A Geological Itinerary through the

Spanish Pyrenees(Written for the Summer Field Meeting, September 1961)

by J. M. RIGS

Received 26 June 1961

1. INTRODUCTION

GEOGRAPHICALLY the 'Pyrenees' denotes a mountain chain which extendsfrom east to west, and separating Spain from France. The frontierapproximately coincides with the line of the watershed. The SpanishPyrenees are much more complex topographically than the French side,being wider and the descent from the axial line to the plains being com­pleted more gradually across a series of parallel mountain chains. TheFrench Pyrenees are much narrower, and consequently more precipitous.

The area sharing the same fundamental geological history with thePyrenees, both in sedimentation and tectonics, extends much farther to thewest, always with the same mountainous character, although gentler in

QSCHEMATIC STRUCTURAL

~~ MAP Of SPAIN-# a Area. of Utt'o or no~ 1I0.t~ercynlan s.dimentation

~~ and foldln9

, k::::':1 Area. of 1I0st.hercynianI.dimentatlon and Ailline'0'din9

9 I 190, 300Km•

Fig. I. Geological boundaries in Portugal omitted

359

360 J. M. RIOS

some parts than in the Pyrenees proper. In this sense the Pyrenees extendwell into the Asturian region along the Bay of Biscay, and include most ofthe ranges known as the Cantabrian Mountains.

The party will travel through the Pyrenees from east to west, just reach­ing the eastern part of the Cantabrian Mountains but leaving out theeasternmost end of the chain. The route will cut across the different ele­ments, from the Ebro Valley to the basement, several times. The structureof the chain is varied, complex but very well exposed. It is possible tofollow clearly the change from simple to complicated series and back againto simple ones, and finally to see them develop into a very thick andcomplex succession. Most of the important changes affect the Cretaceousformations, and to a lesser degree the Eocene system. The route will alsoenable the tectonic evolution to be demonstrated.

Intense exploration for oil started recently and is yielding a rapid in­crease in our knowledge. Apart from the fact that many of the results arekept unpublished and that a detailed analysis would be too long, thefollowing account aims to be only a general unified summary, uncom­plicated by new discoveries still imperfectly digested.

2. GEOLOGICAL mSTORY OF THE IBERIAN PENINSULA

(a) Pre-Hercynian History

The pre-Hercynian history of the Peninsula appears to have beenrelatively uniform. A regime of marine sedimentation was dominant andon the whole fairly continuous from the Cambrian until well into theUpper Carboniferous.

Our present limited knowledge suggests that the Hercynian foldingphases affected the whole of the area occupied by the present Peninsula ina similar manner. The Peninsula was during these times part of a muchmore extensive palaeo-geographical unit, an ancient continent which laterwas broken up.

(b) Post-Hercynian History

Following the close of the Hercynian orogeny at the end of the Carbon­iferous, the Peninsular Region became divided into two regions whosesubsequent history was essentially different. The western region, boundedto the south by the Guadalquivir fault (or flexure) which originated at thistime, and to the east by assumed flexures (hidden today under more recentsediments), shows from then on a continental development except for afew, short-lived marine invasions (mainly the Cretaceous transgression).The southern (south of the Guadalquivir fault) and eastern region (east ofthe flexures) began to function again as a geosyncline, and became the siteof more or less thick, sometimes very thick, deposits, either of continentalor marine character.

THE SPANISH PYRENEES 361

The complicated problems of the Betic Chains (to the south-east of theGuadalquivir fault) will not be considered here; it will suffice to say thattheir sedimentary development is varied and complex, although notessentially different from the rest of the peninsular areas of Alpine sedi­mentation. But their orogenic history is more prolonged (the folding lastswell into the Miocene) and also more violent. Among the Alpine foldsystems in Spain, the Betic are by far the most complicated, even reachinga true Alpine tectonic category.

