Correspondence

Trigo Unit and the transitional area between the External and Middle Subbetic is located to the south of these local faults.

There are Triassic materials clearly rooted in the Middle Subbetic overlying the External Subbetic units (e.g. Sierra del Ahillo, Cambil-Cabra de Santo Cristo, Sierra de Gaena). Some of these External Subbetic units appear locally as tectonic windows underneath the Triassic materials, e.g. the Sierra de Orbes and north of Huelma, in the south of Ja6n province.

Finally, the External Subbetic is present only towards the front of the fold-thrust belt, from Montellano (south-east of Seville) until Alicante, along more than 480 km and never over the Middle Subbetic to the south. Why are there not more important remnants of the erosion-resistant Jurassic strata over the Middle Subbetic if the External Subbetic has overthrust it?

In summary, the data presented by Blankenship do not lead to her palaeogeographic interpretation of a single Mesozoic basin bounded to the north and south by platformal margins. The stratigraphy, structural and regional data support the traditional interpretation in which the original Mesozoic basin consisted of one important platform (Prebetic Zone) to the north and a large basin to the south, in which appeared an isolated platform (External Subbetic) between two basins: the Intermediate Units to the north and the Middle Subbetic to the south. This more complex Mesozoic

palaeogeography is similar to that of nearby alpine areas such as the Rift, Alps and Apennines.

Jose M. Molina and Pedro A. Ruiz-Ortiz Departamento de EstratigraHa y Paleontologia,

Facultad de Ciencias Experimentales, Campus Universitario, 23071 Jadn, Spain

References Blankenship, C. L. (1992) Structure and palaeogeography of the

External Betic Cordillera, southern Spain Mar. Petrol GeoL 9, 256-264

Martin-Algarra, A. (1987) Evolucion geoldgica alpina del contacto entre las Zonas Internas y Las Zonas Externas de la Cordillera B~tica PhD Thesis, University of Granada, 1171 pp

Martin-Algarra, A., Ruiz-Ortiz, P. A. and Vera, J. A. (1992) Factors controlling Cretaceous Turbidite deposition in the Betic Cordillera Rev. Soc. GeoL Esp. 5, 53-80

Molina, J. M. (1987) An~lisis de facies del Mesozoico en el Subbetico Externo (provincia de Cdrdoba y Sur de Ja~n) PhD Thesis, University of Granada, 518 pp

Nieto, L. M. (1990) An&lisis de facies del Mesozoico en la transicidn Subbetico Externo-Subb~tico Medio al Sur de la Provincia de Ja6n (Cordilleras Beticas) MSc Thesis, University of Granada

Ruiz-Ortiz, P. A. (1980) Analisis de facies del Mesozoico de las Unidades Intermedias (entre Castril-prov. de Granada y Jaen) PhD Thesis, University of Granada, 272 pp

Ruiz-Ortiz, P. A. (1983) A carbonate submarine fan in a fault controlled basin of the Upper Jurassic, Betic Cordillera, southern Spain Sedimentology 38, 33-48

Sanz de Galdeano, C. (1973) Geologia de la transversal Jaen- Frailes (provincia de Jaen) PhD Thesis, University of Granada, 274 pp

Comments on 'Structure and palaeogeography of the External Betic Cordillera, southern Spain" by C. L. Blankenship

The paper discussed here proposes a new hypothesis about the organization of the External Zones of the Betic Cordillera. These External Zones are made up of the Prebetic and the Subbetic Zones. The Prebetic Zone was situated near the coasts of the Mesozoic sea in the Iberian Massif, and the Subbetic Zone corresponded to a more typical pelagic basin.

The traditional division of the Subbetic accepts several domains that, in the present situation, from north to south, are: (a) the Intermediate Units (IU) between the Prebetic and Subbetic, corresponding to a basinal realm; (b) the External Subbetic (ES), corresponding to a shallow pelagic area; (c) the Median Subbetic (MS), a basinal realm; and (d) the Internal Subbetic (IS), which was in general a shallow pelagic area.

The division of the Subbetic by Blankenship presents two major realms: the 'Basinal Subbetic' ('BS') and the 'Plafformal Subbetic ('PS'). The BS is formed of the IU and the MS, and appears adjacent to the Prebetic. The PS, overthrusting the BS, is formed by the ES and IS which, in this hypothesis, constitutes a single unit. The palaeogeography of the Mesozoic basin of the External Zones is therefore considerably simplified to a basinal

realm and a shallow area situated in a more internal position.

Data on which the hypothesis is based

This division is based on the interpretation of three seismic profiles (BT-8A, 82-32 and 82-31) and on the proposed geological interpretation of a region, south of the Valdepefias de Ja6n, where the existence of an important thrust, or a transitional area between the ES and MS, is discussed.

