Distributary Channel Network Research Articles

Palaeohydrology of the Fazzan Basin and surrounding regions: The last 7 million years

We have integrated information on topography, geology and geomorphology with the results of targeted fieldwork in order to develop a chronology for the development of Lake Megafazzan, a giant lake that has periodically existed in the Fazzan Basin since the late Miocene. The development of the basin can be best understood by considering the main geological and geomorphological events that occurred thought Libya during this period and thus an overview of the palaeohydrology of all Libya is also presented. The origin of the Fazzan Basin appears to lie in the Late Miocene. At this time Libya was dominated by two large rivers systems that flowed into the Mediterranean Sea, the Sahabi River draining central and eastern Libya and the Wadi Nashu River draining much of western Libya. As the Miocene progressed the region become increasingly affected by volcanic activity on its northern and eastern margin that appears to have blocked the River Nashu in Late Miocene or early Messinian times forming a sizeable closed basin in the Fazzan within which proto-Lake Megafazzan would have developed during humid periods. The fall in base level associated with the Messinian desiccation of the Mediterranean Sea promoted down-cutting and extension of river systems throughout much of Libya. To the south of the proto Fazzan Basin the Sahabi River tributary know as Wadi Barjuj appears to have expanded its headwaters westwards. The channel now terminates at Al Haruj al Aswad. We interpret this as a suggestion that Wadi Barjuj was blocked by the progressive development of Al Haruj al Aswad. K/Ar dating of lava flows suggests that this occurred between 4 and 2 Ma. This event would have increased the size of the closed basin in the Fazzan by about half, producing a catchment close to its current size (~ 350,000 km 2). The Fazzan Basin contains a wealth of Pleistocene to recent palaeolake sediment outcrops and shorelines. Dating of these features demonstrates evidence of lacustrine conditions during numerous interglacials spanning a period greater than 420 ka. The middle to late Pleistocene interglacials were humid enough to produce a giant lake of about 135,000 km 2 that we have called Lake Megafazzan. Later lake phases were smaller, the interglacials less humid, developing lakes of a few thousand square kilometres. In parallel with these palaeohydrological developments in the Fazzan Basin, change was occurring in other parts of Libya. The Lower Pliocene sea level rise caused sediments to infill much of the Messinian channel system. As this was occurring, subsidence in the Al Kufrah Basin caused expansion of the Al Kufrah River system at the expense of the River Sahabi. By the Pleistocene, the Al Kufrah River dominated the palaeohydrology of eastern Libya and had developed a very large inland delta in its northern reaches that exhibited a complex distributary channel network which at times fed substantial lakes in the Sirt Basin. At this time Libya was a veritable lake district during humid periods with about 10% of the country underwater.

Late-Holocene evolution of the Mahakam delta, East Kalimantan, Indonesia

The late-Holocene Mahakam delta, located along the tropical eastern shore of Kalimantan, Indonesia, is considered to be a textbook example of a mixed tide–fluvial dominated delta system. The delta prograded about 60 km during the past 5000 years, which led to the development of a distinct network of distributary and tidal channels. Wave action is low due the limited fetch in the narrow strait of Makassar. Mahakam River discharge is about a quarter of the Mississippi River discharge and is characterized by absence of flood surges. Therefore, natural levees, crevasse splays and avulsions are absent in the delta plain. For the past four decennia, both modern and ancient Mahakam delta deposits have been studied in detail in order to better understand subsurface Miocene and Tertiary Mahakam deposits, which host large volumes of hydrocarbons. This study focuses on the dynamics and stratal patterns of delta plain, delta-front platform deposits and suspended sediments. Due to the predominance of semi-diurnal tides and the associated flow reversals, depositional patterns are highly variable which has resulted in the formation of characteristic sand–mud couplets. The distribution of the sand–mud couplets found in this study differs from previously proposed conceptual models. They are limited to the fluvial domain and form in the distributary channels (lateral channel bar) or at the fluvial dominated delta-front platform, which flanks the mouth bar deposits in offshore direction. The sand–mud couplets which formed as delta-front platform and lateral channel bar deposits are similar and can only be identified based on their 14C age. The sand content decreases significantly towards the tidal dominated areas due to limitation in transport capacity. Turbidity measurements taken in front of the river mouth also show rapid settlement of river plume sediments. Some 22 new AMS 14C dates show that late Holocene sea level history resembles the eustatic sea level curve giving a first approximation of the Late Holocene sea level history for East Kalimantan. The dates suggest that the progradational delta system evolved under conditions of slowly rising sea level, which compares well to the eustatic sea level curve. In addition, calculated averaged deposition rates of the sand–mud couplets indicate that deposition is driven by the spring–neap tide cycles instead of the daily tidal cycle.