3. THE GEOLOGICAL SETTING OF THE PYRENEES

The Pyrenees can be defined geologically as those regions which make upthe Pyrenaic orogen-that is the regions which, after the Hercynian fold­ing, had the same geosynclinal history, and which together were subjectedto a more or less uniform style of folding during the Alpine orogeny,building a continuous mountain chain. These folded sediments range fromPermian to late Oligocene and rest on the Hercynian basement which isexposed almost continuously in the axial zone. This chain forms thePyrenees proper and also its direct prolongations which share the sametrends and display the same type of general structure.

The geological history of the Pyrenees is also shared to a varying degreeby other chains (Iberian and Catalonian Coastal Ranges), but they are nottrue extensions but separate branches. The sediments of the Alpine geo­syncline were deposited over a much wider area than the Pyrenees proper,extending to the south, for example, as far as the Ebro Valley where theyare covered by Oligocene-Miocene sediments. The structure and history ofthe Pyrenees are tied up closely with that of the Ebro Valley so that onecannot separately study both regions.

4. GENERAL GEOGRAPHICAL AND GEOLOGICAL CHARACTER

OF THE EBRO VALLEY

(a) Geographical Description

The Ebro Valley, in its present geographical expression, occupies thenorth-east corner of the Iberian Peninsula (Fig. 1). It has a triangular shapeand is confined on the north by the Pyrenean chains along the French­Spanish border, on the east by the Catalonian Coastal Ranges, and on thesouth-west by the Iberian Mountain Ranges.

(b) Geological Description

The triangle formed by the Ebro Basin proper corresponds to a deepdepression filled by Eocene and Oligocene sediments which, depending ontheir more marginal or their more central location within the depression,

362 J. M. RIOS

appear today highly, little, or not at all folded, and are partially covered bya relatively thin sheet of Miocene deposits.

The mountain ranges that frame the basin are essentially built up by amore or less complete Mesozoic sedimentary sequence which is folded, andthrough which the Palaeozoic basement locally appears.

The Eocene-Oligocene formations, before they were largely removed byerosion, also spread over part of the area occupied by the framing moun­tain ranges and some isolated outcrops are still preserved. Similarly, thereare some ramifications of the marginal Mesozoic chains of mountainswhich outcrop among the Eocene-Oligocene sediments of the Ebro Valley.

The three marginal chains are more or less of the same age, and origin­ated during the Alpine phases which took place from late Eocene until theend of the Oligocene. The Pyrenean Cordillera is the most important andits tectonic category varies between that of the more violent Jurassic foldmovements and that of the more moderate Alpine types; the Iberian Rangeis of an intermediate category and corresponds to the typical Jurassicfolding; the Catalonian Coastal Range displays more moderate foldingand in its constitution vertical movements played a more important partthan folding.

The Palaeozoic basement displays intense folding which took placeduring the Hercynian orogeny in Upper Carboniferous time.

Both in the Pyrenean and Catalonian Coastal Chains the Palaeozoicoutcrops contain granitic intrusions which correspond to synorogenicHercynian magmatic phases. Their regional metamorphic influence issmall, at least compared with the metamorphic effects of the coeval graniticintrusions in the western half of the Peninsula which were very importantboth in intensity and extent in the later geological history of the Peninsula.

(c) Geological History of the Ebro Valley

(i) Pre-Hercynian History and Hercynian Intrusions. The present know­ledge of the sedimentary and tectonic pre-Hercynian and Hercyniandevelopment of this region is both imperfect and incomplete. It has beenbuilt up as a synthesis of a series of observations from outcrops which areoften discontinuous and distant from one another. All our knowledgepoints to it being essentially similar to that of the western half of thePeninsula. What, then, might be the reason for the difference in the post­Hercynian development?

Most probably it lies in the fact that the Hercynian orogeny in theWestern Zone developed a very intense magmatic activity, predominantlyof an acid granitic type, which caused a very intensive and extensive meta­morphism. As a consequence the Western Zone became stabilised, in­capable of further folding and tectonically dead, except in a regime offracturing.

