Mineralogy, geochemistry and uses of the mordenite–bentonite ash-tuff beds of Los Escullos, Almería, Spain

The mordenite ore deposit of Los Escullos has a surface area of 10⁶ m² with an average thickness of 5 m and estimated reserves of 7,500,000 tons of mordenite–bentonite. It is made up of horizontal layers of interbedded epiclastic tuffs with volcanic bentonitised materials which have been subjected to hydromagmatic activity. The layers are essentially composed of bentonite and mordenite with lesser amounts of quartz, cristobalite, biotite, plagioclase, chlorite, amphiboles, titanomagnetite, ilmenite and calcite. The harder layers display a higher proportion of plagioclase crystals and are enriched in Al₂O₃, CaO, Fe₂O₃, TiO₂, P₂O₅, Cu, Zn, Co, Cr, Ni and V, while the more altered layers contain larger contents of SiO₂, K₂O and Y. The amount of sodium increases (from 2% to 4%) relative to depth. Alteration processes resulted in a reduction in the contents of CaO, K₂O and MnO and increase in Na₂O and MgO. The beds of volcanic ash-tuffs have been devitrified by hydrothermal solutions giving rise to bentonites and sodium- and silica-rich residual fluids which have partly crystallized as mordenite and cristobalite. The raw material (mordenite–bentonite) can be improved removing biotite (magnetic separation) and plagioclase and quartz (by floating methods); however, the mordenite–bentonite mineral assemblage is practically impossible to separate due to the size of the crystals (average 0.5 μm under SEM–EDAX). In turn, this upgraded raw material has very useful properties (total area=520 m²/g and cation exchange capacity=70 meq/100 g) which may make it suitable for use in absorption processes (e.g. deodorization, cationic exchange), catalysis and molecular sieving.     

The great historical 1755 earthquake. Effects and damage in Spain

The access to the complete reports sent in answer of a Royal enquire relative to the 1755 earthquake (Archivo Historico Nacional, 1756) provided us with information more comprehensive than that included in the analysis of such documents prepared by the Spanish Royal Academy of History in 1756 (Real Academia de la Historia, 1756). With such data, we have made a new study of the shock centred mainly in the following points: number of victims, source parameters (including moment magnitude and epicentral location),intensity map and seismogeological effects compared with those expected from the EMS-98. Relevant results from this analysis are: a new intensity attenuation law for earthquakes from the region of the 1755 shocks and first approximation damage probability matrices for buildings of the types common in the XVII to XIX centuries affected by large, distant earthquakes.     

Deep seismic soundings in southern Spain

Summary In 1974 and 1975 deep seismic sounding experiments were carried out in the area of the Betic Cordillera in southern Spain. A network of crustal seismic profiles was established with shotpoints at sea close to Cádiz, Adra and Cartagena and on land at Alquife near Guadix. The lengths of the profiles range from 50 km near Alquife to 440 km for the main profile between Cádiz and Cartagena parallel to the strike of the Betic Cordillera. The main profile was supplemented by a reversed recording line close to the Mediterranean coast between Adra and Cartagena and another one perpendicular to the main tectonic strike from Adra towards the north. The first interpretation of the data indicates considerable variation in the crustal thickness. A preliminary inversion leads to a three-layered model of the crust. The mean compressional velocity is about 5.1 km/s down to a depth of 4 km. Below this the velocity is 6.13 km/s from 4 to 16 km where it increases to 7.14 km/s. TheP _n-velocity is 8.18 km/s. The crust-mantle boundary is reached at a depth of 27 km near Cartagena and lies 32 km deep near Adra, Underneath the gravity minimum of the Betic Cordillera the crust-mantle boundary is found at a depth of about 36 km. Below the Betic zone a pronounced zone of low velocity with 7.7 km/s seems to exist in the depth range from 40 to 60 km.Contribution No. 4, Grupo de Trabajo de Perfiles Sismicos, Comisión Española del Proyecto Geodinámico, C.S.I.C., Madrid, Spain.Contribution No. 139, Geophysical Institute, University of Karlsruhe, Hertzstrasse 16, D-75 Karlsruhe 21, Germany.Contribution No. 217, Institut de Physique du Globe, Université Paris VI, 4 Place Jussieu, F-75230 Paris Cedex 05, France.Contribution No. 11, Instituto y Observatorio de Marina, San Fernando (Cádiz), Spain.Contribution No. 163, Institute of Geophysics, Swiss Federal Institute of Technology, ETH-Hönggerberg, CH-8093 Zürich, Switzerland.     

