Synchronous detachment folds and coeval sedimentation in the Prepyrenean External Sierras (Spain): a...

Sedirnentology (1994) 41, 1001-1024

Synchronous detachment folds and coeval sedimentation in the Prepyrenean External Sierras (Spain): a case study for a tectonic origin of sequences and

systems tracts

H. MILLAN, M. AURELL* and A. MELENDEZ

Dpto. Geologia, Universidad de Zaragoza, 50009-Zaragoza, Spain

ABSTRACT

This paper reports on the structural and sedimentary evolution of the middle to late Eocene of the Prepyrenean External Sierras (southern Pyrenees, Spain). The initiation, duration and kinematics of a set of growth structures that developed in a shallow marine depositional setting is documented. The detailed analysis of the syntectonic marine sediments not only confirms the already known east to west progression of deformation, but also reveals the continued growth of the early formed structures as later ones propagate towards the foreland. The sedimentary units coevally deposited with these growth structures are arranged in four depositional sequences. Their boundaries correspond to flooding surfaces which grade basinwards into correlative conformities. They are also indicated by the presence of both angular unconformities and onlap geometries. Each depositional sequence generally consists of two systems tracts. The lower one, or transgressive systems tract, is formed by up to 400 m of azoic marls deposited in outer ramp areas. The upper one, or highstand systems tract, mainly consists of shallow siliciclastic and carbonate facies, up to 200 m thick, deposited in middle to inner ramp areas. These depositional sequences are interpreted to be controlled by regional tectonic pulses. An increase of tectonic activity resulted in the flooding of the basin and in the subsequent deposition of a thick succession of nearly azoic blue marls (i.e. transgressive systems tract). The overlying highstand systems tract developed following periods of diminished tectonism, with the consequent growth and progradation of shallow carbonate platform facies.

INTRODUCTION

Some recent works have dealt with the relationships between thrusting structures and their simultaneous sedimentation, especially with those related to fault- bend folds and fault-propagation folds (Martinez et al., 1988; Medwedeff, 1989; Zoetemeijer & Sassi, 1992). Nevertheless, there is little literature about detachment folds and coeval sedimentary deposits. These type of folds develop due to shortening above a detachment level and, unlike other fault-related folds, are not associated directly with ramps (Dahlstrom, 1970; Jamison, 1987, 1992).

If compressive deformation is submarine and sedimentation rate is high in relation to uplift rate, the preserved syntectonic, or growth, sediments will provide valuable information about structural and stratigraphical relationships, and hence about the timing,

*To whom correspondence should be addressed.

kinematics and rate of fold growth (Medwedeff, 1989). These characteristics are displayed by the marine Tertiary deposits of the Prepyrenean External Sierras, where sedimentation and synchronous detachment folding, along with excellent exposure, record the syntectonic evolution of the area.

This paper documents the character and evolution of a set of intrabasinal growth anticlines which developed during the Eocene and shows how they controlled the facies and thickness distribution of the coeval marine sedimentary units. The data suggest that sequences and systems tracts (i.e. third order cycles of Haq et al., 1987) may result from a non- linear variation of accommodation induced by local tectonics. Our results show that the folding of these Eocene anticlines (and, consequently, the accommodation created in the basin) was not a continuous process but included intermediate episodes of

1001

1002 H. Millan et al.

tectonic quiescence. Depositional sequences consist of stacked transgressive and highstand systems tracts. The lower transgressive systems tract is formed by a thick wedge of azoic marls and is related to a period of increased synsedimentary tectonic activity, whereas the upper highstand systems tract mainly consists of shallow marine units and developed when this tectonic activity diminished.

GEOLOGICAL SETTING

Structural remarks

The Prepyrenean External Sierras are located in the southern Spanish Pyrenees and represent the emer- gent front of the basement-involved Gavarnie thrust sheet (Seguret, 1972; lower thrust sheet, Muiioz et al., 1986; Muiioz, 1992). The Gavarnie sheet trends WNW-ESE and separates the overthrusted and almost undeformed Ebro Basin to the south from the piggy-back Jaca Basin (Ori & Friend, 1984; Labaume ef al., 1985) to the north (Fig. I). According to Labaume et al. (1985) the Meso-Cenozoic series of this unit was detached at the Triassic level with a southward movement related to the Gavarnie basament thrust.

The studied sedimentary units were deposited in the southern part of the Jaca Basin. This basin is the westernmost compartment of the South Pyrenean foreland basin. During the history of the latter, the geometry and infill of the sedimentary trough was mainly controlled by tectonism which caused tectonic subsidence related to sheet loading and subcrustal forces (Puigdefabregas et al., 1992). The Jaca Basin constitutes a synclinorium characterized by an array of compressive WNE-ESE striking structures with the exception of a series of N-S trending folds that outcrop in the southern margin of the basin, above the footwall of the Gavarnie unit. The Jaca Basin mostly developed during the Palaeogene. Its depo- centre was nearly parallel to the orogen and migrated southward by means of detachment and imbrication of the cover rocks, in response to the advance of the deformation front (Labaume et al., 1985; Barnolas et al., 1991).

In the studied area, the toe of the south directed Gavarnie thrust sheet consists of very reduced Meso- zoic series overlain by a much thicker Palaeogene one, both becoming thicker and more complete to the north and east. The initial stages of deformation affecting the External Sierras are recorded in one of their most distinctive features: the N-S trending folds (Fig. 1). The earliest N-S structures have grown since

early Lutetian and are located in the easternmost sector of the External Sierras.

The first attempts to explain the structure and evolution of the Sierras were given by Mallada (1878), Selzer (1934) and Almela & Rios (1951). The last of these proposed a westward migration of the deformation which, in turn, caused the so-called 'N-S folds' to be younger to the west. Puigdefabregas (1975) presented the first observations on the relationships between tectonic and coeval sedimentation, and proposed a clockwise rotation of the Gavarnie Nappe on the basis of palaeogeographi- cal incongruities. Recent results of palaeomag- netic (Hogan, 1993) and structural investigations (McElroy, 1990; Millan ef al., 1992) also document a clockwise rotation for the External Sierras and the adjacent Jaca Basin. Magneto-tectonic work in the western and central sectors of the External Sierras within the Arguis Formation, shows that the hangingwall has rotated at least 30" (E. Pueyo et ul., unpubl. observation).

The N-S trending folds of the External Sierras reveal the south to south-westward advance of deformation in the western sector of the south Pyrenean foreland basin during the Eocene and Oligocene times. Their present attitude is due to the progressive rotation of these originally oblique structures which are likely to be related to southerly directed thrust sheets. This assumption is based on data from the western end of the External Sierras where model experiments show that equivalent folds are un- ambiguously linked to the aforementioned type of thrusts (Millan & Pocovi, 1993).

