Upper succession of cenozoic sedimentary facies in the southern part of the Phrae Basin, Northern Thailand

Abstract

The Cenozoic intermontane Phrae basin is located in Changwat Phrae, Northern Thailand. The basin lies between latitudes 18 degree to 18 degree 35' N and longitudes 100 degree to 100 degree 20' E. The shape of basin is ellipse with NNE-SSW of approximately 60 km. long and 15 km. wide. The northern end of the basin is bifurcated. The basin covers about 1,100 sq.km. The study area occupies the southern half portion of the basin covering an area of approximately 300 sq.km. This research aims at analysing the Cenozoic an area of sedimentary facies in the upper succession of the Phrae basin in order to reconstruct the depositional environment and the evolution of the sedimentary basin. The data employed in this study is composed of 12 drill-holes with wire-line geophysical logs of gamma ray, caliper, and density. The 2-D seismic survey data of 156 line-km. is also employed in the study. The Phrae basin is classified as a fault-bounded basin. The Phrae-Thoen fault is the main fault that strongly influenced the development of the basin. The Phrae-Thoen fault trends northeast-southwest and displays a sigmoidal shape. The thickness of the Cenozoic sedimentary sequence within the Phrae basin is about 1,500 m. The sedimentary facies of the upper part of the Phrae basin are generally characterised as alluvial fan, fluviatile, lacustrine facies associated with fluviatile facies and peat swamp facies. The regional and local tectonics are believed to be the major controlling factors of the variation in the depositional environment. At least five tectonic events can be recognised in the sedimentary sequence.