Neotectonic effect of Late Quaternary Deposits of the Changhua Coastal Plain in the Frontal Arc-Continent Collision Belt of Central Taiwan

Detailed analysis of the sequence stratigraphy in 30 cores from the Late Quaternary deposits of the Changhua Coastal Plain identifies two sequences (an older S2 and a younger S1) since 40 Ka and provides a record of the depositional history of a region situated in the frontal arc-continent collision belt of Taiwan. The Changhua Coastal Plain, bounded to the west by the Taiwan Strait and to the east by the Pakua Tableland, is part of the foreland basin of western Taiwan, located between the Dadu River (northern boundary) and Chuoshuei River. The Pakua tableland comprises a frontal deformation as an anticline in the thrust-and-fold belt. A wide variety of Late Quaternary depositional environments, from alluvial fan to fluvial, estuarine, marginal marine and shallow shelf (offshore-transition, this environment is not found in S2) can be identified from the sedimentary cores. Transgressive (lower TST and upper TST) and highstand (HST) deposits can be differentiated. Except alluvial fans were deposited along the western front of the Pakua Tableland during movement along the blind thrust (Changhua Fault), and a local sag basin was produced. No other deposits in the seaward area are recognized between 20 and 16 ka, or in the landward area between 20 and 10 ka. Correlation with Late Quaternary (post-40 ka) sea-level fluctuations shows that sedimentation of last interglacial and late last glacial reflects the cycles of sea-level fall and rise. Constrained by C14 dating, S1 (thickness is between 40 to 60 m and average is about 50 m) was deposited between 16 ka to recent and S2 (average preservation thickness is about 45 m) was deposited from 40 to 20 ka. During S1 deposition, the average basin subsidence rate is about 1.1 mm/yr and sedimentation rate is about 3.1 mm/yr. During S2 deposition, the average regional basin subsidence rate is about 2.8 mm/yr and sedimentation rate is about 2.3 mm/yr. In this study can find that the basin subsidence rate during S2 deposition is about 2.5 times of S1, obviously, in the period of S2 deposition is much faster than S1. This result indicates that the uplifting rate of Pakua tableland is not constant. Between 40 to 20 ka, the uplifting of the tableland should be much faster than 20 ka to recent. At the same time, the main fluvial channel of old Chuohsuei River was confined to the north of present Chuoshuei River and located at the central part of Changhua Coastal Plain since the Southern Pakua tableland was not uplifted completely. The main channel of Chuoshuei River might have migrated to the south following the uplift of Pakua tableland.