Stratigraphy

The sedimentary record of the External Sierras is characterized by a stratigraphical pile alternating between competent and incompetent units (Fig. 2). The lowermost incompetent unit involved in the nonbasement deformation is the late Triassic succession of evaporites, mark and dolomites, constituting the main detachment level of this realm. The second unit is made up of 90 m of late Cretaceous limestones (Adra6n-Bona Formation), up to 70m of Garum- nian sandstones, mudstones and limestones (Tremp Formation), overlain by a succession of Lutetian shallow platform sequences up to 800 m thick (Guara Formation). These are overlain by 1 km of Bartonian to lowermost Priabonian incompetent strata (see Fig. 2B), mainly marls with some interbedded limestones (Arguis Formation) which pass up to shallow marine

Folding and sedimentation, southern Pyrenees I003


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and transitional siliciclastic deposits (Belsue-Atares Formation). The fourth and last unit comprises late Eocene-Oligocene continental clays, sandstones and conglomerates of the Campodarbe Formation (Fig.

The sediments studied herein correspond to the third package including both the Arguis Formation, the Belsue-Atares Formation and its western lateral equivalent, the so-called Yeste-ArrCs Formation (terminology from Puigdefabregas, 1975). They span from latest Lutetian to early Priabonian (Canudo et al., 1988, 1991) and are bounded by two regional

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unconformities, the lower one located on top of the shallow marine carbonates of the Guara Formation and the upper one at the bottom of the continental Campodarbe Formation. These unconformities are the boundaries of the Depositional Sequence 3 of McElroy (1990) which can be correlated with the previously defined Milany-Campodarbe Sequence (Puigdefabregas & Souquet, 1986). Further division of this sequence into higher order unconformity- bounded units in the External Sierras has been proposed by Canudo et al. (1991), Toledo (1991) and Montes Santiago (1992).

METHODOLOGY

Our results are based on the mapping of the sedimentary units and sedimentological facies analysis. Over 10 km of strata in 12 separated sections were measured and studied along some 40 km of the frontal Pyrenean thrust belt, in an area located in the central part of the External Sierras between Lusera and Villalangua. The middle portion of the External Sierras fold and thrust belt displays a suite of well exposed examples of N-S striking synsedimentary folds; these are, from east to west, the Gabardiella, Lusera, Pic0 del Aguila, BentuC de Rasal and Rasal anticlines (see Fig. 3 for location of sections and anticlines).

Facies description included textural classification and identification of fossil groups or trace fossils, other carbonate components and detritic grains. Field-work also included description of sedimentary architectures and measurement of palaeocurrent directions, lateral stratigraphical correlation studies and mapping of structures and syntectonic deposits along the thrust front. Local facies analysis and measurement of intermediate sections in order to evaluate possible lateral changes in facies and thickness were also carried out. Interpretations of depositional environments were based on bedding geometry and lateral continuity, fossil content, texture and composition.

DEPOSITIONAL SEQUENCES: DESCRIPTION AND

SEDIMENTOLOGICAL INTERPRETATION

Nature of the depositional sequences Figure 4 shows a synthetic representation of the measured sections showing the main pattern of distribution of thickness and facies. Four sedimentary

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units (i.e. DS. I-IV) have been distinguished and correlated across a section nearly transverse to the basin which displays the transition from inner areas located to the east to outer areas towards the west.

Based on both the magnetostratigraphical analysis carried out by Hogan (1993) in the Arguis and Salinas sections and the available palaeontological data (Canudo et al., 1988, 1991; Toledo, 1991), the time span of each sedimentary unit ranges between 0.8 to 1.7 Myr (time-scale from Haq et al., 1987). Boundaries between sedimentary units correspond to important breaks in the sedimentation and show sharp lithological and sedimentological changes. Moreover, in eastern proximal areas, some sequence boundaries are locally indicated by the presence of angular unconformities. Onlap geometries observed in the sediments overlying these unconformities are also significant features associated with these boundaries. These unconformities grade basinwards into correlative conformities (Fig. 4). According to the observed time span and taking into account the nature of the boundaries of these sedimentary units, they can be defined as depositional sequences (e.g. Haq et a/., 1987).

As a whole, each depositional sequence consists of a lower azoic marly unit and an upper carbonate unit bearing abundant benthic fossils. The lateral extent of carbonate units overlying the marly facies is progressively larger in younger sequences, with progradation advancing from east to west. Siliciclastic components also increase according to the same pattern and they become clearly dominant in the uppermost depositional sequence (Fig. 4). The main stratigraphical and sedimentological features observed in the four distinguished depositional sequences are outlined below.

Sequence I: blue mark and glauconitic sandy mark

Lower boundary and age

The lower boundary of the lowermost depositional sequence is an important unconformity which is locally represented by an encrusted and ferruginous surface on top of the massive bioclastic limestones of the Guara Formation. The unconformity is generally overlain by sandy marls containing abundant glauconite comprising rounded grains up to 2 mm in diam- eter. This is a stratigraphically condensed level, which can be very reduced in thickness or even absent at some localities (Hogan, 1993). No evidence for sub- aerial exposure was found in the studied area on top

of the limestones of the Guara Formation, and the unconformity has been interpreted as a drowning unconformity.

The age of this depositional sequence is well con- strained by both palaeontological and magnetostratigraphical data. According to Canudo et al. (1988, 1991), the glauconitic marls are very rich in planktonic foraminifera, which correspond to the Higginsi and lower part of the Pomeroli Biozones (latest Lutetian to early Bartonian). In agreement with these data, the boundary between the nannoplancton biozones NP-16 and NP-17 (i.e. early Bartonian) was identified as the top of the sequence (Toledo, 1991). Based on correlation of magnetostratigraphical data with the time-scale of Berggren et al. (1985), Hogan (1993) proposed an age of about 44.0 Ma for the transition between the Guara Formation and glauconitic mark (1.e. prior to Chron 19) and 43.0 Ma for the end of the glauconitic sedimentation (lower part of the sequence in the Arguis section).

Thickness and litholog,v

The thickness of this depositional sequence is highly variable, from 0 m in the Pic0 del Aguila and Gabardiella anticlines to up to 400 m in the Belsue syncline (Fig. 5). From Arguis to Rasal the thickness is more homogeneous and ranges between 100 and 250 m.

