stratigraphy and k ar ages across the west flank of the northeast iceland axial rift zone in relation to the 7 ma volcano tectonic reorganization of iceland lithostratigraphy magnetostratigraphy and new k ar ages from the region between eyjafjordur and skjalfandi north central iceland reflect tectonic reorganization of crustal accretion zone activity 7 ma two flood basalt piles are in unconformable contact along this western flank of the northeast iceland axial rift zone neiarz the older ranges in age from 9 5 to 13 ma and is largely composed of tholeiite flows in eastern dalsmynni the upper to middle portions of this older basaltic pile define a 15 35 se dipping monoclinal flexure developed 6 7 ma during early development of the present neiarz lavas of a flood basalt group younger than about 6 5 ma were deposited unconformably on the older flexured basalt pile these postflexure age flows mostly consist of compound doleritic basalts and olivine tholeiites possible tillites are found in the upper portion of this sequence the unconformity between the older and younger basalt piles represents a major structural temporal and lithological boundary in north central iceland zeolite zones appear subhorizontal through the dalsmynni flexure zone and have been largely reset subsequent to flexuring by westward onlap of the younger basalt sequence onto the older subsided and flexured flows radiometrie ages regional isochron patterns and timing constraints for tjornes fracture zone related shear deformation indicate the model of a singular 130 km eastward jump of the axial rift zone in north iceland 6 7 ma is too simple crustal accretion older than 12 ma along a proto neiarz is apparently required indicating that northern iceland has had multiple branched crustal accretion zones up until about 7 ma at which time a single spreading zone along the present neiarz developed an eastward shift of axial rift zone activity 6 7 ma in southwestern iceland is time correlative with reorganization of spreading activity in northern iceland our proposed 6 7 ma reorganization of axial rift zone activity on land is time correlative with an independently proposed increase in discharge from the iceland mantle plume

evidence for thin oceanic crust on the extinct aegir ridge norwegian basin ne atlantic derived from satellite gravity inversion 1 satellite gravity inversion incorporating a lithosphere thermal gravity correction has been used to map crustal thickness and lithosphere thinning factor for the ne atlantic predicted oceanic crustal thicknesses in the norwegian basin are between 4 and 7 km on the extinct aegir ridge increasing to 9 14 km at the margins consistent with volcanic margin continental breakup at the end of the paleocene the observation from gravity inversion and seismic refraction studies of thin oceanic crust produced by the aegir ridge in the oligocene may have implications for the temporal evolution of asthenosphere temperature under the ne atlantic during the tertiary thin oligocene oceanic crust may imply cool normal asthenosphere temperatures during the oligocene in contrast to elevated asthenosphere temperatures in the paleocene and miocene recent as indicated by the formation of volcanic margins and iceland respectively

downfan transformation from turbidity currents to debris flows at a channel to lobe transitional zone the lower pleistocene otadai formation boso peninsula japan transformation of turbidity currents into debris flows across a channel to lobe transition zone was recognized on the basis of lateral mapping of a falling stage systems tract deposit in the lower pleistocene otadai formation submarine fan sequence on the boso peninsula japan the most proximal outcrops comprise 1 to 4 m thick turbidites which are laterally equivalent to debrites encased in turbidites in outcrops farther down flow such turbidites and encased debrites were deposited from single flow events the most distal outcrops comprise only thinner 2 to 40 cm thick turbidites debrites contain many siltstone clasts and finer grained sediments which are characterized by deep water faunas and clay mineral composition similar to those of interbedded hemipelagites therefore the transformation of turbidity currents into debris flows is interpreted to have occurred in response to the incorporation of many siltstone clasts and finer grained sediment particles into the precursor turbidity currents through erosion of muddy substrates subsequently turbulence in the precursor turbidity currents was suppressed and near bed flow with higher sediment concentration developed intense erosion of muddy substrates is interpreted to have occurred in response to the increase in intensity of turbulence in the precursor turbidity currents at the mouths of middle fan channels the precursor turbidity currents or turbidity currents generated by dilution of debris flows produced a turbidite deposit which was subsequently overrun by the debris flow in a proximal lobe environment the debris flows are interpreted to have subsequently been transformed into turbidity currents which are recorded by turbidite deposition in the more distal lobe environment debrites can characterize the proximal portion of an attached lobe deposit in a channel to lobe transition zone such debrites can be significant heterogeneities for fluid flows in sand prone successions from middle fan channel to attached lobe deposits

the morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae experimental studies of subcritical unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves flow depth was 0 11 m mean flow velocities were 0 86 1 0 m s 1 shear velocities were 0 058 0 71 m s 1 and dimensionless shear stresses were 0 56 0 87 bedwaves are asymmetrical in profile and range from 0 75 to 11 mm in height mainly 2 6 mm from 0 7 to 1 3 m in wavelength and have mean celerities of 10 mm s 1 flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs high resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave

the role of subduction induced subsidence in the evolution of the karoo basin abstract the stratigraphic record indicates that during the late carboniferous to early triassic south africa experienced an episode of large scale subsidence that resulted in the development of the karoo basin the southern margin of the region was contemporaneously dominated by an episode of compressional tectonics and there is evidence for subduction of the paleo pacific plate beneath the karoo basin we propose that the long wavelength component of the basin subsidence resulted from the deflection of the lithosphere due to mantle flow coupled to the adjacent subduction this was followed by a recovery phase during which the entire region was uplifted on cessation of subduction and subsequent loss of dynamic support for subsidence contributing to the present day high elevation topography of south africa numerical models of mantle convection are presented and show that a shallow dipping slab is able to reconcile the observed long wavelength subsidence of the basin a significant component of the n

sedimentology stratigraphic occurrence and origin of linked debrites in the west crocker formation oligo miocene sabah nw borneo abstract the west crocker formation oligocene early miocene nw borneo consists of a large 20 000 km2 submarine fan deposited as part of an accretionary complex a range of gravity flow deposits are observed the most significant of which are mud poor massive sandstones interpreted as turbidites and clast rich muddy sandstones and sandy mudstones interpreted as debrites an upward transition from turbidite to debrite is commonly observed with the contact being either gradational and planar or sharp and highly erosive based on their repeated vertical relationship and the nature of the contact between them these intervals are interpreted as being deposited from one flow event which consisted of two distinct flow phases fully turbulent turbidity current and weakly turbulent to laminar debris flow the associated bed is called a co genetic turbidite debrite with the upper debrite interval termed a linked debrite linked debrites are best developed in the non channellised parts of the fan system and are absent to poorly developed in the proximal channel levee and distal basin floor environments due to outcrop limitations the genesis of linked debrites within the west crocker formation is unclear based on clast size and type it seems likely that a weakly turbulent to laminar debris flow flow phase was present when the flow event entered the basin a change in flow behaviour may have led to deposition of a sand rich unit with turbidite characteristics which was subsequently overlain by a mud rich unit with debrite characteristics flow transformation may have been enhanced by the disintegration and incorporation into the flow of muddy clasts derived from the upstream channel floor channel mouth or from channel levee collapse lack of preservation of this debrite in proximal areas may indicate either bypass of this flow phase or that the available outcrops fail to capture the debris flow entry point establishing robust sedimentological criteria from a variety of datasets may lead to the increasing recognition of co genetic turbidite debrite beds and an increased appreciation of the importance of bipartite flows in the transport and deposition of sediments in deepwater environments

crustal structure of iceland and greenland from receiver function studies 1 we used data from both permanent and temporary seismic networks on iceland and greenland to investigate the crustal thickness by partly reinterpreting earlier data p receiver functions and adding s receiver functions to better constrain the results we obtained good conversions from the moho and also crustal multiples in both iceland and greenland the central ice covered part of greenland has an average crustal thickness of 40 km typical for a craton at the edges of greenland the crustal thickness decreases to 30 40 km on the east coast of greenland where the track of the iceland plume is thought to have affected the lithosphere the crustal thickness is only 24 26 km in contrast to previous studies we find that the crustal thickness in the east and the northwest coastal regions of iceland is more than 40 km similar to beneath the active volcanic region in the southwest region of iceland and along the mid ocean ridge the crustal thickness is only 25 km or less also in contrast to earlier receiver function interpretations which deduced a broad crust mantle transition zone for iceland we find indications for a normal sharper moho beneath a number of sites

convolute laminations and load structures in turbidites as indicators of flow reflections and decelerations against bounding slopes examples from the marnoso arenacea formation northern italy and annot sandstones south eastern france abstract this work discusses the significance of particular types of soft sediment deformations very common within turbidite deposits namely convolute laminations and load structures detailed facies analyses of the foredeep turbidites in the marnoso arenacea formation northern italy and annot sandstones south eastern france show that these deformational structures tend to increase near morphological obstacles concomitantly with contained reflected beds the lateral and vertical distribution of convolute laminae and load structures as well as their geometry has a well defined depositional logic related to flow decelerations and reflections against bounding slopes this evidence suggests an interaction between fine grained sediment and the presence of morphologic relief and impulsive and cyclic wave loadings which are produced by flow impacts or reflected bores and internal waves related to impinging bipartite turbidity currents

origin and chemical evolution of formation waters from silurian devonian strata in the illinois basin usa abstract a suite of formation water samples from silurian devonian reservoirs in the illinois basin has been analyzed for major minor and trace element concentrations and for h o and sr isotopic compositions in order to interpret origin of salinity and geochemical evolution of brine compositions in this evaporite and shale poor cratonic basin although chloride concentrations range from 2000 to 137 000 mg l cl br ratios 291 18 are consistent with those of seawater or seawater evaporated short of halite saturation cl br 292 thus during silurian devonian time subaerially evaporated penesaline brine entered the subsurface where it was chemically modified through brine rock interactions cation br ratios and mineralogy of associated strata indicate that na and k were depleted through interaction with clay minerals ca was enriched and mg depleted by dolomitization and sr was enriched as a result of caco3 recrystallization and dolomitization although significant dilution of the modified brine with meteoric water is supported by d 18o covariance original marine waters have not been completely expelled from silurian devonian strata hydrogen and oxygen isotopes exhibit covariant relations with cation and anion concentrations implying that isotopic exchange between h2o and minerals has not greatly influenced the d 18o trend brine 87 sr 86 sr ratios range from 0 7092 to 0 7108 when these ratios are plotted versus 1 sr a two component mixing trend is suggested although sr concentrations have experienced local diagenetic modification a 87sr enriched fluid may have accompanied petroleum migration from new albany shales into adjacent silurian devonian carbonates where it mixed with remnant evaporated seawater this event probably preceded the influx of meteoric water as gdd and 18o are not correlated with sr isotopic compositions of formation waters

origin and evolution of the kolbeinsey ridge and iceland plateau n atlantic variations in crustal structure along the 700 km long krise7 refraction reflection and gravity profile straddling 66 5 n across the iceland shelf iceland plateau and western norway basin confirm that extinct spreading centers coexisted with the now extinct aegir ridge prior to the initiation of the kolbeinsey ridge at 26 ma the western 300 km of the profile across the iceland shelf formed by rifting at the kolbeinsey ridge whereas the eastern 400 km across the iceland plateau and the western norway basin formed by earlier rifting possibly containing slivers of older oceanic or continental crust rifted off the central e greenland margin along with the jan mayen ridge crustal thickness increases gradually across the iceland shelf from 12 to 13 km near the kolbeinsey ridge to 24 28 km near the eastern shelf edge decreasing abruptly across the shelf edge to 12 13 km the iceland plateau has crustal thickness ranging from 12 to 15 km decreasing to 5 8 km across the western norway basin and 4 5 km at the aegir ridge we suggest that high velocity lower crustal domes and corresponding gravity highs across the iceland plateau mark the location of extinct rift axes that coexisted with the aegir ridge similar lower crustal domes are associated with the currently active rift segments within iceland and the kolbeinsey ridge