THE SPANISH PYRENEES 363

In the Eastern Zone, on the contrary, the magmatic phenomena musthave been much less intense. Among the mountain ranges which frame theEbro Valley, the Pyrenees show granitic Hercynian intrusions (Figs. 2 and3) but their extent compared with that in the area occupied by the pre­Hercynian formations in the Western Zone of the Peninsula is muchsmaller and their metamorphic action much less.

The Catalonian Coastal Chain shows a more important Hercynianmagmatic activity. The Iberian chains do not show any granitic outcrops,but it should be remembered that, generally speaking, the Mesozoic andCaenozoic cover is much more complete, the erosion surfaces here beingat a higher stratigraphic level than in the Pyrenees.

(ii) Post-Hercynian Mobility. The conclusion that the Hercynian graniticintrusions were much less important in the eastern half than in the westernhalf of the Peninsula, is more due to inference than direct observation. It isbased on the fact that the eastern part of the Peninsula did not becomeimmobilised as did the western one. On the contrary, it was able to functionanew as an orogenic area, with prolonged and deep subsidence, theaccumulation of new and sometimes very thick marine and continentaldeposits, and, finally, the creation of important systems of folding. It didnot acquire the rigidity, which in the western part is due to strong andextensive metamorphism and crystallisation.

MONTSECH AREA

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364 J. M. RIOS

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(iii) Triassic-Jurassic History (Uniformity). If we examine the characterof the post-Hercynian formations which fill the Ebro Valley a salientfact strikes us: the sediments are patchy and variable, particularly abovethe base of the Cretaceous.

After the Hercynian folding was completed and until the end of Jurassic,the sediments of the Permian, Trias and Jurassic are uniform to a certaindegree! in their respective lithologies, and somewhat less so in the thicknessof their units which is more variable from place to place. All throughthis period there was relative uniformity in the depositional basins andno great regional variations.

(iv) Cretaceous History (Mobility and Variety). From the late Jurassicuntil the Middle Cretaceous, the depositional basins acquired a greatmobility, although the pattern was very variable. Some areas developedas great deeps and very thick and continuous sedimentary series weredeposited; others emerged and a greater or lesser part of their previousdeposits fell prey to erosion; others oscillated vertically which gave riseto a very irregular and discontinuous series.

1 To a certain degree because compared to conditions in other areas, for example in Africaand America. all geologic conditions are extremely variable in Spain. This country is extraordinarilyrich in quantity and variety of stratigraphic and tectonic situations, as a consequence of th e rapidityof changes, Spain , in great areas, is a geological mosaic.

THE SPANISH PYRENEES 365

The stratigraphy of the Upper Jurassic-Middle Cretaceous is extremelyvaried and complex. Some series are complete and entirely marine; someare continental sediments, occasionally interrupted by one or more marineepisodes. In other areas this was a period of erosion and a greater or lesserpart of the Jurassic, and even Triassic and Permo-Triassic rocks dis­appeared, and the Palaeozoic basement was exposed.

In addition to the mobility of the basins from the end of the Jurassiconward and the variety in character of the sediments, there were also dis­cordances and even the formation of basal conglomerates-all phenomenasuggesting a mild phase of folding at this time.

It is difficult to discern whether these movements were due to verticaldisplacements of blocks within the Palaeozoic basement, or to its tectonicre-formation in ample folds; or perhaps, as a final cause, to deep magmaticmovements. But it is a fact that after the end of the Jurassic all the area ofthe present Ebro Valley and marginal chains experienced a deep structuralmodification. The previous, rather uniform bottom became divided intocompartments by a series of elevations separating small local deeps. Theseelevations of the basement, known in the Spanish geological literature asthe 'Macizo Aragones', 'Macizo del Ampurdan', etc. ('Ebro Masse','Ampurdan Schwelle', etc., of the German geologists who first showed theirexistence), received very sparse and fragmentary sedimentation or none atall. The deeps received thick sedimentary deposits in complete and con­tinuous series. There were also zones of intermediate characteristics.