Rivers turned to rock: Late Quaternary alluvial induration influencing the behaviour and morphology of an anabranching river in the Australian monsoon tropics

Late Quaternary alluvial induration has greatly influenced contemporary channel morphology on the anabranching Gilbert River in the monsoon tropics of the Gulf of Carpentaria. The Gilbert, one of a number of rivers in this region, has contributed to an extensive system of coalescing low-gradient and partly indurated riverine plains. Extensive channel sands were deposited by enhanced flow conditions during marine oxygen isotope (OI) Stage 5. Subsequent flow declined, probably associated with increased aridity, however, enhanced runoff recurred again in OI Stages 4–3 (∼65–50 ka). Aridity then capped these plains with 4–7 m of mud. A widespread network of sandy distributary channels was incised into this muddy surface from sometime after the Last Glacial Maximum (LGM) to the mid Holocene during a fluvial episode more active than the present but less so than those of OI Stages 5 and 3. This network is still partly active but with channel avulsion and abandonment now occurring largely proximal to the main Gilbert flow path. A tropical climate and reactive catchment lithology have enhanced chemical weathering and lithification of alluvium along the river resulting in the formation of small rapids, waterfalls and inset gorges, features characteristic more of bedrock than alluvial systems. Thermoluminescence (TL) and comparative optically stimulated luminescence (OSL) ages of the sediments are presented along with U/Th ages of pedogenic calcrete and Fe/Mn oxyhydroxide/ oxide accumulations. They show that calcrete precipitated during the Late Quaternary at times similar to those that favoured ferricrete formation, possibly because of an alternating wet–dry climate. Intense chemical alteration of the alluvium leading to induration appears to have prevailed for much of the Late Quaternary but, probably due to exceptional dryness, not during the LGM. The result has been restricted channel migration and a reduced capacity for the channel to adjust and accommodate sudden changes in bedload. Consequent avulsions have caused local stream powers to increase by an order of magnitude, inducing knickpoint erosion, local incision and the sudden influx of additional bedload that has triggered further avulsions. The Gilbert River, while less energetic than its Pleistocene ancestors, is clearly an avulsive system, and emphasizes the importance in some tropical rivers of alluvial induration for reinforcing the banks, generating nickpoints, reworking sediment and thereby developing and maintaining an indurated and anabranching river style.

An integrated approach to flood hazard assessment on alluvial fans using numerical modeling, field mapping, and remote sensing

Millions of people in the western United States live near the dynamic, distributary channel networks of alluvial fans where flood behavior is complex and poorly constrained. Here we test a new comprehensive approach to alluvial-fan flood hazard assessment that uses four complementary methods: two-dimensional raster-based hydraulic modeling, satellite-image change detection, field-based mapping of recent flood inundation, and surficial geologic mapping. Each of these methods provides spatial detail lacking in the standard method and each provides critical information for a comprehensive assessment. Our numerical model simultaneously solves the continuity equation and Manning's equation (Chow, 1959) using an implicit numerical method. It provides a robust numerical tool for predicting flood flows using the large, high-resolution Digital Elevation Models (DEMs) necessary to resolve the numerous small channels on the typical alluvial fan. Inundation extents and flow depths of historic floods can be reconstructed with the numerical model and validated against field- and satellite-based flood maps. A probabilistic flood hazard map can also be constructed by modeling multiple flood events with a range of specified discharges. This map can be used in conjunction with a surficial geologic map to further refine floodplain delineation on fans. To test the accuracy of the numerical model, we compared model predictions of flood inundation and flow depths against field- and satellite-based flood maps for two recent extreme events on the southern Tortolita and Harquahala piedmonts in Arizona. Model predictions match the field- and satellite-based maps closely. Probabilistic flood hazard maps based on the 10 yr, 100 yr, and maximum floods were also constructed for the study areas using stream gage records and paleoflood deposits. The resulting maps predict spatially complex flood hazards that strongly reflect small-scale topography and are consistent with surficial geology. In contrast, FEMA Flood Insurance Rate Maps (FIRMs) based on the FAN model predict uniformly high flood risk across the study areas without regard for small-scale topography and surficial geology.

