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Development of the Lower Central Plain of Thailand with History of Human Settlements: Evidence from Pollen, Spores and Diatoms




Author(s)

Wickanet Songtham, Sumalee Musika, Dallas C. Mildenhall, Ursula A. Cochran and Darya Kojevnikova

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DOI:10.17265/2328-2193/2015.02.004

The lower central plain of Thailand is characterized by a flat, low-lying area in the central region bound by mountain to the west, north and east and connected to a coastal shoreline of the Gulf of Thailand to the south as a sedimentary basin containing a thick sequence of Quaternary sediments. The present study focused on the Bangkok Clay, a dark grey to black, soft marine clay sandwiched between thin layers of peat and fluvial sediments. The Bangkok Clay is about 25 meters thick in the middle portion of the shoreline, tapering towards the basin margin to the west, north and east forming a half lens-shaped, dark grey clay body. The Bangkok Clay contains abundant and diverse organic remains including vertebrates, invertebrates, plants and microscopic algae. Palynological analysis has yielded diverse assemblages of pollen, spores and diatoms. The pollen and spores include several types of Avicenniaceae, Brownlowia sp., Combretaceae, Cyperaceae, Myrtaceae, Pinaceae, Poaceae, Rhizophoraceae, Sonneratiaceae, Xylocarpus sp., Acrostichum spp., Gleicheniaceae and Stenochlaena palustris. In addition, coastal diatoms derived from the palynological slides include Diploneis elliptica, Tryblionella cocconeiformis, Coscinodiscus radiata, Nitzchia sp., Tryblionella cf. compressa, Coscinodiscus sp., Paralia sulcata, Actinocyclus sp., Triceratium reticulum, Surirella sp., Cocconeis costata, and Thalassiosira sp. The Bangkok Clay was, therefore, formed in intertidal and shallow subtidal environments under a tropical moist climate. The Bangkok Clay is sandwiched between two thin peat layers. The basal peat developed through continuous accumulation of plant debris along the shoreline during the marine transgression of the Late Pleistocene to Early Holocene (ca. 8,000-7,000 years ago) whereas the upper peat formed during the Early Holocene to present day marine regression. Migration back and forth of the mangrove vegetation is recorded in these sediments as sea-level changed. A reddish-brown, mottled, stiff clay beneath the basal peat is devoid of pollen whereas the dark grey, silty clay layers overlying the upper peat, developed by a fluvial system migrating as the marine regression receded, contains palynomorphs from terrestrial environments including freshwater swamps. The pollen and spores include Ceratopteris thalictroides, Cocos sp., Cyperaceae, Ipomoea sp., Jussiaea sp., Lagerstroemia spp., Poaceae, Polygonum sp., Typhaangustifolia, Nymphaea sp., and Lygodium sp. Even though the lower central plain area developed after ca. 8,000-7,000 years ago, no evidence of human settlements has been observed until around 2,000 years ago. There are many ancient Dvaravati towns along the 8,000-7,000 year-old shoreline and further inland, except around the 8,000-7,000 year-old and younger paleo-gulf. Around 2,000 years ago when the Dvaravati people had settled their communities along the 8,000-7,000 year-old shoreline and inland, the marine regression moved to more or less the present position leaving swampy areas with dense sedge-dominant vegetation where no evidence of human settlements have been found. Our research suggests that pollen, spores and diatoms are important tools differentiating fluvial and marine sediments allowing us to trace the history of sea-level changes closely related to human settlements. However, detailed age determinations for the two peat layers are strongly recommended for tracing the spatial and temporal development of the shoreline to understand the lower central plain development and history of human settlements.

Bangkok clay, Ayutthaya paleo-gulf, Dvaravati, central Thailand.