older crust underlies iceland summary the oldest rocks outcropping in northwest iceland are 16 myr old and in east iceland 13 myr the full plate spreading rate in this region during the cenozoic has been 2c m a 1 and thus these rocks are expected to be separated by 290 km they are however 500 km apart the conclusion is inescapable that an expanse of older crust 210 km wide underlies iceland submerged beneath younger lavas this conclusion is independent of any considerations regarding spreading ridge migrations jumps the simultaneous existence of multiple active ridges three dimensionality or subsidence of the lava pile such complexities bear on the distribution and age of the older crust but not on its existence or its width if it is entirely oceanic its maximum age is most likely 26 37 ma it is at least 150 km in north south extent but may taper and extend beneath south iceland part of it might be continental a southerly extension of the jan mayen microcontinent this older crust contributes significantly to crustal thickness beneath iceland and the 40 km local thickness measured seismically is thus probably an overestimate of present day steady state crustal production at iceland

onset of submarine debris flow deposition far from original giant landslide submarine landslides can generate sediment laden flows whose scale is impressive individual flow deposits have been mapped that extend for 1 500 km offshore from northwest africa these are the longest run out sediment density flow deposits yet documented on earth this contribution analyses one of these deposits which contains ten times the mass of sediment transported annually by all of the world s rivers understanding how this type of submarine flow evolves is a significant problem because they are extremely difficult to monitor directly previous work has shown how progressive disintegration of landslide blocks can generate debris flow the deposit of which extends downslope from the original landslide we provide evidence that submarine flows can produce giant debris flow deposits that start several hundred kilometres from the original landslide encased within deposits of a more dilute flow type called turbidity current very little sediment was deposited across the intervening large expanse of sea floor where the flow was locally very erosive sediment deposition was finally triggered by a remarkably small but abrupt decrease in sea floor gradient from 0 05 degrees to 0 01 degrees this debris flow was probably generated by flow transformation from the decelerating turbidity current the alternative is that non channelized debris flow left almost no trace of its passage across one hundred kilometres of flat 0 2 degrees to 0 05 degrees sea floor our work shows that initially well mixed and highly erosive submarine flows can produce extensive debris flow deposits beyond subtle slope breaks located far out in the deep ocean

treitel ridge a unique inside corner hogback on the west flank of extinct aegir spreading ridge norway basin abstract detailed shipborne mapping sidescan sonar swath bathymetry seismic reflection 3 5 khz profiling gravity and magnetics of the extinct aegir spreading axis ca 55 25 ma in the norway basin revealed a highly unusual in a mor setting narrow 130 km long asymmetrical basement hogback treitel ridge tr on the southern west flank of the extinct rift close to its junction with the iceland faeroe aseismic ridge and the putative paleo iceland hotspot center the ridge seems to continue north another 130 km in the form of a wider more irregular basement rise for a combined length of ca 260 km the southern narrow part of tr approximately following or slightly younger than chron 18n ca 39 ma attains a basement relief of ca 1000 m in the south its summit descending gradually 1 7 100 towards the ne although an east flank conjugate appears generally lacking along both northern and southern parts of tr asymmetry of the southern portion steeper slope facing towards the extinct rift valley suggests volcanic formation and axially dipping normal faulting at or near the active plate boundary the possibility of continental crust extending close to the southern end of tr may also be important because some of this crust may have been remelted at depth contributing magma to feed tr eruptions southern tr may record a brief 0 1 1 m y inside corner tectonic event probably a flexurally compensated normal faulted offset in effect a mega abyssal hill we suggest tr was probably not an expression of change in iceland hotspot activity or somehow related to nearby continental crust but rather represents the volcano tectonic response of the aegir accretion axis at the time it became the eastern edge of the new jan mayen micro plate to extinction of the mid labrador sea spreading center as proposed by nunns 1983 the annexation of the greenland plate by the north american plate forced a plate boundary reorganization east of greenland with northward propagation of the mid atlantic ridge rift along the greenland margin to form the jan mayen micro plate simultaneously two ca 107 trending transforms developed from the previously 146 trending faeroes fracture zone both tr formation and fracture zone trend changes may record aegir ridge response to this major plate kinematic reorganization because tr was formed during a short interval spreading along the mid labrador sea ridge may have ceased abruptly as well

mixed sand mud bedforms produced by transient turbulent flows in the fringe of submarine fans indicators of flow transformation the fringe of fine grained deep marine systems often exhibits complex sedimentary facies and facies associations because the presence of clay promotes the development of transient turbulent flows with complex depositional properties relatively little is known about the variation of current induced sedimentary structures found within these facies this study provides the first comprehensive description and interpretation of mixed sandstone mudstone bedforms observed in the fringe of the mud rich submarine fan that makes up the aberystwyth grits group and borth mudstone formation wales united kingdom using textural and structural descriptions 158 bedforms in sediment gravity flow deposits were characterized into three main types classic sandy current ripples large current ripples and low amplitude bed waves the sandy current ripples comprise clean sandstone with average heights and lengths of 11 mm and 141 mm respectively the large current ripples are composed of mixed sandstone mudstone and possess greater dimensions than the sandy current ripples with an average height of 19 mm and an average length of 274 mm the low amplitude bed waves are long thin bedforms composed commonly of mixed sandstone mudstone with an average height and length of 10 mm and 354 mm respectively the large current ripples and low amplitude bed waves are strikingly similar to experimental bedforms produced under decelerating mixed sand mud flows and are interpreted to form beneath transitional flows with enhanced and attenuated near bed turbulence respectively from the fringe to the distal fringe of the fan the dominant bedform type changed from sandy current ripples via large current ripples to low amplitude bed waves suggesting that the flows changed from turbulent to increasingly turbulence modulated it is proposed that the flow reynolds number reduced reflecting this flow transformation from a combination of constant or decreasing flow height flow deceleration from sediment deposition and increasing flow viscosity due to the shear thinning nature of clay rich suspensions large current ripples and low amplitude bed waves are likely to be common in the fringe of other submarine fans the presence and spatial trends in mixed sand mud bedform types may be an important tool in interpreting fan fringe environments

topographic controls on the development of contemporaneous but contrasting basin floor depositional architectures abstract sediment laden gravity driven flow deposits on the basin floor are typically considered to form either discrete lobes that stack compensationally or packages of laterally extensive beds commonly termed sheets these end member stacking patterns are documented in several basinfills however whether they can coexist in a single basin or there are intermediate or transitional stacking patterns is poorly understood an analysis of depositional architecture and stacking patterns along a 70 km dip oriented transect in the upper broto turbidite system jaca basin south central pyrenees spain which displays disparate stacking patterns in contemporaneous strata is presented proximal and medial deposits are characterized by discrete packages of clean sandstones with sharp bed tops which exhibit predictable lateral and longitudinal facies changes and are interpreted as lobes distal deposits comprise both relatively clean sandstones and hybrid beds that do not stack to form lobes instead localized relatively thick hybrid beds are inferred to have inhibited the development of lobes hybrid beds developed under flows which were deflected and entrained carbonate mud substrate off a carbonate slope that bounded the basin to the south evidence for this interpretation includes 1 divergent paleoflow indicators and hummock like features in individual beds 2 a decrease in hybrid bed thickness and abundance away from the lateral confining slope 3 a carbonate rich upper division not seen in more proximal turbidites the study demonstrates the co occurrence of different styles of basin floor stacking patterns in the same stratigraphic interval and suggests that characterization of deep water systems as either lobes or sheets is a false dichotomy

the stratigraphic record and processes of turbidity current transformation across deep marine lobes sedimentary facies in the distal parts of deep marine lobes can diverge significantly from those predicted by classical turbidite models and sedimentological processes in these environments are poorly understood this gap may be bridged using outcrop studies and theoretical models in the skoorsteenberg formation south africa a downstream transition from thickly bedded turbidite sandstones to argillaceous internally layered hybrid beds is observed the hybrid beds have a characteristic stratigraphic and spatial distribution being associated with bed successions which generally coarsen and thicken upward reflecting deposition on the fringes of lobes in a dominantly progradational system using a detailed characterization of bed types including grain size grain fabric and mineralogical analyses a process model for flow evolution is developed this is explored using a numerical suspension capacity model for radially spreading and decelerating turbidity currents the new model shows how decelerating sediment suspensions can reach a critical suspension capacity threshold beyond which grains are not supported by fluid turbulence sand and silt particles settling together with flocculated clay may form low yield strength cohesive flows development of these higher concentration lower boundary layer flows inhibits transfer of turbulent kinetic energy into the upper parts of the flow ultimately resulting in catastrophic loss of turbulence and collapse of the upper part of the flow advection distances of the now transitional to laminar flow are relatively long several kilometres suggesting relatively slow dewatering several hours of the low yield strength flows the catastrophic loss of turbulence accounts for the presence of such beds in other fine grained systems without invoking external controls or large scale flow partitioning and also explains the abrupt pinch out of all divisions of these sandstones estimation of the point of flow transformation is a useful tool in the prediction of heterogeneity distribution in subsurface systems

multiple scale characterization of sand rich distal lobe deposit variability examples from the annot sandstones formation eocene oligocene se france abstract the present paper investigates the outcrop scale architecture of submarine gravity flow deposits in both moderately to highly confined settings a significant dataset was gathered on the sand rich confined siliciclastic turbidite system of the annot sandstone formation se france we focused on thick sedimentary bodies formed by depletive gravity flows in relatively distal settings sand rich lobes the purpose was to investigate the architectural variability of their small scale components beds and bedsets and consequently to evaluate the associated heterogeneities facies changes grain size sedimentary features sediment thickness etc therefore we provide a set of detailed descriptions on well exposed outcrops through very high resolution correlation panels built thanks to multiple vertical logs these turbidite system end members have rarely been described in such details and have often been considered as laterally continuous and homogeneous on the basis of the hierarchical subdivisions of turbidite lobes results show a high complexity in internal architecture from lobe bed to lobe scale whereas large scale scenery seismic scale is characterized by apparently homogeneous tabular and very extensive sandbodies thus corresponding to the classical definition of sheet sands this complexity implies heterogeneities not only in terms of sediment thickness changes abrupt pinch outs and variable scale compensation features but also in terms of facies and associated sedimentary features distributions tractive structures internal scours with associated by pass freezed mudclast rich layers etc this facies variability is controlled by the interplay between internal erosive bedforms i e large scours and constructive bedforms such as dome shaped or laterally stacked bedsets that are related to local topographic controls and compensation processes variability then appears both laterally and vertically besides results also showed small scale evidences of interactions between turbidite flows and basin floor only expressed at lobe bed to lobe element scale especially on the basin sides where paleoslope is subject to erosion and remobilisation and where flows can undergo significant changes concerning both seismic and reservoir implications although those deposits are characterized by a sheet like geometry this high internal variability necessarily implies heterogeneities in terms of petrophysical characteristics porosity permeability that may have a significant impact on fluid circulation these elements lead us to reconsider both sedimentary processes involved in their building and also reservoir models that can be established on field analogues

