a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

the upper jurassic kimmeridgian fluvial aeolian systems of the southern neuquen basin argentina abstract the kimmeridgian quebrada del sapo formation in the southernmost neuquen basin in argentina represents a succession up to 40 m thick of coarse to fine grained fluvial deposits overlain by aeolian deposits these fluvial aeolian deposits reflect a significant palaeogeographic change in the basin and are related to a major tectonically enhanced relative sea level fall the fluvial section is dominated by braided channel fine grained ephemeral and sheetflood deposits aeolian facies are dominated by dune deposits with minor sandsheet and interdune units changes in the nature of both fluvial and aeolian sedimentation within the studied area suggest a regional variability of accommodation sediment supply conditions the regional changes of the aeolian succession likely reflect different relative positions within a major erg in the upwind margin of the erg a shallow water table promoted water lain sedimentation in interdune areas whereas in the central parts of the erg dry sediment accumulation took place above the regional water table level the vertical transition observed in the quebrada del sapo formation from fluvial to aeolian deposits may be the result of a local climatic change to drier conditions due to the development of a climatic barrier imposed by growth of a magmatic arc to the west alternatively the vertical transition could be related to a lowering of the water table associated with the compartmentalization of the basin during a period of low sea level

interaction between aeolian fluvial and playa environments in the permian upper rotliegend group uk southern north sea the permian upper rotliegend group in offshore uk quadrants 42 43 47 and 48 comprises a sequence of mixed aeolian fluvial playa deposits these deposits are up to 300 m thick and contain a record of the interaction between desert fluvial systems and adjacent aeolian and playa environments the relative dominance of water vs wind transport and deposition in this stratigraphic package was a function of fluctuations in the discharge of ephemeral fluvial systems and changes in water table playa level driven by a combination of climatic change and syndepositional tectonics the rotliegend sedimentary record is punctuated by numerous surfaces recording erosion by wind and water the origin of these surfaces is mostly climatic with periods of increased runoff resulting in fluvial incision especially near active faults during periods of reduced runoff wind erosion of fluvial deposits occurred with fluvially derived sand being reworked into expanding aeolian dune fields wind erosion also occurred as a rising water table isolated dunes from their sediment supply resulting in deflation of dunes down to the water table these surfaces formed in a basin that was subsiding thus even in a background of overall increasing accommodation space climatically driven falls in the water table allowed for periods of erosion the occurrence of significant erosion especially near syndepositional fault zones resulted in a sedimentary record that shows pronounced lateral as well as vertical facies variations

the vast proto tibetan plateau new constraints from paleogene hoh xil basin abstract the surface uplift of the tibetan plateau is the key boundary condition in many cenozoic geological events ranging from global cooling to changes of asian environments during cenozoic however poorly constrained timing for the uplift of tibetan plateau makes these interpretations highly debatable here we report results from sedimentology detrital zircon u pb and lu hf isotopic compositions and paleomagnetic signatures from both the eastern and western hoh xil basins of north central tibetan plateau sedimentary lithofacies and facies associations analyzed in the western hoh xil basin indicate they were deposited in a braided fluvial system and alluvial fan similar with the fenghuoshan group eastern hoh xil basin provenance analyses from conglomerate clast compositions paleocurrent orientations and detrital zircon u pb and lu hf isotopic compositions document sediments in both western and eastern basins were derived from the qiangtang and lhasa blocks these observations in combination with comparative paleomagnetic results imply that hoh xil basin was a single wide basin during paleogene the period of hoh xil basin deposition was coeval with significant period of the early cenozoic uplift and erosion of the qiangtang and lhasa blocks these observations not only reinforce the suggestion that the qiangtang and lhasa blocks were uplifted during eocene to form a proto tibetan plateau but also imply that the proto tibet plateau is vast in areal extent the large dimension and high elevation of the proto tibetan plateau probably contributed to the global cooling during the early eocene

reconstruction of palaeoenvironmental conditions of the vaca muerta formation in the southern part of the neuquen basin tithonian valanginian evidences of initial short lived development of anoxia abstract the vaca muerta formation is a famous source rock being largely present in argentina the formation was examined in the southern part of the neuquen basin through clay mineral assemblage composition as well as organic and inorganic geochemical characteristics in total 259 samples were analyzed on both sides of the huincul arch major e w structure that divided the southern part of the basin into two depocentres south to this structure picun leufu area the total organic carbon content toc can reach 20 but is generally ranging from 0 1 to 6 to the north covunco area average toc does not exceed 0 5 clay mineral assemblages indicate an evolution of the sources of clastic supply through time in response to evolving weathering erosion at the regional scale redox sensitive trace element distribution shows that the particulate iron shuttle process operated with an initially euxinic depositional environment that evolved rapidly to oxygenated seawater and suboxic sediments our results suggest an episodically restricted circulation pattern that occurred at the beginning of the deposition of the vaca muerta formation highest toc content

architecture and development of the alluvial sediments of the upper jurassic tordillo formation in the canada ancha valley northern neuquen basin argentina the upper jurassic tordillo formation at canada ancha area northern neuquen basin argentina comprises a multi stage suit of predominantly alluvial sediments that is heterolithic in nature in that suit several lithofacies architectural elements and bounding surfaces of different order have been identified and their lateral and vertical distribution characterized this analysis allowed the differentiation of 3 main units lower middle and upper 20 subunits c 1 to c 20 and the characterization of their alluvial styles the lower unit which comprises subunits c 1 to c 4 is mainly formed by fine to medium grained sandstones which become medium to coarse grained towards the top these sandstones characterize settings ranging from floodplains with isolated unconfined flows to more complex vertically stacked multistorey sheet sandstones of braided fluvial systems the middle unit c 5 to c 10 is dominated by pale browngrey fine to coarse grained sands and medium size subangular to angular conglomerates which reflect amalgamated complexes of sandstone sheets and downstream accretion macroforms remarkably this alluvial sedimentation was episodically punctuated by volcaniclastic flows the upper unit c 11 to c 20 consists of finer sediments mainly pink to white fine to medium grained sandstones and red to green siltstones towards the top bioturbation becomes important and also the presence of volcanosedimentary flows is noticeable fluvial settings include braided sheet sandstones with waning flood deposits evolving to isolated high sinuosity fluvial systems with flash flood deposits at the top of this unit facies may suggest marine influence vertical changes in the fluvial style result from both climatic and tectonic controls a semiarid to arid climate and the active tectonism linked to the eastward migration of the andean volcanic arc determined major bounding surfaces fluvial style evolution and the presence of the volcano sedimentary deposits different stages of high and low subsidence rates has been deduced from the vertical stacking of sediments