In most of the studied sections, the sequence mainly consists of unburrowed and nearly azoic blue marls. Quartz silt and fine bioclasts (echinoids, bivalves, benthic foraminifera) are locally recognized. Planktonic foraminifera are the most abundant fossil group in the lower part of the sequence. Decimetre to metre thick beds bearing an important amount of glauconite and quartz detrital grains are found interlayered with these blue marls. The detrital components increase in the BelsuC section and are also abundant between Bentue de Rasal and Rasal. To the east, in the Lusera section, the sequence is thicker and a number of cross bedded siliciclastic levels up to 15 m thick are found interfingered with the sandy marls. These siliciclastic levels form coarsening and thickening upward sequences.

Depositional setting

The blue marls represent sedimentation in a poorly circulated and relatively deep marine environment, located below storm wave base level. Interlayered glauconitic levels indicate low rates of sedimentation.

Folding und sedimentation, southern Pyrenecs 1009

Fig. 5. Distribution of depositional sequences in the Belsue syncline (Be, Belsut: village). The view is from the top of the Pic0 del Aguila anticline to the east (Lu, L6sera anticline). Sequence boundaries 1, 2 and 3 as in Fig. 1. Numbers 0 to 4 mark the upper boundary of the Guara Formation and the lower boundary of the Campodarbe Formation (i.e. Ca), respectively. The top of the first and second competent levels in DS. I1 are also indicated (a and b, respectively).

Fig. 6. The angular unconformity between DS. I and DS. I1 east of Lhsera village (i.e. west flank of the Gabardiella anticline).

The coarsening upward siliciclastic sequences are interpreted as shallow marine bars corresponding to a deltaic complex. Distal counterparts of this deltaic complex are the sandy mark located on top of the sequence. There is no indication of any depositional slope between these sandy marls and the shallow marine siliciclastic bars, and therefore the depositional setting can be defined as a low angle ramp.

Sequence 11: bryozoan platform

Lower boundury and age

The sequence boundary is a low angle angular unconformity east to Lusera (Fig. 6) where it is also marked by a sharp transition between the last cross bedded siliciclastic level and the overlying bioclastic marls.

Westwards, the unconformity is also indicated by onlap geometries in both flanks of the Pic0 del Aguila and Bentue de Rasal anticlines. Between Arguis and Rasal, the boundary was placed in the last glauconitic and sandy level found on top of the previous sequence. This level marks a sharp and significant lithological change which is not observed west to Rasal, where exposure is poorer and the marly lithology is far more homogeneous (i.e. basinwards correlative conformity).

According to the palaeontological data of Canudo et al. (1988, 1991), this sequence spans the middle and upper part of the Bartoniap (i.e. Pomeroli Biozone p p . ) . The Bartonian-Priabonian boundary (i.e. Middle-Late Eocene boundary) is located near the top of the sequence, around the ‘Bryozoan bed’

1010 H. Millun et al

Fig. 7. Distribution of depositional sequences in the Arguis syncline (Ar, Arguis village). The view is from the top of the Pic0 del Aguila anticline to the west (BR, the Bentut de Rasal anticline). Legend of sequence boundaries as in Fig. 5.

Fig. 8. Distribution of depositional sequences in the Lusera anticline (Lu, Lusera village). The view is from west to east (Ga, the Gabardiella anticline). Legend of sequence boundaries as in Fig. 5. Notice the outcropping beds on the right side of the photograph which correspond to the first competent level on DS. 11. These beds are overlain by a slight angular unconformity in the west flank of the Lusera anticline.

(terminology from Puigdefabregas, 1975; see also Montes Santiago, 1992).

Thickness und lithology

The maximum thickness of the sequence is 500 m in the Arguis syncline (Fig. 7). This thickness is halved in the crests of the Bentue de Rasal and Pic0 del Aguila anticlines, and is considerably reduced in the Gabardiella anticline (less than 50 m). A thickness of

300400 m is found west to Bentue and in the Belsut syncline.

The lower part of the sequence consists of nearly azoic and unburrowed blue mads bearing scarce benthic fossils, silty grains of quartz and plant remains. Fossil content and bioturbation increases in the upper part of the sequence, where abundant decimetre thick interlayers of bioclastic and siliciclastic graded beds are present. These are made up of debris of bryozoans, bivalves (Pectinids, Oysters),

Folding and sedimentation, southern Pyrenees 101 1

benthic foraminifera (Nummulites, Operculina) and echinoids. These beds can be amalgamated, forming thick carbonate successions, also associated with burrowed fossil wackestones. They can be grouped in three successive levels (Fig. 4). The lower level is restricted to the Belsue syncline and is bounded above by an angular unconformity which can be observed west to the Lusera anticline (Fig. 8). The intermediate level extends further west, into the Arguis syncline. Its top is located near Arguis village and consists of a 10 m thick siliciclastic and bioclastic bed formed by successive graded beds (b in Fig. 7). The third carbonate level prograded more to the west, and extends to Rasal (Fig. 9). This is the thickest carbonate level in this sequence (up to 20 m) and it was informally named the ‘Bryozoan bed’ (Puigdefabregas, 1975). West of Rasal the ‘Bryozoan bed’ laterally grades into a 0.2 m thick unit, which contains considerable glauconite and scarce, but well preserved skeletons of bryozoans, echinoids and bivalves.


4 s in the previous sequence, the depositional setting can be defined as a low angle slope carbonate ramp. The blue mark were deposited in a relatively deep marine environment, located in the outer part of the ramp, below storm wave base level. Carbonate-rich levels, which contain graded beds interpreted as storm deposits, were deposited in the middle and inner parts of the ramp. The transition between the middle and outer ramp can be observed near Rasal (i.e. between sections 4 and 5 in Fig. 4), where the depositional slope of the ramp is around 5”, in agreement with the predictions of Kenter (1990) for muddy carbonate platform flanks.

Sequence 111: pectinid platform

Lower boundary and age

West of Rasal (i.e. section 4, Fig. 4), the sequence boundary is represented by the above described glauconitic level. This glauconitic level pinches out basinwards, where the transition between the second and third depositional sequences is a correlative conformity located in a very homogeneous marly succession. East of Rasal, the boundary is marked by a sharp lithological change between bioclastic levels (i.e. ‘Bryozoan bed’) and a thick succession of nearly azoic blue marls. These marly successions display an

onlap geometry on the ‘Bryozoan bed’ in the west flank of both the Bentue de Rasal and Rasal anticlines (Figs 4 and 9). According to Montes Santiago (1992), this unconformity has a regional extent and can be correlated some 25 km eastward into the ‘Campodarbe discordance’, which is a very apparent angular unconformity, located near Campodarbe.

