EVOLUTION OF THE PLEISTOCENE PALAEO-KAMBINARU DELTA; EASTERN SUMBA, EAST NUSA TENGGARA, INDONESIA

The Kambinaru River system drains much of the eastern portion of the island of Sumba in East Nusa Tenggara, Indonesia and has done so since at least the mid-Pleistocene. Sourced in mixed igneous and sedimentary strata of the Matawai La Pawu highlands in the southern part of the island, the Kambinaru delivers a large volume of siliciclastic sediment to the northeastern, carbonate-dominated coast of the island. The Kambinaru mouth debouches into the Savu Sea where it forms a seasonally wave-dominated deltaic complex. The Kambinaru river / delta system incises deeply into adjacent, uplifted, coral reef terraces. The oldest dated terraces (~ 1,000,000 years) in the Waingapu area now occur approximately 475 metres above sea level with 6 younger terraces stepping down towards present-day sea level. Sediments deposited in thePalaeo-Kambinaru delta consist medium to very coarse-grained volcanogenic sand, gravel, cobbles and boulders. Clastic sediment units commonly occur interstratified with sandy / gravelly carbonate beds characterized by abundant corals, bivalves, and gastropods. Bored limestone pebbles, bioclasts (echinoids, gastropods, oysters, corals, pectinids, and other bivalves) and oyster-encrusted pebbles attest to marine incursions that penetrated deep into the Kambinaru valley. Deltaic deposits have been dated via both their position relative to well-dated coral terrace units as well as via radiocarbon dates on shell material. The Palaeo-Kambinaru appears to have prograded northward synchronous with coral reef terrace evolution until ~45,000 years ago. At this point the Kambinaru Valley experienced a significant marine incursion, with bioturbated fossiliferous sandstone units occurring well to the south (landward) of temporally equivalent terrace margins. Large-scale sediment deformation features in fluvial and deltaic sedimentary successions suggest that this marine incursion was tectonically induced. Fluvial input was strongly episodic with most sediment delivered to the delta during seasonal high-flow events. Palaeo-Kambinaru sedimentary successions have a very low proportion of fine-grained sediment (i.e. silt and clay). Low turbidity and temporally restricted sediment input facilitated co-occurrence of coral-rich carbonate communities in a coarse-clastic deltaic complex.