MICROMORPHOLOGY, MINERALOGY, AND GEOCHEMISTRY OF SEDIMENTS AT THE THAM LOD ROCK SHELTER ARCHAEOLOGICAL SITE IN MAE HONG SON, THAILAND: SUGGESTIONS OF A LATE PLEISTOCENE CLIMATE

摘要

Sediments from an archaeological pit at the Tham Lod rock shelter in Mae Hong Son, Thailand, reveal sparse palaeoenvironment and palaeoclimate information in the north-western region of Thailand and Southeast Asia. This study is a pioneering work for the site. Micromorphological, mineralogical, and geochemical methods were applied for analyses. The sediment profile was divided into two zones: the upper 0-338 cm depth and the lower 338-450 cm depth. Intense chemical weathering occurred in the lower zone, indicated by low quantities of kaolinite and high chemical index of alteration (CIA, 64.55-73.36%), reflecting greater humidity. The upper zone yielded mild CIA (47.21%) and contained ubiquitous smectite, indicating drier conditions. Overall sediments (along the profile, 0-450 cm depth) were affected by leaching and thin section analyses suggested a wet/dry cycle during diagenesis under humid tropical conditions. Micromorphological and geochemical analyses indicated that mudstone, sandstone, and limestone were major parent materials of sediments deposited mainly in a fluvial environment in the lower zone, which changed to a colluvial environment in the upper zone. The palaeoclimate during 12,770-33,340 yr BP was in the Last Glacial Maximum Period, characterized by drier and cooler than the present climate. The period 33,340 yr BP was wet and cool. Evidence of prehistoric human occupation, including bone and charcoal fragments, appeared in sediments in the upper zone, suggesting animal hunting for survival.