oolitic sandbody depositional models and geometries mississippian of southwest britain implications for petroleum exploration in carbonate ramp settings abstract a 1000 m thick early mississippian carbonate supersequence the carboniferous limestone of southwest britain consists of three third order depositional sequences these comprise parasequences in various configurations and the whole forms a carbonate ramp stack within this framework five major oolitic carbonate sandbodies developed a castell coch limestone b stowe oolite c brofiscin oolite d gully oolite and e high tor limestone the depositional regime was storm and wave dominated throughout and the major sandbodies represent a range of progradational carbonate beaches barriers and detached subtidal shoals analysis of the three dimensional shapes and distribution of these five examples shows that they evolved to produce three major carbonate sandbody geometries a strings b sheets and c wedges these geometries are characterised using the five field examples and offered as a template which may assist in the exploration and reservoir modelling of petroleum rich high energy ramp systems progradation for up to 40 km of barrier islands stowe oolite and beach ridge plains gully oolite formation generated strings and thick sheets individually up to 10 20 m thick thin shoreface retreat carbonate packstone grainstone sheets up to 5 m thick high tor limestone developed during transgressions as veneers across flooding surfaces these are comparable with sheet sands developed in siliciclastic shelf depositional systems progradation for up to 30 km and vertical aggradation of shoreline detached oolite shoals castell coch limestone brofiscin oolite generated basinwards expanding or thinning wedges up to 30 m thick tectonically controlled stacking of strandplain sheets produced a composite carbonate sandbody up to 80 m thick gully oolite the intrinsic sedimentary and extrinsic eustacy tectonism climate factors which controlled these sandbody geometries are addressed establishing the positions of the sandbodies accurately within depositional sequences allows them to be located within inferred seismic sequence geometries and provides one possible solution to the difficult problem of predicting carbonate facies distribution in subtle stratigraphic plays in this ramp system the most homogeneous sandbodies up to 30 m grainstones with greatest reservoir facies potential are represented by shoal belt wedges potential grainstone reservoir facies in the prograding shorelines are limited to the upper parts of individual shoreface sequences max 10 m grainstones for shoreline carbonate sandbodies the greatest reservoir and stratigraphic trapping potential exists in the earliest ramp parasequences where enveloping offshore sediments are siliciclastic mudstones in later stages potential seals are likely to be less reliable low porosity outer ramp carbonates

transgressive sand bodies on the maresme continental shelf western mediterranean sea abstract three large scale elongate asymmetrical sand bodies lie beyond the shoreface of the straight sandy coast of maresme barcelona in the northwestern mediterranean sea they are found on the continental shelf at water depths of 15 30 m 35 80 m and 95 113 m they are typically 12 20 m thick 1000 2000 m wide and trend parallel or subparallel to the coast for a distance of up to 24 km slopes on the steep offshore side vary between 3 and 20 while the shoreward slopes are gentler the shallowest sand body is modern in dynamic equilibrium with present conditions the other two however are relict bodies built up during the last postglacial rise in sea level development of these sand bodies occurs in the shoreface environment and in a space time evolution they show a punctuated but nearly continuous nature in the way that their preservation during the holocene transgression occurs only in specific locations on the continental shelf they begin growing on pleistocene scarps with offshore slopes and their growth pattern is mostly controlled by the sediment supply flow distribution and rate of sea level rise the formation of these sandy bodies is related to winter storms and southwesterly flows which cause large volumes of sediment to be transported downshelf and offshore by downwelling bottom currents generated by coastal set up the result is sand movement onto and across the sand body surface down to water depths of 15 20 m the thick sandy lenses of the maresme shelf are interpreted as topographically induced lee side sand bodies and not primarily as sand ridges in the classic sense the occurrence of these bodies on a low energy shelf may be significant for the interpretation of the geological record

accommodation and sediment supply controls on clastic parasequences a meta analysis this study combines data from many published case studies to undertake a quantitative characterization of clastic parasequences with the aim to determine how accommodation sediment supply and autogenic sediment storage dynamics are recorded in their sedimentary architecture and stacking patterns results of this study are used to critically evaluate the validity of paradigms and models that are routinely used to explain and predict trends in the anatomy and arrangement of parasequences data on 957 parasequences from 62 case studies of clastic shallow water successions were coded in a relational database which includes outcrop and subsurface datasets of ancient and quaternary examples these units cover the preserved records of both river dominated deltas and wave dominated coasts representing shoreline transits over a breadth of timescales likely of both local and regional extent the role of extant accommodation rates of creation of accommodation and rates of sediment supply in determining parasequence architecture is assessed through analysis of relationships between i proxies of these variables at different scales rates of aggradation and progradation facies belt shoreline trajectories systems tract type parasequence set stacking patterns parasequence progradation angle and stratigraphic rise size of feeder rivers and ii parameters that describe the geometry and stacking style of parasequences and associated shallow water sand bodies statistical analyses of database outputs indicate which proxies of accommodation sediment supply and accommodation sediment supply ratio are significant as predictors of parasequence architecture and allow for interpretations of the importance of allogenic and autogenic factors the principal results of this study reveal the following i parasequence thickness varies as a function of water depth accommodation generation and erosional truncation and these variations are also reflected across types of systems tracts and parasequence sets ii the dip length of parasequence sand bodies demonstrates scaling with measures of accommodation sediment supply ratio at multiple scales partly in relation to the possible effect of sediment supply on progradation rates iii in systems tracts stratigraphic trends in parasequence stacking due to autogenic mechanisms or to acceleration or deceleration in relative sea level fluctuations are not revealed quantitatively iv some association is seen between the abundance of deltaic or river dominated parasequences and progradational stacking v positive but modest correlation is observed between measures of river system size and the dip length of shallow marine parasequence sand bodies the resulting insights can be applied to guide sequence stratigraphic interpretations of the rock record and the characterization of sub seismic stratigraphic architectures of subsurface successions

