South-central Pyrenees Research Articles

Depositional wedge tops: interaction between low basal friction external orogenic wedges and flexural foreland basins

AbstractGrowth structures develop mainly across depositional wedge tops where the top of the distal orogenic wedge lies below local base level within the flexural foreland basin. Documented examples indicate that optimum conditions for the development of a depositional wedge top occur when the dip of the upper surface of the outer orogenic wedge (α) is low (0–1°), regardless of the taper angle (α+β), where β is the inclination of the base of the wedge. Low α angles are associated with the presence of low friction décollements, usually salt. Therefore, depositional wedge tops develop most commonly on low basal friction wedges. In natural examples such as the northern Apennines, the South Central Pyrenees and the Zagros, it is shown that whereas α is consistently low, β can vary considerably (1–9°), being principally controlled by flexure of the lower plate. In addition, the distribution of deformation in the wedge is complex. The applicability of the critical wedge model to low basal friction external orogenic zones is examined. Neither taper angles, nor the complex distribution of internal deformation nor the often associated forward slip of the whole wedge, can be satisfactorily explained by the established critical wedge model. As weak detachments such as salt cannot support any significant topography, it follows that low basal friction wedges cannot attain a critical state and, therefore, the critical wedge model cannot be satisfactorily applied to these external orogenic systems. This model, however, explains well the behaviour of external orogenic wedges with high basal friction. In the analysis of orogenic wedges, separate analysis of the angles α and β proves more useful than the critical taper angle α+β.

A new middle to late Eocene continental chronostratigraphy from NE Spain

The calibration of the European Paleogene mammal biochronology with the international reference geochronological scale is unconstrained due to the lack of direct marine–continental correlations and insufficient robust magnetostratigraphic data for mammal fossil-bearing sequences. The currently accepted European continental biochronology argues for a correlation of the Paleogene MP14 to MP16 mammal reference levels with the Bartonian stage, and of the MP17 to MP20 levels with the Priabonian. However, new magnetostratigraphic data of the continental sediments of the south-central Pyrenees substantially challenge this chronology. Our new results from the La Pobla de Segur Basin and Sierra de Sis have been integrated into the framework of previous marine–continental stratigraphic correlations and magnetostratigraphic data of the south-central Pyrenees. In this new chronology, MP14 and MP15 reference levels correspond to the Lutetian, whereas MP16 and early MP17 correlate with the early Bartonian. This calibration represents a significant revision of currently accepted marine–continental correlations in Europe, but is not in conflict with the available biostratigraphic data of the south Pyrenean Ebro foreland basin and other western European reference basins.

High-resolution correlation patterns in the turbidite systems of the Hecho Group (South-Central Pyrenees, Spain)

Detailed bed-by-bed correlations in sheetlike lobes and basin-plain elements of the Hecho Group were carried out through a hierarchical decomposition based on three orders of correlation. This procedure provides the most detailed correlation framework with chronostratigraphic significance. First-order decomposition is based on physical correlation by mapping of major megaturbidites. Second-order decomposition consists in matching a series of marker beds within first-order units, leading to the definition of units several metres thick. Finally, third-order decomposition relies on the matching of serial features within second-order packages (number, vertical arrangement and facies features of beds), and further testing the coherence of the downstream evolution of facies in single beds. Sheetlike lobes and basin plain are closely related and display a distinctive sheet-like geometry. Basin-plain beds are deposited by the same flows that build the sheetlike lobes, with the existence of a transitional facies change from the lobe to the basin-plain. Thickening of some beds towards the basin plain due to ponding of the fine-grained (silt and mud) divisions, a diagnostic feature of the basin plain, controls the sheet-like stacking pattern by balancing the wedging-out of thinner lobe beds upcurrent from, or within, the basin plain. As a consequence, the whole area can be geomorphically classified as a basin plain, which comprises both lobe and basin plain facies associations.

Magnetic Stratigraphy from Deep Clastic Turbidites: An Example from the Eocene Hecho Group (Southern Pyrenees)