(v) Regions of deposition of thick and continuous Cretaceous sedimenta­tion. As can be seen in the map (Fig. 1), the special regions characterised bythe accumulation of thick sediments are distributed at present along themargins of the Ebro Valley and are a part of the Sierras which frame thevalley. These areas are as follows:

(a) Montsech Area is characterised above all by the extreme differencewhich exists between the complexity of its almost complete stratigraphicsequences and the simplicity of the neighbouring ones to the east, southand west where many of the members are lacking, and also by the greatdifference in thicknesses between formations within and outside the area.There is important diapirism all around the margins, especially on thesouth side. The diapiric material belongs to the Keuper. An unknownnumber of diapiric manifestations are hidden by Oligocene sediments.

(b) Fosa Cantabrica is characterised by the exceptional accumulation ofCretaceous sediments (both Lower and Upper) which contrasts with theneighbouring areas, especially to the east. This area is also characterised byits intense diapirism-the most important on Spanish soil and most of it ofthe salt dome type.

(c) Sierra de la Demanda is characterised by the thickness of its LowerCretaceous with respect to that of surrounding zones, and also by its

366 J. M. RIOS

continental character. The Keuper around the area is hardly diapiricat all.

(d) Maestrazgo Area has more or less the same characteristics but, in theLower Cretaceous, marine facies are represented along with the continentalones.

(e) Montserrat. This area is not a part of the marginal chains but belongsto the Ebro Valley itself, and typifies some of the facies changes whichoccur there. It is altogether different and much less important than theprevious areas, both in extent and in thicknesses involved. It is a typicalexample of variability in the Palaeocene inside the Ebro Valley proper,otherwise so monotonous in all its characteristics. There are many moreareas of the same character along the inner border of the marginal chains.

Of these regions only (a) and (b) will be included in our visit.(vi) Geological History 0/ Post-Cretaceous Time. The deposits of the

Albian already show a trend to a greater uniformity of character in thesediments of most regions. Immediately afterwards there was a greatCretaceous transgression which began in the Cenomanian and persisteduntil the end of the Senonian. The Upper Cretaceous sediments are ratheruniform in their lithological character in all the marginal areas of the EbroValley, but less so in their thicknesses.

The transition from Cretaceous to Eocene was marked by a generalwithdrawal of the seas which became permanent for the lower or southernhalf of the Ebro Valley, south of a line which constitutes the median lineof the triangle going from the north-west corner to the middle point of theCatalonian Coastal Range. North of this line the sedimentation soonreverted to marine conditions, whereas to the south of it the continentalregime persisted. The thickness of the Eocene sediments is great in bothfacies, and also variable, especially in the marine examples.

At the end of the Eocene began the great Alpine folding which was tocreate the mountain chains which today frame the Ebro Valley. The rise ofthe Catalonian Coastal Range isolated the Ebro Valley from now on fromall communication with the open Mediterranean Sea. The Ebro Valley hadjust been born.

All the framing mountain chains completed their orogenic developmentduring Oligocene times and enclosed inside their walls the Ebro Basin,which filledup gradually with considerable, though variable, thicknesses ofOligocene lacustrine deposits. With the Oligocene the orogenic history ofthe Ebro Valley and marginal chains drew to its end. After the end of theOligocene there was mass lifting of the marginal mountain ranges relativeto the deposits that fill up the Ebro Valley. The Pyrenees have risen as achain of mountains, and this newly created range stimulated erosion.

It is important to focus attention on the characteristics of these twoareas, the intermediate areas being much more simple. Both of them will be

THE SPANISH PYRENEES 367

visited and their understanding will be made easier and more complete iftheir general outlines are previously known.

THE AREAS TO BE VISITED

(i) Their Importance in the Search for Oil. Areas (a), (b), (c) and (d),because of their greater thicknesses and more continuous sedimentaryhistory, constitute distinctive zones within the general sedimentary settingof the Ebro Valley. They show lateral changes in lithology, pinchouts ofstrata, marginal facies developments, and a greater abundance of sand­stones and they are the zones with the most hopeful prospects for oil withinthe region as a whole. Some of them, for example (a), contain importantlimestone reefs. Others, for example areas (c) and (d), contain sand andsandstone beds of very good porosity in the Albian stage. All of them showmore or less abundant oil seepages, particularly areas (a) and (b). Oilprospectors will look with special interest at these areas and at their con­tacts with neighbouring districts. This does not mean, however, that therest of the Ebro Valley is excluded from oil possibilities; it means only thatthese areas are the outstanding ones.

