M. J. Mayayo

Geochemistry of Precambrian and Paleozoic siliciclastic rocks from the Iberian Range (NE Spain): implications for source-area weathering, sorting, provenance, and tectonic setting.

Abstract The major and trace element chemical composition of Precambrian and Paleozoic shales and sandstones from the Iberian Range (Spain) has been investigated to determine the provenance and tectonic setting of these rocks, as well as to appraise the influence of the weathering, hydraulic sorting and recycling processes upon source rock signature. The samples studied belong to a prograde sequence from diagenesis to anchizonal grade, and are mainly composed of quartz, illite/mica phases and/or kaolinite, with chlorite, feldspars, and carbonates as minor components. The major element distribution (Chemical Index of Alteration (CIA) indices) reflects that recycling processes have been important in homogenizing the composition of shales and sandstones. The higher CIA indices observed in the Silurian and Devonian shales (80 and 85, respectively) compared with those of Precambrian to Ordovician and the Carboniferous age (about 70) indicate that their source area underwent more intense chemical weathering processes, possibly due to climatic and/or tectonic variations. Alternatively, the source area of the Silurian and Devonian shales may have been composed of recycled sedimentary materials. Mineral fractionation is mainly observed in coarser rocks through zircon, apatite and xenotime accumulation, although the shales are not free of these phases. The zircon content of the fine-grained rocks is not high enough to affect rare earth element (REE) contents, but phosphate minerals (apatite and xenotime) at least partially control the REE distribution. The influence of apatite and xenotime are slightly higher in the Precambrian shales as the phosphate concentration produces a decrease in the La/Sm ratios. Chondrite-normalized REE patterns and negative Eu anomaly size of the studied rocks are similar to that of Post-Archean Australian shales (PAAS) indicating that they originally come from a differentiated silicic source. The higher REE contents in the studied shales in relation to PAAS indicate that recycling processes in the Iberian Range sources were probably more intense than that of PAAS. The slight differences among the REE patterns of the different groups of shales probably do not reflect changes in source-area composition, but instead suggest variations in mineral sorting, chemical weathering and/or sediment recycling. The Th/Sc, Co/Th, Cr/Th, Cr/V, V/Ni ratios support a primitive silicic source for these rocks, with the higher Th/Sc and lower Co/Th in Cambrian and Ordovician shales indicating a higher proportion of felsic material in their primitive source area. The compositional maturity of analyzed sandstones is typical of cratonic environments and their La, Sc, Th and Zr contents reflect their passive continental margin setting.

Sedimentary facies distribution and genesis of a recent carbonate-rich saline lake: Gallocanta Lake, Iberian Chain, NE Spain

Abstract The study focuses on the Holocene sedimentary infill of the Gallocanta lacustrine basin in the Iberian Chain, NE Spain. The Gallocanta lake is a saline wetland with a maximum length of 7.5 km and a maximum width of 2.85 km. The water depth varies significantly, from a maximum depth of 2 m to completely dry. In the central areas (central subenvironment) sapropels and salts develop, with halite, gypsum, anhydrite, dolomite, aragonite, calcite, magnesite, and lesser amounts of quartz and clay minerals. Cyanobacteria filaments are related to the aragonite and dolomite crystals. The marginal subenvironment either has a gradual or a sharp change from that of the central subenvironment. An inner area with desiccated light grey lutites is present in this marginal subenvironment. In SEM and X-ray diffraction analyses, quartz, clay minerals, aragonite, calcite and small quantities of dolomite, gypsum, anhydrite and halite can be identified. This inner area is surrounded by an external fringe composed of light brown lutites and a high concentration of Salicornia meadwod and microbial mats. This zone is only occasionally submerged and contains sandy and conglomerate islets. Active palustrine areas are flood zones, where grey lutites with a significant quantity of vegetation, such as reeds, are common. In general, this entire sector is being modified by human action. Five sedimentary facies have been defined, and this has enabled the identification of three distinct stages in the general evolution of the basin. The first stage is an alluvial period, developing during an arid climate. After this, a more humid stage facilitated the installation of a shallow carbonate-rich lake. A reduction of the water level, probably due to a more arid stage, induces a salinity increase of the lacustrine brine and the change to the third stage, which corresponds to the present conditions. The water level experienced frequent oscillations, and alternations between humid conditions with a high production of organic matter, which favors carbonate formation, and water level dropdowns, even to total dryness, with saline sedimentation.

