Abstract: Many carbonate reservoirs are located on top, or down the flanks, of extant structural highs or syndepositional palaeo-highs. This study examines diagenesis in Pennsylvanian oolitic reservoirs close to the crest and down the flank of a long-lived anticline. It illustrates that the position of the best reservoir quality shifted back and forth during successive diagenetic events. Cement stratigraphy shows that early diagenesis did not enhance reservoir character significantly. Most oomoldic porosity formed penecontemporaneously with compaction. Fluid-inclusion and stable isotope data indicate that late cements precipitated during burial conditions by refluxing brines and later hydrothermal fluids. After initial burial, greater permeability existed downdip, where smaller amounts of early meteoric cement allowed for compaction. Subsequent reflux cementation initially degraded downdip reservoirs preferentially and then progressed updip, resulting in relatively uniform reservoir porosity. Later hydrothermal events are most important in affecting the distribution of the highest quality present-day reservoir. Highest porosity is preserved in wells down the flanks of the structure, where hydrothermal cements are not as prevalent. Understanding the effect of diagenesis on location of the best reservoir in relation to palaeotopographical and structural highs allows for the prediction of reservoir quality using seismic and mapping data typically available in the subsurface.

Abstract This paper constrains fluid flow and chemistry in Miocene dolomites of Spain, where dolomitization has been ascribed to ascending freshwater–mesohaline mixing. End-Miocene dolomite formed as replacement and cement with the same widespread cathodoluminescence. Fluid inclusion final melting temperatures of ice ( T m ice : −0.2 to −2.3 °C) indicate mixing of freshwater and evaporated seawater. δ 18 O and δ 13 C data mostly show positive covariation, and only some have variable δ 13 C and invariant δ 18 O, arguing that mixing was more important than sulphate reduction. Data range from +0.9 to +6.0‰ for δ 18 O and from −4.5 to +3.0‰ for δ 13 C (VPDB (Vienna Pee Dee Belemnite)). Lower stratigraphic units are more depleted isotopically than upper units, suggesting upwards flow of freshwater. 87 Sr/ 86 Sr values (0.70866–0.70904) range from less than to greater than late Miocene seawater. δ 18 O, δ 13 C and Sr analyses show that freshwater interacted with basement, confirming injection of freshwater from below. Upwards flow of freshwater, driven by low density and hydraulic head, created fluid mixing and CO 2 degassing. Comparison of La Molata dolomite to other dolomites of the western Mediterranean suggests that ascending freshwater–mesohaline mixing may be widespread, and that local composition of basement is not the primary driver of dolomitization. The model is broadly applicable to carbonates adjacent to highs, where freshwater discharged into slightly evaporated seawater.

Abstract The localization and heterogeneity of carbonate oil and gas reservoirs are commonly controlled by extensive diagenetic alteration. Mississippian (Osagean–Meramecian) strata in SE Kansas are investigated to determine structural, relative sea-level, diagenetic and depositional controls on stratigraphy, lithofacies distribution and reservoir character. This project shows how karst horizons and fractured zones can provide preferred conduits for hydrothermal porosity enhancement. Thus, enhanced porosity in karst horizons may have a late origin, with chemically aggressive hydrothermal fluids following preferred pathways of fluid flow. Lithofacies include echinoderm-rich bioclastic wacke–packstone, sponge-spicule-rich packstone, dolomitic bioclastic wackestone, argillaceous dolomite, tripolitic chert and chert breccia. Four cores are used to construct a 10 mile-long SW–NE-trending cross-section, showing three genetic units deposited on a mostly south-facing distally steepened ramp, with periods of upwelling. Paragenesis reveals that early and late dissolution enhances porosity in chert and carbonate facies. Fluid inclusion microthermometry from megaquartz and baroque dolomite reveals variable but increasing homogenization temperatures (70–160 °C) and increasing salinity through time. The best reservoirs may be controlled by depositional setting that led to large amounts of chert, alteration associated with subaerial exposure, and a hydrological and structural setting that led to enhanced hydrothermal fluid flow for later dissolution.

