mixing of siliciclastic and carbonate sediments in shallow shelf environments the inhibiting effect that siliciclastic material has on carbonate secreting organisms has lead to the generalization that sediments composed of mixtures of carbonate and siliciclastic material should rarely form however many modern and ancient shelf deposits contain a spectrum of sediments that are of mixed composition the processes responsible for this mixing can be grouped into four categories 1 punctuated mixing where sporadic storms and other extreme periodic events transfer sediments from one depositional environment to another 2 facies mixing where sediments are mixed along the diffuse boundaries between contrasting facies 3 in situ mixing where the carbonate fraction consists of the autochthonous or parautochthonous death assemblages of calcareous organisms that accumulated on or within siliciclastic substrates and 4 source mixing where admixtures are formed by the uplift and erosion of nearby carbonate source terranes the allochemical constituents of mixed sediments are both coralgal and foram mollusc in composition the foram mollusc assemblage is the most common because of the effects of increased turbidity unstable substrates and the clogging of filter feeding mechanisms associated with a siliciclastic influx

two dimensional modeling of exogenic mass transfer at the calabrian active margin southern italy uplift caused by crustal growth at active plate margins triggers an exogenic mass transfer from the mountain ranges into arc associated basins this mass transfer comprises weathering and erosion of the source terranes differentiation and transport of particles and finally deposition the hydraulic populations produced by chemical dissolution and mechanical abrasion possess different recycling potentials coarse grained particles are piled up in forearc accretionary wedges and recycled by subduction with the oceanic lithosphere fine grained particles and the chemical load may leave the exogenic cycle by suspension and solution their recycling potential is low for the calabrian active margin a composite exogenic mass balance has been determined sedimentation rates were calculated by two dimensional modeling of the forearc sedimentary wedge based on meteor airgun sections erosion rates for the calabrian massif were evaluated by a morphometric volume balance the total erosion of the calabrian massif is 178 m ma about 28 m ma 16 is removed by chemical dissolution and particle dispersion the mean sedimentation rate in the forearc basin is 137 m ma the remaining mass deficiency of 13 m ma 7 is referred to sediment bypass to the abyssal plain through submarine canyons

a model for carbonate to terrigenous clastic sequences the development of carbonate sequences that grade toward a source region into terrigenous clastic sediment is poorly understood in both carbonate and terrigenous clastic regimes environments of rapid sedimentation have comparable rates of deposition 102 to 102 5 cm 1 000 yr slowest continuous rates are also comparable 100 5 to 101 cm 1 000 yr because of the inhibiting effect of terrigenous deposition on carbonate skeletal production the slowest rates in a transect tend to occur where mixed lithologies are deposited midway between coarse clastic and pure carbonate sediments this leads to a bimodal distribution of rapid depositional rates and invariably to the development of a basin in the intervening area to develop a model we combine concepts of depositional rate with knowledge of the middle ordovician carbonate to terrigenous clastic sequence of the southern appalachians deposition began on a shallow water ramp sloping gently eastward but everywhere within the photic zone an episode of tectonic subsidence differentiated this ramp into a shallow shelf on the northwest and a deep starved basin on the southeast separated by a linear shelf margin with organic build ups subsequent development of the sequence consisted of the filling of the deep basin first with pelagites then with turbidites during which the carbonate shelf was isolated from terrigenous deposition once the basin filled terrigenous deposition began to affect the shelf inhibition of carbonate deposition near the shelf edge led to formation of successively smaller basins within the old shelf area after the initial pulse of tectonic subsidence deposition of the sequence can be explained by sediment loading subsidence thus the ordovician sequence coupled with the concept of bimodal distribution of sedimentation rates in carbonate to terrigenous clastic facies patterns allows development of a model for basins adjacent to active tectonic lands five model stages are derived by application of calculated deposition rates and sediment loading along a transect away from the terrigenous source these stages are 1 an initial carbonate ramp stage followed by 2 a carbonate shelf starved deep basin stage 3 a carbonate shelf turbidite basin terrigenous shelf stage 4 a carbonate shelf narrowed and shallowed basin broad terrigenous shelf stage and finally 5 a stage in which the initial basin is filled a subsequent starved basin forms on the old carbonate shelf and a new carbonate shelf margin forms a few to tens of kilometres farther from the terrigenous source the model should be useful in predicting depositional facies in analogous basins

the paradox of drowned reefs and carbonate platforms shallow water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation and benthonic carbonate production ceases drowned platforms are common in the geologic record but they present a paradox if one considers rates of processes involved during the early holocene when sea level rose at rates of 6 000 to 10 000 m yr mm 1 000 yr most reefs and platforms were outpaced by the rising sea during the late holocene with sea level rising 500 to 3 000 m yr in the atlantic caribbean area reefs and platforms started to recover built to sea level and prograded seaward 1 000 m yr is thus a conservative estimate of the average growth potential of modern reefs and platforms independently accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year the growth potential of 1 000 m yr exceeds any relative rise of sea level caused by long term processes in the geologic record newly formed ocean crust subsides at a maximum of 250 m yr basin subsidence averages 10 to 100 m yr and sea level rises due to increased sea floor spreading amount to less than 10 m yr rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning the geologic record shows examples of both of these processes global mass extinctions of reefs and platforms occurred in the middle cretaceous eustatic rise due to submarine volcanism or desiccation of a small ocean basin and the late devonian global crisis of ocean environment extraterrestrial cause drowning controlled by regional tectonics prevailed in the jurassic and early cretaceous of the tethyan realm and the drowning of mesozoic platforms in the western north atlantic seems to have been dictated by plate tectonic drift to higher latitudes

temperature subsidence and hydrocarbon maturation in extensional basins a finite element model the thermo mechanical history of extensional basins is of great importance in determining their hydrocarbon potential because of the great variation of geologic conditions a flexible method is needed to model this history a one dimensional finite element method is particularly suited for this purpose its use shows that the temperature distribution is out of equilibrium if the rate of sedimentation exceeds 100 m ma 300 ft ma favorable conditions for hydrocarbon production in three models are found between 100 and 225 c but this is not a general rule in areas of reasonably flat interfaces the method yields excellent results at very little cost it shows the interplay among sedimentation temperature and subsidence more clearly than has been possible heret fore

fringing reef growth on a terrigenous mud foundation fantome island central great barrier reef australia the leeward fringing reef at fantome island central great barrier reef province is a carbonate body which has developed under the influence of terrigenous sedimentation the reef flat is up to 1000 m wide and is surfaced by mobile sand and gravel with almost all live corals restricted to the seaward rim the reef slope has coral columns and heads on the upper part but below 5 m water depth it is a muddy substrate with scattered mounds of branching corals three high recovery cores show the reef is up to 10 m thick and developed over a gently sloping terrace of weathered pleistocene alluvium three post glacial stratigraphic units are recognised i carbonate reef top unit of coral rudstone and framestone including sinuluriu spiculite 2 lower slope unit of coral floatstone in a terrigenous muddy matrix and 3 transgressive basal unit of skeletal arkosic sand the acid insoluble content of matrix and of individual corals increases downwards coral growth rates decrease downwards reflecting slower growth in muddier environments radiocarbon dating shows that the reef prograded seaward at almost stable sea level an average vertical accumulation rate of 6 7 mm yr is indicated two age reversals are interpreted as material transported by storms or by erosion in response to a late holocene sea level fall the carbonate reef top unit has developed adjacent to and is environmentally compatible with a muddy terrigenous lower slope unit terrigenous influx has not changed during the holocene and terrigenous content of sediments is controlled by deposition on the reef slope of fine sediment winnowed from the reef flat and concentration of coarse sediment in the transgressive basal sheet

