INTEGRATING SEDIMENTOLOGICAL AND PALYNOLOGICAL OBSERVATIONS TO DEDUCE TECTONIC AND CLIMATIC CONTROLS ON THE DEPOSITIONAL ENVIRONMENTS OF THE PRODUCTIVE SERIES

Clare Davies¹, Mike Simmons², Stephen Vincent¹, Mark Allen¹, Keith Richards³, Andy Morton⁴,
and Elmira Aliyeva^5
1 CASP, University of Cambridge, UK
² formerly CASP, now Neftex Petroleum Consultants Ltd, UK
³ KrA Stratigraphic, Deganwy, UK
⁴ Heavy Mineral Research Associates, Leicestershire, UK
⁵ Geological Institute of Azerbaijan Academy of Sciences, Baku, Azerbaijan

Sediments of the Neogene Productive Series in Azerbaijan, particularly in the offshore, contain the most economically important intervals in the South Caspian Basin. Equivalent strata crop-out in Azerbaijan, allowing sedimentological and structural analysis through the complete succession.

Previous sedimentological analysis of the Productive Series, for the Apsheron region (Reynolds et al., 1998), interpreted a fluvio-deltaic system with fluvial to delta front settings. Detailed sedimentological observation by the authors has resulted in the discovery of desiccation cracks, palaeosol features and palynology assemblages representing floodplain deposition. These observations suggest that fluvial overbank deposits and other associated fluvial deposits (e.g. ephemeral or ox-bow lakes) may have been more common than previously recognised, for example within the NKG and especially in the upper Productive Series. The Kirmaky Suite shows a more pronounced fluctuation of depositional setting than previously envisaged, ranging from true lacustrine-deltaic to fluvial overbank.

The Caspian basin was fed by a number of sediment sources with varying sand composition and potential reservoir quality. These included the palaeo-Volga, Kura and Amu Darya rivers, as well as those fed from the Greater Caucasus. Petrographic and heavy mineral analysis has been used to discriminate between these systems within the Azeri sector of the basin, thus allowing the definition of variations in sandstone quality and provenance.

Facies of the Productive Series in the Apsheron area are dominated by single and amalgamated sheet sandstones and channelised sandstone bodies with interbedded heterolithics. These indicate deposition from high flow regime flood events and deposition within shallow channels, with fines indicating deposition from lakes and/or overbank flows onto the floodplain respectively. The succession in this region becomes more mud prone through time. This was previously considered to relate to an increasingly lacustrine depositional setting. Following detailed sedimentological and palynological analysis it is more likely to result from a combination of mechanisms occurring on the floodplain. These include changes in the location of the dominant fluvial system (possibly tectonically controlled), changes in source area, due to the growth of the Greater Caucasus, and changes in the climate, which became cooler and more arid in the basin’s palaeo-Volga catchment area, reducing runoff.

The Kura system, on the western margin of the basin, shows a trend to becoming more sand prone through the equivalent succession of the Productive Series. Sedimentary observations for the uppermost part of the succession differ from those on the Apsheron peninsula in being dominated by laterally extensive, amalgamated fluvial channel sandstones. Combining the sedimentological observations with palynology data suggests that the fluvial floodplain in the Kura region was rarely exposed with few desiccation cracks, palaeosols and limited weathering and removal of pollen taxa. The taxa present show the same overall trend of cooling and increasing aridity affecting the basin’s palaeo-Kura catchment area.

Heavy mineral techniques identify changes in provenance that may be tectonically and/or climatically controlled. Palynological analysis allows mudstone rich heterolithics that represent deposition within ephemeral ponds on the fluvial floodplain to be discerned from more persistent and extensive freshwater ponds, as well as the identification of brackish, lacustrine mudstone deposition.

Using these techniques, combined with outcrop sedimentology, we conclude that the most numerous and extensive incursions of the Caspian Sea, and hence true lacustrine deposition, occurred during the lower Productive Series (dominantly the Kirmaky Suite). Heavy mineral data show an influx of sand from the Greater Caucasus at the base of the Fasila (Pereriva) Suite in the Apsheron area. A significant Greater Caucasus contribution persists through the rest of the Productive Series sediments. The mudstone dominated top to the Productive Series in the Apsheron region represents overbank floodplain deposition, with minimal coarser sand-grade channelised sedimentation, rather than lake level rise.