Abstract Outcrop-based, reservoir-analog models are important tools for assessing reservoir potential and efficient schemes for exploitation in the subsurface. A new outcrop reservoir-analog model is documented herein for Upper Miocene deep-water sediment-gravity-flow and hemipelagic deposits within the Agua Amarga basin, southeast Spain. This study demonstrates that large volumes of resedimented deposits exhibiting high ratios of potential reservoir to baffle facies (net to gross) accumulate where funneling topographic features focus sediment-gravity flows from the long linear dimension of a carbonate platform into a confined channel (focused flow). Where topographic funneling features are absent, and where a short linear dimension of the carbonate platform margin is available as a source of sediment-gravity flows, deposits accumulate with lower volumes and high proportions of baffle facies (dispersed flow). Extensive outcrops in the Agua Amarga basin allow for characterization of facies and facies architecture using measured sections, photomosaics, and core-plug petrophysical data. Petra™ and Petrel™ were used for correlation, data integration, and static geomodeling to create a reservoir-analog model that synthesized geological observations noted in outcrop. Facies modeled as reservoir units consist of graded fine-to very coarse-grained skeletal packstones and fine-to very coarse-grained breccias. Graded skeletal packstone facies exhibit a mean porosity and corresponding permeability of 30.5% and 136 mD; breccia facies exhibit a mean porosity and corresponding permeability of 30% and 65 mD. Facies modeled as baffle units consist of foraminiferal, volcaniclastic foraminiferal, and skeletal foraminiferal wacke-packstones. These planktonic foraminifera-rich facies exhibit a mean porosity and corresponding permeability of 36% and 12 mD. Paleotopography, in conjunction with sea-level history, largely controls the geometry, lateral continuity, and volume of a given reservoir body. Paleotopographic differences that lead to focused flow versus dispersed flow result in markedly different reservoir properties. Static model volumetric results reveal that compared to the dispersed-flow system, deposits within the focused-flow system have greater reservoir to baffle facies volume ratios (0.70 compared to 0.09), and greater reservoir facies bulk volumes (46.5 million m 3 compared to 18.6 million m 3 ). Further, the ratio of reservoir facies bulk volume to linear dimension of the shelf margin supplying both the focused-flow and dispersed-flow systems is similar, suggesting that deep-water reservoir volume may be predictable on the basis of the linear dimension of the shelf margin. Finally, interrogating modeled reservoir facies for different connected volume scenarios offers significant insight relevant to subsurface exploitation strategy and supports observations noted in the field.

Abstract Cambrian–Ordovician Arbuckle Group rocks in Kansas occur entirely in the subsurface and are absent only in areas of northeastern and northwestern Kansas and over ancient uplifts and buried Precambrian highs. During Arbuckle deposition, Kansas was located approximately between 20 and 308 south of the equator and south of the Transcontinental arch. Because of the lack of biostratigraphic data and a chronostratigraphic framework, correlation of Arbuckle Group subunits has relied predominantly on lithologic character and insoluble residues. Core studies reveal shallow-water, carbonate-dominated depositional facies that are stacked in vertical cycles (ranging from less than 1 m [<3.3 ft] to several meters thick) and cycle sets. Eight depositional facies predominate: (1) clotted algal boundstone (subtidal conditions) with porosities less than 6% and permeabilities less than 0.1 md; (2) muddy to grainy laminated algal boundstones (subtidal to peritidal conditions); muddy textures exhibit porosities generally less than 6% and permeabilities less than 0.1 md, and grainy textures represent some of the best reservoir rock ranging in porosity up to 32% and permeability up to 1500 md; (3) peloidal packstone-grainstone (subtidal to peritidal conditions) with porosities from 0 to 4% and permeabilities generally below 0.005 md; (4) mixed packstone-grainstone (subtidal to peritidal conditions) with porosities from 6 to 18% and permeabilities from 0.1 to 50 md; (5) ooid packstone-grainstone (subtidal to peritidal conditions) with porosities from 11 to 30% and permeabilities from 10 to 1500 md; (6) wackestone-mudstone (restricted subtidal to peritidal conditions) with porosities from 0 to 17% and permeabilities from less than 0.0001 to 1000 md; (7) intra-Arbuckle shale (low-energy subtidal to peritidal and, perhaps, supratidal conditions); and (8) intraclastic conglomerate and breccia, fracture-fill shale, and chert in variable abundances. The abundance of intercrystalline, moldic, fenestral, and vuggy porosity is related to depo-sitional facies, early diagenesis, and dolomitization and not necessarily to karst influence from the upper super-Sauk subaerial exposure surface. Arbuckle reservoirs historically have been viewed as fracture-controlled karstic reservoirs with porosity and permeability influenced by basement structural patterns and subaerial exposure. Although fractures and karst influence production in some Arbuckle reservoirs, the presence of reservoirs where water drive is minimal or absent indicates the dominance of matrix porosity. The Arbuckle in Kansas can be characterized using three end-member reservoir architectures, representing fracture-, karst-, and matrix-dominated architectural systems. Lithofacies and stratal packaging of reservoir and nonreservoir strata exert an important influence in all three reservoir architectures.

