Proc 2nd Intern Symp Devonian Syst 591-595 OUTLINE OF PA LEOGEO GRAPHICAL RECONSTRUCTION OF VIETNAM AND A DJA CEN T REGIONS DURING THE DEVONIAN To n g -D zu y Th a n h a n d A N g u y en D in h H o e b st r a c t During the Devonian, eastern and southern Vietnam and adjacent areas in Laos and Kam puchea were the site o f the Cathaysian L and, a late Caledonian positive area A marginal transition zone lay to the west, and a shelf zone was present in the central area W ithin the shelf zone a small Early Devonian intrashelf basin is thought to have been present since Silurian time This basin is characterized by euxinic, fine grained terrigenous sediments containing a mainly pelagic fauna O ther parts o f the shelf received shallow w ater carbonate-terrigenous sediments containing rich, level-bottom faunas of corals, brachiopods, pelecypods, crinoids and conodonts, etc Acidic volcanic rocks are only seen in the marginal zone The climate o f the area during Lochkovian was tropical and sem i-arid; it became hum id during Pragian-L ate Devonian time In t r o d u c t io n Research into the Devonian o f Vietnam began early in this century New data have accumulated in recent years but little attention was directed towards paleogeography until 1986 when a preliminary account was presented (Thanh et a l, 1986a) This synthesis has shown that Devonian seas persisted over the northern and western regions throughout the Devonian, whereas areas to the south and east were emergent and are assumed to have form ed part o f the Cathaysian Land (Fig 1) In the submerged area, shelf facies with some small islands and an intrashelf basin has been discriminated from a marginal transitional zone in the west; the latter has been referred to (Hoe, 1986) as the ‘Central Zone of the Southeast Asian Devonian Field’ The environm ent o f deposition within the marginal zone is still poorly known from isolated outcrops at Muongte (NW Vietnam), around Paklay (NW Laos), near Loei (NE Thailand), in SW Kampuchea and SW Vietnam where Devonian strata are represented by thick bedded limestones, clastic sediments, shales, and siliceous shales with intercalations o f acidic volcanics and tuffs Occasional sp ecim en s o f c o ls (F a v o site s, C r a s sia lv e o lite s ) , brachiopods (Howellella, A tryp a , Camarotoechia) and bivalves occur in isolated outcrops in NW Laos and NW Vietnam Siliceous skeletal rem ains o f radiolaria and sponges collected in SW Kampuchea and SW Vietnam are consistent with rather deep water conditions The marginal zone may have persisted throughout Devonian time as a future shelf zone but compelling data to support this has not been obtained The Cathaysian Land is believed to have emerged as a consequence of widening and joining together of the former Indosinian islands of the South Indochina peninsula and islands in SE China (Hainan, SE Kwangtung, etc.) (cf Liu, 1962) Terrestrial conditions prevailed in the southern and eastern parts of the region throughout the Devonian, flank ing the shelf zone Clastic materials were derived from this land region and from some small intrashelf islands; the style of sedimentation was closely connected with events in the land areas The largest body o f data bearing on Devonian history of the region has been obtained from the northern Indochina peninsula, where shelf conditions persisted throughout the period Many biostratigraphic correlations have been pro posed in the past 20 years and some im portant paléontologie monographs have been published Though some o f this new d a ta is still open to discussion, a generalized p a leogeographic pattern for the shelf zone can be made out (Table 1) Ge L o c h k o v ia n Pa l e o g e o g r a ph y ( F i g 1) Lochkovian nearshore redbeds bearing fish remains, plant imprints and the brachiopod Lingula were widespread over the southern and western parts of the shelf zone adja cent to the C athaysian L and In the intrashelf basin, which perhaps had been in existence already during the Silurian, calm euxenic conditions prevailed characterized by dark fine grained mudstones, dark shales and siliceous shales yielding graptolites (Monograptus) and some plant fragments A few grey marl lenses have produced rare, small crinoid remains, inarticulate brachiopods (Lingula) and articulate brachiopods (Schellwienella) Small shallow water, level bottom faunules have been obtained from the latest Lochkovian of the northeast margin o f the basin; these have produced the brachiopods Pugnacina, “H ysterolites”, Schellwienella, Strophodonta and Howellella as well as Favosites and other corals P r a g ia n (F i g 2) The intrashelf basin became narrower during the P ra gian and was restricted to the area between the Songma and Namthuen islands Euxinic dark shales, siliceous shales and fine grained mudstones accumulated; these have yielded numerous tentaculitids (Nowakia and Styliolina) along with 'D epartm ent of Historical Geology, H anoi University, Dongda, H anoi, Vietnam 591 o l o g y a n d 592 Thanh and Hoe Fig Paleogeographical reconstruction of N Indochina peninsula during Pragian Fig Paleogeographical reconstruction of Indochina peninsula during Lochkovian Notes of the figures â i â ã ■ © ( from a © i ^r IQ IS n °i to o cc asio n al trilo b ite s (P lagiolaria) an d cep h alo p o d s (Erbenoceras) The intrashelf basin had disappeared by the end o f the Pragian The Lochkovian redbeds pass up into Pragian shallow water grey shales, sandstones and mudstones with num er ous crinoid fragments and gastropods Brachiopods, spe cifically the 'Euryspirifer to n kin en sis’ fauna discussed below, occur in abundance at some localities near the region where interfingering of carbonate and terrigenous deposits occurs An area of shallow water carbonate-terrigenous depo sition, characterised by grey thin bedded limestones, marls and calcareous shales, occurs in the northernmost part of the sh elf zone T he shelly fa u n a o f th is a re a , th e n°s) © —• 11 II d Z l 17 Notes to Figures 1-5 Islands: 1) Sikan-Yunnan, 2) Songma, 3) Phuboat, 4) Namthuen, 5) Paklay, 6) Yunkai Other symbols: 7) Subconti nental Red Beds, 8) Intrashelf basin, 9) Terrigeneous depositional area, 10) Area of terrigenous-carbonate intercalation, 11) Area of sil iceous—carbonate deposits, 12) Area of carbonate deposits, (13) Area of subcontinental deposits (Grey Beds), 14) Area of siliceous shales, sil iceous limestones which may bear manganese ore, 15) Tidal clastic deposits, 16) Emergent land, 17) Marginal zone Fig Paleogeographical reconstruction of N Indochina peninsula during Zlikhovian-Eifelian Devonian Paleogeography, Vietnam 593 Marginal Siage zo ne SHELF (M odif ied ZONE N o rih from Center n o n c/epos///on) Qty3S/e/?i/û- f f j a i7 s Z Frasnian ef al 1986) S o uth , jl/c?//77t?t0/(?p/ỵ\ftbt/naj Famen man T D Z Thanh ls - S // / c ,S h K f f - PalmtaleplS> yunn*no//,na Quasiendothi yP arathuram m m TJSPiriJ i v e -f i a n e n eo u s o' * Ca/ceota Zl i cho vian r i , oi d S s; 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