Coarse Debris Research Articles

Seawater intakes for desalination plants

The seawater intake has to ensure sufficient seawater in terms of quantity and quality independently from the type of desalination plant (RO, MED, MSF) installed downstream. The best seawater quality can be reached by beach wells, but in these cases the amount of water which can be extracted from each well is limited by the earth formation, and therefore the amount of water available by beach wells is very often far below the demand of the desalination plant. In these cases the developer has the choice between (1) an intake from deep seawater with the advantage to have less polluted seawater and the disadvantage of high investment cost which normally limits this type of seawater intake to 20,000 m 3/h; (2) open seawater intakes with the advantage of low investment cost but the disadvantage of biologically more active water which requires more efforts to treat the seawater. Offshore seawater intakes require a submerged pre-screening device minimizing the amount of sand sucked into the pipeline and ensuring that no particles able to damage or block the pump can enter. A description with pros and cons of the different available pre-screening devices will be given in the presentation, with special consideration of how the different types influence the total project cost. Open seawater intakes have to handle much higher amounts of coarse debris as well as micro organisms. The different kinds of available open seawater intakes will be presented in the presentation as well, demonstrating how efficiently and economically the different intake types fulfil this requirement.

Microstructural alterations through heat treatment and its influence on wear response of a silicon containing zinc based alloy under different test conditions

Observations pertaining to the influence of microstructural alterations brought about through heat treatment on the sliding wear behaviour of a zinc based alloy comprising of silicon have been analysed in this study. The effects of sliding conditions such as pressure and speed on the wear response of the alloy in as cast and heat treated conditions have also been investigated. The as cast alloy revealed dendritic structure consisting of primary α, eutectoid α + η, and ϵ phase. Silicon was present in the alloy microstructure as discrete particles. Heat treatment caused breaking of the dendritic structure and more homogeneous distribution of various microconstituents without affecting the morphology and mode of distribution of the silicon particles. The heat treated alloy attained superior wear response as compared with the as cast one especially under more severe wear conditions. Wear rate versus pressure plots revealed two slopes wherein the slope was low at low pressures and increased considerably beyond a critical pressure. The critical pressure decreased with speed while it was more for the heat treated alloy. The wear behaviour of the specimens deteriorated with pressure and speed. High wear rates were supplemented with severe surface/subsurface damage and coarse debris formation and vice versa. Changing microstructural features of the regions at different depths below the wear surface were attributed to the changing degree of deformation they experienced during wear. The wear behaviour of the specimens has been explained in terms of specific characteristics of various phases such as lubricating and load carrying capability, thermal stability and cracking tendency. Typical characteristics of worn surfaces/subsurface regions and debris further supplemented the specific wear behaviour of the alloy in different test/material conditions.

The rock-avalanche of February 1995 at Claix (French Alps)

The sedimentology of a rock-avalanche involving ca. 7·10 3 m 3, that happened on February 1995 at Claix, the French Alps, is analysed here. The talus shows a proximal depression due to the impact of the debris, then a grooved area with few, mainly sandy silt deposits and a downslope accumulation of coarse debris that reaches up to 3 m in thickness. The material underwent mass movement as demonstrated by the steep distal front, the strong fabric strength and the thrust planes. The greatest part of the deposits consists of a clast-supported to partially openwork debris with angular clasts over a sheared matrix-supported basal layer. The largest blocks, often more than 1 m in length, are concentrated at the surface and the front. The overall sedimentological characteristics are similar to those of the small dry grain flows that occur usually on rockfall talus, except for the abundance of fine-grained particles that results from intense cataclasis during flow. It is suggested that the deposits of such medium-scale rock-avalanches, probably not uncommon in seismic regions, may have been confused in previous studies with other slope mechanisms such as debris flow and rockfall.

