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UNIVERSIDAD DE GRANADA   

Departamento de Estratigrafía y Paleontología

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Artículo publicado con motivo del International Symposium Epicontinental Triassic (Halle, Alemania):

Zentralblatt für Geologie und Paläontologie. Teil I, Jahrgang 1998, Heft 9-10, 1009-1031. EPICONTINENTAL TRIASSIC. G.H. Bachmann and I. Lerche (editors). E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, 2000. ISBN 3-510-66014-5

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      Epicontinental Triassic of the Southern Iberian Continental Margin (Betic Cordillera, Spain)

Alberto Pérez-López

Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, 18071-Granada, Spain.

  

Abstract

This paper presents the geology of the epicontinental Triassic within the Southern Iberian Continental Margin (Betic Cordillera). Emphasis is placed on those problems posed by the study of the Betic Cordillera Triassic. The characteristics of the epicontinental Triassic and its tectonic setting are described.

In Triassic outcrops folds and nappes are quite frecuent. In many cases Triassic rocks are part of an olistostromic complex developed during the orogeny in the Miocene over the entire cordillera.

Two carbonate formations have been distinguished: an Anisian-Carnian Muschelkalk facies (Majanillos Formation), which is composed of limestones, and especially marls in the upper part; and another of Norian age (Zamoranos Formation), composed of bedded dolomites and a red detrital member, with volcanic rock debris, which is intercalated in the lower dolomites. The Keuper is represented by five units which are of Carnian-Norian age. The lower unit displays alternations of shales, gypsum, dolomites and sandstones. The two intermediate units consist of sands and shales, respectively, whereas in the two upper units there is a predominance of gypsums.

Finally, several considerations are made regarding the sequence stratigraphy and, specifically, the transgressive cycles corresponding to the Majanillos Formation and to the Zamoranos Formation, which is the most expansive unit.

1. Introduction

Triassic deposits constitute the beginning of the alpine sedimentation cycle inmuch of the Iberian Penisula. The three lithostratigraphic units equivalent to the Germanic Triassic Groups can be identified in almost all of the Triassic basins: Buntsandstein, Muschelkalk and Keuper. However, each basin displays variations in lithofacies and biofacies which are sufficiently significant to make different palaeogeographical units distinguishable (Sopeña et. al. 1985; Virgili et al. 1977; Sopeña et al. 1988).

In the Betic Cordillera (Southern Iberian Peninsula) there are many epicontinental Triassic outcrops (Fig. 1) with facies that are similar to the those described in other regions of the Peninsula. However, stratigraphic and palaeogeographical studies of Triassic rocks in this region encounter a number of difficulties which are due to the tectonic conditions in these outcrops. It is important to keep in mind the tectonic evolution of the Triassic deposits in order to understand such difficulties.

1.1 Tectonic characteristics of the Betic Cordillera

In terms of geography, the Betic Cordillera is the range of mountains occupying the Southern Iberian Peninsula from Cadiz to Alicante (Fig. 1). This range, as well as the Rif (Northern Africa) is the westernmost segment of the alpine System, and displays a number of characterics which are typical of alpine cordilleras (Fontboté and Vera 1984): a mesozoic preorogenic evolution with an extensional tectonic that causes a significant differentiation of paleogeographical domains; a compressional structure characterised by the superposition of allochthonous units (nappes); and, finally, the presence of dynamothermal metamorphism processes.

The cordillera is formed by two large geological zones: External Zones and Internal Zones. The External Zones (according to the definition given by Auboin 1965) consist of a basement which is not visible at the surface, and a cover of Mesozoic and Tertiary deposits in the Southern Iberian Paleomargin. The oldest rocks of the cover are Triassic, since the Paleozoic rocks belong to the basement. The structure of the cover is due to a detachment tectonics which gives way to folds and overthrust folds (e.g. Azema et al. 1979; García Hernández et al. 1980). The Internal Zones consisting of basement and cover rocks, which in many cases are metamorphic, are structured in large nappes. During the Mesozoic, these rocks constituted a more southern domain independent from the Southern Iberian Paleomargin (e.g. Durand Delga and Fontboté 1980; Comas and García Dueñas 1988).

