The Silurian Black (Hot) Shales of the Cantabrian Arc, Northern Spain: Biostratigraphy, Chemostratigraphy & Palaeoenvironmental Reconstruction

.Tim Ferriday BSc. (Hons) Basin Dynamics Research Group, Earth Sciences and Geography, Keele University, Keele, Staffordshire, ST5 5BG

Upper Ordovician

Lower/Middle Silurian

Lower Devoinian

Sea-Level

Total Organic Carbon (TOC)

Gamma Ray (API)

Conclusions

References

Overview (project outline) What are ‘hot’ shales - Where are they found?

The Maps! What are they showing?

Reasons for anoxia and genesis:

Silurian

Devonian

488.3Ma

443.7Ma

416.0Ma

0

5

10

15

%T

OC

Grey regions showing source rock areas

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 ofthe 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.

2. Rapid transgressions as a result of the huge amounts of fresh water released at the margins of the Hirnantian Ice Mass. This preventing then contamination of continental sediments and producingthe clean shale deposits.

3. The freshwater masses may have led to fragmentation of the water column as the freshwater isless dense than the underlying saline rich water. Effectively acting as a cap to the water column preventing mixing of the oxygen depleted zones thus excelling the anoxia.

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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.

Total Organic Carbon %

Gam

ma R

ay (A

PI)

The peaks in the Gamma Ray log correspond to

the rapid increases in organic content. The first

peak is in excess of 700API. There is a direct

link between the organic content and the fixing

of uranium. Hence the shales are termed ‘hot’

in terms of the uranium content. Taken from

Luning et al (2005)

0000000000000000000000

0

100

200

400

500

600

700

444Ma

436Ma

400Ma

200API

3% TOC

Gamma Ray (API)

The first peak is showing a TOC value in excess

of 15%. This rapid increase in organic content

corresponds to the genesis of the firs

t Silurian

‘hot’ shales during the Rhuddanian. The second

peak during the Telychian leads to the

secondary ‘hot’ shale from Luning et al (2005)

Sea-le

vel

+

-

Ordovician

Cambrian

Tely

ch

ian

ho

t sh

ale

Rh

ud

dan

ian

ho

t sh

ale

Sea Level

Representing the marine transgressions and

regressions through time. A generalized curve

representing Palaeozoic sea-level fluctuations

after Schandelmeier and Reynolds (1997). Note

the ‘hot’ shale genesis at times of global

transgression

The Silurian Black (Hot) Shale deposits of the peri-gondwanan 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 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.

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?

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.

Gondwana

Baltica

Laur

entia

Siberia

Kaza

kstania

‘Hot’ Shales

MESOZOIC-TERTIARY COVER

PISUERGA CARRION PROVINCE/Unit

PICOS DE EUROPA PROVINCE/Unit

NAPPE (PONGA NAPPE) PROVINCE/Unit

CENTRAL ASTURIAN COALFIELD PROVINCE

Aramo Unit

La Sobia-Bodon Unit

Esla-Valsurvio Unit

Somiedo-Corrcillas Unit

GRANITOIDS

UNCONFORMABLE STEPHANIAN

NARCEA ANTIFORM (PRECAMBRIAN)

FOLD AND NAPPE PROVINCE UNDIFFERENTIATED PALAEOZOICOF THE CANTABRIAN ZONE

50km

BAY OF BISCAYN

SP

Equator

Rheic Ocean

Iape

tus

Oce

an

Gondwana

Baltica

Laur

entia

Siberia

Kaza

kstania

Equator

Rheic Ocean

Iape

tus

Oce

an

SP

Gondwana

Rheic Ocean

SP

Equato

r

Laurasia

Panthalassa O

cean

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.

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. TakenFrom Lunings ‘blackshale.com’accessed 2010.