Assessment of gully erosion rates through interviews and measurements: a case study from northern Ethiopia

Gullying has been widespread in the Ethiopian Highlands during the 20th century. It threatens the soil resource, lowers crop yields in intergully areas through enhanced drainage and desiccation, and aggravates flooding and reservoir siltation. Knowing the age and rates of gully development during the last few decades will help explain the reasons for current land degradation. In the absence of historical written or photographic documentation, the AGERTIM method (Assessment of Gully Erosion Rates Through Interviews and Measurements) has been developed. It comprises measurements of contemporary gully volumes, monitoring of gully evolution over several years and semi‐structured interview techniques. Gully erosion rates in the Dogu'a Tembien District, Tigray, Ethiopia, were estimated in three representative case‐study areas. In Dingilet, gullying started around 1965 after gradual environmental changes (removal of vegetation from cropland in the catchment and eucalyptus plantation in the valley bottom); rill‐like incisions grew into a gully, which increased rapidly in the drier period between 1977 and 1990. The estimated evolution of the total gully volume in the other areas show patterns similar to those of the Dingilet gully. Average gully erosion rate over the last 50 years is 6·2 t ha^?1 a^?1. Since 1995, no new gullies have developed in the study area. Area‐specific short‐term gully erosion rates are now on average 1·1 t ha^?1 a^?1. The successful application of the AGERTIM method requires an understanding of the geomorphology of the study area and an integration of the researchers with the rural society. It reveals that rapid gully development in the study area is some 50 years old and is mainly caused by human‐induced environmental degradation. Under the present‐day conditions of ‘normal’ rain and catchment‐wide soil and water conservation, gully erosion rates are decreasing. Copyright © 2006 John Wiley & Sons, Ltd.     

Analysis of Post-Failure Slope Movements within the Framework of Hazard and Risk Analysis

The paper first recalls briefly a methodological framework to assess landslide hazard and risk analysis in terms of predisposition, triggering andrevealing factors. This framework, that reflects the mechanisms involved in the landslide, is based on the Geotechnical Characterisation of slope movements proposed by Vaunat et al. (1994) and Leroueil et al. (1996). The Geotechnical Characterisation can be schematized by a 3-D matrix having the following axes: types of movement; types ofmaterial; and the four stages of movement: pre-failure, failure, post-failure andreactivation. For each relevant element of this 3-D matrix, there is a characterisationsheet including: the controlling laws and parameters, the predisposition factors, thetriggering or aggravating factors, the revealing factors and the consequences of the movement. The paper focuses afterwards on the post-failure stage, which generallyis the most destructive, and on the mobility index. It is shown that this laterindex can be described as the product of sub-indices associated with failure, brittlenessof the material, ability of the soil to develop pore pressures, geometry of the moving soil mass and characteristics of the terrain. It is also shown how these aspectscan be incorporated into the Geotechnical characterisation of slope movements. This seems to provide a rational basis for examining slope movements at the post-failure stage and assessing associated risks.     

Présentation d'un capteur classeur de particules à lame unique et d'un capteur absolu permettant de l'étalonner

Résumé Dans le capteur monolame à un seul jet proposé ici, l'air est projeté obliquement sur la lame de verre en amont du jet; ce capteur permet ainsi d'obtenir un prélèvement de particules classées suivant leurs dimensions sur une grande surface d'une seule lame. Par ailleurs ce capteur, en plexiglas, permet de surveiller la lame en cours de prélèvement. Les coefficients de captation pour des gouttelettes de différentes dimensions ont été déterminés en comparant les prélèvements obtenus avec le capteur monolame à ceux obtenus avec un capteur à deux jets identiques en série, réalisé afin de permettre un comptage de toutes les gouttelettes déposées sur les parois intérieures. On a de la même façon déterminé les coefficients de captation d'un cascade impactor. Les coefficients du capteur monolame et du cascade impactor sont trouvés assez peu différents, et leurs valeurs restent comprises entre 40 et 86% pour des gouttelettes de 1 à 30 de rayon. Enfin un paragraphe de cette communication indique comment peut être utilisé le capteur monolame lors de prélèvements dans un nuage.

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Summary In the monoslide and monojet impactor here proposed, the air is obliquely projected on the slide, up the jet; thus this impactor gives samples of particles classified according to their sizes on a large surface of only one slide. On another way, this impactor, made of plexiglas, allows the supervision of the slide during the sampling. The different size droplets sampling coefficients have been determined by comparison of samples obtained with an impactor having two identical jets in series, this impactor being realized in order to permit a counting of all the droplets settled in the interior walls. On the same way the sampling coefficients of a cascade impactor have been determined. The monoslide impactor coefficients and these of the cascade impactor have been found nearly identical and their values remain included between 40 and 86% for 1 to 30 radius droplets. Lastly a paragraph of this paper shows how the monoslide impactor can be used when sampling in a cloud.

