Recent Geology of the Chotts of Tunisia

James W. MAYOR, Jr. Woods Hole Oceanographic Institution

RECENT GEOLOGY OF THE CHOTTS OF TUNISIA

The region of the large Chotts of Tunisia, Chott el Djerid, Chott el Fedjedj and Chott el Rharsa, are widely accepted as the location of Lake Tritonis of Greek Mythology. Apollonius of Rhodes1 describes the stranding of the Argo on a Libyan beach and her later passage out of the Tritonian lagoon, south along the shore and north along the west coast of the island of Djerba. This story has much detail that is 2 in accord with the geological record. Diodorus Siculus identifies the island of Hespera within the marsh Tritonis as the site of the great, peninsular, Amazonian city of Cherronesus. The island was of great size and full of friut bearing trees and multitudes of flocks and herds. It is difficult to associate this marsh Tritonis with 3 the Chotts of today. Herodotus speaks of the river Triton which empties into Lake 2017 Tritonis of the Argonauts and around which lived several tribes. Paul Borchardt4 placed Plato's Atlantis in the region of the Chotts and Mavor,5 who wrote of the Biblioteca, identification of Atlantis with the Aegean island of Thera, accepts the probability

that Berber tribal names and other North African history influenced the story of ULPGC.

Atlantis recorded in Egypt. The miniature fresco discovered on Thera in 1973 por implies extensive intercourse between Libya and Greece in the Bronze Age and

supports this thesis. Lake Triton occupies a prominent place in myth but archeologi realizada cal evidence is scant. The historical interpretations of the ancient Greek writings must accord with the geological record. This article summarizes and interprets this record. Digitalización Extensive geomorphological investigations made during petroleum exploration autores. have established the main features of the Chott development during the Quaternary 6 los period (about one million years ago to present). The Chotts are an area of subsi dence that reached its present topography by the late Tertiary during the tectonic movements which formed the Atlas mountains. Since then, some local subsidence documento, has taken place, notably in Chott el Rharsa which is today 21 meters below sea level. Del © There is a North-South flexure or fault between the Chott el Fedjedj and the Gulf of Gabes forming the eastern boundary of the Tertiary basin but there is no evidence of

Quaternary tectonic activity there. There is agreement among geologists that tectonism ceased during the time of the Acheulian culture, about 50,000 years ago. The Chott el Djerid and Chott el Fedjedj both have their minimum elevations well above sea level and are gradually deepening through erosion. There are two exit channels which discharge overflow through the Oudref sill, a rock barrier with minimum height 45 meters above sea level, and into the Gulf of Gabes. These channels follow the routes of the Oued el Akarit and Oued el Melah. The glacis sloping toward the large Chotts have but a few meters of sediment covering the bedrock and there is no sign of deformationsince late Acheulian times. It is unlikely that any significant subsidence could be masked by sediment which has lead

83 R. Coque and A. Jauzein6 to conclude that tectonism since the end of the Acheulian is unlikely. This conclusion denies the likelihood of an earthquake that could have drained the Chotts suddenly or that the Chott region was an estuary of the Mediter ranean since Tertiary times. As to sudden flooding of the Chotts over the 45 meter high barrier, there is a possible mechanism. This would be by tsunami. The tsunamis generated by the collapse of Thera in the 15th century B. C. might have propagated to the Tunisian coast through air-blast resonance. 5 If Euripides' play, Hippolytus, is the story of the Thera catastrophe, then the sea passed over the isthmus of Corinth 70 meters above sea level and five kilometers wide. But there is no conceivable mechanism for sudden drainage that fits the geological observations and there is a complete lack of marine fossils found in the fauna of the Chotts. These circum stances make a catastrophic event extremely unlikely. The Quaternary history of the Chotts is well explained by bio-climatic instability. This view considers that there has been and continues to be a systematic seasonal alternation between a winter phase of partial flooding from a saline water layer on top of the artesian water saturating the basin fill beneath the Chotts, and a summer 2017 phase of evaporation during which a salt cover is laid on the surface and eroded by wind. Su per posed on this seasonal cycle are cycles which have long time periods Biblioteca, including at least two during which the climate was different fromthat today, when the Chott was in equilibrium as a lagoon. There is evidence in the Chott el Djerid ULPGC. that there were springs and a possible rainy period between 8000 and 15000 years por ago when the water level was several meters higer than it is today. Cardium shells around the edge of the lagoon define these wet periods. The large closed depressions realizada in the Chotts were formed by interruption in the cycle of alternating sedimentation and erosion. The theory of a fluctuating Quaternary climate and an increasing aridity Digitalización of climate in recent times is substantiated by the studies of the landsform and

associated deposits over 15 years. autores.

The bio-climatic balance of the region of Chotts is delicate and the difference los between humid and dry conditions has not necessarily been large, possibly as little as a variation in the frequency of torrential rains. There are no catastrophic events in documento,

the geological record, only slow change of humidity and the gradual drying and Del flooding of the Chotts. © Eustatic rise of sea level is not a possible agent of floodingof the Chotts over the Oudref sill during the past 35,000 years. The curve of world-wide eustatic sea level for this period, 7 shows a minimum level about 130 meters below that at present occurring 15,000 to 18,000 years ago and no local sea level change in height more than two meters above that at present. There has been no change in sea level at Gabes within the past 2000 years.8 The world-wide eustatic level reached its present point about 4000 years ago.

84 REFERENCES

1 Apollonius of Rhodes, "The Voyage of the Argo," Penguin 1959. 2 Diodorus Siculus, Book lll, 52-54. 3 Herodotus, "The History;" Tudor, N. Y. 1928. 4 Borchardt, P., "Nordafrika und die natlirlichen Reichtiimer von Atlantis," Petermanns Mitteilungen, 74 Jahr gang 1928, Gocha. 5 Mavor, J. W., Jr., "Voyage To Atlantis," Putnam, N. Y. 1969. 6 Coque, R. and A. Jauzein, "The Geomorphology and Quaternary Geology of Tunisia," in "Guidebook to the Geology and History of Tunisia," Petroleum Exploration Society of Libya. Ninth Annual Field Conference, 1967. 7 Milliman, J. D. and K. 0. Emery, "Sea Levels During the Past 35,000 years," Science Vol 162, 6 Dec. 1968. 8 Flemming, N. C., "Archeological Evidence for Eustatic Change of Sea Level and Earth Movements in the Western Mediterranean During the last 2000 years," Geological Society of America, Special Paper 109, 1969. 9 Balout, L., "Prehistoire de l'Afrique du Nord," Arts et Metiers, Paris . 2017

. ' Biblioteca, .. ..• " , ... ' . ULPGC. por '• Chott el Djerid ', ,,

, realizada ..'-," '·;-., Digitalización autores.

...... , Shore line of lagoon los

--•• Boundary of present-day Chott

85 r ,.. I

RANCE -- CHCRB lJ1:-:"·�....;,.

Flg.13 DISTRIBUTION OF CARDIUM DEPOSITS AROUND THE CHOTTS ( SOUTHERN TUNISIA)

y;n,1,... o;u---• 2017 uo.,,11,,1,1•:c---• s.,,,-�n,,r1•,,.1,.1.,., __ Biblioteca,

Table 2: Reproduced from Coque and Jauzein6 . ULPGC. por realizada

. . Cap.5it!n O ...... oo o,./0 z I . ····· ...... 0 O• �i : �'...... • ······ ...... ·, ,··· 0

9 Table 3: The Capsien Maghreb (About 8,000 to 5,000 B. C.) (Reproduced from Balout ).