ISSN 1028334X, Doklady Earth Sciences, 2009, Vol. 429, No. 8, pp. 1278–1283. © Pleiades Publishing, Ltd., 2009. Original Russian Text © V.V. Yudin, V.S. Vishnevskaya, D.V. Kurilov, 2009, published in Doklady Akademii Nauk, 2009, Vol. 429, No. 1, pp. 89–93.
GEOLOGY
Ophiolitic Radiolarites of the Crimea and Their Significance for Geodynamics of Mesotethys V. V. Yudina, V. S. Vishnevskayab, and D. V. Kurilovb Presented by Academician V.E. Khain December 25, 2008 Received March 11, 2008
Abstract—In southern Crimea, at the southeastern foot of the Demerdzhi Mountain, at the base of conglom erates, aged from the Callovian to the Kimmeridgian, there were revealed pebbles of radiolarian cherts, from which using hydrofluoric acid, Triassic radiolarians of fine and satisfactory preservation were identified. Among them were Podobursa primitiva Tekin, Picapora robusta Kozur et Mostler, Spinotriassocampe carnica Kozur et Mostler, etc., by which the age of Crimean radiolarites was dated as Triassic (Ladinian–Early Norian). DOI: 10.1134/S1028334X09080091
Abyssal radiolarites from molasse pebbles, the age of which was dated as Middle–Late Triassic enable us to propose the time of formation of the first layer of the paleoceanic crust, which is presumably located at a large distance from sources of materials on land. The datings of radiolarites from molasse in the Bitak and Demerdzhi foredeeps geologically confirm the paleo magnetic and geodynamic reconstructions, which indicated that a substantial part of the Mesotethys abyssal part with oceanic crust was recorded in the Mesozoic, north and south of the Mountainous Crimea terrane. Judging by the age of the molasse that is synchronous to convergence, the northern fragment of the Mesotethys between Laurasia and Crimea ter rane reduced in the period from the Early Jurassic to Early Cretaceous. Subduction of the Mesotethys southern fragment between Crimea and Anatolia refers to Callovian–Oxfordian time with collision in the Kimmeridgian–Tithonian. Ophiolites play an important role in paleogeody manic reconstructions. According to G. Steinmann, the ophiolite triad consists of serpentinized ultra mafics of the mantle, gabbrobasalts of the basalt layer, and abyssal radiolarites of the upper sedimentary layer of the oceanic crust. Studies of deepwater sedimenta tion and experiments have shown that at a depth of 3.7 km (lysoclin), the rate of calcium carbonate solu
a Ukrainian State Geological Prospecting Institute, pr. Kirova 47/2, Simferopol, 95017 Ukraine; email:
[email protected] b Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russia
tion increases markedly due to a rise in the hydrostatic pressure, partial pressure of carbonic acid gas, and fall in temperature. At depths of 4.5–5 km, the socalled carbonate compensation depth, carbonatebearing sediments dissolve completely. Consequently, far away from continents, where terrigenous material from land does not arrive, the ocean floor is covered with deep water silts and radiolarites, composed of insolluble SiO2, clay minerals, and tektites. Fragments of ophiolites encountered in the conti nental crust among mélange rocks of collision sutures are a geological feature pointing to subducted paleoce anic crust. The age of this crust may be defined by iso topic and magnetostratigraphic dating of basalts, or by radiolarian evidence. When ancient collisional orogens are eroded, ophi olites are usually redeposited in foredeeps. Therefore, fragments of ophiolite rocks identified in molassic psephites might indicate their wider distribution in the ancient denudation section than in the recent one [4, 9, 10]. This particularly refers to radiolarites, which during dislocation metamorphism were converted into jasper and jasperoid. Due to high hardness, they are better preserved during transfer than igneous rocks, which are ground into a pelitic fraction. The composi tion of orogenic psephite fragments in molassic units of foredeeps is an important indicator of the presence of a suture zone nearby. For example, consider the foredeeps in the Urals, Tien Shan, Zagros, and other regions, where molasse contains abundant radiolarite fragments [4, 9]. Two Mesozoic collisional sutures, varying in age and inclination trend, covered with Cretaceous–Cen ozoic rocks, are identified in the Crimea. There are a
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northerly dipping Piedmond structure of Jurassic– Early Cretaceous age and a Late Jurassic southern Crimea suture dipping southward [3, 5]. Ophiolitic fragments, structures of intense tangential compres sion, and dislocation metamorphism were recorded in the suturerelated zone of the Piedmont joint. The ophiolite triad is represented by radiolarites, ultra mafics, and mafics, which compose the paleoceanic crust. These rocks are also present in pebbles, which are synchronous with convergence of molasse in fore deeps. Collision joints border diachronous plates and ter ranes. The results of paleomagnetic research, obtained in independent laboratories of different countries, suggested that they are characterized by varying pale olatitudes, which allow us to judge about the width of the paleoceans. Recumbent flanks (autochthon) of sutures in passive margins contain orogenic forma tions and structures. In the Crimea, the Bitak and Demerdzhi foredeeps are among such structures. Their psephites contain radiolarite pebbles, which substantiate the considerable width of subducted pale oceans, according to paleomagnetic reconstructions [6, 8, 9]. The content of radiolarite pebbles in outcrops is negligible amounting to several percent. However, their presence, as such, is of vital genetic importance in paleoreconstructions. The intense dislocation metamorphism of radi olarites has made it difficult to define taxonomic spe ciation of radiolarians in thin sections under the microscope. Therefore, rocks were dissolved using hydrofluoric acid, which is a standard method for such rocks, allow preparation of radiolarian skeletons until they acquire a natural appearance. To remove the minor carbonate component, the samples were provi sionally treated with acetic acid. This allowed us to identify wellpreserved radiolarians and reliably date their age under the electron microscope as Upper– Middle–Late Triassic. Radiolarian pebbles have been recovered in con glomerates of two molasse complexes in the Bitak and Demerdzhi foredeeps in the Mountaneous and Pied mond Crimea. The complexes are autonomous, vary in age, and have different source areas of terrigenous material: respectively the northern Piedmont and the southern Crimea suturerelated zones. The Bitak molasse complex is exposed in the Sim feropol southern suburbs and identified as a formation of the same name [1]. It occurs subvertically and con sists of polymictic conglomerates, gravelstones, and sandstones more than 3 km thick (Fig. 1, map and the upper section). The stratum formed a large allochtho nous Simferopol thrust faultrelated anticline, which inverted the initially synclinal structure of the Bitak foredeep. A wide band of the overthrust Simferopol DOKLADY EARTH SCIENCES
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mélange occurs south of molasse outcrops, and far ther, no analogous conglomerates are revealed. Palin spastic reconstructions of the anticline made it evident that the 3km molasse stratum was devoid of a south ward pinching out zone. Hence, the outer half of the foredeep must lie within a deep underthrust; other wise, its structural model would not be balanced out. Macrofaunal finds in sandstone dated the age of the Bitak Formation as Early–Middle Jurassic (Toar cian–Bajocian). The orogenic complex is overlain with an angular unconformity by polymictic conglom erates of the Bairaklin Formation. Earlier, its age was assumed to be Late Jurassic, but the latest data referred to the Earliest Cretaceous. Among Bairaklin conglomerates, there are ophiolite pebbles. An over thrust suturerelated mélange with dynamometamor phic schists and ultramafic and mafic clastoliths was recovered by drilling under the Cretaceous–Cenozoic mantle, several kilometers north of Simferopol [5]. The composition of fragments in the molasse is similar to that of the mélange rocks and suggests that the material from the suture zone is supplied to the north. In 1992, in the upper part of the Bitak conglomer ate section, situated near the rescue station of a reser voir, we discovered fragments of jasper and siliceous shale abundant in radiolarians. Pebbles were red and brownish red, occasionally black, highly siliceous, and almost completely devoid of carbonates; the structure was fine and microlaminated. In some cases, silicides contain minor amounts of poorly preserved Hedber gella planktonic forams. The size of radiolarites ranges from fractions of a centimeter to 10–15 cm. During field studies it was found that the majority of radiolar ian samples were disintegrated, elongated acquiring a “pencil” shape, and undeterminable because of dislo cation metamorphism and intense microlamination. In some cases, pebbles contain slightly disturbed vari eties, which are used for age dating. Radiolarians were recovered by means of thinned hydrofluoric acid. The presence of marking species in the taxonomic list provided precise dating of the Late Jurassic–Early Cretaceous age for radiolarites [12]. The dating of radiolarites from the upper part of the section of the Bitak Formation has widened the age range of molasse formation, from the Toarcian to the Early Cretaceous inclusive and allowed us to assume a longer duration of subduction process of the fragment of the Mesotethyan oceanic crust. According to paleomagnetic reconstructions, the width of the Paleocean between the Mountaneous Crimea terrane and Laurasia in the Middle Jurassic was 1000–2000 km and gradually decreased until the end of the Early Cretaceous. Continuation of the con vergence in the Late Jurassic–Early Cretaceous is confirmed by active synchronous marginal volcanism
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Fig. 2. Triassic (Ladinian–Lower Norian) radiolarians from Tapshan conglomerates of the Demerdzhi Mountain (Southern Crimea). (1–2) Astrocentrus ? sp., 280×, 150×; (3–4) Welirella cf. weveri Dumitrica, Kozur et Mostler, 400×; (5–6) Staurocontium ? trispinosum (Kozur et Mostler), 200×, 280×; (7) Pentaspongodiscus ? sp., 270×; (8) Plafkerium sp. Cf. P. hindei Pessagno, 300×; (9) Gorgansium ? sp., 180×; (10–11) Triassobipedis sp., 280×, 180×; (12) Deflandrellium sp., 280×; (13) Palaeoscenidiidae ?, 120×; (14) Welirella cf. weveri Dumitrica, Kozur et Mostler, 400×; (15) twisted spine of Hindeosphaeridae ?, 200×; (16) Pseudostylosphaera ? sp., 150×; (17–18) Triactoma ? sp., 150×, 180×; (19) Gorgansium ? sp., 280×; (20) Pseudosty losphaera ? sp., 180×; (21–22) Gen. et sp. Indet., 300×, 280×; (23) Triassocampe sp., 200×. All the figures are Samples 147=2=94. Radiolarian collections are kept in the Russian Center of Micropaleontological Reference Collections.
