Depositional age and tectonic environment of the Gouap banded iron formations from the Nyong group,SW Cameroon:Insights from isotopic,geochemical and geochronological studies of drillcore samples

The discovery of the Gouap banded iron formations(BIFs)-hosted iron mineralization in the northwestern of the Nyong Group(Ntem Complex)in southwestern Cameroon provides unique insights into the geology of this region.In this contribution,we firstly report detailed study of geochemistry,isotopic and geochronology of well preserved samples of the Gouap BIFs collected from diamond drillcores.The Gouap BIFs consist mainly of amphibole BIFs and amphibole-pyrite BIFs characterized by dominant Fe₂O₃+SiO₂contents and variable contents of CaO,MgO and SO₃,consistent with the presence of amphibole,chlorite,epidote and pyrite,formed during amphibolite facies metamorphism and overprinted hydrothermal event.The amphibole–pyrite BIFs are typically enriched in trace and rare earth elements(REE)compared to the amphibole BIFs,suggesting the influence of detrital materials as well as secondary hydrothermal alteration.The Post Archean Australian Shale(PAAS)-normalized REE–Y profiles of the Gouap BIFs display positive La,Eu anomalies,weak negative Ce anomalies,indicating a mixture of low-temperature hydrothermal fluids and relatively oxic conditions probably under relative shallow seawater.We present here the first isotopic data of BIFs within the Ntem Complex.Theδ³⁰Si_NBS28values of the quartz from the Gouap BIFs vary from-1.5‰to-0.3‰and from-0.8‰to-0.9‰for the amphibole BIFs and amphibole–pyrite BIFs,respectively.The quartz hasδ¹⁸OV-SMOW values of 6.8‰–9.5‰(amphibole BIFs)and 9.2‰–10.6‰(amphibole–pyrite BIFs).The magnetite from the Gouap BIFs showsδ¹⁸O values ranging from-3.5‰to-1.8‰and from-3‰to-1.7‰for the amphibole BIFs and amphibole–pyrite BIFs,respectively.Moreover,the pyrite grains in the amphibole–pyrite BIFs displayδ34S values of 1.1‰–1.8‰.All isotopic data of the Gouap BIFs confirm that they might have precipitated from low-temperature hydrothermal fluids with detrital input distant from the volcanic activity.According to their geochemical and isotopic characteristics,we propose that the Gouap BIFs belong to the Superior type.In situ U–Pb zircon dating of BIFs was conducted to assess the BIF depositional age based on strong evidence of zircon in thin section.The Gouap BIFs were probably deposited at 2422±50 Ma in a region where sediments extended from continental shelf to deep-water environments along craton margins like the Caue Formation of the Minas Supergroup,Brazil.The studied BIFs have experienced regional hydrothermal activity and metamorphism at 2089±8.3 Ma during the Eburnean–Transamazonian orogeny.These findings suggest a physical continuity between the protocratonic masses of both Sao Francisco and Congo continents in the Rhyacian Period.     

Petrographic and geochemical characterization of weathered materials developed on BIF from the Mamelles iron ore deposit in the Nyong unit,South-West Cameroon

Acta Geochimica - Iron ore deposits hosted by Precambrian banded iron formation (BIF) are the most important source of mineable iron. In Cameroon, they are located in the southern part of the...     

Rise and Fall of an Egyptian Oasis: Artesian Flow,Irrigation Soils,and Historical Agricultural Development in El‐Deir,Kharga Depression,Western Desert of Egypt

The present study examines the geoarchaeological history of an oasis in Kharga Depression in central Egypt. El‐Deir is renowned for its Ptolemaic temple and Roman fortress on the road from former Hibis (Kharga) to the Nile Valley. During the survey, spring mounds and irrigation soils belonging to an ancient agricultural zone were discovered, and further documented by ceramics found on the site. Our methodology combines the geomorphological interpretation of landforms (especially yardangs) with ceramics and ¹⁴C‐dated charcoal to distinguish and date former agricultural areas in El‐Deir. The results show that the oasis experienced several phases of soil accretion and destruction through time. Playa sediments were deposited in the humid early Holocene and severely eroded by deflation before the onset of irrigated agriculture between Pharaonic and Persian times. Very fast vertical soil accretion occurred in the Ptolemaic period, but irrigation soils were later destroyed during the Roman period by a combination of wind deflation and flash floods (second to fourth century A.D.), suggesting a period of climate instability. The case of El‐Deir invites reevaluation of constructive agencies for the development of irrigated land and destructive agencies as limiting factors for the sustainability of agricultural practices in late antiquity.     

