Dolomite formation in coastal Al-Khiran, Kuwait Arabian Gulf — a re-examination of the sabkha model

Abstract

Recent dolomites are found in the subtidal, intertidal and supratidal sediments of the lagoons and sabkhas of coastal Al-Khiran, Kuwait. Two stages of diagenetic dolomite formation have been recognised in the sabkhas; one formed during the earlier lagoonal (transgressive) phase and the other during the later sabkha phase after progradation has commenced. In Al-Khiran, dolomite forms under geochemical conditions within the following ranges: (a) a Mg 2+ Ca 2+ molar ratio of 5.3 to 6.1 (in the lagoons) and 3.6 to 14 (in sabkhas); (b) reduced levels of dissolved SO 4 2− concentration, either through microbial sulphate reduction and/or achievement of seasonal mild schizohaline conditions (as in the lagoons) or by minor gypsum precipitation within the sediments; (c) a reducing environment in association with diagenetic pyrite formation; (d) a chloride concentration between 0.62 mol Cl − kg −1 (in lagoons) and 2.15−2.95 mol Cl − kg −1 (in sabkhas); (e) temperatures between 12°C and 35°C, and a pH between 6.4 and 8.4. In those areas of present-day optimum dolomite formation in the sabkha, the near-surface groundwater brines (the potential “dolomitizing” fluid) do not show major changes in chemical composition with time. The question of the dominant mechanism of dolomite formation (i.e. primary precipitation versus diagenetic alteration of host aragonite) is left open, but the balance of evidence (chemical and petrographic) favours a primary origin. Due to the coastal configuration and topographic barriers in the form of chenier ridges, marine flood recharge of the sabkhas and flooding frequencies (which control flow rates) are curtailed. The highest dolomite percentage is confined to intertidal channel areas, and in the absence of an efficient hydrodynamic flow mechanism, the volume of dolomite formed in the sabkha remains low (4 to 10% on average). Away from these areas, the intensity of dolomitization is reduced, even though the porosity and permeability of the host sediments are favourable. This occurrence is rather different from other well known areas of dolomite formation, as in the Caribbean and the southern Arabian Gulf (where the dolomite content is an order of magnitude greater), despite a similar host sediment stratigraphy and mineralogy. In Al-Khiran, it has not been possible to demonstrate a quantitative relationship between the dolomite content in the sediments and solution chemistry variables. It is surmised that the physical setting of the area, not solution chemistry, may be the major control on how much dolomite forms in the sabkha.