Alluvial terrace preservation in the Wet Tropics, northeast Queensland, Australia

Abstract

Alluvial terraces provide a record of aggradation and incision and are studied to understand river response to changes in climate, tectonic activity, sea level, and factors internal to the river system. Terraces form in all climatic regions and in a range of geomorphic settings; however, relatively few studies have been undertaken in tectonically stable settings in the tropics. The preservation of alluvial terraces in a valley is driven by lateral channel adjustments, vertical incision, aggradation, and channel stability, processes that can be further understood through examining catchment force–resistance frameworks. This study maps and classifies terraces using soil type, surface elevation, sedimentology, and optically stimulated luminescence dating across five tropical catchments in northeast Queensland, Australia. This allowed for the identification of two terraces across the study catchments (T1, T2). The T1 terrace was abandoned ~13.9ka with its subsequent removal occurring until ~7.4ka. Abandonment of the T2 terrace occurred ~4.9ka with removal occurring until ~1.2ka. Differences in the spatial preservation of these terraces were described using an index of terrace preservation (TPI). Assessments of terrace remnant configuration highlighted three main types of terraces: paired, unpaired, and disconnected, indicating the importance of different processes driving preservation. Regional-scale variability in TPI was not strongly correlated with catchment-scale surrogate variables for drivers of terrace erosion and resistance. However, catchment-specific relationships between TPI and erosion–resistance variables were evident and are used here to explain the dominant processes driving preservation in these tropical settings. This study provides an important insight into terrace preservation in the tectonically stable, humid tropics and provides a framework for future research linking the timing of fluvial response to palaeoclimate change.