The surface sediment distribution was highly regular in the UGOT. Three types of sediment were observed in the UGOT, i.e.,silt, sandy silt, and silty sand (Fig. 3A). The fine-grained silt sediment, which mainly covers the Chao Phraya River mouth area, is
either dark gray or gray olive in color with a strong rotten egg smell. The coarsegrained silty sand sediment can be found southeast of the study area, which is characterized by abundant shell fragments. The dark gray sandy silt sediment with some shell fragments is distributed between the two previously mentioned areas. The Mz increases from 8.6 lm to values >110 lm in a seaward direction (Fig. 3B). The results on oceanographical data in the gulf during March 1993 show that clay, sandy clay, and sand sediment are distributed from north to south in the UGOT (Srisuksawad et al., 1997) but has now become silt, sandy silt, and silty sand. Sediments near the Mae Klong River mouth area are sandy silt based from our results, whereas a previous study has reported it to be clay (Srisuksawad et al., 1997). Emery and Niino (1963)showed the sediment distribution of the gulf, but they considered that the amount of calcium carbonate was too low to affect the grain size parameters in the gulf of Thailand and thus analyzed
the whole sample with calcium carbonate. However, the UGOT is abundant in shell fragments, which are calcium carbonate-rich and are significant in determining the grain size composition of sediments (Fig. 3B). The total sediment color in the study of Emery
and Niino also show that samples near the Chao Phraya River mouth is yellow brown, whereas the adjacent bank is olive brown. The averaged Mz of sediments collected from the Chao Phraya River and Bang Pakong River is 55 lm (Fig. 3B).