Hydrogeology of Brahmaputra Basin, India

The Ganga-Brahmaputra river system together forms one of the largest deltas in the world comprising some 59570 sq km. The waterpower resources of the Brahmaputra have been presumed to be the fourth biggest in the world being 19.83 x 103 m3s1. The entire lower portion of the Brahmaputra consists of a vast network of distributary channels, which are dry in the cold season but are inundated during monsoon. The catchment area of the entire river is about 580,000 sq km, out of which 195,000 sq km lies in India. The maximum discharge as measured at Pandu in 1962 was of the order of 72800 m3 s-1 while the minimum was 1750 m3 s-1 in 1968. The drainage pattern in the valley is of antecedent type while the yazoo drainage pattern is most significant over the composite flood plain to the south of the Brahmaputra.
 The Brahmaputra valley is covered by Recent alluvium throughout its stretch except a few isolated sedimentary hills in the upper Assam, inselbergs/bornhardt of gneissic hills in the Darrang, Kamrup and Goalpara districts and a few inlying patches of Older Alluvium in the Darrang and Goalpara districts.
 The basin is very unstable. The present configuration of the basin is the result of uplift and subsidence of the Precambrian crystalline landmasses. Four geotectonic provinces can be delineated in the N-E India through which the Brahmaputra flows. These are bounded by major tectonic lineaments such as the basement E-W trending Dauki fault, a NE-SW trending structural feature of imbricate thrusts known as 'belt of Schuppen' and the NW-SE trending Mishmi thrust.
 Hydrogeologically, the Brahmaputra basin can be divided into two distinct categories, viz(a) dissected alluvial plain and (b) the inselberg zone. The first category is rep resented in the flood plain extending from the south of Sub-Himalayan piedmont fan zone in the north to right upto the main rock promontory of Garo Hills and Shillong Plateau. The inselberg zone is characterized by fractured, jointed and weathered ancient crystalline rocks with interhill narrow valley plains, consisting of thin to occasionally thick piles of assorted sediments.
 From the subsurface lithological data, two broad groups of aquifers are identified. These are i) shallow water table and ii) deeper water table or confined ones, separated by a system of aquicludes. The shallow aquifer materials, in general, consist of white to greyish white, fine grained micaceous sand and the thickness ranges from 1.2 to 10.3 m. The sand and clay ratio varies from 1: 2.5 to 1:26. The bedrock occurs at depth ranges of 30.4 to 39.5 m. The materials of the deeper aquifers comprise grey to greyish white, fine to medium grained sand. The sand and clay ratio varies from 1:2 to 1:7. The effective size of the aquifer materials varies from 0.125 to 0.062 mm with uniformity co-efficient around 4.00, porosity 38 to 42%, co-efficient of permeability 304 to 390 galls per day/0.3m2. The ground water is mildly alkaline with pH value 6.5 to 8.5, chloride 10 to 40 ppm, bi-carbonate 50 to 350 ppm, iron content ranges from a fraction of a ppm to 50 ppm. Total dissolved solids are low, hardness as CaCo3 50 to 300 ppm, specific conductance at 25 °C 150 to 650 mhos/cm. The yield from shallow aquifers is 1440 litres to 33750 litres/hour and for deeper aquifers ~ 1700 litres/hour at a drawdown of 13.41 m, specific capacity 21 litres/minute. The temperatures of ground water are 23°-25° C during winter, 24°-26° C during pre-monsoon and 27°- 28° C during peak monsoon. The general hydraulic gradient in the north bank is 1:800 whereas in the south bank it is 1: 300-400 The Tertiary sediments yield a range of water from 200 to 300 l.p.m whereas the yield from the Older Alluvium is 500 to 700 1.p.m. The estimated transmissibility and co-efficient of storage is of the order of ~ 800 1.p.m/ m and 8.2 x 10-3 respectively. Depths to water levels range from 5.3 to 10m below land surface (b.l.s). In the Younger or Newer Alluvium, ground water occurs both under water table and confined conditions. Depths to water levels vary from ground level to 10 m b.l.s. Depth to water ranges from 6 m b.l.s. to 2 m above land surface. The yield of the deep tubewells ranges from 2 to 4 kl/minute for a drawdown of 3 m to 6 m. The transmissibility of the aquifers varies from 69 to 1600 l.p.m/m and the storage co-fficient is of the order of 3.52 x 10-2.