petrology and geochemistry of siliciclastic rocks of mixed feldspathic and ophiolitic provenance in the northern apennines italy provenance in clastic sedimentary rocks in the northern apennines is determined by integrated petrographic and geochemical criteria thus mafic poor arkoses in the gottero formation senonian paleocene deposited prior to the alpine orogeny were derived from granitic provenance in the sardinian corsican massif in the gottero formation chemical criteria distinguish between very low input of detrital mafic material and co c and ni added entirely by adsorption on clay minerals post alpine orogeny feldspathic sandstones in the macigno formation mid oligocene lower miocene and cervarola formation lower miocene reflect a more diverse provenance of granitic metamorphic ophiolitic and sedimentary rocks in the apennines and alps the macigno and cervarola rocks are distinguished by serpentine gabbro and diabase fragments and higher chlorite contents than in the gottero rocks chemically the macigno and cervarola rocks contain distinctive signatures expressed in higher mgo co cr and ni contents of the mafic ultramafic detritus a wider range and more scattered cr and ni variation and higher niggli ti for a given niggli al alk content the mgo co cr and ni abundances indicate up to 10 ultramafic ophiolitic detritus in the macigno and cervarola sandstones and up to 17 ophiolitic detritus in associated shales estimated by comparison of mixtures of feldspathic sands with ligurian peridotites up to 10 basaltic ophiolitic detritus in these sediments is suggested by cao and fe2o3 total abundances petrographic data indicate that a major part of the detritus in the macigno and cervarola formations is from alpine granitic basement of calc alkaline granite to tonalite plutons and gneisses but apart from higher ba in macigno and cervarola sandstones than in the gottero sandstones this is difficult to identify chemically the trace elements cr ni ba rb ce la y co cu ga pb and zn generally have positive correlations with niggli al alk a chemical surrogate measure of sheet mineral clays micas and chlorites content whereas sr which is associated with carbonate and feldspar shows negative correlation with al alk an olistostrome unit contains the most weathered detritus but otherwise shows similar chemical variation to the other formations methods to 1 identify separately mafic and ultramafic inputs 2 recognize in shales co cr and ni derived from ophiolitic sources and non ophiolitic sources 3 estimate the minimum mafic ultramafic proportion in a sediment may have general application elsewhere in sedimentary and metasedimentary rocks

linked turbidite debrite resulting from recent sahara slide headwall reactivation the northwest african margin has been affected by numerous large scale landslides during the late quaternary this study focuses on a recent collapse of the sahara slide headwall and characterises the resulting flow deposit core and seismic data from the base of the upper headwall reveal the presence of blocky slide debris comprising heavily deformed hemipelagic slope sediments the blocky slide debris spilled over a lower headwall 60 km downslope and formed a thick transparent debris flow unit cores recovered 200 250 km farther downslope contain a surficial turbidite that is interpreted to be linked to the headwall collapse event based on timing and composition one core located approximately 200 km from the headwall scar c13 contains debrite encased in turbidite the debrite comprises sheared and contorted hemipelagic mudstone clasts similar as those seen in the vicinity of the sahara slide headwall and lacks matrix this debrite pinches out laterally within 25 km of c13 whereas the accompanying turbidite can be correlated across 700 km of the northwest african margin the linked turbidite debrite bed is interpreted to have formed through recent failure of the steep sahara slide headwall that either 1 generated both a debris flow and a turbidity current almost simultaneously or 2 generated a debris flow which with entrainment of water and progressive dilution led to formation of an accompanying turbidity current

determining flow directions in turbidites an integrated sedimentological and magnetic fabric study of the miocene marnoso arenacea formation northern apennines italy abstract depositional models of turbidity flows require estimating paleocurrent directions using sedimentological indicators such as flute and ripple marks but these are not always present in outcrop sections or drill cores in this study we apply the anisotropy of magnetic susceptibility ams as an alternative tool to estimate paleocurrent directions in a case study turbiditic system the marnoso arenacea foredeep turbidites of miocene age exposed in the northern apennines of italy different depositional facies have been sampled for ams and additional rock magnetic analyses we observed a good agreement between paleocurrent directions from flute casts at the base of turbidite beds and mean directions of maximum magnetic susceptibility axes in organized facies such as massive and laminated sandstones even if a relatively small but apparently consistent offset of 15 20 seems to be present highly dispersed ams fabrics were instead observed in disordered facies such as convoluted and undulated sandstones as well as debrites this strong correlation between hydrodynamic regimes of depositional facies and ams data represents a novel contribution and confirms the validity of the ams method to estimate flow directions in the absence of sedimentological indicators finally paleomagnetic analyses from the literature were used to reconstruct the paleogeography of the marnoso arenacea basin and make inferences about the origin and direction of transport of the sediments at the basin scale

stratigraphy and depositional setting of slurry and contained reflected beds in the marnoso arenacea formation langhian serravallian northern apennines italy this work presents the stratigraphy and facies analysis of an interval of about 2500 m in the langhian and serravallian stratigraphic succession of the foredeep turbidites of the marnoso arenacea formation a high resolution stratigraphic analysis was performed by measuring seven stratigraphic logs between the sillaro and marecchia lines 60 km apart for a total thickness of about 6700 m the data suggest that the stratigraphy and depositional setting of the studied interval was influenced by syndepositional structural deformations the studied stratigraphic succession has been subdivided into five informal stratigraphic units on the basis of how structurally controlled topographic highs and depocentres a consequence of thrust propagation change over time these physiographic changes of the foredeep basin have also been reconstructed through the progressive appearance and disappearance of thrust related mass transport complexes and of five bed types interpreted as being related to structurally controlled basin morphology apart from bouma like type 4 beds type 1 tripartite beds characterized by an internal slurry unit tend to increase especially in structurally controlled stratigraphic units where intrabasinal topographic highs and depocentres with slope changes favour both mud erosion and decelerations type 2 beds with an internal slump type chaotic unit characterize the basal boundary of structurally controlled stratigraphic units and are interpreted as indicating tectonic uplift type 3 beds are contained reflected beds that indicate different degrees of basin confinement while type 5 are thin and fine grained beds deposited by dilute reflected turbulent flows able to rise up the topographic highs the vertical and lateral distribution of these beds has been used to understand the synsedimentary structural control of the studied stratigraphic succession represented in the marnoso arenacea formation by subtle topographic highs and depocentres created by thrust propagation folds and emplacements of large mass transport complexes

rheologic controls on inter rifting deformation of the northern volcanic zone iceland article i nfo extensional rifts are characterized by significant lateral variations in crustal rheology as lithospheric material cools and advects away from the rift axes nevertheless most models used for modeling of crustal movements consist of horizontal layers with uniform properties this paper explores the role spatial variation of rheological properties play in modifying the style of surface deformation at rifts by comparing finite element model predictions to inferred inter rifting surface deformation in iceland s northern volcanic zone crustal deformation has been observed by satellite radar interferometry insar and gps measurements extension is observed across the entire northern rift zone whereas subsidence occurs in two distinct areas corresponding to mapped fissure swarms where major rifting took place in 1975 1984 and 1874 1876 respectively our models indicate that the observed inter rifting plate spreading deformation field is controlled by rheological variations within the en echelon arrangement of fissure segments and that a central ridge axes model does not apply uniform stretching across a plate boundary zone where fissure swarms are weaker than the surrounding crust and reflect the surface expression of the rift reproduces the characteristics of the deformation field the most realistic fissure swarm structure consists of a wedge of

autogenic controls on hybrid bed distribution in submarine lobe complexes abstract hybrid beds the deposits of sediment gravity flows that show evidence for more than one flow regime turbulent transitional and or laminar have been recognized as important components of submarine lobe deposits a wide range of hybrid bed types have been documented however quantitative analysis of the stratigraphic and geographic distribution of these enigmatic bed types is rare here extensive exposures integrated with research borehole data from unit a of the laingsburg formation and fan 4 of the skoorsteenberg formation ecca group south africa provide the opportunity to examine geographical and stratigraphic patterns over a range of hierarchical scales for this purpose 23 000 individual beds have been evaluated for deposit type and bed thickness on average hybrid beds make up data from unit a laingsburg formation show hybrid beds are less common in the basal a 1 and top a 6 subunits compared to a 2 a 5 in both core data sets the bases and tops of some lobe complexes a 2 a 3 and a 5 7 are observed to be slightly enriched in hybrid beds whereas others a 5 1 a 5 5 and a 6 1 show no hybrid beds in their bases which does not conform to expected allogenically driven distributions that predict more hybrid beds during the initiation of lobe complexes instead the occurrence and distribution of hybrid beds in lobe complexes are interpreted to be controlled by autogenic processes including flow transformation processes on the basin floor meaning enrichment in frontal lobe fringe settings therefore the 1d distribution of hybrid beds in lobe complexes reflects the dominant stacking pattern of lobes within a lobe complex with enrichment at the base and top of lobe complexes due to overall progradational to retrogradational stacking patterns individual lobes show a wide range of hybrid bed distributions due to stacking patterns of the component lobe elements these findings highlight the importance of autogenic processes rather than allogenic controls in the distribution of hybrid beds which has implications for reservoir evaluation and the assessment of lobe stacking patterns in 1d core data sets

linked debrites in sand rich turbidite systems origin and significance abstract the outer parts of a number of small late jurassic sandy deep water fans in the northern north sea are dominated by the stacked deposits of co genetic sandy and muddy gravity flows sharp based structureless and dewatered sandstone beds are directly overlain by mudclast breccias that are often rich in terrestrial plant fragments and capped by thin laminated sandstones pseudonodular siltstones and mudstones the contacts between the clast rich breccias and the underlying sandstones are typically highly irregular with evidence for liquefaction and upward sand injection the breccias contain fragments up to metre scale of exotic lithologies surrounded by a matrix that is extremely heterogeneous and strewn with multiphase and variably sheared sand injections and scattered coarse and very coarse sand grains often coarser than in the immediately underlying sand bed markov chain analysis establishes that the breccias consistently overlie sandstones and the character of the breccias and their external contacts rule out a post depositional origin via in situ liquefaction intrastratal flowage or bed amalgamation and disruption the breccias are interpreted as debrites that rode on a water rich sand bed just deposited by a co genetic concentrated gravity current as such they are referred to as linked debrites to distinguish them from debrites emplaced in the absence of a precursor sand bed the distinction is important because these linked debris flows can achieve significant mobility through entrainment of both water and sediment from beneath and they ride on a low friction carpet of liquefied sand this explains the paradox presented by fan fringes in which there are common debrites when conventional thinking might predict that deposits of low concentration gravity currents should be more important here in fact evidence for transport by low concentration turbidity currents is rare in these systems several possible mechanisms might explain the formation of linked flows but the ultimate source of both sandy and clast rich flow components must be in shallower water on the basin margin the debrites are not triggered from distal slopes flow partitioning may have occurred by upslope erosion and retardation of the mudclast charged portion of an erosional sandy density current partial flow transformation of a precursor debris flow and or hydraulic segregation and reconcentration of the flaky clasts and carbonaceous matter during transport linked debrites are not restricted to small sand rich fans and similar mechanisms may be responsible for the long runout of debris flows in other systems the recognition of a distinct class of linked debrites is of wider importance for facies prediction reservoir heterogeneity and even carbon fluxes and sequestration on continental margins

frequent shifts of the volcanic zone in iceland the heterogeneous and locally anomalously thick icelandic upper crust cannot be fully explained with current models of crustal construction a model that invokes frequent shifts of the volcanic zone accreting plate boundary is proposed as an alternative three important parameters which result from a complex history of shifting determine the upper crustal structure of iceland 1 the time interval between two successive shifts 2 the scale of lateral displacements of the volcanic zone and 3 the intensity of volcanism at each volcanic zone

deposits of flows transitional between turbidity current and debris flow the relationship between submarine sediment gravity flows and the character of their deposits is poorly understood annular flume experiments were used to investigate the depositional dynamics and deposits of waning sediment laden flows decelerating fast 3 m s flows with fixed sand content 10 vol and variable mud content 0 17 vol resulted in only four deposit types clean sand with a mud cap that resembled a turbidity current deposit turbidite formed if the flow was turbulent when deposition began or if the muddy fluid had insufficient strength to suspend the sand the clean sand could contain structures if mud content was low 300 s ungraded muddy sand with a mud cap that resembled a debris flow deposit debrite formed if the flow became laminar before sand could deposit clean sand overlain by ungraded muddy sand and a mud cap formed either from a transitional flow or by late stage settling of sand from a muddy suspension these deposits resemble enigmatic submarine flow deposits called linked debrite turbidites the experiments provide a basis for inferring flow type from deposit character for submarine sediment laden flows

