Tectonophysics 282 (1977) 303-329


Martínez A.(1), Rivero L.(2) and Casas A. (2)

(1)Geological Consultant. Enamorats 132, 2on 1a, 08026-Barcelona

(2) Departament Geoquímica, Petrologia i Prospecció Geològica. Universitat de Barcelona. c/ Martí i

Franqués s/n, 08071-Barcelona

Received after reviewers: July 1996, accepted 13 May 1997



Combined surface, subsurface and gravity data, were used to determine the deep structure of the thrust belt in the southeaster Pyrenees. The Ripollés and La Garrotxa area is a part of the oil exploration Cardona permit. Hydrocarbons pervade the concession with numerous oil seeps, abandoned oil mines, and gas or condensate shows in the two wells associated with the overthrust belt. The source rock is the Armàncies Fm., (Upper Cuisian-Lower Lutetian) consist of alternating layers of organic-rich shales and thin limestones, this formation outcrops in the Cadí Thrust Sheet. In the Ripollés area, the Cadí unit overlies the Serrat unit with a hanging wall ramp geometry, deduced from the Serrat-1 well. The structure of the La Garrotxa area is more complex, consists of a duplexs foldeds by antiformal stacks structures with basement rocks in their cores. The north boundary of these antiformal stacks represents an inversion of pre-existent extensional faults. The basement rocks involved in the structure represent short-cuts formed in the footwall of the extensional faults. A negative anomaly is showed in theses antiformals stacks. These anomalies are interpreted as a combination of salt accumulation, evaporites an sedimentary rocks, under the antiformal stacks. In the Serrat-1 well, the Serrat Unit is made up of a thick Middle Eocene evaporitic sequence. But the gravity and seismic data evidence the presence of the non evaporitic sedimentary rocks in the northern part of the Serrat Unit. These sediments corresponds to the footwall Cadí serie, so the probably existence of the Armàncies Fm. in the Serrat Unit is a very good target for a oil prospecting. The existence of the previous extensional faults, only in the La Garrotxa area, determine the difference in the deep structure between the Ripollés and La Garrotxa areas.



The many surface indications of petroleum and the existence of a good source rock (Armàncies formation) have interested oil companies in prospecting the south-eastern Pyrenees. Nevertheless, duplexes and imbricated thrust systems form some of the most complex hydrocarbon traps in overthrust belts. The structure in the south-eastern Pyrenees, especially in the La Garrotxa area, is very complicated and its interpretation poses many problems. The causes are: frequent facies changes, incorporation of the foreland basins into the thrust sheets, and the presence of non-outcropping lower units. The main unit (Cadí Thrust Sheet) was defined in internal reports of Union Texas España Inc., the most recent oil company to operate in this area. The company used geophysical data and boreholes, because there was not sufficient surface exposure. Problems that hinder the subsurface interpretation of this area: include the low seismic coverage, its poor resolution, the existence of only two boreholes in the allochthonous units, and the secrecy imposed by the oil industry. Now the oil companies have given their licenses up and we are able to present this information.

The main aim of this study is to give an interpretation of the gravity lows observed in the La Garrotxa area and check a possible northern continuation of sedimentary materials covered under basement thrust sheets. This is important because these anomalies are mainly located over structures with Paleozoic core materials, and many sedimentary rocks in the footwall, which probably would be a good target for new oil exploration. An important question for the companies operating in the area is whether there are sedimentary rocks below these anticlines. For this purpose the Bestrecà-1 borehole was drilled but it stopped before it reached the reflectors observed in the seismic lines.



The south-eastern sector of the Pyrenees consists of a stack of south-vergent units including both sedimentary cover and basement (Fig. 1). These units overthusted the last foreland basin (Ebro basin) which consists of Lower Eocene-Oligocene sediments. Two main thrust systems can be distinguished (Muñoz et al. 1986): the upper units with Mesozoic sediments and a smaller Lower Eocene series, which detachment level is the Keuper, and the lower units contain basement and sedimentary cover. In the area of this work, only the lower units are found. The most important is the Cadí Thrust Sheet, made up of a very thick Lower-Middle Eocene series and Paleocene sediments discordant on the basement (Puigdefábregas et al. 1986; Vergés & Martínez 1988; Clavell et al. 1988). Toward the east (outside the study area) Mesozoic limestones of the upper units outcrop (Bac Grillera and Empordà thrust sheets). The upper units began to overthrust during the Upper Cretaceous and the lower ones reached their position during Oligocene times (Vergés, 1993).

Taking into account differences in their geological structure, the studied area can be divided in two sub-areas: The Ripollès and La Garrotxa areas. The limit between them is located between Sant Pau de Seguries and the Oix fault. The Cadí Thrust Sheet outcrops in both areas.

In the Ripollès area the Cadí Thrust Sheet is folded in a E-W trending syncline (Ripoll Syncline). In the northernmost part of the zone basement outcrops in a culmination constituted by a stack of basement and Paleocene thrust sheets, so-called Freser Antiformal Stack, (Muñoz et al. 1986), limited by the Ribes-Camprodon Thrust.

La Garrotxa area is characterized by the existence of many antiformal stacks trending E-W, with basement rocks in their core and some Ypresian limestone levels in duplex structures. North and south flanks of La Garrotxa antiformal stacks are asymmetric. The south flank is build by many duplex structures constituted by Ypresian limestones trending to the south. The north flank is constituted by thick series of the same age but with marl facies. The contact between north and south flank is a substractive fault. During the compression this fault was inverted overlaying marl sediments over platform limestones (Martínez et al., 1989). These normal faults were active since Lower Ypresian time.