Carboniferous–Permian facies and tectono-stratigraphic successions of the glacially influenced and rifted Carnarvon Basin, western Australia

The Carnarvon Basin of Western Australia is a rift basin that contains a thick (up to 5 km) succession of late Carboniferous–early Permian glacially influenced marine sedimentary strata. These rocks accumulated in near-polar paleolatitudes along the uplifted and glaciated margin of the west Australian Shield (Pilbara Craton). Three stratigraphic successions (I, II, III) can be recognised, each characterised by distinct facies associations that record different stages in the tectonic evolution of the basin and associated changes in the rate of basin subsidence and sediment accommodation. A lowermost succession (I) comprises rapidly deposited (30 m/Ma) glacially influenced marine strata (Lyons Group) containing palynomorphs of Westphalian–Tastubian (early Sakmarian) and possibly older age. Strata are dominated by subaqueously deposited sediment gravity flow facies. Succession II is composed of richly fossiliferous cool water shales (Callythara and Cordalia formations) that record much reduced sedimentation rates (2 m/Ma). In turn, shales are overlain by an uppermost succession (III) of shallow marine, wave- and storm-influenced sandstone (Moogooloo Sandstone). Comparison with other rift basin fills indicates that Succession I likely records initial basin infilling where abundant coarse debris was produced by faulting and glaciation of the adjacent Pilbara Craton. Shales of Succession II mark a phase of ‘sediment underfilling’ characterized by rapid tectonic subsidence, an increase in relative sea level and reduced sediment supply. Shallow water sandstone facies of Succession III record a late stage in the tectonic cycle when subsidence rates had decreased and sediment supply outpaced accommodation. Such successions, where found in other late Paleozoic basins, are widely interpreted in terms of glacioeustatically driven changes in sea level resulting from deglaciation events across Gondwana. Instead, the three successions within the Carnarvon Basin are argued to reflect a dominantly tectonic control on sedimentation and preservation.

Sediment transportation and deposition by rain-triggered lahars at Merapi Volcano, Central Java, Indonesia

Abstract A lahar is a general term for a rapidly flowing mixture of rock debris and water (other than normal streamflow) from a volcano and refers to the moving flow. Located in the populated area of Central Java, the stratovolcano Merapi (2965 m) is prone to lahar generation, due to three main factors: (1) millions of cubic meters of pyroclastic deposits are the product of frequent pyroclastic flows, which have occurred on 2- to 4-year intervals; (2) rainfall intensity is high (often 40 mm in 2 h on average) during the rainy season from November to April; and (3) drainage pattern is very dense. Following the 22 November 1994 eruption of Merapi, 31 rain-triggered lahar events were recorded in the Boyong River between December 1994 and May 1996. On Merapi's slopes, instantaneous sediment concentration at any given time of the lahars varies widely over time and space. Lahars are transient sediment-water flows whose properties are unsteady, so that the sediment load fluctuates during the flow. The boundary between the flow types (debris flow, with sediment concentration >60% volume, or hyperconcentrated flow, with sediment concentration ranges from 20% to 60% volume) may fluctuate within the flow itself. Grain-size distribution, physical composition of sediments, shear stress, yield stress, and water temperature play each a role on this boundary. Natural self-damming and rapid breakout are partly responsible for the sediment variations of the flows. Debris-flow phases at Merapi typically last a few minutes to 10 min, and are often restricted to the lahar front. Debris-flow surges are sometimes preceded and always followed by longer hyperconcentrated flow phases. As a result, mean sediment concentration of the lahars is low, commonly from 20% to 50% volume. Besides, transient normal streamflow phases (sediment concentration Low sediment load and frequent transient flows in the Merapi channels may result from at least three factors: (1) several breaks-in-slope along the channel increase the deposition rate of sediment, and hinder the bulking capacity of the lahars; (2) source material is mainly coarse debris of “Merapi-type” block-and-ash flows. Consequently, the remobilization of coarse debris is more difficult and the clast deposition is accelerated; (3) variations of rainfall intensity over time and space, common in tropical monsoon rainfall, also influence the sediment load variations of the lahars. Sedimentologic analyses of the lahar deposits in the Boyong River at Merapi encompass clast-supported and matrix-supported debris-flow deposits, hyperconcentrated flow deposits, and streamflow deposits. The stratigraphic succession of massive and stratified beds observed immediately after any given lahar event in the Boyong River indicates that the sediment concentration varies widely over time and space during a single lahar event. Sedimentation rate varies from 3 to 4.5 cm/min during relatively long-lived surges to as much as 20 cm/min during short-lived surges. These results indicate that the sediment load fluctuates during lahar flow, further demonstrating that lahars are transient sediment-water flows with unsteady flow properties.

