Abstract

In the regions considered, the distribution of the Devonian is as follows: 1. Lower Devonian, present only in a few marginal areas; 2. Middle Devonian, sporadic; 3. Upper Devonian, fairly widespread. The Devonian deposits are everywhere of a transgressive character. On the old Iranian craton they represent the first signs of the more extensive Carboniferous transgression.

In the Elburz range of Iran and in the whole of the northern part of the country, only Upper Devonian strata are known, as largely infralittoral deposits. Westward from the Central Elburz, the Devonian is predominantly represented by the Famennian. The thick successions of sandstones and red shales, considered until 1960 to be Devonian Old Red Sandstone, are in fact of Cambrian age. The Devonian covers them discomformably, but without marked angular unconformity.

In eastern Iran subsidence was greater, the transgression advanced across Silurian deposits and may have begun at the end of the Middle Devonian. Both the Frasnian and the Famennian are well documented as regards brachiopods and cephalopods. The sedimentation is more pelagic, particularly in the southern part of the area.

In northwestern Afghanistan, as in northern Iran, there is infralittoral Upper Devonian represented predominantly by Frasnian with brachiopods. It is transgressive across either the red beds or the crystalline basement.

The Koh-i-Baba range in Afghanistan is situated on the northern edge of a basin in the Hazarajat, which contains beds with fossils representing all three Devonian series. These strata are largely calcareous, and up to 2800 metres thick. In Koh-i-Baba itself however, is a calcareous-arenaceous succession several hundred metres thick, in which both the Frasnian and the Famennian are represented.

In Badakhshan, the calcareous Devonian is metamorphosed, but contains occasional Frasnian fossils. In Chitral all three series of the Devonian are represented, but they belong to different tectonic zones.