The Quaternary alluvial and coastal plains in Thailand covered approximately forty percent of the whole area of the country. These deposits are principal areas for the production of rice, the staple food of the country. The capital and major cities are mainly located on these Quaternary sedimentary plains. The Quaternary deposits are also of importance for economic mineral exploration, construction materials, groundwater, for evaluation of land use and rural development planning and also for coastal development projects. A complete stratigraphic sequence of Quaternary age probably developed in the Central Plain and these sedimentary formations can be classified into four distinct categories based on their histories, lithology, morphology, fauna and depositional environments. These are the fluviatile environment, the coastal environment, aeolian deposits and laterite. Block-faulting appear to be controlled by tectonic movements which affected also the changes in base level and erosional processes. These were followed by infilling of Quaternary alluvial sediments into the larger valley basin systems. Changes in climate and environmental processes were involved during these tectonic movements. Aeolian activity possibly occurred during the glacial period which coincided with drier conditions in this region. Weathering in humid tropical climate is generally responsible for laterization. The marine transgressionregression during Late Pleistocene-Holocene is also acceptable according to the available evidences. Quaternary stratigraphy ofThailand is in an embryonic state ofdevelopment therefore it seems most desirable that attempts to set up local time stratigraphic classifications based on various types of lithogenetic sequences be continued in a variety of geographic areas. Before a valid Quaternary stratigraphy of Thailand can be established, more detail studies together with accurate absolute age determinations are needed.
The part of Mangahao River that traverses the Tararua Range, upstream from Kakariki, was captured at some time in the Holocene Epoch (about 5,000-10,000 years B.P.) from the Hukanui Stream, which was formerly a major tributary ofthe Mangatainoka River. The captured segment now delivers high floods more than an order of magnitude greater than the mean annual flow of the Mangahao, and its diversion from the Hukanui Stream was a large drainage change. The capture site is on the line of the active Wellington Fault. The main factors that caused the capture were: (1) dextral displacement of the captured segment relative to the former Mangahao River; (2) in sequent erosion along the crush zone of the fault by a tributary of the former Mangahao; (3) aggradation of the floodplain of the captured segment during the Last Stadial of the Last Glaciation; and (4) easy lateral erosion of the fault crush zone by the captured segment during early Holocene time. The Mangahao provided a shorter route than the Mangatainoka to the local base-level (head of Manawatu Gorge) and the newly diverted water quickly excavated a deep ravine that now has a flood-plain 45 m lower than the early Holocene flood-plain of the Hukanui Stream at Kakariki.
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