In the study area both basement and sedimentary cover outcrop. The sedimentary materials form a series of more than 2000 m: Eocene rocks in the Cadí Unit and Eocene-Oligocene in the Ebro Basin. The main lithologic units recognized in this area are (Fig. 3):

Basement: Limestones, marbles and slates of the Upper Paleozoic prevail in the Freser antiformal stack. Granites and Lower Paleozoic slates (Silurian, Devonian and Carboniferous) are found in the antiformal stack cores of La Garrotxa. To the north of the Ribes-Camprodón Thrust Lower Paleozoic (Cambro-Ordovician) metamorphic rocks are exposed.


Garumnian Facies: Continental materials from the Upper Cretaceous to Paleocene ages. The lower part consists of red clay sediments; the upper one is a lacustrian series.



Cadí and Sagnari formations (Mey et al. 1968, Pallí 1972): Platform limestones and other distal facies (marls and limestone-marls) from the Lower Ypresian age. In the La Garrotxa antiformal stacks limestone duplexes of the Cadí formation are found. Toward the north there are the transition facies and Sagnari marls. The platform limestones drilled by boreholes in the Ebro Basin were defined as Perafita Fm. of Upper Ypresian age.

Corones formation (Estévez 1973): This is a group of two limestone layers separated by a lower Upper Ypresian detritic continental layer (Giménez 1993). It belongs to the La Garrotxa antiformal stacks.

Armàncies formation (Pallí 1972): Grey marls (Middle Upper Ypresian-Lower Lutetian) with abundant organic material, limestone-marl and marl alternations. This is the best source rock in the southern Pyrenees (Permanyer et al. 1988). In the eastern area of the Pyrenees (outside the study area) this formation changes to platform limestones (Penya formation).


Campdevànol and Vallfogona formations (Pallí 1972, Gich 1969): Turbiditic Lower Lutetian formations. The Campdevànol formation outcrops on the northern flank of the Ripoll Syncline and the Vallfogona formation on the southern flank. The Vallfogona formation differs from the Campdevànol formation in that the former has evaporite layers.

Beuda gypsum (Pallí 1972) and Serrat evaporites (Martínez et al. 1989): Beuda gypsum outcrops on the northern flank of the Ripoll syncline, overlying the Campdevànol turbidites, but on the southern flank it appears sandwiched in the Vallfogona formation. The Serrat borehole shows more than 2,000 m of evaporites, alternately made up of marls, anhydrites and salts, named the evaporites of Serrat, with an approximate age of Lower Lutetian. This has been interpreted as the evaporitic basin depocenter (Martínez et al. 1989, Vergés et al. 1992, Vergés 1993).

Bellmunt formation (Gich 1969): Continental red clays, sandstones, and conglomerate in channel-fills of the Bellmunt Formation Middle and Upper Lutetian were deposited at the same time as the Ripoll Syncline folding attachment to the Cadí Unit took place. Both are found in the hanging wall and foot wall of the Vallfogona Thrust.




During the 60's and early 70's some oil exploration boreholes were drilled in this area without subsurface control (Clavell 1992) using only surface criteria, mainly at the top of anticlines in the Ebro basin. During the 70's the seismic works began, and culminated with the drilling of only two boreholes in the allochthonous units: Serrat-1, almost 3,000 m, and Bestrecà-1, 1,200 m deep.

Riudaura-1 (Clavell 1992, Lanaja 1970) (Fig. 4): Drilled in December 1964 by SEPE-CIEPSA. It is located in the Ebro basin and cuts all the Eocene sequence before reaching the basement a depth at 2,345 m. Its most important feature is the evidence of a small series (500 m) of Lower and Middle Eocene materials, in the limestone platform facies. This contrasts with the 2,000 m outcropping of equivalent sediments but in deeper facies. This well produced only humid and condensed gas. The upper part cuts the continental materials of the Bellmunt and Coubet formations. From 800 m to 1,400 m marls interbedded with marly-limestones, and in the lower part evaporites with similar characteristics to Vallfogona Fm., are found. From 1,400 m to 1,800 m evaporitic materials with some marly intercalations prevail. The last 500 m are formed by a carbonate layer (limestones and dolomites) with some marl intercalations in the middle part. The upper part was defined as Perafita Fm., equivalent in age to the Armàncies marls and limestones of La Penya Fm. The lower level, called Orpí or Cadí limestones, is equivalent to the Sagnari marls. Its bottom consists of some meters of Garumnian red sediments and a Hercynian granite basement.

Serrat-1 (Clavell 1992) (Fig. 5): Drilled in 1987 by Union Texas Spain España Inc., it was the first borehole made in the allochthonous units of the eastern Pyrenees and reached a depth of 2,985 m. The most significant results obtained from this borehole are the evidence that the Vallfogona Thrust was much shallower than predicted (Martínez et al. 1989), and also it confirmed the existence of a very thick evaporitic basin. In the upper part of the borehole red materials of Bellmunt Fm. and part of the turbiditic material from Campdevànol Fm. were found. At 750 m they met a sequence of alternating marls and evaporites (point 1) and from 1,050 m (point 2) an abrupt change of the dipmeter was observed. This point corresponds to the Vallfogona Thrust. Between points 3 and 4 (1,300-1,550 m) they drilled through salts. Between 4 and 5, there was a prevalence of evaporitic rocks in relation to the marl materials and after point 5 (2,000 m) there was practically a massive anhidritic layer until the point 6 (at 2,985 m) where marly sediments and some limestones layers appeared again. This evaporitic serie is called Serrat Evaporite Fm. and is considered to be the depocenter of the Lutetian of a evaporitic basin.

