Regional gravity and auxiliary geophysical measurements in the Songkhla, Yala and Pattani provinces in the peninsular Thailand and their geological interpretation

Abstract: A regional gravity survey and VES measurements were carried out in Songkla-Yala-Pattani provinces of Peninsular Thailand, an area between latitudes 6º 15'N and 7º 15'N and longitudes 100º 15'E and 101º 30'E. In addition the aeromagnetic maps of the area have been analyzed qualitatively as well as quantitatively in this thesis, an analysis that has not been undertaken before this. Peninsular Thailand is the long, north-south trending, narrow strip of about 1000 km joining mainland Southeast Asia in the north and the Malay peninsula in the south. The objective of this study is to add to the geological information in the area, for example, the extents of geological formations, the depths of sedimentary basins. Little about the geological picture of the area has been known because of sparsity and deep weathering and lateritising of exposures. Altogether, more than 600 gravity points and some 620 linekilometres with station spacings of 0.5 km and 2.0 km were measured. The VES measurements comprised 27 sounding points with maximum AB/2 of 1 km in 2 subareas (Saba Yoi basin and Chana basin). In addition to these, density measurement on rocks from 26 sites and the field determination of alluvium density on one selected site were carried out. An absolute Bouguer anomaly map was prepared. It shows a strike of the gravity anomaly (N20E) that agrees with the geological map. However the gravity results show high Bouguer anomalies in many places where on the geological map there is Quaternary rock and Triassic rock. These anomalies have been interpreted as due to very shallow granitic basement which has not been known before. The extent of sedimentary basins, determined from low anomaly areas, I.e. the Saba Yoi basin. Hat Yai basin, Chana basin and Yarang basin were found to be smaller than the ones drawn on the geological map. The Triassic mountain range on the west of Saba Yoi appears as low anomaly region and was interpreted as a Triassic/Carboniferous basin with a depth extent of 10 km. The limestone outcrops to the southwest of Saba Yoi and west of Changwat Yala appear to follow trends of faults defined by closely spaced gravity contours. There are limited regions of granitic basement, here assumed to be slabs at about 12.6 km depth and confined to the northern part of the area. in which the magnetic content (<0.5 vol.%) seems to be locally higher than normal in the basement. Vertical electrical sounding has been proved to be a viable for determining shallow sedimentary sequences about 100 m to 300 m deep within a sedimentary basin. Geoelectrical stratification has been derived for the Chana and Saba Yoi basins.