Jaca Basin Research Articles

La sédimentologie et les foraminifères bartoniens et priaboniens des coupes d'arguis (Prépyrénées aragonaises, Espagne). Incidence sur la corrélation des biozones à la limite Bartonien/Priabonien

The sedimentological study of Arguis anticline in the Jaca basin (south-central Pyrenees, Spain), has shown the overall progradation of a deltaic system, which progressively overlap the structure since Bartonian. The Arguis Marl and Belsué-Atarés Formation which correspond to these sediments contain 206 species of benthic foraminifera. They represent outer and inner shelf depositional environments. Planktonic foraminifera define Morozovella lehneri (with uncertainty), Truncorotaloides rohri s. l., Dentoglobigerina eocaena and Globigerinatheka semiinvoluta zones. Interregional correlations put forward substantial differences between different regions of the Tethys. In central and western Europe as well as in the Arguis sections, N. fabianii and N. garnieri, which mark Priabonian, probably appear coevally with G. semiinvoluta. In Egypt, this event occurs later in the same zone. In the first region, bartonian nummulites ( N. perforatus, N. millecaput, etc.) disappear before that zone, whereas in Armenia they seem to coexist with N. fabianii. These uncertainties show that the definition of the Bartonian/Priabonian boundary should be preceded by the revision of numerous sections, in order to better establish the correlation of biozones of these stages.

Transverse fold evolution in the External Sierra, southern Pyrenees, Spain

Fault-slip data are used to reconstruct varying tectonic regimes associated with transverse fold development along the eastern and southern margins of the Jaca basin, southern Pyrenees, Spain. The Spanish Pyrenean foreland consists of thrust sheets and leading-edge décollement folds which developed within piggyback basins. Guara Formation limestones on the margins of the Jaca basin were deposited synchronously with deformation and are exposed in the External Sierra. Within the transverse folds, principal shortening axes determined from P and T dihedra plots of fault-slip data show a shift from steep shortening in stratigraphically older beds to NNE–SSW horizontal shortening in younger beds. Older strata are characterized by extensional faults interpreted to result from halotectonic (salt tectonics) deformation, whereas younger strata are characterized by contraction and strike-slip faults interpreted to result from thrust sheet emplacement. The interpretation of the timing for the shortening axes in the younger strata is supported by the observation that these axes are parallel to shortening axes determined from finite strain analysis, calcite twins, and regional thrusting directions determined from fault-related folds and slickenlines. This study shows that fault population analysis in syntectonic strata provides an opportunity to constrain kinematic evolution during orogeny.

The South Pyrenean Eocene carbonate megabreccias revisited: new interpretation based on evidence from the Pamplona Basin

The South Pyrenean Foreland Basin contains numerous units of Eocene carbonate megabreccias intercalated with siliciclastic turbidites and derived by resedimentation of shallow-marine carbonate platforms. Previous studies were limited mainly to the foreland eastern part, known as the Jaca Basin. The present study from the Pamplona Basin, a western part of the foreland trough, sheds new light on the origin and regional significance of these South Pyrenean Eocene carbonate megabreccias (SPECMs). The number of the SPECM units in the foreland basin is higher than previously recognized and their age is somewhat older than originally assumed. The SPECM units appear to occur as time–stratigraphic clusters, which can be correlated with the relative sea-level lowstands and linked with phases of tectonic activity. The megabreccias were derived from a carbonate-platform system hosted by the foreland basin's southern (passive) margin. The episodic instability and mass wasting were triggered by phases of structural steepening (forebulge uplift) accompanied by high-magnitude earthquakes, with the former causing platform emergence, increased load stresses and excess pore-water pressure in the carbonate ramp. The SPECM deposits were emplaced by cohesive debris flows evolving into high-density turbidite currents. An ideal SPECM unit consists of (1) an immature, homogeneous debrite in the proximal part; (2) a differentiated, bipartite debrite and turbidite in the medial part; and (3) an incomplete, base-missing debrite overlain by turbidite, or a turbidite alone, in the distal part. The debrite component volumetrically predominates in the SPECM units, and the original terms `megaturbidite' and `seismoturbidite' thus seem to be inappropriate for these deposits.

