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The Urgonian facies of Central Asia
Cretaceous Research (1990) 11,253-260
The Urgonian facies of Central Asia V. A. Prosorovsky Leningrad University, Geological Facuky, Universitetskaya nabereznaya, 7/9; Leningrad 199034, USSR Accepted 20April 1990
The mass presence of orbitolinid tests in fossiliferous limestones is a palaeontological criterion for the establishment of the Urgonian facies in Central Asia. The stratigraphic distribution of the facies encompasses the Barremian and Aptian stages. Typical Urgonian deposits are widespread in Turkmenistan, Iran and Afghanistan. They were formed in large reef systems on the shelf of marginal seas of the northern periphery of Tethys. KEY WORDS: Urgonian facies; orbimlinids; reefs; Cretaceous; Itauterivian; Barremian; Aptian; Tethys; Central Asia; Caucasus.
The term 'Urgonian' had a rather specific stratigraphic connotation from the moment it was established (d'Orbigny, 1847), and it seems that this approach should also apply to coeval strata elsewhere. This is in agreement with the decision of the Lyon Colloquium (1963) where it was recommended that the term 'Urgonian facies' should be reserved only for limestones with the rudistids (Toucasia) of Barremian and Aptian age (Rat, 1965). The main propositions of the Grenoble Colloquium (1965) are hardly rational. According to these it is possible to ascribe any Lower Cretaceous formations of organic origin on carbonate platforms to Urgonian facies. The replacement of the word 'facies' by 'biosedimentary system' (Kotetishvili, 1979, 1986; Rat & Pascal, 1979) hardly changes anything of substance, although the latter term evidently reflects the variety of Urgonian lithotypes more accurately. It is unsatisfactory also to consider that the presence of rudists in micritic limestones is the main criterion for the recognition of the Urgonian facies. According to my observations, this is only one of a large variety of facies in the Lower Cretaceous carbonate complex. Besides, an acceptance of the presence of rudists as a main indication of Urgonian can lead to a loss of precision for the term. A consequence of this is, for example, that the boundary with the Lusitanian (Choffat, 1885) is unrecognizable, with the result that a mass of rock more than 1000m thick in western Turkrnenia, and ranging in age from Oxfordian to Albian, is called by one name. In such a case 'Urgonian facies' or 'Urgonian biosedimentary system' is applied to any fossiliferous carbonates in which there are the rudists (i.e., encompassing the Upper Jurassic-Maastrichtian). In order to impose stratigraphic limits on the use of 'Urgonian' it is necessary to choose palaeontological criteria for its ~cognition which are present everywhere, or almost everywhere, and can be recognized easily. The distribution of rudists in Lower Cretaceous biogenic carbonate complexes is concentrated in Central Asia as lenses comparatively limited in area and situated in the rear slopes of reef masses. The coral or algal bioherms which are nttjacent to them on one side and the oolitic limestones of trans-reef lagoons on the other are more variable in size than the rudistid buildups. But the coral and algal bioherms, in common with the 0195-6671190/030253 + 08 $03.00/0
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V.A. Prosorovsky
rudistid masses, occur only sporadically in the Urgonian, with the result that they cannot always determine its presence. The main criterion for the establishment of Urgonian facies is the presence of orbitolinids which are found everywhere. They are easy to recognize and, hence, do not need to be very precisely analysed before they can be identified. Acceptance of this palaeontological criterion corresponds quite closely to the definition of 'Urgonian facies' by Gignoux (1952, p. 368, translation) as follows: " . . . zoogenic limestones,.., consisting of accumulation of more or less redeposited fragments of foraminifers (miliolinas, orbitolinas), corals, hydromedusae, bryozoans, rudistids, nerineas, etc.". There is a similar definition in the Lexique Stratigraphique (1957). When typical Urgonian limestones are considered to contain orbitolinids, the lower limit of their development can be restricted to the Barremian. As for the upper boundary, it is generally not higher than Aptian in the Mediterranean, and hence is in agreement with the position adopted by the Lyon Colloquium (1963). Only in central Afghanistan does this facies perhaps also include the Albian, but the dating of the rocks there is not very reliable at present. Representatives of other groups of the Urgonian biota (e.g., echinoids and brachiopods) are rather limited in diversity (Chernov et al., 1980), long-ranging, and unlike the orbitolinids are not ubiquitous. They reflect particular environmental conditions, appearing in Central Asia from the base of the Hauterivian. Ih Central Asia the Urgonian facies is extremely widespread (Figure 1) and outcrop localities on the western plunge of the Maliy Balkhan ridge may be regarded as standards. They closely correspond to its classic determination according to the variety of oryctocoenoses present. They are also very widely exposed in the Kopetdag and its spurs so that it is possible to analyse facies and the ecology and taphonomy of fossils in detail (Kuzmichova & Prosorovsky, 1968; Prosorovsky & II'in, 1972; Verba & Prosorovsky, 1972; Bugrova et al., 1987). The Cretaceous of the