Evolution of an intra-plate rift basin:
the Latest Jurassic-Early Cretaceous

Cameros Basin (Northwest Iberian Ranges,
North Spain)

R. Mas1, M. I. Benito1, J. Arribas1, A. Alonso2,
M. E. Arribas1, K. C Lohmann3, L. González-Acebrón1,

J. Hernán1, E. Quijada1, P. Suárez1 and S. Omodeo1

1 Universidad Complutense Madrid-CSIC,
ramonmas@geo.ucm.es

2 Universidad A Coruña, angelami@udc.es

3 Universidad Michigan, kacey@umich.edu

CONTENTS

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Geo-Guías, 8

Introduction to the Cameros Basin ............................................................
The Mesozoic Substrate of the Cameros Basin ........................................
The Cameros Basin fill (Late Jurassic - Early Cretaceous) ......................
Evolution of the Cameros Basin: Basin Model ........................................
Geological itinerary ..................................................................................
First day: S-N Cross-section to the Cameros at the depocentral area ......

Stop 1. Renieblas: The southern border of the Cameros Basin ..........
Stop 2. Villar del Río-Yanguas: Berriasian lacustrine facies ..............
Stop 3. Yanguas: Late Berriasian-Barremian fluvial systems.

Hydrothermal metamorphism ......................................................
Stop 4. Enciso: Barremian-Aptian shallow lakes. Dinosaur footprints
Stop 5. Peñalmonte: The Cameros Northern Thrust. Panoramic view

Second day: Main features of the eastern sector of the Cameros Basin ..
Stop 6. San Felices: Panoramic view of the eastern basin..................
Stop 7. Valdemadera: Berriasian shallow carbonate-saline lakes ......
Stop 8. Cervera-Grávalos: Late Berriasian-Early Albian cross-sec-

tion ................................................................................................
Stop 9. Préjano: Aptian-Albian fluvial-deltaic deposits at the foot

wall ................................................................................................
Third day: N-S cross section to the basin in the western sector ..............

Stop 10. Leza Valley: Barremian-Early Aptian lacustrine facies........
Stop 11. Torrecilla en Cameros: Late Jurassic pre-rift coral reefs......
Stop 12. Villoslada: Tithonian-Berriasian fluvial-lacustrine sequences
Stop 13. Montenegro-Sta. Inés Pass: the Marine Jurassic substrate ..
Stop 14. Santa Inés Pass-Cidones: Southwest cross-section of the

alluvial syn-rift basin fill ..............................................................
Stop 15. Picofrentes: tar sands at the base of the Utrillas Fm ..................
Acknowledgements....................................................................................
References..................................................................................................



– 119 –

Introduction to the Basin

The main objectives of this field trip to the Cameros Basin (Fig. 1)
are to visit seismic scale outcrops, and to analyse the stratigraphy and
sedimentology of pre-rift marine deposits and syn-rift lacustrine, flu-
vial and alluvial depositional systems. The trip will go through three
transects to the Cameros Basin, along which, vertical and lateral varia-
tions of the sedimentary record and both, extensional and contractional
tectonic structures, will be observed. The hydrocarbon geology of this
area (reservoirs, source rocks, oil seeps, and thermal events) will be
also discussed.

The Cameros Basin is located in the Northwest of the Iberian Chain
(Fig. 1A). This is one of the basins forming part of the Mesozoic
Iberian Rift System or Iberian Basin (Mas et al., 1993; Guimerà et al.,

Figure 1. A. Geological map of the Cameros Basin. A-A’, B-B’ and C-C’:
Location of cross-sections of Fig. 1B. B. A-A’, B-B’, and C-C’: geological
cross-sections through the Cameros basin. D-D’: partial restoration of cross
sections A-A’ and B-B’ at the previous state of basin inversion. See Fig. 1A for 

location. Modified from Guimerà et al. (1995).



1995; Salas et al., 2001; Mas et al., 2002). In the Iberian Basin two rift-
ing cycles can be distinguished from the late Permian to the late
Cretaceous (Alvaro et al., 1979; Vilas et al., 1982; Salas and Casas,
1993; Roca et al., 1994; Salas et al., 2001; Mas et al., 2004): 1) a
Permo-Triassic Rift, Late Permian to Hettangian; and 2) a Latest
Jurassic-Early Cretaceous Rift, Kimmeridgian (southeastward) or
Tithonian (northwestward) to Early Albian (Fig. 2). During the Latest
Jurassic-Early Cretaceous Rift cycle four sub-basins up to 3-6.5 km
thick developed in the Iberian Rifting, being the Cameros Basin the
most subsident (Mas et al., 1993). The Iberian Rift System was inverted
during the Paleogene generating the Iberian Chain (Salas et al., 2001),
which is a fold-and-thrust belt whose overall structure is defined by
two NW-SE trending arches. 

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Figure 2. A. Rift cycles and Post-rift stages on the evolution of the Iberian
Basin (modified from Salas et al., 2001). B. Idealized and synthetic
sketch/lithostratigraphic section showing the latest Paleozoic and Mesozoic
stratigraphic units at the Cameros area (North Iberian Range). From Mas et al. 

(2003).

The Cameros tectonic Unit includes the Cameros Basin fill, its
Mesozoic substratum and the Variscan basement, which crops out to
the northwest and southeast. The structure of the Cameros Unit is char-
acterized by (1) a major newly formed north-verging thrust sheet,
which has a horizontal displacement of up to 30 km and overrides the
Tertiary Ebro Basin; and (2) a conjugate, south-verging imbricate fan
fold-and-thrust system that encroaches on the Almazán Basin (Fig. 1B,
B-B’; Mas et al., 1993; Guimerà et al., 1995; Mas et al., 2002).