According to the available palaeontological data (Canudo et al., 1988, 1991), this sequence corre- sponds to the lower part of the early Priabonian, i.e. Semiinvoluta biozone p.p.

Thickness and lithology

Maximum thickness of this sequence is reached west of the Rasal anticline, where it varies between 500 and 700 m. Thicknesses of about 400 m are found in the more eastern synclines. The most reduced thickness of the sequence is recorded in the Gabardiella and Pic0 del Aguila anticlines (c. 100 m).

The lower part of the sequence consists of azoic and poorly burrowed mark which are overlain by shallow carbonate facies (Fig. 4). Thicker and well developed carbonate facies are found between Bentue and Rasal, where the upper part of the sequence consists of coarsening and shoaling upward parasequence sets, between 10 and 20m thick. Elemental parasequences are formed by couplets of fossiliferous marls and decimetre to metre thick wackestones to packstones bearing well preserved skeletons of pectinids, echinoids, bryozoans, oysters, benthic foraminifera and solitary corals. This facies sharply grades to the east into the more marly facies recorded in the Arguis syncline. Laterally consistent decimetre thick beds formed by skeletal packstone are commonly found interlayered with these marls. Benthic foraminifera are the predominant fossils in these beds. The proportion of these skeletal beds over the mark increases eastwards in the BelsuC syncline, where cross bedded siliciclastic lobes up to 5 m thick are also recognized. East of Belsue (Lusera, Gabardiella) most of the sequence consists of cross bedded siliciclastic facies (i.e. BelsuC-Atares Formation).


The depositional setting may be defined as a low angle carbonate ramp. Marly facies represent outer ramp facies deposited below storm wave base level. Carbonate facies in the middle part of the ramp developed above storm level and consist of a highly

1012 H. Millan et a].

Folding and sedimentation, southern P.vrenees 1013

productive benthic community. Fast creation of accommodation in this middle part of the ramp resulted in repeated flooding of the basin. Progressive shallowing of the platform via rapid carbonate production resulted in the filling of this accommodation, showing the arrangement of facies in parasequences and parasequence sets. The inner ramp facies found to the east represent more restricted marine environments, with marly deposition and interbedded storm beds. Clastic input related to a deltaic complex increases eastwards.

Sequence IV: siliciclastic platform and coral growth

Boundaries and age

The lower boundary of this sequence is marked by a significant lithological change across the studied area. From Bent& to BelsuC villages it roughly coincides with the boundary between the Arguis Formation and Belsue-Atares Formation. In these instances, cross bedded siliciclastic facies and associated sandy marls are found overlying the carbonate skeletal facies of the previous sequence. East of BelsuC, the boundary is located in the upper part of the Belsue- Atares Formation (sections 10-12, Fig. 4). West of Rasal, the sequence boundary is also represented by a sharp lithological change, and blue marls overlie the fossiliferous levels located on top of the previous sequence. The sequence boundary is a correlative conformity westwards, in the Villalangua section (section 1, Fig. 4).

The upper boundary is an important unconformity and defines a major sequence boundary in the frontal Pyrenean thrust belt (Puigdefabregas & Souquet, 1986; McElroy, 1990; Canudo et al., 1991; Montes Santiago, 1992). The unconformity represents the marine-fluvial transition in the studied area (i.e. Belsue-AtarCs Formation and Campodarbe For- mation boundary). The data presented by Hogan (1993) show that this transition took place at the same time in separated sections (i.e. 39.6 Ma in the Salinas and Arguis sections; sections 2 and 9) with no evidence for lateral changes between the BelsuC-

Atares and the Campodarbe formations in this middle part of the External Sierras. Canudo et al. (1 988) also postulate the presence of a sedimentary hiatus between both formations.

The available palaeontological data are provided by the Arguis section, where Sequence IV corre- sponds to the BelsuC-Atares Formation. Canudo et al. (1988, 1991) placed the early-late Priabonian boundary around the boundary between the Belsue- Atares Formation and Campodarbe Formation in the Arguis section, and therefore Sequence IV was deposited during the upper part of the early Priabonian. However, Hogan (1991) dated the sandstones of the BelsuC-Atares Formation in the Arguis section at 40.3-39.6 Ma, i.e. a younger age, from the latest Bartonian to the early Priabonian (time-scale from Berggren et al., 1985).

Thickness and lithology

Maximum thicknesses of 200 and 300m are found near Arguis and Rasal, respectively, whereas the thinner record of the sequence (c. 50m) is found in the Gabardiella, Pic0 del Aguila and Bentue anticlines.

Three separate facies are recognized. The sequence is formed by sandy mads and cross bedded sandstones arranged in coarsening upward parasequences in the east (BelsuC-Atares Formation). The second group of facies appears westward of the west flank of the Bentue de Rasal anticline, where marls and siliciclastic facies are still dominant, but they consist of tabular beds with planar or low angle lamination and frequent preservation of symmetric ripples (Yeste-Arres Formation; Puigdefabregas, 1375). Interlayered bioclastic beds of bivalves and benthic foraminifera are also present. The third group of facies are carbonates located west of Rasal. They consist of blue marls and interlayered skeletal graded beds, these sharply grading into a coral reef level. This level displays a lenticular or biohermal geometry showing maximum thickness between Rasal and Yeste (3040 m). The facies consists of poorly sorted

Fig. 9. Aerial photograph the Bentue de Rasal anticline. The sketch shows the previously folded thrust sheet. Sequence boundaries are indicated by numbers (see Fig. 1 for legend). Notice the fan of beds recorded in the west flank of the anticline at the bottom of both Sequences I1 and 111. Both onlap geometries over the ‘Bryozoan bed’ (i.e., number 2) and the thinning of the Campodarbe Formation strata onto the crest of the aticline can be also observed. Legend of sedimentary units: 1 Keuper facies, 2 Adraen-Bona and Tremp formations, 3 Guara Formation, 4 Arguis and Belue-Atarks formations, 5 Campodarbe Formation.


boundstones and floatstones of branching and massive hermatipic corals, bryozoans, calcareous algae, bivalves and benthic foraminifera. This reefal level can be sharply overlain by the siliciclastic facies of the Yeste-Arres Formation (section 2, Fig. 4). However, in the areas where the reefal level is well developed, it may also display an intermediate transitional facies with bioclastic levels rich in coral debris that interfinger with the siliciclastic beds of the Yeste-Arres Formation (section 4, Fig. 4).

