Anadon, 1989

Palaeogeography, PaIaeoclimatology, Palaeoecology, 70 (1989): 7-28
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
7
LACUSTRINE OIL-SHALE BASINS IN TERTIARY GRABENS FROM
NE SPAIN (WESTERN EUROPEAN RIFT SYSTEM)
P. A N A D O N 1, L. C A B R E R A 2, R. J U L I A 1, E. R O C A 2 a n d L. R O S E L L 2
l Institut "Jaume Almera", C.S.I.C., c~ Mart$ i Franquds s/n. 08028 Barcelona (Spain)
2Faculty of Geology, University of Barcelona, 08028 Barcelona (Spain)
(Received June 15, 1987; accepted October 1, 1987)
Abstract
Anad6n, P., Cabrera, L., Juli&, R., Roca, E. and Rosell, L., 1989. Lacustrine oil-shale basins in Tertiary grabens from
NE Spain (Western European rift system). Palaeogeogr., Palaeoclimatol., Palaeoecol., 70: 7-28.
In the NE Iberian Plate, the convergent motion and collision of the European, Iberian and African plates resulted in
the development of Paleogene compressional features (strike-slip systems, thrust-fold belts) and late Oligocene to
mainly Neogene extensional structures which are superimposed on the former. These extensional structures are
represented by horsts, half grabens and tilted blocks, often developed in connection with preexisting, inherited faults.
From the late Oligocene and during the Neogene both strike-slip and extensional regimes alternated and coexisted,
giving rise to a number of fault-bounded basins. The deposition of organic-rich facies, and in particular oil-shales,
took place in the lacustrine complexes developed in some of these basins: Campins Basin (late Oligocene); Ribesalbes
and Rubielos de Mora basins (early middle Miocene); Libros Basin (late Miocene) and Cerdanya Basin (late
Miocene).
Deep lacustrine sequences ranging from 100 up to 250 m thick were deposited in all the above mentioned basins. The
organic-rich sequences are characterized by thin lamination, absence of bioturbation, rare benthonic fauna (if
present, it is supplied from other parts of the basin) and excellent preservation of "exotic" fossils (plant leaves,
insects, amphibians). The suitable depth conditions needed for permanent stratification in the water bodies and
anoxia in the deeper parts of the lakes, were due essentially to increasing subsidence outstripping deposition. Intense
tectonic activity is recorded by the occurrence of olisthostromes and slumps affecting the lacustrine deposits as well
as by the syntectonic structures recorded in the basin-fill successions.
The late Oligocene late Miocene paleoclimatic regimes, ranging from warm tropical to subtropical conditions,
were favourable for the development of permanently stratified lakes. Moreover chemical ectogenic meromixis may
have contributed in some cases to the establishment of permanent stratification.
The Eastern Iberian rift system, makes up the southernmost part of the larger Western European rift system. The
occurrence of lacustrine sediments ranging from the late Eocene to late Miocene, is frequent in the fault-bounded
basins of this complex rift system and lacustrine oil-shale deposits have been recorded in several of these basins. Thus
the Western European rift system presents an interesting model of intracontinental rifting in a foreland platform
setting, where favourable conditions for organic rich deposits often took place. The high potential of this kind of
tectonic settings for lacustrine oil-shale exploration must be stressed.
Introduction
A n y k i n d of r o c k able e i t h e r to provide
c o m m e r c i a l oil p r o d u c t s b y h e a t i n g ( S t a c h et
al., 1982) o r t o y i e l d o i l i n c o m m e r c i a l a m o u n t s
u p o n p y r o l y s i s ( T i s s o t a n d W e l t e , 1984) is o f t e n
n a m e d o i l - s h a l e . T h i s t e r m is n o t a p p l i e d t o a n y
0031-0182/89/$03.50
specific k i n d o f s e d i m e n t a r y r o c k , so a w i d e
r a n g e of l i t h o l o g i e s a r e i n c l u d e d i n it. U s u a l l y
c o n s i d e r e d as a n early stage of the transformation from oil-prone organic matter into
m a t u r e oil, o i l - s h a l e d e p o s i t s h a v e b e e n rec o r d e d f r o m a v a r i e t y of p a l e o e n v i r o n m e n t s
r a n g i n g f r o m m a r i n e to n o n - m a r i n e . A s a
© 1989 Elsevier Science Publishers B.V.
consequence of the discovery of large oil fields
which have yielded oil derived from lacustrine
source rocks (Powell, 1986), special attention
has been recently focused on oil-shales that
originated in lacustrine environments.
The Phanerozoic record of lacustrine oilshales is quite wide, and includes Paleozoic to
Cenozoic basins developed in diverse tectonosedimentary settings (i.e. Donovan, 1975; Eugster and Hardie, 1975) although special emphasis is often put on the rift setting (Robbins,
1983).
This paper is concerned with the overall
description of some Tertiary oil-shale lacustrine basins from NE Spain. The common
general megasequential trends recorded in
these basins, as well as the setting of the
studied basins within the Western Europe
intracontinental rift system, are stressed.
Geological setting
During the Tertiary, tectonic evolution in
the Iberian Plate was largely influenced by its
intermediate location between the convergent
and colliding European and African plates
(Vegas and Banda, 1982). Two large thrust-fold
belts developed in relation to the active
margins of the Iberian Plate: The Pyrennees to
the North (mainly Paleogene in age) and the
Betic Ranges to the South (mainly Neogene).
Most of the tectonic structures observed in the
internal parts of Iberia record the development
of intraplate deformation linked to the evolution of the mentioned active margins (Vegas
and Banda, 1982).
In the NE Iberian Plate, continental collision between Iberia and Europe resulted in the
development of large Eocene-Oligocene compressional features. The Pyrenean fold-thrust
belt and its late southern foreland basin (Ebro
Basin) are the most important structural
features in the area (Fig.l). Other important
structures which resulted from Paleogene compression are the Iberian Range (a cover fold
and thrust system determined by major basement faults) and the Catalan Coastal Range
strike-slip system (Guimer~, 1984; AnadSn et
al., 1985). Between the Iberian Range and the
Catalan Coastal Range (C.C.R.) an array of
E-W folds and thrusts developed. Extensive
cover folding and thrusting took place in this
unit (Linking Zone) during the Paleogene
(Guimer&, 1984).
At the end of the "compressive" evolutionary stage in NE Iberia, restricted late Oligocene rift processes took place, probably linked
to local transtensional situations produced in
the NE part of the Catalan Coastal Range
strike-slip system. A small basin with a relatively thick infill records this early rifting in
the Campins area (AnadSn, 1986). This episode
was followed by a later transpression which
resulted in the overthrusting of the alluvial
and lacustrine sequences by a basement slice.
