HOLOCENE INTERTIDAL CALCIUM CARBONATE CEMENTATION, QATAR, PERSIAN GULF

TLDR

Cemented layers in Qatar’s shallow lagoonal, intertidal, and supratidal sediments are composed of high‑magnesian calcite and aragonite, forming today from sea‑derived waters and helping explain crystal fabrics in older limestones. Magnesian calcite forms both as a primary precipitate and by replacing aragonite through dissolution and reprecipitation, producing two main cemented layer types—1 m‑thick beachrock units on exposed beaches and thin sheets beneath intertidal flats formed by hardening of friable lumps. Radiocarbon dating limits the age of these cements to less than ~4,500 years, and the cementation process accounts for crystal fabrics observed in older limestones.

Abstract

SUMMARY Cemented layers in the shallow lagoonal, intertidal, and supratidal sediments surrounding the Qatar Peninsula, Persian Gulf, contain high‐magnesian calcite and aragonite cements. The magnesian calcite appears to occur both as a primary precipitate and as a replacement of aragonite. These cements are believed to be forming today from sea‐derived waters. Radiocarbon dating sets an upper age limit of about 4,500 years. Two major types of cemented layer can be distinguished: beachrocks form variably cemented units about 1 m thick at the surface of freely draining exposed beaches: cemented sheets a few cm thick form beneath the surface of broad intertidal sand flats by the growth, hardening, and coalescence of small friable lumps near low‐tide level. Replacement of aragonite matrix, druse, and pellets by microcrystalline magnesian calcite is accomplished by dissolution and reprecipitation; detail is obliterated but the gross features of the original fabric are preserved. The process helps to explain the crystal fabrics of many older limestones.