The Cameros Basin is atypical, if compared to other basins of the
Chain. It is a synclinal basin (Mas et al., 1993; Guimerà et al., 1995)
with no major fault bounding during its development (Fig. 1B, A-A’).
It is interpreted as an extensional-ramp basin (Mas et al., 1993;
Guimerà et al., 1995; Mas et al., 2002). This basin is also the only one
of the Mesozoic basins in the Chain in which deposits have been par-
tially affected by metamorphism. A large proportion of deposits of the
eastern sector of the basin has been affected by low-grade to very low-
grade metamorphism. Findings based on a detailed study of the basin
combining mineralogical, petrological, geochemical and structural
approaches, have defined the metamorphism as hydrothermal and alo-
chemical (Casquet et al., 1992; Barrenechea et al., 1995, 2000; Alonso-
Azcárate et al., 1995, 1999; 2001; Mantilla et al., 1998, 2002;
González-Acebrón et al., 2010). The most important conclusions to be
drawn from these studies are: (1) metamorphism displays very clear
thermal inversions in the depocentral areas, and the grade reached is
influenced by the changes in permeability and composition of the sed-
iments rather than by the burial depth; (2) metamorphism postdates the
basin fill and comprises two different hydrothermal events: The first
one dated as Late Albian to Coniacian (-106 to -86 Ma), and a second
event dated as Lower to Mid Eocene; (3) the metamorphic conditions
range from low-grade or epizone to very low-grade or anchizone, with
a maximum temperature of 350-370ºC and a maximum pressure of 1
kbar, at the first metamorphic peak.

The Mesozoic Substrate of the Cameros Basin

The Late Permian and Mesozoic evolution of the Iberian Rift
System has been divided into four major rift cycles and post-rift stages
(Salas et al., 2001) (Fig. 2A). In the NW sector of the Iberian Chain
four megacycles or depositional supersequences have been distin-
guished: the Late Permian to Triassic Megacycle 1; the Early to Late
Jurassic Megacycle 2; the Latest Jurassic to Early Cretaceous
Megacycle 3; and the Late Cretaceous Megacycle 4 (Guimerà et al.,
2004; Fig. 2B). The first two megacycles correspond to the “Mesozoic”
substrate of the Cameros Basin. During the Late Permian-Triassic
Megacycle 1, the development of the Iberian Basin started with the
reactivation of late-Variscan faults (Vegas and Banda, 1982). As a con-
sequence, Late Permian and Triassic deposits unconformably overlie
the Variscan basement. The Early to Late Jurassic Megacycle 2 was
dominated by thermal subsidence and very extensive carbonate plat-
forms developed (Alonso and Mas, 1990; Salas et al., 2001).

Several depositional sequences have been identified in this mega-
cycle at the northwestern sector of the Iberian Chain, the last of which,
the Early Kimmeridgian Sequence, generally constitutes the substra-
tum of the Latest Jurassic-Early Cretaceous sin-rift sequences of the
Cameros Basin (Fig. 2B). The Early Kimmeridgian was a period of
global sea level rise (e.g. Haq et al., 1987), which is apparent through-
out the Iberian and North Tethys domain. Sedimentation in the Iberian
Basin mainly occurred in a shallow carbonate storm-dominated ramp
which was opened southeastwards to the Tethys and northwards to the
boreal realm (Alonso and Mas, 1990; Bádenas and Aurell, 2001; Benito

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et al., 2005). The connection between both domains was located in an
epicontinental seaway (the Soria Seaway, Bulard, 1972), generated
between the Iberian and the Ebro massifs. Sedimentation in this seaway
was characterized by the development of coral reefs of the Torrecilla en
Cameros Fm. The growth geometry of these reefs was different
towards the northern and southern parts of the basin, because it was
controlled by both the prevailing positive eustatic conditions during
Kimmeridgian times, and by the basin tectonics (Alonso et al., 1986-
1987; Benito et al., 2001, Benito and Mas, 2002, 2006).

The Cameros Basin fill (Rift cycle 2): The Latest
Jurassic - Early Cretaceous Megacycle 3

The basin fill (Rift 2; Fig. 2A), which extended from the Tithonian
to the Early Albian, corresponds to a large cycle or megasequence
bounded by two main unconformities (Figs. 2B, 3). The stratigraphic
gap of the lower unconformity is more important in the northern part of
the basin than in the central and southern areas (Fig. 3) (Mas et al.,
1993; Benito and Mas, 2002; Arribas et al., 2003). The upper limit is
the intra-Albian unconformity bounding the base of the Late
Cretaceous Megacycle 4 of the Iberian Ranges (Figs. 2B, 3), when
large carbonate platforms occupied the Iberian realm (Alonso et al.,
1989; Alonso et al., 1993). 

– 122 –

Figure 3. Simplified stratigraphy of the Depositional Sequences (DS) which fill
the Cameros basin. Modified from Guimerà et al. (1995).



