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The Silurian Black (Hot) Shales of the Cantabrian Arc, Northern Spain:
Biostratigraphy, Chemostratigraphy & Palaeoenvironmental Reconstruction
Tim Ferriday. BSc. (Hons)
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Basin Dynamics Research Group, Earth Sciences and Geography, Keele University, Keele, Staffordshire, ST5 5BG
What are ‘hot’ shales - Where are they found?
The Silurian Black (Hot) Shale deposits of the perigondwanan region are one of the most important
source rocks for hydrocarbons world-wide. Despite
intensive research, questions regarding the genesis
and the palaeoenvironmental, palaeogeographic
factors controlling the formation of these Silurian black
‘hot’ shales are still a matter of ongoing debate.
An in depth palaeoenvironmental reconstruction
combined with the high-resolution biostratigraphic and
chemostratiographic data will provide new insights into
the genesis of the Cantabrian black ‘hot’ shale
deposits. A comparison of the results of this research
project with data available from other Silurian
Gondwanan black ‘hot’ shale deposits will lead to a
progressive, state-of-the-art genetic model and an
advanced understanding of the factors controlling the
deposition and genesis of the restricted black ‘hot’
shales of the Northern Gondwanan Area.nnnnnnnnnnn
The 'hot' shale is termed so because of the high gamma radiation content, that of the insoluable U4+ (reduced
from soluable U6+ found in seawater, as result of the anoxia). The Gamma values may exceed that of 700+
API (American Petroleum Institute) units in the 'hot' shale horizons. Yet the 'lean' shale surrounding is
commonly 90-120+. In oxygenated conditions U6+ is dissolved in seawater. Under the anoxic black 'hot'
shale conditions the U6+ is reduced to U4+ which is insoluble and is therefore precipitated.
The enrichments in authegenic U4+ in Silurian 'hot' shales therefore coincide with high organic content
and anoxic phases. The Silurian ‘hot’ shales were deposited along the majority of the northern rim
of Gondwana. The palaeo-reconstructions (below) the central globe shows the locations of the
‘hot’ shale and infers where else they may have been deposited.
Reasons for anoxia and genesis:
Rheic O
and
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sgr alized ations
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ner fluctu . Note
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Leting theugh timoic seaynolds ( s of g
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Th 5%. T s to t the R c h i a n g et a
of 1 espond during e Te l y Lunin
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corr shales n g t h ale fro
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‘hot k d u r ‘hot’ sh
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‘Hot’ S
Siberia
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Sea
Upper Ordovician
444Ma
3.
The freshwater masses may have led to
fragmentation of the water column as the freshwater is
less dense than the underlying saline rich water.
Effectively acting as a cap to the water column nnn
preventing mixing of the oxygen depleted zones thus
excelling the anoxia.nnnnnnnnnnnnnnnnnnnnnnnnnnn.
-
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Conclusions
The project after successfully
completing a number of field seasons and
cataloguing/analysing a substantial data set
(regarding the geochemistry and palynology) will determine
whether the shales of the Cantabrian region are indeed ‘hot’ or not. If they
are ‘hot’ the shales can be correlated to those of the North African basins. Modelling
of the ‘hot’ shale genesis and factors controlling the formation of the shales within Cantabria
will bring new insight to a topic poorly understood. If the shales of the Cantabrian region do not
express the ‘hot’ shale properties, why not?
0
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4. Finally the upwelling of nutrient rich
water along the northern rim of
Gondwana, this resulting in a high
bioproductivity. This organic rich
event is reflected in the high bio
content in the shales. Large
amounts of nutrient rich water
flooded the plains of
northern Africa the waters
were over saturated,
an excess of organic
matter that could
not be
oxygenated.
Lower Devoinian
400Ma
a
ani
2. Rapid transgressions as a result of the huge nnnnn
amounts of fresh water released at the margins of the
Hirnantian Ice Mass. This preventing thenn nnnnnn
contamination of continental sediments and producing
the clean shale deposits.ngggggggggggggggggggggg.
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1. The palaeorelief generated by the Ordovician
glacial processes meant that vast island belts formed
as a result of the sea-level rise. The island belts acted
as flow barriers in turn preventing the reoxygenation of
the shelfal waters. This created a perfect environment
for the anoxic phases and furthermore reduction U6+
to U4+ and the locking of organic content at the
sediment-water interface.nnnnnnnnnnnnnnnnnnnnnnnn
SP
an
UNDIFFERENTIATED PALAEOZOIC
OF THE CANTABRIAN ZONE
tor
Baltica
O
ce
FOLD AND NAPPE PROVINCE
Equ
a
us
PICOS DE EUROPA PROVINCE/Unit
PISUERGA CARRION PROVINCE/Unit
The three globes below are reconstructions of the continent
configurations for the Late Ordovician, Silurian and Lower Devonian.
The first globe (bottom left) represents the configuration of the
Late Ordovician at the time when the Hirnantian glaciation was
at its maximum. It shows the glacial maximum taken from
Monod et al (2003).
The central globe is of most importance as it shows the
global configuration at the time of ‘hot’ shale genesis.
The ‘hot’ shales are clearly marked upon the
northern rim of Gondwana. Note how the
Hirnantian ice masses have retreated. This
lead to a massive global transgression
and ultimately the genesis of the Silurian
‘hot’ shales.
The third globe (top right) represents
the global configuration of the
Lower Devonian. Gondwana
has migrated north and has
collided with Laurasia
leading to the closure of
the Rheic Ocean and
the formation of the
Variscan
Orogeny.
Ia
pe
t
50km
CENTRAL ASTURIAN COALFIELD PROVINCE
NAPPE (PONGA NAPPE) PROVINCE/Unit
Gondwana
The Maps! What are they showing?
tia
Somiedo-Corrcillas Unit
Esla-Valsurvio Unit
La Sobia-Bodon Unit
Aramo Unit
Siberia
La
ure
n
NARCEA ANTIFORM (PRECAMBRIAN)
GRANITOIDS
Lower/Middle Silurian
436Ma
an
ce
N
BAY OF BISCAY
MESOZOIC-TERTIARY COVER
UNCONFORMABLE STEPHANIAN
R
ean
c
O
heic
aO
Project area overview
Figures from left to right. Graptolitic ‘hot’
shales from the Algerian Sahara.
Silurian black shale expressing
Graptolites. Finally the Algerian‘hot’
shales in weatherd form. Taken
From Lunings ‘blackshale.com’
accessed 2010.
La
ss
a la
n th
Pa
Overview (project outline)
References
LÜNING, S., SHAHIN, Y., LOYDELL, D.K., AL-RABI, H.T., MASRI, A., TARAWNEH, B. and KOLONIC, S., 2005. Anatomy of a world
-class source rock: Distribution and depositional model of Silurian organic rich shales in Jordan and implications for hydrocarbon potential.
American Association of Petrolium Geologists, 89(10), 1397-1427.
MONOD, O., KOZLU, H., GHIENNE,J.F., DEAN, W.T., GUNAY, Y., LE HERISSE, A. and PARIS, F., 2003. Late Ordovician glaciation in southern Turkey. Terra
Nova, 15, 249-257.