M. Marzo

Eastern Pyrenees and related foreland basins: pre-, syn- and post-collisional crustal-scale cross-sections

Abstract A new crustal-scale cross-section through the Eastern Pyrenees shows a minimum of 125 km of total shortening across the belt. Convergence rates of 6 mm/yr (during early and middle Eocene time) between the northern domain of the Iberian plate and Europe can be evaluated from calculated shortening rates in both sides of the orogen. Two stages of orogenic growth can be determined in the Eastern Pyrenean transect. A first stage (from Early Cretaceous to middle Lutetian time) is characterized by a low topography, submarine emplacement of the thrust front, fast rates of south-directed shortening up to 5 mm/yr and widespread marine foreland deposition. This stage is also characterized by equivalent amounts of mountain erosion and detrital foreland accumulation. A second stage (middle Lutetian to late Oligocene) is marked by an increase in structural relief, subaerial emplacement, a decrease in shortening rates and widespread continental sedimentation. This leads towards a non-equilibrium condition in which mountain erosion is almost three times the foreland basin accumulation, leading to a large by-pass of sediments towards the Atlantic before the final endorrheic stage of the basin. Erosion rates based on area conservation between middle Lutetian and present day sections in a two-dimensional calculation indicate an average of 0.15 mm/yr. This rise is lower than middle Lutetian to early Miocene rock uplift rates in the Eastern Pyrenees, which account for 0.2–0.35 mm/yr, suggesting that erosion has been discontinuous through time. Inferred maximum river incision rates since the middle Miocene opening of the Ebro Basin towards the Mediterranean Sea account for less than 0.1 mm/yr.

Oligocene to Middle Miocene evolution of the central Catalan margin (northwestern Mediterranean)

Abstract The Catalan margin belongs to the northern sector of the margin that separates the extremely thinned continental crust of the Valencia trough from the undeformed or thickened crust of the Iberian microplate. The integration of the available onshore and offshore data (mapping, stratigraphic sections, seismic profiles and well data) enables the present structure of the central Catalan margin to be identified as a major northwest-directed, thick-skinned thrust sheet compartmentalised in a system of NE–SW oriented horsts and grabens. The geometry and depositional features of the Tertiary successions denote that this structure resulted from two successive evolutionary episodes. The first episode was compressional and gave rise to the emplacement of the Catalan major thrust sheet; the second was extensional, linked to the opening of the Valencia trough. The compressional (pre-extensional) episode is recorded by Lower to lower Upper Oligocene deposits sedimented in a piggyback basin that developed in the present-day offshore. The pre-rift signature of these sedimentary successions is clear (they show constant thickness, are cut by all the normal faults, etc.) and their facies distribution indicates a mostly southwestward spreading of the sediments. The extensional episode includes the syn-rift and post-rift stages. The syn-rift stage resulted in the present horst-and-graben structure of the Catalan margin and it is recorded by the uppermost Oligocene–lower Burdigalian (Lower Miocene) sediments restricted to the graben troughs. The post-rift stage started in the late Langhian and has continued until the present. It is characterised by the attenuation of the tectonic activity, the sediment spreading over the horsts (most of them were completely overlapped during this stage) and no sediment thickness changes near the faults. The upper Burdigalian and lower Langhian sequences record that the transition between the syn-rift and post-rift stages was not sharp. These units show intermediate features since they concealed some minor faults and overlapped horsts, but were still affected by most of the major normal faults. The onshore and offshore tectono-sedimentary and palaeogeographic evolution show that: (1) the northwestern Mediterranean basin existed during the early to middle Oligocene as an extensional basin in the Gulf of Lions and as a contractional piggyback basin in the northeastern Valencia trough; (2) the extension started in the early Oligocene in southern France and migrated progressively southwestward, affecting the northeast Valencia trough during the latest Oligocene–Aquitanian.

Three-dimensional reconstruction of geological surfaces: An example of growth strata and turbidite systems from the Ainsa basin (Pyrenees, Spain)

The external and internal geometry of four turbidite systems outcropping around the Buil syncline (Ainsa basin, Spanish Pyrenees) has been reconstructed with reservoir-scale resolution in three dimensions (3-D). The irregular geometry of the syncline and the resolution required for the reconstruction cannot be resolved with cross sections. Therefore, reconstruction has been carried out with a new methodology that applies a 3-D dip-domain geometrical model and 3-D restoration techniques to achieve reservoir-scale resolution in kilometric-scale reconstructions. This methodology is aimed at resolving 3-D geometries in folded areas and regions with variable thickness stratigraphy.The 3-D reconstruction of the Buil syncline reveals the synsedimentary growth of an intrabasinal anticline and the foreland lithospheric flexure associated with tectonic loading north of the Ainsa basin.