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

dynamic stratigraphy of an evaporite to red bed sequence gipskeuper triassic southwest german basin abstract the dynamics of evaporite accumulation are studied in the gipskeuper triassic of the southwest german epicontinental basin based on a hierarchical three level stratigraphic analysis 1 among the various stratification types the following are most important a thin carbonate intervals with marine faunas b massive sulfates with selenite indicating very shallow subaqueous gypsum precipitation c thin bedded gypsarenites with a variety of sedimentary structures suggesting clastic evaporite accumulation during high energy events gypsum tempestites d laminated gypsum with spectacular tepee structures testifying inter to supratidal settings e green grey and reddish claystones with nodular gypsum indicating evaporitic playa mudflats 2 facies sequences occur in several varieties and commonly form 1 4 m thick transgressive regressive cycles with shallowing upwards trends the long recognized carbonate marker beds represent short term trans or ingressions into the almost flat keuper basin highly disturbed tepee horizons form widely correlatable regressive peaks these sequences form evaporite equivalents to the ubiquitous shallowing upward cycles known from carbonate systems 3 the basin fill consists of a small scale cycles being vertically stacked upon each other to form the basic building block of the layer cake architecture of the gipskeuper succession the stacking of small scale cycles is superimposed onto a larger scale regressive cycle which records a general shift from restricted marginal marine to hypersaline marine to the more continental red bed deposition of the overlying keuper this hierarchy of cycles probably reflects various orders of eustatic sealevel fluctuations the gipskeuper thus exemplifies dynamics of evaporite accumulation and allows us to understand some of the processes that produce a layer cake stratigraphy its cycle hierarchy is a common motif in the entire southwest german basin fill and appears typical for epicontinental basins

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

cross strata and bedforms of probable transitional dune to upper stage plane bed origin from a late precambrian fluvial sandstone northern norway a late precambrian fluvial sandstone sequence in northern norway is dominated by large scale cross sets that show either lenticular or tabular geometries in the streamwise sections the lenticular sets interdigitate and in places show nearly symmetrical formsets the tabular sets are in places solitary but are mainly grouped in cosets in both cross set types the cross strata range from concave up to sigmoidal in shape with the latter variety comprising subhorizontal to gently inclined topset strata with parting lineation that merge uninterruptedly downflow into the steeper 10 2 foresets within the cross sets the geometry and dip azimuths of the foresets are conspicuously consistent although the concave up and sigmoidal strata commonly alternate downcurrent the cross strata characteristics suggest flood stage deposition from relatively high velocity steady currents heavily laden with suspended sand both cross set types are interpreted as representing bedforms generated by flow in the dune to upper stage plane bed transition the lenticular cross sets probably represent periodic dunes but it is far less clear whether the long bedforms represented by the tabular sets should be classified as dunes or rather as solitary to quasi periodic bars

sedimentation and preservation of the miocene atacama gravels in the pedernales chanaral area northern chile climatic or tectonic control in recent years longitudinal changes on the thin thick skinned tectonic styles of the central andes has been intensively discussed while other studies have considered the role of mass transfers on the unloading the orogen and on the stress regime along the plate interface arising from changes on the volume of sediment arriving into the peru chile trench the search for paleo climate records is therefore crucial for our understanding of the history of the central andes in this paper we focus on the atacama gravels an extensive blanket of miocene continental deposits filling a neogene paleo valley system along the southern atacama desert in northern chile an east west transect between pedernales and chanaral 26o30 s enabled us to carry out a sedimentological and tectonic study of the atacama gravels based on logging and field observations along the rio salado canyon new 39ar 40ar ages obtained on intercalated and overlying ignimbrites date the beginning of the atacama gravels sedimentation at around the oligocene miocene boundary and cessation of sedimentation in the late miocene thirteen lithofacies included within five facies associations a1 to a5 were identified depositional environments vary from proximal alluvial fan a1 a2 in the precordillera through ephemeral fluvial a3 a4 to distal playa lake a5 in the coastal cordillera no evidences of syn sedimentary deformation have been found showing that the change from sediment removal to sediment preservation cannot be explained by tectonic causes and climate change appears to be the dominant controlling factor of sediment preservation a progressive change from semi arid towards hyper arid climatic conditions during the miocene led to a reduction on the transport capacity of the fluvial system and sediment preservation along the paleovalley system formed during the oligocene

characteristics and evolution of the segmentation of the mid atlantic ridge between 20 n and 24 n during the last 10 million years abstract high resolution bathymetry and geophysical data collected along the slow spreading axis and flanks of the mid atlantic ridge between 20 n and 24 n reveal the characteristics and history of different wavelengths of segmentation during the last 10 m y the bathymetric data exhibit a morphotectonic pattern dominated by ridge normal and oblique bathymetric lows that partition the ridge flanks into rhomb shaped areas of relatively high elevation at least four different types of oblique bathymetric lows have been identified which represent the off axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge axis segmentation one group of oblique structures is represented by two deep ridge normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis these oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis while maintaining a constant offset of the latter two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge parallel bathymetric lows symmetric about the ridge axis both types of oblique structures frequently change their orientation from normal to subparallel to the ridge axis and appear to merge and diverge off axis these oblique depressions are characterized by positive filtered mantle bouguer anomalies high magnetizations complex magnetic anomaly patterns and possible exposure of mantle lithologies the ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis with propagation rates varying from 0 to 25 km m y 1 the last and smallest discontinuities observed in this area correspond to small ridge axis offsets and off axis traces identified by alignments of the terminations of abutting abyssal hills the ridge flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt the centres of the rhomb shaped areas correspond to maximum crust production and thin lithosphere and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites we propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years

anatomy and facies distribution of terminal lobes in ephemeral fluvial successions jurassic tordillo formation neuquen basin argentina in terminal fluvial fan systems characteristic proximal to distal variations in sedimentary architectures are recognized to arise from progressive downstream loss of water discharge related to both infiltration and evaporation this work aims to elucidate downstream trends in facies and architecture across the medial and distal zones of terminal fan systems which record transitions from deposits of channel elements to lobe like and sheet like elements this is achieved via a detailed characterization of ancient ephemeral fluvial deposits of the well exposed kimmeridgian tordillo formation neuquen basin argentina the fine sand prone and silt prone succession associated with the medial to distal sectors of the system has been studied to understand relationships between depositional processes and resulting architectures facies and architectural element analyses and quantification of resulting sedimentological data at multiple scales have been undertaken to characterize sedimentary facies facies transitions bed types architectural elements and larger scale architectural styles eight bed types with distinct internal facies transitions are defined and interpreted in terms of different types of flood events channelized and non channelized architectural elements are defined based on their constituent bed types and their external geometry the most common elements are terminal lobes which are composite bodies within which largely unconfined sandy deposits are stacked in a compensational manner a hierarchical arrangement of internal components is recognized proximal feeder channel avulsion events likely controlled the evolution of terminal lobe elements and their spatiotemporal shifts stratigraphic relations between architectural elements record system wide trends whereby a proximal sector dominated by channel elements passes downstream via a gradational transition to a medial sector dominated by sandy terminal lobe elements which in turn passes further downstream to a distal sector dominated by silty terminal lobe margin and fringing deposits this work enhances current understanding of the stratigraphic record of terminal fluvial systems at multiple scales and provides insight that can be applied to predict the facies and architectural complexity of terminal fluvial successions