southwest of Soviet Central Asia is represented mainly by marine deposits. These are subdivided into three formational associations which are successively (upwards): Neocomian carbonates (Berriasian-Barremian); middle Cretaceous sandy-argillaceous and glauconite-bearing (Barremian-Turonian); Senonian white chalk (Coniacian-Maastrichtian). Lagoonal and continental redbeds play a major part in the northern Kara-Kum and Amu-Daryan syneclise. The Neocomian formations transgressively overlap older deposits including those of the Palaeozoic (Figure 2). They are represented mainly by carbonate rocks in the Kopetdag and Maliy and Bolshoy Balkhan; there are cyclic sandy-argillaceous rock masses among them. Northwards and eastwards, marine deposits first become lagoonal (Cubadag, southern Kara-Kum) and then continental red beds (Tuarkyr, Mangyshlak). Zoogenic rocks, most of which contain an abundance of reef-building organisms, first appear in the Neocomian formational association, beginning at least with the base of the Hauterivian (Bacharden Formation of the Kopetdag, Arlanian Formation of the Bolshoy Balkhan). In the upper Hauterivian-lower Barremian they are present on a more massive scal~'. They reach their maximum development in the upper part of this stratigraphic interval and are represented by orbitolinid (Urgonian) limestones practically everywhere in a very wide area encompassing south and central Turkmenia. There is replacement by other facies along the south periclinal part of the Tuarkyr structure, the north of the Central Kara-Kum Arch and the Amu-Daryan syneclise (Figure 3). Overall, the Urgonian facies in Central Asia is a complex system of different carbonate rocks representing open shelf
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deposition to the south of the Eurasian continent. Their thicknesses range from 350 m in the south to almost zero in the north and the various associations consist of orbitolinid, coral, coral-algal, rudistid and often bioherm limestones, oyster beds and oolitic, detrital, pelitomorph and other limestones• There are intercalations of clays, siltstones, sandstones and.rarely conglomerates, mainly of marine origin• On the other hand, the widespread fossils are closely similar, indeed practically identical, to assemblages in older deposits. Most species of brachiopods, bivalves, echinoids and other invertebrates (including many endemics) range throughout the upper part of the Hauterivian-lower BarremiaJa interval. In this sense the Central Asian material confirms Kotetischvili's (1986) conclusion thai a common biosedimentary system existed during the Hauterivian and Barremian. On the other hand,
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the orbitolinids, unknown earlier, alJpear in very large numbers within this system. These include Orbitolina delicata Hens., O. discoidea Gras, O./ent/cu/ar/s Blum., O. con/coform/s Mamont., D/ctyoconus arab/cus Hens., D. valnutensis Carsoy. Almost everywhere a specific assemblage of echinoids occurs within the upper part of orbitolind beds. This consists of Heteraster spp., Toxaster ricardeaui d'Orb., T. arg//aceus d'Orb., T. exitis Lor., Pygurus romani Lamb., P. g////eroni Des. Thus, in my opinion, despite the similarities, the composition of fossils of the Urgonian facies in Central Asia is clearly different from that of the underlying strata. This allows the facies to be readily recognized in both natural and artificial outcrops and in borehole cores. Some of the Urgonian changes are now determinable in Turkmenia./In the very southern-most outcrop zone in the Kopetdag (near the Iranian border) there are complex and large reef systems, perhaps even of barrier type. Reef cores, reef slope facies, inter-reef lagoons and the outer part of trans-reef lagoons can be recognized with confidence. They are formed mainly by scleractinian and algal colonies. The organization and zonation of colonial forms can be readily observed, and sometimes the distribution of other fossils in the reef system is also easy to see. Northwards in the Bolshoy Balkhan and occasionally in the Cubadag it is possible to observe trans-reef lagoonal facies with isolated patch-reefs formed by coral, algal and rudistid buildups, and also oyster and brachiopod banks. Further north, in the south of the Tuarkyr district, foreshore slopes of reef lagoons with coral meadows and brachiopod, gastropod and oyster banks are exposed and are transitional to continental deposits. It is difficult to establish the precise stratigraphic position of the Urgonian facies in Central Asia because of the absence of ammonites. The wide distribution of orbitolinids and representatives of the echinoid genus Heteraster (H. couloni Ag., H. cf. elegans Rouch., H. magnus Poretzc.) at its base confirm its placement within the Barremian, but it is not clear which part because typical Hauterivian species of benthonic organisms are found much lower. The Urgonian deposits are overlain by beds with the ammonites Imerites and Colchidites, which correspond to the upper half of the upper Barremian substage"(part of Imerites giraudi Zone of Georgia). Hence, the distribution of the-Urgonian facies in Turkmenia perhaps does not extend above the middle Barremian. The formation of the Urgonian carbonate deposits probably took place on the shallow shelf of large marginal seas of the nor,t,hern periphery of Tethys during a transgressive cycle which began in the early Hauterivian and ended in the middle-
