An Electrostratigraphic Study of the Hsinying-Matou Coastal Plain, Southwestern Taiwan

Abstract

One hundred and two vertical electric soundings were conducted in the Hsinying-Matou coastal plain in southwestern Taiwan, for the study of electrostratigraphy and the changes in sedimentary environments. The sounding data were interpreted using the 1-D inversion method

The results indicate that the strata from the surface to 180 meters depth can be divided into three electrostratigraphic units. They are designated, from top to bottom, as the T, S, and R facies.

The T- facies is a combination of thin layers of resistivity ranging from 5 to 45 ohm-m, and is 10 to 60 meters thick. The S- facies is characterized by thick layers of resistivity lower than 5 ohm-m, and is 80 to more than 130 meters thick in the west but absent in the east. The R- facies is characterized by thick layers of resistivity ranging from 5 to 50 ohm-m. These three units are similar in lithology; they are mainly composed of clay, mud, silt, and fine sand. The S- facies was sedimented in lagoonal, beach, and shallow marine environments. The T and the R facies were sedimented in non-marine environments in the east, and in lagoonal and estuarine environments in the west. The resistivity of stratum is controlled mainly by the salinity of formation water where the water is saline, and by the grain size and the amount of clay contained where the water is fresh.

It is inferred that the period from 40 to 8 Ka is transgressive dominant, and the period since 8 Ka is regressive dominant. The .mouth of the Tsengwenchi stream was located about 20 Km east of the present coastline at 9 Ka. The Tsengwenchi stream developed westwardly since 9 Ka till recent times, and then changed southwestwardly to the present location.

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