Research Herman Zevenberg: Facies and Sequence analysis of the Boltaña anticline in the Spanish central pyrenees

The Boltaña Anticline was the subject of our fieldstudy of six weeks in the summer of 1997. Detailed sections were made of the west and east flanks of the antiform, which consist of platform and slope sediments of middle Eocene age. Also many samples were taken to study the contents in thin sections for later analysis. The purpose was to correlate the two sections with sequence stratigraphy analysis, and to find out more about the origin of the Boltaña antiform and the regional geological setting. The trends and sequences give evidence of changes in relative sea level and carbonate production (accommodation space) in the investigated area. Faulting is probably also an important factor. In the two lithological different sections many sequences could be recognized and the two sections could be correlated. With this investigation of the detailed sections, the Boltaña region could be placed better in a regional geological perspective. Also several hypothesis are presented for the origin of the Boltaña antiform and for the origin of the turbiditic sediments in the west section, although further research is needed.

1 INTRODUCTION

1.1 Purpose of fieldwork

As part of the HD630 course ‘Fieldwork Sedimentology/Stratigraphy’ at the faculty of Earth Sciences at the Free University Amsterdam, fieldwork was carried out in the Boltaña antiform region in the central Spanish Pyrenees (Fig. 1.). The antiform consists of middle Eocene marine platform and slope facies with different environmental developments in the east and west sections and was studied along the road Ainsa-Torla (N 260) between Boltaña and Janovas. The east section was also partly studied on the road to Sabiñanigo (A 1604). On each of the flanks detailed sections were made and samples taken. From the hart of the antiform sections were taken until the point that no further sequences could be recognized, and only deeper water sediments were deposited. This resulted in two logs of nearly one km stratigraphy each. The east section is divided in several parts because of several structural complications. The purpose of this fieldwork was to correlate the two different flanks with the help of sequence stratigraphy. Another purpose of the fieldstudy was to place the antiform in a regional geological setting and to find out the origin of the antiform itself together with M. de Cock who did the structural part of the fieldwork, and of the turbiditic sediments in the east section. By doing this, the research could be spread to nearby areas. More west of the antiform itself, some extra fieldwork was carried out to find out the origin of the Boltaña antiform and the turbidite deposits. Maurice de Cock, who was also working in the same area at the same time, was investigating the structural development of the Boltaña antiform.

1.2 Fieldwork methods and materials

The Spanish topographic maps of Boltaña (30-10 (211)) and Broto (30-9 (178)) by Cartografía Militar de España were used for orientation in the field (scale 1:50.000).

We used photocopies in the field to note the locations of the different parts of the sections. Aerial photographs (211A 10-12, 211B 12-14, 211C 11-13) were used to determine the locations of the best sections with the least number of important faults, and to try to determine the distribution of the turbidite deposits west of the anticline itself. The sections were mostly described along roads and rivers because the sections were easy accessible and there were few gaps. Only in the most eastern part of the east section we had to find parts of the stratigraphic column off-road. Also major slumping and faulting disturbed the east section.

In the field two detailed sections were made of the Boltaña antiform using millimeter logpaper. Every bed in the field was measured and drawn on the log (scale 1:100) together with the natural appearance in the field, the lithology and sedimentary structures. On the log there was also space to describe the grainsize of the quartz, notation for sedimentary structures, bioturbation, fossil contents, thickness trends, grainsize trends, color and peculiarities. For every few meters or at least in every bed, the percentages of the different large Foraminifera was estimated. For determination of the fossil content, the large Foraminifera and the grainsize a 10x handlens was used. From the logs, condensed stratigraphic columns were produced (scale 1:1.000).

From the hills opposite of the Boltaña antiform many photographs were taken, which were later glued together, making it possible to sketch each sequence on the photographs. These sequences were numbered and also drawn in the condensed stratigraphic columns. From several perspectives several drawings of parts of the antiform were made which served as an aid with the photographs to draw and number the sequences. With this we could already determine the correlation at the bottom of the two sections. A complicating factor is that because of the sections are not perpendicular to the lithology, the bottom of the two sections are much closer to each other at the time of deposition than the top of the section.

From every bed samples were taken for later analysis. From the thicker beds mostly three samples were taken; one at the bottom, one in the middle and one at the top. This resulted at the end of the fieldwork in about 400 samples. At first instance 59 thin sections were made to make a first correlation between the sections together with the drawn sequences from the photographs. Later 26 additional thin sections were made to make the correlation. Also 6 soft samples were investigated.