dependence of the flexural rigidity of the continental lithosphere on rheology and temperature while the first order mechanical properties of oceanic and continental lithosphere are well defined and explain the observed increase in lithospheric flexural strength with thermal age the temporal variation of continental flexural rigidity following loading and its dependence on lithospheric composition and temperature structure is only just beginning to be appreciated we describe here the results of a quantitative theological model in which laboratory determined rock deformation data and transienttemperature distributions resulting from intra plate basin formation within continental interiors have been integrated continental flexural rigidity is shown to be controlled by crustal thickness lithospheric thickness through the temperature structure and the interaction of these factors during basin formation for thermally young lithosphere the flexural rigidity is dominated by the quartzo feldspathic theology of the crust while for the thermally older lithosphere it is dominated by the olivine rheology of the mantle flexural data through thermo mechanical models of lithospheric deformation provide a powerful constraint on both the transient and steady state temperature structure of the lithosphere

tectonic and sea level controls on nontropical neogene limestones in new zealand modern shelf carbonates and neogene on land equivalents have formed upon and adjacent to the new zealand sector of the convergent australia pacific plate boundary they are exclusively skeletal hashes dominated by bryozoans bivalves barnacles and foraminifers typical of nontropical foramol carbonates tectonism has influenced carbonate formation by determining terrigenous sediment supply rates and therefore the dilution factor and has affected the structure of the sea floor by forming imbricate thrust ridges and fault bounded platforms that support the carbonates the neogene limestones are more abundant closer to the plate boundary than their modern counterparts reflecting an increase in tectonism with time in the late pliocene sea level movements combined with uplift of the forearc and transform basins to form widespread limestones only during the glacials

reconstruction of tertiary progradation and clinoform development on the new jersey passive margin by 2 d backstripping we have reconstructed the oligocene to middle miocene paleobathymetry and stratigraphy of the new jersey margin using a modified backstripping technique by analyzing the geometry of the margin through time we investigate its response to fluctuating sea level changing climate and variable sediment supply during the tertiary the reconstructions reveal a change in the margin morphology from a more steeply dipping 1 300 to 1 500 carbonate ramp in the eocene to a flatter shelf with a sharp shelf edge at present this was accomplished by an increase in the terrigenous sediment supply that filled available accommodation and caused progradation across the margin we link the increase in sediment flux with climatic cooling rather than tectonic processes the progradation is evidenced by a series of clinoforms whose formation was modulated by sea level and which extend over 100 km across the shelf the height and dip of the clinoforms increased as they extended onto the deeper parts of the earlier ramp the miocene clinoform rollovers at the new jersey margin had water depths of60 130 m and are interpreted as the edge of a new continental shelf built over the older ramp sea level fall was probably insufficient to drive the miocene shorelines past the shelf breaks thus measurements of sea level amplitude based upon coastal onlap over the clinoforms are not reliable 1999 elsevier science b v all rights reserved

eustatic controls on carbonate facies in reservoirs and seals associated with mesozoic hydrocaron fields of the arabian gulf and the gulf of mexico abstract abundant subsurface data for mesozoic and cenozoic sections of the us gulf coast and the middle east make it possible to track the relationship of shelf carbonates and evaporites associated with minor amounts of clastics to eustasy our contention is that because the sedimentary deposition for both regions was in part controlled by gentle tectonic subsidence punctuated by eustatic variations the major hydrocarbon fields which occur here can be related to sea level behaviur at the time of deposition of the reservoir sections and their early diagenesis with the exception of chalks most of these carbonate hydrocarbon fields can be related to highstand system tracts and include the following 1 1 keep up reservoirs with sheet like geometry formed when carbonate accumulation matched sea level rise aggrading to form shoaling upward cycles during sea level highstands 2 2 give up and catch up reservoirs with lense like carbonate geometry formed on drowned shelves during and following rapid sea level rises often downslope from carbonate margins give up reservoirs occur when carbonate accumulation was unable to match sea level rise and catch up reservoirs form where carbonate accumulation was initially unable to keep pace with the sea level rise but then aggraded to sea level 3 3 reservoirs with the prograded discontinuous clinoform geometry of the platform margin formed during stillstands by carbonate accumulation that not only kept up with the sea level rise but also advanced in a seaward direction source rocks for these carbonate reservoirs are often formed during rapid sea level rises while the reservoir seals are usually transgressive shales dense transgressive limestones and or highstand evaporites

synthetic stratigraphy of carbonate platform and basin systems a forward numerical model of the carbonate platform and basin depositional system aids in understanding the roles played by different variables in controlling the growth patterns of carbonate platforms and thus their internal facies architecture the numerical model simulates the two dimensional development of a carbonate platform and adjacent basin by successively calculating and depositing discrete sediment budgets generated over short time periods 10 sup 4 yr and displaying the results in synthetic cross sections through the carbonate buildup a series of model experiments shows that the effect of changing the eustatic sea level is largely dependent on the rate of subsidence in general sea level fluctuations tend to restrict progradation of the platform by producing periods of exposure of the platform top that decrease the overall sediment budget available for deposition on the marginal slope either increasing the amplitude or decreasing the period of the sea level cycle also restricts progradation which suggests that the higher order short period sea level cycles that are evident in the rock record may play an important role in controlling the cross sectional geometry of carbonate platforms forward models such as the one presented here may provide a basis for creating an inverse model that could be used tomore closely constrain the variables such as sediment production rate subsidence and sea level history that led to the cross sectional geometry observed in the field or in a seismic section less

the effect of rift shoulder erosion on stratal patterns at passive margins implications for sequence stratigraphy abstract numerical modelling indicates that the erosion of uplifted rift flanks at passive margins has a profound effect on offshore stratigraphic patterns flexural uplift due to isostatic rebound in response to erosion extends far into the basin and causes uplift of the shelf as a result the contemporaneously deposited sedimentary wedge displays a characteristic offlap pattern when the rift shoulder is largely eroded onlap promoting mechanisms related to cooling of the lithosphere enable sediments to onlap onto the basin margin the initial offlapping and subsequent onlapping strata form one complete second order depositional sequence comprising a shelf margin transgressive and highstand systems tract the modelling inferences are in broad agreement with stratal patterns and basin geometries observed on the u s east coast the southeastern brazilian and southeastern australian passive margins and the transantarctic mountains ross sea shelf system the initial offlaps caused by erosion of rift shoulders have important implications for the deriviation of eustatic signals from coastal onlap patterns

