Middle Triassic (Anisian) Paleobiogeography of Shan Plateau

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1 Universities Research Journal 2011, Vol. 4, No. 5 Middle Triassic (Anisian) Paleobiogeography of Shan Plateau Ohnmar Soe Yin Abstract Recent advances in Middle Triassic paleobiogeography of Shan Plateau are based on the special use of ammonites and conodonts. The Kondeik Limestone and the cherty limestone unit of the Shan Plateau include ammonites such as Balatonites, Judicarites, Ptychites, Meekoceras, Danubites, Ceratites, Flexoptychites, Paraceratites, Beyrichites, Frechites, Bulogites, Noetlingites, Pseudohollandites, Gymnotoceras, Paracrochordiceras, Proptychites and conodonts such as Gladiogondolella, Epigondolella and Hindeodella. The commonly found genus Paraceratites is reported from Italy, Hungary, Turkey, Nepal, China, Myanmar, Laos, Cambodia and Malaysia. Judging upon the known paleogeographic distribution of Paraceratites during Middle Triassic time, the genus is considered to be an equatorial distribution along the Tethyan region. Another moderately found genus Balatonites is the most diagnostic genus of this time span and appears essentially as a low paleolatitude marker and it is known from N.America, Europe and Asia. During Middle Triassic, the paleogeographic position of Shan Plateau at low latitudies appeared as a part of Sibumasu or as a microcontinent near Indochina. Key words: Middle Triassic, paleobiogeography, Shan Plateau, Tethyan region Introduction The Middle Triassic (Anisian) ammonite fauna of the Shan Plateau is well represented by the Kondeik and Indawgaing ammonite faunas. A total of 19 species belonging to 16 genera can be discriminated in the present study. These two ammonite faunas exhibit a commendable degree of resemblance in faunal composition and taxonomic diversity in comparison with coeval faunas of other regions such as northwestern America, central Europe, Himalaya and southeastern Asia. The present work makes an attempt, based on generic level comparison of the fauna, to elucidate paleobiogeographic and paleogeographic bearings of the Shan Plateau Middle Triassic (Anisain) ammonite fauna in the context of Triassic paleogeographic frameworks and scenarios recently put forward by various investigators. Demonstrator, Dr, Department of Geology, Taungoo University

2 146 Universities Research Journal 2011, Vol. 4, No. 5 Faunal Affinity and Paleobiogeography of the Kondeik Fauna The Kondeik fauna consists mainly of Middle Triassic (Anisian) ammonites occurring in the Kondeik Limestone which is composed of layered, dark grey to graphite-black limestone containing thin intercalation of black chert in the form of nodules and layers. Associated with the ammonites are open marine pelagic forms such as thin-shelled bivalves, radiolarians, a few pteropods and some conodonts. The Kondeik fauna consists mainly of Middle Triassic (Anisian) ammonites occurring in the Kondeik Limestone which is composed of layered, dark grey to graphite-black limestone containing thin intercalation of black chert in the form of nodules and layers. Associated with the ammonites are open marine pelagic forms such as thin-shelled bivalves, radiolarians, a few pteropods and some conodonts. The ammonite fauna from Kondeik Limestone was previously reported by Gramann et al. (1972). It consists of Acrochordiceras sp., Paraceratites thuillieri (Oppel,1863), Paraceratites cf. thuillieri (Oppel,1863) and Flexoptychites manhendra (Diener,1895). The Kondeik fauna of the present work is found constituted of ammonites such as Balatonites cf. balatonicus,?judicarites sp., Ptychites cf. cochleatus, Meekoceras sp., Danubites cf. dritarashtra, Ceratites sp., Flexoptychites cf. mahendra, Paraceratites cf. thuillieri and Paraceratites sp. indet; conodont such as Gladiogondolella cf. tethydis and?epigondolella sp. They form successive ammonoid biozones, namely, Balatonites cf. balatonicus zone, Ptychites cf. cochleatus zone and Paraceratites cf. thuillieri zone in ascending order. From the faunal comparison of Kondeik fauna with those of other regions, it is found out that the genera Paraceratites, Acrochordiceras and Balatonites, among others, of the Kondeik fauna have already been reported from almost all areas of ammonite faunal realms, such as Northwestern Nevada, Italy, Hungary, Turkey, Iran, Nepal, south China, Laos, Cambodia and Malaysia. These three genera can thus be considered as cosmopolitan faunal elements having least paleobiogeographic value. However, the genus Balatonites has been recognized as the most diagnostic genus of this time span and essentially as a low paleolatitude marker ( Bucher, 2002 ). On the other hand, the genus Ptychites that ocurrs in common only in areas of Central and Eastern Tethyan realms are characteristically present in the