The thin sections were studied with a stereomicroscope with magnifications ranging from 10x to 60x. The thin sections first were classified in type of limestone (limemudstone, wackestone, grainstone, floatstone, rudstone and packstone). Then the numbers and species of the larger Foraminifera (Assilina, Nummulites, Discocyclina, Alveolina, Miliolids, Rotalia) were identified. When the grainsize/roundness/sorting differed from the mean it was noted and also the presence of quartz with estimated percentages, other fossils, heavy minerals and carbonate clasts. All this information was put on ‘Administration Cards’ and later the most important information such as the Foraminifera and quartz percentages were added to the condensed stratigraphic columns of the sections. Biozones were later determined for each thin section. This information was used together with the field information to make the final correlation between the sections.

1.3 Regional geology

The Pyrenees is a mountain chain extending for about 1500 km from Northwest Spain to Southern France. The mountains are formed as part of the Alpine orogeny as a result of relative movement of the European and the Iberian plate. Deformation took place from the upper Cretaceous until in the Miocene because of the subduction of the undercrust and subcrustal lithosphere of the Iberian plate under the European plate. During the early phase of the orogenesis a forelanddeep was formed. The deformation fronts migrated towards the forelandbasins. Because of that the forelandbasins migrated as well.

The central Pyrenees is subdivided in three structural units divided by major thrusts. From north to south: ‘North Pyrenean Thrust System’, ‘Axial zone’ and ‘South Pyrenean Thrust System. Our research area is located in the South Pyrenean Thrust zone. This zone consists of south dipping Mesozoic thrust sheets. In this perspective the north-south Boltaña antiform is a strange phenomenon (see chapter 4.2 for discussion). The Boltaña region is on the border of the Jaca basin and the Tremp-Graus basin (see fig 1.).

In the South Pyrenean Thrust zone during the early Mesozoic nonmarine clastic sediments, volcanic material, shallow marine carbonates and evaporites were deposited. The evaporites from the Triassic were to play an important role in the forming of certain structures in the Pyrenees. Then about 2500m marine shallow water carbonates were deposited before the beginning of the orogenesis. A major transgression took place in the Eocene inundating the forelandbasins. An average of 4000m of flysch was deposited in the forelandbasins that moved south with the thrust belt. At the margins of the forelandbasins about 1500m carbonate platform deposits were formed. These are also found in the Boltaña region.

Fig.1 Geologic sketch map showing the setting of the Jaca and Tremp-Graus basins and the Boltaña antiform in the south central Pyrenees (modified after Barnolas & Teixell, 1994). Top is North.

2 FACIES AND SEQUENCE ANALYSIS

2.1 Introduction

Sequences can be described as depositional system tracts. These are combinations of lithofacies, which are deposited during the same period and under more or less the same circumstances. The term ‘system tracts’ is based on relative sea level change, the accompanying deposits and accommodation space.

Three types of system tracts are recognised (van Wagoner et al., 1988);

-Transgressive system tract (TST):

The sequence begins with rapid relative sea level rise and aggradation of the carbonate platform. This characterizes a transgressive system tract.

The termination of a TST is the Maximum Flooding Surface (MFS) which is generally followed by a HST.

-Highstand system tract (HST)

The relative sea level remains the same; the only way the platform can grow is by progradation of the platform in lateral directions.

-Lowstand system tract (LST

This system tract occurs when rapid sea level drop occurs and exposes the shelf edge. Erosion takes place at the platform. The LST is not always present or is unrecognizable; then the HST is followed directly by a TST separated by a sequence boundary (SB).

A combination of tectonics (creating accommodation space) and eustatic sea level fluctuations can result in a cyclic alternation of secondary TST’s and HST’s, also called ‘parasequences’.

A description of a parasequence is made in paragraph 2.4

Fig.2 Example of a sequence eroded in the field in the west section.

2.2 Large Foraminifera

Large benthonic Foraminifera were used together with other information obtained in the field to reconstruct the paleo-environment and to correlate the two sections. In the setting of a carbonate platform, every specific depositional environment has its own assemblage of larger Foraminifera dependent on the place on the platform or slope. Due to changes in relative sealevel, carbonate production and accommodation space those biofacies tend to shift. With the percentages data of certain Foraminifera and other data in the section, sequences could be recognised. The trends and the Foraminifera data were most valuable.

In a supposed mainly ‘open shoal’ carbonate platform setting, the Foraminifer Alveolina sp. is mainly present at the open intertidal part of the platform. Nummulites sp., Assilina sp. and Discocyclina sp. are mainly present in the open shoal until the offshore part of the carbonate platform. These Foraminifera are Tertiary and are present in the entire studied section which is from middle Eocene age.

In general the Imperforate foraminifera (i.e. Alveolina and Miliolids) indicate a near shore environment while the Perforate foraminifera (i.e. Assilina and Nummulites) live in a deeper environment. If these two groups occur both at the same time in the section then there was no barrier. When a barrier was present, both groups are not present together in the rocks, because the groups were separated.