the calcarenite di gravina formation in matera southern italy new insights for coarse grained large scale cross bedded bodies encased in offshore deposits excellent outcrops of the upper pliocene lower pleistocene calcarenite di gravina around matera southern italy provide continuous exposure of coarse grained clastic basin margin shoreline to offshore facies among these facies the most conspicuous and volumetrically important are the transition slope deposits that form large scale high angle cross bedded lithosomes these are laterally extensive parallel with the paleoshoreline and show seaward progradation we interpret them to represent avalanches of sediment swept out onto a depositional slope below wave base from the shoreface zone by storm waves and wind driven currents three types of building blocks are recognized based on bedding patterns and facies architecture embryonic parasequences mature parasequences and simple sequences parasequences formed during stillstands of sea level and simple sequences during high frequency cycles of relative change of sea level these building blocks are stacked in a backstepping configuration and onlap onto cretaceous limestone substrate backstepping is believed to be due to a tectonically forced transgression that is punctuated by high frequency cycles of sea level modern analogs for these building blocks are the holocene prograding prisms detected in high resolution seismic lines of the mediterranean shelves the reflection patterns of these seismic units resemble the bedding architecture of the matera simple sequences and parasequences and show similarities of shape size position and orientation of coastal setting and direction of progradation comparison of the holocene prisms and the pliocene pleistocene accretional units in matera indicates that they may represent the same begin page 662 genetic process progradation of clastic prisms below the wave base level the matera accretional units also show similarities with other examples of laterally extensive large scale cross bedded sand bodies encased in offshore deposits such as some sandbanks differentiating between these laterally extensive and seaward prograding cross bedded sand bodies and some coarse grained gilbert type deltas is difficult however if interpretations are based only on two dimensional 2 d outcrops in dip section without high resolution data it is also difficult to distinguish between transition slope and prograding shoreface lithosomes this difficulty may be acute where only seismic or well log data are available however certain other architectural characteristics such as stacking and preservation of facies belts and position of bounding surfaces as well as differences in associated sedimentary structures and fossil content may be used for interpretation the matera example provides a mechanism for emplacement in offshore settings of elongate and strandline parallel sand and gravel deposits that prograde seaward and that preserve a coarsening upward internal succession this article offers an interpretation for other ancient examples of large scale cross bedded lithosomes encased in offshore deposits

clinoforms and clinoform systems review and dynamic classification scheme for shorelines subaqueous deltas shelf edges and continental margins abstract clinoforms are inclined and normally basinward dipping horizons developed over a range of spatial and temporal scales in both siliciclastic and carbonatic systems the study of clinoform successions underpins sequence stratigraphy and all efforts to reconstruct the relative partitioning of reservoir seal and source rocks along shoreline to basin floor profiles here we review clinoform research and propose a more systematic description and classification of clinoforms this is a crucial step to improve predictions of facies and lithology distribution within shoreline to continental shelf and abyssal plain successions together with the genesis drivers and dynamics of their constituent sedimentary units four basic clinoform types are here distinguished in delta shorelines lacustrine and marine environments on the basis of their overall spatial and temporal scale morphology outbuilding dynamic and geodynamic and depositional setting 1 2 delta scale clinoforms which in turns are sub divided into shoreline and delta scale subaqueous clinoforms 3 shelf edge clinoforms and 4 continental margin clinoforms delta scale clinoform sets are tens of metres high and typically represent 1 103 kyr with progradation rates ranging from 1 000 100 000 m kyr for shorelines and subaerial deltas to 100 20 000 m kyr for subaqueous deltas shelf edge clinoform sets are hundreds of metres high and are nucleated and accreted in 0 1 20 myr usual progradation rates of 1 100 m kyr by successive cross shelf transits of delta scale clinoforms continental margin clinoform sets are thousands of metres high hallmark key geodynamic crustal boundaries e g continent ocean transition and slowly prograde basinwards in ca 5 100 myr with typical rates of 0 1 10 m kyr as a consequence of the very different progradation rates and of the difficulty of large scale clinothems to backstep during transgressions shorelines are the most dynamic clinoforms with regards to position continental margins the least and shelf edges are intermediate shortly after a transgression therefore the four clinoform types may prograde synchronously along shoreline to abyssal plain transects forming compound clinoform systems during the subsequent regressive cycle however due to the dissimilarity in progradation rates different clinoform types will normally merge progressively with each other giving rise to hybrid clinoforms e g shelf edge deltas and fewer depositional breaks in slope are distinguished along a single shoreline to abyssal plain transect overall all clinoform systems are the result of the dynamic evolution of compound and hybrid clinoforms along a temporal and spatial continuum regulated by the cyclical backstepping of the smaller scale system within natural progradational retrogradational cycles of larger scale clinothem outbuilding all clinothem types may show either an accretionary active or draping passive style depending on the proximity to the sediment source draping clinothems are nearly always composed of condensed fine grained sediments while actively accreting clinothems can be composed of predominantly coarse grained i e reservoir forming or predominantly fine grained i e non reservoir lithotypes a novel hierarchical classification scheme for both recent and ancient clinoforms is here proposed consisting of 12 classes the four basic clinoform types delta scale shoreline delta scale subaqueous shelf edge and continental margin are sub divided into eight accretionary active and draping passive sub types 8 division each accretionary sub type is then sub divided into a sandstone prone and mudstone prone variant 12 division which can be at least tentatively predicted on the basis of the clinoform morphology even in the absence of direct stratigraphic logs

a pliocene cliff line around the giza pyramids plateau egypt abstract escarpments bordering the giza pyramids plateau represent the cliff line of a pliocene transgression up the pre nile eonile valley geomorphologically a limestone cliff can be distinguished from a slip block shore associated with a distinct fining up sequence differences in bedrock lithology and in structure joint pattern faults are morphogenetic controls the pre pliocene morphology with escarpments that allowed the development of later cliff lines is related to deep incision of the eonile drainage system caused by the desiccation of the mediterranean sea during the messinian

