The internal structure of spring mounds of Nefzaoua oasis in Tunisia: Formation of an original geomorphic structure

Abstract

Spring mounds are specific geomorphological landforms in arid or semi‐arid environments associated with playas and artesian springs. Spring mounds are found worldwide, especially in the great Artesian Basin of Australia and in North American and Egyptian deserts. They result from an exceptional succession of climatic, geomorphological and hydrogeological conditions, with processes that follow each other in a specific order. In Tunisia, in the arid zone, ca. 126 spring mounds have been identified in Nefzaoua province alone, especially in the oasis east of Chott el Jerid. They have a conical shape that ranges from 200 to more than 2000 m in diameter and 3–30 m tall, and their centre is hollowed out by an artesian spring of fresh water. Palm groves (Phoenix dactylifera) spread out at the foot of each mound. The springs have dried up because of the proliferation of borehole wells. Because of the low electric conductivity (EC) of the sediments (<1 mS cm−1) at their base, spring mounds have been excavated and used as a soil amendment to expand new palm plantations. This excavation allows for analysis of their internal structure, which has never been observed well. In the present study, fine analysis of sediment layers in four sections of two representative mounds showed that vegetation had trapped sediment at their base. Fine strata of variable texture alternating with variable calcium carbonate, gypsum or organic carbon contents suggest a clear limnic origin. Many redoximorphic features, sometimes associated with the presence of old roots, suggest variation in the water level in the centre of the mound. The mounds are capped by a thick layer of indurated gypsum, which helps them resist hydric and aeolian erosion. The origin of the sediment components is discussed.