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V.A. Prosorovsky
late Barremian. The relief of the sea bottom during this period became more and more complicated from the north, where it was a submerged plain, to the south, where seamounts were fixed by organic structures. The maximum development of reefs took place in the regressive phase of the cycle (Verba & Prosorovsky, 1973). The transgression spread over the considerable extent of the Turanian platform and reached the basin of the Vachsh. The cessation of Urgonian sedimentation was connected at first with the abrupt uplift of a large part of Soviet Central Asia, evidently as a result of a late stage of renewed Kimmerian folding. A major transgression followed. It began as a result of rapid subsidence and led to the accumulation of deep sea deposits in a large zone in the south (Arvazian Formation and its analogues). Conditions suitable for the formation of orbitolinid limestones ceased to exist. The Turkmenian depositional basin was, however, only part of an enormous submerged area which included northern and central Iran (Tigran and DjampSchuran Formations, the Park Series and others) and southern Afghanistan (Alavi-Naini, 1972; Stampfli, 1978; Stocklin & Setudehia, 1972; Geologia Afghanistana, 1980). Urgonian facies here include not only the Barremian but also the Aptian, and perhaps even younger Cretaceous in Afghanistan. The systematic composition of Berriasian-Barremian biocoenoses in Central Asia is evidence that this region belonged to the Mediterranean palaeogeographic region. In-my opinion its provincial zonation changed during this period (Figure 4). The composition of the benthonic upper Hauterivian fauna which contains many endemic brachiopods, bivalves and echinoids (Psilothyris oglan!iensis Moiss., Cydothyris firusensis Lobatsch., C. kerisensis Moiss., Cyrtothyris pseudosella Lobatsch., Sellithyris plicata Kvah. & Lobatsch., Turkmenia balkhanensis Krimh., Neithea daghestanica Renng., Diplopodia vassilievskii Vial., Pseudodiplopodia transcaspia Vial.) points to the isolation of the Central Asiatic province with the Kopetdag and Bolshoy Balkhan districts (Figure 3) by the late Hauterivian, not by the Late Cretaceous (Bobkova & Luppov, 1969). It is interesting that there are many endemic Exo~Drra(E. turkmenica Lupp., E. balkhanica Pros., E. geokderensis Pros.), together with the benthonic fauna mentioned above, in the north-west (the Bolshoy Balkhan, Cubadag, Tuarkyr) which are absent in the Kopetdag, Iran and Afghanistan. The Urgonian biocoenoses in Central Asia are very similar to those of Western Europe, Transcaucasia, the Carpathians, North Iran and South Afghanistan (Marchenko, 1962; Prosorovsky et al., 1961; Bogdanova & Lobatschova, 1966;
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259
Urgonian facies of Central Asia
Chernov et al., 1980; Kotetischvili, 1986). As a result, it is necessary to refer, in my opinion, to an Alpine-Central Asiatic province in the Barremian instead of the Alpine-Caucasian one proposed by Kotetischvili (1982). It consolidated the shelf of the northern periphery of the Tethys except in submerged areas of the Crimea, North Caucasus and perhaps Georgia (where there are no orbitolinids). It is necessary to distinguish the Central Asiatic subprovince within this province where the importance of the endemics is even greater. There were also the Kopetdag and Bolshoy Balkhan domains, as in Hauterivian. The particular character of the biocoenosis of the latter during the Barremian was increased by the appearance of the large rock-forming Meandropsinidae-Balkhania balkhanensis Mamont. These were discovered last year along with the bivalve Turkmenia in the Urgonian limestones of eastern Azerbaidzhan. The biocoenosis of Soviet Central Asian seas altered at the end of the Barremian because of tectonic and climatic changes. As a result new middle Cretaceous formations accumulated (Figure 2). Sandy-argillaceous deposits of the late Barremian aquatic realm contain ammonite casts and shells, planktonic foraminifers, and rather specific assemblages of bivalves, brachiopods and echinoids which were unknown in this region in older deposits. Kotetischvili (1986) included the late Barremian Central Asiatic depositional area in the Alpine-Caucasian province (Alpine-Central Asiatic here). The seas of the southern parts of Western Europe, the Caucasus and the Georgian block in the Transcaucasus were also part of this province. The comparative poverty of the ammonite composition makes possible the recognition of an independent Turkmenian subprovince (Kotetischvili, 1986) (Figure 5) which can be subdivided into Bolshoy Balkhan and West and East Kopetdag domains according to the specific composition of the benthonic fauna. The former is characterized by containing plenty of morphologically varied large, thick-shelled bivalves (Gervillia, lsognomon, Litschkovitrigonia, Iotrigonia, Quadratotrigonia, Astarte, Sphaera). There are many endemics among them including Litschkovitrigonia ingushensis alta Pros., L. tuarkyrica Pros., lotrigonia abichi renngarteni Mordv., and Astarte karajmanica Pros. The biocoenosis of the second domain is rather similar but differs in being less varied and in Lopha sp. being more important. The East Kopetdag domain is characterized by the presence of only rare thin-shelled bivalves. The increase in depth of the sea from the northwest to the southwest was probably the main cause of this differentiation.