- Miliolidae: Because of the difficulties identifying the different species in this family, they are not further described. In the described section they have a low abundance. The Miliolinids are relatively small and difficult to recognize in the field. See Fig. 3 and 4.

Fig.3 Example of the family Miliolidae Pyrgo sp. ; magn: 80X

Fig.4 Example of the family Miliolidae Quinqueloculina sp. ; magn: 80X

Fig.5 Example of family Alveolinidae Fig.6 Example of family Alveolinidae
Glomalveolina sp. ; magn: 50X Alveolina cf. coudurensis ; magn: 20X
Axial section Axial section

- Alveolinidae: Like the Miliolinids this family is not easily identified to the species level. This family is relatively very abundant in the sections. Alveolinidae are abundant in lagoons and intertidal parts of a carbonate platform, in a reef – lagoon setting or in the open shoal part of carbonate platforms in an ‘open shoal’ setting. Alveolinids are usually long and elongated and are usually white. See Fig. 5 and 6.

- Discocyclina sp.: This Foraminifer is small and lens shaped and has a thin equatorial layer. Usually it has a relatively dark color and is easy recognizable also in the field. See Fig. 7. It is relatively abundant in parts of the sections. It is abundant in the open shoal and beginning of the slope in an ‘open shoal’ setting of carbonate platforms. In a setting with reefs, Discocyclina is not very common but can also be present in the open shoal and beginning of the slope.

Fig. 7 Discocyclina dispansa ; magn: 20X ; vertical section

- Assilina sp.: Together with Nummulites sp. it has different shapes mainly dependent on the waterdepth. The big flat specimens lived near shore, and the more lens shaped specimens lived in deeper water. The only difference with Nummulites sp. is its internal evolute chamber structure. See Fig. 8. Assilina sp. is very abundant in many parts of the studied sections. In vertical sections of the relatively big Assilina sp. is very easy to recognize, also in the field.

Fig. 8 Assilina sp. ; magn: 25X ; horizontal section and vertical section

- Nummulites sp.: Together with Assilina sp. it has different shapes mainly dependent on the waterdepth. The big flat specimens lived near shore, and the more lens shaped specimens lived in deeper water. The only difference with Assilina sp. is its internal involute chamber structure. See Fig. 8. In the field in the vertical sections Nummulites sp. are very easy to identify.

Fig. 9 Nummulites sp. ; magn: 12,5X and 20X ; horizontal section and vertical section

- Rotalia sp.: This small foraminifer occurs in two forms (Geel, pers. Comm). One form
lives relatively deep on the shelf and has a brown shell. The second form has a relatively thick shell and lives in very shallow water near the end of the reef zone. See fig. 10.

Fig. 10. Rotalia sp. ; magn: 40X ; horizontal section.

- Orbitolites sp.: This foraminifer is found mainly behind coastal barriers, normally associated with Assilina and Miliolids. See fig. 11.

Fig 11. Orbitilites sp. ; magn:20X ; vertical section.

- Some other Foraminifera were also found, but in very small numbers. These are:
Operculina sp. (See Fig. 12), Nodosaria sp., Actinocyclina sp. and Amphistegina sp.

Fig. 12. Operculina sp. ; magn: 12,5X ; horizontal section.

2.3 Other fossils and trace fossils

- Micraster sp. (see Fig. 13). An echinoderm that is heart shaped. This fossil occurs mostly in the carbonate layers with ball-shaped structures. These are most common in the east section of the antiform, but occur as well in the west section. The echinoderm is 3 to 6 centimeters wide. We found about 40 specimens.

Fig. 13. Example of a micraster. This is a Micraster cortestudinarium from the Cretaceous.

- The Arthropod Xanthopsis dufouri (see Fig. 14). This crab occurs is the same carbonates as the Micraster sp. About 20 crabs were found; many of them at the same place on a hill south of the sections on the other side of the river. Distinction can be made between the male and the female crabs because of difference in size of the pincers. The males have one very big pincer and one small pincer. The females have two small pincers and are generally smaller. The crabs are well preserved, so they are probably found in situ. The crabs are between 5 and 10 centimeters wide.

Fig. 14. The crab Xanthopsis dufouri found in Boltaña. Female and Male specimens.

- Echinoderm which is not further identified. This irregular sea urchin is very big, about 15 centimeters. We found one perfect specimen and parts of three more specimens. These echinoderms like the micraster sp. and the crabs were only found in the carbonates with ball-shaped structures.

- Several bivalves of several different species were found in the described sections. These are not further identified because of most of them only the inside cast was preserved. The only bivalve identified is Chlamys sp. (see Fig. 15). The bivalve is 3 centimeters wide.

Fig. 15. Chlamys sp.

