Lower and Middle Devonian marine deposits of Central Kazakhstan

Abstract The Lower and Middle Devonian marine sequence of Central Kazakhstan, in a highly fossiliferous clastic facies, forms a continuous sequence with the Silurian. The Lower Devonian is divided into two units - the Balkhash Land and Sardjal divisions, tentatively correlated with Gedinnian, Siegenian, and Emsian. In the Balkhash Land unit, Devonian Heliophyllum, Stenophyllum, Anoplotheca, Euryspirifer inelegans (Hall), Howellella mercuri (Gos.) etc., make their appearance, in association with Silurian species. In the Sardjal division Acrospirifer primaevus kasachstanica Kap., Rhytistrophia beckii (Hall), Crotalocephalus gibbus hexaspinus Z. Max., etc. occur. In the Middle Devonian, the Kazakh, Besoba and Aidarly divisions, tentatively correlated with “Lower Eifelian”, “Upper Eifelian” and Givetian, have been distinguished. In the Lower Devonian genera and species as Hexacrinus, Decacrinus, Isorthis pertermedius (Schn.), and E. supraspeciosus Lotze appear. In the “Upper Eifelian” Calceola sandalina Lam., Grypophyllum striatum Spassky, and Striatopora schandiensis Dubat. are diagnostic. The Givetian Stage is characterized by the presence of the Mucrospirifer mucronatus (Hall) group of brachiopods. In the Lower Devonian and “Lower Eifelian”, the marine basin occupied a vast territory. By the end of the “Lower Eifelian”, the sea had regressed so that a major part of the region was emergent. Marine sedimentation continued through the Middle Devonian only in the southeast. The marine basin of central Kazakhstan was a peculiar palaeozoogeographical province, comprising western European, North American, and local faunas. The brachiopod fauna in Lower Devonian and “Lower Eifelian” differs sharply from the contemporaneous fauna of the Urals, Kuznetzk basin and central Asia. Not until the “Upper Eifelian” is the fauna comparable with contemporaneous associations of one of the other regions mentioned above.