A paleomagnetic study of a 2200-meter thick section of clastic turbidites from the Eocene Hecho group (southcentral Pyrenees, Spain) allows defining its magnetostratigraphic record. Natural remanent magnetization is carried by up to three components: a viscous low-temperature component; a second component unblocked between 300°C and 345°C, likely carried by iron-sulphides; and a third component which demagnetizes at temperatures higher than 345°C and is likely carried by magnetite. The second and third components may display opposite polarities at the same site. The “magnetite” component delineates different polarity zones and has a more consistent behavior along section if compared with the “iron-sulphide” component, which displays either a normal or a reverse direction without any stratigraphic consistency along section. The bulk of iron sulphides are interpreted to be secondary in origin and to carry diagenetic overprints acquired at different times after deposition and the “magnetite” component is taken as the characteristic primary magnetization. As supported by biostratigraphy, the section is correlated from chrons C20r to C18n.2n (Lutetian-Bartonian transition), which indicates a mean sediment accumulation rate of about 52 cm/ky for the studied section. The new chronostratigraphy allows constraining the age of the upper Hecho Group (Banaston and Jaca allogroups) to an unprecedented level and is consistent with previous magnetostratigraphic work in younger sediments from the Jaca Basin. Deep clastic sedimentary systems should not be neglected as a target for magnetostratigraphic studies despite diagenetic growth of secondary minerals may mask the primary signal.

ABSTRACT: Controls on the Variability of the Turbidite Systems in the Ainsa Slope Complex (South-Central Pyrenees, Spain)

The Ainsa slope complex developed during the lower and middle Eocene in a foredeep to piggy-back setting in south-central Pyrenees. It is preserved as a 40 Km long, 30 Km wide and 4000 m thick, mudstone-dominated clastic wedge, which includes coarser grained turbidite lithosomes. The latter correspond mostly to channel fills and associated overbank and splay deposits. Our research in the Ainsa slope complex has produced a stratigraphic and structural dataset resolving from the basin fill to the reservoir scale. From the analysis of this dataset we offer a scheme of stratigraphic subdivision which covers the entire range of scales mentioned above plus tabulated parameters and related graphs for each of the elements in the scheme. The basin fill is first divided into four major depositional cycles (2-5 m.y. each) which are bounded by unconformities and occur stepped several kilometers towards the foreland due to the propagation of the thrust front. Within a major depositional cycle, several turbidite systems occur which are bounded below by unconformities and above by a mudstone cap. From system to system, in a younging direction, there is an increase in N/G and reductions on stratigraphic thickness, amount of forelandwards migration of system axis, volume of bypass facies and maximum sediment caliber. These changes can all be explained by an overall decrease in gradient from base to top within a major depositional cycle and fit into a model characterized by progressively reduced rates of forelimb rotation along thrustrelated frontal anticlines.

ABSTRACT: Depositional Controls and Sequence Stratigraphic Significance of Debrites on Prograding, Draping, and Retrograding Carbonate Slopes, Cretaceous of South-Central Pyrenees, Spain

ABSTRACT: Depositional Controls and Sequence Stratigraphic Significance of Debrites on Prograding, Draping, and Retrograding Carbonate Slopes, Cretaceous of South-Central Pyrenees, Spain

Depositional processes, triggering mechanisms and sediment composition of carbonate gravity flow deposits: examples from the Late Cretaceous of the south-central Pyrenees, Spain

Cenomanian through Coniacian strata near the town of Sopeira in the south-central Pyrenees (northern Spain) are composed of a variety of autochthonous and allochthonous carbonate slope lithologies that are divided into six depositional sequences based on facies distribution patterns and stratal relationships. The sequences record three major phases of platform margin evolution: rifting, burial, and exhumation. During the first phase (sequences UK-1, UK-2, UK-3, UK-4, and lower UK-5), deposition occurred on the edge of a wrench basin, and a normal fault located beneath the platform margin strongly influenced slope evolution. Background hemipelagic sediments on the slope were commonly redeposited by submarine slumps and slides. More intense reworking resulted in matrix-supported, slope-derived megaconglomerates (debrites). During the Cenomanian and Turonian, seismically triggered debris flows originated at the platform margin, bypassed the upper slope, and were deposited on the lower slope as polymictic, clast-supported, matrix-rich megabreccias. The megabreccias form channelized and sheet-like bodies with erosional basal surfaces. Shallow carbonate environments backstepped during the Late Turonian and Coniacian, but displacement along the fault at this time resulted in the development of a steep submarine scarp and the exposure of Cenomanian and Lower Turonian strata to submarine erosion. Matrix-poor, margin-derived megabreccias form a thick talus pile at the base of the scarp. Some of the breccias were transported into the basin as debris falls, forming sheet-like beds. Marl eventually buried the Coniacian scarp in sequence UK-5, resulting in the second major phase of platform slope evolution. The slope profile at this time was relatively gentle, and redeposited material is less common. In the third phase (sequence UK-6), tectonically induced bankward erosion during the Santonian resulted in a high (greater than 800 m) erosional scarp with a regional east–west trend that was subsequently onlapped by siliciclastic turbidites. Rejuvenation of erosion in the same vicinity suggests that long-term tectonism controlled the position of the slope, rates of erosion, and sediment type on the slope. Sediment gravity flow processes are laterally and temporally related. Submarine slide and slump deposits commonly grade laterally downslope into slope-derived megaconglomerates. Debris flows that originated at the platform margin appear to have initiated slumps, slides, and other debris flows on the slope. Debris fall deposits are commonly capped by coarse, graded, lithoclastic packstones that may represent turbidites generated by the debris falls. Sediment fabric exerted a profound impact on depositional processes, distribution of facies, and morphology of the slope. Fine-grained, mud-rich, lower slope deposits were unstable at even moderate slope angles, and have been extensively redeposited. Redeposition of grain-rich, upper slope facies was triggered by syndepositional seismic activity and upslope migration of instability and erosion. In the presence of mud, the transport mechanisms are typically cohesive debris flows, which were able to carry material onto the lower slope and into the basin. When no mud was available, rock falls and debris falls were the dominant sediment gravity flows, and their deposits are restricted to a position on the hanging wall proximal to the fault.