- In the sections we also found four small black teeth fragments. Possible they are fish teeth. The smallest is about one millimeter wide and the biggest is about 4 millimeters wide.

- In the top of the west section in the last sequence near the village of Janovas, we found several strange tubes build with foraminifer skeletons. The tubes are about four centimeters wide. The inside diameter is about 2 centimeters. The tubes were probably build by some kind of worm-like organism that used the tube as protection. The foraminifers are a little obliquely cemented together to form the tube.

- Very big and round Nummulites sp were found in Liguere de Ara, which is located about 5 kilometers west of Janovas which is the west end of our sections. The Nummulites sp. was found in an extensive area with marls. These marls lie probably stratigraphically on top of our west section. The foraminifer has a diameter of two till two and a half centimeter and is about one centimeter thick.

- Also outside our section area, we found many corals and other associated reef organisms on a hill directly north east of the village Boltaña. We found them in relatively soft material. The area consists of marls with occasionally turbidites and lies stratigraphically on top of a thick turbidite layers of our east section. We think the reef material is part of such a turbidite layer and that it comes from far away. The Ostrea sp. we found shows evidence of transport, the delicate corals show little evidence of transport, but were possibly moved in large blocks. The area has a very dense shrub cover, so we could not determine if the material is really part of a turbidite. The material consists of at least ten different species of coral, several oysters Ostrea sp., possibly some sponges, several crinoids, three echinoderm spines and a gastropod.

Also many big flat foraminifers (Nummulites sp. and Assilina sp.) till four centimeters wide and several millimeters thick were found.

- Dacycladacea were found in the thin sections. These green algae live in low energy environments. They are most abundant in warm shallow waters behind reefs (Genot, 1991).

- Serpula sp.: Worm tubes, they are present in the thin sections. They live on a hard substrate.

- Ditrupa sp.: Another type of worm tube that lives on soft or no substrate.

- Lithothamnium: This encrusting coraline algae is present in our thin sections and was probably most abundant in shallow waters (<30m).

- Planktonic foraminifera are also present in some of the thin sections. The exact species of the planktonic foraminifera were not determined.

2.4 Description of a Parasequence

On the west side on the Boltaña antiform many sequences are very good visible in the field. Many Parasequences are also present. We studied one of the parasequences (Fig. 16) also in the thin-sections. The thin-sections we studied are 275a, 275b 277 and 278b.

275a Biozone V (See paragraph 3.1 for a description of the biozones)

275b Biozone III

277 Biozone IV

278b Biozone V

The base of the parasequence consists of thin-bedded marls. Upwards more and more carbonate layers are present (gradual thickening upwards trend). This trend leads to thick to massive ‘layers’ of carbonate without marl. This thickening upward trend is the HST part of the parasequence. Then the trend switches to a thinning upwards trend; the TST. More and thicker marl layers are present upwards until there are no carbonate layers left. The grain size of the quartz is coarsening upwards in the HST part and fining upwards in the TST part of the parasequence.

All the paleobiological information (Fig. 16) derived from the thin-sections perfectly fits with this HST/TST model. The Miliolids, Nummulites sp., Assilina sp. and Discocyclina sp. increase to the transition between HST and TST then decrease again. The planktonic foraminifera decrease and increase in exactly the opposite way.

Fig 16. Details of the parasequence. From left to right: the erosion profile, lithology, grainsize, quartz percentage, Miliolids, Alveolina sp., Nummulites sp., Assilina sp., Discocyclina sp., planktonic foraminifera and the biozone.

3 RESULTS AND INTERPRETATION

3.1 Interpretation of the thin-sections

To determine the different environments represented in our thin sections, the foraminifera were plotted in the sections (see Appendix II and III). Each difference in the abundance of the foraminifera is the result of differences in the environment. On this basis we could determine six biofacies which are described below and each represent a different depositional environment. This was done using the different abundance of the different foraminifera, the large-scale regressive and transgressive trends and the information about the foraminifera present in the literature.

The foraminifera abundance of the used species are plotted. (See Fig. 17)

Also very important is the abundance of Perforate and Imperforate foraminifera. This can be used to determine whether there was a barrier present at that time or not (see chapter 2.2). Sometimes subfacies are recognised to express this difference.

Fig. 17. Paleoecological distribution of some foraminifera present in the central Pyrenees (modified after Luterbacher, 1984).

Biofacies Ia:

Beach

The environment is extreme sandy (Qtz content > 40% in most cases). The quartz-grainsize is medium to very coarse. No fauna is present, only a few miliolids and shellfragments.

Thin sections no.: HJ36a, HJ 55b, HJ 261a, HJ 273a, HJ 290a, and HJ 292.

Biofacies Ib:

Lagoon.

