5
Emplacement of a passive-margin evaporitic allochthon in the Betic Cordillera of Spain Joan F. Flinch* Department of Geology and Geophysics, Rice University, Houston, Texas 77251-1892 Albert W. Bally Shengu Wu* ABSTRACT The Triassic section of the External domain of the Betic Cor- dillera is rootless and thus allochthonous. The frontal accretionary wedge of the Guadalquivir allochthon consists dominantly of Tri- assic evaporites and red beds forming a melange with Upper Cre- taceous–Paleogene deep-water sedimentary rocks. Throughout the unit, Jurassic rocks are absent. It is here proposed that the wide- spread Triassic evaporites of the Guadalquivir allochthon were originally emplaced as gravitational allochthonous masses in a pas- sive-margin setting much like the widespread allochthonous salt of the Texas-Louisiana Gulf of Mexico. Thus, on the Betic passive margin, allochthonous evaporites were first emplaced during the Late Cretaceous–Paleogene. Later, during Neogene time, these evaporites were overthrusted as an accretionary wedge to form the Guadalquivir allochthon. A schematic reconstructed position of the Guadalquivir allochthon places the original passive-margin alloch- thon in its continental-slope setting. REGIONAL SETTING The Gibraltar arc, the westernmost orogenic loop of the Al- pine-Mediterranean system, connects the Betic Cordillera to the north (Fig. 1) with the Rif Cordillera of Morocco to the south. The Gibraltar arc overthrusts the passive margins and the Hercynian basement of the African and Iberian plates. The Gibraltar arc can be subdivided into External and Internal domains. The External domain includes sedimentary allochthonous units derived from the south Iberian and north African passive margins of the Tethys (e.g., Dercourt et al., 1989). The structure of the External domain is the result of a piggyback emplacement sequence; that is, the upper units were emplaced first, and the later emplacement of the lower units deformed the upper units from underneath (see Fig. 1). The first unit to be emplaced was the accretionary wedge (i.e., Prerifaine nappe and Guadalquivir allochthon); the underlying, more land- ward, passive-margin units were emplaced afterward. This sequence of emplacement led to widespread structural enveloping of the over- lying accretionary wedge in Spain as well as in Morocco (Flinch, 1993). The Internal domain comprises various metamorphic and sed- imentary units that were mostly derived from an independent Al- bora ´n platform that was separate from both Europe and Africa (Andrieux et al., 1971). Neogene extension associated with the col- lapse of the Albora ´n Sea (see map of Fig. 1), and late inversion and transpressional tectonics modified the overall compressional devel- opment of the Gibraltar arc (Platt and Vissers, 1989; Garcı ´a-Duen ˜as et al., 1992; Flinch, 1993). Frontal Tectono-Sedimentary Complexes The frontal part of the Gibraltar arc consists of a rootless and chaotic mixture of Triassic, Cretaceous, Paleogene, and Neogene sedimentary units, overlying the less deformed sedimentary cover of the Iberian and Moroccan mesetas. The Prerifaine nappe of the Rif (Bruderer and Le ´vy, 1954) and the Guadalquivir allochthon of the Betic Cordillera (Fig. 1) (Perconig, 1960 –1962) represent, respec- tively, the southern and northern parts of this frontal tectono-sed- imentary complex. Previous authors interpreted these units as either olistostrome or melange (e.g., Perconig, 1960 –1962; Vidal, 1977). Bourgois (1978) described the Guadalquivir allochthon as a perfect melange formed of polygenic breccias in a gypsum cement, parts of the Subbetic unit, and scaly clays with blocks. On seismic profiles the frontal complex of the Gibraltar arc appears indeed as an accretionary complex (Flinch and Bally, 1991; Flinch, 1993) (Fig. 2). The structure of this accretionary wedge is very similar in Spain and Morocco, but Triassic evaporites are much more extensive in the Betic Cordillera. TRIASSIC ROCKS OF THE EXTERNAL DOMAIN Stratigraphy Wells penetrating the frontal part of the Betic Cordillera (e.g., Instituto Geolo ´gico y Minero de Espan ˜a, 1987) demonstrate the superposition of allochthonous Triassic evaporites and Cretaceous deep-water sedimentary rocks onto the Mesozoic autochthonous platform (Fig. 1). Poor surface exposures and the complex defor- mation prevent the establishment of a credible stratigraphy of the allochthonous Triassic. The Guadalquivir allochthon overlies lower Tortonian to Serravallian ‘‘infra-nappe’’ sandstones and is in turn onlapped and overlain by uppermost Miocene to Pliocene-Pleisto- cene ‘‘supra-nappe’’ clastic rocks (Fig. 1). The Guadalquivir alloch- thon involves a Triassic shale, anhydrite, gypsum, salt, marlstone, and a minor amount of diabase, interbedded with Lower Cretaceous marly limestone, marlstone, and sandstone and Upper Cretaceous– Paleocene pelagic facies, locally referred to as ‘‘Capas Rojas’’ (Per- conig, 1960 –1962; Chauve, 1962; Baena and Jerez, 1982). The Guadalquivir allochthon also involves Miocene marlstones ranging from Aquitanian to Tortonian (Curto ´ and Matı ´as, 1986–1987). Some wells have reported layers of salt along the Guadalquivir Basin (see Fig. 1) (Instituto Geolo ´gico y Minero de Espan ˜a, 1987). Salt is also reported in surface outcrops and wells (Instituto Geo- lo ´gico y Minero de Espan ˜a, 1987) that pierced autochthonous dia- pirs of the Gulf of Cadiz (Fig. 2). Structure Well data confirmed that the Triassic evaporites constitute large allochthonous masses (see cross section in Fig. 1). The well Bornos-1 (see Fig. 1 for location) encountered ;2500 m of inter- mixed Triassic salt, shale, and anhydrite, with massive salt layers more than 100 m thick (Instituto Geolo ´gico y Minero de Espan ˜a, 1987). The unit also crops out in the form of conspicuous evaporitic ridges in the western Betic Cordillera (Cadiz area), as can be rec- ognized from geologic maps (Junta de Andalucı ´a, 1985). Because of its plastic nature, the internal structure of the Tria- ssic rocks is quite complex; structures range from kilometres-wide sheets to centimetre- or decimetre-size blocks interbedded within marlstones. At a mesoscopic scale, the structure of the Triassic rocks is characterized by multiple decollement tectonics, disharmonic *Present addresses: Flinch—Departamento de Geologı ´a, Lagoven, Caracas 1010A-889, Venezuela; Wu—Total Minatome Corporation, Box 4326, Houston, Texas 77210-4326. Geology; January 1996; v. 24; no. 1; p. 67–70; 3 figures. 67 on December 17, 2014 geology.gsapubs.org Downloaded from

