Thermal maturity of coal. The huminite/vitrinite values indicate lignite to sub-bituminous B for the Mukah coals. The high concentration of NSO compound (65%) obtained from the EOM supports the low to moderate thermal maturity of the Mukah coals (Fig. 8a). Concerning the biomarker distributions of homohopanes C31 to C32 (m/z 191) and their frequencies, most of the samples being studied display a relative dominance of the “R” over the “S” epimers, indicating a low to moderate degree of thermal maturity22,23. The ratio of C32 homohopanes 22S/(22S + 22R) values range from 0.28 to 0.56 (mean 0.46), suggesting an increase in thermal maturity from the upper to lower coal seams of the Mukah coals. The ratios of C29 ββ/(ββ + αα) sterane ranging from 0.03 to 0.50 (mean 0.31) indicate low thermal maturity. The cross plot of 20S/(20S + 20R) steranes C29 and C32 22S/(22S + 22R) homohopanes (Fig. 8b) shows that the samples being studied were immature to early oil window for the Mukah coals and consistent with the CPI values from odd to even carbon ratios of n-alkanes and Pr/n-C17 vs. Ph/n-C18 22,30.
Paleomires condition. The vertical profile (Fig. 9) shows the Mukah coals enriched with brighter coals, huminite maceral, and clarite from the base to the top seams. The richness of huminite maceral implies the existence of oxygen-deficient and water-saturated conditions in the precursor mire. The apparent lack of dull coal and extremely low inertinite content (<10%) further suggests minimal wildfires or burning and oxidation of the peat. The uncommon occurrences of pyrite and alginite maceral in the coals being studied suggest peat accumulated in freshwater mires with little or no marine influence. Furthermore, the abundance of 80% angiosperm pollen from the coals being studied supports the depositional setting of the Mukah coals as freshwater peat swamp. Generally, arborescent types of plants are predominant, suggesting the characteristic of typified bog facies in the Mukah coals.
The plot of S vs. TOC (Fig. 10) may further describe the development of peat in freshwater mires within topogenous to ombrogenous mire facies, as suggested by31. The vertical changes in hydrological regimes (Fig. 9) under which the precursor mires accumulated indicate that the peat-forming mires began with predominantly ombrotrophic mires and eventually ended in rheotrophic mires because of the moderate to rapid rise of the water table during peat accumulation. Temporary development of rheotrophic–ombrotrophic mires may have occurred because of the water table fluctuations. Here, the mires were fed by rainfall and groundwater levels that formed during low to moderate water floods. In most cases, the intervals suggest that waterlogged and rheotrophic origins are related to the occurrence of increased mineral matter, which was probably influenced by the clastic influx from the fluvial input into the mires with rising water table levels. Furthermore, the biomarker distribution and concentration of n-alkanes, high Pr/n-C17, low Ph/n-C18, high CPI, high C29/C30 hopane, high Tm/Ts, high tetracyclic, and high C29 sterane support the fact that the source of the organic matter in the Mukah coals is mainly terrestrial. In addition, the plots of Pr/n-C17 vs. Ph/n-C18 and atomic ratios of H/C and O/C support the interpretation of organic matter source input, within the sub-oxic to oxic (mainly) condition32  and mainly from Type III kerogens and mixed Type II/III. The ratio of C/N is reported to be greater than 20 and 40 for the investigated samples, further representing the organic input from higher plants32.
Peat-forming vegetation. References of peat-forming vegetation were made to the known ecological data of ancient peats in Malaysia and Brunei indicated by33,34,35.36,37.
The vertical profile (Fig. 4) appears to be predominated by the freshwater peat swamp communities. Meanwhile, the riparian, strand forest, ferns, and mangrove swamp vegetation, which are present in low frequencies, support the ecology within the main lowland forest. The rich preservation of angiosperm pollens suggests that mostly terrigenous inputs are fed into the organic matter in dense and lowland forest vegetation. Overall, the plant communities appear to be represented mainly within freshwater-influenced swamps. The presence of Calamus type, Barringtonia, Palaquium, Pandanus, and Stenochlaena palustris in a few coals being studied represent a former riparian fringe type of forest. The occurrence of mangrove pollens in sections MC12 and MC01 in small quantities suggests the mangrove pollens were within proximity to the peat swamp forest communities. In contrast, no indication of mangrove influence is recorded in section MC05. Although pteridophytic spores of monolete smooth and Stenochlaena palustris are found in the entire coals being studied, they are not abundant, suggesting that the plant types were slightly influenced by fern taxa. Moreover, the significant occurrence of Casuarina type in the middle part of section MC12 and the small amount of pollen in most of the samples suggests that the Mukah coals were deposited close to the sea under the influence of the strand forest.
The assemblage list recovered in this study is similar to that provided by2. Compared with their work, this study yielded approximately similar assemblages of palynomorphs, but in slightly different proportions. The overwhelming presence of Casuarina type and Calamus type found in Mukah coals further suggests that the peat swamp vegetation was closely linked to Kerapah/Kerangas peat forest and marginally bordered by rattan2 even though these pollens were found restricted to certain seam intervals of the coals being studied.