Depositional set I ing

The coarsening upward sandstone sequences of the Belsue-Atares Formation correspond to successive delta lobes prograding on to prodelta maris. Accord- ing to palaeocurrent data, prevailing deltaic progradation was to the north-west. However, if the above mentioned clockwise rotation of the External Sierras is considered, this progradation was actually directed to the west. This restored direction of progradation is more congruent with the Eocene palaeogeography of the overall Jaca Basin (see also Puigdefabregas, 1975). The sandstones of the Yeste-ArrCs Formation were deposited in a shallow marine to littoral environment. Evidence of wave reworking in a roughly NW-SE direction is also found in this unit. These facies grade basinwards into the coralline facies, located on the middle ramp, above storm wave base level. As in previous sequences, the outer ramp facies are represented by blue marls.

THE N-S TRENDING ANTICLINES: DESCRIPTION AND KINEMATICS

Structural style and development

The structural position of the studied N-S trending folds (all of them situated over a north dipping footwall ramp) and the excellent outcrop control on subsurface structure as a result of deep erosion combine to provide a complete representation of these folds. Mapping and facies analysis of the area gives both sufficient data to constrain a unique structural and kinematic interpretation and an accurate idea of the prevailing structural style.

The most distinctive feature of these anticlines is the isoclinal folding of the detachment unit in their core. The concentric geometry of the folds along with the isoclinal folding of the detachment allow their definition as concentric lift-off detachment folds (terminology based on Mitra & Namson, 1989). Note

that there is a remarkably well exposed folded thrust sheet in the core of the BentuC de Rasal anticline. This previous thrust affected the base of the Eocene marls (Arguis Formation) and the late folding of both the hangingwall and the footwall gave rise to a particular type of double hinge anticline (Fig. 9).

The Rasal anticline is the westernmost high amplitude N-S trending fold. It is an asymmetric parallel fold with a steeper west facing forelimb and a more gently dipping backlimb. It was mainly active after the deposition of the Campodarbe Formation, which was detached from the crest of the anticline along a decollement level located at the bottom of this sequence. The high amplitude of this fold along with the absence of any field evidence of a ramp associated with it suggest that this structure is also a detachment fold.

The Gabardiella and Lusera anticlines also present a different fold style and fold shape. The Gabardiella anticline is a growth structure that represents the tip line fold of the eastward verging Gabardiella backthrust. The competent unit of the fold depicts a parallel, tight form offset by a strongly dipping thrust that places a hangingwall ramp in Triassic strata over a footwall ramp in Lutetian strata (Guara Forma- tion). To the south, the Gabardiella backthrust dips at roughly 35" to the west and places a hangingwall flat in Triassic strata over a footwall ramp in the Belsut-AtarCs Formation, indicating a southward increase in displacement. The Lusera anticline is a NW-SE trending minor fold-thrust that branches to the east with the Gabardiella backthrust. A complete exposure of this south-west verging fault- propagation fold can be seen 3 km south of the study area, where this structure has a hectometer broad geometry with a tip point located at the bottom of the Lutetian limestones, implying relatively low angle displacement.

In the examples described so far the existence of a predominant fold-thrust style (i.e. detachment folds) has been recognized. Considering that the first and the lower part of the second unit (i.e. Adratn-Bona and Tremp formations, Fig. 2A) are pre-growth strata in the External Sierras, the occurrence of different foldhhrust styles, at least during the first stages of deformation, is a function of both the mechanical stratigraphy and the stress path (which in turn depends on burial and regional tectonics) of these rock packages (Jamison, 1992). .

Two geological aspects favour the development of detachment folds in fold-thrust belts: the existence of a ductile decollement level (Jamison, 1987; Mitra &

Folding and sedimentation, southern Pyrenees 1015

r 1

p%. . . . . . . . . . . . . . . . . . . . . .......................... . . . . . . . . . . . . . * . . . . . . . . . . . . . . . . . . . . . . . . ....., . . . . . . . . , . . . .. , , . . . . . . . . . . . . . , , , .:: ............................. . . . . . . . . . . . . . . \. ..... .... . . . . ................................ J

Fig. 10. Model showing three stages in the evolution of the growth of a class 1 detachment fold with strata deposited at high sedimentation rate during deformation. For the specific case simulated here, folding was initiated before the deposition of the growth strata, as is seen at the Arguis and Bentue de Rasal anticlines at the onset of Sequence I. The loose line of the shortening profile has been drawn considering the syntectonic character of the strata. See text for further explanation.

Namson, 1989) and, in general, a low depth of burial (Jamison, 1992). The combination of both factors is present in the External Sierras. On the one hand, the upper Triassic deposits have the appropriate rheo- logical properties to behave as a decollement zone and, on the other hand, the majority of the N-S trending folds nucleated under a thin water column and continued growing at relatively shallow depths.

Kinematics

According to Suppe et al. (1992), an axial surface that moves with respect to the rock during fold

development is called an active axial surface. In contrast, the surface fixed to the material is an inactive or fixed axial surface. This has a direct application to detachment fold kinematics. A detachment anticline may grow in three different ways: with a fixed anticline axial surface (class l), with an active one moving toward the foreland or to the hinterland (class 2), or with a combination of both (class 3). In other words, the structure may develop with a fixed tip point (i.e. stick point), or a foreward or backward moving tip point (J.A. Mdiioz, pers. comm.). As a result of this, the shortening linked to a class 2 detachment fold is just limited to one side of the fold,


r I I

Fig. 11. Model showing three stages in the evolution of the growth of a class 2 detachment fold with strata deposited at high sedimentation rate during deformation. Note that the earliest hinge zones and their associated syntectonic deposits are progressively involved in the limb of the structure. The loose line of the shortening profile has been drawn considering the syntectonic character of the strata. See text for further explanation.

while that involved in a class 1 detachment fold affects both sides of the structure. This type of behaviour is consistent with the experimental modelling designed by Blay el al. (1977) to simulate the development of large scale folds in a bedded sedimentary succession underlain by a less competent unit.

As with thrust related folds, the kinematic history of detachment folds may be unravelled thanks to the occurrence of associated syntectonic deposits. Recon- struction of kinematics can be reliably accomplished because the geometry and attitude of growth strata not only depends on erosion and sedimentation rates,

but also on the type and rate of deformation. As a consequence, growth structures typically display wedge shaped patterns, in which sedimentation may take place over limbs or the crest of the growth fold.

Assuming n o erosion and that the sedimentation rate exceeds the rate of fold growth on both flanks of the fold, the syntectonic sequences of a n ideal class 1 detachment fold will onlap onto the anticline, covering the structure and giving a syxnnietrical pattern relative to the fixed axial surface of the fold (Fig. 10). In this case, growth strata will display a convex upwards profile which in turn will become increas-


ingly wider with time. Taking into account the aforementioned premises, the latter would be caused by the simultaneous deposition and folding of syntectonic sediments on both flanks of the anticline. Both the Pic0 del Aguila and the Bentue de Rasal anticlines clearly reflect this type of geometry, and they can therefore be classified as class 1 detachment anticlines (see also Figs 1 and 4).

