Penecontemporary Dolomite in the Persian Gulf1

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Abstract
Carbonate sediments dominate the shallow waters along the arid southwest side of the Persian Gulf. In the more protected parts of the west coast of the Qatar Peninsula, the processes of near-shore sedimentatiohave created lagoons and embayments with high chlorinity (30-35 g/1) and reduced tidal range; they are separated from the normal Gulf waters (22-24 g/1) with their average 4-foot tides, by many miles of sea less than two fathoms deep. The lowest-lying parts of the lagoonal shores are fringed by salt flats—”sebkhas”—varying in width from a few tens of yards to several miles. The sebkhas pass seawards into the intertidal zone, commonly via an intermediate algal flat. This is just covered by normal high tides; but only with favourable winds can occasional spring tides reach far onto the sebkha surface which is a few inches higher. Sedimentation is gradually filling the lagoons by the seaward advance of the environmental belts, so that sebkha sediment overlies stromatolitic algal laminae, and these are underlain by intertidal muddy sands composed of pelleted lime-muds, resting on lagoonal lirrie-muds. The chlorinity of the pore waters increases landwards and upwards in response to surface evaporation losses. It increases rapidly within the algal flat (50-130 g/1), where small selenite crystals form beneath the higher, landward parts. Together with the continuing precipitation of aragonite, this causes an increase in the Mg/Ca ratio of the pore waters from the normal marine value of 3 in the lagoon to over 10 at the sebkha edge. Within the sebkha, the ratio falls gradually to below 5, while the chlorinity continues to rise slowly to over 150 g/1. The water table is close to the sebkha surface, and beneath the uppermost layer subject it to large daily temperature changes the wet sediment reaches well over 40°C. in summer time. Its pH is low (around 6.7) and decreases downwards. These warm magnesium-rich brines cause diagenetic changes in the aragonite sebkha sediment. Dolomite appears. It occurs as a stiff, sticky, tan or tan-grey mud composed of rhombs 1-5 microns in size. Associated with it are turbid flattened crystals of gypsum up to five inches across, enclosing, displacing and replacing aragonite sediment. Depositional textures tend to become obscured, but macroscopic and microscopic relic structures and the changing chemistry of the pore waters make it clear that both the dolomite and the associated coarse platy gypsum are replacing aragonite. They increase in abundance away from the lagoon until they make up the bulk of the sebkha sediment. The dolomite normally appears a few inches beneath the surface, increases rapidly, and almost disappears again in a more irregular fashion within a depth of two to four feet. Carbon 14 determinations on two dolomite samples collected within 9 to 18 inches of a sebkha surface gave ages of 2670 and 3310 years, confirming that the dolomitisation is a penecontemporary phenomenon related to the present sedimentary environment.

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