25th International Meeting on Organic Geochemistry IMOG 2011

P-200
Palaeoenvironmental change reflected by carbon isotopes and
palynology in the early Silurian of the Tanezzuft Formation,
Libya
Mohamed Elkelani 1 , Gert Jan Reichart 1 , Jaap Sinninghe Damsté 1,3 , Zwier Smeenk 2 ,
Wolfram Kurschner 2 , Henk Brinkhuis 2,4 , Peter Nederlof 1,5
1 Utrecht Universities, Faculty of Geosciences, Department of Earth Sciences-Organic Geochemistry.,
Utrecht, Netherlands, 2 Utrecht Universities, Institute of Environmental Biology, Faculty of Science,
Laboratory of Palaeobotany and Palynology, Utrecht, Netherlands, 3 NIOZ Royal Netherlands institute for
sea Research., Texel, Netherlands, 4 Utrecht Universities, Institute of Environmental Biology, Biomarine
Sciences, Laboratory of Palaeobotany and Palynology., Utrecht, Netherlands, 5 Shell Exploration &
Production., Rijswijk, Netherlands (corresponding author:m.elkelani@geo.uu.nl)
The glaciations at the end of the Ordovician attracted
interest not only because of the associated major
extinction event, but also because they resulted in
widespread graptolitic black shales deposition.
Deposition of this organic-rich (‗hot‘) shale unit in the
North African/ Arabian region was restricted to the
earliest Silurian Rhuddanian stage and is currently the
source of almost all Palaeozoic oils in North Africa
and the Middle East. Two cores covering the early
Silurian Tanezzuft Formation from the Ghadames and
Murzuq Basins in western Libya were studied for the
palynological composition and carbon isotopic
composition of the bulk organic matter (δ 13 Corg).
Informal acritarch assemblage biozones were
identified based on the known ranges of key marker
taxa and on overall palynomorph assemblages
calibrated with high-resolution graptolite stratigraphy.
Unfortunately data on graptolites are scarce in Libya.
The palynomorph assemblages from the Tanezzuft
Formation appear to be largely influenced by periodic
environmental changes recorded from early to late
Llandovery. The marine acritarchs flora is
accompanied by common non-marine miospores and
cryptospores suggesting a proximal position and
significant admixing of land-derived material into the
marine setting. Some similarities are evident between
this Rhuddanian assemblage and those of Saudi
Arabia and British acritarch. This high-resolution
biostratigraphy for the same records across the early
Silurian (Llandoverian) allows comparison the δ 13 Corg
signatures between basins with events recorded from
elsewhere, such as Argiles Principales Formation
(Tunisia), Cornwallis Island (Canadaian Arctic), and
Dob‘s Linn (Scotland). The δ 13 Corg record from the
deeper-water successions on Cornwallis Island, and
Dob‘s Linn shows evidence of a significant positive
δ 13 Corg excursion in the upper Aeronian and weak
positive shifts in the mid-Rhuddanian and uppermost
Rhuddanian and the global environmental effect of
the early Rhuddanian and late Aeronian glaciations.
The base of the Tanezzfut Formation is marked by a
conspicuous high gamma ray intensity interval with a
high total organic content (TOC wt%), which can be
traced along the North African Gondwana margin.
This formation represents a progressive,
transgression, first drowning the deepest parts of the
palaeodepressions, but at a later stage also extending
to the basins palaeohighs. The δ 13 Corg record shows a
significant positive excursion during the onset of the
hot shale (Rhuddanian) deposition in the Ghadames
Basin with a rapid onset and gradually recovery. The
δ 13 Corg record from the Murzuq Basin, in contrast,
shows no clear isotope excursion. Since the hot shale
in Murzuq Basin is also much thinner than that of
Ghadames, the lack of a carbon isotope excursion at
the Murzuq Basin suggests a shift in the black shale
deposition from the central part of the basin to the
margin. Comparison of the biomarker signatures
between the two basins suggests a much more
intense biodegradation in Murzuq Basin compared to
Ghadames Basin.
The lowermost Tanezzuft Formation (hot shale) is
characterized by low abundances of acritarchs
assemblages and some prasinphytes occur in time of
high stable isotope values and dominated by what
was probably originally well preserved AOM, which is
typical of laminated black shales with (Type II) oil
prone kerogen deposited in distal stratified shelf sea
basin with dysoxic-anoxic depositional conditions.
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