Chapter 7 - Petrological and Geochemical Constraints on Provenance, Paleoweathering, and Tectonic Setting of Clastic Sediments From the Neogene Lambir and Sibuti Formations, Northwest Borneo

Abstract

The petrography and geochemistry of clastic sediments from the Neogene Lambir and Sibuti Formations, Northwest (NW) Borneo, were investigated to infer the provenance and tectonic setting. Thirty samples from Sibuti (n=3) and Lambir (n=27) Formations were analyzed for geochemistry using inductively coupled plasma mass spectrometry. Petrography and X-ray diffraction results show that the sandstones are quartz dominant with minor amounts of feldspar and lithic fragments with muscovite, zircon, rutile, and opaque grains of accessory minerals. Geochemically, the clastics are classified as quartz arenite, litharenites, arkoses, wackes, and Fe-sands. Compared with upper continental crust, a high percentage of SiO2 content is recorded among all the samples along with Al2O3 and variable CaO content, due to the presence of calcareous cement and calcareous fossils. Compared to other rock types, quartz arenite from the Lambir Formation has the highest SiO2 (95.03wt%) and lowest Al2O3 content (1.88wt%), which reflects high sediment maturity. High Chemical Index of Alteration values of Sibuti (67–74; n=3) and Lambir (66–85, n=27) Formations indicate that the source area had undergone moderate to intensive chemical weathering. This is supported by higher Plagioclase Index of Alteration values (71–98). Al2O3/TiO2, low Cr/V, Th/Sc versus Sc plot, trace element ratios, and light rare earth element enrichment, flat heavy rare earth element chondrite normalized pattern associated with negative Eu anomaly (Eu/Eu∗=0.59–0.76) and (Gd/Yb)cn ratio <2, reflecting dominance of an upper crustal felsic (recycled) source. The major element-based provenance discriminant function shows that the sediments were derived from felsic source rocks of recycled provenance. La/Th versus Hf, Zr/Sc versus Th/Sc, and Al2O3-TiO2-Zr ternary plots show that the sediments are well sorted and were recycled. Tectonic discrimination diagrams based on major and trace elements suggest that the sediments were derived from predominantly continental to passive margin settings, associated with the rifting of the proto-South China Sea in Early to Middle Miocene times. This observation is consistent with the depositional history of the Lambir and Sibuti sediments and the geological evolution of NW Borneo at that time.