Paleobiogeographic Zoning of the Basins of the Caucasus in the Early Jurassic-Bajocian by Ammonites
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saqarTvelos mecnierebaTa erovnuli akademiis moambe, t. 6, #1, 2012 BULLETIN OF THE GEORGIAN NATIONAL ACADEMY OF SCIENCES, vol. 6, no. 1, 2012 Geology Paleobiogeographic Zoning of the Basins of the Caucasus in the Early Jurassic-Bajocian by Ammonites Mirian Topchishvili* and Tamaz Lominadze** * Academy Member, A.Janelidze Institute of Geology, I. Javakhishvili Tbilisi State University ** Department of Geology, Georgian Technical University, Tbilisi ABSTRACT. On the basis of study of the ecology and evolution of Early Jurassic-Bajocian ammonites of the Caucasus, the routes of migration of these organisms and areas of their dispersal were established. The paleobiogeographic boundaries were specified and the existence of 4 palaeobiogeographic regions on the territory of the Caucasus in the Early Jurassic-Aalenian time was verified: 1. The Lesser Caucasian, in its southern part; 2. The Southern Caucasus intermountain area including the Dzirula massif; 3. The Southern slope of the Greater Caucasus including the territories of Georgia and Azerbaijan; 4. The Northern Caucasus. At the end of the Bajocian age considerable differentiation of ammonite fauna took place. It led to the appearance of new families and genera. Ranking of the earlier distinguished palaeobiogeographic areas to the subprovinces and also the existence of the Nakhichevan subprovince are justified. © 2012 Bull. Georg. Natl. Acad. Sci. Key words: palaeobiogeography, Early Jurassic, Bajocian, Caucasus, ammonites. Over the past few decades our knowledge of received. They are of major importance in revealing Jurassic ammonites has increased considerably. Due general patterns of their geographic differentiation. to numerous studies of this important group of fauna, Taxa of palaeobiogeographical subdivisions used the possibility of solving the problems of biostrati- in this study are determined by the rank of groups of graphy and paleobiogeography appeared. The ammonites characteristic of them. In particular, changes in ammonite fauna in space and time were biogeographic unit of the highest rank, i.e. an area traced on the basis of extensive material. covering an extensive territory of land or sea that Numerous researches were dedicated to Lower differs from the adjacent one by the presence or Jurassic-Bajocian palaeobiogeographical structures. disappearance of superfamilies and genera. Biogeo- However, information on the palaeobiogeography of graphical unit of the second rank is a province, which this geological time interval in the Caucasus can be is a part of the area characterized by species and found only in [1, 2]. subspecies and more fractionated subdivisions Recently new data on the Early and Middle belonging to the provinces, such as subprovinces Jurassic ammonites of the South Caucasus were and districts. © 2012 Bull. Georg. Natl. Acad. Sci. 104 Mirian Topchishvili and Tamaz Lominadze Based on the study of ammonite fauna and bio- Arieticeras, Liparoceras, the Middle European facies analysis, in the Caucasus in the Early Jurassic Tropidoceras, mainly by the Mediterranean Pleuro- and Aalenian 4 regions are distinguished: 1) the ceras and by widely spread Amaltheus. Lesser Caucasian, located in its southern part; 2) the The ammonite complexes of the Toarcian and South Caucasian intermountain area including the Aalenian differ in a restricted amount of ammonites, salient of the Dzirula crystalline basement; 3) the belonging mainly to the Mediterranean Callirhyl- Southern slope of the Greater Caucasus including loceras, Peronoceras, Harpoceras, Phymatoceras the territories of Georgia and Azerbaijan; 4) the and to the Middle European Grammoceras, Pseudo- Northern Caucasus. These paleobiogeographical grammoceras, Dumortieria, Leioceras, Costileio- units are considered in this paper (Fig. 1). ceras and Ludwigia. Geographic differentiation of ammonite fauna in It is clear from the above list that the ratio of the Early Jurassic and Aalenian was not sufficiently ammonite genera in some epochs changes. Ap- expressed and manifested mainly at the species level. parently, in the Sinemurian and Toarcian ages, At that time mainly the same genera were widespread. marine conditions on the territory of the Lesser This, of course, complicates the distinguishing of a Caucasus region were favorable for ammonite fauna palaeobiogeographical unit that is larger than a habitat. province. The Caucasian marine basin in the Early Species composition of genera is typical of the Jurassic and Aalenian was a part of the Mediterranean Mediterranean and Middle European provinces. The province and was located in its northeastern part. penetration of Middle European ammonites into the Marine basins of North-Western