Abstract During the Carboniferous a carbonate shelf covered areas of the central U.S, including Kansas, with the outer shelf and shelf margin intermittently extending through southern Kansas and northern Oklahoma. The regional setting resulted in deposition of relatively laterally continuous carbonate and siliciclastic facies belts. Areally sparse subsurface well data and surface exposures have led to the interpretation that most structures on the craton are simply shallow draped anticlines and associated synclines primarily reflecting general aspects of regional structure and depositional and erosional heterogeneity. Models that assume a broad continuous shelf relegate local structure to being minor or non-existent. However, our recent examination of subsurface data, 3-D seismic, and rock properties for oil fields from the Middle Mississippian shelf margin, Middle Pennsylvanian mid-shelf, and Late Pennsylvanian lower shelf indicate that regional- and kilometer-scale structures (e.g., faults, fractures, lineaments) segmented the shelf and shelf-margin areas in Kansas, primarily along Precambrian structures that were reactivated throughout the Phanerozoic. Movement on faults resulted in segmentation expressed as rhombic-shaped structural blocks (1-10s km) with subtle variations in relief (generally meter to ~ 70 m) and slope (near zero to upwards of 2-3 m/km). Regional, down-to-basin block faulting produced linear shelf edges and segmentation of the ramp and shelf profile repeatedly during the Carboniferous. The association of stratal packages and rock properties with structural elements argues that structure exerted continued, but episodic, influence and affected sediment accommodation, depositional patterns, paleotopography, weathering intensity, diagenesis, and later fluid movement, including hydrocarbon emplacement. Results from our study of the “stable” shelf carbonates of the Midcontinent indicate that tectonic events may have had far-reaching effects and caused structural deformation in the interiors of cratons. Sedimentologic and stratigraphic analyses in such settings can benefit by evaluating the possible influence of subtle faulting and fault reactivation on depositional and diagenetic patterns that can significantly influence rock properties and reservoir development.