the action of wind and water in a mid cretaceous subtropical erg margin system close to the variscan iberian massif spain aeolian processes and ephemeral water influx from the variscan iberian massif to the mid cretaceous outer back erg margin system in eastern iberia led to deposition and erosion of aeolian dunes and the formation of desert pavements remains of aeolian dunes encased in ephemeral fluvial deposits aeolian pods demonstrate intense erosion of windblown deposits by sudden water fluxes the alternating activity of wind and water led to a variety of facies associations such as deflation lags desert pavements aeolian dunes pebbles scattered throughout dune strata aeolian sandsheets aeolian deposits with bimodal grain size distributions mud playa ephemeral floodplain pebble sand and cobble sand bedload stream pebble cobble sand sheet flood sand bedload stream debris flow and hyperconcentrated flow deposits sediment in this desert system underwent transport by wind and water and reworking in a variety of sub environments the nearby variscan iberian massif supplied quartzite pebbles as part of mass flows pebbles and cobbles were concentrated in deflation lags eroded and polished by wind driven sands facets and ventifacts and incorporated by rolling into the toesets of aeolian dunes the back erg depositional system comprises an outer back erg close to the variscan highlands and an inner back erg close to the central erg area the inner back erg developed on a structural high and is characterized by mud playa deposits interbedded with aeolian and ephemeral channel deposits in the inner back erg area ephemeral wadis desiccated after occasional floods were mud cracked and overrun episodically by aeolian dunes subsequent floods eroded the aeolian dunes and mud cracked surfaces resulting in largely structureless sandstones with boulder size mudstone intraclasts floods spread over the margins of ephemeral channels and eroded surrounding aeolian dunes the remaining dunes were colonized occasionally by plants and their roots penetrated into the flooded aeolian sands upon desiccation deflation resulted in lags of coarser grained sediments a renewed windblown supply led to aeolian sandsheet accumulation in topographic wadi depressions synsedimentary tectonics caused the outer back erg system to experience enhanced generation of accommodation space allowing the accumulation of aeolian dune sands ephemeral water flow to the outer back erg area supplied pebbles eroded aeolian dunes and produced hyperconcentrated flow deposits fluidization and liquefaction generated gravel pockets and recumbent folds dune damming after sporadic rains the case of the namib desert monsoonal water discharge thar desert and meltwater fluxes from glaciated mountains taklamakan desert are three potential non exclusive analogues for the ephemeral water influx and the generation of hyperconcentrated flows in the cretaceous desert margin system an increase in relief driven by the aptian anti clockwise rotation of iberia led to an altitude sufficient for the development of orographic rains and snowfall which fed melt water fluxes to the desert margin system quartzite conglomerates and sands dominantly consisting of quartz and well preserved feldspar grains which are also observed in older cretaceous strata indicate an arid climate and the mechanical weathering of precambrian and palaeozoic metamorphic sediments and felsic igneous rocks unroofing of much of the cover of sedimentary rocks in the variscan iberian massif must therefore have taken place in pre cretaceous times

new constraints on the jurassic cretaceous boundary in the high andes using high precision u pb data abstract the jurassic cretaceous j k boundary is poorly constrained and is the only phanerozoic system boundary that lacks an internationally accepted reference stratigraphic section gssp precise radio isotopic u pb data are unavailable for the earliest stage of the cretaceous the berriasian the age of the jurassic cretaceous boundary was based on several assumptions including the relative duration of ammonite zones the constant spreading rates of magnetic anomalies and the extrapolation of rb sr or k ar isotopic data this paper discusses a site in an andean basin of western gondwana showing the j k boundary interval with geographically widespread nannofossil markers which are here uniquely combined with precise radiometric dates the recent finding of a sequence of marine black shales in the high andes of argentina interbedded with ash fall tuffs provides important constraints on this boundary this succession bears calcareous nannofossils and ammonites which allow correlation with well established tethyan floras and faunas in the northern hemisphere the tithonian berriasian transition in the andes was recognized on the basis of ammonite zones and nannofossil bioevents for the first time in the southern hemisphere the new ages obtained are 137 9 0 9 ma by sensitive high resolution ion microprobe shrimp and 139 55 0 09 0 18 ma by chemical abrasion isotope dilution thermal ionization mass spectrometry tims near the base of the berriasian these new ages can be interpreted in two different ways the first alternative would indicate that the present geological time table is correct and the fossil levels should be late berriasian the second alternative is that the j k boundary is 5 ma younger than the recently published geological time scale the authors support the last alternative and propose that the j k boundary should be close to 140 ma

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

sequence stratigraphy of a tidally dominated carbonate siliciclastic ramp the tithonian early berriasian of the southern neuquen basin argentina the tithonian berriasian vaca muerta carrin cura and picun leufu formations in the southern neuquen basin were deposited on a tidally dominated mixed carbonate siliciclastic ramp basinal outer middle shallow and back ramp facies associations are recognized and a sequence stratigraphic analysis reveals that the ramp record consists of three shallowing upwards sequences ti1 ti2 and ti3 set within a lower order progradational cycle a higher order of cyclicity is superimposed on to the middle ti2 sequence the majority of the ramp facies belong to the transgressive and highstand systems tracts however at the base of ti2 a lowstand systems tract is identified characterized by a basal unconformity and an abrupt basinward shift of the shallow marine lithofacies transgressive systems tracts were characterized by slow sedimentation rates and rapid sea level rises that affected carbonate productivity highstand systems tracts show the greatest carbonate productivity and an increased progradation rate on account of a reduction in accommodation space generation palaeogeography played a major role in the development of the depositional systems partial isolation from the pacific ocean reflecting the growth of the andean magmatic arc and geographic restriction due to tectonic inversion in the central part of the basin resulted in a meso macrotidal regime that produced a tidally dominated sedimentary record in the shallow and back ramp environments coeval anoxic conditions in the central part of the neuquen basin favoured distal ramp and basinal black shale deposition during episodes of relative sea level rise

sandy high energy flood sedimentation some criteria for recognition with an example from the devonian of s w england abstract many modern sand deposits resulting from episodic floods in semi arid zones are dominated by parallel laminations but parallel laminated sands are not a major feature in perennial streams single flood events may deposit over 1 5 m of parallel laminated sand hence ancient parallel laminated sand dominated alluvial deposits may be the product of ephemeral flows the trentishoe formation hangman sandstone group middle devonian of north devon consists in part of laterally extensive beds of parallel laminated fine grained red sandstones that build multistorey sand bodies individual beds are little more than 1 m thick but may extend laterally for hundreds of metres a vertical sequence of erosion surface sparse silt clasts parallel laminations silt drape is often found suggesting high flow stage deposition of plane bedded sand followed by a very rapid waning of flow with silt fallout in the final stages of the flood this sequence bears marked similarity to certain modern ephemeral flow sands

from alluvial fan to playa an upper jurassic ephemeral fluvial system neuquen basin argentina abstract the upper jurassic fill of the back arc neuquen basin includes a lowstand wedge known as the tordillo formation the studied deposits crop out along a n s oriented belt that runs parallel to the andean magmatic arc they are limited to the south by the east west oriented positive structure of the huincul arch formed as a result of upper jurassic tectonic inversion the tordillo deposits were formed in an arid fluvial dominated system characterised by systematic downstream changes in architectural style a gravelly and sandy bedload fluvial system is recognised in the southern upstream sector the reduced thickness and the coarse grain size suggest steep gradients excess of bedload supply and a low subsidence rate thicker and finer grained deposits prevail to the north and northeast they were formed under arid conditions in a wadi sand flat playa fluvial system this distal facies association indicates increased accommodation owing to high rates of subsidence relative to coarse siliciclastic sedimentation rates these low gradient deposits are characterised by cyclic alternations of mud dominated and sand dominated packages interpreted as high and low accommodation systems tracts the overall fining upward stacking pattern of the tordillo formation suggests a change towards higher accommodation rates this is accompanied by frequent development of soil horizons and darker primary and reworked pyroclastic deposits these attributes indicate a stronger explosive volcanic activity associated with increased precipitation and high water table emplacement towards the end of the tordillo lowstand wedge

