Late Quaternary geology of the Lower Central Plain, Thailand
Introduction
The Central Plain, a broad, flat, low-lying area of Thailand, is divided into upper and lower parts. The Upper Central Plain originates from where the Ping, Wang, Yom and Nan Rivers flowing from the north, combine to form the Chao Phraya River in the Pak Nam Pho district, Nakhon Sawan province. Around this confluence a number of monadnocks stand over the plain. The Lower Central Plain begins at Chainat, where the Chao Phraya River flows southward through a flat and featureless plain until it reaches the Gulf of Thailand, Chao Phraya River and its tributaries created the broad depositional surface with its well-defined meander belts. The evolution of an extensive flat deltaic area can also be recognized, in which swampy, marshy, and tidal flats occur.
Lower Central Plain is crucial to the economy of Thailand because the major cities are located in this plain, including the capital and principal port Bangkok, former capital Ayutthaya, and the rice bowls of Thailand such as Chainat, Angthong, Sing Buri and Suphan Buri. Agriculture is important throughout the plain with paddy rice and fruits, while aquaculture is practised along the coast. The main distributary rivers on the plain are also important for trade, transport and infrastructure and link the major towns.
This plain has some of the most densely populated urban areas. Sedimentary successions forming the plain are known to contain vast quantities of groundwater. The plain is also considered to be a potential area for hydrocarbon accumulation and a source of construction materials, particularly sand. However, the plain is still one of the most serious environmental impact areas of the country in terms of flooding and subsiding.
Geological data for the Lower Central Plain, particularly the subsurface geology, was investigated by the Groundwater Division of the Department of Mineral Resources, whereas the Quaternary sediments are now being studied in various parts of the Lower Central Plain by the Geological Survey Division of the same department. This paper presents new geological information, together with data obtained from other studies on the Lower Central Plain, including the Chao Phraya delta which is a major geomorphological feature of the area. The Late Quaternary geological history and deposits of the Chao Phraya delta are also described. This geologic information will be significant for the future development of urban areas in the Lower Central Plain.
Section snippets
Location
The northern boundary of the Lower Central Plain is delineated in the Chainat province (15°15′N, 100°15′E) where the Chao Phraya River passed the monadnocks and flowed southward through the flat, low-lying plain until reaching the Gulf of Thailand at Samut Prakarn (14°30′N, 100°30′E). The elevation of the plain ranges from 15 m A.S.L. at Chainat to 2.5 m A.S.L. at Ayutthaya and 1.5 m A.S.L. at Bangkok which is about 25 km north of the Gulf of Thailand (Fig. 1).
The southern edge of the plain is
General geology of the plain
The geology of Lower Central Plain reflects the structure of the depositional basement, basement rocks and pre-Quaternary geology, and the Quaternary sediments were deposited in a broad, structural basin.
Quaternary geology
The Quaternary geology of the Lower Central Plain has been neglected and is poorly understood because of the thick accumulation of unconsolidated sediments and inaccessibility of subsurface data. The first general outline of Quaternary deposits in this plain was produced by Brown and others in 1951 (Thiramongkol, 1983). Subsequently, Alexseev and Takaya, 1967, Takaya, 1968, Takaya, 1971, Takaya, 1972) have described the lithology and landforms of the unconsolidated sediments in the Central
Late Quaternary geology
Most of the well known geological information in the Lower Central Plain relates to the Late Pleistocene to Holocene epochs. The sedimentary succession and depositional environments have been obtained from shallow boreholes, open pits and groundwater well samples, and are classified into fluvial, alluvial, tidal flat and deltaic environments.
Conclusions and recommendations
The present geologic information concerning the Chao Phraya delta can be summarized in the following statements.
- 1.
The term Chao Phraya delta is defined mainly on the basis of the Holocene depositional environment, while the Pleistocene delta is questionable, particularly during the Late Pleistocene when geologic records indicated a regression of sea level in Thailand.
- 2.
The Holocene Chao Phraya delta is of composite origin with distinct parts dominated by fluvial and tidal processes; each has its
Acknowledgements
The author is grateful to Mr Chaiyan Hinthong, Director of the Geological Survey Division, for his encouragement. Appreciation is extended to Dr Assanee Meesook for reading the manuscript. Special thanks to Mr Apichard Lumjuan, Mr Kittisak Sonklin and Mr Watcharin Phimonphan for drawing and drafting. Thanks are due to Mrs Pranee Silarak for typing the paper.
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