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Page 1: Emplacement of a passive-margin evaporitic allochthon in the Betic Cordillera of Spain

Emplacement of a passive-margin evaporitic allochthonin the Betic Cordillera of SpainJoan F. Flinch*

Department of Geology and Geophysics, Rice University, Houston, Texas 77251-1892Albert W. BallyShengu Wu*

ABSTRACTThe Triassic section of the External domain of the Betic Cor-

dillera is rootless and thus allochthonous. The frontal accretionarywedge of the Guadalquivir allochthon consists dominantly of Tri-assic evaporites and red beds forming a melange with Upper Cre-taceous–Paleogene deep-water sedimentary rocks. Throughout theunit, Jurassic rocks are absent. It is here proposed that the wide-spread Triassic evaporites of the Guadalquivir allochthon wereoriginally emplaced as gravitational allochthonous masses in a pas-sive-margin setting much like the widespread allochthonous salt ofthe Texas-Louisiana Gulf of Mexico. Thus, on the Betic passivemargin, allochthonous evaporites were first emplaced during theLate Cretaceous–Paleogene. Later, during Neogene time, theseevaporites were overthrusted as an accretionary wedge to form theGuadalquivir allochthon. A schematic reconstructed position of theGuadalquivir allochthon places the original passive-margin alloch-thon in its continental-slope setting.

REGIONAL SETTINGThe Gibraltar arc, the westernmost orogenic loop of the Al-

pine-Mediterranean system, connects the Betic Cordillera to thenorth (Fig. 1) with the Rif Cordillera of Morocco to the south. TheGibraltar arc overthrusts the passive margins and the Hercynianbasement of the African and Iberian plates. The Gibraltar arc canbe subdivided into External and Internal domains. The Externaldomain includes sedimentary allochthonous units derived from thesouth Iberian and north African passive margins of the Tethys (e.g.,Dercourt et al., 1989). The structure of the External domain is theresult of a piggyback emplacement sequence; that is, the upper unitswere emplaced first, and the later emplacement of the lower unitsdeformed the upper units from underneath (see Fig. 1). The firstunit to be emplaced was the accretionary wedge (i.e., Prerifainenappe and Guadalquivir allochthon); the underlying, more land-ward, passive-margin units were emplaced afterward. This sequenceof emplacement led to widespread structural enveloping of the over-lying accretionary wedge in Spain as well as in Morocco (Flinch,1993).