In contrast, during the development of a class 2 detachment fold, the earliest hinge zones and their associated syntectonic deposits will be progressively involved in a limb of the anticline. This, regardless of the symmetry or asymmetry of the detachment fold, will give rise to peculiar changes in the facies and thickness distribution of the synsedimentary sequences relative to the latest axial surface of the fold (Fig. 11). Although the stratigraphical pile thickness gradually diminishes towards the west, Fig. 4 shows a distinctive thinning of Sequence I1 between the Rasal anticline hinge and its corresponding western syncline hinge and, furthermore, Sequences 111 and IV clearly thicken mainly to the west of the anticline hinge. These geometric features not only indicate the syntectonic character of the Rasal anticline, but also suggest this structure to be an asymmetric class 2 detachment fold.

Timing Even though there is an overall east to west progression of deformation in the External Sierras (Almela & Rios 1951; Puigdefabregas 1975), a detailed study of the Eocene syntectonic sediments reveals continuing and simultaneous growth of the early formed structures as later ones propagate towards the foreland.

This type of behaviour has been predicted by Blay et al. (1977) and Dixon & Liu (1992) experimenting with analogue models. Synchronous thrusting and sedimentation has also been described in the eastern sector of the central Pyrenees deposits within the Cadi thrust sheet (Martinez et a/., 1988). It has been also described to explain the presence of ‘out of sequence’ thrusts, and imbricate stacks on the fore- limbs and crests of folds and structural culminations (Boyer, 1992).

In the studied folds, one of the factors that could explain their simultaneous movement is the existence of the Triassic decollement horizon that facilitates the transfer of displacement below the folds. Another factor could be the lack of important structural discontinuities between them that could transfer shortening out of the folding.

SEDIMENTARY AND STRUCTURAL EVOLUTION

Evolution of the N-S trending anticlines

Figure 12 shows a set of schematic restored profiles representing the successive setting of the middle and late Eocene sedimentary units. These profiles, along with the cross section presented in Fig. 4, show the syntectonic character of each sequence in relation to its associated growth fold. The initiation and duration of the growth structures as well as the syntectonic attitude of their related growth strata are shown in Fig. 13. Note that initiation of the synsedimentary activity of at least two of the studied folds, i.e. Pic0 del Aguila and Bentue de Rasal anticlines, was prior to deformation of Sequence I, as indicated by growth strata of the Guara Formation.

Figure 13 also shows that the Gabardiella and Pic0 del Aguila anticlines depict simultaneous and ‘continuous’ activity during the whole recorded period. This activity reaches the earlier deposits of the Campodarbe Formation. A slight angular unconformity, located within Sequence I1 to the west of Lusera village, represents the initiation of the movement of the Lusera anticline (see also Fig. 8). The part of Sequence 111 related to the Lusera anticline progressively loses its syntectonic character, this is, however, preserved on the Gabardiella anticline.

Growth of the BentuC de Rasal anticline is recorded after cessation of the displacement of the thrust fault that off-sets Sequence I and perhaps the lowest strata of Sequence 11. The fold maintains its growth at least until the deposition of the Campodarbe Formation lower units. Lastly, the syntectonic activity of the Rasal anticline has been recognized, during the deposition of Sequences 11, I11 and IV.

Sedimentary evolution

An important feature observed in the studied sequences is that, as a whole, each one of them consists of two stacked units. The lower unit is formed by up to 400 m of azoic mark deposited in outer ramp areas, below storm wave base. The upper one mainly consists of shallow siliciclastic and carbonate facies, up to 200m thick, deposited above storm wave base level (i.e. middle to inner ramp areas). As explained above, the boundaries of these sequences generally correspond to flooding surfaces, as it shows the sharp transition between the shallow facies and the overlying azoic mark


3 4 5 6

Fig. 12. Sedimentary evolution shown in four successive steps. Schematic restored profiles represent the successive setting the middle and late Eocene sedimentary units (Sequences I to IV). The arrowheads indicate periods of growing of the anticli structures (the lateral migration of the hinges of the anticlines is due to the restoration of the profiles). Unornamented are in Sequences 1 and 11 correspond to marine water. Numbers in the lower part of the figure indicate the studied sections (s Fig. 3 for location).

Another feature that should be pointed out is the geometry of the sedimentary bodies which include the shallow carbonate and siliciclastic facies. They are located on top of each one of the four distinguished depositional sequences and their basinwards progradation is progressively larger in younger sequences (Fig. 4). On the other hand, the thickness of this shallow facies is more constant and widely distributed than the underlying blue marls, which encom- passes the larger thickness variations, as clearly shown by the presence of both onlap geometries and fans of beds which pinch out toward the crest of the growing anticlines. These onlap geometries would reflect the syntectonic activity of the anticlines. Con- versely, and assuming relatively steady sedimentary rates for both marly and carbonate units, the more homogeneous distribution of thickness in the overlying carbonate sedimentary bodies would indicate a lower influence by the anticlines creating differential accommodation in the sedimentary basin.

In our model, the rate of anticline growth is not constant when a single depositional sequence is considered. In fact, the onset of a sequence is marked by a sharp increase in the tectonic activity, resulting in a rapid creation of accommodation in the basin (i.e.

flooding of the sedimentary basin), which is lat filled up because the high sedimentary rates (i.e. bll marls). Basinward progradation of shallow facies the second episode takes place in subsequent periol of decrease in the tectonic activity, resulting in lower and more homogeneously distributed creatic of accommodation.

In the proposed model, the drowning unconforn ties or flooding surfaces at the onset of the sequenc as well as the growth anticlines act as tector markers in relation to creation of new accomm dation in the basin due to tectonic activity. T: former is related to the tilting of the whole southe platform due to both the load produced in t hinterland by the thickening of the crust ai to subcrustdl forces (Puidefabregas rt a/., 199 Barnolas & Teixell, 1992). The growth anticlines a oblique structures that register a part of the shorte ing associated with the thrust sheet moveme (Millan et al., 1992).

