Lake Megafezzan had a maximum lake area of 130,000±7000 km2 and a catchment area of 350,000 km2. The main river systems that feed the giant lake rise in the highlands of the Tasilli N’Ajjer and Hamada Mangueni. Of the three megalakes only lake Megafezzan has a catchment that resides entirely within the Sahara, and thus is the only lake that provides an unambiguous long-term record of climate change in the central Sahara.
Figure 3.1 SRTM DEM showing the main geomorphological features of the Fezzan (Image by Nick Drake)
In the middle of the last century Desio (1942) and Bellair and Pauphilet (1959) discovered palaeolake sediments in the Fezzan consisting of limestones sometimes containing fossil molluscs (Figure 3.2).
|Figure 3.2. An outcrop of Lake Megafezzan sediments in Fezzan, Libya (image © Dr. Simon Armitage, University of Oxford). Five limestone (humid)/Silt (arid) depositional cycles were are evident in this outcrop near Tamessa (photograph by Simon Armitage) |
These deposits were first subjected to detailed study by Petit-Maire et al, (1980) who found evidence for a lake centred on the lowest part of the Wadi Shati and dated to the last interglacial using U/Th methods. More recently Thiedig et al, (2000), White et al, (2001), Brooks et al, (2003a); Thiedig and Geyh, (2004); Drake et al, 2006; McLaren et al, 2006; White et al 2006, Armitage et al, (2007) and Drake et al, (2008) 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 Quaternary (Figure 3.2). We have termed this lake ‘Lake Megafezzan’ and size estimates vary between 76,250 (Brooks et al, 2003a) to 150,000 km2 (Thiedig et al, 2000).
Four different types of lacustrine deposit have been recognised:
The Al Mahruqah Formation
Extensive sequences of inter-bedded limestones and sandstones known as the Al Mahruqah Formation are found at numerous locations throughout much of the Fezzan (Thiedig et al, 2000; Thiedig and Geyh 2004; Armitage et al, 2007, Drake et al,2008). Six stratigraphic sections were measured in detail. Sections are composed of cycles that consist of greenish or reddish sandstone that exhibit evidence of soil development and other evidence of sub-aerial exposure including root casts, gypsum pseudomorphs, desiccation cracks, rootlets and bioturbation (Figure 3.4).
Figure 3.4. Bioturbation by rootlets and burrows in lacustrine sediments (photography by Simon Armitage)
These sediments are abruptly overlain by limestone that indicate the onset of lacustrine conditions (Figure 3.5).
Figure 3.5: Laminated lacustrine limestones underlain by green sands. (Photograph by Simon Armitage.)
The Bir az Zallaf site (Figure 3.6) provides a good example section of the Al Mahruqah Formation (Drake el al, 2008). It exhibits many representative features of the stratigraphy of the six sites investigated so far, showing four sandstone and limestone cycles. The flooding surface at the top of section dates by OSL to 420± 2.4 ka while the lower limestone layers are beyond the range to the method (Armitage et al, 2007).
Figure 3.6. Top: Stratagraphic section of the Al Mahruqah Formation at Bir az Zallaf. Below: Exposure of lacustrine sediments at Bir az Zallaf (image by Nick Drake and photography by Simon Armitage.)
Armitage et al (2007) and Thiedig and Geyh (2004) have dated the uppermost limestones using U/Th and OSL methods. They find a peak frequency of about 240, 350 and 420 ka, thus the timing of palaeolake sediment deposition events appears to correspond to interglacials. The presence of limestone beds below this suggests that the three older glacial/interglacial cycles have been preserved in the Bir az Zallaf section (Figure 3.6). We are currently investigating these sediments using magnetostratigraphy.
The Al Mahruqah Formation represents deposits associated with a very large lake that we have termed Lake Megafezzan (Figure 3.1). GIS analysis of the DEM shows that the maximum size the lake could attain before outflow occurs over the catchments rim is 130,000±7000km2 (Drake el al, submitted).