A relationship of cause and effect is shown in the existence of thesespecial areas and in the concentration of the plastic and saliferous materialsof the Keuper at their margins. This concentration must be attributed tothe gravitational pressure due to the accumulation in them of greaterthicknesses of sediments. The materials of the Keuper escaped from under­neath in search of areas of lesser gravitational pressure, and migratedgradually towards the marginal zones. The manner in which the Keuperfound its way to the surface, and the time, continuity or lack of continuity,of the upward movement depended on the particular characteristics ofeach area. In Area (a), the Keuper broke out and flowed over the surfacewherever the sedimentary cover was very weak. The upward movementseems to have been reinforced by, and restricted to the period of, theAlpine Movements. In Area (b), the upward movement started when thegreat accumulation of Lower Cretaceous sediments was completed andpersisted with variable intensity until the end of the Oligocene. The saltsand clays of the Keuper ascended in a series of very deeply rooted domesthrough great thicknesses of sediment. The movement was much moreindependent of tectonics than in the first area. The domes developedhaphazardly on monoclines, on synclines, or on anticlines, and, withinthese, either on the culminations or on the flanks. The upward movementwas predominantly gravitational. Areas (c) and (d) are much less wellknown from these viewpoints. The salt tectonics, if any, are much moresubdued.

This diapiric interplay also has its interest and importance in oil pros­pecting. The diapiric materials may form barriers to migration, and thus

368 J. M. RIDS

exert an influence on the formation or displacement of oil accumulations.On the other hand, the diapirs may be, in themselves, petroliferous struc­tures, particularly the salt domes.

(ii) The Montsech Area (a). The sedimentary evolution along this de­pression is clearly shown in the cross-sections of Fig. 4. Although thesesections have been derived from different sources their legends have beenunified. The thicknesses cannot be exactly compared since cross-sections I,3, 4 and 7 are only schematic. Nevertheless, all of them have been adaptedmore or less to the same scale.

Section I shows the sedimentary situation immediately to the east andoutside the depression. The series includes only a very thin representativeof Upper Cretaceous lying between the Eocene and the Keuper. TheMesozoic strata are extremely schematic.

The Upper Cretaceous is present in all cross-sections, but the thicknessand stratigraphic complexity increase to the west, and diminish to thesouth and south-west.

Section 8, similar to section I, shows a very schematic sedimentarysequence made up of only a very thin part of the Upper Cretaceous lyingbetween the Eocene and the Keuper.

CROSS-SECTIONS OF MONTSECH AREA

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Fig. 4

THE SPANISH PYRENEES 369

The Lower Cretaceous makes its appearance between section 2, whereit is still absent, and section 3. It reaches its maximum development insection 4. The thickness and complexity is already smaller in sections 5 and6, and following the same trend as the Upper Cretaceous, the LowerCretaceous wedges out towards the south. Section 7 corresponds to thelast outcrops of the Lower Cretaceous in the west; it is completely absentin section 8.

The Lias is present, but very thin, in the zone traversed by section 2. Itreaches a normal development in 4 to 7 and also wedges out to the southand west.

From all these observations we infer that a special zone of sedimentationwas built up during the Lower Cretaceous (Fig. 2) as a deep depressionwith steep slopes giving rise to a thick accumulation of marine deposits.This basin was limited to the south by a continental mass or shelf ('Macizodel Ebro') and to the east and west by two salients ('Umbral del Ampurdan'and 'Umbral Alto Aragones'). Communication with the open seas musthave been on the northern side, over the not yet existing Pyrenees. Theseshelves must have been created at the end of the Jurassic and erosion musthave abraded the Jurassic sediments previously deposited on them.

