LITHOFACIES ZONATION OF LOWER PLIOCENE PRODUCTIVE SERIES SEDIMENTS, SOUTH CASPIAN BASIN

Elmira Aliyeva and Dadash Huseynov
Institute of Geology, Azerbaijan National Academy of Sciences

Intensive orogenic movements adjacent to South Caspian basin folded areas and submersion of its central parts at the end of Pontian-beginning of Early Pliocene, led to dramatic PaleoCaspian Sea level fall (from 600 to 1500 m). This initiated deposition of the Lower Pliocene Productive Series within an enclosed basin, concentrated in the area of South Caspian, following separation from East ParaTethys from about 5.5 Ma. The Productive Series (PS) consists of rhythmically bedded fluvial-deltaic-lacustrine sediments deposited in the isolated South Caspian basin by several large river systems and reaching up to 7 km in thickness in the central part of the basin.

Productive Series sediments accumulated in several different geotectonic environments - periclinal trough (Absheron Peninsula), intermountain depression (Lower Kura depression), foreland depressions (PreTalysh-Elburs trough, PreCaspian region, and the Jeyrankechmez depression), and also in the platform setting. The latter is represented by facies of river canyons and valleys, mainly formed by PaleoVolga, which cut its main channel in the platform region of the Middle Caspian.

There are several lithological types of PS deposits – Absheron, Gobustan, Lower Kura, PreCaspian. At the same time it is necessary to note that all lithofacies zones have similar features – accumulation of thick terrigenous sediments, lack of mollusk fauna and scarcity of microfaunal composition.

The comparison of PS sediments in these different areas has been done from outcrop and collected samples. Except for the Absheron type, all other types are characterized from the Surakhany Suite only.

ABSHERON TYPE

The Absheron type of PS sediments (the so called disthene - ilmenite province) cover almost all of the Absheron Peninsula, the Absheron archipelago and the northern part of the Baku archipelago, with a maximum thickness of up to 5000m in the South-Absheron trough. 

The thickness of rhythmical alternating sands, silts and shales varies from several meters to several hundreds of meters. The number of production horizons in the some fields reaches up to 40-50. Absheron type sediments consists of 9 hydrocarbon-bearing suits - Kala, Pre Kirmaki, Kirmaki, Post Kirmaki Sand, Post Kirmaki Clay, Pereriva, Balakhany, Sabunchi, Surakhany suites progressively onlapping to the north-west.

Recent studies have proven that sediments of the Absheron lithological type are mainly sourced by a PaleoVolga river from the Russian platform. This has been proven by a similar mineralogical composition with the modern Volga River sediments - high proportions of quartz, up to 95%, and the presence of disthene, ilmenite, tourmaline and garnet.

Space distribution of Absheron type sediments is significantly changed in separate stages of PS accumulation and is dependent on Caspian Sea level fluctuations which have resulted in basinward/landward facial shifts and wide depositional environment changes from fluvial, deltaic to lacustrine setting.

GOBUSTAN TYPE 

Gobustan type PS sediments are called the “zirconium-epidote province” due to a predominance of these two minerals in the mineralogical composition. Within PS deposits of the Gobustan type there are two distinguishable subtypes - Donguzdyk and East-Gobustan.

Donguzdyk series is developed in Central Gobustan along the north-west zone of Jeyrankechmez depression and is from 200 to 400 m in thickness. The zone is 4-5 km in width and 25-30 km in lateral extent. It is represented by the alternation of calcareous sand – clay rich layers with coarse-grained sandstones containing pebbles and the alternation of brown loams with conglomerates. To the south the Donguzdyk series gradually passes into the East-Gobustan subtype.

East-Gobustan subtype is distinguished by a predominance of fine-grained sediments. The sediments of this subtype occur in the Jeirankechmez depression, the Alyat-Pirsagat zone and offshore fields located near Gobustan.

East-Gobustan sediments are subdivide into 6 suits: Solakhay, Duvany, Clay, Miajik, Clayey sand, Sandy-clay suites. The sediments are poorly sorted and weakly differentiated from each other. Mixed types of deposits – clayey-sandy, sandy-silty prevail. The thickness of sandstone beds sometimes reach up to 20 m, with a total thickness of 2200-2500 m. Amalgamated channelised rippled sandstones and evidence of subaeral exposure demonstrate sedimentation in fluvial setting.

