Studies of Sedimentary Facies, Stratigraphy, and Deformation Structures of the Chelungpu Fault Zone on Cores from Drilled Wells in Fengyuan and Nantou, Central Taiwan

Abstract

Stratigraphy, sedimentary facies, and deformation characteristics of drill cores from Fengyuan and Noutou wells reveal important attributes for the westmost portion of active Chelungpu thrust fault and contrasting deformation mechanisms between the north and south ends. Stratigraphy of the Fengyuan well (including BH-l and BH-IA boreholes) is composed of three major units, including the upper Miocene to Pliocene Kueichulin Formation (455.4-224.7 m), the Pliocene Chinshui Shale (224.7-3.9 m) and recent terrace deposits (3.9-0 m). The Kueichulin Formation comprises three upward coarsening, tide-dominated delta parasequences with sandstone and sandstone-Shale alternations. The Chinshui Shale is dominated by shallow marine facies with siltstone, mudstone and fine-grained sandstone. Shallow marine facies are occasionally intercalated with tide-dominated delta deposits. Terrace deposits are characterized by paleosol, yellowish mud, mottled leaching soil and thin pebble layers. The Chi-Chi earthquake slip zone is located at a transgressive deposit, which is also the sequence boundary between the Kueichulin Formation and the Chinshui Shale. Other two major brecciated shear zones are also the parasequence boundaries within the Kueichulin Formation. Stratigraphic sequence of the Nantou well (CLF -2) is composed of the Pleistocene Toukoshan Formation (211.9-177 m) in the f,the Chinshui Shale (177-8.7 m) and terrace deposits (8.7-0 m) in the hanging- erates and yellowish fine-grained deposits with drifted pebbles, an indication of braided fluvial deposits. The Chinshui Shale comprises alternating shallow marine and tidal deposits. The shallow marine face is dominated by mudstone, siltstone and fine-grained sandstone with moderate to high degree of bioturbation. Terrace deposits are characterized by yellowish gray mud, pebble layer, and mottled paleosol. Overall, shear zones in the Nantou well is characterized by foliated gouge or breccia as opposed to breccia or gouge of random fabrics in the Fengyuan well.