depositional features of co genetic turbidite debrite beds and possible mechanisms for their formation in distal lobated bodies beyond the base of slope ulleung basin east sea japan sea abstract a detailed analysis of the mr1 11 12 khz sonar images chirp 2 7 khz profiles core sediments and 14 c ages from the latest quaternary lobated bodies lb on the deep 2100 m water depth basin plain of the western ulleung basin reveals depositional features and context of co genetic turbidite debrite beds in the lb on the distal setting and a plausible mechanism for generating these beds eight lb are present in the basin plain ca 30 60 km beyond the base of slope and were generally deposited retrogressively older and more distal lb 1 4 have large dimensions 27 km long 15 25 km wide in contrast younger and less distal lb 5 8 are small 8 8 31 5 km long 1 2 12 km the muddier larger lb 1 and 2 were most likely originated from the relatively large scale sediment failures on the muddy upper slopes 350 400 m water depth between 18 5 and 20 0 cal ka b p on other hand the sandier smaller lb 6 and 7 were deposited between 17 0 and 17 5 cal ka b p probably by the relatively small scale sediment failures on the sandy uppermost slope shallower than 350 400 m water depth in lb 1 a lower sandy mud turbidite is transitional upward into an upper mud matrix debrite having small rounded mud clasts in contrast lb 6 exhibits a gradual upward change from a lower clay poor sandy turbidite to an upper clay rich sand matrix debrite with large interlocking mud clasts each mixed bed in lb 1 and 6 can represent a co genetic or linked turbidite debrite bed formed by the down dip flow transformation from turbidity current to debris flow at a point during the same event the abundant large scale up to 20 25 m deep 3 5 km long erosions of fine grained substrates near the base of slope area suggest that the down dip transformation was driven by incorporation of mud via erosional bulking by turbidity currents reaching the base of slope area the different clay sand content and size shape of mud clasts in the co genetic turbidite debrite beds between lb 1 and 6 imply that the composition sandy or muddy and dimension of initial turbidity current approaching the base of slope the volume of eroded masses from the fine grained substrates and the transport distance of sediment gravity flows from the erosional areas of muddy substrates can be all important factors controlling the depositional styles of co genetic turbidite debrite beds

distribution and origin of hybrid beds in sand rich submarine fans of the tanqua depocentre karoo basin south africa abstract this study documents the stratigraphic and palaeogeographic distribution of hybrid event beds that comprise both debris flow cohesive and turbidity current non cohesive deposits this is the first study of such beds in a submarine fan system to combine outcrop and research borehole control and uses a dataset from the skoorsteenberg formation of the tanqua depocentre in the karoo basin south africa three types of 0 1 1 0 m thick hybrid beds are observed which have a basal weakly graded fine grained sandstone turbidite division overlain by a division of variable composition that can comprise 1 poorly sorted carbonaceous rich material supported by a mud rich and micaceous sand matrix 2 poorly sorted mudstone clasts in a mud rich sand silt matrix or 3 gravel grade rounded mudstone clasts in a well sorted mud poor sandstone matrix these upper divisions are interpreted respectively as 1 the deposit of a debris flow most likely derived from shelf edge collapse 2 the deposit of a debris flow most likely developed through flow transformation from turbidity current that eroded a muddy substrate and 3 from a turbidity current with mudstone clasts transported towards the rear of the flow all three hybrid bed types are found concentrated at the fringes of lobes that were deposited during fan initiation and growth the basinward stepping of successive lobes means that the hybrid beds are concentrated at the base of stratigraphic successions in medial and distal fan settings hybrid beds are absent in proximal fan positions and rare and thin in landward stepping lobes deposited during fan retreat this distribution is interpreted to reflect the enhanced amounts of erosion and availability of mud along the transport route during early lowstands of sea level therefore hybrid beds can be used to indicate a fan fringe setting infer lobe stacking patterns and have a sequence stratigraphic significance

a sub crustal piercing point for north atlantic reconstructions and tectonic implications plate tectonic reconstructions are usually constrained by the correlation of lineaments of surface geology and crustal structures this procedure is however largely dependent on and complicated by assumptions on crustal structure and thinning and the identification of the continent ocean transition we identify two geophysically and geometrically similar upper mantle structures in the north atlantic and suggest that these represent remnants of the same caledonian collision event the identification of this structural lineament provides a sub crustal piercing point and hence a novel opportunity to tie plate tectonic reconstructions further this structure coincides with the location of some major tectonic events of the north atlantic post orogenic evolution such as the occurrence of the iceland melt anomaly and the separation of the jan mayen microcontinent we suggest that this inherited orogenic structure played a major role in the control of north atlantic tectonic processes

anatomy of turbidites and linked debrites based on long distance 120 x 30 km bed correlation marnoso arenacea formation northern apennines italy much of our understanding of submarine sediment laden density flows that transport very large volumes ca 1 to 100 km3 of sediment into the deep ocean comes from careful analysis of their deposits direct monitoring of these destructive and relatively inaccessible and infrequent flows is problematic in order to understand how submarine sediment laden density flows evolve in space and time lateral changes within individual flow deposits need to be documented the geometry of beds and lithofacies intervals can be used to test existing depositional models and to assess the validity of experimental and numerical modelling of submarine flow events this study of the miocene marnoso arenacea formation italy provides the most extensive correlation of individual turbidity current and submarine debris flow deposits yet achieved in any ancient sequence one hundred and nine sections were logged through a ca 30 m thick interval of time equivalent strata between the contessa mega bed and an overlying columbine marker bed correlations extend for 120 km along the axis of the foreland basin in a direction parallel to flow and for 30 km across the foredeep outcrop as a result of post depositional thrust faulting and shortening this represents an across flow distance of over 60 km at the time of deposition the correlation of beds containing thick 40 cm sandstone intervals are documented almost all thick beds extend across the entire outcrop area most becoming thinly bedded 40 cm in distal sections palaeocurrent directions for flow deposits are sub parallel and indicate confinement by the lateral margins of the elongate foredeep flows were able to traverse the basin in opposing directions suggesting a basin plain with a very low gradient small fractional changes in stratal thickness define several depocentres on either side of the verghereto high area the extensive bed continuity and limited evidence for flow defection suggest that intrabasinal bathymetric relief was subtle substantially less than the thickness of flows thick beds contain two distinct types of sandstone ungraded mud rich sandstone intervals record evidence of en masse debrite deposition graded mud poor sandstone intervals are inferred to result from progressive grain by grain turbidite deposition clast rich muddy sandstone intervals pinch out abruptly in downflow and crossflow directions in a fashion consistent with en masse debrite deposition the tapered shape of mud poor sandstone intervals is consistent with an origin through progressive grain by grain turbidite deposition most correlated beds comprise both turbidite and debrite sandstone intervals intrabed transitions from exclusive turbidite sandstone to turbidite sandstone overlain by debrite sandstone are common in the downflow and crossflow directions this spatial arrangement suggests either i bypass of an initial debris flow past proximal sections ii localized input of debris flows away from available sections or iii generation of debris flows by transformation of turbidity currents on the basin plain because of seafloor erosion and or abrupt flow deceleration a single submarine flow event can comprise multiple flow phases and deposit a bed with complex lateral changes between mud rich and mud poor sandstone

upper pleistocene turbidite sand beds and chaotic silt beds in the channelized distal outer fan lobes of the mississippi fan cores from a mississippi outer fan depositional lobe demonstrate that sublobes at the distal edge contain a complex local network of channelized turbidite beds of graded sand and debris flow beds of chaotic silt off lobe basin plains lack siliciclastic coarse grained beds the basin plain mud facies exhibit low acoustic backscatter on seamarc ia sidescan sonar images whereas high acoustic backscatter characteristic of the lobe sand and silt facies the depth of the first sand silt layer correlates with relative backscatter intensity and stratigraphic age of the distal sublobes i e shallowest sand highest backscatter and youngest sublobe the high proportion 50 of chaotic silt compared to graded sand in the distal outer fan sublobes may be related to the unstable muddy canyon wall source areas of the extensive mississippi delta fed basin slope a predominace of chaotic silt in cores or outcrops from outer fan lobes thus may predict similar settings for ancient fans

submarine transitional flow deposits in the paleogene gulf of mexico gravity driven flows on the seafloor are the largest yet least well understood sediment transport agents on earth recent exploration wells in ultradeep basins have revealed the presence of large sandy submarine fan systems of enigmatic facies types many hundreds of kilometers from paleocoastlines these sedimentary deposits often defy conventional turbidite or debrite interpretations having a character suggestive of deposition from flows with transient turbulent laminar rheologies in the wilcox formation gulf of mexico inferred transitional flow deposits have distinctive stratigraphic stacking patterns from fine grained debrites to coarser grained turbidites the vertical sequence of beds is here inferred to reflect the longitudinal bed distribution in response to lobe progradation and demonstrates a transition from well mixed turbulent flow to progressively more rheologically stratified flow and eventually to fully laminar flow the progressive development of internal rheological boundaries resulted in a high concentration but fluidal basal layer and an upper quasi laminar layer with an overriding sheared dilute turbidity current the long runout of the flows is linked to their high silt and clay content it is most likely flow expansion at the channel lobe transition that drives flow transformation this process based model may be applicable to many deep water settings and provides a framework within which to interpret the stratigraphic and spatial distribution of these complex deposits

tb or not tb banding in turbidite sandstones recognition and interpretation of sedimentary structures is fundamental to understanding sedimentary processes banded sandstones are an enigmatic sedimentary facies comprising alternating mud rich as matrix and or mud clasts and cleaner sand layers the juxtaposition of hydrodynamically different grain sizes contradicts established models of cleaner sand bedform development here outcrop subsurface core and petrographic data from three deep water systems with well constrained paleogeographic contexts are used to describe the range of sedimentary textures bedform morphologies and facies associations and to quantify the mud content of banding banding can occur in any part of a bed base middle or top but it typically overlies a structureless basal sandstone or mud clast conglomerate lag and is overlain by clean parallel laminated sandstone and or ripple cross lamination banding morphology ranges from sub parallel to bedforms that comprise low angle laminae with discontinuous lenses of mudstone or asymmetric bedforms comprising steeply dipping foresets that transition downstream into low amplitude bedwaves or steeply dipping ripple like bedforms with heterolithic foresets this style of banding is interpreted as a range of bedforms that form progressively in the upper stage plane bed flow regime via tractional reworking beneath mud laden transitional plug flows the balance of cohesive and turbulent forces and the rate of flow deceleration aggradation rate govern the style of deposit banded sandstones and linked debrites are rarely found juxtaposed together in the same bed because they are distributed preferentially in proximal and distal settings respectively understanding the origins of banding in turbidite sandstones the conditions under which it forms and its distribution across deep water systems and relationship to linked debrites is important for it to be used effectively as a tool to interpret the geological record

hybrid event beds dominated by transitional flow facies character distribution and significance in the maastrichtian springar formation north west voring basin norwegian sea hybrid event beds comprising clay poor and clay rich sandstone are abundant in maastrichtian aged sandstones of the springar formation in the north west voring basin norwegian sea this study focuses on an interval informally referred to as the lower sandstone which has been penetrated in five wells that are distributed along a 140 km downstream transect systematic variations in bed style within this stratigraphic interval are used to infer variation in flow behaviour in relatively proximal and distal settings although individual beds were not correlated the lower sandstone shows an overall reduction of total thickness bed amalgamation sand to mud ratio and grain size in distal wells turbidites dominated by clay poor sandstone are at their most common in relatively proximal wells whereas hybrid event beds are at their most common in distal wells hybrid event beds typically comprise a basal clay poor sandstone non stratified or stratified overlain by banded sandstone with clay rich non stratified sandstone at the bed top the dominant type of clay poor sandstone at the base of these beds varies spatially non stratified sandstone is thickest and most common proximally whereas stratified sandstones become dominant in distal wells stratified and banded sandstone record progressive deposition of the hybrid event bed thus the facies succession within hybrid event beds records the longitudinal heterogeneity of flow behaviour within the depositional boundary layer this layer changed from non cohesive at the front through a region of transitional behaviour fluctuating non cohesive and cohesive flow to cohesive behaviour at the rear further spatial variation in the dominant type of clay poor sandstone at the bed base suggests that the front of the flow remained non cohesive and evolved from high concentration and turbulence suppressed to increasingly turbulent flow this is thought to occur in response to deposition and declining sediment fall out this research may be applicable to other hybrid event bed prone systems and emphasises the dynamic nature of hybrid flows this article is protected by copyright all rights reserved

