The sedimentology of the Lower Fars Formation (Miocene) of Northern Iraq

Abstract

This thesis incorporates a study of the petrology and sedimentology of rocks of the Gachsaran Formation (formerly lower Fars) (Miocene) in northern Iraq. Field sttdy led to the recognition of three basic lithofacies: mudrock, limestone and gypsum-anhydrite, which are arranged into cycles. The lithofacies are interpreted as the deposits of subtidal, intertidal and supratidal environments by comparison with Recent sediments, particularly those developing along the Trucial Coast, Arabian Gulf. Petrological studies show that the gypsum is secondary after anhydrite. Several types of sulphate textures are distinguished, each of which is described and its origin interpreted. The dominant nodular nature of the sulphates and their occurrence in repeated mudrock, limestone, sulphate cycles are consistent with a supratidal (sabkha) origin. Geochemical data were also used to aid in the interpretation of the depositional environment and diagenesis of the sulphates. Detailed petrographic examination of the limestones has led to the distinction of twelve carbonate-lithofacies. The most common types are peloidal and bioclastic limestones as well as oolitic and oncolitic varieties. Stromatolites and cryptalgal laminites are also present and their significance is discussed. In general the limestones are interpreted as deposited in shallow suibtidal to intertidal situations. Diagenetic events in the limestones include early and late cementation, dissolution and some dolomitization. Early diagenetic cementation appears to have been widespread. Two types of dolomite are recognised: an early non-rhombohedral variety resembling micrite, and a much coarser, later diagenetic dolomite. Geochemical data indicate diagenesis in both open and closed systems. The limestones possess average Fe and Mn contents, but Mg is relatively low compared with published limestone analyses. The very high content of Sr in some of the carbonate rocks is taken to indicate that aragonite has been an important early mineral in these rocks. The Na content, a reflection of salinity, indicates hypersalinity, especially for the dolostones. With regard to the mudrocks, illite is the dominant clay mineral with montmorillonite, mixed-layer clays and kaolinite less common. On the basis of colour, mineralogy and type of Foraminifera present, two types of mudrock are recognised: a green variety with benthic Foraminifera, other fossils and higher kaolinite, interpreted as deposited in hypersaline lagoons, and a red variety with planktonic Foraminifera, and a lower kaolinite content, interpreted as of relatively deep water origin. Cycles of mudrock - limestone- gypsum, limestone - gypsum and mudrock - gypsum are interpreted as sabkha cycles produced through the progradation of sabkhas over intertidal and aubtidal sediments. A consideration of the tectonic context of the Mesopotamian Basin suggests that vertical tectonic movements may well have been an important factor in Gachsaran deposition and the development of the cyclicity, as well as eustatic sea-level changes through variations in the extent of polar glaciation and plate tectonic movements.