The Fazzan Basin, or Fezzan Basin, is a large

endorheic basin An endorheic basin ( ; also endoreic basin and endorreic basin) is a drainage basin that normally retains water and allows no outflow to other external bodies of water (e.g. rivers and oceans); instead, the water drainage flows into permanent ...

in Libya. It has no outlet to the sea and contains large areas of desert or semi-arid land. It is one of two basins in southern

Libya Libya, officially the State of Libya, is a country in the Maghreb region of North Africa. It borders the Mediterranean Sea to the north, Egypt to Egypt–Libya border, the east, Sudan to Libya–Sudan border, the southeast, Chad to Chad–L ...

on the northern flanks of the

Tibesti Mountains The Tibesti Mountains are a mountain range in the central Sahara, primarily located in the extreme north of Chad, with a small portion located in southern Libya. The highest peak in the range, Emi Koussi, lies to the south at a height of and i ...

in the central

Sahara desert The Sahara (, ) is a desert spanning across North Africa. With an area of , it is the largest hot desert in the world and the list of deserts by area, third-largest desert overall, smaller only than the deserts of Antarctica and the northern Ar ...

, the other being the

Kufra Basin Kufra () is a basinBertarelli (1929), p. 514. and oasis group in the Kufra District of southeastern Cyrenaica in Libya. At the end of the 19th century, Kufra became the centre and holy place of the Senussi order. It also played a minor role in t ...

, further to the east.

Formation

The Fazzan Basin is situated on the junction between two

tectonic plates Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

. Collision between these occurred in the

Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...

period and caused thickening of the Earth's crust, which then downwarped under its own weight to form a depression in the ground, the Fazzan Basin. Since then, there has been a deposition of "continental intercalaire" and other continental rocks, and large quantities of water have been trapped in underground

aquifer An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...

s. An outcrop of

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

occurs between the Fazzan and the Kufra Basins, and both are overlaid by sand. The climate of this region has varied greatly in the past, with pluvial and dry periods alternating; the current arid phase averages less than of precipitation per year. During its geologic history, the Fazzan Basin has on at least four occasions during the

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

been inundated with water to form a large lake. On each occasion, a thick layer of limestone was deposited. Armitage et al. (2007) estimated that two of these events occurred earlier than 400,000 years ago, another one was in

MIS 11 Marine Isotope Stage 11 or MIS 11 is a Marine Isotope Stage in the geologic temperature record, covering the interglacial period between 424,000 and 374,000 years ago. It corresponds to the Hoxnian Stage in Britain. Interglacial periods which occ ...

(~400,000 years ago) and a further one in

MIS 5 Marine Isotope Stage 5 or MIS 5 is a marine isotope stage in the geologic temperature record, between 130,000 and 80,000 years ago. Sub-stage MIS 5e corresponds to the Last Interglacial, also called the Eemian (in Europe) or Sangamonian (in N ...

(between 130,000 and 80,000 years ago). Geye and Thiedig (2008) recognise further lake sedimentation periods in

MIS 9 Marine Isotope Stage 9 (MIS 9) was an interglacial (warm) Marine Isotope Stage. It was the last period of the Lower Paleolithic. Estimates of its dating vary. It lasted from 337,000 to 300,000 years ago according to Lisiecki and Raymo's 2005 LR04 ...

and MIS 7. They have also estimated that the lake extended to in MIS 11, and this was when it was at its maximum size. By MIS 5, the lake was reduced to and less than during the most recent iteration in the

Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

. However, Armitage et al. (2007), citing Brooks et al. (2003), put the Holocene lake area at . Although there is some disagreement about the exact timing and the size of Lake Megafazzan, both authorities agree that there has been no

lacustrine A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from t ...

activity between MIS 5 and the Holocene. The MIS 5 deposits include fossils of ''

Cerastoderma glaucum ''Cerastoderma glaucum'', the lagoon cockle, is a species of saltwater clam, a marine bivalve mollusc in the family Cardiidae, the cockles. This species is found along the coasts of Europe and North Africa, including the Mediterranean and Blac ...

'', which indicates that the water was probably

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

Lake Megafazzan

During the

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

, Libya was drained by two large rivers systems, both flowing into the

Mediterranean Sea The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern Eur ...

, the Wadi Nashu River in the west and the

Sahabi River The Companions of the Prophet () were the Muslim disciples and followers of the Islamic prophet Muhammad who saw or met him during his lifetime. The companions played a major role in Muslim battles, society, hadith narration, and governance ...

in the centre and east. In the late Miocene and early

Messinian The Messinian is in the geologic timescale the last age or uppermost stage of the Miocene. It spans the time between 7.246 ± 0.005 Ma and 5.333 ± 0.005 Ma (million years ago). It follows the Tortonian and is followed by the Zanclean, the fir ...

, increased volcanic activity in northeastern Libya resulted in the damming back of the Wadi Nashu, which caused water to accumulate in the Fazzan Basin, forming the proto-Lake Megafazzan during humid periods.

References

{{Reflist Endorheic basins of Africa Sedimentary basins of Africa Geology of Libya