Bestrecà-1 (Fig. 6): Drilled in February 1991 by Union Texas España Inc. It was the second borehole carried out in the allochthonous units; its main objective was to determine the basement depth in the la Garrotxa antiformal stacks. It was drilled obtaining a continuous core and reached a depth of 1,200 m. The upper part consists of marly sediments of the Armàncies Fm. Between 300 and 370 m there were part of the Cadí limestones Fm. and Garumnian red materials, with a very low thickness due to tectonic contacts associated to the Oix fault. The rest of the borehole shows granites with some deformation. The borehole stopped at 1,200 m although very clear deeper reflectors appeared in the corresponding seismic line (EXP88-02, Fig. 16)




In the studied area exists a great structural complication. We made the Bouguer anomaly map and after we separated this anomaly in two components, the regional anomaly and the residual anomaly. This process was made using a polynomial regression and adjusted a second degree polynomial for the regional field. The residual map include the gravity effect of the nearest surface structures. Comparing this map (Fig.7) with the geological map (Fig. 2) some anomalies were impossible to justify. We studied this anomaly using 2D gravity modeling and we have made four different models in a N-S over different seismic lines. 2D gravity modeling is adequate in this case, due to we can extend lateral continuity of every models more than 2 or 3 Km, and the difference in front of the 2 1/2 D is practically zero. We include the surface geology, gravity, boreholes and seismic data, in order to obtain a more accurate models.


Gravity Study

The gravity map was configured taking into account 1987 measurements. These points have a fairly irregular distribution, caused by the important relief of the area. Most of the measured stations are located along highways, forest roads and seismic lines.

The gravity data used in this study comes from different sources. Some were taken from academic works (Villarroya 1982, Rivero 1993), others from various surveys of Union Texas España Inc. These are also data from an ITGE (Intituto Tecnológico y Geominero de España) campaign and another from the Gravity Map of Catalunya (Casas et al. 1986). The gravity networks used are those set up by Casas et al. (1986); these bases are tied to the Fundamental Gravimetric Network of the Spain. This belongs to International Gravity Stadarization Network (IGSN-1971). In cases where campaigns used different bases from this system, connections were between these bases and those of the above mentioned system (Rivero 1989, Rivero 1993). These bases were used to calculate the observed gravity.

The World Geodetic System (WGS'84), which includes the total mass of the atmosphere, was used to calculate the normal gravity at sea level. The positioning of stations in latitude and longitude was carried out in various ways. The longest error introduced by the positioning of the stations is 0.024 mGal. The topography of the gravity stations was also evaluated in various ways. The longest error derived from the station height measurement is 1.7 mGal. Which does not significantly affects the calculation of the Bouguer anomaly in a regional gravity study (675 km2).

Perhaps the most important correction that should be made to the data in a area with significant topography, is the topographical correction. This was carried out from the measuring point to 167 km., and was differently evaluated according to the distances from the measurement point (Table-1).

It is interesting to note that the maximum terrain correction is the topographical correction between 1,529 m and 22 km. This is because most points are in the sector with smallest heights, next to the greatest relief.

The reduction density used was 2.67 g/cm3 , the mean value of the upper crust. Once the normal gravity and the observed gravity have been calculated, all corrections have been made, the difference between them is the Bouguer anomaly.

With these data the Bouguer anomaly map was traced. This map contains all the gravity anomaly effects generated by the crustal structure at Moho level and other effects generated by shallower local structures with enough density contrast. As our objective was to define the subsurface structure we decomposed the Bouguer anomaly into a regional component (generated by deep structure) and a residual component (generated by shallow bodies). This separation was carried out by assimilating the regional anomaly to a second degree polynomial surface in accordance to observations in the large scale Bouguer anomaly maps (Casas et al. 1986, Rivero 1993). In this process we used gravity measurements external to the area, in order to obtain a better regional anomaly.



Qualitative analysis of the residual anomaly map

This map has a large number of fairly well defined gravity anomalies (Fig. 7). An E-W trend in the gravity anomalies can be observed in the map southern sector. This alignment is possibly related to one of the main structures in the area, the Vallfogona Thrust. Associated with this structure are some gypsum outcrops. This material has a very low density (2.2 g/cm3) but at depth it is transformed into anhydrites (2.90 g/cm3). This great variability in density and the possible accumulations of these materials due to the Vallfogona structure probably generate these relative gravity highs. In the Castellfollit area the structure is very complicated because of significant NW-SE faults.

To the N of the Vallfogona Thrust lies the Ripoll Syncline area. Due to the detritic material that is found in the syncline, a gravity low would be expected, but this is not so. In certain places these outcrops coincide with positive anomalies, in other places, the maximum negative values are approximately -2 mGal. This effect was probably caused by the accumulation of anhydrites under the Ripoll syncline, detected in the Serrat-1 borehole.

In the northwestern part of the map there is a steep gravity gradient toward positive values. This gravity high coincides with the Paleozoic outcrop to the north of the Ribes-Campordon Thrust. These materials have basically high density (2.72 g/cm3) Cambro-Ordovician slates.

The most significant feature is the gravity low observed in the NE sector of the map reaching values down to -10 mGal. This anomaly is over the Ormoier and Montmajor antiformal stacks. These two structures are intensely folded and contain materials like the Sagnari Fm. and granites as has been determined the Bestrecà-1 borehole. The low densities of these lithologies probably contribute to this significant anomaly.



Density Study

An accurate gravity model requires knowledge of the densities of the main geological units so as to limit the large number of possible models that explain the presence of a single anomaly only.

There are two problems in determining densities: First, the rocks lose porosity when are compacted, density varying with porosity and compressibility; second, the formations could change, making it difficult to attribute mean densities.

To evaluate the density of each geological unit we used two systems: a direct method using Archimedes principle and an indirect method from borehole density logs, whenever such logs exist.

