Structural and Stratigraphic Evolution of the West Jaca Thrust-Top Basin, Spanish Pyrenees

Journal of the Geological Society, London, Vol. 147, 1990, pp. 177-184, 6 figs, 1 table. Printed in Northern Ireland

Structural and stratigraphic evolution of the West Jaca thrust-top basin, Spanish Pyrenees

JONATHAN P. TURNER University of Bristol, Department of Geology, Wills Memorial Building, Bristol BS8 lRJ, UK Present address: Nederlandse Aardolie Maatschappij B. V., Schepersmaat 2, Postbus 28000, 94ooHH Assen, Netherlands

Abstrad: Thrust-top basins form and fill during their detachment from the foreland margin by active thrust sheets. Strike-normal (N-S) sections from the West Jaca thrust-top basin (Southwest Pyrenees) tothe Ebro basinshow that the Pena flexure, the local expression of thethrust front, can be interpreted as a passive-roof duplex. The duplex accommodatedup to 7 km bulk shortening at the tip of the Gavarnie thrust sheet. Unravelling the timing of uplift and sedimentation in the West Jaca basin has revealed that development of the Pena flexure represented the climactic phase of deformation. Lateral (E-W) diachroneity of gross stratigraphy and thrust emergence throughout the southwest Pyreneessuggests that the thrust front propagated both toward the foreland and westward, parallel to the orogen axis.

TheJaca thrust-top basinis the westernmost of several (e.g. Ten Haaf 1966; Rose11 & Puigdefabregas 1975; Mutti sedimentary basins along the foreland margin of the South 1977; Puigdefabregas & van Wet 1978; Friend et al. 1979; Pyrenees that were filled contemporaneously with their Hirst & Nichols 1986; Farrell et al. 1987). detachment by south-moving thrust sheets (Puigdefabregas The Jacabasin provides an instructiveexample of the 1975; Nijman 1981; On & Friend 1984). The eastern part of tectono-stratigraphicrelafionships typical of thrust-top the Jaca basinis bounded along its southern edge by the basins. The setting of the basin is illustrated in Fig. 1 which Exterior Sierras, a prominentrange of hillscomprising shows that it occupies much of the central and western part Mesozoicand lower Palaeogene limestone and marl. The of the Gavarnie thrust sheet. After its detachment from a Sierras constitute the emergent tip of the Gavarnie thrust pre-Triassicbasement in the late Eocene,the Jaca basin sheet and, despite somevariation in interpretation, their filled while being displaced southward before the locking of structure is relatively well understood. the thrust sheet in the early Miocene. This history was first A lack of exposed major faults along the thrust front worked out by Soler & Puigdefabregas(1970) who zone of the West Jaca basin makes it a more problematic investigated in detail the structure of the Exterior Sierras. structure to elucidate and hasled to the regionbeing More recently, balanced sections spanning the width of the neglected. Indeed, some tectonic mapshave incorrectly Pyrenees(e.g. Deramond et al. 1984;Williams & Fischer implied that a buried thrust trends obliquely to strike, across 1984; Labaume et al. 1985; Seguret & Daignieres1986) the long axis of the West Jaca basin,and dies out to have been constructed along linesthat cross the Graus basin northwest (e.g. Choukroune & Seguret 1973). and the East Jaca basin. These sections all demonstrate that Inthis contribution, a solution is proposed for the development of the South Pyrenean thrust sytem was structure of the West Jaca basin thrust front at this western dominated by piggybacka thrusting sequence; that is, extremity of the SouthPyrenees. It shownis how propagation into the undetached footwalls of thrusts causing diachronous development of the regional thrust system the thrust front to migrateprogressively toward the during the latter stages of Pyrenean orogeny is reflected in foreland. the gross stratigraphy of the West Jaca basin, and indeedthe South Pyrenean thrust-top basins in general. Stmtigraphy and structure Establishingreliablea chrono-and biostratigraphyin The Jaca basin in its South