TEM study of mineral transformations in fired carbonated clays: relevance to brick making

Abstract This study uses transmission electron microscopy (TEM) and analytical electron microscopy (AEM) supported by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to investigate the mineralogical and textural changes produced in carbonated clays by firing. Sample bars were prepared using raw clays composed of quartz, illite and carbonates with minor amounts of smectite, chlorite, feldspars and Fe oxides. The raw samples were then fired at temperatures of between 800 and 1050°C. The XRD data show that increases in firing temperature result in dehydroxylation of clay minerals, carbonate decomposition and the formation of Ca-bearing silicates (e.g. gehlenite, wollastonite, pyroxenes and anorthite). The sizes of the Ca-silicate crystals make the use of the SEM inappropriate since they lie below the resolution threshold. However, TEM/AEM do provide the required textural and compositional characterization, revealing that there is a broad range of pyroxene compositions, some of which resemble fassaite, and that Ca/Mg ratios increase with temperature. The TEM also shows significant dehydroxylation and vitrification of the clay-rich matrix at T of ~800°C. Observed mineralogical and textural changes probably occurred in a system with a local disequilibrium much like small-scale, high-temperature metamorphic reactions (i.e. pyrometamorphism). The importance of these results is that they enable the selection of more appropriate raw clay composition and firing dynamics (temperature, firing duration and cooling rate) for both the brickmaking industry and brick conservation in the field of cultural heritage.

Genesis of kaolinite from Albian sedimentary deposits of the Iberian Range (NE Spain): analysis by XRD, SEM and TEM

Abstract The kaolinite from Albian sedimentary deposits (Escucha and Utrillas Formations) of the Iberian Range (Spain) have been investigated. This research has shown the presence of different types of kaolinites (detrital and diagenetic) along with micaceous phases in these deposits. Detrital kaolinites show anhedral morphology, low crystallinity and a degree of ordering as well as the presence of interstratified smectite layers. They constitute the matrix of the claystones and siltstones and were probably formed as a consequence of intense weathering processes in the source area during the warm period of the early Cretaceous. Diagenetic kaolinites have been recognized in the sandstones and siltstones, with kaolinite growing between ‘expanded’ mica flakes and vermiform and euhedral kaolinite forming the matrix. They have euhedral morphologies, high crystallinity and a high degree of ordering. They grew in situ as a response to incipient diagenesis by K-feldspar dissolution and/or organic acid-rich fluids derived from the maturation of organic matter in shales.

Variations in the chemistry of smectites from the Calatayud Basin (NE Spain)

Abstract Smectites of sedimentary series from a playa lake system from Calatayud Basin have been studied by XRD and TEM chemical microanalyses. Clay particle microanalyses show complete continuity between dioctahedral aluminous and trioctahedral magnesian smectites. Aluminous smectites have been classified as montmorillonites and ferribeidellites, both with compositional variability. They represent detrital phases resulting from weathering in the source area. Compositionally-intermediate smectites (beidellite-saponite and montmorillonite-stevensite) seem to correspond to weighted mean compositions of di- and trioctahedral phases stacked together in the same particle. They represent intermediate transitional stages from detrital to authigenic smectites in more distal basin facies. Trioctahedral smectites, although called saponites and stevensites, are actually random mixed-layer kerolite-Mg-smectite with a high percentage of expandable layers. They represent authigenic clays formed from solutions with high pH and Mg content as a result of evaporative concentration. The variability in composition of smectites, both within the same sample and amongst different samples, is a possible consequence of the heterogeneity in the local chemical environment, and so, equilibrium may be only reached locally in a large variety of microsystems.

Assessment of technological properties of calcareous and non-calcareous clays used for the brick-making industry of Zaragoza (Spain)

The granulometric, mineralogical, and geochemical features of two different types of clayey raw materials (carbonated clays, CC; and non-carbonated clays, NCC) used for the brickmaking industry near Zaragoza (Spain) have been studied. Their particle-size distribution suggests that they may be classified as silty clays and are suitable for use as thin-walled hollow bricks. For technological testing, prismatic bars obtained by extrusion under vacuum conditions from raw materials were fired from 800 to 1050 °C. Technological determinations, such as linear shrinkage, bulk density, hydric tests, and an accelerated decay test, indicate that both CC and NCC are suitable to be used as bricks for building, although bricks made of NCC would be of better quality. In addition, pore-size distribution suggests that the durability of the highest temperature fired products will be higher because of an increase both in the dominant pore access radius and in the percentage of larger pore sizes.

