Cuaderno,s de Geología Ibérica 158N: 0378-102X ¡998. número 24, 277-291
Biostratigraphic characterization ofLower Jurassic deposits of Georgia by ammonites
Caracterización bioestratigráfica de los materiales del Jurásico Inferior de Georgia por medio de ammonites
M. ToPCHísnv¡L¡ Geological Institute of the Academy of Sciences, 380093, Tbilisi, Georgia
ABSTRACT
The Lower Jurassic deposits playing important role in the geological struc- ture of Georgia are considered here according to structural-facial zones. The characteristic of each zone is accompanied by description of the most typieal ant) paleontologically well-substantiated lithological-stratigraphic units. The pa- per also argues the problem of interrelation between Lower Jurassic ant) un- derlying Devonian-Triassic formations. On the ground of Ammonite associa- tions the presence of nearly alí LowerJurassic stages, several zones of standard scale ant) also regional stratigraphic subdivisions has been recognized.
Key words: LowerJurassic, Ammonite associations, Biostratigraphy, Cau- casus, Georgia.
RESUMEN
Los materiales del Jurásico Inferior de Georgia presentan una gran impor- tancia en el contexto de la estructura geológica de este país. En el presente tra- bajo se estudian estos materiales subdividiéndolos en zonas de carácter fa- cial-estructural. Los rasgos particulares de cada zona se acompañan de la descripción de las unidades litoestratigráficas más típicas y mejor justificadas
277 M. TopchishviIi Biostratigraplíic characterization ofLower Jurassic deposits por datos paleontológicos. Se discute asimismo la problemática de la interre- lación entre las unidades del Jurásico Inferior y las formaciones subyacentes del Devónico-Triásico. El análisis de las asociaciones de ammonites ha permitido caracterizar la presencia de la mayoría de los pisos del Jurásico Inferior; algu- nas de las zonas de la escala estratigráfica estándar, y algunas subdivisiones es- tratigráficas regionales de carácter más local.
Palabras clave: Jurásico Inferior, Asociaciones de ammonites, Bioestrati- grafía, Cáucaso, Georgia.
INTRODUCTION
The Lower Jurassic deposits are developed irregularly on the territory of Georgia. Their outcrops are rather widespread within the Pold system of the Greater Caucasus - in the Main Range. Kazbegi-Lagodekhi, Chkhalta-Laila and Gagra-Djava zones (Gamkrelidze, 1984) (Fig. 1). They are somewhat less spread within the Transcaucasian Intermontane area - Georgian Block (Dziru- la massif) and within the PoId system of the Lesser Caucasus - in the Artvin- Bolnisi (Khrami massif) and Locki-Karabakh (Locki massif) structural-facial zones. These deposits are represented by various facies with ah the intermediate varieties - from deep-water marine to coastal ant) continental. Magmatic activity has been proceeded in both effusive ant) intrusive forms. The Lower Jurassic deposits contain a wide range of valuable minerals, both ore ant) non-metallic that have important practical application. Conse- quently, it is quite clear the great interest revealed by many investigators to- wards these deposits the stratigraphic study of which has more than a centena- rian history in Georgia. The investigations of the first period had episodical character ant) actually were very general by nature. Duc to the extremely rare finds of fossils the sub- division of the Lower Jurassic sediments was based mainly en lithological fe- atures and stratigraphic succession. The beginning of a systematic study of the Lower Jurassic stratieranhv in Georgia was based by works of Gamkrelidze (1933, 1940), Djane-Lidze( 1946), Kakhadze (1947), Nutsubidze (1972) and others. The author of the present paper has led, for many years, systematic works on detail stratigraphic diffe- rentiation of Lower Jurassic deposits based en monographic studies of Am- monite assemblages. New finds of Ammonites allowed us to define more exactly the stratigraphy of these deposits and to establish, for the first time, their belonging to certain stratigraphic leveN of the standard scale