Pyrenean context continental molasse sequences is notoriously difficult and in The earlyMesozoic history of the Pyrenees involved most cases only a lithostratigraphic framework is possible. left-lateral movement of the Iberian microplatealong an As aresult of the lateral impersistence of synorogenic oceanic transform fault, an offshore extension of the present lithofacies, theJaca and Ebro basinsprove to be no North Pyrenean Fault (Fig. 1) (van der Voo 1%9; Le Pichon exception to the general rule. The lithostratigraphy used in et al. 1970; Choukroune et al. 1973). The climactic episode thispaper (Table1) isderived from that of Soler & of orogeny in the Palaeogene, however, was dominated by Puigdefabregas (1970) with the modification that their three roughlyhead-on collision between Iberia and Europe divisions of the Campodarbe Formation receive new names (Mattauer & Henry 1974). During this time, the South based on type localities in the study area. In the absence of Pyrenean thrust sheets developed (Seguret 1972). Following new biostratigraphic data, the ages of Formations given by recognition of these thrust sheets, the region has become Puigdefabregas (1975) are also followed here. wellknown for displaying the close relationship between The distribution ofprincipal stratigraphic units and structural evolution and sedimentation in an orogenicsetting structures in the West Jaca basin is shown in Fig. 2. Several 177

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-47"

Fig. 1. Location of the main South Pyrenean thrust sheets and tectonic settingof the South Pyrenean thrust-top basins. Thrust sheet boundaries are shownwith a solid box ornament irrespectiveof age and are taken from Seguret (1972). The tracesof thrusts depictedwith solid barbs cut pre-Oligocene rocks; those with open barbs cut Oligo-Miocene rocks. Modified from Seguret (1972,fig. 6),Choukroune & Seguret (1973) and Deramond et al. (1984, fig. 4).

compound anticlines immediately to the north of the basin flexureemphasizes the importance of the flexureas a (e.g. the Sierras de Orba, Leyre andAlaiz) consist of structural divide. The Penaflexure is composed of mid exposures of pre-molassic Cretaceous and Palaeogene Oligocene to lowerMiocene conglomerate and sandstone limestone. The West Jaca basin itself may be divided into that becomes gentler in dip southward, further from the severalsub-basins, defined here asdistinctive areas of influence of Pyrenean thrusts. former subsidence that were isolated depocentres within a largerhost basin (Turner 1988). The sub-basins contain a thick Oligocene to lower Miocene sequence and are located Sequence of thrust development around the periphery of a central thrusted and folded area The earliest major structure to form in the study area was of upperEocene to mid Oligocenerocks. Exposed and the thrust culmination of the Sierra de Orba (Thrust 1: Figs buried faults define the margins of the sub-basins. The 2 and 3). The age of the culmination is constrained as latest absence of closely spaced thrusts and folds south of the Pena Eocene by its deformation of upper Eocene marine marls

Table 1. Stratigraphic subdivision of the Jaca basin molasse sequence

~~~~ After Soler & After Puigdefabregas Puigdefabregas Age (1970) (1975) This study

Bu rdigalian Uncastillo Fm Uncastillo Fm Uncastillo Fm Uncastillo Burdigalian MioceneUncastillo Fm Aquitanian B ernues Fm BernuesBernues Fm Fm Chattian Bernues Fm UCampodarbe Fm Petilla Member

L Rupelian J CampordarbeMemberRuestaCampodarbe M FmAnzanigo Fm Fm

Priabonian EoceneCampodarbe Fm L Campodarbe Fm GaiardonMember (dominantly marine B artonian facies) Bartonian

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Fig. 2. Geology of the West Jaca basin showing also the locationsof sections givenin Fig. 3. Circled numbers refer to the sequence of thrust development. Modifiedfrom maps by the Instituto Geolgico y Minero de Espana (1972,1974,197&, b), Puigdefabregas (1975) and Castiella et al. (1978). For details of stratigraphic ages see Table 1.

