TY - JOUR
T1 - Organic petrography and geochemistry of inertinite-rich mudstones, Jakobsstigen Formation, upper Jurassic, northeast Greenland: Indications of forest fires and variations in relative sea-level
AU - Bojesen-Koefoed, Jørgen A.
AU - Petersen, Henrik I.
AU - Surlyk, Finn
AU - Vosgerau, Henrik
PY - 1997/12
Y1 - 1997/12
N2 - 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% Rm. 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.
AB - 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% Rm. 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.
U2 - 10.1016/S0166-5162(97)00030-X
DO - 10.1016/S0166-5162(97)00030-X
M3 - Article
VL - 34
SP - 345
EP - 370
JO - International Journal of Coal Geology
JF - International Journal of Coal Geology
SN - 0166-5162
IS - 3-4
ER -