fluvial processes and cyclicity in terminal fan deposits an example from the late devonian of southwest ireland abstract the gun point formation is a sandstone dominated sequence approximately 1 5 2 5 km thick which forms part of a major 6 km late devonian alluvial basin fill succession in southwest ireland it is interpreted as the deposits of a southerly flowing terminal fan system which operated under a semi arid climate the proximal zone was characterized by low sinuosity gravelly feeder channels which passed downstream into a medial zone dominated by a sandy braidplain further south in the distal zone phases of high energy flood activity were centred on transient low sinuosity ephemeral channels and were separated by more quiescent periods of shallow sheetflooding subaerial exposure and non deposition sedimentary parameters such as set thickness coset thickness channel sand body density and maximum sand body thickness all exhibit regular cyclic fluctuations with a periodicity of 160 180 m the cycles are interpreted as evidence of regular discharge variations which may have been a response to the 412 ka orbital eccentricity cycle a second periodicity of approximately 700 m has also been detected and is thought to reflect long term variations in basin subsidence

an example of a recent accretion on the mid atlantic ridge the snake pit neovolcanic ridge mark area 23 22 n abstract the inner rift vailey of the mark area mid atlantic ridge at the kane fracture zone was investigated with the submersible nautile at the latitude of the snake pit hydrothermal field 23 22 n a transect across the axial valley shows a stepwise increase in the sediment thickness away from the axis these steps probably reflect major magmatic and tectonic events in the evolution of the inner floor dives on the summit of the snake pit neovolcanic ridge constrain a generalized geological map and document relations between hydrothermal activity magmatism and faulting the neovolcanic ridge was constructed by the build up of several hundred metres of pillow lava flows its summit is intensely fissured and cut by an axial graben about 100 m wide three hydrothermal sites were recognized within the axial graben the eastern site the most active is structurally related to the eastern fault scarp the central site is located within the graben the western site associated with the western fault scarp is more complex and was constructed by two successive hydrothermal events the new data suggest a model of evolution for the inner floor of the rift valley involving alternating stages of magmatism and volcanism which produce ridges and troughs preserved outside of the axial domain these volcano tectonic cycles have an approximate duration of 100 000 years we discuss variations in the morphology of the mar along strike in the light of this model of evolution

evolution of miocene fluvial environments eastern potwar plateau northern pakistan the miocene pliocene siwalik group records changing fluvial environments in the himalayan foreland basin the nagri and dhok pathan formations of this group in the eastern potwar plateau northern pakistan comprise relatively thick tens of metres sandstone bodies and mudstones that contain thinner sandstone bodies metres thick and palaeosols thick sandstone bodies extend for kilometres normal to palaeoflow and are composed of large scale stratasets storeys stacked laterally and vertically adjacent to each other sandstone bodies represent single or superimposed braided channel belts and large scale stratasets represent channel bars and fills channel belts had widths of km bankfull discharges on the order of 103 cumecs and braiding parameter up to about 3 individual channel segments had bankfull widths maximum depths and slopes on the order of 102 m 101 m and 10 4 respectively and sinuosities around 1 1 these rivers are comparable to many of those flowing over the megafans of the modern indo gangetic basin and a similar depositional setting is likely thin sandstone bodies within mudstone sequences extend laterally for on the order of 102 m and have lobe wedge sheet and channel form geometries they represent crevasse splays levees and floodplain channels mudstones are relatively bioturbated disrupted and represent mainly floodbasin and lacustrine deposition mudstones and sandstones are extremely disrupted in places showing evidence of prolonged pedogenesis these mature palaeosols are m thick and extend laterally for km lateral and vertical variations in the nature of their horizons apparently depend mainly on deposition rate the 500 m thick nagri formation has a greater proportion and thicker sandstone bodies than the overlying 700 m thick dhok pathan formation the thick sandstone bodies and their large scale stratasets thicken and coarsen through the nagri formation then thin and fine at the base of the dhok pathan formation compacted deposition rates increase with sandstone proportion 0 53 mm year for nagri 0 24 mm year for dhok pathan and palaeosols are not as well developed where deposition rates are high within both formations there are 100 m scale variations representing on the order of 105 years in the proportion and thickness of thick sandstone bodies and tens of m scale alternations of thick sandstone bodies and mudstone sandstone strata that represent on the order of 104 years formation scale stratal variations extend across the potwar plateau for at least 100 km although they may be diachronous however 100 m and smaller scale variations can only be traced laterally for up to tens of km alluvial architecture models indicate that increases in the proportion and thickness of thick sandstone bodies can be explained by increasing channel belt sizes mainly average deposition rate and avulsion frequency on a megafan comparable in size to modern examples 100 m scale variations in thick sandstone body proportion and thickness could result from regional shifts in the position of major channels possibly associated with fan lobes on a single megafan or with separate megafans however such variations could also be related to local changes in subsidence rate or changes in sediment supply to the megafan system formation scale and 100 m scale stratal variations are probably associated with interelated changes in tectonic uplift sediment supply and basin subsidence increased rates of hinterland uplift sediment supply and basin subsidence recorded by the nagri formation may have resulted in diversion of a relatively large river to the area alternatively changing river sizes and sediment supply rates may be related to climate changes affecting the hinterland possibly linked to tectonic uplift climate during deposition of the siwalik group was monsoonal although the deposits contain no direct evidence for climate change independent evidence indicates global cooling throughout the miocene and the possibility of glacial periods e g around 10 8 ma corresponding to base of nagri formation if the higher himalayas were periodically glaciated a mechanism would exist for varying sediment supply to megafans on time scales of 104 105 years although eustatic sea level changes are related to global climatic change they are not directly related to siwalik stratigraphic changes because the shoreline was many 100 km away during the miocene

late carboniferous cyclothems and alluvial paleovalleys in the sydney basin nova scotia cyclothems are documented for the first time in alluvial to coastal plain strata west phalian d to stephanian of the sydney basin nova scotia well developed cyclothems commence with an extensive carbonaceous lime stone or shale with a restricted marine biota that indicates maximal flooding succeeding gray detrital bay and lake fills with distributary channel deposits are overlain by stacked channel deposits paleovalley fills that mark an abrupt basinward facies shift and a profound change in alluvial style channel fills and red vertisols were deposited on an alluvial plain above the filled valleys and the paleovalley is equated with thick vertisols in adjacent inter fluve areas the cyclothems are capped by gray coastal deposits with economic coals agglutinated foraminifera are present in seat earths beneath the coals which formed as a result of a brief stillstand during transgression red mudstones pass laterally into gray detrital bay fills in regions of greater subsidence associated with faulted basement blocks poorly developed cyclothems are bounded by extensive but split coals and autogenic effects partially obscure allogenic patterns mean cyclothem duration approximates 200 ka

