Intermontane basins of Northern Thailand

Tertiary

Lacustrine or fluviatile sediments are known in six intermontane basins of Northern Thailand and probably underlie a seventh. At Maemo and Maechang near Lampang, clay, shale, sand, and marl beds of the Maesot series a number of feet thick contain lignite-bearing sediments. Similar sediments have been reported in the Maerim Valley north of Chiangmai. In the Maefang basin tilted carbonaceous sediments have been reported in drilling records.

The Krabi series, comparable in age to the Maesot series, have been found in eight basins in the Peninsula. These are at Khiansa, southwest of Suratthani; at Hin Ru, east of Phangnga; at Hin Nak, west of Krabi; at Krabi; at Simpun, northeast of Krabi; at Kantang, southwest of Thapthiang; at Bukit Arang, south of Sadao; and at Betong. The sediments, all of which include lignite beds, are best known near Krabi. Recent alluvium covers most of the series, restricting outcrops to stream beds and beaches.

Terrace Deposits and Quaternary Alluvium

These deposits are present in almost every stream valley of Thailand. Alluvium underlies the entire Central Valley--the Upper Plain and the Bangkok Plain--drained by the Čhaophraya River. Isolated areas of thick alluvium are also scattered over Khorat, in the larger stream valleys of northwestern Thailand, and at various places along the coastal plain around the Thai Gulf, especially long the East Coast of the Peninsula. The alluvial deposits in the Peninsula and western Thailand, which contain placers of tin and tungsten, are of special geological importance.

Most of the terrace and alluvial deposits are less than 150 feet thick. Low dissected terraces along the West Coast of the Peninsula lie 10 to 40 feet above sea level and reach a maximum reported thickness of about 50 feet. Along the East Coast of the Peninsula, however, the alluvium is as much as 100 feet thick, and in the Bangkok Plain, near Bangkok, drillings more than 1,000 feet deep have failed to reach consolidated rocks. Structural basins in northwestern Thailand may contain as much or more debris of Pleistocene and Recent age.

The Quaternary deposits are almost all of stream or beach origin. The Bangkok Plain doubtless includes beds of deltaic character and is known to contain marine or estuarine marly beds and sands and gravels. Wherever the Quaternary deposits are exposed, they comprise sand, gravel, silt, and clay.

Characteristically associated with many planated surfaces, marine and fluvial terraces, and elevated mesas or plateaus, are residual capping layers of laterite which are generally mottled red and brown and are vesicular or pisolitic. Laterite may be formed from rocks diverse in geologic age and in physical or chemical character. The stage of development of laterites in Thailand is apparently related directly to the physiographic age and position of the surface on which they form. Young laterites characterize young low-level terraces.