Eustatic knickpoint dynamics in an uplifting sequence of coral reef terraces, Sumba Island, Indonesia

An emerged coral reef terrace sequence flanks the northern coast of Sumba Island in Indonesia. The sequence was created by the joint effects of uplift and Quaternary sea level oscillations. Since its emergence, it undergoes chemical erosion, which is facilitated by its carbonate lithology. The morphology is dissected by multiple catchments drained by deep canyons, whose stream profiles display several knickpoints. We applied a multi-methods approach using high-resolution topographic data (Pleiades imagery), geomorphological analysis and denudation rates derived from Cl-36 cosmogenic nuclide concentrations to characterize the dynamics of these coastal drainages. We specifically investigate (1) the channel response to eustatic forcing, by analyzing knickpoint dynamics along a longitudinal river profile, and (2) the control of internal and external forcing over the catchment-scale erosion. Firstly, knickpoints form at the distal edge of the emergent reef at each regressive stage following a sea level highstand. The knickpoint propagates and rotates upward by regressive erosion. Knickpoint propagation rate decreases sharply over time until it becomes negligible before the next sea-level rise, which implies a marked decline in knickpoint retreat rate before its resorption. We attribute the inability of the river at Cape Laundi to fully propagate eustatic oscillations to its low stream discharge and low sediment supply, related to its small drainage area, to the karstic nature of the coral reef terraces composing the sequence, and to the high frequency of eustatic forcing. Secondly, average denudation rates calculated from Cl-36 cosmogenic nuclide concentrations of sands collected at the outlet of five catchments draining the sequence amount to 69 +/- 8 mm.ka(-1). We emphasize the role of canyon areas in driving the denudation of the major catchments. However, no first-order correlation has been observed between catchment metrics, uplift rates and denudation rates. Finally, our correlations between catchment finite eroded volumes and uplift rates highlight the tendency of catchments to widen with low uplift rate and to lengthen while incising the sequence deeply with higher uplift rates. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study focuses on a emerged coral reef terrace sequence along the northern coast of Sumba Island in Indonesia, investigating the dynamics of coastal drainages through various methods and analyzing the morphology of the area.