Abstract This study analyzes the three-dimensional variability of a 20-meter-thick section of Pennsylvanian (Missourian) strata over a 600 km 2 area of northeastern Kansas, USA. It hypothesizes that sea-level changes interact with subtle variations in paleotopography to influence the heterogeneity of potential reservoir systems in mixed carbonate-siliciclastic systems, commonly producing build-and-fill sequences. For this analysis, ten lithofacies were identified: (1) phylloid algal boundstone-packstone, (2) skeletal wackestone-packstone, (3) peloidal, skeletal packstone, (4) sandy, skeletal grainstone-packstone, (5) oolite grainstone-packstone, (6) Osagia-brachiopod packstone, (7) fossiliferous siltstone, (8) lenticular bedded-laminated siltstone and fine sandstone, (9) organic-rich mudstone and coal, and (10) massive mudstone. Each facies can be related to depositional environment and base-level changes to develop a sequence stratigraphy consisting of three sequence boundaries and two flooding surfaces. Within this framework, eighteen localities are used to develop a three-dimensional framework of the stratigraphy and paleotopography. The studied strata illustrate the model of “build-and-fill”. In this example, phylloid algal mounds produce initial relief, and many of the later carbonate and siliciclastic deposits are focused into subtle paleotopographic lows, responding to factors related to energy, source, and accommodation, eventually filling the paleotopography. After initial buildup of the phylloid algal mounds, marine and nonmarine siliciclastics, with characteristics of both deltaic lobes and valley fills, were focused into low areas between mounds. After a sea-level rise, oolitic carbonates formed on highs and phylloid algal facies accumulated in lows. A shift in the source direction of siliciclastics resulted from flooding or filling of preexisting paleotopographic lows. Fine-grained siliciclastics were concentrated in paleotopographic low areas and resulted in clay-rich phylloid algal carbonates that would have made poor reservoirs. In areas more distant from siliciclastic influx, phylloid algal facies with better reservoir potential formed in topographic lows. After another relative fall in sea level, marine carbonates and siliciclastics were concentrated in paleotopographic low areas. After the next relative rise in sea level, there is little thickness or facies variation in phylloid algal limestone throughout the study area because: (1) substrate paleotopography had been subdued by filling, and (2) no siliciclastics were deposited in the area. Widespread subaerial exposure and erosion during a final relative fall in sea level resulted in redevelopment of variable paleotopography. Build-and-fill sequences, such as these, are well known in other surface and subsurface examples. Initial relief is built by folding or faulting, differential compaction, erosion, or deposition of relief-building facies, such as phylloid algal and carbonate grainstone reservoir facies, or siliciclastic wedges. Relief is filled through deposition of reservoir-facies siliciclastics, phylloid algal facies, and grainy carbonates, as well as nonreservoir facies, resulting in complex heterogeneity.

Abstract Computer stratigraphic simulation models provide a quantitative means to evaluate and understand complex interactions of sedimentary depositional systems. People in the geosciences are quickly advancing in their ability to acquire and interpret large data sets, resulting in major advances in understanding earth systems. Simulation is a natural outcome of these advances, as is the need to integrate and process this information. This volume provides a collection of 26 papers that describe and illustrate the application of some of the latest approaches to stratigraphic-sedimentologic modeling. This paper serves as an overview of these papers, classifies modeling, reviews current issues of modeling, and evaluates possible future modeling directions and opportunities. We have recognized several different approaches to modeling and present a rational classification for these model types, illustrated here and in the volume by diverse examples. Despite varying philosophies and methodologies of their creators, most models consist of three essential components: (1) input, (2) engine, and (3) output. Our results suggest that models have a sound observational basis (input) and logical foundation (engine), both of which use ever-improving quantitative knowledge of geologic systems. Roles of modeling include: (1) encouraging accuracy and precision in data collection and process interpretation (Slingerland et al., 1994); (2) providing a means to quantitatively test interpretations of the roles of various driving mechanisms to produce sedimentary packages; (3) predicting or extrapolating results into areas of limited control; (4) affording mechanisms for enhanced multidisciplinary integration and communication; (5) gaining new insights to offer nonintuitive results regarding the interaction of parameters; and (6) helping focus future studies to resolve specific problems. The future of modeling is dependent upon fully using improved computational methods and machines, refining quantitative geologic observations and interpretations, and developing rigorous, quantitative approaches to testing, calibrating, verifying, and comparing models.

Abstract This document provides a record of the discussions conducted by the Geological Observations and Parameterizations (GOP) group, which sought to define and address important questions concerning the parameterization of geological observations for use in stratigraphic simulations. The discussions of this group, which was intermittently composed of a combination of model-building, model-using, and nonmodeling geologists, centered on the type and quality of common geological observations, the scales of these observations, and the accurate representation of observations in geological models. The underlying concerns were whether all relevant processes are accounted for in existing models, and whether modelers are using the available models correctly, inputting realistic values and ranges of values at the proper scales. Our fundamental questions include the following: How closely is the variability of geological observations reflected in stratigraphic models? If such variability is well represented within a model, how well does a simulation predict away from points of control? If it does not do this well, is it the input parameters that are not reflecting the natural variability of the system, or does the fault lie with the model itself? If the model itself is the limiting factor, how well do we really need to constrain input parameters? How well do we understand the nonlinear characteristics of natural systems, and how are these characteristics introduced into models?

