The Mai Sai basin and the Wiang Nong Lom swamp lie north of, and adjacent to, the northeast-trending Mae Chan fault, which is a left -lateral strike-slip fault in northernmost Thailand. The sediment in the Mae Sai basin is about 600 meters thick. The Mae Sai basin has an anticlinal fold below a 390-meter depth. This fold is buried by broadly downwarped sediment. In the Wiang Nong Lom swamp, sediment is about 170 meters thick. Radiocarbon ages from a 2.7-meter auger hole at Ko Mae Mai in the Wiang Nong Lom swamp show that 0.5 meter of sediment accumulated in the past 218 years. This sediment rests on an older stiff clay. The stiff clay is Early Holocene in age. The age of this clay 1.05 meters below the sediment surface is 9,830 years. The stiff clay has hematite staining and a total organic carbon content of less than 0.6 per cent. This suggests that it has been sub-aerially exposed prior to inundation about 220 years ago. Much of the sedimentary section at a site on the north edge of the swamp is older than Early Holocene and recent sediment there is no older than a few hundred years. A hiatus of deposition occurred when the site was dry. Time of inundation at this site does not agree with the legendary AD 460 earthquake and submergence of Yonok, but the age does allow that this site may have been dry and habitable in the interval from 1786 back to BC 9270. The sedimentation rate for clayey sediment at this site during the past 200 years is about 2.2 millimeters per year. This compares to similar high rates in the lake at Phayao in northern Thailand.