Sedimentary environment of Tangenshan sandstone in southwestern Taiwan and its tectonic implications

The most widely distributed shallow marine sandy formations in the western foothills of Taiwan is the Guizhulin Formation of late Middle Miocene to early Pliocene age, along with its contemporaneous strata. Its facies transition is also the most prominent, exhibiting an overall trend of increasing sedimentary environment depth from north to south. The lower part of the Guizhulin Formation in northern and central Taiwan, is of late Middle Miocene age and can be correlated to the Tangenshan Sandstone in southwestern Taiwan. According to numerous previous studies, in terms of tectonic evolution for the western foothills terrain, the former have been regarded as the accumulation in the early stage of the foreland basin (syn-orogenic) to the north of the Peikang High, whereas the latter represents the deposits in the passive continental margin stage (pre-orogenic) to the south of the Peikang High. In order to understand the sedimentological distinction between the lower part of the Guizhulin Formation and Tangenshan Sandstone, this study reconstructs and analyzes stratigraphic columns and lithofacies of the Tangenshan Sandstone, which is approximately 380 meters thick and exposed in the hanging wall of the Chutouchi Fault in the Kueitanchi section in the Tainan area, southwestern Taiwan. Observations and records of trace fossils were also utilized to comprehensively reveal the variations in sedimentary environment and the characteristics of depositional facies in the study area. The aim of this study is to investigate the tectonic implication for the variations in the sedimentary environment based on the sedimentology study results. Based on the descriptions in the field, the lithofacies can be categorized into one mudstone facies, two siltstone facies, and three sandstone facies. According to the lithofacies association, they can be further subdivided into channel deposits and non-channel deposits, indicating two distinct sedimentary features. It is inferred that the sedimentary environment during the deposition was primarily an offshore depositional system dominated by storm wave and possible turbidity currents. Lithofacies association indicates that the thick sandstone layers in the study area were transported deeper in the continental shelf by turbidity currents induced by storms. These currents carried coarse-grained sediments below the average storm wave base, resulting in the formation of submarine channels and finer-grained non-channel deposits. The overall sedimentary characteristics resemble those of shallow water turbidite deposits in terms of features and mechanisms. Furthermore, with comparison with previous research results, it is inferred that the deposition of the Tangenshan Sandstone in the study area was in an atypical passive continental margin environment but rather influenced by pre-orogenic normal faulting, leading to an increase in thickness but decrease in sedimentary environment depth of the contemporaneous Tangenshan Sandstone deposits from the west (basin margin) to the east (basin center).