Y. Sánchez-Moya

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.

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.

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.