and its lack of effect on lower Oligocene molasse. It is likely whether they are unique structures formed during a single that following the development of the hinterland-dipping episode in the history of a thrust system (e.g. Jones 1982; imbricatefan (nomenclature of Boyer & Elliot(1982)) of Vann et al. 1986). They point tothe apparent rarity of the culmination, further thrustingexperienced resistance structures characteristic of mountain fronts in the internal thatwas eventually resolved by the formation of a steep parts of orogenic belts, where there has been a piggyback backthrust of less than 1km total displacement (Fig. 3b). sequence of thrust development in whichthe thrust front has The next generation of thrusting was concentrated along formerlypropagated through the orogenhinterland. The the E-W trending frontal ramp that may be traced from the imbricatefans of the Sierras de Orba and Leyre, and Sierra de Leyre, westward for 44 km, to the western extent especially the backthrusted ‘popup’ geometry of the Sierra of the Sierra de Alaiz (Thrust 2: Figs 2 and 3). This thrust de Orba (Fig. 3b), are particularly characteristic of thrust episodeis dated as latest Eocene, largely on the basis of fronts described from elsewhere (e.g. Morley 1986; Vann et abundant slumphorizons and flowrolls concentrated in al. 1986), although they now occursome distance behind the upper Eocene and lower Ruesta Member rocks along the finalposition of the mountain front. Theseculminations northern margin of the West Jaca basin, and in the Izaga probably represent ‘frozen’palaeothrust frontsthat sub-basin. These syndepositional deformation structures developed during the latter part of the Eocene, which have occurin muddy lacustrine sediments. Kelling & Williams sincebeen incorporated into the thrustsystem by further (1966)suggest that slumps and flowrollscan be reliable propagation in the Oligocene. indicators of palaeoseismicityslowlyin accumulated The Ruesta thrustposesses a frontal ramp (Thrust3: lacustrinesequences. Figure 2depicts the Leyre-Alaiz Figs 2 and 3) extending for 15 km in a E-W direction and thrust cutting rocks of the mid-Oligocene Petilla Member, was the earliest thrust to significantly affect the configuration implying that the thrustformed after Petilla Member of the West Jaca basin. Above the westernlateral deposition. It will be shown, however, that the thrust has a termination of the Ruesta thrust is a sub-area characterized history of reactivation subsequent to its initiationin the by severaltransversely-striking normal faults interpreted latest Eocene. here as collapse structures (Figs 2 and 3a). This sub-area, Recent workers on mountain fronts havequestioned the Ruesta fault zone, reflects extension in the hangingwall

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interpreted as a passive-roof duplex accommodating up to 7 km (locally 45%) bulk shortening between the West Jaca basin and the Ebro basin foreland. Passive-roofduplexes wereoriginally described by Jones (1982),and later by Charlesworth & Gagnon (1985)and Price (1986), on the basisof evidencefrom the Canadian RockyMountain 0 5 10 15 foothills. The duplexes evolve by a process of progressive km (V-H) footwall detachment, with accretion of detached slivers to the thrust sheet in a sequence initially outlined by Boyer & Elliott(1982). The distinctive feature of a passive-roof duplex, however, isthat the roof thrust displays a backthrust sense of displacement, in contrast to the foreland-displacing roof thrust characteristic of most duplexes. The northward-verging roof thrust of the Pena flexure passive-roofduplex emerged during Bernues Formation - deposition, therebyinducing substantial alluvial fan km (V-H) developmentalong its southern flanks (Turner 1988). (C) However, erosion of the Ruesta Member above the Pena flexure, as evidenced by a thinned RuestaMember sequence (Figs 3b andc) and atop Ruesta Memberunconformity (Figs 2 and 4), suggests that it was a zone of uplift and tilting from the earlyOligocene. Assuming that this embryonic development of the Penaflexure took place above a detachment, then the early Oligocene thrust tip must have propagated at least as far south as the flexure. From the early Oligocene