Spatial distribution of rock fragments in cultivated soils in northern Ethiopia as affected by lateral and vertical displacement processes

Although, in semi-arid environments, rock fragments at the soil surface and within the topsoil play an important role in desertification control, little is known about their spatial distribution. Therefore, this study analyses spatial patterns of rock fragment cover along catenas, and the vertical variations in volumetric rock fragment content in soil profiles in the highlands of Tigray, northern Ethiopia. Natural and anthropogenic processes inducing these patterns are assessed. Volumetric rock fragment content ( R V) was analysed in 10 soil pits. All rock fragments were extracted, and their volumes determined by pedo-stratigraphic unit, size and lithology. The rock fragment cover ( R C) was determined by the point-count method using vertical photographs of the soil surface. The following processes contribute to the vertical variability of R V: (1) in Vertisols, upsqueezing as a consequence of swell-shrink cycles (argillipedoturbation) is responsible for high R C at the soil surface; (2) large rock fragments (>7.5 cm) are rapidly brought to the soil surface by kinetic sieving through tillage, even in the case of continuous fine sediment deposition, what may result in a rock fragment rich subsoil, underlying a thick soil layer (up to 80 cm) without large rock fragments and a topsoil with a high R C at the surface; (3) Skeletic Regosols at the foot of cliffs show no systematic vertical rock fragment distributions. As to rock fragment cover along the catena, some fundamental differences appear between the basalt and limestone substrate. On the basalt catena with slope gradients between 0.06 and 0.42 m m −1, R C is high everywhere (57–85%) and is unrelated to slope gradient. Vertical processes such as kinetic sieving through ploughing and argillipedoturbation determine the rock fragment distribution at the soil surface. In limestone areas, argillipedoturbation is less active and R C is positively correlated with slope gradient ( R 2=0.74; n=6; P<0.05), when only the rock fragments (0.5–2.0 cm across) are considered. Including the larger (>2 cm) and more kinetic sieving sensitive rock fragments lowers R 2 to 0.46 ( n=6; n.s.). Preconditions for the present spatial distribution of rock fragments at the soil surface in the study area are the occurrence of two geomorphic processes in the past: (1) mudflows depositing diamictons including much coarse debris and (2) intense water erosion, which occurred after deforestation and exposed rock fragments at or near to the soil surface. Generally, the balance between lateral and vertical movements of rock fragments now controls the spatial distribution of R C. With respect to the lateral displacement processes: (1) lateral transport over the soil surface by trampling, tillage and concentrated runoff, especially on steep slopes, and (2) rockfall from the cliffs. Vertical supply of rock fragments to the soil surface is caused by (1) selective runoff erosion and the development of erosion pavements, (2) tillage induced kinetic sieving, bringing preferentially large rock fragments (>7.5 cm) to the soil surface, and (3) argillipedoturbation in Vertisol areas.

The chemical weathering regime of Kärkevagge, arctic–alpine Sweden

Kärkevagge is a valley located in Swedish Lapland at approximately 68°N and represents an arctic–alpine landscape. It is a presently periglacial, glaciated trough incised into essentially horizontal metamorphic rocks, some of which are presumably pyrite-rich. A set of coordinated studies was undertaken to investigate the nature of chemical weathering and pedogenesis in the valley and upon the abutting ridges. August 1996 water quality measures reveal considerable spatial variation in solute totals with the highest Total Dissolved Solute abundances being correlated with high sulfate abundances. Ridge-crest soils exhibited poor horizonation, but more extensive development of secondary clay minerals developed in situ than was found in valley-flank and valley-bottom soils. Valley soils exhibited multiple thin horizons, many of which were buried, and are taken to reflect great paraglacial and periglacial instability. Favorable microenvironments in the valley permit significant development of Spodosols. Coarse debris along and across the valley bears both weathering rinds and rock coatings. Rock coatings in the valley include several types of iron films, sulfate crusts, carbonate skins, and heavy metal skins. Kärkevagge represents a mild arctic environment, which does not preclude substantial chemical weathering in locations where abundant pyrite-rich bedrock and water coincide. This weathering follows pathways which are eminently expectable given that weathering of the pyrite-rich rock permits generation of sulfuric acid which, in turn, weathers muscovite mica and calcite in local schists and marble, respectively. Zones of intense chemical weathering also generate clearly visible deposits of gypsum and iron sulfate deposits such as jarosite. Not all arctic and/or alpine environments are likely to be so active chemically, but the results from Kärkevagge clearly show that dismissal of chemical weathering in cold regions on the basis of presumed first principles is erroneous. Consequently, chemical weathering in such environments merits substantially more attention than it has hitherto received.

