Characterization of organic matter and depositional environment of the Jurassic small sedimentary basins exposed in the northwest onshore area of Sri Lanka

抄録

The Jurassic sediments in two small basins formed on the Precambrian metamorphic terrain of Sri Lanka, which had been located in the eastern Gondwanaland in mid-latitudes of the southern hemisphere, were investigated to make clear the origin of organic matter (OM) and its depositional environment. Total organic carbon (TOC) contents are high (3.05 – 5.10%) in brown to black color mudstones in the Jurassic Andigama Basin, while the grey color sandy sediments in the Jurassic Tabbowa Basin generally have very low TOC (0.04 – 0.17%). The Andigama mudstones are thermally immature, and the Tabbowa sediments and relational Miocene Aruwakkalu limestone are moderately matured in the oil generation stage. The sediment samples from the Jurassic Andigama Basin show a large proportion of middle-chain n-alkanes (nC₂₁-C₂₅), enriched C₂₉ steranes, high C / N ratios (16.3 – 37.8), very low total sulfur (TS < 0.001%) and higher pristane / phytane ratios (Pr / Ph = 2.1 – 3.0). These results suggest that the sediments were deposited in freshwater swamp under oxic condition. Abundant retene, simonellite, perylene, 1-methylphenanthrene, 1, 7-dimethylphenanthrene and 1, 2, 7- / 1, 2, 9-trimethylphenanthrenes in the Andigama mudstones indicate that the OMs were mainly originated from gymnosperm with fungi. On the other hand, the Jurassic sandy sediments in the Tabbowa Basin have predominant nC₁₆-C₂₁ alkanes with a minor peak of waxy n-alkanes (nC₂₉, nC₃₁, nC₃₃), very low TS (< 0.001%), abundant C₂₇ steranes and higher Ts / (Ts + Tm) [Ts: 17 α (H)-22, 29, 30-trisnorhopane, Tm: 18 α (H)-22, 29, 30-trisnorhopane] triterpane ratios, which suggesting the OMs from algal origin with a significant terrestrial higher plants. The Miocene Aruwakkalu limestone shows very low TOC (0.06%) and algal origin. The sediment samples from the relational Pleistocene Aruwakkalu Red Earth show a high unresolved complex mixture (UCM), and high values of carbon preference index (CPI = 4.78) and average chain length (ACL = 29.9), which indicate the OM from oxidized and / or biodegraded ancient terrestrial plant.