In addition to these two large geological zones it is important to consider the Depression of the Guadalquivir which is a foreland basin (e.g. Sanz de Galdeano and Vera 1992). This domain is especially interesting because of the presence of Triassic rocks redeposited during the Miocene.

During the alpine orogeny the deposits constituting the meridional margin of the Iberian Plate, and which now constitute the cover of the External Zones, evolved from the tectonic point of view in a way analogous to that of other alpine margins (Fig. 4; Vera 1988). This margin was structured in various palaeogeographical domains which now constitute two main tectonic zones spanning in an ENE direction: the Subbetic Zone and the Prebetic Zone. The Prebetic, palaeographically nearer to the Hercynian massif, displays mainly marine sediments more shallow than the Subbetic, or even continental sediments. Between the Subbetic and the Prebetic a domain of mixed stratigraphic characteristics can be distinguished, as well as an intermediate position between both, which constitutes the so-called "Intermediate Domain".

1.2 Triassic lithotypes

Three main types of Triassic deposits can be distinguished in the Betic Cordillera (Fig. 1.B): alpine, continental (redbeds) and epicontinental Triassic.

In the Internal Zones there are carbonate alpine facies, which can be recognised above all in the Alpujárride Complex; the Continental Triassic consists of detrital facies (Verrucano type) which appears in the Maláguide Complex (e.g. Azema et al. 1979).

The epicontinental Triassic outcrops primarily in the External Zones over more than 400 km, although epicontinental facies also exist in certain units of the Internal Zone (López Garrido et al. 1997).

On the other hand, on the south side of the Meseta, another Triassic outcrop can be seen displaying continental facies (redbeds), which have been called "Hespérico" (Virgili et al. 1977), "Mesético" (Busnardo 1975) or the Chiclana de Segura Formation (López Garrido and Rodríguez Estrella 1970).

In this paper an overview will be given of those epicontinental Triassic facies which outcrop in the External Zones, especially in the Subbetic Zone, and of the problems which have been posed on the basis of the data published in recent years. It should be kept in mind that many of these questions remain open to new interpretations and that a number of difficulties have yet to be resolved.

1.3 Epicontinental Triassic deposits and geodynamic context

Epicontinental Triassic deposits constitute in general the main detachment level of the tectonic units formed during the alpine orogeny as a result of their tectonic position (Fig. 2), their plastic characteristics (shales and evaporites) and their substantial thickness ( García Dueñas 1969; Sanz de Galdeano 1973; Azema et al. 1979; Rivas et al. 1979).

Furthermore, in the Jurassic and the Cretaceous, during the stage of the most important rifting, the Triassic rocks were expanding, breaking and becoming thinner while Jurassic and Cretaceous sediments were depositing (Fig. 2.3, 2.4). These Triassic rocks were being displaced by means of tectonic and diapiric mecanisms, thus being redeposited in certain cases on the Cretaceous basin. Therefore, before the important orogenic movements were produced in the Miocene, the Triassic deposits had already been mobilised and, in some cases, deformed and redeposited.

As a result of these mecanisms, Triassic outcropping occurs in a notably displaced and fractured manner, and displays a mixture of different tectonic units (Fig. 2.5). This is what renders difficult the study of its stratigraphy, as well as the characterisation of the various palaeogeographical units.

2. Lithostratigraphic units and sedimentary interpretation

In the Betic epicontinental Triassic the differentiation of the three large groups of lithological units (Buntsandstein, Muschelkalk and Keuper) had already been established by the end of the last century (for instance Bertrand and Kilian 1889). Several peculiar differences were observed regarding the Triassic in the Central European Basin (Blumenthal 1927). Later, enough authores studied Triassic outcrops in the External Zones (e.g. López Garrido, 1971; García-Rossel 1973; Sanz de Galdeano 1973; Cruz-Sanjulián, 1974; Peyre 1974; Busnardo 1975).

Recently, Pérez-López (1991) and Pérez-López et al. (1992) have distinguished in the central area of the External Zones (Fig. 3) a carbonate formation of Muschelkalk facies (Majanillos Formation), five detrital evaporitic formations, which constitute the Jaen Keuper Group, and finally, one upper carbonate formation of Norian age (Zamoranos Formation). The presence of the Bundsandstein (Anisian) in the Subbetic domain is still being debated and the evaporites of the Rhaetian are difficult to recognize.

