Anhydrite diagenesis in a vegetated sabkha, Al-Khiran, Kuwait, Arabian Gulf

Abstract

In the Al-Khiran sabkha, supratidal anhydrite occurs in three distinct morphological forms: (a) hair-cream anhydrite which occurs exclusively in association with the halophyte Halocnemum strobilaceum; and (b) nodular/laminated cottage cheese anhydrite which is associated with a bacterial-cyanobacterial mat. The former occurs as a massive layer up to 4 cm thick with the ahydrite initially pseudomorphing gypsum crystals: while in the latter the layers are up to 7 cm thick and the anhydrite occurs as microcrystalline laths giving an aphanitic texture; (c) sub-aqueous anhydrite formed by the alteration of prismatic gypsum crystals growing in drainage channels. On the death of the halophyte the gypsum (G) _rlhar2; anhydrite (A) metastable equilibrium is upset and the hair-cream anhydrite undergoes a reverse transformation to microcrystalline alabastrine secondary gypsum followed by gypcrust formation. The above changes result in a series of characteristic diagenetic textures that were studied in detail by scanning electron microscopy (SEM). The major diagenetic mechanism during the G _rlhar2; A transformation is a dissolution-reprecipitation process that is only partially controlled by physico-chemical factors. A climatic and biological influence also plays a significant role in this sometimes annual change, depending on the rainfall intensity. In contrast, type (b) shows better preservation potential. The intimate relationship between the plant and its “soil water” requirements results in a particular hydrodynamic behaviour of the capillary groundwater brines in the vadose zone. Hence, it is inferred that the individual diagenetic stages are also correlatable with the changing hydrological conditions of the sabkha. Type (c) represents a rare occurrence in sabkhas and is more akin to some deeper water occurrences, albeit on a very small scale. Anhydrite is always formed after a gypsum precursor, and under predictable physico-chemical conditions (an average annual temperature of > 30°C and brine chlorinity of > 4 mol Cl − kg −1). Although it is confined to the vegetated zone of the marine sabkha, stable isotope analysis indicates a dominantly continental origin for the groundwaters and coexisting sulphate minerals. On a regional scale, the vegetated Kuwait sabkhas appear to occupy a climatic position intermediate between the classic UAE sabkhas and those of the Mediterranean region. Al-Khiran is the northernmost limit of the evaporite belt containing anhydrite in the Arabian Gulf. The palaeoclimatic and sedimentological implications of this distribution may be of potential value in the rock-record.