposit lobe estimated lobe volumes are 80 80 and 100 km3 respectively the old post shield landslide scar is an amphitheatre 20 25 km wide partly submarine now completely filled with younger materials age width relationships for tenerife s coastal platform plus onshore geological constraints suggest an age of ca 3 ma for the old collapse young landslides are all less than 560 ka old the la orotava and icod slides involved failures of slabs of subaerial flank to form the subaerial la orotava and icod valleys offshore they excavated troughs by sudden loading and basal erosion of older slide debris the onshore east dorsal slide also triggered secondary failure of older debris offshore the slab like geometry of young failures was controlled by weak layers deep drainage channels and flank truncation by marine erosion the partly submarine geometry of the older amphitheatre reflects the absence of these features relatively low h l ratios for the young slides are attributed to filling of the slope break at the base of the submarine edifice by old landslide materials low aspect ratios of the failed slabs and channelling within troughs post shield landslides on tenerife correlate with major falls in sea level reflecting increased rates of volcanism and coastal erosion and reduced support for the flank landslide head zones have strongly influenced the pattern of volcanism on tenerife providing sites for major volcanic centres

tectonic evolution of the emergent part of a volcanic ocean island lanzarote canary islands abstract a reconstruction of the structural evolution of lanzarote confirms the existence of two principal stages of construction of the island separated by a main erosional event the first pre erosional is the shield stage miocene pliocene in age which is divided in three stratigraphic groups where different formations can be distinguished using unconformities mappable over the whole area of outcrop in the second post erosional stage developed during the quaternary nine stratigraphic levels are tentatively identified and are grouped in two phases of volcanic activity shields phase and pyroclastic cones phase data from 580 photogeologic lineaments as well as field data from 204 fault planes 195 with sense of movement 273 dykes and 45 volcanic alignments are presented the tectonic style of the deformations is brittle and polyphasic strike slip faults are dominant but normal and reverse faults are also present both the so called atlantic and african directions are well developed in trends of strike slip faults and volcanic indicators at least two deformation phases can be identified from the geometrical analysis of faults and from the stress tensor calculated using lineations on fault planes the reconstructed stress field responsible for the measured deformations changed orientation during the evolution of the island of lanzarote it is suggested that a permutation between principal stress axes 1 and 3 occurred subparallel to the length of the island between the pre erosional and post erosional stages

238u 230th radioactive disequilibria in historic lavas from the canary islands and genetic implications abstract lava samples from 10 historic or recent eruptions on la palma tenerife and lanzarote canary islands have been analyzed for u and th contents and th sr and o isotopes the sample suite ranges from mantle derived tholeiites and basanites to evolved phonolites a phonolite from la palma has identical th isotope ratio as basanite from the same 1949 eruption and was most probably formed by crystal fractionation of a basanitic magma in contrast the th isotope ratios in the teide phonolites tenerife slightly decrease with differentiation these latter were produced either by crystal fractionation in magma pockets isolated for about 35 000 y or by an assimilation fractional crystallization afc process involving old syenites as contaminants the sr and o isotope ratios show very small variations from 0 7030 to 0 7032 and 5 2 to 5 9 respectively in the analyzed samples except in the most evolved phonolite from tenerife but th isotope ratios are significantly different between the three islands the 230 th 232 th ratios vary from 1 1 in la palma to 0 94 1 01 in lanzarote and to 0 9 or lower in tenerife a possible interpretation of this variation is mixing of melts derived from an undepleted mantle source similar in composition to the tristan da cunha mantle plume with melts of the suboceanic lithosphere the proportion of the plume derived magmas would be higher below the center of the archipelago a variably metasomatized mantle source beneath each island is an alternative explanation the few available 3 he 4 he ratios are correlated with 230 th 232 th ratios and are compatible with both these models they indicate the existence of a mantle source or a metasomatic component with a high th u and th u he ratios below the canary islands

