Abstract Holocene rivers have a high degree of morphological variability, and many researchers see a continuum of channel forms that are transitional between end-member planforms such as braided, meandering, or straight. Individual rivers also show a high degree of longitudinal and vertical (through time) variability, as a result of changes in base level, climate, tectonics, tributary contribution, and/or valley slope. This high degree of variability in plan-view morphology of Holocene rivers is generally not reflected in interpretations of ancient fluvial deposits, which continue to be interpreted as meandering or braided. Many of these interpretations are suspect because of myths about modern rivers and because the characteristics described bear little relationship to plan-view morphology. Modern fluvial facies models are built around the nature and type of channel bars. Knowledge of the architecture of bar forms and their occurrence in Holocene river channels has been greatly enhanced in the past two decades with data gained from ground-penetrating radar (GPR) studies. In spite of our expanded data base for modern rivers and their deposits we still need additional data on width-to-thickness ratios and architecture of deposits of aggrading river systems. A review of the literature suggests that data on channel bar deposits is lacking in most studies of ancient fluvial deposits. The few research efforts that describe paleochannel bars in 3D and 2D exposures of ancient fluvial deposits suggest that there was considerable variability in paleochannel form within a given formation. Our focus in the study of ancient fluvial deposits should shift from interpreting planforms to description and interpretation of preserved bar forms and how river systems evolved. In this regard many ancient fluvial sheet sandstones need to be re-examined.

Abstract The goal of this research is to develop a predictive model for use in hydrocarbon exploration and risk analysis/mitigation that permits estimation of the sealing capacity of marine mudrocks. Research to date has concentrated on Cretaceous shales of the Western Interior foreland basin in Colorado and Wyoming and Eocene shales in the Ventura basin, California. This research focuses on Eocene rocks of the central Pyrenean foreland basin, Spain. Outcrop mudrock samples were collected from areas around Ainsa, Broto, Undues, and Anso in northeastern Spain. Geologically the areas sampled include the Ainsa basin slope, Broto base of slope, and the Jaca outer fan and basin plain in the south-central Pyrenees. Extensive research to define stratigraphic relationships, depositional environments, basin type, and sandstone body geometry have been undertaken, however the nature and sealing capacity of included mudrocks have been largely ignored. The Eocene Ainsa basin, from which most of our samples were taken, originated as a foredeep ahead of a thrust ramp and evolved into a piggy back setting as the thrust migrated basinward. This foredeep was filled with up to 4000m of slope sandstones and mudrocks, had a source area to the southeast and a basin plain (the Jaca basin) to the northeast. Sealing capacity of these mudrocks was determined by mercury injection-capillary pressure (MICP) measurements conducted by Poro-Technology on 43 samples representing a range of slope, base of slope, and basin floor environments. Capillary pressure curves generated during mercury injection have been used to evaluate sealing capacity by equating it to the pressure required to achieve 10%mercury saturation. Pore throat diameter was determined from data generated during the MICP analyses. Bioturbation was characterized on a qualitative scale of 0 to 6. Mean quartz grain size was determined by measuring the apparent long axes of thirty quartz grains per sample. Total organic carbon (TOC) and CaCO 3 were determined by analytical techniques in the soils laboratory at Colorado State University Samples analyzed range from laminated and bioturbated, calcareous mudstones to silty biomicrites and calcareous siltstones. Some samples contain thin, normally graded, laminations of silt-size quartz grains. MICP values at 10%saturation range from 500 PSIA to more than 60,000 PSIA. Despite the high degree of variability in samples from a single outcrop and from each sampled area there is a general correspondence with geographic and sequence stratigraphic setting. Furthermore, significant correlations exist between MICP and sample porosity, pore aperture diameter, standard deviation of pore diameter, and TOC. As expected sample porosity, pore diameter and standard deviation of pore diameter are inversely related to sealing capacity. TOC is directly related to sealing capacity. Degree of bioturbation is more variable in samples with low sealing capacity and is generally lower in samples with high sealing capacity. There are no apparent correlations between MICP and permeability, grain density, average quartz grain size, standard deviation of quartz grain size, quartz grain roundness, standard deviation of quartz grain roundness, or CaCO 3 content. From a depositional setting and sequence stratigraphic viewpoint proximal slope deposits have the lowest average sealing capacity, highest porosity and permeability, highest pore aperture diameter and most poorly sorted pore diameters, highest overall grain size and degree of bioturbation, and lowest TOC. Basin floor deposits have the highest average sealing capacity, lowest porosity and permeability, smallest pore aperture diameter, and best sorted pore diameters, lowest overall grain size and degree of bioturbation, and highest TOC. Compositional variables, other than TOC, are less important than textural variables in determining the sealing capacity of these mudrocks. Factors that influence the relative sealing capacity of these carbonate-rich mudstones are generally similar to those for non-calcareous shales from other foreland basins and include overall grain size, degree of disruption of depositional fabric by bioturbation, pore throat size and sorting, TOC, and depositional setting within the basin. The variables that most strongly favor high sealing capacity are most likely associated with deposits of deep water anoxic environments, hence the common association between good seals and upper transgressive systems tract deposits and condensed sections.

