Devonian Sediments have been studied in south west of England for first time by Murrchison and Sedgwick, in 1839 in Devonshire and that is why Murrchison has named Devonian, sediments which have been located between Silurian and Carboniferous sediments. Estimated time interval for this period is 50 million years.
Generally it can be indentified two important facieses in this period, first one is lagoonal facies and the other one is marine facies. Lagoonal Facies is mainly composed of red sandstone which geologists have named them as old red sandstones. Their thickness sometimes reaches to several thousand meters and they have mainly cross – bedding. Old red sandstone have been found and extended into England, Scandinavian, Ireland, North America, east of Greenland, west of Spitsbergen and also in north west of Russia, Belgium and finally to center of Poland (generally in lands of north Hemisphere). Because these sediments are pertaining to nonmarine environments (lagoonal), so the fossils which have been found in them are mainly flora fossils, armoured fishes and Bivalves and marine fossils such as Brachiopod, Trilobite, Cephalopod and Echinoid have not been observed in them.
Devonian in Iran
Week point of Paleontology has been resulted in finding ambiguities in paleogeography in lower and middle Devonian, in contrast to upper Devonian. Imagination of most of geologists is that condition of Paleogeography in Iran, in beginning of Devonian, has been similar to late of Silurian that means Caledonian land has been located in north and north west of Iran and east of Iran has been covered by a shallow sea which have been impressed by vertical and upwards movements in Caledonian event. That is why lower Devonian rocks have been observed in vast parts of Iran and middle Devonian rocks have been located on older rocks as transgressive and with disconformity. In late Devonian, extent of Caledonian land has been decreases and so upper Devonian deposits have been deposited with much extent and after a sedimentary gap, in many areas of Iran. Nevertheless in some areas of Central Iran (Tabas, Kerman, Ardakan and …) and eastern Alborz (Khoshyeilagh, Bojnoured and …) there are red clastic strata (Padeha Formation) which have been attributed to early Devonian because of Stratigraphic position. But, early Devonian has not been confirmed certainly and even studies of Palynology demonstrate that these sandstones are pertaining to late Devonian. In case of certainty about age of early Devonian for red sandstones (Padeha formation), there can be uncertainty about strata pertaining to middle Devonian too. Besides strata pertaining to lower Devonian, presence of fossils such as Brachiopod, Coral, Conodont and…, pertaining to Eifelian – Josean epochs, demonstrates that marine conditions have been begun since middle Devonian and has reached to maximum amount in upper Devonian. However studying of marine Palynomorphs was resulted in this belief by Ghavidel Sivaki that unlimited sea has been regressed from all over parts of Iran platform in early and middle Devonian and this emersion has been lasted to late Devonian.
One of characteristics of Devonian in Iran is permanence and iteration of movements; depend on Caledonian event which has been sometimes accompanied with basic volcanic lava and sometimes with emplacement of intrusive bodies (Syenites of Marand – Julfa). Based on results from palynology and description of stratigraphic characteristics, Devonian rocks of Iran can be put in two groups; clastic rocks pertaining to early Devonian and mainly carbonate rocks in middle - late Devonian.