evolution of a plio pleistocene volcanogenic alluvial fan the puye formation jemez mountains new mexico the plio pleistocene puye formation north central new mexico contains 15 km 3 of volcaniclastic alluvial sediments deposited in response to rift margin volcanism associated with the later evolution of the tschicoma volcanic center northeastern jemez mountains stratigraphic and sedimentologic evidence indicates that aggradation of puye deposits was nearly continuous and contemporaneous with graben development and establishment of the rio grande as the through flowing axial drainage system in this region much of the puye sediments debouched into an adjacent subsiding depocenter resulting in near complete preservation of this wedge shaped volcanogenic fan primary pyroclastic and reworked facies in the puye formation exhibit distinctive 5 to 30 m scale cyclicity directly related to volcanic activity in the northeastern jemez highlands reworked pyroclastic and conglomeratic sequences deposited by high energy braided streams sediment charged sheet floods sediment gravity flows and in shallow lakes reflect markedly increased sediment loads during and following explosive eruptions individual sequences change markedly in character with increased distance from the source area analysis of these abrupt lateral and vertical variations reveals that puye sedimentation was controlled by the influences of semiarid climate and syndepositional tectonism although the associations that most characterize the fan resulted largely from explosive volcanism cessation of fan growth is attributed to waning sediment supply as a result of volcanic quiescence and to the onset of basinwide pedimentation associated with downcutting of the ancestral rio grande the puye fan therefore provides a detailed record of rift basin alluvial sedimentation complete with closely spaced tephrostratigraphic control and it records the growth and partial denudation of high standing northeastern jemez mountains volcanoes

malargue group maastrichtian danian deposits in the neuquen andes argentina implications for the onset of the first atlantic transgression related to western gondwana break up abstract the discovery of marine to brackish and fresh water carbonates in the inner agrio fold and thrust belt at pichaihue neuquen argentina located to the west of the andean orogenic front imposes important constraints on the paleogeography of the first atlantic transgression in the neuquen basin related to the break up of western gondwana the constraints on the timing and areal extent of these deposits shed light on the early uplift history of the southern andes these limestones are part of the maastrichtian danian malargue group which was previously only known from its exposures in the extra andean area representing foreland basin deposits the presence of stromatolites oncoids serpulids bivalves and gastropods as well as silicified stems of macrophytes indicates a shallow marine partially brackish environment associated with non marine deposits these strata are interfingered with and overlie distal tuffs and proximal pyroclastic flows whose geochemical characteristics point to a magmatic arc source shrimp u pb dating of volcanic zircons of these tuffs yielded an age of 64 3 0 9 ma that confirms the correlation to the maastrichtian paleocene marine transgression from the atlantic ocean the change in the paleoslope of the basin from pacific ocean transgressions to this atlantic transgression is related to the uplift and deformation of the agrio fold and thrust belt the pichaihue limestone is unconformably deposited on volcanic agglomerates which in turn unconformably overlie early cretaceous deposits based on these data it is confirmed that the cretaceous uplift of the andes was episodic at these latitudes with a first pulse in the cenomanian and a second one in pre maastrichtian times the episodic uplift is also related to an eastward migration of the thrust front and the volcanic arc related to a previously proposed shallowing of the subduction zone these episodes were controlled by the western gondwana break up and the beginning of absolute motion of south america toward the west

three dimensional gravity study of the mid atlantic ridge evolution of the segmentation between 28 and 29 n during the last 10 m y during the high resolution survey sara segmentation ancienne de la ride atlantique sea beam bathymetry magnetic gravity and seismic reflection data were collected on the flanks of the mid atlantic ridge between 28 and 29 n this survey was designed to provide off axis information up to approximately 10 m y and to complement a detailed on axis survey carried out in 1988 and 1989 sempere et al 1990 between the atlantis and kane fracture zones a previous gravity study had revealed the existence of bulls eye shaped gravity lows centered on the axial segments and gravity highs centered on the non transform discontinuities lin et al 1990 we carried out a three dimensional calculation of the mantle bouguer anomaly in order to investigate if the axial pattern of circular anomaly lows can be followed on the flanks of the spreading center the off axis gravity anomalies are characterized by anomaly lows over the centers of the segments and anomaly highs over the discontinuities after correcting for the gravity effect of lithospheric cooling away from the ridge the segmentation configuration determined from gravity data appears to be very similar to that deduced from bathymetry off axis bathymetry is characterized by southward pointing v shaped basins which indicate that the traces of the nontransform discontinuities do not follow plate motion flow lines and that each ridge segment advances and retreats continuously however the gravity trace of the discontinuities is always slightly offset with respect to the bathymetric lows northward on the western flank and southward on the eastern flank the sense of offset between the gravity and bathymetric traces appears to be related to the right stepping nature of the axial discontinuities

facies architecture and heterogeneity of the fluvial aeolian reservoirs of the sergi formation upper jurassic reconcavo basin ne brazil abstract the sergi formation upper jurassic represents the main hydrocarbon reservoir of the reconcavo basin brazil the basal vertical facies succession of the sergi formation comprises reservoirs formed by a complex fluvio aeolian lacustrine interaction facies architecture and detailed petrophysical analysis of these reservoirs have enhanced the understanding of heterogeneity at a variety of scales and has allowed the development of predictive models that describes the range of styles of mixed fluvial aeolian reservoirs at megascopic scale the reservoirs are predominantly composed of sand bodies deposited by fluvial channel and aeolian facies associations regional flooding surfaces and sequence boundaries are their main flow barriers the regional flooding surfaces are composed of fine grained sediments deposited by lacustrine facies associations and the sequence boundaries act as flow barriers due to mechanically infiltrated clays based on its geometrical relations reservoirs linked to fluvial aeolian lacustrine interaction formed two types of reservoirs at macroscopic scale i with good lateral continuity of aeolian packages and relatively simple stratigraphic correlation and ii of highly compartmentalized aeolian packages with complex stratigraphic correlation and truncation by fluvial deposits mesoscopic heterogeneity reflects lithofacies sedimentary structures and lamina scale variability within aeolian and fluvial facies associations

architecture of coastal and alluvial deposits in an extensional basin the carboniferous joggins formation of eastern canada abstract the joggins formation was deposited in the cumberland basin which experienced rapid mid carboniferous subsidence on bounding faults a 600 m measured section of coastal and alluvial plain strata comprises cycles tens to hundreds of metres thick the cycles commence with coal and fossiliferous limestone siltstone intervals interpreted as widespread flooding events these intervals are overlain by coarsening upward successions capped by planar based sandstone mounds up to 100 m in width that represent the progradation of small river generated delta lobes into a standing body of open water developed during transgression the overlying strata contain sand rich heterolithic packages 1 8 m thick that are associated with channel bodies 2 3 m thick and 10 50 m wide drifted plant debris calamites groves and erect lycopsid trees are preserved within these predominantly green grey heterolithic sediments which were deposited on a coastal wetland or deltaic plain traversed by channel systems the cycles conclude with red siltstones containing calcareous nodules that are interbedded with thin sandstones and associated with both single storey channel bodies 1 1 5 m thick and 2 3 m wide and larger multistorey channels 3 6 m thick with incised margins numerous channel bodies at the same level suggest that multiple channel anastomosed river systems were developed on a well drained floodplain many minor flooding surfaces divide the strata into parasequences with dominantly progradational and aggradational stacking patterns multistorey channel bodies are relatively thin fine grained and modestly incised and palaeosols are immature and cumulative the abundance and prominence of flooding surfaces suggests that base level rise was enhanced whereas the lack of evidence for abrupt basinward stepping of facies belts coupled with the absence of strong fluvial incision and mature palaeosols suggests that base level fall was suppressed these architectural features are considered to reflect a tectonic architectural signature in accordance with the high subsidence basinal setting evidence for restricted marine influence and variation in floral assemblages suggests modulation by eustatic and climatic effects although their relative importance is uncertain