wings mushrooms and christmas trees the carbonate seismic geomorphology of central luconia miocene present offshore sarawak northwest borneo central luconia is a geological province of the sarawak basin offshore northwest borneo characterized by extensive development of miocene to holocene carbonate buildups the buildups are up to 2 km 1 mi thick and reveal complex seismic geomorphologies including out building in building carbonate siliciclastic intercalations and coalescence of neighboring carbonate bodies intermittent carbonate and siliciclastic deposition is interpreted to have been governed by frequently oscillating sea level and variable siliciclastic input location of the buildups with respect to deltaic facies and the seismic expressions of marginal carbonate strata locally known as wings are used here to classify carbonate seismic geomorphologies buildups encompassed by deltaic topsets are said to reside onshelf and are flanked by marginal carbonate deposits conformable with the onlapping siliciclastics and interpreted as thin carbonate platforms deposited in situ over prominent erosional unconformities and flooding surfaces such marginal facies are described on the basis of their appearance as open wings in contrast buildups downlapped by prodelta foresets or bottomsets or those draped by pelagic sediments are said to reside offshelf and are characterized by high relief steeply dipping marginal carbonate strata discordant with respect to the relatively flat lying platform carbonates as well as the onlapping siliciclastics these marginal strata have an appearance of closed wings and are interpreted as slope aprons composed of carbonate material derived from the productive platform tops and deposited off platform in deeper water carbonate geometries range from contracting upward pinnacles to expanding upward platforms in onshelf settings whereas buildups located offshelf have mostly contracting upward or aggrading morphologies buildups deposited onshelf have generally maintained a similar height of depositional slopes as they evolved reflecting rates of background siliciclastic deposition approximating those of carbonate growth in contrast depositional slopes of offshelf buildups have become progressively higher owing to limited siliciclastic input observations presented here on carbonate geomorphology and adjacent siliciclastic stratigraphy provide important clues for the understanding of the geology of the province because carbonates and siliciclastics intercalate frequently siliciclastic biostratigraphy can be used to precisely date and correlate geographically remote carbonate reservoir intervals and intrareservoir depositional and diagenetic events such as marine transgressive or exposure surfaces or karst networks conversely basin paleobathymetry can be measured by the size of carbonate clinoforms and the thickness of buildups established over flooded siliciclastic surfaces deposition of low relief carbonate platforms over siliciclastic substrates indicates shallow marine conditions as opposed to the prevalent notion of kilometer scale paleobathymetry these interpretations have important implications for understanding of the sedimentology and lithology of the ambient basinal siliciclastics with fundamental applications to hydrocarbon exploration

unconformities and porosity development in carbonate strata ideas from a hedberg conference the purpose of this paper is to highlight ideas from a hedberg conference concerning detection of unconformities and associated porosity in carbonate strata many types of information including cores and outcrops seismic data eustatic sea level curves wireline logs biostratigraphic data stable isotope trends cycle stacking patterns and tectonic and basin evolution models can be used to predict and or detect subaerial unconformities all methods of detecting subaerial exposure with these types of information have limits and pitfalls detailed analysis of the stratigraphy sedimentology and diagenetic history of a rock sequence from outcrop or core data is considered the most reliable means of detecting subaerial exposure though an integrated approach using all a ailable information is recommended the relationship between subaerial exposure and subsurface porosity in carbonates is not simple or easily predicted subsurface porosity is the product of many factors operating during deposition subaerial exposure and burial several concepts should be recognized when evaluating porosity associated with subaerial unconformities 1 carbonate diagenesis during subaerial exposure rearranges pore networks but usually does not increase the total porosity near surface dissolution can simply lower a topographic surface without increasing porosity within the host limestone in many cases total porosity in carbonates under unconformities is actually reduced during subaerial exposure 2 permeability is likely to change more than porosity during subaerial exposure increasing in some c ses but decreasing in others 3 pore systems evolve with time during subaerial exposure short periods of subaerial exposure 10 000 400 000 yr are often associated with greater porosity than long intervals of subaerial exposure 1 20 million yr prolonged subaerial exposure may change permeability less than porosity because high permeability karst related conduits can form quickly and persist for millions of years 4 many carbonates subjected to subaerial exposure have little or no porosity in the deep subsurface because compaction cementation and stratal collapse reduce porosity during burial 5 unconformity related diagenesis may enhance reservoir potential by creating pore systems that are resistant to compaction during deeper burial 6 vugs caverns and breccias can fo m in the deep subsurface because of dissolution by basinal fluids independent of subaerial exposure 7 lithologic changes at unconformities like shales overlying carbonates can influence the flow of subsurface fluids during end page 857 deeper burial resulting in deep burial dissolution localized along unconformities if subaerial exposure and exposure surfaces are integrated into a sequence stratigraphic paleogeographic and climatic framework systematic and hopefully predictable patterns will emerge however we do not fully understand the complex interplay between the factors critical to creating and preserving porosity in carbonates additional research is needed to quantify critical processes and products so we can reliably predict porosity associated with subaerial exposure during exploration in frontier basins and on a smaller scale during reservoir analysis

results of exploration in mesozoic shelf edge carbonates baltimore canyon basin the late jurassic early cretaceous carbonate shelf margins of the baltimore canyon basin were drilled in three exploration wells by shell offshore inc and partners drilled during 1983 1984 the wells were in record setting water depths maximum of 2119 m the most updip well ocs a 0336 1 tested two structures it penetrated thick oolitic grainstones of an aptian bar complex and deeper in section a rollover anticline of lower kimmeridgian lime mudstone dominated shelf interior facies intercalated sandstones in the lower structure gave minor gas shows which were the only hydrocarbons found in the three wells the ocs a 0337 1 well 5 5 km farther downdip tested a mesa penetrating sponge and tubiphytes rich chalky allodapic wackestone drilling below the mesa penetrated upper kimmeridgian berriasian back reef and lower kimmeridgian foreslope deposits the most downdip well ocs a 0317 1 1 km seaward in strike projection tested a pinnacle reef the well was sited on the back side of the reef 49 m below the reef crest it encountered a cap of sponge and tubiphytes rich limestone overlying hexacoral and stromatoporoid skeletal grainstones and boundstones

computer simulation of the influence of basin physiography on condensed section deposition and maximum flooding abstract sequence stratigraphic analysis commonly involves the correlation of condensed sections which are widely used as chronostratigraphic markers condensed sections represent periods of very slow deposition due to sediment starvation and have generally been equated with the maximum flooding surface which is most commonly inferred to occur at sometime between the maximum rate of relative base level rise and the highest point of relative base level however similar sediment starvation can be produced by a rapid landward shift in the shoreline over large distances during the early stages of a base level rise over a very low gradient surface condensed section deposition therefore does not always correspond to a period between the maximum rate of relative base level rise and the highest point of relative base level but can be initiated earlier than the maximum rate of relative base level rise or conclude after the highest point of relative base level the initial timing and the duration of condensed section deposition is therefore influenced by basin physiography basin physiography can also contribute to a diachronous maximum flooding horizon lying at different levels throughout a basin because the rate of change in accommodation during fluctuating base level is partially dependent on the slope of the basin surface these principles are illustrated in the paper by computer simulations

