25th International Meeting on Organic Geochemistry IMOG 2011

P-210
The preservation of vascular plant biomolecules in a 155 million
years old sedimentary deposit. Case of the Flogidarry Shale
Member (Isle of Skye, Scotland)
Apolline Lefort, Yann Hautevelle, Bernard Lathuillière, Vincent Huault
UMR7566 G2R, CNRS, Nancy Université, Vandoeuvre-lès-Nancy, France (corresponding
author:yann.hautevelle@g2r.uhp-nancy.fr)
The Oxfordian/Kimmeridgian Flodigarry
Shale Member (Isle of Skye, Scotland) represents a
continuous succession of silty claystones and nodular
limestones. This outcrop may be a potential stratotype
(GSSP) for the Oxfordian/Kimmeridgian boundary.
So, many studies were conducted on these deposits,
covering the fields of lithology, biostratigraphy (using
both micro and macrofossils), sedimentology, isotopic
geochemistry and magnetostratigraphy (Wierzbowski
et al., 2006). Although these shales are particularly
rich in organic carbon, available data about their
organic molecular composition are very scarce. We
sampled all beds of the argillaceous succession in
order to conduct molecular analyses of the soluble
organic matter. Originally, the main objective was to
trace and document any evolution of the molecular
signature in these shales which could be related to
the Oxfordian/Kimmeridgian boundary.
The molecular compounds preserved in
these deposits shows that the organic component of
the Flogidarry Shale Member is a mixture of both
marine (algae, plankton, bacteria) and terrestrial
(vascular plants) organic matter. Excepted for one
single bed, the terrestrial contribution is clearly
important. This can be linked to the palaeogeography
of Scotland during the Oxfordian/Kimmeridgian
transition. Indeed, the Flogidarry area was located in
a narrow sea channel between the Hebrides and the
Irish massif. In regard to this palaeogeographic
background, it is not surprising to record a substantial
terrestrial molecular biomarkers supply to this
sedimentary succession.
No significant evolution of the molecular
signature is recognizable from one layer to another,
especially at the Oxfordian/Kimmeridgian boundary.
From a molecular point of view, the
exceptional preservation of the organic matter
remains the most interesting characteristic of these
sedimentary rocks deposited 155 million years ago.
Indeed, the Flogidarry Shale Member is characterised
by the abundance of diasterenes, hopenes, the large
predominance of ββ-hopanes over the αβ-hopanes as
well as the absence of the R epimers for
homohopanes. This distribution demonstrates the
thermal immaturity of the Flogidarry Shale Member.
Most interesting are the still functionalised
biomolecules (meaning molecules having escaped
diagenetic transformations since biosynthesis) like
cadinol, ferruginol and podocarpatrienol. This clearly
proves the excellent preservation of this sedimentary
organic matter deposited some 155 million years ago.
The preservation of such bioterpenoids in
sedimentary rocks as old as Jurassic is clearly
uncommon. Their recognition in deposits older than
50 million years are very scarce because they are
metastable and thus rapidly degraded during the first
steps of diagenesis. In fact, to our knowledge, there is
only one report of biomolecules preserved in
sedimentary deposits older than Kimmeridgian, dated
from Callovian (Marynovski & Zaton, 2010). Thus, this
makes the Flogidarry Shale Member the second
oldest sedimentary deposit to contain well-preserved
bioterpenoids and emphasizes the exceptional
preservation conditions associated to this outcrop.
These well-preserved biomolecules suggest
a palaeovegetation largely composed of pinophytes
on the nearest emerged lands. After the plant
biomarkers distribution, these landmasses were
probably covered by pinophytes forests especially
composed on Cupressaceae, Taxodiaceae or
Cheirolepidiaceae. This observation is moreover in
accordance with palynological studies (Riding &
Thomas, 1997).
Marynowski & Zaton (2010). Applied Geochemistry
25, 933-946.
Wierzbowski et al. (2006). Jurassica, Volumen IV, 17-
33.
Riding & Thomas (1997). Scottish Journal of Geology
33, 59-74.
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