a lower cretaceous clastic slope succession livingston island antarctica sand body characteristics depositional processes and implications for slope apron depositional models the early cretaceous fill of the forearc intra arc byers basin includes a 600 to 900 m thick interval of marine slope apron deposits the president beaches formation this is a predominantly argillaceous succession within which coarser grained deposits are largely confined to lenticular packages of low width thickness ratios the entire formation was deposited in mid to late berriasian times coincident with a pulse of regional arc expansion at minimum mean accumulation rates of 120 225 mm 1000 years 1 the mudstones are finely laminated with a restricted benthic macrofauna and minimal bioturbation indicating relatively inhospitable sea floor environments sand rich packages occur as 7 to 30 m thick channel fill units composed chiefly of classical medium grained turbidites in some cases associated with complex high concentration turbidity current deposits and minor mud rich debrites these sand bodies are apparently elongate along normal to the nw facing palaeoslope implied by slump fold axes and the strike of the volcanic arc similarly palaeocurrent indices show a consistent arc parallel ne directed trend suggesting that transport processes were strongly influenced by the structural grain of the irregular slope morphology slope instability is recorded by widespread slump and soft sediment collapse features promoted by a combination of steep sea floor gradients and relatively high rates of sedimentation a lack of systematic vertical facies trends indicates that this was not a progradational or well organized system despite high rates of sediment supply however the strong systematic relationship between palaeocurrents and palaeoslope suggests a promising basis for evaluating organization in otherwise poorly ordered slope apron depositional systems

on balanced and unbalanced accommodation peat accumulation ratios in the cretaceous coals from gates formation western canada and their sequence stratigraphic significance coal composition was investigated by means of photometric and maceral analyses on closely spaced lithotype based strip samples over the full thickness of several paralic coal seams from the cretaceous gates formation of the western canadian sedimentary basin the aim of this investigation was to test various methods of identifying accommodation trends in coal and use them to refine sequence stratigraphic interpretation of continental sediments conventional sequence stratigraphy derives its subdivisions and significant surfaces from the records left by relative sea level oscillations these records either do not project into the continental realm or are difficult to recognise in clastic non marine sediments paralic coal seams have been selected to study this problem because they are not entirely removed from marine influence and compared with most inorganic deposits coal has stored a greater wealth of information that can be analysed at a higher level of resolution the study has led to the identification of five new surfaces with chronostratigraphic potential in the sequence stratigraphic analysis of non marine sediments two of these surfaces called paludification surface pas and terrestrialisation surface tes occur at the bases of the investigated coal seams while two other surfaces referred to as non marine flooding surface nfs and give up transgressive surface guts form the tops of the coal the fifth and probably most important new surface called the accommodation reversal surface ars is independent of any particular facies and may either coincide with some of the other surfaces or occur separately the proportion of detrital minerals has been used as the chief discriminator between different mire types and accommodation trends other useful indicators of mire type and peat dispersal have been the proportions of sporinite and inertodetrinite as well as some derived maceral and or mineral ratios e g the groundwater influence index and the tissue preservation index isometamorphic variations of telovitrinite reflectance and fluorescence as well as their coefficients of variation were also found to contribute to the identification of cyclic shifts between balanced and unbalanced accommodation peat accumulation ratios some of these cycles which are backed up by clastic stratigraphy appear to correspond to the development of shallowing upward and deepening upward parasequences superimposed high frequency low amplitude perturbations in the coal cycles relate to smaller scale accommodation cycles of sub parasequence level not always recognised in non marine strata these sub parasequence coal cycles do not always continue the shallowing upward trend typical of conventional parasequences several coals were found to contain stacks of small scale cycles with upward increasing accommodation signatures either in their lower or upper halves or over the whole seam section

sequence stratigraphy of nonmarine jurassic cretaceous rocks central cordilleran foreland basin system an analysis of the upper jurassic lower cretaceous morrison and cedar mountain formations of utah and colorado has resulted in a general sequence stratigraphic model for nonmarine rocks in this model nonmarine deposition is governed by changes in basin accommodation development and corresponding shifts in depositional base level these fluctuations result in deposition of systematically varying facies and architectural elements that allow nonmarine depositional sequences to be recognized internally nonmarine depositional sequences comprise three systems tracts degradational transitional and aggradational which are analogous to the lowstand transgressive and highstand systems tracts of marine depositional sequences degradational systems tracts overlie sequence bounding unconformities and consist of relatively coarse grained low sinuosity fluvial deposits that are either contained within incised valleys or deposited as widespread thin sheets above shallow erosion surfaces transitional systems tracts represent an increase in basin accommodation development following degradational systems tract deposition they are marked by the transition from laterally continuous low sinuosity fluvial channel sandstones and conglomerates to lenticular and ribbon like meandering and anastomosing channel sandstones aggradational systems tracts are characterized by meandering anastomosing channel sandstones and abundant fine grained overbank and lacustrine deposits the upper jurassic lower cretaceous nonmarine rocks of the study area contain three depositional sequences the first of these the uj 1 sequence consists primarily of an aggradational systems tract overlain by a sequence bounding unconformity however the lower parts of this sequence are transitional with older marine rocks and can be considered the late stages of a marine highstand systems tract the upper morrison sequence uj 2 consists of degradational transitional and aggradational systems tracts above the uj 2 sequence are a sequence bounding unconformity and degradational and transitional systems tracts of the lk 1 sequence represented by the buckhorn conglomerate the buckhorn is overlain by a sequence bounding unconformity and transitional aggradational systems tracts of the lk 2 sequence that is composed of the upper part of the cedar mountain formation the upper jurassic lower cretaceous sequences in utah and colorado can be traced regionally and correlated with nonmarine depositional sequences in central utah and sequences that contain nonmarine transitional and marine rocks in central wyoming these sequences were deposited in the back bulge forebulge and distal foredeep depozones of the late jurassic early cretaceous foreland basin system

mesoproterozoic delta systems of the acurua formation chapada diamantina brazil abstract facies analysis distinct depositional paleoflow directions and grain size ranges suggest that different fluvial systems fed coeval distinct independent river dominated and tide dominated braid delta systems in the mesoproterozoic acurua formation the river dominated delta system is characterized by a vertical stacking of parasequences up to 30 m which are characterized by coarsening and thickening upward trends b gradationally based vertical succession of prodelta delta front distributary channels and floodplain c grain size of sandstone from delta front and distributary channels varies respectively from very fine to medium and medium to coarse d lacking of wave or tide reworking suggests delta development in a relatively low energy marine basin e paleocurrent readings indicate delta front sandstone bodies shifting laterally towards ne to se directions the tide dominated delta system is characterized by a vertical stacking of parasequences up to 25 m which are characterized by coarsening and thickening upward trends b gradationally based vertical succession of prodelta delta front and tide influenced distributary channels c grain size of sandstone from delta front and tide influenced distributary channels varies respectively from very fine to coarse and fine to granular d abundance of tide influenced sedimentary structures suggest tidal dominated delta development e consistent bidirectional paleocurrents with northwards ebb tide direction and no lobe shifting the studied braid delta systems belong to a highstand system tract and as such they exhibit classic parasequence sets with progradational stacking pattern the facies association variability and overall characteristics of these mesoproteozoic delta systems are compatible with delta systems of all ages however contrary to typically phanerozoic deltas which are characterized by progradational clinoforms the coalescence of the studied braid delta facies associations promoted fairly sheet like geometry limited by marine flooding surface that more likely resulted from the response of deposition on shallow and wide epeiric sea

the sequence stratigraphy of fluvial depositional systems the role of floodplain sediment storage abstract the currently used model for alluvial sequence stratigraphy has been criticised miall 1991 and an alternative model is offered here in this model alluviation mainly takes place during transgressive and not highstand phases when accommodation space allows sediment storage the high capacity for some river systems to store floodplain sediments is a critical consideration the effects of climatic and intrinsic geomorphic changes during fluvial deposition are likely to greatly complicate simple base level controlled sequence models and recognising higher order fourth and fifth order cycles will be particularly difficult

