Los depósitos aluviales sintectónicos de la Pobla de Segur: alogrupos y su significado tectonoestratigráfico

El análisis estratigráfico y estructural de los conglomerados finieocenos y oligocenos de La Pobla de Segur (Formación de Collegats) localizados en los Pirineos centrales, ha permitido correlacionar diferentes episodios de sedimentación y de estructuración tectónica. Los materiales que constituyen el relleno de la cuenca intramontañosa de La Pobla de Segur (facies de abanico aluvial y deltaico, con intercalaciones lacustres subordinadas) reposan discordantemente sobre un substrato Mesozoico previamente deformado y estan organizados en una serie de cuñas clásticas que alcanzan un espesor acumulativo mínimo de 3500 m. El reconocimiento de superficies de discontinuidad, cartografiables a lo largo de toda la cuenca, ha permitido la definición de 5 alogrupos (Pessonada, Ermita, Pallaresa, Senterada y Antist) que a su vez pueden subdividirse en aloformaciones o secuencias. Las relaciones estructurales entre el suhstrato y los depósitos aluviales han permitido establecer la cronologia de la deformación compresiva tardía que afectó a este sector de la cadena sur-pirenaica y precisar el orígen claramente tectónico de las discontinuidades que constituyen los límites de los alogrupos.

Características estratigráficas y sedimentológicas del 'Garumniense' en el sector de Ager (Pre-Pirineo, Lleida).

La sucesión estratigráfica de los depósitos continentales de la ' cuenca de Ager corresponde a sedimentos lacustres (Fm Massana), depósitos fluviales y de llanura de inundación (Fm. Figuerola), sedimentos lacustres (Fm. Milla) y una sucesión de materiales palustres, lacustres, fluviales, de lagunas evaponticas y aun mareales (Complejo de Perauba). Todos esos materiales fueron depositados en los diferentes medios sedimentarios citados que se desarrollaron en esa zona entre el Maastrichtiense y el Paleógeno inferior. En concreto, el tránsito entre el Cretácico y el Terciario (m)se produjo en uno de esos medios sedimentarios. Anteriormente se habia correlacionado ese límite con un nivel litoestratigráfico concreto (carbonatos de la Fm. Milla), pero actualmente se conoce que ese evento se produce en el tercio superior de la unidad tem'gena de la Fm. Figuerola. Los datos de paleomagnetismo sugieren que los dinosaunos y las carófitas cretácicas empezaron a desaparacer, en esta zona, unos dos millones de años antes que sucediera el evento asociado al límite K/T.

Origin of a restraining bend in an evolving strike-slip system: The Cenozoic As Pontes basin (NW Spain)

The As Pontes basin (12 km2), NW Iberian Peninsula, is bounded by a double restraining bend of a dextral strike-slip fault, which is related to the western onshore end of the Pyrenean belt. Surface and subsurface data obtained from intensive coal exploration and mining in the basin since the 1960s together with additional structural and stratigraphic sequence analysis allowed us to determine the geometric relationships between tectonic structures and stratigraphic markers. The small size of the basin and the large amount of quality data make the As Pontes basin a unique natural laboratory for improving our understanding of the origin and evolution of restraining bends. The double restraining bend is the end stage of the structural evolution of a compressive underlapping stepover, where the basin was formed. During the first stage (stepover stage), which began ca. 30 Ma ago (latest Rupelian) and lasted 3.4 My, two small isolated basins bounded by thrusts and normal faults were formed. For 1.3 My, the strike-slip faults, which defined the stepover, grew towards each other until joining and forming the double restraining bend, which bounds one large As Pontes basin (transition stage). The history of the basin was controlled by the activity of the double restraining bend for a further 3.4 My (restraining bend stage) and ended in mid-Aquitanian times (ca. 22 Ma).

Estratigrafía del Paleógeno en la zona de tránsito entre la Cordillera Prelitoral Catalana y el Prepirineo

La serie de Eoceno y Paleoceno de El Far (provincia de Gerona), presenta un gran interés por servir de punto de enlace entre el Paleógeno de la Cordillera Prelitoral Catalana (en la zona de la Plana de Vic) y el Prepirineo. Es por ello que lieinos considerado interesante el pulilicar una serie detallada que sirva de jalón para poder correlacionar el Eoceno de la Plana de Vic con el de La Salut, Bellmunt y Puigsacalm, sobre todo en los tramos inferiores. Ambas series eocénicas poseen diferencias muy notables en su litología (cainbios laterales de facies) y potencia (aumento pogresivo hacia el surco paleógeno prepirenaico), lo que influye en la misma fauna...

La situación estratigráfica de las 'margas de Bañolas' (Almela y Ríos, 1943)

Se demuestra que las 'margas de Baolas' (ALMELA y Ros 1943) y las margas que se sitan encima de la 'caliza de Tavertet' (REGUAN1967) entre Vic y El Far son dos formaciones distintas. Estas ltimas que se pueden llamar 'margas de El Coll de Malla', son de edad Biarritzense inferior, mientras que las 'margas de Baolas' son posiblemente del Luteciense inferior o medio.

Las capas de 'Bulimus gerundensis' Vidal de la región de Utiel (Valencia)

Se da cuenta de nuevos hallazgos del Bulimus gerundensis, Vidal, en la región de Utiel, estudiandose su situación estratigráfica, su relación con el Microcodium y su posible utilización paleogeográfica.