Discussion on syntectonic burial and post-tectonic exhumation of the southern Pyrenees foreland fold-thrust belt

A. González , C. Arenas & G. Pardo write: Coney et al. 1996 considered that Late Eocene-Early Miocene continental deposits (conglomerates and related sediments) show an 'unusual distribution' in the south Pyrenean Thrust Belt. These authors proposed that the foreland Ebro Basin 'was filled and then backfilled to bury the flanking thrust belts on its margins, overlapping the southern margin of the Axial Zone'. As a consequence, they found that the youngest conglomerates were deposited furthest north, closer to the Axial Zone from which sediments were shed southward. Thus the youngest conglomerates should contain exclusively material derived from the Axial Zone, and this does seem to be valid for the Upper Thrust Sheets domain (fig. 3 of Coney et al.). However, from observations in that areally limited domain they extrapolated general conclusions, and proposed that not only was complete burial under syntectonic deposits a general feature of the entire southern flank of the Pyrenees, but it was also 'typical of the entire Ebro Basin and its surrounding mountain ranges'. This generalization contradicts much of our own data and paleogeographic reconstructions, and also information from many other geologists. Previous stratigraphic and palaeogeographic studies in the Jaca Basin, west of the Upper Thrust Sheets domain ( Puigdàfabregas 1975 ; Friend et al. 1989 ; Montes 1992 ) and in the Pyrenean margin of the Ebro Basin, west of the Sierras Marginales (Hirst & Nichols 1986; Nichols 1987 , 1989 ; Arenas 1993 ; Arenas & Pardo 1994) demonstrate that the conclusions of Coney et al. cannot be extended to those

The Ansó transect of the southern Pyrenees: basement and cover thrust geometries

A balanced geological section through the western central Pyrenees allows the Alpine geometry of Hercynian basement to be determined. This shows marked differences with other transects of the southern part of the chain. Basement-involved thrust sheets do not form an antiformal stack, but are imbricated and define steps separated by frontal culmination limbs. Basement thrusts propagate upwards and have formed distinct thrust systems in the cover. In this transect, the North Pyrenean Zone is bounded to the south by gently dipping thrusts of large displacement, and the Axial Zone massif, at its western termination area, is a flat-topped culmination caused by a single thrust fault, the Gavarnie thrust. This thrust emerges to the south in the interior of the syntectonic Jaca Basin, the geometry of which is also controlled by underlying basement thrusts. Various generations of structures can be correlated with distinct episodes of syntectonic sedimentation. A sequential restoration of the section illustrates how successive foreland sedimentary wedges were generated and progressively involved in deformation. Tectonic shortening is 48 km, almost half of that estimated further to the east for the southern Pyrenees. The structure deduced at the ECORS-Pyrenees seismic section cannot be translated to this transect.

Discussion on palaeostresses associated with thrust sheet emplacement and related folding in the southern central Pyrenees, Huesca, Spain Journal, Vol. 152, 1995, 353–356

J. P. Turner & P. L. Hancock write: We congratulate Martínez-Peña, Casas-Sainz and Millán-Garrido (1995) on their palaeostress analysis of a complex suite of mesostructures contained in several south Pyrenean thrust sheets. We agree that, in spite of the difficulty in interpreting the multi-phase deformation of thrust belts, analysis of structures from which palaeostress may be inferred yields useful information not apparent from a macro-structural study. Like Martínez-Peña et al. (1995), we also found that the folded Oligo-Miocene molassic sedimentary rocks of the Jaca thrust-top basin contain remarkably few faults. Indeed, explaining how a near-surface structural unit has travelled 30 km or more over such an irregular tectonic topography yet without being faulted is a challenging structural question. However we are suprised that Martínez-Peña et al. (1995) found in their palaeostress results from the Cretaceous and Palaeocene carbonates to the north of the molasse basin, that: There are no great differences between stress tensors found in the hanging wall and the footwall of thrusts’. Our experience (Turner & Hancock 1989) in that part of the Jaca basin contiguous with and immediately to the west of their study area is that the assemblages of small-scale brittle structures in the immediate (within 1-3 km) hanging walls and footwalls of thrusts are distinctively different, recording locally contrasting stress regimes. In particular, our study showed that the immediate footwalls of thrusts experienced extension parallel to the strike of thrusts before they underwent extension normal to the strike of thrusts.

