Aquitaine Basin Research Articles

Preliminary report on Middle Coniacian-Lower Santonian Ammonite distribution from the Basque-Cantabrian Basin (Northern Spain)

The distribution of the ammonite faunas from Nidáguila, Turzo, Villamrtín, West Villamartín, Torme and Losa Valley sections (Burgos Province) have been studied. Six ammonite assemblages have been identified from the Middle Coniacian to the Lower Santonian. They have been compared with the northern Spain ammonite zones of Wiedmann (1960, 1964, 1979a, 1979b) and Wiedmann & Kauffman (1978), and with the standard zones established by Kennedy (1984, 1987) for the Coniacian and the Santonian respectively, in the type area (Aquitaine Basin).

The present-day stress regime of the southwestern part of the Aquitaine basin, France, as indicated by oil well data

In the Aquitaine Basin, there is little information available for estimating subsurface in situ stress magnitudes, but a small number of oil industry drilling leak-off tests suggest that σ h approaches 80% of σ v in magnitude. Horizontal principal stress orientations have been obtained by analysing drill ‘breakout ovalization’ in 55 wells. However, there is no clearcut directional homogeneity to the indicated stress trajectories and ovalization commonly parallels well deviation. Around Bordeaux, σ H exhibits a consistent NW-SE orientation. As one approaches the Pyrenees, the situation becomes less definitive. NE-SW σ H orientations are widely present and NW-SE σ H orientations also occur. ‘High-grading’ the data does not improve the picture significantly. This lack of directional homogeneity in areas adjacent to the Pyrenees is not completely understood. The data points could straddle a sinuous boundary between two stress provinces, but this explanation is not favoured. Alternatively, they may indicate that there is some local structural control of stress orientations. Whatever the cause, there are grounds for suspecting that horizontal stress anisotropy is weakly developed in this region today.

Attempts to detect open fractures and non-sealing faults with dipmeter logs

Abstract If enough stress-generated breakouts have been identified on four-arm dipmeter logs to map regional horizontal stress trajectories, it may be possible to use anomalous orientations to identify open fractures and non-sealing faults. These act as free surfaces and, if they were oriented obliquely to regional stresses, deflect them locally. This type of analysis is simple to apply and appears to offer a novel way to obtain key information required for optimizing the production of oil and gas. In offshore eastern Canada, the regional stress regime is well established and local deflections appear to be caused by near-by open fractures and non-sealing faults. In the Aquitaine Basin in France the regional stress signature is not homogeneous. Though some anomalous orientations appear to be caused by open fractures and faults, it is clear that many are not. Hence the method may not be applicable everywhere. Possible reasons include differences in the σ H :σ h ratios of the two areas and the geotectonic settings.

Une riche faune d'échinoides de L'éocène D'angoumé (Sud-Aquitaine, France)

A fauna of echinoids is recorded for the first time in Angoumé (South-Aquitaine, Landes) from beds of upper Bartonian age (zone P14 of Blow, zone NP17 of Martini). It contains 27 species belonging to 11 families and 15 genera. Seven species, coming respectively from the north of the Aquitaine Basin, the province of Barcelone, the Vicentin and the French Alps, are new for the south of the Aquitaine Basin. Among them, Clypeaster profundus is one of the oldest species of that genus. This fauna, that of Biarritz (la Gourèpe) excluded, is the richest in the Bas-Adour and Chalosse area. The big Holectypoida and Cassiduloida are prominent. The genera of these echinoids show a distinct specific diversity: it is possible to divide the Echinolampas species in two groups of sympatric species; owing to its anatomical characters, one of these is supposed to have lived buried deeper in the sediment than the other.

Consequences of foreland basin development on thinned continental lithosphere: Application to the Aquitaine basin (SW France)

We present an analysis of the consequences of foreland basin development on thinned continental lithosphere, inherited from pre-orogenic phases of extension. Bathymetry at the transition from pre-orogenic extensional basin to foreland basin and compaction of pre-orogenic sediments contribute to the accommodation space for foreland basin sediments and thrust loads. In addition, the extension-induced transient thermal state of the lithosphere, results in ongoing thermal subsidence, and a flexural rigidity which changes through time. Quantitative modelling of the phase of extension and the foreland basin stage of the Aquitaine basin (southern France) shows that the inherited transient thermal state of the lithosphere contributes significantly to (1) the total foreland basin depth and width, (2) the post-compressional subsidence history, and (3) the cratonward onlap pattern. Accounting for the thermo-mechanical effects of pre-orogenic extension significantly reduces the estimates of both the flexural rigidity (30–43% for the Aquitaine basin) and the required topographic or thrust load (40% for the Aquitaine basin) at foreland basins. Emplacement of thrust loads below sea level, as expected in a pre-orogenic extensional basin setting, further reduces the required topographic load. This sheds light on the wide range of flexural rigidity values reported for continental lithosphere from foreland basin modelling studies, and explains, in many instances, the inferred ‘hidden load’ or subsurface load in flexural modelling studies at foreland basins. The present study has shown that pre-orogenic extension phases significantly affect the record of vertical motion and the stratigraphy of the Aquitaine basin and is probably important for foreland basin evolution in general.