The Internal domain comprises various metamorphic and sed-imentary units that were mostly derived from an independent Al-boran platform that was separate from both Europe and Africa(Andrieux et al., 1971). Neogene extension associated with the col-lapse of the Alboran Sea (see map of Fig. 1), and late inversion andtranspressional tectonics modified the overall compressional devel-opment of the Gibraltar arc (Platt and Vissers, 1989; Garcıa-Duenaset al., 1992; Flinch, 1993).

Frontal Tectono-Sedimentary ComplexesThe frontal part of the Gibraltar arc consists of a rootless and

chaotic mixture of Triassic, Cretaceous, Paleogene, and Neogenesedimentary units, overlying the less deformed sedimentary cover of

the Iberian and Moroccan mesetas. The Prerifaine nappe of the Rif(Bruderer and Levy, 1954) and the Guadalquivir allochthon of theBetic Cordillera (Fig. 1) (Perconig, 1960–1962) represent, respec-tively, the southern and northern parts of this frontal tectono-sed-imentary complex. Previous authors interpreted these units as eitherolistostrome or melange (e.g., Perconig, 1960–1962; Vidal, 1977).Bourgois (1978) described the Guadalquivir allochthon as a perfectmelange formed of polygenic breccias in a gypsum cement, parts ofthe Subbetic unit, and scaly clays with blocks.

On seismic profiles the frontal complex of the Gibraltar arcappears indeed as an accretionary complex (Flinch and Bally, 1991;Flinch, 1993) (Fig. 2). The structure of this accretionary wedge isvery similar in Spain and Morocco, but Triassic evaporites are muchmore extensive in the Betic Cordillera.

TRIASSIC ROCKS OF THE EXTERNAL DOMAINStratigraphy

Wells penetrating the frontal part of the Betic Cordillera (e.g.,Instituto Geologico y Minero de Espana, 1987) demonstrate thesuperposition of allochthonous Triassic evaporites and Cretaceousdeep-water sedimentary rocks onto the Mesozoic autochthonousplatform (Fig. 1). Poor surface exposures and the complex defor-mation prevent the establishment of a credible stratigraphy of theallochthonous Triassic. The Guadalquivir allochthon overlies lowerTortonian to Serravallian ‘‘infra-nappe’’ sandstones and is in turnonlapped and overlain by uppermost Miocene to Pliocene-Pleisto-cene ‘‘supra-nappe’’ clastic rocks (Fig. 1). The Guadalquivir alloch-thon involves a Triassic shale, anhydrite, gypsum, salt, marlstone,and a minor amount of diabase, interbedded with Lower Cretaceousmarly limestone, marlstone, and sandstone and Upper Cretaceous–Paleocene pelagic facies, locally referred to as ‘‘Capas Rojas’’ (Per-conig, 1960–1962; Chauve, 1962; Baena and Jerez, 1982). TheGuadalquivir allochthon also involves Miocene marlstones rangingfrom Aquitanian to Tortonian (Curto and Matıas, 1986–1987).

Some wells have reported layers of salt along the GuadalquivirBasin (see Fig. 1) (Instituto Geologico y Minero de Espana, 1987).Salt is also reported in surface outcrops and wells (Instituto Geo-logico y Minero de Espana, 1987) that pierced autochthonous dia-pirs of the Gulf of Cadiz (Fig. 2).

StructureWell data confirmed that the Triassic evaporites constitute

large allochthonous masses (see cross section in Fig. 1). The wellBornos-1 (see Fig. 1 for location) encountered ;2500 m of inter-mixed Triassic salt, shale, and anhydrite, with massive salt layersmore than 100 m thick (Instituto Geologico y Minero de Espana,1987). The unit also crops out in the form of conspicuous evaporiticridges in the western Betic Cordillera (Cadiz area), as can be rec-ognized from geologic maps (Junta de Andalucıa, 1985).

Because of its plastic nature, the internal structure of the Tria-ssic rocks is quite complex; structures range from kilometres-widesheets to centimetre- or decimetre-size blocks interbedded withinmarlstones. At a mesoscopic scale, the structure of the Triassic rocksis characterized by multiple decollement tectonics, disharmonic

*Present addresses: Flinch—Departamento de Geologıa, Lagoven,Caracas 1010A-889, Venezuela; Wu—Total Minatome Corporation, Box4326, Houston, Texas 77210-4326.

