The Persian Gulf, which is a shallow marginal sea of the Indian Ocean, is an excellent model for the study of some ancient troughs. It is bordered on the west by the Arabian Precambrian shield and on the east by the Persian Tertiary fold mountains. Persia is an area of extensive continental deposition. It is bordered by a narrow submarine shelf. The deeper trough of the Persian Gulf lying along the Persian Coast seaward of the shelf is floored by marly sediments. East of this, the Arabian shelf is covered with skeletal calcarenites and calcilutites. To the northwest is the Mesopotamian alluvial plain and deltaic lobe. Arabia is bordered on the Persian Gulf littoral by a coastal complex of carbonate environments. Barrier islands, tidal deltas (the site of oolitic calcarenite formation) and reefs protect lagoons where calcilutites, pelletal-calcarenites and calcilutites and skeletal calcarenites and calcilutites are forming. There are Mangrove swamps, extensive algal flats and broad intertidal flats bordering the lagoons and landward sides of the islands. A wide coastal plain, the sabkha, borders the mainland. Here evaporation and reactions between the saline waters percolating from the lagoons, and calcium carbonate deposited during a seaward regression, leads to the production of evaporitic minerals including anhydrite, celestite, dolomite, gypsum and halite. Inland, wide dune sand areas pass into the outwash plains skirting the mountain rim of Arabia.
