Genesis and paleoenvironment of the paralic neogene coals in the Mukah-Balingian area, central Sarawak Basin, Malaysia / Nor Syazwani Zainal Abidin

Nor Syazwani , Zainal Abidin (2021) Genesis and paleoenvironment of the paralic neogene coals in the Mukah-Balingian area, central Sarawak Basin, Malaysia / Nor Syazwani Zainal Abidin. PhD thesis, Universiti Malaya.

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Abstract

A study utilizing high-resolution macroscopic, microscopic, and geochemical analyses was conducted on the Neogene Mukah – Balingian coals in Sarawak, Malaysia, emphasizing on intra-seams and whole seam variations of the coals, and their associated sedimentary rocks. The analyses were performed on 90 coal samples to describe changes in the coal facies, peat development and its precursors, their associations in sequence stratigraphy, and depositional conditions of the peat/coal. The Early Miocene to Middle Miocene Mukah coal, Balingian Formation is characterised by bright, banded bright, banded dull, and dull coal lithotypes. Humotelinite (average range 41.2% – 60.8% m.m.f), resinite (average range 1.3% – 10.3% m.m.f), semifusinite (average range 0.7% – 2.3% m.m.f) and clarite microlithotypes (30.4% – 84.5% c.m.f) are predominantly observed throughout the studied coals. The mineral matter and carbominerite are found in low amounts (< 20%). Low sulphur, low ash yield, atomic ratios of hydrogen/carbon (mean 1.10 wt. %), oxygen/carbon (mean 0.90 wt. %), and carbon/nitrogen (mean 38.20 wt. %), and biomarker distributions support the source of organic matter of the Mukah coals from mainly terrestrial input sources under sub-oxic (mainly) to oxic within freshwater mires. A complete succession from topogenous to ombrogenous was identical for peat evolution. The basal section of the coal bench is dominated by humocollinite-liptinite rich coal or humocollinite-mineral matter-rich coal, while the middle section is characterised by humotelinite-rich coal and overlain by humotelinite-liptinite rich coal. The temporary development of rheotrophic – ombrotrophic mires occurred due to water table fluctuations that strongly depend on the ever-wet climate changes and syndepositional tectonics during the Neogene, which resulted in a balanced to high peat accumulation and preservation. A maximum thickness of 35m of the peat deposits is postulated to have accumulated between a period of 10,000 to 175,000 years, in order to produce the Mukah coals. The Mukah coals are suggested to be deposited in a coastal- flood plain within an upper (mainly) to lower delta plain setting. Within this setting, telmatic coal facies is more prone and could be influenced by the transgressive (TST) to initial highstand (HST) cycles. The Late Pliocene Balingian coal of the Liang Formation is characterised by bright, banded bright, and banded dull coals lithotypes. Humodetrinite (average range of 40.9% – 60.0% m.m.f), suberinite (average range of 2.1% – 12.0% m.m.f), semifusinite (average range of 0.1% – 1.9% m.m.f) and clarite microlithotypes (25.3% – 79.4% on c.m.f) are mainly observed throughout the studied coals. Mineral matter and carbominerite were found in low amounts in most of the samples. Peat doming is a significant factor resulting in upward brightness, dominance of humodetrinite over humotelinite, low contents of inertinite, detrital mineral, ash, and sulphur, and also scarcity of alginite and pyrite minerals. This is indicative of mainly freshwater paleomires and terrestrial input sources during peat accumulation and is further supported by petrographic indices, atomic ratio, and biomarker data. Mesotrophic to ombrotrophic (mainly) paleomires were also inferred. A complete succession of domed peat is characterized by humodetrinite-rich coal in the basal section, humotelinite-liptinite-rich coal in the middle section, and humodetrinite-rich coal in the top section. The thick coals in ombrotrophic mires reflect an extremely convex-matured dome, while thin coals represent a low-gradient dome within mesotrophic – ombrotrophic mires. The lack of basin subsidence, seasonal dry/wet climates, and water level fluctuations resulted in a low to high accumulation and peat preservation rate during peat growth. Peat was originally 10 to 70m thick and evolved between 20,000 and 350,000 years in a lower delta plain environment. The dome is interpreted to be protected from marine water flooding and lacustrine influence by the presence of adjacent vertically stacked shoreface deposits. The peat formation of the Balingian coal is thus suggested to be formed mainly in telmatic mires during the late lowstand (LST) and initial transgressive (TST) cycles. Based on the analyzed coals in this study, the compositional variations, geochemistry, and thicknesses of the coal seams have resulted in different characters, architecture, and history of the Neogene peat-to-coal evolution in the Mukah – Balingian areas, Sarawak.

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