Reconstruction of terrigenous organic matter transport by biomarker analysis of Miocene sandy to muddy shallow water deposits of Brunei Darussalam

抄録

The late Miocene Miri and middle Miocene Belait Formations of North Borneo exposed in Brunei Darussalam, are characterised as muddy offshore and sandy tidal-shoreface shallow marine environments, respectively, deposited by progradational deltaic sedimentary systems that developed due to rapid uplifting of the Borneo hinterland. Within the tidal-shoreface environment, tides were found to enhance organic matter (OM) deposition and preservation in sandstones, contrary to the belief that only mudstones can preserve OM (Mayer, 1994). Hence, we analysed terrigenous biomarkers in shoreface to offshore facies to reconstruct how tidal processes can affect OM deposition in sandstones. A total of 29 mudstone samples were collected from both formations, while an additional 16 samples were collected from tide-dominant shoreface sandstones. Biomarkers were analysed using GC/MS. Based on TOC% and terrigenous biomarkers such as leaf wax n-alkanes and perylene as well as redox indicators such as pristane/phytane ratios, decreasing seaward trends in terrigenous OM are seen and is concordant with general trends. However, gymnosperm-derived diterpenoids increased in the late Miocene, suggesting an increase of conifers in the Bornean hinterland possibly linked to increasing altitudes. The increase of angiosperm-derived cadalene in the late Miocene sediments also suggests the radiance of certain angiosperms. Sandstones created by tides exhibit either OM-rich cross bedding or wavy bedding of mud and sand alternations. The OM-rich sands have lower aquatic plant contents but slightly higher carbon preference index (CPI), which are indicators of maturity and source organisms, while wavy bedded sands have slightly higher marine OM contents. The presence of terrigenous OM preserved in tidal-sandstones provides evidence that sandstones can become important global carbon sinks.[References]Mayer, L.M. 1994. Relationships between mineral surfaces and organic carbon concentrations in soils and sediments: Chemical Geology, v. 114, p. 347–363