Arguments hydrogéochimiques en faveur de Trias évaporitique non affleurant dans le massif du Djurdjura (dorsale kabyle, élément des Maghrébides) Hydrogeochemical arguments for a non out cropping Triassic formation in the Djurdjura massif (Kabyle ridge, an element of the Maghrebides range)

Résumé

L'analyse des eaux pose à nouveau la question de l'importance des faciès gypso-salifères dans le Trias de la dorsale kabyle du Djurdjura. Cette dernière est un élément de l'orogène alpin périméditerranéen où le Trias est décrit comme étant formé essentiellement de grès et de pélites avec des niveaux marneux et dolomitiques. Dans les régions telliennes, plus méridionales, il est représenté par des formations marno-gypsifères de grande épaisseur (Trias tellien), en position tectonique constamment anormale. On ne rencontre généralement pas de formations évaporitiques en surface dans le massif du Djurdjura. Les analyses chimiques de la majorité des sources le confirment. Toutefois la source de Tinzert, dont l'impluvium est constitué essentiellement de calcaires montre un cortège d'éléments d'origine évaporitique (fortes teneurs en chlorures, sodium et sulfates et la présence du strontium) et plaide pour la présence de niveaux évaporitiques du Trias, à la base de l'aquifère drainé par cette source. Le rapport Sr²⁺/Ca²⁺ (en ‰) de la source de Tinzert (3 < Sr²⁺/Ca²⁺ < 8,7) qui est nettement supérieur à celui des autres sources (Sr²⁺/Ca²⁺ < 1,5) le confirme.

Mots-clés:

• Djurdjura,
• Maghrebides,
• Evaporites,
• Trias,
• Hydrochimie,
• Source,
• Strontium

Abstract

The Kabyle ridge of Djurdjura (Algeria, DURAND-DELGA, 1969) is bordered by the basement to the north, which overthrusts it, and flyschs to the south which it overthrusts. Its structure corresponds to highly piled dipping thrust sheets. N140 transversal faults laterally cut them. The different structural units, mainly calcareous, which form the inner, central and southern ridge (FLANDRIN, 1952; FLANDRIN et al., 1952; COUTELLE, 1979; GELARD, 1979 and NAAK, 1988), isolated by low permeability layers, constitute independent drainage units (ABDESSELAM, 1995). The massive and thick lower Liassic and Eocene limestones are highly karstified and constitute the main aquifers (ABDESSELAM et al., 1990).

FALLOT (1942) summarises the stratigraphy of the Djurdjura Triassic layers: conglomerates and red sandstones (Permo-Triassic), pale sandstones (Werfenian), dolomites and vermiculate limestones (Muschelkalk), variegated marls and cellular dolomites (Keuper), limestones (Rhetian).

The lithostratigraphy of the Triassic of the Kabyle ridge is very differentiated from one unit to another (FLANDRIN, 1952; GELARD, 1979). This fact reflects an early sedimentary segmentation of the tethysian passive margin from the north to the south. The Kabylian Triassic (inner zones) contains gypsum and salty formations only very rarely. Thus, in the Djurdjura, evaporites are very rarely mentioned at the surface (LAMBERT, 1947; FLANDRIN,1952; COUTELLE, 1979; GELARD, 1979; NAAK, 1988; TEFIANI et al., 1991; DURAND-DELGA and TEFIANI, 1994), when the early authors (FALLOT, 1942; GIGNOUX, 1946) suspected the presence of a tectonised salty Triassic in the Djurdjura.

Overall, the evaporitic outcrops are insignificant, compared to the Triassic ones of the Djurdjura. The Triassic layers however frequently bear vacuolar dolomites and cellular dolomites associated with variegated clays that can have been accompanied with gypsum and salt, which can have migrated with or without tectonic compressions.

Water analysis is used to answer the question of the importance of gypsum and salty formations in the Kabylian Triassic.

In an area where gypsum and salty layers rarely outcrop, chemical analyses of spring waters are used to demonstrate their importance in the Triassic of the Kabylian Dorsale of the Djurdjura. A hydrochemical study was carried out on the main perennial springs of the whole Djurdjura : chemical type, Sr content, Sr²⁺ /Ca²⁺ molar ratio.

