Total Sediment Volume Research Articles

Estimation of sedimentation rates under Mediterranean conditions deduced from the Mishmar Ayyalon Reservoir, Israel

Fischhendler, I., Enzel, Y., Gvirtzman, H. 2003. Estimation of sedimentation rates under Mediterranean conditions deduced from the Mishmar Ayyalon Reservoir, Israel. Isr. J. Earth Sci. 52: 21–29. The Mishmar Ayyalon Reservoir was constructed in 1955 and its watershed drains the upper basins of Nahal Ayyalon and Nahal Nachshon (160 km). It has a water storage capacity of 7.3 × 10 m and is the only reservoir located within the western-flowing drainage basins that drains the mountain backbone of central Israel. Because the reservoir has never overflowed, the sediments that accumulated on its floor during the last 44 years provide a unique record of sedimentation rates and total sedimentation. A pit 3.3 m deep was dug into the sediment and variable-thickness layers, associated with specific years or flood events, were identified. Based on the sediment that was exposed in the pit and considering the original volume–elevation data of the reservoir, we have estimated the total sediment volume. Specific sediment layers were correlated with specific storms (and their resulting floods), based on three distinctive historical marker beds. The total sediment volume, which has been deposited by at least 22 significant floods, was calculated to be about 456,000 m. Approximately 30% of the sediment reached the reservoir during the two largest consecutive floods of the winter of 1963/4. Approximately 70% of the sediment was produced by a small number of very large floods. The mean annual specific suspended sediment yield for the catchment of the reservoir is 114 t km year, which is a typical sediment yield for land under minimal cultivation.

ASSESSING SEDIMENTATION ISSUES WITHIN AGING FLOOD CONTROL RESERVOIRS IN OKLAHOMA1

Since 1948, the USDA‐NRCS has constructed nearly 11,000 flood control dams across the United States, and many of the reservoirs are rapidly filling with sediment. To rehabilitate these structures, the impounded sediment must be assessed to determine the volume of accumulated sediment and the potential hazard this sediment may pose if reintroduced to the environment. An assessment of sedimentation issues within two reservoirs, Sugar Creek No. 12, Hinton, Oklahoma, and Sergeant Major No. 4, Cheyenne, Oklahoma, is presented. Sediment cores obtained using a vibracoring system were composed of alternating layers of gravel, sand, silt, and clay. Stratigraphic analysis coupled with 137Cs dating techniques enabled the discrimination of pre‐construction sediment from post‐construction deposition. An acoustic profiling system was unencumbered by the relatively shallow water depth at Sugar Creek No. 12 and the seismic horizons agreed well with the sediment core data. Total sediment volume determined from the acoustic survey and the sediment core data for comparable areas differed by only 1.4 percent. The seismic profiling system worked well in the relatively deeper Jake of Sergeant Major No. 4 and showed good correspondence to the collected core data. Detailed chemical analyses showed that overall sediment quality was good at both locations and that chemical composition was spatially invariant. Implementation of these techniques will aid action agencies such as the USDA‐NRCS in their assessment and effective management of aging flood control reservoirs.

Holocene storage of siliciclastic sediment around islands on the middle shelf of the Great Barrier Reef Platform, north‐east Australia

ABSTRACTThick sequences of sediment surround the Whitsunday Islands on the middle shelf of the Great Barrier Reef (GBR) Platform. Much of this sediment is siliciclastic material deposited since the sea‐level highstand at around 6·5 ka. This raises a mass balance dilemma because modern terrigenous discharge to the GBR Platform is restricted to the inner shelf. Shallow seismic profiles and sediment samples were collected over 450 km2 around the Whitsunday Islands to quantify the mass of siliciclastic sediment for a dynamic model of the shelf. The sea floor and pre‐Holocene surfaces were mapped using 4584 stations along the seismic profiles and a graphical computer program. The total volume of sediment between these two surfaces is 3·67 ± 0·45 × 109 m3. This volume is composed of buried reefs (0·13 ± 0·01 × 109 m3), medium‐ (0·70 ± 0·30 × 109 m3) and fine‐grained shoals (2·84 ± 0·35 × 109 m3). The volume estimates combined with measurements of carbonate concentration, density and porosity indicate that 1850 ± 380 Mt of Holocene siliciclastic sediment surround the Whitsunday Islands in medium‐ (510 ± 225 Mt) and fine‐grained shoals (1340 ± 155 Mt). The total mass of siliciclastic material is 1·7–2·6 times that stored in Cleveland Bay, a similar sized repository on the inner shelf. A simple numerical model has been constructed to explain this large quantity of Holocene siliciclastic sediment. The model results in the appropriate siliciclastic mass next to the Whitsunday Islands by integrating regional shelf processes over time. Unlike the present day, rivers discharged sediment to the middle shelf during the early Holocene. This material was subsequently focused by northward transport into the vicinity of the islands, a geomorphologically complex region that serves as a sediment trap. Although direct riverine inputs to the middle shelf have stopped during the present sea‐level highstand, previously deposited siliciclastic sediment is continually being winnowed from the middle shelf and redeposited next to the Whitsunday Islands. The transport and distribution of siliciclastic sediment on the GBR Platform is thus influenced significantly by storage around islands on the middle shelf.

