Alfonso Sopeña

Palaeoflood record of the Tagus River (Central Spain) during the Late Pleistocene and Holocene

Abstract Palaeoflood hydrology of the Tagus River (Central Spain) was reconstructed from slackwater deposits and palaeostage indicators in two canyon reaches located at El Puente del Arzobispo in the central part of the catchment ( 35,000 km 2 in drainage area) and in Alcantara, at the lower part of the catchment ( 52,000 km 2 in drainage area) near the Portuguese border. The palaeoflood record, with more than 80 flood events, shows clusters of floods at specific periods from 9440 to 9210 14 C yr BP (8540–8110 BC), 8500 to 8000 14 C yr BP (7500–7000 BC), ∼6750 14 C yr BP (∼5000 BC), 1200 to 950 14 C yr BP (AD 785–1205), ∼410 14 C yr BP (AD 1450–1500), 170 to 80 14 C yr BP (AD 1670–1950). The largest flood(s) occurred during the periods 9440 to 9210 14 C yr BP, ∼6750 14 C yr BP and 1200 to 950 14 C yr BP reaching minimum discharge estimates of between 4000 and 4100 m 3 s −1 in the El Puente del Arzobispo reach and 13,700– 15,000 m 3 s −1 in the Alcantara reach. These periods with increased flood magnitude and/or frequency in the Tagus River are strongly related to increased moisture influx and winter precipitation in the Iberian Peninsula, especially in the western zone. Proxy records sensitive to winter precipitation such as lake levels and vegetation changes (indicated by pollen records) are in good agreement with the clusters of floods found in the Tagus River. This flood variability seems to correspond to changes in the prevailing atmospheric circulation pattern affecting the Iberian Peninsula.

Sedimentology of high-stage flood deposits of the Tagus River, Central Spain

Abstract This paper details the sedimentology of high-stage flood deposits, with the definition of sedimentary environments and their characteristic sequences, along two bedrock reaches of the Tagus River (Central Spain). High-stage flood deposits accumulated in bedrock canyons include slackwater flood deposits (SWD) and other types of deposits located at flow separation zones and associated with slow-moving flow ( −1 ). These flood deposits are common indirect indicators of flood stages used in palaeoflood studies for estimating the discharges associated with Quaternary floods. Depositional environments of flood deposits include (1) channel widening, (2) canyon expansion, (3) bedrock obstacles, and (4) backflooded areas along tributary streams. These flood deposits can be found associated with other non-fluvial environments, namely aeolian reworked and slope washflow facies. Channel widening, due to flood stage variations, comprises internal and external zones of the channel margins, and their characteristic sequences contain similar facies to those of alluvial floodplains. Canyon expansion environments favour vertical accretion of slackwater units and the development of flood deposit benches, which contain four sequences related to bench elevation and distance from the channels main thread of flow. At the lee side of bedrock obstacles, characteristic sedimentary sequences are dominated by reverse flow structures (e.g. climbing ripples migrating upstream) due to eddies with a high sand concentration. Flood deposits located within tributary mouths contain typical sequences of reworked floodplain deposits. Backflooding of tributaries during flood stages produces deposition from suspension of sand, silt and clay within three sequences characterised by non-structure or parallel lamination and intense bioturbation. A better understanding of the flood deposit sequences may contribute to the characterisation of flood magnitudes and flood hydraulics and can also be applied to some ancient depositional environments.

Chapter 31 - Permian and Triassic rift basins of the Iberian Peninsula

Abstract The Iberian Peninsula is a good example of late Hercynian structures that influenced Mesozoic sedimentation. Complex graben systems, evolving along the fracture systems during late Hercynian movements were filled by hundreds of meters of red beds during the Permian and Triassic, the final stages of Hercynian movements were accompanied by several magmatic events. This paper deals with some well studied areas of the Iberian Peninsula (Cantabrian Mountains, Pyrenees, Catalo-nian ranges, Central System margins, Iberian ranges and southern margin of the Iberian massif) where basin evolution and tectonic activity are clearly related. A model for the evolution of the Iberian ranges is proposed based in Alvaro et al. (1979) with three distinct periods: (1) a compressive regime related to late Hercynian movements and the formation of Pangea, which occurred from the Late Carboniferous to Early Permian (Autunian); (2) a less-well-defined period when Saxonian facies were deposited; and (3) a distensive regime resulting in rift-basin development and Buntsandstein sedimentation, followed by marine sediments as the Tethys Sea encroached on the margins of the Iberian massif. This third period lasted from the Late Permian (Thuringian) to, at least, the Early Jurassic.