onward, therefore, deposition in the main part of the West Jaca basin must have occurred in a basin detached from its 'basement', that is, in a thrust-top basin. Further evidence for the existence of a passive-roof duplex beneath the Penaflexure basedis on the following relationships. (1) The presence of an upward-steepening roof thrust to the duplex provides an explanation for the progressive offlap Fig. 3. Strike-parallel (a) and strike-normal (b)-(d) sections unconformity that occurs throughout the PetillaMember through the West Jaca basin. Locations of section lines are given in and Bernues Formation sediments along the southern flank Fig. 2 and circled numbers referto the sequence of thrust of the Pena flexure (Fig. 5). Thoughconsiderably less development shown also in Fig.2. The solid teeth in (a) mark the complex than the progressive unconformities described from hangingwalls of thrust faults. The simultaneous shorteningand the eastern Ebro basin by Riba (1976) and Anadon et al. sedimentation characteristicof thrust-top basins means that the (1986), the Pena flexure unconformity reflects a comparable Oligo-Miocene stratigraphy of the sections is not requiredto uplift history. The offlap of the flexure indicates a history of line-length balance. Ornamentin (b-d) as in (a); e-bulk extension accelerated uplift and rotation as underthrusting from the (negative extension indicates net compression). north created a southward(forelandward) tilt above the backthrust. The presence of foreland-tilted sequenceshas above the lateral termination of the Ruesta thrust. Among a frequently been used as a criterion for postulating buried host of relationships demonstrating contemporaneous fault basin margin duplexes (e.g. Williams 1985). The most rapid movementand sedimentation, an intra-Ruesta Member period of uplift may be deduced by considering the rate of angularunconformity in the Ruesta fault zone (Fig.4) change of dip through the sedimentary succession. Figure 5 records thrust activity duringdeposition of the Ruesta shows that a 18" dip change (from 89" to 71") accompanies a Member.Gentle folding of PetillaMember rocks in the 2300 m thickness of the Petilla Member. A 56" change of dip Bailosyncline (Fig. 2), in the hangingwall of the Ruesta (from 71" to IS"), however, accompanies only 630 m of the thrust, indicates that the thrust remained active after the Bernues Formation, suggesting that the rate of uplift and start of Petilla Member deposition. landsurface rotation wasmost rapid during Bernues Further advance of the thrust front southward led to Formation deposition. uplift and tilting above the Sanguesa thrust ramp (Thrust 4: (2)Puigdefabregas & Soler (1973) andNichols (1987) Figs 2and 3)from the beginning of PetillaMember have demonstrated that, only 15 km east along strike from deposition, as constrained by a top Ruesta Member angular the Penaflexure, there is substantial shortening(up to unconformity at Sanguesa(Figs 2and 4). The Sanguesa 14 km) across the Exterior Sierras. Despite this shortening thrust may be traced for 60 km along strike, terminating at further east, the transition from the West Jaca basin to the its western tip above the Alaiz-Ujue buried ramp. Ebro basin is marked at the present surface by only a single The climax of deformation in the West Jaca basin was backthrust with less than 2 km displacement. Furthermore, represented by the development of the Pena flexure (Thrust no major lateral structure seperating the Pena flexure from structure 5: Figs 2 and 3), the last and most forelandward the Exterior Sierras hasbeen recognized. It is therefore major thrust structure to form in the study area.The necessary to incorporate a comparable amount of shortening sectionsin Figs 3b and 3c illustrate that the flexure is into sections that cross the Pena flexure.

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e.4. Scaled schematic logs of the Oligo-Miocene fill of the West Jaca basin located within a mapof the basin derived from that shown in Fig. 2. The logs depict an increase in thicknessof the sequence anda decrease in the ageof mega-facies transitions and local unconformities (indicated by wavy lines) toward the western part of the basin. Map ornament as in 2, Fig. S, Sanguesa.