Stratigraphic placement of the Santonian-Campanian boundary (Upper Cretaceous) in the North American Gulf Coastal Plain and Western Interior, with implications to global geochronology

Recent micro- and macropaleontological research in the northern Aquitaine Basin of France, the stratotypic area for the Upper Cretaceous Santonian and Campanian Stages, was used to resolve a long-standing discrepancy in the stratigraphic position of the Santonian-Campanian boundary in east-central Texas as based upon zonations of planktonic foraminifers versus ammonites. On the basis of the presence in Texas of species of ammonites restricted to the lower reaches of the basal Campanian Placenticeras bidorsatum Zone of the Aquitaine Basin, the Santonian-Campanian boundary in Texas is significantly lower than has generally been considered by micropaleontologists. Specifically, it is within the foraminifer-based Globotruncana (= Marginotruncana ) concavata Zone ( sensu lato ). Such placement involves middle reaches of the type Austin Chalk, perhaps within the Dessau Chalk Member. An assumption that underlies the following conclusions is that the boundaries of the G. concavata Zone as recognized in European Tethys, the Atlantic and Caribbean basins, and North American Gulf Coast represent approximately synchronous stratigraphic levels. The following represent additional conclusions of pertinence to global chronology of the Cretaceous. (1) Rather than being younger than the Globotruncana concavata Zone, the Placenticeras bidorsatum Zone should be thought of as being contemporary with at least the younger half of the G. concavata (species total range) Zone. (2) Most of the Merchantville and Magothy Formations on the Atlantic seaboard of New Jersey and Delaware were deposited during early Campanian time, rather than principally during the Santonian. (3) The traditional placement of the Santonian-Campanian boundary in the Western Interior of North America slightly below the zone of Scaphites hippocrepis I is essentially correct. (4) The chronology of the Niobrara Formation of the North American Western Interior as based upon planktonic foraminifers is shown to be in serious error; specifically, it has been considered much too young. (5) Paleomagnetically sampled parts of the Niobrara Formation probably correlate with upper reaches of the “Cretaceous Long Normal Interval”, below magnetochron 33R. (6) Although the Santonian Stage surely represents a surprisingly brief interval of earth history, the Coniacian-Santonian boundary probably should be placed below the G. concavata Zone. (7) The Santonian-Campanian boundary slightly post-dates initiation of the last general Cretaceous regression of the North American Western Interior Seaway. (8) The beginning of the North American Aquilan Land Mammal “Age” represents earliest Campanian time. (9) Rather than representing an earliest Campanian geomagnetic event, the temporal span of global magnetochron 33R equates with approximately the second quarter of the total duration of the Campanian.

Epipaleolithic and Mesolithic adaptations in Cantabrian Spain and Pyrenean France

The period of deglaciation from ca. 13,000 to ca. 9000 B.P. along the northern edge of the Cantabrian Cordillera and Pyrenees was characterized by marked climatic and environmental oscillations, culminating in the establishment of interglacial conditions. While along the Cantabrian coast, late Upper Paleolithic groups had long been developing diversified systems of adaptation, fully exploiting the wide range of food resources of that narrow but ecologically varied region (notably red deer and marine mollusks), Magdalenian hunters along the southern edge of the Aquitaine basin were becoming increasingly specialized in the hunting of one medium-size game species, reindeer. Thus, while the artifact industries and artistic traditions of the two adjacent regions along the forty-third parallel developed along similar lines in the Magdalenian and Azilian, and despite a common montane specialization in ibex hunting, the changes that came with the end of the Last Glacial affected the human groups of the two regions very differently, as reflected in the early Mesolithic records of Vasco-Cantabria and Gascony, respectively.

