Late Quaternary mass-wasting records and formation of fan terraces in the Chen-yeo-lan and Lao-nung catchments, central-southern Taiwan

Development of fan terraces in incision-dominated river settings documents episodic increases of sediment supply from upland catchments. In the non-glaciated, active mountains of Taiwan, such increases in sediment supply result typically from increases in landslide/debris flow activity. Studying fan terraces, therefore, helps understand the history and nature of mass-wasting processes eroding the mountains. This study explores fan-terrace successions developed at tributary mouths of the Chen-yeo-lan and Lao-nung Rivers in central-southern Taiwan. Judging from the spatial variations of sizes, heights, sequences and sedimentation of the fan terraces, the magnitude and frequency of mass-wasting activities appear to vary from tributary catchment to catchment. Two types of tributary catchment are differentiated, which can be located in close proximity to each other. One has been subject to extremely large but infrequent mass-wasting events, as a result of which high and/or large fan terraces with 100–200 m thick debris-flow/fluvial gravels have been formed. Seven examples of this type of fan terrace have yielded radiocarbon dates of >40 ka, ∼18.4 ka, ∼13.9 ka, 8.9–9.5 ka, 3.9–4.8 ka, ∼3.3 ka, and ∼1.4 ka in age. With the availability of multiple dates, this type of fan terrace can be shown to have been created in no more than 1000 years, from the aggradation of thick gravel sequences to subsequent incision. The second type of catchment has been subject to relatively minor but frequent mass-wasting events. Due to repeated incision after each such event, the resultant fan terraces are relatively low and limited in size but have formed over lengthy periods. Although located in the same geological and geomorphic setting, the dated fan-forming mass-wasting events in the Chen-yeo-lan catchment do not appear to have occurred synchronously with those in the Lao-nung catchment. Nor could the succession of these events, or their origins, be clearly linked to known paleo-climate data. These findings highlight the importance of internal factors (e.g., rock-mass strength) in determining the timing, magnitude, and frequency of mass-wasting events in the studied mountains, which are subject to active rock uplift, highly active seismicity, and intense rainfall.