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 succ essions of offshore to shoreface heteroliths, sandstone and rare fores hore sandstones. The units are separated by thin, laterally extensive sheets of terrigenous carbonaceous mudstones, which have been subjecte d to organic petrographic and geochemical studies. The mudstones are t hermally immature, with maturities corresponding to R-0, in the range 0.35-0.50%. The mudstones contain very high proportions of allochthono us inertinite, subordinate huminite, char and negligible proportions o f liptinite. Inertinite reflectance distributions are markedly bimodal , with maxima at approximately 1.73 and 4.91% R-m. Both pyrolysis yiel ds and solvent extract yields are low. The distributions of n-alkanes are markedly light-end skewed and show a pronounced predominance of ev en-numbered compounds in the lower carbon number range. Biomarker-dist ributions feature a dominance of C-29-steranes, slight enhancement of extended hopanes and alpha beta beta-steranes, low proportion of tricy clic triterpanes and very low hopane/sterane ratios. Sedimentological, organic petrographical and geochemical evidence suggests that the reg ular alternation between marine and terrestrial depositional environme nts during deposition of the Jakobsstigen Formation was related to low -amplitude, high-frequency changes in relative sea-level and local cli mate. The mudstones were deposited during early rise of relative sea-l evel in shallow, flat-bottomed lakes or lagoons on a broad coastal pla in The lakes acted as traps for fine elastic sediment and for predomin antly 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 halophil ic microorganisms. Similarly, saline porewaters excluded. higher plant vegetation from emergent areas. Upon continued rise of the relative s ea-level, the lakes were gradually flooded and their deposits became c overed by sandy shallow marine sediments. The larger areas covered by shallow marine waters during-periods of high relative sea-levelled to a more humid local climate and to lower frequency of wildfires. During falling relative sea-level, the marine deposits were eroded and parti ally removed and the cycle subsequently repeated upon renewed rise in relative sea-level. Hence, minor changes in relative sea-level gave ri se to the regular alternation of two vastly different depositional env ironments, as well as to marked variations in local climate. (C) 1997 Elsevier Science B.V.