Seismic profiles Profile BT-8A only cuts the IU and the Guadalquivir units. In addition, an autochthonous or parautoch- thonous basement with a structure of reverse faults appears between numbers 300 and 400 of the profile - - that is, similar to that found in the Cazorla sector of the Prebetic, about 100 km east.

Profile 82-32 mainly crosses IU strata and slightly cuts the ES overthrusting the IU. However, this profile does not touch the southern border of the Valdepefias de Ja6n tectonic window, where another unit of the ES

518 M a r i n e and P e t r o l e u m G e o l o g y , 1993, Vo l 10, O c t o b e r

Correspondence

T r i a s l i c Jura l l l i c C rtit oGeoul Quofernory . . . . . . . . . . . . . . . . . . Str~tigraphRa COntact . . . . . . UncQnfor mJty

T ~ t o n l ¢ ContQ(;t . . . . Normal Faul t ; Diapi~ Contact R l v i r l l Fault

v v T h r u l t ( nappe contact) I ~ ~ l ~ I i Po l i t ion of the Bian~.en.hip' i S u ppoll4~l T h r u l t

~ / ~ it T ran l i t l ona l area b o t w o l n E, and M. Subb41tic i

Figure1 Simplified geological map of the Valdepefias de Ja6n sector. Based on SanzdeGaldeano(1973)

appears (Vent i squero-Sier ra del Trigo unit; Sanz de Galdeano, 1973), which is the centre of the debate on the relations between the ES and MS.

Profile 82-31, mainly located in the Valdepefias de Jadn window, reveals part of the IU and some Triassic and Jurassic rocks of the ES (Ventisquero 'unit '). The interpretation proposed by Blankenship seems to suggest that the ES thins out towards the south, whereas the facts show just the opposite (see our longer B - B ' cross-section of the same profile in Figure 2).

Moreover, this profile does not reach the sector where there is doubt about whether the ES overthrusts the MS or there is a gradual passage from one to the other.

In conclusion, these lines cannot be used to demon- strate the proposed continuity between the IU and the MS, nor the supposed structural position as a klippe of the ES units on the MS in this area. In fact, the structural interpretation proposed by Blankenship for the whole External Zone is not factually supported by seismic evidence.

Marine and Petroleum Geology, 1993, Vol 10, October 519

Correspondence

C

i I / i -- ... \ \ I I " % , ,

k \ \ # Martina ~I~F

0 2km

B B' I Trm~ion~

Voldepe6as de Jean (E S ) vCtjq, Mo~-ales t ) area

W. Pondera | " " / - - - ~ ~ ,,¢,1 At

Jurassic I > L I N E 8 2 - 31 < I

C p C Valley at 1 Transitional area

Valdepefias de '~' Bo~eta Oiapir ( M . S ) daen W. Ventisquero

Jurassic

(LLO

[•]Lim. FII~Z~E~ Limest°nes ~ 4 - - - A p t i a n - Albian

V / I r-- - - - -4 Mor's Triassic Dogger- M a i m Cretaceous

Liassic

I I Jurassic

( I .U . ) = I n t e r m e d i a t e U n i t s , ( E . S . ) = E x t e r r ~ l S u b . , (M .S . ) = M e d i a n S u b .

,~, P o s i t i o n o f t h e B l a n k e n s h i p ' s T h r u s t ( E . S . a b o v e M . S . )

Figure 2 Geological cross-sections of the Valdepef ias de Ja@n area. Cross-section B - B ' is superposed to the line 82-31, but it is not the best to show the structure near Va ldepefas. The posi t ion of these cross-sections is shown in Figure 1

Region south of Valdepe~as de Ja#n In this sector, Figure 3 of the paper under discussion shows a southward thrust that would separate the ES (Ventisquero 'unit') from the MS (Sierra del Trigo). The caption to this figure indicates that it is based on several papers, but this continuous thrust is not shown in any of them (one of the citations corresponds to an area totally different to that of Figure 3). Only Sanz de Galdeano (1973) and Molina C~mara (1987) show several reverse faults, coinciding with the position of the thrust mentioned in Figure 3, but not a continuous thrust. These are several reverse faults verging south

that disappear laterally, thus allowing observation at several points of the southward continuation of the Ventisquero stratigraphic series (see our Figures 1 and 2). This so-called continuous thrust is an interpretation by Blankenship that, in our opinion, is clearly contra- dictory to what can be observed in the field.

In addition, the boundary between the ES and MS series is located further south. Thus, directly south of the so-called thrust, there are still outcrops of condensed Jurassic deposits which do not belong to the MS but to the ES. These are series similar to those of the Ventisquero 'unit'. Even further south (see our

520 Marine and Petroleum Geology, 1993, Vol 10, October

Figures 1 and 2) the Jurassic limestone series begins to intercalate marls in the Upper Liassic and increasingly abundant limestone and marly limestone beds with flint nodules. Close to this area radiolarites appear (all of the foregoing in continuous outcrop) and the MS then begins.