Abstract A predictive model to estimate the distribution, sealing capacity and petrophysical properties of shale seals and flow barriers will significantly reduce the risks associated with hydrocarbon exploration and exploitation. Such a sequence stratigraphy-based predictive model must be grounded in outcrop and field analogs, such as this examination of the sealing capacity, petrophysical properties and distribution of Upper Cretaceous Lewis marine shales in two wells from south-central Wyoming. The measured sealing capacity of these shales varies with textural and compositional factors that allow division of the Lewis Shale depositional sequence six argillaceous microfacies. Each microfacies displays distinctive compositional and petrophysical properties and occupies a well-defined sequence stratigraphic position including transgressive, highstand, and condensed section deposits, with characteristic seal and seismic properties. The microfacies, in order of greatest seal capacity to least, are phosphatic shales, pyritic fissile shales, silty shales, silty calcareous shales, silty calcareous mudstones, and bioturbated argillaceous siltstones. The most promising seals, the phosphatic and pyritic shales, belong to the condensed section and uppermost transgressive systems tract. The phosphatic shale is also characterized by the highest content of both total organic carbon (TOC) and authigenic minerals. Interestingly, neither of these two high sealing capacity microfacies shows more detrital clay than other microfacies. The microfacies with lower sealing capacities belong to the highstand systems tract and are generally poorer in iron-rich minerals than the better sealing microfacies. Petrophysical properties, including high bulk density, shear velocity, Young’s modulus and shear modulus, distinguish the best sealing microfacies from highstand systems tract microfacies with poorer seal capacity. This correspondence between sealing capacity and petrophysical properties suggests that seismic data may have good potential as a tool for seal evaluation.

Abstract The sequence stratigraphic model, with its initial emphasis on eustatically driven controls on sedimentary sequences, has generated considerable interest in the ultimate controls on alluvial successions. Arguments regarding controls on alluvial sequences have been ongoing for many decades. Proposed controls include global (eustatic) or local base-level fluctuation, climate, tectonics, and sediment supply. Since sediment supply is in general a function of one or more of the other three controls, the possible number can be reduced to three. Most models and explanations for fluvial successions are too simplistic. They attempt to explain these successions on the basis of a single controlling factor. Additionally the models commonly fail to take into account modern geomorphic concepts of complexity and ignore the fact that controls other than base-level fall can produce incision and that base-level lowering does not always result in incision and rejuvenation of a fluvial system. They also fail to realize that base-level fluctuations may have their greatest effect only in the lower reaches of a fluvial system and that the amounts of sediment produced by incision alone cannot account for the volume of sediment observed in most stratigraphic sequences. For these reasons it is difficult to justify the application of systems tracts designations to upstream portions of fluvial valley-fill successions. Field and analog experiment studies demonstrate the difficulty of distinguishing various controls in fluvial systems because (1) similar erosional and depositional features and sequences can be produced by different processes and vice versa and (2) different levels of sensitivity may result in a minor, a major, or no response of a system to an extrinsic perturbation.

Abstract Valleys are formed by both erosional and tectonic forces, although the former is the most common. Most valleys form by channel incision, and they follow an evolutionary sequence of deepening and widening that is controlled by the lithologic and structural character of the valley perimeter and the erosive power of the river that is eroding the valley. Usually valleys are single features that are contained by valley walls, but on alluvial plains a complex anastomosing network of multiple valleys can develop as a result of channel avulsion. In general, valley dimensions increase down valley, and older valleys are larger than younger valleys, but the main characteristic is great variability both in cross section and longitudinally. Unlike stream channels formed in relatively homogeneous alluvium that reflect hydrologic and hydraulic variables, valley morphology also reflects lithologic and structural controls. Valley-fill deposits should also reflect this variability to a lesser extent, as channel morphology varies in response to variations of valley slope and to tributary influences.