The origin of the Sierra de Aracena Hollows in the Sierra Morena, Huelva, Andalucia, Spain

Hollows in the Sierra de Aracena, part of western sector of Sierra Morena region (Huelva, Spain), are geoecologically unusual macroforms. They are underlain by deeply weathered bedrock but have eutrophic soils with distinctive vegetation. Paleosols with very dark colours, a predominance of smectites and large amounts of total and free iron occur on the floors on the hollows. An evolutionary model is proposed for the hollows, involving differential weathering during the Mesozoic on plutonic and amphibolitic rocks, alpine tectonic activity followed by Quaternary erosion and exhumation leading to formation of erosional terraces.     

Soil-water behaviour of sandy soils under semi-arid conditions in the Duero Basin (Spain)

This paper examines the soil-water behaviour of sandy soils representative of a broad area in the Duero river basin (Spain), with semi-arid bioclimatic characteristics. Soil-moisture measurements of the first 25 cm of soil profile were taken using time domain reflectometry (TDR) methodology. The infiltration process was studied by means of field experiments combining rainfall simulations and TDR techniques. Finally, a set of hydrodynamic variables was calculated in the laboratory (i.e. available soil-water content, soil-water-retention curves and hydraulic conductivity).The average available soil-water content is 0·07 cm³ cm⁻³, in accordance with the predominance of the sandy textural fraction (85%). The rainfall simulation experiments show that infiltration prevails over overland flow, with a steady-state infiltration rate higher than rainfall application (40mmh⁻¹) in most cases; a low water-retention capacity was also observed. The evolution of soil-moisture over time indicates a close dependence on intra-annual rainfall variability due to the low water-retention capacity of sandy soils. With a maximum water-storage capacity of around 0·18 cm³ cm⁻³, the soils studied show high sensitivity to dry spells and may remain with <50% of the available soil-water content for several months during the year.     

RUNOFF PRODUCTION AND EROSION PROCESSES ON A DEHESA IN WESTERN SPAIN*

ABSTRACT. Runoff generation and soil erosion were investigated at the Guadalperalón experimental watershed (western Spain), within the land‐use system known as dehesa, or open, managed evergreen forests. Season and type of surface were found to control runoff and soil‐loss rates. Five soil units were selected as representative of surface types found in the study area: hillslope grass, bottom grass, tree cover, sheep trails, and shrub cover. Measurements were made in various conditions with simulated rainfall to gain an idea of the annual variation in runoff and soil loss. Important seasonal differences were noted due to surface cover and moisture content of soil, but erosion rates were determined primarily by runoff. Surfaces covered with grass and shrubs always showed less erosion; surfaces covered with holm oaks showed higher runoff rates, due to the hydrophobic character of the soils. Concentrations of runoff sediment during the simulations confirmed that erosion rates at the study site depended directly on the sediment available on the soil surface.     