La estratigrafía del Mioceno entre Altafulla y Torredembarra (província de Tarragona)

Se da la estratigrafa de dos secciones del Mioceno en el sector de Altafulla-Torredembarra. Las dos secciones se caracterizan por su elevado contenido en COsCa.

La estratigrafía y la fauna de la sección de Francás (Tarragona)

Se da la estratigrafia y la dispersión de la fauna en la sección de Francás (Tarragona). La edad de la seccin no es inferior al Tortoniense.

Presencia de Caliche Fósil en el Buntsandstein del valle del Congost (Provincia de Barcelona)

Se señala la presencia de caliche fósil en el Triásico inferior y el cual es interpretado como caliche inicialmente maduro producido en cortas exposiciones subáreas en condiciones semiáridas. Se deducen das tipos de ambientes de acuerdo con los dos modelos $e secuencias litolgicas en las que se encuentra el caliche: partes distale de glacis de acumulación y 'playas', y diques y charcas en una llanura aluvial meandriforme.

Firmground ichnofacies recording high-frequency marine flooding events (Langhian transgression, Vallè-Penedès Basin, Spain)

The decapod burrow Spongeliomorpha sudolica occurs associated with transgressive firmgrounds in the transition between Aragonian continental red beds and Langhian marine units in some of the inner sectors of the Vallès-Penedès Basin. This ichnospecies designates branching burrow systems with scratch marks in the walls produced by marine crustacean decapods. The occurrence of Spongeliomorpha represents an example of theGlossifungites ichnofacies. The several horizons where the traces are found are intercalated with continental red beds a few meters below the main transgressive surface, which is overlain by fossiliferous marine sandstones. The Spongeliomorpha-bioturbated layers record short, high frequency marine flooding surfaces that may be related either to actual sea-level changes or to variations in tectonic subsidence or sediment input. In any case, these flooding events punctuated the early phases of the Langhian transgression in the basin.

Sedimentos prodeltaicos en el Delta emergido del Llobregat

La unidad de limos intermedios del delta del Llobregat corresponde a un ambiente prodeltaico en el que se han identificado, por comparación al modelo actual, subambientes de prodelta proximal, medio y medio-distal. Su agua intersticial es sinsedimentaria, hallándose en la actualidad parcialmente diluida.

Comparison between detailed digital and conventional soil maps of an area with complex geology

Since different pedologists will draw different soil maps of a same area, it is important to compare the differences between mapping by specialists and mapping techniques, as for example currently intensively discussed Digital Soil Mapping. Four detailed soil maps (scale 1:10.000) of a 182-ha sugarcane farm in the county of Rafard, São Paulo State, Brazil, were compared. The area has a large variation of soil formation factors. The maps were drawn independently by four soil scientists and compared with a fifth map obtained by a digital soil mapping technique. All pedologists were given the same set of information. As many field expeditions and soil pits as required by each surveyor were provided to define the mapping units (MUs). For the Digital Soil Map (DSM), spectral data were extracted from Landsat 5 Thematic Mapper (TM) imagery as well as six terrain attributes from the topographic map of the area. These data were summarized by principal component analysis to generate the map designs of groups through Fuzzy K-means clustering. Field observations were made to identify the soils in the MUs and classify them according to the Brazilian Soil Classification System (BSCS). To compare the conventional and digital (DSM) soil maps, they were crossed pairwise to generate confusion matrices that were mapped. The categorical analysis at each classification level of the BSCS showed that the agreement between the maps decreased towards the lower levels of classification and the great influence of the surveyor on both the mapping and definition of MUs in the soil map. The average correspondence between the conventional and DSM maps was similar. Therefore, the method used to obtain the DSM yielded similar results to those obtained by the conventional technique, while providing additional information about the landscape of each soil, useful for applications in future surveys of similar areas.

New stratigraphic data from the Lower Penninic between the Adula nappe and the Gotthard massif and consequences for the tectonics and the paleogeography of the Central Alps

New stratigraphic data along a profile from the Helvetic Gotthard Massif to the remnants of the North Penninic Basin in eastern Ticino and Graubunden are presented. The stratigraphic record together with existing geochemical and structural data, motivate a new interpretation of the fossil European distal margin. We introduce a new group of Triassic facies, the North-Penninic-Triassic (NPT), which is characterised by the Ladinian "dolomie bicolori". The NPT was located in-between the Briançonnais carbonate platform and the Helvetic lands. The observed horizontal transition, coupled with the stratigraphic superposition of an Helvetic Liassic on a Briaçonnais Triassic in the Luzzone-Terri nappe, links, prior to Jurassic rifting, the Briançonnais paleogeographic domain at the Helvetic Margin, south of the Gotthard. Our observations suggest that the Jurassic rifting separated the Briançonnais domain from the Helvetic margin by complex and protracted extension. The syn-rift stratigraphic record in the Adula nappe and surroundings suggests the presence of a diffuse rising area with only moderately subsiding basins above a thinned continental and proto-oceanic crust. Strong subsidence occurred in a second phase following protracted extension and the resulting delamination of the rising area. The stratigraphic coherency in the Adula's Mesozoic questions the idea of a lithospheric mélange in the eclogitic Adula nappe, which is more likely to be a coherent alpine tectonic unit. The structural and stratigraphic observations in the Piz Terri-Lunschania zone suggest the activity of syn-rift detachments. During the alpine collision these faults are reactivated (and inverted) and played a major role in allowing the Adula subduction, the "Penninic Thrust" above it and in creating the structural complexity of the Central Alps.