Cameros Basin Research Articles

Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history: gypsum pseudomorphs of the Tera Group, Tithonian-Berriasian, Cameros Basin

Abstract This study reports a complex fluid and thermal history using petrography, electron microprobe, isotopic analysis and fluid inclusions in replacement minerals within gypsum pseudomorphs in Tithonian-Berriasian lacustrine deposits in Northern Spain. Limestones and dolostones, formed in the alkaline lakes, contain lenticularly shaped gypsum pseudomorphs, considered to form in an evaporative lake. The gypsum was replaced by quartz and non-ferroan calcite (Ca-2), which partially replaces the quartz. Quartz contains solid inclusions of a preexisting non-ferroan calcite (Ca-1), anhydrite and celestine. High homogenization temperatures (T h) values and inconsistent thermometric behaviour within secondary fluid inclusion assemblages in quartz (147–351°C) and calcite (108–352°C) indicate high temperatures after precipitation and entrapment of lower temperature FIAs. Th are in the same range as other reequilibrated fluid inclusions from quartz veins in the same area that are related to Cretaceous hydrothermalism. Gypsum was replaced by anhydrite, likely during early burial. Later, anhydrite was partially replaced by Ca-1 associated with intermediate burial temperatures. Afterward, both anhydrite and Ca-1 were partially replaced by quartz and this by Ca-2. All were affected during higher temperature hydrothermalism and a CO2-H2O fluid. Progressive heating and hydrothermal pulses, involving a CO2-H2O fluid, produce the reequilibration of the FIAs, which was followed by uplift and cooling.

Diagenetic albitization in the Tera Group, Cameros Basin (NE Spain) recorded by trace elements and spectral cathodoluminescence

This paper deals with the diagenetic albitization of both plagioclases and K-feldspars in the Tithonian fluvial sandstones of a rift basin (Cameros Basin). The sandstones in the lower part of the rift record have not suffered this albitization process. A clear relationship is observed between sodium contents, as the main element of some feldspars and their cathodoluminescence (CL) color (the higher the sodium content, the lower is their CL intensity). In conclusion, albitization processes are detectable by decreased CL intensities and changes in the CL spectra.In addition, very different trace element compositions are obtained by laser ablation when comparing trace elements of non-albitized feldspars in sandstones of the lower part of the rift record with those of albitized feldspars in sandstones of the infill top. Non-albitized K-feldspars show Rb, Sr, Ba and Pb contents of up to 1000ppm. In contrast, very flat profiles of trace element contents (<250ppm) are recorded in albitized feldspars (both K-feldspars and plagioclases). Thus, albitization implies feldspars impoverished in trace elements, including REE, which suggests that albitization is a dissolution and reprecipitation process. Further, albitized plagioclases show higher REE contents than albitized K-feldspars. We report here that REE patterns partly depend on the initial composition of the feldspar (K-feldspar or plagioclase) as a useful geochemical criterion for distinguishing albitized detrital plagioclases from albitized detrital K-feldspars.CL spectra from non-albitized and albitized K-feldspars and plagioclases revealed marked differences. Non-albitized K-feldspars present blue (main emission band at 460nm) and brownish CL colors (590nm), sometimes in the same grain. Brownish colors are related to weathering processes. The primary blue emission is related to Al–O−–Al centers, enhanced probably by Al incorporation due to the coupled substitution of Ba2++Al3+↔M++Si4+. Weathered K-feldspars present 4.8 times lower Ba content than fresh blue luminescent ones. The brownish colors are related to the external border or fractured grain zones, altered by weathering processes. Therefore, the observed 590nm emission is assumed to be caused by structural defects resulting from weathering and alteration.Albitized K-feldspars are usually weak luminescent with a typical CL emission band at 620nm. Sometimes, relicts of the original blue luminescence (460nm band) are still present. The leaching of probably both Al and Ba can be responsible for the decrease in the blue band. The characteristic 620nm band is also dominant in albitized weak luminescent plagioclases. Two additional emission bands at 440nm (Al–O−–Al center) and 565nm (Mn2+) occur, when albitized plagioclases preserved their original CL characteristics (green CL color). Another spectral peak at ca. 720nm can be explained by Fe3+ activation due to Fe3+–Al3+ substitution. The spectral CL measurements indicate that changes in luminescence due to albitization (620nm emission) seem to be more related to structural defects than to trace element activation or quenching.