     

The granite hosted gold deposit of Moulin de Chéni (Saint-Yrieix district,Massif Central,France): petrographic,structural, fluid inclusion and oxygen isotope constraints

The Moulin de Chéni orogenic gold deposit is the only granite-hosted deposit of the Saint-Yrieix district, French Massif Central. It occurs in 338±1.5 Ma-old peraluminous leucogranites and is characterized by intense microfracturing and bleaching of the granite in relation to pervasive sulfide crystallization. Formation of quartz veins and gold deposition occurred in two successive stages: an early mesozonal stage of quartz-sulfide (Fe-As-S) deposition, usually devoid of gold and a late epizonal stage of base metal and gold deposition. Both stages postdate peak metamorphism and granite intrusion. The genesis of the deposit is the result of four successive fluid events: (1) Percolation of aqueous-carbonic metamorphic fluids under an assumed lithostatic regime of 400–450 °C, at a maximum depth of 13 km; (2) Formation of the main quartz lodes with coeval K-alteration and introduction of As and S from aqueous-carbonic fluids percolating along regional faults. Arsenopyrite and pyrite deposition was linked to the alteration of Fe-silicates into K-feldspar and phengite at near-constant iron content in the bulk granite. Temperature was similar to that of the preceding stage, but pressure decreased to 100–50 MPa, suggesting rapid uplift of the basement up to 7.5 km depth; (3) The resulting extensional tectonic leads to the deposition of gold, boulangerite, galena and sphalerite in brecciated arsenopyrite and pyrite from aqueous fluids during a mixing process. Temperature and salinity decrease from 280 to 140 °C and 8.1 wt% eq. NaCl to 1.6 wt% eq. NaCl, respectively; (4) Sealing of the late fault system by barren comb quartz which precipitated from dilute meteoric aqueous fluids (1.6 wt% eq. NaCl to 0.9 wt% eq. NaCl) under hydrostatic conditions at 200–150 °C.Editorial handling: B. Lehmann     

Mineralogy and geochemical behaviour during weathering of greenstone belt under tropical dry conditions in the extreme North Cameroon (Central Africa)

Mineralogy, major, trace and rare earth elements of a weathering profile developed on tertiary greenstone belt in the extreme North Cameroon are reported. The aim of which was to investigate mineralogical evolution and element mobilization and redistribution during weathering under dry tropical climate. The weathering profile consists of four main horizons: (1) a spheroidal weathering zone constituted by a corestone–shell complex, (2) a C horizon, (3) a Bw horizon and an Ah horizon. The results indicate that nontronite, a Fe-rich smectite, is the exclusive clay mineral formed in the exfoliated shells and the C horizon. It is associated with kaolinite in the upper horizons. The coexistence of these two clay minerals induced a decrease of CEC and pH which becomes neutral. The weathering index (WI) values reveal that weathering becomes more and more intensive from the corestone up to Bw horizon, which is the most weathered horizon in the weathering profile. Mass balance calculations, using Th as immobile element, indicate that Ti is quite mobile and that Al and Fe are relatively enriched at the bottom and strongly leached at the top of the profile. Alkalis and alkaline earth elements are strong leached through out the profile, except Ca which displays similar trend as Al and Fe. The same goes for LILE (Cs, Sr), TTE (Cr, Co, Ni) and HSFE (Y, Nb, Hf). In opposite, REE are depleted at the bottom and enriched in the upper horizons, with more enrichment for LREE than for HREE. It appears that weathering of greenstone belt causes a fractionation of HREE and induces a concentration of LREEs. Ce and Eu anomalies display opposite behaviour.     

High‐ to ultrahigh‐temperature metamorphism in the lower crust: An example resulting from Hikurangi Plateau collision and slab rollback in New Zealand

Lower crustal xenoliths erupted from an intraplate diatreme reveal that a portion of the New Zealand Gondwana margin experienced high‐temperature (HT) to ultrahigh‐temperature (UHT) granulite facies metamorphism just after flat slab subduction ceased at c. 110–105 Ma. P–T calculations for garnet–orthopyroxene‐bearing felsic granulite xenoliths indicate equilibration at ~815 to 910°C and 0.7 to 0.8 GPa, with garnet‐bearing mafic granulite xenoliths yielding at least 900°C. Supporting evidence for the attainment of HT and UHT conditions in felsic granulite comes from re‐integration of exsolution in feldspar (~900–950°C at 0.8 GPa), Ti‐in‐zircon thermometry on Y‐depleted overgrowths on detrital zircon grains (932°C ± 24°C at aTiO₂ = 0.8 ± 0.2), and correlation of observed assemblages and mineral compositions with thermodynamic modelling results (≥850°C at 0.7 to 0.8 GPa). The thin zircon overgrowths, which were mainly targeted by drilling through the cores of grains, yield a U–Pb pooled age of 91.7 ± 2.0 Ma. The cause of Late Cretaceous HT‐UHT metamorphism on the Zealandia Gondwana margin is attributed to collision and partial subduction of the buoyant oceanic Hikurangi Plateau in the Early Cretaceous. The halt of subduction caused the fore‐running shallowly dipping slab to rollback towards the trench position and permitted the upper mantle to rapidly increase the geothermal gradient through the base of the extending (former) accretionary prism. This sequence of events provides a mechanism for achieving regional HT–UHT conditions in the lower crust with little or no sign of this event at the surface.     

