John S. Bridge

Three-dimensional model of alluvial stratigraphy; theory and applications

ABSTRACT A three-dimensional model of alluvial stratigraphy has been developed to simulate the spatial distribution, proportion, and connectedness of coarse-grained channel-belt deposits in alluvial strata as a function of channel-belt width, floodplain width, bankfull channel depth, channel-belt and overbank sedimentation rates, avulsion location and period, compaction, and tectonism (tilting and faulting). In this model, a floodplain surface of variable width and length is occupied by a single channel belt. Changes in floodplain topography are produced by spatial and temporal variation of channel-belt and floodplain deposition rates and by compaction and local or regional tectonism. The location and timing of avulsions are determined by local changes in floodplain slope relative to channel-b lt slope and by flood magnitude and frequency. The diverted channel belt follows the locus of maximum floodplain slope. At the end of each simulation, architectural parameters are calculated, including channel-deposit proportion and connectedness and the dimensions of channel-belt sandstone bodies. Three-dimensional perspective diagrams, mesh surfaces, and two-dimensional stratigraphic sections can be plotted to illustrate depositional surfaces (time planes) and the location and geometry of coarse-grained channel-belt deposits within finer-grained overbank deposits. The model predicts that channel-belt proportion and connectedness and dimensions of sandstone bodies vary as a function of distance from avulsion points and cross-section orientation. Upstream from avulsion points, sandstone bodies have low width/thickness ratios because of aggradation in a fixed channel belt. Immediately downstream from avulsion points, channel belts tend to be connected, resulting in sandstone bodies with high width/thickness ratios. Avulsion sequences develop where points of avulsion shift up valley with a progressive decrease in avulsion period. Such sequences may produce successions in which channel-belt proportion and connectedness vary vertically with a cyclic period of 103 to 105 years. Down-valley increases in aggradation rate or down-valley decreases in floodplain slope (for example, associated with a rise in base level) may result in an increase in channel-belt proportion and connectedness because of high avulsion frequencies in down-valley regions of the floodplain. Down-valley decreases in aggradation rate (as in alluvial fans, foreland basins, and during base-level fall) may result in high avulsion frequencies in up-valley parts of the floodplain. Tectonic tilting and faulting locally increase avulsion probabilities, and channel belts generally shift toward areas of maximum subsidence. Under certain conditions, however, depositional topography may cause channels to shift away from areas of maximum subsidence. Channel-deposit proportion and connectedness are gen rally high near downthrown areas of the floodplain, but distribution (clustering) of channel belts may not be a reliable indicator of fault geometry or displacement. Models of alluvial architecture that consider only sediment accumulation rate as the main controlling factor are oversimplified. The three-dimensional model presented here predicts many of the features of channel behavior observed in modern rivers, but there is a pressing need for better models and adequate natural data to test them.

A model for the entrainment and transport of sediment grains of mixed sizes, shapes, and densities

A model for the entrainment and bed load transport of sediment grains of different sizes, shapes and densities by a unidirectional turbulent flow is developed in terms of (1) sediment types available for transport; (2) the mean and turbulent fluctuating values of fluid forces acting upon the sediment grains; and (3) the nature of the interaction between turbulent fluid forces and available sediment, resulting in entrainment and transport of grains as bed load or in suspension. The behavior of the model is explored extensively, and compared with natural data from flumes and rivers. The predicted threshold of entrainment of individual size fractions within a mixed-size bed agrees well with observations as long as the pivoting angle is specified appropriately as a function of grain size. The rate and size distribution of bed load transport generally agrees well with natural data as long as effective bed shear stress in the presence of bed forms can be defined, bed load transport measurements are reliable, and the size distribution of available sediment is accurately specified.

Interpreting the dimensions of ancient fluvial channel bars, channels, and channel belts from wireline-logs and cores

A primary objective in exploration for and development of fluvial reservoirs is determining the thickness and width of sandstone-conglomerate bodies (mainly channel-belt deposits). Most of the existing techniques for estimating the dimensions of fluvial reservoirs have major drawbacks. A fresh approach to the problem is made using recent theoretical, experimental, and field studies. This new approach involves (1) new models for the lateral and vertical variation of lithofacies and petrophysical-log response of river-channel deposits with explicit recognition of the different superimposed scales of strata, (2) distinction among single and superimposed channel bars, channels, and channel belts, (3) interpretation of maximum paleochannel depth from the thickness of channel bars and the thickness of sets of cross-strata formed by dunes, and (4) evaluation of various methods for estimation of widths of sandstone-conglomerate bodies that represent either single or connected channel belts (outcrop analogs; correlation of sandstone-conglomerate bodies between wells; use of empirical equations relating channel depth, channel width, and channel-belt width; theoretical models; and three-dimensional seismic data).

