CONDITIONS OF LOWER PLIOCENE DEPOSITS ACCUMULATION IN KURA GULF OF CASPIAN LACUSTRINE BASIN

A.M. Suleymanov
State Oil Company of Azerbaijan Republic (SOCAR)
Azerbaijan Scientific Research and Project Institute of Oil, Aga-Neymatulla st. 39, Az 1033, Azerbaijan

As a result of rising of the surrounding territories, at the end of Pontian the Caspian Sea basin loses connection with East ParaTethys and become lake. From this time the Caspian basin level varies independently and depends on changes of climatic conditions, tectonical regime of basin and surrounding its territories. To the beginning of the Early Pliocene the Caspian lake level was 600 m below the level of World Ocean.

At the beginning of Early Pliocene the uninterrupted sedimentation continued only in the central and northwest parts of the South-Caspian trough. As results of sharp declined of erosion base the water flows of paleoVolga, paleoKura and paleoUzboya are directed to the central part of the South-Caspian depression. This allows to gradual rising of level and extension of the Caspian lacustrine basin. To the end of first haft of the Early Pliocene the falling of the basin level was occurred and the north part of the South-Caspian depression covered by delta plain sediments of PaleoVolga, where sandy suite “Fasila” developed. Within Lower Kura trough the XX horizon of Productive Series (PS), represented by conglomerates, sandstones and clays, is the analogue of this suite.

At the second half of Lower Pliocene the uninterrupted-interrupted rise of the level Caspian lake take place. PaleoVolga deltas remove in northern and PaleoKura in northwest directions as well. The Absheron facies of PS formed in the northwest part of the South-Caspian depression in the deltafront conditions. In Kura gulf of fluvial-delta deposits of PaleoKura occupied the central part of Lower Kura trough, where its thickness reaches 3000 m. There are distinguished about 20 sandy-clayey horizons, between which clearly defined clayey divisions are absent. The variability of lithological composition of sandy horizons on the section and on strike is the result of direction changes of distributing channels in delta plain of PaleoKura. SE part of Kura gulf and adjacent part of Baku archipelago are within prodelta zone of PaleoKura, where clays and silts are accumulated.

In flank and northwest parts of Kura intermountain trough in sedimentation process the paleorivers played essential role, which are drainage of the Greater and Lesser Caucasus. The most coarse-fragmental terrigenous sediments of these rivers are deposited at the foothills, and the more small ones formed the alluvial plains. On the SW flank of Kura trough the following paleorivers - Araks, Karkarchai, Terter, Akstafachai and others took part in formation of alluvial plain. Their erosive cuts at the foothills of Lesser Caucasus reached 500-600 m.

In the northwest part and on the northeast flank of trough of the Lower Pliocene deposits was deposited in the continental conditions. River fans like paleorivers as Gabyrry, Alazan, Gekchai, Girdimanchai and others small rivers within Kura and Gabyrry interfluve and in the Ajinour trough formed the Shirak strata, in the structure of which together with Lower Pliocene took part conglomerates, sandstones and clays of Pontian and Meotian stages. In the SW directions due to relief falling of the south slope of Greater Caucasus the role of the coarse-fragmental sediments is significantly reduced and in subaqual conditions formed the Lengebiz facies of PS, where Karanaur, Lengebiz and Kalamadin sandy-clayey suites are distinguished.

Within NE part of LowerKura trough and Jeirankechmez depression the upper part of PS was formed by terrigenous sediments, bringing by paleorivers which are drainaged SE subsidence of Greater Caucasus. The coarse-grained Donguzduk and sandy-clayey Pirsagat facies are distinguished here. Within the last one the Pirsagat, Khamamdagh, Solokhai and Duvanny sandy horizons are distinguished.

In Akchagylian stage the reconstruction of connection of the Caspian with East ParaTethys basin, allowed the sharp rise of basin level and water salinity increase. The Akchagylian sea level exceeded the level of the PS basin up to 600-700 m. As a result of sharp increase of erosion base the introducing of terrigenous material in the SE part of Kura trough sharply decreased and in the conditions of non-compensated sagging the clayey deposits of small thickness accumulated. At the same time in the NW part of vast marine gulf due to continued rising of surrounding mountain, especially of the south slope of Greater Caucasus, the sandy-clayey and more coarse-fragmental sediments with thickness up to 700-800 m, are accumulated.