A model of permafrost formation and occurrences in the intracontinental mountains

The proposed method of permafrost distribution mapping entails the use of a computer-based model that takes into account the specific conditions of arid mountains for permafrost formation and occurrence. In the intracontinental arid mountains, incoming short-wave solar radiation, mean annual temperature and evaporation are the main influences on the distribution of permafrost. Coarse debris is widespread in the high mountains and greatly influences the thermal state of the ground. The model can compute heat transfer in the coarse debris, which is considered a porous body, and it can be used for permafrost distribution mapping with various degrees of spatial resolution. As a result, the GIS-based map of permafrost distribution for the Bolshaya Almatinka River basin, Kazakhstan, was generated.

Drainage of late Wisconsin glacial lakes and the morphology and late quaternary stratigraphy of the New Jersey–southern New England continental shelf and slope

Abstract We propose that late Wisconsin deposition and erosion (Hudson Shelf and Block Island valleys) on the shelf and slope from New Jersey to southern New England were a consequence of the catastrophic drainage of glacial lakes behind terminal moraine systems and the huge volume of water stored beneath the Laurentian ice sheet and subsequent erosion of the lake sediments by flash floods. The morphology imparted by glaciation regulated the discharge associated with the ablation of the glaciers. Associated with the deposits west of Hudson Shelf Valley are the remains of mammoth and mastodon which were transported from their living habitats along the lake shores to their present burial sites on the shelf. The floods also triggered gravity flows on the upper continental slope which made possible the transportation of coarse debris over hundreds of km into the deep-sea. That these catastrophic flood morphologies can still be recognized on the middle to outer shelf suggest that much of its surface was little modified during the late Pleistocene/Holocene transgression. Thus the late Pleistocene/Holocene transgression may have been characterized by short periods when sea level rose rapidly allowing for the preservation of relict features.

Post-glacial colluvium in western Norway: depositional processes, facies and palaeoclimatic record

The postglacial Quaternary colluvial systems in western Norway are arrays of steep fans, often coalescing into aprons, developed along the slopes of valley sides and fjord margins. The coarse debris, derived from weathered gneissic bedrock and its glacial-till mantle, varies from highly immature to mature. The depositional processes are mainly avalanches, ranging from rockfalls and debrisflows to snowflows, but include also waterflow and debris creep. The mechanics and sedimentary products of these processes are discussed, with special emphasis on snow avalanches, whose role as an agent of debris transport is little-known to sedimentologists. The subsequent analysis of sedimentary successions is focused on colluvial-fan deltas, which are very specific, yet little-studied, coastal depositional systems. The stratigraphic variation and depositional architecture of the colluvial facies assemblages, constrained by abundant radiometric dates, are used to decipher the signal of regional climatic changes from the sedimentary record. The stratigraphic data from two dozen local colluvial successions are compiled and further compared with other types of regional palaeoclimatic proxy record. The analysis suggests that the colluvial systems, although dependent upon local geomorphic conditions, have acted as highly sensitive recorders of regional climatic changes. The study as a whole demonstrates that colluvial depositional systems are an interesting and important frontier of clastic sedimentology.

Analysis of debris flow surges using the theory of uniformly progressive flow

Coarse debris flows develop surges with distinct longitudinal sorting. Although highly unsteady, such flow often appears to attain a steady-state condition, moving over long distances with approximately constant velocity and maximum depth. Typically, a steep, bouldery front is followed by an accumulation of liquid slurry, which in turn decays into a dilute tail. Such sorting has long been recognized by field workers, but its influence on the dynamic behaviour of debris flow surges has not yet been fully clarified by analysis. A simple model is presented, using the theory of uniformly progressive flow and incorporating zoned longitudinal variation in rheology. It is shown that non-homogeneity can cause very significant magnification of the peak discharge, depending on the slope angle and on the length of the frontal boulder concentration. The shape of the surge flow profiles is determined not only by the rheology of the retained material, but by the longitudinal variation of material characteristics. As a result, excessive reliance on laboratory-derived rheological constitutive relationships is not advisable. Models of debris flow surges should be non-homogeneous and able to incorporate zones of contrasting rheology. Copyright © 2000 John Wiley & Sons, Ltd.