diagenetic history of pipe creek jr reef silurian north central indiana u s a abstract calcite cements in the silurian ludlovian pipe creek jr reef north central indiana are compositionally zoned with characteristic minor element concentrations and stable isotope signatures and were precipitated in different diagenetic environments superposition and crosscutting relationships allow us to group cement zones and to relate them to the sequence stratigraphic evolution of the reef pipe creek jr reef grew in normal marine waters with the reef top high greater than 50 m above the platform floor flank facies are volumetrically important and are preserved largely as limestone in contrast to most dolomitized silurian reefs in the midcontinent syndepositional marine cements fill primary porosity and synsedimentary fractures and are interlayered with marine internal sediment now low magnesium calcite their isotopic compositions are similar to those of depositional grains and cements estimated to have precipitated from ludlovian sea waters depositional porosity was reduced by 75 by the precipitation of these syndepositional cements which stabilized the steeply dipping flank slope postdepositional clear calcite cements are interpreted as shallow phreatic and burial cements on the basis of their relationship to periods of karstification and fracturing shallow phreatic cements with concentric cathodoluminescent cl zonation precipitated in primary pores and are postdated by fractures and caves filled with middle devonian sandstone cl zonal boundaries are sharp and some near a major stratigraphic unconformity show evidence of dissolution the volumetric abundance of the individual cl zones varies in the reef indicating a complex superposition of waters of varying chemistry and rock water interaction that are probably related to relative sea level changes this important aspect of the reef stratigraphy is recorded only by the diagenetic succession because evidence of earlier sea level changes is removed by a major later regional unconformity burial cements are the youngest diagenetic feature recognized and they rest conformably or unconformably over older cements they exhibit both concentric cl zonation and sectoral zoning they are ferroan to nonferroan and they contain thin sulfide zones along growth band boundaries their isotopic compositions do not overlap with shallow phreatic or marine cement values degraded oil postdates burial cements and is composed of the same sterane class as the devonian age antrim shale the probable source rock this source contrasts with that of reef reservoirs in the michigan basin where silurian strata are commonly the hydrocarbon source

crust generated above the iceland mantle plume from continental rift to oceanic spreading center since the north atlantic continental breakup in the early tertiary the process of rifting above a mantle plume has produced large thicknesses of igneous crust we report results of an integrated offshore onshore seismic study of the crust and upper mantle along a transect of the aseismic faroe iceland ridge between the continental fragment beneath the faroe islands and the present day spreading center in northeast iceland normal incidence seismic data provide an image of the uppermost crust which is complemented by a velocity model from streamer refraction analyses these data together image four sedimentary basins up to 400 m deep along the crest of the ridge the streamer refraction velocity model in turn forms the uppermost section of a full crustal velocity model derived from travel time modeling of air gun and explosive data the compressional to shear wave velocity ratio in the crust of the faroe iceland ridge is 1 77 0 02 similar to that of eastern iceland the moho beneath the faroe iceland ridge lies at a depth of 25 30 km crust of 25 30 km thickness requires an upper mantle potential temperature elevated 200 250 c above normal if formed by passive adiabatic decompression melting of the mantle or a somewhat less elevated temperature if as is likely there is a component of active convection in the underlying mantle plume core

the norway basin revisited from continental breakup to spreading ridge extinction abstract we re evaluate the structure and spreading evolution of the southeastern norway basin nb based on a new high resolution aeromagnetic survey nb 07 the survey covers a complete oceanic spreading segment from the continental oceanic transition of the more margin off norway to the aborted aegir ridge the new survey documents a transform margin an orthogonally rifted segment and an oblique sheared volcanic margin formed during the onset of breakup observed from the east jan mayen fracture zone to the faroe platform the detailed fabric of the nb revealed by the new data indicates that two distinct tectonic phases have reshaped the basin before the cessation of seafloor spreading and abortion of the aegir ridge in the late oligocene after continental breakup phase i from c24 to c21r 52 to 49 ma marks the earliest phase of spreading probably initiated in the central and outer part of the more basin during this period competing oceanic segments led to the formation of overlapping systems and pseudo fault development we observe a significant change in the nb s oceanic spreading system in the late early eocene and based on observations from surrounding areas we suggest that this is a record of a major tectonic event in the norwegian greenland sea around c21r 49 47 9 ma during phase ii from c21r to c10 48 to 28 ma of nb s development spreading rates decreased spreading direction changed and the number of faulting with large displacement increased leading to the formation of unexpected n s oriented oceanic fracture zones the fan shaped development of the spreading system initiated around c21r 49 47 9 ma instead of c18 c17 40 38 ma or c24 53 3 52 3 ma as previously proposed these new observations were used to re evaluate the tectonic evolution of the norwegian greenland sea and discuss some implications on the syn and post breakup development of the surrounding continental margins and the evolution of the jan mayen microcontinent

glacial isostatic adjustment and the viscosity structure underlying the vatnajokull ice cap iceland we examine the dependence of glacial isostatic adjustment gia due to changes in the vatnajokull ice cap iceland on the underlying viscosity structure iceland offers a unique case study for gia research with a thinner elastic lithosphere underlain by a low viscosity zone or asthenosphere as opposed to regions such as fennoscandia or north america described by a thicker lithosphere while not necessarily featuring an asthenosphere

holocene tectonic uplift of the st peter and st paul rocks equatorial atlantic consistent with emplacement by extrusion abstract st peter and st paul rocks spspr are emergent portions of a peridotitic mass within the st paul fracture zone of the equatorial mid atlantic ridge where local seismicity is dominated by strike slip mechanisms we used subtidal red algae in living position death assemblages in intertidal conglomerates and marine terraces as sea level markers the highest deposits correspond to a marine terrace with a shoreline angle at 8 1 m asl that formed ca 6400 6200 cal yr bp after correction for the sea level changes derived by glacioisostatic modelling ams and radiometric 14 c age measurements on the holocene sea level markers indicate emergence during the last 6600 a at a minimum average rate of 1 5 mm a we propose that uplift resulted from peridotite emplacement by extrusion rather than volcanic activity or fault slip with the implication that the ridge has been subject to compression during the holocene

deposition of ungraded muds from high density non turbulent turbidity currents turbidity current deposits of mud have recently been found up to 10m in thickness resulting from single depositional events1 2 turbidites 1 5 m thick from the madeira abyssal plain3 4 display long segments with little vertical change of grain size5 6 this suggests that the currents froze in some way making them unable to sort sediment here we suggest that a decline in flow speed of these currents led to a concentration gradient in silt close to the bed that was sufficiently steep to stratify the flow a concentration gradient of 18 kg m 3 over 1 5 m height gives a layer richardson number above the critical value of 0 25 for damping of turbulence we propose that with turbulence severely damped a dense suspension of fine material consolidates like a static suspension while continuing to flow downslope the current changes from a fully turbulent flow to a decelerating flow with suppressed turbulence and increasing density in the latter state interparticle forces prevent differential settling of the coarser grains

frontal and lateral submarine lobe fringes comparing sedimentary facies architecture and flow processes submarine lobe fringe deposits form heterolithic successions that may include a high proportion of hybrid beds the identification of lobe fringe successions aids interpretation of paleogeographic setting and the degree of basin confinement here for the first time the sedimentological and architectural differences between frontal and lateral lobe fringe deposits are investigated extensive outcrop and core data from fan 4 skoorsteenberg formation karoo basin south africa allow the rates and style of facies changes from axis to fringe settings of lobes and lobe complexes in both down dip frontal and across strike lateral directions to be tightly constrained over an 800 km2 study area fan 4 comprises three sand prone divisions that form compensationally stacked lobe complexes separated by thick packages of thin bedded siltstone and sandstone intercalated with muddy siltstone interpreted as the fringes of lobe complexes lobe fringe facies associations comprise i thick bedded structureless or planar laminated sandstones that pinch and swell and are associated with underlying debrites ii argillaceous and mudclast rich hybrid beds and iii current ripple laminated sandstones and siltstones typically frontal fringes contain high proportions of hybrid beds and transition from thick bedded sandstones over length scales of 1 to 2 km in contrast lateral fringe deposits tend to comprise current ripple laminated sandstones that transition to thick bedded sandstones in the lobe axis over several kilometers variability of primary flow processes are interpreted to control the documented differences in facies association preferential deposition of hybrid beds in frontal fringe positions is related to the dominantly downstream momentum of the high density core of the flow in contrast the ripple laminated thin beds in lateral fringe positions are interpreted to be deposited by more dilute low density parts of the flows the development of recognition criteria to distinguish between frontal and lateral lobe fringe successions is critical to improving paleogeographic reconstructions of submarine fans at outcrop and in the subsurface and will help to reduce uncertainty during hydrocarbon field appraisal and development

rheological complexity in sediment gravity flows forced to decelerate against a confining slope braux se france abstract hybrid event beds are now recognized as an important component of many deep sea fan and sheet systems they are interpreted to record the passage of rheologically complex sediment gravity currents hybrid flows that comprise turbulent transitional and or laminar zones hitherto the development of hybrid flow character has mainly been recognized in system fringes and attributed to distal and lateral flow transformations and or declining turbulence energy expressed over lateral scales of several kilometers or more however new field data show that deposition from hybrid flows can occur relatively proximally where flows meet confining topography turbidity currents primed to transform to hybrid flows by up dip erosion and incorporation of clay may be forced to do so by rapid slope induced decelerations within 1 km of the slope local flow transformation and deposition of hybrid event beds offer an alternative explanation for unusual facies developed at the foot of flow confining seafloor slopes

crustal structure beneath the faroe islands and the faroe iceland ridge abstract we have mapped the transition from the continental faroe block the faroe islands and surrounding shelf to the thickened oceanic crust of the faroe iceland ridge in the north atlantic using the results of a detailed sea to land seismic profile with wide angle to normal incidence recordings of explosive and airgun shots fired at sea along the faroe iceland ridge interpretation of all available seismic and gravity data indicates that this aseismic ridge is composed of 30 3 km thick oceanic crust with a gradual transition to ancient continental crust from 100 to 40 km northwest of the faroe islands close to the shelf edge this confirms that the crust beneath the faroe islands which may be up to 46 km thick comprises continental material in agreement with previous seismic and geochemical results results suggest that the upper 5 2 0 7 km of the faroe crust consists of tertiary basalts generated during continental breakup overlying the continental crust beneath the lower crust where seismic constraint is poor may exhibit high seismic velocities 7 1 7 6 km s 1 which we attribute to underplating or intrusion by mafic melts during continental breakup in the early tertiary

spatial variations in the composition of turbidites due to hydrodynamic fractionation 1 this study uses measurements from physical experiments to document turbidity currents which are density currents composed of suspended sediment and water to be effective at hydrodynamically fractionating minerals on the basis of grain density and grain shape alone resulting in large scale spatial variations in the composition of their deposits while grain composition varies spatially the population sampled at any one location is hydrodynamically equivalent spatial variations in composition of the deposits are modeled using exponential decay functions which are based on initial concentration of grain types and their respective differences in settling velocity we further discuss implications of this process for addressing practical geophysical problems in which mineralogical distributions are important such as provenance and geochronology studies subsurface imaging and predicting bulk properties of subsurface reservoirs