a new felsic cone sheet swarm in the central atlantic islands the cone sheet swarm of boa vista cape verde abstract the island of boa vista is one of the oceanic islands with the largest amount of felsic rocks reported in the world 50 of the total outcropping igneous rocks of boa vista the felsic rocks are trachytes and phonolites generated within the second main stage of the volcanic activity that lasted about four million years three consecutive felsic episodes are distinguished the first occurred between 14 5 ma and 13 5 ma the second between 13 6 ma and 12 0 ma and the third and last between 12 0 ma and 10 4 ma their geochemical composition reveals the presence of crystal fractionation processes with participation of feldspars and amphibole as well as different accessory phases involved in each episode the boa vista felsic cone sheet swarm the youngest third episode is formed by several hundreds of peralkaline ne phonolitic sheets distributed within a circular surface about 11 km in radius the cone sheets conform a single general pattern characterized by a decreasing dip outward the structure together with a constant dip of each individual sheet the average inclination of sheets is around 40 in the sectors next to the centre of the structure whereas it is only about 30 next to its periphery the magmatic focus of the sheets is located over 3 km deep almost coinciding with the convergence centre of a contemporary basic radial dike swarm

hydrodynamic behavior of kangan gas capped deep confined aquifer in iran the kangan aquifer ka is located beneath the kangan gas reservoir kgr 2 885 m below the ground surface the gas reservoir formations are classified into nine non gas reservoir units and eight gas reservoir units based on the porosity water and gas saturation lithology and gas production potential using the logs of 36 production wells the gas reservoir units are composed of limestone and dolomite whereas the non gas reservoir units consist of compacted limestone and dolomite gypsum and shale the lithology of ka is the same as kgr with a total dissolved solid of 333 000 mg l the source of aquifer water is evaporated seawater the static pressure on the gas water contact gwc was 244 atm before gas production but it has continuously decreased during 15 years of gas production resulting in a 50 m uprising of the gwc and the expansion of ka water and intergranular water inside the gas reservoir the general flow direction of the ka is toward the northern coast of the persian gulf due to the migration of water to the overlying formations via a trust fault the ka is a gas capped deep confined aquifer gcdca with special characteristics differing from a shallow confined aquifer the main characteristics of a gcdca are unsaturated intergranular water below the confining layers no direct contact of the water table gwc with the confining layers no vertical flow via the cap rock permanent uprising of the gwc during gas production and permanent descend of gwc during water exploitation

morphology and micro fabrics of weathering features on gyprock exposures in a semiarid environment ebro tertiary basin ne spain abstract gyprock is a common rock in evaporitic formations which have a wide distribution across all continents however outcrops of gyprock are rather limited occurring mainly in low rainfall areas i e southwest of usa italy spain gyprock is one of the most common lithology in the ebro tertiary basin here semiarid conditions propitiate the development of specific weathering processes and landforms with significant implications for understanding the genesis of soils and the geomorphological evolution of these areas gypsum crystallization within the pores and fissures of gyprock close to the surface leads to volume increase and pressures with the consequent weathering of this rock as a result considerable porosity develops producing peculiar morphologies on gyprock outcrops domes and decimetre scale blisters specific microscopic features voids with plain walls horse shoe pores and serrated pores as well as microscopic fabrics grating fabric skeletal fabric fan fabric and botryoidal fabric this paper presents a morphological characterization of the material forming the various types of weathering features from microscopic to mesoscopic scale the macromorphology and micromorphology of those features were used to interpret their genesis

depositional facies textural characteristics and reservoir properties of dolomites in frobisher alida interval in southwest north dakota the mississippian frobisher alida interval is an upward shoaling cycle that began with open marine sedimentation and culminated with the deposition of a widespread sabkha salina evaporite this cycle is the most prolific oil producing interval in the north dakota portion of the williston basin most frobisher alida production in the southern williston basin is from dolomite reservoirs the six major facies defined in this paper are lithologic suites that represent sediments and precipitates deposited in similar environments dolomite formed as a replacement of muddy sediments deposited in three facies ii iv and v facies ii sediments consisted primarily of muds deposited in offshore lagoons facies iv sediments were composed of burrowed muds deposited in a restricted marine environment in front of a barrier bar and island complex fossiliferous muds which formed in an open marine environment were the dominant sediments deposited in facies v the original sediment textures strongly influenced the textures and distribution of replacement dolomite some dolomite zones display relatively uniform textural and petrophysical properties over large portions of southwest north dakota porous dolomite zones with larger average crystal sizes display better producing characteristics than zones with smaller average crystal sizes the average crystal size in frobisher alida dolomites ranges from about 8 to 44 m most large frobisher alida dolomite reservoirs in southwest north dakota occur in combination structural stratigraphic hydrodynamic traps stratigraphic traps in dolomites are found where facies changes occur or where an updip decrease in the dolomitization intensity formed lower permeability rocks a regional hydrodynamic gradient aids stratigraphic trapping in many dolomite zones that would not trap oil under hydrostatic conditions