This seale was compiled with due regard for ¡he results of ¡he International eolloquiums in Lu- xemburg (¡962, ¡976), Moscow (l967), Budapest<1969), Stutgart (1977). etc. (see Decision 1978 and Tbe Jurassic..., 1982). cuaderuos ele Geología Ibérica ¡998. número 24, 277-291 278 M. Tupchishvili Biostratigraphic characterization ofLower Jurassic depusits
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a.”0a’4 o.a .-— ~ -~ ~ a zO5~E~~ ~ ~ n.a >2 ~2 -~ ~ .1 ‘4 ~ ~ .2 —, o ~ o — ‘4 ~.. o .E‘-o—~ -e a-a N ‘5 ~ ‘5 - Cosa -e ‘4 a - ‘4 ~-‘—‘a 2 ~ >, a O— o a o o ~>t o E t ~ ,E .g ~ EN O -~ ‘o E 0 0 Cuacte-rnoc de Geología Ibérica 279 ¡998, núnien, 24. 277-291 M. Topchishvili Biostratig,-aphic characterization of Lower ,Jurassic deposits SINEMURIAN DEPOSITS The most ancient sediments containing ammonite remains are of Sinemu- rian age (Fig.2). They are widespread within the Southern slope of the Greater Caucasus. Here, in the north-western pan, in the territory of Abkhazia Sine- murian deposits make up the Chkhalta-Laila zone. Here, they transgressively overlie the Paleozoic basement. By their granulometric composition Sinemurian deposits can be divided into two parts. The lower one is represented by con- glomerates, gravelites, coarse-grained sandstones and sandy-aleurolitous shales 50-150 m thick. The upper part is mainly made up of the interchange of shales, sandstones ant) aleurolites with rare intercalations of gravelites ant) conglome- rates with total thickness up to 450 m. Volcanogenic rocks of this sequence form the flows of quartzkeratophyres and, less frequently, thin layers of their tuffs. In shales ant) sandstones have been found numerous Bivalve ant) some ammonites-Verrnieeras aif scyl/a (Reyn.), Epideroceras cf. steinmanni (Haug) and Oxynoticeras sp. (Bukia, Kolossovskaya & Abamelik, 1971). The finds of these ammonites give evidence of the presence of both lower ant) upper subs- tages of the Sinemurian stage. To the south, in the Gagra-Ojava zone, Sine- murian deposits are exposed in the core of the large Chedim anticlinal foId. They consist of fine- and coarse-grained budinaged sandstones (500 m) with in- tercalations ant) lenses of small-pebbled conglomerates. Locally, the sandstones interchange with argillites which towards the top replace them totally. Within the argillaceous part of the section (200 m) there appear layers of marís and Ii- mestones wirh rich remains of Ammonites: Jnraphy!!ites ste!la (Sow.), Rads- tc¿-kice,-as cf. buvignieri (d’Orb.), ¡‘altechioceras boehmi (Hug), ¡‘. elicitun2 Buck., ¡‘. noN/e Truem. & Will., P. rothpletzi (Bose), Epideroceras loí’io/i (Hug), E. steinmanni (Hug), E. tehedimicuin Topch. (Topchishvili, 1982). In the basin of the river of Eastern Gumista analogous sediments contain .Iuraphylhtes stella (Sow.), EChiocel-as í-aricostattíni (Ziet.), Leptechioceras rneigeni (Hug), L. nodo//anam (d’Orb.). AII these forms, except the first two ones, are characteristic of the Upper Sinemurian-Raricostatum Zone. However, relatively large thickness of the described rocks (700 m) gives us the ground to include the Lower Sinemurian substage in this sequence as well. Fío. 2—Correlation of generalized slratigraphic columns of LowerJurassic deposits of Georgia.l-sha- les; 2-fine-grained sandstone and argillites: 3-quartz-arkosic coarse-grained sandstones: 4-conglomerates and graveistones; 5-marís; 6-limestones: 7-lenses of lirnestone; 8-lenses of congloínera¡e: 9-II -volcaiiic formations: 9—basic eornposi¡ion, 1 O—interrned ate composition, 1 1 —ac dic cnmpnsi¡ion; 1 2—Paleuznie gra— nitoids: 1 3—levels of ainmonite finds. Fío. 2.—Currelaeión de las columnas estratigráficas generales de los materiales del Jurásien Inferiur de Georgia. 1: Pizanas: 2: Limolitas y areniscas de grano fino; 3: Areniscas cuarcíferas y arcósicas dc gra- no grueso; 4: Conglomerados y gravas; 5: Margas; 6: Calizas en bancos: 7: Calizas en capas lenticula- íes; 8: Conglomerados en tramos lent¡culaíes; 9—II: Formaciones volcánicas: 9: 1-ocas de composición básica: ¡0: rocas de composición inlermnedia; II: rocas de composición ácida; ¡2: Granitoides del Pa- leozoico; ¡3. Niveles con registro de a¡nmonites. C?uaelcroox dc Geología Ibérica 1998. númcru 24, 277-291 280 M. Tupchishvili Biostratigraphic chatacierization of Lower .Jurassic deposits Co E tu o-. tE a, jo,Co Co E o ‘EN! zo o 0<0 di, “Co 20 <¾. III’ o- 0188000 Cuadernos de Geología tbé‘lea 281 199X. número 24. 