DISCUSSION On the origin of the sequences and systems tracts According to the sequence stratigraphy approa, (e.g. Haq et al., 1987), the lower blue marls and t

Folding and sedimentation, southern Pyrenee3 1019

SEQUENCE IV

SEQUENCE 111

SEQUENCE II

SEQUENCE I

7 8 3 10 11 12 .- -- --,---

Fig. 12. continued

W E

Fig. 13. Summary of the evolution of the growth structures (legend of the anticlines as in Fig. 3). The areas where an onlap geometry in the syntectonic growth strata was observed are enhanced (pattern 1). Pattern 2 indicates notable thinning of the strata with local onlap geometries on the crests of the anticlines. The angular unconformities observed in the eastern anticlines are also located ( 3 ) . Time scale according to Haq et al. (1987).

upper shallow carbonate and siliciclastic facies may flooding surfaces and therefore sequence bound- correspond to the transgressive and highstand sys- aries and transgressive surfaces merge such that the tems tracts, respectively. Sequence boundaries are lowstand systems tract is absent.


Distribution of facies in depositional basins resulting in stacked depositional sequences and systems tracts is governed by the relationship between the variation of the accommodation (i.e. relative sea level changes) and the sedimentary supply and/or carbonate production (e.g. Aurell, 1991). Studies in many ancient sedimentary basins have shown that the variation of the accommodation is not a linear function but it has a cyclical variation of several orders of magnitude. These cycles have been classified from first to fifth order according to its magnitude. High frequency cyclicity with orders of magnitude of 0.1- 0.2 to 0.01-0.02 Myr (i.e. fourth and fifth order cyclicity, Mitchum & Van Wagoner, 1991) has long been attributed to climatic sea level changes caused by Milankovitch effects (e.g. Hays et al., 1976). Longer term relative rises or falls of sea level (i.e. first or second order cycles) have been related to the world-wide redistribution of continent and oceans (e.g. Heller & Angenvine, 1985). The discussion is more open when the origin of the so called third order cycles is considered. These cycles are at the scale of 1 Myr and may generate depositional sequences. Alternative explanations have been offered to explain the origin of these sequences, either due to world-wide eustatic changes (e.g. Haq et al., 1987) or due to tectonics at different scales (e.g. Cloetingh, 1988).

In this paper we have reported on an example of tectonically induced third order cycles, which result from non-linear tectonics that cause sedimentation of a set of depositional sequences. Sequence boundaries are commonly represented by flooding surfaces. Depositional sequences consist of two members. The lower member is formed by a thick wedge of azoic mark that were deposited during a period of relatively high tectonic activity (i.e. transgressive systems tract). The upper member developed when this activity diminished, with the consequent growing and progradation of shallow carbonate platform facies (i.e. highstand systems tract). Accommodation rates are only higher than sedimentary rates at the onset of the sequences, resulting in both the flooding of the platforms and their setting below storm wave base level, where sedimentation of blue mark becomes predominant.

According to Verges & Riba (1991), during the growth of a slightly vergent anticline, syntectonic strata folded within the adjacent syncline will depict a simple fan shape. Considering no erosion and depending on the relationship between uplift rate (Ur ) and sedimentation rate (Sr), growth strata will

be deposited in an offlap attitude if Ur>Sr. Con- versely, if Sr> Ur, growth strata will display an onlap geometry or will become thinner when passing over the crest of the fold. Changes produced in the relationship between UrlSr will cause the superposition of different geometries among syntectonic sets of beds. In the studied area, the observed syntectonic features related to the growth of the detachment anticlines show that during the development of the sequences, accumulation rates were generally higher than the fold uplift rates.

As stated before, in our model we assume steady sedimentary rates into each depositional sequence. The diminished deformation of the second stage (i.e. highstand systems tract) of each sequence represents a slowing of deformation. However, applying an alternative ‘autocyclical model’, it is possible that, within each depositional sequence, the clasticl carbonate units were deposited much faster than were the marly units. In this case, steady deformation would be reflected by reduced deformation in the clasticlcarbonate units. Actually, there is insufficient time control on the sequences to be able to show that there are no significant changes in the rate of deposition within each sequence. Nevertheless, this alternative model fails to explain the generation of the sequence boundaries at the scale of the third order cycles (i.e. depositional sequences). The magnitude of the flooding of the basin at the onset of the sequences cannot be precisely measured, but an estimation based on the facies analysis shows that this flooding was, at least, of some 50m (i.e. a sharp change from shallow platform carbonates to marls deposited below storm wave base level). It is unlikely that this deepening of the basin originated by steady subsidence. Note that the ‘autocyclical model’ was originally defined to explain some metre thick, shallowing upward sequences (Ginsburg, 1971).

Eustatic third order cycles

In the previous section we discussed the role of tectonics in generating sequences boundaries and systems tracts. Non-linear local tectonics have been proposed herein as the main cause of the observed third order cycles. Several previous works studying the nature and origin of the depositional sequences and sequence boundaries in the Eocene south- western Pyrenean foreland basin hive also shown that they are intimately related to thrust tectonics (e.g. Garrido-Megias, 1973; Mutti et al., 1985;


A B

Fig. 14. Comparison between the timing of the depositional sequences defined herein (A) and the Haq et al. (1987) third order cycles (B).

Puigdefabregas & Souquet, 1986; Remacha et al., 1987; Pujadas et al., 1989; McElroy, 1990; De Boer et al., 1991; Barnolas & Teixell, 1992; Remacha & Oms, 1992; Puigdefabregas et al., 1992).

Further discussion can be reported on the possible effects in the sedimentary record on other extrabasinal factors of control such as third order eustatic cycles. In a recent review, Vail et al. (1991) state that third order tectonic event (i.e. folding, faulting and diapirism) ‘may enhance or subdue sequences and systems tract boundaries, but does not create them’ as it shows that ‘the ages of sequence boundaries when dated at the minimum hiatus at their correlative conformities match the age of the global falls and not the plate tectonic event causing the tectonism’ (Vail et al., 1991, p. 619-620). Some recent works have reported on the apparent matching between some of the timing of the Pyrenean marine Palaeogene sequences and the eustatic third order cycles proposed in the Haq et al. (1987) cycle chart. Such a coincidence suggested that eustacy was the driving mechanism for the Palaeogene sequences in the south Pyrenean foreland basin (e.g. Luterbacher et al., 1991).

Figure 14 shows a comparison between the sequences deduced in our work and the Haq et al. (1987) third order cycles. There is a close match between the number of sequences proposed during the same time span in both instances. Moreover, as the available palaeontological data in the External Sierras are not conclusive, some correlation between the sequence boundaries deduced herein and the

boundaries of the Haq et al. (1 987) third order cycles is possible (see also Canudo et al., 1991; Toledo, 1991). However, this is not a conclusive argument to prove the role of eustacy in generating the different stacked sequences. On the contrary, the systematic linking between flooding surfaces (i.e. sequence boundaries) and an overall increase in tectonism supports the hypothesis of a tectonic origin for the distinguished depositional sequences.

