A. Casas

Mesozoic extensional tectonics, stratigraphy and crustal evolution during the Alpine cycle of the eastern Iberian basin

Abstract Sequence stratigraphy, subsidence analysis and the integration of the basin fill data allow to the identification of four successive evolutionary stages in the basins of the eastern Iberian margin during Mesozoic extension: (1) Triassic rift (Late Permian-Hettangian); (2) Early and Middle Jurassic postrift (Sinemurian-Oxfordian); (3) Late Jurassic and Early Cretaceous rift (Kimmeridgian-middle Albian); and (4) Late Cretaceous postrift (late Albian-Maastrichtian). The present-day crustal structure of the eastern Iberian Range and evidence of its evolution are deduced from the analysis of a new gravity map and other geophysical data. A regional gravity low along the Iberian Range is interpreted in terms of crustal thickening beneath the orogen as a result of the collision between the Ebro block and the Iberian plate during the Paleogene. The boundary between these blocks coincides with a magnetic lineament that is interpreted as an inherited signature of the thinning that occurred during Mesozoic extension. A geodynamic scenario for the crustal evolution of the eastern part of the Iberian Peninsula, based on the evaluation of Mesozoic extensional tectonics and an analysis of the available geophysical data, includes three main successive evolutionary stages: 1. (1) Mesozoic crustal thinning; 2. (2) Paleogene crustal thickening; 3. (3) Neogene crustal thinning. Based on new geological and geophysical arguments, this study offers an alternative and a more complete geodynamic history of the eastern Iberian basins related to the evolution of the Iberian plate and the Central-North Atlantic.

Analysis of inlet and outlet industrial wastewater effluents by means of benchtop total reflection X-ray fluorescence spectrometry

The monitoring of heavy metals in industrial waste water effluents is an important activity in many laboratories. Of special interest is the screening of elemental composition of inlet effluents and quantitative analysis of outlet effluents to study the efficiency of chemical treatment process to eliminate metals and to comply with current established concentration limits, respectively. In this sense, fast analytical methodologies which entail simple sample preparation are desired. In the present work, the possibilities and drawbacks of a benchtop total reflection X-ray fluorescence spectrometer (TXRF) for the rapid and simple determination of some inorganic impurities (As, Ba, Cd, Cu, Cr, Sn, Fe, Mn, Ni, Pb, Se and Zn) in inlet and outlet industrial waste water effluents from metallurgical and tanning leather factories have been tested. An evaluation of different simple sample treatments is presented and it is followed by a discussion of spectral and chemical matrix effects when dealing with this type of samples. Analytical figures of merit such as accuracy, precision and limits of detection have also been carefully studied. Finally, the data obtained by direct TXRF analysis has been compared to that obtained by ICP-OES/ICP-MS after a microwave digestion.

Deep structure of the Betic Cordillera derived from the interpretation of a complete Bouguer anomaly map

Abstract A complete Bouguer-anomaly map of the Betic Cordillera and surrounding areas has been compiled from 2995 land-gravity observations. The measurements used to construct the map come from previous surveys as well as from a survey carried out specially for this project. A large negative anomaly reaching −145 mGal is centered on the intra-mountain basins of Loja-Granada and Baza-Guadix, and systematically displaced from the region of greatest topographic relief. This gravity low is interpreted as produced by the thickeming of the earths crust at a principal contact between two domains of continental crust having different origins and geological evolution. Three gravity profiles selected to investigate the deep structure of the crust in the Betic Cordillera are presented and discussed.

INTEGRATED GRAVITY AND SEISMIC INTERPRETATION OF DUPLEX STRUCTURES AND IMBRICATE THRUST SYSTEMS IN THE SOUTHEASTERN PYRENEES (NE SPAIN)

Abstract Combined surface, subsurface and gravity data were used to determine the deep structure of the thrust belt in the southeastern Pyrenees. The Ripolles and La Garrotxa areas are part of the Cardona oil exploration permit. Hydrocarbons pervade the concession with numerous oil seeps, abandoned oil wells, and gas or condensate are in these two wells associated with the overthrust belt. The source rocks, the Armancies formation (upper Cuisian-lower Lutetian), consist of alternating layers of organic-rich shales and thin limestones; this formation outcrops in the Cadi thrust sheet. In the Ripolles area, the Cadi unit overlies the Serrat unit with a hanging-wall ramp geometry, as deduced from the Serrat-1 well. The structure of the Garrotxa area is more complex. It consists of a duplex structure folded by antiformal stack structures with basement rocks in their cores. The northern boundary of these antiformal stacks represents an inversion of pre-existent extensional faults. The basement rocks involved in the structure represent shortcuts formed in the footwall of the extensional faults. A negative anomaly is shown in these antiformal stacks. These anomalies are interpreted as a combination of salt accumulation, evaporites and sedimentary rocks, under the antiformal stacks. In the Serrat-1 well, the Serrat unit is made up of a thick Middle Eocene evaporitic sequence. But the gravity and seismic data evidence the presence of non-evaporitic sedimentary rocks in the northern part of the Serrat unit. These sediments correspond to the footwall of the Cadi series, so the probable existence of the Armancies formation in the Serrat unit is a very good target for oil prospecting. The existence of the previous extensional faults only in the Garrotxa area, determines the difference in the deep structure between the Ripolles and Garrotxa areas.