Shallow, restricted environment with the first foraminifera present: Miliolids. The environment is very sandy (Qtz content > 20%). The quartz-grainsize is ranging from medium to coarse. Together with the miliolids sometimes bryozoa and dacycladacea are present. In some thin sections Nummulites sp. are also present, but these are not ‘in situ'

Thin sections no.: HJ37a, HJ 41, HJ 48, HJ 251a, HJ 251c, and HJ 292.

Biofacies IIa:

Bay; restricted reef environment with sand.

The environment is very sandy (Qtz content > 25%). The quartz grainsize is medium to very coarse. For the first time more then one species of foraminifera is present: Together with miliolids, Alveolina sp. is often present in the thin sections. Sometimes also Rotalia sp. and Nummulites sp. can be found. Other organisms are Bryozoa, Dacycladacea, Micraster sp. and gastropods as well as shell fragments.

Thin sections no.: HJ6b, HJ 20b, HJ 215a, HJ 217, HJ 225, HJ 232, HJ 239b, HJ 240b, HJ 244b, HJ 246a, HJ 247b, HJ 247c, HJ 248a, HJ 250b, HJ 252c, and HJ 296.

Biofacies IIb:

Restricted reef environment without sand.

This is the same environment as biozone 2a. The only difference is the Quartz content, which is less then 25%. In biozone 2a there is probably more supply of sand. The location of

biozone 2a/2b is however the same in our environmental models. (Many) Miliolids and/or Alveolina sp. are present in the thin sections. Sometimes Rotalia sp. and Nummulites sp.

Thin sections no.: HJ228b, HJ 229b, HJ 252a, and HJ 265.

Biofacies III:

Carbonate shoal; healthy open shelf environment.

The quartz content is commonly < 25%, with Quartz grainsizes ranging from medium to coarse. The first Perforate foraminifera are present like Assilina sp. and Discocyclina sp together with Nummulites sp. Imperforate species like Alveolina sp. an Miliolids are still present. There are still no planktonic foraminifera. Other organisms found in the thin sections and in the field are a single Orbitolites sp., bryozoa and shell fragments.

Thin sections no.: HJ11a, HJ 57a, HJ 234a, HJ 235, HJ 254a, HJ 275b and HJ 253b.

Biofacies IV:

Upper slope.

The quartz content is commonly < 25%, with quartz grainsizes ranging from fine to medium. Sometimes more sand is present due to (proximal) turbidites. Perforate foraminifera such as Discocyclina sp. and Assilina sp. can be found. No Alveolina sp. and Miliolids are present anymore in this deeper environment. Other organisms and foraminifera in this environment are bryozoa, Ditrupa sp., serpulids, Micraster sp., a single Actinocyclina sp. and Operculina sp. as well as shell fragments.

Thin sections no.: HJ1, HJ 23b, HJ 25a, HJ 27b, HJ 61, HJ 67, HJ 69, HJ 197, HJ 214b, and HJ 259a.

Biofacies V:

Deeper slope.

The quartz content is commonly < 10%, with quartz grainsizes ranging from very fine to fine. The first planktonic foraminifera are present together with Discocyclina sp. Assilina sp. is not present anymore. Other organisms and foraminifera are Operculina sp., bryozoa, micraster sp. and shell fragments.

Thin sections no.: HJ69, HJ 188, HJ 202b, HJ 208, HJ 275a, HJ 278b, and HJ 285a.

Biofacies VI:

Proximal/Distal; deep open marine environment

The quartz content is commonly < 10%, with quartz grainsizes are max. very fine. Sometimes more sand is present due to (distal) turbidites. In this environment, only planktonic foraminifera are present. Other organisms found are Micraster sp. and a single crab.

Thin sections no.: HJ93b, HJ 92, (HJ 90b), HJ 86a, HJ 81, HJ 52, HJ 35a, HJ 103, HJ 155, HJ 174, HJ 179b, HJ 186, HJ 164, HJ 165, HJ 167b, HJ 200, HJ 277, HJ 283, and HJ 285c.

3.2 Interpretation of the stratigraphic sections

Section I west

'Section I (west)' is like 'section II (east) of Middle Eocene age. Section I has a total length of approximately 1100 meters. The base consists of middle to thick-bedded carbonates, interbedded with a marl layer. These sediments are deposited in a deeper marine environment, which is proved by abundant planktonic foraminifera. Some burrows are present. There is a 'shallowing upwards trend' visible; this trend is gradual although some parasequences (TST) are present. The shallowing up trend is proved by the existence of a platformslope facies: bally carbonates, preceded by deeper marine sediments. At 200 Meters above the base, the first miliolids 'in situ' occur and also Discocyclina sp., Nummulites sp. and Assilina sp. This gives evidence for a shallower environment; Looking at the biozones, there is a shift in the first 360 meters from biozone 6 to biozone 2 (=protected environment, containing Alveolina sp. and or miliolids). In the next 200 meters, there are only small changes in environment:

Biozone 1A en 2 occurs. Only in two cases the environment is not (so much) protected.