A budget of marine and terrigenous sediments, Hanalei Bay, Kauai, Hawaiian Islands

Abstract The sediment budget of Hanalei Bay on the north shore of Kauai was calculated using sedimentological and geophysical methods. The calculations of the budget subsequently allowed an interpretation of the Holocene history of the bay. The bay sediments are easily separated into marine (carbonate) and terrigenous (siliciclastic) grains. Surficial sediments are dominated by carbonate grains (∼70%) of coralline algae, coral, and mollusc fragments as well as foraminifera, Halimeda , bryozoa, and echinoderm tests. However, siliciclastic grains (e.g. olivine, plagioclase, volcanic lithics) from the Hanalei River watershed draining shield volcanic highlands are the most common individual grain type (∼27%) and form a zone of high concentration from the mouth of the Hanalei River into the center of the bay. Flooding in the bay by the post-glacial sea-level rise began soon after 11.7 kyears. The resulting marine environment caused the net deposition of 45.5±1.5×10 6 m 3 of sediment in the bay and approximately 33.7±11.2×10 6 m 3 of sediment on the Hanalei coastal plain. The total volume of carbonate sediment stored in the bay and coastal plain is greater than the volume likely to have been produced exclusively within the bay during the same time. Calculations indicate that approximately 2490 m 3 year −1 have been imported into the bay or coastal plain and deposited since 11,700 years ago. The majority of this sediment influx is likely delivered from the east by the strong tradewind-driven littoral currents that characterize Kauai's north shore. Net carbonate sediment deposition in Hanalei Bay peaked at a rate of 15,500 m 3 year −1 between 5000 and 3000 years ago (when sea level may have been 2 m above present) diminishing to 3890 m 3 year −1 from 1000 years ago to the present. This influx is likely to have played a significant role in the mid to late Holocene progradation of the Hanalei shoreline.

Debris flow in the upper Isarco valley, Italy – 14 August 1998

The stretch of the Isarco valley between the localities of Fortezza and Mules (South Tyrol, Italy) is an area prone to debris flows due to the particular combination of climate, morphology and geological conditions. On 14 August 1998 the steep side streams located on the lateral slopes of the valley were swollen due to heavy rainfall and water issuing from joints in the rock mass. This resulted in debris flows which caused damage and resulted in the death of five people on a highway passing through the valley. This rainfall event is compared with previous similar occasions. The paper also assesses empirical formulae developed to estimate the total sediment volume that may be mobilised in a torrent catchment during an extreme rainstorm.

Post-glacial sediments of the Californian shelf from Cape San Martin to the US–Mexico border