The tectonic model exposed herein clearly contra- dicts the one previously proposed by Toledo (1991) for the External Sierras at the same time span. Toledo (1991) assumed tectonics to be a process controlling features such as the geometry of the basin or the relative thicknesses of the systems tracts. Superimposed world-wide eustatic changes (i.e. third order cycles TA. 3.5, 3.6. and 4.1 in the Haq et al., 1987, cycle chart; see Fig. 14), however, were respon- sible for the development of the successive depositional sequences. A similar model was proposed by Seguret (1991) in the nearby Eocene Tremp Basin.

Our tectonic model is, nevertheless, in agreement with the one recently proposed by Barnolas & Teixell (1 992) for the early and middle Eocene of the overall Jaca Basin. These authors also postulate the alternation of periods of high tectonic activity (related to the setting of the successive southern Pyrenean thrust sheets) generating both drowning unconformities in the shallow carbonate platform and onlap geometries in the overlain deep marine sediments (i.e. glauconitic marls, turbidites). The following period of tectonic quiescence resulted in the aggradation and growth of the carbonate platforms. A correlation between a set of Palaeogene depositional sequences with the emplacement of the southern Pyrenean thrust sheets was previously proposed by Puigdefabregas et al. (1986) east of the studied area.

Further similar research on the shallow marine sediments of the middle and late Eocene in other separate areas of the Jaca Basin or in other realms of the Pyrenean foreland basin may show the geo- graphical extent of the depositional sequences identified in the External Sierras. This research will give more information about the extension of the tectonic agent which created the depositional sequences identified herein.

Orbital forcing

In addition to the tectonic and/or eustatic control at the scale of the third order cycles, the possible role of orbital forcing resulting in larger order eustatic cycles


(i.e. fourth and fifth order cycles) can be also considered. The control of the thrust sheet movements on the character of the sedimentary fill in the nearby Tremp-Graus Basin (South Central Pyrenees) was documented by De Boer et al. (1991). These authors also reported on the influence of possible climatic changes at the Milancovitch scale (i.e. 20 kyr preces- sion cycle and 100 kyr eccentricity cycle) causing variation in sedimentary supply, which resulted in the alternation of coarse and fine grained intervals.

In the studied sediments there are several pieces of evidence that show the presence of larger order cyclicity superimposed on the tectonic, third order cyclicity. The more apparent example is shown in the highstand systems tract of Sequence 111 in the sections located between Bentue de Rasal and Rasal, which display coarsening and shoaling upward parasequence sets, between 10 and 20m thick. Elemental parasequences are formed by couplets of fossiliferous marls and decimetre to metre thick fos- silliferous wackestones to packstones. Rapid creation of accommodation resulted in repeated flooding of the basin. Progressive shoaling of the platform induced by fast carbonate production resulted in the filling of this accommodation. Note that these parasequences are located in the part of the sequence that is considered to be in a period of tectonic calm (i.e. highstand systems tract). This observation re- inforces the idea of an extrabasinal control for the creation of this fast and cyclical accommodation.

CONCLUSIONS

1. The evolution of a set of growth structures observed in the central part of the External Sierras (southern Pyrenees) which developed during the Middle-Late Eocene in a shallow marine depositional setting has been documented. The detailed analysis of the syntectonic marine sediments not only confirms the already known east to west progression of deformation, but also reveals the continued growth of the early formed structures as later ones propagate towards the foreland. 2. Two styles of growth of the Eocene anticlines have been described. Most of them (i.e. Pic0 del Aguila, Bentue de Rasal) developed with a fixed tip point (class 1 detachment fold). The Rasal anticline is an example of a foreward moving tip point (class 2 detachment fold). The syntectonic sequences covering the class 1 detachment fold onlap onto the anticline, covering the structure and depicting a

symmetrical pattern relative to the fixed axial surfaa of the fold. In contrast, during the development of s class 2 detachment fold, the earliest hinge zones anc their associated syntectonic deposits are progressivelj involved in a limb of the anticline, giving rise tc distinctive changes on the facies and thickness distri. bution of the synsedimentary sequences relative tc the latest axial surface of the fold. 3. This paper also identifies four depositional sequences for the Middle-Late Eocene of the Exter- nal Sierras. These depositional sequences span from latest Lutetian to early Priabonian and have a length ranging between 0.8 and 1.7 Myr. Sequence boundaries are represented by angular unconformities, flooding surfaces and basinward correlative conformities. As a whole, each depositional sequence consists of two stacked units: the lower unit, or transgressive systems tract, is formed by up to 400 m of azoic marls. The upper one, or highstand systems tract, mainly consists of shallow siliciclastic and carbonate facies, up to 200 m thick. 4. The depositional setting of these sequences has been deduced from a facies analysis across a nearly transverse section some 40 km long. Each sequence developed on a distinctive low angle carbonate ramp. Outer ramp areas are dominated by nearly azoic blue marls, whereas middle and inner ramp facies consist of skeletal wackestones to packstones, coral boundstones and shallow siliciclastics. The steepest slope on this ramp was found to be around 5" and concen- trated in the transition area between the middle and outer ramp of Sequence 11. 5. Stacked sequences of about 1 Myr duration. which consist of a lower transgressive systems traci and an upper highstand systems tract, are caused by local tectonic pulses. An increase in the tectonic activity results in flooding of the basin and in the subsequent deposition of a thick succession of nearly azoic blue mark (i.e. transgressive systems tract) The upper member developed in periods of tectonic quiescence, with the consequent growth and progradation of shallow carbonate platform facies (i.e highstand systems tract). 6. The possible role of extrabasinal factors control. ling the sedimentation is also considered. There is nc definite indication about the influence of third ordei world-wide eustatic cycles in the genesis of sequencer and systems tracts. However, the local presence ol parasequences and parasequence sets on the shallocl sediments of highstand systems tracts (periods 01 tectonic calm) suggests the idea of possible highei order cycles (i.e. orbital forced eustatic cycles at thc


scale of 20 and 100 kyr). However, further research is necessary to provide more information about the timing and lateral extent of these cycles.

ACKNOWLEDGMENTS

We are indebted to A. Pocovi and J. I. Canudo for many helpful discussions a n d suggestions. Construc- tive reviews by D. W. Burbank, J. A. Muiioz and T. Pedley are gratefully acknowledged. Financial support was provided by Research Project CICYT GE0191-0924 (Ministerio de Education y Ciencia, Spain) and CONAI (Diputacion General de Aragon).

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(Manuscript received I March 1993; revision accepted 7 March 1994)

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