Provenance of Cenozoic sedimentary rocks of northern Borneo
Marco van Hattum, completed PhD
New petrographic and geochronological studies on Cenozoic sandstones from northern Borneo suggest that great volumes of clastic material have been derived from local (Borneo and nearby SE Asia) rather than distal Asian and Himalayan source areas since at least the Eocene. Cenozoic sediments of northern Borneo consist of two major successions. During the Paleogene, deep marine turbidites were deposited in an accretionary wedge setting. After a major orogenic event in the Early Miocene subsidence resumed, and during the Neogene fluvio-deltaic and shallow marine sediments were deposited with a drainage pattern similar to the present-day configuration. This study concentrates on the extensive Eocene-Oligocene turbidites of the Crocker Formation and related formations.
Compositional and textural observations on both the light and heavy mineral fractions are in apparent conflict with each other. The framework petrography of the Paleogene turbiditic sandstones suggest a relatively mature derivation from an intermediate to quartzose recycled source. It also suggests that most of the sediment has been originally derived from a granitic source lithology, with some input of volcanic and ophiolitic material. Although the older parts of the turbidite succession tend to be more quartzose and mature, the textural maturity of all these sandstones however is quite low, especially in terms of clast rounding.
The preserved heavy mineral types tend to be the stable and ultrastable minerals, and zircon and tourmaline are most dominant. This also suggests that a granitic source lithology may have been of great importance. The presence of fresh chromian spinel indicates input from an ophiolitic source. Although the less stable heavy minerals are not preserved, the observed heavy minerals tend to show few signs of abrasion due to transport. The detrital zircons have often been preserved as perfectly euhedral crystals.
U/Pb SHRIMP ages of the detrital zircons show that the zircons are mostly igneous Mesozoic. The age distribution in the samples is surprisingly diverse compared to the uniform composition of the sandstones. Most ages are consistent with relatively proximal derivation from granites in Kalimantan and Peninsular Malaysia, rather than distal sources in Indochina.
It is inferred that the extensive Eocene-Oligocene Crocker Fan has been derived from relatively nearby source areas, controlled by local tectonics and tropical climate. Detrital modes of low-latitude tropical sandstones show a compositional maturity that is not easily reconciled with U/Pb SHRIMP ages of detrital zircons, sediment texture, and heavy mineral assemblages, even though the variety of the latter can be extensively depleted by weathering. Detrital modes may not be the most appropriate method to apply to low-latitude tropical sandstones as it shows a signal that not only reflects provenance terrane but also climatic modification of the sandstones.