Intra- and Extrabasinal Controls on Mid-Tertiary Sandstones and Conglomerates, Jaca Basin, Northern Spain: ABSTRACT

Intra- and Extrabasinal Controls on Mid-Tertiary Sandstones and Conglomerates, Jaca Basin, Northern Spain: ABSTRACT

Platform sedimentation and collapse in a carbonate-dominated margin of a foreland basin (Jaca basin, Eocene, southern Pyrenees)

The Eocene Jaca basin is a foreland basin with well-exposed carbonate platforms in the distal margin. This margin underwent alternating periods of stable platform growth and platform drowning. Periods of drowning were accompanied by large-scale collapses with generation of shelf-edge truncations and resedimentation of carbonate megabreccias into a terrigenous turbiditic trough. These features caused a stepped retreat of the platforms and are related to episodic variations of tectonic loading in the hinterland and correlative flexural bending of the distal basin.

Relationships between thrusting and joint systems in the Jaca thrust-top basin, Spanish Pyrenees

The Oligo-Miocene rocks of the West Jaca thrust-top basin and adjacent parts of the Ebro basin are cut by up to eight sets of joints and allied mesofractures. The fractures belong to three groups that can be distinguished on the basis of their relative ages and geometry. An older group of joints strikes normal or subnormal to the Pyrenean mountain front and is restricted to subareas (here called front-normal joint domains) coincident with the immediate footwalls of thrusts. Joints striking parallel to a buried lateral ramp characterize a lateral ramp joint domain. Younger joints striking parallel or subparallel to the mountain front occur throughout most of the West Jaca and Ebro basins, and define front-parallel joint domains. The joint domains appear to reflect the geometry and evolution of thrust sheets. Joints in front-normal domains were formed during stretching of footwalls as a result of their loading by overriding thrust sheets. Stretching above a lateral ramp is thought to be responsible for the development of joints in the lateral ramp domain. Joints in the front-parallel domains of the West Jaca basin are related to stretching in growth folds that were amplifying during salt doming. Front-parallel joints in the Ebro basin are attributed to stretching of a foreland basin sequence above a basement flexure related to thrust loading.

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

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) to the Ebro basin show that the Pena flexure, the local expression of the thrust front, can be interpreted as a passive-roof duplex. The duplex accommodated up 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 Pyrenees suggests that the thrust front propagated both toward the foreland and westward, parallel to the orogen axis.

Interpretación geodinámica de la vertiente centro-occidental surpirenaica (cuencas de Jaca-Tremp)

Durante el Eoceno y Oligoceno en las cuencas de Jaca y Tremp se produce un sistema de cabalgamientos imbricados ("piggy-back sequence") contemporáneos con el relleno sedimentario de las mismas. Toda la sedimentación eocena está condicionada por las rampas anticlinales formadas, originándose una facies de talud sintectónica en los flancos suaves o traseros de cada cabalgamiento (calizas y margas de Burgui), Entre estas rampas anticlinales se crean una serie de surcos en los cuales se depositan diferentes sistemas turbidíticos, En estos sistema se inercalan conjuntos de megacapas carbonatadas que se canalizan a lo largo de dichos surcos. Estos conjuntos migran claramente hacia el sur, observándose también, en las megacapas que los componen, un desplazamiento oeste a este. Esta disposición es un reflejo del movimiento de los cabalgamientos, al emplazarse antes en el oeste de la cuenca de Jaca para irse desplazando en el tiempo hacia el sur y este de la misma. El material carbonatado que forma las megacapas procedería de una plataforma eocena septentrional, localizada en el borde activo de la cuenca, así como de la erosión de las rampas anticlinales creadas simultáneamente. El sistema cabalgante imbricado, que afecta a la cobertera, está ligado y superpuesto a un gran sistema "dúplex" en el basamento. Este último buza hacia el noroeste, es de tipo "hinterland dipping" y su raíz o "branch line" se localiza en una falla intrapaleozoica en la actual zona axial. El sistema tectónico formado se origina por una compresión NNW-SSE, debida, posiblemente, al movimiento dextrógiro relativo entre las placas Europea e Ibérica.