Quantitative interpretation of sedimentary structures formed by river dunes

ABSTRACT New experimental data from a range of flumes and rivers have been used to test and develop a theoretical model for the thickness of sets of cross strata formed by the migration of subaqueous dunes. The distribution of dune height can be calculated directly from the distribution of cross-set thickness, and vice versa, independently of aggradation rate. Mean dune height is approximately 2.9 (± 0.7) mean cross-set thickness. The new models relating dune height to cross-set thickness have been tested successfully with data from flumes and from the Calamus and Mississippi Rivers. Furthermore, it is well known that dune height can be related approximately to formative flow depth. Although prediction of flow depth from cross-set thickness and dune height is imprecise, it provides a useful complement to other methods of estimating flow depth from sedimentary information.

Paleochannel patterns inferred from alluvial deposits; a critical evaluation

ABSTRACT Interpretation of paleochannel patterns from ancient alluvium involves reconstruction of sinuosity and degree of braiding. It is emphasized here that the geometry, flow, and sedimentary processes of the different channel patterns form a continuum, such that similarities between their deposits are more significant than differences. While many commonly cited sedimentary criteria for distinguishing channel patterns are shown to be invalid, the following are considered useful: 1) proportion of channel fills relative to lateral-accretion deposits, which increases with degree of braiding; 2) mean grain size of channel fills relative to lateral-accretion deposits, which decreases with increasing sinuosity; 3) paleocurrent variance, which may indicate sinuosity; and 4) bankfull discharge, slo e, and width/depth of paleochannels throughout the channel belt, as long as they can be quantitatively reconstructed from channel-bar and channel-fill deposits. Utilization of these criteria requires examination of completely preserved sections of channel belts and overbank deposits in large outcrops. Presently existing facies models do not convey the diversity of different channel types and facies; those that are not three-dimensional and lack scales and paleocurrent information are of limited value.

Geometry, Lithofacies, and Spatial Distribution of Cretaceous Fluvial Sandstone Bodies, San Jorge Basin, Argentina: Outcrop Analog for the Hydrocarbon-Bearing Chubut Group

ABSTRACT Fluvial deposits of the Cretaceous Chubut Group, San Jorge Basin, Argentina, were studied in outcrop to provide analogs for adjacent subsurface hydrocarbon-bearing strata. Outcrops were described using photomosaics and detailed sedimentological logs. Particular attention was paid to describing the geometry (e.g., width, thickness), lithofacies, and spatial distribution of sandstone bodies. Sediment accumulation rates were calculated using radiometric ages obtained from the tuffs and ignimbrites that are an important component of these strata. Interpretation of depositional environment included quantitative reconstruction of the geometry, hydraulics, and mode of migration of paleochannels. The proportion, connectedness, and spatial distribution of channel-belt sandstone bodies were interpreted using alluvial stratigraphy models. Sandstone bodies are generally meters thick and tens to hundreds of meters across (normal to paleocurrent direction). Channel-form sandstone bodies represent channel bars and fills within channel belts, whereas sandstone sheets, wedges, and lenses represent the deposits of overbank sheet floods, levees, and crevasse splays, respectively. Most of the rivers were single-channel and sinuous (sinuosity less than 1.2), but there were also braided rivers. The rivers flowed eastward and were perennial. Individual channel widths were on the order of tens of meters (mainly 35 to 65 m) and maximum channel depths were on the order of meters (mainly 2 to 6 m). The thickest and widest sandstone bodies (up to 16 m thick and in excess of 1 km wide) represent the largest channel belts or superimposed channel belts. Inasmuch as the proportion of channel-belt deposits is generally less than 0.5, most channel belts are unconnected. Channel-deposit proportion varies laterally and vertically on a 100-m scale. These variations are related to changes in the dimensions of channel belts, but they may also be related to variations in the deposition rate, floodplain width, and the timing and location of avulsions. These factors may in turn be related to intrinsic fluvial processes, tectonic tilting of the floodplain, or variations in sediment supply related to climate, tectonism, and igneous activity. Thickness and orientation of the sandstone bodies are similar to those interpreted from adjacent subsurface data. However, the width of subsurface sandstone bodies estimated from well-to-well correlation is greater than measured in outcrop. This discrepancy is because: (1) subsurface sandstone-body width less than the well spacing (typically 300 m) cannot be resolved; (2) the width of some of the subsurface sandstone bodies may be overestimated in well-to-well correlation; and (3) the full extent of the widest sandstone bodies cannot be observed in the smaller outcrops.

