Evolution and environment of the global community maturation (GCM) cycle of Alveolina species in Eocene deposits of the Ziarat Formation (Soltanieh Mountains)

Up to now, the global community maturation cycle of Eocene deposits on the basis of Alveolina species was hitherto unknown from Iran. In this research, three evolutionary lines were identified based on phylogenetic and environmental parameters, represented by Alveolina ellipsoidalis (Schwager), Alveolina subpyrenaica (Leymerie), Alveolina moussoulensis (Hottinger), Alveolina laxa (Hottinger), Alveolina ilerdensis (Hottinger), Alveolina canavari (Checchia-Rispoli).  The phylogenetic and environmental parameters of Alveolina species, are revealed in a part of the fourth phase of the global community maturation (GCM) cycle within the five shallow benthic zones (SBZ7-SBZ11) in the western Alborz zone (Soltanieh Mountains).        

The material was collected from two sedimentary successions in Gheynarjeh and Baghdareh localities. The Gheynarjeh section of 52 m thick, in close proximity of Gheynarjeh village is exposed at 19 km southeast of Zanjan. The Baghdareh section of 85 m thick is measured approximately 1.5 km of the southwest of Baghdareh village and ca. 30 km west of Khoram Dareh. The section is situated about 90 km southeast of the town of Zanjan. This study is based on the samples collected of tow continuously outcropping stratigraphic sections (Baghdareh and Gheynarjeh) with special emphasis on alveolinids. The Alveolina assemblages were studied on about 80 oriented random thin-sections since the indurate carbonate rocks do not allow specimens to be freed from their encasing sediment. Systematic determinations of Alveolina species were adopted using Hottinger (1960), Drobne (1977), and Sirel & Acar (2008).

The global community maturation (GCM) cycle of Eocene deposits based on species of Alveolina from the Ilerdian-Cuisian shallow marine successions in the western Alborz (Soltanieh Mountains) are represented by 3 evolutionary lines into three groups:1- Ellipsoidalis group with evolutionary trend of Alveolina ellipsoidalis and Alveolina moussoulensis, associated with species of Alveolina aragonensis, Alveolina cf. dedolia, Alveolina pisiformis, Alveolina subpyrenaica and Alveolina tumida. 2- Subpyrenaica group consists of Alveolina ilerdensis and Alveolina subpyrenaica evolutionary line that is together with ovoidal species such as Alveolina sakaryensis and Alveolina decipensis. 3- Canavarii group is represented by Alveolina canavarii - Alveolina laxa evolutionary lineage with the first occurrence of Alveolina bayburtensis species. Therefore, the lower part of the fourth phase of the global community maturation (GCM) cycle (Hottinger 2001) is distinguished by the dominance of A-forms of Alveolina species with life strategy (r-mode) in the three evolutionary lineages, as explained above.