The sedimentation of the evaporates in Sebkha Ndghamcha: testimony of a aridification of the climate in the Tafolian
by
Ould Sabar, Mohamed Salem
Département de Géologie, FST de Nouakchott, BP 5026, Mauritanie
Coauthors: Ahmedou Ould Mahfoudh (Département de Géologie, FST de Nouakchott, BP 5026, Mauritanie), Raoul Caruba (IRIM, Université de Nice-Sophia Antipolis, France).

The Ndgamcha sebkha (18°15 ' - 19°10 ' N; 15°25 ' - 16°10 ' W) is located in the Senegalo-Mauritanian sedimentary basin at around fifty kilometres north of Nouakchott, capital of Mauritania (fig. 1). It is a vast oval basin along the littoral, c. 80 km long and 50 km wide and, on average, with an area of 4 000 km2 located mainly under the mean level of the sea.

The Ndghamcha sebkha (fig. 2) is currently subdivided in two principal zones (Elouard and Faure, 1967-1972; Elouard, et al., 1969; Hébrard, 1973; Maglione and Carn, 1976; Carite, 1980; Caruba and Dars, 1991; Ould Sabar et al., 1997; Ould Woyssatte, 1997; Ould Sabar, 2001):

1- an active and easily flooded zone (at a level lower than 0 m) formed of a gypso-argilo-salted crust covering, in the western part of the sebkha, a layer of blackish mud and in the northern part, of an alternation of salt and clay forming an active sebkha drowned in a seasonal varved gypsum environment; 2- a dry inactive or fossil zone with a gypsum base under diverse facies: varved, crystalline and saccharoid.

A synthetic geological cut of the formations of this sebkha makes it possible to summarize the geological evolution of the latter since the Nouakchottian, from top to bottom:

4 - 0 to 1.60 m: recrystallised gypsum and varved or saccharoid gypsum....1.60 m 3 – 1.60 to 1.80 m: shells of Cardium edule with large gypsum crystals around the shells....0.20 m 2 – 1.80 to 2.40 m: shelly sand (falun) more marine downwards....0.60 m 1 – 2.40 to 3.00 m: shells of Cardium ringens locally cemented by a beige calcareous sandstone....0.60 m Based on ages this outcrop can be summarised as follows: Tafolian 3 - 0 to 1.60 m: gypsum....1.60 m Nouakchottian 2 – 1.60 to 2.40 m: shelly sand (falun)....0.80 m Inchirian 1 – 2,40 to 3,00 m: locally cemented shells....0.60 m This cut allows us, in addition to other field observations (red dunes, calcareous-clayey sediments with diatoms and lacustrine gastropods), to summarize the geological evolution of this sebkha since the Inchirian (120,000 to 30,000 yr BP ):

- between 120,000 and 30,000 yrs BP, during the Inchirian transgression, the sea covers all the littoral zone and a vast gulf in the Tafoli (where the modern Ndghamcha sebkha is), up to 120 km inland. Coastal rivers were flowing (humid climate) in this gulf (5 - 10 m depth). A rich fauna of Molluscs and one Echinoderm (Radiorotula orbiculus) lived there.

- between 30,000 and 10,000 yrs BP, the sea retreated beyond –100 m and the climate became arid.

- This contributed to the building up again of vast dune massifs (ergs) in western and southern Mauritania (Amoukrouz, Akchar, Azefal, etc.). This is the Ogolian regression or the Ogolian.

- between 10,000 and 7,000 yrs BP, the sea slowly progressed to the modern coastline. A humid phase influenced all the Sahelian countries: this is the Tchadian during which there was rubefaction of the Ogolian dunes and recharge of the water tables such as that of Trarza.

- around 7,000 yrs BP, the sea carries out a transgression from the west towards the east: it is the Nouakchottian (7,000 to 4,000 years BP). During the maximum of this transgression (5,500 –5,000 years BP), the sea forms several small gulfs in between dune hollows (Gouds and Aftouts) and two broad gulfs one in north, the gulf of Tafoli (with the current site of the sebkha Ndghamcha), the other in the south, the gulf of the delta of the River Senegal.

In the gulf of Tafoli (90 km inland), the contribution of fresh water from the continent supports the installation of a mangrove and the proliferation of some species (Crassostrea gasar, Tympanotonus fuscatus), while the abundance of food, the sunning and the shallow depth support the blooming of other species (Anadara senilis, Cerastoderma edule, etc).

- Around 4,000 years BP, the sea starts a regression called the Tafolian (4,000 to 2,000 years BP) during which the gulf of Tafoli gradually transforms into a lagoon because of the construction, by the North-South sand drift, of a littoral sandy barrier.

The climate continues its aridification and the fast over-saturation in salt of the lagoon involves the brutal death of all molluscs, the deposit of seasonal gypsum and the formation of the sebkha. The rhythmic sedimentation of gypsum indicates a regular succession of periods of strong evaporation in the dry season with gypsum depositing and of wetter periods during which clays and marls settle.

The Ndghamcha sebkha ceased functioning as an evaporating basin when the sandy barrier had well isolated it from the sea. The last brines and the salted water tables localised in the western and north-western parts are responsible for some sodium chloride deposits.

Currently, the wind causes deflation in the sebkha and one notes wind rehandlings thereafter, that is to say starting either from the Ogolian old red dunes (red dunes built during arid the Ogolian: 30,000 to 10,000 years BP), or from the gypsum (gypsum dunes).

References: Carite D. (1980). - Le gypse en Mauritanie occidentale. Lithos, 2, 22-36, 13 fig., 18 photos.

Caruba R. and Dars R. (1991). - Géologie de la Mauritanie. CRDP Nice., 321 p, 149 fig., 19 pl. , ISS Nktt, Univ. Nice-Sophia Antipolis.

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Maglione G. and Carn M. (1976). - Données géochimiques préliminaires sur la sebkha de Nrhamcha (Mauritanie). Ass. Sénégal. Et. Quatern. Ouest afr., Bull. Liaison, Sénégal, n°49, déc. 1976, pp. 45-56.

Ould Sabar M. S. et al. (1997). - Livret-guide de l'excursion B (Parc National du Banc d'Arguin (Mauritanie)). Coll. Nouakchott ''Changements aux marges des déserts en Afrique depuis 135 000 ans : implications pour l'Eau, le Carbone et l'Homme.'' PICG 349, PICG 404, Commission Carbone de l'INQUA, Univ. De Nouakchott, Univ. De Marseille II, CNRS et ORSTOM., 35p., 12 fig.

Ould Sabar M. S. (2001). - Structure, mécanisme et projet d'aménagement future de la sebkha Ndghamcha (Mauritanie). Thèse d'Univ., Nice-Sophia Antipolis-Fac. Sci., 116 p., 45 fig., 24 tabl., 8 annexes.

Ould Woyssatte M. (1997). - Recherches sur les minéralisations (gypse et sel) liées à l'eau dans la sebkha Ndghamcha (au nord de Nouakchott-Mauritanie). Diplôme d'Université (D.U.), Univ. Nice-Sophia Antipolis, 104 p., 42 fig., 3 annexes, 2 pl. photos.

Date received: January 27, 2004