Previous workers of the Californian shelf from Cape San Martin to the US–Mexico border have assembled compilations of high-resolution acoustic seismic profiling data from a number of primary sources. To examine these data more effectively the region has been divided into four areas for convenience in mapping, and partly in response to natural geographic divisions. These data, plus additional detailed profiling off San Onofre south of Dana Point, have made possible calculations of the total volume of unconsolidated post-glacial sediments on this narrow, active transform margin shelf. The San Onofre survey shows that there are several well-defined buried terraces cut into the bedrock surface. These occur at about −20, −30 to −35, −40 to −50, and −60 to −80 m below present datum. Dating reported in the literature suggests that the bulk of the sediments are Holocene. As sea level shifts from glacial to interglacial, much of the sediment stored on the shelf at high stand is reworked to deeper water during the fall to the following low stand. Previous studies suggest that for the large deposits off the Santa Maria, Santa Ynez and Ventura–Santa Clara Rivers, some sediment may remain after a sea-level cycle. The estimated total Holocene (past 10,000 years) river discharge for the region, based on records of the past century, is about 130 km 3. Thus the total shelf deposit of ca. 60 km 3 represents about one-half of the Holocene siliciclastic contribution. However, the 60 km 3 includes basal sediments in the three large river lenses that are probably pre-Holocene. A more probable Holocene-contributed volume is about 30 km 3. Previous studies show that biogenic and fluvial contributions are generally <5% and 25–30% of the shelf sediments, so that the dominantly sandy shelf sediments of probable Holocene age approximate the coarse fraction of the river. The bulk of the fluvial contribution passes via lateral longshore transport into the adjacent basins via submarine canyons. Little sediment from the areas of relatively high input passes to the starved areas which remain as non-depositional, low-deposition rate, or erosional surfaces. The major portion of the shelf is sediment-starved since only a few small ephemeral streams enter most of the cliffed coastal areas, and rocky surfaces are exposed in the inner and outer shelf.

Fluvial response to sea‐level changes: a quantitative analogue, experimental approach

ABSTRACT Quantitative evaluation of fluvial response to allogenic controls is crucial for further progress in understanding the stratigraphic record in terms of processes that control landscape evolution. For instance, without quantitative insight into time lags that are known to exist between sea‐level change and fluvial response, there is no way to relate fluvial stratigraphy to the sea‐level curve. It is difficult to put firm constraints on these time‐lag relationships on the basis of empirical studies. Therefore, we have started to quantify time‐averaged erosion and deposition in the fluvial and offshore realms in response to sea‐level change by means of analogue modelling in a 4 × 8‐m flume tank. The rate of sea‐level change was chosen as an independent variable, with other factors such as sediment supply, discharge and initial geometry kept constant over the course of 18 experiments. Our experimental results support the common view that neither fall nor rise in sea level affects the upstream fluvial system instantaneously. An important cause for the delayed fluvial response is that a certain amount of time is required to connect initial incisions on the newly emergent shelf (canyons) with the fluvial valley. Lowering of the fluvial longitudinal profile starts only after the connection of an active shelf canyon with the fluvial valley; until that moment the profile remains steady. We quantified the process of connection and introduced the quantity ‘connection rate’. It controlled, in conjunction with the rate of sea‐level fall: (1) the amount of fluvial degradation during sea‐level fall; (2) the total sediment volume that bypasses the shelf edge; (3) the percentage of fluvial relative to shelf sediment in the lowstand delta; (4) the volume of the transgressive systems tract and (5) the amount of diachroneity along the sequence boundary. Our experiments demonstrate also that the sequence‐stratigraphic concept is difficult to apply to continental successions, even when these successions have been deposited within the influence of sea level.

The characteristics of woody debris and sediment distribution in headwater streams, southeastern Alaska

Large woody debris (LWD), fine woody debris (FWD), fine organic debris (FOD), and sediment deposition were measured in 15 steep headwater streams with five management and disturbance regimes. Clear-cut channels logged in 1995 contained large accumulations of logging residue that initially provided sites for sediment storage. Half of the LWD in clear-cut channels was recruited during and immediately after logging. Woody debris from logging activities remains in young growth conifer channels 37 years after logging. Numbers of LWD in clear-cut and young conifer channels were significantly higher than in old-growth channels, although numbers of FWD pieces were not significantly different because of higher recruitment from old-growth stands. Channels that experienced recent (1979 and (or) 1993) and earlier (1961 and (or) 1979) scour and runout of landslides and debris flows contained less LWD and FWD, although large volumes of LWD and FWD were found in deposition zones. The volumes of sediment stored in young alder and recent landslide channels were higher than in the other channels. Because of the recruitment of LWD and FWD from young alder stands, the ratio of sediment stored behind woody debris to total sediment volume was higher in young alder channels compared with recent landslide channels. Numbers of LWD and FWD pieces in all streams were significantly correlated with the volumes of sediment stored behind woody debris. Timber harvesting and soil mass movement influence the recruitment, distribution, and accumulation of woody debris in headwater streams; this modifies sediment storage and transport in headwater channels.

