The Kufrah paleodrainage system in Libya: A past connection to the Mediterranean Sea?

International audiencePaillou et al. (2009) mapped a 900 km-long paleodrainage system in eastern Libya, the Kufrah River, that could have linked the southern Kufrah Basin to the Mediterranean coast through the Sirt Basin, possibly as long ago as the Middle Miocene. We study here the potential connection between the terminal part of the Kufrah River and the Mediterranean Sea through the Wadi Sahabi paleochannel, which may have constituted the northern extension of the lower Kufrah River paleodrainage system. New analysis of SRTM-derived topography combined with Synthetic Aperture Radar images from the Japanese PALSAR orbital sensor allowed the mapping of seven main paleochannels located west of the Kufrah River, each of which is likely to have formed a tributary that supplied water and sediment to the main paleodrainage system. The northernmost four paleochannels probably originated from the Al Haruj relief, a Pliocene alkaline basaltic intracontinental volcanic field, and potentially connected to the Wadi Sahabi paleochannel. The remaining three paleochannels are in the more southerly location of the Sarir Calanscio, North-East of the Tibesti mountains, and barely present a topographic signature in SRTM data. They end in the dunes of the Calanscio Sand Sea, forming alluvial fans. The most southern paleochannel, known as Wadi Behar Belama, was previously mapped by Pachur (1996) using LANDSAT-TM images, and was interpreted by Osborne et al. (2008) as representing part of an uninterrupted sediment pathway from the Tibesti mountains to the Mediterranean Sea. Processing of SRTM topographic data revealed local depressions which allow to connect the seven paleochannels and possibly the terminal alluvial fan of the Kufrah River to the Wadi Sahabi paleochannel, through a 400 km-long, south-north oriented, paleocorridor. These new findings support our previous hypothesis that proposed a connection between the lower Kufrah River in the region of the Sarir Dalmah and the Wadi Sahabi paleochannel, which connected to the Mediterranean Sea. Including the newly mapped paleochannels, the Kufrah River paleowatershed, at its maximum extent, would have covered more than 400,000 km2, representing close to a quarter of the surface area of Libya

Long-term flood controls on semi-arid river form: evidence from the Sabie and Olifants rivers, eastern South Africa

Rivers in the Kruger National Park, eastern South Africa, are characterised by bedrock-influenced ‘macrochannels’ containing variable alluvial thicknesses and riparian vegetation assemblages. Evidence from the Sabie and Olifants rivers suggests that flows up to moderate floods (<3500 m3 s-1) tend to result in net alluviation, with sediments gradually covering the underlying bedrock. More extreme floods strip alluvium and erode bedrock, effectively exerting the primary control over long-term river morphologic development. On the Olifants River, post-flood aerial LIDAR imagery reveals that the 2012 extreme flood (~14000 m3 s-1) resulted in extensive stripping of stored alluvial sediment, exposing and eroding the underlying weathered bedrock. On the Sabie River, preliminary optically stimulated luminescence ages for remnant alluvium are all less than 1000 years, highlighting typical timescales of sediment storage. Together, these results suggest that while periods of general alluviation occur on these systems, long-term river development results from extreme flood-generated bedrock erosion

The behaviour of thin spherical shells under a variety of load actions

The subject matter of the thesis concerns the analytical and experimental investigation of the elastic behaviour of shell structures, in particular that of spherical shells, under axi-symmetric, asymmetric, and unsymmetric load systems. Chapter I presents a critical survey of the relevant literature in the form of a dissertation. In view of the lack of a survey of this type it was felt desirable to present this in considerable detail so as to provide an up-to-date reference of the shell field. The survey clearly shows the need for an analytical procedure capable of handling all types of unsymmetrical load systems, and also establishes the lack of published experimental work. The paucity of the latter is most surprising in relation to the voluminous literature on the analysis of simple symmetrical load cases. The plan of the research undertaken was, in consequence, designed to fill these gaps. Chapter II presents a unified analytical approach based on the linear concepts of the shallow shell theory in which all load actions are considered as built up of the four basic actions of radial and tangential load, bending and twisting moments. Their evaluation is to the author's knowledge the first presentation of the unified approach to the analysis of such a complete range of load actions. Further, analytical consideration is given to the correlation of this unified shallow shell approach with the general shell theory. It is shown that neglecting certain second order terms the general shell equations reduce to those of the shallow shell. The chapter culminates in the presentation of an Influence Line Method which, utilizing the permissibility of superposition in linear analysis, provides a ready approach to the solution of any type of unsymmetrical load action. In Chapter III a number of load actions are analysed by means of the Influence Line Method, These examples, in the main, have been selected from a range for which conventional theoretical solutions are available. It is shown that good agreement is obtained in all cases between the Influence Line and conventional solutions. The experimental investigations are described in Chapter IV covering the examination of the four basic load actions and certain selected composite actions. Some seventy tests were carried out covering a variety of radial and tangential area and ring loads, bending and twisting moments, and their combinations, applied directly to the continuous shell and to rigid inserts Incorporated in the shell wall. The experiments were carried out on shallow shell models of 60in radius, of 1/4 , 1/2 and 1 in thickness and on a complete spherical shell 13ft. in diam. which was a 1/10 scale model of the Dounreay Nuclear Reactor Containment Building. The experimental and theoretical results are compared and fully discussed in Chapter V. It is generally shown that good agreement la obtained, fully substantiating the proposed analytical methods and their underlying concepts. Chapter VI summarizes the main findings of the investigation regarding the basic aspects and their application to design analysis. A Bibliography and Author's Index is provided in Chapter VII, followed in Chapter VIII by eight appendices giving the details of analyses considered in the thesis

A comparison of multiple luminescence chronometers at Voordrag, South Africa

A suite of 10 samples collected from an 11 m thick colluvial sequence at Voordrag, KwaZulu-Natal, South Africa, have been used to undertake a comparison of different luminescence methods. Good agreement is found between single grain quartz optically stimulated luminescence (OSL) and single grain K-feldspar post-infrared infrared-stimulated luminescence (post-IR IRSL) ages, with the exception of the basal samples where the quartz OSL signal is saturated. Multiple grain quartz OSL consistently yields ages older than single grain OSL methods. Multiple grain feldspar ages derived from the IRSL50 signal are underestimated due to anomalous fading. A previously published radiocarbon chronology yields ages that are younger than those from single grain quartz OSL and post-IR IRSL, and this is most likely due to contamination with younger carbon. Identifying the effect of saturation on the quartz OSL signal remains challenging when quartz is dated in isolation. However, using a paired quartz/feldspar dating approach is an effective way of identifying the impact of saturation on the OSL signal