physical sedimentation processes on a mid proterozoic 1800 ma shelf the reynolds range group arunta block central australia abstract shallow marine siliciclastic and carbonate deposits belonging to the 1800 ma old reynolds range group central australia represent a transgressive sequence deposited on a low gradient shelf that was affected by longshore currents tidal currents storm wave action and fair weather wave action the sediments comprise a 1100 m thick sequence in which three main depositional facies associations can be distinguished i a near shore and inner shelf quartz arenite facies association is overlain by ii a middle shelf mudstone facies association which contains lenses of iii a carbonate facies association the quartz arenite facies association mainly occurs along the northeast side of the range and grades into the mudstone facies association towards the southwest this suggests that the palaeo shelf extended towards the southwest which is in accordance with a concentration of southwest directed palaeo current directions within the quartz arenite facies a sudden increase in thickness of quartz arenite in the mount thomas area suggests this area was the locus for fluvial sediment input vertical and lateral stratigraphic transitions between the facies associations are generally gradational which may reflect coexistence of all facies on the shelf and small net relative sea level changes the quartz arenite facies association contains tide influenced bidirectional small scale trough cross bedded tabular cross bedded and parallel laminated sets that were deposited on the shoreface these are generally overlain by large scale trough and tabular cross bedded sets which were deposited on the inner shelf and record a southeast directed longshore current numerous truncation surfaces indicate that frequent storms eroded the shoreface and carried large volumes of sediment onto the shelf the mudstone facies association consists of alternating mudstone beds and generally thin sheetlike quartz sandstone beds that lack grading and traction structures they represent storm induced suspension deposits laid down on the middle shelf inferred water depths 100 m longshore and tidal currents appear to have had little influence in this environment stromatolite bioherms and rippled marls and grainstones constitute the carbonate facies association which formed on the middle shelf in shallow marine areas of low siliciclastic sediment input ripples in storm sands throughout the mudstone facies association and a lack of turbidites reflect the shallow depths and low gradient of the shelf slope a general lack of tidal deposits and the non barred nature of the palaeo coastline indicate a small tidal range 0 4 m the unidirectional southeast directed longshore current appears to have only influenced sedimentation on the inner shelf which suggests it resulted from prevailing westerly winds the described facies in the reynolds range group occur in strongly deformed and highly metamorphosed rocks and the paper serves as an example that even from such tectonised rocks valuable sedimentological information can be extracted

shallow water eruption fed mafic pyroclastic deposits along a paleoproterozoic coastline kangerluluk volcano sedimentary sequence southeast greenland abstract the 200 300 m thick volcano sedimentary sequence at kangerluluk is part of the psammite zone one of four major zones which constitute the 1 8 ga ketilidian orogen in south greenland three lithofacies are emphasized in the study 1 the conglomerate sandstone 2 the volcanic and 3 the pyroclastic lithofacies the 2 40 m thick conglomerate sandstone lithofacies represents a subaerial to subaqueous fan delta deposit matrix and clast supported conglomerates are interpreted as debris flow and longitudinal gravel bar deposits erosive based conglomerate channel fills attest to stream incision trough crossbedded sandstone interpreted as lunate megaripples planar bedded sandstone indicative of upper flow regime bar top sands and small scale trough crossbeds reflecting ripples follow up section form collectively with the conglomerate 0 40 2 50 m thick fining upward sequences the sandstone dominated unit up section from the conglomerates and composed of planar and low angle crossbeds minor ripples and graded beds as well as mudstone is indicative of a lower shoreface deposit below normal wave base the clastic sedimentary rocks are suggestive of a fan delta setting the 100 200 m thick volcanic lithofacies composed of pillowed and pillow brecciated lava flows is consistent with shallow water deposition interstratification of lava flows with both conglomerate sandstone and pyroclastic lithofacies intrusion of dykes into volcaniclastic rocks and peperite formation accentuate contemporaneity between volcanism and sedimentation and is a common feature of island arcs the 1 50 m thick pyroclastic lithofacies with sharp depositional contacts to the overlying volcanic and underlying conglomerate sandstone lithofacies was emplaced in a subaqueous setting the lithofacies is divided into a planar to crossbedded tuff lapilli tuff and a bedded lapilli tuff breccia whereby both deposits are inferred to result from shallow water surtseyan type eruptions the 5 15 m thick bedded lapilli tuff breccia with abundant bomb sag structures and graded beds is considered a result of subaqueous eruptions strong enough to form an insulating steam cupola characterized by ballistically emplaced bombs that rapidly collapsed allowing for transport via mass flow processes the deposits are considered proximal to the vent the 2 50 cm thick planar to crossbedded tuff lapilli tuff featuring abundant euhedral and broken crystals of feldspar 2cm and minor pyroxene 1 cm are massive graded crossbedded and stratified the planar but laterally discontinuous beds characterized by abundant low angle scours are interpreted as low to high concentration sediment gravity flows produced directly from subaqueous tephra jets that collapsed due to massive water ingestion local breccia size pyroclasts disrupting beds are interpreted as bomb sags the mafic eruption fed surtseyan type deposits postulated to be a subaqueous counterpart of cold subaerial base surges originate from subaqueous tuff cones formed along a rugged volcanic dominated shoreline featuring high energy fan deltas

geometry and facies of tertiary clinothems spitsbergen sandstone clinothems from the battfjellet formation palaeogene on spitsbergen are locally exceptionally well preserved along depositional dip parallel mountainsides the clinothems are more than 1 km wide and more than 100 m thick superposition of several sandstone clinothems separated by mudstones reflects repeated shoreline progradation and transgression deposition took place partly on post transgressional depositional shelves and partly by contributing seaward sloping wedges or clinothems to a ramp progradation shorelines dominated both by mouth bar and shoreface environments have been identified the clinothems are organized into an overall progradational architecture with a geometry having features in common with progradational seismic facies