3 Universities Research Journal 2011, Vol. 4, No Kondeik fauna. It should be here noted that non of the faunal element characteristic to the western Tethys is recorded in the Kondeik fauna. One noteworthy point reached as a result of present work is that Flexoptychites that has not been found in other regions occurs in Myanmar and Nepal. It is possible to consider this genus endemic to Myanmar and Nepal within the Tethyan region. The associated conodont fauna such as Gladiogondolella tethydis is almost entirely restricted to the Middle Triassic (Mosher, L.C., 1968). Ishii and Nogami (1966) reported some elements of the G. tethydis fauna in Southeast Asia at the eastern extremity of the Tethys depositional trough. Summing up the above faunal comparison and correlation, it becomes apparent that the Kondeik fauna clearly exhibits Tethyan aspects and closer affinity to the Central Tethys, particularly to Nepl Himalaya, because of the common occurrence of Ptychites and Flexoptychites. On account of the presence of Balatonites in the fauna, it is possible to assumed that the Kondeik fauna would have developed at an area of low latitudes. Faunal Affinity and Paleobiogeography of the Indawgaing Fauna The Triassic ammonite fauna from the cherty limestone unit of the Plateau Limestone of Indawgaing area exhibits strong similarities to the fauna from the Kondeik Limestone of Kalaw-Aungban area. The cherty limestone unit consists of dark grey or black-coloured, well-bedded micritic limestones, interbedded with pinkish or purplish-coloured silicified limestone beds, or interlayered with black chert layers, patches and nodules. Some ammonites such as Ptychites cf. cochleatus, Ptychites cf. asura, Ceratites (Paraceratites) cf. thuillieri, Ceratites (Paraceratites) cf. himalayanus, Danubites sp. and Protrachyceras sp. have already been described from the cherty limestone unit by Zaw Win (1991) together with a few straight nautiloid cephalopods (Michelinoceras sp.). The present study revealed that the cherty limestone unit of the Indawgaing area contains a rich Anisian fauna, predominantly of ammonites and conodonts. The ammonites discriminated in this work are Paraceratites cf. thuillieri, Paraceratites cf. himalayanus, Paraceratites sp. indet., Ceratites sp. indet., Meekoceras sp., Beyrichites srikanta, Flexoptychites cf. mahendra, Ptychites cf. cochleatus, Ptychites cf. rugifer, Danubites sp., Frechites sp., Bulogites cf. mojsavori, Noetlingites sp., Billingsites cf. cordeyi, Gymnotoceras sp. and?paracrochordiceras sp.

4 148 Universities Research Journal 2011, Vol. 4, No. 5 This fauna is assigned to as the Ptychites-Paraceratites Assemblage representing middle-late Anisian age. Conodont such as Hindeodella sp. is also recovered. Hindeodella is a characteristic genus in the Triassic conodont assemblage, particularly abundant in Kelantan, Malaysia (Igo, H. et al., 1966). The present fauna is dominated and characterized by the genus Paraceratites which is cosmopolitan and one of the most common ammonoids in the Middle Triassic (Anisian) rocks. It has already been recorded in Tethyan regions such as Italy, Hungary, Turkey, Iran, South China, Laos, eastern Cambodia, Malaysia, and also in the Mid-continent areas such as western Nevada and northeastern British Columbia. However, the genera such as Ptychites, Flexoptychites, Pseudohollandites and Beyrichites, on the other hand, characteristically constitute the Indawgaing fauna. These genera have been recorded mainly in the Eastern Tethyan and partly in the Central Tethyan areas. From this point, it is justifiable to assume that the present Indawgaing ammonite fauna has biogeographically a close affinity to those of the Eastern Tethyan areas (northern Thailand and Malaysia) and Central Tethyan areas (particularly Nepal Himalaya). Paleogeographic Consideration Because of geographical proximity and similar faunal composition, the Kondeik and Indawgaing ammonite faunas can be considered as a single fauna representing the Middle Triassic of the Shan Plateau. The Indawgaing fauna would have flourished under quiet pelagic marine condition in a relatively deep basinal area when the basin-slope-shelf margin phase was taken place during Middle-Late Triassic (Zaw Win, 2004). He stated that the similar depositional and biotic history would have occurred in Thigaungdaung-Kondeik area where similar stratigraphy and fauna have been noted. In other words, the western margin of Shan Plateau (such as Nahkam, Maymyo, Wundin Sakarn, Lungyaw-Indawgaing, Kinda, Kondeik and Kamawkala) would have been in a deep marine basinal realm during Middle Triassic time. Recent advances in Triassic paleobiogeography of ammonites and conodonts make it possible to discuss the Triassic paleogeography in greater detail. Various studies of time and space distribution of ammonoid faunas revealed that they were nearly cosmopolitan in the Early Devonian but became progressively more and more provincial in the Mesozoic.

