The results of this nannofossil analysis supply essential information for determining the formation boundaries in the upper Pliocene to Pleistocene. These results also verify the existence of a repetition fossil zone.
The TCDP well-A was sunk through the soft fine-grain muddy sandstone and mudstone dominated formations of the Pliocene and Pleistocene in the Taichung area. This study determines methods for providing core preservation in wells at fault zones and establishes a nannofossil biostratigraphy for the integrated Taiwan Chelungpu-fault Drilling Project (TCDP). Good core fabrics are useful for core description and sampling. In this present study, over 400 meters of subsurface cores were covered in resin and slabbed. Digitized images were created for all the core fabrics. More than 150 rock samples were analyzed for nannofossils to give a detailed appraisal of the biostratigraphic column of TCDP well-A.
A fossil zone at a depth interval of 431 - 869 m is a NN16 - 18 biozone. This zone is within the Cholan Formation, a lithologic stratigraphy in northern and central Taiwan. The depth interval 883 - 1226 m is NN15, and is within the Chinshui Shale. The Chelungpu fault is composed of five major shear zones. These are all found at depth within the marine Chinshui Shale. At a depth interval of 1293.37 - 1710 m is a NN12 - 14 biozone; this interval is within the Kueichulin Formation.
Interestingly, both the interval beneath 1714 m and the nannofossil zone near the well bottom are NN16 - 18 (Cholan Formation), indicating a repeat of the Cholan Formation. The lowest fossil zone is also abundant in secondary reworked fossils in its assemblages. Hence, the repetition of the younger fossil zone, NN16 - 18, at the bottom of the well verifies the subsurface position of the Sanyi Fault and indicates that TCDP well-A must have passed through it.