All of these formations display significant variations in their lithofacies and thickness, depending on which palaeogeographical unit is concerned (Martín Algarra 1987; Martín Algarra et al. 1995). 

2.1 Bundsandstein

This unit displays outcrops in very few places. It has been definitively identified only below the Muschelkalk in the northernmost sector of the basin (External Prebetic), near the Meseta, and in several places in the province of Murcia (Southern Internal Prebetic) also below the Muschelkalk carbonates. It consists of red and grey shales, thin layers of gypsum and sandstones with mud cracks. This should be considered the upper part of the Buntsandstein, interpreted here as coastal plain deposits. However, this unit may be equivalent to the Middle Muschelkalk of the Iberian Range (e.g. López-Gómez 1985), especially in the outcrops of the Southern Internal Prebetic.

2.2 Muschelkalk

The Muschelkalk deposits consist of a carbonate succession generally dominated by marlstones towards the upper part (Fig. 3). They display an overall thining-upward trend.

Three main members can be distinguished in the Majanillos Formation in the Subbetic Zone: a lower one (Member 1), 20-40m thick, consisting of thick limestone and dolomite beds, with intervals of approximately 10m of thin-bedded marly limestone; and a higher member (Member 2), up to 50 m thick, and consisting of a more marly succession with intercalations of bioclastic limestone. In some outcrops a third upper member can be seen (Member 3), formed by mainly marls and shales with intercalations of thin carbonate which pass upwards into Keuper gypsums and shales.

The lower member has been interpreted in general as a deepening sequence in which subtidal and ramp deposits can be identified (Pérez-López 1998). The middle member presents a predominance of marly facies deposited on a shallow platform on which there are frecuent storm deposits. The facies belonging to the higher member display lagoon and tidal flat deposits which correspond to a muddy shallowing-upward sequence.

2.3 Keuper

The Keuper is characterised by multicoloured and red shales, sandstones, gypsum and sometimes by basic intrusive. Although none of the outcrops display a complete section of Keuper, five different lithostratigraphic units have been distinguished in Subbetic Zone (Jaén Keuper Group). The thicknesses and facies of these units vary from the Internal Subbetic to the Prebetic Zone. The units can be correlated with the formations that outcrop in the Eastern Iberian Peninsula (Ortí Cabo 1974). Despite their having different denominations, a decision has been made to use the following abbreviations: K1, K2, K3, K4 and K5 (Pérez-López 1991).

2.3.1 Unit K1 (100-200 m)

Consists of multicoloured shales with thin intercalations of carbonates, gypsum and fine-grained sandstones. Local lignites also appear. The sandstones present parallel and cross-lamination. Ripples on the top of some layers are also frequent, and in some cases erosive surfaces, plant debris (especially conifers) and mud pebbles can be observed.

Associated with the sediments of this unit, there are apparently halite deposits, which, though they do not outcrop, can be inferred from the many saline gullies which drain these materials. The marine origin of these salts and sulphates is known to exist in various Spanish basins (Ortí and Pueyo 1983; Ortí 1987; Utrilla et al. 1987; Ortí 1990; Ortí et al. 1994).

The facies of this unit can be attributed to a fluvial-coastal systems tract, with ample development of lakes and salt pans (Pérez-López and López Chicano 1989).

2.3.2 Unit K2 (25-60 m)

This formation is characterised by a predominance of thick sandstone beds with interbedded claystones. The sandstone beds show trough and planar cross stratification and contain one or several erosive surfaces. Other facies, commonly in the middle and upper part of this unit, are those with parallel-laminated sandstones and low-angle cross-bedded sandtones, which sometimes display deformed cross-lamination or small-scale current structures (ripples). Plant debris and mud pebbles are frecuent in some layers.

These deposits are channel infill from an extensive fluvial system having wide and shallow watercourses with the development of large expanses of sand bars. Also important are the deposits related to the high-flow regime (sheet floods), which developed over broad areas and are characteristic of ephemeral floods (e.g. McKee et al. 1967; Tunbridge 1981, 1984). This unit corresponds to the deposits of a terminal alluvial system (Pérez-López 1991).