Morphodynamic Modelling of Intertidal Sediment Transport in Morecambe Bay

The construction of a morphodynamic model for the intertidal region of Morecambe Bay, England, and its calibration and validation using extensive sediment volume change data measured by remote sensing, is described. A properly validated sediment transport model of the bay would be of interest for coastal defence, management and economic purposes. The morphodynamic model consisted of a set of linked modules for predicting 2D depth-averaged tidal currents, waves, sediment transport rates and sediment budgets. The intertidal bathymetry was updated at regular intervals and the currents, waves and sediment transport patterns recomputed using the new bathymetry. Tidal input filtering using representative spring and neap tidal cycles was used to reduce computational requirements. Sediment volume changes during the period 1992–1997 were measured over the intertidal zone at a resolution of 240m using the waterline method employing remote sensing and hydrodynamic modelling. The total volume of sediment eroded was −102±16×106m3, and the total volume accreted was 107±15×106m3.Tidal asymmetry is shown to be the dominant agent of sediment transport in the intertidal zone, with waves being of secondary importance. A close correlation was found between the computed directions of tidal asymmetry and observed sediment transport paths. It was necessary to use the remotely sensed data for calibration of the sediment transport model as well as for validation. The model showed some limited capability for predicting gross changes in bathymetry, but there were substantial differences between the predicted and observed patterns of sediment movement. The possibility of improvement by a further incorporation of the spatially and temporally extensive field data into the modelling process is discussed.

Designing efficient sampling schemes for reconnaissance surveys of contaminated bed sediments in water courses

AbstractA method for designing efficient sampling schemes for reconnaissance surveys of contaminated bed sediments in water courses is presented. The method can be used in networks of water courses, for instance to estimate the total volume of bed sediment of a defined quality class. The water courses must be digitised as arcs in a Geographical Information System.The method comprises six steps: (1) stratifying the water courses; (2) choosing a variogram; (3) calculating the parameters of the variance model; (4) choosing a compositing scheme; (5) choosing the values for the cost-model parameters; and (6) optimising the sampling scheme. The method is demonstrated with a survey of the main water courses in the reclaimed areas of Oostelijk Flevoland and Zuidelijk Flevoland.

Morphology and Late Quaternary sedimentation on the North Faeroes slope and abyssal plain, North Atlantic

Whilst the interplay between tectonics, glaciation and oceanography is reasonably well understood on some high-latitude margins, relatively little is known about the nature of Quaternary processes affecting the continental margin north of the Faeroe Islands. We therefore present side-scan sonar (GLORIA and TOBI) and 3.5 kHz data in combination with a new regional bathymetry in order to characterise and analyse the margin's physiography and the sedimentary processes that have occurred in the area during the Late Quaternary. Seven acoustic facies are identified and interpreted, in combination with published core records. The margin is subject to differing sedimentation regimes over glacial/interglacial cycles, relating to the changing importance of contouritic, hemipelagic, glacimarine and mass-movement processes. Mass movement is much more extensive than previously thought, with a total sediment volume of >2000 km3, similar to that of the Storegga Slide (5500 km3) and may occur primarily at glacial terminations. The downslope limit of instabilities is associated with a series of channels, focusing the deposition of distal, turbiditic sediments into the regional deep beyond. Together with glacigenic debris flows from the western margin of the North Sea Fan, mass-movement processes are identified to have affected 52–70% of the North Faeroes margin area. In contrast, distal glacimarine sedimentation, alternating with hemipelagic and contouritic development, is found on the remaining 30–48% of the margin during glacials, and hemipelagic and contouritic sedimentation are active across 90% of the margin in interglacial conditions. Despite a lack of any local, direct glacial influence, these observations on the relative importance of sedimentation processes are comparable with other glacially influenced passive margins.

Evidence of bioturbation in the Cap-Ferret Canyon in the deep northeastern Atlantic

In the sedimentary column, a combined quantification of burrows and macrobenthic community provides evidence of bioturbation features in the submarine canyon of Cap-Ferret between 2000 and 3000 m depth. An image-processing technique allows accurate quantification of burrow volumes with depth in the sedimentary column. The major bioturbation mode seems to be different in the channel compared to the interfluve. Macrobenthic activity is more inclined to mix the sediment in the channel in response to increased organic matter supplies. Sediment mixing leads to burrow destruction in the upper mixed layer of sediment in the canyon. Burrows are better preserved on the interfluve where mixing is slower. Under the mixed zone, the volume of recorded burrows is higher when sedimentation rate increases, as in the upper canyon. In this transition layer, the burrow volume is estimated to be between 3 and 64% of the total sediment volume depending on the sediment depth. The fill-down of numerous burrows with surface sediment by bioregeneration suggests that anaerobic degradation of fresh organic matter is dominant in this canyon. In the sedimentary column, the negative relationship between carbonate content and macrobenthic abundance confirms that carbonate dissolution is largely influenced by bioturbation.

