Magnetostratigraphy of the Phu Thok formation at Phu Thok and Phu Wua areas, Changwat Nong Khai

The Phu Thok red sandstone rocks, which expose at Khao Phu Thok and Phu Wua, Changwat Nong Khai, which have general attitude following the NW-trending regional structure which is fault-bound. They consist of two distinct sedimentological units of which the first is thickly-bedded, reddish brown, fine- to medium- grained (arkosic) sandstone with large-scale cross-beddings. The second is purplish to brownish red, mostly calcareous, very fine-grained sandstone (and some siltstone) with wavy structures. The total thickness of the whole Phu thok sequence is about 205 m. The oriented samples collected by protable core drilling machine from two continuous sections of the sandstone at Khao Phu Thok and Phu Wua, were measured by digital spinner magnetometer. The results indicate that the secondary magnetism is completely demagnetized in temperature range from 350° to 500℃ or may be up to 650℃. The magnetostratigraphy of the Phu Thok sequence constructed from 351 specimens clearly indicate seven normal and seven reverse polarity bands. In addition, this magnetostratigraphy is well correlated with the Standard Geomagnetic Polarity Time Scale at time of Early Cretaceous (Valanginian to Hauterivian). The Mean VGP (Virtual Geomagnetic Pole) of the Phu Thok rocks is calculated from 225 significant specimens. It is 61.9°N, 189.9°E with K = 1006.0, A95 = 1.8 and is quite similar to the overall VGP of the average Khorat Group. This implies the little or no movement of the Indochina micro-Plate until Phu Thok rocks were deposited (at paleolatitude of 17.8°±1.2°N, in part of the Indochina microplate). Moreover, the northeast declination of primary magnetism in the rocks possibly advocates the clockwise rotation of the Indochina taking place after the cessation of Phu Thok deposit.