The existence of these massifs is, so far, purely speculative and aninference from the analysis of the cross-sections and facies, but the geo­physical studies and the few wells drilled to date in the Ebro Valley tend toconfirm their existence.

The analysis of the character and thicknesses of the sediments which fillthe depression shows a migration of the line of greatest depths towards thesouth from the Lower Cretaceous onwards.

The down-sinking of the massifs or shelves began gradually immediatelyafter the Lower Cretaceous. The most durable and persistent of them allwas the Ebro Massif as all the sediments of the Upper Cretaceous thin, orperhaps wedge out completely, in a southerly direction.

The greater weight of the sediments on the centre of the depression musthave expelled the Keuper laterally from underneath and it accumulatedtowards the south where the sedimentary cover was much weaker. At thebeginning of the Alpine folding, and with its help, the Keuper must havebroken easily through the thin cover of sediments, favouring the foldsoverthrusts, and an extensive though not very intensive diapirism tookplace.

(iii) The Cantabrian Trough (b) (Fig. 4). The evolution of the sedimentsin this depression is clearly shown in cross-sections 1 to 5 in Fig. 5. Theyare all of them directly comparable, with all vertical and horizontaldistances drawn to the same scale.

Cross-section 1, the most easterly, shows how the Mesozoic sequence inthe region immediately to the east is made up of only a thin Upper Cre-

370 J. M. RIOS

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taceous representative which lies directly over the Trias and under theEocene. Section 1 is similar (although the Upper Cretaceous sequence ismore complete) to the ones on both sides of the Montsech Area (sections Iand 8 in Fig. 4), and corresponds to the general stratigraphic sequence ofthe Pyrenees to the east and south of the Montsechs, and between theMontsechs and the Fosa Cantabrica.

In section 2 (Fig. 5) not only is the Upper Cretaceous much thicker thanin section I, but there is already a thick sequence of Lower Cretaceous andin addition the Jurassic (mostly Lias) makes its appearance between theLower Cretaceous and the Trias. Both of them, Lias and Lower Cre­taceous, make their appearance immediately to the west of section 1, withthin sequences at the beginning, but rapidly increasing, until the LowerCretaceous soon exceeds 1500-2000 m.

Section 3 shows the zone of maximum development of the LowerCretaceous.

In section 4, a reduction of thickness in the Lower Cretaceous approach­ing normal regional values can be observed. The Upper Cretaceous, on thecontrary, reaches its maximum stratigraphic complexity, which is verygreat, and also its maximum thickness.

THE SPANISH PYRENEES 371

Section 5 shows a reduction of the thicknesses of both Lower and UpperCretaceous to normal regional values.

Fig. 3 shows the position of the deep depression which corresponds tothe thickest Lower Cretaceous section.

As happens all along the Pyrenees, the axis of the depression migratesfrom its original position during the Lower Cretaceous towards the south.We also observe the disposition of the salt-domes along a line parallel tothe margin of the depression but well inside its borders. These domes, ascan be seen in Sections 3 and 4 of Fig. 5, are very deeply seated.

The upward movement of the Keuper is essentially due to gravitationalforces and was started, once the thick deposits of the Lower Cretaceouswere laid down, at the same time as the axis of the sedimentary depressionmigrated towards the south, corresponding with the thickening of theUpper Cretaceous sediments in this direction.

The salt-domes originate along the line of very rapid thinning of LowerCretaceous sediments to the south and east. This line must perforce corres­pond to a big fault, or rather a series of parallel faults en echelon, in thePalaeozoic basement underneath.

The upward movement persisted during the Eocene and Oligocene.Some of the salt-domes outcrop today in Oligocene areas; some in areas ofthe Upper or Lower Cretaceous. The location of the diapirs is independentof fold tectonics.

EDITOR'S NOTE. This account was prepared and distributed to those membersattending the Field Meeting. The Report of the Meeting will be published inPart 4 of this Volume.