STRATIGRAPHICAL DISSECTION OF THE SEDIMENTS OF THE CLOSED OLIGOCENE BASIN OF AZERBAIJAN

S.A. Shikhlinsky
AzNIPIneft, SOCAR, Baku, Azerbaijan

Serious contradictions exist in regard of the subdivisions of Oligocene of stratotypical region of Anglo-Paris basin and contradictory interpretation is due to formal reasons. Initially, the compound of Oligocene was distinguished by the stages, such as Lathdorfian, Rupelian and Khattian, it was natural to see the triple divisions (lower, middle and upper Oligocene). However, such a detailed dissection withstood the geological practice. As an example, we can mention the territory of Azerbaijan, where Oligocene practically divides into two subdivisions: lower and upper.

The position which Latdorfian fills is quite uncertain – it is placed either in the upper Eocene (Hardenbol, Bergren, 1978) or in the top of Upper Eocene – the Lower Oligocene (Martini, 1979).

The main point is the study of plankton microorganisms from the Oligocene sediments of oceanic basins, which proved that there is only one level of large scale change of plankton inside the Oligocene. Therefore, only two subdivisions can be distinguished in the compound of Oligocene. That is why the present scientific literature contains mainly two points of view. According to the first point, the Oligocene does not divide into subdivisions. Basically, it expresses a simple drift from the necessity to solve the issue of Oligocene subdivisions. According to the second point, more widespread one, Oligocene includes two subdivisions – lower and upper, the boundary between them goes either by the bottom of zone G.ampliapertura or by the top of the mentioned zone.

As we see, regarding the division of Oligocene into subdivisions, several different opinions exist. But it would be a rude mistake to assume this as a primitive chaos. It is necessary now to set an indicator of popularity of both opinions. But we get a completely different picture. Views of some Anglo-Paris basin geologists stand aside, stating the orthodox opinion. Most of the specialists of Mediterranean, Caribbean, Pacific, Atlantic and Indian basins share common views on the stratigraphic scale of Oligocene, applying in the stratigraphic surveys the Mediterranean scheme. Actually, it has become more of an international stratigraphic scale of Oligocene (Krashennikov, 1964, 1969, 1977, 1978, 1982; Shikhlinsky, 1984, 1988, 1989, 1990, 1996, 1997, 2002, 2003).

The stratoregion of the Stampian stage is the suburbs of Stamp town located in the north of France.

The bottom of the stage is quite clear, as it is underlined by gypsum and marl of ludey stage from the upper Eocene. In terms of geological development, stampian stage can be referred to single sedimentation cycle. It begins with lagoon green clays, changing by Sannoisian limestone, where elements of marine fauna appear. The upper Stampian is composed of sand, limestone and shells with lagoon and lake fauna. These sediments are characterized with numerous benthos forams and their change by the section most probably reflects variations of bionomic environment. The Stampian stage is correlated with microfauna zones G.tapuriensis, G.sellii and G.ampliapertura (Cavelier, 1981, Krasheninnikov, 1982).

Fooks (1894) offered to name Khattian stage for the sediments of the upper Oligocene, as default section and named section of Kassel and Bunde (territory of Germany) and Orma in the Paris basin. Further, Herges in 1957 considered it rational to regard the section of Doberg quarry to be neostratotype of Khatt stage, where the latter has been lithologically represented by sand and sandstone with subordinate interlayers of shell limestone, detritus limestone and marls. From the fauna point of view Khattian stage is characterized by mollusks, ostracodas and benthos forams. Bergren (1969) correlates Khattian stage with microfauna zones G.amplapertura and G.opima.

In Azerbaijan, Oligocene corresponds to the lower subsuite of Maykopian formation. The beginning of Oligocene is fixed due to the new cycle of sedimentation of sediments, which are of more terrigenous character and less carbonaceous than the underlying sediments of the Upper Eocene. Such change of sediments had followed the intensive uplifts in the end of the Upper Eocene, when the island sizes of the onshore increased and rough relief appeared, contributing to the destruction of rocks and enriching with the terrigenous components of the discussed sediments.

The intensive folding became the reason for isolation of Maikopian basin from the world ocean and then compared to the almost closed internal basin the mode could be defined as peculiar. Due to such limitation, the oceanic fauna of forams could not intrude inside and the remaining forms were deprived of normal development at such conditions.

In the area of the Great Caucasus, sediments of the Lower Oligocene are developed in the clay facies and expressed by clays with jarosite coating with interlayers of sandstone and sands with the fauna from the Lower Oligocene age. The character of deposit shows that they were lying at the Upper Eocene areas at considerable depth, although normal marine regime was sustained. The development of the current ground foraminifera fauna proves this fact if considered in the below of the above mentioned sediments. Stratigraphically, upper sediments not only lose carbonaceous properties, but are deprived of ground fauna as a result of hydrosulphide contamination; only fish remains are present.

In the Lesser Caucasus, sediments of the Lower Oligocene inconsistently deposit at the underlying layers and rocks are more carbonaceous than at the southern side of the Great Caucasus. Along with that, at some time the pleasant conditions allowed development of mollusk fauna. The character of deposit shows that at the beginning of Maikopian transgression, ingressive phenomena had place, as hereby without any interlayer of sandstone and conglomerate, slaty clays with mollusk fauna Planorbella and series of benthos forams were deposited. Upper, the gap of sediments slightly change as a result of frequent vibration of the basin bed and interlayers of sandstones with Caucasina oligocaenica appear. Hereby, the specifically well developed are representatives of Rotalia species which prove the shallow-onshore character of the basin. Besides, the appearance of some eurigall forms Elphidium, Bolivina and Nonion prove that physical and chemical character of basin slightly changed in the direction of salinity decrease.

The vertical differentiation of the representatives of the above mentioned species allows to highlight in the section of Oligocene a several of microfauna lones, which have specifically local character. In order to perform a direct correlation of these sediments with synchronous sediments of Anglo-Paris basin is practically impossible. Due to this reason, the dissection of Oligocene of the Caspian region into the Stampian and Khattian stages contain quite conventional character.