High-resolution sequence stratigraphic analysis 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 provides a 25 ka record of the depositional history of a region situated in the frontal arc-continent collision belt of Taiwan. Twenty-five lithofacies and nine facies associations are recognized. An alluvial plain was the dominant feature during a pronounced sea-level fall at 25–20 ka ago. Between 20 and 10 ka, alluvial fans were deposited along the western front of the Pakua Tableland during movement along the 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. Between 16 and 6 ka, the Changhua Coastal Plain experienced rising sea level and retrogradational deposition. From 16 to 10 ka, estuarine deposits (lower transgressive systems tract) were deposited, and the base of these deposits defines the Last Glacial Maximum (LGM) unconformity (sequence boundary), now preserved in four regions to the east of the plain. From 10 to 6 ka, shelf, shoreface, and lagoonal deposits (upper transgressive systems tract) were laid either on top of the estuarine deposits or directly on the LGM unconformity surface. By about 7–6 ka, sea level had reached its peak (maximum flooding surface), and shelf and marginal marine sediments covered most of the study area. After 6 ka, the highstand was dominated by progradation of offshore-transition, shoreface, lagoonal, tidal channel, tidal flat and alluvial plain deposits. One-thousand-year-old fluvial deposits in incised channels are preserved in the central part of the Changhua Coastal Plain. Since then, fluvial channels shifted southward to the present-day Chuoshuei River. The depositional patterns in this tectonically active area during the last interglacial period reflect the complex interplay between high-frequency sea-level fluctuations, tectonics (subsidence and uplift), and autocyclic processes. In the seaward area, however, fluctuations in sea level were clearly the dominant factor in controlling the nature of depositional facies.