Abstract The purpose of inversion is to determine the limits of prediction for model testing and risk assessment given the state of our knowledge (knowledge in the form of model assumptions, accuracy, and precision, and in the form of data distributions, types, accuracy, and precision). Inversion is a tool for making decisions. Should we do inversion? Are current models and assumptions sufficient to solve posed problems, or do we need to develop a new model? Are currently available data sufficient, or do we need to collect more or different data? And how do we decide? The above plethora of queries arose during the committee discussions as general concerns, as did a large number of more specific questions. We record in Appendix 1 all of the questions that arose because the committee deliberations did not allow time for complete resolution of all issues. We recorded all of the questions in the hope that some, at least, will be answered in the future. The question list of Appendix 1 is far too long to address in detail, so the committee focused on attempts to provide some pragmatic rules of operation to guide the actual technical use of inverse modeling. Although the rules set down in this paper are not a universal panacea, they do represent considerable practical experience with inverse models. As such, they are useful guides as one struggles to determine the strengths and weaknesses of inverse modeling.

Abstract A procedure determines the relative importance of uncertainties in input information and in multiple parameter estimation to all outputs from two-dimensional basin modeling codes. The procedure does not rely on Monte Carlo methods, but on some simple properties of the cumulative probability distribution of output variations related to uncertainties. As a consequence, only a couple of computer trials are needed to evaluate the relationship of the variability of outputs to input uncertainties. The procedure is applied to a two-dimensional cross section with evolution of the section with time. Attention is focused first on mainly geologic input uncertainties, and then on uncertainties of thermal factors and of hydrocarbon kinetic factors. Each group is initially taken separately, and then all three groups of uncertainties are combined and used simultaneously. The influence of each group of uncertainties on a suite of different outputs from the basin model is explored at different times across the evolving section. At each lime step, the relative sensitivity is examined of the uncertainty in a specified output to each group of input uncertainties, as is the relative importance of the uncertainty in a specified input to the suite of all outputs at each time step. In addition, the global relative importance of input uncertainties to output variabilities is considered, thereby providing a measure of output uncertainty effects, no matter where and when they occur, as a consequence of input uncertainties. This work enables one to assess which inputs need to be more tightly constrained, and also to determine by how large a factor they need to be better constrained if the uncertainties on a suite of specified outputs are to remain within given tolerance limits. The advantage to this rapid procedure is that one can focus more quickly on those factors of dominance in controlling, say overpressure development or hydrocarbon charge in a basin, without having to spend an inordinate amount of time, effort, or financial or staff resources on providing narrower limits of uncertainty to those input factors that provide but little change in output uncertainties.

Abstract Stratigraphic inversion is a quantitative technique that extracts values of process parameters, such as tectonic movement, lithosphere strength, sea level change, sediment supply, and basin topography, from stratigraphic data. A stratigraphic inverse model contains (1) a forward model that simulates stratigraphy through the operation of a set of input process parameters and algorithms that describe the behavior of the stratigraphic process-response system; (2) a set of observed data that are comparable in type and form to forward model predictions; and (3) a set of equations and algorithms that compare the values of forward model predictions with observations, and simultaneously adjust values of all forward model parameters to create a better match between predictions and observations. The inverse model iteratively reduces differences between forward model predictions and observations until a best match is achieved. The model calculates the degree of accuracy and uncertainty of values of stratigraphic predictions. Constructing an inverse model requires the following steps: (1) selecting a stratigraphic forward model; (2) designing simple mathematical functions that most accurately describe the real stratigraphic processes that operated in a basin and that make inversion computationally possible; (3) measuring data that correspond in type to the output of that forward model and transcribing those data into the form of a numerical vector; (4) selecting an appropriate parameter optimization algorithm; and (5) building a stratigraphic inverse model that connects components of steps 1–4. One purpose of stratigraphic inverse modeling is predicting stratigraphic attributes (e.g., facies, geometry, distribution, volume) with calculated estimates of accuracy and uncertainty. Once the range of parameter values is calculated by the inverse model, a population of forward models may be run that should contain the true stratigraphy. The population of forward models is used to predict the geographic and stratigraphic positions and extent of potential reservoir and source and seal rocks. We show an example of accurate stratigraphic predictions using inverse modeling of the Mesa Verde Group, San Juan basin, Colorado and New Mexico, United States.

