Salt and Strike-Slip Tectonics as MainDrivers in the Structural Evolution of theBasque-Cantabrian Basin, Spain

1. Salt and Strike-Slip Tectonics as Main
Drivers in the Structural Evolution of the
Basque-Cantabrian Basin, Spain
By: MOHAMMED ELSAYED

2. CONTENT
1. INTRODUCTION
2. GEOLOGICAL SETTING
3. STRATIGRAPHY OF THE BCB
4. STRUCTURE OF THE BCB
5. SALT TECTONICS
6. BCB SALT TECTONICS AND HYDROCARBON EXPLORATION
7. TECTONIC EVOLUTION OF THE BCB
8. CONCLUSIONS

3. INTRODUCTION
• The Basque-Cantabrian Basin (BCB) has been the subject of numerous
structural and sedimentary studies.
• The aim of this work is to provide a regional geological overview of
the basin through four geological cross sections, where the main
structural features of the salt tectonics that affect the basin are
described.

4. GEOLOGICAL SETTING
• The Pyrenean chain can be divided into two main sectors: the main
Pyrenees and the BCB, also called the western Pyrenees.
• The BCB (Fig. 1) represents the western continuation of the Pyrenees
to the west of the Pamplona fault, and is mainly composed of
materials of Mesozoic and Tertiary sedimentary rocks.

5. • The Pyrenees is a double vergent structural Alpine orogen formed by the
convergence of the Afro-Iberian and European plates during the Tertiary.
• Prior to this collision, the rotation of the Iberian Peninsula resulted in the opening
of the Bay of Biscay and in forming a rift system with continuous sedimentation
from Triassic to Cretaceous times.
• The main structural elements of the BCB are depicted in Fig. 2, and are
highlighted in the cross sections. The Santander or Cantabrian block represents
the westernmost end of the Mesozoic BCB outcrops. It lies to the west of the N-S
striking Ramales fault. The southern area of the Cantabrian block, known as the
Burgalesa platform, is a large area with its own entity and distinguishing features.
• The Navarra-Alava trough is located south of the Bilbao anticline. Generally, it
presents a smooth south dipping structure and the greatest sediment thickness.
• The Basque arc, located between the Basque coast and the Navarra-Alava trough,
is the area of greatest structural complexity.
• It is structurally formed by a trend of major NW-SE folds like the Bilbao anticline,
Biscay syncline and the Biscay northern anticline.

6. FIG. 2 Geological map of the Basque-Cantabrian Basin. Based on IGME geological maps and own work. A large-
scale image of this figure is available in the book website.

7. STRATIGRAPHY OF THE BCB
• The stratigraphy of the BCB can be divided into large depositional
sequences separated by regional unconformities related to the
tectonic evolution of the basin (Figs. 3 and 4).
• The Mesozoic history begins in the BCB with the development of a rift
phase during Permian-Triassic times.

9. STRUCTURE OF THE BCB
• In order to describe the structure of the BCB, four cross-sections were
built in the generic dip NNE-SSW direction (Fig. 5).

10. FIG. 5 Regional geological cross sections of the Basque-Cantabrian Basin. Inset shows the location of the sections.
Chronostratigraphy and color scale for the different sequences is detailed in Fig. 3. A large-scale image of the
sections is available in the book website.

11. Cross Section 1: Amaya-Ayoluengo-Leva
Anticline-Matienzo-Ajo
• The SW thrust front of the BCB ramps the Cretaceous and Tertiary of the
Duero foreland basin, emerging in the surface, partially covered by the
syntectonic Miocene deposits.
• This structure is affected by a reverse fault with south vergence, like other
striking WNW-ESE basement thrusted highs of the area.
• Due to the existence of a NE-SW striking thrust, the Jurassic of the Amaya
hangingwall overlies the footwall’s Weald-Utrillas.
• The Humada anticline is a WNW-ESE trending fold, cored by a reverse fault,
that represents an inversion structure.
• During the Late Jurassic-Early Cretaceous rift stage, this structure was
cored by a normal fault where the northern hangingwall accommodated a
thicker sedimentary section. The main normal fault was inverted as a
reverse fault during the Tertiary.

12. Cross Section 2: Ubierna-Poza de la Sal-
Villarcayo-Castro Urdiales
• In the southernmost sector of this section below the Duero foreland
basin, three oil exploration wells were drilled in the San Pedro
Palaeozoic high, encountering a thin Cretaceous section
unconformable over Permo-Triassic materials.
• The variable thickness of the Permian-Triassic sediments suggests
that these were deposited in a half-graben basin.
• the WNW-ESE bounding faults of these small troughs were inverted
during the Tertiary compression as reverse faults.

13. Cross Section 3: Treviño-Bilbao Anticline-
Guernika
• Due to the narrowing of the BCB to the east, this cross section is shorter
than the previous ones, with a total length of 100 km.
• The S thrust front of the BCB crops out in Pancorbo with a displacement
over the Ebro foreland basin of more than
• 15 km.
• The structure in this part of the Navarra-Alava trough is quite simple,
generally south dipping, only deformed by the Salinas de Añana and
Murguia diapirs.
• The Bilbao anticline marks, in fact, a change in the structural vergence in
the BCB. This fold has been interpreted to represent the western
continuation of the Leiza and North Pyrenean faults.