Ubari Sand Sea limestones and sands
Numerous palaeolake deposits are found in the Ubari Sand Sea (Figure 3.7). The older deposits have experienced extensive deflation and form isolated yardangs or terraces protruding from under the dunes. Exposed sections reveal a stratigraphy remarkably similar to the Bir az Zallaf formation and we feel they provide evidence of Lake Megafezzan beneath the Ubari Sand Sea.
Figure 3.7. Terraces of palaeolake sediments outcropping in an inter-dune basin in the Ubari Sand Sea (photography by Nick Drake)
A more recent lacustrine phase is represented by deposits found at the base of the inter-dune depressions. These sediments consist of sandy peat that sometimes contains Melanoides tuberculata and less commonly Bulinus truncatus, indicating a perennial lacustrine environment. Three samples from adjacent basins were radiocarbon dated to 9.12(+0.09-0.14), 8.42(+0.04-0.12) and 6.69(+0.03-0.05) ka suggesting lacustrine conditions throughout the early Holocene (Drake el al, in prep).
The Wadi Shati Coquinas
Numerous small (1-100 meters in length) coquina deposits, largely consisting of Cerastoderma glaucum shells (Figure 3.9), are found within the lower portions of Wadi ash Shati, the lowest part of the Fezzan Basin.
Figure 3.9 Coquina of Cerastoderma glaucumfrom Wadi Shati (photograph by Simon Armitage)
OSL dating of three coquinasamples yielded ages from c.100-115ka (Armitage et al, 2007). These are in reasonable agreement with a U-series ages from a mollusk shells within the coquinas of 128ka (Theidig et al., 2000) and the peak frequency of ~135ka from 21 ages obtained by Petit-Maire et al. (1980) All studies suggesting a lacustrine interval during the last interglacial with a lake of about 1730 km2 restricted to the lowest point in Wadi Shati.
The Wadi al-Hayat Sediments
The Wadi al-Hayat is an extensive river system that drains the south-eastern flanks of the Tasilli n’ Ajjer and the Accacus Mountains, the Uan Kasa Valley and the north and western flanks of the Messak Sataffat (Figure 3.10).
|Figure 3.10 Map of the Wadi al-Hayat and Ubari Sand Sea study area showing a simplified representation of its geomorphology. The rough location of sample sites, spring and beach deposits are marked (image by Nick Drake). |
In its lower reaches the Wadi is bounded on one side by the escarpment of the Messak and on the other by the Ubari Sand Sea (Figure 3.10). Restricted outcrops of palaeolake deposits are found in both these regions. More recent lacustrine deposits are found in the thalweg of the valley (Figures 2.10).
|Figure 3.11. Lake sediments intercollated with fluvial gravels and sands at the base of the escarpment near el-Grafia (photograph by Nick Drake). |
The lake sediments at the base of the escarpment intermix with the sands and gravels and gradually pinch out within the alluvial fans at the base of the escarpment (Figure 2.11). The stratigraphy of this deposit was dated using both OSL and radiocarbon techniques. Four OSL date from sands bracketing the lacustrine layers provide ages ranging from 11.3±0.9 to 9.2±0.5 ka (Armitage et al.,2007) (Figure 3.12 section A). Radiocarbon dating of a humic layer provided a radiocarbon age of 7400±50 BP with a calibration curve intercept of 8190 and 1 Sigma calibrated ranges of 8300 to 8260 and 8210 to 8170 BP (Table 1). There appears to be a discrepancy between dating techniques, though the error bars of the dates overlap and suggest that a large lake was found in Wadi al-Hayat in the early Holocene sometime between 11 and 8 ka (Drake el al, in prep).