deglaciation sequences in the permo carboniferous karoo and kalahari basins of southern africa a tool in the analysis of cyclic glaciomarine basin fills the late westphalian to artinskian glaciomarine deposits of the karoo and kalahari basins of southern africa consist of massive and stratified diamictite mudrock with ice rafted material sandstone silty rhythmite shale and subordinate conglomerate forming a cyclic succession recognizable across both basins a complete cycle comprises a resistant basal unit of apparently massive diamictite overlain by softer bedded stratified diamictite sandstone and mudrock with a total thickness of as much as 350 m four major cycles are observed each separated by bounding surfaces lateral facies changes are present in some cycles the massive diamictites formed as aprons and fans in front of the ice grounding line whereas the stratified diamictites represent more distal debris flow fans the sandstones originated in different environments as turbidite sands small subaqueous outwash channel sands and delta front sands the rhythmites and mudrock represent blanket deposits derived from turbid meltwater plumes cycles represent deglaciation sequences which formed during ice retreat phases caused by eustatic changes in the karoo and kalahari basins evidence for shorter term fluctuation of the ice margin is present within the major advance retreat cycles hardly any sediment was deposited during lowstand ice sheet expansion whereas a deglaciation sequence was laid down during a sea level rise and ice margin retreat with the volume of meltwater and sediment input depending on temporary stillstands of the ice margin during the retreat phase the duration of the cycles is between 9 and 11 ma suggesting major global tectono eustatic events smaller cycles probably linked to orbital forcing were superimposed on the longer term events a sequence stratigraphic approach using the stacking of deglaciation sequences with the ice margin advance phases forming bounding surfaces can be a tool in the framework analysis of ancient glaciomarine basin fills

high resolution stratigraphic analysis of a turbidite system tanqua karoo basin south africa abstract the tanqua submarine fan complex is located in the southwestern karoo basin of south africa and comprises four basin floor fans fans 1 4 and one slope fan fan 5 fan 3 exhibits an extensively exposed turbidite system which enables a detailed investigation of its architecture and growth history a fan wide stratigraphic correlation has enabled the construction of a deterministic 3d fan model illustrated by correlation panels and isopach maps the stratigraphy of fan 3 comprises 10 depositional units interpreted as six sandy fan growth phases separated by four intervals of reduced deposition the clinoformal geometry of the units defines fan 3 as a prograding fan characterized by three depositional phases the initial phase of turbidite sedimentation is represented by basal scours and deposition of thin bedded channel fill and sheet deposits the second phase is characterized by widespread channeling moderate overbank deposition and significant mid and outer fan construction the final phase is marked by bypass of the proximal axial channel system overbank aggradation and reduced progradation on the mid and distal fan

a sedimentological process based approach to depositional reservoir quality of deep marine sandstones an example from the springar formation northwestern voring basin norwegian sea reservoir properties of deep marine sandstones deposited by various sediment gravity flow types in the northwestern voring basin norwegian sea are characterized using detailed sedimentological and petrographic analyses the sandstones are quartz arenites and subarkoses having similar mineralogical compositions a low degree of diagenetic modification and low to moderate volumes of quartz cement up to 6 however significant textural variability is observed for the various sandstone types which were subdivided based upon their interpreted processes of deposition in the studied wells there is a predominance of turbidites deposited by high density turbidity currents hdts and a range of hybrid event beds hebs hdts are coarser grained and contain less detrital clay than the various hebs in addition the dominant grain size of hebs decreases down the depositional profile whilst the content of detrital clay increases porosities of the various bed types span the same interval but overall the hdts and proximal hebs have a higher proportion of samples with porosities higher than 20 compared to distal hebs permeabilities in contrast are significantly different with the hdts having permeabilities approximately two orders of magnitude higher than clay rich hebs a comparison of the observed variability in composition and textural parameters with porosity and permeability indicates that sedimentological flow processes control the depositional reservoir quality of deep marine sandstones understanding these processes therefore enhances our ability to predict distribution of reservoir quality in deep marine systems

the structure and kinematics of substrate entrainment into high concentration sandy turbidites a field example from the gorgoglione flysch of southern italy sandy turbidites commonly show evidence for significant dynamic coupling with their substrate the resulting deformation can be described using structural kinematic methods linked to palaeoflow indicators to better understand the links between flow and entrainment processes a field example from the syn orogenic gorgoglione flysch a succession of upper miocene turbidites deposited into a deforming array of thrust top basins in the southern apennine thrust belt italy is described the succession contains metre scale packages of alternating sandy turbidites and shales but is notable for containing 100 m thick massive sandbodies these are structureless apart from sporadic horizons of aligned mud clasts commonly the substrate beneath the massive sandbodies is deformed with minor folds and thrusts verging in the direction of palaeoflow determined from tool marks and flutes at the base of these sandbodies structural studies from the base of a selected massive sandbody have identified that the substrate mud has been injected upwards with flames sheared over in the direction of palaeoflow thus the substrate has deformed and become entrained during emplacement of the massive sandy body at some locations the substrate can be traced into the overlying deposit with substrate clay beds becoming boudinaged and entrained into the sandbody analysis of the orientation of the mud clasts indicates that this bed disruption and incorporation into the sandy massive bed turbidite was an organized viscous process these features indicate that significant shear stress was partitioned out of the flow and onto the substrate the incorporation and disruption of substrate into the sandbody suggest that post disruption strains increase upwards implying that displacement gradients increased into the flow these behaviours showing variations in strain partitioning between the flow and its substrate are explored in terms of evolving flow dynamics and substrate rheology

prediction of hydrocarbon recovery from turbidite sandstones with linked debrite facies numerical flow simulation studies abstract a series of two dimensional numerical flow simulations were carried out to investigate the production characteristics of a sheet sandstone bed with a linked debrite interval a deterministic geological model was used based on a two dimensional representation of a bed from the marnoso arenacea formation the model was 60 km long and 1 m thick and contained three zones arranged in a vertical facies arrangement typical of many linked debrite beds i a lower coarse to medium grained clean turbidite sandstone interval ii a middle muddy sandstone debrite interval iii an upper fine grained clean laminated sandstone interval simulation involved only a 3 km long sector of the model with one injector well and one production well placed 1 km apart in the middle of the sector model the simulated sector was moved progressively down the length of the bed in 1 km steps sampling different parts of the bed with different facies proportions the petrophysical properties of the debrite interval were varied to produce different porosity permeability cases all other modelling parameters including the upper and lower interval petrophysics were kept constant results indicate that in most cases key production parameters such as cumulative oil production with time and water cut are proportional to the volume of movable oil between the wells this relationship does not hold however for cases with relatively low values of debrite porosity 0 15 and permeability kh 100 md where the debrite interval accounts for more than 20 of the interwell volume in these models production efficiency declines systematically with reducing reservoir quality and increasing debrite percentage resulting in relatively low oil production and early water breakthrough

can liquefied debris flows deposit clean sand over large areas of sea floor field evidence from the marnoso arenacea formation italian apennines the marnoso arenacea formation in the italian apennines is the only ancient rock sequence where individual submarine sediment density flow deposits have been mapped out in detail for over 100 km bed correlations provide new insight into how submarine flows deposit sand because bed architecture and sandstone shape provide an independent test of depositional process models this test is important because it can be difficult or impossible to infer depositional process unambiguously from characteristics seen at just one outcrop especially for massive clean sandstone intervals whose origin has been controversial beds have three different types of geometries facies tracts in downflow oriented transects facies tracts 1 and 2 contain clean graded and ungraded massive sandstone deposited incrementally by turbidity currents and these intervals taper relatively gradually downflow mud rich sand deposited by cohesive debris flow occurs in the distal part of facies tract 2 facies tract 3 contains clean sandstone with a distinctive swirly fabric formed by patches of coarser and better sorted grains that most likely records pervasive liquefaction this type of clean sandstone can extend for up to 30 km before pinching out relatively abruptly this abrupt pinch out suggests that this clean sand was deposited by debris flow in some beds there are downflow transitions from turbidite sandstone into clean debrite sandstone suggesting that debris flows formed by transformation from high density turbidity currents however outsize clasts in one particular debrite are too large and dense to have been carried by an initial turbidity current suggesting that this debris flow ran out for at least 15 km field data indicate that liquefied debris flows can sometimes deposit clean sand over large 10 to 30 km expanses of sea floor and that these clean debrite sand layers can terminate abruptly

reversing buoyancy in turbidity currents developing a hypothesis for flow transformation and for deposit facies and architecture a turbidity current that contains fresher or otherwise less dense water than its surroundings may initially be denser than the ambient and propagate as a bottom hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient seawater it is proposed that this reversal in buoyancy may be a significant mechanism controlling the structure and facies of turbiditic deposits buoyancy reversal followed by lofting may directly affect the relative distribution of fine and coarse material in the deposit while buoyancy reversal itself may mediate the transformation between dilute and highly concentrated suspension flows particularly in distal regions and thus lead to the formation of complex turbiditic beds in particular the generation of distal co genetic debrites may be expected similar transformations occur within dilute pyroclastic density currents where a mobile basal concentrated flow termed a surge derived pyroclastic flow develops through rapid sedimentation from the suspended load of the overlying surge the physical mechanisms involved in these processes are discussed leading to the proposal of some associated facies models these are compared with field data from the northern apennines with some striking similarities being noted as well as some differences on the basis of this discussion some directions are suggested for future experimental and modelling work on the topic

subduction mega shear systems and late palaeozoic basin development in the african segment of gondwana basins within the african sector of gondwana contain a late palaeozoic to early mesozoic gondwana sequence unconformably overlying precambrian basement in the interior and mid palaeozoic strata along the palaeo pacific margin small sea board pacific basins form an exception in having a carboniferous to early permian fill overlying devonian metasediments and intrusives the late palaeozoic geographic and tectonic changes in the region followed four well defined consecutive events which can also be traced outside the study area during the late devonian to early carboniferous period up to 330 ma accretion of microplates along the patagonian margin of gondwana resulted in the evolution of the pacific basins thermal uplift of the gondwana crust and extensive erosion causing a break in the stratigraphic record characterised the period between 300 and 330 ma at the end of this period the gondwana ice sheet was well established over the uplands the period 260 300 ma evidenced the release of the gondwana heat and thermal subsidence caused widespread basin formation late carboniferous transpressive strike slip basins e g sierra australes colorado karoo falklands ellsworth central transantarctic mountains in which thick glacial deposits accumulated formed inboard of the palaeo pacific margin in the continental interior the formation of zambesi type rift and extensional strike slip basins were controlled by large mega shear systems whereas rare intracratonic thermal subsidence basins formed locally in the late permian the tectonic regime changed to compressional largely due to northwest directed subduction along the palaeo pacific margin the orogenic cycle between 240 and 260 ma resulted in the formation of the gondwana fold belt and overall north south crustal shortening with strike slip motions and regional uplift within the interior the gondwana fold belt developed along a probable weak crustal zone wedged in between the cratons and an overthickened marginal crustal belt subject to dextral transpressive motions associated with the orogenic cycle was the formation of mega shear systems one of which falklands east africa tethys shear split the supercontinent in the permo triassic into a west and an east gondwana by a slight clockwise rotation of east gondwana a supradetachment basin formed along the tethyan margin and northward displacement of madagascar west falkland and the gondwana fold belt occurred relative to a southward motion of africa

the full range of turbidite bed thickness patterns in submarine lobes controls and implications a widely misused criterion to interpret lobe deposits in submarine fan systems at outcrop and in core and well logs is a thickening and or coarsening upward profile lobe deposits from the laingsburg depocentre sw karoo basin demonstrate that a full range of bed thickness patterns exists within lobes when lobes are defined by their laterally extensive bounding surfaces that are marked by abrupt facies changes five types of bed stacking patterns are identified thickening upward thinning upward thickening then thinning upward thinning then thickening upward and constant the abrupt bounding surfaces are interpreted to record avulsion of feeder distributive channels the stratigraphic bed thickness pattern preserved in a lobe reflects the internal organization of smaller scale lobe elements rather than lobe wide initiation and progradation as implied by a thickening upward only pattern the full range of bed thickness patterns in lobes can be used to understand the stacking of lobe elements the evolution of sediment deposition through time and space and the relative movement of depocentres