Turbidite depositional architecture across three interconnected deep‐water basins on the north‐west African margin

ABSTRACT The Moroccan Turbidite System (MTS) on the north‐west African margin extends 1500 km from the head of the Agadir Canyon to the Madeira Abyssal Plain, making it one of the longest turbidite systems in the world. The MTS consists of three interconnected deep‐water basins, the Seine Abyssal Plain (SAP), the Agadir Basin and the Madeira Abyssal Plain (MAP), connected by a network of distributary channels. Excellent core control has enabled individual turbidites to be correlated between all three basins, giving a detailed insight into the turbidite depositional architecture of a system with multiple source areas and complex morphology. Large‐volume (> 100 km3) turbidites, sourced from the Morocco Shelf, show a relatively simple architecture in the Madeira and Seine Abyssal Plains. Sandy bases form distinct lobes or wedges that thin rapidly away from the basin margin and are overlain by ponded basin‐wide muds. However, in the Agadir Basin, the turbidite fill is more complex owing to a combination of multiple source areas and large variations in turbidite volume. A single, very large turbidity current (200–300 km3 of sediment) deposited most of its sandy load within the Agadir Basin, but still had sufficient energy to carry most of the mud fraction 500 km further downslope to the MAP. Large turbidity currents (100–150 km3 of sediment) deposit most of their sand and mud fraction within the Agadir Basin, but also transport some of their load westwards to the MAP. Small turbidity currents (< 35 km3 of sediment) are wholly confined within the Agadir Basin, and their deposits pinch out on the basin floor. Turbidity currents flowing beyond the Agadir Basin pass through a large distributary channel system. Individual turbidites correlated across this channel system show major variations in the mineralogy of the sand fraction, whereas the geochemistry and micropalaeontology of the mud fraction remain very similar. This is interpreted as evidence for separation of the flow, with a sand‐rich, erosive, basal layer confined within the channel system, overlain by an unconfined layer of suspended mud. Large‐volume turbidites within the MTS were deposited at oxygen isotope stage boundaries, during periods of rapid sea‐level change and do not appear to be specifically connected to sea‐level lowstands or highstands. This contrasts with the classic fan model, which suggests that most turbidites are deposited during lowstands of sea level. In addition, the three largest turbidites on the MAP were deposited during the largest fluctuations in sea level, suggesting a link between the volume of sediment input and the magnitude of sea‐level change.

The continental margin off east Tasmania and Gippsland: structure and development using new multibeam sonar data

