IGCP 610 First Plenary Meeting and Field Trip, Tbilisi, Georgia, 12-19 October 2013
, Sorokin V.M.
, Bezrodnykh Yu.P.
M.V. Lomonosov Moscow State University, Moscow, 119992, Russia
АО «Мorinzhgeologiya», Riga, Latvia
Keywords: Caspian Sea, late Pleistocene, cores, transgressive deposits, regressive sediments, radiocarbon dating, paleogeography
The event scheme of the Caspian Late Pleistocene includes Late Khazar (Late Khazar and Girkan transgressive stages) and Khvalynian (Early Khvalynian and Late Khvalynian transgressive stages) transgressive epochs, separated by the Atel regression. Many questions of a paleogeography of these transgressive and regressive events (quantity of stages, age, environmental conditions, correlation with events in the neighboring territories and with climatic events) are debatable. The main part of a material on a problem is received as a result of researches on the coasts of the Caspian
Sea. Therefore undoubted interest in researchers is caused by drilling materials on the bottom of the
Caspian Sea. We studied the cores of the wells (sequences of the boreholes) up to 100 m in depth drilled during engineering-geological researches in the Northern Caspian Sea. They opened thickness of late Pleistocene deposits with difficult construction. Studying of the structure of the section and spatial correlation of lithological-stratigraphical thicknesses are executed by means of seismoacoustic profiling and static sounding. The sediments of the cores are studied by lithological, malakofaunistic and geochronological methods. Among mollusks we mainly focused on the brackish water species of genus
Eichw. Members of this genus are endemic and index species for the Caspian Sea. This genus is known for its high evolutionary rates at the species and subspecies levels, which highlight its significance for the stratification of the marine Pleistocene of the Caspian and for paleogeographic reconstructions of the Caspian basins.
The Late Khazar transgression developed in two independent stages, separated with a regression.
Sea-level of the earlier Late Khazar basin, according to sediment spatial distribution, did not exceed -10m, and its area was much bigger (more?) than the modern Caspian. Malacologic fauna represent a faunal assemblage with crassoidal
and characteristic species
. Its distinguishing feature is large dimensions and massiveness of shells, especially common for southern parts of the Caspian. Common gigantism of shells, high carbonate content in the sediment and presence of oolites indicated the warm climate of the Late Khazar. Warm waters of the basin, obviously, had higher salinity than in the modern Caspian: between 10-12‰ for the northern and up to 14-15‰ for the southern Caspian Sea (Yanina, 2012). Early Late Khazar transgression stage was followed by a regression. It was evidenced by hiatuses in marine sedimentation
(Dagestan), and wash-outs and soil formation (Volga River valley) (Svitoch, Yanina, 1997).
Traces of the second transgressive Late Khazar stage are not preserved at the Caspian coasts. Based on drill sites materials from the north-western part of the Caspian region, G.I. Popov (1983) reconstructed a freshened brackish water basin. He defined it as an independent Girkan transgression of the Caspian, which took place after the Late Khazar transgression and was separated from the
Khvalynian transgression by the Atel regression. The basin was inhabited by “Khvalynian-like” fauna, traces of which have not been recorded elsewhere. A number of researchers strongly objected to this concept.
The structure of the cores studied by us showed that between two regressive thicknesses (Chernoyarsk and Atel) two thicknesses of the marine deposits are located. The lower part of the late
Khazar deposits is most fully presented on northern structures. Sandy sediments in the basis of the studied cuts (sequences) are characterized by domination of fresh-water species (
IGCP 610 First Plenary Meeting and Field Trip, Tbilisi, Georgia, 12-19 October 2013 nalis, Lymnaea stagnalis, Valvata sp
.) with impurity of the slightly brackish water species. They characterize the shallow warm-water almost fresh-water basin existing on the place of modern central part of the Northern Caspian Sea. Sediments of higher intervals contain the malakofaunistic complex, which feature - a combination of two thermophilic species belonging to different types of fauna:
and also rare
D. surachanica, D. vulgaris
, with obviousness testifying to late Khazar age of the complex. It characterizes the brackish water shallow warm-water basin with inflow of fresh waters. The described layers reflect different phases of development of the late Khazar transgression.
The following stage of development of the Caspian Sea is presented by sandy-clay deposits. Feature of its faunistic complex is the abundance of
the occurrence of
and species of Khazar type
D. pallasi, D. shuraosenica, D. subcrassa
. According to materials of G.I.
in combination with
are the "face" for the Girkan fauna.