gypsum karst features as evidence of diapiric processes in the betic cordillera southern spain in the triassic evaporite deposits of the betic cordillera halokinesis has played a fundamental role in the morphodynamic evolution of gypsum karst as evidenced by the morphology and distribution of dolines and poljes by data obtained from caves and from the types of water associated with evaporite domes distribution patterns of major exokarstic forms display a subcircular array with collapse sinkholes located in the centre of the domes and solution drawdown dolines on the outer limits of the diapiric forms in addition the least mineralized waters are situated in association with the caprock gypsum hypersaline waters which are also hotter drain deeper flow systems that flow out at the edges of the halokinetic structures the morphodynamic and hydrogeological information constitute a valuable tool for the correct interpretation of the tectonic structure of such complex areas

natural and human induced sinkholes in gypsum terrain and associated environmental problems in ne spain the central ebro basin comprises thick evaporite materials whose high solubility produces typically karstic landforms the sinkhole morphology developed in the overlying alluvium has been studied using gravimetry and ground penetrating radar gpr on stream terraces as well as analyzing the evolution of sinkhole morphologies observed in aerial photographs taken in 1928 1957 and 1985 the sinkhole morphologies give some idea of possible subsurface processes as well as an indication of the final mechanisms involve in sinkhole development on stream terraces and cover pediments the most commonly encountered dolines are bowl shaped in their morphology with both diffuse and scarped edges in contrast dolines developed in the gypsiferous silt infilled valleys have a funnel and well shaped morphology the diffuse edged bowl shaped dolines are developed through the progressive subsidence of the alluvial cover due to washing down of alluvial particles through small voids and cracks into deeper subsurface caves resulting in a decrease alluvial density future compaction of the alluvial cover will produce surface subsidences this type of dolines are associated with negative gravity anomalies in contrast the scarped edge dolines are formed by the sudden collapse of a cavity roof the cavities and cracks formed in the gypsum karst may migrate to the surface through the alluvial deposits by piping and they may subsequently collapse in this instance the cavities can be detected by both gravity and gpr anomalies where the voids are not deeper than 4 5 m from the surface these processes forming sinkholes can be enhanced by man induced changes in the groundwater hydrologic regime by both inflows due to irrigation ditch losses or pipe leakages and by outflows from pumping activities

river response to quaternary subsidence due to evaporite solution gallego river ebro basin spain abstract the stream terrace evolution of the gallego river during the quaternary was controlled by both climatic change and subsidence quaternary terrace deposits overlying tertiary clay and limestone are between 2 and 5 m thick whereas above evaporite formations the alluvial deposits may be as much as 110 m thick chronologically the first period of alluvial thickening involved the stream terraces t2 105 m above the present thalweg t3 95 m and t4 85 m which have been dated by paleomagnetic reversals as matuyama pre 780 000 the second subsidence period affected the deposits of the stream terrace t9 30 dated as brunhes post 780 000 in both thickening periods the subsidence was due to solution of the underlying evaporite formations halite and gypsum presumably during intervals of high water flow in the proposed model the valley subsidence was balanced by stream aggradation maintaining the river equilibrium profile the subsidence recorded in the alluvial deposits shows a complex spatial and temporal evolutionary pattern and total subsidence was assumed to be equal to the alluvial thickening for each subsidence period reaching up to 165 and 25 m respectively

equation for defining hydrodynamic oil water contact surface and an alternative approach structure surface transformation for mapping hydrodynamic traps abstract flowing groundwater beneath an oil accumulation causes the oil water contact owc to be tilted in this study we present an equation defining the surface of a tilted owc z o w c z o w c 0 w w o h w 0 h w where zowc zowc 0 are the depth of the owc at a given location and a reference location e g a well location respectively w o are the density of formation water and oil respectively hw hw 0 are the hydraulic head at the given location and the reference location respectively this equation can be used directly to delineate the tilted owc of an oil accumulation it can also be employed to map hydrodynamic traps based on the equation an alternative approach essentially the same as hubbert s 1953 minimum energy level methodology for mapping hydrodynamic traps is derived from an easy to understand concept structure surface transformation a field example from the arabian basin is presented to illustrate the proposed approach for mapping hydrodynamic traps and the significance of hydrodynamic effect on trap size and volume for this particular example the traps boundaries and volumes are significantly enlarged owing to a favorable hydrodynamic condition

