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B. French Indo-China.
Geological maps of Indo-China show much of its western and southern parts covered by a formation usually referred to by French writers as "Les Grès supérieurs", "Les Grès continentaux", or simply as "Le Revêtement posttriassique ". A long band stretches completely across the Upper Laos, between the frontiers of the Southern Shan States and Siam on the west and the Mekong on the east, with various extensive outliers to the north-west and south-east. Large isolated patches continue down the Mekong valley, from the neighbourhood of Vientiane until, coalescing in the Lower Laos, they mantle the whole country between the Central Chain and the Mekong and surround the basalt plateau of Boloven. These exposures are continuous with those of Cambodia, where the formation builds practically the whole of the higher ground which wraps around the central plain with its great lakes. Remnants of the covering are also found in the eastern parts of the territory, particularly in Cochin China and Southern Annam, and, again, far to the north, where there are patches on both flanks of the delta of the Red River in Eastern Tongking. As a general rule the formation consists of red or grey sandstones, sometimes containing a little salt in their more northern exposures, with beds of conglomerate and red, shaly clay. It is often very thick, as for example in the Mekong valley where it is estimated at over 1,000 metres (3,280 feet). For the main part horizontal and undisturbed, yet undoubtedly discordant on the Trias in its clearest sections, a Rhetic or Jurassic age is generally attributed to it. This is confirmed by the presence of Olozamites latieri in Eastern Tongking, of Goniomya bisinuata Mans. in a red conglomeratic sandstone of the upper part of the Red Beds of the Northern Laos and by the fact that in the Lower Laos sandstones and red shales pass locally upwards into Hettangian marls with Polymorphites Jamesoni.1
Late Mesozoic Continental Formations.
The recent discoveries of J. H. Hoffet in the Lower Laos of red sandstones and "terrain rouge" containing bones of the Senonian dinosaur Mandchurosaurus together with those of Titanosaurus and various species of lamellibranchs, belonging to the genera Trigonoides and Plicatounio lead to some extension of the previously accepted
1 Bull. Soc. geol. France, 5th Serie, 4th Vol., pp. 110, 137, 142, 143, (1934),
Cretaceous Red Beds
in the Lower Laos.
upper limits of the beds just described, and also entails a modifica tion of existing ideas of the distribution of land and sea in IndoChina and Siam at the close of the Mesozoic era. These Senonian rocks lie above another group, described as "Grès et Poudingues supérieurs " with "terrain rouge", and sandstones with intercalations of limestones containing Liassic fossils. They are brackish and lagunary formations believed to have been deposited on the borders of a marine gulf, the limits of which are broadly defined in M. Hoffet's papers.1
While it cannot be doubted that the Red Beds of Upper Mesozoic age cross the Laotian frontier into Southern Yunnan and probably into the Southern Shan States, just as it would Red Beds of Triassic be idle to deny that the Cretaceous rocks of the Red Basin of Szechuan may not cross for some small distance into Northern Yunnan, it is not with such formations that the folded, rock salt-bearing Red Beds of Central Yunnan are to be compared. In Indo-China, as in Szechuan, a lower series of Red Beds of Triassic age exists and it is amongst these that our true homologues are to be found. J. Fromaget has summarized the lithological types of the Indo-Chinese Trias as follows: The Triassic facies vary with the position and with the age of the sediments. Detrital deposits, arkoses, conglomerates, with or without "terrain rouge", are abundant at the base of the Trias and in all the littoral sediments of the period; at the top they mark the passage into the lagunary regime of the Rhætic and of the Lias. Shales of a more or less sandy character and sandstones with Myophorias occur on the edges of the marine depressions, the axes of which are occupied by shales and limestones with cephalopods, brachiopods, gasteropods and more rarely, anthozoa "2 Describing the continental Rhætic, Liassic and Jurassic deposits it is shown that they only continue a set of conditions already initiated in Triassic times, the sandy deposits of which, of a reddish facies, are well known.3
In cases, therefore, where the younger "terrain rouge" displays the same lithological characters as the Triassic rocks themselves,
1 C. R. Ac. Sc., Tome 202, No. 22, pp. 1867-69, (1936).
Bull. Serv. géol. Indo-Chine, Vol. XXIV, Fasc. 1, (1937).
2 Bull. Serv. géol. Indo-Chine, Vol. XIX, Fasc. 1, p. 17, (1931).
it is not surprising to learn that the relations of the two groups are far from being distinctly clear, everywhere.1
The great extent to which such red, sandy formations occur in the Trias of Indo-China, is perhaps somewhat over-shadowed by the intensive studies which French geologists have rightly devoted to the elucidation of their marine equivalents with their abundant faunas, but a consideration of such tables as Fromaget's "Essai de Synchronisation des Formations géologiques dans le Nord de L'Indo-Chine centrale "2 or of the same writer's 'Résumé de la Répartition des Indosinias "3, will show that they are to be found in most parts of the country from Cochin China and Cambodia in the south, to Western Tongking and the Northern Laos at the other extreme. Only two cases can be briefly dealt with here, from Central Annam and the Northern Laos, respectively.