The use of density logs was very limited, because only in the well Serrat-1 this register was made (Fig. 5). The outputs gotten by these methods outside of the area (Rivero 1989), can be extrapolated in some formations like the Orpí and Cadí limestones, which have great continuity and could be checked using seismic reflection profiles. The main advantage of this method is that the density is measured "in situ," and therefore, the diagenetic process is taken into account.

A summary table of densities formation and the methods used to obtain them, is show in the Table-2. We have data from three sources: studies carried out by Union Texas España Inc., academic studies (Villarroya 1982, Rivero 1989 and Rivero 1993, Hernández 1991), and from boreholes Serrat-1, Bestrecà-1, Jabali-1 and Puigreig-1 (Jabali-1 and Puigreig-1 are located outside of the studied area).

The most used density-log was that of Serrat-1 (Fig. 5). This borehole has some lithology variability, specially in the upper part, although in deeper areas anhydrites prevail. The density log record starts at 900 m and five lithologies can be recognized. They are characterized this in the seismic profiles, specially in the lines UTC-8 and UTC-101 (figs. 8 and 10) where these horizons have been marked with the numbers 1 to 6 (Table-3).

The evaporites drilled by the Serrat-1 borehole are hidden throughout the Pyrenees. Because of their plasticity and ability to move, it is difficult extrapolate beyond reasonable limits. So we only used these densities for the models carried out on the seismic lines UTC-8 and UTC-101, near to the borehole. For the other models we simplified these density data: the simplifications are described in each model.





Profile UTC-8

This profile is based on the UTC-8 seismic line which has a good resolution. All available boreholes were projected to this profile, one in the foreland basin and another in the Cadí Thrust Sheet. This section is locate in the southern part of the Cadí Thrust in the contact zone with the Ebro Basin (fig-2). The Cadí Thrust Sheet consists of the Ripoll Syncline, with a northern flank composed of the thick Lower-Middle Eocene sequences there are almost vertical or inverted. On the northern flank the Corones, Armàncies and Campdevànol formations outcrop and there are discontinuous outcrops of Beuda gypsum. The syncline core is formed by the Bellmunt Fm. continental sediments and on the southern flank the turbiditic materials outcrop with some levels of Vallfogona Fm.

Seismic data

It runs NW-SE (Fig. 2), and in practice this line is restricted to the Ripoll syncline and a small part of the Ebro basin. The first analysis of this line did not reveal the shallow position of the Vallfogona thrust. In Figure 8 the line is shown without interpretation, and interpreted with the data from the boreholes, using the reference numbers that we described in the borehole (Fig. 5). Due to the precision in the subsurface structure control, this post-stack migration line was considered to be the most objective result both in determining the geological structure and in defining the materials. The criteria defined in this seismic line, both for the structure and for the material densities, were exported to other lines which were more difficult to interpret. In this line, as in the majority of them, the reflectors, which are slightly northerly inclined and represent the autochthonous Eocene carbonate sequences, can be observed, as can be observed the syncline structure (Ripoll syncline) in the upper block of the Vallfogona Thrust. However, the interpretation of the borehole data, met the base of the Cadí Unit at point 2 some 1,000 m deep, with the Serrat evaporites remaining in the lower block which forms another thrust sheet (Serrat Unit).


Gravity modeling

The control supplied by the Serrat-1 borehole and the seismic lines allowed us to construct a well-defined 2D density model; the existence of a density-log permitted considerable limitation of the variables in the modeling process.

Whenever possible the densities that appear in Table-2 were used, although the density of some bodies does deserve comments. The detritic materials of the Bellmunt and Coubet formations (bodies "A" and "C") have an approximate density of 2.60 g/cm3 mean of these materials. The Banyoles marl Fm. ("B") has 2.45 g/cm3. The upper anhydrites drilled by Serrat-1 have a mean density of 2.75 g/cm3, the same as the intermediate one (bodies "E" and "G"). The lower anhidritic unit is almost clay-free and therefore heavier reaching 2.90 g/cm3 (body "H"). Salty facies (body "F") could have some clay content, which increases to their density 2.15 g/cm3. Body "M" represents the sediments of the Armàncies, Corones and Sagnari formations grouped together to ease the modeling process: The mean density of the group is 2.56 g/cm3. The Garumnian sediments have a density of 2.67 g/cm3. The Beuda evaporites have a density of 2.77 g/cm3, although it is difficult to attribute one single density to this body. The density of body "D" was problematic. In the Serrat-1 density-log between points " 1" and "2", there is a group of materials which mean density is 2.72 g/cm3, which is neither the density of the Campdevànol formation (2.58 g/cm3 at surface), nor the Vallfogona formation (2.60 g/cm3). We divided this body into three parts, one to the south constituted for Vallfogona Fm. rocks, one to the north constituted for Campdevànol Fm. materials and between them a denser area (2.72 g/cm3), which probably contains more anhydrite. Metamorphic basement (body "J") have a very regular density in every sectors of the area, approximately 2.72 g/cm3. The final model shows a mean error of 0.07 mGal.

We assume that the positive anomaly in the profile center is generated by the anhidritic material drilled in the Serrat-1 borehole, the negative value in the south responded to the thickness of the Bellmunt and Coubet detritic formations, and the Banyoles Marls Fm.