Les Loxoconchidae (Ostracodes) a la limite oligo-miocène en Aquitaine: évolution, adaptation et biostratigraphie

Species belonging to the genera Loxoconcha, Loxocorniculum, Paracytheromorpha and Sagmatocythere have been studied in shallow marine to brackish palaeoenvironments of the Aquitaine Basin; ranging in age from the Middle Oligocene (Stampian) to the Lower Miocene (Burdigalian). Microevolutionary and regional stratigraphic information is inferred from temporal distributions of these species, the changes of their population structure, the changes of the means and standard deviations of some measured parameters and their ornamentation. All the changes recorded here seem to be specific responses to climatic and palaeogeographical modifications that took place between the Middle Oligocene and Lower Miocene. Four new species are described: Loxoconcha clinata nov. sp., L. estotiensis nov. sp., Loxocorniculum meridionalis nov. sp., Sagmatocythere escornebeouensis nov. sp.

Tectonic subsidence of the Aquitaine Basin since Cretaceous times

Tectonic subsidence of the Aquitaine Basin since Cretaceous times

Main results of the ECORS Pyrenees profile

The 250 km long, deep Seismic reflection survey across the Pyrenees from one foreland (Ebro Basin) to the other (Aquitaine Basin) was designed to study the structure of the deformed Mesozoic boundary between the Iberian and European Plates. The results are: 1.(1) the entire crust shows well-defined reflectors with a general fan-shape geometry;2.(2) a deep layered domain is evident above the Moho discontinuity;3.(3) near the surface in the external domains, reflectors accurately define the geometry of major thrusts and structures affecting the Mesozoic and Cenozoic cover of the Pyrenees;4.(4) the middle crust shows two major sets of Seismic events interpreted as Hercynian structures.Using these results, a balanced structural section of the Pyrenean belt is proposed. The mode of thickening of the crust during the Alpine orogeny is briefly discussed; only the Iberian crust was significantly thickened by a southward stacking of slabs.

Comparative Stratigraphy and Subsidence History of Mesozoic Rift Basins of North Atlantic (1)

Mesozoic rift basins around the North Atlantic all possess many of the following Upper Jurassic to Lower Cretaceous (Oxfordian to Albian) stratigraphic elements: (1) an early Oxfordian hiatus, (2) backfill deposits in Upper Jurassic paleovalleys, (3) multiple Tithonian transgressive-regressive sequences, (4) early Berriasian regressive sequences, (5) a sub-Valanginian unconformity overlain by coarse siliciclastic sandstones and conglomerates, (6) Valanginian to Hauterivian fining-upward sequences, (7) condensed Barremian sequences, and (8) prominent Aptian unconformities. More local features include abrupt Kimmeridgian deepening (Lusitanian basin, Portugal), Albian deepening (Cantabrian and Aquitaine basins, northern Spain and southern France), and late Albian-early Cenom nian source rejuvenation (Cantabrian basin, northern Iberia). Decompaction and backstripping of sediments from well and outcrop sections in six basins also reveal broad similarities in trends of rates of apparent basement subsidence. Relatively high rates (50-100 m/m.y.) characterize onset of rift phases in the Triassic and the Kimmeridgian. Locally, in the Lusitanian basin of Portugal, Kimmeridgian rates reach about 250 m/m.y. In the Cantabrian and Aquitaine basins, a third phase of rapid subsidence characterizes the Aptian-Albian interval after the main phase of continental breakup around Iberia. This phase may be due to transtensional stresses as Iberia continued to rotate relative to Europe, tensional intraplate stresses, or gravity driven partial collapse of the new continental marg n. The North Atlantic Mesozoic basins share many features because they had a common origin controlled by intracontinental rifting and subsequent separation of Europe and North America. Pulses of extension generally are reflected in the stratigraphy of all of the basins. Superimposed on this record are the effects of eustatic sea level variations. The main difference between the Canadian Jeanne d'Arc basin and Iberian basins is the significantly greater Early and Middle Jurassic basement subsidence rate in the former (about 40 m/m.y. vs. 0-10 m/m.y.). This difference may result from prolonged extension on the Canadian side, perhaps due to northward transfer of shear stress from left-lateral transform motion between the Grand Banks and northwestern Africa.

