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Structural Styles Offshore Málaga Province, Alborán Basin, Spain

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The study area is located in the westernmost part of the Neogene Alborán basin. The sedimentary cover deposited above the Alborán allochthonous domain (igneous-metamorphic Palaeozoic-Mesozoic complex) ranges from early Miocene to Pleistocene. Structural styles have been defined using available regional seismic sections integrated with geological data from two exploratory wells drilled in 1983 and 1986. Main structural styles are as follows: Oligocene-Early Miocene normal faulting affecting acoustic basement and sedimentary column, early Miocene gravitational listric faulting, upper Miocene gravitational listric faulting and Late Miocene-Quaternary mud diapirism. These mud diapirs are associated to early Miocene overpressured shales.
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71st EAGE Conference & Exhibition — Amsterdam, The Netherlands, 8 - 11 June 2009
W028
Structural Styles Offshore Málaga Province,
Alborán Basin, Spain
C.G. Giraldo* (Repsol) & J.V. Vargas (Repsol)
SUMMARY
The study area is located in the westernmost part of the Neogene Alborán basin. The sedimentary cover
deposited above the Alborán allochthonous domain (igneous-metamorphic Palaeozoic-Mesozoic complex)
ranges from early Miocene to Pleistocene. Structural styles have been defined using available regional
seismic sections integrated with geological data from two exploratory wells drilled in 1983 and 1986.
Main structural styles are as follows: Oligocene - Early Miocene normal faulting affecting acoustic
basement and sedimentary column, early Miocene gravitational listric faulting, upper Miocene
gravitational listric faulting and Late Miocene - Quaternary mud diapirism. These mud diapirs are
associated to early Miocene overpressured shales.
71st EAGE Conference & Exhibition — Amsterdam, The Netherlands, 8 - 11 June 2009
Introduction
The extensional (“back-arc”) Alborán basin is located in an overall convergent plate tectonic
setting between the Eurasian plate to the north and the African plate to the south (Comas et al.
1999).The main objective of this paper is to illustrate the dominant structural styles and
timing of tectonic episodes in the westernmost part of the Alborán Sea (offshore Málaga
province of southern Spain; Figure 1).
Figure 1 Geologic map of the Alborán area (from Soto et al. 2003) showing location of figure
2.
From a geodynamic point of view this Neogene extensional “back-arc” basin has been
developed above the Alborán allochthonous domain, an igneous-metamorphic complex well
documented in the outcrops of the inner part of the Betic - Rif mountain chain (Comas et al.,
1999). The stratigraphic cover of the Alborán basin ranges from early Miocene (over-
pressured shales) to Pleistocene. Soto et al. (2003) concluded that mud diapirism, triggered by
neogene tectonic compression and sediment overburden, took place since the Middle
Miocene. Mud diapirism and associated mud volcanoes occurred only in the western part of
the Alborán basin and they have been mapped over more than 200 km between Morocco and
Spain in deep water following an approximately North-South trend. Soto et al. (2003) also
suggest that listric normal faulting occurred during Middle Miocene time. Examples of Early
to Middle Miocene outer shelf-slope outcrops have been documented by Serrano et al. (2006)
near the city of Málaga (Figure 2).
Available Data
Structural styles have been documented from:
-Seismic interpretation of 2D and 3D seismic data (3 and 4).
-Geologic data from Andalucía G1 and Alborán A-1 exploration wells.
-Bibliographic review of published papers and internal reports: Comas et al. (1999),
Davison (2005), McClay et al. (2000), Serrano et al. (2006) and Soto et al. (2003).
71st EAGE Conference & Exhibition — Amsterdam, The Netherlands, 8 - 11 June 2009
Geometry and deformation styles
Seismic interpretation and integration with geologic was used to define timing and geometry
of the structural units:
- “Thick-skin” late Oligocene ? – Early Miocene normal (“back-arc”) faulting affecting the
Alborán “basement” and early Neogene sedimentary cover (Figures 3 and 4). This
deformation style is associated to crustal extension in a general Eurasian - African plate
convergence setting (Soto et al. 2003).
Figure 2 Location of Early to Middle Miocene outcrops, “mud-diapir” trend and features
associated to gravity-driven deformations (“extension” and “toe-thrust” anticlines).
Figure 3 Seismic section across Alborán A-1 well showing “thick-skin” normal faulting and
Early Miocene listric faulting.
