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CUPP-COUTUNN CAVE, TURKMENISTAN

Cupp-Coutunn, the longest limestone cave in the former Soviet Union, is located in the Kugitang region of eastern Turkmenistan, close to the borders with Uzbekistan and Afghanistan. The cave is internationally famous for its mineralogy, both in the number of mineral species and in the great variety of its speleothem forms. In 1991, Cupp-Coutunn and its associated caves were placed on the Global Indicative List of Geological Sites (GILGES) by a working group of the World Heritage Convention.

The Cupp-Coutunn cave system (Figure 1: Maltsev & Self, 1992) is located on the southwestern flank of the Kugitangtau ridge, which runs north-south for some 50 km and attains an altitude of 3137 m. Low hills to the north connect the Kugitangtau ridge to the southwestern end of the Tien Shan mountain chain, from which it appears

Topographically as an outlier. The Kugitangtau is an anticlinal dome with a granite batholith intruded into its core. A major fault, the Eastern Kugitang Upthrust, along the north—south major axis of the anticline, separates the uplifted Kugitangtau from the Uzbek plain to the east. The Kugitangtau cuesta presents a scarp face over 1 km high, topped with a near-vertical wall of upper Jurassic limestone. The dip slope to the west is a gentle 7° to 15°, deeply dissected by canyons. This dip slope is cut by a second major fault, subparallel and of opposite throw to the Eastern Kugitang Upthrust, leaving the central spine of the ridge as a horst block.

The caves of southern Kugitangtau show a complex history. They developed as an extensive phreatic maze, following the then horizontal bedding in the limestone on several levels, probably in late Cretaceous times. These passages were subsequently abandoned as conduits for groundwater flow and infilled with argillaceous sediments. Neogene uplift and tilting of the strata rejuvenated the caves. Detrital deposits were locally eroded by groundwater introduced via a north—south fault set, though in places the consolidated remains of this ancient filling are retained as the walls or roof of the cave. New passage development was restricted to invasive tubes in the immediate vicinity of these faults. The air-filled passages were then invaded by thermal water entering from another (Chilgas) fault set, with chemical alteration of the cave wall rocks and sediments. Alteration effects include recrystallization of the limestone and replacement of carbonates by other minerals, mostly silicates and sulfides. Small amounts of hydrothermal calcite and fluorite were locally deposited within the cave, the calcite sometimes containing microscopic inclusions of metal sulfides. The thermal fluids do not appear to have been particularly aggressive, as no significant passage enlargement has been noted. Stable isotope analysis of rock and mineral samples suggests that the thermal water was an evolved basinal brine (Bottrell, Crowley & Self 2001).

During the mid-Quaternary, there was some movement of the Chilgas faults. This caused the displacement of cave passages crossed by these faults, and collapse in some of the larger airfilled passages. The modern phase of development is marked by condensation corrosion of the cave walls and roof the reworking of material from the thermal phase, and the deposition of speleothems. For the most part, the caves are independent of drainage from the surface, though some canyons have cut their beds deep enough to intersect the caves. However, the Kugitang region is semi-arid and significant quantities of water enter the caves about once per decade, locally introducing alluvium or reworking the older sediments.

The international importance of the Cupp-Coutunn cave system is in its speleothem mineralogy, which has been entirely deposited during the post-thermal phase of the caves’ development. Some very rare minerals have been formed by the reworking of material from the thermal phase, whether this material was deposited in the cave itself or as vein minerals in fractures crossing the cave. Particularly interesting are zinc aluminosilicates, which are coloured green by trace quantities of nickel. Sauconite was the first to be identified, and then the very rare fraipontite (with only a few occurrences known worldwide, this is its first report as a cave mineral). Si and A1 mobility is due to acidic gases released from a red, clay-like coating (ochre) on hydrothermally altered wall rock. Ochre is a variable but stratified material, with sulfate-reducing bacteria in the lower layers and oxidizing bacteria in the outer layers, ft is more like a soil than a classic corrosion residue and various secondary oxide and silicate minerals have been identified

Within it (Maltsev, Korshunov & Semikolennykh, 1997). Hydrogen sulfide produced within the ocker is oxidized to sulfuric acid, which in turn attacks crystals of thermal phase fluorite. Gaseous hydrogen fluoride is released into the cave air, the evidence for this being tiny dark purple crystals of fluorite that have been found growing on calcite helictites and on euhedral gypsum crystals (Maltsev & Korshunov, 1998).

Cupp-Coutunn has spectacular displays of speleothems of the more common cave minerals. Where surface water infiltrates from canyon floors or from north—south faults, there are calcite stalactites, stalagmites, and even shields. A particularly beautiful banded flowstone (marble onyx) was commercially mined from 1970 until 1982, with three mine adits cut into the cave to provide easier access. The mining was finally halted following a campaign led by Moscow and Ashkhabad cavers. Aragonite is a main speleothem mineral in some parts of the cave, often in association with calcite. This association produces some of the most photogenic speleothems, such as quill anthodite bushes and multi-corallite splays (Figure 2). Helictites are abundant, sometimes with euhedral crystals of clear gypsum or blue celestite growing on their sides. Gypsum is found in most parts of the cave, as beards, flowers, crusts, chandeliers, and as large hollow stalagmites. Rare speleothem forms include solid crystalline stalagmites of gypsum, aragonite pseudo-stalactites, and macro-crystalline calcite stalactites.

Cupp-Coutunn/Promeszutochnaya (combined length 56 km) is part of a more extensive karst system that is still largely filled with ancient sediments. Nearby, and recognized as part of the Cupp-Coutunn system, is Geophyzicheskaya (length 4.5 km, with spectacular gypsum chandeliers) and Hashm-Oyeek (length 7 km, first reported by Diodorus Siculus c. 40 BC). Dozens of other cave fragments have been found in neighbouring canyons throughout southern Kugitangtau. The area was once very popular with Russian cavers, but there have been few expeditions since Turkmenistan became an independent country.

Vladimir Maltsev’s team (Moscow) was pre-eminent in both the exploration and the scientific study of the caves during the late Soviet period. Much of our modern understanding of ontogeny is based on speleothem observations made in these caves.

CHARLES ANTHONY SELF

Works Cited

Bottrell, S. H., Crowley, S. & Self, C. A. 2001. Invasion of a karst aquifer by hydrothermal fluids: evidence from stable isotopic compositions of cave mineralization. Geonulds, 1:103-21 Maltsev, V. A. & Korshunov, V. A. 1998. Geochemistry of fluorite and related features of the Kugitangtou ridge caves, Turkmenistan. Journal of Cave and Karst Studies, 60(3):151-55 Maltsev, V. A., Korshunov, V. A. & Semikolennykh, A. A. 1997. Cave

Chemolithotrophic soils.

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Literatura: Encyclopedia of Caves and Karst Science