Hydrate distribution off Vancouver Island from multifrequency single‐channel seismic reflection data

On the northern Cascadia margin, single channel seismic data clearly image the bottom‐simulating reflector (BSR) that marks the base of the hydrate stability zone. Airguns with three distinct source frequencies (30, 75, and 150 Hz) were recorded along several coincident lines, and at all frequencies the BSR appears as a single pulse. BSR reflection coefficients are slightly larger for the 30‐Hz data than for the 75‐ or 150‐Hz data. Multi‐frequency analysis indicates that the BSR is due to a single sharp interface at the base of the hydrate zone, whereas the velocity structure away from the interface must be gradationally‐varying so as to produce no separate reflection and no tuning effects at the 75‐ and 150‐Hz frequencies. In contrast, the seafloor reflection in some areas has significantly larger amplitudes for data recorded with the 75‐Hz source, relative to the 30‐Hz source. Synthetic seismogram analysis of the two data sets shows that the high amplitudes can be produced by a 2‐m‐thick high‐velocity layer at the seafloor, with velocities and densities corresponding to those of carbonate. Using the 75‐Hz source, seafloor and BSR amplitudes and reflection coefficients were measured over a tight grid of lines, nominally spaced at 200 m. Maximum BSR reflection coefficients of 0.15–0.18 were observed beneath topographic highs, while maximum seafloor reflection coefficients of up to 0.5–0.6 were found on the flanks of the topographic highs. This suggests that topography provides a major control on the flow of methane‐bearing fluids. Amplitude measurements, converted to values of velocity above the BSR, were used to estimate hydrate concentrations, which reaches a maximum of 15–18% of the total sediment volume with mean values near 5–10%. Over the 100 km2 survey area, the volume of methane gas in the hydrate reservoir is estimated as 6.4 × l010 m3, or 2.1 trillion cubic feet.

Storm-driven variability of the beach-nearshore profile at Duck, North Carolina, USA, 1981–1991

The US Army Corps of Engineers' Field Research Facility (FRF) at Duck, North Carolina, has collected approximately biweekly beach-nearshore profile data to 8-m depth and associated wave data since 1981. Sediment budget analysis was used to examine the medium-scale (years to a decade) variability of the beach-nearshore profile from 1981 to 1991. Significant changes occurred during four groups of energetic storm events during February/March of 1983, 1987, 1989 and December 1989. Each group was comprised of at least two storms within a period of less than 39 days both with H mo > 4 m. During each storm group, offshore sediment movement caused a distinct outer bar to migrate offshore and grow in size resulting in an abrupt increase in the volume of sediment on the upper shoreface. The net profile changes were much larger than the changes due to single storms and the cumulative effect of the storms can be considered as one ‘event’. During these events, the first storm appears to have a destabilizing effect on the profile which has insufficient time to recover before the second (and subsequent) storm (s). As a result, several storms in quick succession are able to have a large impact on the morphology. The intervening periods between the groups of storm events (termed fairweather conditions) lasted up to 4 years. They are characterized by slow, but steady sediment redistribution (averaging 33 m 3 m −1 year −1) from the upper shoreface (> 5 m depth) toward the shore, while the total sediment volume was effectively constant. The onshore feed of sediment was not significantly affected by individual storms during the fairweather conditions. These two processes of (1) morphologic change during groups of storm events and (2) the steady onshore feed of sediments from the shoreface during fairweather conditions appear to play an important role on medium- and long-term profile evolution at least at Duck.