Observed Behaviour of Laterally Expanded Stone Column in Soft Soil

The paper describes the behavior of remolded kaolin clay reinforced by stone column as investigated in laboratory. The installation of stone column was simulated by performing lateral expansion at different rates within hollow cylindrical remolded kaolin specimens initially subjected to K0 consolidation path. These specimens were, then, subjected to classical consolidated undrained triaxial tests while recording excess pore pressure values. The objective of the experimental programme was to quantify the effects of consolidation stress and stone column on the undrained Young’s modulus and shear strength of kaolin clay. Obtained results showed a significant improvement in Young’s modulus when the cavity expansion ratio and the consolidation stress increase. It was also found that the increase in undrained shear strength of improved kaolin clay mainly occurred at lower consolidation stress. Another important finding of this study is that the ratio of undrained Young’s modulus to undrained shear stress increases when the consolidation stress decreases. Finally, the paper presents a model developed for the design of stone columns.     

Buried and Submerged Greek Archaeological Coastal Structures and Artifacts as Gauges to Measure Late Holocene Seafloor Subsidence off Calabria,Italy

This synthesis integrates recently acquired archaeological and geological data with earlier documented observations to shed light on the subsidence of ancient Greek coastal facilities in southern Italy. These are now positioned between former shorelines and inner shelf sectors at five Calabrian margin localities. Submergence of coastal to inner shelf facilities has resulted in part from sea‐level rise by about 2 m associated with glacio‐hydro‐isostatic factors since archaic to classic Greek time. This phenomenon alone, however, does not explain the wide variation of measured subsidence rates from site‐to‐site. The marked lowering of coastal site substrates by seismo‐tectonic activity (including extensional fault motion), stratal readjustments at depth, and compaction of underlying sediment sequences is significant. Four of the subsided facilities are positioned near emerged Calabrian areas where prevailing Holocene average annual land uplift rates range to ˜1.0 mm/yr; at the fifth, near Hipponion, terrains have risen by nearly 2 mm/yr. In marked contrast, submerged and/or buried structures record the following late Holocene long‐term average rates of coastal margin subsidence: Sybaris‐Thuri on the Taranto Gulf margin (˜0.5–1.0 mm/yr); Hipponion‐Vibo Valentia along the Tyrrhenian coast (˜0.8 to ˜3.2 mm/yr); and Locri‐Epizefiri, Kaulonia, and Capo Colonna on Calabria's Ionian margin (˜1.6, ˜1.6–2.4, and ˜4.0 mm/yr, respectively). © 2012 Wiley Periodicals, Inc.     

First evidence of Campanian radiolarians in Turkey and implications for the tectonic setting of the Upper Antalya Nappes

The definition and inventory of the upper units of the Antalya Nappes or “Calcareous Antalya Nappes” (CAN) are still a matter of controversies and often conflicting interpretations. In the Gedeller type locality, we logged a new succession that sheds light on the detailed stratigraphy of the Upper Antalya Nappes. The lower part of the series corresponds to the uppermost part of the Kemer Gorge Nappe and is overthrust by the Ordovician Seydişehir Formation of the Tahtalı Dağ Nappe. The newly described Gedeller Formation belongs to the Kemer Gorge Nappe and is represented by Campanian (Upper Cretaceous) Scaglia-type pelagic limestones, which yielded radiolarians of the Amphipyndax pseudoconulus Zone. It is demonstrated that the “Calcareous Antalya Nappes” are composed of three different nappes, the Kemer Gorge, Bakırlı and the Tahtalı Dağ nappes, all of them belonging to the Upper Antalya Nappes system.     

International Association of Geoanalysts' Protocol for the Certification of Geological and Environmental Reference Materials: A Supplement

The International Association of Geoanalysts (IAG) published a protocol for the certification of reference materials in close accord with the International Organisation for Standardisation (ISO) guidelines (Kane et al. 2003). This article supplements that protocol, providing additional discussion of best approaches for pre-selecting laboratories for participation in certification projects. This discussion also makes a distinction between inter-laboratory certifications, where n = 15 is the general standard, and expert laboratory certifications, where a much smaller number of laboratories will be deemed qualified to provide data of the quality needed for certification.