14. Cross Section 4: Urbasa-Aitzgorri-Azpeitia
• The Sierra de Cantabria thrust front is a complex structure, with E-W
oriented faults and narrow folds. The sole thrust detachment is
located in the Triassic section and climbs up through the Mesozoic
and Tertiary deposits towards the south.
• The thrust front is not exposed in the surface, and probably continues
through a buried thrust flat located within the Oligocene formations
• The Aitzgorri anticline has a very steep southern flank affected by
high-angle normal and reverse faults, which contact the Purbeck and
Keuper formations. It has been interpreted as an inverted diapiric
structure thrusted towards the north during the Tertiary.

15. SALT TECTONICS
• The BCB salt basin is an outstanding area to observe and study salt tectonics and
sedimentation relationship.
• The presence of saline highs and lows during Late Jurassic-Early Cretaceous
greatly conditioned the geometry of the basin and its subsequent sedimentation.
• The Triassic sediments were deposited in an extensional graben-shaped basin
bounded by the Bilbao and Ventaniella faults, generated by a simple sinistral
shear stress during the Permian to Early Triassic.
• A great variety of salt geometries can be recognized in the BCB: salt walls, rafts,
saline anticlines, salt pillows and diapirs.
• Diapirs occurred northwards, while to the S (Burgalesa platform), the Triassic
evaporites depict salt pillow geometries.
• A schematic map (Fig. 10) depicts the present salt distribution in the BCB and the
actual thickness.

17. BCB SALT TECTONICS AND HYDROCARBON
EXPLORATION
• The existence of salt Triassic formations has played an important role in the
geochemical evolution and the maturation of the source rocks.
• The salt has a thermal conductivity from two to four times greater than
other sedimentary rocks.
• The salt chimney effect, due to the high thermal conductivity of the salt,
leads to low temperatures below the salt and high temperatures above,
having a significant effect on petroleum generation.
• the thermal maturity of sediments increases as the lateral distance from a
diapir decreases.
• The timing of salt movement is very important in order to simulate the
correct thermal and maturity history in the vicinity of a salt dome.

18. • in the pinch-outs and presalt plays, the Triassic salt acts as the top and
lateral seal of some traps.
• In spite of the existence of some effective source rocks (i.e., Stephanian,
Liassic, Late Jurassic, Middle Cretaceous) and reservoirs (i.e., Purbeck
sandstones, Jurassic and Cretaceous fractured carbonates) (Fig. 2), no
significant results have been achieved in the BCB. This is probably due to
the inversion and Alpine compression that affected the initial hydrocarbon
accumulations, breaching the traps and producing the hydrocarbon
washing and dismigration.
• Salt tectonics was important in the generation of various trap models,
which are traditionally explored in the BCB (e.g.,see several hydrocarbon
fields in Fig. 5).

19. TECTONIC EVOLUTION OF THE BCB
1. Late Jurassic-Early Cretaceous Rift Extension
• The Permian-Triassic rift produced the first BCB configuration as well
as the Triassic salt sedimentation.
• The major structural faults are oriented in a strike WNW-ESE with
associated NE-SW transversal structures.
• Inherited faults were reactivated during this rift stage and were
inverted later during the Tertiary compression.

21. 2. Tectonic Inversion and Tertiary Compression
• This contractive stage was characterized by a reverse, both right-lateral and
left-lateral reactivation of WNW-ESE and E-W striking inherited faults.
• The tectonic evolution of the BCB during the Tertiary compression was
conditioned by the inheritance of the previous rift geometry.
• Some of the main rift faults as well as other salt geometries, like rollers or
rafts, have been inverted during the compressional Alpine phase.
• Some of the NW-SE oriented thrusts show arch-shaped prolongations with
a NE-SW direction. This geometry could be interpreted as lateral ramps of
the main thrusts.

23. 3. Evolutionary Models of the BCB
• Several models have been proposed for the interpretation of the
tectonic evolution of the BCB.
• A thick-skinned tectonic model involving the basement is widely used
to explain the structure in some southern and western areas.
• In the western Cantabrian Mountains, the basement is involved in
low-angle thrusts without a visible detachment level between the
Paleozoic basement and the Mesozoic cover.

25. CONCLUSIONS
• The BCB is characterized by active salt tectonics during the Late Jurassic-
Lower Cretaceous that controlled the depositional geometry of their
sedimentary sequences, as well as the formation and evolution of the main
compressive structures during the Tertiary.
• The major extensional faults of the Late Jurassic-Early Cretaceous rift phase
were oriented in a strike WNW-ESE with NE-SW transversal structures
which, along with other salt structures such as listric faults, rollers or rafts,
evolved to strikeslip faults or thrusts during the Alpine compression.
• The existence of thick salt formations and their tectonic evolution has
played an important role in some of the key petroleum system elements,
such as hydrocarbon trap generation, geochemical evolution and the
maturation of source rocks.

26. THANK YOU