The formation and inversion of the western Greater Caucasus Basin and the uplift of the western Greater Caucasus

The southern slope of the western Greater Caucasus is dominated by outcrops of up to 17.6 km of Jurassic to Eocene sediments. These were deposited in the western segment of an extensional, or more likely transtensional, basin termed the Greater Caucasus Basin. The basin developed between the crystalline core of the Greater Caucasus (including the Fore Range and Main Range zones) to the north and the northern part of the Transcaucasus (the Rioni and Kartli basins, and Dziruli Massif) and its offshore equivalent, the Shatskiy Ridge, to the south. Its fill is characterised by siliciclastic turbidite, calciturbidite and hemipelagic mudstone and marl deposits, with a volcanic component in the Early and Middle Jurassic and mid Cretaceous. Sediment thickness variations indicate episodic basin forming events beginning in the Sinemurian, with the highest subsidence rates occurring in the Aalenian. Opening of the South Caspian Basin, a possible along-strike equivalent to this basin also occurred in the Early to Middle Jurassic. Small outcrops of strongly deformed Devonian-Triassic sediments, known as the Dizi Series, also outcrop on the southern slope of the range in Georgia. The tectonic setting of the Dizi Series is disputed, although a proto-Greater Caucasus Basin rift setting is plausible.

The basement of the centre of the Greater Caucasus Basin is not exposed, but is thought to be extended continental crust. The basement of the northern flank of basin forms the crystalline core of the western Greater Caucasus, whilst the southern flank is exposed in the Dziruli Massif. These two areas of outcrop share many similarities and comprise Gondwana-derived, predominantly Early to mid Paleozoic crystalline protolith and island arc and ophiolitic material that were metamorphosed and intruded, most likely during their Variscan accretion to the southern margin of Laurasia. The basement areas are overlain by thin successions of Late Carboniferous to Cenozoic strata that are punctuated by numerous unconformities. This would suggest that the crystalline basement to the flanks of the Greater Caucasus Basin have been at or near surface for much of the Late Paleozoic to Cenozoic.

Jurassic to Eocene sediments that are preserved along the northern flank of the crystalline core of the western Greater Caucasus are up to 3.8 km thick and are dominated by shallow-water siliciclastic sediments derived from the Russian and Scythian platforms to the north. They were deposited on the northern shelf of the Greater Caucasus Basin. Late Jurassic to Eocene sediments in the northern Transcaucasus are up to 2.5 km thick and are dominated by shallow-water carbonates. These were deposited on the southern shelf of the Greater Caucasus Basin. The paucity of siliciclastic material in this region is due to the isolated nature of the northern Transcaucasus between the Greater Caucasus Basin to the north and the proto or actual Eastern Black Sea to the south.

Oligocene and younger sediments are not preserved in the region of the former Greater Caucasus Basin. On the former shelves of the basin, the Eocene-Oligocene boundary is marked by the development of the mudstone-dominated Maykop Series, as it is across much of eastern Paratethys. At its type-section on the Belaya River, the base of the series is conformable and is marked by a switch from carbonate to clastic deposition; farther to the west a hiatus is more typical. Olistostromes mark this hiatus at a number of outcrops on either side of the Russian western Greater Caucasus. Olistoliths are typically dominated by Paleogene and Late Cretaceous sediments, although on the northern side of the range meta-igneous and metasedimentary basement clasts are also present. Unpublished subsurface data from hydrocarbon exploration in the adjacent Indolo-Kuban Basin record a number of olistostrome units are present in the Oligocene succession and that much of the lower part of the Maykop Series may be missing at outcrop due to pinch out and onlap onto active north-vergent structures.

The palaeontological analysis of the Eocene-Oligocene boundary indicates a marked increase in nannofossil reworking. In West Georgia, levels of reworking increase from ~1% below to up to ~30% 10 m above the boundary and continue in the range of ~10-65% throughout the rest of the Maykop Series. Similar or higher levels of nannofossil and microfossil reworking in the Maykop Series have been reported by earlier workers from elsewhere along the flanks of the Greater Caucasus.

Field data derived from the coarse clastic components of the Maykop Series show: 1) divergent palaeocurrents away from the axis of the western Greater Caucasus, and 2) heavy mineral compositions on either side of the range that are similar to each other and to Jurassic-Eocene sandstones in the western Greater Caucasus itself. These observations demonstrate a shared western Greater Caucasus source for these sediments and hence date the minimum age of clastic derivation from parts of the range. On the southern side of the range these sediments are as old as the early Early Oligocene, whilst on the northern side of the range they are probably Late Oligocene in age. However, even where sandstones are not present in the Early Oligocene part of the series, basal olistostrome deposition and the increase in the degree of nannofossil and microfossil reworking constrain the timing of initial uplift and sediment derivation from the western Greater Caucasus to the earliest Oligocene. The large volume of derived sediment and the presence of reworked plant material indicate that this sediment source was in part subaerial. Submarine deformation presumably commenced prior to the earliest Oligocene.

Other workers have identified a phase of Late Eocene or latest Eocene – earliest Oligocene deformation in the western Greater Caucasus and the flexure of adjacent regions that are consistent with the evidence of Greater Caucasus Basin inversion and western Greater Caucasus uplift presented above. The cause of inversion and uplift is thought to be the initiation of Arabia-Eurasia collision.

Petrography, provenance, fission track and U-Pb zircon dating studies indicate that Oligo-Miocene sediments derived from the western Greater Caucasus comprises a mixture of first-cycle material from the crystalline core of the range and recycled material from the inverting Greater Caucasus Basin. Along strike variations in the composition of sediments derived from the range are controlled, in part, by variations in the age and nature of material being reworked from the Greater Caucasus Basin. Evidence for Oligo-Miocene sediments on the southern flank of the former Greater Caucasus Basin derived from the crystalline core of the range formerly on its northern flank is incompatible with recent geodynamic models that propose that the Greater Caucasus Basin was open, oceanic and subducting northward underneath the core of the western Greater Caucasus until c. 5 Ma ago.