2.3.3 Unit K3 (50-80 m)

In this unit two members have been distinguished: a clay member and a sand member. In the clay member a predominance of red claystone with red nodular gypsum can be observed. There is also a number of thin beds consisting of greenish and red sanstone.

The sand member is less thick (1-8 m). In this member multicoloured shales appear with thick beds of pale red or pink fine-grained sandstone, with a predominance of ripples and horizontal lamination. In many cases there are thinning-upward sequences.

The deposits of this unit, essentially claystones with gypsums and some dolomites, are interpreted as a saline mud flat with environments of sabkha and lagoonal deposits.

The sandstones, which have a greater proportion of clay and display ripples, suggest a relationship between the mud flat and a sand flat with distal facies, that correspond to the terminal-fan deposits.

2.3.4 Unit K4 (5-30 m)

This unit presents sections of less thickness in the Subbetic Zone, although it is well represented in the Prebetic Zone. It consists of clay with a large percentage of red nodular gypsum, although laminated gypsum can be found locally. The abundance of nodular gypsums and red clay permits this unit to be interpreted as sabkha deposits of a coastal plain.

2.3.5 Unit K5 (50-70m)

This unit consists of stratiform masses of white and grey laminar gypsums. In the higher part of the unit there are dolomites which can reach 25 m in thickness. In some cases it is formed by thin-bedded dolomites with evaporitic pseudomorphs, while in other cases the upper part is more massive and crystalline, corresponding to recrystallized dolomites.

This unit is similar in facies to the evaporitic deposits of the Upper Evaporitic Series of Keuper facies (Ortí 1974), in which halite deposits have also been found in subsurface sections and certain outcrops (Ortí and Pérez-López 1994).

The evaporites, claystones and dolomites of this unit correspond to the deposits which are situated on the coast, in relation to marshes, salt pans or saline lagoons where the precipitation of the evaporites or carbonates took place. This implies extremely restricted coastal or marine facies.

2.4 Upper Carbonate Unit

This unit has been recognised from the Prebetic Zone to the Internal Subbetic Zone, though with variable thickness and different facies. It corresponds to the upper carbonate formation of the Triassic, the so-called Zamoramos Formation in the Subbetic Zone (Pérez-López et al. 1992).

Subbetic outcrops of this carbonate Formation are decametric blocks, which are limited by tectonic contacts and float on shales. For this reason, these carbonates have been confused on previous occasions with Muschelkalk carbonates due to the fact that they had never been dated and their stratigraphic situation was difficult. This Formation outcrops in its stratigraphic position above the K5 unit exclusively in some outcrops of the Eastern External Prebetic (Pérez-López et al. 1996).

It consists of bedded limestones and dolomites (25-45 m thick) with a peculiar red detrital intercalation (0.1-1.5 m thick) in the lowest part. In the lower part of the carbonates there are bioclastic and oolitic limestones, whereas in the upper part bioturbated limestones, flat pebble breccia and dolomites with stromatolitic laminations can be observed. These facies are interpreted as shallow platform and tidal flat deposits.

The detrital deposits consist of red shales, sandstones, volcanic rock debris and high concentrations of iron oxides (Hematite). This red intercalation is interpreted as coastal-continental complex deposits: fluvial, mud flat and lake deposits with pedogenic processes.

2.5 Carcelen Anhidrite (Triassic-Liassic transition)

The Carcelen Anhidrite (Ortí 1987) is situated above the carbonates of the Zamoranos Formation (Fig. 3). It is an evaporitic unit which presents gypsums, carbonates and shales in its outcrops. In the subsoil, however, this unit is formed by anhidrite, shales, and dolomite beds with important intercalations of halite. This unit outcrops in several places of the central part of the Subbetic Zone and in the Eastern External Prebetic Zone, although it is very difficut to recognize because some of its facies are like K5 Unit facies.

It possesses a substantial development in the subsoil, as has already been observed in the eastern sector of the Betic Cordillera and, especially, in other adjacent basins such as the region of Valencia-Cuenca (Ortí 1990).

This unit is interpreted as deposits in a palaeogeographical location similar to that of the K5 unit which are related to lagoonal environments and coastal sabkhas.