Abstract Stratigraphic modeling involves a multidimensional parameter fitting problem where a large number of free model parameters have to be adjusted for the model to match observational data. This task can be viewed as an optimization problem, which here is addressed using a genetic algorithm. The iterative trying-and-checking process, usually done manually, is thereby automated. We apply this method for the automatic construction of sea level and subsidence curves for two simple toy models. We also address the problem of distinguishing the sea level variations vs. subsidence variations, and we give an example of a simulation involving carbonates from Mallorca, Spain.

Abstract Numerical-statistical algorithms are used to model end-member grain-size distributions of pelagic and hemipelagic siliciclastic sediments of the Arabian Sea. The grain-size distributions of sediments from the Oman continental slope, the Owen Ridge, the Pakistan continental slope, and the Indus Fan can be adequately described as mixtures of three end members. The spatial variation in relative contribution of the end members is interpreted in terms of transport processes and provenance. In the western Arabian Sea, deposition is dominated by two end members that represent "proximal" and "distal" eolian dust. A third end member, which dominates the deposits of the middle Indus Fan, represents fluvial mud deposited from low-density turbidity currents (lutite flows). At any given location, the temporal changes in the relative contribution of the end members can be interpreted in terms of climate change. The ratio of contributions of the two eolian end members (i.e., the grain-size distribution of the eolian dust) on the Owen Ridge (NIOP492) reflects the strength of the summer monsoon. Deposition on the upper Indus Fan (NIOP458) is dominated by "distal" eolian dust and fluvial mud. The ratio of contributions of eolian and fluvial sediment reflects continental aridity. The ratio of contributions of the two eolian end members (i.e., the grain-size distribution of the eolian dust) on the upper Indus Fan reflects the strength of the winter monsoon. Our reconstruction of the late Quaternary variations in Arabian Sea monsoon climate corresponds well with interpretations of the loess-paleosol sequences on the Chinese Loess Plateau. In both areas, the bulk of the annual precipitation is confined to the summer monsoon season. Intensification of the summer monsoon during interglacials, which has been identified as the principal control on pedogenesis on the Loess Plateau, also explains increased discharge of Indus River-derived muds to the northern Arabian Sea. Independent evidence for summer monsoon strength, provided by the eolian grain-size record of the western Arabian Sea, fully supports this conclusion. The strength of the summer monsoon thus provides an aridity forcing mechanism for both the Arabian Sea and the Loess Plateau. The grain size of the eolian dust in the northern Arabian Sea and on the Loess Plateau indicates intensified winter monsoons during glacials.

Abstract A stratigraphic simulator called phil. (Process- and History- Integrated Layers) documented the history of a seismically defined cross section through the Baltimore Canyon Trough, offshore New Jersey. This cross section helps to constrain a sea level curve from 30 Ma to the present. The interval from 18 to 11 Ma is especially well defined and is considered a type section for this time interval. The stratigraphy was modeled with empirically derived algorithms that produced a resulting distribution of sediment that were compared at 2980 points along depth converted horizons. The result deviated by an average of 36 m from the observed depths and explains 90.6% of the total variation in depth. This was accomplished by introducing a predominantly siliciclastic sediment supply that was distributed by a mixture of traction and suspension sedimentation processes that varied through time. The traction and suspension mixture varied as the distance to the fluvial source varied with respect to the section. Although the Haq et al. (1988) curve served as a good initial sea level history, it proved to have problems when it was applied to reproducing a stratigraphic record for this section, as well as other sections of equivalent age from North America, Southeast Asia, and Africa. The long-term trends tended to be highly over exaggerated. phtl allowed us to produce a new curve that significantly improves the match between the stratigraphy and the model. phil simulation results allow us to extend the understanding of this cross section by predicting lithofacies distributions and associated physical properties, and systems tract boundaries. The resulting physical properties were used as input to a seismic model. Simulation provides values for tectonic subsidence and sedimentation supply rates, carbonate production rates, definition of stability conditions, erosion rates, and the development of water level history that integrates all the active processes. Simulation also is an important tool for visualizing the development of structural and stratigraphic features.