Passive-roof duplexes of various sues have also been underthrusting of the overlying stationary foreland wedge by described from thefrontal Himalayan ranges of Pakistan the hinterland thrust system. (Banks & Warburton 1986), thesouthern Norwegian At its western end, the Pena flexure dies out across a Caledonides (Morley 1987) and the Exterior Sierras of the zone separating the Olleta sub-basin from the central areaof South Pyrenees (Nichols 1987). They are thought here to Ruesta Member and Petilla Member rocks to the east. The form at orogen margins where, during the waning stages of location of this structural divide, interpreted as a buried thrust propagation and below a critical depth to detachment ramp, is marked by arcuate fold axial traces and an (which would vary according to the angle of initiation of the anticlockwiseswinging of local structuraltrends (Fig. 2). backthrust, the gliding quality of the detachment horizon Additionally, there is a 3 km increase in thickness of the and the density of the hangingwall rocks),the most Oligo-Miocene sequence tothe westof the Alaiz-Ujue economic mechanism of duplex development is to form an buried ramp, in the Olleta sub-basin. emergent backthrust. Along such orogen margins, the Following the formation of the Tafalla thrust (Thrust 6: relatively thin deforming cover means that the overburden Fig. 2) in the southwest corner of the West Jaca basin, resisting the formation of an incipient backthrust is small development of the regional thrust system was complete. andeasy tounderthrust. The backthrust evolves by Despite the subsequent waning of thrust propagation from

Uncastillo Formation

Fig. 5. Section through the Pena flexure showing stratigraphic geometryof the S 012 unconformity. offlap progressive The N line of section is located at the eastern km backthrust. flexure Pena end of the

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the beginning of the Miocene, orogenic stressesstill during the climactic episode of deformation of the basin. required some form of structural expression. There is Along the Pena flexure, E-W diachroneity of emergence of evidence to suggest thatone response to these residual the backthrust is indicated by the ages of first appearance of stresses was a reversion of thrusting toward the orogen such conglomerates. These occur at the base of the Bernues hinterland. Forexample, angular unconformities beneath Formation (mid-Oligocene) in the east and at the top of the the Bernuesand Uncastillo Formations in the Izaga BernuesFormation (late Oligocene) in the west.In the sub-basin (Fig. 4) record at least two episodes of thrusting, Olleta sub-basin, an early Miocene(top Bernues Formation) to the north of the Leyre-Alaiz thrust, duringthe late localunconformity andlower Uncastillo Formation Oligocene. More spectacular, however, was the reactivation conglomerates are preceeded by a great thickness of of the Sierra de Alaiz culmination, a late Oligocene or early pre-climactic fluvial sandstone (Fig. 4) reflecting prolonged Miocene event resulting in a 45" anticlockwise rotation of subsidence and sedimentation before the encroachment of the culmination about a pivot point at its eastern end, above thrusting into the area. The distribution of local the Alaiz-Ujue buried ramp. Field mapping and borehole unconformities and the timing of megafacies transitions thus data from the Navarra Diputacion shows that reactivation records the dynamic nature of orogenic contraction and was a consequence of aneastward deterioration in the contemporaneous sedimentationin thrust-top basins. gliding quality of the Keuper detachment horizon beneath In addition tothe lateral diachroneity inferred from the Sierra de Alaiz (J. del Valle: pers. comm.,1985). An unconformitiesand mega-facies changes, the following analogousprocess of 'synchronousthrusting' in the lateral variations are revealed by the sections in Fig. 3. hinterland hasalso beendescribed from thesouthern (1) The depth to detachment (thickness of the deforming Norwegian Caledonides (Morley 1987) and from the western sedimentary cover) increaseswestward from 2.0 to Moroccan Rif (Morley 1988). In these examples the cause 6.8 km. was attributed tothe ever-increasingdistance between (2) There is a westwardincrease in mean thrust ramp orogenic sutures and foreland thrust tips. Where orogenic spacing from about 2 to 20 km. contraction is minimal, suchas in the West Jaca basin during (3) Bulk shortening decreases from 30% in the east to 7% the