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Paleozoic Calcareous Algae from Southern Tien Shan, Uzbekistan, Central Asia

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Paleozoic Calcareous Algae from Southern Tien Shan, Uzbekistan, Central Asia GEOLOGICA BELGICA (2003) 6/3-4: 97-117 PALEOZOIC CALCAREOUS ALGAE FROM SOUTHERN TIEN SHAN, UZBEKISTAN, CENTRAL ASIA Rimma KHODJANYAZOVN & Bernard MAMET2 1. Institute ofGeology and Geophysics, UzbekAkademy ofS ciences, Khodjibaev St., 49, Tashkent, 700041, Uzbekistan. 2. Laboratoire de Geologie, Sciences de la Terre et de I' Environnement, CP 160102, Universite Lib re de Bruxelles, 50, av. FD.Roosevelt, Bruxelles, 8.1050. (4 figures, 5 plates) ABSTRACT. This paper is the first illustration of Late Ordovician- Early Silurian, Early Devonian and Middle Car­ boniferous- Early Permian calcareous marine algae from Uzbekistan. Chlorophyta are very prolific and diverse. They are associated with some cyanophytes and rhodophytes. Taxa have been previously recorded. The flora is typically Central Tethyan and compares easily with that of Central and Southern Urals. Most algae have a wide stratigraphic range, are facies sensitive, and thus have only local age significance. KEYWORDS: Paleozoic, calcareous algae, Central Asia. RESUME. Cet article constitue la premiere illustration d' Algues calcaires marines paleozo'lques de l'Ouzbekistan (Ordovici'en Superieur/Silurien lnferieur, Devonien Inferieur, Carbonitere Moyen/ Permien lnferieur). Les Chlorophytes sont abondantes et diversifiees. Elles sont associees a quelques Cyanophytes et Rhodophytes. Tous les taxa ont ete anterieurement decrits. La flore est typique de la Tethys Centrale et se compare facilement a celle de l'Oural Central et Meridional. La plupart des Algues ont une repartition etendue en age, sont sensibles a l' environnement, et n'ont done qu'une valeur stratigraphique locale. MOTS-CLEFS: Paleozo'lque, Algues calcaires, Asie Centrale. 1. Introduction sented by claystones, siltstones and sandstones (Saltovskaya, 1981). The microflora is poorly repre­ Marine paleozoic deposits are widely distributed in South­ sented. More favourable conditions were found in the ern Tien Shan (Uzbekistan, Kirgizstan). Well-exposed Late Ordovician, which consists in part of carbonate rocks (Balakin and Bensh, 1975) sections and abundant fossils deposited in shallow marine environment. The Upper permit a detailed stratigraphic zonation (fusulines, Ordovician crops out in the Zeravshan Range as a con­ brachiopods, corals, conodonts) (Fig.l ). In Paleozoic his­ tinuous 30 km sublatitudinally directed belt. They form tory, succeeded several periods when the Tien Shan re­ the Shachriomon Formation, the name of the pass where gion was covered by shallow warm water seas with wide­ these deposits are well-exposed with a continuous tran­ spread carbonate sedimentation. Calcareous algae were an sition into the Lower Silurian. The rich assemblages of important component and sometimes led to the construc­ brachiopods, tabulates, t1ilobites, graptolites allowed rec­ tion of build-ups. Surprisingly these microfossils have re­ ognition of the Caradocian and the Ashgilian (Fig.2A). mained one of the least studied groups in Central Asia. To The Caradocian is composed of claystones, siltstones with fill this gap we present here a sh01t review of the microflora intercalations of sandstones. The Ashgilian deposits are observed at different intervals of the Paleozoic. characterized by the appearence of volcanic-clastic rocks, tuffs, tuff sandstones, tuff gritstones with numerous chert fragments. Upwards, volcanic clastic deposits give way 2. Late Ordovician - Early Silurian to sandstones, sandy limestones and organic sandy and clayey limestones. The stage ends in diagenic dolomites. In many mountain ranges, Ordovician deposits are ex­ These carbonates rocks (about 50 m) have been given posed as separate isolated tectonic blocks. The Lower the local name of Archalig Beds. The total thickness of and Middle Ordovician is chiefly terrigenous and repre- the Ordovician at the Shachriomon Pass is 420 m. 98 RIMMA KHODJANYAZOYA & B ERNARD MAMET RUSSIA Bishkek • --­KIRGIZ~ · KAZAKHSTAN Bishkek • AFGANISTAN Figure 1. Locati on of the Karachatyr, Zeravshan and Nuratau Ridges ( I- Shachriomon Mts Pass, 2- Kitab, 3- Shakhtau, 4-Michin Syncline, 5- Kyskl ysay) V) >- :z V) ;>< :z V) 0 V) ::!; w 0 fol ::!; fol w (.!) ;::: 0 w ;::: 0 :z "...l (.!) m:Z 1- c{ w "...l (/) < ::.:: f- c{ < 0~ 1- 0 m wu 0 >- m ~ u f- ~ :I: m ::c >- (/) ~ r:o- ~ 1- V) 1- 0 - ...0 =1- :3 ... 1-= :3 z I / J I ) 4 97/103 <( I I z a: I I 4 97/98 :::!.; w I I er oS~~ :::J 6 I 49 7/97 ...J 0 "'"' I / z ii5 z <( ::5 i 5 4 97194 ...J ~ s H1 -M (j) 0 1-- r- :::J u - L / / s H1-A (j) =i 0 0 :r < er m E 4 98/15 s H2-A w ~ ~ 0 r- u... 5 "' z . - )<:: <( z 4 97/88 z :J E i? <( (3 z . .. 