5 Universities Research Journal 2011, Vol. 4, No The present work follows the paleogeographic classification scheme of the Middle Triassic period proposed by Bando (1980) and Ishibashi et al. (1994). They classified as the Mid-continents, Western Tethys, Central Tethys and Eastern Tethys, respectively. The Mid-continents Province such as northwestern Nevada and northeastern British Columbia is marked by the common occurrence of Protrachyceras, Frechites, Paraceratites, Bulogites, Balatonites, Acrochordiceras and Paracrochordiceras. The Western Tethys represented by Italy, Hungary, Turkey, Iran is typified by the presence of dominant Paraceratites, Bulogites, Balatonites and Acrochordiceras. The Central Tethyan Province consisting of Nepal and China yields diverse genera of ammonites including Protrachyceras, Paraceratites, Ptychites, Flexoptychites, Pseudohollandites, Bulogites, Balatonites, Paracrochordiceras, Danubites, Beyrichites and Acrochordiceras. The Eastern Tethys Province such as Thailand, Laos, Cambodia, Malaysia and Timor is marked by the common occurrence of Frechites, Paraceratites, Ptychites, Beyrichites, Balatonites, Acrochordiceras and Paracrochordiceras (Table 1). Eleven genera of the Central Tethys (especially Nepal) are identical with the Middle Triassic ammonites of the Shan Plateau, Myanmar. Only four genera of the Western Tethys are similar to the Middle Triassic ammonites of the Shan Plateau and seven genera to the Eastern Tethys and Mid-continent. Faunal comparison of the present work reveals that the early Middle Triassic (early Anisian) fauna of the Shan Plateau is characterized by the presence of Meekoceras spp. that have not been reported in other provinces. In the middle Anisian, the Shan Plateau contains two Tethyan genera, Bulogites and Beyrichites, of which Bulogites is of the Western and Central Tethys affinities, but Beyrichites the Eastern Tethys affinity. In the late Anisian, generic diversity of the Shan Plateau fauna is greater than those of the early and middle Anisian. In this time, the Shan Plateu fauna is characterized by the genera Frechites, Ceratites, Ptychites, Flexoptychites and Pseudohollandites and much more closely similar to the Central and Eastern Tethys affinities. In brief, although palaeobiogeographic affinity of Shan Plateau cannot be defined clearly in early Anisian, it can be regarded as a Tethyan affinity in the middle Anisian and then exhibits greater similarities to the Central and Eastern Tethys affinities in late Anisian. In addition, the Ladinian of the Shan Plateau fauna is characterized by Protrachyceras resembling the Central Tethys affinities.according to this faunal

6 150 Universities Research Journal 2011, Vol. 4, No. 5 comparison, it is justifiable to regard that the Shan Plateau of Myanmar would have been much closer to the Central Tethys. The Shan Plateau represented by the Kondeik and Indawgaing faunas can be regarded as the main part of the Sibumasu or Shan-Thai block which would have been in the Mesotethys with the Indochina block in the northeast and the Lahsa and western Burma block in the southwest. The exact paleogeographic position of Shan Plateau during the Middle Triassic is still uncertain, but it is supposed to have been located at low latitudes of the tropical area, judging from the geographic position of Shan-Thai block on the paleogeographic maps reconstructed by Stampfli & Borel (2002) (Figure 1). The Cimmerian continent containing the Sibumasu or Shan-Thai block rifted from the Gondwanaland in Early Permian time, and collided and amalgamated to Proto-Asia by late Triassic time (Metcalfe,1999) (Figure 2). In this connection, the Shan Plateau would be most possibly as part of the southwestern fringe of the Sibumasu block of the Cimmerian continent influenced by tropical to subtropical deeper / pelagic marine condition of Meso-Tethys. Table (1) Summary of the comparison of faunal affinities between the Shan Plateau Middle Triassic ammonite fauna and other paleogeographic provinces Mid continent Western Tethys Central Tethys Eastern Tethys Myanmar Protrachyceras Frechites Bulogites Balatonites* Acrochordiceras* Paracrochor -diceras Bulogites Balatonites* Acrochordiceras* Protrachyceras Ptychites** Balatonites* Acrochordiceras* Flexoptychites** Pseudohollandit -es Bulogites Paracrochordic Frechites Ptychites** Balatonites* Beyrichites** Paracrochordi -ceras Protrachyceras Frechites Ptychites** Balatonites Acrochordiceras* Flexoptychites** Paracrochordiceras?Gymnotoceras Pseudohollandites Noetlingites