Depositional Sequences

The Latest Jurassic-Early Cretaceous megacycle o supersequence
filled the basin through eight depositional sequences bounded by
unconformities (Mas et al., 1993; Mas et al., 1997; Mas et al., 1998;
Martín i Closas and Alonso Millán, 1998; Mas et al., 2002; Arribas et
al., 2003) (Fig. 3). This sedimentary record consists mainly of conti-
nental sediments corresponding to alluvial and lacustrine systems, with
some marine influence (Guiraud and Seguret, 1985; Gómez Fernández,
1992; Alonso and Mas, 1993; Quijada et al., 2010; Suarez-González et
al., 2010; Figs. 3, 4). In general, all the depositional sequences are very
thick, especially DS 2 and DS 3 of Tithonian-Berriasian age (more than
2000m of sediments each); and DS 7 and DS 8 of Barremian-lower
Albian age (up to 2500 and 2800m, respectively). Sequences are usu-
ally arranged into sedimentary cycles starting with alluvial clastic
deposits, which evolve upwards to lacustrine limestones (Figs. 3, 4).
Paleocurrent data (Fig. 4) and petrofacies composition of siliciclastic
intervals at the Southwestern basin margin, reveal that the siliciclastic
input mainly came from the Iberian Massif (Mas et al., 1997; Arribas
et al., 2003; 2007).

Chronostratigraphic data are not yet complete due to poor fossil
record and local metamorphism. The main data are based on charo-
phytes, ostracods, palynological associations for the continental record,
and a few marine calcareous algae (mainly Dasycladal) from the
marine incursions (Brenner, 1976; Guiraud and Seguret, 1985;
Schudack, 1987; Clemente et al., 1991; Alonso and Mas, 1993; Martín
i Closas and Alonso Millán, 1998; Schudack and Schudack, 2009;
Suárez-González et al., 2010). Ages for the different sequences are
shown in figure 3.

Facies and Sedimentary Environments

Proximal facies are commonly developed towards the southwest
margin (Figs. 3, 4). These deposits are essentially formed by alluvial
facies associations of sandstones and conglomerates corresponding to
braided systems. The more distal deposits, mainly lacustrine carbonates
and shales, are generally displaced towards the northeastern margin
(Fig. 4). One of the more characteristic facies associations correspond
to lacustrine-palustrine cycles of relatively permanent shallow carbon-
ate lakes, which occasionally accumulated major quantities of organic
matter. Another distal facies association is carbonate-evaporite
laminites, corresponding to shallow ephemeral lakes (Fig. 4). Locally,
laminated carbonate facies are associated with black-shales, which pre-
sumably corresponds to shallow lacustrine areas with high organic mat-
ter input. Between the areas of proximal alluvial systems and those of
distal carbonate lakes, meandering fluvial systems developed (Fig. 4). 

In some of the sequences there is evidence of sporadic marine
incursions from the Southeast (Tethys), especially during global high
sea level intervals (e.g. Alonso y Mas, 1988; 1993) as the Tithonian-
Berriasian and the Barremian-Aptian (Figs. 3, 4). These incursions pro-
duced the development of coastal lakes containing benthic forams and

– 123 –



Dasycladal algae (Alonso and Mas, 1993; Suarez-González et al.,
2010). In the siliciclastic units (DS 3 and DS 8) the sea incursions lead
to tidal-influenced depositional environments (Quijada et al., 2010).

– 124 –

Figure 4. Schematic palaeogeographic evolution of the Cameros Basin. BU:
Burgos, LO: Logroño; SO: Soria. Modified from Mas et al. (1997).

Geometry of the basin fill

The different depositional sequences that filled the basin lie uncor-
fomably on the Mesozoic substratum (Fig. 1B). The architecture of
basin infill suggests a progressive migration of the depocentres of the
successive depositional sequences towards the North, lying in on-lap
with respect to the previous sequences. Thus, the maximum thicknesses
are achieved in the Northeastern area of the basin (9500 m) (Mas et al.,
1993; 2002), with a maximum vertical thickness of up to 6.5 km (Fig.
2B). It should be noted that no major faults have been found in the
Nothern margin of the basin to explain such a large thickness. In the



southern part of the basin the first sequences (DS 1 to DS 3, Tithonian-
Berriasian) lie in on-lap with respect to the Kimmeridgian reefal com-
plexes of the Torrecilla en Cameros Fm. In the northern area, however,
DS 7 (Late Barremian-Early Aptian) sequence lies unconformably on
the Kimmeridgian Torrecilla Fm. (Fig. 1B).

Evolution of the Cameros Basin: Basin Model

The Cameros Basin is a synclinal basin and no major faults
bounded it during its development (Fig. 1A). The idea of a major exten-
sional fault that surfaced at the northern boundary of the basin can be
rejected because of the great continuity of the Jurassic substratum of
the basin. In turn, the northward migration of the depocenters of the
successive depositional sequences and its lapping on the pre-basin
Mesozoic substratum (mainly, marine Jurassic) (Fig. 1B), may be
explained by assuming that the basin formed over a roughly south dip-
ping ramp in a horizontal extensional fault (Fig. 1B). That is the
Cameros Basin corresponds to an extensional-ramp basin (Mas et al.,
1993; Guimerà et al., 1995; Mas et al., 2002). Indeed, in scale models
(McClay, 1990; Roure et al., 1992), similar patterns of infill units in a
synclinal basin have been obtained by horizontal-extensional faults
containing ramp segments over which the basin was developed (Mas et
al., 2003: Fig. 22). 

– 125 –

Figure 5. Schematic evolutionary model for the Cameros Basin. Modified from
Mas et al. (2003).



GEOLOGICAL ITINERARY

The maps (Figs. 6, 7) show the locations of the places to be visited
during the fieldtrip (stops 1 to 15). Figure 8 includes the chrono- and
litho-stratigraphic location of each stop. The explanation of these loca-
tions is given on the following pages.