Abstract Observations of modern and ancient sedimentary basins indicate that the shoreface and the depositional shelf break (DSB) can range from coincident to more than 100 km apart. Most conceptual and numerical models of sequence formation do not adequately separate these two features. The model used here incorporates independent calculation of the position of both the shoreface and the DSB; however, at present the lack of an adequate understanding of long-term shoreline response to sea level and other environmental change is a serious limitation to our understanding of the genesis of continental margin stratigraphy. When independent movements of the shoreface and the depositional shelf break were incorporated into the model, both ravinement surfaces and regressive surfaces of erosion developed in the simulations. This response attests to the importance of distinguishing these two features and to the important role of the physiographic break at the shoreface. In model results, the shoreface and DSB do not always respond similarly to sea level fluctuations. The relationship between shoreface and DSB movements differs depending upon whether they are geographically separated. As a result, defining sequence boundaries and systems tracts can be difficult. The extent of the transgressive systems tract, in particular, is a problem because progradation of the DSB begins partway through the shoreline transgression. This mismatch between the shoreface and DSB predicted by the model has not previously been noted. Fluvial erosion of the coastal plain develops progressively as the shoreface advances, indicating a progressive development of the sequence boundary. Onlap onto the front of the previous regressive shorefaces occurs primarily during the transgressive systems tract. Systems tracts therefore should be based on stratigraphic and lithological distinctions in the rock record, and not be tied to an interpretive model. To accurately calculate vertical motions and resultant stratigraphy for high-frequency eustatic fluctuations, isostatic adjustment and erosion to an equilibrium profile are modeled as time-dependent processes. These models show that the form of sequences changes with the frequency of eustatic fluctuations. Similarly, the erosion rate has a major influence on stratal relationships. The rate of isostatic compensation can alter whether a sea level cycle generates a sharp-based or gradational-based shoreface. The type of base at prograding shoreface successions can change within sequences. Thus, the sequence boundary should be placed at the top of regressive shoreface packages. Continued isostatic or compactional subsidence following the deposition of depositional delta lobes can explain the formation of flooding surfaces and parasequences.

