James M. Coleman

Brahmaputra River : Channel processes and sedimentation

Abstract The Ganges and Brahmaputra rivers combined have formed one of the largest deltas in the world, comprising some 23,000 sq. miles. These rivers originate within the Himalayan Mountains and drain an enormous land area before entering East Pakistan. Individually, each of the rivers discharges in excess of 2.5 million cusecs of water during flood, and combined they carry nearly 6 million cusecs of water to the Bay of Bengal, nearly three times the amount borne by the Mississippi River. Having such a large drainage area, the rivers are also heavily charged with sediment, transporting approximately 13 million tons of suspended sediment per day during flood. The large discharge and heavy sediment load cause the rivers to be extremely unstable, and the channels are constantly migrating laterally. Within Recent times both rivers have occupied and abandoned numerous river courses. The Brahmaputra followed a route some 60 miles to the east of its present course only 200 years ago. The long-term patterns of river migration indicate that the Ganges has been migrating eastward, whereas the preferred migration of the Brahmaputra is westward. These movements are obviously controlled by major faults or fractures in the earths crust. The Brahmaputra River displays a braided pattern in plan view, and short-term channel migration is quite drastic, with rates of movement as high as 2,600 ft. a year being common. The rate of rise and fall of the river, the number and position of major channels active during flood, the formation and movement of large bedforms, cohesion and variability in composition of bank material, and intensity of bank slumping are some of the factors responsible for controlling the bankline configuration and movement. The most significant bankline modifications take place during falling-river stage, when excess sediment is deposited as bars within the channel, causing a change in local flow direction and migration of the thalweg. Studies of bedform patterns and migration indicate that during a single flood cycle the bed configuration undergoes a definite sequence of changes. During low-river stage small bedforms (WH = 1–5 ft.) are present and migrate downstream at an average rate of 400ft./24 h. As discharge increases, the bedforms grow in size (WH = 5–20 ft.), and surface turbulence assumes a regular pattern on the water surface. During maximum-flood period some of the bedforms attain gigantic size, with heights up to 50 ft., and migrate downstream at rates as high as 2,000 ft./24 h. At other positions in the channel a plane bed exists, and surface turbulence patterns become oriented parallel to the channel axis. Although quite different in scale, this sequence is similar to that reported in flume studies. During low-river stage trenches on the various exposed bars revealed the type of bedding formed by migration of the various bedforms. Crossbedding measurements at these stations were recorded and compared to sand body trends. The combined Bengal Basin rivers deliver some 1 billion tons of suspended sediment a year to the Bay of Bengal, yet map and photo comparisons indicate that the shoreline has remained quite stable. Most of the sediment brought to the bay bypasses the bar and continues on into deeper water through a canyon called the Swatch of No Ground. Thus in deep water a subaqueous delta is being formed that dwarfs the subaerial delta, one of the largest in the world.

Variations in Morphology of Major River Deltas as Functions of Ocean Wave and River Discharge Regimes

Procedures were developed to evaluate the relative contribution of riverine versus marine forces to the construction of river deltas. Seven deltas, the Mississippi (U.S.A.), Danube (Rumania), Ebro (Spain), Niger (Nigeria), Nile (Egypt), Sao Francisco (Brazil), and Senegal (Senegal), were found to represent a spectrum of delta types reflecting process regimes ranging from fluvial-dominated, low-wave-energy, (Mississippi) to wave-dominated, low-fluvial-influence (Senegal). Deltas at the river-dominated end of the spectrum are characterized by highly irregular and protruding shorelines, a sparsity of wave-built features, and low lateral continuity of sands. Wave-dominated deltas exhibit straight shorelines characterized by well-developed barriers and beach ridges with high l teral continuity of sands. The configuration and landform suite characteristic of any given delta depend to a considerable degree on the wave power adjacent to the shore and on river discharge relative to wave forces. Nearshore wave power is not correlative with deep-water wave power but, owing to frictional attenuation, is also a function of the subaqueous slope. River-dominated shoreline configurations result only when the river is able to build flat offshore profiles; where the subaqueous slope is steep, wave-built shoreline landforms dominate the delta.