Controls on peat formation. Following the coal-forming models38,39,40,41,42,43,44,45, the authors surmised that different characteristics, architecture, and thicknesses of coal seams are governed by the height of the mire water table, which can be correlated with the transgressive–regressive cycles and, hence, may reflect different system tracts and the significance of key surface of sequence-stratigraphic development in coal-bearing strata. Since the Mukah coals were deposited in a low-lying coastal setting, the groundwater and seawater are hydrologically connected. Thus, the rise in relative sea level can cause the groundwater table to rise in coastal mires, resulting in accommodation space for peat accumulation and upstream deposition of siliciclastics in the study area as the accumulation of peat requires a balance between the rate of accommodation space and peat accumulation (AR/PPR).
Overall, the Mukah coal formation initially occurred at a low water table that allowed the peat to form in peat-forming mires. The peat growth progressed within an overall rising water table (base level). During the peat formation, the creation of accommodation space was moderate to high in response to the rate of change in the base level. A gradual decrease in the rate is expected at the end of peat growth, which allowed the peat to accumulate and maintain a steady pace with the rise in the water table (Fig. 9). In some cases, in particular the upper section of the Mukah coals, the coal formation occurred in the mires during a high-water table, which resulted in high peat accumulation, followed by stable water table conditions, and balanced peat accumulation (Fig. 9). Since the Mukah coals were located in a coastal setting, peat accumulation and preservation were, therefore, associated with the sea-level rise in the paleo-mires, as indicated by the balanced to high AR/PPR. The balance in AR/PPR may cause partially expose paleo-peat bodies in the mires, whereas high AR/PPR may cause the drowning and inundations of paleo-peat bodies. Therefore, it is suggested that the proposed vertical variations in the lithotypes, microlithotypes, and macerals have been controlled by fluctuations in the groundwater level in ancient freshwater mires, thereby resulting in different accommodation/peat preservation rates (Fig. 9). High huminite, low inertinite, and high humite and clarite contents in most of the coals being studied indicate permanently water-saturated peat with balanced to high accommodation creation46. The increased liptinite content in the middle section of the Mukah coals indicates a loss of biomass and poor preservation of woody tissues. This may reflect the high preservation of herbaceous vegetation (unstructured material) in the mires. The moderate detrital-mineral matter content in topogenous peats may imply the association of fluvial input with the mires when peat accumulation was unable to keep up with the high rates of accommodation.
In this study, a large number of thin and clean coal seams topped by the tidal flat Begrih Formation7 indicates that frequent changes in the peat-forming mires occurred during the accumulation of peat, as evidenced by the temporary development of paleo-peat bodies from ombrotrophic to mesotrophic to rheotrophic and vice versa (Fig. 9). Based on the compositional variations of macerals, sub-macerals, and detrital-mineral matter, these changes were controlled by the moderate to high eustatic rise in sea level that led to a continuous rise in the base level of the region and subsequent rise in the water table of the coastal plain. The variations of coal lithotypes from being coarsely banded to dull in nature support intermittent moderate to high flooding of the fluvial areas, resulting in moderate to high diversification of the macerals/submacerals and moderate to no amount of mineral matter (Fig. 9). Once the mires were formed, they were shielded from substantial clastic deposition, as shown by the moderate to low amounts of clastic material in the seams (Fig. 9). On the basis of these observations, it is suggested that the anticipated stability of the tectonic setting should be excluded during the development of peat. This is supported by the study conducted by47, which showed that shifts in the base-level from the Middle Miocene to the Pliocene were highly influenced by eustasy, whereas during the Late Oligocene to Early Miocene, the effects of global sea-level changes were significantly obscured by major tectonic movements. Moreover, the syn-collision between Luconia Block–Dangerous Grounds and Borneo in a rapidly subsiding basin that influenced the formation of Early Miocene Mukah coals2 may be more important locally within the depositional basin in controlling the properties of coal seams (i.e., thickness, composition, continuity, and geometry)48 and is comparable according to structural characteristics and overall subsidence that occurred on land within the Sarawak basin49.
The water table fluctuations in the study, however, are strongly dependent on climate changes, as the influence of relative sea level on the position of regional water tables diminishes further inland38,50,51. The compositional variations of coal lithotypes in Mukah coals have most likely been affected by climatic conditions as the changes may be attributed to the relative sea-level variables of the region. In this study, the obvious absence of major of fire-generated inertinite coal seams, uncommon dull coal, and dulling-upward succession from the base to top coal seams could be related to the ever-wet climate during the Neogene, as the coals being studied may have experienced a widespread rise in rainfall52,53,54,55, which corresponded to the Asian monsoon from the earliest Miocene onward. As previously reported, the structured huminite maceral content (humotelinite) is predominant in the Mukah coals; thus, this observation supports permanently water-saturated mires with a minimum oxidation level during peat accumulation and was influenced by the rise in the water table. Furthermore, the high content of humite-clarite in the entire coals being studied implies that the paleomires have experienced a prolonged rise in sea level, as the signatures are typical of “transgressive” coal46. From the petrographic evidence, the Mukah coal peat mires evolved on a wet, low-relief coastal plain in low-lying areas with continuously intermittent moderate to high flooding. The relationship between low inertinite (<10 vol. %), moderate to high Rdit ratio (m/z 123), palynology signatures, and low IV factor suggests that a moderate to high-water table had influenced the development of peat in the wet mires while the climate was probably more wet during these periods. The scarcity of gymnospermous pollen (Podocorpus) in the coals being studied may further indicate a low contribution from vegetation in the drier or upland areas. This is further supported by2, who reported the abundance of Casuarina-type pollen associated with common occurrences of Dacrydium in the Kerapah peat swamps, indicating an extremely wet climate. The presence of Casuarina type in high frequency is also the case of this study.