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

emplacement of deep crustal and mantle rocks on the west median valley wall of the mark area mar 23 n abstract exposures of deep crustal and mantle rocks within the axial rift valley characterize accretion processes in the slow spreading mid atlantic ridge nine dives of the submersible nautile explored the western axial valley wall in the northern cell of the mark area mid atlantic ridge kane fracture zone where peridotite and gabbro outcrops had been previously reported in order to constrain the structure and determine emplacement mechanisms the ridge transform intersection massif on the western wall shows a section of gabbros from 6000 to 2500 mbsl locally overlain by metabasalts and metadolerites capped by slightly weathered basalts the morphology is controlled by ridge parallel faults dipping moderately 40 65 to the east transform parallel scarps present in the northernmost dives become rare toward the south brittle deformation along moderately dipping fault scarps produced a dense microcrack network filled with greenschist facies minerals chlorite actinolite epidote quartz locally overprinting high temperature ductile deformation fabrics the small hill located at 23 20 in the western wall shows good exposures of serpentinized peridotite between 3700 and 3100 mbsl above 3100 mbsl the summit of the hill is composed of pillow basalts and sediments the peridotite outerops display a strong schistosity dipping 20 40 to the east parallel to striated normal fault planes some steeper east facing fault scarps truncate the lower dipping fault surfaces the serpentinites deriving from a harzburgitic protolith are cut by rare dikelets of highly differentiated gabbro these new data combined with previous results of alvin dives are used to draw a generalized geological map of the western axial valley wall this map suggests a variation in the thickness of crustal units and composition along strike in the northern cell of the mark area the gabbro body disappears toward the south where basalts appear to directly overlie the mantle peridotites which are cut by isolated gabbroic dikelets low angle stretching affecting this heterogeneous lithosphere is probably responsible for the exposure of gabbros in the intersection massif and of peridotites further south

segmentation of the mid atlantic ridge between 24 n and 30 40 n the mid atlantic ridge mar extends for over 12 000 km from iceland to the bouvet triple junction in the south atlantic most of our knowledge of the geology of this slow spreading centre comes either from widely spaced low resolution echo sounding lines or from detailed studies of small areas using deep towed instruments or submarines high resolution multibeam bathy metry1 2 has been insufficient to define the variability of accretionary processes along this plate boundary here we present the results of a recent investigation of a 800 km long section of the mar carried out using the multibeam echo sounder seabeam3 analysis of these data reveals the scale and nature of the segmentation of the mar between the kane fracture zone 24 n and latitude 30 40 n

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

an allostratigraphic approach to a non marine basin the north western part of cretaceous kyongsang basin se korea the north western part of the cretaceous kyongsang basin south east korea comprises alluvial deposits of conglomerate gravelly sandstone sandstone and mudstone which can be grouped into four allomembers bounded by stratigraphic discontinuities the discontinuities trend nw se and are marked by distinct facies transitions abrupt emplacement of conglomerate and thin but persistent mudstone beds sedimentary facies and architectural analyses reveal that each allomember formed a depositional system of fluvial channel networks draining toward the south east with alluvial fans on the northern margin each allomember can be characterized by distinctive architecture of channel fills clast composition of conglomerate and sandstone mudstone ratio successive units show an eastward shift in the locus of deposition suggesting basinward relocations of alluvial systems such variations with time and space are interpreted to reflect changes in accommodation space and sediment supply during basin evolution probably caused by fault movements this study shows that detailed mapping combined with architectural analysis and the establishment of alluvial allostratigraphy can help assess changes in alluvial systems and structural development of the basin

the jurassic succession in west central argentina stratal patterns sequences and paleogeographic evolution abstract jurassic rocks in west central argentina are predominantly marine and marginal marine siliciclastics associated with prominent but volumetrically subordinate carbonates and evaporites facies developments were ruled by paleogeographic persistence within the southern infratropical latitude belt within 40 50 s and by siliciclastic derivation from the patagonian hinterland to the southwest minor volcanic and volcaniclastic supply arrived from the west out of a magmatic belt related to circum pacific convergence timing of the main marine flooding events and general correspondence with the high rank stratal packaging recorded in western north america suggest that global eustasy was also a factor in controlling the local stratigraphic record early jurassic sedimentation occurred within a series of semi isolated depocentres linked to fault bounded triassic troughs the sinemurian toarcian deposits record depocentre expansion and coalescence these trends were coeval with progressively more widespread marine invasions from the northwest and west leading to an elongate marine seaway which connected central patagonia with the pacific domain during the aalenian bajocian the region was subject to a more subdued tectonism and the foreland side of the basin became fringed by an extensive clastic embankment bathonian and early callovian were times when coarse clastics prograded into the basin while the marine embayment shrank as a result of stepwise forced regressions during late callovian to oxfordian globally rising sea level the depocentre witnessed the appearance of cosmopolitan invertebrates and a stratal pattern of basin widening and depositional underbalance that promoted cratonward onlap and inception of widespread carbonate deposition ooidal coralline carbonate development was terminated after a relatively sudden messinian style event that desiccated a large tract of the andean basin and favored massive precipitation of anhydrite in the course of the kimmeridgian the evaporite basin was largely flooded by siliciclastics and turned into a broad and featureless mudflat salina complex linked to a widespread erg and to an ephemeral drainage system by the tithonian at a period of peak oceanic stand marine connection was reinstated and recorded as the most widespread jurassic transgression across the neuquen aconcagua embayment shelfal deposition consisted of molluscs and ooid dominated carbonate terraces that grew in pulses tuned to eustatic fluctuations like in other prolific petroleum provinces around the world the tithonian basinal strata involve widespread euxinic deposits featuring unusually high organic content

alluvial fans and fluvial fans at the margins of continental sedimentary basins geomorphic and sedimentological distinction for geo energy exploration and development alluvial fans and fluvial fans are the most common depositional landforms along the margins of continental sedimentary basins their occurrence is determined by the area relief and hydrology of the catchment which ultimately control the relative ratios of sediment v flow discharge and runoff regimes fundamental morphological and process distinctions exist between these two kinds of deposystem which are seen as essentially different facies associations and internal architectures for the corresponding deposits alluvial fans commonly develop over short radial distances hundreds of metres to a few kilometres and are constructed by ephemeral flash flow events that lead to poor organization of the sedimentary facies and overall architecture of the corresponding deposits by contrast fluvial mega fans are fed by proper river systems which aggrade much larger volumes of clastic sediment over distances of up to a few hundred kilometres distinct channel belt and overbank domains are developed with a marked heterogeneity in the distribution of sedimentary facies represented by hierarchically well organized fluvial deposits the general properties of alluvial and fluvial fans reflect the different potentials for the corresponding successions to host economically attractive oil and gas resources and the different approaches required in exploration and prediction

