Morphogenesis of the Holocene coastal landforms on Sumba Island, Indonesia

Coastal terraces, whether erosional, depositional, or bio-constructed, have been widely used to quantify Late Cenozoic vertical deformation and relative sea-level changes. Here, we review and investigate the Holocene coastal terraces of Sumba Island, Indonesia. We describe fifteen coastal sites, evenly distributed between the South and North coasts. The terraces are Holocene age, based on ¹⁴C and U/Th ages. We show that the preservation of coastal terraces in Sumba is favoured on the leeward coast (North). Holocene and older Pleistocene coastal terraces are almost absent on the windward coast (South), although modern coral reefs almost ubiquitously thrive. The morphology and the nature of the bedrock varies between the two sides of the island: while sequences of earlier Pleistocene coral reef limestones dominate in the North, Miocene/Pliocene tuffs and pelagic carbonates outcrop on the southern coast. The elevations of the Holocene inner edges range from 2.4 ± 0.7 to 6.9 ± 0.7 m above the present mean sea level. Despite disparate uplift rates, the three preserved windward terraces (out of the seven sites investigated) generally have a higher inner edge than leeward terraces. We conclude that neither glacio-eustatic oscillations, vertical deformation rates nor hydrodynamics by themselves can explain the variability in inner edge elevations on Sumba, but only a complex combination of these processes. Finally, our study serves as a solid basis for future investigation of the Holocene landforms of the island of Sumba and other similar tropical coastal zones.