More generalized rifting processes related to
either tensional or transtensional situations,
took place in NE Iberia mainly during the
Neogene and gave rise to structures superimposed on the earlier compressive ones. Thus
~
N
-'-"<l___a.__~ PYRI:^,- ~
~ . ~
'Te /VE A N
(~
AXIAL ZON"E
Cerdanyaj.~j
Campini~ '
e
R
o
e A S \ ~
Barcelona
Penedes-Valles
~- Graben
lOOKm
J
Teruel
Graben
~ ~.~'--"~ Ribesalbes
~ ' " " Basin
I'~
Neogene Grabens
Fig.1. Rift basins developed in the Northeastern Iberian
Peninsula. Numbers refer to the studied lacustrine oilshale basins. Encircled letters correspond to the main
grabens mentioned in the text. E=L'Empord&. S = L a
Selva. V = Valls-Reus. B = Baix Ebre. M = Maestrat.
C = Calatayud-Daroca.
horsts, half-grabens and tilted blocks developed extensively, often in relation to preexisting inherited faults (Fig.l). The Cerdanya,
Empord&, La Selva, Vall~s-Pened~s, VallsReus, Baix Ebre, Maestrat, Teruel, CalatayudDaroca, Rubielos de Mora grabens and halfgrabens are some of the structures related with
the Neogene rifting. Neogene rifting in the NE
Iberian Plate was closely influenced by the
preexisting basement faults and it developed
mainly in the Catalan Coastal Range and in
the Iberian Range. During the early Miocene
(Aquitanian-Burdigalian),
rift
processes
spread along the present NE Iberian margin
and extended towards the Iberian Range. In
the mentioned areas the transition from the
Paleogene compressional to the Neogene
mainly extensional regime was gradual (Guimere, 1984). Thus, all along the Neogene,
tensional and transtensional conditions alternated and coexisted with transpressive conditions, depending upon the orientation of the
faults in relation with the tectonic stress field
and the evolutionary trends of the latter
(Guimer&, 1984). In some cases this fact is well
recorded by tectonic macrostructures (i.e.
Calatayud-Daroca Basin). In most of cases,
however, this persistently changing, transitional situation is just recorded by microstructures and is not so evident on a large scale
(Guimer&, 1984). The Catalan Coastal Range
and Iberian Range rift basins evolved with
diverse alternatives until the latest Pliocene.
Rift processes also developed in the Eastern
Pyrenees and neighbouring areas during the
late Oligocene-early Miocene (Calvet, 1986). A
later graben basin generation took place in the
Eastern Pyrenees during the late Mioceneearliest Pliocene, in relation to the strikeslip motion along large basement faults. Thus
the Cerdanya basin developed as a half graben
linked to the SW horsetail end of the La Tet
strike-slip fault (Roca, 1986; Cabrera et al.,
1988).
Lacustrine complexes developed very often
in some of the fault bounded basins. Paleoenvironmental conditions favourable to the development of organic-rich facies, and oil-shales in
particular, took place in some of these lacustrine complexes. Thus organic-rich sediments
occur in the lacustrine systems whose evolution
was linked to the Campins (late Oligocene),
Ribesalbes, Rubielos de Mora (early Miocene),
Libros and Cerdanya (late Miocene) basins.
Tertiary oil-shale basins in the NE
Iberian P e n i n s u l a
Campins Basin (Paleogene strike-slip fault
system of the Catalan Coastal Range)
The late Oligocene deposits in Campins
consist of alluvial and lacustrine sequences, up
to 700m thick, which crop out in a small
(3 km 2) area close to the Vall~s fault (Fig.2).
This N N E - S S W striking basement fault, belongs to the Paleogene strike-slip fault system
of the C.C.R. Convergent wrenching along the
-o ox\ \ \
+1
Quaternary
- ~ Miocene alluvial
CAMPINS BASIN
~
Upper alluvial unit
[~
Paleozoic
metasediments
~
Lacustrine unit
[~
Granitoids
[ ~ ~ Lower alluvial unit
deposits
Fig.2. Geological sketch of the upper Oligocene deposits of
Campins. The Vall~s fault, which belongs to the Catalan
Coastal Range Paleogene strike-slip system, acted as a
normal fault during the Neogene.
10
Vall~s fault resulted in the overthrusting of the
Oligocene deposits by a basement slice of
Paleozoic metamorphic rocks (Anad6n, 1986).
During the Neogene, the major faults of the
Paleogene strike-slip fault system of the C.C.R.
acted as normal faults which gave rise to
grabens. Upper Miocene alluvial deposits unconformably overlie both the Campins Oligocene deposits and the overthrusting basement
slice. In broad outline the Oligocene deposits of
Campins are the remnants of an earlier late
Oligocene basin of u n k n o w n areal extent, now
included in the later Neogene Vall~s-Pened~s
graben.
The first description of the entire Oligocene
sequence was made by Almera (1907). Later
studies (Anad6n, 1973, 1986; Anadbn and
Villalta, 1975) enabled one to establish the
main stratigraphic and sedimentological features of the Oligocene sequence (Fig.3). On the
basis of these works three main stratigraphic
units can be distinguished in the Campins
Basin: Lower alluvial unit, Intermediate lacustrine unit and Upper alluvial unit.
- - Lower alluvial unit. This unit is mainly
composed of variegated arkosic coarse sands
with interbedded red mudstones and conglomerates. These deposits overlie granitoids. Their
thickness ranges from 40 m, in the southernmost outcrops, up to 400 m in the sector nearest
to the Vall~s fault. This abrupt thickness
change is probably related to the burial of an
earlier, preexisting active fault.
- - Intermediate lacustrine unit. The lower
part of this unit, up to 20 m thick is formed by
variegated sandstones, mudstones and limestones. Coal beds, up to 0.25m thick and
travertines are also locally present. These
lower deposits have been interpreted as shallow fresh-water lacustrine deposits (Anad6n,
1986) and are overlain by a deeper lacustrine
sequence up to 150 m thick. During this "deep"
lacustrine episode terrigenous and carbonate
facies developed. Oil-shales were deposited
also during this evolutionary stage. Deep
lacustrine facies are overlain by lacustrine
marls and limestones, deposited during a later
shallow lacustrine stage.
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Road
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---'~-I
Granitoids
\
Arkosic sandstones I
I~.',,.~:l and conglomerates
LACUSTRINE FACIES
\
\ F?
//
~
Siltstones with
interbedded sandst.
~
O i l - s h a l e s , d o l o s t o n e s I~
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and limestones
Mudstones, limestones \
:.4~?
minor sandstones
Coal
T Travertine
g Charophytes
fl~ Macrophytes
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,-~ Ostracods
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Fig.3. Simplified lithological sections of t h e upper Oligocene s e q u e n c e s in t h e C a m p i n s Basin.
- - Upper alluvial unit. This is formed of red
arkosic sandstones and conglomerates, up to
120 m thick. They overlie the upper shallow
lacustrine facies of the intermediate unit.
Two main facies assemblages can be differentiated in the "deep" lacustrine sequence: clastic
facies and carbonate facies. The clastic facies
assemblage which crops out in the NE area, is
mainly formed by massive and laminated siltstones up to 5 m thick which alternate with
arkosic sandstones and microconglomerate
beds up to 1.5m thick. The coarse-grained
ll
sediments display massive or graded bedding.
Neither scours nor channels have been observed. These deposits were formed by mass
flow processes (Anad6n, 1986). This clastic
facies passes laterally southwestward into a
mudstone-dominated sequence with minor
interbedded sandstones and carbonates which
in turn pass laterally into the carbonate facies.