3. Biostratigraphy

When the Triassic is studied on the basis of the large number of outcrops which together compose, as it were, a kind of "puzzle", there is a scarcity of fossils that make age determination of the various lithostratigraphic units possible. This introduces difficulties regarding the correlation of units and the study of Triassic stratigraphy with the required exactitude or completeness.

In general, the record is quite poor and the degree of preservation of the fossils is frecuently deficient (Márquez-Aliaga 1985). Nevertheless, in spite of the fact that many areas have not been sampled, or properly studied, new age determinations have permitted in recent years a better understanding of Triassic stratigraphy.

The Upper Buntsandstein has been dated in the Prebetic Zone with an assemblage of Ladinian polynomorphs (Besems 1983). However, in the Southern Prebetic Zone the so-called Buntsandstein or the Middle Muschelkalk, that is, the unit of shales which is situated below the Muschelkalk carbonates, is probably of Anisian age. This is because the lower part of the Muschelkalk carbonates is Anisian (Goy and Martínez 1996; Goy and Pérez-López 1996). In the upper part of the same Muschelkalk carbonates, conodonts, as well as an assemblage of bivalves typical of the Ladian age, have been found (Pérez-López et al. 1991).

The coastal-continental deposits (shales, sands and evaporites) present plant debris (Equisetites arenaceus) and Carnian pollen (Camerosporites secatus, Patinaporites densus and Vallasporites ignacii) in the K1 and K3 units. On the other hand, it is interpreted that the Unit K5 may be Norian due to its stratigraphic position.

In the upper carbonate unit (Zamoranos Formation), situated above the K5 unit, Norian pollen has been found. Finally, have been dated as Rhaetian some Carcelen Anhidrite shales.

A fossil record, therefore, exists from the Anisian to the Rhaetian, although it is not a very precise one. No datings appear to have been done of the Anisian in coastal-continental deposits that can be attrituted to the Buntsandstein or Middle Muschelkalk. Furthermore, in a number of shale outcrops situated below Muschelkalk carbonates Carnian pollen has been found. This suggests that the shales in many cases occupy a non-stratigraphic position below a carbonate tectonic unit, thus corresponding to the Keuper.

4. Sequence Stratigraphy

Garrido-Megías and Villena (1977) establish a sequence stratigraphy for the Triassic of the Iberian Penisula. However, the boundaries of certain sequences are difficult to establish for certain Triassic basins on the Peninsula.

A sequence stratigraphy can be established for the epicontinental Triassic in the Betic Cordillera, although not without certain difficulties which are due to the fact that the starting point is a stratigraphy reconstructed on the basis of incomplete sections. In the stratigraphic succession visual lacunae and faults exist whose importance is not easy to estimate. A preliminary interpretation has been made regarding several of the boundaries of the possible depositional sequences (Pérez-López and Fernández 1992).

In this paper we have described the most important discontinuities, and also those parasequence associations with a specific polarity in facies evolution (Fig. 3). These associations correspond to cyclostratigraphic units (García et al. 1989).

4.1 Discontinuities

In general, in the entire Triassic succession numerous surfaces can be observed to have characteristics which can be related with discontinuities. Problems arise in attempts to quantify or assess the importance of these discontinuities that offer so few biostratigraphic data. Until now what has been attempted is the recognition of the possible boundaries of depositional sequences and, as a result of such recognition, four significant discontinuities have been identified.

4.1.1 DM1 and DM2 in the Muschelkalk

In the Majanillos Formation a number of discontinuities and paraconformities can be observed, normally corresponding to accumulation of shells and iron oxides, and sometimes to erosive surfaces. However, only two of these discontinuities are important, namely the ones that are the cyclostratigraphic boundaries which are related to sea level changes. The first (DM1) corresponds to a hardground with ceratites, where the Anian-Ladinian boundary is situated. The second (DM2), which is difficult to recognize, is located on a surface which displays abundant iron oxides with a high concentration of brachiopods and bivalves. This second discontinuity is the basal boundary of a marly section which passes upwards into Keuper shales.

4.1.2 DK2 in the Keuper

In the Jaen Keuper Group many discontinuities and/or diastems can be observed in association with erosive surfaces, calcretes and surfaces rich in iron oxide. One of these surfaces is recognised as a regional sedimentary break it is located at the base of the K2 unit. The facies display an evolution which is quite different both underneath and over the discontinuity (Pérez-López 1996). In the K1 unit succession no very significant facies evolution can be observed. However, from this discontinuity to the top there is a progressive facies evolution from Unit K2 to Unit K5: first there is a predominance of sandstones, and then, successively of claystones, gypsums and carbonates.

