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Journal of the Geological Society; 1995; v. 152; issue.2; p. 259-268;
DOI: 10.1144/gsjgs.152.2.0259
© 1995 Geological Society of London

Article

Brackish-water and evaporitic Ca–Mg carbonates in the Holocene lacustrine/deltaic deposits of southern Mesopotamia

A. A. M. AQRAWI*

Geo-Archaeology Research Unit, Centre for Revival of The Arabian Scientific Heritage, University of Baghdad, Jadiriya, Baghdad, Iraq

Authigenic dolomite, Mg-calcite and calcite occur in the Holocene near-surface marshland (Ahwar) sediments and some subsurface strata of southern Mesopotamia. Dolomite has been detected in evaporitic and brackish/marine sedimentary units of both the siliciclastic and carbonate sequences, whereas high-Mg calcite and low-Mg calcite are mainly restricted to brackish/marine and lacustrine carbonate units respectively.

The dolomite, which is polygenetic, is highly calcian and has the composition Ca1.12–1.22 Mg0.88–0.78 (CO3)2. The calcite is of two distinct phases, low-Mg and high-Mg, consisting of 5 mole% or less and 10 mole% or more MgCO3 respectively. The dolomite samples show a spread of {delta}18O values from –8{per thousand} PDB for detrital dolomite, –3 to –6.5{per thousand} for inferred dilute brackish samples, –1 to +l{per thousand} for mixing zone samples and +3 to +5{per thousand} for evaporitic types.

The influence of sea-level fluctuations during the Holocene evolution of the Tigris–Euphrates Delta has resulted in the formation of different sedimentary environments of mixing marine and fresh waters favourable for Ca–Mg carbonates formation. Evaporitic environments were the main source for dolomite formed in the early Holocene.

Key Words: Mesopotamia • brackish-water environment • evaporites • carbonates