Answer to the comment of Casas et al. about González Acebrón et al.’s (2011) paper

The comment of Casas et al. concerns the paper published by Gonzalez-Acebron et a1. (201 1) and refers mostly to their model for the structural and thermal events in the sedimentary record (Late Jurassic-Eady Cretaceous) from the Cameros Basin. Their model differs significantly from the interpretations previously published by Mas et a1. (1993). Guimera et a1. (1995. 20M) and Mas and Salas (2002). Their main concern appears to be the difference of opinion about the previously published structural evolution of the basin, not the results of our new study. Rather than debate the earlier, more appropriate papers, they have chosen this our marginally related paper to air their dif­ ferent view of the structural interpretations. Our paper did not concern evidence for interpretations of the structural evolution of this basin. which had already been published and peer-reviewed. We clearly stated that we had excluded their model for consideration relative to our new data, why we had done so, and referred to the appropriate peer­ reviewed literature. Thus, we must question whether this is

Comment on González-Acebrón et al. Criteria for the recognition of localization and timing of multiple events of hydrothermal alteration in sandstones illustrated by petrographic, fluid inclusion, and isotopic analysis of the Tera Group, Northern Spain Int J Earth Sciences (2011) 100:1811–1826

The paper by Gonzalez-Acebron et al. (2011) deals with both regional and methodological problems about the timing of thermal events in one of the best-known and controversial basins in the Iberian microplate: the Late Jurassic–Early Cretaceous Cameros Basin. The aforementioned paper contains invaluable data about the thermal evolution of the basin, and it is an important contribution to the understanding of the post-sedimentary, pre-inversion processes in this strongly subsiding basin. It will undoubtedly add new evidence to the knowledge of hydrothermal processes in general and particularly to the paleothermal evolution of the basin. However, in our opinion, there are some problems involving important matters within the paper, namely (1) the interpretation of the data obtained, which are too constrained within a univocal line of thinking without considering other simpler and more viable options that in the discussed paper are aprioristically rejected. (2) The second problem deals with referencing to previous papers and acknowledgement of previous works by other authors. We must say that this aspect does not refer to this paper alone, but it is recently becoming a commonplace for many scientific papers in this field. On the side of the references to previous works, it must be stated that:

Exceptional preservation processes of 3D dinosaur footprint casts in Costalomo (Lower Cretaceous, Cameros Basin, Spain)

Terra Nova, 24, 136–141, 2012AbstractTheropod dinosaur footprints at the Costalomo tracksite (Pinilla de los Moros Formation, Upper Hauterivian–Lower Barremian, western Cameros Basin, Salas de los Infantes, Burgos Province, Spain) show unusual preservation of a previously undescribed nature. The footprints occur as casts (positive epireliefs) at the top of a sandstone bed and preserve exceptional details of the top of the digit and claw morphology, and of digital interactions with the sediment during penetration and extraction from the sediment. Footprint formation and preservation occurred in the following stages: (1) The dinosaur stepped on a thin (4–8 cm thick) mud layer, its foot sinking to contact an underlying sand layer (channel fill); (2) voids left in the cohesive mud after foot withdrawal were later filled with sand; (3) subsequent deposition, burial and Alpine compression indurated the muds and the sands of both the footprint casts and the underlying channel sand layer; and (4) modern erosion exposed the footprint casts, by removing the mud above the sandstone.

