The age, palaeoclimate, palaeovegetation, coal seam architecture/mire types, paleodepositional environments and thermal maturity of syn-collision paralic coal from Mukah, Sarawak, Malaysia

Abstract

The Mukah coal accumulated in the Balingian Formation where the time-stratigraphic position is poorly defined by fauna, though a probable Late Miocene age has always been assigned to this formation. Samples collected in the present study that yielded an abundance of Casuarina pollen associated with occurrences of Dacrydium, Stenochlaena palustris, Florschuetzia levipoli and also Stenochlaena areolaris spores, compare closely to zone PR9 of the palynological zonation of the Malay Basin, and can be tied to depositional sequences of Malay Basin Seismic sequences I2000/I3000, indicating an Early Miocene age for the studied coal. The Early Miocene age shows that the Mukah coal was formed during the collision between Luconia Block–Dangerous Grounds with the Borneo that lasted from Late Eocene to late Early Miocene. The rapid increase of deposition base-level caused by the collision is clearly reflected by the architecture of the Mukah coal seams that were generally thin, and also by the reverse order of the paleo-peat bodies.The studied coal samples contained large amounts of detrohuminite without the enrichment of liptinite group macerals, usually explained by the dominance of herbaceous plants in the paleomires. However, the pollen assemblages recovered in the present study provide no support for a ‘herbaceous’ swamp, instead the peats were likely formed mainly from forest vegetation based on the recovery of dominant arboreal pollen assemblages in the present study. Palynomorph assemblages recovered in the present study, with abundance of Casuarina pollen associated with common occurrences of Dacrydium, strongly suggest Kerangas vegetation and Kerapah type peat swamps, indicating a very wet climate. The occurrence of rattan and Pandanus pollens shows that the kerapah type peat swamps were locally bordered by rattan and Pandanus swamps. This shows that the prevailing use of coal petrography to discern the type of vegetation that was present in the peat-forming mires may lead to wrong conclusions. Therefore, a multi-disciplinary approach must be applied for a more accurate and reliable spatial interpretation of the type of vegetation that was present in the peat-forming mires.Consistent with the low sulphur content, evidence from the palynomorph assemblages recovered from the coal seams shows that the coal-forming peat was deposited in freshwater mires with little or no marine influence, despite the fact that the accumulation of the Mukah coal-forming peat took place within a coastal plain setting. Moreover, the fauna present in the host rock formation also suggested a brackish-water environment of deposition.