Aragonitic, Supratidal Encrustations on the Trucial Coast, Persian Gulf

Abstract

Strontium-rich aragonite crusts exhibiting various morphologies coat both discreet particles and beach rock within the supratidal zone of the Trucial Coast. They are best developed in protected lagoonal settings, although polished “pelagosite” crusts are common within the “splash zone” exposed to wave action.

The morphology, both of coated grains and associated aragonite sheets, changes from smooth to highly irregular as one crosses into the upper parts of the supratidal zone. The latter frequently have dripstone morphologies. Micro-dripstones are developed on the under surfaces of many beach rock layers and are composed partly of detrital sediment which is deposited on the roof of small caverns by a rising water table to produce “sedimentary dripstone”.

Although many of these aragonitic encrustations resemble lithified algal stromatolites, their dripstone morphologies, nanostructures and invariably lithified character strongly suggests that they are formed by physico-chemical precipitation from sea water. Criteria are given which may permit the distinction between supratidal tufas (here termed “coniatolites”), lithified stromatolites, and cave tufas.