Turtles from the Lower Cretaceous of the Cameros Basin (Iberian Range, Spain)

Knowledge of the turtle fauna from the Lower Cretaceous of the Iberian Peninsula has been very limited until now. There are several fossil sites where Lower Cretaceous associations of continental vertebrates have been found. Although turtles have been identified in some of them, most of these specimens have not been studied, so the diversity is unknown. Among all these findings, the turtles from the Cameros Basin are considered particularly relevant, both in their abundance and diversity. Their study has allowed the identification of several taxa. At least one representative of Solemydidae and three taxa of Eucryptodira are recognized. This study establishes kinship and biogeographic relationships between the taxa in Cameros with those found in other Spanish fossil sites and with those of other European regions.

New Trigonioidoidea (Bivalvia; Unionoida) from the Early Cretaceous of Spain

Abstract: Two taxa belonging to the Trigonioidoidea (Order Unionoida) are described from the Early Cretaceous of Spain. Nippononaia (Paranippononaia) camerana subgen. et sp. nov. is described from the Aptian of the Cameros Basin of the north-west Iberian Range. Subnippononaia fordi Barker et al., 1997 is described from new material from the Calizas de la Huérguina Formation (Late Barremian) from Las Hoyas and Buenache de la Sierra, Cuenca Province, and Subnippononaia is raised to generic status. The stratigraphy and palaeoenvironment of the Cornago G exposure of the Enciso Group where Nippononaia (Paranippononaia) camerana originates is interpreted as a fresh to brackish water system.

Criteria for recognition of localization and timing of multiple events of hydrothermal alteration in sandstones illustrated by petrographic, fluid inclusion, and isotopic analysis of the Tera Group, Northern Spain

Stratigraphic relations, detailed petrography, microthermometry of fluid inclusions, and fine-scale isotopic analysis of diagenetic phases indicate a complex thermal history in Tithonian fluvial sandstones and lacustrine limestones of the Tera Group (North Spain). Two different thermal events have been recognized and characterized, which are likely associated with hydrothermal events that affected the Cameros Basin during the mid-Cretaceous and the Eocene. Multiple stages of quartz cementation were identified using scanning electron microscope cathodoluminescence on sandstones and fracture fills. Primary fluid inclusions reveal homogenization temperatures (Th) from 195 to 350°C in the quartz cements of extensional fracture fillings. The high variability of Th data in each particular fluid inclusion assemblage is related to natural reequilibration of the fluid inclusions, probably due to Cretaceous hydrothermal metamorphism. Some secondary fluid inclusion assemblages show very consistent data (Th = 281–305°C) and are considered not to have reequilibrated. They are likely related to an Eocene hydrothermal event or to a retrograde stage of the Cretaceous hydrothermalism. This approach shows how multiple thermal events can be discriminated. A very steep thermal gradient of 97–214°C/km can be deduced from δ18O values of ferroan calcites (δ18O −14.2/−11.8‰ V-PDB) that postdate quartz cements in fracture fillings. Furthermore, illite crystallinity data (anchizone–epizone boundary) are out of equilibrium with high fluid inclusion Th. These observations are consistent with heat-flux related to short-lived events of hydrothermal alteration focused by permeability contrasts, rather than to regional heat-flux associated with dynamo-thermal metamorphism. These results illustrate how thermal data from fracture systems can yield thermal histories markedly different from host-rock values, a finding indicative of hydrothermal fluid flow.

Sand provenance and implications for paleodrainage in a rifted basin: the Tera Group (N. Spain)