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.

Braided stream and flood plain architecture: the Rio Vero Formation, Spanish Pyrenees

Early- to middle-Miocene fluvial sandstones of the Rio Vero Formation were studied, in an area around the town of Barbastro, south central Pyrenees Spain. The outstanding quality of outcrops in this area allows a three-dimensional study of architectural elements. Six architectural elements are recognised, described in detail, and interpreted from three key localities. Seven main lithofacies were identified and sub-divided into gravelly, sandy and fine-grained lithofacies. The architectural elements and lithofacies have been combined with a hierarchy of depositional bounding surfaces to fully interpret the evolution of the depositional system at the meso- and macro-scale. Not only the different architectural elements and lithofacies of the complete braided fluvial system, but also the lateral variation of the architectural elements were emphasised in this study. Differential tectonic movements, seasonal climate change, and their effect on vertical and lateral evolution of the area were the main control on basin sedimentation, channel interconnection, palaeocurrent patterns, and consequently the fluvial architecture. The presence of lateral ramp anticlines caused the fluvial system to be laterally restricted, with the main channel-belts being located in the areas of highest subsidence and lowest topography. Intervening topographic highs acted as both flood plains and lateral barriers between the main channel systems. The proposed depositional model comprises broad, low-sinuosity, perennial, but seasonal moderate-energy streams. The sandstone architecture is dominated by channel-fill and sheet sands, and associated simple and more complex bars. Adjacent to the main channel-belts fine-grained sandstones, siltstones and immature paleosols occur. The along-strike relationship between major fluvial systems and their outlets into a foreland basin has important implications for the infill of the basin and the modelling of fluvial systems along mountain belt fronts.

New dinosaur sites correlated with Upper Maastrichtian pelagic deposits in the Spanish Pyrenees: implications for the dinosaur extinction pattern in Europe

Six new dinosaurs sites have been found close to the Cretaceous/Tertiary boundary in Arén (south-central Pyrenees, Huesca, Spain) in coastal and non-marine deposits of the Arén and Tremp Formations. The sites contain articulated remains (skull elements, vertebrae, hind-limb bones) and isolated teeth and bones of hadrosaurids, three types of theropod teeth, one sauropod, at least seven types of eggshells (six ornithoid types and one Megaloolithidae probably from a sauropod), remains of other vertebrates, and four charophyte species. The fossil-bearing rocks have been correlated with marine sediments containing planktonic foraminifera from the uppermost Maastrichtian Abathomphalus mayaroensis Biozone. These rich and diversified dinosaur assemblages enable more accurate dating of the faunal changes that took place during the Maastrichtian in Europe and support the hypothesis of a sudden dinosaur extinction at the Cretaceous/Tertiary boundary.

ABSTRACT: Tectonically Induced Distorsion Of Stratigraphic Cycles. Example Of The Arguis Anticline In The South Central Pyrenees (Spain)

ABSTRACT: Tectonically Induced Distorsion Of Stratigraphic Cycles. Example Of The Arguis Anticline In The South Central Pyrenees (Spain)

Tectonic, eustatic and environmental controls on mid‐Cretaceous carbonate platform deposition, south‐central Pyrenees, Spain