Geology; January 1996; v. 24; no. 1; p. 67–70; 3 figures. 67

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folding, shear fabrics, and gypsum veins (Flinch, 1993). Conven-tional thrust or inversion tectonics do not fully explain all theseobservations, and the terms ‘‘olistostrome’’ and ‘‘melange’’ do notreally address the problem of the provenance of large evaporiticmasses mixed with incomplete Mesozoic pelagic sequences. Ourhypothesis offers another choice for the origin of the Guadalquivirallochthon.

MODEL OF EMPLACEMENT: A PASSIVE-MARGINALLOCHTHON BECOMES A FOLDED-BELT ALLOCHTHONAllochthonous Salt on Passive Continental Margins

In this model we suggest that the passive margins of southernIberia may have been the sites of widespread allochthonous evapor-itic sheets similar to those reported in the Gulf of Mexico (e.g.,Worrall and Snelson, 1989; Wu, et al., 1990; Wu, 1993).

Offshore Louisiana and Texas, the emplacement of large al-lochthonous masses of salt follows a very well defined evolution thattypically begins in a continental-slope environment. First, diapirsrise to near the surface of the slope and begin to flow downslope,leading to a pronounced asymmetry. Eventually, the salt spreads

downdip to form widespread tongues. When the tongues are dis-connected from the salt feeder, they form allochthonous sheets (Wuet al., 1990). This last stage is associated with extension overlyingand landward of the salt (Worrall and Snelson, 1989;Wu et al., 1990;Wu, 1993). Locally, extensional basins, referred to as ‘‘mini-basins,’’developed on top of the allochthonous sheet, because of evaporitewithdrawal, and are characterized by listric normal faults and veryhigh subsidence rates (Worrall and Snelson, 1989).

Application to the Betic CordilleraThe initial development of Triassic salt diapirs at the Betic

margin is observed in the Gulf of Cadiz (Fig. 2). This region con-stitutes the only preserved segment of the most proximal part of thesouth Iberian Mesozoic passive margin. Much of the remainder ofthe margin has been involved in thrust sheets and allochthonousunits of the Betic Cordillera. As we suggested above, during theBetic passive-margin phase in Late Cretaceous to Paleogene time,allochthonous Triassic evaporites were emplaced within youngersedimentary rocks (Fig. 3A). Geologic maps of the Betic Cordillera(Junta de Andalucıa, 1985) show lower Eocene strata unconform-

Figure 1. Tectonic sketch map of Betic Cordillera, showing distribution of Triassic rocks (simplified from Junta de Andalucıa,1985). Most of Triassic section is detached from underneath and constitutes huge allochthonous masses. Cross section ofwestern Betic Cordillera is based on well logs (Instituto Geologico y Minero de Espana, 1987) and surface data from Juntade Andalucıa (1985).

68 GEOLOGY, January 1996

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ably overlying Triassic evaporites, suggesting that the emplacementof the Triassic was already completed at that time. During the Neo-gene, this primary allochthonous mass was transported to the northto become the secondary allochthon of the Guadalquivir accretion-ary wedge (Fig. 3B).

Our hypothesis explains the presence of large volumes of Tri-assic rocks and the general absence of Jurassic strata within theGuadalquivir allochthon. Because allochthonous salt masses on pas-sive margins are intimately associated with listric normal faults, our

model also suggests that some of the Cretaceous normal faultingreported from the Betic Cordillera (Vera, 1981; Banks and War-burton, 1991) may be interpreted as synsedimentary growth faultsthat do not involve the basement.

A geologic map of the Betic Cordillera (Junta de Andalucıa,1985) reveals the widespread surface of exposure of the Triassicrocks (Fig. 1).Wells (Instituto Geologico yMinero de Espana, 1987)demonstrate that much of the Triassic consists of rootless evaporiticsheets. The maximum thickness of these allochthonous evaporites is

Figure 2. Cross section showing accretionary wedge in Gulf of Cadiz (i.e., Guadalquivir allochthon) and salt diapir associated with preservedsegment of Mesozoic passive margin.

Figure 3. Hypothetical restoration of Betic margin. A: End of the Paleogene (before Neogene compression). B: After thrusting and collision.Initiation of oceanic crust is indicated by seaward-dipping reflectors.

GEOLOGY, January 1996 69

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;2.5 km in the western Betic Cordillera. In contrast, offshore Lou-isiana, the Sigsbee salt nappe may reach an estimated thickness of;7 km and may extend over;200 km (Worrall and Snelson, 1989),substantially more than the dimensions of the Guadalquivir evapo-ritic nappe of the western Betic Cordillera, which is ;70 km wide.

