Upper Jurassic Reefs from the Russian Western Caucasus: Implications for the Eastern Black Sea

Exposures of Upper Jurassic reef outcrops in the Russian western Caucasus provide excellent field analogues for possible reef-complex reservoir targets imaged on seismic reflection data from the northern Shatskiy Ridge, eastern Black Sea. The reefs at outcrop can be generally grouped into coral-dominated, siliceous sponge-microbialite and microbialite types. Coral-dominated reefs occur as isolated patchy and massive forms, and can be subdivided into higher-diversity and low-diversity types. The former developed at shallow-water platform margins and in platform interiors, whilst the latter occurred in deeper-water mid-shelf settings. Siliceous sponge-microbialite and microbialite reefs occur as lenses and mounds that were restricted to deeper-water mid-outer shelf environments. The reefs developed on two Late Jurassic carbonate platforms in the Russian western Caucasus: the north Caucasus platform to the north and South Adler platform to the south. These platforms were separated by a deep marine (Greater Caucasus) basin, along the margins of which thick shallow-water coral-dominated reefs formed. The southwestern margin of the north Caucasus platform probably represented a reef barrier-slope-basinal system that was structurally controlled. At the northeastern margin of this platform, in the Laba River region, inner ramp coral-dominated reefs pass northwards into deeper-water siliceous sponge- and microbialite-dominated reefs. The South Adler platform may extend offshore into the eastern Black Sea and the coral-dominated reefs that crop out at its northern margin form potential exploration analogues. The palaeowater depth of the possible isolated reef complexes imaged on the Shatskiy Ridge is unclear. If they were deposited in shallow-water, the coral-dominated reefs examined in the north Caucasus or South Adler carbonate platforms may form suitable analogues. Alternatively, if they were developed in deeper water they may be equivalent to the siliceous sponge and microbialite reefs examined in the Laba River region.