in the Lowland Crimea, by isotopic datings of dyna mometamorphic sericite in the Piedmont suture, and by Lower Cretaceous rocks revealed in the Simferopol mélange, which is linked with the Piedmont suture. Therefore, age dating of radiolarites is quite possible and the model of the geodynamic evolution of the region is correct [6, 9, 10]. The Demerdzhi molasse complex, Middle–Late Jurassic in age, is exhibited in local outcrops along the southern slope of the Crimea Mountains [1]. Super imposed Cenozoic overthrusts and gravigene distur bances of the complex were complicated by a series of flakes and folds situated in large olistoliths of Moun tainous Crimea and Massandra olistostromes [6–8]. Conglomerates pinch out in the northwesterly direc tion. The lithological composition of conglomerate pebbles suggests a southern source of terrigenous material [1]. The most complete section of the molasses com plex is situated 6 km to the northeast from the town of DOKLADY EARTH SCIENCES
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Alushta, at the southeastern slope of the South Demerdzhi Mountain, in the TapshanGya gully (in latest maps, Edyfler). The section is represented by a thick (more than 2 km) sequence of polymictic con glomerates, gravelstones, and sandstones, stratigraph ically overlain by Tithonian limestones. The geological structure of the Demerdzhi Moun tain area is fairly complicated (Fig. 1). Conglomerates form an isolated olistostrome massif shifted from the south in the Early Cretaceous. In the Neogene the massif was remobilized by overthrust disturbances, and at present, it is underlain by a regional Piedmont mélange. Mixtite is composed of disintegrated flysch rocks of the Tavrida Formation of Late Triassic–Early Jurassic age and partly of Middle Jurassic and Early Cretaceous sandstones and clays [8]. Beneath the mélange there is Tavrida flysch detached by gently dip ping thrusts and crumpled into intense, even isoclinal and plunging, folds of southern vergence (Fig. 1, lower section).
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The base of the conglomerate stratum in the Taps han–Gya gully contains the most ancient part of the section with fauna of Callovian–Oxfordian age (Fig. 1). In 1993, we discovered in it pebbles of greenstone rocks, basalts, and sedimentary silicites: siliceous shales, jasper, and jasperoids. Pebbles discovered in the Tapshen Formation are from 1–5 to 20 cm, gray, dark gray, and black in color, have a high Si composition, and are abundant in radiolarians. Intense dyna mometamorphism and brecciation makes it almost impossible to determine the taxonomic identity of radiolarians in thin sections. Therefore, the method of chemical preparation, with the application of hydrofluoric acid, was used, which allowed us to identify Triassic radiolarians of fine and satisfactory preservation [2]. Among them were the following: Podobursa primitive Tekin, Pica pora robusta Kozur et Mostler, Spinotriassocampe car nica Kozur et Mostler, and others that allowed us to determine precisely the age of radiolarites as Carnian (Fig. 2). The radiolarite sequence also contains Pan tanellium ex gr. browni Pessagno et Blome, Podobursa sp. cf. P. turriformis Tekin, Triactoma cf. acythis (De Wever), Tubospongopallium ? tornatum Tekin et Mos tler, Plafkerium sp. cf. P. hindei Pessagno, Staurocon tium ? trispinosum (Kozur et Mostler), Welirella cf. weveri Dumitrica, Kozur et Mostler, Xiphosphaera? fistulata Carter, which are widely spread in the Ladin ian–Lower Norian interval (uppermost Middle–Late Triassic, (Fig. 2)). The association of pebbles at the base of the Demerdzhi section of conglomerates in the Tapshan Formation indicates erosion of bathyal and abyssal rocks, which had earlier deposited on the oceanic crust, situated farther south. This provided the basis for distinguishing the South Crimea collisional struc ture, inclined to the south, and now is evidently situ ated within the Black Sea region [5, 6]. Pebbles of sedimentary, metamorphic, and igneous rocks, up to Early Riphean granites, described in the literature, which reflect erosion of the ancient conti nental crust, occur in the section stratigraphically higher than Demerdzhi conglomerates. Therefore, the longterm formation of the Demerdzhi molasse might have proceeded simultaneously with the Crimea and Anatolia convergence accompanied by a complete subduction of the oceanic crust. This was followed by a collision of the terranes and formation by the Late Jurassic of the South Crimea suture.Conglomerates, similar in age and composition, are present farther along the east and west trend at the base of large olis toliths derived from Tithonian limestones. In the east ern Crimea, analogous conglomerates with radiolar ian pebbles transformed into jasperoids are known in