277-291 -I M. Topchishvili Biostratigraphic <-haracterization ofLower Jurassic depusits Eastwards Sinernurian deposits outerop in Svaneti where they take part iii the composition of both Gagra-Djava ant) Chkhalta-Laila zones. Here they form the limbs of two large en echelon anticlinal structures in the cores of which the pre-Jurassic formations of Dizi series are exposed. Sinemurian de- posits (400 m thick) are represented by conglomerates, gravelites, coarse-grai- ned quartz sandstones and shales interchanged with thin-grained sandstones and aleurolites.The presence of lenses of limestones, pyroclasties of quartz albi- tophyres, charred remnants of trunks, stems and leaves is characteristic of the- se rocks. With the lower part of th¡s sequence are linked the finds of Early Si- nemurian Arietites sp. (aff. bisuk-atus Brug.), Megarietites sp., ¡‘rimarietites sp., Coroniceras sp., Vermiceras sp. aff.spiratissimum (Quenst.) (Síavin, So- miii & Morgunov, 1962), Vermiceras spiratissimum (Quenst.), V.scyfla (Reyn.), y. spiratissimum latesella Erb. (Chikhradze, 1967). The upper part contains Late Sinemurian ¡‘altechioceras sp. (Topchishvili, 1987). The distribution of our paleontological material through the section alío- wed, to a certain extent, to detail the relationships between Lower Jurassic ant) underlying Devonian-Triassic rocks of the Dizi sedes. This problem is known to have given rise to very controversial opinions among investigators. Some of them (Kakhadze, 1947; Gamkrelidze, 1957; Adamia, 1968 and others) suppo- sed that in certain places the basal layers of the Lower Jurassic he transgressi- vely on different horizons of the Dizi series, but locally the latter quite confor- mably, without any gap, pass into LoweT iurassic. The woi-ks by Siavin, Somin & Morgunov (1962), Belov & Somin (1964) showed, on the basis of ah avai- lable ¡naterials, that in late Triassic or Early Jurassic Svaneti underwent abso- lute uphift ant) Jurassic formatiotis everywhere occur transgressively upon the different horizons of the Devonian-Triassic. The existence of the hiatus between the Devonian-Triassic Dizi series ant) Lower Jurassic has been confirmed by our faunistical finds (Topchishvili, 1987). such as Arnioceras geornetí-u-oides Erb., A. cf. miseíabile (Quenst.), dis- covered ir> the vicinity of the ridge of Bakild at 60 ir> up frorn the sole of Sine- murian deposits. These fossils represent guide forms of the middle pan of the Lower Sinemurian - the zone of Semicostatum. Underlying beds of 60 m thick correspond, most likely, by their stratigraphic position to the lowerrnost zone of the Sinemurian stage - (he Bueklandi Zone. Apparently, there is no ground here to assume the presence of the Hettangian stage in the base of the Jurassic sec- tion. The same situation is observed in the valley of the Inguri river where over tbe deposits of the Dizi series containing Late Triassic spores ant) pohlen occur layers with Sinemurian Bivalves. Consequently, even when Lower Jurassic im- mediately lies on the Upper Triassic, the absence, probably everywhere, of the Hettangian stage evidences of the stratigraphic gap between them. To the north-east, in the Kakhetian pan of the Kazbegi-Lagodekhi zone, at the base of the Lower Jurassic section there is a sequence (700 m) composed of thin- to coarse-grained sandstones, gravelstones, conglomerates interchanging with shales ant) andesite-dacitic ant) albitophyric pyroclastics. Lenses of Ii- c nadernor de Geología Ibérica ¡998, número 24, 277-291 282 M. Tup.hishvili 8iostratigraphic characterization ofLuwer Jurassic depusits mestones are also encountered in which Arietites bisulcatus (Brug.), Coron¡- ceras sp. md., l/erm¡Ceras aff sp¡ratíssímum (Quenst.), Arnioceras sp., Euas- teroceras sp. ant) Paltechioceras cf. elle ¡tum Buck. are found. These ammoní- tes allowing to admit the Sinemurian age of containing rocks were collected from the upperrnost part (200 m) of the sequence. Its lower part must also co- rrespond to Sinemurian ant), possibly, the top of Hettangian stage. On the Georgian block Sinemurian