GEOCHEMICAL CHARACTERIZATION OF CENOMANIAN/TURONIAN BLACK SHALES FROM THE TARFAYA BASIN (SW MOROCCO):
RELATIONSHIPS BETWEEN PALAEOENVIRONMENTAL CONDITIONS AND EARLY SULPHURIZATION OF SEDIMENTARY ORGANIC MATTER#
S. Kolonica*, J.S. Sinninghe Damstéb, M.E. Böttcherc, M.M.M. Kuypersc, W Kuhntd, B. Beckmanna, G. Scheedere and T. Wagnera
aUniversity of Bremen, Faculty of Geosciences, PO Box 330440, 28334 Bremen, Germany.
bNetherlands Institute for Sea Research (NIOZ), Marine Biogeochmistry and Toxicology, Texel-Netherlands.
cMax Planck Institute (MPI) Bremen, Department of Biogeochemistry, Germany.
dUniversity of Kiel, Department of Geosciences, Germany.
eGerman Federal Institute for Geosciences and Natural Resources Hannover, Germany.
*Corresponding author: email email@example.com
# tabulated data presented in this paper is available under
Organic geochemical and petrological investigations were carried out on Cenomanian/Turonian black shales from three sample sites in the Tarfaya Basin (SW Morocco) to characterize the sedimentary organic matter. These black shales have a variable bulk and molecular geochemical composition reflecting changes in the quantity and quality of the organic matter. High TOC contents (up to 18wt%) and hydrogen indices between 400 and 800 (mgHC/gTOC) indicate hydrogen-rich organic matter (Type I-II kerogen) which qualifies these laminated black shale sequences as excellent oil-prone source rocks. Low Tmax values obtained from Rock-Eval pyrolysis (404-425°C) confirm an immature to early mature level of thermal maturation.
Organic petrological studies indicate that the kerogen is almost
entirely composed of bituminite particles. These unstructured organic aggregates
were most probably formed by intensive restructuring of labile biopolymers (lipids
and/or carbohydrates), with the incorporation of sulphur into the kerogen during
early diagenesis. Total lipid analyses performed after desulphurization of the
total extract shows that the biomarkers mostly comprise short-chain n-alkanes
(C16-C22) and long-chain (C25-C35)
n-alkanes with no obvious odd-over-even predominance, together with steranes,
hopanoids and acyclic isoprenoids. The presence of isorenieratene derivatives
originating from green sulphur bacteria indicates that dissolved sulphide had
reached the photic zone at shallow water depths (~100m) during times of deposition.
These conditions probably favoured intensive sulphurization of the organic matter.
Flash pyrolysis GC-MS analysis of the kerogen indicates the aliphatic nature
of the bulk organic carbon. The vast majority of pyrolysis products are sulphur-containing
components such as alkylthiophenes, alkenylthiophenes and alkybenzothiophenes.
Abundant sulphurization of the Tarfaya Basin kerogen resulted from excess sulphide
and metabolizable organic matter combined with a limited availability of iron
during early diagenesis. The observed variability in the intensity of OM sulphurization
could be attributed to sea level-driven fluctuations in the palaeoenvironment