Sedimentological studies of some tertitary sediments of Li Basin, Changwat Lamphun

Abstract

The Cenozoic intermontane Li basin is located in the southern part of Changwat Lamphun, northern Thailand. The basin has a elliptical shape with longitudinal axis oriented in the north/northwest-south/southeast direction. The maximum width is about 18 kms. Area. The Li basin is surrounding by high mountain ranges of the pre-Cenozoic rocks in three sides, namely, the east, the south and the west, and open in the north. The data employed in this study composed of 616 borehole data of totally 47.56 kms. In length with the lithological description. Amongst these data, 228 boreholes geophysical logs are available, namely, gamma-ray, neutron, long spacing density, bed resolution density and caliper. Besides, the paleontological and palynological reports are also available. Within the Li basin, attention has been focusing upon five coal-bearing sub-basin where data are available, namely, Ban Pu, Ban Hong, Ban Mae Long, Ban Pa Kha and Ban Na Sai-Ban Mae Wang, covering approximately 12, 15, 10, 8.5 and 14 square kms. Area, respectively. The thickness of Tertiary sedimentary sequences within the Li basin is greater than 500 metres in the deepest part. The major structural trends in the vicinity of the Li basin lie in north-south direction, northwest-southeast direction with conjugated north/northeast-south/southwest direction, clearly recognized in the southern and the western parts of the basin. The major structures of the Li basin compose of a series of grabens, half-grabens or tilted fault-block and horsts which are comformabel to this regional trend. These faults are believed to control the development of basin as well as the sedimentation within the basin. The extension force in northern Thailand is believed to have caused by the collision of India with Southeast Asia, and interactions of major faults zones in this area. The overall sedimentary facies of the Li basin especially in sub-basinal areas are generally characterized as alluvial fan, lacustrine facies associated with meandering fluviatile facies, peat swamp facies, lacustrine facies associated with braided fluviatile facies, peat swamp facies with lateral facies change to lacustrine and meandering fluviatile facies in ascending order. The sedimentary units and sedimentary facies of the Cenozoic sediments within the Li basin show variation of lateral facies change. The regional and local tectonics are believed to be the major controlling factors of the variation in the depositional environment. During Late Eocene (?) to Late Oligocene, sediments from surrounding basement rocks began to deposit in the lowermost part of the Li basin. The sediments were deposited in the lacustrine environment, underlain by thin alluvium sediments of basement pebbles, Afterthat, the peat was accumulated in the continuous subsiding basin. In Early Eocene, the area was strongly subsided, and lacustrine sediments were deposited. Later on, the basin was continuously subsided with different subsiding rates. The peat accumulation was frequently interrupted by clay deposition. The Late Miocene unconformity is a major regional unconformity which is related to the late stage of the external force in this region, and the termination of tilted block-faulting.