The Quaternary ostracod assemblages of the Apsheron archipelago

Introduction

Apsheron archipelago is one of the largest oils producing regions of Azerbaijan in the Caspian Sea. Oil fields have been under development in this area since 19^th century.  Many scholars obtained information on the regional ostracod fauna (Agalarova et al, 1961; Mandelstam et al, 1962; Maev et al, 1977; Mamedov et al, 1989; Javadova, 1989, etc).

Ostracods are excellent environmental indicators; thus, every geological time has its specific fauna (Boomer et al,, 2005, Boomer, 2012, Gofman, 1966, etc). They are numerous in the Caspian Sea and can live at various temperature and salinity. Therefore, ostracods were used to date sediments while oil well drilling. In the result of wide-scale test drilling in the water area of Apsheron archipelago samples were collected from numerous wells and ostracod fauna, stratigraphy and lithofacies features of the Quaternary deposits were studied in the region. The main study target was paleoreconstructions in Apsheron archipelago area based on the Quaternary faunal complexes of ostracods.

Methods

The work is based on the materials the first author obtained while employed in the State Oil Company of Azerbaijan. It is officially permitted to use the materials for scientific publications. As oil companies drilled for production, complete coring was not always performed.  Samples for ostracod study were collected from many wells in the water area of Apsheron archipelago. In total, over 3000 samples from over 300 wells were analyzed. Specimens for ostracod fauna study were sieved through 0.063 mm sieve. Logs, seismic materials, lithological study and analyzed faunal complexes were used to date sediments. Maps with well locations are drafted in СоrelDraw, scale 1:200 000.

Results

Total about 130 ostracod species were found in the Quaternary deposits of Apsheron archipelago. In the early Apsheronian the following genera were widely represented among ostracods: Amnicythere, Euxinocythere, Loxoconcha, Paracyprideis, Candona and other. In the mid-Apsheronian ostracods are represented by species from freshwater and brackish genera Amnicythere, Loxoconcha, Cyprideis, Ilyocypris, Paracyprideis and other. In the late Apsheronian considerable freshening of the Caspian water is registered along with its shrinkage. Judging by the faunal composition this freshening resulted from climatic cooling. Emerging gravels and conglomerates can be explained not only by regression but by intensified tectonic and volcanic activity at the end of the Apsheronian. This is expressed by the occurring thick interbeds of volcanic ash in the upper Apsheronian. Many representatives of genera Caspiocypris Mandelstam, 1956 and Amnicythere extinct in the late Apsheronian time. The Bakuvian unconformity overlaps the Apsheronian deposits in Apsheron archipelago. In the Bakuvian time, brackish species prevail in ostracod fauna: Bacunella, Caspiocypris, Paracyprideis and Amnicythere. The lower Khazarian unconformity on the eroded Bakuvian surface and older deposits, along with faunal differences between the Bakuvian and lower Khazarian marine deposits clearly show the deep Caspian regression between the Bakuvian and early Khazarian times. The deposits of this Caspian regression named the Chelekenian are not established within Apsheron archipelago and the adjacent area. Among ostracods the species of genera Candona were widely distributed, the species preferring relatively low salinity, along with brackish specimens of genera Amnicythere and Euxinocythere. In the beginning of the upper Khazaian time Euxinocythere beata (Stepanaitys), Amnicythere periculosa (Stepanaitys), A. hildae (Stepanaitys), A. lunata (Stepanaitys), Scoloconcha edita (Schneider), and other first emerge. At the end of the upper Khazarian the Caspian regression occurs due to climatic cooling. The Lower Khvalynian unconformity within Apsheron archipelago shows this. One may assume Apsheron archipelago used to be part of Apsheron peninsula. Ostracod fauna species composition considerably changed during that period and main Khvalynian faunal complexes formed. The Post-Khvalynian New-Caspian – modern time in the Caspian history is characterized by frequent small fluctuations of the sea level and its salinity. Within Apsheron archipelago in places the deposits of that time have limited distribution and are represented by shallow-water clays, sandy-muddy formations with shells of Cardium edulе, Mytlilaster lineatus, Didacna crassa crassa, D. baeri, Monodacna caspia and other. As for ostracods, the following are established: Amnicythere caspia (Livental), A. striatocostata (Schweyer), A. quinquetuberculata (Schweyer), along with numerous foraminifera Ammonia beccarii (Linnaeus). The compositions of ostracod assemblages from the New-Caspian and modern deposits are very close.

Conclusion

The given study resulted in the discovery of about 130 ostracod species, stratigraphical sectional plane is prepared on the basis of ostracod fauna, index species are identified for each Quaternary horizon and the map of deposits in Apsheron archipelago is drafted. The highest diversity of ostracod species is specific for the Apsheronian and Khazarian deposits. The comparative analysis of ostracod complexes shows the highest difference between the assemblages from the Apsheronian and Bakuvian supra-horizons and between the Khazarian and Khvalynian assemblages. The facies and thickness analysis of the deposits in Apsheron archipelago shows that all its basic structural components existed by the beginning of the Quaternary time. The intensive growth of anticlinal highs occurred at that time, in places attended by erupting mud volcanoes. The facies thickness and faunal distribution analysis shows that in the Quaternary time the area of Apsheron archipelago was characterized by a rather dynamic paleoenvironmental setting.