 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; July 2006; v. 34; no. 7; p. 589–592; doi: 10.1130/G21939.1; 5 figures; Data Repository item 2006111. 589 Southeast Asian sediments not from Asia: Provenance and geochronology of north Borneo sandstones Marco W.A. van Hattum Robert Hall    SE Asia Research Group, Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK April L. Pickard School of Earth & Geographical Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA 6109, Australia Gary J. Nichols SE Asia Research Group, Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK ABSTRACT Eocene–lower Miocene sandstones of the Crocker turbidite fan of north Borneo were derived from nearby Borneo and southeastern Asian sources, rather than distant Asian sources eroded after India-Eurasia collision. They are compositionally mature due to trop- ical weathering, but are mostly first-cycle sandstones derived from granitic rocks and subordinate metamorphic, sedimentary, and ophiolitic rocks. Detrital zircon ages range from Archean to Eocene, and the majority are Mesozoic. The most important source areas were Cretaceous granites of the Schwaner Mountains in southwest Borneo during the Eocene, and Permian–Triassic granites and Proterozoic basement of the Malay-Thai Tin Belt during the Oligocene. Keywords: Southeast Asia, Borneo, turbidites, provenance, zircon geochronology. INTRODUCTION The Crocker Fan of north Borneo (Fig. 1) is the largest volume of Paleogene sediment in a single basin in Southeast Asia, and was deposited during early stages of India-Asia collision. This has suggested to some (Hall, 1996; Hamilton, 1979; Hutchison, 1996; Me ´- tivier et al., 1999) that sediments were derived from sources in Asia elevated by the collision and transported to the Sunda Shelf by major rivers, whereas others (Hall, 2002; Hall and Morley, 2004) argue for local sources. The Crocker Fan formed on the south side of the Proto–South China Sea at an active sub- duction margin (Hazebroek and Tan, 1993; Hutchison, 1996; Tongkul, 1989). It was de- posited between the Eocene and early Mio- cene (Fig. 2), but different formations in the fan are not precisely dated because of the pau- city of microfossils. It overlies an older se- quence of deep-marine sediments, the Upper Cretaceous–Eocene Rajang Group, represent- ed by the Sapulut Formation in Sabah, Malay- sia. Hutchison (1996) reported a major uncon- formity at the base, due to the Eocene Sarawak orogeny. The fan includes the middle Eocene Trusmadi Formation, the upper Eocene–Oligocene Crocker Formation, and the Oligocene–lower Miocene Temburong Formation (Fig. 2). Deep-marine sedimenta- tion stopped in the early Miocene when the Proto–South China Sea was eliminated by subduction beneath Borneo (Hamilton, 1979; Hutchison, 2004; Taylor and Hayes, 1983). The resulting Sabah orogeny (Hutchison, 1996) deformed and exposed sediments of the Crocker Fan; today a part is offshore beneath thick Neogene sediments (Fig. 1). METHODS Unweathered sandstones were collected in Sabah for provenance analysis. Detrital modes of 97 sandstones were determined, and heavy mineral contents and zircon varieties were de- termined for 53 of these samples. Detrital zir- cons from five samples for which the strati- graphic age was known, selected from different levels in the fan, were dated using sensitive high-resolution ion microprobe. (Results are provided in the GSA Data Repository 1 .) SANDSTONE COMPOSITION Quartz in sandstones derived from plutonic, volcanic, metamorphic, or sedimentary rocks can be used as a provenance indicator (Petti- john et al., 1987). The Crocker Fan sandstones consist mostly of monocrystalline plutonic quartz. Metamorphic quartz is less common. Rare volcanic quartz is restricted to the oldest sandstones. QFL (quartz, feldspar, lithics; Fig. 3) and QmFLt (monocrystalline quartz, feld- spar, total lithics) plots (Dickinson et al., 1983) based on the Gazzi-Dickinson point- counting method show the quartzose nature of the sandstones and suggest a recycled orogen- ic source. Feldspar rarely forms more than 10% of the sandstones, and is predominantly potassium feldspar, indicating an acidic plu- 1 GSA Data Repository item 2006111, sandstone modal, heavy mineral, and chemical compositions; varietal distributions; and zircon U-Pb ages deter- mined by SHRIMP analysis, is available online at www.geosociety.org/pubs/ft2006.htm, or on request from editing@geosociety.org or Documents Secre- tary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. tonic source. Radiolarian chert fragments are always present, forming as much as 5% of the grains. Other lithic grains include serpentinite, rare acid volcanic rocks, granite, and schist. HEAVY MINERALOGY Zircon and tourmaline typically make up 70% of the heavy mineral assemblages in the Crocker Fan sandstones (Fig. 4). Their abundance indicates erosion from acid pluton- ic rocks and their unabraded shapes suggest a nearby source area. Other common heavy minerals are apatite, chrome spinel, garnet, and rutile. Apatite would normally be abun- dant in material derived from acid plutonic rocks, but it is often absent and those grains present are often pitted or partially dissolved. This suggests that the abundance of apatite re- flects weathering rather than provenance. Ap- atite is stable during burial but susceptible to acidic weathering, which is common in humid tropical settings (Morton, 1986). Common chrome spinel, showing very little abrasion, indicates a local ophiolitic source. Garnet abundance is variable, from 0% to 50%, and its association with rutile (average 6%) sug- gests a metamorphic source. Less common heavy minerals include pyroxene, amphibole, monazite, staurolite, and cassiterite. Cassiter- ite is brittle and easily destroyed during trans- port, and indicates a nearby acid plutonic source. Index values of chrome spinel–zircon (CZi) and garnet-zircon (GZi) are useful for prove- nance interpretation, because they are inde- pendent of the effects of hydraulic sorting and chemical destruction (Morton and Hallsworth, 1994, 1999). In Sabah the CZi value indicates the amount of ophiolitic relative to acid ig- neous material, and the GZi value indicates the relative amounts of metamorphic and acid igneous material. CZi values of the Crocker and Trusmadi Formations are typically be- tween 2 and 7, showing a small but consistent input of ophiolitic material. The GZi values range from 0 to 60, indicating variable input of metamorphic material. Other indices were less useful for interpretation; for example, the apatite-tourmaline index value reflects the loss of apatite during tropical weathering.