Europe were territory of the Lesser Caucasus probably took place identified as the Middle European province [2]. from Central Europe along the northern margin of the The complex of Early Jurassic and Aalenian Tethys, via North Anatolia. ammonite fauna of the Caucasus is fairly abundant. In the Sinemurian age, the sea invading from the Its members are distributed unevenly. A great number Lesser Caucasus basin through a wide strait on the of ammonite genera of the Sinemurian age of the Early territory of the South Caucasian intermountain area Jurassic epoch are recorded in the marine basin reached the Dzirula massif. During the Early Jurassic situated in the northern part of the Lesser Caucasus and Aalenian, the land on both sides of the strait was region. Here, in the Sinemurian age shallow marine highly elevated [6]. conditions with normal water temperature and sali- The Sinemurian ammonite fauna of the Dzirula nity favorable for intensive development of vital massif is considerably poorer than that of the Lesser processes were established. In the organic world the Caucasus. It is represented by only three genera - ammonites of the following genera play an important Vermiceras, Arnioceras and Microderoceras. role: the Mediterranean- Partshiceras, Eoderoceras The biocenosis of the Pliensbachian of the Dzirula and Epideroceras, the Middle European - Canava- massif is composed mainly of brachiopods and rites, Arietites Coroniceras, Paracoroniceras and ammonites. An overwhelming majority of this fauna Metophioceras, as well as Vermiceras, Arnioceras, developed in the shallow part of the marine basin. As Oxynoticeras, Echioceras, Microderoceras, found a part of taphocoenosis, the ammonites belong to both in the Middle European and Mediterranean the Mediterranean Juraphyllites, Calliphylloceras, provinces and predominantly the Middle European - Arieticeras, Pleurocerass, Fuciniceras and to the Gleviceras and Paltechioceras [3-5]. Middle European Crucilobiceras, Phricodoceras, Ammonites of the Pliensbachian age are more Uptonia, Polymorphites, Acanthopleuroceras, exhausted and represented by the Mediterranean Tropidoceras and Pseudogrammoceras [7, 8]. Bull. Georg. Natl. Acad. Sci., vol. 6, no. 1, 2012 Paleobiogeographic Zoning of the Basins of the Caucasus ... 105 Fig. 1. Scheme of palaeobiogeographic zoning of the Caucasus in the Early Jurassic-Bajocian. The regions: I.The Lesser Caucasus; II.The Southern Caucasus intermountain area; III. The Southern slope of the Greater Caucasus; IV.The Northern Caucasus In the Toarcian- Early Aalenian the sedimentary of the Middle European and Mediterranean provinces environments inherited from the Late Pliensbachian as well as the genera common for both biogeographic dominate. The abundance of fauna, represented by units were spread. Migration routes and dispersal of different groups, indicates optimal conditions for their ammonites passed through Northern Anatolia by the existence. strait located in the territory of the South Caucasian The ammonite complex of these epochs in the intermountain area. South Caucasian intermountain area, within the limits In the Sinemurian, water penetrated into the of the Dzirula massif basin, is rather rich and diverse territory of the region of the Southern slope of the in genera and species composition. In the Toarcian Greater Caucasus. At that time a normal hydrochemical age the Mediterranean genera Calliphylloceras, regime favorable for the dispersal of a rich complex of Lytoceras, Harpoceras, Phymatoceras, Hammato- ammonites (42 species) was established in the basin. ceras, Peronoceras, Catacoeloceras, Pseudolio- The complex is represented by the ammonite genera ceras, Polyplectus, Praehaploceras prevail and the found in the Mediterranean and Middle European Middle European Hildoceras, Grammoceras, provinces - Phylloceras, Partschiceras, Juaphyllites, Pleydellia also occur. Arietites, Coroniceras, Vermiceras, Arnioceras, The Aalenian age is represented predominantly Euasteroceras, Oxynoticeras, Gleviceras , Radsto- by the Mediterranean Tatrophylloceras, Lytoceras, ckiceras, Echioceras, Paltechioceras, Leptechio- Hudlestonia, Planammatoceras, Erycites and the ceras [9, 3, 5, 10]. Middle European Costileioceras and Leioceras. In the Pliensbachian age the Mediterranean Thus, in the given ammonite complex the genera Partshiceras and Audaxlytoceras, Arieticeras, the Bull. Georg. Natl. Acad. Sci., vol. 6, no. 1, 2012 106 Mirian Topchishvili and Tamaz Lominadze Middle European Zetoceras, Tropidoceras, Uptonia and Brodieia were found. and a cosmopolitan Amaltheus occur. For the Aalenian age the Middle European In the Toarcian age the ammonites reached their Leioceras, Costileioceras, Staufenia, Ludwigia, bloom. The number of genera and species (27 genera Brasilia, Graphoceras and predominantly the and 80 species) increased.