71st EAGE Conference & Exhibition — Amsterdam, The Netherlands, 8 - 11 June 2009
- “Thin-skin” Early Miocene listric system indicating a gravitational transport to the south-
east (basinward). This gravity-driven deformation was sealed by the middle Miocene deep-
water sedimentary column (Figure 3). The detachment level is located in the lower part of
the Early Miocene sedimentary column.
- “Thin-skin” Late Miocene listric system detached above the over-pressured Early
Miocene shales and sealed by the Pliocene - Quaternary deep-water sedimentary column.
Extensional features and “toe-thrusts” of this gravity driven folded-belt indicate a southeast
vergency transport (Figures 4 and 5).
- Late Miocene – Quaternary mud diapirism triggered by the over-pressured early Miocene
shales. Mud diapirism ocurred simultaneously to the Late Miocene and Pliocene-Quaternary
sedimentary process (Figure 4). The diapirs intruded the post-Early Miocene geologic column
and also the Late Miocene listric detachment system. Mud-diapir trends as well as “toe-
thrust” anticlines and “extensional” features associated to gravitational listric systems are
shown in figure 2.
Figure 4 Seismic section across Andalucía – G1 well showing “thick-skin” deformation, Late
Miocene listric faulting and mud diapirism.
Figure 5 Schematic diagram showing “thick-skin” and “thin-skin” deformations.
71st EAGE Conference & Exhibition — Amsterdam, The Netherlands, 8 - 11 June 2009
Conclusions
Based on 2D and 3D seismic data the following structural styles have been differentiated in
the westernmost part of the Alborán basin:
- “Thick-skin” normal faulting (Late Oligocene ? - Early Miocene).
- “Thin-skin” toe-thrust deformation during Early Miocene and Late Miocene time
(two “gravitational” episodes). An analogous structural situation has been documented in the
Mahakam delta by McClay et al. (2000) and X. Legrand (pers. comm.), as well as in offshore
Mauritania (Davison, 2005).
- Mud diapirism (Late Miocene to Quaternary): triggered by overpressured Early
Miocene shales.
Acknowledgements
We want to thank the Spain Exploration Department of Repsol for authorization to present
this paper. Thanks also to Juan I. Soto (Granada University, Spain) and Xavier Legrand
(Repsol) for fruitful discussions.
References
Comas, M.C., Platt, J.P., Soto, J.I. and A. Watts (1999). The origin and tectonic history of the
Alborán basin: insights from Leg 161 results. In: Proceedings of the Ocean Drilling Program,
Scientific Results (C. Zahn, M.C. Comas and A. Klaus, eds.), College Station, Texas,
161:555–580.
Davison, I. (2005). Central Atlantic margin of North West Africa: Geology and hydrocarbon
potential (Morocco to Guinea), Journal of African Earth Sciences, 43:254-274.
McClay, K., Dooley, T., Ferguson, A. and J. Poblet (2000). Tectonic evolution of the Sanga
Sanga Block, Mahakam Delta, Kalimantan , Indonesia, AAPG Bulletin V. 84 , Nº 6 : 765-
786.
Serrano, F., Sanz de Galdeano, C., El Kadiri, K., Guerra- Merchán, A., López-Garrido, A.,
Martín-Martín, M. and R. Hilla (2006). Oligocene – early Miocene transgressive cover of the
Betic-Rift Internal zone. Revision of its geologic significance, Eclogae Geol. Helv. 99: 237-
253.
Soto, J.I., Comas M.C. and A. Talukder (2003). Evolution of the Mud Diapirism in the
Alboran Sea (Western Mediterranean), AAPG International Conference & Exhibition,
Extended abstract.
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The origin and tectonic history of the Alborán basin: insights from Leg 161 results
  • M C Comas
  • J P Platt
  • J I Soto
  • A Watts
Comas, M.C., Platt, J.P., Soto, J.I. and A. Watts (1999). The origin and tectonic history of the Alborán basin: insights from Leg 161 results. In: Proceedings of the Ocean Drilling Program, Scientific Results (C. Zahn, M.C. Comas and A. Klaus, eds.), College Station, Texas, 161:555-580.
Evolution of the Mud Diapirism in the Alboran Sea
  • J I Soto
  • M C Comas
  • A Talukder
Soto, J.I., Comas M.C. and A. Talukder (2003). Evolution of the Mud Diapirism in the Alboran Sea (Western Mediterranean), AAPG International Conference & Exhibition, Extended abstract.