Physical Processes and Fine-grained Sediment Dynamics, Coast of Surinam, South America

ABSTRACT The prograding Holocene mud wedge between the Amazon and Orinoco Rivers in the trade wind belt of northeastern South America provides a modern-day example of muds accumulating under moderate wave-energy conditions. Gigantic shore-attached mudbanks (10 km 20 km), composed partly of thixotropic fluid-mud gel, front this coast every 30-60 km to form a buffer to wave attack and a temporary storage for fine-grained sediments. This mesotidal coast (tide range 2.0 m) with gentle offshore slope (0.0006) allows the exposure twice a day of extensive tidal flat deposits, which are backed by mangrove swamps on a well-developed chenier-plain complex. Field experiments were conducted in Surinam du ing 1975 and 1977 to provide new information on process-form relationships in this interesting but unusual muddy environment. Simultaneous measurements of waves, currents, tide elevation, suspended-sediment concentration, and variations in mud density show that soft intertidal and subtidal muds are suspended at both tide and wave frequency. Suspended-sediment concentrations typically exceed 1,000 mg/l at the surface as incoming solitary-like waves partially disperse fluid mud into overlying water on a falling or rising tide. Redeposition of mud may occur near time of high tide. The strong attenuation of shallow-water waves by these muds provides conditions that are favorable for further sedimentation. High concentrations of suspended fluid mud, together with solitary-like waves from the northeast throughout the year, can lead to extraordinarily high net sediment transport rates in the nearshore zone. Calculations based on solitary-wave theory and on data obtained from this study indicate that 15-65 106 m3 of mud can move along shore each year without involving breaking waves, the concept of radiation stress and a nearshore circulation cell, or bedload transport. Farther offshore, outside the zone of wave dominance, wind-driven currents and the Guiana Current combine to transport muds to the northwest, consistent with the observed direction of mudflat migration.

Morphology of a submarine slide, Kitimat Arm, British Columbia

A digitally acquired, scale-corrected side-scan sonar survey yielded high-resolution imagery of a submarine landslide in British Columbia. The landslide, in a fjord-head setting at Kitimat, was last active in 1975 and created a wide area of deformed sea floor. The sediment failure involved shallow rotational movements on the slopes of a fjord-head delta, marginal tearing, translational sliding, compressional folding, and block gliding of fjord-bottom marine clays. The slide is shallow and elongate and appears to have been produced by failure in mobile, low-strength sediments.

Wetland Loss in World Deltas

Abstract Geologic and geomorphic data on 42 world deltas were compiled for a NASA-sponsored research project. Satellite images from 14 of these deltas (Danube, Ganges, Brahmaputra, Indus, Mahanadi, Mangoky, McKenzie, Mississippi, Niger, Nile, Shatt el Arab, Volga, Huang He [Yellow], Yukon, and Zambezi) were analyzed for delta plain wetland loss caused by natural causes and conversion of wetlands for agricultural and industrial use. These analyses indicated that a total of 15,845 km2 of wetlands have been irreversibly lost during the past 14 years and the average rate of loss is 95 km2/y. If a similar trend is present in the other deltas, a total wetland loss in the delta plains of the 42 deltas would be on the order 364,000 km2 over the past 15 to 20 years.

Active Slides and Flows in Underconsolidated Marine Sediments on the Slopes of the Mississippi Delta

On the continental shelves off large deltas, rapid progradation and deposition result in highly underconsolidated marine sediments. These deposits, which are often also rich in interstitial methane gas, can be subject to widespread and active mass movement downslope. For example, the submarine slopes of the Mississippi River delta are affected by a variety of sediment instability processes. Geologic and geophysical surveys using side-scan sonar, subbottom profilers, and precision depth recorders have been completed for the entire subaqueous delta. Survey lines were spaced at 240-m intervals, and water depths ranged from 5 m to 20 m. Bottom morphology, including sediment deformations indicative of instability, has been mapped at a scale of 1:12,000, and large-area, scale-corrected sonar mosaics have been constructed. The features identified include collapse depressions, bottleneck slides, shallow rotational slides, mudflow gullies, overlapping mudflow lobes, and a wide variety of faults. The slides and mudflows are extremely active, and movement rates of several hundred metres per year have been recorded. Damage to offshore oil and gas pipelines and platforms has occurred. Also, the concept of slow, continuous deltaic progradation must be modified to include the effects of these processes. For example, on the shelf, normal settling of suspended clays averages only a few millimetres per year, whereas at the front of the delta slope more than 30 m of sediment has been deposited by mudflows and slides since 1875.