sequence stratigraphy of the siliciclastic east puolanka group the palaeoproterozoic kainuu belt finland abstract the 2300 2600 m thick palaeoproterozoic east puolanka group within the central fennoscandian shield records four major transgressions on the cratonic margin within the approximate time period 2 25 2 10 ga stacking of siliciclastic facies in parasequences and parasequence sets provides data to evaluate oscillation of relative sea level and subsidence on different temporal scales the lowermost part of the passive margin prism is characterized by alluvial plain to shallow marine sediments deposited in incised valleys the succeeding highstand period is recorded by ca 250 m of progradational parasequence sets of predominantly rippled and horizontally laminated sandstones representing stacked wave dominated shoreline units in sequence 1 capped by a hiatus or in some places by a subaerial lava as relative sea level rose again sand rich barrier beach complexes developed with microtidal lagoons and inlets corresponding to a retrogradational parasequence set this was followed by a highstand period with aggradation and progradation of alluvial plain and coastal sediments grading up into wave tide influenced shoreline deposits in sequence 2 in sequence 3 the succeeding mudstones represent tidal flat deposits in a back barrier region with continued transgression the parasequences stacked retrogradationally each flooding episode being recorded by increasingly deeper water deposits above low angle cross bedded sandstones of the swash zones the succeeding highstand progradation is represented by alluvial plain deposits the next transgressive systems tract overlying an inferred erosional ravinement surface is recorded by a retrogradational parasequence set dominated by low angle cross stratified swash zone deposits in sequence 4 the large scale trough cross bed sets in these parasequences represent sand shoals and sheets of the inner shelf system the overall major transgression recorded in the lowermost part of the palaeoproterozoic cratonic margin succession was related to first to second order sea level changes probably due to increasing regional thermal subsidence of the lithosphere following partial continental breakup the stratigraphic evolution can be related to changes of relative sea level with a frequency of ca 25 million years probably propagated by episodic thermal subsidence the parasequences identified here are related to high frequency cycles of relative sea level change due to low magnitude eustatic oscillations

is barrier island morphology a function of tidal and wave regime abstract classification of barrier island morphology stems from the seminal work of m o hayes and others which linked island shape to tidal range and wave height and defined coastal energy regimes i e wave dominated mixed energy tide dominated if true this general relationship represents a process based framework to link modern and ancient systems and is key for determining paleomorphodynamic relationships here we present a new semi global database of barrier islands and spits n 702 shape parameters aspect circularity and roundness are used to quantify island boundary shape and assess potential correlation with coastal energy regime using global wave and tide models in adopting the original energy classification as originally put forth i e wave dominated wave influenced mixed tide influenced mixed tide dominated results show that wave dominated islands have statistically different mean shape values from those in the mixed energy fields but the two mixed energy designations are not distinct from each other furthermore each energy regime field contains a wide range of island shapes with no clear trends present linear regression modeling shows that tidal range and wave height account for

regional diagenetic trends in the lower cretaceous muddy sandstone powder river basin lower cretaceous sandstones of the muddy formation have produced a number of significant hydrocarbon reservoirs in the powder river basin these reservoir sandstones are accumulated in a variety of fluvial deltaic and shallow marine environments the porosity and permeability of muddy sandstones are affected significantly by the amount and composition of diagenetic clay minerals diagenetic clay mineral assemblages within the muddy sandstones are time controlled oldest muddy sandstones sedimentologic zones 5 and 6 are characterized by a kaolinite chlorite illite quartz assemblage with some smectite intermediate muddy sandstones sedimentologic zones 3 and 4 are characterized by an illite smectite assemblage with sporadic chlorite quartz and kaolinite youngest muddy sandstones sedimentologic zones 1 and 2 are characterized by a kaolinite quartz assemblage

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

the sedimentary record of the post glacial transgression on the gulf of cadiz continental shelf southwest spain abstract four seismic units ta to td have been identified from the analysis and interpretation of high resolution seismic profiles in a sector of the gulf of cadiz shelf they constitute a composite seismic unit attributed to the transgressive systems tract tst of the last depositional sequence the earliest transgressive unit was deposited on the outer shelf and represents the distal facies of a coastal deposit a process of shelf partitioning seems to have occurred during the formation of the three later transgressive parasequences the shelf sector offshore from the guadiana river mouth was a high energy environment dominated by storm events by contrast the shelf sector offshore from the donana national park was a low gradient shelf where large barrier island and lagoon systems were formed the sparse occurrence of marine deposits within the tst is a consequence of the episodic nature of the sea level rise the generation and preservation of these transgressive deposits results from the interaction of the following controlling processes 1 relative sea level rise the formation of coastal transgressive deposits is related to intervals of reduced sea level rise or stillstands within a period of continuous sea level rise those sea level changes were probably driven by short term periods of colder climate during the late pleistocene holocene deglaciation the younger dryas interval is the most widely recognised climatic event of this type but probably other events of similar characteristics had the same effect on the glacio eustatic sea level rise 2 sediment supply changes short lived climatic events probably also involved changes in the sediment supply to the continental shelf due to changes in erosion rates and river regimes of the river basin hinterlands 3 effect of the paleophysiography paleogeography the sedimentary environments differentiation processes are associated to the formation of an exposed coastal promontory offshore from the guadiana river mouth and a semi protected embayment offshore from donana national park this coastal configuration was controlled by previous tectonic evolution and influenced the non uniform landward migration of the coastline

rollover drowning and discontinuous retreat distinct modes of barrier response to sea level rise arising from a simple morphodynamic model we construct a simple morphodynamic model to investigate the long term dynamic evolution of a coastal barrier system experiencing sea level rise using a simplified barrier geometry the model includes a dynamic shoreface profile that can be out of equilibrium and explicitly treats barrier sediment overwash as a flux with barrier behavior primarily controlled by the maximum potential overwash flux and the rate of shoreface response the modeled barrier system demonstrates four primary behaviors height drowning width drowning constant landward retreat and a periodic retreat height drowning occurs when overwash fluxes are insufficient to maintain the landward migration rate required to keep pace with sea level rise on the other hand width drowning occurs when the shoreface response rate is insufficient to maintain the barrier geometry during overwash driven landward migration during periodic barrier retreat the barrier experiences oscillating periods of rapid overwash followed by periods of relative stability as the shoreface resteepens this periodic retreat which occurs even with a constant sea level rise rate arises when overwash rates and shoreface response rates are large and of similar magnitude we explore the occurrence of these behaviors across a wide range of parameter values and find that in addition to the maximum overwash flux and the shoreface response rate barrier response can be particularly sensitive to the sea level rise rate and back barrier lagoon slope overall our findings contrast with previous research which has primarily associated complex barrier behavior with changes in external forcing such as sea level rise rate sediment supply or back barrier geometry