deposits extend along the south-western periphery of the Dzirula massif where they transgressively overlap the volcano- continental sequence of the so-called «lower tuffites» (from a few m up to 700 m thick) ant) crystalline rocks of the Paleozoic basement. The question of the age of the «lower tuffites» remains still problematie. We confine their age by the limits of the Hettangian stage. This assumption is confirmed, on one hand, by the stratigraphic position of the suite (it underlies the deposits with Early Jurassic Ammonites) ant) on the other hand, by finds in its basal layers the remnants of fossil flora whose lower age range does not ex- ceed Jurassic limits (Svanidze, 1971). The greater part of the Sinemurian here is represented by fine-grained mi- caceous sandstones (up to 100 m) with flora remnants which almost everyw- here are underlain by quartz-arkosic coarse-grained sandstones (25 m) with ten- ses of middle-pebbled conglomerates in the base. The micaceous sandstones contain Arnioceras ceratitoides mcxicanum Erb., Verm ¡ceras sp. and Miero- deroceras sp. Rich complex of ammonite remains substantiates the Sinemurian age of de- posits in the Locki-Karabakh zone where they, in the ibrm of a narrow nearly uninterrupted band, endose the Locki massif. Here, the Sinemurian is repre- sented by a rather uniform sequence (up to 3000 m) of fine-grained micaceous sandstones overlying conformably Hettangian coarse-grained quartz sandstones with conglomerates in the base. One of the sections of Sinemurian deposits which is most thoroughly fau- nistically characterized, is located in the basin of the river of Gulmagometchai (western peripbery of (he rnassif). Ibis section is composed of: 1. dark-grey fine-grained micaceous sandstones and sandy aleurolites with lenses of ferru- ginous calcareous rocks and numerous remains of flora detritus (160 m). In sandstones were found ammonites: Verm ¡ceras ultraspiratum Fuc., V. spíra- tissimum (Quenst.), V.franeisci Fue., y. solarioldes Cos., V. cf. scylla (Reyn.), Metophioceras cord¡er¡ (Can.), Arnioceras kr¡d¡o¡des (Hyatt) and others (Top- chishvili, 1989); 2. grey fine-grained sandstones and aleurolites (60 m) with Ar- nioceras ceratitoides (Quenst.), A. harpoides Erb., A. italicum Fue. ( Top- chishvili, 1989). These ammonites indicate here the presence of two Sinemurian zones- Bucklandi ant) Semicostatum. In the uppennost layers of sandstones the presence of Mk-roderoceras cf. b¡rchi (Sow.) was determined (Panov, 1978). In the central part of the northern periphery of the Locki massif,at the vici- nity of Ortaposta, the lower layers composed of micaceous-feldspar-quartz 283 Cuadernos¡998, númcrode Geología24, 277-29tIbérica M. Topchishvili B¿ost,-atigraphic characterization of Lower .Iu,assic depusits sandstones contain the following Early Sinemurian forms: Epammonites cf. la- t¡sulcata (Quenst.), Vermiceras aff. suprasp¡ratum (Wahn.) and Metophioceras cord¡e¡-i b¡furata Nout. More upper horizons contain Microderoceras cf. bir- chi (Sow.), M. sp. (ex gr. aff. bin-h¡ Sow.) ant) Late Sinemurian Oxynot¡ceras cf. doris (Reyn.). Here the presence of the uppermost part of the Sinemurian stage is confirmed by finds of Echioceras rarkostatum (Ziet.) aud Epideroce- ras steinmanni (Hug) (Zesashvili, 1967). Irt the westem periphery of (he massif, along the river of Moshevani and in the lower reaches of its right tributary at the different levels of the 120 m-thick sequence of micaceous sandstones were found early Sinemurian Vermié-eras cf sp¡rat¡ssimum (Quenst.), Microdenx-e¡-as cf. biíc-h¡ (Sow.) and late Sinemurian ¡‘altechioceras el¡c¡tum Buck., P. dignatum Truem. & Will., ¡‘. í-othpletz¡ (Bose), Epideruceras ste¡nmann¡ (Hug) and E. lorioli (Hug) (Topchishvili, 1989). The last ammonite association corresponds to the uppermost Sinemu- rian-Raricostatum Zone. PLIENSEACHIAN DEPOSITS Pliensbachian deposits are spread more widely (han underlying ones ant), as a rule, they are not rich in ammonites that, in most cases, prevents their division into substages. This is especially true of the FoId system of the Greater Cau- casus. The only exception is the Abkhazian pan of the Gagra-Djava zone. Here, te lower