Patterns and drivers of asteroid abundances and assemblages on the continental margin of Atlantic Canada

Although continental shelf and slope environments typically exhibit high epifaunal biomass and have been subjected to increasing fishing pressure, ecological information on assemblages of non‐commercial invertebrate species from subtidal and bathyal areas remains limited. Sea stars (Echinodermata: Asteroidea), which are known to influence communities through their feeding habits, have received less attention than structural taxa such as corals and sponges. To better understand the ecological roles of asteroids on continental shelves, we investigated ~30 species and assessed their distributions and co‐occurrence with other benthic invertebrates on the shelf and slope of Eastern Canada. Using fisheries data and in situ video footage, we compiled a large dataset covering ~600,000 km² that included over 350,000 individual asteroid records (37–2243 m depth). Multivariate analyses revealed geographically distinct asteroid assemblages, with a maximal overall density at 400–500 m and the highest diversity at 500–700 m. The most abundant and densely occurring species was Ctenodiscus crispatus. We found that asteroids associate with corals, sponges, bivalves, and other echinoderms, and that depth and substrate influence these assemblages. We identified species likely to affect coexisting organisms by their burrowing behavior that can disrupt epi‐ and infauna (C. crispatus) and through predation on ecologically important corals (Hippasteria phrygiana). In addition to providing baseline distribution and ecological information for many bathyal asteroid species in the Northwest Atlantic, this work highlights the abundance and diversified roles of asteroids within continental shelf and slope ecosystems.     

Gully-head morphology and implications for gully development on abandoned fields in a semi-arid environment,Sierra de Gata,southeast Spain

In this paper we examine whether gully-head morphology can be used as an indicator for gully development and, hence, for sediment production. A survey was conducted at five hillslopes in the Sierra de Gata where different types of channel heads occur close to each other. The survey included measurements of morphologic and pedologic properties, ground surface, channel and catchment characteristics of every gully head present (n = 59). On the basis of the observed morphologies, the heads were subdivided into four types: gradual, transitional (a short inclined section), abrupt and rilled-abrupt. The analyses showed that it is possible to explain the differences of gully heads and the role of some environmental factors on the basis of their morphologies, at least for the gradual and the abrupt types. The results suggested that steep headcuts (abrupt) were formed from secondary headcuts in the channel, which migrated upstream. The abrupt headcuts were always formed in more than one soil layer of which one was a resistant (stony) layer. However, shear strength measurements (at saturation) showed that the top layer was not always the most resistant one. Width–depth relationships indicated that gradual type headcuts were controlled by fluvial processes and abrupt headcuts by a combination of fluvial and mass-wasting processes. Gradual types occurred more downslope than the abrupt types suggesting that the incisions started by fluvial processes and migrated upwards when knickpoints developed in the channel. The rilled-abrupt types are still actively retreating. Thus, the abrupt types correspond to slower retreat rates. Abrupt gully heads may deteriorate into transitional types when plunge-pool erosion becomes less effective. The conceptual model is supported by data from ephemeral gullies in two other study areas (Sierra de la Torrecilla, Spain, and Alentejo, Portugal). Copyright © 1999 John Wiley & Sons, Ltd.     

Spatial patterns of soil water balance on intensively cultivated hillslopes in a semi‐arid environment: the impact of rock fragments and soil thickness

During past decades, a diverse system of subsistence agriculture in south‐east Spain (annual rainfall of less than 300 mm) has been overturned in favour of large‐scale plantations of almond trees without consideration for topography and related spatial patterns in soil hydrological properties. The objective of this paper is to investigate the spatial pattern in soil physical properties induced by this cultivation system, and to highlight its impact on the water balance. Soil properties were recorded along hillslopes with shallow soils developed on slates and greywackes in the upper part of the Guadelentin drainage basin (Murcia region). Frequent tillage of these almond plantations covering entire hillslopes has resulted in denudation by tillage erosion on the topographic convexities, as well as transport of rock fragments and fine earth along the slopes. These processes have created a systematic spatial pattern of soil thickness and rock fragment content: shallow and stony soils on the topographic convexities and deep soils with a rock fragment mulch in the concavities at the foot of the slopes. At the same time, a negative relationship between rock fragment content and fine earth bulk density was observed. The impact of this spatial pattern in soil properties on the water balance was evaluated using the PATTERN one‐dimensional hydrological and plant growth model. The model simulates the water balance of soil profiles covering the observed variation in soil thickness, stoniness and bulk density. The model results indicate that the highest rates of infiltration, evaporation and drainage, as well as the lowest rates of overland flow are restricted to shallow soils on the hilltops. In contrast, the deeper soils in the valley bottoms produce a more stable moisture regime than shallower soils, which tend to saturate and dry out quickly. These model results are in agreement with the spatial patterns of almond productivity: an asymptotic increase with soil thickness. Copyright © 2000 John Wiley & Sons, Ltd.     

Stress–strain model for clays with anisotropic void ratio distribution

The porosity of soils is considered to be a directional measure and its distribution is characterized by a functional form. This form has been used to extend the critical state soil mechanics framework to include the effects of structure in soils. A new internal plastic energy dissipation formulation has been proposed to account for fabric arrangement. New expressions for the yield locus, and the plastic stress–strain response of structural soils have been derived. The applicability of the concepts to model the plastic stress–strain behaviour of a number of soils is illustrated. The advantage of the new model is very well identified in modelling the stress–strain behaviour of K₀ consolidated and natural clays. © 1998 John Wiley & Sons, Ltd.