Effects of punctuated sediment supply on valley-floor landforms and sediment transport

Research Article| December 01, 2000 Effects of punctuated sediment supply on valley-floor landforms and sediment transport Daniel J. Miller; Daniel J. Miller 1Earth Systems Institute, 1314 North East 43rd Street, Seattle, Washington 98105, USA Search for other works by this author on: GSW Google Scholar Lee E. Benda Lee E. Benda 1Earth Systems Institute, 1314 North East 43rd Street, Seattle, Washington 98105, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (12): 1814–1824. https://doi.org/10.1130/0016-7606(2000)112<1814:EOPSSO>2.0.CO;2 Article history received: 18 Feb 1999 rev-recd: 15 Sep 1999 accepted: 15 Nov 1999 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Daniel J. Miller, Lee E. Benda; Effects of punctuated sediment supply on valley-floor landforms and sediment transport. GSA Bulletin 2000;; 112 (12): 1814–1824. doi: https://doi.org/10.1130/0016-7606(2000)112<1814:EOPSSO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Large, infrequent fluxes of sediment to streams by mass wasting are intrinsic to the erosion regime of mountain drainage basins. To elucidate the role of mass wasting in the construction and evolution of steep land channel environments, it is crucial that we identify the processes involved and recognize their legacy on the valley floor. In the winter of 1996, nine storm-triggered debris flows carried ∼18 000 m3 of coarse debris into the upper reaches of the South Fork of Gate Creek (Oregon Cascade Range) during flood flow. Analysis of resulting channel morphologies and bed textures shows that the sediment moved downstream as a wave-like pulse or pulses, overwhelming the channel and causing it to braid, with flooding and alluvial deposition over the valley floor. Downstream progression of the sediment wave resulted in vertical accretion of the valley floor with sediment carried as bedload, the maximum depth of valley-floor burial being set by the amplitude of the wave. Passage of the wave left a channel incised to bedrock, inset between coarse-grained alluvial terraces. This study examines the genesis of these features at Gate Creek and points to such terraces as field indicators of massive episodic influxes of sediment and the associated formation of fluvially transported sediment waves. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Relict periglacial slope deposits in the Hex River Mountains, South Africa: observations and palaeoenvironmental implications

Shattered bedrock near the summit of Matroosberg, Western Cape Mountains, is closely associated with several blockstreams which grade downslope into broad terraces. With an increase in slope angles the regolith grades into broad bands of coarse but sorted material parallel to the maximum gradient. The width of the coarse stripes increases in the downslope direction, where they merge into openwork block deposits several tens of metres long and wide. Whereas the terraces near the summit plateau closely resemble gelifluction sheets, the large sorted stripes and coarse debris mantles are analogous to the stone-banked terraces described for the Colorado Front Range. The landforms are primarily related to seasonal frost creep at the Last Glacial Maximum (LGM) when mean annual temperatures are estimated to have been around 0°C, i.e., winter temperatures 7–8°C lower than present. Snowcover must have been limited to allow deep frost penetration.

Field observations of a debris flow event in the Dolomites

A debris flow event occurred in June 1997 in the Dolomites (Eastern Alps, Italy). The phenomenon was directly observed in the field and recorded by a video camera near its initiation area. The debris flow originated shortly after an intense rainstorm (25 mm in 30 min) whose runoff mobilised the loose coarse debris that filled the bottom of the channel in its upper part. The analysis of the steep headwater basin indicates a very short concentration time (9–14 min) that fits the quick hydrological response observed in the field. The debris flow mobilisation was not contemporaneous with the arrival of the peak water discharge in the initiation area probably due to the time required for the saturation of the highly conductive channel-bed material. Channel cross-section measurements taken along the flow channel indicate debris flow peak velocity and discharge ranging from 3.1 to 9.0 m/s and from 23 to 71 m 3/s, respectively. Samples collected immediately after deposition were used to determine the water content and bulk density of the material. Channel scouring, fines enrichment and transported volume increase testify erosion and entrainment of material along the flow channel. Field estimates of the rheological properties based on open channel flow of Bingham fluid indicate a yield strength of 5000±400 Pa and relatively low viscosity (60–326 Pa s), probably due to a high percentage of fines (approx. 30%).