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260
V.A. Prosorovsky
References Alavi-Naini, M. 1972. Etude g6ologique de la region de Diam. GeologicalSurvey oflran, Report 23, 288 PP. Bobkova, N. N. & Luppov, N. P. 1969. Osobennosti Sredneasiatskoy posdnemelovoy paleozoogeographieheskoy provincii. In Stratigraphia verchnegopaleozoya i mesozoyayuznich biogeographicheskich provinciy, pp. 193-203 (Moskva). [In Russian.] Bogdanova, T. N. & Lobatschova, S. V. 1966. Fauna neocoma Kopetdaga. Leningrad, Trudy Vsesoyuznogo Geologicheskogolnstituta, 130, 140 pp. [In Russian.] Bugrova, I. Yu., Verba, Yu. L., Preobrazhensky, M. B. & Prosorovsky, V. A. 1987. Nizhnemelovoy rifovy komplex Borshogo Balkhana. In Geologicheskoestroenie Turkmenistana, 79-103 (Askhabad). [In Russian.] Chemov, V. G., Yanin, B. T. & Golovinova, M. A. et al. 1980. Urgomkie otlozhenia Sovetskich Karpat, 183 pp. Moskva. [In Russian.] Choffat, P. 1885. Ddscription de la faune jurassique du Portugal. Mollusques lamellibranches,pars 1,394 pp. (Paris). D'Orbigny, A. 1847. PaldontalogieFranf~aise. Terrains Crdtads, vol. 4, 668 pp. (Paris). "Geologia Afghanistana" 1980. Geologia i polesnye iskopaemye Afghanistana, 1. Geologia, 379 pp. (Moskva). [In Russian.] Gignoux, M. 1952. Stratigraphicheskaya geologia, 638 pp. (Moskva). [In Russian.] Kotetischvili, E. V. 1979. K biostratigraphii verchnebarremstdch otlozheniy Grusii. Soobschenia Akademii Nauk GrusinskoySSR, 95, (2), 389-392, [In Russian.] Kotetischvili, E. V. 1982. Zonalnaya stratigraphia nizhnemelovych otlozheniy Grusii i paleogeographia Kaukazskay chasti Sredizemnonurrskoy oblasti v rannem melu. Avtoreferat doktorskoy dissertacii, Tbilisi, 32 pp. [In Russian.] Kotefischvili, E. V. 1986. Zonalnaya stratigraphia nizhnemelovych otlozheniy Grasii i paleozoogeographia rannemelovych basseyruroSredizemnomorskoy oblasti. Tbilisi, 160 pp. [In Russian.] Kuzfnichova, E. L & Prosorovsk'y, V. A. 1968. Organogennie postroiki v barremskychotlozheniyach Zapadnoy Turkanenii. Referat doklada. Bulletin Moskovskogo ObschestvaIspytateley Prirodi; Otdelenie Geologii, 1968/1. [In Russian.] Lexique Stratigraphique International, 1957. 1. Europe, 4a, VI, France, Belgique, Pays-Bus, Luxembourg, Cretacl, Centre National de la Scientifique, Paris, 361-367. Marchenko, V. I. 1962. Stratigraphia, litologia i facii neocoma Kopetdaga. Leningrad; Trudy Vsesoyusnogo GeologicheskogoInstituta, 78, 326 pp. [In Russian.] Prosorovsky, V. A., Korotkov, V. A. & Mamontova, E. V. et al. 1961. Neokom Zapadnoy Turkmenii. Leningrad, Trudy VsesoyusnogoGeologicheskogoInstituta, 51, 233 pp. [In Russian.] Prosorovsky, V. A. & Win, V. D. 1972. Organogennye postroiki v nizhnem barreme Turkmenii. Trudy Vsesojusnogo Nauchno-issledovatelskogo Geologo-raz'oedochnogoNeftjyanogo Instituta, 122, 16-24. [In Russian]. Rat, P. 1965. Rapport sur les facies urgoniens. Colloque mr le Cr~tac2 inferieur (Lyon, 1963). M~moire BRGM 34, 157-159. Rat, P. & Pascal, A. 1979. De l'etageaux syst~mes biosedimentaires urgoniens. G(obios, M(moire Special 3, 385-399. Stampfli, G. M. 1978. Etude glologfque g~n(rale de l'Elburz oriental au S de Gonbad-e-Qabus lran N-E. Th/~se no. 1868, Universit6 de Gen/:ve, 192 pp. St6cklin, J. & Setudehia, A. O. 1972. Iran. Lexique Stratigraphique International, Asie. CNRS, VIII, lb, 376 pp. Verba, Yu. L. & Prosorovsky, V. A. 1972. Hoterivskye i barremskye otlozhenia Malogo Balchana. Vesmik Leningradskogo Universiteta 6, 51-63. [In Russian.] Verba, Yu. L. & Prosorovsky, V. A. 1973. O regressii v seredine barremskogo veka na territorii Zapadnoy Turkmenii. In Litologia i paleogeographia, 1, 75-86 (Leningradsky universitet). [In Russian.]