The carbonate facies assemblage is formed
by a complex arrangement of carbonate mudstones (marls), massive and laminated mudstones, oil-shales, limestones and dolostones.
Limestones and dolostones make up thin beds,
up to 20 cm thick, which are interbedded in
the marl-dominated successions. Nevertheless,
intervals up to a few meters thick of thinbedded carbonates with mudstone intercalations have been recorded. Limestones and
dolostones are formed mainly by micrite, and
sometimes are laminated. Peloid grainstones to
packstones, up to 1 cm thick are also recorded
in the carbonate facies.
Rhythmites (calcite-organic rich clay couplets) are locally present. Abundant plant
leaves and minor insects and fish remains have
been recorded from the laminated mudstones
and marls (Almera, 1907; Anad6n, 1973).
The uppermost levels of the carbonate facies
consist of carbonate mudstones and thin limestones with abundant mollusks, ostracods and
charophyte remains. This succession has been
interpreted as shallow lacustrine deposits
(Anad6n, 1986). Oil-shales from the carbonate
facies of the lacustrine unit comprise carbonate marls and mudstones. Laminated
mudstones (paper-shales) up to 30 cm thick
alternate with faintly laminated carbonate
mudstones and marls up to 2 m thick. The total
oil-shale unit thickness probably is ca. 150 m.
No extensive analyses have been made on the
oil-shales of this basin. Laminated mudstones
(paper-shales) have a high organic carbon
content (up to 11.5°//o TOC). Oil yield, recorded
from some samples of marls and mudstones are
respectively 62.1 1/MT (6% after, Almera, 1907)
and 50 1/MT (I.G.M.E., 1981a).
The overall described basin-fill megasequence records an early basin formation phase,
with related alluvial deposits, followed by the
development of a shallow lacustrine environment which evolved into a deep lacustrine
environment. The sedimentary features indicate that, in this phase, the lake was meromictic and organic-rich sediments were formed.
Coarse-grained terrigenous deposits accumulated in marginal zones. They were formed by
mass flow processes. The upper parts in the
studied sequence record a new shallow water
sedimentation phase and a later alluvial phase.
Ribesalbes-Alcora Basin (Iberian Chain)
The Miocene Ribesalbes-Alcora Basin
(Fig.4) is a complex graben belonging to the
Neogene rift system superimposed on the
preexisting structural features of the SE Iberia
Chain. The basin (up to 150 km 1) is bounded by
ENE-WSW to NNE-SSW normal faults. The
Mesozoic outcrops which surround the Ribesalbes-Alcora basin are formed mainly by a
Jurassic-Cretaceous carbonate-dominated sequence up to 1000 m thick. Minor outcrops of
Triassic red beds, carbonates and evaporites
are also present. Several Mesozoic faultbounded blocks crop out within the graben.
Two main sequences can be distinguished in
the sedimentary infill of this basin (Agusti et
al., 1988). The lower sequence is formed by
alluvial and lacustrine deposits up to 600 m
thick which originated through the early-middle Miocene (Ribesalbes sequence; Fig.5). The
upper sequence consists of alluvial deposits up
to 200 m thick, probably formed during the
middle and late? Miocene (Alcora sequence). A
continuous transition without major unconformities has been observed between the two
Neogene sequences near Araya. In the
southern margin of the basin the basal levels of
the Ribesalbes sequence unconformably overlie Cretaceous limestones whereas in the
northern part of the graben, conglomerates of
the Alcora sequence overlie the Mesozoic
rocks. This means the present outcrop boundaries do not record the original margins of the
sedimentary basin.
In the Ribesalbes sequence (outcrops near
la
B) Dolostones,
mudstones
Mesozoic
D) Olisthostrome
Fig.4. Geological
sequence.
sketch of the Miocene
Quaternary
E) Limestones
Ribesalbes-Alcora
the village) five main units can be distinguished (Anadon, 1983a; Agusti et al., 1988).
This sequence (Fig.5) from base to top consists
Of:
Unit A. This consists of up to 300 m thick
pebble to large boulder breccias of Mesozoic
limestone clasts with minor interbedded red
sandstones and mudstones. Their poor sorting
and strongly disorganized fabric, lack of internal stratification and clear erosive basal contacts justify its interpretation
as mass flow
deposits. Minor channelised
stream flow deposits have also been observed.
Unit B. Constituted by a succession, up to
100 m thick of laminated
dolostones
with
interbedded thin layers of massive or laminated dark brown mudstones and sandstones.
The contact between A and B units is poorly
exposed. Dolostones
are formed by Mg-poor
dolomite.
Minor calcite and aragonite
are
sandstones
Basin. A-E
refer to the units of the lower Miocene
Ribesalbes
present in variable amounts. Early diagenetic
opal-CT has also been recorded. Organic-rich
horizons are present in the laminated dolostones and in the mudstone beds. Olistholiths
of Cretaceous
rocks and slumped horizons
have been recorded.
Well preserved abundant plant leaves, insects and minor amphibian skeletons and bird
feathers are found in this unit. This fact, as
well as the fine lamination in the rocks, and
the absence of benthonic fauna and bioturbation point to anoxic bottom conditions in a
meromictic lake phase.
Oil-shale facies from this unit includes two
rock types. Organic-rich laminated mudstones
have a mean oil yield value of 87 l/MT and a
maximum record of 250 l/MT. Other rocks
(mainly dolostones and dolomitic marls) show
mean values of 10.8 l/MT (IGME, 1981b). Dolostones and dolomitic marls cropping-out near
13
13,
n3
20
m
O
=~
Sand
, , ,
1.8
d
~'J_-,:-~ i
rn
10
15
f
10
~
P:::/
o o <:3 o ~
~ o
,o0
,~.
Laminated
dolostones
~
Laminated
mudstones
[-~
p~
~
~
0
Breccias
-~
5
A
1
~
~ Macrophyte
leaves
~,' Insects
~-" O s t r a c o d s
../2 Slump
Massive
mudstones
~
W a v e ripples
Marls
~
Current r i p p l e s
Sandstones
.--'~ g l a s e r str.
0
Fig.5. Synthetic stratigraphic log of the Ribesalbes sequence and detailed sedimentological logs (b and c) of the main
lacustrine units.
Ribesalbes show a TOC c ont e nt ranging from 1
to 15%.
Unit C. Yellow to gray massive and laminated mudstones with interbedded dolostones
and sandstones. Dolostones, locally displaying
slump folds, are similar to those from the
underlying unit and form packets up to 10 m
thick. Sandstone intervals, up to 1.5 m thick
with thin interbedded mudstones comprise
horizontally stratified and wave ripple crosslaminated beds. This unit, up to 90 m thick, is
capped by a 3 m thick coarsening upward
sequence, grading from mudstones alternating
with thin sandstones in the base to crossbedded sandstones and conglomerates on top.
The C unit records a significant increase of
detrital inputs to the lake in comparison with
the underlying B unit. Shallow water sedimentation stages are recorded by the wave-rippled
sandstones pointing to continuous or intermittent agitation episodes in a shallow lake floor.