a review of wave dominated clastic shoreline deposits abstract barrier islands beaches wave dominated deltas and chenier plains probably form more than one fifth of present coasts and constitute an even larger proportion of the subsiding lowland coasts likely to be preserved in the stratigraphic record the geometry vertical sequences and composition of their deposits are the result of 1 shoreline behaviour whether transgressive regressive or stable 2 physical processes particularly the relative intensities of waves and tides 3 climate 4 global tectonic setting and 5 topography of underlying deposits published interpretations indicate that equivalent deposits are abundant in the geological record sandstones are mostly quartzose cemented by carbonates and quartz and have sheet or linear geometries ancient deposits can provide evidence of palaeogeography and some are economically significant particularly as hydrocarbon reservoir rocks four end member sedimentation models are proposed for the interpretation of ancient deposits 1 transgressive sheet sands associated with deltas 2 non deltaic transgressive sheet sands 3 regressive sheet sands and 4 linear sand bodies probable depositional settings geometries characteristics of deposits and palaeocurrent patterns are discussed

1 66 ga old metamorphosed pbcu deposit in sargipali eastern india manifestations of tidal flat environment and sedex type genesis abstract the metasedimentary rocks and the unusual pb cu mineralization of the sargipali mine area are described and discussed to elucidate the processes involved in their deposition important lines of evidences used for the interpretation of an inter to supratidal flat environment are 1 a general pattern of a shallowing upward sequence from coarse clastics arenaceous through finer clastics argillaceous to carbonate 2 the occurrence of metaconglomerate trough cross bedding and wave ripples within the arenaceous lithostratigraphic unit 3 intercalations of quartzite carbonate skarn dolomite and convolution of chert laminae within the argillaceous lithostratigraphic unit and 4 intercalation of quartz mica chlorite schist laminae within the carbonate lithostratigraphic unit the ores seems to belong to the same environment furthermore in the assessment of the origin of the sargipali ores it is recognized that the orebodies not only represent a sedimentary setting but also show a high pb content and an older age of mineralization than that of deformation metamorphism and granite intrusion along with the lack of stockwork zones wall rock alteration subaerial volcanism and the presence of chert and tourmalinite these features are consistent with a sedex type origin the idea of this origin finds support by yet another observation i e the ore sulphur was mainly derived from the inorganic reduction of the seawater sulphate at more than 300 c while the ore metals were leached from the 238u 204pb enriched singhbhum soda granite of 2200 ma the present study may help in conceiving a new possibility that the sediment hosted pb rich deposits of mvt origin can also be evaluated in terms of the sedex model

middle miocene palaeoenvironments of the niger delta abstract in order to recognize the palaeoenvironments of the deltaic sequence of the middle miocene oil bearing e2 0 reservoir in the kolo creek field investigations of sediment grain size textures structures trace fossils mineralogical composition diagenetic imprints palynodebris types palynomorphs and foraminifera have been carried out on materials from two boreholes using this multidisciplinary approach the following have been identified in the reservoir ten lithofacies two palynofacies associations a total of 103 palynomorph taxa and a low diversity assemblage of benthic foraminifera these criteria have provided information on the positions of the deposits relative to the palaeocoastline as well as the physical and ecological conditions within the delta the overall depositional conditions were found to be broadly similar to those in the present day niger delta although the climatic conditions were somewhat drier in the middle miocene as reflected in the abundance of the grass pollen graminidites seven or possibly eight deltaic sub environments of deposition have been recognized based on an interpretation of gamma ray and resistivity log profiles as well as sedimentological and microfossil data from cores of two boreholes in the field these depositional settings are the distributary channel fill tidal channel fill lagoon tidal flat lagoonal delta flood tidal delta in the lower delta plain the coastal barrier and the delta fringe and prodelta in the outer delta the two palynofacies associations a c and b d which emerged from average linkage cluster analysis of the palynodebris are environmentally significant

stratigraphic and sedimentary records of the rift to drift evolution of the northern north china craton at the paleo to mesoproterozoic transition article i nfo this paper presents a stratigraphic and sedimentary study of late paleoproterozoic to early mesoproterozoic successions of the northern margin of the north china craton including the changcheng and nankou groups a reconstruction of the tectono sedimentary history of these two groups demonstrates that the northern north china craton experienced a two stage evolution as recorded by the changcheng group early synrift subsidence and sedimentation were controlled by normal faults and accompanied by volcanism in contrast a late postrift stage was characterized by broad subsidence and widespread occurrence of shallow marine clastic and carbonate rocks a transgressive unconformity is identified and together with its time equivalent conformable contact clearly separates the synrift and postrift sequences the diachronous unconformity can be related to continental rifting and the separation of the north china craton from the adjacent india craton at roughly 1600 ma and is thus interpreted as a breakup unconformity this age of ca 1600 ma is constrained by newly reported precise shrimp u pb zircon ages of transgressive deposits in the overlying mesoproterozoic dahongyu and gaoyuzhuang formations just coincident with the paleoproterozoic to mesoproterozoic transition the 1600 ma time estimate for the tectonic departure of the north china craton from the india craton is comparable with the timing for the breakup of columbia a pre rodinia supercontinent containing almost all of the earth s continental blocks

anatomy of a compound delta from the post glacial transgressive record in the adriatic sea abstract on the mediterranean continental shelves the post glacial transgressive succession is a complex picture composed of seaward progradations related to sea level stillstands and or increased sediment supply to the coasts and minor flooding surfaces associated with phases of enhanced rates of sea level rise among late pleistocene examples major mid shelf progradations have been related to the short term climatic reversal of the younger dryas event a period during which the combination of increased sediment supply from rivers and reduced rates of sea level rise promoted the formation of progradations up to tens meter thick while the documentation of coastal and subaqueous progradations recording the younger dryas interval is widely reported in literature the model of compound progradation within transgressive deposits has not yet been proposed here we present the documentation of a deltaic system where both delta front sands and related fine grained subaqueous progradations prodeltaic to shallow marine have been preserved the paleo gargano compound delta pgcd formed offshore the modern gargano promontory southern adriatic sea and is composed of a coastal coarse grained delta of reduced thickness and a muddy subaqueous clinoform up to 30 m thick the pgcd probably the first worldwide documentation of a compound delta within the transgressive record provides the opportunity to investigate the processes controlling the formation of a compound delta system during an overall sea level rise and the factors that allowed its preservation the finding of the pgcd provides the opportunity for a comparison with modern worldwide compound systems

