Palaeocurrent direction estimated in Mesozoic redbeds of the Khorat Plateau, Lao PDR, Indochina Block using anisotropy of magnetic susceptibility

Abstract

Mesozoic redbeds in the western part of central and southern Laos are the most abundant rocks of the Khorat Plateau. In earlier studies, palaeocurrent directions of the Mesozoic redbeds in the Khorat Plateau were mapped, but for the Upper Cretaceous sediments no palaeocurrent data was presented. The purpose of the present study is to test previous models of sedimentary source areas and propose palaeocurrent flow directions for Upper Cretaceous sediments in the Khorat Plateau. This we do by characterization of the rocks on the basis of their magnetic properties and anisotropy parameters. Integrated magnetic properties, including susceptibility, NRM, Q-value, and S-ratio, have been used for classification of the stratigraphies of the redbeds. A clear difference in magnetic properties was observed between the Jurassic and Cretaceous rocks, while some sites of the Lower and Upper Cretaceous were similar. From magnetic analyses the Champasak redbeds could be defined statigraphically as an unconformity beneath the Khorat Group. On the basis of the AMS interpretation, the rocks can be divided into undeformed and deformed rocks. The palaeocurrent directions in the undeformed redbeds are predominantly from the NNE and NE of the Khorat Basin. For the Upper Jurassic-Lower Cretaceous redbeds, we argue that the sources rocks of the sediments were located in the northern and eastern margin of the Khorat Plateau, probably in the Truong Son Belt zone in the northeastern Laos and central Vietnam. While, the source rock of the Upper Cretaceous redbeds in the Khorat Plateau was located in the Qinling Belt in China or in the Loei-Luang Prabang Fold Belt in northeastern Thailand and northern Laos.