substage (400 m) consists of argillites, usually calcareous, with interlayers of the limestones and marís. Considerable role in its formation play volcanogenic rocks of spilite-quartz-keratophyric composition and ac- companying silís. The lower part of this sequence contains Early Pliensbachian ¡‘latypleu¡-oceras va-¡scoi Par., Uptonia angustata (Quenst.), Tropidoceras mas.s-eanum (d’Orb.) (Topchishvili, 1982) ant) in the upper part was found Arieticeras cf. algovianum (Opp.) indicating the late Pliensbachian age of host rocks. The lower Ammonite complex is included by us in the regional Masse- anum Zone (Table 1). Overlying rocks (500-550 m thick) are mainly represented by argillites, usually calcareous, with interlayers of aleurolites ant) thin-grained sandsto- nes. Considerable part of these deposits contains Amaltheus ma¡-garitatus (Montf.) ant) A. stokesi (Sow.). Sornewhat northwards, in the eastern part of the Chkhalta-Laila zone, the Pliensbachian stage contains, together with terrigenous rocks, common limes- tone lenses, sometimes of rather considerable dimensions. The length of some of them exceeds 1km. Here, only Upper Pliensbakhian is characterized by ammonite fossils. The presence of this substage is confinned by finds in shales of Late Pliensbachian ammonites: ¡‘hylloceras hebe¡-hnurn (Reyn.), P. zetes (Monest.), Amaltheus margaritatus Montf. Westwards, in the upper reaches of thc Atsgara river and in the basin of ¡te Cucele-o,os de Geolcgía ibérica ¡998. número 24. 277-291 284 M. Topchishvili Biostratigraphic chaíacterization cfLower Ju,assic deposits TABLE 1 .—Ammonite zonal subdivision of the Lower Jurassic deposits of Ge- orgia. TABLA 1 -—Subdivisión zonal de ammonoideos en los materiales del Jurásico Inferior de Georgia. General stratigraphic seale (Dedsion..., 1978) — REGIONAL BIOSTRATIGRAPHIC UNITS AND eE -—~ TYPYCAL AMMONITE COMPLEXES ZONE o,m DumorU&ie Dumorteria Domonlefla pbeodoCodlosa (•ranj, O-mann (S.eb). Qbtaqcoi levesquel Ievesqu.¡ Ben,. fl.rloua•ttek. D.bMtchefl Ben,, bsnoomi (LIC,) etc. ao, Gramn,oc,ras Gran,mooo,res &anocans thoo-’saense (dOtb.), «strMtwn(Sow.). G.ctqua&s o. 2, thouarsense tl,ouarsense tum (8ng). Psoudor.q,moceres tuffactsuoo (Bay¡•) etc 0 Heugla Peroncoeras Wacoe~ce~n ,~uhoWqn (d’Orbj. p~,m,oe..ss sutannatwn o vsnabI)is (Y.t Si. P.5b.daú.,,(So~.j. tfi~cernbI*a,s(Bwugj, fl.d,sablev~ o(o Híldocoras subamiamun, n$Fo,o.. Ph>qnatoce’ssd-na’tonens.Booek., PO*flnn 1~- bifrons (Naos,). Psnense s’Oóts¡Asaz). I4au~act kflb{4Orb,) etc. Harpoceras Haq>ocns laidier- Hsroocns Bzibi river the amount of calcareous material considerably decreases ant) the Pliensbachian stage consists mainly of shales (600 m) with Amaltheus marga- ritatus Montf., Paltarpites argutus Buek. in the upper pan of the sequence. Its lower part is quite conventionally attributed to the Lower Pliensbachian. The described terrigene-carbonate deposits conformably pass into pyro- clastics of andesite-basaltic composition with thin layers of shales of total 285 Cuadernos de Geología Ibérica 1998, número 24, 277-29 1 M. Topchishvili Biostratigraphk.- Cuadernos ~íeGeellogía Ibérica 1998, número 24. 277-291 286 M. Topchishvili fiiusti-atigraphic characterization ofLuwer Jurassic deposits In the north-eastem periphery of the Dzirula massif the limestones are re- placed by argillites ant) marly clays (up to 125 m) withAma/theus margar¡tatus Montf. Pliensbachian deposits of the Locki massif can be subdivit)ed into two parts: the lower part - micaceous fine-grained sandstones (90 m) with Trop¡- doceras cf. masseanum (t)’Orb.), and (he upper one - argillites intercalated with sandstones ant) aleurolites with relies of Amaltheus margaritatus Montf. TOARCIAN DEPOSJTS Oeposits of this stage are, as a míe, more abundant in fossil remains than those of Pliensbachian. In the FoId system zone of the Greater Caucasus these sediments are characterized by a considerable increase of sandstone layers. In the north-eastern part of the Greater Caucasus, in the Fore Range zone (the territory of Tusheti) the Toarcian stage is subdivided into two substages. The lower substage (700 m) consists of aleurolitie shales with sandstone inter- calations and