Fluvial-fan and fluvial siliciclastic strata, developed during the rifting that generated the Cameros Basin (North Spain), record important provenance changes that reveal source areas compositions and locations, paleodrainage evolution and rift patterns. The Tera Group represents the first rifting stage in the Cameros Basin, containing fluvial-fan sediments at the lower part of the sedimentary fill that evolve to fluvial and lacustrine systems in the upper part of the record. Our quantitative sandstone petrographic analysis evidences the presence of three main petrofacies related closely to the rift basin evolution. At the base of the sedimentary succession, Petrofacies 1 (quartzolithic) indicates that the fluvial-fans source areas included Juras¬sic marine carbonates and older siliciclastic Mesozoic units, as well as metamorphic supplies from the West Asturian Leonese Zone (WALZ). Variscan basement sources of this metamorphic area (WALZ) were more abundant in the upper fluvial record (Petrofacies 2, quartzofeldspathic). Further, the influence of plutonic source areas with a mixed potassic and calcium-sodium composition is also recorded, probably related to the Central Iberian Zone (CIZ). In addition, a local sedimentary input was active during the fluvial and lacustrine stages (Petrofacies 2 and 3, both quartzofeldspathic), as a function of the palaeogeographical position of the Jurassic marine rocks and the level of erosion reached. Plutonic rock fragments have not been observed in the Tera Group sandstones of the western part of the basin. Thus, deeper erosion of the basement in the eastern Cameros Basin is suggested. The provenance evolution from quartzolithic to quartzofeldspathic petrofacies registered in Tera Group siliciclastic deposits is due to the higher influence of transversal supplies during the fluvial-fan stage (quartzolithic) to more important axial inputs during the fluvial stage (quartzofeldspathic). This provenance change represents the evolution from an undissected rift shoulder stage to more advanced stages of rifting (dissected rift shoulder) and during the beginning of a provenance cycle in a rifted basin.

Role of sandstone provenance in the diagenetic albitization of feldspars: A case study of the Jurassic Tera Group sandstones (Cameros Basin, NE Spain)

The Cameros Basin (Iberian Chain, NE Spain) formed during the latest Jurassic–Early Cretaceous rifting stage in an extensional regime characterized by high subsidence rates. Its sedimentary infill (thicker than 6000 m) has been subdivided into eight depositional sequences (DS) mainly composed of continental sediments. DS 1 and DS 2 represent the first rifting stage (Tera Group, Tithonian), mainly formed by fluvial and lacustrine sediments. Sandstone petrofacies evolved from quartz-sedimentolithic in DS 1 to quartz-feldspathic in DS 2 due to the rifting process. In DS 2, three different types of detrital feldspars (K-feldspars, albites and polysynthetic plagioclases) with similar sodium-rich compositions (mean: Ab 94.0 An 4.5 Or 1.5) can be recognized. Chemically pure nonluminescent albites (Ab > 99%) are common. In DS 2, diagenetic albitization of both plagioclases and K-feldspars is inferred from conventional microscopy observations, cathodoluminescence and electron microprobe analyses. DS 1 contains few plagioclase grains, which show no evidence of transformation into albite. Although the albitization is characterized as diagenetic it seems to be provenance-controlled since it affects the units showing higher original plagioclase/K-feldspar ratio (DS 2), due to the greater influence of plutonic and metamorphic source areas in DS 2. Possible Na sources are: (1) the percolation of moderate to high salinity residual brines from related alkaline lakes developed at top of DS 2 in the eastern sector of the basin, (2) clay mineral reactions (sodium smectite to illite and chlorite) indicated by mudstone composition in the interlayered mudstones, and (3) the replacement of detrital sodium plagioclases by carbonate. These three sources can be complementary.

Probable palaeogeographic influences of the Lower Cretaceous Iberian rifting phase in the Eastern Cameros Basin (Spain) on dinosaur trackway orientations