The coarsening upward sequence on top of this
unit must be interpreted as a deltaic mouth bar
sequence. Deeper sedimentation phases, alternating with the shallow ones, are recorded by
the thick laminated mudstones and the interbedded laminated dolostone intervals. The
latter point to a recurrence of the anoxic
bottom conditions which prevailed in the
underlying unit.
Unit D. Formed by an olisthostrome of
disorganized, heteromictic large blocks of Cretaceous limestones, up to 70 m thick. Blocks of
several m 3 in volume are abundant and several
large blocks have cross sections of 20 x 5 m.
Unit E. Thin bedded limestones with thin
interbedded mudstones, up to 25 m thick. The
limestones contain abundant ostracods and
14
charophytes and sometimes are laminated.
This mainly carbonate sequence has been
interpreted as formed in a shallow open
offshore lacustrine environment.
In the Araya zone extensive outcrops of C
unit are present, but the olisthostromic unit is
not found. There, the Alcora sequence overlies
the C unit.
The overall described Ribesalbes megasequence indicates a basin (graben) formation
phase with related coarse mass-flow dominated
alluvial fan deposits overlain by a thick
lacustrine sequence formed in a meromictic
lake where carbonate deposits were predominant. Minor and episodic sandy underflows
have been also recorded. Olisthostromes and
slumped horizons reflect tectonic activity in
the area during this deep lacustrine stage.
Later the lacustrine system experienced water
level fluctuations and detrital inputs increased. Shallow and deep water conditions
alternated. This phase ended with a deltaic
episode. Renewed tectonic activity led to the
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The Rubielos de Mora Basin is an e a r l y middle Miocene half-graben developed in the
SE Iberian Chain. The basin is bounded by
E N E - W S W to N E - S W striking faults and
shows a southern steep, fault bounded
margin related with E N E - W S W faults. The
Miocene deposits unconformably overlie the
Cretaceous substratum along the n o r t h e r n
margin, but in some places the contact is
faulted (Fig.6).
The Cretaceous outcrops which surround
the Rubielos de Mora Basin consist mainly of
a lower thick red-bed sequence (sandstones
and mudstones) overlain by thick marine
limestones with minor interbedded dolostones,
carbonate mudstones and sandstones. The
Miocene basin-fill sequence is made up to over
J ~-.~- ~
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k"
o " ' "~ ~':.2 .'- :,6; -: ~~ ~
o
Rubielos de Mora Basin (Iberian Chain)
es)" 'r~
o o °~'~,-,
o
emplacement of an olisthostromic unit 70 m
thick followed by a shallow water-carbonate
sedimentation phase.
cycloa
Sandstones and
"~L • • j C 2 conglomerates
O ~
I
[~
Quaternary
[~
Upper Miocene
Laminated
i
C1 mudatonea
~
/Major fault .~Syncllne Fl~'l Cretec'ou.
Fig.6. G e o l o g i c a l s k e t c h o f t h e M i o c e n e R u b i e l o s d e M o r a B a s i n .
~
.,,
,
~
O I'''l
B
Limestones.
mudetonea end
sandstones
Sendet°neS%ndmudatones
A
conglomerates
15
800 m thick of terrigenous and minor carbonate deposits of alluvial and lacustrine origin.
Three main depositional units have been
defined and they record three different, successive evolutionary stages of the basin infilling
(AnadSn, 1983a).
(A) Lower unit: Formed by up to 300 m of
sandstones with minor interbedded mudstones
and conglomerates. This unit records an early
evolutionary stage of the basin, largely dominated by alluvial sedimentation.
(B) Middle unit: This unit, up to 70 m thick,
mainly consists of lacustrine limestones with
interbedded mudstones, sandstones and brown
coals (lignites). Limestones show a high content of bioclastic remains (gastropods, bivalves, ostracods and charophytes). This unit
records a shallow lacustrine stage.
(C) Upper unit: This unit (Fig.7) is up to
300 m thick and has been divided into three
major facies assemblages (AnadSn et al., 1988).
C1. This facies assemblage is up to 250 m
thick and consists mainly of laminated
mudstones (oil-shales) and interbedded
rhythmite beds showing a very scarce or
tAI
E
4on laminated
mudstone
'Laminated
mudstone
02
Limestone
Sandstone
;onglomerate
L
Brown coal
( lignite )
lOOm
Fig.7. Synthetic stratigraphie logs of the lower Miocene
deposits from the Rubielos de Mora Basin.
absent bioturbation. The laminated mudstones have a high organic matter content
and delicately preserved fossil remains
(leaves, insects, amphibia, etc.) are often
recorded. The rhythmites consist of alternating laminae of carbonate (mainly aragonite)
and clays.
C2. This assemblage mainly consists of
sandstones, mudstones and conglomerates
up to 150 m thick. These terrigenous deposits
interfinger with the C1 and C3 minor units.
•C3. This facies assemblage is up to 200 m
thick and is mainly formed, in the western
part of the basin, by cyclical sequences.
These sequences consist of organic-poor,
non-laminated mudstones and marls cyclically alternating with thinly laminated facies:
sandy mudstones, bioclastic laminae, oilshales, rhythmites (carbonate-clay, varvelike couplets) and marls.
Along the SE margin of the basin, slumps,
olistholiths and chaotic breccias occur interfingering with the C1 facies assemblage. The
former record strong tectonic activity during
sedimentation of unit C.
The facies assemblages in unit C record
diverse paleoenvironmental conditions, from
alluvial (unit C2) to lacustrine (C1 and C3).
When compared with unit B, the features of C1
and C3 deposits (oil-shales and thinly laminated rhythmites) reflect the occurrence of
relatively "deeper" lacustrine conditions.
C1 facies record the development of sequences formed in open (offshore) lacustrine
areas in a meromictic lake, where anoxic
bottom water conditions were persistent and
laterally continuous. Cyclical sequences (C3
unit) record alternating oxic-anoxic bottom
conditions in marginal zones of this meromictic lake, due to cyclical changes in lake water
volume. This feature is recorded in the cyclical
sequences by mineralogical changes and facies
transitions. Thus low-Mg calcite and nonstoichiometric (Mg-poor) dolomite are the dominant carbonate minerals in the non-laminated
facies, whereas variable amounts of low-Mg
calcite, high-Mg calcite, aragonite and nonstoichiometric dolomite have been recorded in
16
most of the laminated facies. Low-Mg calcite is
the main carbonate mineral in sandy mudstones and bioclastic laminae which occur at
the lower part of laminated intervals in the
cycles.
Strong tectonic subsidence (not balanced by
sedimentation and taking place in an active
rift setting) and palaeoclimate were the most
important features favourable to the establishment of a meromictic lake.
The oil-shale and rhythmite deposits show
the same general features in the C1 and C3
facies assemblages. The oil-shale deposits consist of organic-rich, brown to dark grey shales.