stresses beneath a ramping thrust sheet abstract there is a systematic progression of both the timing of motion of thrust sheets and the spacing of thrust ramps during the development of some thrust dominated foreland fold and thrust belts on the basis of these observations we suggest that a regional mechanical process that controls frontal ramp formation may dominate local effects such as stratigraphic pinchouts and interaction with basement to investigate this process we have developed elastic plane strain models of the state of stress in the footwall of the last formed or most forward thrust ramp these models include the load of an overriding thrust sheet and a synorogenic sedimentary cover the effect of emplacing a thrust sheet or developing a synorogenic sedimentary cover above an incipient thrust sheet is to stabilize the footwall toward the hinterland with respect to failure and thereby move failure more to the foreland failure is localized where there is a favorable trade off between the spatial rates at which mean stress and differential stress decrease toward the foreland the models also predict that frontal ramps nucleate well above the basal decollement and propagate upward and downward eventually linking with the regional decollement fault trajectories predicted by using a coulomb criterion are subhorizontal in the hinterland portion of the model and curve upward to the foreland resulting in a curved flat and ramp geometry these analyses show that each successive thrust sheet would be thinner as well as shorter than the previous thrust sheet based on these results we propose a model for the formation of the first thin skinned thrust sheet in the external portions of orogenic belts the synorogenic sediments that form during uplift of the internal zones of mountain belts will cover the preorogenic strata to the foreland as deformation progresses to the foreland and deformation becomes more shallow and more brittle in nature the preorogenic strata will interact with the synorogenic strata and failure will occur creating the first brittle thrust sheet the location of this first thrust will be controlled by the mechanisms outlined in our models to the foreland of this point each sucessive thrust fault will localize as a result of the combined effects of an overriding thrust sheet and synorogenic deposits

out of our depth on the impossibility of fathoming eustasy from the stratigraphic record the evolution of a sedimentary basin s gross morphology depends upon 1 basement movement 2 sediment accumulation and compaction and 3 variations of eustasy with time thus if an accurate history of eustasy could be derived it would aid in our understanding of basin stratigraphy and help in the prediction of sediment geometry from described areas techniques which attempt to determine the magnitude of eustatic sea level excursions include the measurement of 1 the amount of sedimentary onlap onto the continental margins with and without the use of hypsometric curves 2 the thickness of marine sedimentary cycles and the elevation and distance between indicators of old strandlines 3 the perturbations on individual thermo tectonic subsidence curves and stacked crustal subsidence curves 4 the variations in deep ocean oxygen isotopes found in sediments and 5 the size of variables used in graphical and numerical simulations of basin fill in terms of tectonic behavior rates of sediment accumulation and eustasy which invert the problem to date a combination of the use of relative sea level charts derived from sediment onlap of the continental margin with dimensioning by oxygen isotopes responding to glacial events offers the best potential for relative tectono eustatic sea level curves but even this method can not produce a unique solution for absolute eustatic variations mathematical modelling shows that at best it is possible to obtain only the sum of tectonic basement subsidence and sea level variations from the above methods and at worst not even that simple a combination thus every proposed scheme to measure eustatic sea level excursions assumes some behavior for two of the three underlying processes tectonic movement of the basement sedimentary accumulation and eustatic sea level each scheme then determines the third process relative to the assumed model behavior of the other two it would seem that like king canute we cannot command the sea though we can still use undifferentiated relative tectono eustatic sea level curves to generate a family of solutions which are the product of a variety of absolute sea level curves and tectonic models each solution has then to be assessed in terms of geologic setting and the hypotheses it generates

sources of periplatform carbonates northwest providence channel bahamas abstract sources and quantities of periplatform carbonate sediments have been determined in northwest providence channel bahamas by an end member method employing geochemical textural petrographic and sem data the holocene sediment over most of this open seaway 200 to 2 000 m deep between great bahama bank and little bahama bank is a drape of largely fine grained lime mud 50 cm thick shallow platform sources contribute 75 90 percent the remainder is planktonic foraminifera pteropods and coccoliths mud 62 m from each bank and the planktonic source are each geochemically distinct and can be quantified the two shallow platform sources dominate sedimentation in the deep basin trend surface maps of mud derived from each bank reveal no windward leeward asymmetry in cont ast to reported sand transport patterns this indicates that the fines are swept off both platforms by storm and tidal exchange concentric distribution of fines relative to source indicates no offset by currents suggesting that fecal pelleting must rapidly remove fine sediment from the water column canyon valleys and intercanyon highs have equal thicknesses of holocene sediment which indicates that pelagic processes of deposition presently dominate over gravity flow processes confirmation of this depositional process is that the deposition rate increased from 2 cm 1 000 years to 10 cm 1 000 years when postglacial sea level flooded the bank tops thus 80 percent of the present sedimentation rate results from bank top contribution furthermore the calculated mass of mud traceable to little bahama bank agrees excellently with the independent estimate of overproduction there in cores bank sediments resulting from the most recent sea level highstand sharply overlie planktonic sediments writing a sea level history in alternating stable and unstable mineral suites

testing eustatic curves based on baltimore canyon neogene stratigraphy an example application of basin fill simulation basin fill simulation techniques provide scientists in both research and exploration with an important new tool the researcher obtains a tool for investigating the interaction of the major controls on basin scale stratigraphy subsidence sediment supply and eustasy the explorationist gains a tool for better understanding the hydrocarbon potential of an area through integrating data from various sources and testing an interpretation for spatial and temporal consistency as a result a researcher or explorationist may be able to refine a basin interpretation to a degree that might not have been possible if he or she had considered only a present day rendering of the basin basin modeling can be divided into three major categories structural stratigraphic and rock fluid properties evolution structural and stratigraphic modeling can be conducted back through time using backstripping techniques all three types of modeling can be conducted forward through time using simulation techniques several modeling systems combine backstripping procedures to obtain basin geometries with forward simulation to predict rock and fluid properties each category of basin modeling has its advantages and limitations when used together these techniques complement one another in this paper we illustrate the benefits of combining stratigraphic modeling techniques by presenting a research oriented case study involving the neogene sediments of the baltimore canyon trough offshore new jersey we designed this case study to evaluate several published sea level curves stratigraphic backstripping and simulation techniques were used to model a dip oriented cross section for which present day stratigraphy is well documented each sea level curve was used in a separate modeling run comparing presently accepted and modeled stratigraphic patterns allowed us to draw conclusions about the different eustatic curves in the final section we present several ways in which basin fill simulation can address exploration problems

mesozoic carbonate platforms and banks of the eastern north american margin abstract the jurassic lower cretaceous carbonate platforms and banks form a discontinuous belt extending over distance of 6000 km from the grand banks up to the bahamas six types of carbonate buildups are recognized and document the variability of depositional paleo oceanographic and tectonic processes on the eastern north american margin the composition of the carbonates closely resembles the recent deposits of the western great bahama bank since oolitic shoals were present near to the shelf edge and skeletal peloid wackestones and biomicrites were deposited in the inner part of the platform coralstromatoporoid and sponge bioherms were only rare constituents of the carbonate banks the thickness of carbonate buildups progressively increases southward along the margin attaining a thickness of more than 5 km on the bahamas the carbonate platforms also become younger southwards which is thought to reflect the northward movement of the north american plate of less than 1 5 cm yr the carbonate platforms were seeded over the continental basement following the taphrogenic period of plate tectonics building of carbonate ramps which characterized the early jurassic has began during the transitional period between continental rifting and early drift the second stage in construction of the carbonate platforms and offshore banks proceeded mainly after separation of the continental plates