The Gallup Sandstone represents one of a few regressive facies of a succession of Upper Cretaceous regressive-transgressive clastic wedges in northwest New Mexico. In the Gallup sag, the formation overlies the Mancos Shale and underlies and intertongues with the Crevasse Canyon Formation, and may be grouped vertically into lower sandstone-dominated, middle coal-bearing, and upper sandstone-dominated lithofacies intervals. The lower sandstone-dominated lithofacies interval represents delta-front, barrier, and tidal environments. This lithofacies grades upward into the coal-bearing lithofacies interval that represents delta-plain and coastal-plain environments. Peat coals were formed in swamps in both interdistributary and floodplain areas; however, the thickest accumulations were in swamps associated with detrital-free, abandoned channel, and interchannel areas in the lower and upper delta-plain environments. The coal-bearing lithofacies is overlain unconformably by the upper sandstone-dominated lithofacies interval, which is interpreted as deposits of an alluvial valley drained by braided rivers passing downstream into rivers having meandering and straight channels. These lithofacies intervals record the evolution of a northeast-prograding fluvial system that drained the coastal plain and alluvial valley along the southwestern margin of the Western Interior, Late Cretaceous seaway.

Abstract Several hundred papers on fluvial sedimentology have appeared since the first fluvial conference 9 years ago (Calgary, 1977). Many variants of existing models have been published, including several sub-models of high-sinuosity rivers based on sediment load grain size, and the anastomosed model has become well documented and widely known. The main advance in the study of low-sinuosity rivers is the recognition of mid-channel macroform bar complexes comparable in their medium- to long-term geomorphic significance to point bars. Increasing attention is gradually being paid to the overbank environment, with the development of floodplain facies models, and an improved understanding of the conditions necessary for coal formation. Few new syntheses of facies models have been attempted. The current research trend is toward documentation of three-dimensional architecture, both on the local scale (channels and complex bars) and on the regional scale, using two- and three-dimensional lateral outcrop profiles and employing chronostratigraphic markers to tie sections together.

Abstract The development of three separate devices at Birkbeck College, London, and their deployment in a gravel-bed stream allows us to pinpoint entrainment thresholds, to measure bedload continuously and to quantify the ingress of matrix fines. This provides a comprehensive picture of processes and gives clear indication of the reasons why gravel beds appear to react unpredictably to flood flows and why a universal bedload equation has yet to be developed. We show that ubiquitous bed microforms (e.g., pebble clusters) delay initial motion so that stream power is 5 times greater than it is when bedload transport ceases. We also show that low-flow ingress of matrix fines increases the difference to 16 times by increasing shear strength of the stream bed. Our continuous monitoring devices indicate a regular pulsation of bedload transport (mean wave period 1.7 hrs) that cannot be attributed to deficiencies in the samplers because it is corroborated by distinctly different and independent instruments; the pulses are not explained by changes in hydraulic parameters. We suggest that they are kinematic waves of mobile bed particles, the first dynamic manifestation of this phenomenon in a natural stream. Our data give a clear indication of those facets of water-sediment interaction that must be investigated in order to arrive at a general bedload transport equation for gravel-bed streams, and the data highlight the naive assumptions currently made in paleohydraulics when using clast size as an indicator of paleo-flow conditions.

Abstract An electromagnetic coil placed in the bed of a stream can provide instantaneous real-time electronic data on the motion of coarse, naturally-magnetic, bed material. Data collected in May 1982 show that sediment transport of material larger than 32 mm occurred on the bed during falling stage, began somewhat gradually, and ended nearly instantaneously. Because transport data were not averaged, transport waves of material larger than coarse pebbles can be seen on the record. During early parts of the transport event, the period of the waves was less than 1 min. By the end of the event, waves were occurring at approximately 6-8 min intervals. In 1985, the device detected the motion of coarse pebbles and cobbles in one part of a stream, while other parts of the stream showed no motion. If background noise problems can be overcome, the device has the potential to provide insight into the nature and location of pebble and cobble transport on the bed of gravelly streams and the conditions which lead to that transport.

Abstract Flow dynamics at river channel confluences can be characterized by six major regions of flow stagnation, flow deflection, flow separation, maximum velocity, flow recovery and distinct shear layers. The dominant controls upon the magnitude of these regions are shown to be the junction angle and the ratio of discharges between the confluent channels. Through the combined use of scaled laboratory modelling and an analysis of field evidence, the dynamics of flow are found to produce a confluence morphology which consists of avalanche faces at the mouth of each confluent channel, a deep central scour and a bar within the separation zone. Tracing of sediment in both laboratory and natural channels reveals distinct sediment pathways within the junction which can be explained through the model of flow dynamics. A knowledge of confluence flow dynamics is important when assessing channel design criteria, junction bed morphology and ancient confluence sediments.

