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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/1/177/4890660/gsjgs.147.1.0177.pdf by guest on 29 September 2021 178 TURNER J. P. 3" -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 larger host 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/1/177/4890660/gsjgs.147.1.0177.pdf by guest on 29 September 2021 EV OL UT ION OF THRUST-TOP BASINTHRUST-TOP OF EVOLUTION 179 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; imbricate fan (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

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