The Petroleum Evaluation Workstation--A New Tool for Volumetric Estimates of Generated and Migrated HCs in Sedimentary Basins: ABSTRACT

The authors can achieve a better evaluation of hydrocarbon prospects by means of organic geochemical analyses when volumetric estimates are done. Such geochemical information can be provided by a petroleum evaluation workstation composed of a modified Rock-Eval apparatus connected to a personal computer incorporating related software modules. Using the standard data given by the Rock-Eval (S{sub 1}, S{sub 2}, T{sub max}, IH, etc.) associated with the processing of the S{sub 2} peak signals, the petroleum evaluation workstation provides better location of source rocks, more precise determination of source rock OM types, and the computation of their efficiency transformation ratio (ETR). Likewise, these parameters combined with geological data (net thickness of source rocks, geothermal data etc.) give access to a rapid volumetric estimation of the initial generative capacity (IGC) of the source rocks, the generated hydrocarbons (GHC), and the migrated hydrocarbons (MHC). The energy activation distribution of the representative source OM can also be determined, as well as the quantities of oil and gas they can generate in a sedimentary basin. The workstation has been successfully used in the Aquitaine basin where the computed data were displayed on different specific maps. It appears from these results that the geochemical workstation makesmore » a better assessment of petroleum prospects.« less

Uranium and organic matter in a paleodeltaic environment; the Coutras Deposit (Gironde, France)

The Coutras uranium deposit located north of the Aquitaine basin (France) was discovered by COGEMA in 1980. It is hosted by middle Eocene arenaceous unconsolidated sediments, at the surface (the Jean Vincent area) and down to 100 m depth (the Le Fieu area). The detrital components of the sediments are mainly quartz, feldspars, and granitic and metamorphic rock fragments. The host sediments originated from the French Massif Central to the east. Orebodies are black tabular lenses occurring in sediments deposited in a delta plain environment where plant debris is abundant. Preservation of ligneous vegetation during burial established a reducing environment capable of concentrating uranium.There is a good correlation between uranium, total sulfur, and organic carbon. Uranium is mainly fixed in organics as organo-uranyl compounds. Uranium minerals detected include coffinite, pitchblende, and secondary hexavalent U minerals. Accumulation of uranium was favored by a permeable, organic-rich, and slightly acid environment. Adsorption processes probably enhanced ore formation. The lack of oxidized tongues may be the result of oxygen-depleted uranium-bearing solutions when they reached the organic-rich zone. The Coutras uranium deposit is then a good example of a tabular-type deposit which was deposited during early diagenesis.

Modelling of structures representing the South Armorican suture

The modelling of magnetic structures, which constitute a linear “marker” extending 600 km from the continental shelf off South Brittany to the northern part of the Aquitaine Basin, has led to the recognition of a major crustal fault zone; this is confirmed by the existence of other geophysical markers (gravity and seismicity). They all can be interpreted in the light of an inter-plate suture model.

The Ecors Pyrenean deep seismic profile reflection data and the overall structure of an orogenic belt

The 250‐km‐long deep seismic survey across the Pyrenees from the Aquitaine basin to the Ebro basin was sponsored by French and Spanish organizations, and was carried out in 1985 and 1986. This first seismic survey of an entire orogenic belt crosses the deformed boundary between the Iberian and European plates, the geometry of which has been greatly debated during a recent past. The main results are summarized as follows: (1) The entire crust shows well‐defined reflectors with a general fan shape geometry. (2) The Iberian crust seems to be thicker than the European one. Both are limited by a well‐defined deep layered material above the Moho discontinuity. (3) Only the Iberian crust was significantly thickened by a southward overstacking of slabs. (4) Near the surface in the external domains, reflectors define accurately the geometry of major thrusts and structures affecting the Mesozoic and Cenozoic cover of the Pyrenees. Using ECORS data (Etude Continentale et Océanique par Réflexion et Réfraction Sismique), the structure of the studied section of the belt can be modeled considering the nature and continuity of reflectors beneath the surface trace of the North Pyrenean fault considered as the initial vertical boundary between Iberian and European plates.

Dinoflagellate cysts of theWetzeliella‐complex from Eocene sediments of the Aquitaine Basin, southwestern France

Abstract Dinoflagellate cysts of the Wetzeliella‐complex belonging to the genera Kisselovia (sensu Lentin and Williams, 1976), Rhombodinium, Wetzeliella and Wilsonidium are reported for the first time from Eocene sediments of the Aquitaine Basin, southwestern France. Ten species are illustrated, six of which are described as new: Kisselovia aculeata, K. columna, Rhombodinium translucidum, R. rugosum, Wetzeliella uncinata, and Wilsonidium compactum. The assemblages illustrate the wide range of ornamentation and archeopyle morphology within the Wetzeliella‐complex. Kisselovia aculeata combines a penitabular arrangement of processes with a soleiform archeopyle. Paratabulation in Rhombodinium rugosum is outlined by parasutural folds. Wetzeliella uncinata has a unique archeopyle type, combining an adnate perioperculum with a shorter, free endoperculum. The species of Kisselovia display a para‐quadra paratabulation pattern.