Sedimentation processes and facies on a semi-vegetated talus, Lousteau, southwestern France

A two-year survey of clast displacements on a small rockfall talus at Lousteau, southwestern France, shows that creep reaches up to 19 cm/month near the talus apex, whereas the distal segment remains stable. Movements are not distributed homogeneously but concentrate in distinct areas because of the deflection of clasts by vegetation patches. Sections across the talus reveal stratified deposits, thought to result from the lateral shifting of the tongues of very mobile debris. Poorly sorted gravel layers alternate with coarser debris corresponding to former pavements developed where sedimentation is reduced due to the protective effect of vegetation. Similar deposits have been found in larger, semi-vegetated taluses from the forest belt of the French Alps. Interactions between vegetation development and sedimentation possibly controlled by the climatic changes during postglacial times, may be responsible for their stratification. Copyright © 1999 John Wiley & Sons, Ltd.

A rock glacier/debris‐covered glacier system at galena creek, absaroka mountains, wyoming

At Galena Creek, the ice/debris system has alternated between rock glacier and debris‐covered glacier modes due to changes in the size of the accumulation area. In the rock glacier mode, coarse debris is concentrated at the base of couloirs along the lower edge of a small, steep accumulation area where it becomes part of the surface debris mantle of the rock glacier. In the debris‐covered glacier mode, debris is primarily incorporated englacially within an expanded accumulation area. Englacial debris originating on the sides of the cirque is concentrated at the margins of the glacier; debris originating from the cirque headwall is concentrated where it emerges in the ablation area and forms a debris cover. As the equilibrium line rises during warm periods, the debris‐covered glacier is transformed into a rock glacier. The marginal englacial debris is exposed progressively up‐glacier resulting in ice‐cored debris ridges which extend into the cirque and are analogous to the ice‐cored lateral moraines of many subpolar alpine glaciers. The debris‐covered terminus continues to flow as a rock glacier lobe. The debris ridges/lateral moraines record previous positions of a debris‐covered glacier margin and can be used to map past accumulation areas and to infer the position of the associated glacier terminus. At least three of these features are cross‐cut by the rock glacier. Age estimates for three glacial advances are made using reasonable surface velocities and the distance between a rock glacier lobe and the inferred position of its associated glacier terminus. The estimated ages correlate with regional alpine glacier chronologies. These observations, which are not unique to the Absaroka Range, have general utility for understanding how changes in the relative flux of ice and debris control the morphology of the resulting deposits and for placing rock glacier deposits within alpine glacier chronologies.

Postglacial colluvium in western Norway: depositional processes, facies and palaeoclimatic record

The postglacial Quaternary colluvial systems in western Norway are arrays of steep fans, often coalescing into aprons, developed along the slopes of valley sides and fjord margins. The coarse debris, derived from weathered gneissic bedrock and its glacial‐till mantle, varies from highly immature to mature. The depositional processes are mainly avalanches, ranging from rockfalls and debrisflows to snowflows, but include also waterflow and debris creep. The mechanics and sedimentary products of these processes are discussed, with special emphasis on snow avalanches, whose role as an agent of debris transport is little‐known to sedimentologists. The subsequent analysis of sedimentary successions is focused on colluvial‐fan deltas, which are very specific, yet little‐studied, coastal depositional systems. The stratigraphic variation and depositional architecture of the colluvial facies assemblages, constrained by abundant radiometric dates, are used to decipher the signal of regional climatic changes from the sedimentary record. The stratigraphic data from two dozen local colluvial successions are compiled and further compared with other types of regional palaeoclimatic proxy record. The analysis suggests that the colluvial systems, although dependent upon local geomorphic conditions, have acted as highly sensitive recorders of regional climatic changes. The study as a whole demonstrates that colluvial depositional systems are an interesting and important frontier of clastic sedimentology.

The structure and sedimentology of relict talus, Trotternish, northern Skye, Scotland

Sections up to 3·5 m deep cut through the upper rectilinear segment of relict, vegetated talus slopes at the foot of the Trotternish escarpment reveal stacked debris-flow deposits intercalated with occasional slopewash horizons and buried organic soils. Radiocarbon dating of buried soil horizons indicates that reworking of sediment by debris flows predates 5·9–5·6 Cal ka BP, and has been intermittently active throughout the late Holocene. Particle size analyses of 18 bulk samples from these units indicates that c. 27–30 per cent of the talus deposit is composed of fine (<2 mm) sediment. Sedimentological comparison with tills excludes a glacigenic origin for the talus debris, and the angularity of constituent clasts suggests that in situ weathering has been insignificant in generating fine material. We conclude that the fine sediment within the talus is derived primarily by granular weathering of the rockwall, with syndepositional accumulation of both fine and coarse debris, implying that c. 27–30 per cent of rockwall retreat since deglaciation reflects granular weathering rather than rockfall. The abundance of fines within the talus deposits is inferred to have been of critical importance in facilitating build-up of porewater pressures during rainstorms, leading to episodic failure and flow of debris on the upper parts of the slope. A wider implication of these findings is that the mechanical properties of talus slopes cannot be regarded as those of free-draining accumulations of coarse clasts, and that models that treat talus slopes as such have limited value in explaining their form and evolution. Our findings lend support to models that envisage the upper straight slope on talus accumulations as the product of mass-transport as well as rockfall, and indicate that episodic debris flow has been the primary agent of mass-transport at this site. © 1998 John Wiley & Sons, Ltd.