Remants of Mini-BasinsContacts that omit stratigraphic section, juxtaposing Triassic

with Cretaceous or Paleogene pelagic sedimentary units having lo-cally preserved rollover structures, offer the most widespread evi-dence for extensional mini-basins overlying the allochthonous Tri-assic of the Betic Cordillera (Flinch, 1993). In the Gulf of Mexico,such supra-allochthonous extensional and/or withdrawal basins arewell preserved, but in the Betic Cordillera, these basins may havebeen subsequently inverted by Neogene compression and carriedpiggyback on the Guadalquivir allochthon. Thus, we can hypothe-size the presence of deformed Cretaceous-Paleogene mini-basins ofthe passive-margin allochthon; these basins would differ in originfrom the commonly extensional Neogene supranappe basins relatedto the extensional collapse of the accretionary wedge (Flinch andBally, 1991; Flinch, 1993).

IMPLICATIONS FOR THE PASSIVE-MARGINRECONSTRUCTION OF THE BETIC CORDILLERA

Published well-log data (Instituto Geologico y Minero de Es-pana, 1987) and geologic maps (Junta de Andalucıa, 1985) for thewestern Betic Cordillera suggest repeated carbonate sequences (seeFig. 1) thrust onto the Gulf of Cadiz–Algarve platform. In the cen-tral Betic Cordillera, seismic data indicate superposition of severalbasinal and platform paleogeographic domains (e.g., Garcıa-Her-nandez et al., 1980; Martın-Algarra, 1987; Blankenship, 1992).

A nondeformed analogue of the Mesozoic southern Iberianpassive margin could be a hybrid that would resemble in part theGulf of Mexico and in part the Nova Scotia passive margin as de-scribed by Friedenreich (1987). The Nova Scotia margin shows di-mensions and a facies distribution that may be compared to therestored Betic margin (see Fig. 3A).

Our proposed hypothetical reconstruction (Fig. 3) can be de-scribed as follows: Triassic half-grabens superimposed on the Her-cynian basement of the Iberian Meseta represent the initial riftingof the margin. The salt was probably deposited in a synrift setting.Much of the seismic data fromMorocco (Heyman, 1989; Zizi, 1994,personal commun.) and Nova Scotia (Welsink et al., 1989) havesuggested that the salt is primarily a synrift deposit (see alsoFig. 3A). A widespread Lower Jurassic carbonate platform occupiedthe south Iberian margin. The carbonate platform eventually brokeup into platform and basin domains during Middle Jurassic time asthe region evolved into a passive margin (Garcıa-Hernandez et al.,1980; Vera, 1983; Martın-Algarra, 1987). In our model, allochtho-nous salt deposits occupy the distal positions of passive margins(Fig. 3a). A tentative hypothetic reconstruction of the Externalzones of the central Betic Cordillera suggests northward displace-ment of the Guadalquivir allochthon on the order of 150–200 km(Fig. 3).

CONCLUSIONSAllochthonous Triassic evaporites occupy extensive areas of the

frontal tectono-sedimentary units of the Gibraltar arc; they are par-ticularly prominent in the Guadalquivir allochthon. It is suggestedthat the Triassic salt and associated gypsum of the Guadalquivirallochthon were originally emplaced within Mesozoic marine sedi-mentary units in a manner comparable to the allochthonous salt ofthe Gulf of Mexico. A primary passive-margin allochthonous Tri-assic sequence was tectonically emplaced within pelagic Cretaceous

to Paleocene slope and deep-water sedimentary rocks. This passive-margin unit was then carried about 200 km northward to form theNeogene Guadalquivir allochthon. The Guadalquivir allochthonwas emplaced first and then enveloped, i.e., deformed from under-neath, by the Subbetic and Prebetic thrust sheets.

ACKNOWLEDGMENTSSupported by a Fulbright felllowship provided by the Spanish Ministry of Education and

Science. Repsol Exploracion provided seismic data from the Gulf of Cadiz. We thank R.Buffler, H. T. Mullins, and W. Ryan for helpful reviews.

REFERENCES CITEDAndrieux, J., Fontbote, J. M., and Mattauer, M., 1971, Sur un modele explicatif de l’arc de

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Zona Betica (s. str.): Madrid, Coleccion Informe, Instituto Geologico y Minero deEspana, 256 p.

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Manuscript received June 7, 1995Revised manuscript received September 28, 1995Manuscript accepted October 10, 1995

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Geology

doi: 10.1130/0091-7613(1996)024<0067:EOAPME>2.3.CO;2 1996;24;67-70Geology

 Joan F. Flinch, Albert W. Bally and Shengu Wu SpainEmplacement of a passive-margin evaporitic allochthon in the Betic Cordillera of  

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