The Cameros Basin sedimentary infill comprises a large, essentially continental, megasequence ranging from the Tithonian (Upper Jurassic) to the Albian (Lower Cretaceous). It occupies an area of some 5500 km 2, and is home to around 300 dinosaur tracksites. Most of these tracksites are found in either the Huérteles Formation, which is part of the Oncala Group (Berriasian), or the Enciso Group (Lower Aptian), both of which represent early Cretaceous lacustrine episodes. Dinosaur trackways (n = 1170) from both episodes were analysed in order to establish the preferred direction of dinosaur movement, and to determine whether these movements were influenced by the palaeogeographic and palaeoenvironmental conditions of the area. The Huérteles Formation is interpreted as a complex record of alluvial plain systems distally connected with a playa-lake. Its dinosaur tracksites are distributed throughout its alluvial plain facies and trackways show two preferential unidirectional orientations: 1) NW, more or less parallel to the distribution of the facies belt and 2) NNE. The Enciso Group is represented by a wide and shallow lacustrine system connected with marine environments towards the SE (Iberian Basin realm) and in close proximity to marine settings to the NW (Basque–Cantabrian Basin realm), rendering the Cameros Basin as the only continental connection between the Ebro and Iberian Massifs. The fluctuating (but always shallow) water level of the system with frequent desiccations probably allowed dinosaurs to pass through the lake basin. As a consequence of these paleogeographic restrictions, the dinosaur trackways generally show a bidirectional NE–SW orientation. Despite the temporal and geographical differences between the Huérteles Formation and Enciso Group, the ichnocenoses of both are dominated by theropod dinosaur trackways (85% as a mean value). This is probably explained by these dinosaurs being more active than others, a consequence of their searching/hunting behaviour.

Protopleurobema: a new genus of freshwater bivalve from the lower Cretaceous of the Cameros Basin (NW Spain)

A new genus of fossil bivalve from the Lower Cretaceous of the Cameros Basin is described and designated Protopleurobema. As well as the type material of Unio numantinus (Palacios and Sanchez, 1885), we examined 805 specimens discovered in deposits comprising unit D of the lithological Urbion Group of Upper Barremian-Lower Aptian age. This new taxon is compared here to all the available closely related Mesozoic genera described in the literature. A large number of juvenile and adult specimens of Protopleurobema numantina (Palacios and Sanchez, 1885) were found in what was interpreted as a palaeoecological association. The presence of bivalves (Unionoida) and gastropods of the genus Viviparus indicate the freshwater nature of the environment.

Sauropod tracks of the Cameros Basin (Spain): Identification, trackway patterns and changes over the Jurassic-Cretaceous

Sauropod tracks make up only about 2% of the Cameros Basin ichnocenosis, but they are present over the entire time span represented by the Cameros sediments. The makers of these tracks are identified in terms of their associated trackway pattern as either wide or narrow-gauge morphotypes. Narrow-gauge trackways dominate the Tithonian-Berriasian interval. Wide-gauge trackways become notably more common after the Berriasian, although narrow-gauge trackways are still present and dominate the Cameros ecosystems even during the Aptian. At this time, an interesting equilibrium between titanosauriform and non-titanosauriform sauropod trackways is evident, although the latter are somewhat more common. A review of the Iberian sauropod bone record suggests that Turiasauria + Euhelopidae, Rebbachisauridae and Titanosauriformes are the three groups mainly responsible for the Cameros Basin sauropod ichnocenosis.

A reassessment of the Pteraichnus ichnospecies from the Early Cretaceous of Soria Province, Spain

ABSTRACT The Cameros Basin, distributed over Soria, La Rioja and Burgos provinces in NE Spain, is apparently one of the richest sources of pterosaur footprints, with six ichnospecies of Pteraichnus named to date. The Cameros Basin exposes 9 km thickness of Late Jurassic and Lower Cretaceous continental deposits that are richly fossiliferous. Of the six alloformations in this sequence, the Huérteles Alloformation is a succession of alluvial siliclastics and lacustrine limestones dated as Tithonian-Berriasian in age. The pterosaur footprints are described, and compared with related forms from elsewhere. Of the six ichnospecies named from the Cameros Basin, Pteraichnus palacieisaenzi and P. cidacoi are nomina dubia because they lack holotypes housed in a public institution. Further, P. manueli and P. vetustior cannot be characterized at present, and so are also regarded as nomina dubia until diagnostic characters may be identified. P. longipodus and P. parvus are probably valid taxa, distinct from P. saltwashensis and P. stokesi, named previously from North America. Of the four supposed pterosaurian ichnogenera, Pteraichnus is valid, and Purbeckopus and Haenamichnus may be valid, but Agadirichnus is a nomen dubium because it is poorly characterized and lacks type specimens.