The dynamic of organic carbon in South Cameroon: fluxes in a tropical river system and a lake system as a varying sink on a glacial–interglacial time scale

In the first attempt to estimate both (i) a bulk carbon flux in a tropical river system (mainly Sanaga River) and (ii) their palaeoenvironmental implications from the Last Glacial Maximum (LGM) to the present, this study presents a synthetic approach based on the combined use of modern evaluation of fluxes and estuarine biodegradation in the tropical river system Sanaga and nearby Douala Bay rivers, and of sedimentation rates of a well studied marine shelf and lake system (Barombi-Mbo). In the lake Barombi-Mbo, the Holocene transfer of particulate carbon (96.6×10 3 t) is very close to the mass fixed presently in soil catchments (117×10 3 t). A complete process of stored carbon consumption would require some 10 4 years, namely the Holocene period. During the last 20,000 years, variations in the sediment organic matter can be explained by the change of the vegetation cover, particularly with the substitution of open environments by forests. The global sedimentation was slow between ca. 18,000 and 10,000 years BP and increased after 12,000 years. But the carbon sedimentation rate remains fairly constant as the carbon content is higher in the LGM deposits. Such LGM carbon concentrations are probably explained by the input of coarse debris by rough floods and by a less degraded organic matter as a result of the cooling of the climate. Today, the total transport of dissolved and particulate organic carbon of the Sanaga and Douala Bay rivers to the Guinea Gulf is estimated as 0.62 to 0.79×10 6 t C yr −1. Based on 50% biodegradation at the estuarine interface, the loss of organic matter per unit of land is evaluated around 8.8 t C km −2 yr −1. Marine oceanic records of the carbon sedimentation rate reflect with difficulty the major palaeoenvironmental changes according to interfering hydrodynamic factors. The greatest input of organic carbon during warm marine biozones would be balanced by higher concentrations during the LGM resulting in a nearly homogenous carbon transfer during the last 20,000 years. Such results might be largely representative of tropical river system as the contrasting vegetal cover (savanna and forest) of the Sanaga basin reflected as well the majority of the intertropical ecosystem. Thus, an estimate of the Holocene transfer to the ocean up to four times the present carbon stored in soil of the surrounding continent implicates that the Holocene shelf was a significant organic carbon sink. Although the sources of the Sanaga River are located in a mountain region, a significant floodplain is not found downstream. This results in a significant altitudinal factor in the carbon fluxes to the ocean.

Recovery of fresh debris of different sizes in density fractions of two contrasting soils

Fresh plant residues are often identified with the light organic matter obtained by fractionation in a dense liquid, but the extent to which residues can be extracted efficiently by densimetric methods has not been widely studied. This paper presents the results of two experiments in which 14C-labelled straw was mixed with two soils of differing texture, and the mixture was subjected to densimetric fractionation. In the first experiment, soil was mixed with bulk milled straw, and in the second, with straw of different sizes (> 200 μm, 200-50 μm, 50-20 μm and < 20 μm). The recovery of straw in the light fraction was low, and decreased with decreasing straw size, reaching a minimum (less than 20 %) for straw < 20 μm. Except for straw < 20 μm, the recovery in the light fraction was lower in soil richest in silt and clay. In the absence of soil (blanks), the recovery of fine straw in the light fraction was lower than in the presence of soil, suggesting that the recovery of fine straw in the light fraction is partly due to its association with light coarse debris. Extractable and polytungstate-soluble fractions accounted for a small proportion of the 14C-activity. These results suggest that densimetric methods are not efficient for recovering fresh plant residues, except in the case of large residues in coarse-textured soils.