Slide 234a and 235 contain besides Alveolina sp. and/or miliolids also Nummulites sp.,

Discocyclina sp. and/or Assilina sp.; This is biozone 3, a shallow, open shelf environment.

In the context however, this biozone is nearer to the shore then biozone 3 normally is. There is no reef present and biozone model 2 is used to explain the HST.

In these 200 meters of section, the lithology consists of mostly middle bedded sandy carbonates; sometimes burrows and Micraster sp. are present together with some pyrite and shellfragments. At 580 Meters above the base, a change occurs. The environment first deepens en then shallows again. (Shift Biozone 2 to biozone 6, then back to biozone 1).

This trend occurs in ca 200 meters of the section. The lithology is middle to thick-bedded sandy carbonates with sometimes shellfragments, some Micraster sp. and pyrite (see also Appendix II section 1 west). At ca. 200 meters beneath the top of this section the environment dramatically deepens (biozone 1A to biozone 6), after which a gradual shallowing upwards trend (via parasequences, see also paragraph 2.4) from biozone 6 to biozone 3 occurs. After this shallowing upwards trend, the environment finally deepens (the platform is drowned, biozone 6 with abundant plankton occurs). The lithology consists of interbedded very thin-bedded marls and thick-bedded carbonates with gradual changes via thickening/thinning upward and coarsening/fining upward in the parasequences. In the thick-bedded carbonates, burrows and shellfragments are present, sometimes Micraster sp. The top of the section consists of very thin-bedded marls underlining the final drowning of the platform.

Section II East

The section has a total length of ca. 700 meters and is of middle Eocene age. The base is formed in a shallow, protected environment (biozone 2). In the basal 130 meters is a gradual deepening trend present until biozone 6. The lithology consists of middle to thick bedded carbonates interbedded with thin bedded marls (see also Appendix III section II East). The bedding type is mainly small-scale crossbedding. A few burrows, Micraster sp. and shellfragments are present. After this first trend the platform dramatically progrades (biozone 6 to biozone 1A)

Then there are 100 meters of section with a relatively static environment with biozone 1A and 2 are present. The lithology is mainly middle to thick bedded, sandy carbonate to sandstone with small-scale crossbedding. A few Micraster sp., burrows and shellfragments are present.

At 220 meters from the base a new change occurs, the gradual deepening and final drowning of the platform. (Change from biozone 1A to biozone 6 with abundant planktonic foraminifera). This deepening trend is extra underlined by the presence of glauconite and a

'platformslope facies' or 'bally carbonates'. The carbonates on the platform slope are stretched by 'slope gravity' and become bally. The lithology consists of middle to thick-bedded carbonates, with some thin bedded marl layers. The top of the concordant section consists of thin-bedded marls. (Final drowning also underlined by these marls).

Discordantly deposited on the marls are massflows and mainly turbidites, consisting of coarse middle to thick-bedded sandstones and conglomerates/breccia. Barnolas & Teixell interpret these massflows/turbidites as a submarine canyonfill.

3.3 Correlation and discussion of the stratigraphic sections

Dividing the east section in three megatrends and the west section in 5 megatrends can correlate the two sections.

Megatrend 1: A very gradual shallowing trend in the west section and an overall shallowing trend in the east section.

Megatrend 2: A TST and followed by a HST can be recognised in both sections however the strength of these trends is different. These trends represent an overall shallowing upward trend in the west section and a rapid deepening in the east section.

Megatrend 3 starts on both sections with a TST. However, in the east section this TST is the final trend in the section resulting in the drowning of the platform. In the west section, megatrend 3 continues (containing several HST’s and TST's) and is a period of stable shallow platform-conditions (biozone 1a/2).

Megatrend 4 is only visible in the west section and contains a deepening and afterwards a shallowing upward trend.

Megatrend 5 is the final trend visible in the west section, starting with several parasequences and resulting in the final drowning of the west section (a deepening/drowning trend).

The west and East sections first were correlated using field photographs and drawings. In a later stadium, the HST-TST trends from the thin-sections and the bedding trends have been used to make a final correlation (See Appendix IV).

The first three units (1a, 1b, and 1c) are data from correlations made in the field (See Appendix IV). Looking at the HST’s and TST's in the first three units a big HST in the West section correlates with smaller HST’s and TST’s in the east section. The big HST at the base of the west section however is not certain, and only based on the absolute decreasing amount of planktonic foraminifera.

