STRATIGRAPHY AND SEDIMENTOLOGY OF OIL-GAS BASINS
2012 / 1
THE RECALL TO THE ARTICLE BY R. GALLAGHER ‘THE ICE AGE RISE AND FALL OF THE PONTO CASPIAN; ANCIENT MARINERS AND THE ASIATIC MEDITERRANEAN’
The article in question covers a broad range of issues related to the development history of the Caspian and Black Sea basins. The author advocates the idea of the existence of one whole ice sheet that blocked the river flows in Western Siberia and of the ice sheet in Eastern Siberia resulting in the development of a gigantic periglacial lake in Western Siberia that was fed by also by the water streams from the glacial lakes in the Altai.
The spillway went via the Turgay (the rift at 125 m) to the Aral Sea and farther into the Caspian. The level of the latter had gone up thanks to the water influx at about 15,000 years ago – the so-called Khvalyn Horizon Transgression occurred. In the opinion of the author, so signify the strand lines along the Caspian shores at elevations of up to 125 m. The level of the Black Sea also reached high points (up to 150 or 220 m in Figure 8). Let us consider these generations of the author without touching on the other issues also covered in the article.
Many detailed research works including those reliant on the geophysical and new geo-chronometric methods (especially OSL) have emerged in the 15 years since the emergence of the idea of the gigantic ice sheets. There are many Russian-language works that probably are not always accessible for overseas researchers; there are research papers in the English language, too, however. Modern perceptions have it that there was no united Barents-Kara Sea Ice Sheet 15,000 years ago but that there were individual ice shelves. The Barents-Kara Ice Sheet was too small in the period LGM (about 20 thousand years ago) to block the Western Siberian Rivers (Mangeruda et al., 2004, Astakhov et al., 1999 et al.).
It should be mentioned that there existed large ice sheets blocking the firths of the rivers Yenisei, Ob, Pechora and Mezen during the Quaternary Period; there were the overflows
Lomonosov State University of Moscow
from the periglacial lakes into the Caspian Sea. But that was too early, for instance, 90–80 thousand years ago, for the Western Siberian Lake. These data are based on numerous OSL datings as well as on the fragments of the strand lines at elevations of approximately 60 m (Mangeruda et al., 2004).
The water from the periglacial lake over-flowed into the Caspian via the Turgay Depression, the elevation of which approximates 125 m right now though we should not forget that the elevation was different at the overflow time because it was filled up with the alluvial and proluvial faeces afterwards. The foot of the former sound lies at the approximate depth of 55 m and, more likely than not, it is this value that was the sille of the Western Siberian Lake’s discharge outlet that reached the Aral Sea via the Turgay.
The author of the present recall advocates the water overflow idea and believes, in particular, that the so-called Western Siberian Crests are the analogue of a kind of the Berov Knobs that are widespread in the North Caspian (Badyukova, 2010). But the overflow did not happen 15,000 years ago as well as it did not happen at such a quite high lake level and it was not catastrophic in nature as can be deduced from the lithological nature of the sediments that make up these relief forms (sands, sandy loams and clay loams).
A few words need to be said about the Khvalyn Transgression… Right now, opinions differ as to its age. Not everyone by far believes that its age equals 15,000 years as the author states. For instance, Leontyev et al. (1977), Fedorov (1978), Rychagov (1997), Badyukova (2007) et al. tend to think that the Khvalyn Transgression occurred earlier, within the range of 70–40 thousand years ago. The author might have considered quoting those opinions as in this case the paleogeographical reconstructions would change considerably.
The author’s data about the Caspian Sea coastlines are the main source of perplexity. It has long been known that there is a series of elevated coastal terraces (Leontyev, Mamedov et al., 1988, Aleskerov et al., 1989, etc.) along the Western Caspian Coast. All of them are manifest and stand out for their geomorphological features as well as the faunal com-plexes including the well-studied molluscs of the Didacna genus (Svitoch et al., 1997, Yanina, 2005). The highest terrace – that of the Baku Tier – is greatly offset with elevations ranging from 200 m to 260 m in the welt areas and going below the sea level in the depressions. The Khazar Terraces are less offset though are also elevated considerably. The age of the Baku Sediments is about 500,000 years in accordance with the thermo-luminescent method examination findings while that of the Khazar Sediments is approximately 300,000 years. The article also quotes the datings for the elevated terraces, namely, 16.7, 14.3 and 28.5 thousand years, which cannot be accepted given the offset nature of the bench levels, the nature of the greatly decayed sediments and the malacofauna make-up.
The detailed studies of the key sections in Azerbaijan, Dagestan, the North Caspian and the Eastern Coast done by many researchers give a reason to ascertain that the level of the Caspian Sea did not exceed 50 metres above sea level (the maximum value for the Khvalyn Transgression). Nowhere except the tectonically active Western Coast of the Caspian are higher shoreline levels registered. It is important to note that there is a gradual lowering of the coeval coastlines as they increase the offset distance from the Caucasus; for instance, at the Manych the coastline of the Khvalyn age is at the 50 m mark. In the North Caspian and on the Eastern Coast the same coastline is at approximately the same elevation. And there are no higher coastlines at all!
Now, a few words about the Black Sea… Without going into a discussion about the development history of this basin that has been the subject of a great many research works of late, we would simply note that there are no coastlines at the 150 m and even 220 m there. The history of the Black Sea basin cannot appropriately be reconstructed in such bold strokes and dabs. After all, there are many serious enough publications covering this region. Yes, it is true that some aspects of the paleogeographical reconstructions are debated currently but all the debates take into account the geological, geomorphological and other data to this extent or other. To repeat it, there are no such quite high coastlines in the Black Sea. There are the elevated (15–17 m) Karan-gat Age (approximately 100,000 years ago) terraces in a number of regions and there is also the low (2–4 m) little Holocenic New Black Sea terrace.
Lastly, as to the possibility of the Cerastoderma glaucum penetration into the Caspian Sea, the structural analysis of the sediments that fill up the Manych Depression made it possible to presume that it is now filled with the lagoonal-alluvial-proluvial sediments (Badyukova, 2008). This ‘stopper’ was regularly eroded with water flows when the erosion basis was lowering at the times of the Black Sea and Caspian regressions, so, the 26 m mark is not the discharge outlet sille but, rather, the height of the batardeau as it is now. Regrettably, it is a widespread fallacy to carry out paleogeographical reconstructions using modern-day topography. There was a paleo shut-in in Manych at the beginning of the Holocene that is filled with the sediments now but via which Cerastoderma glaucum could migrate into the Aral Sea.
STRATIGRAPHY AND SEDIMENTOLOGY OF OIL-GAS BASINS
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