Stratigraphy and Environmental Sedimentology of Lower Fars Group (Miocene), South-Southwest Iran

Abstract

On the basis of surface and subsurface geologic data, the lower Fars Group (Miocene) in the Zagros geosyncline of south-southwestern Iran can be divided into four major stratigraphic units: (1) anhydrite-salty facies (Gachsaran Formation); (2) sandy-silty facies (Razak Formation); (3) gypsum-anhydrite facies (Namaki Formation); and (4) massive salty facies (Qeshm Formation). The names Namaki and Qeshm are assigned to two distinct units with different lithologies within the southeastern extension of the lower Fars Group. Surface and subsurface reference sections are established to illustrate the distribution of these units both vertically and horizontally. During Tertiary time, subsidence of the Zagros geosyncline diminished considerably. The folded belt or miogeosynclinal portion of the Zagros geosyncline, by late Oligocene and early Miocene time, had developed into a large shallow trough which, by middle Miocene time, had formed local lagoonal and evaporite traps in which the lower Fars chemical evaporite series was deposited. The broad, shallow, foreland trough which was the site of lower Fars deposition extended from extreme southeastern Zagros northwestward to the eastern limit of the Mediterranean trough. On the basis of field evidence, the source of sediments for the lower Fars Group could be chemical salt-anhydrite precipitation directly from sea water. The detrital-clastic sequence within the Namaki Formation possibly had its source in the Oman basin on the east. Sedimentologic evidences indicate that the source area for the Razak Formation was the emerging eugeosynclinal portion of the Zagros north-northeast of the site of Razak deposition. An independent disharmonic gliding movement of the beds occurred in the lower Fars Group, mainly in the Gachsaran and Namaki Formations. The major effect of this disharmony was westward displacement of the surface axes of the anticlines.