the transition between sheet like lobe and basin plain turbidites in the hecho basin south central pyrenees spain genetic facies analysis based on bed by bed correlations from sheet like lobes to the basin plain in some hecho turbidite systems demonstrates that at least 50 of the flows building the sheet like lobes kept moving downcurrent to the closely related basin plain and underwent flow transformations interpreted to have resulted from interaction with topography at the basin margin s that gave rise to specific facies these facies form a new facies tract that replaces the fine grained group of turbidite facies very fine sand to mud and characterizes the basin plain beds beds in the sheet like lobes evolve downcurrent in a way that is predictable by the existing turbidite facies tract models whereas 36 of basin plain beds which account for 78 of the basin plain volume do not the latter have deposits from high density turbidity currents at their bases and typically complete basin floor coverage the new facies tract developed when the flows obliquely encountered the southern foreland margin ramp at the ramp the lower sand laden and high density part of the larger flows was deflected evolving downcurrent along the ramp trend the upper part of the flow more dilute and thicker was reflected from the foreland margin as a train of declining undular bores moving hydraulic jumps subsequent reflections generated against the flanking margins in the closed basin led to ponding which resulted in an overall sheet like stacking pattern across sheet like lobes and the basin plain and is the diagnostic feature of the distal element in the lower sand rich stages of the turbidite systems calcilutites on top of beds interpreted until now as hemipelagites show field evidence of having a turbiditic origin hemiturbidites thus forming the facies capping the new facies tract

beds comprising debrite sandwiched within co genetic turbidite origin and widespread occurrence in distal depositional environments co genetic debrite turbidite beds occur in a variety of modern and ancient turbidite systems their basic character is distinctive an ungraded muddy sandstone interval is encased within mud poor graded sandstone siltstone and mudstone the muddy sandstone interval preserves evidence of en masse deposition and is thus termed a debrite the mud poor sandstone siltstone and mudstone show features indicating progressive layer by layer deposition and are thus called a turbidite palaeocurrent indicators ubiquitous stratigraphic association and the position of hemipelagic intervals demonstrate that debrite and enclosing turbidite originate in the same event detailed field observations are presented for co genetic debrite turbidite beds in three widespread sequences of variable age the miocene marnoso arenacea formation in the italian apennines the silurian aberystwyth grits in wales and quaternary deposits of the agadir basin offshore morocco deposition of these sequences occurred in similar unchannellized basin plain settings co genetic debrite turbidite beds were deposited from longitudinally segregated flow events comprising both debris flow and forerunning turbidity current it is most likely that the debris flow was generated by relatively shallow few tens of centimetres erosion of mud rich sea floor sediment changes in the settling behaviour of sand grains from a muddy fluid as flows decelerated may also have contributed to debrite deposition the association with distal settings results from the ubiquitous presence of muddy deposits in such locations which may be eroded and disaggregated to form a cohesive debris flow debrite intervals may be extensive 26 x 10 km in the marnoso arenacea formation and are not restricted to basin margins such long debris flow run out on low gradient sea floor 0 1 may simply be due to low yield strength 50 pa of the debris water mixture this study emphasizes that multiple flow types and transformations between flow types can occur within the distal parts of submarine flow events

geodynamics of rift plume interaction in iceland as constrained by new 40ar 39ar and in situ u pb zircon ages abstract the interaction between a rift zone and a mantle plume leads to exceptional situations in iceland where the island is 1 5 wider than expected given the north atlantic spreading rate in order to give a better idea of the timeframe of this evolution we present 32 new 40 ar 39 ar and in situ u pb dating on zircon from 16 volcanic systems located from the west to east coasts of iceland the north iceland rift zone nirz was initiated at least 12 ma ago furthermore during these last 12 ma the nirz half spreading rate was between 0 7 and 1 2 cm yr and it propagated to the south at a rate of 1 0 1 2 cm yr the excess width of iceland can thus not be explained by faster spreading rate in the past here we discuss a model that explains the 200 km excess of crust taking into account the eastward relocation of the rift zone and corresponding older crustal capture over the course of iceland s geological history the most recent rift relocation is dated at approximately 6 ma at snaefellsnes peninsula in the west whereas the oldest volcanic systems 15 13 ma from the extreme north east of iceland were most likely generated at the kolbeinsey ridge north of iceland rather than in the nirz itself the need for rift relocations and crustal capture to explain the width of iceland strongly suggests that during rift plume interaction the mantle plume plays an active role it forces the active rift zone to be frequently relocated by rift jumps above its center leaving inactive rift zones as older synclines in the geological record this result in an eastward position of the rift zone in iceland relative to the north atlantic ridge and it can be predicted that in a few tens of millions of years the mid atlantic ridge and the icelandic plume may become decoupled

architecture of shoreface to deep water systems in segmented rift systems evidence from the southern margin of the sogn graben northern north sea the spatial distribution of syn rift deposits and temporal evolution of sandy gravity flows in the context of rift segmentation are poorly constrained based on conventional core description well log analysis and 3d seismic interpretation this study focuses on the upper jurassic successions in the southern margin of the sogn graben northern north sea thirteen facies associations are classified in order to discuss the architecture of shoreface gravity driven submarine fans and basin plain deposits on the scale of centimeters to meters in the eastern part of the study area gamma ray log stacking patterns suggest that northeast southwest oriented shoreface successions underwent progradational to retrogradational deposition during the oxfordian age lithological description of conventional cores indicates that large submarine fan system formed by debris flow dominated channels and turbidity flow dominated lobe complexes covers a broad 22 20 km depositional area the entry of regional drainages and the location of sandy gravity flow deposits were controlled by first order rift segmentation whereas the route of submarine canyon feeder system and the orientation of submarine fan lobes were determined by second order rift segmentation thus both first order and second order rift segmentation should be considered when determining the sedimentary facies and depositional architecture of sandy gravity flow deposits in individual rift segments

permian passive margin submarine fan complex karoo basin south africa possible model to gulf of mexico abstract the tectonic development of the southwestern part of the intracratonic karoo basin was related to a subduction zone and an orogenic fold belt however the entire lower permian ecca group basin fill succession exhibits the depositional characteristics of a passive margin setting the subbasins that developed during subsidence accumulated large submarine fan complexes that were succeeded by a river dominated deltaic system the subbasin fill in the west southwestern area tanqua karoo is approximately 250 m thick and is comprised of six arenaceous submarine fan systems separated by basinal shales the uppermost fan system is overlain by prodelta shales and siltstones followed by delta front and delta plain sediments the fan complex displays middle and outer fan deposits over an area of about 650 km2 the individual fans vary in maximum thickness from 24 to over 60 m the cyclicity of the fan deposition is believed to have been controlled by fluctuations in relative sea level with an average periodicity of about 100 000 years paleocurrent directions vary between the six units reflecting changes in deltaic point source locations between sse and wsw the sixth fan system is located to the south downlaps onto the last of the five sequences and may represent a slope fan with major slumps and channels the strata are not affected by folding or vegetation permitting three dimensional viewing of channel fills and levee overbank relationships lateral migration of channels and the transition from channelized to non channelized lobe and sheet deposits the sediments of the different subenvironments seem to have their own typical sedimentary structures which makes it possible to start defining the placing of thin bedded turbidites and their potential relationships to channel fill deposits the gentle tectonic development of the southwestern karoo basin as is reflected in the distribution of the lower shale formations low sand shale ratio and overall fine grain size throughout the entire submarine fan deltaic mega sequence reflects a passive margin setting these well exposed outcrops contain many sedimentary characteristics that can be related to those of the mississippi delta and fan therefore this area can provide important elements that will be useful for the development of models of deep water sands in the northern margin of the gulf of mexico

short length scale variability of hybrid event beds and its applied significance abstract hybrid event beds hebs are a type of deep water sediment gravity flow deposit that generally comprise a basal clean sandstone overlain by a variety of muddier and less permeable sandy facies they are thought to be emplaced by combinations of turbidity currents transitional flows and debris flows all as part of the same transport event to date a number of studies have highlighted the common presence of hebs mainly in the outer and marginal parts of deep water systems where they replace beds composed dominantly of clean sand up dip and or axially over scales of km to 10 s km in addition to these broad patterns important yet poorly understood short length facies changes over metres to 100 s m occur modifying the overall texture and reservoir characteristics at or beneath typical spacing of production wells the nature and origin of the short length scale transitions is here addressed in four well exposed heb prone outcrops the cretaceous paleocene gottero sandstone and the miocene cilento flysch both in italy the carboniferous mam tor sandstone in northern england and the carboniferous basal ross sandstone formation western ireland a series of detailed correlation panels show marked lateral variations in internal bed make up for most of the hybrid event beds studied this variability typically involves lateral changes in the proportions of the cleaner basal sandstone and the overlying muddy sandstone division that occur without substantial change in the overall event bed thickness the variability is inferred to reflect the complex fingering between the up dip sandstone dominated part of the event bed and the down dip linked debrite due to internal erosion ploughing of the debrite into the basal clean sand where the upper part of the bed is dominated by large mudstone rafts these may have foundered into liquefied sand and been injected and partly fragmented by the sand intrusions the variable thickness and continuity of the basal clean sandstones have important implications for reservoir characterisation significant variability in bed character at interwell scale can be anticipated rugose contacts between the intra bed facies divisions may impact on drainage and sweep efficiency during hydrocarbon production

post glacial permian stratigraphy and geography of southern and central africa boundary conditions for climatic modelling abstract the post glacial stratigraphy of southern and central africa comprises mudrocks with subordinate sandstones and coal measures belonging to the ecca group overlain by fossiliferous mudstones and sandstones of the beaufort group madumabisa mudstone of south central africa preserved in the karoo congo and extreme western margin of the paranabasins the kalahari zambezi and east africa malagasy basin complexes and the nyasa basin and range complex the palaeogeography of the region was controlled by the stable elevated kalahari and congo cratons a ne trending weak zone between the cratons site of the kalahari zambezi malagasy basin complexes a nnw trending weak zone between the cratons and the mozambique mobile belt site of the nyasa basin and range complex and an e w mobile belt along the southern margin of the continent site of the foreland karoo basin the post glacial artinskian landscape which was inherited from the permo carboniferous glaciation consisted of a rugged interior plateau and shallow seas and seaways in the south and west the early late permian landscape was still dominated by an uneven coastline in the south and freshwater lakes in the congo basin and along the kalahari zambezi malagasy drainage systems a marine incursion occurred in the malagasy basin relief on the interior plateau was steadily denuded by kazanian times large lakes covered the karoo and paranabasins whereas along the coastal region all shallow seas were destroyed by an orogen freshwater lakes in the interior had reached their maximum extent the permo triassic landscape shows an increase in the interior relief due to renewed graben formation with lakes becoming smaller and more desiccated the surface area of southwestern gondwana covered by water reached a maximum of 34 during the early late permian and decreased to about 6 at the permo triassic boundary climatic zones shifted southeastwards as southwestern gondwana drifted into lower latitudes during the permian the ice margin started to retreat in central africa during the sakmarian but the marine ice sheet in the karoo basin collapsed only in the artinskian deglaciation was followed by a cool wet climate suitable for the formation of peatlands in central and south central africa this climate was followed by temperate to warm seasonal conditions which became progressively wetter with less seasonal fluctuation towards the east malagasy basin and drier towards the west these conditions lasted until the end of the kazanian in the karoo basin the cool wet climate of the artinskian became more temperate and also lasted until the end of the kazanian when warm semi arid conditions expanded over the entire southern and central africa the geographic controlling factors for permian climatic change were the interplay of latitudinal setting and continental evolution formation of orographic barriers and loss of coastal seas

