Electrostratigraphic Study of the Tainan Coastal Plain, Southwestern Taiwan

Abstract

In order to study the electrostratigraphy, the depositional environments and the change of shoreline, a geoelectric survey was carried out on the Tainan coastal plain. The DC resistivity method was used, and the sounding data were interpreted by the 1-D inversion method. 

Based on the electric resistivities and the thicknesses of the layers derived from the sounding data, three electrostratigraphic units are found and specified. They are designated as the T, R, and S fades corresponding to the strata from the surface to 200 meters in depth. The T-facies is characterized by layers of resistivity ranging from 12 to 400 ohm-m at the top of the layer sequence. This unit is less than 100 meters thick and is widely distributed in the area studied. The S-facies is characterized by a thick layer or layers of resistivity less than 5 ohm-m, and is generally accompanied with thin layers of resistivity less than 12 ohm-m. The S-facies is 50 to 200 meters thick and is mostly overlain by the T -facies in the western and the central parts of the coastal plain, and is absent in the eastern part. The Rfacies is characterized by a thick layer or layers of resistivity ranging from 12 to 70 ohm-m. It is overlain by the T-facies in the east and by the S-facies in the west. The R-facies intertongues with the S-facies in the north-center and is absent in the south-center. The T-facies was deposited in terrestrial and estuarine environments during the Holocene. The S-facies was deposited in marine environments in the late Pleistocene to Holocene. The upper part of the R-facies in the east was deposited in terrestrial and estuarine environments in the late Pleistocene. These three units are similar in lithology being mainly composed of layers of clay, mud, silt, and fine sand.

A marine transgression occurred when the strata between the depths of 200 and 40 meters were deposited in the area studied except for the Tainan tableland, and was then followed by a marine regression. At the peak period of the marine transgression, the shoreline reached the line connecting Anding, Hsinhua, and the east of Gueizen. Since the beginning of the regression, the shoreline has retreated about 22 km westward in the north, and about 14 km westward in the south. The Tainan tableland was formed by an anticlinal uplifting beginning at the time when the stratum at 80 meters depth was deposited.

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