Geology of Thailand and her Surroundings

Abstract

This is a critical review of the geology of the region. The stratigraphy is discussed in dividing the terrain into three tectonic units as shown in fig. 1, and schematized as indicated in the table on page 6. The best Palaeozoic sequence can be seen in the west zone. There however, the Devonian system is unknown, but present in the east flank of the Pak Lay anticline in the median zone.
The Khorat series in the Khorat plateau in the east which contains dicotyledonous plants must be Cretaceous, instead of Jurasso-Triassic. It is quite probable for it to extend easterly into the Indosinias supérieures of South Laos which yielded Senonian Reptiles and non-marine Trigonicides faunule and southerly into Grès continentales of Cambodia which is also Upper Cretaceous, if not Palaeogene. Thus it is certain at the least that marine Jurassic and Triassic formations should be eliminated out of the continental Khorat series.
The Triassic formation in the median zone is accompanied by eruptive rocks, like the Pahang volcanic series, but it is known paralic in North Laos. In the west zone there are Triassic and Jurassic formations devoid of volcanics. They are strongly folded, while the Cretaceous (?) red sandstone of Tenasserim is gently monoclinal.
It is probable that the old Cimmerian folding in the median zone has confined the flooding of the Jurassic sea to the west zone and that crustal movements have taken place in the still older periods on the west side of the Indosinian massif. Nevertheless it is quite warranted that the folded mountains through the middle-west of Thailand has not been established by the Cretaceous period. Older rocks are intruded by granitic rocks, but not the Khorat series as well as the Tenasserim sandstone. They are both continental, and slightly undulated or simply inclined with low angles. Thus the vast terrain turned out land in the Palaeogene. The influence of the Himalayan orogeny was weak in this part of Asia.