Petrography of Subduction Zone Sandstones from Nias Island, Indonesia

Abstract

ABSTRACT Rocks on Nias Island, Indonesia can be divided into two tectonostratigraphic units: (1) highly deformed late Oligocene-early Miocene trench deposits (tectonic melange); and (2) Miocene-Pliocene trench slope deposits. Modal point counts of 24 representative samples of sandstones, half from the melange, and half from the slope strata, reveal that they are rich in quartz and sedimentary/metasedimentary lithic fragments. Melange and slope sandstone points are closely clustered on the QFL diagram, however several other differences serve to distinguish them petrographically. Trench slope sandstones contain significant amounts of K-feldspar, polycrystalline quartz, glauconite, and carbonate grains and cement, and the lithic fragment population averages Lv21 Lm 3 Ls26. Melange sandstones contain only small amounts of polycrystalline quartz, insignificant amounts of K-feldspar and glauconite, are cemented by phyllosilicate minerals rather than calcite, and the lithic population averages Lv37 Lm17 Ls46. Similar differences may characterize trench/trench slope deposits elsewhere. The quartzose nature of the Nias sediments indicates that they were derived from the west coast of Sumatra, where early Tertiary quartz-rich sediments and Paleozoic/Mesozoic metamorphic and plutonic rocks are exposed. Nias sandstones have much lower contents of volcanic lithic grains than do most other arc-derived sandstones. This may in part be due to the nonvolcanic source terrane on the west coast of Sumatra, and may also be due to volcanic inactivity in the late Oligocene and paleodrainage away from the trench.