Typology and formation of pothole along the Mekong river and erosional remnant in sandstone of Khorat group, Ubon Ratchathani province, northeastern Thailand

Abstract

Phachan Samphanbok Geopark showcases fascinating erosional features, such as a multitude of potholes in the Mekong River. Additionally, it is home to the largest pedestal rock in Thailand, located in Ubon Ratchathani province in the northeastern part of the country, near the border between Thailand and Laos PDR. The formation of potholes is attributed to the erosive force of swirling sediment within fractures caused by vortex currents. In this research study on pothole evolution, the potholes in the Mekong River were categorized into two main groups: structural control potholes and non-structural control potholes. These types were further divided into five groups: ovoid potholes, lateral potholes, open lateral potholes, hierarchical potholes, and coalesced potholes. To analyze pothole characteristics, pebbles within sandstone beds were counted, and samples of pothole grinders were collected for composition comparison. The study revealed that a higher proportion of grinders in a pothole increases the likelihood of its occurrence. Stratigraphic columns were constructed in the study area to describe lithofacies and sedimentary structures. Pothole occurrence was found to be related to pebbly sandstone and trough cross bedding, which are associated with hierarchical potholes. A drone was utilized to capture aerial photos in a grid pattern, enabling the estimation of pothole density and categorization into three zones: high-density, moderate-density, and low-density pothole zones. Several factors were identified as controlling pothole occurrence, including horizontal bedding, the narrow width of the river leading to increased flow velocity, and the location situated in the cutbank zone. The formation of the giant pedestal rock is attributed to ancient gullies eroding into vertical fractures during the last tectonic event. The study of lithofacies indicates a paleoenvironment characterized by a braided river, with paleocurrents flowing in a southwest direction. Overall, this research emphasizes the significance of promoting the Phachan Samphanbok Geopark as a world-class geological heritage site. It also provides suggestions for the local communities and recommendations for future risk management in the Phachan and Samphanbok areas.