the impact of milankovitch scale climatic forcing on sediment supply abstract stratigraphy is the main technique to predict the occurrence of potential reservoirs and seals in the oil industry stratigraphic tools used are stratigraphic observations from drilled wells rock samples and wireline logs and seismic interpretation nowadays computer modelling allows scenario forecasting of the sequence to be expected application of these tools indicates that the key input parameters are initial basin topography subsidence eustasy and sediment supply the first three parameters are usually well constrained or can readily be deduced from well and seismic data however the fourth sediment supply is loosely defined and because of that it is used to calibrate the model in the present article the controls on sediment supply are reviewed and the impact of in particular milankovitch scale climatic forcing on sediment supply is tested on the neogene of the niger drainage and delta system sediment supply is controlled by a complex interplay of geographic and climatic factors such as 1 relief or mountain height 2 the size and temporal storage capacity of the drainage area 3 overall climate 4 amount and seasonality of precipitation and 5 vegetation cover studies in geomorphology have indicated that although complex computer models can be made to predict erosional product for the present a large scale sediment supply prediction should be based on the combination of palaeogeographical palaeoclimatological and erosion potential maps more detailed sediment supply predictions should include milankovitch scale variations the climatic impact of these variations is mainly felt when we are in a transition between two climates then the area will experience regular climatic changes from e g wet rainforest to dry monsoonal climate or low to high sediment supply respectively this concept was tested in the forward modelling package stratagem on the neogene of the niger delta four scenarios were evaluated using various combinations of large and milankovitch scale sediment supply curves combined with global or milankovitch scale eustatic sea level curves the results confirm the impact of both large scale and milankovitch scale climatic forcing on sediment supply however in an icehouse world the various scenarios also suggest that the effect of eustatic sea level fluctuations overshadows the impact of milankovitch scale sediment supply by contrast in a greenhouse world the impact of milankovitch scale sediment supply is expected to be more significant in the absence of significant sea level fluctuations

synthetic passive margin stratigraphy synthetic stratigraphic cross sections are derived mathematically for a variety of simple conditions the variables considered in the mathematical model include variations in sea level rate of tectonic subsidence rate of sedimentation and rate of erosion derived stratigraphic relationships include unconformities correlative conformities and disconformities coastal onlap coastal toplap erosional truncation pinch out and sigmoidal progradational clinoforms an important conclusion is that the rate of erosion is a dominant variable in determining the type of stratigraphic section observed the proposed approach may provide the basis for either a forward or inverse modeling of seismic stratigraphic sections

cyclic facies architecture as a key to depositional controls in a distal foredeep campanian mesaverde group wyoming usa the mesaverde group consists of a thick wedge of fluvial littoral deltaic and shallow marine clastics shed into the cretaceous western interior seaway of north america the western parts of the seaway lay within the strongly subsiding foredeep of the active sevier fold and thrust belt further to the west the study area is located east of the axis of maximum subsidence and is thus in a favourable position to record competing effects of eustasy sediment supply and thrust load induced subsidence facies and sequence analysis carried out on high quality outcrop and well log data led to the recognition of a complex depositional cycle hierarchy within the typical storm and wave dominated inner shelf shoreface strand plain and delta systems of the mesaverde

automatic calibration of stratigraphic forward models for predicting reservoir presence in exploration understanding and predicting reservoir presence and characteristics at regional to basin scales is important for evaluating risk and uncertainty in hydrocarbon exploration simulating reservoir distribution within a basin by a stratigraphic forward model enables the integration of available prior information with fundamental geologic processes embedded in the numerical model stratigraphic forward model predictions can be significantly improved by calibrating the models to independent constraints such as thicknesses from seismic or well data a three dimensional basin scale stratigraphic forward modeling tool is coupled with an inversion algorithm the inversion algorithm is a modification of the neighborhood algorithm a type of genetic algorithm which is designed to sample complex multimodal objective functions and is parallelized on computer clusters to accelerate convergence the process generates a set of representative geological models that are consistent with prior ranges for uncertain parameters calibration constraints and associated tolerance thresholds the workflow is first demonstrated on two data sets a synthetic example based on a clastic passive margin and a real hydrocarbon exploration example for slope and basin floor stratigraphic traps in the neocomian lower cretaceous of the west siberian basin the analysis of calibrated models provides constraints on stratigraphic controls and allows prediction of locations with higher potential to develop stratigraphic traps these locations are related to complex interactions between paleobathymetry subsidence eustatic fluctuations characteristics of sediment input sources and sediment transport parameters results show the potential of stratigraphic forward modeling combined with inverse methods as an additional tool to support conventional play based exploration and reservoir presence prediction

two dimensional synthetic seismic and log cross sections from stratigraphic forward models in an effort to fully use deterministic stratigraphic forward modeling techniques in subsurface stratigraphic analysis we developed a computer interface to routinely create synthetic logs and one dimensional and two dimensional 2 d seismic responses from 2 d stratigraphic simulations each 2 d stratigraphic model can contain up to 200 timelines defining age equivalent stratigraphic layers with laterally variable lithofacies and depths synthetic gamma ray density and velocity responses are calculated for the simulated lithofacies using user specified rock and fluid properties vertically incident synthetic seismograms are created using calculated reflection coefficients and user defined input wavelets because stratigraphic simulations provide chronostratigraphy as a nown log correlations and the interpolated seismic geometries follow timelines exactly the power of this technique as an aid in sequence stratigraphic interpretations is shown from three examples 1 a simulation of an idealized clastic system assuming constant clastic input and sinusoidal fourth order and third order sea level variations 2 a detailed simulation of one third order carbonate depositional sequence lower middle san andres formation from the northwest shelf permian basin and 3 simulations of the permian mixed clastic carbonate infill of the midland basin some general conclusions from these examples include the following 1 seismic and well log defined topsets foresets bottomsets and related event terminations can be directly related to relative sea level fluctuations 2 exxon type sequence boundaries i e unconformities are not necessarily seismically imageable and their identification on well logs is not straightforward and 3 lateral variations in amplitude related to lithofacies variations can be modeled synthetic logs and seismic sections from stratigraphic forward models such as these may be useful in constraining interpretations of subsurface data and thus aiding the prediction of reservoir and seal

onlap offlap and the origin of unconformity bounded depositional sequences christie blick n 1991 onlap ofttap and the origin of unconformity bounded depositional sequences mar geol 97 35 56 unconformity bounded depositional sequences represent the fundamental building blocks of sedimentary successions they are typically characterized by onlap at the base and by offlap at the top and they tend to be markedly asymmetrical with onlap accounting for a larger part of any cycle of sedimentation than offlap offlap cannot be attributed solely to erosional truncation but instead implies that sequence boundaries develop over a finite interval of time depositional sequences are commonly associated with a cyclic arrangement of facies but transgressive regressive cycles are out of phase with respect to sequence boundaries which in down dip locations are both overlain and underlain by progradational deposits and hence form during times of regression of the shoreline these observations are used to develop some ideas about the origin of unconformity bounded sequences with reference to the inter related roles of changes in depositional base level and sediment supply in particular it is shown than onlap and offlap are due to lateral migration of a line of critical bypassing defined so as to incorporate the effects of sediment loading and compaction as well as the rate of change of elevation with respect to sea level downward shifts in onlap may be achieved by either an increase in the rate of eustatic fall or a decrease in the rate of tectonic subsidence and it is premature to assume that eustatic and tectonic controls on sea level may be distinguished solely on the basis of the frequency of depositional cyclicity small shifts in the position of onlap can also be produced by changes in sediment supply and more attention needs to be paid to the influence of sediment supply in the development of minor boundaries unconformities related to eustatic fluctuations are thought to correspond approximately to times of relatively rapid sea level fall inflection points but questions remain about the existence of possible leads and lags of up to cycle and hence about the degree to which sequence boundaries of eustatic origin may vary in age both within a given basin and from one basin to another

