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 Two sand ridges on the New Jersey continental shelf were cored to determine their lithologic characteristics and possible modes of deposition. The ridge at location 1A is a nearshore, shoreface-connected sand ridge within 3 mi (4.8 km) of shore and in less than 65 ft (20.3 m) of water. The ridge at location 2 is 25 mi (40 km) from shore and in 80-115 ft (25-36 m) of water. Three main lithologic units are present in the cores from both ridges. The cores presented in this manuscript have sedimentary characteristics representative of these lithologic units and display the key differences between ridges observed in cores. The three main lithologic units are (from bottom to top): (1) "nonfossiliferous" sand and mud, (2) shell-rich poorly sorted sand and mud, and (3) upper ridge sand. The "nonfossiliferous" unit contains no macrofauna, but has traces of microfauna, massive-appearing sand layers, laminated muds, and some pebbly sand layers. The shell-rich unit contains numerous shell fragments and is predominantly bioturbated. Carbon-14 age determinations from the shell-rich unit in the nearshore ridge range from 6130 ± 120 years BP to 6360 ± 90 BP; those from the midshelf ridge range from 12,480 ± 155 years BP to 13,240 ± 180 BP. The upper ridge sand unit consists of stacked beds ranging in thickness from 1 to 28 in. (2.5-70 cm). Within the upper ridge sand unit, most beds in the nearshore ridge are up to 9 in. (22.5 cm) thick, whereas most beds in the midshelf ridge are slightly thicker, 12 in. (30 cm) or less. Within the upper ridge sand unit, both ridges have alternating laminated and nonlaminated (bioturbated) layers, contain fine- to medium-grained sand, and generally coarsen upward. The nearshore ridge has a slightly coarser range of mean grain size (150-400 μ) than that of the midshelf ridge (140 to 360 μ). The youngest carbon-14 age determinations from the upper ridge sand unit are 1480 ± 170 years BP from the nearshore ridge and 1155 ± 85 BP from the midshelf ridge. Results from this study support the hypothesis that both the nearshore and midshelf ridges are being actively modified and possibly formed at present sea level. These conclusions support past theories on the origins of the nearshore ridges (Duane et al., 1972), but indicate that the midshelf ridge has a much more "dynamic" posttransgressional depositional history than previously surmised.

Abstract As an introductory step, this paper defines continental shelves and briefly discusses their origin and evolution. Most of the paper is concerned with the large-scale tidal and storm-driven fluid circulation patterns of the continental shelves and the manner in which these flows entrain and move sediment. It is essential to understand these circulation patterns in order to understand the distribution of facies on continental shelves. However, oceanic currents on a rotating planet are complex and their pattern is not intuitively obvious. Therefore, a considerable portion of the chapter is devoted to an analysis of the mechanisms of shelf flow, and the importance of these mechanisms in determining shelf sediment transport. Storm-driven and tidal currents are considered in turn. The shoreface and inner shelf together constitute a gateway through which all shelf sediments must pass, and the complex flows of the shoreface and inner shelf are described in detail. Finally, fluid and sediment dynamics at the shelf edge are reviewed.

Abstract The storm and tidal currents that sweep the surfaces of continental shelves imprint a variety of morphologic and textural patterns on these surfaces. As the surfaces aggrade, the grain size gradients and bedform arrays become the textures, structures and stratification patterns of the resulting sedimentary sequences. This paper describes textural gradients and bedform arrays characteristic of shelf surfaces, and the process of strata formation.

Abstract This paper reviews studies of Woodbine--Eagle Ford reservoir sandstones from the subsurface of East Texas and evaluates shelf sand depositional models in the light of recent studies of fluid and sediment dynamics on modern shelves. The application of fluid and sediment dynamical principles has reaffirmed some shelf depositional models, traditionally applied to the East Texas basin, but modifies or discredits others; in these cases, new models are proposed. Three distinct types of reservoir-quality shelf sandstones can be recognized in these studies; (1) sand ridge deposits, (2) tabular or sheet sandstones, and, (3) lenticular (topographically controlled) sandstones. This preliminary classification is based on external sand body geometries, facies associations and facies distributions. Sand ridge deposits occur at Kurten Field as stacked, en echelon, linear sandstone bodies deposited on the muddy shelf of the east side of the Cretaceous Interior Seaway. Sandstone bodies are asymmetric in cross-section with steeper eastern flanks and are elongated in a north-south direction. Sand ridge deposits at Kurten Field occur stratigraphically adjacent to deposits of the Harris Delta. Sand ridge deposition probably occurred in an inner to middle shelf environment during small scale transgressive episodes,} possibly associated with the abandonment of delta lobes (autocyclic transgression). Intermittent, alongshelf, geostrophic flows appear to be the most likely mechanism of sand transport and deposition. Tabular shelf sandstones occur in the lower Woodbine at Damascus Field as a complex of single to multistory thin beds within a dominantly shale section. Cores display stacked, massive to laminated, fining-upward sandstone sequences, with abundant soft-sediment deformation and primary structures indicating rapid sedimentation. Sandstones form a series of thin sheet-like deposits elongated across the strike of the paleoshoreline. Sanddeposition took place in an inner to middle shelf environment, during a general period of shoreline regression. Deposition is suggested to have occurred in localized zones of alongshelf flow deceleration and expansion during storms. Bouma-like vertical sequences of primary structures in Damascus sandstones indicate that these beds are tempestites (i.e. suspension deposits produced by storm flows). Lenticular shelf sandstones are present in the uppermost Eagle Ford (Sub-Clarksville) section in Grimes County, Texas. Fining-upward sandstone sequences consist of amalgamated, massive to cross-stratified beds with erosional bases, overlain by bioturbated shaley sandstones. Individual sandstone bodies have restricted areal extents and deposition appears to have been controlled by local, salt-related topographic lows. These Sub-Clarksville sands were apparently deposited during a regional transgression which succeeded a phase of sea level stillstand. Remobilization of the substrate by wind-forced storm currents during transgression appears to have formed broad erosional surfaces, accompanied by deposition of sands swept into zones of local flow deceleration. Transgressive sand ridges may have formed contemporaneously on other parts of the late Eagle Ford shelf.