Evolution of Buntsandstein fluvial sedimentation in the northwest Iberian Ranges (central Spain)

ABSTRACT The Buntsandstein sediments in central Spain (Permian and Triassic) are mainly continental redbeds that form the base of a sequence that passes up into siliciclastic and carbonate tidal sediments, the Muschelkalk facies. Buntsandstein sedimentation began with conglomeratic and sandy fluvial facies whose detailed sedimentological analysis is the main subject of this work. Massive sheets are the most important conglomerate facies; they were formed by deposition as longitudinal bars. Channel-fill conglomerates are the second most important facies. Lateral accretion deposits also occur in the conglomerates; they evolved from longitudinal bars because of decreasing flow or because the bars had reached a certain height. Sandstones are composed of eleven different facies. Large linguoid or transverse bars were the main bedforms in this environment. These bedforms may be related downstream to large, oblique troughs developed when the tabular bedform grew to a certain size or when a tabular bedform reached another one that acted as an obstacle. Compound and composite compound bars consisting of large sets of trough cross-bedding with scoured, wavy bases are al o recognized in the sandstone facies. Two major cycles occur in the complete conglomerate sequence which correspond to different physiographic models. The lower cycle was mainly formed by channels and bars that were smaller than those in the upper cycle. The upper cycle is more extensive than the lower one and has characteristics that indicate a higher stability. Paleocurrents for both cycles have a north-northeasterly trend. The vertical evolution of these sediments is clearly related to the different fault systems that controlled their distribution. The paleogeography of the sandstone facies, together with paleocurrents (perpendicular to the conglomerate paleoccurents) indicate deposition from a separate dispersal system that extensively overlies the previous one. This unit, which can be followed extensively over most of the Iberian Ranges and even outside the Iberian Ranges, represents the growth of a fluvial braidplain over a large area of Iberia during the Lower Triassic.

Extensional tectonics in the central Iberian Peninsula during the Variscan to Alpine transition

Abstract The passage from the Variscan cycle to the Early Alpine framework in the central part of the Iberian Peninsula can be explained in terms of a transitional process involving four clearly differentiated tectonic episodes. 1. (1) A first Variscan compressional stage (VI, Middle Devonian to Early Carboniferous) dominated by compressional conditions leading to the building-up of the orogenic edifice. The stress regime was relevant to what might be called “Variscan-type” compression (E-W-oriented). This stage was characterized by major Himalayan-type tectonics with frontal nappes, thrusts, overturned folds, lateral transcurrent ramps, and localized anatectic magmatism. Minor synorogenic extension and plutonism was also recorded during this stage in the Tormes Granitic Dome. 2. (2) A second Variscan stage (V2, Early to Middle Carboniferous) was characterized by increasing extensional conditions leading to widespread plutonism (adamellites, granodiorites). Wanning compressional conditions were restricted to the eastern and southern realms of central Iberia (the eastern part of the Spanish Central System, and the Toledo Mountains). 3. (3) A third stage, here defined as Late Variscan (LV), developed from Middle Carboniferous to Early Permian, as a result of N-S late-orogenic extension. This episode is relevant to detachment tectonics and the gravitational collapse of the Variscan orogenic edifice under combined simple/ pure-shear conditions. Plutonism (granites and leucogranites) was still of major importance. Early Permian andesitic to dacitic volcanism and sedimentary basins developed within the eastern part of the Spanish Central System. 4. (4) A fourth stage, here defined as Early Alpine (EA, Early Permian to Triassic) marks the onset of the Alpine framework. This stage was characterized by what might be called an “Early Alpine-type” regional stress regime i.e. E-W extension and N-S compression, within a simple-shear model, and resulted in the configuration of the Iberian Peninsula into two contrasted realms: a western inherited Variscan block, and an eastern Alpine block subjected to post-orogenic extension. Elements developed during this event include N-S high-angle normal faults, NW-SE and NE-SW conjugate strike-slip faulting, and asymmetric rifting involving listric low-angle detachments.

Evolving fluvial architecture during a marine transgression: Upper Buntsandstein, Triassic, central Spain

Abstract The sedimentation in central Spain during the Late Anisian (Middle Triassic), (Limos y Areniscas de Rillo Formation), recorded the evolution from an alluvial environment into a coastal one with a diversity of environments. This evolution was the result of one of the episodes of the Tethys sea transgression over Iberia. During the first and second stages of this evolution, mixed-load, low sinuosity channels evolved upwards into a distal braidplain with ephemeral run-off and poorly defined channels. During a third stage, the characteristics of the sediments indicate the existence of both continental and marine waters in a transitional zone with seasonal hydrological fluctuations. The fourth stage records the sedimentation in an upper intertidal to supratidal environment in a semiarid coastline with the presence of thin cryptalgal sheets and chicken-wire textures. The final evolution consists of an environment with a high percentage of fluvial channel deposits, probably related to a meandering pattern. This evolution has been correlated with the depositional sequences established for the NE of Spain where the Anisian transgressive event resulted in the formation of an inter- to supratidal carbonate ramp. So, lowstand, transgressive and highstand system tracts from depositional sequence-1, have been recognized in the centre of Spain.