Large-scale facies sequences in alluvial overbank environments

ABSTRACT Gradational coarsening-upwards or fining-upwards sequences (order of meters thick) in overbank environments can be produced by progressive crevasse-splay/levee progradation or abandonment while the channel belt remains unchanged in position. In contrast, meter-scale facies sequences related to periodic channel-bell avulsion may show relatively abrupt changes in facies and paleocurrent direction. Simulation models suggest that the number of such avulsion-related facies changes in any overbank sequence bounded above and below by channel-belt deposits is on the order of 1 to 10; this is confirmed using an ancient example. In this example the average thickness of each avulsion-bounded overbank facies sequence was 3 m: assuming an average recurrence interval for avulsions of about 1,000 ye rs, the mean overbank deposition rate was about 3 mm per year.

Fluid and sediment dynamics of upper stage plane beds

To understand more fully the fluid and sediment dynamics of upper stage plane beds, laboratory experiments were conducted using mobile and fixed beds where turbulent motions of fluid and sediment were measured using laser anemometry. Bed-elevation fluctuations on mobile upper stage plane beds reveal millimeter-high bed waves. Vertical profiles of flow velocity, mixing length, and eddy viscosity (diffusivity) are represented well by the law of the wall. For the mobile bed, von Karman/s κ ≈ 0.33 and equivalent sand roughness to mean bed-grain size varies from 9 to 17 because of the presence of bed load and low-relief bed waves. For fixed beds with no sediment transport, κ ≈ 0.41 and equivalent sand roughness is equal to the mean bedgrain size. The decrease in κ for mobile beds is related to the relative motion of grains and fluid. Mobile-bed turbulence intensities are greater than those for sediment-free fixed beds because of enhanced wake formation from the lee side of near-bed grains and low-relief bed waves. Sediment diffusivities es calculated in a similar way to fluid diffusivities e indicate that es≈e. Sediment diffusivities calculated using the equilibrium balance between upward diffusion and downward settling of sediment are similar to e in near-bed regions (y/d 0.3, suggesting that larger, more energetic turbulent eddies are responsible for sediment suspension higher in the flow.

Ground penetrating radar: application to sandbody geometry and heterogeneity studies

Abstract Ground penetrating radar (GPR) offers a high-resolution, shallow subsurface profiling technique for use in sedimentological and reservoir analogue studies. GPR is similar to seismic reflection profiling but uses electromagnetic radiation in the 50 to 500 MHz frequency range (in geological applications). By using these relatively high frequencies, high resolution data can be obtained. Short duration pulses of electromagnetic energy are transmitted into the ground, reflected from interfaces across which there are abrupt changes in dielectric properties, and are detected by a receiver. These received signals are displayed in nanoseconds two-way time and may be recorded digitally allowing subsequent processing. Some 1000 m of 2D GPR profiles were collected from a modern point bar on the Madison River, Montana USA and have been interpreted using an approach similar to seismic stratigraphic analysis. This has allowed identification of a number of radar sequences and radar facies. Radar sequence boundaries are identified by reflector terminations (onlap, downlap, toplap and erosional truncation) and represent episodes of erosion during the development of the point bar. In contrast, radar sequences and their component radar facies record phases of accretion of the point bar. Each radar sequence is linked to a discrete accretionary unit that can be mapped on the surface of the point bar. Mapping of the radar sequences and radar facies has allowed quantification of their 3D geometry.

A revised model for water flow, sediment transport, bed topography and grain size sorting in natural river bends

A revised model for the interaction of water flow, bed topography, and the rate and mean grain size of bed load in river bends is presented and compared with a large range of observational data. The model represents a modification of the approaches used by F. Engelund, J. S. Bridge, G. Parker, S. Ikeda, and coworkers. Model predictions generally agree with data from the River South Esk, Muddy Creek, River Dommel, and Hookes laboratory channel. This simple model apparently performs at least as well as more complicated flow models. To make further improvements it is necessary to more accurately specify the interaction between local bed shear stress, sediment transport, and the local bed configuration.