0 I 0 0 w - I :::E ..,.N CD ~ u (j) 0 er 0 I ) - <( 0: <( 2 4 97/220 > :r ' / ' I u 0 0 <> .. u J 0 <( 0 :r ., , (J) ,.. ' .. er - l 0 z <( z --~-i :z E ~ -- <( a: > 0 u :;:: 0:: M .) 4 96/50 0 I <( 0 --_-, (j) z - <( ~ .. CD <( u --- -:1 28 2A Figure 2. Strati graphi c secti ons: A. Zeravshan Ridge, Ordovician/Silurian (AR.-Archalig, MIN.- Minkuchar beds); B. Nuratau Ridge, Bashkiri an/Moscovan. PALEOZOIC CALCAREOUS ALGAE FROM SOUTHERN TIEN SHAN, UzBEKISTAN 99 The Archalig Beds contain abundant and very diverse ded carbonates and carbonate siliceous rocks accumu­ fauna and an algal microflora, that has rock-building im­ lated on shelf and continental slope. pOitance. Among them Dinwrphosiphon is the most abun­ dant (D. rectangulare Hoeg, D. magnum Gnilovskaya, The Madmon Formation (about 800 m) is largely made D. diadromum Gnilovskaya). Diverse vermiporellid oc­ up of massive and partly of layered, light and dark col­ cur (Vermiporella cf. V. inconstance Hoeg, V cf. V oured, aphanitic, crystalline limestones. Fossils show ir­ borealis Hoeg, V acerosa Gnilovskaya, Vaff regular distribution and occur usually in a form of scat­ .wesenbergensis Moskalenko) mixed with Rhabdoporella tered clusters or small layers. The massive facies contain pachyderma Rothpletz. Small Cyanobacteria nodules are branching, cabbage -shaped colonies of tabulate corals, formed by Girvanella problematica Nicholson et colonies of stromatoporoids and compounds of rugose Etheridge. Red algae are scarce with the exception of corals. This indicates a reef genesis for that part of the Furcatoporella coa/ita Gnilovskaya. formation. Layers of limestone breccia are observed in the lower part while in the upper part there are many Tremendous fossil abundance is found in the Lower stromatactoid structures. Llandovery deposits, named the Minkuchar Beds. This unit consists of organic limestones (about 20 m). The [n the lower part of this unit, a few detrital limestone extent of the Minkuchar Beds is about 17 km; along the layers yield rare algae among numerous stromatoporoids, strike they are replaced by dolomites. Upwards they are tabulate and rugose corals. Cyanobacteria are represented overlain by a thick member of dolomites, the South by rare Girvanella problematica N icholson et Etheridge. Sumsar Formation (Middle and Upper Llandovery). Nodular Codiaceans include Hedstroemia halimedoidea Rothpletz and Bevocastria conglobata Garwood. The Minkuchar and Archalig microflora are quite simi­ Dasycladaceans are restricted to lssinella devonica lar. Dimorphosiphons (D. magnum Gnilovskaya, D. Reitlinger, I. grandis Chuvashov in assosiation with rectangulare Hoeg) occur rather frequently, but they do stromatoporoids. not form mass accumulations as observed in the under­ lying beds. Numerous nodules with Girvanella The Khodzhakurgan Formation (about 800 m) conform­ problematica Nicholson et Etheridge are present in the ably overlies the Madmon limestones and consists of lay­ limestone mixed with frequent Vermiporella cf. Vfragilis ered, dark-coloured, silicified turbidites with shales and Stolley, V acerosa Gnilovskaya, Anticostiporella black chert partings. Abundant and diverse fauna repre­ vaurealensis Mamet et Roux and minute Hoeegonites sp. sented both pelagic and reworked benthic organisms. On the basis of paleontological data and changes in lithol­ Although the Upper Silurian in Central Asia contains ogy it is subdivided into Zinzilban, Norbonak, Dzhaus, shallow marine carbonate facies, any identifiable mate­ and Obisafit Beds. Reworked microflora is present in rial has not been recognized up to now. Zinzilban, Norbonak and Obisafit Beds of medium lay­ ered grey-coloured limestones. These limestones with rough bedding planes are non persistent along the strike 3. Early Devonian grading into small bioaccumulations ( l-3 m). Fossils are irregulary distributed in the form of clusters of The Devonian is widely distributed in Central Asia. Two brachiopods, corals, and crinoids. different types of succession are distinguished. The first is composed of volcanic-carbonates and it is typical of Microflora of the Zinzilban Beds are not very diverse. the Middle Tien Shan. The second, mainly of carbonate lssinella devonica Reitlinger, /. grandis Chuvashov, facies, is typical of the Southern Tien Shan. Material for lssinellina(?) sp. are numerous, while fragments of Devonian calcareous algae was collected in the Kitab Udoteaceans are less common. A few axial sections per­ National Geological Park situated in the western part of mit recognition of Pseudopaleoporella lummatonensis the Zeravshan Range (Fig.3A). Continuous sections of (Elliott) and Wagonella(?) sp. the Lower and Middle Devonian with abundant and di­ verse fauna crop out (Kim et al. 1978). Later this section The Norbonak Beds are notable for abundant and diverse was investigated in detail and chosen as a Lower Devonian algae. The most prolific microflora is observed in the
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