various concretions of clayed siderite. From the ammonite fauna collected here we determined Catacoeloceras sp., Dactylioceras sp., Harpo- ceras fa/cifer (Sow.), U. mulgrav¡um (Y. & B.), 1-lildaites serpentinus (Rein.), Orthilda¡tes orthus Buck. and ¡-lildoceras b¡frons Brug. The upper Toarcian (900 m) is composed of fine-, middle- and coarse-grained sandstones alterna- ting with shales which locally contain small lenses of limestones and Ammonite associations: Haug¡a cf. variabills (d’Orb.), Harpoceras cf. subplanatum (Opp.), Po/yp/ectus discoides (Ziet.), Grammoceras thouarsense (d’Orb.), G. str¡atulum (Sow.), O. penestriatulum Buck., O. cf. quadratum (Haug), O. sub- quadratum Buck., Pseudogrammoceras cf.jhl/ac¡osum (Bayle), P. cotteswoldia Buck., ¡‘. thusheth¡cum Topch. (Topchishvili, 1990), ¡‘Ieydellia cf. aalensis (Ziet.) and Hammatoceras cf ¡ns¡gne (Schub.). The similar composition Toarcian deposits have farther west, in Khevsureti. Here they are made up of an irregular alternation of fine-grained sandstones ant) shales with clayed sideritic concretions. The age has been established by finds of Dactylloceras sp., Dernkmannia sp., Haug¡a sp., Orammoceras thouarsen- se (d’Orb.), Dumorfier¡a cf. ble¡cher¡ Ben., D. gundershofens¡s (Haug), O. branco¡ Ben., D. radiosa (Seeb.), D. pseudoradiosa Bran., O. cf. tahulata Buck., D. tnoorei (Lyc.), O. mata-a (Dum.) and ¡‘leydellla subcompta (Bran.). On the Southern slope, in the eastern part of the Kazbegi-Lagodekhi zone (Mountainous Kacheti) Toarcian is mainly represented by thick - bedded, in places massive, sandstones with interlayers ant) beds of shales. The sandstones are locally calcareous with faintly expressed flysch figures. Peronoceras sp. and Catulloceras sp. found respectively at the base and in the top of a 600 m thick sequence give possibility of assuming here both the lower and upper Toarcian. To the north-west, in the area between the rivers of Didkhevi ant) Ion, the role of shales gradually increases ant) here they prevail over sandsto- Cuadernos de Geología Ibérica 287 1998, número 24. 277-291 M, Tupchishvili Biostratigraphic cha,ac.terization ofLower ,Iurassic deposit.s nes. At the same time, the thickness of the sequence also increases reaching here 800 m. Onty late Toarcian ammonites have been stated in these rocks: Grammoceras thouarsense (d’Orb.), ¡‘seudogrammoce¡-as cf.saemanní (Dum.), Dumortieria b/eicheri Ben., D. gundershofens¡s (Haug), D. maurel (Lyc.), D. subundulata (Bran.), D. tabulata Buck., D. exigua Buck., D. cf. costula (Rein.), ¡‘leydell¡a subcompta (Bran.) ant) E. ermita (Buck.). Early Toarcian Ammonites appear in the western part of this zone, in Nort- hern Svaneti. These are Maconiceras sp., Hildoceras sp. ant) H. cf? sublev¡son¡ Fuc. found by us in the 600 m thick sequence of sliales ant) sandstones. The up- permost pan of this sequence (1100 m) is characterized by increased amount of sandstones, locally calcareous. Almost the whole section contains fossil rem- nants most of which are represented by guide forms undoubtedly indicating the presence of the upper Toarcian-Thouarsense ant) Levesquei Zones. To confirm this assumption the following list of fossils is given below: Gramn2oceí-as cf. striatulum (Sow.), ¡‘seudogrammoceras cf. saemanni (Dum.), P. subregale Fin., P.fallaciosum (Bayle), P. cf. mue/Ieri (Denck.) (Topchishvili & Lobja- nidze, 1977), PIeydellia aalensis (Ziet.), Dumortiejia exigua Buck., D. mactra (Dum.) ant) D. bleicheri Ben. In the Chkhalta-Laila zone in Abkhazia the Toarcian stage includes an outcrop of shales showing a thin ant) rather consistent sequence (400 m) of striated sandy-aleurolitic shales with interlayers of aleurolites ant) fine-grained sandstones. The amount of the latter is very changeable - sometimes they al- most disappear, at other times they altemate with shales. This sequence also contains interlayers of tuffs ant) tuffites of spilitic ant) quartz-keratophyric composition. Within the shales has been found Haí-poceras cf.falcífer (Sow.) and the striated varieties contain Calliphylloceí-as cf. avevronense (Men.), C. ex. gr. supraliasicum (Fomp.), Harpoceras subplanatum (Opp.), Grammoceras cf. thoua¡-sense (d’Orb.) and Denckmannia sp. In the southernmost pad of the Southern slope of the Greater Caucasus To- arcian deposits are developed in the Gagra-Djava zone. Lithologically they are composed of an altemation of sandstones ant) argillites with predominance of the former. Argillites usually contain remnants of charred detritus. The thick- ness of sandstone beds ranges from a few cm to tens of m. The surfiices of bed- ding are characterized by the presence of flysch figures. Sandstones in places turn into microconglomerates ant) breccia beds composed of unrounded frag- ments of shales. It is a rather thick (800-900 m) flyschoid sequence which at its different levels contains Harpoceras falcifer (50w.), 1-lildaites aff.serpentinus (Rein.), G-ammoceras penestriatum Buck., G. cf. thouarsense (d’Orb.), Du- mortíeria mclCtra (Dum.), ¡‘/eydellia lotharingh.a (Bran.) ant) P. crinita (Buck.). The Toarcian stage in the Ozirula massif is identified within (he 30 m thick ~equenceof red organogenit- limestoncs~ Its- presence is- substantiated -by finds of the following ammonites: I-Iaípoceí-asft¡lc~fer (Sow.), Cata.-oeloeeras raquinianuin (d’Orb.), Peronoceras subarmatum (Y.& E.), Hildoceras bíjtons (Brug.), Phvmatoceras comense evoluta (Renz.), ¡‘raehaploceras zwieselei Cu Monest., Grammoceras thouarsense (d’Orb.), G. quadratum (Haug), ¡‘seudo- grarnmoceras saemanni (Dum.), ¡‘/eydel/ia aatensis (Ziet.), Dumortieria le- vesquel (d’Orb.), D. gunders/wfensis Buck., Hammatoceras speciosum Jan. Somewhat poorer Toarcian deposits are charaeterized by fossils in the north-eastem periphery of the Dzirula massif. Here, in the argillitie strata (90 m) were fount) only Grammoceras thouarsense (d’Orb.) ant) ¡‘seudolioceras cf. lythense (Y.et B.). In the Locki massif Toarcian embraces a part of the flyschoid sequence with Peronoeerasfibulatum (Sow.), Harpoceras cf.falcífer (Sow.), Grammo- ceras cf. penestriatum Buck., Hildoceras lateplicata Nout., ¡‘hymatoceras cf. narbonense (Buck.), ¡‘seudogrammoceras cf. fallaciosum (Bayle), Dumortieria str,atulocosta (Quenst.). CONCLUSIONS Ammonites in Georgia are spread nearly through the whole area of the dis- tribution of Lower Jurassic deposits. Considerable bulk of species come from Sinemurian, upper Pliensbachian and Toarcian deposits. In the lower Pliens- bachian (he amount of species notably decreases. In general, here ammonites are distingushed by great variety of composition. The presence of many ar- chistratigraphic genera and species among which index-ammonites are en- countered as well, allows to specify in Georgia some zones of Western Europe (Table 1) ant), in some cases, regional stratigraphic subdivisions. The oldest zone established within the Lower Jurassic is the lowermost Si- nemurian - Bucklandi Zone. It is specified by the rich complex of ammonites belonging to the following species: Arietites, Vermiceras, Metopliloceras ant) Epammonites. The strata with fossils of the Bucklandi Zone are overlain by deposits con- taining Arnioceras ceraritoides (Quenst.), A. harpoides Erb., A. italicum Fuc., etc., permitting to establish here the second standard zone of Sinemurian: - Se- micostatum Zone. The uppermost zone of the Lower Sinemurian - Turnen Zone can be co- rreLated with the strata containing Microderoceras cf. birchi (Sow.) distin- guished by the finds of Ammonites of the same name. Especially reliably is specified by ammonites the roof of Sinemurian which is represented by Raricostatum Zone. It is presented by species of the following genera: ¡‘altechioceras, Leptechioceras, Echioceras and Epideroceras. Upwards the Late Sinemurian ammonite association is replaced by another one which forms the regional Masseanum Zone identified by us. It is composed by Uptonia angustata (Quenst.), Tropidoceras masseanum (d’Orb.) ¡‘laty- pleuroceras variscol Par, ant) some other forms. Very numerous are finds of ammonites in the upper part of the Pliensba- chian stage. Especially frequent among them are Amaltheus margaritatus Cuadernos de Geología Ibérica 289 998, número 24, 277-291 M. Tupchish viIi Biustratigraphic characte,ization of Lower Ju,assic deposits Montf. ant) A. subnodosus (Y.