The Early Stage of the Burmese-Malayan Geosyncline

The history of thi s geosyncline is divided into three stages: (1) Devonian and older ; (2) late Devonian to middle Triassic; and (3) la ter Mesozoic period. The so called Pre-Cambrian metamorphic rocks require a further confirmation of their ages Upper Cambrian is the oldest dated sediment of th e geosyncline. Three facies named Shihtien, Naungkangyi, and Thungson, can be distinguished in the Ordovician formation. In the later Palaeozoic sequence carbonate-rich and carbonate-poor facies altern ate twice or more with intercalations of graptolites and tentaculites facies and other clastic facies . Broadly speaking, the early stage was the age of epirogenic undulations, whil e the second stage was the age of embryonic foldings by which the geosyncline was partly modified, and at length it was wholly deformed and consolidated by Mesozoic orogenies. In the Thai-Malayan Peninsula a Carboniferous fauna was first described from Phattalung by Reed (1920), but for a long time little was known of the older history (Alexander, 1956). In the last 15 years, however, all of the Palaeozoic Systems were documented by fossil evidence in the southern area of this zone. Therefore, the "Sino-Burmese-Thailand-Malayan geosyncline" was abbreviated to the "Burmese Malayan geosyncline" (Kobayashi, 1964). In my opinion it is relevant to divide the history of its development into three stages. Main events in the early stage were wide and gentle movements of the geo syncline, causing oscillations of shore line in the Devonian and older periods as revealed in the gradual variations of sedimentation and local discordances. In the middle stage, from late Devonian to middle Triassic, local crustal disturbances and igneous activities took place, but they were not strong enough to change the principal outline of the geosyncline. During the Mesozoic Era, however, the geosyncline suffered strong orogenic activity accompanied by the emplacement of granites through which the whole geosyncline has become emergent and consolidated. CAMBRIAN SEDIMENTS AND THE SO-CALLED PRECAMBRIAN ROCKS Upper Cambrian trilobites occur in shales at Paoshan and Mangshih, west Yunnan, in the north, and in the Tarutao sandstone at Tarutao island just to the north of the Thailand-Malaysian boundary (Sun, 1939, Kobayashi, 1957, Lu, 1964). In the Langkawi Islands to the south of the boundary the thick "Machinchang