Prof. Jacob has described how from the latitude of Hué on the eastern side of the Annamite Cordillera, northwards as far as Vinh (a direct distance of approximately 190 miles) Continental Trias of and following the coast, even as far as Thanhhoa (a further 80 miles), the Trias is represented by conglomerates, sandstones and red clays, sometimes with coal seams as at the locality of Ha-tinh. Throughout this region it consists of an unfossiliferous, transgressive complex which further to the north-west still, passes into another folded complex of clays, reddish-purple shales and subordinate sandstones, described by Ch. Jacob and L. Dussault as occurring in the Tran-Ninh region of the Laos.4
North of Vinh, in the Hoang Mai district, though the Lower Trias still consists of conglomerates and red sandstones, fossiliferous marine deposite of Virglorian and Ladinian age occur but further west towards the Annamite Cordillera, these once more pass laterally into sandy shales and red clays.
In the Upper Laos, west of the Nam On valley, owing to the plunging of the prevailing anticlinal structures towards the north, the surface mantle of the "grès rouges salifères", comes eventually to outcrop over the greater part of the extreme north-west and, as Prof. Jacob has foreseen, probably extends into the Southern
Red Beds of the Upper
1 Géologie et Mines de la France d'outre mer: p. 417, (1932).
3 Bull. Soc. géol. France, 5th Ser., Vol. IV, (1934).
4 Géologie et Mines de la France d'outre mer: p. 410, (1932).
5 Bull. Serv. géol. Indo-Chine, Vol. XVI, Fasc. 2, fig. 20, (1927).
Shan States and into Southern Yunnan.1 In the Nam On valley itself and particularly at Phong Saly, a series of shales with thin coal seams has yielded a flora which although containing Rhætic forms is made remarkable by the abundance of other archaic species and these have led to its being placed in the Norian. These plant beds are the lateral equivalents of the base of the local Red Beds, the lower portions of which are consequently believed to be of the same age. The lowest members of the Red Beds are generally red clays with red or white sandstones and above them follow thick beds of red sandstone alternating with thin layers of clay. Green layers, or more frequently green stains, as well as beds of pebbles also occur, especially towards the base. The formation is salt-bearing but the salt is only found in impregnations which seem to be more abundant or even localised in the lower argillaceous parts. The red sediments are nearly horizontal and no discordance breaks the monotony of their stratification, nevertheless, in descending the series, a more and more marked tendency to folding developes, a phenomenon already met with in the Upper Red Beds of Central Yunnan. The formation has only yielded two fossils, the first a phyllopod, Estheria Zeilli Mans., which Mansuy considered as extremely close to E. mangaliensis Rupert Jones, of the Wardha-Gondwana basin of the Indian Peninsula, which occurs in the salt-bearing clays of the base; the second, Goniomya bisinuata Mans. from a conglomeratic red sandstone of the upper part of the Red Beds. J. Fromaget has discussed this fossil and concludes that its horizon is to be placed in the Rhætic, with the admission that the formation of the Red Beds could have continued into the Lias and even beyond.2
There is thus some correspondence between at least the lower part of the Laotian Red Beds and the upper horizons of the Red Beds of Central Yunnan, but this does not complete the analogy, for there is more than a suspicion of an age relationship between slightly older groups in both regions, as will now be seen.
Above the fossiliferous Upper Permian rocks of the Luang Prabang neighbourhood and particularly at Pou Say, there follows a series of variegated clays, often full of pebbles The Luang Prabang of Fusulina limestone, above which succesDicynodon. sively and concordantly come red, argillaceous
1 Bull. Serv. géol. Indo-Chine, Vol. XIII, Fasc. 4, p. 64, (1925).
sandstones and green, quartzose sandstones, passing by weathering into white and rose tints. From a band of green sandstone immediately above the variegated clays a reptilian skull was obtained by H. Counillon in 1892.1 It was described by J. Repelin in 1923 as Dicynodon incisivum and compared with D. orientalis Huxley from the Panchet Series of the Indian Gondwanas.2 If this comparison may be taken as a criterion of age then the beds from which this vertebrate comes are to be regarded as belonging to the Lower Trias, though to J. Fromaget they are the lateral equivalents of marine Carnian-Norian and Norian horizons.3 P. L. Yuan and C. C. Young, however, state that "Dicynodon" incisivum is clearly referable to Lystrosaurus, adding that Huxley's D. orientalis from the Panchets is probably also a member of the same genus. On the other hand, J. Piveteau, who has made the most recent examination of the Luang Prabang skull, identifies it with the South African Dicynodon lacerticeps Owen.5 In spite of these conflicting opinions it is not apparent that any change need be made in the presumed Lower Triassic age of the specimen.
Summarizing this brief review, it is seen that in addition to "Les Grès supérieurs" of Rhætic or post-Rhætic age and of "Les Grès inférieurs" or Terrain rouge inférieur ", usually assigned to the Norian, there is incontestable evidence of the existence in various parts of French Indo-China of still lower. horizons of Triassic Red Beds (in spite of the frequency of lateral marine facies), with which the lower parts of the folded Red Beds of Central Yunnan may properly be compared.
In a résumé published while this paper was in the press, J. Fromaget has divided the enormously thick covering of continental, lagunary and subcontinental deposits (the Indosinias) of Central Indo-China, as follows:
1. A Lower Series of sandstones and effusive rocks, dating from the end of the Hercynian phase to the Carnian.
2. A Middle Series constituting the "Terrain Rouge Inférieur "
which, in general, comprises two elements: one versi
1 Bull. Serv. géol. Indo-Chine, Vol. XIII, Fasc. IV, p. 55, (1924.)
3" Contribution à l'étude structurale du sud-est de l'Asie.", p. 15, (1934).
5 C. B. Soc. géol. France, Fasc. 6, pp. 70-72, 15th March, (1937).