11) Palynology of shallow boreholes from the Volga delta (EAGE Baku Oct 2008)

KEITH RICHARDS(1) AND NATALIYA BOLIKHOVSKAYA(2)

(1) KrA Stratigraphic, Deganwy, UK

(2) Dept of Geography, Moscow State University, Russian Federation 

Talk presented at the EAGE International Conference on Petroleum Geology and the Hydrocarbon Potential of the Caspian and Black Sea Regions (Baku, Azerbaijan October 6-8, 2008)

Results of palynological study of shallow boreholes from the Holocene of the Volga Delta, Russia

This study describes the results of palynological analysis from several shallow cores from the Volga Delta. The cores were collected during a joint field program organised by Moscow Sate University and the Technical University of Delft in 2006. 25 cores were taken on the lower delta to delta front region of the Volga, each in the region of 10m in length. The sediments collected are mainly of Holocene age, with some probable Late Pleistocene deposits also represented. The main objective of the study was to assess Holocene Caspian Sea level change and vegetational change in the lower delta region using palynological analysis. The results of the study can be compared to the Pliocene “paleo”- Volga , notably the Productive Series of Azerbaijan, which is a major oil and gas producing region.  

Palynological results are available from 8 cores, all taken from the Astrakhan Biosphere Reserve, close to the Damchik Research Station. All cores studied contained rich recovery of palynomorphs: pollen, spores, algae and dinoflagellate cysts (dinocysts). The pollen and spore component gives an indication of vegetation change on land, particularly in relation to climate. The algal bodies and dinocysts present illustrate the relative importance of freshwater and brackish (Caspian) influences. Radiocarbon (C14) dates are available from all cores. The palynological results are able to demonstrate subtle changes in depositional setting within the Volga Delta. Late Pleistocene to Early Holocene deposits typically consist of reddish- brown clays, which indicate a period of delta inactivity, with widespread sub-aerial exposure. These sediments are usually characterised palynologically by the presence of frequent fungal bodies (spores and hyphae) and by locally frequent freshwater algae. Channel deposits, identified by shallow seismic, typically contain frequent reworking, mainly of Mesozoic age. 

Palynological studies from the Caspian Sea include those of Marret et al. (2004) and Leroy et al. (2007). The former described several new genera and species of dinoflagellate cysts from the region, whereas the latter describes mainly environmental changes (river influences and salinity changes) during the late Holocene. The dinocysts described by Marret et al. (2004) include Impagidinium caspienense, Caspidinium rugosum and several variants of the Spiniferites cruciformis “complex”. These dinocysts are restricted to brackish water bodies with low salinity, more or less within the range of 5 to 12 ‰, and are found in the open waters of the Caspian Sea , but also in the low salinity waters close to the modern Volga Delta. All of these forms were recorded in the sediment cores from the Lower Volga . It is assumed that those intervals of the Damchik cores that contain these brackish dinocysts were deposited in a distal delta front (“Avandelta”) setting, similar to the present-day “kultuks”, or brackish embayments. Other samples contain fewer dinocysts, but increased numbers of freshwater algae, notably Pediastrum, which are assumed to indicate proximal delta front, freshwater deposits. The pollen and spore (terrestrial) components include types such as grass (Gramineae), sedge, (Cyperaceae) and various Compositae such as Artemisia, typically an indicator of steppe or similar dry grassland vegetation. Pollen from salt-marsh, or similar dry-adapted, salt tolerant vegetation is also frequent, mainly from the Chenopodiaceae family. Relative proportions of these types permit a reconstruction of the main vegetation types present on the delta during the Holocene.  

The palynological results from the Damchik cores indicate at least three main phases of deltaic deposition during the Holocene. The first occurred after approximately 7000 BP and is marked by a “flood” of brackish dinocysts (mainly Spiniferites cruciformis) in the sediment cores. This is overlain by a “prograde” or similar sediment package, usually typified by increased freshwater components. The second deltaic phase occurs after c.3000 BP and consists of several short-lived transgressive / regressive episodes, each marked by dinocyst “floods” overlain by intervals with increased freshwater influence. The main dinocyst present in this section is Impagidinium caspienense. This younger phase of deltaic activity includes the so-called “Derbent” lowstand episode which began at about 1500 BP. The palynological data do not independently identify the Derbent event, probably as it would have been a period of non-deposition in the Volga Delta region. Most boreholes also show an indication of rising Caspian Sea levels in the very recent past, within the uppermost metre or so of the sediment cores. This is likely to represent the increase in Caspian Sea levels in recent historical times, since c.1980.  

The second deltaic phase (post c.3000 BP) is marked in the palynological record by an increased presence of Pinus (pine) pollen, derived from the hinterland of the Volga catchment, and also by an increase in the abundance and diversity of Mesozoic reworking (mainly dinocysts). This suggests that the Volga River was active at this time, and was bringing in both reworked and contemporaneous sediments into the site of deposition. The same samples also show evidence for a rise in Caspian Sea level. The palynological data therefore support the suggestion that the outflow of freshwater from the Volga River is a major contributor to Caspian Sea level rise.   

Other frequent pollen types present include Salix (willow) and Tilia (lime). The former occurs mainly on natural levees between the main Volga River channels at the present time, and its distribution in the sedimentary record gives a proxy of channel migration over time. The latter is frequent in the “mid”-Holocene and is likely to represent a climatic optimum during that period. Pollen from emergent reeds (e.g. Typha and Sparganium) and submerged aquatic plants (e.g. Potamogeton) are also frequent in the sedimentary record and give an indication of the relative extent of reedbeds and open water habitats.  

The results of the palynological study of the Holocene Volga Delta have been supplemented by “modern” pollen studies from around the Caspian region. In particular, modern “analogue” sedimentary environments have been sampled and analysed from the Volga Delta itself, and also from lagoonal and isolated saline lake regions of Kazakhstan . These give a “pollen profile” of the true modern-day deltaic and lacustrine environments from the region.  

Marret, F., Leroy, S.A.G., Chalié, F. and Gasse, F. [2004] New organic-walled dinoflagellate cysts from recent sediments of the central Asian seas. Review of Palaeobotany and Palynology, 129, 1-20.  

Leroy, S.A.G., Marret, F., Gibert, E., Chalié, F., Reyss, J.-L. and Arpe, K. [2007] River inflow and salinity changes in the Caspian Sea during the last 5500 years. Quarternary Science Reviews, 26, (25-28), 3359-3383.

Click here to go back to palynology page...

Click here to go to more Talks & Posters...