Peat accumulation rates vary considerably, particularly with respect to geographical latitudes43. It is proposed in this study that the average peat accumulation rate for low-lying Holocene peats at low latitudes (<10°) ranges from ~2 to ~5 mm/year. According to56, the compaction ratio to form low-rank coal seams from their original peats varies between 1.4:1 and 30:1. A ratio of 10:1 has been most commonly used57,58,59,60. Considering that the coals being studied are low in rank and accumulated in a tropical climate, a ratio of 10:1 and peat accumulation rates of 2–5mm/year have been used as conservative estimations, resulting in the original thickness of the peat deposits in the Mukah coals of the Balingian Formation to be between 5 and 35 m and that records peat-forming environments were established between 10,000 and 175,000 years ago. The calculation is strongly dependent on the assumed peat–coal ratio; thus, it shows that the deposition of peat for the Mukah coals occurred within a short time interval.
The Mukah coals in a sequence-stratigraphic context. Some of the key sequence-stratigraphic surfaces have been proposed in the paralic coal beds being studied of the Balingian Formation (Fig. 9). The coal that initially formed at a low water table, overlies the paludification surface (PaS); meanwhile, the gradual termination of the peat represents a give-up transgressive surface (GUTS) in response to an increasing accommodation rate. Furthermore, the coal bed overlies a terrestrialization surface (TeS) at its initiation of peat formation in response to a decreasing accommodation rate, resulting in a GUTS during a stable water table. An accommodation reversal surface (ARS) was identified during the transition of accommodation from decreasing to increasing (balanced to high AR/PPR) or increasing to decreasing (high to balanced AR/PPR).
In this study, the creation of peat accumulation is observed from the difference in the thickness of the coals, which could be linked to the period of the base level changes in the mires. A shorter period of rising base level is assumed to have occurred in the middle and top parts of the section. The lowest coal seam (05/01) within the lower stratal section (MC05) is thicker than the upper and middle coal seam, suggesting that the initial transgressive stage occurred during the formation of the peat38, as the increasing base level increases the rate of accommodation, resulting in the formation of a moderately thick coal bed (Fig. 11). Meanwhile, the development of the middle (05/02) and upper (05/03) coal seams in section MC05 is suggested to occur during the middle transgressive stage (Fig. 11), as their thicknesses are relatively thin and isolated owing to the high accommodation rate during peat accumulation in the mires38. The increase in the rate of change in the base level is supported by the transition to a coarsening-upward unit, which indicates the gradual decrease in the growth of the peat-forming environment into a more clay- and sand-rich deposition (Fig. 11). The greater thickness of the middle section coal at MC12 indicates an overall increase in base level with a moderate water table in the initial stage, and that a late transgressive stage (Fig. 11) may have influenced the formation of peat in the mires38. From the associated sedimentary deposits within the coals, the overall fining upward succession further supports an upward decrease in depositional energy in the mires, thereby decreasing the rate of change in the base level. The occurrences of thin coal seams in the upper section (MC01) indicate that shifts in the base level have occurred over a relatively short period under stable peat-forming conditions (Fig. 11). The continuous formation of thin coal seams of the upper stratal section may suggest that the initial highstand stage (Fig. 11) controlled the low to moderate accommodation rate during the accumulation of peat in the mires38. The overall fining upward cycle indicates that hydraulic energy is low during deposition, thereby supporting the interpretation of gradual overgrowth peat in the mires.
In this study, the overall rising water table level during the growth of paleo-peat bodies suggests that the Mukah coals are deposited in a fluvial system. In the sequence-stratigraphic framework, the architecture of the Mukah coals from the lower to upper section could represent the transition from transgressive (TST) to initial highstand (HST) cycles (Fig. 11). This interpretation is supported by the high content of humite/vitrite + clarite in the entire coals being studied (Fig. 11), which corresponds to a transgressive event influencing the growth of peat in the mires49. The interpretation may also be linked to Sarawak’s stratigraphic framework analysis61, the detailed offshore reservoir geological assessment in the Balingian province62,63, and the paleogeographic study of the Balingian Formation7, which suggested that shales and coals of offshore Cycles I–II were deposited in an overall transgressive environment61,62,63, while two deposition stages, comprising an early transgressive event, followed by a late regressive episode, influenced the onshore Balingian Formation7.