4.1.3 DZ1 in the Zamoranos Formation

As has already been said, the carbonate Zamoranos Formation is characterised by the intercalation of red detrital deposits. The lower boundary of these deposits corresponds to a more or less developed karstic surface, situated on the top of the lower carbonates. This surface has been interpreted as a discontinuity (DZ1) which separates continental upper deposits from marine lower deposits.

4.2 Cyclostratigraphic units

Four cyclostratigraphic units have been identified on the basis of the main discontinuities, in comparison with the sequence stratigraphy established in other basins on the Penisula, and according to facies evolution.

1. Cyclostratigraphic unit B-M. The lower boundary of this unit never becomes visible either because it always corresponds to a tectonic contact or because the section is incomplete. Its upper boundary is the DM1 discontinuity. This sequence comprises an evolution from coastal-plain facies to the distal facies of a carbonate ramp (deepening-upward sequence).

2. Cyclostratigraphic unit M. Its lower and upper boundaries are, respectively, the DM1 and DM2 discontinuities. In this cyclostratigraphic unit, a shallow-deep facies alternation can be observed, which corresponds to an association of parasequence with vertical aggradation.

3. Cyclostratigraphic unit M-K1. This unit comprises the final member of the Majanillos formation and the first unit of the Jaen Keuper Group (Unit K1). The lower boundary corresponds to the DM2 discontinuity and the upper boundary to the DK2 discontinuity. In this unit a progradation of the sedimentary environments has been interpreted. In the lower part there are very shallow marine carbonates. Afterwards, there is a progressive increase in shales and coastal gypsums, and a predominance of fluvial sandstones in the upper part.

4. Cyclostratigraphic unit K2-Z. The boundaries of this unit are the DK2 and DZ1 discontinuities. The sequence comprises units K2 through K5 and the lower part of the Zamoranos Formation carbonates. It is represented mainly by sediments of a transgressive depositional system, beginning with fluvial-coastal deposits and ending with very shallow marine deposits (Pérez-López 1996).

Correlation between the depostional sequences and the third order cycles (Haq et al. 1987) has been attempted, although with certain reservations, and as a possibility open to new interpretations (Pérez-López and Fernández 1992; Pérez-López 1996). It is also very probable that comparisons cannot be made with all of the global eustatic cycle boundaries due to the significant effect of the tectonic control over Triassic deposits.

Lithostratigraphic correlations with other regions appear to be of greaterinterest, but in this case in terms of parasequence associations or of transgressive/regressive cycles. It can thus be observed that over most of the Peninsula two distinct carbonate units and one intermediate detrital unit are distinguishable in the Muschelkalk (e.g. Virgili et al. 1977; López-Gómez 1985). However, only one Muschelkalk carbonate unit, and below it a possible detrital unit, are identifiable in the Betic Cordillera (Fig. 4).

 This carbonate unit corresponds to a transgressive cycle which in the Betic Cordillera starts in the Anisian and continues until reaching the Carnian, which is where the carbonate deposits of Muschelkalk facies occupy the maximum area. In the thicker stratigraphic sections after the trangressive stage, there is an aggradation interval which is followed by a regressive stage.

The second transgressive cycle, related to marine deposits, is even more extensive, even shorter in terms of time and, therefore, faster. The carbonate deposits corresponding to this transgression occupy a greater area in the Betic Cordillera and coincide with those of an identical age in most of the Peninsula (Calvet et al. 1998). We are referring to the Zamoranos Formation carbonates, of Norian age, which can be correlated to the Imon Formation and, very probably, with the Isabena Formation (Calvet et al. 1990).

Finally, it can be observed that the thicknesses of the sequences are, roughly, inversely proportional to the number of years they involve. The rate of sedimentation of the detrital materials ranges from 5-10 cm per 1000 years. For the marine carbonate facies of the Majanillos formation, the rate is 1-2 cm per 1000 years, while it is less than 0.5 for the Zamoranos formation. The rapid subsidences which can be inferred are related to the rifting stage. This stage is characterised by the deposit of clastic sediments (Hubbard 1988), with abundant sands in the case of the betic basin, as also occurs in other basins on the Peninsula in which there was a crustal extension (Sopeña et al. 1988; López-Gómez and Arche 1992; López-Gómez and Arche 1993).