Quantified vertical motions and tectonic evolution of the SE Pyrenean foreland basin

Abstract Local isostatic backstripping analysis is performed across the eastern part of the Ebro foreland basin between the Pyrenees and the Catalan Coastal Ranges. The subsidence analysis is based on two well-dated field-based sections and four oil-wells aligned parallel to the tectonic transport direction of the eastern Pyrenean orogen. The marine infill of the foreland basin is separated into four, third-order, transgressive-regressive depositional cycles. The first and second depositional cycles are located in the Ripoll piggy-back basin and the third and fourth ones are located south of the syn-depositional emergent Vall-fogona thrust. Subsidence curves display a typical convex-up shape with inflection points recording the onset of rapid tectonic subsidence. Inflection points coincide roughly with the base of depositional cycles. Rates of tectonic subsidence are less than 0.1 mm a−1 in distal parts of the basin and up to 0.53 mm a−1 in proximal parts during second depositional cycle. Younger depositional cycles show maximum rates of tectonic subsidence of 0.26 mm a−1. The locus of subsidence within the basin migrated southward at a rate of c. 10 mm a−1. This flexural wave crossed the complete Ebro foreland basin in 10–11 Ma. The intraplate Catalan Coastal Ranges at the southeastern margin of the Ebro foreland basin produced an increase of tectonic subsidence rate at 41.5 Ma. Maximum rates of tectonic subsidence coincide with deep-marine infill of the basin, maximum rates of shortening and thrust front advance, and low topographic relief orogenic wedge. Transgressive-regressive depositional cycles can be controlled partly by reductions of available space within the basin during tectonic thickening of the sedimentary pile by layer parallel shortening, folding and thrusting. Although much less constrained, an approximation of post-thrusting exhumation and isostatic and tectonic uplift, as well as a first determination of possible amounts of eroded material of parts of the Ebro basin illustrate the impact of post-depositional erosion and uplift on the foreland.

Growth strata in foreland settings

Abstract The accurate analysis of growth strata has revealed their significance to unravel both fold kinematics and timing of deformation in both compressive and extensive settings. The increasing acquisition of 3-D multichannel seismic lines (with few tens of meters of resolution) in complex tectonic areas reveals complex interplay between growing structures and deposition either in marine or continental environments. Interestingly, growth strata reveal similar relationships in both compressive folds linked to thrusts at depth and folds related to propagating normal faults in extensive regimes. To completely document the complete kinematic history of an individual fault or a group of faults, it is necessary to combine studies at different scales to integrate the findings revealed by field work (meters of resolution), and multichannel seismic lines results. The margins of the Ebro Basin to the south of the Pyrenees have been the focus of abundant studies of syntectonic deposits. This is because of its special long-term evolution from a foreland to an intermontane basin and a final open basin to the Mediterranean Sea in Neogene times. The complete comprehension of growth strata in these natural laboratories is essential for our present needs in both petroleum exploration and earthquake prediction.

Stratigraphy and sedimentology of the Middle Ordovician Hawaz Formation (Murzuq Basin, Libya)