three armed rifts or masked radial pattern of eruptive fissures the intriguing case of el hierro volcano canary islands abstract using new surface structural data as well as subsurface structural data obtained from seventeen water galleries we provide a comprehensive model of the volcano tectonic evolution of el hierro canary islands we have identified measured and analysed more than 1700 volcano structural elements including vents eruptive fissures dykes and faults the new data provide important information on the main structural patterns of the island and on its stress and strain fields all of which are crucial for reliable hazard assessments we conducted temporal and spatial analyses of the main structural elements focusing on their relative age and association with the three main cycles in the construction of the island the tinor edifice the el golfo las playas edifice and the rift volcanism a radial strike distribution which can be related to constructive episodes is observed in the on land structures a similar strike distribution is seen in the submarine eruptive fissures which are radial with respect to the centre of the island however the volcano structural elements identified onshore and reflecting the entire volcano tectonic evolution of the island also show a predominant ne sw strike which coincides with the main regional trend of the canary archipelago as a whole two other dominant directions of structural elements n s and wnw ese are evident from the establishment of the el golfo las playas edifice during the second constructive cycle we suggest that the radial striking structures reflect comparatively uniform stress fields during the constructive episodes mainly conditioned by the combination of overburden pressure gravitational spreading and magma induced stresses in each of the volcanic edifices by contrast in the shallower parts of the edifice the ne sw n s and wnw ese striking structures reflect local stress fields related to the formation of mega landslides and masking the general and regional radial patterns

solution and recrystallisation processes and associated landforms in gypsum outcrops of sicily abstract four small areas of messinian upper miocene age gypsum outcropping in western sicily are described messinian age evaporites are found in sicily over a 1000 km 2 area here gypsum outcrops extensively as a consequence of soil erosion induced by human impact geomorphological maps show how the rocky surfaces are characterized by a wide range of forms there are large medium small and microsized forms which can be identified as belonging to different morphotypes the morphotypes can be classified into two main categories those that originated by solution and those that originated through recrystallisation four areas illustrated by geomorphological maps were specifically chosen to describe a type of medium sized form dome like hills these medium sized forms are covered by a mosaic of smaller forms related to both the previous categories different types of karren and of expansion forms the types of karren can be explained as the results of the solution process under different hydro dynamical behaviour the dome like hills and other related expansion forms are more difficult to understand these expansion forms can be explained by the same process that leads to the development of gypsum tumuli the outcrops of gypsum lacking soil cover and influenced by alternating seasonal water conditions of surplus and deficit are affected by both solution and recrystallisation processes during the wet season the water soaks into the rocky mass filling all the fissures and pores of the outer rocky layer from a few centimetres to some metres below the surface during the dry season there is a capillary upward motion of the water solution near the surface gypsum precipitates from the oversaturated solution increasing the crystal size or forming new crystals in this way during the dry season there is a pressure increase in the outer gypsum layers which is responsible for the development of a gypsum weathering crust and characterised by many different forms such as gypsum tumuli pressure ridges pressure humps and other related small forms the crust may also lead to the development of mega tumuli and dome like hills from the morphostructural point of view the dome like hills do not seem to be controlled by the strike dip or fissuring of the gypsum beds their evolution seems to be linked to the fact that on most of the dome surfaces the weathering crust is evolving through a nearly isotropic field of stresses resulting in volume increase in the outer gypsum layer

gypsum karstification induced subsidence effects on alluvial systems and derived geohazards calatayud graben iberian range spain abstract the karstification of the tertiary gypsum formations in the calatayud graben has given rise to synsedimentary and postsedimentary subsidence phenomena in both the neogene sediments of the endorheic graben fill and in the quaternary alluvial sediments deposited under exorheic conditions in the so called maluenda and perejiles areas covering 4 4 and 12 km 2 respectively the neogene sedimentary units stratigraphically above the gypsum have subsided due to the karstification of the underlying evaporites the maximum subsidence in both areas has reached at least 200 m the jalon jiloca perejiles alluvial system has been affected by karstic subsidence during its quaternary evolution ten stepped alluvial levels pediment terrace have been identified as a consequence of synsedimentary karstic subsidence the deposit of an alluvial level can be locally thickened changing in a short distance from less than 10 m to more than 100 m in thickness thus the deposit of an individual terrace can be locally superimposed on the deposits of older terraces the palaeokarst recorded in the sediments of the different alluvial levels shows how the karstic subsidence has controlled the dynamics of the sedimentary system the alluvial karstic subsidence is currently active in the flood plains affecting fluvial channels buildings communication ways irrigation ditches and the calatayud gypsum scarp