direct measurement of magnetic reversal polarity boundaries in a cross section of oceanic crust magnetic field measurements made by submersible define the cross sectional geometry of a magnetic polarity rever sal boundary and the vertical variation of crustal magnetization in upper oceanic crust measured polarity boundaries show a systematic pattern of shallow dip towards the spreading axis within the upper extrusive lavas and steeper dip in the lower extrusive lavas this geometry is a consequence of the em placement of extrusive lava at a midocean ridge reversal boundary geometry and magnetization estimates are used to calculate the magnetic contribution of the extrusive lava se quence to the overlying marine magnetic anomaly signal from the forward modeling the highly magnetized extrusive lavas contribute the majority 50 75 of the observed sea surface magnetic anomaly consistent with the extrusive crust forming the primary source layer for young marine magnetic anomalies

climatic cyclicity and accommodation space in arid to semi arid depositional systems an example from the rotliegend group of the uk southern north sea

aeolian fluvial interactions and high resolution sequence stratigraphy of a non marine lowstand wedge the avile member of the agrio formation lower cretaceous central neuquen basin argentina abstract accumulation within the unconformity based hauterivian avile sandstone oftheneuque nbasin argentina wascharacterizedbyacloseinteractionbetween uvialandaeolianprocessesdevelopedafteramajorrelativesea leveldropthatalmostcompletely desiccated the entire basin and juxtaposed these non marinedeposits on shallow and deep marine facies aeolian deposits within the avile memberincludedune a1 andsandsheet a2 unitsthatcharacterizethelowerpart of the unit fluvial deposits comprise distal ood units f1 interbeddedwith aeolian dune deposits in the middle part of the succession and low f2 and high sinuosity f3 channels associated with oodplain deposits f4 towards the top the internal characteristics of the aeolian system indicate thatits accumulation was strongly controlled by water table dynamics with thedevelopment of multiple horizontal de ation super surfaces that truncate dunedeposits and form the basal boundary of ood deposits and sand sheet units along term wetting upward trend is recorded throughout the entire unit with anincrease in uvial activity towards the top and the development of a morepermanent uvial system overlying a major erosion surface interpreted as asequence boundary the upward increase in water table in uence might berelated to relative sea level rise which controlled the position of the water tableand allowed the accumulation of tabular aeolian units bounded by horizontalde ation surfaces this high frequency eustatically driven process actedtogether with a long term climatic change towards wetter conditions keywords aeolian uvial interactions argentina avile sandstone creta ceous neuque n basin water table introductionseveral authors have described intertonguingbetween aeolian and uvial deposits in differentbasins suggesting that the interaction betweenthese two processes is common in both modernenvironments langford 1989 and the rockrecord langford chan 1989 herries 1993 thelowercretaceous hauterivian avile sand stone member of the agrio formation is excep tionally well exposed in the central part of theneuque n basin it consists of aeolian and uvialdeposits intercalated between ammonite bearingshales of the lower and upper members of theagrioformation fig 1 ithasbeeninterpretedasa lowstand wedge produced by a major relativesea level drop legarreta gulisano 1989 although the avile sandstone has been dis cussed in terms of its depositional environments veiga vergani 1993 and aeolian processes

depositional character and preservation potential of coarse grained sediments deposited by flood events in hyper arid braided channels in the rift valley arava israel abstract four coarse grained braided channels undergoing rapid deposition in the hyper arid arava rift valley have been studied to determine the dynamics of strata deposition by monitored flood events and to ascertain the sedimentologic character of these event deposits the systems span granule sand to sandy gravel bed material having longitudinal bars with simultaneously increasing grains size slope and bed relief scour and fill were monitored along cross sections using scour chains and repeat surveys while event strata fill were described with reference to sediment calibre type of support grading thickness and location unlike many previous investigations of gravel sedimentology the source of information for this study involves not only stratigraphic description but also observation of the concomitant dynamics of the channel bed as expressed by the extent of event deposition the texture of the deposited sediment generally coarsens in sympathy with the surface bed texture of the studied channels the thickness of event deposited sediments increases as the magnitude of flow events increases the stratigraphic record is characterized by 1 sub parallel stratification 2 no grading 3 clast supported gravel infilled with sand granule matrices and 4 rapid sedimentation rates this character is hypothesized to mirror the elevated bedload fluxes in this hyper arid fluvial setting lacking densely vegetated banks the proximal braidplain of this arid environment often contains no extensive fine textured floodplain successions the depth of fill as well as the extent of strata obliteration increase with flood magnitude explaining the increased preservation potential of event strata with increase in flood magnitude it also explains why most event strata including large ones are only partially preserved

sandstone body structures and ephemeral stream processes in the dinosaur canyon member moenave formation lower jurassic utah u s a abstract studies of fluvial sandstone body structures in the lower jurassic dinosaur canyon member suggest a threefold subdivision of the ephemeral stream deposits sandstone sheets with interbedded siltstones are less than 1 m thick and laterally extensive for hundreds of metres they are interpreted as sheetflood deposits simple channel sandstone bodies are a few metres thick and a few tens of metres wide they reflect solitary channel incision episodic migration and plugging multistorey channel sandstone bodies are a few metres thick and laterally extensive for hundreds of metres they are composed of several channel shaped storeys and exhibit only local incision the multistorey sandstone bodies are interpreted as braided ephemeral stream deposits two sandstone sheet subtypes with grooves and mounds respectively are interpreted as intermediate between the sheetflood deposits and solitary incised channel deposits on one hand and between sheetflood deposits and braided stream deposits on the other hand the solitary channels and braided streams are accordingly interpreted to be initiated from sheetfloods through differential erosion and differential deposition respectively this model of channel evolution from sheetfloods is probably applicable to other semiarid and arid fluvial environments dominated by surface runoff

sedimentology and stratigraphy of cenozoic deposits in the kagizman tuzluca basin northeastern turkey the kagizman tuzluca basin is located in the northeastern anatolia to the east of the intersection point near karliova of the major north and east anatolian fault systems this intermontane basin displays a thick sequence over 2000 m of mostly terrestrial deposits represented by repetitive alternations of the lake and fluvial environments ranging from late eocene oligocene to middle late miocene a marine incursion only mappable in the southeastern margin of the basin deposited limestones and sandy limestones rich in marine mollusks and nummulites in particular n fichteli that constrain an early oligocene age for this marine unit kagan fm the terrestrial basin fill deposits show different thicknesses throughout the basin due to irregular bottom topography and tectonic configuration of the basin margins the thickest deposits were accumulated along the different margins of the basin which received high quantities of siliciclastics from meandering river alluvial and coastal fans fan delta gilbert type delta and wave worked fluvial delta climate changes have also driven the development of lake environments during distinct depositional periods siliciclastic dominated overfilled lakes halikislak and kizilkaya formations and chemical dominated underfilled lakes turabi and tuzluca formations were formed they have been also classified as newark type and fundy type lakes respectively fluvial systems evolved from high energy meandering rivers deposited under humid climate gungoren formation to low energy meandering rivers resulted from arid and semiarid climates cincavat formation the transitional intervals from ephemeral river dry mudflat cincavat formation to saline pan lake tuzluca formation indicate wadi sand flat playa fluvial systems the chronostratigraphic constrains of the entire sequence remain poor and so far solely based on vertebrate fossil assemblages the evaporitic tuzluca formation would be middle miocene in age due to its stratigraphic position over the cincavat formation