Geological interpretation

The integration of subsurface data (seismic lines, gravity and boreholes) with surface geology shows that the basal thrust of the Cadí Thrust Sheet was shallower than previously supposed (Fig. 9). The lower block has a very thick sequence of evaporitic materials of the Serrat Unit. In the upper block and in the northernmost area of the cross-section Corones and Armàncies Fm. sediments have a high section-off angle, while in the southernmost area, under the Ripoll syncline, Campdevànol and Vallfogona Fm. over flat-lying. These two formations, which outcrop on the north and south flanks of the Ripoll syncline, are separated by a third unit observed in the Serrat-1 borehole (tract 1-2). Their age cannot be specified, it was interpreted as a lateral facies change of the turbiditic materials (Campdevànol Fm.) to the north. In the southern part they change laterally to turbidites with evaporitic layers (Vallfogona Fm.). In this profile the lower unit is made up exclusively of Serrat Unit evaporites (Martínez et al. 1989). The décollement level is at the base of the evaporites and meets the Vallfogona Thrust. The Ebro basin is to the south of the Vallfogona Thrust. The basal part of the Ebro basin consists of a Lower-Middle Eocene limestone layer dipping to the north and bending below the allochthonous units under their weight (Clavell et al. 1988). The rest of the autochthonous series consists of a thin evaporitic layer equivalent to the Serrat Unit, and a continental-marine Middle Eocene sequences.



Profile UTC-101

Section running NW-SE (Fig. 2), which shows the Serrat-1 borehole. It begins in the north in the antiformal stack sheets of the Upper Paleozoic and Garumnian which is the Freser antiformal stack (Muñoz et al. 1986). Toward the south Garumnian and Eocene sediments, inverted or strongly deepening to the south, are found until they form the Ripoll syncline. The Vallfogona Fm. outcrops on the south flank in the upper block of the Vallfogona Thrust. Continental sediments from the Ebro Basin outcrop in the lower block.

Seismic data

This profile is longer than the UTC-8 line which implies a major degree of interpretation in the north area, due to the seismic noise under the most important elevations is clearly greater (Figs. 2 and 10). The general structure is similar to that of the UTC-8 line. The reflectors that represent the materials of the Ebro basin are observed as far as below the outcrop of the basement materials. The Serrat-1 borehole was projected with reference numbers (Fig. 5). The interpreted structure is similar to UTC-8, in the south part. The reflector corresponding to the Vallfogona Thrust, which can be followed with difficulty toward the north, is located at point 2 of the borehole. The quality of the line diminishes toward the north of the Freser antiformal stack.


Gravity modeling

The southern sector of the density model of seismic line UTC-101 is similar to the density model of the UTC-8 line, but some differences are found in the northern sector. The interpretation consisted in taking the representative levels deduced from the UTC-8 though profile and extend this to UTC-101. There is a zone in the northern sector which has not been examined thoroughly. We observed certain antiformal structures in the seismic line. We began by attributing Lower Paleozoic density (2.72 g/cm3) to these antiformal structures, which generates excessively positive anomalies, but were unable to justify it, nor could it be justified using Upper Paleozoic materials (2.67 g/cm3). Finally, we postulated the existence of two bodies, an upper body of density (2.72 g/cm3) that could be Lower Paleozoic and a lower body of density 2.60 g/cm3. Using these parameters a good fit was obtained with a mean error of 0.16 mGal.


Geological Interpretation

This profile depicts the general structure of the Ripollès area from the foreland basin to the internal part of the range. The southern part, like profile UTC-8, shows the southern front of the allochthonous units, where the Cadí Thrust Sheet lies above the evaporitic Serrat sequence (Fig. 12). In the northern part it shows the structure of the thrust sheets of the Freser antiformal stack consisting of Paleozoic materials, limited to the north by the Ribes-Camprodón Thrust (Muñoz et al. 1986). The positioning of this antiformal stack produced the Ripoll syncline bending (Vergés 1993). Interpretation of the gravity data seems to confirm the existence of some low-density sedimentary layers below the Paleozoic outcrops, which had already been suggested but not demonstrated in previous works (Martínez et al. 1989; Rivero 1993; Vergés 1993). These materials are located to the north, below the evaporitic series found in the Serrat-1 borehole, which correspond to the Lower-Middle Eocene series of the Serrat Unit and constituted part of the lower block of the Cadí Thrust Sheet. At surface these materials, the Sagnari, Corones and Armàncies Fms. outcrop in the Cadí Unit. Gravity interpretation (Fig. 11) suggests that these rocks are basically marly sediments and thus the presence of Armàncies Fm. in these materials, which are in the hangingwall ramp of the basal thrust. This could create a favorable oil-reservoir structure. The facies change from the platform deposits exposed in the Ebro basin to the basinal facies of the Serrat Unit probably took place in the ramp area.



Profile UTC-6

This section is located in the western part of the La Garrotxa stacks and crosses the main gravity low of this area (Figs. 2 & 7). To the north it begins in the lower Paleozoic materials and in the Freser stack, separated from the Ribes-Camprodón thrust. Toward the south this seismic line cuts the Lower Eocene materials, these materials form a NE-SW syncline and a high compound for the Puig Ou antiformal stack and western end of the Montmajor antiformal stack. These antiformal stack consists of limestone duplexes of the Cadí and Corones Fms. and basement materials in the nucleus. This is interpreted as a tectonic inversion of a syn-depositional normal fault. Above these materials there are the Armàncies Fm. and the continental sediments of the Bellmunt Fm. The section ends to the south in the core of the Ripoll Syncline.

Seismic data

This post-stack migrated line shows reflectors of the autochthonous structure below the antiformal structure (Puig d'Ou antiformal stack) and in the southern part of the Ripoll syncline (Fig. 13). On the southern flank of the antiformal structure some wedge-shaped reflective domains can be observed. The seismic facies of the antiformal core is fairly homogeneous, but some reflectors bending anticlinally can be observed. The high quality of this line shows that the autochthonous rocks are cut by the upper block and form a foot-wall ramp.