Contribution of deep seismic profiling to the knowledge of the lower crust in France and neighbouring areas

In France and neighbouring areas, the crust is essentially formed of segments of the Variscan orogen (380-300 Ma) and older formations. Cratonic basins began to form in the Permian, but extensive subsidence only occurred from the Lias in basins such as the Paris and Aquitaine basins, and also along the margin of the Alpine Tethys ocean. The Pyrénées and Alps range eventually formed during the Tertiary as a result of the collision between the European and African continental plates. Large grabens were also rifted in the Rhine and Rhone valleys. Deep seismic profiles show remarkable features, common to the crust of the whole area, although it underwent such a complicated geological history. The most striking feature is the occurrence of conspicuous flat reflections densely distributed in the lower part of the crust below a comparatively transparent upper crust. These reflections, hereafter called laminations, have been found to occur on most profiles within a large area covering Western Europe, although they vary in depth, thickness and energy. Dipping reflections have frequently been observed in the upper crust. Some of them extend across the lower crust and even penetrate into the upper mantle. They may be reasonably interpreted as Variscan compressional features by correlation with surface geology and comparison with similar features found in deep seismic profiles from other regions. The origin of the flat laminations which generally lie in the lower part of the crust is more controversial. This paper attempts to use the well-known geological history of the region to place some constraints on the hypotheses which may be advanced to explain the formation of these laminations. These features may be explained by the following tectonothermal events: 1. (1) Compression which prevailed during the Variscan orogeny may have formed the laminations as a result of delamination and shearing near the bottom of the crust, associated with melting, intrusion and metamorphism during the different stages of the crustal evolution. 2. (2) The thermal equilibration of the upper mantle which occurred in the Permian, at the end of the Variscan orogeny, may have caused metamorphism and magmatism in the overlying crust. 3. (3) A regional thermal event which might be associated with subordinate extension occurred during the rifting of the Alpine Tethys and the initiation of Mesozoic cratonic basins in Late Triassic-Middle Jurassic times. It may also have caused metamorphism and magmatism in the lower crust. While the first model implies conservation of the crust material, the other two suggest that the crust-mantle transition has been somewhat uplifted since the end of the Variscan orogeny. The latter hypothesis is one way to explain the undeniable occurrence of seismic events below the present Moho reflection. The above three models are not mutually exclusive, and all these phenomena may well have affected the crust to some extent. However, the data at hand are still insufficient to determine what parts tectonic and nontectonic processes have played in the creation of the laminations in the lower crust and the Moho. Further experiments are needed to check these hypotheses.

Comparative Subsidence in the Aquitaine and Ebro Basins: ABSTRACT

Comparative Subsidence in the Aquitaine and Ebro Basins: ABSTRACT

Comparison between the Galicia and Aquitaine margins

There is a great similarity between the Galicia margin and the Aquitaine margin (i.e. the Aquitaine Basin and the North Pyrenean zone). In particular, the timing of the rifting as well as the resulting structures (exposure of perodotite, tilted fault blocks, normal and transverse faults, half grabens) are practically the same. This comparison leads to the interpretation that the northern side of the Pyrenean fold belt is inherited from the Mesozoic deep passive margin of Europe. The Cenozoic thrust sheets and reverse faults of the northern Pyrenees zone are tentatively interpreted as former rift structures (tilted blocks and listric faults) which were removed during the convergence of the Iberian and European plates during the Paleocene and Eocene.

The influence of the evolution of the Pyrenees on adjacent basins

Important thicknesses of syn- and post-orogenic sediments are observed in the southern part of the Aquitaine Basin and in the north of the Ebro Basin. Their deposition cannot be explained only as a result of the extensional history of the Early Cretaceous, thus the influence of the compression associated with the Pyrenees formation upon the subsidence of these basins is studied. Europe and Iberia are considered to behave as two semi-infinite elastic plates. The evolution through time of the sedimentary thicknesses is explained by the deflections of these two plates induced not only by the topographic load of the Pyrenees but also by additional forces (10 12 N/m) that may have their origin in the formation of a cold lithospheric root during the shortening. A series of calculations is presented for an approximately north-south profile across the Aquitaine Basin, the Pyrenees and the Ebro Basin; a good fit for the shape of the basins and the gravity anomalies was obtained, with flexural rigidities of 1.5·10 23. Nm for the northern plate and the outer part of the southern plate, and 1.4·10 21 Nm for the inner part, of the southern plate.