These occur either as very thin laminae
alternating with clastic accumulations of ostracods and Potamogeton seeds (C3 unit) or as
individual beds up to 7 m thick. Carbonaceous
vegetal debris, including very well preserved
fossil leaves (which are often concentrated in
thin beds of laminae), are abundant. Occasionally thin shelled ostracods, rare gastropod
shells, dropstones and Potamogeton seeds are
scattered in the mudstone. Articulated skeletons of Salamandrinae have been also recorded
(AnadSn et al., 1988).
The average organic matter content in the
oil-shales ranges from 20 to 40 1/MT, with a
maximum content of 70 1/MT (IGME, 1980).
Libros Basin (Teruel graben, Iberian Chain)
The N N E - S S W Teruel graben, dissects older
N W - S E structural trends in the Iberian Chain,
over at least 100 km, with a median width of
about 20 km (Fig.8). Although the sedimentary
infill began in the early Miocene the main
faulting and tilting phase took place from the
middle to the late Pliocene (Moissenet, 1980).
The Neogene deposits in the Teruel graben
consist mainly of alluvial red beds, and lacustrine white limestones and gypsum (Gautier et
al., 1972). The graben-fill sequence ranges from
lower-middle Miocene to lower Pleistocene
(Adrover et al., 1978; Moissenet, 1982). The red
bed dominated successions in the basin margins pass laterally to carbonate and gypsum
lacustrine deposits in the axial graben zones
Fig.8. Geological sketch of the Neogene Teruel graben.
The distribution of the Libros Gypsum is shown.
(Fig.9). Main lacustrine expansion phases took
place during the latest Miocene and early
Pliocene. Minor expansion phases have been
recorded in Vallesian (early-late Miocene) and
Aragonian (early-middle Miocene). In general
the carbonate lacustrine deposits show shallow water and/or paludine features. Open
lacustrine carbonate facies are mainly formed
by well bedded laminated limestones. The more
frequent nearshore or paludal carbonate sequences are mainly formed by highly bioturbated limestones showing frequent pedological
features, with travertines or coal interbedded
(AnadSn, 1983b). Gypsum units, except for
Vallesian gypsum of the Libros area, have been
deposited in shallow lakes and in the surrounding mud flats.
In the Libros area (20 km SSW from Teruel),
a gypsum-dominated sequence of Vallesian
17
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Libros
Gypsum
Fig.9. Synthetic stratigraphic framework of the Teruel Graben infill showing the successive evolutionary lacustrine stages.
(early late Miocene) age contains oil-shales.
The basin fill sequence in the Libros area
(Fig.10) is formed by the following units:
(a) Lower detrital unit (lower Miocene). This
unit consists of more t han 150 m of alluvial
conglomerates, red sandstones and mudstones.
Conglomerates are more a b u n d a n t at the base
of the succession. The upper part of this unit is
made up of red mudstones, white marls and
thin limestones and gypsum. This part of the
sequence originated in shallow water lacustrine environments and in their surrounding
mudflats.
(b) Bolage Limestones (Aragonian, middle
Miocene). This unit is made up of 50-80 m of
thin-bedded limestones and marls. Limestones
are frequently laminated and bear abundant
gastropods, charophytes and o s t r a c o d s (wackestones). Burrowing is rare. In some southern
basin areas coals are also present. Northward,
this unit is formed of thick bedded limestones
showing abundant bioturbation traces. This
unit was deposited in a fresh water lacustrine
environment. Open lacustrine facies, characterized by finely laminated wackestones and biomicrites (mudstones) are found in the LibrosMinas area, whereas in the surrounding zones,
paludal and shallow lacustrine facies occur.
(c) Libros-Minas Gypsum (Vallesian, upper
Miocene). The lower part of the sequence, up to
50 m thick, is formed of limestones and mudstones with interbedded gypsum layers up to
1 m thick. The upper part of the sequence, up to
70 m thick, is mainly composed of laminated
gypsum with some interbedded limestones and
mudstones. Limestone beds, up to 1.5 m thick,
consist mainly of wackestones with charophyte stems, gastropods and ostracods. Cross
and convolute bedding are present in some
limestone beds. Large scale, irregular replacements of the skeletal limestone by anhedral
gypsum have been observed. Laminated mudstones up to 10 m thick are frequently found
alternating with limestones and interbedded
between gypsum beds in the upper part of the
sequence.
The laminated mudstones are organic-rich,
containing a b u n d a n t diatoms and sponge spicules. Well preserved remains of amphibians,
birds, snakes and leaves have been also recorded. Diatoms found in the laminated mudstones suggest alkaline, fresh to slightly saline
water (Margalef, 1947).
Gypsum successions consist of thin bedded,
laminated gypsum. The laminae are formed by
microlenticular gypsum associated with dia-
18
I
I
]
I
I
I
J
I
I
I
- A - -
Laminated
gypsum
^--
-J~rUI•
"
Skeletal
limestones
l
h I. h i . ^
A .~'
A'
^
A .f~ A
A'
A
^
A
A
.a
Calcisiltites
('Marls')
A _ A _ -~
Mudstones.
Laminated
mudstones
^
A
..L
=
•
I
J
I
I
--A
J'Z
n
=
1
I
I
I
,
1
*Slump"
"Oil-shales"
s
Native
sulphur
I
--A~
--A --A--A-
Fig.10. Synthetic stratigraphic log in the Libros sequence
and detailed log of the oil-shale bearing facies.
toms and sponge spicules and anhedral, microcrystalline gypsum. The latter usually occur as
disrupted bands. In some places diagenetic
gypsum with a variety of textures replaces the
finely laminated facies. Slumps affecting gypsum and limestones packets are common.
Irregular nodules and bands of native sulphur are frequently found in gypsum beds and
more frequently in limestones interbedded in
mudstone and gypsum rich intervals. In this
case carbonate replacements of gypsum have
been observed. They are probably related to
the reactions leading to the formation of native
sulphur.
Oil-shales from the Libros-Minas unit are
formed by dark brown to black laminated
mudstones up to 10 m thick. The mean oil yield
of the oil-shales from well samples ranges from
20 to 70 1/MT, although maximum, isolated
values up to 180 1/MT have been recorded
(IGME, 1981a,c) TOC from oil-shale outcrops
range from 1 to 2.6%.
(d) Upper detrital unit (Vallesian-?Turolian,
upper Miocene). Formed by skeletal limestones
which are overlain by red mudstones with
interbedded sandstones. This sequence, up to
50 m thick may be interpreted as shallow water
lacustrine deposits overlain by alluvial deposits, recording a major retreat of the lacustrine systems in the Teruel graben.
(e) Santa Barbara Limestone (Turolian,
uppermost Miocene). It consists of up to 50 m
thick of shallow water lacustrine limestones
with abundant fresh water gastropods. Travertines also occur.
The overall basin fill sequence from the
Libros area indicates a basin formation phase,
with related alluvial deposits followed by the
development of a shallow lacustrine system
which evolved to deeper lacustrine conditions
(Libros-Minas unit). The Neogene sequence in
the Libros area ends with shallow water
lacustrine and alluvial deposits.