Probably, the described by us thickness belong to Girkan transgression. The radio-carbon datings
(> 40 thousand years ago) can testify to it. The conclusion about the taxonomical status of this transgression and its age can be made only after dating of this thickness, and also spreading it late
Khazar thickness, by methods, which opportunities extend for the end of middle Pleistocene and the beginning of late Pleistocene.
Beginning of the late Pleistocene was characterized by the warm interglacial epoch (MIS 5), the
Mikulino interglaciation on the Russian Plain. Late Khazar and Girkan transgressions corresponded to this warm epoch.
The end of the Khazar stage of the Caspian development was marked by the deep Atel regression.
During this time vast areas of the Caspian shelf were exposed. Out of all continental formations of regressive Caspian epochs, sediments of this one are the most widely spread and are found at all
Caspian coasts (Svitoch, Yanina, 1997). Atel regressive sediments are well expressed in the cores.
They transformed by subaeral environment, deposits include the fresh-water mollusks and the vegetable remains. In the Lower Volga region in the base of the regressive layer there are Akhtuba deposits, first distinguished by G.I. Goretskiy (1958) and referred to as a periglacial formation. They represent a perfect marking horizon in the Lower Volga sections, often with sharp wedges penetrating the underlying sediments. These wedges and winterkill fractures in the base of Akhtuba sands are syngenetic to them, and represent the convincing proof of the severe climatic conditions at the time of deposition and permafrost development. It is obvious, that upon approaching the cooling maximum (MIS 4, early Valday glaciations on the Russian plain), the transgressive development of the Caspian was disrupted by the Atel regression under cold and dry climate.
The Atel regression was followed by the Khvalynian transgression with the most significant sealevel rise in the late Pleistocene history of the Caspian. Unlike earlier transgressions, traces of
Khvalynian Sea development are found at all Caspian coasts. The maximum sea-level can be easily traced by the distinct marked relief found with rare exceptions, at levels of 46-50 m absolute scale throughout the entire perimeter of the paleobasin. Almost all researchers who studied the Caspian history agree that there are two Khvalynian transgressions – early and late Khvalynian, separated by the Enotaevka regression.
Drilling material and datings, obtained from shells and organic matter from sediment cores, are of great interest. The Khvalynian section on the bigger square of the Northern Caspian Sea begins by a layer of a coarse-grained marine sediments with faunal material, among them the community of
C age is from 28,5 to 31,5 ky; the calibrated dates are from
33,5 to 36,5 ky. Thickness of deep-water clay deposits with inclusion of typical Khvalynian fauna of
blocks it. According to structure of the cores and properties of malakofauna (
Didacna protracta submedia
Dreissena rostriformis compressa
), this thickness answers the most deep-water Khvalynian basin. Dating of this interval by
C method gave the age about 27 ky ago (the calibrated dates about 32 ky ago) (Bezrodnykh et al., 2004, 2013). Evidently, the first transgressive stage of the Khvalynian transgression took place during the warming of the
MIS 3, in the Valday interstadial.
IGCP 610 First Plenary Meeting and Field Trip, Tbilisi, Georgia, 12-19 October 2013
Above it in the number of boreholes (mainly in the southern part of the Northern Caspian Sea) accumulation of sandy and dusty sediments was noted. Obviously, they correspond to fall of level of the Khvalynian Sea. Age interval of this event was from 22 to 19 thousand years ago. Apparently, this fall of level answers the last glacial maximum (MIS 2, late Valday glaciation). In northern areas of the Caspian Sea this stage is fixed in the cores structure by washouts.
This pack of deposits is blocked in the southern part by a sandy-clay sediments with thick up to 2 m; clay deposits of brown coloring with thickness up to 5 m lie on them with washout. This clay thickness is marked in the all cores. It is characterized
by Didacna parallella
C datings its age interval is 17645-16075. The authors have no consensus about paleogeographic treatment (early or late Khvalynian transgression) of this thickness. Transgression development occurred during the degradation of the late Valday glaciation.
The Khvalynian cycle of deposits comes to the end with marine-deltaic sediments in the northern part of the Northern Caspian Sea and sandy sediments in its southern part. On the deposits, answering to Mangyshlak regression (sapropel and peat), the
C dating between 8,8-9,6 ky ago (the calibrated dates 9,9-10,9 ky) are received. It corresponded to the climate continentalization of the early
Drilling materials on the water area (bottom) of the Caspian Sea show, what not all events of the late Pleistocene history of the Caspian Sea are reflected in the deposits of the coasts. Actual problem is correlation of the Caspian thicknesses opened by boreholes, and deposits exposed in the coastal sections. Only this way will lead us to the fullest interpretation of the Pleistocene history of the Caspian Sea.
The work is partly performed with financial support of RFBR (Grant 13-05-00242).
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