late Oligocene, it may be more favourable to reactivate in the west. a pre-existing structure than to advance the thrust front (4) There is a westward change in the attitude of the basal further. detachment from a foreland-tilt to a hinterland-tilt. Two processes are considered to have been the principal ones controlling lateral variations in the thrust system: Diachroneity of deformation (1) the westward (i.e. laterally) migrating deformation front The geometric evolution of the West Jaca basin thrust in the South Pyrenees; system conforms largely with the classical Canadian Rocky (2) the waningof orogenic contractionfrom the early Mountainmodel that invokes a piggyback, foreland- Oligocene onward. migrating sequence of thrusting. North-south development The inference that thrusting commenced earlier in the of thrusts accounts for most N-S variations in the age of east follows from observations showing that the eastern part thrusts and structural style, as revealed by the sections in of the basin experienced the transition from subsidence and Fig. 3. netsedimentation to upliftand net erosion (topographic However, the varying ages of local unconformities and inversion) earlier than the western part. For example, the megafacies transitions in the West Jaca basin record lateral Ruesta fault zone was a region of sediment accumulation for (E-W)diachroneity of deformation.The mostsignificant roughly 5 Ma or less (based on the Tertiary subdivision of local unconformities and the main mega-facies divisions are Harland et al. (1982)) before its topographic inversion at the shown on the summarylogs of Fig. 4. Generally, the end of the deposition of the Ruesta Member. Contrastingly, unconformities can only be traced for up to 10 km laterally. the Olleta sub-basinexperienced approximately 14Ma of They are interpreted as products of an actively deforming relatively undisturbed subsidence and sedimentation before depositional surface experiencing tilting and erosion as the topographicinversion during deposition of the Uncastillo basinmoved overa near-surface, ramp-flat tectonic Formation.The thickestand most complete sequence topography. therefore accumulatedin the Olleta sub-basinwhere In the West Jaca basin the earliest local unconformity, of subsidence was most prolonged. Consequently, the thickest earlyOligocene (mid-RuestaMember) age, occursin the deforming cover was in the western part of the West Jaca Ruesta fault zone where it is accompanied by a change from basinwhere the detachment horizonhad subsided, lacustrine to fluvial sediments (Fig. 4). Sustained lacustrine uninhibited, to its greatest depth. sedimentation takes placein the topographicallylowest Suppe(1985) notedthat, in the Front Ranges of the parts of a basin and this transition toward a fluvial Canadian Rockies, reductions in thickness of the deforming environmentrecords the firstsigns of uplift,with the pile occur where there is an increase in thrust frequency. encroachment of the Ruesta thrust. Further advance of the Several other workers have described similar relationships in thrust front beneaththe Ruesta fault zone,subsequently which the depth to detachment controls thrust ramp spacing spreading southand west,is evidenced bymid- to late (e.g.Bombolakis 1986; Morley 1987). In the WestJaca Oligocene (top Ruesta Memberto top Petilla Member) local basin, the westward increase in thickness of the deforming unconformities in the central part of the basin. In addition, covermay also account for the greater N-S separation lacustrine environments, widespread during Petilla Member between thrust ramps toward the west (compare the sections deposition, weredisplaced southwestwards by increased in Figs 3b and d). fluvial sedimentation inresponse to upliftalong the During the closingstages of Pyreneanorogenesis, Sanguesa thrust. The first influxes to the basin of alluvial fan westwardmigration of the deformation front through the conglomerate generally indicate local thrust emergence WestJaca basin was accompanied by decreasing contrac-

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I basinW. Jaca E. JacaAinsa basin baus E LIE Uncas tillo basin basin E -. Formation. . . . .

/h...... A

Fig. 6. Stratigraphic and facies relationships in the South Pyrenean thrust-top I basins; the line of section is shown in Fig. 1. The section emphasizes the highly diachronous style ofEocene to Miocene sedimentation along the length of the Southwest Pyrenees. Modified from Puigdefabregas et al. (1975, fig. 1).

