The Palaeohydrology and Palaeoclimate of Lake Megafezzan: A Giant Palaeolake in the Fezzan Basin, Libyan Sahara

 

 

Figure 1: An outcrop of Lake Megafezzan sediments in Fezzan, Libya (image © Dr. Simon Armitage, University of Oxford).

 

 

Introduction

The Fezzan forms a large closed basin that contains numerous ancient palaeolake sediment outcrops and shorelines (Figure 1). Palaeolake sediments containing fossil molluscs were discovered in the Wadi Shati region of the Fezzan early this century. They were first subjected to detailed study by Petit-Maire et al., (1980) who postulated the existence of a lake centered on the lowest part of the Wadi Shati Basin that was dated to the last interglacial using U/Th methods. More recently Thiedig et al., (2000), White et al., (2001), Brooks et al., (2003) and Drake et al., (in press) have discovered palaeolake sediments in other parts of the Fezzan and it has been recognized that much of the Fezzan forms a giant closed basin in which a very large lake existed at numerous times during the last 380 ka. We have termed this lake ‘Lake Megafezzan’ and size estimates vary between 76,250 (Brooks et al., 2003) to 150,000 km2 (Thiedig et al., 2000).

 

It has become increasingly evident form the studies outlined above that the Fezzan contains a wealth of palaeohydrological, palaeoenvironmental and palaeoclimatalogical information in a region (the Sahara) that is generally thought to be largely devoid of it. The Great Man Made River Authority (GMMRA) have funded further research that will enhance our understanding of this important region.

 

Aims and Methods

The aim of this research is to investigate Lake Megafezzan in order to further our understanding of climate change in the Fezzan, determine periods of recharge of ground waters and enhance our understanding of the palaeohydrology of the region.

 

To achieve this remote sensing and digital elevation model (DEM) analysis was used to map lake sediments and shorelines throughout the Fezzan. Furthermore, possible outflow channels that connect Lake Megafezzan to the Sirte Basin have been located.

 

We then investigated these newly discovered areas in the field. The stratigraphy and form of sites was mapped. Samples of siltstone, sandstone, limestone, various duricrusts and palaeosols were collected in the field for analysis of the sediment microfabric and degree, types of, and stages of post depositional digenetic alteration. These data will aid in the understanding of sedimentary processes post-depositional and geochemical environments.

 

Samples were also collected for OSL dating. OSL techniques allow assessment of the time elapsed since quartz grains were exposed to sunlight (Figure 2). This gives the burial age of a sedimentary deposit. OSL can be used to date samples with ages between 0 and 200ka. Samples older than 200ka will be dated using palaeomagnetism. The method enables dating of the rocks by comparison with apparent polar overpaths and the geomagnetic polarity timescale. Over sixty samples have been collected for dating and are currently being processed.

 

 

Figure 2: Gamma spectometer collecting field dosimetry data in a coquina of Cerastoderma glaucum (image © Dr. Simon Armitage, University of Oxford).

 

 

Results

Sediments and Stratigraphy

Six stratigraphic sections were measured in detail in different parts of the Fezzan basin (Figure 3). These sections are composed of cycles that consist of greenish or reddish sandstone at the base that changes laterally into siltstones that exhibit evidence of soil development and other evidence of sub-aerial exposure including root casts, gypsum pseudomorphs, desiccation cracks now filled by silica and calcite (Figure 4). These sediments are abruptly overlain by laminated limestone (Figure 5) with evidence of rootlets and bioturbation (Figure 6). At least five of these depositional cycles were identified in a single section, suggesting at least five humid arid cycles. Samples from these deposits are currently being analysed.

 

 

Figure 3: Exposure of lacustrine sediments. Note rock shelter cut beneath hard upper limestone horizon in centre of phoograph (image © Dr. Simon Armitage, University of Oxford).

 

 

 

 

Figure 4. Lacustrine sands that exhibit later reworking by channels with subsequent root cast development (image © Dr. Nick Drake, King’s College London).

 

 

 

Figure 5: Laminated lacustrine limestones underlain by green sands (image © Dr. Simon Armitage, University of Oxford).