The Middle Ordovician Hawaz Formation is a 200-m (660-ft)-thick succession made up of fine-grained quartzarenites displaying a variable degree of bioturbation. It records the deposition in a large-scale, low-gradient estuary, which was partially controlled by tectonic extension. The upper boundary of the formation is marked by two erosion surfaces (unconformities U1 and U2), related to the Late Ordovician glaciation. The U1 and U2 erosion surfaces generated a pronounced paleotopography that controlled the deposition of the Upper Ordovician sequences. Tectonism influenced the paleogeography, although faults were unimportant from the point of view of sedimentary thickness. Tectonic subsidence was moderate, and accumulation rates were low. Physiography favored tidal power, especially during transgressive episodes, when the coastal embayment was flooded. We defined 11 lithofacies, forming 6 facies associations. These associations are subtidal sandstones; storm-reworked, shoreface sandstones; shoreface-to-beach sandstones; channel-sandstone bodies; nearshore to inner-platform sandstones; and K-bentonites. Trace-fossil assemblages match Skolithos and Cruziana archetypal ichnofacies. On the basis of the dominant facies associations and ichnofacies, we divided the formation into three informal units, from base to top: HW.1, HW.2 and HW.3. Periodically, volcanic ash was supplied to the basin from distal eruptive centers and was preserved as thin beds of K-bentonite interstratified with the shoreface sandstones, but not with the tidal-dominated sandstones. We divided the Hawaz Formation into five third-order depositional sequences. Lowstand deposits were not identified. The lower boundaries of transgressive systems tracts are tidal ravinement surfaces or sequence boundaries, whereas the upper boundaries are flooding surfaces. The transgressive systems tracts are constituted by early transgressive tidal deposits separated by a wave ravinement surface from the late transgressive storm-dominated deposits. Highstand systems tracts consist of bioturbated shoreface-to-beach sandstones, which record seaward, shoreline progradation. Emilio obtained his Ph.D. in geology from the Universitat de Barcelona in 1988. Since then, he has been a lecturer in basin analysis and petroleum geology. He has been involved in several research projects on sedimentology and basin analysis in Spain, northern Africa, Antarctica and South America. His present-day research interests include three-dimensional modeling of sedimentary bodies and reservoirs. A professor of stratigraphy at the Universitat de Barcelona, M. Marzos research interest focuses on the application of clastic sedimentology, sequence stratigraphy, reservoir modeling, and basin analysis to the exploration and production of hydrocarbons. He has been involved in several research projects funded by oil companies in southern Europe, the North Sea, South America, and northern Africa. Jordi M. de Gibert received his Ph.D. from the Universitat de Barcelona in 1996. After a period at the University of Utah, he returned to Barcelona in 1999, where he currently holds a position as a tenure-track lecturer. His interests and areas of expertise include trace fossils, their paleobiological significance, and their implications for understanding ancient depositional environments. K. Tawengi received a B.Sc. degree in geology from Alfateh University, Libya, in 1984, and an M.Sc. degree in sedimentology and stratigraphy from Durham University, England, in 1996. He worked as an explorationist with Agip Oil Company in Libya from 1985 to 2000 and since then has worked as a senior exploration member with REPSOL Exploration in Murzuq S.A. His main fields of interest are sedimentology, stratigraphy, and subsurface geology. A. Khoja graduated in 1972 from the University of Libya. He received a diploma in petroleum geosciences from Oxford Polytechnic (1991) and an M.Sc. degree from Oxford Brookes University (1993). He joined the National Oil Corporation of Libya in 1972 and is presently the regional studies superintendent in the National Oil Exploration Department. Nestor obtained his degree in geology from Cordoba University (Argentina) in 1982 and his postgraduate in petroleum geology in the University of Cuyo, Argentina. He joined YPF in 1984 and worked in their exploratory department in Mendoza, Plaza Huincul, and Neuquen until the year 2000. Subsequently, he became Libya team leader for REPSOL- YPF in Madrid. He is currently director of exploration and production in Brazil.

Palaeo-elevation and effective elastic thickness evolution at mountain ranges: inferences from flexural modelling in the Eastern Pyrenees and Ebro Basin

Abstract The results are presented of a two-dimensional flexural modelling study of the lithosphere underlying the southern Pyrenees and the Ebro Basin. The modelling is based on a crustal-scale balanced cross-section along a profile through the north-eastern part of the Iberian plate. Two time slices of the structural evolutions of the mountain chain are modelled: the present day configuration and the configuration at Middle Lutetian time (47 Ma), where important structural and sedimentological changes are observed. The flexure model incorporates lateral variations in the effective elastic thickness (EET) of the lithosphere. The present day deflection in the profile is simulated using boundary forces and a northward decreasing EET that varies from 30 to 11 km. Models for Middle Lutetian times indicate EET values of 26-18 km in the northern part of the profile, assuming that the EET at the distal margin of the Ebro Basin has not significantly changed since Middle Lutetian times. These higher values for the EET at Middle Lutetian times suggest that the effect of the Cretaceous extensional phase on the present day flexural rigidity is small and, therfore, the inferred northward decreasing rigidity is predominantly related to the Pyrenean collision. Flexural modelling provides also constraints for the palaeo-elevation of the inner part of the chain. Including the assumption that the EET at the distal margin of the Ebro Basin has not significantly changed since Middle Lutetian times, the model predicts a maximum palaeo-elevation of ∼2000 m, which is in agreement with geological observations concerning the relation between basin-fill and palaeo-elevation.

The siliciclastic Permian-Triassic deposits in Central and Northeastern Iberian Peninsula (Iberian, Ebro and Catalan Basins): a proposal for correlation

The siliciclastic deposits of the Iberian, Ebro and Catalan Basins have been described for more than a century, but facies similarities and a lack of biostratigaphic data have difficulted the correlation of the local stratigraphic units in a general framework up to now. Combining pollen and spores data, the identification of the regional unconformities and hiatuses and the quantitative analysis of the subsidence by backstripping methods, a new correlation scheme for these facies is proposed.

Stratigraphic solution sets for determining the roles of sediment supply, subsidence, and sea level on transgressions and regressions

Although identifying unequivocally the cause of transgressions and regressions from the rock record can be difficult, we can place limits on the possible combinations of sediment supply, subsidence, and sea level that could generate an observed Stratigraphic sequence. Using simple geometric models that estimate maximum rates of change in the three controlling variables, and combining these with other limits derived from field observations, we can plot a solution set to define the most likely range of variations in these three primary controls on shoreline migration. This approach shows the amount each controlling factor must change alone, or in concert, to account for the stratigraphy. These quantitative values can be used to decide what is geologically reasonable and what is not. Although a unique solution may not be available, we can calculate the required parameters for the observed shoreline migrations, which provide a basis for informed interpretations.

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.