The mineralogical composition is characterized by a considerable decrease in the quartz content in comparison with the Absheron type. An increased prevalence of feldspar and sedimentary rock fragments in light fraction, and epidote, zircon, garnet, tourmaline in the heavy fraction also occurs. Similar mineral compositions are typical for sediments of the Southern slope of Greater Caucasus which indicates a Greater Caucasus provenance for the formation of Gobustan type PS deposits. Several small river systems, including a PaleoPirsagat river, may have supplied terrigeneous material of this composition.

LOWER KURA TYPE

The Lower Kura type PS sediments occur in the Lower Kura intermountain depression, bounded by mountain ranges of the Greater and Lesser Caucasus. The boundary between sediments of Lower Kura and East-Gobustan types occurs between the ridges of Boyuk and Kichik Kharami.

The total thickness of deposits of the Lower Kura type is 3500-4000 m. Sedimentary clastic material input into the Lower Kura succession was provided by two large river arteries – PaleoKura, which had supplied the terrigeneous material from Kura lowland, Great and Lesser Caucasus and PaleoAraks brought sediments from Talysh mountain ranges.

The succession is represented by alternations of silty-clayey-sandy deposits with sand bed thicknesses of up to 20 m. According to observations of the Lower Kura sediments exposed at Babazanan and Kichik Kharami, which are coeval to the Surakhany Suite in the Apsheron lithofacial zone, the stratigraphic architecture is dominated by ~ 10 m thick, amalgamated, channelised sandstones and overbank sediments.

Mineralogically there is a considerable increase in the percentage of the fine-grained sediment fraction and a decrease in amount of quartz compared with the Absheron type sediments. The increased amount of feldspar, pyroxene and hornblende indicates volcanic rocks in the provenance area. 

Along the northern edge of this lithofacial zone the Lengebiz subtype of Lower Kura sediments is developed, which is represented by the combination of the Donguzdyk, East-Gobustan and the lower Kura lithofacies types. Along with fine-grained sediments there are also sands and conglomerates deposited with a total thickness of about 1000 m.

PRECASPIAN TYPE 

The Precaspian type is developed along the north-eastern slope of the Greater Caucasus and is lithologically represented by silty sands, pebbles and conglomerates. The sediments of this type are mainly sourced by the several small mountain rivers from the Greater Caucasus. Stratigraphically these deposits are analogous to the Surakhany Suite of Absheron lithofacies type. There appears a clear zonation of the distribution of two types of sedimentary material – sandy-clayey sediments in the south-east (Gyzylburun type) and pebbly – in the north-west (Guba type).

The sandy-clayey Gyzylburun succession is mainly composed of fine-grained sediments, the sandstones are subordinate and mostly fine-grained with red gravels occurring at the base of the succession. The succession is up to 2000 m in maximum thickness. The number of sand bodies increases up section, with thickness variations from several cm to 1-2 m. Several nice examples of stacked channels filled by fine grained sands and desiccated surfaces prove deposition in fluvial environment.

The mineral composition of the light fraction shows feldspar and sedimentary fragments predominate, while in the heavy fraction –magnetite, ilmenite and micas are common. In general there is a trend towards a gradual decreasing thickness of this succession from the south-east to the north-west, with the Gyzylburun type of PS sediments passing into the Guba type.

The Guba lithofacies type is represented by a succession of poorly sorted pebbles interbedded with sandstones, up to coarse grained, and conglomerates. The size of pebbles varies from small, less than 2 cm, to boulders up to 1 m in diameter. However, small and medium size pebbles predominate. Petrographically the pebbles are composed of Jurassic sandstones, limestones, slates, Cretaceous limestones and Cretaceous-Eocene marls. The light fraction is mainly represented by feldspars and sedimentary fragments. 

ACKNOWLEDGMENTS

The authors greatly acknowledge Drs. Clare Davies and Stephen Vincent from Department of Earth Sciences, University of Cambridge (CASP) for the help in carrying out of this research.

REFERENCES

Geology of USSR. 1972. Volume XLVII “Azerbaijan, Geological descriptions”. Publishing House “Nedra”, Moscow, p. 520.

Geology of Azerbaijan. 1998. Volume “Lithology”. Publishing House “Nafta-Press”, Baku, p.480.