Henrik Vosgerau

Relative sea-level changes recorded by paralic liptinite-enriched coal facies cycles, Middle Jurassic Muslingebjerg Formation, Hochstetter Forland, Northeast Greenland

Abstract The Middle Jurassic Muslingebjerg Formation of Hochstetter Forland, Northeast Greenland, consists of a succession of shallow marine sandstones encasing four coal seams formed in low-lying coastal mires. The seams are up to 3.45 m thick and contain coal facies cycles with high liptinite contents (up to 70 vol%). The cycles generally start with huminite-rich bright or banded coal and end with dull coal or clayey coal/coaly claystone. The dull coals are characterized by a high proportion of liptinite, commonly resinite, and often mineral matter. Liptodetrinite, resinite and cutinite dominate the liptinite maceral group. Similarity in the relative proportions of the liptinite macerals in the bright and dull coals and the general association of bright coal and dull coal with a high resinite concentration suggest, that the dull coals represent a residue after selective removal of ligno-cellulosic tissues in the precursor peat of primarily the bright lithotype. The clayey coal/coaly claystone lithotypes have high contents of mineral matter and commonly contain alginite and pyrite. Occurrence of pyrite in levels with high diasterane C 27 C 29 ratios and sometimes also alginite in the clayey coal/coaly claystone lithotypes, indicates a marine influence during flooding of the mires. This suggests a causal link between base-level rise in the coastal mires and relative sea-level rise. Accommodation space available for thick peat accumulations was governed by overall relative sea-level rise and the dulling-upward cycles record outpacing of peat accumulation due to accelerated rise in base level/relative sea level. The Muslingebjerg Formation consists of four depositional sequences, beginning with a thick coal seam resting on a sequence boundary. Peat accumulation reflects onset of base-level rise in the coastal area and the coals represent the lower part of the transgressive systems tract. The overlying shoreface sandstones form the upper part of the transgressive systems tract and a progradational highstand systems tract. Alternatively the two lower seams in the succession may form a transgressive-regressive couplet separated by a lagoonal sandstone split. In this case a sequence boundary occurs at the top of the upper regressive seam. The dulling-upward cycles within the coal seams represent parasequences or possibly higher order sequences. Minimum age-estimates suggest that they accumulated over 4000–11000 yr.

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

Abstract 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 R 0 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 C 29 -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.