At the base both sections have an overall shallowing upward trend and together with the field data, they can be relative easy correlated. The correlation of megatrend 2 can be explained by a platform, moving to the west: In this case the west section shallows, while the east section deepens. Megatrend 3 is a continuation of this process. Stable shallow platform conditions in the west section, while the east section continues deepening and finally drowns.

After having two more megatrends, the west section also drowns. Megatrends 4 and 5 cannot be correlated because in the east section part of the section has been eroded and filled with turbidites.

In general whole the section can be explained by a platform shifting to the west. But this west shift is alternating with small margin shifts to the east (progradational shifts) Evidence for this are the disappearing beds both to the east and to the west in the antiform (see fig. 18).

Fig. 18. Lateral disappearing beds in the antiform. Magnification just west of the top of the antiform, looking to the north.

In subsection I of the east section we found evidence of a progressive unconformity (Bertotti, pers. comm. 1997). This can be used as evidence for a fast deepening of the environment due to tectonics. The progressive unconformity occurred in the marls just below the discordance with the turbidites on the road to Sabiñanigo.

3.4 Results and discussion of additional research

In the field we found as part of our east section a big deposit of turbidites. We could determine the direction of flow at several locations in the section by using flute casts and other sedimentary structures associated with turbiditic deposits. The sediments have the same nature as the turbidites on the other side of the Boltaña antiform (Barnolas, A. & A. Teixell, 1994). We measured the direction of the paleocurrents in the turbidite section in the east.

Results of the measurements in the east section near Boltaña (in degrees):

130
110
124
135
130
130
140
104 average: 123

The average direction of the paleocurrents is south-east.

The direction of the paleocurrents in the turbidites west of the antiform is west-northwest or east-southeast. The exact direction of the flow was not clear because of bad preserved flutecasts.

To investigate the possibility that the turbidites on both sides of the Boltaña antiform have the same origin, more measurements at the other side of the anticline are needed. Barnolas, A. & A. Teixell investigated the west turbidite section and they say the carbonate platform collapsed and there was an incision of a submarine canyon of at least 800 meters deep.

We found more to the south in subsection I in the east section much more stratigraphy before the turbidite section started. This can be evidence the east turbidites section is also formed in a canyon. Just below the discordance, progressive unconformity occurred (see paragraph 3.3) and this can be evidence of a tectonic event that also could have caused the massive turbidite deposits.

We think it is possible that the nortwest-southeast canyon formed before the anticline formed. When the anticline formed afterwards, the middle part of the turbidite canyon was eroded and the sections of turbidites on both sides of the anticline were left. Maurice de cock concludes in his paper that the antiform developed due to salt doming and thrusting at the same time. He describes the massive turbidite deposits as a result of the doming.

If the two turbidite sections are not of the same origin (two stream-directions in the turbidites in stead of one stream-direction), the eastern turbidites can be a normal turbidite section and not a single canyonfill conform the conclusions of M. de Cock.

The Boltaña antiform itself is a north-south structure in an east-west mountainrange. This is strange because all other geological structures in the Pyrenees are east-west structures because of the north-south compression during the forming of the Pyrenees. Maurice de cock concludes in his paper that the antiform developed due to salt doming and thrusting at the same time.

4 CONCLUSIONS

In both sections (East and West flanks of the Boltaña anticline) the same overall trend is present: At the base of both sections deeper marine sediments, then a shallowing upward trend to a reef/restricted environment. After that the environment deepens again until the platform finally drowns. The West section starts deepening much earlier then the East section probably due to faulting.

The correlation of the two sections has many difficulties. The east section is much more compact than the west section. When the environment is deeper marine at the west section, the east section is shallow marine thus resulting in very different sediments and differences in thickness. (See Appendices II and III)

The two sections can still be correlated using field-photographs and using sequences (see paragraph 1.2 Fieldwork methods and materials). The final correlation of the different sections is shown in Appendix IV.

To make a clear overview we give a summary of the events:

First the base of the sections were deposited with deepwater platform deposits in the west and shallow platform deposits in the east.
Then the platform shifted to the west probably due to tectonic causes. Deep water deposits in the east and shallow water platform deposits in the west. Occasionally the platform margin shifted more to the east probably due to small relative sea level changes.
Then the platform drowned. Marl deposits indicating deep water in the west; the east section is not complete due to erosion by the turbidites.
To make a final conclusion on the origin of the east and west turbidite deposits more research is needed to investigate whether the two turbidite sections are deposited in one ‘canyon’ or deposited on both sides of the antiform independent from each other. This is necessary to determine if these deposits formed before or as a consequence of the forming of the antiform.
The antiform formed probably due to a combination of salt doming and thrusting (M. de Cock, 1998).
This part of the Boltaña region was uplifted as a consequence of the forming of the Pyrenees.
Erosion of the mountain-chain exposed the sections of the Boltaña antiform.