Geomorphic effects of large debris flows on channel morphology at North Fork Mountain, eastern West Virginia, USA

Extreme rainfall in June 1949 and November 1985 triggered numerous large debris flows on the steep slopes of North Fork Mountain, eastern West Virginia. Detailed mapping at four sites and field observations of several others indicate that the debris flows began in steep hillslope hollows, propagated downslope through the channel system, eroded channel sediment, produced complex distributions of deposits in lower gradient channels, and delivered sediment to floodwaters beyond the debris-flow termini. Based on the distribution of deposits and eroded surfaces, up to four zones were identified with each debris flow: an upper failure zone, a middle transport/erosion zone, a lower deposition zone, and a sediment-laden floodwater zone immediately downstream from the debris-flow terminus. Geomorphic effects of the debris flows in these zones are spatially variable. The initiation of debris flows in the failure zones and passage through the transport/erosion zones are characterized by degradation; 2300 to 17 000 m3 of sediment was eroded from these zones. The total volume of channel erosion in the transport/erosion zones was 1·3 to 1·5 times greater than the total volume of sediment that initially failed, indicating that the debris flows were effective erosion agents as they travelled through the transport/erosion zones. The overall response in the deposition zones was aggradation. However, up to 43 per cent of the sediment delivered to these zones was eroded by floodwaters from joining tributaries immediately after debris-flow deposition. This sediment was incorporated into floodwaters downstream from the debris-flow termini causing considerable erosion and deposition in these channels. © 1998 John Wiley & Sons, Ltd.

Natural gas hydrates on the southeast U.S. margin: Constraints from full waveform and travel time inversions of wide‐angle seismic data

Strong bottom‐simulating reflectors (BSR) have been mapped over a region of approximately 50,000 km2 on the southeastern U.S. margin and have been associated with possible abundance of natural gas hydrates. In June 1992, coincident single‐channel seismic and wide‐angle ocean bottom seismic data were acquired in the region, focusing on the Blake Ridge and the Carolina Rise. Wide‐angle reflections from the BSRs were clearly observed at offsets up to ∼6 km. Joint travel time inversion was conducted with wide‐angle and vertical‐incidence data in order to explore possible regional variation, and the resultant two‐dimensional average velocity models imply higher background velocities on the Carolina Rise. Full waveform inversion was then performed to determine the seismic origin of the BSRs. The best fit model shows a similar low velocity (∼1.4 km/s) beneath the BSR at both sites, indicating trapped free gas with low saturation (&lt;10%). The inversion results also indicate that a thin, high‐velocity wedge, with a maximum velocity of ∼2.3 km/s, is present just above the Blake Ridge BSR. Sediment reflectivities were also calculated, and higher reflectivities are observed on the Carolina Rise. An increase in reflectivity below the BSR seems to correspond to the gas‐bearing zone at both sites. Concentration of hydrates were estimated based on these velocity models. Whereas average hydrate concentration of 3% of the total sediment volume is suggested for the lower half of the hydrate stability zone at the Blake Ridge, only a very low average concentration of hydrate can be expected at the Carolina Rise. The hydrates seem to be concentrated near the base of the hydrate stability zone, and the maximum hydrate concentration is estimated as ∼20% at the Blake Ridge and ∼7% at the Carolina Rise, both of which are too high to be explained by in situ biogenic activity only and require some secondary accumulation mechanism. It is suggested that hydrate recycling caused by the stability field migration may have effectively condensed hydrates at both sites. Additional enhancement by upward fluid expulsion may also be viable for the Blake Ridge.

Mass transfer between eastern Tien Shan and adjacent basins (central Asia): constraints on regional tectonics and topography

SUMMARY Using depths and ages derived from isopachs, drill-holes or cross-sections, it is possible to reconstruct the space-time depositional history of a sedimentary basin. Drill holes and cross-sections give the local sedimentation history, while isopachs allow the definition of the spatial distribution of the sediments. Assuming several simple hypotheses, such as similarity of the strata and regional applicability of results derived from local analyses, one can reconstruct balanced maps of the solid (or grain) volumes, and hence the mass of sediments deposited during several time intervals since the Palaeogene. Applying this method to the Tarim and Dzungar basins (NW China), we estimate the total Cenozoic solid-phase volume and mass of sediments stored to be 1358 f 520 x lo3 km3 (36.7 f 14 x 1017 kg) and 172 f 56 x lo3 km3 (4.6 f 1.5 x 1017 kg) respectively. The reconstruction also enables us to detect two main pulses in the sedimentation. The first, around 17 Ma, affected only the northern part of the Tarim Basin (also known as the Kucha or Kuche Depression) at the foot of the Tien Shan Mountains and supports the idea that the presently active shortening regime in that range started at that time. The second, 5 to 6 Ma, affected most of the depositional areas of the region and may have an even greater geographical extent. Assuming local isostasy, we estimate the volume of shortening induced by the rotation of the Tarim block relative to Siberia and stored in the range and adjacent basins to be between 1.15 x lo6 and 4.23 x lo6 km3. This correspbnds to a clockwise rotation of between 2.5 and 8.7. We use these results in two simple models of self-similar growth of pyramidal topographies that approximately fit the eastern Tien Shan.

