Stratigraphy and morphogenesis of Pleistocene coral reefs at Tambolaka (Sumba Island, Indonesia)

The fossil record of Quaternary reef systems, as expressed in uplifted regions by sequences of stacked terraces, has been extensively used to either understand their morphodynamics, or to unravel sea level variations. Yet, because these two aspects are intimately linked, Quaternary reef analysis is often underdetermined because the analysis often focuses on single sequences, along one-dimensional profiles. Here, we take advantage of the lateral variations of coral reef sequences by documenting the morphological variations of the reef sequence on Sumba Island. Near Tambolaka, Northwest Sumba, we analyzed a reef transect, topography, and associated sedimentological record to obtain a precise coral reef stratigraphy and geomorphic patterns that can be compared to the well documented eastern counterpart. In Tambolaka, the reef sequence displays four lower layers of bedded chalky limestone units with a weakly cemented sandy matrix, which we attribute to the Middle Miocene to Pliocene Wakabukak formation based on calcareous nannofossils and planktonic foraminifers. The uppermost layer is a calcretized reefal limestone unit with a well-lithified sandy matrix, which we attribute to the Plio-Pleistocene reef sequence of the Kalianga formation. Seven marine terraces imprint the regional morphology, four of which we correlate with MIS 5e, MIS 7e, MIS 9e, MIS 11c terraces of Cape Laundi, Northeast Sumba. When scrutinized at the light of numerical models of reef development, these results indicate that the morphodynamics of reefal sequences is strongly impacted by the tectonic evolution, even at local scales. The geodynamic context sets both the extrinsic -morphology of the basement, hydrodynamics- conditions of reef development, but also the intrinsic properties - reef growth rate in particular. While the morphodynamic evolution of the sequence is at first order representative of the interplay between uplift rates and sea level oscillations, the detailed assemblage of the reef units drastically varies along the coastline.