Some remarks concerning the deep lacustrine deposits must be stressed: The laminated
mudstones (oil-shales) have a high organiccarbon content, bear well preserved "exotic
fossils" and show lack of bioturbation. These
features suggest they originated under anoxic
bottom conditions in a meromictic lake. The
crossbedding in the limestones that alternate
with the oil-shales suggests resedimentation of
skeletal carbonates from littoral zones. In
addition they do not show bioturbation structures. Gypsum beds have been deposited in a
subaqueous (lacustrine) environment as is
proved by the lithological characteristics and
fossil content (diatoms, sponges, etc.). Slumps
and bioclast reworking suggest that both
limestones and gypsum have been deposited in
an open lacustrine environment although clear
evidences of meromixis cannot be deduced
from these lithofacies.
19
dance of slumped horizons in the Libros-Minas
sequence. On the other hand stratification of
the lake waters could have been reinforced by
ectogenic meromixis.
The gypsum sedimentation probably corresponds to high-sulphate (despite low totalsalinity) lake water phases. High-sulphate
content in the lake water probably was obtained by incoming surface and subsurface
waters from neighbouring Keuper gypsum
outcrops. Neither a high evaporation rate nor
high-salinity are needed to explain gypsum
precipitation in lakes, since gypsum solubility
in pure water is 2.41%o. Thus high-sulphate
water phases would have alternated with lowsulphate water ones. In the latter, laminated
mudstones were deposited in the open lacustrine areas under meromictic, low sedimentation-rate conditions. These phases alternate
with stages of resedimentation of the skeletal
carbonates originated in littoral zones.
The meromictic conditions suggested for
explaining the Libros gypsum lacustrine sequence could be attained by a relative deepening of the lake due to subsidence outstripping
deposition. This subsidence was probably tectonic in origin, as is suggested by the abun-
Cerdanya Basin (Eastern Pyrenees)
The Cerdanya Basin is an ENE-WSW oriented half-graben which lies over Paleozoic
rocks in the Eastern Pyrenees (Fig.ll). The
Cerdanya Basin is a horsetail structure located
in the northwestern block of the La Tet fault at
its southern termination (Roca, 1986; Cabrera
et al., 1988).
The basin shows a clear asymmetry between
the S and SE, and N margins. Subvertical fault
scarps bound the half-graben at the southern
and southeastern margins while Neogene sediments directly overlap Paleozoic rocks along
the northern margin.
The Neogene infill in the Cerdanya Basin is
essentially siliciclastic and consists of 1000 m
thick (Pous et al., 1986) muddy, sandy and
0
..o ..T'..':':'~..:.- .
.:~-...~::.~.....
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.
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LACUSTRINE
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DEPOSITS
DEPTH ~ +
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ALLUVIAL
/
io:.:....::¢:o
.
.
.
.
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0
2
4
6Kin
PROXIMAL
DEPOSITS
Granitic source a r e a
Non granitic s o u r c e a r e a
DISTAL
Fig.11. Geological setting of the Cerdanya Basin (Eastern Pyrenees). N . B . = Neogene basins. P . F . = Paleogene foreland basin
infill. P . T . = M e s o z o i c - P a l e o g e n e
thrust-sheets. P . B . = P a l e o z o i c
basement. Paleocurrent trends and paleoenvironmental
distribution during deposition of the lower Neogene unit.
20
conglomeratic alluvial fan to fluvial sequences. Diatomites and thin lignite seams
linked respectively to open and marginal
lacustrine paleoenvironments are also recorded.
Two main stratigraphic units have been
identified in the Cerdanya Basin (Roca, 1986;
Cabrera et al., 1988) (Fig.12). The lower Neogene unit dated as middle-late Miocene (Vallesian) and the upper Neogene unit dated as
earliest Pliocene. Alluvial fan, lacustrine fan
delta, and deep lacustrine facies are widely
developed in the lower unit. The upper unit
records the final evolutionary stages in the
LLI
Arkosic
sandstones
Conglomerates
Mudstones and
diatomites
Fig.12. Synthetic stratigraphic log of the late Neogene
Cerdanya Basin.
basin. Thick alluvial fan red beds and minor
shallow lacustrine deposits make up the sedimentary record of this stage, which took place
under a tensional regime (Roca, 1986).
The open lacustrine facies of the lower
Neogene unit crop out mainly in the western
Cerdanya Basin. In this basin sector three
main lithological assemblages are recorded
(Fig.12) (Julia, 1984):
(A) Lower terrigenous assemblage: It consists of pale-brown, yellowish and greyish
gravels, arkosic sands and mudstones, up to
100 m thick. Thin lignite seams occur locally.
(B) Middle diatomitic assemblage: It is
composed of up to 250 m thick fine blue-grey to
pale-gray organic mudstones and diatomites.
Interbedded sand and ostracod laminae occur
in this unit. Scattered C a - F e phosphate rich
beds or isolated nodules are present at several
levels. Diatomitic facies show a fine lamination
which may be related to seasonal varve sedimentation reported by Margalef (1983).
(C) Upper terrigenous assemblage: This is
formed of grey to brown mudstones, sandstones
and conglomerates about 100 m thick. Lignite
beds have been also recorded in this unit.
The three above-mentioned assemblages may
grade laterally one into the other and their
spatial distribution as well as their lithological
features are closely linked to the source areas
(Roca, 1986). The lower assemblage records the
development of early alluvial fan and shallow
lacustrine conditions, which evolved into
the deep lacustrine conditions recorded by the
Middle diatomitic assemblage. At the top of
the "deep" lacustrine sequence a sharp generalized fan delta progradation, which infilled
the basin, has been recorded.
The widespread occurrence of diatomites in
the open deep lacustrine deposits suggest high
pH, silica and sodium-rich surface water conditions. Seasonal fluctuations in surface water
temperature may be recorded by the diatomitic
laminae couplets defined by Melosira morphology changes (Margalef and Marras~, 1985).
The absence of widespread bioturbation, the
very well preserved plant and insect remains
and the high organic carbon content (up to 5%
21
netic phosphates (anapaite and fairfieldite) are
consistent with reducing anoxic conditions,
and also indicate low calcium concentrations
in the sediment interstitial waters (Nriagu and
Dell, 1974).
TOC) in the shales and diatomites, show poorly
oxygenated to rather anoxic bottom water
conditions.
The oil-shales consist mainly of grey mudstones and finely laminated (varved) diatomites. These facies have yielded a very rich fossil
assemblage which includes plant leaves, palynomorphs, ostracods, insects, and rare, poorly
preserved fish skeletons. Major carbonate deposits are absent but minor, varying amounts
of calcite and scarce siderite have been recorded in the mudstone facies. Early diagenetic phosphates (widespread single or amalgamated nodules and very scarce thin veins)
occur at several levels in the organic-rich
mudstones and diatomites. The early diage-
Campins
General characteristic features of the
studied basins
The tectonosedimentary evolutionary trends
recorded in the studied basin-fill sequences are
quite similar. In all cases, the graben fill
commences with an alluvial unit recording an
early stage of rift basin generation (Fig.13).
The dominant lithology in this unit (breccias,
conglomerates or sandstones) depends on the
Rubielos
de Mora
Ribesalbes
Libros
L2,
I
l j
I
I I
~,'^,~
5YY4
,a
-_-_:
-12
~1
Cerdan
"%-r~T i
,,,,.,,,
J
G
_/'Z_
I
^L ^ ^ ,GjT_O
I
.