Cenomanian–Turonian strata of the south‐central Pyrenees in northern Spain contain three prograding carbonate sequences that record interactions among tectonics, sea level, environment and sediment fabric in controlling sequence development. Sequence UK‐1 (Lower to Upper Cenomanian) contains distinct lagoonal, back‐margin, margin, slope and basin facies, and was deposited on a broad, flat shelf adjacent to a deep basin. The lack of reef‐constructing organisms resulted in a gently dipping ramp morphology for the margin and slope. Sequence UK‐2 (Upper Cenomanian) contains similar shallow‐water facies belts, but syndepositional tectonic modification of the margin resulted in a steep slope and deposition of carbonate megabreccias. Sequence UK‐3 (Lower to Middle Turonian) records a shift from benthic to pelagic deposition, as the shallow platform was drowned in response to a eustatic sea‐level rise, coupled with increased organic productivity. Sequences UK‐1 to UK‐3 are subdivided into lowstand, transgressive and highstand systems tracts based on stratal geometries and facies distribution patterns. The same lithologies (e.g. megabreccias) commonly occur in more than one systems tract, indicating that: (1) the depositional system responded to more than just sea‐level fluctuations; and (2) similar processes occurred during different times throughout sequence development. These sequences illustrate the complexity of carbonate platform dynamics that influence sequence architecture. Rift tectonics and flexural subsidence played a major role in controlling the location of the platform margin, maintaining a steep slope gradient through syndepositional faulting, enhancing slope instability and erosion, and influencing depositional processes, stratal relationships and lithofacies distribution on the slope. Sea‐level variations (eustatic and relative) strongly influenced the timing of sequence and parasequence boundary formation, controlled changes in accommodation and promoted platform drowning (in conjunction with other factors). Physico‐chemical and climatic conditions were responsible for reducing carbonate production rates and inducing platform drowning. Finally, a mud‐rich sediment fabric affected platform morphology, growth geometries (aggradation vs. progradation) and facies distribution patterns.

ABSTRACT: Compartmentalisation of an Inverted Lower Cretaceous extensional basin, south central Pyrenees, Spain

ABSTRACT: Compartmentalisation of an Inverted Lower Cretaceous extensional basin, south central Pyrenees, Spain

The role of sediment supply in controlling alluvial architecture: an example from the Spanish Pyrenees

This study isolates the controls on the alluvial architecture of two interfingering late Lutetian fluvial systems preserved at the northern margin of the Tremp-Graus Basin, in the south central Pyrenees. One of these, a northerly, mountain-belt-derived, high-gradient, bedload-dominated gravelly system, displays marked variations in its alluvial architecture, with systematic changes in the width, depth, internal facies arrangement and stacking pattern of its channel bodies. The other, an easterly, basin-axis-derived, lower gradient, poorly drained alluvial system, displays no discernible systematic variations in alluvial architecture. A common control that would similarly affect both of the interfingering fluvial systems cannot be responsible for their contrasting architectural patterns. Variations in base level, despite the contemporaneous shoreline being only 18 km downstream, climate, or the tectonic rotation of the study area are therefore discounted. Fluctuations in sediment supply to the northerly derived system resulting from the episodic uplift of its drainage basin are instead proposed, although intrinsic controls cannot be totally discounted. Similar sediment flux variations in the headwaters of the easterly derived system are envisaged, but its longer, lower-gradient transport path was effective in dampening out these fluctuations upstream of the study area.

The succession of Vertebratesand Plants across the Cretaceous-Tertiary boundary in the Tremp Formation, Ager valley (South-central Pyrenees, Spain)

The Tremp Formation red beds in the Ager valley (Fontllonga section, Lleida, Spain) have yielded plants (macrorests, palynomorphs) and vertebrates (teeth, bones, eggshells and footprints) at different levels from Early Maastrichtian to Early Palaeocene. A decrease in diversity affected both, plants and vertebrates, but not synchronously. Plant diversity decreases early in the Maastrichtian, while the change in vertebrate assemblages (sudden extinction of the dinosaurs) occurs later on, at the Cretaceous-Tertiary (K/T) boundary. This pattern agrees with the record of France and China, but contrasts with that of North American Western Interim; where both changes coincide with the K/T boundary. [RESUME] - Les depots continentaux de la Formation Tremp dans la vallge d'Ager (coupe de Fontllonga, Lleida, Spain) ont fourni des fossiles de plantes (palynomorphes et macrorestes) et de vertebras (ossements, dents, oeufs et traces) dates du Maastrichtien inferieur-Paleocene inferieur. Les deux groupes montrent une reduction de la diversity mais celle-ci n'est pas synchrone: la crise floraie a eu lieu pendant le Maastrichtien, et celle des vertebras terrestres s'est passee plus tard, a la limite K/T. Ceci est en accord avec les donn~es de la France et de la Chine, mais contraste avec celles du Western Interior de l'Amerique du Nord ou les deux changements coincident avec la limite Cretace-Tertiaire.