Early Cretaceous fish trails from La Rioja, Spain

We report a new find of the fish trail Undichna unisulca in fluvial sediments of the Lower Cretaceous Oliván Group of the Cameros Basin (Aptian-Albian; Valtrujal, La Rioja, northern Spain). It is the second Mesozoic record of Undichna unisulca after the first discovery in the Lower Cretaceous of central Spain (Las Hoyas, Cuenca, central Spain). The trails are characterized by single sinusoidal waves which in this case are often surrounded by well-preserved lateral levees that rule out an undertrail hypothesis. The continental depositional setting, a floodplain fluvial system, reflects the known broad palaeoenvironmental distribution of Undichna and confirms its occurrences in freshwater settings.

Early Cretaceous non-marine bivalves from the Cameros and Basque-Cantabrian basins of Spain

Large unionids from the Weald facies of northern Spain are described. Three hundred seventeen specimens belonging to the Order Unionoida have been studied. Two species from the family Margaritiferidae were identified from the Cameros Basin. Margaritifera idubedae (Palacios and Sanchez, 1885) was recorded from the Urbion Group (Hauterivian-Barremian in age), being a very frequent bivalve in these fluvial sediments. Margaritifera valdensis (Mantell, 1844) was collected from the Enciso Group (Aptian in age) and is an uncommon bivalve in this lacustrine environment. A new genus with a new species called Protoanodonta conchae Delvene and Araujo, family Unionidae, is described from the Basque-Cantabrian Basin. Samples came from the Viviparus Bed Member (Vega de Pas Formation, Hauterivian-Barremian), interpreted as freshwater environment lacustrine in origin.

Characterizing the Mesozoic extension direction in the northern Iberian plate margin by anisotropy of magnetic susceptibility (AMS)

We present the results of anisotropy of magnetic susceptibility (AMS) analyses carried out on weakly deformed Triassic red beds, Jurassic and Lower Cretaceous limestones, sandstones and shales from several northern Iberian Mesozoic basins located near the northern Iberian plate margin (Santander, Cabuérniga and Polientes basins in the western part of the Basque–Cantabrian basin, and Bilbao and Aralar basins in the central–eastern part; Cameros basin in the northernmost part of the Iberian chain). A well-defined magnetic lineation of tectonic origin is found at most of the sampled sites, and is interpreted as the stretching direction resulting from the extensional deformation characterizing this period. During the Mesozoic, the northern margin of the Iberian plate underwent two rifting stages, the Triassic and Late Jurassic–Early Cretaceous (Kimmeridgian–Albian) rifting events. Both the Triassic and the period of the Late Jurassic–Early Cretaceous rifting event with maximum subsidence rates (i.e. until the Barremian) are characterized by a NE–SW extensional direction in the main subsidence areas. This indicates that NW–SE master faults controlled basin subsidence and sedimentation through these periods in the northern Iberia plate margin basins. Conversely, during Jurassic and Aptian times a lower activity of NW–SE faults, together with more important extension along a NW–SE direction can be inferred from AMS analysis. This study indicates the power of AMS analysis in basin analysis and geodynamic reconstructions even in subsequently inverted basins.

Multidisciplinary approach to an extensional syncline model for the Mesozoic Cameros Basin (N Spain)

