Late Middle Miocene volcanism in Northwest Borneo, Southeast Asia: Implications for tectonics, paleoclimate and stratigraphic marker

Abstract

Explosive volcanic events often produce pyroclastic materials that can be recognized from the geological record. These discrete pyroclastics form regional marker beds. Here we report the occurrence of a tephra layer interbedded within very thick coal beds near Mukah, Sarawak, Borneo. Traceable for tens of kilometers in the Mukah area of Sarawak, this tephra layer can be considered as regional stratigraphic marker with precise chronostratigraphic control. Systematic sedimentological, mineralogical, geochemical and zircon U-Pb geochronological studies have revealed a major effusive volcanic event during the latest Middle Miocene, presumably contemporaneous and/or related to a magmatic event of an earlier phase of the Mt. Kinabalu pluton or magmatism in West Sarawak or East Sabah. The volcanic event had promoted catastrophic flooding of coastal swamps and fallout from the ash clouds that formed a regionally monotonous tephra layer across the Serravallian-Tortonian boundary. In conjunction with the regional occurrences of trap rocks, structural trends and known tectonic events, we constrained the regional depositional environments, and climate. The tephra layer was deposited in a coastal plain-swamp,seasonal, shallow, high-moderate energy fluvial channel-lacustrine environmental setting, wherein atmospheric fallout and eroded material from regoliths formed over older basement and volcanic rocks of the hinterland which were mixed to produce the tephra layer. This tephra layer is sandwiched between the very thick coal beds. A pre-existent volcanic chamber that was active for a long time, also experienced periodic explosive activity from probably the same magma chamber and conduit and including a major explosive activity that recycled early-formed crystals and felsic magma (rhyolite-dacite) during the major effusive event are also recognized. Our findings provide robust evidence for the prevalence of intensive chemical weathering under a wet-humid climate, and relative tectonic quiescence before the major effusive event, and the existence of vast, monotonously gently-sloping coastal plains and luxuriant vegetation akin to the present.

A new upper Paleogene to Neogene stratigraphy for Sarawak and Labuan in northwestern Borneo: Paleogeography of the eastern Sundaland margin

“…Murtaza et al (2018) concluded a Late Pliocene to Pleistocene age for the Liang Formation based on palynology. More recently, a tephra deposit within the coalfield has been dated by Ramkumar et al (2018) and revealed a latest Middle Miocene age (c. 12 Ma), thus indicating a slightly older age for the sequence than previously assumed, and a correlation with the Liang Formation in the northern Miri Zone/Brunei therefore seems inappropriate.…”

“…2b and 3. The age of the Liang Formation in the Mukah-Balingian province is latest Middle Miocene as defined by the interbedded tephra layer reported by Ramkumar et al (2018).…”

“…Here we consider the previously reported unconformity to represent a sequence boundary. We placed the Begrih Formation in the Middle Miocene (Fig. 3) based on recent dating of the underlying Balingian and overlying Liang Formations by Murtaza et al (2018) and Ramkumar et al (2018).…”

Reply to Discussion: Hennig-Breitfeld, J., H.T. Breitfeld, R. Hall, M. BouDagher-Fadel, and M. Thirlwall. 2019. A new upper Paleogene to Neogene stratigraphy for Sarawak and Labuan in northwestern Borneo: Paleogeography of the eastern Sundaland margin. Earth-Science Reviews 190, 1–32

“…The Balingian Formation was initially considered to be Late Miocene based on foraminifera assemblages (Liechti et al, 1960) resulting in an estimated Early Pliocene and Late Pliocene to Pleistocene age for the overlying Begrih and Liang Formations respectively. However, recent U-Pb dating of zircons from a tuff layer in the Liang Formation (Ramkumar et al, 2018) yielded 11.4 ± 0.4 to 11.8 ± 0.8 Ma mean ages (late Middle Miocene) which enabled more precise stratigraphic positioning for the whole Mukah-Balingian sequence and highlights the importance of absolute age dating of critical horizons.…”

Relict hydrocarbon seeps in the Oligocene–Miocene Subis carbonate platform, Malaysia: Implications on hydrocarbon generation and migration pathways and potential sealing by shale gouging

“…The whole Borneo Island sits within the Sundaland region (Metcalfe, 2011). Although Borneo is not one of the most active regions of Sundaland, with only sparse earthquake and volcanic events today, it had a violent tectonic history (Mathew et al, 2016, b;Ramkumar et al, 2018). The basement of Borneo, at the western interior zone, consists of Palaeozoic and Mesozoic igneous, sedimentary, and metamorphic rocks that behaved as a craton during the middle to late Tertiary (Hall, 1996).…”