holocene fluvial coastal interactions on a mixed sand and sand and gravel beach system north canterbury new zealand abstract fluvio coastal interactions are examined on a progradational sand and mixed sand and gravel beach sequence between the ashley and kowai rivers pegasus bay new zealand this coastal system presents an example of a wave dominated environment energetic enough to deal with the sediment supply derived from the rivers but where the coast is still prograding progradation occurs because of the inability of the marine system to evacuate wave eroded sediment from the bay floor this results in gradual nearshore aggradation until nearshore storage is filled and sediment is finally transported landward to form a new beach ridge this type of coastal system can be identified by a diagnostic morphological assemblage comprising a small river coastal morphology sensu zenkovich 1967 processes of coastal development oliver and boyd edinburgh backed by a sequence of beach ridges sediment fractionation shape and size sorting is identified as the primary effect of erosional processes on a mixed sand and gravel beach fractionation is capable of converting sandy beaches with minor gravel components to graveldominated beaches gravel is concentrated by the evacuation of sand from the shoreface this occurs dominantly through storm sifting but also occurs in response to normal swash processes this process is very similar to the process of chenier production on mixed mud and sand beaches and fractionation is highlighted as a dominant erosional process on mixed beaches irrespective of grain size

geomorphic response type model for barrier coastlines a regional perspective based on quantitative documentation of historical changes in shoreline position between 1847 and 1991 eight geomorphic response types were established for classifying megascale changes along barrier coastlines 1 lateral movement 2 advance 3 dynamic equilibrium 4 retreat 5 in place narrowing 6 landward rollover 7 breakup and 8 rotational instability long term decades to centuries monitoring of shoreline position over a spatial scale of 10 to 100 km provides a scientific basis for documenting process response relationships that shape regional coastal morphodynamics although megascale shoreline change studies often are lacking this type of information is critical for developing realistic research and management strategies regarding form process relationships in coastal depositional systems the spatial distribution of geomorphic response types is delineated along the barrier coastlines of louisiana mississippi and southern georgia northern florida at megascale the rate of relative sea level rise along these barrier coastlines appears to be one of the major factors controlling the occurrence of geomorphic response types however sediment supply exerts significant influence on shoreline response as well

early carboniferous transgression on a passive continental margin deposition of the kekiktuk conglomerate northeastern brooks range alaska the base of the ellesmerian sequence in the northeastern brooks range is a widespread terrigenous clastic succession assigned to the kekiktuk conglomerate late tournaisian visean the kekiktuk is situated above a regional angular unconformity and is the base of a transgressive succession that is thought to record coastal onlap associated with the formation of a passive continental margin six depositional units have been recognized in the kekiktuk conglomerate unit a consists of clast supported and rare matrix supported pebble and cobble conglomerate with minor interbeds of pebbly sandstone it records deposition at the base of incised paleovalleys from high sediment concentration flows and debris flows units b c and d consist of multistory fining upward cycles that record deposition within incised paleovalleys from low sinuosity bed load dominated streams low to moderate sinuosity bed load dominated streams and high sinuosity mixed load streams respectively unit e consists of multistory fining upward cycles and laminated sandstone with thin conglomerate stringers it records deposition in the distal regions of incised paleovalleys in a variety of marginal mari e settings including fluvial channel channel mouth bar beach ridge and coastal swamp environments unit f consists of horizontally bedded sandstone interbedded sandstone and organic rich mudstone it records deposition over paleotopographic highs on the sub mississippian unconformity from sheetfloods and in associated coastal swamp and mud flat settings relative sea level rise is thought to have initiated fluvial deposition in the northeastern brooks range by the latest tournaisian earliest visean coal bearing fluvial channel fill and flood basin successions and palynologic data suggest that this part of arctic alaska was situated in a humid climatic zone during the early carboniferous the stratigraphic and sedimentologic organization of the kekiktuk conglomerate indicates that once base level rise began to affect the northeastern brooks range local and regional variations in paleotopography along the sub mississippian unconformity exerted primary control on accommodation and consequent distribution of kekiktuk depositional environments the stratigraphic position of the kekiktuk conglomerate above a regional angular unconformity nd below marginal marine and shallow marine shales combined with its limited thickness widespread distribution and suite of paleoenvironments suggests deposition in an upland region situated landward of the tectonic hinge zone along a steadily subsiding passive continental margin thermal subsidence combined with eustatic sea level rise were probably responsible for relative sea level rise in arctic alaska and the resulting regional transgression end page 679

depositional origin and facies variability of a middle triassic barrier island complex peejay field northeastern british columbia middle triassic strata of the peejay field in northeastern british columbia are composed of four regressive shorefaces the youngest having been reworked by tidal inlets tidal inlet sublitharenites and bioclastic grainstones form the best reservoir facies these deposits form a series of shoreline parallel narrow sharp based linear sand bodies that eroded the paleoshoreface the orientation geometry and internal sedimentology of these tidal inlet facies suggest that the paleoshoreline was subjected to a wave dominated paleohydrographic regime determining the processes responsible for forming a specific tidal inlet can provide information regarding waves tides and storms characteristic of the depositional setting knowledge of this paleohydrographic regime can aid in predicting the orientation and internal characteristics of tidal inlet reservoir facies improved predictability of reservoir facies geometry and quality can have direct implications on hydrocarbon exploration and development strategies of these and similarly formed hydrocarbon plays of the triassic in the western canada sedimentary basin and elsewhere