Abstract Although quantitative stratigraphic models have been able to reproduce the gross characteristics of sedimentary successions, most are less successful at reproducing fine stratigraphic details such as marine erosion surfaces, and yield little insight into the dynamics of sediment transport. We have developed a new two-dimensional stratigraphic model that combines a geodynamical model that simulates tectonics, isostasy, compaction, and coastal plain erosion and deposition with a morphodynamical model that simulates marine sediment transport. The morphodynamical model differs from sediment-transport models used in older stratigraphic models in that it allows for both offshore and onshore transport through an estimate of long-term advective and diffusive sediment fluxes, and applies concepts of dynamic equilibrium to the shoreface and continental shelf. This more sophisticated sediment-transport model allows us to simulate the response of stratal geometries and surfaces to changes in hydrodynamic climate, as well as to changes in sea level, tectonics, and sediment supply. In this paper, we simulate a narrow, steep continental margin with relatively high sediment supply (similar to the modem northern California margin) that is undergoing high-amplitude (80 m), high-frequency (40 k.y.) sea level fluctuations. We then compare the effect of varying several parameters on the resulting simulation. These sensitivity tests illustrate the effects of variations in the steepness and erosion rate of the coastal plain, hydrodynamic intensity, and disequilibrium initial conditions, and also a nonsinusoidal "asymmetrical sawtooth" eustatic curve approximately reflecting sea level change over the past 125 k.y. Although quantitative calibration of the model against real hydrodynamic data has not yet been completed, the model responds to changes in input parameters that appear realistic and offers a possible explanation of the patterns observed in real sedimentary successions. Marine and subaerial erosion surfaces are produced at logical times during a sea level cycle, and the shoreface shape changes in ways that resemble real profile adjustments to changes in rates of sea level change, sediment supply, and hydrodynamics. Model results suggest that sediment-transport processes may strongly overprint the stratigraphic record, allowing a considerable variety of sedimentary styles to be produced with identical sea level, tectonic, and sediment-supply histories. In the simulations, sequence thickness and the location and preservation of transgressive and regressive deposits vary with changes in coastal plain behavior and wave intensity. Steeper coastal plains result in reduced subaerial erosion and better shelf preservation. Low rates of subaerial erosion or high wave intensity results in thick, steeply inclined regressive deposits, but poor preservation of transggessive deposits; thicker shelf sections are not necessarily more complete. Clinoforms develop within the model only under conditions of significant disequilibrium; such conditions could occur in nature due to changes in relative sea level that are large relative to rates of sediment supply. These results suggest that factors other than sea level, amount of sediment supply, and tectonics are significant in stratigraphic development and highlight the need for the inclusion of more rigorous sediment-transport dynamics in numerical and conceptual stratigraphic models.

Abstract The shoreface translation model (STM) incorporates advances in the theory for coastal responses to changes in relative sea level, exposing some well-entrenched misconceptions about the formation of transgressive and regressive strata at chronosomal scales. The STM is a mass-conserving, morphological-behavior model that provides added generality to the updated theory by allowing for open sediment budgets (on the shoreface and in the lagoon) and time-dependent changes in shoreface and barrier geometries. Both the theoretical basis and application of the STM give neutral transgression for balanced sediment budgets on gently sloping surfaces undergoing a marine transgression. Under these conditions, no transgressive strata are formed, and the land surface being transgressed is not disturbed en masse. Consequently, shoreface-ravinement surfaces are not necessarily inherent by-products of transgression as assumed previously. Simulated transgressive strata are laid down (aggradational transgressions) only if there is a positive net littoral sediment supply (from deltaic sources or erosion of shoreline promontories), significant deposition in the lagoon (due to trapping of fine marine sediments or direct fluvial inputs), or both. Shoreface-ravinement surfaces are produced only under conditions of negative littoral sediment budgets or if the land surface being transgressed is steeper than the shoreface (degradational transgressions). For negative sediment budgets, simulated shoreface ravinements form on low-gradient surfaces without seaward sediment displacement or genetically related aggradation of the seabed farther offshore. Ravinements also can develop during progressive deepening of the shoreface during transgression and highstands. Simulated highstand ravinements are consistent with, and provide an alternative explanation for, coarse-sand lags found on the lower shoreface of many accommodation-dominated shelves today. Simulated forced regression results in massive in-situ reworking of the highstand shelf surface, inevitably producing a strandplain stratum characterized by (1) an unconformity at its base and (2) shoreface isochrons, as opposed to the landward-dipping, backbarrier isochrons that characterize transgressive barriers (which consist of washover and tidal-delta sand deposits). The revised approach to simulating each of these intrachronosomal-forming processes has significance for sequence models and the interpretation of stratigraphic data at basin-fill scales.

Abstract A numerical model has been developed to investigale the grain size distributions and stratal patterns associated with compressional and extensional fault-related folds in nonmarine settings. First, the basic predicted stratal and facies geometries for a single extensional and thrust-fault-related fold are identified. Then the modeling is extended to situations involving multiple faults and changes in displacement velocity. Finally, the model is invoked to study structural inversion comprising a change from extension to compression. The stratigraphic architecture of facies patterns predicted by the model is sensitive to convergence and sequence of fault activation and may serve as a guide in interpreting the stratigraphic record associated with multiple fault structures. Application of insights from our modeling to a few natural examples of growth faults and sheds light on various interpretations of these structures.