I
~
L:.<,~
I
I
I
A t _t_ ~
I
7-~z-7-j ~ T
/
/
'Ci-i,--_, U_, ~
,~
m
JI
i I
-L--
_~1
J I
±A_
I
[ --I-I
i
I
tl
#t
~
i_
I
I
)o<3o
I
I I
I
I
-±~- ^ - - ±',
",..'~,
~|100
',~
---
Charophytes
m
~
Macrophyte
leaves
Brown coals ( l i g n i t e s )
•
G' M o l l u s k s
Oil-shales
~
~
-
Ostracods
Travertine
100
m
¢~o"Exotic" fossils
,, ( i n s e c t s , amphibians,..)
JZSlump
(~
Olisthostrome
Fig. 13. Overall comparison of the synthetic basin fill sequences recorded in the oil-shale basins. See legends in Figs.5 and 10
for lithologies. Further explanation in the text.
22
nature and rock-types of the surrounding
source areas. The basal alluvial units are
overlain by lacustrine sequences which in
general show shallow lacustrine features in
their lowermost portions. In some basins, coals
are found in these shallow lacustrine sequences (i.e. Campins, Rubielos de Mora,
Libros).
After the stage of rift basin generation,
increasing subsidence without a compensatory
increase in sediment accumulation rate, resulted in a deepening of the lacustrine systems.
Widespread deposition of organic-rich sediments leading to oil shales is recorded in all
the basins during the ~'deep" lacustrine stage
(Fig.13). Intense tectonic activity throughout
this evolutionary stage is recorded in most of
the studied oil-shale sequences by olisthostromes (Ribesalbes, Rubielos de Mora) and
slumps (Campins, Ribesalbes, Rubielos de
Mora, Libros).
Clay, mudstones, marls and carbonates are
the dominant rock-types in the oil-shale units
in Campins, Ribesalbes and Rubielos de Mora.
Gypsum is an important deposit in the Libros
basin together with laminated mudstones and
limestones. In Cerdanya, where carbonate
rocks are very scarce, the oil-shale unit is
formed by laminated mudstones and diatomites.
The lithology of the oil-shales ranges from
clays or mudstones to carbonate rocks. In the
Campins and Ribesalbes basins, although laminated mudstones have the highest oil yield,
carbonate rocks (dolostones in Ribesalbes and
dolostones and limestones in Campins) also
display a high organic matter content. Cerdanya Basin organic-rich deposits (up to 5%
TOC) consist of mudstones and diatomites.
Carbonate minerals from rocks of the oilshale units show a complex variety. Primary
and/or diagenetic carbonates such as calcite,
aragonite, Mg-calcite, dolomite and siderite
have been recorded from the different basins.
Native sulphur in Libros, C a - F e phosphates in
Cerdanya and opal-CT in Ribesalbes are noncarbonate diagenetic minerals which occur in
noticeable amounts.
The hydrochemical characteristics of the
lake water (i.e. salinity, ionic composition)
were different in each case. Two examples of
extreme hydrochemical conditions are the
Libros and Cerdanya basins. In Libros, a high
Ca and sulphate content of the lake waters is
inferred from the gypsum dominated sequence,
whereas in Cerdanya a very low sulphate
content in the waters can be deduced from the
absence of pyrite and the presence of siderite
and iron oxi-hydroxides in the lake sediments.
A typical characteristic of the described oilshales is a well preserved, very fine lamination.
In some cases it is formed by thin alternating
layers of organic matter or organic-rich clays
and carbonate minerals. These rhythmites
(present in Campins, Ribesalbes and Rubielos)
suggest the succession of seasonal and or other
periodic events. Varves, recording seasonal
changes of diatom morphs have been recorded
from the Cerdanya Basin (Margalef and Marras6, 1985).
The preservation of lamination and thin
rhythmites also indicates the absence of benthos. Although in some cases benthic organisms have been found in the laminated oilshale facies, they have been reworked from
littoral zones. The excellent preservation of
'~exotic" (delicate) fossils (amphibians, birds,
etc.) and abundance of leaves, also points to an
absence of benthic organisms living in the
bottom of the lake where oil-shales originated.
The above mentioned features, prove the
origin of the oil-shales in lacustrine basins
with anoxic bottom conditions. Here organic
matter is protected from reworking by benthic
fauna and from aerobic bacterial degradation.
In addition, the high organic matter content
has also allowed "unstable" minerals like
aragonite or Mg-calcite to be preserved
(Lippmann, 1973).
In the studied cases anoxic bottom conditions arose from stratification of water bodies.
Meromictic conditions were attained in the
lacustrine systems at least during the oil shaleformation phases. The meromictic conditions,
in the studied cases, were favoured by a
relative deepening of the lake basins. It is
23
assumed that the suitable depth conditions
needed for meromixis in the water bodies, and
anoxia in the deeper parts of the lakes, were
essentially due to subsidence outstripping
deposition.
Stratification of the lacustrine waters would
be also linked to the climatic conditions. Thus,
despite some strong oscillations which led to
warm temperate conditions, warmer than present, subtropical to tropical regimes were
dominant during most of the late Oligocene
late Miocene time span in the northwestern
Mediterranean area (Sainz de Siria, 1978;
Bessedik, 1985). Changes in precipitation rates
(which in some cases were parallel to the
temperature changes) also took place, resulting probably in alternating wet, semiarid and
even arid episodes. However, local paleogeographic conditions (continentality, rain
shadow effect, altitude ...) also played a major
role in determining the final precipitationevaporation balance.
The NE Spain Tertiary lacustrine oilshale basins in the W e s t e r n Europe rift
basin s y s t e m
The rift basins of the NE Iberian Plate are
part of a larger complex of horsts and grabens
which extends as far as SE Iberia. There a
number of basins, some of which display
features in common with those described here
(i.e. Cenajo Basin, Calvo and Elizaga, 1985),
developed under the influence of the late
evolutionary stages of the Betic thrust belt.
Moreover, the whole Eastern Iberian Plate rift
system makes up the southernmost part of the
large n o r t h - s o u t h rift basin system which
stretches across Western Europe from the
North Sea to the Western Mediterranean
margins (Julivert et al., 1974). The large scale
graben and half-graben complexes included in
this rift system (Lower and Upper Rhine
grabens, Rhone Valley grabens and half grabens, Sardinian graben, Eastern Iberian Plate
rift basins, Fig.14) record the development of
diverse local to regional extensional regimes.
The first rift processes may have developed
during the Eocene in the Rhine graben system
(Illies, 1977; Villemin and Bergerat, 1987).
Later, a spreading towards the south took
place (Fig.15); late Eocene-Oligocene grabens
in the Rhone Valley system (Bergerat, 1982);
late Oligocene and mainly Neogene grabens in
the Eastern Iberian rift basins (Vegas and
Banda, 1982).