post messinian evolution of the southeastern levant basin based on two dimensional stratigraphic simulation abstract the evolution of the southeastern levant basin following the messinian salinity crisis was interpreted using observations derived from seismic sections and well data and from output of a two dimensional forward stratigraphic simulation program sedpak the working assumption was that the thick evaporitic messinian sequence was generated in a shallow water albeit a topographically deep basin the sea level dropped at least 800 m below its present level and may have been as low as 1300 m the latter value representing the reconstructed depth of the top messinian in the deep part of the basin the top of the evaporitic sequence in that part of the basin was virtually horizontal at the end of the messinian crisis and thus may be used as a regional datum subsidence rates measured for the shelf and coastal plain of israel indicate a reduction of these rates for the pleistocene when compared to the pliocene during the pleistocene subsidence rates for the base slope increased and dictated the modern physiography of the margin earlier in the middle to late pliocene small scale salt movements occurred in the deep basin producing the fragmented and irregular shape of the m horizon the seismic event marking the top of the evaporitic sequence and deforming the overlying pliocene section the simulated rate of sediment supply indicates a constant increase over time for the pliocene and the pleistocene probably in response to an increased influx of nile derived along shelf sediment transport

offbank transport of carbonate sands along open leeward bank margins northern bahamas abstract vigorous offbank transport of carbonate sands occurs along the west facing leeward open margins of little and great bahama bank large offbank oriented sand waves thick sand bodies covering reefs and shallow water sands mostly non skeletal obtained from adjacent deep flanks all demonstrate the existence of this shallow to deep pathway of sediment transport a variety of data suggests that transport occurs during storms and not during normal tidal current fluctuations an abundance of composite grains as well as vertical sequences of submarine cemented horizons seen via seismic profiling within the marginal sand bodies indicate cyclic periods of relative quiescence followed by moments of intense sediment movement current meter data obtained during six weeks of low wind activity show that critical threshold velocities for sand transport are only barely exceeded during portions of the tidal cycle these flows coupled with higher frequency bottom currents generated by surface gravity waves winnow the sands but do not provide significant net offbank transport offbank sand transport provides material to the shallow and deep bank margins enabling them to expand during sea level highstands each sea level fluctuation is likely to produce within the shallow margins a vertical sequence of basal reefs covered by a thick non skeletal calcarenite capped by a subaerially exposed diagenetically altered surface shallow water sands carried off the marginal escarpment help to construct deep flanks and large sediment drifts

two models global sea level change and sequence stratigraphic architecture abstract two different conceptual models underlie the application of sequence stratigraphy by exxon stratigraphers and later researchers one the global sea level model gsm relates to presumed sea level behaviour through time the other the sequence stratigraphic model ssm relates to the stratigraphic record produced during a single cycle of sea level change though the two models are inter related they are logically distinct and it is important to test them separately a summary is presented of the nature of the two models and of the nomenclature that is used in their description it is concluded a that the global sea level model comprises an assembly of local relative sea level events which are widely recognisable within their parent sedimentary basin and b that the sequence stratigraphic model is robust and its application is therefore an insightful way to approach the interpretation of sedimentary rocks

tectonics global changes in sea level and their relationship to stratigraphical sequences at the us atlantic continental margin of geological the principal factors that control the extent of seas through geological time are vertical movements of the lithosphere and global changes in sea level the relative height of the sea surface determines the facies and the thickness of sediments that can accumulate in a sedimentary basin backstripping studies show that the primary factors affecting the subsidence of rifted sedimentary basins are thermal contraction following heating and thinning of the lithosphere at the time of rifting and sedimentary loading factors such as compaction palaeobathymetry erosion and global sea level changes also contribute but their combined affects are small compared to those of thermal contraction and sedimentary loading simple models have been constructed which combine the effects of sedimentary loading and thermal contraction with those of compaction sub aerial erosion and global changes in sea level in the models it was assumed that the lithosphere was heated and thinned by stretching at the time of rifting sedimentary loading occurs by flexure of a lithosphere that progressively increases its flexural rigidity with age following rifting and that sediment compaction and bathymetry change across a basin but do not vary significantly with geological time furthermore different assumptions were made on the magnitude of curves of global sea level changes and the relationship between denudation rate and regional elevation the models show that tectonics in the form of thermal contraction of the lithosphere and flexure and slowly varying global changes in sea level can explain a number of the stratigraphic features of the us atlantic continental margin in this paper some of the implications of these results are examined for studies of a sea level changes through geological time and b the maturation history of continental margin basins

testing a seismic interpretation of great bahama bank with a computer simulation sixty million years of carbonate deposition were simulated to test an interpretation of platform development based on seismic data and limited well control from northwestern great bahama bank seismic profiles of the northwestern great bahama bank document the lateral growth potential of isolated platforms that were welded together by progradation to form the modern bank the mechanism we proposed responsible for an evolution from aggradation to progradation was sediment overproduction on the platform the excess of which was transported offbank and which caused a decrease in accommodation space on the marginal slope progradation occurred in pulses that were interpreted to be the result of third order sea level fluctuations to evaluate the proposed mechanism 15 input p rameters were used to model the platform evolution the simulation program which uses empirical relationships to model basin fill successfully reproduced the geometries seen on the seismic lines indicating that the proposed interplay of mechanisms could have built the observed platform architecture the simulation demonstrated in particular that in a setting like the bahamas a basin must be substantially filled before progradation can take place and that sea level changes can drive the pulses of progradation this implies that laterally stacked sequences often contain the record of sea level changes and therefore have potential use in sequence stratigraphy the simulation can also be used to estimate the quantitative importance of individual factors controlling aggradation and progradation in the bahamas we show how close the balance between aggradation and progradation is and how small changes in the rate of subsidence or accumulation can cause immediate switches from aggradation to progradation in particular we show that the rate of subsidence exerts the major control on the timing of progradation more so than basin width carbonate production rates similar to modern rates were required to produce the necessary sediment input for progradation which suggests that carbonate production has been consistently high since the early tertiary repeated exposure and erosion however have decreased the overall accumulation rate the simula ion also suggests that the asymmetric progradation in the bahamas was only possible where there were extreme differences between windward and leeward conditions with a maximum sediment input of 10 from the windward side

trace fossils of the arthropod camptophyllia from the westphalian carboniferous rocks of lancashire uk and their palaeoenvironmental context abstract two arthropod trace fossils are described and analysed from the carboniferous lower westphalian c communis and basal a modiolaris chronozones coal bearing strata of lancashire the biserial trackway diplichnites triassicus consists of five overlapping en echelon sets of 7 9 tracks preserved as epichnia and hypichnia in lacustrine siltstones the trackway suggests subaqueous in phase walking by a multi segmented producer with a body length of 35 40 mm width 17 22 mm and 7 9 appendages curved clustered or laterally repeated hypichnial lobes with transverse striations on the base of ripple cross laminated sandstone are identified as rusophycus versans this trace fossil is interpreted as shallow resting or furrowing burrows of a homopodous arthropod 30 60 mm long 15 30 mm wide and probably the same kind of arthropod as produced d triassicus a review of contemporary arthropod body fossils from lagerstatten in lancashire favours the onisciform or arthropleura like arthropod camptophyllia as a potential producer of both of these trace fossils in a lacustrine palaeoenvironment this study integrates the analysis of sediments trace fossils and body fossils for reconstructing the arthropod biota and ecology in westphalian lacustrine and crevasse splay fluvial palaeoenvironments

