Monadnocks of the Mekong delta: character, caves and evolution

Abstract

Isolated limestone hills and towers near the Gulf of Thailand coast, and granitic monadnocks that extend further inland, protrude through alluvial silts of the Mekong River delta in Vietnam and Cambodia. The form of the granitic monadnocks is guided by bedrock geometry and some exhibit extensive boulder mantles residual from deep weathering within which large boulder cave complexes occur, together with underground stream systems that feed permanent springs. The limestone hills vary in form according to differences in structure, rock purity and hill-base topographic setting and contain vertically-developed vadose invasion caves and basal floodwater stream caves. Evolution of the present day hillbase and stream cave morphology has been influenced by changes in the base level of vadose activity that have been related to changes in relative sea level, most recently as the Mekong delta has formed during the Holocene