TECTONOSTRATIGRAPHY OF THE PARATETHYAN BASINS

Mark Allen¹, Stephen Vincent¹, Clare Davies¹, Arif Ismail-Zadeh², Elmira Aliyeva², Mike Simmons³, 
and Andy Morton^4
1 CASP, Department of Earth Sciences, University of Cambridge, UK
² Geology Institute of Azerbaijan Academy of Sciences, Baku, Azerbaijan
³ formerly CASP, now Neftex Petroleum Consultants Ltd, Reading, UK
⁴ Heavy Mineral Research Associates, Leicestershire, UK

The South Caspian, Eastern and Western Black Sea basins originated as separate rift basins in back-arc settings, north of a Neotethyan subduction zone that dipped northwards under Eurasia. The time of rifting is not well-constrained, but polyphase rifting in the late Mesozoic and early Tertiary is plausible. Major extension and subsidence in the Middle Eocene are suggested by abundant magmatism and thick, rapidly-deposited, deep-water clastic successions along exposed margins of the Eastern Black Sea and South Caspian basins. The beginning of the Arabia-Eurasia collision in the Oligocene ended extension and magmatism, and caused the deposition of organic-rich, anoxic clastic facies in underfilled foreland basins. Strata include the regional hydrocarbon sources rocks of the Oligocene – Lower Miocene Maykop Suite. Anoxia was aided by the lack of good connections with the open ocean system, analogous to the modern Black Sea. In general, water depths decreased and clastics became coarser-grained through the Oligocene and Miocene. This is interpreted to represent the infilling of syn-rift bathymetry, a decreasing component of thermal subsidence and increasing clastic input from rising ranges in the Caucasus and Alborz. This stratigraphy forms the distinctive fill of the “Paratethyan ocean”.

The end-Miocene salinity crisis in the Mediterranean isolated the South Caspian Basin from the world's oceans, and reservoir clastics of the Productive Series were deposited during a resultant lake-level lowstand. Up to ~6 km of Productive Series strata accumulated in ~2 million years: the thickness of this clastic succession over this short time could not result from passive infilling of existing accommodation space. Major tectonic subsidence was generated by flexure at marginal fold and thrust belts in the Alborz and Talysh, and subduction of the South Caspian Basin northwards under the middle Caspian. This occurred at the same time as a regional re-organization of the Arabia-Eurasia collision, itself caused by the construction of the Turkish-Iranian plateau and the migration of deformation into previously unaffected areas. Fluvial, lacustrine and deltaic rocks of the Productive Series were deposited by drainage systems that fed into the South Caspian from all sides. The best quality reservoir sands come from the Palaeo-Volga system in the north of the basin. The sand-prone Pereriva/Fasila Suite represents the start of major sediment input into the South Caspian Basin from the Greater Caucasus. Subsidence and sedimentation were finely-balanced in the north of the basin during Productive Series deposition, such that sub-aerial or shallow lacustrine environments predominated, with no deposition in significant water depths. Productive Series deposition ended with a late Pliocene marine transgression, represented by the Akchagyl Suite. Folding of the South Caspian sediments began before the end of Productive Series deposition - and continues to the present - creating anticlinal traps. Thus reservoir deposition, hydrocarbon generation, trap formation and secondary migration have all occurred in the last few million years.

On the broader scale, the Pliocene South Caspian Basin is distinctive because of rapid tectonic subsidence across a relatively small area of low relief, the match of subsidence by sedimentation, and isolation by active mountain ranges from marine influences. Where are there similar basins? The Eastern Black Sea Basin has a similar history between the late Mesozoic and the Miocene, but there have been no discoveries of Pliocene reservoirs. If such sands exist, their provenance will be crucially important to reservoir quality. Early Tertiary rift basins in northeast China have some features in common with the Paratethyan basins, but lack the major compressional tectonics that led to the reservoirs and traps of the South Caspian Basin.