Geology, lithostratigraphy, petrography, sedimentary structures and depositional environment of marine Jurassic rocks of the Hua Fai Group (HFG) from the Mae Sot area of Tak Province, western Thailand have been studied in detail. Based mainly on five measured sections in the Mae Sot Basin, the group can be divided into three formations: Khun Huai; Doi Yot; and Pha De, in ascending order. Seventeen sedimentary rock units have been established with a total thickness varying from 200 to 832 m. According to the fossil assemblages, the HFG was deposited during Toarcian–Bajocian. Analysis of lithofacies associations in the sedimentary sequences of the group discerns shoreface, fan-delta, protected lagoon, intertidal, subtidal and inner to outer ramp environments with occasional carbonate platforms and reef flats. Toarcian rocks are represented by transgressive–regressive (T–R) cycles that in the Aalenian gradually changed to highest sea level and water depth. During late Aalenian to early Bajocian, sea level was still changing to a transgressive phase, then after early Bajocian, sea level retreated again. The eustatic curves for the Toarcian–early Bajocian in Thailand correspond to the global situation, but differ significantly in the Late Jurassic–Cretaceous, when T–R phases were out of step with the global pattern and instead probably responded to local tectonic movements.
We applied remote sensing techniques and geomorphic index analysis to a study of the NE-SW-striking Thoen Fault, Lampang Basin, northern Thailand. Morphotectonic landforms, formed by normal faulting in the basin, include fault scarps, triangular facets, wine-glass canyons, and a linear mountain front. Along the Thoen Fault, the stream length gradient index records steeper slopes near the mountain front; the index values are possibly related to a normal fault system. Moreover, we obtained low values of the ratio of the valley floor width to valley height (0.44–2.75), and of mountain-front sinuosity (1.11–1.82) along various segments of the fault. These geomorphic indices suggest tectonic activity involving dip-slip displacement on faults. Although the geomorphology and geomorphic indices in the study area indicate active normal faulting, sedimentary units exposed in a trench at Ban Don Fai show no evidence of recent fault movement. In Ban Don Fai trench No. 2, accelerator mass spectrometry radiocarbon (AMS) ages and optically stimulated luminescence (OSL) ages indicate that deposition of the lowest exposed sedimentary unit occurred between 960 and 910 years ago. Therefore, the most recent movement upon the Ban Don Fai segment of the Thoen Fault occurred more than 960 years ago.
Abstract The Khorat Plateau from northeast Thailand, the upstream part of the Mun River flows through clastic sedimentary rocks. A massive amount of sand was transported. We aimed to understand the evolution of fluvial system and to discuss the advantages of two shallow geophysical methods for describing subsurface morphology of modern and paleo-channels. We applied Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) to characterize the lateral, vertical morphological and sedimentary structures of paleo-channels, floodplain and recent point bars. Both methods were interpreted together with on-sites boreholes to describe the physical properties of subsurface sediments. As a result, we concluded that four radar reflection patterns including reflection free, shingled, inclined and hummocky reflections were appropriated to apply as criteria to characterize lateral accretion, the meandering rivers with channel-filled sequence and floodplain were detected from ERT profiles. The changes in resistivity correspond well with differences in particle size and show relationship with ERT lithological classes. Clay, silt, sand, loam and bedrock were classified by the resistivity data. Geometry of paleo-channel embayment and lithological differences can be detected by ERT, whereas GPR provides detail subsurface facies for describing point bar sand deposit better than ERT.
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