Extreme rainfall events in Catalonia. The case of 12 November 1988

A study of the synoptic situation which produced the catastrophic floods of November 1988 in Catalonia (in the northeast of the Iberian Peninsula) is presented. Analyses of the vertical structure, potential instability, precipitable water, and instability index are made through the radiosounding data from Palma, Majorca. It is found that the 1988 situation is included in type I intense convective events in Catalonia (classification obtained from all the events since 1950, (Llasat, 1989)). It was characterized by:

Influence of geological setting on geochemical baselines of trace elements in soils. Application to soils of South–West Spain

A collection of 235 samples were taken from 115 sites (representing a density of 1 sampling site ca. 130 km²) on rural soils derived from the major rock types in the southern Iberian Massif. The geochemical baselines of selected trace elements (As, Co, Cr, Cu, Ni, Pb and Zn) were determined on the < 2 mm soil fraction. The sampling sites were not directly influenced by external pollution. Soil geochemical baseline and threshold values were calculated for each element in two geologically different zones: the Ossa-Morena Zone (OMZ) and the South-Portuguese Zone (SPZ).     

Development of carbonate platforms on an extensional (rifted) margin: the Valanginian–Albian record of the Prebetic of Alicante (SE Spain)

Within the upper Valanginian to upper Albian deposits of the easternmost part of the Prebetic Zone of the Betic Cordillera (Iberian Peninsula), seven lithostratigraphic formations made up of shallow-water carbonate and carbonate-siliciclastic sediments and of outer-platform hemipelagic sediments have been recognized. These formations were deposited in the most distal part of a platform that developed on the Southern Iberian Continental Palaeomargin. The geodynamic context was a margin affected by extensional or transtensional faults that produced tilted blocks. The interval studied records three major second-order transgressive-regressive facies cycles: (I) A late Valanginian to earliest Aptian cycle, mostly represented by hemipelagic and condensed sedimentation, with the development of a tectonically controlled high without sedimentation that separated two sectors with different sedimentary evolution and that ended with an episode of shallow-water carbonate platform development; (II) An earliest to latest Aptian cycle, with a transgressive phase represented by a retrogradational shallow-water carbonate platform capped by a drowning event leading to hemipelagic sedimentation, which was affected by an anoxic event (OAE 1a); the regressive phase is represented by progradation and aggradation of shallow-water carbonate deposits. Finally (III) a latest Aptian to early-late Albian cycle that records the expansion of mixed platform deposits in the entire area, ending with a phase of shallow-water carbonate platform development. Extensional tectonics leading to spatial and temporal changes in subsidence patterns is envisaged as the main control on sedimentation at a local scale, resulting in notable lateral changes in thickness as the main signature. Tectonics exerted a strong control on the distribution of sedimentary environments only during Cycle I. At a higher order, sea-level fluctuations are responsible for sequential organization, and environmental factors determined shallow-water carbonate platform development and demise, as well as oceanic anoxic events. The relevant continuity of the stratigraphic record in the distal part of the Prebetic platform has led to the recognition of events related to cycle boundaries, which result mainly from a combination of tectonics and sea-level changes.     

Functioning of a coastal karstic system with a submarine outlet,in southern Spain

A conceptual model of the functioning of a complex coastal karst aquifer in southern Spain is presented. The system has well developed conduits below sea level and is connected to the Mediterranean Sea. It discharges through two conduits 12 m below sea level, 20 km apart. The Moraig conduit is the main outlet; the discharge to the sea is brackish. The Toix conduit only discharges to the sea during heavy floods and lets in seawater the rest of the time. During the 1999–2000 hydrological year, both conduits were monitored by flowmeters and electrical conductivity-temperature probes. On the basis of the collected data, the hydrological relationship between the functioning of the system and the sea was characterised. The conceptual model assumes the existence of a huge reservoir called an “aquifer reservoir” which is supplied by (1) freshwater from rainfall and (2) seawater flowing into the Toix conduit. In addition, during heavy rainfall events, fast infiltration brings considerable amounts of freshwater into the Moraig conduit. This is typical of a “by-pass” mechanism. Salinity and flow rates were simulated with the use of a rainfall-discharge and a rainfall-salinity model. The simulation of flow and salinity time series can be used for water management purposes.