carbonates and relative changes in sea level abstract in the geologic record some of the most accurate gauges of changes in sea level are the sediment type geometry and diagenesis of carbonate shelves and platforms this is because carbonates frequently occur at or very near sea level and are usually less compacted than siliciclastics world wide changes in relative sea level the sum of eustatic sea level changes sedimentation and crustal movements have occurred repeatedly and cyclicly through geologic time producing characteristic responses in carbonates 1 1 relative rises in sea level usually caused by the cumulative effect of tectonic subsidence and eustatic rise may result in the following 1 1 a drowned carbonate reefs or platforms here carbonate growth potential is exceeded by relative sea level rise and is characterized by shallow water sediments overlain by hardgrounds and or deep water sediments some of which may be condensed sequences 1 2 b platforms where only the fast growing rim and patches of the interior are able to match sea level rise while the remainder of the platform is drowned temporarily 1 3 c platforms which keep up and maintain a flat top at sea level and contain shallow water sediments whose thickness at the least matches the height of the sea level rise if terrigenous supply is limited prograding sheets of shelf carbonate occur frequently capped by supratidal evaporites with prograding shelf margin carbonate clinoforms and turbidites if terrigenous supply is high the shelf carbonates encroach on deltaics 2 2 relative drops in sea level caused by crustal uplift or by subsidence being outpaced by a eustatic drop in sea level cause karst and soil development over shelves and platforms deposition of deep water evaporites in adjacent semi enclosed basins and in open marine basins the deposition of deltaic and aeolian clastics that bypassed the shelf falls are accompanied by platform wide fresh water diagenesis during relative sea level rises marine diagenesis is common in the subtidal portions of the shoaling upward carbonates and fresh water diagenesis and dolomitization and sulphate deposition is common in their intertidal and supratidal portions the stratigraphic significance of these reponses to relative sea level change is that many are tied to eustatic events and so are predictable within a basin of deposition

petroleum geology of the upper jurassic and lower cretaceous baltimore canyon trough western north atlantic ocean numerous hydrocarbon shows including a noncommercial gas and gas condensate accumulation occur in the baltimore canyon trough within sandstone units deposited in prograding coastal plain and transitional marine environments located updip of an oxfordian kimmeridgian carbonate shelf edge the coastal plain and transitional marine facies are overlain by a fine grained deltaic complex dominated by delta plain shales which collectively form a regionally extensive top seal unit this deltaic complex prograded into a back reef lagoon during aggradation of lower kimmeridgian through berriasian shelf margin carbonates wells drilled seaward of the continental shelf edge 1500 m water depth tested large structural stratigraphic closures along the downdip termination of the upper jurassic lower cretaceous carbonate shelf edge but encountered no significant hydrocarbon shows reservoir rocks in these wells consist of 1 oolite grainstone which was deposited within a shoal water complex located at the aptian shelf edge and 2 coral stromatoporoid grainstone and boundstone which formed an aggraded shelf margin complex located at the late kimmeridgian through berriasian shelf edge structural closures having reservoir and top seals are present in both updip and downdip trends hydrocarbon shows in wells along the shelf interior trend indicate the presence of mature source beds at least locally the absence of hydrocarbon shows in downdip carbonate reservoirs and around the schlee dome however suggests charge migration mechanisms within the fetch areas of these objectives have failed failure of charge can be due to 1 absence of mature source rocks 2 absence of migration pathways from source rocks to reservoirs and or 3 absence of top seals at the time of hydrocarbon migration continued development of play concepts in the baltimore canyon trough therefore requires identification and mapping of potential source rock intervals and construction of hydr carbon expulsion models to time hydrocarbon generation relative to trap formation

mathematical modelling of sedimentary basin processes abstract mathematical models of various sedimentary basin processes including hanging wall deformation evaporite deposition carbonate platform sedimentation and sequence development on active domino style fault blocks are presented many of these processes may be formulated using the equation of continuity of an open system as a starting point sedimentary and tectonic processes may be combined into a single mathematical formulation provided that an eulerian coordinate system is used and this ensures that tectonics and sedimentation are modelled as simultaneous rather than sequential processes the resulting algorithms are fast robust and applicable to many geologically very different situations but may be limited to relatively simple examples the conditions under which the models are numerically stable are also easily found the abstract nature of the mathematical models requires that simply determined properties such as palaeo water depth and palaeo slope must be used as proxy facies markers the main limitation of the method is that it is restricted to modelling processes which change slowly with time and space and the method can therefore not cope properly with episodic or chaotic processes

isua supracrustal belt greenland a vestige of a 3 8 ga suprasubduction zone ophiolite and the implications for archean geology abstract the mafic ultramafic units of the 3 8 ga isua supracrustal belt isb in greenland occur in a two armed arcuate zone eastern and western arms and are grouped into two major tectonostratigraphic units based on their lithological and geochemical characteristics 1 undifferentiated amphibolites ua and 2 garbenschiefer amphibolites ga the ua contains all major lithological units of a typical penrose type complete ophiolite sequence the ga is composed dominantly of volcaniclastic and volcanic rocks commonly found in immature island arcs the available geochemical data from ua and ga show distinct differences between the two units compared with the geochemical evolution of some of the well known phanerozoic ophiolites the pillow lavas and associated dikes of the ua show a compositional range that is similar to typical morb type ligurian ophiolites in the western alps apennines and those displayed by lip type caribbean ophiolites the ga is characterized by island arc tholeiite iat to boninite like rocks and defines a magmatic evolution that is comparable to that of suprasubduction zone ssz ophiolites in the mediterranean region our proposed geodynamic model for the isb suggests that the ua was built by primary to differentiated mantle generated melts during seafloor spreading little to moderately affected by subduction processes and that the iat to boninitic like rocks of the ga formed at a later stage by melting from a strongly subduction affected depleted and hydrated mantle our interpretation of the isb is that the ua and ga represent early and late stages respectively in the formation of a ssz ophiolite this implies that phanerozoic type plate tectonic processes such as seafloor spreading and subduction were operating by 3 8 ga in the palaeoarchean

shelf edge delta and reef development on a mixed siliciclastic carbonate margin central great barrier reef the great barrier reef gbr is the world s largest extant mixed silicilastic carbonate margin previous research on the great barrier reef has suggested that the extensive barrier reef system may act as an impermeable barrier and limit the development of delta systems during lowstands but sufficient geophysical data to support this hypothesis are lacking we use dense sparker seismic and sub bottom profiler data to better understand the structure of a large lobe shaped feature 10 km 10 km on the shelf edge of the central gbr and the interactions between siliciclastic and carbonate sedimentary systems interpreted sparker seismic contains prograding clinoforms and suggest that the lobe shaped feature was a river dominated shelf edge delta a delta on the shelf edge implies that the presence of an exposed barrier reef was not a major impediment to deposition and that other adjacent lobe shaped features are also deltaic deposits the shelf edge deltas were deposited onto a broad upper slope terrace that allowed continued progradation and limited incision when sea level fell below the shelf edge delta foresets are commonly colonized by coral reefs but the spatial and temporal relationship between reefs and some deltaic units remains unclear the presence of multiple shelf edge deltas that link to previously mapped burdekin river paleo channels indicates a complex history of sedimentation with the burdekin river delta migrating up to 100 km along the gbr margin during the late quaternary regional bathymetric data suggest that large modern or recent shelf edge deltas are rare on the gbr and that there was a broad range of sedimentary processes operating along the margin of the gbr during periods of low sea level

