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Devonian of Central and Western Afghanistan and Southern Iran
Over the past three years the authors have studied the palaeontology, stratigraphy and palaeogeography of the Palaeozoic sequence of Central and Western Afghanistan, south of the Hindukush. There fossiliferous beds range from Ordovician to Permian.
The Devonian was studied in detail in two areas, one in Western, the other in Central Afghanistan. Other, lesser known Palaeozoic sequences were used for comparison. Corals, brachiopods, trilobites and conodonts substantiate continuous Middle and Upper Devonian deposits in both areas. The presence of Lower Devonian beds could not be established. In Central Afghanistan, however, a sequence of clastic beds, about 2000 metres in thickness, lies between sandy marls of Ludlovian age and Middle Devonian reef limestones. These beds may in part represent the Lower Devonian. In Western Afghanistan, on the other hand, red beds and evaporites lie beneath fossiliferous Middle Devonian. They overlie metamorphic rocks with conglomeratic sequences at the base and probably belong to the Lower Devonian. They are not older than Ludlow.
The Devonian sections of the western and central areas differ in lithology, thickness and fossil content. Western Afghanistan was a shelf with mainly clastic and evaporite deposition at first, followed by fully marine conditions with deposits of dolomites, brachiopod limestones and marls of the Middle and Upper Devonian. In southeastern Afghanistan, however, a great southeasterly trending geosynclinal trough existed. In the northwestern part of this geosyncline, between Kandahar and the area of Kabul, deposition of sandy limestones began in the Ordovician and Silurian, continuing up into the Middle Devonian with an unfossiliferous, clastic sequence. Evaporites and red beds are missing here. In the Middle and Upper Devonian, coarse and fine clastic deposition alternated with that of bedded limestones and reef sediments.
In Central Afghanistan the Upper Devonian is overlain by Carboniferous and Permian beds of geosynclinal facies. In Western Afghanistan the sections studied end with Upper Devonian. In adjacent areas and sections, however, Carboniferous is present in calcareous facies.
The general aspect of the Devonian in Western Afghanistan indicates relationships to the Devonian of Southern Iran.
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