This southward transition of the so-called thrust has already been indicated in previous papers, and is even referred to in the cross-section at the top of Foldout 1 in the paper under discussion, which reproduces a figure by Garcia Hernfindez et al. (1980), where a zigzag line to the right of the word 'Ventisquero' indicates the transition.

Two comments must be made regarding the Aptian- Albian strata located immediately south of the Ventis- quero 'unit':

1. Blankenship has given her thrust an ad hoc curve to leave these materials further south, which is a forced interpretation (see our Figures 1 and 2, cross-section C-C' ) . But, even so, there is also another outcrop indicated with an asterisk in Figure 1 which, in any case, is located north of the so-called thrust.

2. The question is conclusively settled by the fact that the Aptian-Albian sands and shales of the Morales and Carboneros farms, which Blankenship locates in the MS, are in fact in the ES to the south of the so-called thrust (see our Figures 1 and 2). They cannot therefore be used to correlate the MS with the IU (further south, outside the area studied, Aptian-Albian strata do appear in the MS). A study of vitrinite reflectance, proposed as evidence towards the end of Blankenship's paper, could be used to compare IU sediments with those of the ES, which at many points reveal a complicated structure and, moreover, underwent overthrusting by important Triassic masses.

A similar discussion could be held about the Neocomian, which in this sector south of the Valdepefias presents important phenomena of intra- formational breccias, slumping, diapiric intrusions, all common to the ES and the MS. The same situation is found in the Upper Cretaceous, which is the same in the ES and the MS but different to that of the IU.

Cor respondence

It should finally be pointed out that this transition between the ES and MS is also found in other transverses, particularly about 50 km to the west.

Conclusions

The tectonic and stratigraphic data from the area south of Valdepefias de Ja6n totally uphold the rapid but gradual (not abrupt) transition from ES to MS. Although the hypothesis of Blankenship could be tempting in its simplification of the basin, the highly diverse geological data require a consideration of the Betic External Zones as being formed grosso modo by a platform realm for the incompletely homogeneous Prebetic, a trough domain for the IU, a more complex shallow pelagic area for the ES, a similarly complex basin, with intercalated volcanic rocks, corresponding to the MS and a highly complex shallow pelagic area, also with volcanic rocks intercalated in the Jurassic and Neocomian sediments, corresponding to the IS.

Carlos Sanz de Galdeano, Agustin Martin Algarra, Juan Antonio Vera and Pascual Rivas

Instituto Andaluz de Geologia Mediterr4nea, (CSIC- Univ. Granada), Fac. Ciencias,

18071 Granada, Spain

References Blankenship, C. L. (1992) Structure and palaeogeography of the

External Betic Cordillera, southern Spain Mar. Petrol GeoL 9, 256-264

Garcia-Hern~ndez, M., L6pez-Garrido, A. C., Rivas, P., Sanz de Galdeano, C. and Vera, J. A. (1990) Mesozoic palaeo- geographic evolution of the External Zones of the Betic Cordillera GeoL Mijnb. 59, 155-168

Molina C~mara, J. M. (1987) An&lisis de facies del Mesozoico en el Subb6tico Externo (Provincia de C6rdoba y Sur de Ja~n) Thesis, University of Granada, 518 pp

Rivas, P., Sanz de Galdeano, C. and Vera, J. A. (1979) Itinerarios geol6gicos en las Zonas Externas de las Cordilleras Beticas: Granada-Jaen y Cabra-Loja, Publ. University of Granada, 86 pp

Sanz de Galdeano, C. (1973) Geologia de la transversal Jaen- Frailes (provincia de Ja~n) Thesis, University of Granada, 274 pp

Reply to comments on 'Structure and palaeogeography of the External Betic Cordillera, southern Spain' by Sanz de Galdeano et al. and Molina and Ruiz-Ortiz

Both sets of authors state that I have misinterpreted the previous work carried out in the area (what I refer to as 'traditional stratigraphy'). Specifically, they claim that the southern boundary of the External Subbetic Unit is further south than shown in Figure 1 of the subject paper, and that it is not coincident with a major tectonic element. The nature of this boundary is crucial to my proposed cross-section, and thus the palaeo- geographical model. My interpretation of the External

Subbetic Unit as a klippe was based on the hypothesis that the structural style of the Intermediate Unit observed in key seismic profiles (82-31 and 82-32) could be carried further south in the folded belt. In addition, this structural interpretation results in an attractive simplification of an otherwise complex palaeogeography. The balanced cross-section which I constructed to test the feasibility of this hypothesis represents the first published attempt to constrain the structural inter-

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