CATASTROPHIC INPUT TO A SEASONALLY ACTIVE CLASTIC DELTAIC SUCCESSION ON A TROPICAL CARBONATE COAST, NORTHEAST SUMBA, EAST NUSA TENGGARA, INDONESIA (Invited Presentation)

Northeastern Sumba, East Nusa Tengara, Indonesia, is dominated by uplifted Pleistocene coral reef terraces. The only non-carbonate sediment in this part of the island consists of coarse-grained clastic sediment (medium to very coarse-grained volcanogenic sand and gravel) in the Wengga Atas and Aibara areas of greater Waingapu. These deposits occur wedged between adjacent carbonate terrace units. The clastic sand and gravel was likely derived from an Eocene volcanic province in the southeastern part of the island at the headwaters of the Kambaniru drainage system. Coarse, granule, pebble and cobble-dominated lenses incise into moderately to poorly sorted, cross-stratified, fine- to medium-grained sandstone beds characterized by locally dense populations dominated by Ophiomorpha and Schaubcylindrichnus. Barnacles, oysters and corals attached to pebbles and cobbles clearly indicate emplacement in a marine-influenced environment. The coarse gravel units are commonly capped by fine-grained sandstone beds with Cylindrichnus, Skolithos, cf. Rhizocorallium, Teichichnus and Psilonichnus that are interpreted as intertidal flat successions on bars and on the margins of inactive channels. Preliminary radiocarbon dates indicate that the delta was deposited during the late Pleistocene (~21,000-38,000 BP). A thick, moderately well sorted bioclastic sandstone succession with abundant large Ophiomorpha occurs midway through the study interval. Intervals with large (metre-scale) interclasts of finer grained lithologies in a gravel matrix reflect erosion and transportation during seasonal reactivation of the delta. Interbeds of moderately well-sorted, bioturbated sandstone likely reflect seasonal wave reworking between sediment input events. The Waingapu paleo-delta was characterized by flashy discharge and shows evidence of both fluvial and wave dominance. It is interpreted to represent a seasonally-dominated delta associated with the Paleo-Kambinaru River system.