ABSTRACT It has been proposed by Stubblefield and colleagues (this volume) that the sand ridges of the central new Jersey shelf contain a basal muddy sand stratum deposited as a lower shoreface facies during a period of coastal progradation. They conclude that the ridge morphology above the mid-shelf scarp is in part a relict strand plain. The coastal progradation hypothesis for the origin of the lower muddy sand facies is a reasonable one, but to date there is not enough evidence to discriminate among facies. Examination of the inner shelf surface above the mid-shelf scarp reveals topographic, stratigraphic, and grain-size patterns that may be interpreted as being in conflict with the relict strand plain model. We conclude that the ridge topography on the surfaces above the scarp on the New Jersey Shelf is a response to storm flows subsequent to transgression.

ABSTRACT The continental shelf on the western margin of the Cretaceous western interior seaway was a muddy surface which bore abundant northwest-southeast trending sand bodies, as much as 20 m thick and many km long. Important examples are the Medicine Hat Sandstone, the Mosby Sandstone Member of the Belle Fourche Shale, the Shannon and Sussex Sandstone Members of the Cody Shale, and the Duffy Mountain sandstone and the Tocito Sandstone Lentil of the Mancos Shale. These deposits resemble the storm-built and tide-built sand ridges reported from the modem Atlantic Continental Shelf or from the Southern Bight of the North Sea. However, although modem sand ridges may protrude from the Holocene transgressive sand sheet through overlying Holocene mud deposits to be exposed on the present sea floor, no modem examples are known where sand ridges are completely encased in mud, as the Cretaceous examples seem to have been. Hydrodynamical theory suggests that special circumstances may allow the formation of sand bodies from a storm flow regime whose transported load consists of sandy mud. Under normal circumstances, such a transport regime would deposit little clean sand. The sea floor is eroded as storm currents accelerate, but erosion ceases when the boundary layer becomes loaded with as much sediment as the fluid power expenditure will permit (flow reaches capacity). Deposition of a graded bed occurs as the storm wanes and a storm sequence is likely to consist of thin clay beds with basal sand laminae. However, slight topographic irregularities in the shelf floor may result in horizontal velocity gradients, so that the flow undergoes acceleration and deceleration in space as well as in time. Fluid dynamical theory predicts deceleration of flow across topographic highs as well as down their lee sides. The coarsest fraction of the transported load (sand) will be deposited in the zone of deceleration, and deposition will occur throughout the flow event. Relatively thick storm beds (2 to 10 decimeters) can acccumulate in this manner. Enhancement of initial topographic relief results in positive feedback; as the bedform becomes larger, it extracts more sand from the transported load during each successive storm. Individual storm beds may tend to fine upward (waning current grading), but the sequence as a whole is likely to coarsen upwards, reflecting increasing perturbation of flow by the bedform as its amplitude increases. Stability theory suggests that the end product of these processes should be a sequence of regularly spaced sand ridges on the shelf surface. However, sandstone bodies within the Cretaceous shelf deposits are quite localized in stratigraphic position and lateral distribution. Upward-coarsening sequences are a widespread phenomenon in the western interior Cretaceous system, and the sandstone bodies appear to constitute localized sand concentrations within more extensive sandy or silty horizons. Especially widespread upward-coarsening sequences appear to be due to the close coupling between activity in the overthrust belt to the west and sedimentation in the foreland basin. Each thrusting episode increased relief as well as the load on the crust. Initially, the increased relief as well as the load on the crust. Initially, the increased relief resulted in a flood of sediment transported to the shelf on the western margin of the basin so that the shelf became shallower. As it did so, wave scour on the shelf floor increased the amount of bypassing and resulted in the deposition of increasingly coarser sediment. As relief in the hinterland waned, subsidence overtook sedimentation and the shelf subsided. Renewed thursting began the cycle anew. In a second mechanism for the formation of upward-coarsening sequences, tectonic uplift affect portions of the shelf as well. The initiation of Sevier or Laramide structural elements beneath the shelf and the remobilization of other, older structures created submarine topographic highs which caused slight sand enrichment over broad areas. The development of sand-enriched areas in the shelf floor by both mechanisms led to the flow-substrate feedback behavior that built large-scale, elongate bodies of clean sand.