One important question is the depth at which the extensional fault
was located. To get round the problem, it has been suggested that the
extensional fault is located deep inside the Variscan Basement (Fig. 5)
at a depth of 7 to 11 km (Mas et al., 1993; Guimerà et al., 1995). The
Cameros basin could have been produced by the motion of a deep
extensional fault containing two nearly horizontal sections (flats) sep-
arated by an intermediate section dipping to the south (ramp). The
direction of displacement for the hanging wall was S-SW, parallel to
the direction of the basin extension (Fig. 5). Thus, the Cameros Basin
is interpreted as a hanging-wall basin that constitutes a good real equiv-
alent of the extensional-ramp basin scale model of McClay (1990).

Basin inversion took place during the Paleogene to the Early-
Middle Miocene due to Pyrenean compression (Guimerà et al., 1995).
The Cameros Basin appears at present as a pop-up structure bounded
by the Cameros thrust to the North and a minor back-thrust system to
the south (Figs. 1B, 5). The main Northern thrust is a newly formed
fault. Its formation would have been facilitated by: (1) sealing of the
Mesozoic extensional fault beneath the basin as a result of Late
Cretaceous metamorphism, and (2) the presence on the northern flank
of the basin of a potential weakness zone located in the Keuper beds.
From this weak zone, the new thrust could have nucleated and spread
to the North and to the South during deformation. The thrust system to
the South of the basin (Fig. 5) developed from the inversion of the
minor normal faults which bounded the Mesozoic basin towards the
South (Fig. 1B).

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Figure 6. Fieldtrip itinerary and stops.



– 127 –

Figure 7. Location of the fieldtrip stops on the schematic geological map of the
Cameros Basin. Modified from Mas et al. (2003).

Figure 8. Location of the field trip stops in the stratigraphic record of the
Cameros Basin. Modified from Mas et al. (2004).



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FIRST DAY: GENERAL S-N CROSS SECTION
ACROSS THE DEPOCENTRAL AREA OF THE

CAMEROS BASIN.

Stop 1

Renieblas (Soria province): The southern border of the
Cameros Basin

Location: On the road SO-V-1001 between the localities of
Renieblas and Almajano (Soria province). UTM coordinates, datum
WGS84, zone 30 between: x=552988; y=4631257 and x=553342;
y=4631537 (Figs. 6, 7, 8).

Objectives: Introduction to the basin; Early Kimmeridgian coral-
bearing and oolitic pre-rift carbonate sequence. The first syn-rift conti-
nental sequence (DS 1) overlies the marine pre-rift sequence.

Description: Close to Renieblas, at the southern border of the
Cameros Basin, the lower part of Ágreda Fm (first syn-rift sequence
DS 1, Tithonian ) overlies marine carbonates of the Torrecilla en
Cameros Fm (pre-rift Early Kimmeridgian Sequence). The Torrecilla
en Cameros Formation was deposited in a storm-dominated ramp and
comprises coral-bearing and oolitic carbonates (Figs. 9, 10). The
Ágreda Fm corresponds to distal alluvial fan deposits with intervening
conglomeratic paleochannels, calcretes and some shallow lacustrine
carbonate beds.



– 129 –

Figure 9. Simplified stratigraphic section of Torrecilla en Cameros Fm (Early
Kimmeridgian) and the lowermost part of Agreda Fm at Renieblas (Soria sec-

tor). From de Benito and Mas (2002).

Figure 10. Idealized sketch illustrating the sedimentary evolution of the reefal
complexes of the Torrecilla en Cameros Fm in the Soria sector. Modified from

Benito and Mas (2002).



– 130 –

Stop 2

Villar del Río - Yanguas (Soria): DS 3 (Berriasian)
lacustrine facies

Location: On the road SO-615 between the localities of Villar del
Río and Yanguas (Soria province), very close to Yanguas. UTM coor-
dinates, datum WGS84, zone 30 between: x=554967; y=4660455 and
x=554828; y=4660795 (Figs. 6, 7, 8).

Objectives: Lacustrine laminated carbonates, black shales and their
role as hydrocarbons source rocks.

Description: Close to Yanguas, at the depocenter area of the
Cameros Basin, the upper part of DS 3 (Valdeprado Fm, Berriasian) is
composed of thinly laminated carbonates interbedded with black shales
(Fig. 11). Laminated carbonates contain pseudomorphs after sulphates,
stromatolites and veins of calcite, quartz and native sulphur. They are
interpreted as deposited in shallow carbonate-evaporite coastal lakes.
Black shales correspond to lacustrine areas with high organic matter
input where environmental conditions became anoxic.

Figure 11. Partial stratigraphic section of Valdeprado Fm (upper part of DS 3;
Berriasian) at Yanguas (Soria). From Quijada et al. (in press).



– 131 –

Stop 3

Yanguas (Soria): Fluvial systems and source areas in DS
4, DS 5, DS 6 and lower part of DS 7 (Latest Berriasian
to Barremian). Hydrothermal metamorphism of the basin

Location: On the road SO-615 just to the North of the town of
Yanguas. UTM coordinates, datum WGS84, zone 30 between:
x=555077; y=4661593 and x=555343; y=4661778 (Figs. 6, 7, 8).

Objectives: Fluvial systems as hydrocarbons reservoirs.
Hydrothermal events and ages of the metamorphism.

Description: Less than 1 km to the North of the town of Yanguas
there is the contact between the carbonate laminites and black shales of
the Valdeprado Fm, and the fluvial systems of the Urbión Group (Latest
Berriasian to Barremian; DS 4, DS 5, DS6 and lower part of DS 7; Fig.
12). This sector corresponds to the main depocenter area of the basin
and was affected by low grade hydrothermal metamorphism (Fig. 13).
Compositional characteristics of sandstones and deduced diagenetic
processes suggest that these fluvial systems acted as reservoirs of
hydrocarbons; however hydrocarbons were later destroyed due to
metamorphism (Fig. 5).