hybrid event bed character and distribution linked to turbidite system sub environments the north apennine gottero sandstone north west italy this study documents the character and occurrence of hybrid event beds deposited across a range of deep water sub environments in the cretaceous palaeocene gottero system north west italy detailed fieldwork 5200 m of sedimentary logs has shown that hybrid event beds are most abundant in the distal confined basin plain domain 31 of total thickness in more proximal sectors hebs occur within outer fan and mid fan lobes up to 15 of total thickness whereas they are not observed in the inner fan channelised area six hybrid event bed types heb 1 to heb 6 were differentiated mainly on basis of the texture of their muddier and chaotic central division h3 the confined basin plain sector is dominated by thick max 9 57 m average 2 15 m and tabular hybrid event beds heb 1 to heb 4 their h3 division can include very large substrate slabs evidence of extensive auto injection and clast break up and abundant mudstone clasts set in a sandy matrix dispersed clay ca 20 these beds are thought to have been generated by highly energetic flows capable of delaminating the sea floor locally and carrying large rip up clasts for relatively short distances before arresting the unconfined lobes of the mid fan sector are dominated by thinner average 0 38 m hybrid event beds heb 5 and heb 6 their h3 divisions are characterised by floating mudstone clasts and clay enriched matrices dispersed clay 25 with hydraulically fractionated components mica organic matter and clay flocs these hybrid event beds are thought to have been deposited by less energetic flows that underwent early turbulence damping following incorporation of mud at proximal locations and by segregation during transport although there is a tendency to look to external factors to account for hybrid event bed development systems like the gottero imply that intrabasinal factors can also be important specifically the type of substrate available muddy or sandy and where and how erosion is achieved across the system producing specific hybrid event bed expressions and facies tracts this article is protected by copyright all rights reserved

parallel lamination formed by high density turbidity currents experiments were conducted in order to investigate processes of sediment transport and deposition from sustained turbidity currents with initial volume fraction sediment concentration of 0 20 0 25 and 0 35 parallel lamination was formed at bed aggradation rates up to 4 mm s 1 and bedload layer sediment concentration up to 0 36 this observation contradicts the generally held assumption that parallel lamination forms only from low density flows and at low bed aggradation rates this misconception illustrates a poor understanding of threshold values of grain support mechanisms in the bedload layer the present results bring new insight into understanding sediment transport in the deep sea and interpreting the sedimentary record

subaqueous sediment gravity flow deposits practical criteria for their field description and classification a new method for the description and classification of subaqueous sediment gravity flow deposits is proposed the classification scheme employs a convenient letter code and divides deposits individual beds into descriptive categories of two hierarchical levels facies and subfacies facies as the higher rank categories are distinguished chiefly on the basis of sediment type i e bed grain size texture a total of 13 facies have been distinguished g gravel gs gravel sand couplet gys gravelly sand s sand sm sand mud couplet ms mud sand couplet tm silt mud couplet mt mud silt couplet m mud mys muddy sand sym sandy mud myg muddy gravel gym gravelly mud subfacies as the lower rank categories are distinguished within the individual facies on the basis of the bed s internal structures the number of subfacies is unlimited and their labelling code includes particular facies symbols see above preceded by lower case letters denoting specific sedimentary structures and their vertical arrangement subfacies thus refer to the bed s intervals or divisions which are labelled as follows m massive unstratified and ungraded g graded unstratified and graded s plane stratified x cross stratified 1 parallel and or cross laminated q liquefied for example subfacies gsg graded to plane stratified gravel are gravel beds that have a lower graded interval and an upper plane stratified interval subfacies xg cross stratified gravel are gravel beds that are cross stratified throughout subfacies sls plane stratified to laminated sand are sand beds that have a lower plane stratified interval and an upper laminated parallel and or cross laminated interval

textural trends in turbidites and slurry beds from the oligocene flysch of the east carpathians romania deep water sandstone beds of the oligocene fusaru sandstone and lower dysodilic shale exposed in the buza uu valley area of the east carpathian flysch belt romania can be described in terms of the standard turbidite divisions in addition mud rich sand layers are common both as parts of otherwise normal sequences of turbidite divisions and as individual event beds eleven units interpreted as the deposits of individual flows were densely sampled and 87 thin sections were point counted for grain size and mud content s3 ta divisions which form the bulk of most sedimentation units have low internal textural variability but show subtle vertical trends in grain size most commonly coarse tail normal grading is associated with finetail inverse grading the mean grain size can show inverse grading normal grading or a lack of grading but sorting tends to improve upward in most beds fine tail inverse grading is interpreted as resulting from a decreasing effectiveness of trapping of fines during rapid deposition from a turbidity current as the initially high suspended load fallout rate declines if this effect is strong enough the mean grain size can show subtle inverse grading as well thus thick inversely graded intervals in deep water sands lacking traction structures do not necessarily imply waxing flow velocities if the suspendedload fallout rate drops to zero after the deposition of the coarse grain size populations the remaining finer grained flow bypasses and may rework the top of the s3 division forming well sorted coarser grained current structured tt units alternatively the suspended load fallout rate may remain high enough to prevent segregation of fines leading to the deposition of significant amounts of mud along with the sand mud content of the sandstones is bimodal either 3 13 or more than 20 two types of mud rich sandstones were observed coarser grained mud rich sandstones occur towards the upper parts of s3 ta divisions these units were deposited as a result of enhanced trapping of mud particles in the rapidly deposited sediment finer grained mud rich units are interbedded with ripple laminated very fine grained sandy tc divisions during deposition of these units mud floccules were hydraulically equivalent to the very fine sand and silt sized sediment the mud rich sandstones were probably deposited by flows that became transitional between turbidity currents and debris flows during their late stage evolution

mantle thermal structure and active upwelling during continental breakup in the north atlantic seismic reflection and refraction data acquired on four transects spanning the southeast greenland rifted margin and greenland iceland ridge gir provide new constraints on mantle thermal structure and melting processes during continental breakup in the north atlantic maximum igneous crustal thickness varies along the margin from s 30 km in the near hotspot zone 6 500 km from the hotspot track to v18 km in the distal zone 500 1100 km magmatic productivity on summed conjugate margins of the north atlantic decreases through time from 1800 300 to 600 50 km 3 km ma in the near hotspot zone and from 700 200 to 300 50 km 3 km ma in the distal zone comparison of our data with the british faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist joint consideration of crustal thickness and mean crustal seismic velocity suggests that along margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature the thermal anomaly vt at breakup was modest v100 125 c varied little along the margin and transient data along the gir indicate that the potential temperature anomaly 125 50 c and upwelling ratio v4 times passive of the iceland hotspot have remained roughly constant since 56 ma our results are consistent with a plume impact model in which 1 a plume of radius v300 km and vt of v125 c impacted the margin around 61 ma and delivered warm material to distal portions of the margin 2 at breakup 56 ma the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin and 3 by 45 ma both the remaining plume head and the distal warm layer were exhausted with excess magmatism thereafter largely confined to a narrow 6 200 km radius zone immediately above the iceland plume stem alternatively the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre existing thermal anomaly unrelated to the plume fl 2001 elsevier science b v all rights reserved

turbidites facies response to the morphological confinement of a foredeep cervarola sandstones formation miocene northern apennines italy

development of rheological heterogeneity in clay rich high density turbidity currents aptian britannia sandstone member u k continental shelf abstract upper aptian deep water sandstones in the witch ground graben of the north sea have been extensively drilled and cored in the britannia field the upper part of the britannia sandstone member comprises a series of thick 3 8 to 16 5 m sandstone beds that thin and pinch out against a paleoslope formed by the fladen ground spur to the north deposition was from unusually large volume flows that carpeted the deep basin floor and base of the lateral slope with sand the flows were able to heal significant seafloor topography produced during periods of local slope collapse the recognition of widespread early remobilization is new and results in a revised event bed correlation scheme and casts doubt on relationships between spatial patterns of grain size distribution facies distribution and paleoflow used to underpin previous models of deposit emplacement the beds are here interpreted as the deposits of large volume axially dispersing turbidity currents probably some 70 km from source and within 25 to 35 km of their run out limit to the south southeast of the britannia field single event beds contain a wide range of sand rich facies deposited progressively from turbulent suspension and clay rich facies previously termed slurry deposits deposited in cycles of en masse deposition from dilute laminar flow alternating with phases of suspension fallout from turbulent flow along the northern fringe of the system clay rich facies dominate but pass southward over six or seven km into progressively cleaner sand rich facies this trend in facies distribution is at a high angle to the inferred se dispersing paleoflow and is thought to reflect lateral suppression of flow turbulence from flow axes to flow margins controlled by relative changes in flow depth in addition the vertical arrangement of facies within beds always shows sand rich facies at bed bases passing upward often via distinctive banded facies into clay rich facies indicating a temporal flow transformation from dominantly turbulent to dominantly laminar with an intermediate phase of alternating laminar and turbulent flow this temporal transition in inferred flow conditions is similar to the inferred on axis to off axis spatial transition in conditions thus single clay rich sediment gravity flows can evolve in both time and space to develop different rheological zones of turbulent transitional and laminar character in addition certain debritic clay rich facies are now interpreted to result from post depositional remobilization from off the confining slope to the north and must be differentiated from the primary suite of depositional facies

contained reflected turbidity currents from the middle ordovician cloridorme formation quebec canada an alternative to the antidune hypothesis the parautochthonous cloridorme formation is a syn orogenic flysch succession that was deposited in an elongate foredeep basin as mainly lower middle fan outer fan and basin floor deposits the basin floor deposits about 1 5 km thick are confined to members 1 2 and 1 and are characterized by graded thick 1 10 m mud rich calcareous greywacke beds previously interpreted as deposits of concentrated muddy unidirectional turbidity currents that locally generated backset antidune lamination in internally stratified flows the dominant flow directions were from east to west but west to east transport also occurred as seen in the orientation of ripples climbing ripples flutes consistently overturned flames and grain imbrication we believe that the flows that deposited these thick calcareous greywacke beds reversed by roughly 180 one or more times during deposition of the lower sandy part of the beds flow reversals are consistent with the sharp grain size breaks and mud partings within sandy divisions measurement of grain fabric relative to stratification in the most celebrated antidune bedforms indicates flow from west to east thus the bedforms were produced by west to east migration of megaripples not by the upcurrent migration of antidunes the thick muddy beds were deposited by large volume muddy flows that were deflected and reflected from the side slopes and internal topographic highs of a confined basin floor much like the contessa and similar beds of the italian apennines large quantities of suspended mud were ponded above the irregular basin floor and settled to produce the thick silty mudstone caps seen on each bed because of their mode of emplacement we propose that these beds be called contained turbidites

a high precision jacob s staff with improved spatial accuracy and laser sighting capability abstract a new jacob s staff design incorporating a 3d positioning stage and a laser sighting stage is described the first combines a compass and a circular spirit level on a movable bracket and the second introduces a laser able to slide vertically and rotate on a plane parallel to bedding the new design allows greater precision in stratigraphic thickness measurement while restricting the cost and maintaining speed of measurement to levels similar to those of a traditional jacob s staff greater precision is achieved as a result of a improved 3d positioning of the rod through the use of the integrated compass and spirit level holder b more accurate sighting of geological surfaces by tracing with height adjustable rotatable laser c reduced error when shifting the trace of the log laterally i e away from the dip direction within the trace of the laser plane and d improved measurement of bedding dip and direction necessary to orientate the jacob s staff using the rotatable laser the new laser holder design can also be used to verify parallelism of a geological surface with structural dip by creating a visual planar datum in the field and thus allowing determination of surfaces which cut the bedding at an angle e g clinoforms levees erosion surfaces amalgamation surfaces etc stratigraphic thickness measurements and estimates of measurement uncertainty are valuable to many applications of sedimentology and stratigraphy at different scales e g bed statistics reconstruction of palaeotopographies depositional processes at bed scale architectural element analysis especially when a quantitative approach is applied to the analysis of the data the ability to collect larger data sets with improved precision will increase the quality of such studies