Gravity model

The gravity anomaly values are somewhat different in La Garrotxa area than the Ripollès areas. This profile are across the La Garrotxa area. There is a marked gravity low above the Puig Ou antiformal stack area. Which has values reaching -4 mGal and is connected to a great negative anomaly which will be modeled in section EXP-88-02. In accordance with the seismic line the thickness of the evaporite level under the Ripoll Syncline should be less here than in the Ripollès area. We simplified this evaporitic layer into a single body 2.82 g/cm3 of density, the mean value of the anhydrites drilled by Serrat-1. We placed the halites to the north in order to justify the intense gravity low.

The antiformal stack is basically made up of limestones of the Cadí Fm. with 2.65 g/cm3 density. Underlying this we propose the presence of a body (2.63 g/cm3) which was interpreted as granite; although it does not outcrop in the profile, it has been included because the line runs not far from the Bestrecà-1 borehole (Fig. 6).

With these parameters we started the modeling process, assuming only granite under the Puig d'Ou antiformal stack, but it was impossible to compensate it. It was necessary to incorporate materials of lower density, which, in view of observations made in the area are probably marly sediments (Armàncies or Sagnari Fms.) or evaporites with a high salt content (drilled in Serrat-1).

In the first (fig. 14-a) option body "S" has 2.57 g/cm3 density, which is approximately the mean density of the Sagnari, Corones and Armàncies formations. The triangular body observed in the seismic line has a density of 2.62 g/cm3, which could indicate to evaporitic materials with some halite content. The mean error of the model is 0.3 mGal.


The second option (fig 14-b) is rather more complex because it involves two Paleozoic bodies, "J" with granite density and "O" with a density of 2.67 g/cm3, which could be Upper Paleozoic, or a mixture of Lower Paleozoic and granite. Body "I" has density 2.58 g/cm3, which is the same as that of the upper evaporitic group drilled by the Serrat-1 borehole. The mean error of this model is 0.3 mGal.

Geological Interpretation

This profile is the most representative of the general structure in the La Garrotxa area (Fig. 15). To the north the basement materials forms the Freser antiformal stack, limited by the Ribes-Camprodón Thrust. Toward the south there is another antiformal stack (Fig. 2), of Eocene materials with a granite nucleus. The Puig Ou antiformal stack can be observed in the section (Fig. 15). The southern part of the profile shows the Ripoll syncline with the Bellmunt Fm. in its core. Similar with the previous sections, the Cadí thrust sheet is largely superficial, but it presents a much more complex structure than that of the Ripollès area (sections UTC-101 and UTC-8). The existence of these Eocene antiformal stacks with the basement nucleus, was interpreted as short cuts of the tectonic inversion of the Lower Ypresian normal faults (Martínez et al. 1989). The lower unit is again the Serrat unit, observed in the Serrat-1 borehole, but with different materials. The seismic line shows a wedge-shaped morphology of the frontal part of the Serrat Unit south of the Puig d'Ou stack (Fig. 13). The gravity analysis of this area allows two interpretations. One (Fig. 14-b) suggest that this lower thrust sheet consists largely of Paleozoic materials and that this wedge corresponds to the syntectonic sediments of a Lower Ypresian normal fault. This structure implies the existence of two Lower Ypresian normal faults. This structure forms a short cut with basement materials above the sedimentary materials. The second hypothesis (Fig. 14-a) interprets this lower thrust sheet as being similar to section UTC-101, but with fewer evaporitic materials, which are restricted to the wedge observed in the seismic line. We choose the second possibility (Fig. 15) because it is simpler and fits better with the interpretation of section UTC-101. Another factor of the seismic data from the Bestrecà-1 borehole (Fig. 6), which show reflectors between 0.7 and 1.1 sec. (reserved data and unpublished) (Fig. 13), suggest the existence of sedimentary materials below the basement materials. The Bestrecà-1 borehole is located in the eastern part of the Puig d'Ou antiformal stack (Fig. 2) and its results were incorporated in UTC-6 section interpretation. The presence of Lower Ypresian limestones (Cadí Fm.) outcrops in the Cadí Thrust Sheet, implies the continuity between the platform silts of the Ebro basin, the Serrat Unit and the Cadí Unit. In this case marls from the Serrat Unit will correspond only to the Armàncies and Campdevanol formations.




Profile EXP-88-02

Profile EXP-88-02 trend NW-SE and crosses the center of the main gravity low. It starts in the basement (Fig. 2) and cuts the Ormoier antiformal stack toward the south. At the northern end of the Ormoier antiformal stack there are two inverted structures which connect the Sagnari marls Fm. to the north (transition facies between platform and deeper sediments) and the Cadí limestones (of the Ormoier antiformal stack) to the south. The core of the stack consists of granite.

Seismic data

This line is of poor quality (Fig. 16), but the autochthonous reflectors can be seen below a structure that could be interpreted as hangingwall ramp fault of the Serrat unit which folded the Vallfogona thrust.

Gravity Model

Gravity modeling of this seismic line is complicated because the gravity anomaly is very negative and the reflection seismic line limits the underlying structure in which reflectors of the autochthonous Lower Eocene can be observed. The structure and the lithologies make it very difficult to explain this important gravity low. The surface structure is very complex, and we had to simplify this structure to make the gravity model. To the north the Sagnari formation (body "J") is mainly limestone, so we increased its density to 2.61 g/cm3.

For body "L", deduced from seismic data (Fig. 16) and located farther to the north, we first tested Lower Paleozoic and granite densities but neither fit the observed anomaly. We proposed a body of density 2.67 g/cm3, which could be Upper Paleozoic or a mixed body with granite and Lower Paleozoic.