7 Universities Research Journal 2011, Vol. 4, No Mid continent Western Tethys Central Tethys -eras Danubites Beyrichites** Eastern Tethys Myanmar Bulogites Beyrichites**?Judicarites Danubites Proptychites Ceratites Meekoceras * cosmopolitan genus ** Tethyan genus Figure (1) Paleogeographic position of Sibumasu (Shan-Thai) bolck during Middle Triassic plotted on the palaeogeographic map reconstructed by Stampfli & Borel (2002).

8 152 Universities Research Journal 2011, Vol. 4, No. 5 Figure (2) Paleogeographic position of the Shan Plateau on the Cimmerian continent during Triassic period plotted palaeogeographic reconstruction of Asian terranes by Metcalfe (1999) Acknowledgements I am extremely grateful to Rector Dr. Aung Thu for giving me permission to conduct this research work. I would like to thank Professor Dr. Aye Aye Myint, Head of the Department of Geology, Taungoo University, who allowed me to do the present research. I am most deeply indebted to Associate Professor Dr. Zaw Win, Head of the Department of Geology, Hpa-an University, for his valuable suggestions and critically reading the manuscript. References Bando, Y., (1980). On the Otoceratacean ammonoids in the Central Tethys, with a note on their evolution and migration. Memoirs of the Faculty of Education, Kagawa University, Sr. 2, vol. 30, no. 1, Bucher, H., (2002). Early Anisian (Middle Triassic) ammonoid biostratigraphy of northeastern British Columbia. Eclogae geol. Helv. 95, Diener, C., (1895). The Cephalopoda of the Muschelkalk. Paleontologica Indica. Sr. 15, vol. 2, Pt p., pls. 31.

9 Universities Research Journal 2011, Vol. 4, No Gramann, F., Lain, F., Stoppel, D., (1972). Paleontological evidence of Triassic age for limestones from the Southern Shan and Kayah States of Burma. Geologisches Jahrbuch Reihe B, Regionale Geologie Ausland, 6-9, pls Ishibashi, T., Nakornsri, N., Nagai, K., (1994). Permian-Triassic boundary and fauna at Doi Pha Phlung, northern Thailand. Memoirs of the Faculty of Science, vol. 28, no. 2, Metcalfe, I., (1999). The Tethys: how many? how old? how deep? how wide? International Symposium Shallow Tethys (ST) 5, Department of Earth Sciences, University of New England, Armidale, NSW 2351, Australia, 1-5. Oppel, A. ( ). Uber ostindische fossilreste aus den secundaren Ablagerungen von Spiti und Gnari-Khorsam in Tibet. Palaeont. Mitt. Mus. Bayer. Staat., 4, Zaw Win, (1991). Triassic ammonites from the Plateau Limestone, east of Lungyaw and Baukkewzu, Myint-tha and Ywa- ngan townships, Myanmar. Georeports, University of Yangon. vol. 1, no. 1, Zaw Win, (2004). Permian-Triassic Plateau Limestone in Lungyaw-Sakangyi area, Shan State, Its depositional and biotic history. Journal of the Myanmar Academy of Art and Science, vol. 2, no. 5,

Ammonoid diversification in the Middle Triassic: Examples from the Tethys (Eastern Lombardy, Balaton Highland) and the Pacific (Nevada)

Central European Geology, Vol. 57/4, 9 4 (04) DOI: 0.556/CEuGeol.57.04.4. Ammonoid diversification in the Middle Triassic: Examples from the Tethys (Eastern Lombardy, Balaton Highland) and the Pacific