The Urgonian facies of Central Asia V. A. Prosorovsky Leningrad University, Geological Facuky, Universitetskaya nabereznaya, 7/9; Leningrad 199034, USSR Accepted 20April 1990
The mass presence of orbitolinid tests in fossiliferous limestones is a palaeontological criterion for the establishment of the Urgonian facies in Central Asia. The stratigraphic distribution of the facies encompasses the Barremian and Aptian stages. Typical Urgonian deposits are widespread in Turkmenistan, Iran and Afghanistan. They were formed in large reef systems on the shelf of marginal seas of the northern periphery of Tethys. KEY WORDS: Urgonian facies; orbimlinids; reefs; Cretaceous; Itauterivian; Barremian; Aptian; Tethys; Central Asia; Caucasus.
The term 'Urgonian' had a rather specific stratigraphic connotation from the moment it was established (d'Orbigny, 1847), and it seems that this approach should also apply to coeval strata elsewhere. This is in agreement with the decision of the Lyon Colloquium (1963) where it was recommended that the term 'Urgonian facies' should be reserved only for limestones with the rudistids (Toucasia) of Barremian and Aptian age (Rat, 1965). The main propositions of the Grenoble Colloquium (1965) are hardly rational. According to these it is possible to ascribe any Lower Cretaceous formations of organic origin on carbonate platforms to Urgonian facies. The replacement of the word 'facies' by 'biosedimentary system' (Kotetishvili, 1979, 1986; Rat & Pascal, 1979) hardly changes anything of substance, although the latter term evidently reflects the variety of Urgonian lithotypes more accurately. It is unsatisfactory also to consider that the presence of rudists in micritic limestones is the main criterion for the recognition of the Urgonian facies. According to my observations, this is only one of a large variety of facies in the Lower Cretaceous carbonate complex. Besides, an acceptance of the presence of rudists as a main indication of Urgonian can lead to a loss of precision for the term. A consequence of this is, for example, that the boundary with the Lusitanian (Choffat, 1885) is unrecognizable, with the result that a mass of rock more than 1000m thick in western Turkrnenia, and ranging in age from Oxfordian to Albian, is called by one name. In such a case 'Urgonian facies' or 'Urgonian biosedimentary system' is applied to any fossiliferous carbonates in which there are the rudists (i.e., encompassing the Upper Jurassic-Maastrichtian). In order to impose stratigraphic limits on the use of 'Urgonian' it is necessary to choose palaeontological criteria for its ~cognition which are present everywhere, or almost everywhere, and can be recognized easily. The distribution of rudists in Lower Cretaceous biogenic carbonate complexes is concentrated in Central Asia as lenses comparatively limited in area and situated in the rear slopes of reef masses. The coral or algal bioherms which are nttjacent to them on one side and the oolitic limestones of trans-reef lagoons on the other are more variable in size than the rudistid buildups. But the coral and algal bioherms, in common with the 0195-6671190/030253 + 08 $03.00/0
© 1990 Academic Press Limited
254
V.A. Prosorovsky
rudistid masses, occur only sporadically in the Urgonian, with the result that they cannot always determine its presence. The main criterion for the establishment of Urgonian facies is the presence of orbitolinids which are found everywhere. They are easy to recognize and, hence, do not need to be very precisely analysed before they can be identified. Acceptance of this palaeontological criterion corresponds quite closely to the definition of 'Urgonian facies' by Gignoux (1952, p. 368, translation) as follows: " . . . zoogenic limestones,.., consisting of accumulation of more or less redeposited fragments of foraminifers (miliolinas, orbitolinas), corals, hydromedusae, bryozoans, rudistids, nerineas, etc.". There is a similar definition in the Lexique Stratigraphique (1957). When typical Urgonian limestones are considered to contain orbitolinids, the lower limit of their development can be restricted to the Barremian. As for the upper boundary, it is generally not higher than Aptian in the Mediterranean, and hence is in agreement with the position adopted by the Lyon Colloquium (1963). Only in central Afghanistan does this facies perhaps also include the Albian, but the dating of the rocks there is not very reliable at present. Representatives of other groups of the Urgonian biota (e.g., echinoids and brachiopods) are rather limited in diversity (Chernov et al., 1980), long-ranging, and unlike the orbitolinids are not ubiquitous. They reflect particular environmental conditions, appearing in Central Asia from the base of the Hauterivian. Ih Central Asia the Urgonian facies is extremely widespread (Figure 1) and outcrop localities on the western plunge of the Maliy Balkhan ridge may be regarded as standards. They closely correspond to its classic determination according to the variety of oryctocoenoses present. They are also very widely exposed in the Kopetdag and its spurs so that it is possible to analyse facies and the ecology and taphonomy of fossils in detail (Kuzmichova & Prosorovsky, 1968; Prosorovsky & II'in, 1972; Verba & Prosorovsky, 1972; Bugrova et al., 1987). The Cretaceous of the southwest of Soviet Central Asia is represented mainly by marine deposits. These are