Sharks and rays (chondrichthyes) from the Upper Cretaceous red beds of the south-central Pyrenees (Lleida, Spain): indices of an India–Eurasia connection

Six chondrichthyan taxa are recorded in Late Cretaceous (Maastrichtian) red beds of the south-central Pyrenees (Tremp Formation, Lleida, Spain), indicating marine influence in the palaeoenvironment. Two ray species found in Spain have been previously described in India. One ray species conforms to the Gondwanan genus Igdabatis and the endemic Indian species Igdabatis indicus Prasad and Cappetta, 1993 [Prasad, G.V.R., Cappetta, H., 1993. Late Cretaceous selachian from India and the age of the Deccan traps. Palaeontology 36 (1), 231–248.] The other ray from the Tremp Formation is also close to the Indian material assigned to Rhombodus sp. 2. The limited dispersal abilities of benthic rays points to a shallow trans-Tethyan connection between Eurasia and India at the end of the Cretaceous. The data increase the evidence of a biogeographical relationship between Eurasia and Gondwanan continents at the Cretaceous–Tertiary boundary.

Abstract: Chemostratigraphic Transect (Lagoon to Slope) Across a mid-Cretaceous Carbonate Platform, South-central Pyrenees, Spain: The Stratigraphic, Sedimentologic, and Paleoenvironmental Interpretation of a Platform Drowning Event.

Abstract: Chemostratigraphic Transect (Lagoon to Slope) Across a mid-Cretaceous Carbonate Platform, South-central Pyrenees, Spain: The Stratigraphic, Sedimentologic, and Paleoenvironmental Interpretation of a Platform Drowning Event.

Carbonate Platform Drowning and Oceanic Anoxic Events on a Mid-Cretaceous Carbonate Platform, South-Central Pyrenees, Spain

ABSTRACT The Cenomanian-Turonian carbonate platform of the south-central Pyrenees (northern Spain) records an abrupt transition from benthic carbonate deposits to pelagic deposits near the Cenomanian-Turonian (C/T) boundary. This shift coincides with a globally recognized oceanic anoxic event (OAE), characterized by the widespread deposition of organic-rich sediments in deep marine environments. A positive d13C excursion of up 3 is recorded in carbonate and organic carbon deposited near the C/T boundary, and provides evidence of a direct link between the C/T boundary OAE and the demise of shallow benthic carbonate production on the Pyrenean platform. Furthermore, carbon isotope tratigraphy provides a means for dating the platform drowning event on the shelf, where biostratigraphic control is poorly constrained. An analysis of the shape of the carbon excursion suggests a depositional hiatus or a period of condensed sedimentation at the drowning surface on the shelf. Sedimentological, paleontological, and geochemical evidence suggests that the platform was drowned near the C/T boundary as a result of changing environmental conditions. Pelagic-rich strata overlying the drowning surface are highly bioturbated, and concentrations of redox-sensitive elements (Mn, Fe, Cr, Cu, and Zn) indicate deposition under well-oxygenated conditions. The pelagic sediment contains abundant calcispheres, which are interpreted to be opportunistic organisms that inhabit eutrophic, but unstable, environments. Elevated concentrations of Ba, P, and Si (in the form of chert nodules) observed in the pelagic facies are typical of modern and ancient upwelling systems. Petrographic evidence for oxygen deficiency is limited to the lower slope region. The warm mid-Cretaceous clim te and a global rise in sea level resulted in changes in oceanic circulation patterns, increased organic productivity, and a shift in the trophic structure to one that favored planktonic productivity over benthic productivity. Pelagic deposition persisted for approximately 1 to 1.5 m.y., longer than the OAE. The time difference between the end of the OAE and the reestablishment of a shallow, benthic platform is attributed to increased accommodation caused by the Turonian sea-level rise and tectonic subsidence.

Inversion of a transfer system into lateral ramps: an example from the South-Central Pyrenees (Spain)

Inversion of a transfer system into lateral ramps: an example from the South-Central Pyrenees (Spain)

Inversion of a transfer system into lateral ramps: An example from the South-Central Pyrenees (Spain)

Field work in the South-Central Pyrenees suggests that omission contacts (i.e. younger over older rocks) occur at the base of the Cadi unit (Cadi thrust), and pass laterally into thrusts. This change occurs across tear faults which are present in the hangingwall of the Cadi thrust sheet and which controlled the deposition of Upper Cretaceous sediments (Adraen formation). Detailed mapping in the contact area between the Nogueres and Cadi units has shown that the actual thrust geometry in the study area is controlled by preexisting normal and transfer faults which developed in an already compressional context. Lateral ramps or tear faults develop depending on the angle between the pre-existing extensional transfer fault and the thrust transport direction.