Structure and lithological characteristics of the Upper Jurassic Shalbuzdag reef massif (Northeastern Caucasus)

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Resumo

The structure of the Upper Jurassic reef complex of the Shalbuzdag mountain range (southern Dagestan) is considered. It was the western segment of the Shahdag zone of the barrier reef, which separated zones with different types of sedimentation: to the north of it was the area of shallow sedimentation of the carbonate platform of the Greater Caucasus, to the south was the Dibrara trough, where thick strata of carbonate and terrigenous flysch accumulated. In terms of the reef massif has an approximately ring-shaped shape with a diameter of about 4 km. There are several large reef structures on its territory, in the center of which there is a biogenic carbonate dome-shaped core surrounded by a plume of sedimentary layers lying relatively steeply. There are also numerous smaller formations, 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, etc. The inter-reef space is filled with sedimentary rocks, which are mainly products of denudation of reef structures. Gradation cyclicity is often observed in these strata. In the structure of the western edge of the massif, according to the ratio of biogenic and sedimentary deposits, at least 3 large impulses in its formation can be distinguished. The factors that most influenced the formation of the Shalbuzdag reef complex should probably be considered 1) climate changes from humid in the Middle Jurassic to arid in the Late Jurassic, 2) proximity to the transition zone between areas with different tectonic development regimes and sedimentation types, 3) sea level fluctuations of various orders.

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Sobre autores

Yu. Gavrilov

Geological Institute RAS

Autor responsável pela correspondência
Email: yugavrilov@gmail.com
Rússia, Pyzhevsky lane, 7, bld. 1, Moscow, 119017

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2. Fig. 1. Location of the Shalbuzdag mountain range in the structure of the Greater Caucasus. a – location of the Shalbuzdag massif on the relief map of the Caucasus; b, c – reefogenic mountain ranges of the western part of the Shahdag barrier reef zone (b – view from space, c – view from an airplane): b – Mount Shalbuzdag (1), Mount Shahdag (2), Mount Heydar (3); d – contour of the modern structure of the Shalbuzdag mountain range (view from space, GoogleEarth data).

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3. Fig. 2. Reef massif of Mount Shalbuzdag. Views of the massif from different directions: a – from the north, b – from the east, c – from the south, view from the west – see Fig. 10.

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4. Fig. 3. Ring reef structures within the Shalbuzdag massif. The largest reef structures are marked with arrows.

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5. Fig. 4. Bioherm structures of the Shalbuzdag reef complex area (α – biogenic core of the bioherm, β – layers bordering the central part of the bioherm structure (of sedimentary or biogenic origin). a–k – bioherms of various morphologies on the territory of the reef massif; l, m – bioherm structures located outside the massif in the development zone of Middle Jurassic terrigenous deposits (southern slope of Mount Shalbuzdag); n – reef structures buried under the sedimentary cover in the eastern part of the massif.

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6. Fig. 4. Continuation

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7. Fig. 5. The nature of the occurrence and stratification of sediments in the strata of interreef filling. a–z – examples of sedimentary rock outcrops from different parts of the reef massif; the occurrence of layers ranges from horizontal to relatively steep (up to 45° and more), which is not associated with tectonic deformations; irregular bedding patterns are often observed, with layers being unevenly distributed along the strike to the point of wedging out.

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8. Fig. 6. Various types of rocks that make up sedimentary series that fill the interreef space. a–c – cyclites with gradational texture; the lower elements of the cyclites are composed of coarse-grained breccia with large, unrounded fragments of biogenic limestones; g – sandstones with gentle cross-bedding; d, e – cyclic character of relatively fine-grained sediments with gradational texture.

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9. Fig. 7. Carbonate rocks and the remains of reef-building organisms found in them. a, b – organogenic limestones with large inclusions of macrofauna; c, d, d – corals of different species; e – layer with numerous brachiopods; g – fragments of large crinoids; h – bivalves.

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10. Fig. 8. Surfaces of layers with cracks of different morphology and origin. a – surface of layer with polygonal desiccation cracks and extended linear cracks cutting the entire rock unit, having a tectonic nature; b, c – polygonal desiccation cracks developed in a thin (first centimeters) calcareous surface layer; white spots on the surface of the layer are the remains of lichens.

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11. Fig. 9. Bioherm structure with traces of paleokarst development. a – general view of the bioherm, b, c – karst depressions in the upper part of the structure, filled with sedimentary material colored brownish-red by iron hydroxides.

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12. Fig. 10. Western end of the reef massif (with elements of its structure interpretation). a, c – photographs of the object with different types of lighting, which helps to identify the features of its structure, b – view of the northwestern edge of the reef massif. Numbers on the photographs: 1 – a pack of interbedded limestone and marl layers at the base of the Upper Jurassic strata, 2, 4 – dome-shaped lenses of dense biogenic limestones, 3, 5 – stratified sedimentary rock strata, the layers of which abut against limestone lenses, 6 – possibly another lens of limestones (?), crowning section J3.

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