Stop 4

Enciso (La Rioja): Siliciclastic-rich shallow carbonate
lakes in the upper part of DS 7 (Barremian to Early

Aptian). Dinosaur footprints sites of the Cameros Basin

Location: Town of Enciso and the road between Enciso and the
town of Navalsaz (LR-356). UTM coordinates, datum WGS84, zone
30 at x=561500; y=4665272 (Valdecevillo dinosaur ichnites site) and at
x=563571; y=4664075 (Poyales dinosaur ichnites site) (Figs. 6, 7, 8).

Objectives: Dinosaur footprints sites at siliciclastic-rich shallow
carbonate lakes.

Description: The Enciso Gr (upper part of DS 7, Barremian-Early
Aptian) is mainly composed of carbonates and marls, which contain
abundant ostracods and also bivalves and gastropods, that were
deposited in siliciclastic-rich shallow carbonate lakes (Fig. 14).
Deposits of the Enciso Gr contain some of the worldwide known
dinosaur footprints sites, as those we will visit, very close to the town
of Enciso.



– 132 –

Figure 13. Schematic Geological map of the Cameros Basin showing the areas
affected by low and very low grade metamorphism. A. Yanguas - San Pedro
Manrique area. B. El Pégado anticline area. Modified from Mas et al. (2002).

Figure 12. Urbión Group: distribution of the fluvial systems in the central area
of the basin. DS 4 at Yanguas section is indicated with an asterisk. Proximal
facies are dominant towards the W. Modified from Barrenechea et al. (1995).



– 133 –

Figure 14. A. Siliciclastic-rich shallow carbonate lakes facies distribution in the
Enciso Gr (asterisks correspond to stop points); B. Paleogeography of the
upper record of DS 7 (Late Barremian-Early Aptian). A y B = Urbión Gr D,
Aluvial-fluvial systems (A. proximal; B. distal). C y D = Enciso Gr, Carbonate
lacustrine systems (C. with strong siliciclastic influence - Enciso Gr s.s.; D.
with very little siliciclastic influence and marine incursions - Leza Fm; E.
Carbonate shallow marine inner environments - Early Aptian Urgonian car-

bonate platform). Modified from Mas et al. (2008).

Figure 15. Footprints of Theropod dinosaur recorded on palustrine areas of
shallow carbonate lacustrine systems with strong siliciclastic influence. Upper

part of DS 7 (Enciso Group; Late Barremian-Early Aptian).



Stop 5

Peñalmonte (La Rioja): a) Structure of the Cameros
Northern Thrust; b) Late Aptian-Early Albian fluvial to

fluvial-deltaic sequence (DS 8) in the hanging-wall of the
thrust

Location: The Peñalmote Peak. On the forest track that connects the
road between Préjano and Arnedillo with the town of Navalsaz. UTM
coordinates, datum WGS84, zone 30: a) x=564835; y=4671653 and b)
x=563085; y=4672353 (at the bench mark Encineta 1098 m, close to a
place with several antennas) (Figs. 6, 7, 8).

Objectives: The Cameros Northern Thrust and the impressive thick-
ness of the Syn-rift Depositional Sequences of the Cameros Basin.

Description: a) Structure of the Cameros Northern Thrust (Fig. 16).
Impressive view of the Cameros Northern Thrust, in which the Triassic
Keuper evaporites and the marine Jurassic are superimposed to the Late
Aptian-Early Albian syn-rift unit (DS 8) and the Paleogene in the foot-
wall of the thrust. b) Panoramic view of the very thick Barremian-Early
Albian sequences (DS 7 and DS 8) in the hanging-wall of the thrust. DS
7 lies in on-lap with respect to the marine Jurassic and DS 8 lies in on-
lap with respect to DS 7. 

– 134 –



Figure 16. A. Panoramic view of the Cameros Northern Thrust (CNT).
Peñalmonte (La Rioja). B. Sketch of the structure of the CNT at Peñalmonte

high (La Rioja). Modified from Arribas et al. (2009).

– 135 –



SECOND DAY: MAIN CHARACTERISTICS OF
THE EASTERN SECTOR OF THE CAMEROS

BASIN

Stop 6

San Felices (Soria) - Aguilar del Río Alhama (La Rioja):
a) Panoramic view of the eastern general stratigraphic-
section; b) Lacustrine facies in the upper part of DS 2

(Tithonian)

Location: On the road SO-691 between the localities of San Felices
and Aguilar del Río Alhama (Soria province). UTM coordinates, datum
WGS84, zone 30; a) x=580415; y=4644137 and b) x=582380; y
=4646100 (Figs. 6, 7, 8).

Objectives: General stratigraphic record at the eastern part of the
basin. Distal facies for most of the depositional sequences.

Description: a) Panoramic view of the stratigraphic section to be
observed during the day (DS 2 and DS 3, Tithonian-Berriasian) at the
Alhama River valley (Fig. 17). DS 2 is composed of fluvial sandstones
and lutites evolving upwards to carbonate-evaporite lacustrine facies.
DS 3 is composed of thinly laminated and brecciated carbonates and
evaporites. b) Lacustrine carbonate facies at the top of DS 2 (Tithonian)
will be observed in detail (Fig. 18). They are composed of carbonates,
containing abundant stromatolites and calcite and quartz pseudo-
morphs after evaporites. Mud cracks are also frequent.