Figure 12. Sections of inter-bedded lake sediments, sands and gravels at the base of the Messak escarpment in the Wadi el Agial. A) The section at the base of the escarpment west of Ubari (Figure 10). B) The section near el-Grafia (image by Nick Drake)
Lake sediments are also found at the base of the escarpment just south of El Grafia (Figure 3.12 section B). The stratigraphy is similar to the site west of Ubari in that it consists of gravels overlying lake sediments, yet differs in that the section contains horizons of gypsum within the lake sediments, suggesting periodic evaporative concentration of lake waters during a number of different low stands. Armitage et al., (2007) applied OSL dating to a layer of sandy lake sediments and an underlying consolidated sand layer (Figure 3.12 section B). The lake sediments date to 10.8 ka and thus appear to be early Holocene in age like the lake sediments West of Ubari. The underlying sediments are much older and suggest that the lake sediments are uncomfortably underlain by older sands and gravels (Figure 3.12 section B).
Figure 3.13. A simplified representation of the geomorphology of the Lake Megafezzan catchment. The main rivers systems are marked as black lines and sand seas in gray. Small late Pleistocene and Holocene lakes are marked in black while the area covered by Lake Megafezzan during its high stands is marked by gray stripes (image by Nick Drake)
These lake sediments were deposited by a large lake in the Wadi el-Hayat, the area of which was about 2000 km2 (Figure 3.13). The lake would have been flanked by hundreds of smaller lakes in the inter-dune depressions of the Ubari Sand Sea. This relatively small early Holocene lake, coupled with the evidence for a slightly smaller lake in Wadi Shati during the penultimate interglacial, suggests that the last two glacial humid periods in the Fezzan were dryer than earlier Pleistocene humid periods as they did not manage to sustain the giant lake that appears to have existed in earlier interglacials (Drake el al.,2008).
A chain of playas is found strung out along the base of the Wadi al-Hayat (Figure 3.10). They reveal a stratigraphy of sands underlying red lacustrine silts with intergrowths of gypsum near the surface. This stratigraphy indicates that after a period of aridity attested to by the presence of the sands, the Wadi al-Hayat river system became active, lakes became established along its course, depositing red silts and clays, and then gradually dried up in a subsequent arid period, causing the growth of gypsum crystals in the near surface sediments. OSL dating of silts suggests that the lacustrine phase started at 5.9±1 ka while U/Th dating of gypsum indicates that the onset of dry conditions occurred from 4±0.2 ka (Brooks et al., 2003a; Drake et al, 2006).
A gypsiferous spring deposit at Twesh (2.10) provides evidence for a further humid period in the Fezzan Basin with a whole crystal U/Th date of 26.3 ± 3.2. To investigate the period of time over which the crystal precipitated, we dated the outer and inner layers of the crystals using sequential dissolution methods and found that the outer layer was 25.4 ± 6.7 ka and the inner part to 29.1 ± 6.2 ka. This suggests that a humid period started in the Fezzan before 29 ka, that groundwater began to decline and concentrate at about this time and this continued to 25 ka when the spring finally desiccated. This spring deposit is the first and only evidence in the Fezzan for a humid period at this time.
The Fezzan Basin provides a rich palaeoclimate record. Dating of Fezzan Basin lacustrine features demonstrates evidence of palaeolakes during numerous interglacials during the last 750 ka, confirming the view that they are associated with humid periods.
The middle to late Pleistocene interglacials was humid enough to produce a giant lake of at least 130,000 km2 that we have called Lake Megafezzan. Later lake phases were smaller, the interglacials less humid, developing lakes of a few thousand square kilometres. There is evidence for brief periods of aridity during interglacials (eg sometime between 80 and 100 ka), whilst the last glacial appears to have experienced humid episodes at about 47(+17-13) and about 30 ka. Most humid periods correspond to times of increased Saharan insolation suggesting that this is the forcing mechanism.
The last humid phase started at about 10 ka, continued into the Holocene, but was punctuated by brief but abrupt intensifications of aridity that are broadly synchronous with other arid events reported from surrounding regions and are probably linked to monsoon failure.