evolution of the barrier islands of southern long island new york three lines of evidence based on data from more than 400 boreholes and vibrocores have been used to reconstruct the evolution of the barrier islands during the holocene transgression in southern long island new york 1 the holocene transgressive stratigraphic sequence behind the present barriers 2 the stratigraphic patterns of the inner shelf and 3 the morphology of the now buried late pleistocene coastal features the extensive preservation of backbarrier sediments radiocarbon dated between 7000 and 8000 yr bp on the inner shelf of southern long island suggests that the barriers have not retreated by continuous shoreface erosion alone but have also undergone discontinuous retreat by in place drowning of barriers and stepwise retreat of the surf zone such stepwise retreat of the surf zone has prevented the backbarrier sediments from being reworked based on the presence of submerged barrier sand bodies in seismic records it is inferred that about 9000 years ago when the sea stood about 24 m below the present sea level a chain of barriers developed on the present shelf about 7 km offshore of the present barriers with continued sea level rise the 24 m barrier built upward until the sea reached about 15 m msl just prior to 7000 yr bp the barriers were then submerged by the rapidly rising sea and the surf zone shifted rapidly landward to a position about 2 km from the present shoreline the surf zone overstepped to the landward margin of the old lagoon which had become fixed at the steep seaward face of mid wisconsinan or sangamonian coastal barriers during the past 5000 or 6000 years the shoreface has retreated continuously by about 2 km evidence from southern long island and elsewhere in regions of coastal submergence indicates that rapid sea level rise and low sand supply seem to favour the stepwise retreat of barriers whereas slow rates of submergence and a greater supply of sand generally favour continuous shoreface retreat stationary upbuilding or seaward progradation of barriers may occur when supply of sand is great and or submergence is slowed or reversed morphologic highs on the pretransgression surface such as old barrier ridges tend to fix the migrating barrier shoreline during either continuous retreat or stepwise retreat of barriers

eustatic sea level change at the mid pleistocene climate transition new evidence from the shallow marine sediment record of japan abstract based on the excellent sedimentary record we have identified five depositional sequences in the upper part of the omma formation exposed along the japan sea coast of central japan the base of this stratigraphic section is placed just below the top of the jaramillo subchron and the top is located below the brunhes matuyama boundary biostratigraphy supports the interpretation that the lower three sequences correlate with marine oxygen isotope stages mis 28 26 26 24 and 24 22 respectively we have reconstructed a eustatic sea level curve from analyses of sedimentary facies fossil molluscs basin subsidence and sediment accumulation the resulting sea level curve shows that mean eustatic sea level dropped between 1 0 and 0 9 ma and that sea level was lowest at mis 22 which has the heaviest 18o value between mis 28 and 22 the total fall in mean sea level is inferred to be 20 30 m this independent sea level record demonstrates that ice sheet volume rapidly increased at the mid pleistocene climate transition mpt and culminated in mis 22 as inferred from the 18o record the upper two depositional sequences imply the contribution of precession to changes in ice sheet volume would have arisen in mis 21 just after the mpt

seismic stratigraphy and relict coastal sediments off the east coast of australia abstract during 1989 marine geophysical surveys were conducted in three regions within the territorial sea 1 of new south wales nsw to explore for heavy mineral sand deposits the northern and central nsw coastline is a recognised beach placer province for economic deposits of rutile and zircon economic placer deposits of these minerals occur as spatially discrete lenses within coastal sand bodies exploration for marine placers was previously attempted during the late 1960s however with mixed results in order to improve the recognition of potential host sites for placer minerals in marine environments a custom made acquisition system was developed which resulted in digital real time data acquisition linked to a high resolution seismic system the digital data were recorded onto mass storage media and subsequently digitally processed to reproduce geophysical records a literature search suggested that the possibility of preservation of drowned relict coastal sediments was questionable due to reworking by subsequent marine transgressions the improved resolution of sedimentary structures beneath the seabed afforded by processing digital data and interpreted using seismic sequence analysis shows that relict structures persist some of which display features attributed to beach barrier preservation these structures are commonly draped by an apron of sediments deposited during the post glacial transgression relict drowned coastal deposits show various degrees of preservation in the areas surveyed it is proposed that the degree of preservation of relict structures is not dependent on the present coastal landscape preservation may be dominated by conditions operating at the time of deposition and to post depositional processes which uniquely characterise each location

fluvial tidal and storm sedimentation in the chilhowee group lower cambrian northeastern tennessee u s a the lower cambrian chilhowee group of northeastern tennessee consists of the unicoi hampton and erwin formations and is divided into four facies the conglomerate facies occurs only within the lower 200 m of measured section the unicoi formation and consists of fine grained to pebbly quartz wacke with rare thin beds of laminated siltstone low angle to horizontally laminated fine grained sandstone with laminae and lenses of granules and pebbles represents upper flow regime overbank deposition within a braided stream system that was close to a coastline medium scale planar tabular cross stratified conglomerate in which megaripple bedforms are preserved is interpreted as representing deposition in interbar pools of braided channels as flood stage waned and larger bedforms ceased to migrate large scale planar tabular cross stratified conglomerate beds represent migration of large transverse bars within a broad braided stream channel during high flood stage the sandstone facies occurs throughout the chilhowee group and is therefore interbedded with all other facies it consists of mainly medium to very coarse grained subarkosic to arkosic arenite thinly interbedded laminated siltstone and sandstone which may exhibit wavy or lenticular bedding represents deposition during slack water periods between ebb and flood tides large scale planar tabular and trough cross stratification reflects deposition within the deepest areas of subtidal channels whereas medium scale cross stratification represents deposition in shallower water on shoals separating channels fining and thinning upward sequences most likely resulted from the longshore migration of channels and shoals the hummocky facies occurs only in the erwin formation and consists of horizontally laminated to hummocky stratified fine grained arkosic to subarkosic arenite interbedded with equal amounts of bioturbated mudstone it represents deposition between storm and fairweather wave base by combined flow storm currents the quartz arenite facies is characterized by an absence of fine grained units and lithologically consists of a super mature medium to coarse grained quartz arenite large scale planar tabular cross stratification and abundant low angle cross stratification with rare symmetrical ripples lower quartz arenite facies occurs interbedded with the braided fluvial conglomerate facies and was deposited within either a ridge and runnel system or a system of nearshore bars large scale planar tabular cross stratification upper quartz arenite facies which forms the top of two 40 m thick coarsening upward sequences of the type hummocky faciessandstone faciesquartz arenite facies probably represents deposition on sand ridges that formed on a sand starved shelf as transgression caused the detachment and reworking of shoreface channel shoal couplets palaeocurrent data for the chilhowee group are unimodal but widely dispersed from 0 to 180 and exhibit a minor mode to the west the data are interpreted to reflect the influence of longshore tidal and storm currents the ichnofossil assemblage changes upsection from one characterized only by paleophycus to a skolithos ichnofacies and finally to a cruziana ichnofacies the facies sequence biogenic and palaeocurrent data reflect the interaction through time of i non marine and marine processes and 2 transgression coupled with shoreline progradation the chilhowee group represents an overall deepening from terrestrial deposition to a marine shoreface that experienced both longshore and tidal currents and finally to a storm shelf environment that periodically shoaled upward