We tested several gravity models, of which we present only two, due to structural restrictions. In the first (Fig. 17-a) the antiformal core is assumed to have a 2.45 g/cm3 density. Except for halite, which also has a low density, there is no lithology in the area with low density, so this body may be evaporitic material with many halites. If we take densities and thickness of the evaporite section from Serrat-1, this density would be about 50% halite (density 2.15 g/cm3), although in Serrat-1 only 25% halite was drilled in the upper evaporitic layer. Therefore, we need a significant increase in the halite quantity, which could be due to a halite accumulation in the anticline core. Using these parameters the model fits the observed anomalies with a mean error of 0.25 mGal.

In the second model (Fig. 17-b) the gravity low is explained by a thinner accumulation of evaporites (body "N") and another body "O" with material density 2.57 g/cm3, equivalent to the combined Armàncies, Corones and Sagnari fms. This model fits with a mean error of 0.28 mGal.



Geological Interpretation

The interpretation of the general structure (Fig. 18) is similar to that carried out in section UTC-6 (Fig. 15). As in the former case the Ormoier antiformal stack was

interpreted as a short cut of the previous extensive Lower Ypresian structures. These structures caused the incorporation of basement into the Cadí Thrust Sheet. In the north of the Ormoier antiformal stack there are two Lower Ypresian normal faults, which were later tectonically reactivated as thrusts. The marls of the Sagnari Fm. (Lower Ypresian) overthrusted the limestones of the Cadí formation (which are the same age). The deep structure was interpreted as a hangingwall anticline of the Serrat Unit bending the upper structures. This interpretation is based on a series of arguments: the antiformal morphology observed in the seismic line (Fig. 16), the superficiality of the Cadí Thrust Sheet observed in the former sections, and gravity data because low density sediments below the antiformal stack are required (Fig. 17). The gravity data introduce two possible interpretations of the characteristic lithology of the Serrat Unit. The first interpretation (Fig. 17-a) is that the Serrat Unit consists of a very thick sequences of Lower-Middle Eocene evaporites and limestone (Cadí and Perafita Fm.). In the second interpretation (Fig. 17-b), the evaporite thickness is lower and Perafita Limestone Fm. changes to marly sediments (Armàncies and Campdevànol Fm.). The interpretation was chosen (as above) (Fig. 18). The criterion was the regional similarity to other structures and the outcrop in the Cadí Thrust Sheet, to the marly materials of Armàncies and Campdevànol Fms., and to the limestone series of the Corones and Cadí Fms.



In the Ripollès area, the Serrat-1 well shows that the Cadí Thrust Sheet is much shallower than it was expected. This borehole also revealed a lower unit, called the Serrat Thrust Sheet with a thick evaporitic series. The problem is to determine the nature of the series in the northern part of the Serrat Unit. The integration of the gravity and seismic data allows us to better define the characteristics and geometry of the Serrat Unit. It was shown that the Serrat Unit mainly consists of a series of Lower Ypresian-Upper Ypresian and Lower Lutetian marly sediments and in their upper and southern parts of a very thick evaporitic series of the Lower Lutetian (Serrat evaporites equivalent to the Beuda Fm.). This series correspond to the footwall of the Cadí Thrust Sheet, and is therefore assumed to be similar. This suggests the presence of the oil source rock (Armàncies Fm.) in a hangingwall anticline structure of the Serrat Unit below the Freser Antiformal Stack. This structure is sealed by basement and evaporite layers, and the presence of possible reservoirs (Campdevanol Fm. sandstone or fractured limestone of Corones Fm.) there make this structure a good oil prospecting target.

In the La Garrotxa area the structure is similar but more complex. The Cadí Unit comprises a series of antiformal stacks with basement cores. These structures are visible in the seismic lines but resolution is not good enough to distinguish whether all the cores consist of basement. In a previous work (Martínez et al. 1989) these structures were interpreted as short cuts of former normal faults. Now, the seismic and the gravity data allow us to limit the thickness of the antiformal stacks and to define the characteristics of the materials located in the lower block. The factors which permit this interpretation are: the existence of a gravity low located on the antiformal stacks, and the presence of shallow reflectors detected in the Bestrecà-1 borehole (Figs. 13 and 16). The cross-sections of the La Garrotxa area (UTC-6 and EXP-88-02) gives thickness values for the antiformal stack (Cadí Thrust Sheet) which oscillates between 1,000 and 1,500 m: these values are similar to the thickness of the Cadí Thrust Sheet in the Ripollès area. Another conclusion drawn from the cross-sections of La Garrotxa is the existence of a lower unit consisting of Lower Ypresian-Lutetian evaporitic and sedimentary material (marl and limestone), equivalent to the Serrat unit defined in the Ripollès area. The problem is to decide whether this unit contains marl materials similar to the Armàncies formation as in the Ripollès area. There are several models for interpreting the gravity data (figs. 14 and 17): these models vary the thickness of basement, evaporites, limestones and marls. Although there are no sure criteria for choosing one model or another. To opt for a better, two sections were adapted (UTC-6 and EXP-88-02, figs.15 and 18) with the seismic values measured in the Bestrecà-1 borehole. In this model, the Cadí Thrust Sheet has a maximum thickness of 1,500 m in the antiformal stack core. The upper part of the Serrat Unit consists of an evaporitic series (anhydrites and salt) with a thickness varying, from more than 1,000 m to zero. The lower series consists of Lower Ypresian sediments that could be a carbonatic platform or deeper materials, depending on the area, and materials of the Upper Ypresian-lower Lutetian which are basically marls (Armàncies and Campdevànol Fm.) with the exception of the Corones formation (limestone and sandstone).