5 ACKNOWLEDGMENTS

First we want to thank the Free University in Amsterdam for giving us the opportunity to do sedimentological research in the Pyrenees. We want to thank Dr. T. Geel for supervising our fieldwork study and for teaching us the fieldwork methods and also for helping us with the correlation of the sections. Also we want to thank Dr. A. Fortuin, Dr. C. Bierman and Dr. G. Bertotti for helping us in the field. Last but not least we want to thank the rock-laboratory at the Free University in Amsterdam for making the thin sections.
We also want to thank Dr. R. Fraaije for identifying the fossil crabs and E. v.d. Braak for providing the photograph on the cover.

6 REFERENCES

Barnolas, A. & A. Teixell, 1994. Platform sedimentation and collapse in a carbonate-dominated margin of a forelandbasin (Jaca basin, Eocene, southern Pyrenees).Geology, v. 22,p. 1107-1110.

Cock, M de, 1998. Verslag Boltaña anticline. Free University Amsterdam.

Gaemers, P.A.M., 1978. Biostratigraphy, palaeoecology and palaeogeography of the mainly marine Ager formation (Upper Paleocene-Lower Eocene) in the Tremp basin, Central South Pyrenees, Spain. Leidse Geologische Mededelingen., 51: 151-231.

Geel, T., 1990. Microfacies van carbonaatgesteenten. Free University Amsterdam.

Genot, P., 1991. Cenozoic and recent Dacycladales, in: Riding, R. (ed.) Calcareous Algae and Stromatolites, 131-145.

Lunsen, H. A. van. Geology of the Ara-Cinca region, Spanish Pyrenees, Province of Huesca. Publications of the "Geologisch Instituut" of the Utrecht State University.

Wagoner, J.C. van, Posamentier, H.W., Mitchum, R.M., Vail, P.R., Sarg, J.F.,Loutit,T.S. & J. Hardenbol, 1988. An overview of the fundamentals of sequence stratigraphy and key definition. In: Sea-level changes: An integrated approach (ed. by C.K. Wilgus, B.S. Hastings,C.G. St C. Kendall, H.W. Posamrentier, C.A. Ross & J.C. Van Wagoner). Special publication, Society of economic Paleontologists and Mineralogists, Tulsa, 42, 39-45.

APPENDIX V List of thin sections and soft samples

Thin Sections:

Sample number Thin section no. Sample number Thin section no.

HJ 1                   46926                 HJ 214b                 46956
HJ 6b                 46927                 HJ 215a                 46957
HJ 11a               46928                 HJ 217                   48185
HJ 20b               46929                 HJ 225                   48186
HJ 23b               46930                 HJ 228b                 46958
HJ 25a               48175                 HJ 229b                 48187
HJ 27b               48176                 HJ 232                   46959
HJ 33                 46931                 HJ 234a                 48188
HJ 35a               48177                 HJ 235                   48189
HJ 36a               48178                 HJ 239b                 46960
HJ 37a               48179                 HJ 240b                 46961
HJ 41                 46932                 HJ 244b                 48190
HJ 48                 46933                 HJ 247b                 46962
HJ 52                 46934                 HJ 248a                 46963
HJ 55b               48180                 HJ 251c                 46964
HJ 57a               46935                 HJ 252a                 46965
HJ 61                 46936                 HJ 252c                 48191
HJ 67                 46937                 HJ 253b                 46966
HJ 69                 46938                 HJ 254a                 46967
HJ 77                 46939                 HJ 259a                 46968
HJ 81                 46940                 HJ 261a                 46969
HJ 86a               46941                 HJ 265                   46970
HJ 90b              46942                 HJ 268a                 46971
HJ 92                 48181                 HJ 273a                 46972
HJ 93b               46943                 HJ 275a                 48192
HJ 96                 48182                 HJ 275b                 46973
HJ 103               48183                 HJ 277                   48193
HJ 107               46944                 HJ 278b                 48194
HJ 155               46945                 HJ 283a                 46974
HJ 164               46946                 HJ 285a                 46975
HJ 165               46947                 HJ 285c                 46976
HJ 167b             46948                 HJ 290a                 46977
HJ 174               46949                 HJ 292                   46978
HJ 179b             48184                 HJ 296                   46979
HJ 186               46950                 HJ 301b                 46980
HJ 188               46951                 HJ 309                   46981
HJ 197               46952                 HJ 320                   46982
HJ 200               46953                 HJ 326b                 46983
HJ 202b             46954                 HJ 333a                 46984
HJ 208               46955

Soft Samples:

HJ 10
HJ 46
HJ 106a
HJ 176
HJ 236
HJ 287c
HJ 328

APPENDIX I Map