In this paper we explore the possibility of reconstructing the initial geometry of inverted basins using combined geophysical and geological methods: seismic reflection profiles, paleomagnetism, analogue modelling and 3D geometrical analysis. These techniques allow for some unknowns in the geometry and evolution of extensional sedimentary basins (mainly the normal drag–reverse drag geometrical problem) to be solved. The example chosen for checking these techniques is the Cameros Basin (northern Iberian Chain), formed during the Late Jurassic–Early Cretaceous, and inverted during the Tertiary compressional stage. According to the reconstruction proposed, the Cameros Basin can be characterised as a lens-shaped, syncline basin with maximum thickness of deposits of about 8000 m in its depocenter, gradually diminishing toward the North and South. The extensional geometry of the Cameros Basin is defined by: 1) a large-scale growth syncline over the major basement fault, with southward-dipping beds in the northern basin margin, forming a southward-opening sedimentary wedge, with upward-decreasing dip values (large-scale normal drag geometry); 2) roll-over anticlines at the contact between the pre-rift series against the normal fault limiting the basin (smaller-scale reverse drag geometry). The combined syncline–anticline syn-extensional structures were characterised by means of paleomagnetic analysis of a remagnetization acquired at the time of extension. Analogue models indicate that the development of syncline basin geometries in extensional settings, as the one interpreted for the Cameros Basin, is closely related to the presence of a detachment level in the pre-rift sequence above the main basement faults.

New remains of the ankylosaurian dinosaur Polacanthus from the Lower Cretaceous of Soria, Spain

The Zorralbo locality of the eastern Cameros Basin, near Soria, Spain, has produced a diverse dinosaur assemblage from the Lower Cretaceous Golmayo Formation. Ankylosaurs are represented by dorsal vertebrae and ribs, a fragmentary sacrum and ilium, and several types of dermal armour. Most, if not all, of the material probably belongs to a single medium to large-sized adult individual. The Soria remains are referred to Polacanthus on the basis of the presence of conical, ungrooved presacral spines, a sacropelvic shield composed of irregularly arranged bosses and small tubercles, large spined plates, and hollow-based triangular caudal plates with an extended posterior basal edge and a pointed spine. Polacanthus is well known from the Wealden Group (Barremian-Aptian) of the Isle of Wight and from the Weald Clay Group of West Sussex (England). In addition, isolated remains have been reported from the penicontemporaneous formations of the Iberian Peninsula. The Soria outcrop is currently the most productive Polacanthus site outside England. Moreover, it has yielded the oldest record (late Hauterivian to basal Barremian according to charophyte association) of this ankylosaur known to date in Europe. Minor anatomic differences between the Soria material and the taxa P. foxii (type-species) and P. rudgwickensis suggest the presence of a third species of Polacanthus in the Iberian Peninsula, but additional material is needed to confirm this interpretation.

Destruction of a fluvial reservoir by hydrothermal activity (Cameros Basin, Spain)

This study provides an example of a high-quality fluvial hydrocarbon reservoir that was completely destroyed by hydrothermal processes. The reservoir unit was deposited in the Cameros Basin, located in the NW sector of the Iberian Chain (Spain). The basin was filled with clastic fluvial deposits (sandstones and conglomerates) between Late Berriasian and Early Aptian times. Provenance of sands was mainly from coarse crystalline rocks. A humid tropical climate produced intense weathering of K-feldspar during transport from source to basin. Thus, a mineralogically mature rigid framework with high porosity existed at the time of deposition, which would have constituted a high-quality hydrocarbon reservoir. At present however, the porosity of the reservoir is negligible. Porosity was reduced by a sequence of diagenetic processes: (1) mechanical compaction (i.e. crushing of metamorphic lithic grains) and chemical compaction, (2) kaolinite and siderite cementation, and (3) early quartz cementation. Hydrocarbon emplacement probably occurred between phases (2) and (3). A low-grade metamorphic (hydrothermal) event, reaching greenschist facies, took place during the Late Cenomanian. It dramatically reduced the remaining porosity of the reservoir and destroyed the hydrocarbon charge. Hydrothermal processes which affected the sandstones include (1) re-compaction; (2) late quartz cementation and silicification of remaining feldspars; (3) carbonate cementation; (4) chloritization of feldspars, metamorphic lithic fragments and intrabasinal argillaceous grains; and (5) growth of pyrite and chloritoid crystals on argillaceous material of intrabasinal, extrabasinal or even diagenetic origin. Hydrocarbons that migrated to the margins of the basin escaped these hydrothermal modifications and were preserved. The results of this study may be used to predict the diagenetic and hydrothermal evolution of other potential reservoirs in similar tectonic settings.