– 136 –



Figure 17. General panoramic view of the Cameros Basin fill at its eastern sec-
tor. At first plane Magaña Fm and Matute Fm of Tera Gr are observed (DS 2, 

Tithoniense); at second plane Oncala Gr (DS 3, Berriasian).

Figure 18. Characteristic facies associations of the upper part of DS 2 (Matute
Fm; Tithonian) at Aguilar del Río Alhama (Soria) Sector. Carbonate-evaporite
shallowing-upwards sequences deposited shallow lakes (playa- lakes).

Modified from González-Acebrón (2009).

– 137 –



Stop 7

Aguilar del Río Alhama - Valdemadera (La Rioja):
Shallow carbonate-evaporite coastal lakes in DS 3

(Berriasian)

Location: On the road LR-490 between the localities of Aguilar del
Río Alhama and Valdemadera (La Rioja province). UTM coordinates,
datum WGS84, zone 30; a) x=580913; y=4646965 and b) x=580786;
y=4647697 (Figs. 6, 7, 8).

Objectives: Thinly laminated carbonates and evaporites (DS 3,
Berriasian) deposited in shallow coastal-lakes.

Description: a) At the eastern area of the basin DS 3 is up to 2500
m thick (Fig. 17) and it is mainly composed by thinly laminated car-
bonates and gypsum (Fig. 19) interpreted as deposited in shallow
coastal lakes. Carbonates commonly contain abundant pseudomorphs
after evaporites, mud cracks and breccias. b) Intra-formational breccias
and disharmonic folds associated with playa-lake and sabkha deposits
(Fig 19). An impressive general panoramic view of the basin is
observed towards the Pégado anticline (Fig. 20).

– 138 –



Figure 19. Partial stratigraphic sections of the Huérteles Fm (lower part DS 3; 
Berriasian) at Águilar del Río Alhama (Soria). From Quijada et al. (in press).

Figure 20. A splendid panoramic view of the basin is observed looking towards
the Pégado Anticline from the Aguilar del Rio Alhama - Valdemadera road.

– 139 –



Stop 8

Cervera del Río Alhama - Grávalos (La Rioja): General
stratigraphic-section from DS 4 to DS 8 (Latest

Berriasian-Early Albian)

Location: Along the road LR-123 between the localities of Cervera
del Río Alhama and Grávalos (La Rioja). UTM coordinates, datum
WGS84, zone 30: (a) x=584878; y=4658225 (b) x=584116;
y=4660463 (Figs. 6, 7, 8).

Objectives: Carbonate lacustrine systems with siliciclastic influ-
ence (Enciso Group, upper part DS 7) and meandering fluvial systems
(Oliván Group, DS 8).

Description: Cross stratigraphic section of the Cameros Basin sed-
imentary record in its easternmost area: a) carbonate lacustrine systems
with siliciclastic influence (Enciso Gr. upper part of DS 7, Early
Aptian; Fig. 21); b) meandering fluvial systems (DS 8: Oliván Group,
Late Aptian-Early Albian) (Fig. 22).

– 140 –



Figure 21. Siliciclastic inputs (crevasse-splay) deposited over carbonate lacus-
trine facies. Detail of climbing ripples associated with these deposits.

Figure 22. Stratigraphic sections of DS 8 sequence (Late Aptian-Early Albian,
Oliván Gr). Note the reduced thickness at the southeastern Grávalos (GRA). 

Modified from Mas et al. (2009).

– 141 –



Stop 9

Préjano (La Rioja): The Cameros Northern Thrust. a)
Coal rich foot-wall’s thrust fluvial-deltaic deposits (DS 8,
Late Aptian-Early Albian); b) Hanging-wall’s alluvial and

shallow lacustrine to coastal carbonates record in the
Late Barremian-Early Aptian DS 7 (optional)

Location: On the track located just to the South of Préjano, which
connect this locality to the Barranco de las Puertas. UTM coordinates,
datum WGS84, zone 30: (a) x=567332; y=4669493 and (b) x=566728;
y=4669125 (b) (Figs. 6, 7, 8).

Objectives: Fluvial to deltaic deposits with interbedded coal layers
(DS 8) and carbonate coastal-lakes and coastal deposits (DS 7).

Description: The Cameros Northern Thrust:

a) Fluvial to deltaic (tidal influenced) deposits with interbedded
coal layers (Fig. 23) in the foot-wall of the Cameros Northern Thrust
(DS 8, Late Aptian-Early Albian). 

b) Alluvial deposits and carbonate shallow coastal lakes (Fig. 24) in
the hanging-wall of the Cameros Northern Thrust (Jubera and Leza
Fms, DS 7, Late Barremian-Early Aptian; optional).

– 142 –



Figure 23. A. The two last syn-rift depositional sequences of the Cameros
Basin at the foot wall block of the Northern Cameros Thrust: DS 7 (Late
Barremian-Early Aptian) and DS 8 (Late Aptian-Early Albian). DS 7 reaches
up to 50 m in thickness and is mainly composed of continental sandstones and
lutites, marine limestones and marls. DS 8 is up to 300 m thick and is com-
posed of fluvial-deltaic (tide-influenced) sandstones, lutites and coal beds. B.
Characteristic facies of the upper part of DS 8 (Escucha Fm; Late Aptian -
Early Albian) at Préjano (hanging wall block of the Northern Cameros Thrust).

Modified from Arribas et al. (2009).

Figure 24. Detail of a stromatolitic layer in the tidal-influenced cabonate facies
of the Leza Fm (DS 7, Late Barremian-Early Aptian) at Préjano section (La

Rioja).

