Carbonate and clay mineralogy of the Persian Gulf

Abstract

The carbonate and clay mineralogy of 45 widespread Persian Gulf samples was determined. The trends in abundance of both the carbonate and clay minerals roughly parallel the long axis of the Gulf. The carbonate fraction of the relatively coarse-grained, carbonate-rich sediments along the southwest margin (Saudi Arabian coast) consists largely of aragonite and high-Mg calcite. The finergrained sediments along the northeast margin (Iranian coast) are relatively carbonate-poor and the dominant carbonate minerals are low-Mg calcite and dolomite. In individual samples, the carbonate fraction of less-than-sand size usually contains much more low-Mg calcite than the coarser fractions. Dolomite, at least traces of which were found in nearly every samples, is almost exclusively present in the fine fraction of individual samples. Dolomite may make up greater than 50% of the total carbonate fraction in some samples but in most cases is less important. Although primary dolomite has been reported from very shallow water in the Persian Gulf, the dolomite observed in this study is believed to have been derived from dust picked up from outcrops of ancient dolomites and perhaps from local rivers. The mineralogy of the carbonate fraction of the Persian Gulf appears to be controlled by the balance between organic carbonate processes (characterized by abundant aragonite and high-Mg calcite) and the amount of dust and river contribution (characterized by abundant dolomite and low-Mg calcite). The dominant clay mineral found in Persian Gulf sediments is an illite plus an illite-montmorillonite with approximately 10–20% expandable layers. This 10 Å material is followed in abundance by chlorite, a heterogeneous conglomeration of illite-montmorillonite with greater than 40% expandable layers and kaolinite, respectively. In general, the amount of illite plus illite-montmorillonite decreases with respect to chlorite in a direction away from the Iranian shore. The quantity of chlorite with respect to kaolinite decreases also in the same way.