Sedimentology and Paleobathymetry of Neogene Trench-Slope Deposits, Nias Island, Indonesia

Abstract

The Neogene strata exposed on Nias Island, Indonesia consist of Miocene marls, sandstones, and conglomerates of the Nias beds that overlie the Oyo Complex, a tectonic mélange of pre-Miocene age. They provide an excellent opportunity to study ancient trench-slope sedimentation. Lithofacies and vertical sedimentary sequences in the Nias beds are similar to those described from ancient submarine fan environments. The overall vertical sedimentary sequences in the Nias beds describe a coarsening-and thickening-upward trend. The basal portion of the sections consists dominantly of thin-bedded and fine-grained turbidites and marls of Facies D and G. Sandstone-to-shale ratios show a general up-section increase, which is observed as a corresponding up-section increase in Facies A, B, and C sandstones and a decrease in Facies D siltsones and marls. Within the large-scale thickening-and coarsening-upward cycles are smaller-scale thickening- and coarsening-upward and thinning- and fining-upward cycles characteristic of deposition in various submarine fan environments. The general vertical progression of fan facies is basin plain $$\rightarrow$$ outer fan $$\rightarrow$$ midfan. Benthic Foraminifera, generally common and well-preserved throughout all but the basal portions of the Nias beds, provide clear evidence of uplift of the section from lower bathyal/abyssal depths. Samples from the oldest Nias beds contain either a very meager fauna or no calcareous microfossils at all, indicating deposition below the CCD. Lower Miocene assemblages are indicative of depths greater than 2000 meters. Middle bathyal (500-2000 m) species represent the in situ bottom fauna within the upper Miocene strata. The Nias beds are overlain by Pliocene shelf deposits and a Pleistocene coral cap. This implies that the lower Miocene strata were uplifted 4000 meters in about 20 million years, yielding an average uplift rate of $$20 cm/10^{3}$$ yrs. A model of trench-slope sedimentation on Nias suggests that near the base of the trench slope, hemipelagic sedimentation took place below the CCD in basins a few kilometers in length and width. Because the basins were shallower than the trench floor, they were isolated from trench turbidites. As the basins were uplifted tectonically, pelagic carbonate became a more conspicuous component. Terrigenous turbidites began to be delivered to the basins as submarine canyons cut down through the slope. Continued terrigenous sedimentation led to progradation of submarine fan deposits over the older basin-plain deposits. The basin sediments were folded and faulted penecontemporaneously with sedimentation. The dominant source area for the Nias beds was the arc terrane of mainland Sumatra.