Sedimentology of Gachsaran Formation (Lower Fars Series), Southwest Iran

Abstract

The Gachsaran Formation, formerly known as the Lower Fars Series, is exposed extensively in the foothills of the Zagros Mountains in southwest Iran, and forms the caprock at many important oil fields. During recent visits to this area, we were impressed by the almost universal nodular habit of the gypsum-anhydrite rocks, and by the conspicuous cyclic nature of the sediments. This paper presents two field sections which we suggest as typical of the marginal and the central-basin facies. The former was measured near the northern margin of the basin, where the beds at the top of each cycle are red marl, siltstone, or even sandstone, clearly of continental aspect; the latter was measured near the center of the depositional basin, where the evaporitic part of the cycle is dominant. In both sections, the base of each cycle is a marine carbonate rock of demonstrably shallow-water association, which is overlain by nodular gypsum-anhydrite rock, and at the top by dominantly red or variegated marl. The entire sequence is remarkably similar to the shallow-water, marine, supratidal sabkha, and terrestrial environments of the Trucial Oman Coast first described by Shea man in 1963. The Gachsaran Formation records a succession of minor cycles of sedimentation between the fully marine development of the Asmari limestone and the continental redbeds of the Agha Jari Formation.