5. Triassic rocks and tectonics

The various Triassic lithological units of the Betic Cordillera External Zones have reacted differently to tectonic transport in accordance with their lithology and their position in the stratigraphic succession.

The carbonate competent beds of the Muschelkalk generally overlie the Keuper clays, which in turn may sit on top of other Keuper formations. The K2 unit is also superimposed on other upper Keuper units and the K4-K5 unit usually behaves as well like an independent tectonic subunit. The rest of the lithostratigraphic units usually constitute olistoliths inside a olistostrome. This olistostrome affects the Subbetic Zone and reaches out to the Neogene basin of the Guadalquivir. The olistostrome megabreccia is formed of mainly shales belonging to the K3 unit, in addition to scattered blocks of all sizes that come from the remaining Triassic, and sometimes, post-Triassic units.

5.1 Diapiric movements and resedimentation

Starting from the Cretaceous, and even from the Upper Jurassic, Triassic rocks develop alocinetic phenomena and diapirism, and can extrude shales and evapotrites on the sea bed (e.g. Blumenthal 1931; Fallot 1944; Peyre 1960-62; Dupuy de Lome 1965; Foucault 1966, 1971; Sanz de Galdeano 1973; Comas 1978). In fact, in a number of Subbetic Zone sectors there are intraformational breccias and slumps around the diapiric nuclei. These show some alocinetic phenomena that affect Jurassic and Cretaceous deposits (Sanz de Galdeano 1973; Nieto et al. 1992). Even Triassic, as well as Jurassic rocks, can be seen resedimented during the Neocomian (Sanz de Galdeano 1973).

At numerous points there are also some Triassic masses situated between the Middle and Upper Cretaceous (Leclerc 1971; Foucault 1966, 1971; Cruz-Sanjulian 1974, 1976; Comas 1978). There are Triassic deposits as well, which are resedimented in Upper Cretaceous materials in Subbetic units (Sanz de Galdeano 1973).

The main diapiric extrusion and the formation of olistostromic units took place mostly during the main stage of deformations in the External Zones, which began in the Burdigalian (Hermes 1985; Sanz de Galeano and Vera 1992). Especially over important areas of fractures, and locally through old diapiric nuclei, the exit was caused by the push and disorganization which the Subbetic Zone underwent (moving W or NW) due to the advance of the Internal Zones (Sanz de Galdeano 1990). The latest development of the olistostromic units, involving important resedimentation processes (e.g. Mauthe 1970; Bourgois 1975), was produced in the Langhian-Serravallian (Roldán García 1988; Roldán García and García Cortés 1988), and up to the base of the Tortonian.

5.2 Subbetic olistostromic complex

The continuity or extension of the External Zone tectonic units diminishes towards the Guadalquivir Basin (Pérez-López and Sanz de Galdeano 1994). Numerous small blocks are clearly connected to resedimentation processes, included in the Triassic olistostrome (Fig. 5). This means that the olistostromic character is more accentuated towards the Guadalquivir Basin, given the progressive dismemberment and sliding of the Triassic deposits, and of rocks from other more recent ages which are surrounded by the Triassic materials. These redeposits, from older ages, mix with Miocene autochthonous deposits of the foreland basin (Roldán García 1988; García Cortés et al. 1991), and sometimes form interfingerings. In this respect, extensive outcrops like the Triassic ones that appear in many geological maps, correspond, in fact, to Neogene deposits with resedimented shales and gypsums from Triassic rocks.

All of these data have contributed to the proposal of a model in which the epicontinental Triassic units form an olistostromic complex that can be studied on various scales (Fig. 5). The change is gradual from shale olistostromic masses which contain pebbles and small blocks, to the large Triassic masses which include subbetic materials formed mainly by Jurassic and Cretaceous deposits. Some of these subbetic materials are small blocks, but there are others which have a continuity of dozens of kilometres, and which constitute authentic tectonic units with their own internal structure (nappes). On the regional scale, the entire northern sector of the Subbetic Zone, including the Intermediate and certain Prebetic units, appear as a large olistostromic complex which we have called the Subbetic Olistostromic Complex (Pérez-López and Sanz de Galdeano 1994). In this complex, the Triassic shales are frequently the matrix of the Miocene olistostrome, although they are sometimes simply Triassic olistolith.

