Biogeography and Palaeogeography of the Sibumasu Terrane in the Ordovician: a Review
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Ordovician biogeography of Sibumasu 43 Biogeography and palaeogeography of the Sibumasu terrane in the Ordovician: a review R A Fortey and L R M Cocks Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK Key words: biogeography, palaeogeography, Sibumasu, Shan-Thai, Thailand, Burma, China, Aus- tralia, Ordovician, trilobites, brachiopods Abstract as on Figs1 and 2) terrane embraces much of the Malay peninsula, extending northwards into Following an extensive review of previous work, and a con- West Thailand and Burma and southwards into sideration of new faunas from Thailand, the integrity of the western Indonesia (Sumatra) It is bounded to Sibumasu palaeocontinent (Sumatra, Malaysia, West Thai- the east by the Uttaradit-Nan to Raub-Bentong land and Burma) in the early Palaeozoic is reinforced% Its lower Ordovician faunas have been claimed to show affinity sutures, and to the west by what Metcalfe (1992) with North China/Australia, but for the upper Ordovician, termed the Shan boundary (also possibly an an- we describe undoubted similarity to South China, both in cient suture) Thus it comprises a tract of land fossils and sedimentary facies% The possible resolution of some 4000 km long, clearly a continental entity these difficulties is discussed, and the provisional conclu- sions reached that (a) Sibumasu was most similar to South of sufficient size to have acquired its own palae- China; (b) South China and North China may not have been ogeographic signature Several major neigh- so far apart as has been suggested; and (c) Indochina carries bouring terranes have been referred to in rela- a different faunal signal from any of these other terranes% tion to the history of Sibumasu These are (Fig1): the Indochina terrane immediately to the east; the South China terrane, today comprising Introduction many of the most populous regions of China south of the Tsinling (Chin Ling) suture; the The palaeogeography of Australasia is becom- North China terrane (including the northern part ing progressively better understood for the 150 of China and associated regions of Korea); and Ma period following the breakup of Pangaea; the Australian continent, which was already but there remain many unsolved problems in constituted in something like its present form, the Palaeozoic The period with which we are although the Tasman fold-belt to the south (in- concerned, the Ordovician (495-443 Ma), was a cluding New South Wales and much of Victoria) time of general continental dispersal (Scotese and montane Queensland were mobile belts in and McKerrow, 1990) long predating the assem- the Ordovician bly of Pangaea, and the disposition of the con- Here we review published evidence for the tinental masses at this time is still under investi- palaeogeographic position of the Sibumasu gation SE Asia has been divided into a number terrane in the Ordovician, referring to evidence of terranes (e)g), Mitchell, 1981; Burrett et al), derived from fossils and also in relation to the 1990; Metcalfe, 1992), each bounded by major sedimentary sequences developed over this im- faults These terranes have histories which are portant area These fossil faunas have been de- variably decoupled from that of their neigh- scribed in disparate publications over the last bours that is, their contiguity today is no seventy years (many listed in Ingavat-Heimche, guarantee of proximity in the past 1994) This review also provides a salutary re- The Sibumasu (sometimes called Shan-Thai, minder of the uncertainties that still apply to Biogeography and Geological Evolution of SE Asia, pp 43-56 Edited by Robert Hall and Jeremy D Holloway © 1998 Backhuys Publishers, Leiden, The Netherlands 44 R) A) Fortey and L) R) M) Cocks with greater sensitivity to substrate, but whose distribution patterns frequently track those of the trilobites Taken together with various other indicators, these fossils provide a powerful way of discriminating ancient biogeographic prov- inces and hence an indication of ancient ge- ography and plate distribution Both trilobites and brachiopods had different communities related, for example, to water depth, within any given palaeocontinent These have received different descriptions according to biogeographic fashion: in this paper we shall simply refer to them as biofacies The shelf biofacies are, in general most useful to recog- nise faunal similarities of palaeogeographic sig- nificance thus, two shelf faunas identical as to species are likely to have been closely related geographically (Cocks and Fortey, 1982) Even among shelf faunas there were evidently taxa which (at least at generic level) were distributed widely within a single palaeoclimatic belt Since