Distribution and Ultrastructure of Holocene Ooids in the Persian Gulf

Abstract

Ooids occur in many different environments in the Persian Gulf; blackened relict ooids are common at depths of 100 m in the center of the basin while others have been transported to desert environments up to 40 km from the shore. Most Holocene ooids occur in coastal areas in the S parts of the Persian Gulf in which they appear to be forming in a variety of settings. The most spectacular accumulations constitute tidal deltas associated with coastal barrier system in E Abu Dhabi. Other agitated environments include tidal bars situated in wide channels between islands (Yas and Bahrain) and the adjacent Arabian shoreline, and on open tidal flats and beaches in exposed embayments (Sabkha Matti). Although most ooids are forming in these agitated environments, significant quantities are forming within lagoons (Khor Odaid and Jebel Dhanna), and on the SE lee coast of Qatar Peninsula (Umm Said).

The structure of the cortex of ooids collected in these various environments has been studied by light microcope, SEM, micro-probe and other analyses, to evaluate the relationship between ooid structure and environment. Aragonitic nano-grains seem to be associated with organic material in the cortex and occur in most ooids irrespective of environment. Elongate rods (1–2 microns) of aragonite exhibit various orientations. In agitated environments on the crests of bars, or delta levées, these have a statistically tangential orientation with respect to the nucleus, and are often tightly packed or coalescent. In ooids collected from more protected depressions between bars, aragonite rods within the outermost layer of the cortex have haphazard or radial orientation and the fabric is loose. In protected lagoonal settings this radial orientation is well developed and the ooids are often unusually big and irregular in shape.

The relationship between the structure of the external layer of the cortex and the environment in which the ooid was collected indicates that the aragonitic components grow on the nucleus initially with a haphazard or radial orientation, creating a loose fabric. This primary orientation, developed mainly while the ooid is in a relatively protected micro-environment, is subsequently modified to a secondary tangential orientation on the crest of adjacent bars or beaches where crystals are physically compacted to create a dense fabric. Lagoonal ooids tend to retain their primary radial structure and may attain considerable size in the absense of physical abrasion in these protected environments.

Physical conditions favourable to ooid formation would seem to include a bar and protected channel morphology associated with reversible tidal or longshore currents which retain ooids within this favourable system. Evaporitic tidal flats seem to favour the formation of large pisooids whose formation is not related to mechanical movements.

Most Holocene ooids of the Persian Gulf have much in common with the classical Bahamian ooids. They differ mainly in their regional distribution, most forming along the continental shoreline remote from any platform edge, many accumulating in desert environments. Others form in quiet water tidal flats and have much in common with cave pearls. Although the areas of ooid formation at any given moment are small, these grains are readily dispersed. In time a sheet of oolitic sand may develop whose geometry and magnitude will not reflect that of the ooid-forming environment.