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Ichnology of a Seasonally-Dominated Deltaic / Estuarine Complex: The Pleistocene Palaeo-Kambinaru River, Sumba, East Nusa Tengara, Indonesia

Abstract

The Kambinaru River drains the eastern highlands Sumba, East Nusa Tengara, Indonesia. This drainage system has been active since the early Pleistocene & follows the trend of a Pliocene fault complex. Although dominated by volcaniclastic sediment, it incises through a broad uplifted carbonate platform complex. As well, coralliferous reef complexes occur interstratified with fluvially & deltaically deposited clastic sandstone & conglomerate units. Active feeder channels are dominated by coarse, granule, pebble & cobble-dominated lenses which incise into adjacent strata. Barnacles, oysters & corals attached to pebbles & cobbles & bored Neogene carbonate clasts clearly indicate that fluvial feeders were strongly affected by marine influences for many kilometres upstream. Elongate Ophiomorpha, as well as Schaubcylindrichnus& thick-walled Palaeophycuscommonly penetrate gravel-rich channel sandstone beds. The coarse gravel units are commonly capped byfine-grained sandstone beds with Cylindrichnus, Skolithos, cf. Rhizocorallium, Teichichnus &Psilonichnus that are interpreted as intertidal flat successions on bars & on the margins of inactive channels. Baymouth bar & shoreface sandstone complexes lateral to the main channels consist of moderately sorted mixed siliciclastic-carbonate sandstone dominated by locally dense populations of Ophiomorpha, Schaubcylindrichnus, Teichichnus & large Siphonichnus. Scattered pectinid bivalves, ostreids, echinoids vermetid brachiopods & scaphopods occur in these units. Similar to the modern Kambinaru delta, the palaeo-Kambinaru delta was likely highly seasonal, allowing deep invasion of marine waters between fluvial flow events. Feeder channel avulsion within the broad Kambinaru valley allowed coral patch reefs to develop within the deltaic complex. Bioclasts within these patch reefs are commonly highly bored & bioeroded. Flashy discharge resulted in short events of fluvial dominance punctuating longer wave-dominated intervals. This resulted in an overall low proportion of suspended load facilitating both patch reef development as well as the abundance of vertical suspension feeding traces in clastic-dominated successions.