Large benthic foraminifera assemblages and environmental interpretation of Eocene deposits in the Soltanieh Mountains (Western Alborz)

Large benthic foraminifera (LBF) is an informal group of benthic foraminifers that characterized by complicated internal structures and relative large sizes with bathymetric distribution under environmental factors. The main objective of this study is palaeoecological reconstruction of the Eocene shallow- marine carbonate successions based on morphological characters and hydrodynamic behavior of LBF from the Soltanieh Mountains (western Alborz).     

This study has been done on three sedimentary successions from Gheynarjeh and Ghaziabad locations. Gheynarjeh 1, 2 are located at 19 km southeast of the city of Zanjan (close to Gheynarjeh Village) with thickness of 52 m and 34 m respectively. The Ghaziabad section is 25-m thick and located near the Ghaziabad Village (about 25 km south of the city of Zanjan) and about 5 km southwest of the Gheynarjeh 2 section. In total, 50 samples were collected from these sections. After preparation, they were examined in terms of microfossils contents especially LBF. Some samples contained a fair number of well-orientated large benthic foraminifera. We have used relative differences in D/T measurements of LBF and the ratio of the megalospheric (A-form) tests to the microspheric (B-form) tests in palaeoenvironmental interpretation.
Results and

The distributional pattern of LBF and their ecological gradient are represented by four assemblages: 1) Alveolinids assemblages: this assemblage has a high diversity of alveolinids Alveolina ellipsoidalis (Schwager), Alveolina tumida (Hottinger), Alveolina laxa (Hottinger), Alveolina pisiformis (Hottinger), Alveolina rotundata (Hottinger), Alveolina subpyrenaica (Leymerie) associated with smaller porcelaneous foraminifera (mainly miliolids), rare Orbitolites sp. and nummulitids which are represent at maximum depth 40 m under the influence of high energy conditions by wave action in a lagoonal environment. In generally, They were observed down to depths of 80 m (Hottinger 1983, 1997); 2) Nummulitids assemblages: this assemblages is dominated by robust and ovate Nummulites tests on palaeohigh, relatively minor with low relief submerged near fair- weather wave base, with a ratio of A to B forms of 50:1 to 75:1 that affected by wave and current (storm?) action at depth less than 80 m; 3) Encrusting Foraminifera assemblages: The highly abundances of encrusting foraminifera (acevulinids) in this assemblage had constructed the “reefs” at moderate depths in the euphotic zone (40-80 m). In fact, the presence of encrusting foraminifera such as physical barrier may play a major role in decreasing of hydrodynamic energy and 4) Orthophragminids assemblages: this assemblage is dominated by flattened and saddle-like formrs of orthophragminids Discocyclina javana (Verbeek), Discocyclina sp., Discocyclina dispansa sp. cf., Discocyclina dispansa (Sowerby), Orbitoclypeus sp. cf. and Orbitoclypeus marthae associated with nummulitids at a depth range between 80 to 100 m (outer ramp setting).