the Sudaktown and Meganom Cape area, where con glomerates contain Tithonian fauna. The Middle–Late Triassic age of radiolarites is in agreement with the geological evidence and geody namic model of the region evolution. An intricately disturbed distal deepwater Tavrida flysch of Late Trias sic–Early Jurassic age, which underlies the conglom erates, also formed on the oceanic crust [6]. The flysch fragments have a monooligomictic composition, sug gesting that it was formed on the continental slope of the passive margin. The formation of radiolarites pro ceeded in the abyssal environment of carbonate com pensation, hundreds of kilometers farther south, at a place where fine terrigenous material of distal flysch never penetrated. The first layer of the Mesotethyan oceanic crust consisted of sedimentary silicites. Dur ing subduction they were removed from the basalt layer, and after the accretionary prism was eroded, they were redeposited within lower parts of the molasse complex of the Demerdzhi foredeep. CONCLUSION The Late Jurassic–Early Cretaceous and Middle– Late Triassic age of abyssal radiolarites from molasse pebbles allow us to propose the time of formation of the first layer of the paleoceanic crust, located at a large distance from sources of material on land. The dating of radiolarites from the molasse of the Bitak and Demerdzhi foredeeps geologically confirms the paleo magnetic and geodynamic reconstructions, which indicate that a considerable abyssal part of the Mesot ethys with oceanic crust occurred in the Mesozoic, north and south of the Mountainous Crimea terrane [3, 6, 9, 11]. The age of the molasse that is synchro nous to convergence suggests that the Mesotethys northern fragment, between Laurasia and Crimea decreased in the period from the Early Jurassic to the Early Cretaceous. The subduction of the southern fragment of the Mesotethys oceanic crust between Crimea and Anatolia occurred in the Callovian– Oxfordian and the collision happened in the Kim meridgian–Tithonian. REFERENCES 1. Geology of the USSR, Vol. 8: Crimea, Chapter 1: Geolog ical Description, Ed. by M. V. Muratov (Nedra, Mos cow, 1969) [in Russian]. 2. D. V. Kurilov, V. V. Yudin, and V. S. Vishnevskaya, in Paleontological Studies in Ukraine: History, State of the Art, and Prospects. Collected Papers of the Geological Institute, National Academy of Ukraine (Kyiv, 2007), pp. 115–116, 504–507. 3. V. V. Yudin, Dokl. Akad. Nauk 333 (2), 250–252 (1993). DOKLADY EARTH SCIENCES
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OPHIOLITIC RADIOLARITES OF THE CRIMEA AND THEIR SIGNIFICANCE 4. V. V. Yudin, Orogenesis of North Urals and PaiKhoi (Nauka, Yekaterinburg, 1994) [in Russian]. 5. V. V. Yudin, Geol. Zh. (Kiev), No. 3/4, 56–61 (1995). 6. V. V. Yudin, Geol. Zh. (Kiev), No. 3/4, 115–119 (1995). 7. V. V. Yudin, Dokl. Earth Sci. 363A (9), 1230–1235 (1998) [Dokl. Akad. Nauk 363 (5), 666–669 (1998)]. 8. V. V. Yudin, Crimean Geology on the Basis of Geodynam ics. Scientific–Methodical Manual for a Practical Course in Geology (Komi NTs UrO RAN, Syktyvkar. Gos. Univ., Syktyvkar, 2000) [in Russian].
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9. V. V. Yudin, Geodynamics of the Black and Caspian Sea Region (UkrGGRI, Kiev, 2008) [in Russian]. 10. V. V. Yudin, D. V. Kurilov, and V. S. Vishnevskaya, Paleostrat2006. Annual Meeting of the Paleontological Section of MOIP and Moscow Department of the Paleon tological Society. Program and Abstracts of Papers (Mos cow, 2006), pp. 30–31 [in Russian]. 11. V. V. Yudin, L.P. Zonenshain Memorial Conference on Plate Tectonics. Abstracts of Papers, November 17–20, 1993 (Moscow, 1993), pp. 158–159. 12. V. V. Yudin and V. S. Vishnevskaya, L.P. Zonenshain Conference on Plate Tectonics, Abstracts of Papers, November 22–25, 1995 (Moscow, 1995), p. 209.