Strontium is a tracer bound to evaporites. High Sr²⁺ content in waters can only be explained by Celestite dissolution (SrSO⁴), a mineral that is associated with gypsum; Sr is thus a good tracer of the existence of evaporites. The Sr²⁺/Ca²⁺ ionic ratio (molar concentrations) is characteristic of an evaporitic origin if it equals or exceeds 1‰ (BAKALOWICZ 1988). It is higher than 5 ‰ in the Alpine Triassic layers (MEYBECK, 1984).

A hydrochemical study enabled us to determine the water types: calcium bicarbonate for the most, with a second sulfate trend; Tinzert spring has a sodium chloride type (Fig. 2). It is situated at the lowest point to the west of the western Djurdjura, where the ridge is the widest (5 km). Its discharge ranges from 60 l·s^-1 to 2 m3·s^-1. Its water tastes very salty in the low water periods and the concentrations of evaporitic tracers are high: chlorides (> 1000 mg·l-1), sodium (> 500 mg·l^-1), sulfates (> 200 mg·l^-1) and potassium (> 25 mg·l^-1).

During flood periods, because of dilution and quick conduit flows in the upper calcareous zones, the water type becomes calcium bicarbonate. During this period, waters flowing out of the spring are traced by the limestone shallow waters (transit of epikarstic waters). On top and upstream from Tinzert, less than 20 metres to the South, Tala Agouni Lansar displays a very different chemical type, calcium bicarbonate. This fact demonstrates that Tinzert sodium chloride content is acquired by water which stays in the deep saturated zone (Figs. 3 et 4), and that longitudinal faults divide the lithological units into segments.

Analyses of strontium in waters demonstrate that most of the springs (ABDESSELAM, 1995) have low Sr²⁺ contents (0.06-0.23 mg·l^-1). Tinzert spring has a much higher content (0.35-1.83 mg·l^-1 ; Table 1). The use of the Sr²⁺ /Ca²⁺ ratio (‰) enabled us to distinguish among aquifers completely developed in limestones, others related to Triassic sandstones and one related to salty layers. The map of Sr²⁺ /Ca²⁺ ratios indicates that the springs related to Triassic outcrops have the higher values (Fig. 3).

According to the Sr²⁺ /Ca²⁺ ratio (‰), three groups can be distinguished:

- springs related to limestones, with no relationship with Triassic formations, have a low Sr²⁺/Ca²⁺ ratio (< 1‰: Adarchi, Aghvaïlou-1 et 2, Tiksra, Thebout and Mendes; Table 1),

outlets directly limited by red sandstones Triassic formations (Taliouine et Tabourt Lansar), have a Sr2+/Ca2+ ratio ranging between 1 and 1.5 ‰;

Tinzert spring, with a Sr²⁺/Ca²⁺+ ratio that ranges between 3 and 8.77 ‰. This Sr²⁺/Ca²⁺ ratio (> 5‰) classifies this water into the category of waters originating from the alpine Triassic evaporites (MEYBECK, 1984). This high ratio is coupled with the high sulfate, and especially the chloride and sodium content of these waters.

This spring is situated on the trace of a north-east thrust sheet sole that probably includes Triassic in its lower part, which concerns the whole Haïzer massif of about 8 km. The water transit is probably either in the upper thrust sheet, or in the lower one, following the East-West axis lowering of the structure.

The springs of the Djurdjura display well-differentiated hydrochemical responses. Several springs that only drain limestone have a standard calcium bicarbonate chemical type (Sr²⁺/Ca²⁺ < 1 ‰). Other springs (Sr²⁺/Ca²⁺ =1 - 1.5 ‰) are characteristic of waters that have flowed in the sandstone and dolomitic Triassic layers, which are observed on the outcrops. Tinzert spring at least, which drains the middle part of the limestone range (Fig. 4, Sr²⁺/Ca²⁺=3 - 8.77 ‰), is characterised by waters which have transited through the evaporitic Triassic. The waters of Tinzert spring have also high chloride, sodium and sulfate contents.

In the Djurdjura, where evaporitic Triassic formations do not generally outcrop, except in very small lenses, an argument can be made for the existence of deep evaporitic Triassic deposits on the basis of the hydrochemical response of perennial springs.

Keywords:

• Djurdjura,
• Maghrebides,
• evaporites,
• Triassic system,
• hydrochemistry,
• spring,
• strontium