Sibumasu occupied a tropical position in the Ordovician, such pandemic taxa are also distrib- uted in other tropical contemporary sites for example, in Laurentia, Australia and Siberia and their palaeogeographical usefulness is lim- ited Deeper water faunas, for example the Ordovician trilobite cyclopygid biofacies, may have even wider distribution in global terms (Fortey and Cocks, 1986; Cocks and Fortey, 1990) and thus be of little significance in dis- Fig1 Modern map showing the boundaries of the various criminating which continent was close to an- terranes in SE Asia% other However, they are important in a different way because they allow recognition of the mar- gins of these ancient cratonic areas, to which they were confined Also, their orientation understanding much of Lower Palaeozoic geog- should make geographic sense, because deep- raphy, and of the sometimes ambiguous signals water faunas should align with others of their given by biogeographic evidence kind, and generally face outwards towards an- cient oceans Thus the faunal approach we use to deduce the former geography of Sibumasu is Biogeographic methods and materials twofold: (1) comparison of Ordovician shelf fau- nas with their contemporaries in other terranes; The principal evidence for Ordovician biogeog- (2) orientation of biofacies into onshore-off- raphy used in this account comes from trilobites shore gradients In all comparisons identity of and brachiopods, with important additional species is considered more important than ge- contributions from fossil Mollusca, especially neric-level similarity although the latter has nautiloids Trilobites have been used for many often proved useful for more global studies years to discriminate biogeographic areas (e)g), This approach is complemented by compar- Whittington and Hughes, 1972), and they are ing sedimentary sequences in several areas in particularly useful as a highly speciose group different terranes; if there are striking lithological with distinctive morphologies that tend towards and/or sequence similarities this supplies addi- endemicity which is related to, and dependent tional evidence for geographical continuity upon, past continental configurations and cli- However, there are many areas of Sibumasu for matic zones Brachiopods are sessile organisms which the details of the sequences are imper- Ordovician biogeography of Sibumasu 45 Fig2 Differing previous palaeogeographies of Gondwana in the Early Ordovician% A, from Laurie and Burrett, 1992; B, from Cocks and Fortey, 1988 (in the latter North China is not shown; it was imagined then as some distance away)% 46 R) A) Fortey and L) R) M) Cocks fectly known, and we naturally concentrate Lower to lower Middle Ordovician faunas upon those areas that we have studied in the field and which are the best documented The An Ordovician (probably upper Tremadoc) tri- most fully known Ordovician succession is that lobite fauna from the top 100 m of the Tarutao in southern Thailand, Satun Province, of which Formation (Stait et al), 1984) is the earliest Ordo- many details have only recently been available vician yet discovered in Sibumasu Five genera Several different palaeogeographies have been were recognised, and of two named species published (e)g), Fig2) both were newly named All the genera identi- fied are widely distributed in Tremadoc strata, and comparative species were discussed from Ordovician sequence and palaeontology in both palaeotropical China and Laurentia The southern Thailand fauna is not, therefore, biogeographically criti- cal The palaeontology and stratigraphy of the Or- The clastic rocks of the Tarutao Formation are dovician of southern Thailand and the adjacent succeeded by a thick carbonate succession areas of northern Malaysia is now comparatively grouped in the Lower to basal Middle Ordovi- well-known Ordovician rocks crop out over a cian Thung Song Formation (the equivalent stra- discontinuous north-south belt for several hun- ta on Langkawi Island are known as the Lower dred kilometres, and sections have been studied Setul Limestone) These limestones are shallow- particularly in the southern area, to either side of water, peritidal, even partially lagoonal in origin the Thai-Malaysia border Outcrops extend on The fossil faunas are sparse, in spite of excellent to two offshore islands: Tarutao Island on the exposures around the coast of Tarutao Island Thai side of the border, and Langkawi Island on The limestones resemble in lithology many oth- the Malaysian side Earlier studies by Kobayashi er lower-mid Ordovician carbonate platform se- and his colleagues (summarised in Kobayashi quences, e)g), from North China/Korean plate, and Hamada, 1984) tended to be descriptions of central Australia, much of Laurentia and the San spot faunas small collections of a few inver-