Late Quaternary mass-wasting records and formation of alluvial terraces in the actively uplifting Lao-nong catchment, southwestern Taiwan

Abstract:

Although dominated by erosion over long term, the tectonically active mountains of Taiwan commonly contain thick landslide and debris-flow gravels capping hillslopes or forming alluvial terraces. These deposits and their associated landforms serve to study ancient mass-wasting histories and their controls on fluvial processes. This study focuses on the Lao-nong River draining the 1000–3000 m high mountain areas in southwestern Taiwan (current tectonic uplift: 10–20 mm/yr). The Lao-nong River exhibits numerous terraces, many of which are of tributary-fan origins and consist of fluvial/debris-flow gravels 20–200 m thick. The development of such alluvial terraces can be observed during 2009 Typhoon Morakot, which brought record-setting 2000 mm of rain. Triggered by this heavy rain, almost all the tributaries of the Lao-nong generated alluvial fans at their mouths and consequently caused up-to-30 m aggradation along the trunk river.

The Lao-nong has yielded >100 radiocarbon dates (all <20 ka).These data reveal that the formation of the observed alluvial terraces (from deposition of thick gravels to subsequent incision) could proceed very rapidly (<1 ky), and that the trunk river has been characterized by repeated aggradation and incision. This fact highlights the significance of event-driven sediment supply in governing the river behavior, which has strongly obscured the effect of the concurrent tectonic uplift. Among the terraces consisting of thick gravels, the six with 100–200 m-thick gravels are dated~18ka, ~12ka (for two terraces), ~5.5ka, ~3.4ka, and ~1.4 ka. The seven terraces with 50–100 m-thick gravels are dated ~12ka, ~8.3ka, ~5.7ka, ~1.3ka and 0.2–1.6ka (two undated). These dates (and others) suggest the clusters of mass-wasting events around ~12ka, 5.5–5.7 ka, and <1.6 ka. However, note that most of the recorded mass-wasting events vary in time and magnitude among tributary catchments, which implies the controls of internal factors on the timing, magnitude, and frequency of the events. Our field observation suggests that at least some of the large Holocene tributary-fan terraces were sourced from pre-Holocene colluviums.