This olistostrome of the central sector of the Subbetic Zone is even more developed in the western part, where the Jurassic, Cretaceous and Triassic materials of the Subbetic Zone are very disorganized and appear as olistoliths contained within the olistostrome in which Triassic materials are prevalent.

6. Conclusions

A stratigraphy has been established for the epicontinental Triassic despite the difficulties encountered in the age determination of Triassic rocks in the Betic Cordillera, as well as those which derive from the tectonics. The Ladinian Buntsandstein has been identified in the External Prebetic Zone, although it has not been ruled out that in the Subbetic Zone an Anisian Buntsandstein may outcrop. The Muschelkalk (Majanillos Formation) has only one carbonate section, without an intercalated detrital subunit between the two carbonate sections, as occurs in other regions of the Iberian Peninsula. The age of the Muschelkalk is Anisian-Ladian, but also Carnian, because in the northernmost zone (Prebetic Zone) pollen has been determined as Carnian in the middle part of these carbonates. This is the case due to the fact that these sediments began to deposit on the basin margin at a much later date. The Keuper (Jaen Keuper Group) is the thickest unit and constitutes a group of Carnian-Norian age in which five formations can be distinguished. The lower one (unit K1) is mainly formed by clays and evaporites, although the sandstone beds are sometimes important. The second unit (unit K2) consists mainly of sandstones, whereas the third (unit K3) consists of red clays with gypsum. The last two units are gypsums, one consisting of clay with nodular gypsums (unit K4), and the upper one, of gypsums that are laminated (unit K5). Among the K5 unit gypsums, as well as those clays with gypsums belonging to the Rhaetian unit (Carcelen Anhidrite), a Norian carbonate unit has been distinguished which consists of bedded dolomites and limestones with a very typical red detrital intercalation. This unit can be correlated with others on the Iberian Penisula (Imon Formation, Isabena Formation).

In the Muschelkalk facies one deepening and another shallowing-up sequence have been identified and are separated by a succession of facies which constitute the evolution of the depositional system of the ramp to carbonate platform with lagoon. This succession corresponds to a complete cyclostratigraphic unit which is absent in all of the stratigraphic sections.

The lowest part of the Keuper still belongs to the upper cyclostratigraphic unit of the Muschelkalk in which the marine deposits are replaced by coastal deposits. In general, the Keuper facies is interpreted as fluvial-coastal deposits which finally evolve progressively into very shallow marine deposits. The latter facies clearly define another cyclostratigraphic unit in the middle and upper part of the Keuper.

Above the Keuper are the Zamoranos Formation carbonates of the upper cyclostratigraphic unit. This Formation corresponds to the most important transgressive stage of the Norian.

The Rhaetian is mainly represented by the sulphates of the Carcelen Anhidrite unit which belongs to the cyclostratigraphic unit that developed during the Hettangian and is hardly distinguishable in the Betic Cordillera.

The thickness, as well as the characteristics of the units, are strongly connected to the geodynamic context in which these sediments were deposited. These deposits reflect the expansive character of the basin in relation to the beginning of the rifting stage during the Triassic. This is so because the upper units occupy larger and larger areas within the basin. On the other hand, the variations in the thickness of the units, reflect an increase in the accomodation space due to a strong subsidence controlled by the tectonics. This is particularly the case with respect to substantial thickness of the detrital units K1 and K3 and the evaporitic unit K5, in comparison to other units.

From another vantage point, the tectonics in its most important rifting stage during the Jurassic and the Cretaceous, and in its compressive phase during the alpine orogeny, causes fragmentation and expansion processes in the Triassic rocks. It also brings about diapirism and resedimentation processes in the Mesozoic and especially in the Neogene basin, thus giving way to the Olistostromic Subbetic Complex.

Acknowledgements

This study forms part of the results obtained under Research Project PB97--1201, financed by the DGES-IC, and the Research Group RNM 0163 of de Junta de Andalucía.

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