The continental margin facing the southern Tasman Sea off southeastern Australia is of high petroleum and geological significance because it contains Australia’s richest oil province, the Gippsland Basin. Exploration activity in the region has concentrated on the shelfal part of this basin. Off Tasmania the upper margin is host to important deep sea trawl fisheries, and regional geophysical data suggest some long-term petroleum potential.In early 1997, 20,000 km2 of the margin were surveyed by the Australian Geological Survey Organisation, in cooperation with Scripps Institution of Oceanography and the University of Sydney. The SeaBeam 2000 multibeam sonar system of the US RV Melville was used to map the topography and character of the seafloor in unprecedented detail. The SeaBeam 2000 is a 12 kHz, 121-beam echo-sounder that collects both bathymetry and backscatter (sidescan) data across a swath of ~3.4 times the water depth. The survey also collected 3500 km of magnetic and gravity profile data.The margin off Gippsland is dominated by a large embayment 100 km across and floored by a ESE-trending chasm, the Bass Canyon, which is 60 km long and 10-15 km wide. The canyon has cut down about 2 km into the margin and is bounded to the north and south by very steep inner canyon walls 1000 m high. Sediments transported down the canyon debouch at its mouth, at~4000 m depth, and spread onto the abyssal plain via a network of distributary channels and levees. Sediment from the shelf is channelled into the entrance of Bass Canyon by three major, deeply-incised tributary canyons and a number of smaller ones. Bass Canyon has probably acted as a conduit for clastic sediment to the deep sea floor since breakup at ~80 Ma. Seismic profiles over the Bass Canyon complex show that canyon erosion has exposed sections of almost the entire Gippsland Basin sequence down to the Early Cretaceous and underlying basement. Numerous suitable dredge and core sampling sites in the canyons have been identified for a geological sampling program.Off east Tasmania, the survey showed the margin to be generally steep and rugged, with relatively little sediment having been deposited on the continental slope, except in narrow rifts, since breakup. The adjacent abyssal plain is commonly underlain by 2 km of Late Cretaceous-Cainozoic sedimentary section. The continental slope is cut by an extensive system of canyons, some more than 30 km long and 500 m deep. Large fault blocks on the lower slope show structural trends consistent with a NW-NNW rift direction and NE-ENE transfer direction. The upper slope appears to be underlain by a subsided Miocene carbonate shelf. Volcanics, of probable late Cainozoic age, are fairly widespread on the seafloor off northeast Tasmania. Such volcanic terrain includes a number of scattered cones, the largest 450 m high.

An example of 3-D AVO for lithology discrimination in Widuri Field, Asri Basin, Indonesia

Abstract Drilling offshore Southeast Sumatra in the area of the Widuri Field, Asri Basin has demonstrated that the oil producing Upper Gita sandstones within the Lower Miocene Talang Akar formation correlate with bright seismic amplitudes on 3-D stack data. Seismic amplitude maps at the Base Upper Gita horizon are interpreted as either a network of distributary channels or a meandering fluvial system. Channel fill lithologies vary considerably and are difficult to interpret from conventional seismic stack data. Carbonaceous shales, likely deposited during the abandonment phase of channeling, have acoustic impedances similar to sandstones. Dipole sonic log measurements of Poisson's ratio are significantly higher in carbonaceous shales than sandstones. Based on raytraced models, carbonaceous shale has a strongly decreasing AVO while sand has flat AVO response (no change in amplitude with offset). AVO attributes P and G or intercept and gradient inverted from 3-D prestack seismic data are used for detection of carbonaceous shale. Based on mapping, good correlation exists between well data and the AVO product attribute P*G. The stratigraphic model of shale abandonment fill in a meandering fluvial system is interpreted to be supported by the 3-D AVO data. This interpretation will be tested with future drilling.

Small submarine fans on the eastern margin of Corsica: Sedimentary significance and tectonic implications

The small turbidite systems and their network of distributary channels lying on the base of slope and basin off the eastern margin of Corsica exhibit physiographic and acoustic characteristics similar to those encountered in typical deep-sea fans. In most cases, the fans appear to be connected to sinuous canyons that incise into the shelf break and display varying shapes along the continental slope. They begin as large highly meandering erosional troughs. Then they become progressively depositional and bordered by lateral prominent levees and show evidence of branching. In their most distal part the height of levees decrease and lobes appear unchannelled. Seismically, the lobes are characterized in their middle and lower parts by superposition of channel-levee units, showing high amplitude reflectors with poor penetration in channels, and bedded facies with better penetration on levees. Only the most distal domains located beyond the channelized areas of the lobes are formed entirely by high amplitude reflectors with poor penetration that are interpreted as coarse deposits of sand or silt. The majority of these sand-rich fans, and especially those located in the northern sector of the margin, are related to the insular drainage system of Corsica. The sandrich fans located in the southern sector may have been structuraly controlled. By the lack of any obvious actual connexion with insular drainage, they suggest earlier association with high continental relief, subsequently removed by erosion or tectonic collapse. The southernmost canyon of this area (Caprera canyon) records structural trends of the Sardinian crystalline basement, and is also probably related to tectonic movements linked with the opening of the Tyrrhenian Sea.