Tectonic systems tract and depositional architecture of the western border of the Triassic Iberian Trough (central Spain)

Abstract During the Permian and Triassic, a sedimentary wedge developed overlying a listric normal fault on the western border of the Iberian Trough, central Spain. An area was selected for study that provides optimal conditions for a detailed analysis to be made of relationships between geometric strata arrangements, tectonic control and sedimentary responses. The excellent quality of the outcrops allows the observed patterns of strata to be related to fault growth and developmental of the basin. The recognition and mapping of local discontinuities throughout the study area were possible. Packages of similar strata with different angularity can be discerned between the two main discontinuities. Geometrical relationships can be established for depositional unit boundaries. The architectural style and sediment dispersal patterns, interpreted as the results of interactions between sedimentation and tectonics, are integrated into a three-dimensional tectono-sedimentary facies model to explain the basins infilling is interpreted to be tectonic, the rift-related, three-dimensionally linked depositional systems recognized might be defined as tectonic systems tract [Prosser, S., 1993. Rift-related linked depositional systems and their seismic expression. In: Williams, G.D., Dobb, A. (Eds.), Tectonics and Seismic Sequence Stratigraphy. Geol. Soc., London, Spec. Publ. 71, 35–66]. Five such tracts are recognized and related to specific phases of tectonic development.

The Corrubedo beach-lagoon complex, Galicia, Spain: Dynamics, sediments and recent evolution of a mesotidal coastal embayment

Abstract The Corrubedo beach-lagoon complex is situated in an embayment on the northwestern coast of Spain. The 4.1 m tidal range and southwesterly winds are the main agents of sediment transportation. The sediments are hybrid arenites (siliciclastic + bioclastic), with carbonate percentages ranging from 27.36 to 54.10%; silt, clay and organic matter occur in minor amounts. Repeated mapping and sampling of the beach, tidal channels, salt marshes and active and vegetated dunes recorded the vertical facies associations and their temporal evolution. Two peat layers at 0.6 and 1.7 m below the present-day marsh have yielded ages of 2280 ± 60 yrs B.P. and 1045 ± 125 yrs B.P. respectively. The possible causes of the relative sea-level rise are discussed.

Bayesian age modelling applied to palaeoflood geochronologies and the investigation of Holocene flood magnitude and frequency

A new methodology, based on Bayesian age modelling, is presented for the analysis of palaeoflood geochronologies and palaeodischarge data. Bayesian age models were developed, using the Oxcal radiocarbon calibration software, for the geochronologies of three palaeoflood sites in Spain, namely the Gaudalentín, Tagus and Llobregat rivers in SE, Central and NE Spain, respectively. The age–depth plots resulting from the applied Sequence models enabled the construction of flood magnitude-frequency plots through substitution of the original stratigraphic depth data with the associated minimum discharge quantified by hydraulic modelling. The age models presented demonstrate that a Bayesian approach for analyzing Holocene flood magnitude and frequency prevents the loss of geomorphic and hydrologic information inherent in radiocarbon frequency methods previously used in the analysis of palaeoflood data sets. Frequency approaches do not allow proper consideration of flood magnitude information and only incorporate those geomorphic units specifically dated. The Bayesian age models calculate modeled ages for undated units as well, so that all the individual flood events identified in the field can be incorporated and visualized in the data output. The palaeoflood age models therefore illustrate: (1) the age range for clusters of palaeoflood events; (2) the number of events within each flood cluster, with an age estimate and palaeodischarge value for each event; and (3) the potential impact of discharge censoring on the record, for example the role of accommodation space infilling on the quantification of palaeodischarge. The methodology and results are briefly discussed within the wider context of fluvial palaeohydrology, in particular: (1) the role of Bayesian modelling in future fluvial palaeohydrology research; and (2) the value of bedrock gorge sites for investigating past flood–climate relationships, given the problems of deciphering allogenic and autogenic drivers in alluvial sedimentary records.

Le Permien En Espagne

Ces materiaux qui en grande partie etaient jusqu’a maintenant attribues au Trias ou au Carbonifere, existent dans maints endroits de la Peninsule Iberique. Les affleurements sont presque toujours nettement discordants au-dessus du socle hercynien et recouverts aussi en discordance par le Buntsandstein ou par des niveaux plus recents.