tion. This decline in late orogenic contraction is reflected in flexuremarks the location of athrust front dividing the the smaller bulk shortening value that may be inferred from northern thrust-foldbelt from the relativelyundeformed sections across the western part of the basin compared with Ebro basin. The Pena flexure is interpreted as a passive-roof those across the eastern part (Fig. 3b and d). duplex. The E-W contrast in basal detachment attitude, from a (2) The southwarddecrease in age of the West Jaca foreland-tilt to hinterland-tilt,a respectively, is more basin thrust system and N-S variation in structural style, are difficult to explain purely in terms of E-W diachroneity of accounted for piggyback aby sequence of thrust deformation. The tiltsmay reflect lateral variation in the propagation toward the foreland. Lateral (E-W) changes in position of the locus of subsidence, ‘frozen’ at the close of the depth to detachment, thrust ramp spacing and the orogenesis.However, recognition of avery large fault amount of bulk shortening are attributed to the combination system in the Palaeozoic of the South Pyrenees (Camera & of a westward migrating deformation front and decreasing Klimowitz 1985) suggeststhat the attitude of detachments in late orogenic contraction. the Mesozoic and Tertiary mayinstead reflect a (3) Oligo-Miocene advance of the thrust front through tectonically-controlled basement topography. the West Jaca basin is recorded in the gross stratigraphy of The laterallymigrating deformation frontthat locally the basin-fill. In particular, the laterallydiachronous playedsuch an important rolein the structural and deformation front was important controllingin the stratigraphic evolution of the West Jaca basin may also be distribution of local unconformities and the timing of shown to haveinfluenced the development of the South mega-facies transitions. Pyreneanthrust-top basins on aregional scale. Figure 6 (4) During their Palaeogene to early Neogene evolution, illustrates the E-W diachroneity of aregressive marine the SouthPyrenean thrust-top basins were affected bya sequence that ranges in age from mid-Eocene in the Graus thrust frontthat migrated both southward, toward the basin (van Eden 1970) to early Oligocene in the West Jaca foreland, and westward, parallel to the orogen axis. basin(Puigdefabregas 1975). Also shown are aseries of discretealluvial fan conglomerate sequences that become I thank P. L. Hancock for his guidance throughout the course of progressivelyyounger westward, from mid-Eocenein the this work and J. del Valle, B. P. J. Williams and G. D. Williams for Graus basin to lowerMiocene in the West Jaca basin. discussionofspecific topics. The project was fundedby a Despitecomprising a small proportion of granitoidclasts scholarshipfrom Shell International Petroleum Company Limited originally sourced from the Axial Zone (e.g. up to 15% of to whom I am most grateful. E. J. Jolley and S. P. Todd sharpened the bulk clast assemblage in the West Jaca basin is derived this paper through thoughtful reviews. from the Axial Zone), theseconglomerate-filled tectonic valleys are here interpreted to have formed in response to References episodes of essentiallylocal thrust emergence. The decreasingages of the conglomeratestoward the west ANAWN,P., CABRERA,L., COLOMBO,M,, MARZO,M. & RIBA,0. 1986. Syntectonicintraformational unconformities in alluvial fan deposits, suggests that adeformation front responsible for their eastern Ebro basin margin (NE Spain). In: ALLEN, P.A. & HOMEWOOD, emplacement propagated westward,in a series of punc- P. (eds), Foreland Basins, International Association of Sedimentologists tuated displacementevents. Migration of this front, both Special Publication, 8, 259-271. parallel and normal to the long axis of the Pyrenees, exerted BANKS,C. J. & WARBURTON,J. 1986. ‘Passive-roof duplex geometry in the frontal structures of the Kirthar and Sulaiman mountain belts, Pakistan. an important influence on the evolution of the South Journal of Structural Geology, 8,229-231. Pyrenean thrust-top basins. BOMBOLAKIS,E. G. 1986.Thrust-fault mechanics and origin of a frontal ramp. Journal of Structural Geology, 8, 281-290. BOYER, S. E. & ELLIOT~, D.1982. Thrust systems. American Association of Conclusions Petroleum Geologists Bulletin, 66, 1196-1230. CASTIELLA,J., SOLE,J. & DELVALLE, J. 1978. Cartografio geol6gica a partir (1) In the West Jaca basin, the westernmostcompart- de l0 investigaci6ngeol6gica de Navarraa escala 1:25000.Diputaci6n ment of the South Pyrenean thrust-top basins, the Pena Foral de Navarra, Pamplona.

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Received 18 January 1988; revised typescript accepted29 May 1989.

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