Abstract Grain size sorting in channels of the Rio Grande varies with location within ripples and megaripples and within downstream progressions of bedforms. In the natural sand-dominated floodway channel, plane beds and ripples commonly have log-hyperbolic grain size distributions. Under waning flow with minor suspended sediment, late-forming ripples contain unimodal (0.15 mm) log-hyperbolic distributions. Where these ripples have overtaken and mixed with earlier formed migrating coarser grained ripples, bimodal (0.15 and 0.30 mm modes) ripples result. These mixed ripple distributions may be modelled mathematically by mixing hyperbolic distributions. In subjacent dunes formed from mixed ripples overtopping the dunes’ brinks, upper foresets are slightly bimodal. Toesets are also bimodal, but lower foresets are more poorly sorted and approximate one or more hyperbolic distributions. In a 50-m reach of dunes, upper foresets contain progressively more pebbles downstream. Waning flow in the artificial Rio Grande conveyance channel left gravel-dominated straight-crested megaripples with entirely different grain size distributions. Gravel armor developed from the troughs to the crests of the 2.8-m-wavelength megaripples. Both troughs and crests have distributions that exhibit broad “plateaus” from 0.25 to about 10 mm, and both have secondary modes at 0.063 mm. Upper slipfaces have roughly log-hyperbolic distributions, but contain irregular amounts of coarse and fine grains. Lower slipfaces have log-hyperbolic distributions with modes ranging from 1.4 mm upstream to 0.71 mm downstream. The slipfaces deposited during waning flow are better sorted and finer grained than the foresets of the megaripple interiors, which are composed of a mixture of upper and lower slipface grains along with even coarser clasts.

Abstract Significant attributes of the Kosi fan are its flat surface, fine-grained, highly bioturbated sediments, perennial water saturation, and abundant abandoned channel facies. Pre-existing drainages controlled fan shape, and disparate monsoon and dry season discharges cause some channels to be vegetated and muddy while simultaneously containing sandy point bars. Similar fans should be recognizable by isopach-defined wedges, tectonic position, bowing of grain-size isograds away from the orogen, clustering of large channels, and interfingering with interfan facies.

Abstract This study follows the pioneering work of Coleman (1969) and presents the results of a study of a 200-km-long reach of the Brahmaputra River in Bangladesh. Channel pattern and migration has been monitored using LANDSAT imagery and historic maps. Channel pattern in the Brahmaputra is varied. The river is mostly braided but some reaches are anastomosed or meandering. Channel movement is dominated by lateral migration with some minor channel switching and one major avulsion in the last 200 years. There is a hierarchy of channels in the Brahmaputra. First-order channels encompass the whole river and may comprise several second-order channels which, in turn, have third-order channels within them. The channels divide and rejoin around bars which scale with the bankfull width and depth of each channel. Bar types include lateral (point) bars, diagonal bars, medial bars and tributary bars. Bars sometimes coalesce to form semi-permanent bar assemblages. Channel cross sections observed by echo-sounding show a high degree of channel asymmetry and extremely complex cross sections which can be related to bar development. The pattern of deposition is divided into four styles: new mid-channel bars (13 percent), channel abandonment (15 percent), lateral accretion to bank (19 percent) and addition to bars (53 percent). The latter is the most important and is subdivided into three elements of deposition: upstream accretion (14 percent) downstream accretion (29 percent) and flank accretion (57 percent).

Abstract Accretion in meandering rivers does not exclusively occur in point bars. This is demonstrated by the River Tywi, a meandering gravel-bed stream in South Wales, in which deposition occurs at the concave sides of meander bends. These deposits, termed counterpoint bars in this paper, form at sites commonly assumed to be ones of erosion. There are two types of counterpoint bar in the Tywi; fine-grained examples, similar to the concave bank benches of Page and Nanson (1982), and large gravel bars. Gravel counterpoint bars are a little known type. They are bulbous to elongate in plan view, are as much as 30-40 m wide and 60-70 m long. They consist of a bar platform, secondary channel and riffle. Gravel counterpoint bars appear to have a relatively high preservation potential in the Tywi valley. This is suggested by historical maps and the presence of several abandoned examples on the floodplain. These form elongate and bulbous assemblages of two or more bars. Morphology and sedimentary structures are distinct from the more familiar fine-grained examples described from other rivers; however, gravel counterpoint bars may be more difficult to distinguish from gravel point bars if preserved in the geologic record.