simulating transport and deposition of clastic sediments in an elongate basin using the simsafadim clastic program the camarasa artificial lake case study ne spain abstract predicting facies distribution and the stratigraphic architecture of sedimentary basins by process oriented numerical models is nowadays an essential tool in geologic studies they constitute a new approach to predict the geologic heterogeneity and the spatial distribution of the diverse facies generated in a depositional system jointly with the distribution of the physical chemical and petrophysical characteristics of the sedimentary deposits in a quantitative way simsafadim clastic is a 3d process based forward numerical model for the simulation of clastic sediment transport and sedimentation in aquatic systems it simulates the physical process of clastic transport using the advective diffusive and dispersive terms of the transport equation and clastic sediment deposition as a result of a variety of processes the capabilities of simsafadim clastic have been confirmed through the application of the program to a large deep elongated artificial lake the camarasa reservoir in the noguera pallaresa river ne of spain simulation results yield sedimentation rates ranging from 0 04 to 0 09 cm yr 1 close to the dam and from 1 73 to 1 63 cm yr 1 in the upper reservoir section the sample experiment results match well with the observed transport pattern linked to the flow system in camarasa s reservoir near bottom water layer which transports more than 50 of the sediment that is supplied to the reservoir opening and closure of turbine gates and the basin geometry are the main controlling factors on the fluid flow and depositional pattern in the reservoir with a more diversified pattern obtained when an open boundary is defined however the resulting model also shows some limitations of the program as it does consider a stratified water column that is consistently observed in the reservoir refined modeling exercises of the type described in this paper are of potential application to predict and quantify sedimentation patterns allowing the implementation of efficient preservation measures against reservoir siltation

three dimensional stratigraphic sedimentological forward modeling of an aptian carbonate reservoir deposited during the sag stage in the santos basin brazil abstract stratigraphic sedimentological forward modeling is a useful technique to test the role of parameters that may act in the depositional processes of sedimentary rocks the technique was applied to the santos basin brazil using the software program dionisos to test possible depositional scenarios for an aptian section of a lacustrine carbonate platform developed during the late sag stage uppermost pre salt section of the basin different parameters were tested in a set of numerical simulations with the aim of reproducing the carbonate facies spatial distribution and the overall stratigraphic stacking pattern of the platform as observed in the available well data several models were simulated and four of them are discussed in this paper based on the available descriptive data from 10 wells model 3 is considered to produce the best fit to explain the development of the 100 m thick aptian carbonate platform occurrences of grainstones and stromatolites as proximal and intermediate facies respectively and the overall carbonate stacking pattern thus based on 3d stratigraphic sedimentological forward modeling the best set of environmental conditions able to explain the pattern of carbonate deposition observed in the study area are 1 a slow carbonate depositional rate 0 08 mm y over an interval of 2 4 my which is indicative of a larger microbial contribution and 2 lake level oscillations which were essentially induced by arid climatic conditions these results also enable new perspectives for the application of stratigraphic sedimentological forward modeling as a predictive tool for hydrocarbon exploration in carbonates and for the development of depositional models of other sag carbonate platform settings

stratigraphic simulation of sedimentary basins concepts and calibration a two dimensional stratigraphic simulation program based on semiempirical algorithms has been successfully applied to clastic carbonate and mixed clastic carbonate depositional regimes the program may be used to predict reservoir distribution to constrain interpretations of well and seismic data to test exploration scenarios in frontier basins and to evaluate the fundamental controls on basin stratigraphy applications to seismic and well log data sets from main pass u s gulf coast offshore sarawak malaysia and the baltimore canyon u s east coast demonstrate that the program simulates stratigraphy on a basin wide scale as well as on the scale of individual prospects the main pass section is an offlapping sequence of neogene clastics the model simulates 17 m y of geologic history at a 200 000 yr resolution and reproduces gross basin geometry major sequence boundaries depth and location of nearshore marine sands intervals of sediment bypass into deep water shelf margin positions and paleobathymetry three neogene carbonate buildups of central luconia offshore sarawak malaysia were simulated using a sea level history optimized to reproduce the carbonate growth anatomy the model reproduced observed seismic reflection geometries and transgressive aggradational progradational retrogradational and subaerial exposure phases the program has also simulated jurassic pleistocene basin and stratigraphic sequence geometries and mixed clastic carbonate facies distribution along a 300 km regional dip line of the baltimore canyon trough modeled features of interest include a progradational middle jurassic carbonate margin punctuated by clastic deposition during sea level lowstands and a late jurassic early cretaceous aggradational carbonate margin culminating in the development and subsequent drowning of isolated carbonate buildups simulated bathymetries facies relations and subaerial exposure surfaces for the early cretaceous aggradation margin are consistent with detailed petrographic paleontologic and seismic stratigraphic interpretations

models for generation of carbonate cycles computer modeling provides a quantitative approach to a better understanding of actual carbonate cyclic sequences to model carbonate cycles the authors can use water depth dependent sedimentation rate for each facies an initial lag time linear subsidence tidal range and period and amplitude of sea level oscillation about a horizontal datum tidal flat capped cycles up to a few meters thick result from low amplitude sea level oscillation of a few meters and short lag times nonerosive caps reflect sea level lowering being balanced by subsidence and basinward migration of the shoreline not exceeding tidal flat progradation rate when higher amplitude sea level oscillations occur the tidal flats are abandoned on the inner shelf during sea level fall because seaward movement of the strandline outpaces progradation rate of flats increased amplitude also results in sea level falling faster than flats can subside so that disconformities with thick vadose profiles develop high amplitude 100 m or more oscillations result in incipient drowning of platforms and juxtaposition of deep water facies against shallow water facies within cycles sea level falls before the platform can build to the sea level highstand and the shoreline migrates much more rapidly than tidal flats can prograde thus cycles are disconformity bounded and lack tidal flat caps 10 references

probability maps of reservoir presence and sensitivity analysis in stratigraphic forward modeling one of the main objectives of petroleum exploration consists of predicting reservoir location data collected in the basin are used to better understand the sedimentary architecture but are usually insufficient to accurately characterize this architecture three dimensional stratigraphic forward modeling has brought new insights in the understanding of sediment distribution it gives the opportunity to investigate several geological models and to tackle reservoir presence probability however simulation time is a strong limitation to properly taking the uncertainties into account during operational studies here we propose a methodology based on metamodels or surrogate models to perform sensitivity and risk analyses the objective is to reduce the simulation time necessary to quantify the regional impact of the input parameters and to estimate probability maps of reservoir presence the approach consists of building functions that approximate the spatial outputs of the simulator such as sediment thickness or net to gross distributions in the basin and that are fast to evaluate these functions are then called instead of the stratigraphic forward simulator for uncertainty quantification the proposed methodology is applied to a three dimensional synthetic case study considering uncertainty on input parameters related to sediment transport accommodation space and sediment supply the sensitivity analysis quantifies in each location the influence of the parameters on the sediment distribution which can help to better understand the influence of each uncertain process on the basin architecture in addition probability maps of reservoir presence are estimated the proposed approach is a promising trade off between simulation time and information that can be inferred