– 143 –



THIRD DAY: THE NORTHERN BORDER OF THE
CAMEROS BASIN AND A GENERAL N-S CROSS

SECTION ACROSS ITS WESTERN SECTOR

Stop 10

The Leza River Valley (Clavijo-Soto en Cameros, La
Rioja): Geometry of the Cameros Northern Thrust and
Lacustrine facies in DS 7 (Leza Fm; Barremian-Early

Aptian)

Location: On the road LR-250 between the localities of Ribafrecha
and Soto en Cameros (La Rioja). UTM coordinates, datum WGS84,
zone 30; a) x=549232, y=4688363; b) x=548366, y=4684514; and c)
x=547757, y=4682940 (Figs. 6, 7, 8).

Objectives: The Snake’s Head geometry in the Cameros northern
thrust and the lateral facies change between the northern siliciclastic-
poor shallow carbonate lacustrine and palustrine facies and the south-
ern siliciclastic-rich lacustrine carbonates.

Description: a) DS 7 (Barremian-Early Aptian) at the northern flank
of the Snake’s Head structure of the Cameros North Thrust front.
Panoramic view of the Clavijo section (San Prudencio Convent’s ruins)
with alluvial fans from the northern margin of the Cameros Basin
(Urbión Group D: Jubera Fm), which change upwards to carbonate
lacustrine systems (Leza Fm of the Enciso Gr).

b) Panoramic view of active faulting (Fig. 25) during sedimentation
of the alluvial fans and carbonate lacustrine systems of DS 7
(Barremian-Early Aptian ). Carbonate lacustrine facies of the Leza Fm
at the Leza river valley, DS 7) are constituted by shallowing-upwards
sequences, which displays abundant palustrine features at the top, such
as root traces, brecciation, pseudomicrokarst and mud cracks;
Stromatolites, ostracods and charophytes are also frequent (Fig. 26).

c) Panoramic view of the lateral facies change between siliciclastic-
poor northern lacustrine-palustrine carbonates of the Leza Formation
and the southern siliciclastic-rich lacustrine carbonates.

– 144 –



Figure 25. Synsedimentary faults affecting deposits of DS7 (Jubera and Leza 
Fms) at the Leza River Valley.

Figure 26. A. Schematic stratigraphic section of Leza Fm (lower part of Enciso
Gr; Barremian - Early Aptian) at Leza River valley (La Rioja). B.
Paleogeography of the upper part of DS 7 (Barremian-Early Aptian). A y B =
Urbión Gr D, Aluvial-fluvial systems (A. proximal; B. distal). C y D = Enciso
Gr, Carbonate lacustrine systems (C: with strong siliciclastic influence =
Enciso Gr s.s.; D: with very little siliciclastic influence and marine incursions
= Leza Fm; E: Carbonate shallow marine inner environments - Early Aptian
Urgonian carbonate platform). Position of the Leza River valley and Préjano
sections (Leza Fm) are shown in the paleogeographic scheme (LZ and PR,
respectively). Modified from Mas et al. (2008) and Suárez-González et al. (in 

press).

– 145 –



Stop 11

Torrecilla en Cameros (La Rioja): Late Jurassic pre-rift
coral reef complexes as the substrate of the Cameros

Basin

Location: At the Tómalos hermitage, which is located on the road
N-111 (Km 300) close to Torrecilla en Cameros. UTM coordinates,
datum WGS84, zone 30: x=530583; y=4676129 (Figs. 6, 7, 8).

Objectives: The Cameros Basin substrate: the Early Kimmeridgian
coral reef complexes of the Torrecilla en Cameros Fm.

Description: The Torrecilla Reef Complex consists of a fringing
reef composed of several accretionary units.  The first ones were
deposited along a steep margin and display down-lapping and off-lap-
ping geometries (Fig. 27).  In contrast, deposition of the younger accre-
tionary units occurred on a shallow platform where coral reefs were
protected from storm waves by long-shore sand bars (Fig. 28, 29).
Down-stepping geometries and evolution to progressively shallower
environments occurred as a result of a tectonically-forced regression,
as the early Kimmeridgian was a period of rising global sea level,
which is also apparent in other areas of the Iberian Basin.

Figure 27. Late Jurassic prograding reef complexes (Early Kimmeridgian, pre-
rift) at the town of Torrecilla en Cameros. Note the off-lapping and down-lap-
ping geometries. This reefal unit constituted the substrate for the syn-rift
sequences of the Cameros Basin. View from Tómalos hermitage. From Benito

and Mas (2006).

– 146 –



Figure 28. Schematic correlation panel of the stratigraphic sections of the
Torrecilla en Cameros Fm at Torrecilla en Cameros area (La Rioja). The
attached scheme shows the location of each section. Modified from Benito and

Mas (2006).

Figure 29. Palaeogeographic reconstructions of the Torrecilla Reef Complex:
(A) at the development of the fourth accretionary unit. Note that the horizontal
and vertical scale are different, thus, the slope angle is exaggerated. (B) at the
development of the fifth accretionary unit; (C) at the development of the eighth
accretionary unit. Note location of the 4 stratigraphic sections (NR, OR, TH

and RR). From Benito and Mas (2006).

– 147 –



Stop 12

Dam of the Mayor river - Villoslada (La Rioja) -
Montenegro (Soria): Stratigraphic section (from top to

base) of the first syn-rift sequences DS 3, DS 2 and DS 1
(Berriasian to Tithonian)

Location: On the road LR-333/SO-830 between its intersection
with road N-111 and Montenegro de Cameros. Stop at the dam of the
Mayor River close to Villoslada de Cameros. UTM coordinates, datum
WGS84, zone 30: x=527940, y=4662972 (Figs. 6, 7, 8).