An assessment of the effects of sample collection, storage and resuspension on the representativeness of measurements of the effective particle size distribution of fluvial suspended sediment

Flocs/aggregates can comprise a significant proportion of the total volume of suspended sediment in the fluvial environment. In situ measurement of the effective size distribution is frequently precluded on grounds of the cost of appropriate equipment or adverse field conditions. An assessment of the potential for using an alternative and simpler strategy involving collection of bottle samples and subsequent analysis of the samples has been undertaken. A field-portable, laser-backscatter particle size analyser has been used to assess the representativeness of measurements made on-site immediately after sample collection, and subsequently in the laboratory after settling and resuspension. On-site measurements of bottle samples were broadly representative of the in situ effective size distribution. Flocculation occurred within settled samples together with a concomitant increase in the volume mean particle size upon resuspension. The magnitude of this increase was, in general, positively related to settling time. Following resuspension, floc break-up during continuous stirring caused mean particle size to decline towards the initial on-site value.

Flocculation of suspended sediment in rivers of southeastern Canada

Flocculation has been studied extensively within marine science and sanitary engineering and the controlling processes are generally well known. Less research, however, has focused on flocculation within the freshwater river environment. Consequently the occurrence, importance and controlling mechanisms are poorly understood. We examine the suspended sediment characteristics of six rivers in southeastern Canada in order to determine the nature and significance of flocculation in finegrained suspended sediment transport and the factors that may influence it. While flocs may not always comprise the majority of particles in transport, they always represent more than 90% of the total volume of sediment transported. Independent variables that may influence flocculation, such as temperature, pH, major ions, dissolved and particulate organic carbon, suspended solids concentration and bacteria were measured in conjunction with floc size for one of the rivers. Particulate organic carbon concentration, suspended solids concentration and attached bacteria may be important controlling factors of riverine flocculation.

Regional framework, structural and petroleum aspects of rift basins in Niger, Chad and the Central African Republic (C.A.R.)

This paper overviews the regional framework, tectonic, structural and petroleum aspects of rifts in Niger, Chad and the C.A.R. The data base is from mainly proprietary exploration work consisting of some 50,000 kilometres of seismic profiles, 50 exploration wells, one million square kilometres of aeromagnetics coverage and extensive gravity surveys. There have been 13 oil and two oil and gas discoveries. A five phased tectonic history dating from the Pan African orogeny (750−550 MY B.P.) to the present suggests that the Western Central African Rift System (WCAS) with its component West African Rift Subsystem (WAS) and Central African Subsystem (CAS) formed mainly by the mechanical separation of African crustal blocks during the Early Cretaceous. Among the resulting rift basins in Niger, Chad and the C.A.R., seven are in the WAS—Grein, Kafra, Tenere. Tefidet, Termit, Bongor, and N'Dgel Edgi and three, Doba, Doseo, and Salamat are in the CAS. The WAS basins in Niger and Chad are all extensional and contain more than 14,000 m of continental to marine Early Cretaceous to Recent clastic sediments and minor amounts of volcanics. Medium to light oil (20° API–46° API) and gas have been discovered in the Termit basin in reservoir, source and seal beds of Late Cretaceous and Palaeogene age. The most common structural styles are extensional normal fault blocks and transtensional synthetic and antithetic normal fault blocks. The CAS Doba, Doseo and Salamat are extensional to transtensional rift basins containing up to 7500 m of terrestrial mainly Early Cretaceous clastics. Heavy to light oil (15°–39° API) and gas have been discovered in Doba and Doseo basins. Source rocks are Early Cretaceous lacustrine shales, whereas reservoirs and seals are both Early and Late Cretaceous. Dominant structural styles are extensional and transtensional fault blocks, transpressional anticlines and flower structures. The existence of a total rift basin sediment volume of more than one million cubic kilometres with structured reservoir, source and seal rocks favours the generation, migration and entrapment of additional significant volumes of hydrocarbons in many of these basins.