subdivided into three formational associations which are successively (upwards): Neocomian carbonates (Berriasian-Barremian); middle Cretaceous sandy-argillaceous and glauconite-bearing (Barremian-Turonian); Senonian white chalk (Coniacian-Maastrichtian). Lagoonal and continental redbeds play a major part in the northern Kara-Kum and Amu-Daryan syneclise. The Neocomian formations transgressively overlap older deposits including those of the Palaeozoic (Figure 2). They are represented mainly by carbonate rocks in the Kopetdag and Maliy and Bolshoy Balkhan; there are cyclic sandy-argillaceous rock masses among them. Northwards and eastwards, marine deposits first become lagoonal (Cubadag, southern Kara-Kum) and then continental red beds (Tuarkyr, Mangyshlak). Zoogenic rocks, most of which contain an abundance of reef-building organisms, first appear in the Neocomian formational association, beginning at least with the base of the Hauterivian (Bacharden Formation of the Kopetdag, Arlanian Formation of the Bolshoy Balkhan). In the upper Hauterivian-lower Barremian they are present on a more massive scal~'. They reach their maximum development in the upper part of this stratigraphic interval and are represented by orbitolinid (Urgonian) limestones practically everywhere in a very wide area encompassing south and central Turkmenia. There is replacement by other facies along the south periclinal part of the Tuarkyr structure, the north of the Central Kara-Kum Arch and the Amu-Daryan syneclise (Figure 3). Overall, the Urgonian facies in Central Asia is a complex system of different carbonate rocks representing open shelf
255
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deposition to the south of the Eurasian continent. Their thicknesses range from 350 m in the south to almost zero in the north and the various associations consist of orbitolinid, coral, coral-algal, rudistid and often bioherm limestones, oyster beds and oolitic, detrital, pelitomorph and other limestones• There are intercalations of clays, siltstones, sandstones and.rarely conglomerates, mainly of marine origin• On the other hand, the widespread fossils are closely similar, indeed practically identical, to assemblages in older deposits. Most species of brachiopods, bivalves, echinoids and other invertebrates (including many endemics) range throughout the upper part of the Hauterivian-lower BarremiaJa interval. In this sense the Central Asian material confirms Kotetischvili's (1986) conclusion thai a common biosedimentary system existed during the Hauterivian and Barremian. On the other hand,
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the orbitolinids, unknown earlier, alJpear in very large numbers within this system. These include Orbitolina delicata Hens., O. discoidea Gras, O./ent/cu/ar/s Blum., O. con/coform/s Mamont., D/ctyoconus arab/cus Hens., D. valnutensis Carsoy. Almost everywhere a specific assemblage of echinoids occurs within the upper part of orbitolind beds. This consists of Heteraster spp., Toxaster ricardeaui d'Orb., T. arg//aceus d'Orb., T. exitis Lor., Pygurus romani Lamb., P. g////eroni Des. Thus, in my opinion, despite the similarities, the composition of fossils of the Urgonian facies in Central Asia is clearly different from that of the underlying strata. This allows the facies to be readily recognized in both natural and artificial outcrops and in borehole cores. Some of the Urgonian changes are now determinable in Turkmenia./In the very southern-most outcrop zone in the Kopetdag (near the Iranian border) there are complex and large reef systems, perhaps even of barrier type. Reef cores, reef slope facies, inter-reef lagoons and the outer part of trans-reef lagoons can be recognized with confidence. They are formed mainly by scleractinian and algal colonies. The organization and zonation of colonial forms can be readily observed, and sometimes the distribution of other fossils in the reef system is also easy to see. Northwards in the Bolshoy Balkhan and occasionally in the Cubadag it is possible to observe trans-reef lagoonal facies with isolated patch-reefs formed by coral, algal and rudistid buildups, and also oyster and brachiopod banks. Further north, in the south of the Tuarkyr district, foreshore slopes of reef lagoons with coral meadows and brachiopod, gastropod and oyster banks are exposed and are transitional to continental deposits. It is difficult to establish the precise stratigraphic position of the Urgonian facies in Central Asia because of the absence of ammonites. The wide distribution of orbitolinids and representatives of the echinoid genus Heteraster (H. couloni Ag., H. cf. elegans Rouch., H. magnus Poretzc.) at its base confirm its placement within the Barremian, but it is not clear which part because typical Hauterivian species of benthonic organisms are found much lower. The Urgonian deposits are overlain by beds with the ammonites Imerites and Colchidites, which correspond to the upper half of the upper Barremian substage"(part of Imerites giraudi Zone of Georgia). Hence, the distribution of the-Urgonian facies in Turkmenia perhaps does not extend above the middle Barremian. The formation of the Urgonian carbonate deposits probably took place on the shallow shelf of large marginal seas of the nor,t,hern periphery of Tethys during a transgressive cycle which began in the early Hauterivian and ended in the middle-
258
V.A. Prosorovsky