facies architecture and geometry of landward stepping shoreface tongues the upper cretaceous cliff house sandstone mancos canyon south west colorado the campanian cliff house formation represents a series of individually progradational shoreface tongues preserved in an overall landward stepping system in the mancos canyon area the formation consists of four 50 to 55 m thick and 10 to 20 km wide sandstone tongues which pinch out landwards into lower coastal plain and lagoonal deposits of the upper menefee formation and seawards into offshore shales of the lewis shale formation photogrammetric mapping of lithofacies along the steep and well exposed canyon walls was combined with sedimentary facies analysis and mapping of the detailed facies architecture two major facies associations have been identified one comprising the mostly muddy and organic rich facies of lagoonal and lower coastal plain origin and one comprising the sandstone dominated facies of shoreface origin key stratigraphic surfaces were identified by combining the mapped geometry of the lithofacies units with the interpretation of depositional processes the stratigraphic surfaces master ravinement surface shoreface coastal plain contact transgressive surface maximum flooding surface and the sequence boundary allow each major sandstone tongue to be divided into a simple sequence consisting of a basal transgressive system tract tst overlain by a highstand system tract hst within each sandstone tongue a higher frequency cyclicity is evident the high frequency cycles show a complex stacking pattern development and are commonly truncated in the downdip direction by surfaces of regressive marine erosion the complexities of the cliff house sandstone tongues are believed to reflect changes in the rate of sea level rise combined with the responses of the depositional system to these changes synsedimentary compaction causing a thickness increase in the sandstone tongues above intervals of previously uncompacted lagoonal coastal plain sediments also played a role this study of the facies architecture geometry and sequence stratigraphy of the cliff house formation highlights the fact that there may be some problems in applying conventional sequence stratigraphical methods to landward stepping systems in general these difficulties stem from the fact that no single stratigraphic surface can easily be identified and followed from the non marine to the fully marine realm i e from the landward to the basinward pinch out of the sandstone tongues in addition the effects of synsedimentary compaction and changes in the shoreface dynamics are not easily recognized in limited data sets such as from the subsurface

sedimentological aspects of the humber group upper jurassic of the south central graben uk north sea complex structure poorly understood sedimentology and poor biostratigraphic control make the upper jurassic humber group of the south central graben one of the least understood and most complex hydrocarbon reservoirs of the north sea detailed logging of available core from 19 exploration wells has been combined with an improved understanding of the relevance of trace fossils and a recognition of important base level variations to provide a greatly enhanced understanding of the depositional system active within the area at that time a new sedimentological model based upon the distribution of facies and facies associations illustrates that upper jurassic structure and consequent basin geometry were the principal controls upon the distribution of depositional environments rifting and second order transgression controlled the back stepping onlap patterns observed and higher frequency base level fluctuations controlled the internal architecture of individual sandbodies the model presented accounts for features of these deposits that were previously considered anomalous such as the thickness of bioturbated sandstones paucity of foreshore deposits and complex age relationships of sands

stratigraphic setting of transgressive barrier island reservoirs with an example from the triassic halfway formation wembley field alberta canada established models for landward barrier island migration focus primarily on the preservation pattern of transgressive facies in the shoreface which are typically thin and buried beneath finer grained marine lower shoreface to offshore sediments in contrast transgressive barrier island sandstones in the triassic halfway formation of wembley field in alberta are preserved interbedded with and overlain by backbarrier and nonmarine sediments these transgressive barrier sandstones formed from coalescing washover fans during shoreface retreat and were subsequently abandoned as the shoreline stabilized and resumed progradation the abandoned transgressive barrier sandstones were subsequently blanketed by backbarrier and nonmarine sediments as the coastline continued to p ograde abandoned transgressive barrier island sandstones in the halfway formation are 2 6 m thick up to 2 km wide and form paleocoastline parallel trends tens of kilometers in length the trends define the paleolandward limit of transgressive events the updip pinch out of these sandstones in backbarrier mudstones forms a stratigraphic trap for hydrocarbons in wembley field top seal is provided by nonmarine mudstones and evaporites which buried the abandoned transgressive barrier island the sandstone has porosities and permeabilities averaging 11 and 63 md respectively by using well logs and cores to correlate individual parasequences in the halfway formation to their updip termination it is possible to define the extent of associated marine flooding events and therefore identify exploration targets for abandoned transgressive barrier island sandstones

at least three scales of convection in a mantle with strongly temperature dependent viscosity abstract we studied the convective patterns developing at high rayleigh numbers and intermediate viscosity ratios in a fluid with a strongly temperature dependent viscosity within this sluggish lid regime three different scales of convection develop the largest convective scale is cellular with cold downwelling sheets of viscous fluid encasing hotter less viscous parts of the tank within each of those cells develop several typically 3 7 hot 3d upwelling plumes upon impinging under the cold thermal boundary layer each plume in turn generates locally a small ring of cold material which does not reach the bottom of the tank applying those results to the earth s mantle we suggest that the large scale features of mantle convection and the co existence of several scales of convection i e slabs and plumes are produced by thermal convection in a mantle with high rayleigh number and a strongly temperature dependent viscosity material this generates a convective pattern in two large scale cells the pacific and the indo atlantic boxes our experiments further suggest that what has been named the two hot superplumes i e the two seismically slow regions encased within the subduction rings are in fact each constituted of several hot instabilities moreover plumes impacts under the lithosphere should be surrounded by cold rings of small extent the asthenosphere appears therefore as the graveyard of both small scale convection and hot plumes generated in the system