Evolution of Atchafalaya lacustrine deltas, south-central Louisiana

Abstract The entrenched Pleistocene/Holocene Mississippi alluvial valley was cut during the last low sea-level stand, and alluviation of the entrenched valley occurred during the Holocene sea-level rise. Regional Gulf Coast structures (salt domes and faults) have strongly influenced the morphology of the valley. Valley paleotopography has controlled depositional patterns and locations of major river courses, as well as partially isolated large interdistributary basins within the alluvial/deltaic plain. Interdistributary basin formation is further enhanced by distributary channel progradation, switching, and abandonment, in conjunction with compactional subsidence of the alluvial plain surface. Geomorphology and sediment cores from the Atchafalaya Basin, Louisiana, revealed that this basin has alternately accumulated fine-grained organic-rich sediments and terrigenous clastic-dominated meander-belt and lacustrine deposits. Basin subsidence and the presence of an active distributary channel in the basin are major determinants of the lithology of the sedimentary-fill. Prograding lacustrine deltas of the Atchafalaya River are the primary mechanism for filling the Atchafalaya Basin. These upward-coarsening deltas are relatively thin (3 m) but are areally extensive (tens to hundreds of kilometers), and because they occur so frequently during the infilling process, they account for a significant volume of the basin sequence. The Atchafalaya Basin sequence documents lacustrine deltaic episodes dating from the early Holocene (Maringouin and Teche deltas) to the present (Atchafalaya delta). Continued development of the Atchafalaya River will rework portions of the backswamp and lacustrine deposits into meander-belt deposits. Upon complete filling of the Atchafalaya Basin, sediment will mostly bypass the basin and become deposited in the marine delta in Atchafalaya Bay.

Minor sedimentary structures in a prograding distributary

Abstract Minor sedimentary structures were studied in cores taken at the mouth of a small prograding distributary within the Mississippi River delta. The mouth of Johnsons Pass in Garden Island Bay was mapped and the following environments were recognized: subaerial and subaqueous natural levee, channel, distributary mouth bar, interdistributary bay and marsh. Oriented, undisturbed cores were taken from each environment. These cores were split, dried and photographed and the types of minor sedimentary structures within each environment were tabulated. Natural levee deposits contained abundant current ripple laminations, unidirectional cross-laminations, parallel and wavy laminations, distorted layers and burrowed oxidized silty sands. Channel fill deposits consisted of alternating beds of clay and silt containing trough cross-laminations, scour and fill structures and distorted layers. The distributary mouth bar, composed predominantly of silt and sand, is characterized by a variety of small-scale multi-directional cross-laminations and air-heave structures. Two types of interdistributary bay deposits were recognized, homogeneous clay with scattered brackish-water fauna and a predominantly clay section with thin parallel and lenticular laminations and ripple marks. The structures within these two types are a reflection of availability of coarse detritus. Marsh deposits are characterized by the abundance of peat, carbonaceous clays, calcareous nodules and root disturbances. Each environment is characterized by a distinct assemblage of structures. These assemblages form a valuable tool for use in interpreting palaeo-environments in ancient sedimentary rocks.

Disintegrating retrogressive landslides on very‐low‐angle subaqueous slopes, Mississippi delta

Abstract Side‐scan sonar records from the interdistributary bay areas of the Mississippi delta (East Bay, Garden Island Bay, and shallow water areas adjacent to Pass a Loutre) have shown widespread subaqueous disturbance of the bottom sediments. These occur in shallow water and on slopes with very low inclinations (0.01°‐0.45°). The morphology of the features is indicative of mass movement processes involving subsidence and downslope translatory movements. The precise conditions under which failure occurs have not been fully documented, but a conceptual model of potential factor interaction can be formulated.

Morphology and dynamic sedimentology of the eastern Nile delta shelf

Coleman, J.M., Roberts, H.H., Murray, S.P. and Salama, M., 1981. Morphology and dynamic sedimentology of the eastern Nile delta shelf. Mar. Geol., 42: 301-326 Some 13,000 km of bathymetry and overlapping side-scan sonar data were collected over a 4450-km2 area of the eastern Nile River shelf. These data, coupled with more detailed surveys of selected areas, including bottom sampling and current measurements, contribute to the understanding of shelf morphology, lithofacies relationships, and sediment transport processes. Four morphologic zones were defined: Zone 1. Smooth-bottomed, flocculated clays and silty clays extending from the shore-line 40 km seaward to the 25-m contour. Zone 2. A sand belt (5-20 km wide) extending eastward from the Damietta mouth and curving toward the coast at the boundary of the study area. Dominant bottom features were migratory bedforms of various dimensions superimposed on linear sand ridges. Zone 3. Smooth clay to silty-clay bottom topography punctuated with distinct algal mounds. The mounds (up to 10 m relief) are composed of living coralline algae, which contribute coarse debris to the surrounding sediments. Zone 4. Mud diapirs and shelf-edge slumps characterize this distal part of the shelf and the upper slope. Sand on the inner shelf is actively migrating and probably does not represent a relict deposit. The Damietta distributary causes a large-scale perturbation in the mean easterly drift along the Egyptian coast. Unusually strong currents, capable of transporting and reworking large volumes of sand, are associated with an eddy trapped behind this feature. The directionality of these currents and the curvature of the eddy axis correspond well to the distribution pattern of sand on the inner shelf.