 

 

 

Figure 6: Bioturbation by rootlets and burrows in lacustrine sediments (image © Dr. Simon Armitage, University of Oxford).

 

Outflow Channels

Oversized channels are found at two low points on the catchment rim where outflow of lake waters could have occurred during times of high lake level. These sites were investigated in the field for evidence of fluvial sediments and landforms. The northernmost potential outflow site (Figures 7) was found to contain a small and subdued channel with numerous gravel bars that attest to past fluvial activity consisting of angular to sub-rounded sediments. One of these bars was sampled for OSL dating. Outflow discharge of this channel will be estimated using the Colebrook-White equation using b-axis grain size measurements acquired in the field to calculate Darcy-Weisbach friction factor.

 

 

 

 

Figure 7. Three dimensional view of a possible Lake Megafezzan outflow channel created by overlaying the ASTER image on to the SRTM 30m DEM. The channel is located just North of Fougra and is draped over the catchment rim with a meandering form that is evident in both the DEM and the ASTER Imagery (image © Dr. Nick Drake, King’s College London).

 

 

At the southern outflow there are no channel forms , however, extensive sheets of gravels are present. Deposits can reach 3 meters in thickness and consist of very well rounded clasts that sometimes attain large sizes. These characteristics suggest both powerful and persistent transport. The gravel exhibits no imbrication or other evidence of flow direction that would be expected in a most fluvial deposits. Because of this its origin is somewhat enigmatic but in many ways it appears to resemble sediments deposited in perennial braded rivers. The location of the deposit on one of the lowest points the catchment rim suggests that it was formed by outflow of Lake Megafezzan waters into the Sirte Basin. In places these gravels are inter-bedded with basalt lava flows that were sampled for Palaeomagnetic and Argon dating to determine the timing of lake outflow.

 

The Future

The analysis of samples is proceeding in the UK. Results of the field and laboratory research will provide a comprehensive analysis of all the main lacustrine deposits in this unique region will:

 

Acknowledgements: This fieldwork was supported by the generosity of the Great Man Made River Authority (GMMRA), Tripoli, Great Socialist People’s Libyan Arab Jamahiriya (Figure 8). The complex organisation of the fieldwork logistics was managed by Dr. Mustafa Salem and Dr. Ahmed al Hawat. Simon Armitage is funded under NERC EFCHED Contract.

 

 

 

Figure 8: Field facilities provided by ARAMAT and funded by the generosity of GMMRA

 

Great Man Made River Authority

Tripoli, Libya

 

Members of the Research Team

Dr Nick Drake, Department of Geography, Kings College, London, WC2R 2LS. Reader in geography, project co-ordinator and member of the Council of the Society of Libyan Studies.. Nick is a specialist in remote sensing of arid areas, particularly for geomorphological and geoarchaeological applications.

 

Professor Ahmed El-Hawat, University of Garyounis, P.O.Box 1308, Libya. Ahmed is providing expertise in geology, sedimentology and geomorphology of the Sahara and Libya in particular.

 

Professor Mustafa Salem, Geology Department, El-Fatah University, Tripoli, provides expertise in the geology of Libya and logistical control of the field project.

 

Dr Kevin White, Department of Geography, The University of Reading, Whiteknights, Reading RG6 6AB. Age 40. Senior Lecturer in Environmental Remote Sensing and a member of the Council of the Society of Libyan Studies. Kevin is providing expertise in geomorphology, remote sensing and combined satellite imagery/GPS navigation.

 

Dr Simon Armitage, School of Geography & the Environment, University of Oxford, Mansfield Road,

Oxford, OX1 3TB. Simon's research interests include dating desert, coastal and marine sediments using luminescence methods, Quaternary palaeonenvironments and palaeoclimates (especially in deserts).

 

Dr Peter Turner, School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT. Peter is a geologist with expertise in stratigraphy, sedimentology and palaeomagnetism and its applications in the study of continental sedimentation in the Sahara.   

 

Dr Sue McLaren, Department of Geography, University of Leicester, University Road, Leicester, LE1 7RH Lecturer in Geography. Sue is an expert in geomorphology and sedimentology and is co-ordinating isotopic analyses of samples with the NIGL.