Abstract

The Iranian plateau is a triangular area bounded on the north by the Elburz mountain system and on the southwest by the Zagros, the intervening area being arid and having a number of interior basins with no outlet to the ocean. The trends of the bordering mountains and intra-desert areas are in general east-west or northwest-southeast, but north-south trending ranges also exist in central and eastern sections. The country has been brought to this condition by tectonic forces which have faulted and overthrust the bounding ranges and much of the interior and have step-faulted the rocks downward in parts of the east and north. The regional structure is therefore complicated but is intercepted by basins in which more moderate types of folding exist. That the plateau is still in process of elevation is evident from recurrent earthquakes.

The maximum composite thickness of the rocks aggregates nearly 100,000 feet. The formations range in age from possibly pre-Cambrian up to Recent, but the oldest positively known strata are Cambrian. Major unconformities exist, of which that at the base of the Cretaceous is the most intensive and far-reaching, but pre-Triassic, pre-Eocene, pre-Pliocene, and post-Pliocene unconformities are also important.

Cambrian rocks, although known in a single locality in south-central Iran and not at all farther east and north, are 7000 feet thick in southwestern Iran. Ordovician and Silurian rocks are unknown. Devonian strata, mapped in a few places in the Elburz Mountains and central Iran, have a maximum known composite thickness of about 2000 feet. Carboniferous and Permian are likewise present in considerable areas of the Elburz and central Iran; these systems aggregate about 5000 feet thick in places. Triassic rocks are less common but are known to be over 2000 feet in a few places in central-eastern and northern Iran; Jurassic formations aggregate possibly 10,000 feet in the northeast and occur with lesser thickness in wide areas from south-central Iran to the northern frontier and from the Afghan border to central Iran. Cretaceous rocks amount to over 13,000 feet in extreme north-eastern Iran; they outcrop over wide areas north of the Elburz and numerous areas in central Iran. Tertiary rocks are of great thickness, aggregating in one place or another over 40,000 feet, of which more than half the column is locally Miocene. Eocene rocks, locally over 8000 feet thick, are known north of the Elburz and, in a number of interior basins south of these mountains; and cover wide areas in the East. Quaternary deposits consist mainly of outwash gravels but also include loess in the extreme north and deposits of alluvial nature in many places.

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