From the geological cross-sections would be inferred that the Serrat Unit suffered a longer shifting in La Garrotxa than in the Ripollès area. This difference can be solved by the existence of a transitional zone located near the Oix fault, or by the existence of other geological structures not observed in the seismic lines. The main aim of this paper is not develop a structural study, for this reason, the geological cross-sections are not balanced, and therefore is very difficult deduce if the shifting differences observed are related with a shortening of the global structure.

The geological differences between the Ripollès and The La Garrotxa areas are:

1) The presence of Lower Ypresian normal faults in the La Garrotxa area (Fig. 19). These structures were later reactivated as thrusts incorporating material in the Cadí Thrust Sheet. Footwall short cuts across the basement gave rise to antiformal stacks.

2) In the La Garrotxa area the carbonatic platform of the Lower Ypresian age is continuous from the Ebro basin to the Cadí Thrust Sheet. In the Ripollès area, however, the change from carbonatic platform to deeper materials takes place in the lower ramp area of the Serrat Unit. In this way the frontal part of the basal thrust of the Serrat Unit is located in a flat zone at the base of the evaporitic basin. For this reason the evaporitic series of the Serrat Unit thrust sheet thicker in the Ripollés area. On the other hand, in the La Garrotxa area the ramp below the Serrat Unit climbs up from the basal detachment and cuts across the evaporitic series, until it meets the basal thrust of the Cadí Unit.


We are indebted to Dr. Eduard Clavell for his suggestions and advice of oil prospecting, to Dr. Jordi Giménez for his sedimentary advice, to Dr. Jaume Vergés for allowing us to reproduce Fig. 1 of his Ph.D thesis and to Monserrat Obradors the revision of the text.




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Figure 1: Structural map of the NE Iberian peninsula, showing the studied area (Vergés 1993, modified)

Figure 2: Geological map of the Ripollès and The La Garrotxa areas (based on Martínez et al. 1988, Martínez et al. 1994, Muñoz et al. 1994)

Figure 3: Principal lithologic formations and their mean density values.

Figure 4: Riudaura-1 borehole, drilled by SEPE-CIEPSA at 1964, in the Ebro basin.

Figure 5: Serrat-1 borehole, drilled by Union Texas España Inc., in 1987. This was the first borehole in the thrust sheets of the eastern Pyrenees.

Figure 6: Bestrecà-1 borehole, drilled by Union Texas España Inc. in 1991.

Figure 7: Residual gravity map of the area

Figure 8: UTC-8 seismic line: (Top) Uninterpreted seismic line showing the Serrat-1 borehole, with the reference numbers from figure 5. (Bottom) Interpreted seismic line.

Figure 9: Gravity model of the UTC-8 section with the densities of the different polygons.

Figure 10: UTC-101 seismic line: (Top) Uninterpreted seismic line showing the Serrat-1 borehole, with the reference numbers from figure 5. (Bottom) Interpreted seismic line.

Figure 11: Gravity model of section UTC-101.

Figure 12: Geological interpretation of the UTC-101 section, based on the surface, seismic and gravity data.

Figure 13: UTC-6 seismic line: (Top) Uninterpreted seismic line. (Bottom) Interpreted seismic line.

Figure 14: Two gravity models of the UTC-6 section. a) The wedge observed in this seismic line (Figure 13), corresponds to evaporitic materials, the remaining series of this unit are marl and carbonatic materials. b) This interpretation shows three units: (top) Cadí unit, (mid) the wedge materials, Lower Ypresian marls controlled by a normal syntectonic fault, and basement materials, (bottom) the evaporite and limestone compound.

Figure 15: Geological interpretation of the UTC-6 section, based on the seismic (Figure 13) and gravity data (Figure 14-a). In this interpretation the Puig Ou antiformal stack, it is the consequence of a short cut of previous Lower Ypresian normal faults.

Figure 16: EXP-88-02 seismic line; (Top) uninterpreted (bottom) interpreted. In spite of their poor quality, a structure can be seen in the hangingwall anticline. The continuity of certain reflectors toward the north is also noticeable.

Figure 17: Two possible gravity interpretations of section EXP-88-02. a) The lower unit consists of a potent evaporitic series and limestones. b) The unit contains three lithologies: evaporites, marls and limestones.

Figure 18: Geological interpretation of section EXP-88-02, based on the seismic line (Figure 16) and gravity data (Figure 17-b).

Figure 19: Sedimentary pre-bending scheme, showing the differences between the Ripollés and the La Garrotxa areas. The Lower Ypresian normal faults in the La Garrortxa are the main difference between the two areas.





Correction Max. (mGal)

Correction Min. (mGal)

Mean Correction (mGal)

Standard Deviation (%)

0-170 m

Directly in the field (Klingelé 1980)





170-1,529 m

D.T.M. (100 m)





1,529 m -22 km

D.T.M. (1 km)





22-159 km

D.T.M. (5'x3')





Table 1: Topographic correction in the different areas



Density (g/cm3)


Bellmunt Fm.


Density log

Armàncies Fm.


Double weighed

Corones Fm.


Double weighed

Sagnari Fm.


Double weighed

Cadí Fm.


Density log

Orpí Fm.


Density log

Lower Paleozoic


Double weighed

Upper Paleozoic


Double weighed



Double weighed

Banyoles Fm.


Double weighed

Hercinian Granites


Double weighed

Campdevanol Fm.


Double weighed

Anhydrites upper


Density log

Anhydrites middle


Density log

Anhydrites lower


Density log



Density log

Table 2: Density of the formations in the studied area.





Density (g/cm3)

Area between 1-2



Area between 2-3



Area between 3-4



Area between 4-5



Area between 5-6



Table 3: Main lithological layers and densities