Organic petrography and geochemistry of inertinite-rich mudstones, Jakobsstigen Formation, upper Jurassic, northeast Greenland: Indications of forest fires and variations in relative sea-level

The lower-middle Oxfordian Jakobsstigen Formation, Wollaston Forland, northeast Greenland, consists mainly of stacked coarsening-upward successions of offshore to shoreface heteroliths, sandstone and rare foreshore sandstones. The units are separated by thin, laterally extensive sheets of terrigenous carbonaceous mudstones, which have been subjected to organic petrographic and geochemical studies. The mudstones are thermally immature, with maturities corresponding to R₀ in the range 0.35–0.50%. The mudstones contain very high proportions of allochthonous inertinite, subordinate huminite, char and negligible proportions of liptinite. Inertinite reflectance distributions are markedly bimodal, with maxima at approximately 1.73 and 4.91% R_m. Both pyrolysis yields and solvent extract yields are low. The distributions of n-alkanes are markedly light-end skewed and show a pronounced predominance of even-numbered compounds in the lower carbon number range. Biomarker-distributions feature a dominance of C₂₉-steranes, slight enhancement of extended hopanes and αββ-steranes, low proportion of tricyclic triterpanes and very low hopane/sterane ratios. Sedimentological, organic petrographical and geochemical evidence suggests that the regular alternation between marine and terrestrial depositional environments during deposition of the Jakobsstigen Formation was related to low-amplitude, high-frequency changes in relative sea-level and local climate. The mudstones were deposited during early rise of relative sea-level in shallow, flat-bottomed lakes or lagoons on a broad coastal plain. The lakes acted as traps for fine elastic sediment and for predominantly windborne inertinite, generated by wildfires in the hinterland. High rates of evaporation rendered the lakes mildly saline, hampering their colonization by vegetation other than cyanobacteria and halophilic microorganisms. Similarly, saline porewaters excluded higher plant vegetation from emergent areas. Upon continued rise of the relative sea-level, the lakes were gradually flooded and their deposits became covered by sandy shallow marine sediments. The larger areas covered by shallow marine waters during periods of high relative sea-level led to a more humid local climate and to lower frequency of wildfires. During falling relative sea-level, the marine deposits were eroded and partially removed and the cycle subsequently repeated upon renewed rise in relative sea-level. Hence, minor changes in relative sea-level gave rise to the regular alternation of two vastly different depositional environments, as well as to marked variations in local climate.