late Barremian. The relief of the sea bottom during this period became more and more complicated from the north, where it was a submerged plain, to the south, where seamounts were fixed by organic structures. The maximum development of reefs took place in the regressive phase of the cycle (Verba & Prosorovsky, 1973). The transgression spread over the considerable extent of the Turanian platform and reached the basin of the Vachsh. The cessation of Urgonian sedimentation was connected at first with the abrupt uplift of a large part of Soviet Central Asia, evidently as a result of a late stage of renewed Kimmerian folding. A major transgression followed. It began as a result of rapid subsidence and led to the accumulation of deep sea deposits in a large zone in the south (Arvazian Formation and its analogues). Conditions suitable for the formation of orbitolinid limestones ceased to exist. The Turkmenian depositional basin was, however, only part of an enormous submerged area which included northern and central Iran (Tigran and DjampSchuran Formations, the Park Series and others) and southern Afghanistan (Alavi-Naini, 1972; Stampfli, 1978; Stocklin & Setudehia, 1972; Geologia Afghanistana, 1980). Urgonian facies here include not only the Barremian but also the Aptian, and perhaps even younger Cretaceous in Afghanistan. The systematic composition of Berriasian-Barremian biocoenoses in Central Asia is evidence that this region belonged to the Mediterranean palaeogeographic region. In-my opinion its provincial zonation changed during this period (Figure 4). The composition of the benthonic upper Hauterivian fauna which contains many endemic brachiopods, bivalves and echinoids (Psilothyris oglan!iensis Moiss., Cydothyris firusensis Lobatsch., C. kerisensis Moiss., Cyrtothyris pseudosella Lobatsch., Sellithyris plicata Kvah. & Lobatsch., Turkmenia balkhanensis Krimh., Neithea daghestanica Renng., Diplopodia vassilievskii Vial., Pseudodiplopodia transcaspia Vial.) points to the isolation of the Central Asiatic province with the Kopetdag and Bolshoy Balkhan districts (Figure 3) by the late Hauterivian, not by the Late Cretaceous (Bobkova & Luppov, 1969). It is interesting that there are many endemic Exo~Drra(E. turkmenica Lupp., E. balkhanica Pros., E. geokderensis Pros.), together with the benthonic fauna mentioned above, in the north-west (the Bolshoy Balkhan, Cubadag, Tuarkyr) which are absent in the Kopetdag, Iran and Afghanistan. The Urgonian biocoenoses in Central Asia are very similar to those of Western Europe, Transcaucasia, the Carpathians, North Iran and South Afghanistan (Marchenko, 1962; Prosorovsky et al., 1961; Bogdanova & Lobatschova, 1966;
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259
Urgonian facies of Central Asia
Chernov et al., 1980; Kotetischvili, 1986). As a result, it is necessary to refer, in my opinion, to an Alpine-Central Asiatic province in the Barremian instead of the Alpine-Caucasian one proposed by Kotetischvili (1982). It consolidated the shelf of the northern periphery of the Tethys except in submerged areas of the Crimea, North Caucasus and perhaps Georgia (where there are no orbitolinids). It is necessary to distinguish the Central Asiatic subprovince within this province where the importance of the endemics is even greater. There were also the Kopetdag and Bolshoy Balkhan domains, as in Hauterivian. The particular character of the biocoenosis of the latter during the Barremian was increased by the appearance of the large rock-forming Meandropsinidae-Balkhania balkhanensis Mamont. These were discovered last year along with the bivalve Turkmenia in the Urgonian limestones of eastern Azerbaidzhan. The biocoenosis of Soviet Central Asian seas altered at the end of the Barremian because of tectonic and climatic changes. As a result new middle Cretaceous formations accumulated (Figure 2). Sandy-argillaceous deposits of the late Barremian aquatic realm contain ammonite casts and shells, planktonic foraminifers, and rather specific assemblages of bivalves, brachiopods and echinoids which were unknown in this region in older deposits. Kotetischvili (1986) included the late Barremian Central Asiatic depositional area in the Alpine-Caucasian province (Alpine-Central Asiatic here). The seas of the southern parts of Western Europe, the Caucasus and the Georgian block in the Transcaucasus were also part of this province. The comparative poverty of the ammonite composition makes possible the recognition of an independent Turkmenian subprovince (Kotetischvili, 1986) (Figure 5) which can be subdivided into Bolshoy Balkhan and West and East Kopetdag domains according to the specific composition of the benthonic fauna. The former is characterized by containing plenty of morphologically varied large, thick-shelled bivalves (Gervillia, lsognomon, Litschkovitrigonia, Iotrigonia, Quadratotrigonia, Astarte, Sphaera). There are many endemics among them including Litschkovitrigonia ingushensis alta Pros., L. tuarkyrica Pros., lotrigonia abichi renngarteni Mordv., and Astarte karajmanica Pros. The biocoenosis of the second domain is rather similar but differs in being less varied and in Lopha sp. being more important. The East Kopetdag domain is characterized by the presence of only rare thin-shelled bivalves. The increase in depth of the sea from the northwest to the southwest was probably the main cause of this differentiation.
