Structure and Lithological Characteristics of the Upper Jurassic Shalbuzdag Reef Massif (Northeastern Caucasus)

The structure of the Upper Jurassic reef complex of the Shalbuzdag mountainous range (southern Dagestan) is considered. It represents the western segment of the Shakhdag barrier reef separating zones with different types of sedimentation: the shallow sedimentation area of the carbonate platform of the Greater Caucasus to the north and the Dibrara trough accumulating thick carbonate and terrigenous flysch to the south. In the plan view, the reef massif has approximately a ring shape 4 km across. It includes several large reef buildups consisting of a biogenic carbonate dome-shaped core surrounded by a trail of steeply dipping sedimentary layers. There are also numerous smaller structures, bioherms, ranging in size from a few meters to the first tens of meters. Reef-forming fauna is represented by corals, gastropods, brachiopods, various types of algae, and others. The inter-reef space is filled with sedimentary rocks, which are mainly the denudation products of reef structures. These sequences frequently show gradation cyclicity. Based on the relations between biogenic and sedimentary rocks in the western margin of the massif, at least three great pulses can be distinguished in its formation. The formation of the Shalbuzdag reef complex was mainly controlled by the following factors: (1) climate changes from humid in the Middle Jurassic to arid in the Late Jurassic, (2) proximity to the transition zone between regions with different tectonic setting and sedimentation types, 3) sea level fluctuations of different orders.