The three structural provinces of Thailand are Khorat, the Chaophraya depression, and the folded ranges in the north and west, extending south through the Peninsula. The folding of strata has taken place between the Indo-China massif on the east and the Himalayan belt of compression on the west. The latter extends south through Burma and Malaya into Indonesia. Thus, the structural elements in Thailand have originated within the Asiatic continental framework by adjustments to the massif on the east and the compressive forces on the west.
The principal mountain ranges have developed as north-south folds in the sedimentary rocks attributed to the intrusion of batholiths. Much of the overlying strata have been weathered and eroded away. The principal outcrops, beside granite, are hard and very pure limestones and some quartzites. The limestones stand almost vertically, and tower high above the surrounding formations. Because of the very purity of the limestones rain water percolates without leaving any residue of less soluble minerals. Very little organic matter can grow on these limestone rocks, for little soil develops. With the weathering of overlying sedimentary and metamorphic rocks, heavy tin ores have accumulated in the lower portions of the alluvium at the foot of the Peninsula ranges.
The present limited knowledge of the geology of Thailand permits only a very incomplete description of the history. The sedimentary record begins in early Paleozoic time, if the assumption of a Cambrian age for the Phuket series is valid. In black slate of the Phuket series are found granite pebbles and boulders derived from an earlier, possibly pre-Cambrian, granite. The country was the site of marine deposition throughout much of Paleozoic time. Most of the older Paleozoic sedimentary rocks are clastics and now form thick deposits of shale and sandstone or their metamorphic equivalents. The only important exception is the sandstone and limestone of Ordovician age, although thin beds of limestone or marble occur elsewhere in the older rocks. Paleozoic sedimentation culminated in the widespread deposition of the Ratchaburi limestone. Most of the country was elevated and the rocks were regionally metamorphosed by diastrophic movement approximately concurrent with the Saalien and Appalachian orogenies. The Saalien revolution seemingly involved most of Thailand. In the eastern part of the Khorat plateau and Čhaophraya depression the older beds are usually buried, but wherever exposed, the Paleozoic beds are moderately to extensively compressed into north-trending folds. In many places the beds appear isoclinically folded. The massive Ratchaburi limestone in many places lies at flatter angles than the underlying beds, suggesting either that the limestone was deposited nonconformably on the intensively-folded older Paleozoic sedimentary rocks or that the limestone was thrust up over them. Evidence favors overthrusting at the southwest corner of the Maesariang basin in the North.
The extensive intrusion of granitic magma dates from early Mesozoic, perhaps from the late stages of the Saalien orogeny, for slightly disturbed and unmetamorphosed sandstones of the Khorat series (Triassic and Jurassic) overlie granite stocks and batholiths intruded into the Paleozoic sedimentary rocks. The older hornblende-biotite granite now crops out in the hills of Central and Northern Thailand. Accompanying this extensive intrusion were local intrusions of mafic and ultramafic rock.
A younger biotite-muscovite granite crops out widely in the Peninsula and along the Burma frontier. Certainly the folding of the lower part of the Khorat series along the western edge of the Khorat region and the beds of possibly early Cretaceous age at Maesot suggest an orogeny rather late in the Mesozoic. Throughout most of the Mesozoic the country was elevated slightly above the sea and widespread deposition of continental sandstone, shale, and conglomerate of the Khorat series took place. A marine invasion in Jurassic time was apparently limited to the zone along the Burma frontier near the Maesot basin.
At the beginning of Tertiary time most of the country was above sea level and a well-integrated drainage was established. The faulting and folding movements which led to the development of the structural basins characteristic of the North and common in the Peninsula probably began in middle or late Tertiary time. Fluviatile and lacustrine sediments accumulated in the basins of the North; and in the Peninsula, especially along the coast, terrestrial sediments were deposited in association with marine materials. The structural movement probably continued in pace with the accumulation, so that the basin sediments were gently to moderately warped and in places faulted.
Local volcanic and intrusive activity occurred in many scattered localities of the Kingdom, probably in late Tertiary time. This activity resulted in basalt flows and dikes in Čhanthaburi, Trat, Kančhanaburi, and Lampang čhangwat and in isolated diorite stocks cut by later andesite and rhyolite porphyry intrusions in the central part. Toward the end of Tertiary time regional uplift occurred, accompanied by faulting and folding of most of the country.
Throughout Quaternary time to the present the Čhaophraya depression has accumulated sediment supplied to it by tributary streams, which have been grading their courses by erosion in the upper reaches and by alluviation in the lower reaches. The accumulation of Quaternary alluvial deposits in the stream valleys of the Peninsula has produced most of the rich tin deposits.In general, the eastern limits of the belts of orogeny have moved west insofar as the geologic history is known. The Saalien orogeny apparently affected most of Thailand, a Mesozoic orogeny of uncertain age affected most of the western half of the country, and the late Tertiary (Wallachian) orogeny of the central belt of Burma moderately affected the Tertiary beds only along the western frontier. However, high-angle faulting and some renewal of thrusting, together with local eruptions of basaltic lava, did occur throughout the Kingdom, perhaps as a manifestation of the late Tertiary orogeny, elevating Khorat and depressing Tertiary and Quaternary deposits in the intermontane basins of the North and the Peninsula. In comparatively recent times the Peninsula has been tilted slightly northwest, for its east coast on the Thai Gulf has a smooth emergent shoreline backed by a broad coastal plain, whereas the shoreline on the Andaman Sea is marked by strong valleys and prominent headlands or offshore islands such as those of Phuket Bay.
No comments:
Post a Comment