Objectives: Lower Cretaceous black shales as possible hydrocarbon
source rocks.

Description: In this stop we will observe the black shales levels
interbedded with lacustrine limestones of the upper part of DS 3
(Berriasian) and flood-plain fluvial sandstone of DS 2 (Tithonian-
Berriasian) and will be discussed their role as possible hydrocarbons
source rocks (Fig. 30). A general view of the first syn-rift sequences DS
1, DS 2 and DS 3 at the Northwestern area of the Cameros Basin will
be obtained very close to the town of Montenegro.

Figure 30. Partial stratigraphic section of the Valdeprado Fm (upper part DS 3;
Berriasian) at the dam of the Mayor River (La Rioja). From Montenegro -

Villoslada Section.

– 148 –



Stop 13

Montenegro – Santa Inés Pass (Soria): the Marine
Jurassic substrate with a thick Callovian carbonate rhyth-

mite with black shales

Location: the road SO-830 to Vinuesa (Soria) between Montenegro
de Cameros (Soria) and the Santa Inés Pass. UTM coordinates, datum
WGS84, zone 30: x=517369; y=4656152 (Figs. 6, 7, 8).

Objectives: Potential hydrocarbon source rock in the marine
Jurassic substrate of the Cameros Basin. Organic matter-rich layers in
the Callovian rhythmite.

Description: The Callovian Pozalmuro Fm is represented by the
thick rhythmite of limestone and black shales, which were deposited in
an outer carbonate ramp. The role of the black shales as possible source
rock for hydrocarbons will be discussed.

– 149 –



Stop 14

Santa Inés Pass - Cuerda del Pozo water reservoir –
Cidones (Soria): Southwest general stratigraphic section

of the syn-rift basin fill. Sandstone source areas and
petrofacies

Location: Along the roads SO-830 to Vinuesa (Soria) and SO-820
to Cidones. UTM coordinates, datum WGS84, zone 30: a) At the Santa
Inés Pass (Soria) on the road SO-830, x=517796, y=4654016; b) At the
Cuerda del Pozo reservoir, x=524749, y=4636122 (Figs. 6, 7, 8).

Objectives: The syn-rift sequences and sandstone source areas and
petrofacies in the SW Cameros Basin.

Description: a) General view of the upper syn-rift sequences at the
southwest of the Cameros Basin, (DS 6, Barremian and DS 7,
Barremian-Aptian). b) Sandstone composition from the Cameros Basin
sedimentary fill and source areas during the main rifting events (Fig.
31). Petrofacies, diagenetic evolution and quality of potential clastic
reservoirs. The Late Cretaceous Megacycle (post-rift 2) unconformably
overlies the Latest Jurassic-Early Cretaceous Megacycle (syn-rift 2 ).

Figure 31. Evolution of sandstone petrofacies during Late Jurassic - Middle
Albian clastic deposition in the western Cameros Basin and their genetic rela-
tionships with the Iberian Massif zones. Arrows in ternary diagrams show the
evolution of petrofacies through time. Note that marine Jurassic deposits are
poorly represented in the Central Iberian Zone. Qm: Monocrystalline quartz. F:
Feldspar. K: K-feldspar. P: Plagioclase. Lt: Total lithics. Rg: Plutonic/gneissic
rock fragment. Rm: Metamorphic rock fragment. Rs: Sedimentary rock frag-

ment. From Arribas et al. (2003).

– 150 –



Stop 15

Picofrentes: tar sands at the base of the Utrillas Fm

Location: This stop is on the track located to the North of the local-
ity of Fuentetoba (Soria), to the Southeast of the Picofrentes peak.
UTM coordinates, datum WGS84, zone 30: x = 537029; y = 4625918
(Figs. 6, 7, 8).

Objectives: The southern border of the Cameros Basin and the oil
seeps in the Late Albian post-rift sandstones.

Description: The Southern border of the Cameros Basin is mani-
fested by the inversion of the Cameros South extensional Fault (Fig
32). The base of the Late Cretaceous Megacycle (Post-rift Stage 2), the
Utrillas Fm, unconformably overlies the Latest Jurassic-Early
Cretaceous Megacycle (syn-rift basin fill). In this area the Utrillas Fm
is composed of bituminous sandstones (tar sands) interbedded with
mudstone (Fig. 33). As fieldtrip epilogue the Potential Petroleum
Systems in the Cameros area (reservoirs, source rocks, oil seeps, and
thermal events…) will be discussed.

Figure 32. Geological cross-section of the Southern border of the Cameros
Basin at the Picofrentes area. For location see Figs. 1B and 5 for location.

Modified from Guimerà et al. (1995).

Figure 33. The Picofrentes Peak (Soria). Bituminous sandstones (tar sands) at
the base of the Late Cretaceous Megacycle (Utrillas Fm). Fuentetoba-

Picofrentes, Soria.

– 151 –



Acknowledgements

Most of this work summarizes the contributions from an interdisci-
plinary scientific team (sedimentologists, mineralogists, petrologists,
structural geologists, palaeontologists, petroleum geologists) from dif-
ferent research departments which have participated in several projects
funded by Spanish public institutions (CICYT, DGICYT, DGESEIC-
SGPICYT, ...). References of recent founded projects of the “Cameros
Team”: CGL2008-01648; BSCH-UCM (Subv. 2009-2010). Conv.
GR58/08; GI UCM-CM 910429 (Subv. 2008); CGL2005-07445-C03-
02/BTE; GI UCM-CM 910429 (Subv. 2007); GI UCM-CM 910429
(Subv. 2006).

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