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260
V.A. Prosorovsky
References Alavi-Naini, M. 1972. Etude g6ologique de la region de Diam. GeologicalSurvey oflran, Report 23, 288 PP. Bobkova, N. N. & Luppov, N. P. 1969. Osobennosti Sredneasiatskoy posdnemelovoy paleozoogeographieheskoy provincii. In Stratigraphia verchnegopaleozoya i mesozoyayuznich biogeographicheskich provinciy, pp. 193-203 (Moskva). [In Russian.] Bogdanova, T. N. & Lobatschova, S. V. 1966. Fauna neocoma Kopetdaga. Leningrad, Trudy Vsesoyuznogo Geologicheskogolnstituta, 130, 140 pp. [In Russian.] Bugrova, I. Yu., Verba, Yu. L., Preobrazhensky, M. B. & Prosorovsky, V. A. 1987. Nizhnemelovoy rifovy komplex Borshogo Balkhana. In Geologicheskoestroenie Turkmenistana, 79-103 (Askhabad). [In Russian.] Chemov, V. G., Yanin, B. T. & Golovinova, M. A. et al. 1980. Urgomkie otlozhenia Sovetskich Karpat, 183 pp. Moskva. [In Russian.] Choffat, P. 1885. Ddscription de la faune jurassique du Portugal. Mollusques lamellibranches,pars 1,394 pp. (Paris). D'Orbigny, A. 1847. PaldontalogieFranf~aise. Terrains Crdtads, vol. 4, 668 pp. (Paris). "Geologia Afghanistana" 1980. Geologia i polesnye iskopaemye Afghanistana, 1. Geologia, 379 pp. (Moskva). [In Russian.] Gignoux, M. 1952. Stratigraphicheskaya geologia, 638 pp. (Moskva). [In Russian.] Kotetischvili, E. V. 1979. K biostratigraphii verchnebarremstdch otlozheniy Grusii. Soobschenia Akademii Nauk GrusinskoySSR, 95, (2), 389-392, [In Russian.] Kotetischvili, E. V. 1982. Zonalnaya stratigraphia nizhnemelovych otlozheniy Grusii i paleogeographia Kaukazskay chasti Sredizemnonurrskoy oblasti v rannem melu. Avtoreferat doktorskoy dissertacii, Tbilisi, 32 pp. [In Russian.] Kotefischvili, E. V. 1986. Zonalnaya stratigraphia nizhnemelovych otlozheniy Grasii i paleozoogeographia rannemelovych basseyruroSredizemnomorskoy oblasti. Tbilisi, 160 pp. [In Russian.] Kuzfnichova, E. L & Prosorovsk'y, V. A. 1968. Organogennie postroiki v barremskychotlozheniyach Zapadnoy Turkanenii. Referat doklada. Bulletin Moskovskogo ObschestvaIspytateley Prirodi; Otdelenie Geologii, 1968/1. [In Russian.] Lexique Stratigraphique International, 1957. 1. Europe, 4a, VI, France, Belgique, Pays-Bus, Luxembourg, Cretacl, Centre National de la Scientifique, Paris, 361-367. Marchenko, V. I. 1962. Stratigraphia, litologia i facii neocoma Kopetdaga. Leningrad; Trudy Vsesoyusnogo GeologicheskogoInstituta, 78, 326 pp. [In Russian.] Prosorovsky, V. A., Korotkov, V. A. & Mamontova, E. V. et al. 1961. Neokom Zapadnoy Turkmenii. Leningrad, Trudy VsesoyusnogoGeologicheskogoInstituta, 51, 233 pp. [In Russian.] Prosorovsky, V. A. & Win, V. D. 1972. Organogennye postroiki v nizhnem barreme Turkmenii. Trudy Vsesojusnogo Nauchno-issledovatelskogo Geologo-raz'oedochnogoNeftjyanogo Instituta, 122, 16-24. [In Russian]. Rat, P. 1965. Rapport sur les facies urgoniens. Colloque mr le Cr~tac2 inferieur (Lyon, 1963). M~moire BRGM 34, 157-159. Rat, P. & Pascal, A. 1979. De l'etageaux syst~mes biosedimentaires urgoniens. G(obios, M(moire Special 3, 385-399. Stampfli, G. M. 1978. Etude glologfque g~n(rale de l'Elburz oriental au S de Gonbad-e-Qabus lran N-E. Th/~se no. 1868, Universit6 de Gen/:ve, 192 pp. St6cklin, J. & Setudehia, A. O. 1972. Iran. Lexique Stratigraphique International, Asie. CNRS, VIII, lb, 376 pp. Verba, Yu. L. & Prosorovsky, V. A. 1972. Hoterivskye i barremskye otlozhenia Malogo Balchana. Vesmik Leningradskogo Universiteta 6, 51-63. [In Russian.] Verba, Yu. L. & Prosorovsky, V. A. 1973. O regressii v seredine barremskogo veka na territorii Zapadnoy Turkmenii. In Litologia i paleogeographia, 1, 75-86 (Leningradsky universitet). [In Russian.]