An interactive 2D and 3D gravity modeling program for IBM-compatible personal computers

Abstract An Interactive Graphic System for three-dimensional (3D) gravity modeling is presented. The system uses a highly flexible algorithm for the calculation of theoretical anomalies, which permits accurate 3D simulation of most types of geological bodies, and the simultaneous introduction of several bodies. This ability to combine several objects, together with features such as topographic relief, faults, and erosional process, adds a higher degree of realism to the model.

Moho depth structure of the eastern part of the Pyrenean belt derived from gravity data

Abstract This paper proposes a new and updated interpretation of the gravity map of the eastern part of the Pyrenean belt based on the well-established correlation existing between gravity anomalies and crustal structure. A new set of gravity data, collected over the past few years by the University of Barcelona both for academic purposes and hydrocarbon exploration, have been merged with the data previously available. First a quantitative interpretation of the crust’s thickness has been estimated by establishing a local empirical relationship between Bouguer anomaly and the Moho depth calculated from deep seismic refraction and reflection profiles. Then, a spectral analysis of the radial wavenumber of the Bouguer anomaly values using the FFT and a 3D inversion has been performed. Finally, a 2 1 2 D density model from a SE–NW gravity profile linking the southern edge of the coastline with the northern part of the ECORS–Pyrenean profile was also established.

Use of electrical tomography methods to determinate the extension and main migration routes of uncontrolled landfill leachates in fractured areas.

This study focuses on the uses of the electrical tomography and its relationship with hydrochemical data in order to characterize contaminated groundwater flows in fractured aquifers. The studied area is contaminated with different hazardous substances like lyndanes, organochlorinated compounds and benzenes coming from the old non-controlled Sardas landfill. The enormous volumes of wastes filling the landfill have generated a convoluted mixture of leachates. Due to the lack of a landfill liner, the leachates have migrated through the fractured Eocene marls towards the Gallego River. The striking correlation between high concentrations of polluted groundwater and low electrical resistivity of the subsurface (<8Ω·m) allows defining the principal contaminant migration route thanks to the distribution of these conductive anomalies. This mapping verifies that there is intense tectonical-structural control of the leachate migration, because the deep migration presents the same direction as the geological axis fold.

Geophysical exploration on the subsurface geology of La Garrotxa monogenetic volcanic field (NE Iberian Peninsula)

We applied self-potential (SP) and electrical resistivity tomography (ERT) to the exploration of the uppermost part of the substrate geology and shallow structure of La Garrotxa monogenetic volcanic field, part of the European Neogene–Quaternary volcanic province. The aim of the study was to improve knowledge of the shallowest part of the feeding system of these monogenetic volcanoes and of its relationship with the subsurface geology. This study complements previous geophysical studies carried out at a less detailed scale and aimed at identifying deeper structures, and together will constitute the basis to establish volcanic susceptibility in La Garrotxa. SP study complemented previous smaller-scale studies and targeted key areas where ERT could be conducted. The main new results include the generation of resistivity models identifying dykes and faults associated with several monogenetic cones. The combined results confirm that shallow tectonics controlling the distribution of the foci of eruptive activity in this volcanic zone mainly correspond to NNW–SSE and accessorily by NNE–SSW Neogene extensional fissures and faults and concretely show the associated magmatic intrusions. These structures coincide with the deeper ones identified in previous studies, and show that previous Alpine tectonic structures played no apparent role in controlling the loci of this volcanism. Moreover, the results obtained show that the changes in eruption dynamics occurring at different vents located at relatively short distances in this volcanic area are controlled by shallow stratigraphical, structural and hydrogeological differences underneath these monogenetic volcanoes.

Teaching oriented geophysical software

Abstract Interactive teaching techniques encourage students to adopt an active role in their education and should therefore be used at different levels of the teaching sequence. In order to mitigate the lack of educational software for Applied Geophysics, a fully interactive graphic software has been developed. The program is written in Visual Basic with some subroutines in FORTRAN and is designed for IBM-PC microcomputers using a Windows environment. The program offers the majority of the processes involved in geophysical data handling, modelling, tutorials, and instrument simulators.

Geochemical processes controlling groundwater quality under semi arid environment: A case study in central Morocco

Bahira plain is an important area for Morocco due to its agriculture and mining activities. Situated in a sub-arid to arid climate, this plain hosts an aquifer system that represents sequences of carbonates, phosphates, evaporates and alluvial deposits. Groundwater flows from Ganntour plateau (recharge area) to the basin-fill deposits and Zima Lake and Sed Elmejnoun where water evaporates. The objective of this study was to characterize the chemical properties of the groundwater and to assess the processes controlling the groundwaters chemistry. We can divide water samples into three hydrochemical water groups: recharge waters (Ca/Mg-HCO3), transition zone waters (Ca-HCO3-SO4/Cl) and discharge waters (Na-Cl/SO4). Accordingly, compositions of waters are determined by the availability of easily soluble minerals like calcite (Ca-HCO3 dominant), halite (Na-Cl dominant) and gypsum (Ca-SO4 dominant). Cl/Br ratios show that Cl concentration increases from dissolution of natural halite. When groundwater is affected by extreme evaporation Cl/Br ratios may increase up to 1900. High fluoride concentrations are associated with low Ca2+ concentrations (<100mg/L). That means when recharge waters enter the aquifer, it starts dissolving fluorite since the Ca2+ concentration is low. Once groundwater becomes saturated with Ca2+, the immobilization of fluoride is occurring by precipitation of fluoride-rich minerals like fluoro-apatite. According to the environmental isotope (18O and 2H) analyses, they are three potential processes affecting groundwater: 1. Evaporation as verified by low slope value, 2. Water-rock interaction, 3. admixture of waters showed different stable isotope compositions and salinities.