& E.) which in most seetions are found along with the int)ex-speeies of the lower zone of upper Pliensbaehian - Amaltheus stokesi (Sow.). Therefore, it is only possible to single out the Subnodosus re- gional zone. Within the lower Toarcian substage, on the basis of individual finds of Harpoceras ant) 1-1ildoceras species, layers with Harpoceras falcifer- Hl/do- ceras serpenfinus have been singled out. The top of lower Toarcian ant) the base of upper Toarcian are considered as the Subarmatum regional zone which through its range corresponds to two adjacent Bifrons ant) Variabilis standard zones. Itere, within this zonal asso- ciation representatives of Catacoeloceras, Peronoceras, Hildoteras, ¡‘hyma- toceras and ¡‘raehaploceras genera are present. The overlying Thouarsense Zone is distinguished by the rich and diverse as- sociation of ammonites. Itcontains Polyplectus discoides (Ziet.), Grammoceras thouarsense (d’Orb.), G.quadratym (Haug), ¡‘seudogrammoceras ft¿llaciosum (Bayle), etc. Finally, the Levesquei Zone completing the Toarcian stage is substantiated by numerous representatives of the genus Dumortieria. REFERENCES ADAM¡A, SH. (1968): Pre-Jurassicfo¡-rnation ufIhe Caucasus. Metsniereba, Tbilisi. 233 pp. (in Russian). BELov, A. & SOMIN, M. (1964): «On the pre-Jurassic unconformity in Svaneti (Central Caucasus)». Rep. Aead. Sci, uf USSR, 159, Nl: 81-85 (in Russian). BuKIA, 5.; KGLOSSGvSKAYA, O. & ABAMELIK, E. (1971): Geolugical mapand map ofmine- ral deposits of Abkhazian ASSR 1 :50.000 scale. Explanatory note, Moscow. 337 pp. (in Russian). CHIKHRADZE, G. (1967): «New data on stratigraphy of Liassic in Svaneti». Boíl. Acad. Sci.ofGeorgian SSR, XVIII, N3: 627-630 (in Russian). GAMKRELIDZE, P. (1933): «Geological description of the Dzirula and Chkhcrimela river valleys». Bulí. Atad. Sci. ofCeorgian 551<, 1, N2: 107-138 (in Russian). GAMKRELtDZE, P. (1940): «Liassic fauna in the basic shales of Svaneti and Abkhazia». Rep. Georgian branch ofAtad. Sci. uf USSR, 1. N3: 20 1-202 (in Russian). GAMKREI.tDZE, P. (1957): «Main features of tectonic structure of Georgia». Ti-. Geol, Insí., Acad. Sci. of Geoígia, geol. ser., X(XV): 69-98 (in Russian). GAMKREL¡DzE, 1. (1984): «Tectonic structure and Alpine geodynamics of the Caucasus». In: Tectonics and rnetallogeny of the Caucasus. Metsniereba, : 105-184. Tbilisi (in Russian). DEC¡sloN OF THE INTERDEPARTAMENTAL STRATIGRAPH¡c COMMISSION AND ITS PERMANENT COMM¡rrEES (1978): 18, Leningrad, 111 pp. (in Russian). DJANELIOZE, A. (1946): «On ¡he age of red limestones of the Dzirula massif», Bulí. Acad. Sci. of Georgian 551<, VII, N4 :171-174 (in Russian). KAKHADZE, 1. (1947): Georgia in Jurassic time. Tr, Geol. Inst, AcM. Sci. ofGeorgian SSR, III(VIII): Tbilisi. 371 pp. (in Russian). cuadernos de Geología Ibérica ¡998. número 24, 277-291 290 M. Topchishvili 8iostratigraphic characterization of Lower Jurassic deposits KH¡MsmAsíívíLí, N. (1974): Liassic fóuna of eastern Georgia. Metsniereba, Tbilisi. 108 pp. (in Russian). KnuTs¡s¡lvlLl, 0. (1972): Straíigraphy andj&una ofIhe Kvishi suite. Metsniereba, Tbilisi. 81 pp. (in Russian). LOMIZE, M. (1969): Volcanism of the noríh-western Caucasus and its relation with te-ton¡cs. Moscow University, Moscow. 203 pp. (in Russian). NUTSUBIDZE, K. (1972): «Transcaucasia (Georgian SSR). Lower section and Aaleniau». In: Straiigraphy ofthe USSR. Jurassic system. Nedra: 175-183. Moscow (in Russian). PÁNOv, D. (1978): «Regional stratigraphic differentiation of Lower Jurassic and Aatenian de- posits of the Lesser Caucasus». In: Problems of Stratigraphy and historic geology. Moscow University: 153-164. Moscow (in Russian). SvANIDZE, TS. (1971): «Onte age of flora-bering deposits of Lower Jurassic of the Dzirula aud Locki crystalline massifs>~. Tbilisi University, A2(141) :165-169. Tbilisi (in Rus- sían). SLÁvIN, V.; SGM¡N, NI. & MORGUNOV, J. (1962): Pre-Jurassic deposits ofSvaneti. Gostop- tekhizdat : 315-335. Moscow (in Russian). Trw JuRAssíc ZONEs OF TOE USSR (1982): Nauka. Leningrad. 191 pp. (in Russiau). TorcuísHvíLí, M. & LOBJANtOZE, G. (1977): ~ Manuscrito recibido: 30 de Noviembre de 1997 Manuscrito aceptado: 10 de Septiembre de 1998 Cuadernos de Geología Ibérica 291 ¡998, número 24, 277-291