Mineralogical and trace element composition of clay-sized fractions from Albian siliciclastic rocks (Oliete Basin, NE Spain)

Abstract Mineralogical and geochemical techniques have been used to determine the role of minerals in controlling the trace element composition of a set of clay-sized fractions from Albian siliciclastic rocks in NE Spain. These clay-sized fractions are composed of kaolinite and illite, minor quantities of quartz, and accessory heavy minerals. Kaolinite has a smaller crystal size than illite, accounting for its relative concentration in these fractions. The mm-sized heavy minerals are rutile, Fe-Ti- and Fe-oxides, zircon, pyrite, cassiterite, monazite and xenotime. Geochemical data indicate that most of the trace elements are relatively concentrated in clay-sized fractions, except for Zr, Hf, Y and HREE. Statistical treatment shows three different associations in the clay-sized fractions: (1) Rb, Cs and Ba with clay phyllosilicates, especially illite; (2) REE with Th, Y and P phases; and (3) Sc, Cr and V with Ti- and Nb oxides. Therefore, these data do not support the dominant REE control by clay mineralogy that other authors have reported.

Microtexture and genesis of clay minerals from a turbiditic sequence in a Southern Pyrenees foreland basin (Jaca basin, Eocene)

Abstract A set of fine-grained samples from a turbiditic sequence in a Southern Pyrenees foreland basin (Jaca Basin, Eocene) were studied to determine the influence of tectonics (Pyrenean Orogeny) on phyllosilicate recrystallization and infer the grade and basin maturity. The samples from four different outcrops were examined by X-ray diffraction (XRD) and by scanning (SEM) and transmission electron microscopy (TEM) with special emphasis on clay-mineral characterization (e.g. illitic phases). The analysed samples have simple mineral assemblages and consist of detrital quartz, albite and calcite, scarce clay matrix (mainly illite with chlorite), and calcite and dolomite cement. The lack of other phyllosilicates such as mixed-layer illite-smectite (I-S), pyrophyllite, Na-micas, or kaolin minerals is quite remarkable. On the SEM scale, samples (with marl composition) have poorly sorted textures and high detrital contents. In many cases they show bedding and/or cleavage, and in some cases neither is observed. Most of the clay-sized illites show very similar crystallinity and b0 values (determined by XRD) and distributions of crystallite thickness (measured by TEM) in all the outcrops, which is typical of late-diagenesis illites forming under low-pressure conditions. These illites are parallel (or subparallel) to bedding or randomly orientated. They are also characterized by disordered polytypes and low K contents. In some TEM images, a second type of illite has been observed. This secondary illite occurs parallel to cleavage, with thicker crystals (25-35 layers), K contents in the interlayer, and a 2M1 polytype. The pole figure analysis shows that most of the clays have (00l) planes parallel (or subparallel) to bedding although there are abundant clays with random orientation. There is no trend in the clay orientation/disorientation from the south to the north of the basin. All the data indicate that the strain rate associated with the Pyrenean Orogeny has not been recorded in the turbidite sequence controlling the relative orientation of clays, although anchizonal clay crystallization is favoured as a minor process.

Clay mineral assemblages as palaeoclimatic indicators in a shallowing carbonate lacustrine system: Oligocene-Miocene, central Ebro Basin (NE Spain)

Abstract This paper focuses on the clay mineralogy (using XRD, SEM and TEM methods) of the lacustrine ‘‘Calizas de Torrente de Cinca’’ unit that represents the Oligocene-Miocene transition in the central part of the Ebro Basin (NE Spain).Phyllosilicates are mainly detrital although Mgsmectites could have been generated in the lake.Although a temperate, relatively humid climate dominated the source area during the Oligocene-Miocene transition (Chattian-Aquitanian), as deduced by detrital phyllosilicates assemblage, mineralogical vertical trends along with sedimentological studies indicate some changes.Relatively warmer and more humid conditions during the late Chattian, that favoured increasing chemical weathering, were replaced during the early Aquitanian by drier conditions coinciding with the Mi-1 glaciation effects; this change is coeval with a transition from deeper to shallower lacustrine facies. Phyllosilicate association analysis has also permitted an improvement in the palaeogeographical sketch and infers that the Pyrenees are the main source area for the lacustrine system.