Extensional conditions which resulted in
graben formation along the Western Europe
Rift system developed after a gradual transition from a compressional to extensional
regime, and in several cases a strike-slip
regime has been recorded as synchronous or
developed as an intermediate stage (Angelier
and Bergerat, 1983; Guimer~, 1984; Letouzey,
1986). This fact has been related to variations
in the magnitude of the compressive stress,
which could be caused by the changes in the
relative motion of the colliding European and
African plates. Moreover in relation with this
fact, a spatial change in the tectonic regime,
from clearly compressive tectonic conditions
(with generation of thrusts and folds) into a
strike-slip compressive regime (with development of strike-slip processes) and finally into
an extensional regime (with graben generation), has been emphasized by several authors
as a common feature in the foreland platform
regions located in front of the zones which
undergo a global large scale compression
(Taponnier and Molnar, 1976; Letouzey, 1986).
This feature must be considered for the whole
rift basin system evolution. However, it must
be stressed that the extensional situations
along the diverse rift segments have resulted
from more than one regional geotectonic
process. Thus generation of some of the more
marginal basins in the Eastern Iberian rift and
in the Rhone and Provence graben systems
may have been at least partially influenced by
rifting processes linked to the opening of the
NW Mediterranean Basin as a back-arc basin
(Rehault et al., 1984). Anyway, evolution of the
system as a whole has been strongly influenced
by the same global geotectonic process: the
convergent motion and collision of the European and African plates.
24
!
)
/
J
//J"
200
F7~ Alpine thrust belts ~
Rift basins
/<<~" Faults
300Kin
(~) Oil-shale deposits
Fig.14. Location of Tertiary oil-shale basins in the Western Europe intraplate rift system. 1 = Messel (Rhine graben).
Mormoiron and 3 = Al~s (Rhone valley graben system). 4 = Campins. 5 = Cerdanya. 6 = Rubielos de Mora. 7 = Ribesalbes.
8= Libros. 9 = Cenajo (Eastern Iberia rift system). The generation of the diverse rift segments resulted from more than one
geotectonic process. See Fig.15 for explanation of the evolutionary trends.
2=
It is interesting to emphasize that sedimentary and paleoenvironmental conditions suitable for the development of stratified water
lacustrine systems often took place along the
Western Europe Rift system during its evolution. The foreland platform grabens (generated
from the Eocene until present by intraplate
deformation) experienced in several cases evolutionary trends which allowed the deposition
of organic-rich sediments (coals and oil shales)
as well as of other deposits of economic
interest (diatomites, evaporites, sulphur ...).
Coal deposits and lacustrine oil-shales have
been recorded in the Rhine Graben (late
Eocene Messel sequences, Matthes, 1968; Sittler, 1968), in the Rhone Valley basin system
(latest Eocene-early Oligocene sequences in
Al~s, Mormoiron; Triat and Truc, 1974; Truc,
25
OE
AO
EARLY MIOCENE
1978; Bergerat, 1982) and also in the Eastern
Iberian plate rift lacustrine basins described
here. Organic rich deposits occur too in some
of the Prebetic late Miocene lacustrine basins
(I.G.M.E., 1981a; Calvo and Elizaga, 1985).
From this point of view must be stressed the
importance of studying foreland platform graben systems related to intraplate deformation
in order to explore the economic potential of
these tectonosedimentary settings as zones of
production of economic resources. Large scale
intraplate rift systems linked to continental
convergence and collision are also known to
the north of the Himalayan thrust belt (Molnar
and Taponnier, 1975; Ye et al., 1987).
- - _ ...... :~ ___.\
/
Concluding remarks
A number of graben and half graben basins
occur in the NE Iberian Peninsula, related to
transtensional and extensional processes
which took place in the area from the late
Oligocene (Campins Basin) until the late
Miocene (Ribesalbes, Rubielos de Mora, Libros
and Cerdanya basins).
Alluvial fan-fluvial and lacustrine systems
were closely interrelated in the above mentioned basins. Sometimes the latter evolved
into deeper waterbodies prone to the development of oil-shale deposits. Where the whole
sedimentary record of the basin evolution is
preserved, the same overall evolutionary and
megasequential trend has been recorded:
(1) Early rift stage with basin generation and
development of alluvial and shallow lacustrine
environments.
(2) Intense activity along the fractures which
bound the basins resulting in a "deepening" of
the lacustrine systems which became permanently stratified.
(3) Lessening of tectonic activity with the
~
Oil-shale deposits --dLb-Thrusts
Oceanic crust
,~-Oceanic subduction
,,~llllllE, Metamorphism
f / / " Faults
* Volcanism
(i...~ Rift basins
Fig.15. Evolutionary stages of the Western Europe intracontinental rift systemwhichresulted from graben generation in the foreland platform of the Alpine system. Note
that back-arc rift processes probably occurred in the
Western Mediterranean during the late Oligocene-early
Miocene time span. See Fig.14 for a more detailed location
of the oil-shale basins.
26
subsequent infilling of the lacustrine system
and the development of a shallowing upward
sequential trend. Alluvial and shallow lacustrine deposits occur at the top of the sequence.
It is assumed here that the suitable depth
conditions for permanent stratification (meromixis) to develop and anoxia to occur in the
deeper parts of the lakes were linked to a
increasing tectonic subsidence that was not
balanced by an increase in depositional rate.
Tectonic activity is recorded in the studied
lacustrine sequences by the occurrence
of olisthostromes and slumps. However, in
some cases, chemical ectogenic meromixis,
may have contributed to the permanent stratification.
No exclusive relationships between oil-shale
deposition and lake water hydrochemistry can
be established from the studied basins, as a
wide range of conditions in the water column
can be deduced from the sedimentary record.
The occurrence in some of the basins of well
developed cycles (Rubielos de Mora Basin)
and the overall presence of diverse types of
rhythmites (clay-carbonate couplets, diatom
morphs variations) suggest that a detailed
study of these lacustrine sequences may contribute to the knowledge of the Miocene
paleoclimatic conditions in the NW Mediterranean area (i.e. seasonality, middle to long
term paleoclimatic trends, etc.).
The rift basins in the NE Iberian Peninsula
are included in the Eastern Iberian rift system. This, in turn, makes up the southernmost
part of the Western Europe intraplate rift
system, which stretches from the North Sea as
far as the Mediterranean Sea (Fig.14). From
the late Eocene until the late Miocene, lacustrine complexes often developed in the fault
bounded basins included in this large rift
system (Fig.15). During some of the evolutionary stages of these lacustrine systems, oilshale deposits developed. From this point of
view, large scale old intracontinental rift
systems, developed in a foreland platform
setting, can be envisaged as .zones of high
potential interest for lacustrine oil-shale
exploration.
Acknowledgements
We would like to thank M. R. Talbot for
critically reading a first draft of the manuscript and made helpful suggestions.
This work is a contribution to the project
"Modelos de sedimentaci6n lacustre en fosas
neSgenas de la zona meridional de la Cordillera Ib~rica" financed by the Comisi6n Asesora
de Investigaci6n Cientifica y T~cnica and
Consejo Superior de Investigaciones Cientificas (CAICYT-CSIC, ID: 851). One of the
authors (E. R.) benefitted from financial support by the CAICYT project 3170/83.
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