The geochemical evolution of the Catalan potash subbasin, South Pyrenean foreland basin (Spain)

Over the last decade, our studies in ancient evaporitic basins have been based on a detailed study of a single borehole record. The detailed findings in medium- to large-sized evaporitic basins were shadowed with a relevant question: can interpretations from a representative evaporitic record in a single borehole be extended to the whole evaporitic basin? This paper addresses that question; the results obtained are compared with results from another distant point within the basin. The general methodology not only proves its reliability in interpreting the evolution of evaporitic basins from a single borehole but reveals its capability to obtain detailed palaeoenvironmental interpretations.

The chemical evolution of an Upper Eocene evaporitic sequence from the South Pyrenean foreland basin (Spain) has been investigated along the Súria-19 borehole record. Detailed petrographic and mineralogical study, X-ray microanalysis of frozen primary inclusions trapped in halite (Cryo-SEM-EDS), systematic isotopic analysis (δ³⁴S and δ¹⁸O in sulphates) and computer-based evaporation models have been integrated in a multi-proxy methodology. This study revealed that a variable amount of Ca excess is required throughout different parts of the marine Lower Halite Unit (LHU) for sylvite, instead of K–Mg sulphates, to form. This Ca excess is in turn different from that required for the western sector of the same evaporitic basin (Navarrese subbasin). Quick and variable changes in Ca-rich brines or equivalent dolomitization required are explained as internal processes within the basin rather than secular variations in seawater chemistry.

The general hydrological evolution of the Catalan subbasin is explained as a restricted subbasin with a first marine stage in which continental input (up to 50% of total input) had an important control on the geochemistry of the subbasin. A second stage was determined during potash precipitation, in which the subbasin was cut from any seawater input to end up in its last stage as a purely continental evaporitic basin. Coupling evaporation models and analytical results we have obtained the proportions of recycling and their sources, estimated to change from a 100% (total mass of sulphate) Eocene source to 20% Eocene and 80% Triassic (Keuper) towards the latest stage of potash precipitation. The results obtained have been compared with results from the Navarrese subbasin allowing an integrated interpretation of the hydrological evolution of the whole Upper Eocene South Pyrenean basin. Local geochemical variations within the Upper Eocene south Pyrenean basin are explained by the differences in paleogeographical setting of the Navarrese and Catalan subbasins.     

Neogene evaporites in desert volcanic environments: Atacama Desert, northern Chile

The Upper Miocene and Pliocene evaporite deposits of the Atacama Desert of northern Chile (Hilaricos and Soledad Formations) are among the few non‐marine evaporites in which aridity not only formed the deposits, but has also preserved them almost unaltered under near‐surface conditions. These deposits are largely composed of displacive Ca sulphate and halite together with minor amounts of glauberite, thenardite and polyhalite. However, at the base and top of these deposits, there are also beds of gypsum crystal pseudomorphs that originally formed as free‐growth forms within shallow brine bodies, rather than as displacive sediments. The halite is present as interstitial cement, displacive cubes and shallow‐water, bottom‐growth chevron crusts. Most of the calcium sulphate is presently anhydrite, pseudomorphous after gypsum, that was the primary depositional sulphate mineral. The secondary anhydrite formed under early diagenetic conditions after slight burial (some metres) resulting from the effect of strongly evolved pore brines. The anhydrite has been preserved without rehydration during late diagenetic and exhumation stages on account of the arid environment of the Atacama Desert. Both the Hilaricos and the Soledad Formations contain geochemical markers indicating that these Neogene evaporites had a largely non‐marine origin. Bromine content in the halite is very low (few p.p.m.), indicating neither a sedimentological relation with sea water nor the likelihood of direct recycling of prior marine halites. Moreover, the δ³⁴S of sulphates (+4·5‰ to +9‰) also reflects a non‐marine origin, with a strong volcanic influence, although some recycling of Mesozoic marine sulphates cannot be ruled out. δ³⁴S of dissolved sulphate from hot springs and streams in the area commonly displays positive values (+2‰ to +10‰). Leaching of oxidized sulphur and chlorine compounds from volcanoes and epithermal ore bodies, very common in the associated drainage areas, have been the main contribution to the accumulation of evaporites. The sedimentary and diagenetic evolution of the Hilaricos and Soledad evaporites (based on lithofacies analysis) provides information about the palaeohydrological conditions in the Central Depression of northern Chile during the Neogene. In addition, the diagenesis and exhumation history of these evaporites confirms the persistence of strongly arid conditions from Late Miocene until the present. A final phase of tectonism took place permitting the internal drainage to change and open to the sea, resulting in dissolution and removal of a significant portion of these deposits. Despite the extensive dissolution, the remaining evaporites have undergone little late exhumational hydration.     

Conical folds and apparent rotations in paleomagnetism (a case study in the Southern Pyrenees)

The so-called apparent rotation was defined as the angular deviation between a local paleomagnetic direction (after the standard bedding correction) and their corresponding paleomagnetic reference [J. Geophys. Res. 85 (1980) 3659]. In this paper, we make a theoretical exploration on this concept and we conclude that (depending on the number, sequence, orientation and magnitude of the deformation axes that have affected to the rock volume) the apparent rotation may be the addition of a vertical-axis rotation plus a spurious rotation. The later is an error whose origin is the inappropriate application of the bedding correction during the restoration (which does not fit the reverse sequence of deformations). Then, Apparent rot. (s.l.) (δ)=Spurious rot. (θ)+Vertical-axis rot. (β).Conical folds are complex geometries that cannot be restored by using the bedding correction. However, appearance of apparent and spurious rotations has not been studied even though the presence of this kind of folds is very common in fold and thrust belts. In this paper, we show a way to restore these structures and its associated paleomagnetic data by means of forward modelling on a stereographic projection. The modelling has to be based on a good characterization of the geometry (fold axis orientation) and understanding of the kinematics of the fold. General modelling has also allowed us to predict the apparent rotation in conical synclines. Its magnitude depends on the semiapical angle and on the degree of development of the fold; the sense of the rotation (clockwise or counter-clockwise) will depend on the sense of rotation the fold axis.The western External Sierras provide an excellent case study of apparent rotations due to the presence of a conical fold in the footwall (Ebro foreland basin) of the South Pyrenean sole thrust. In addition, a vertical-axis clockwise rotation up to 47° (32° in average) has been detected in the hagingwall. An apparent rotation up to 28° (20° in average) is observed in the footwall of the structure when a simple bedding correction is used. This deviation does not fit with the expected Ebro basin direction (reference) and is caused by the effect of the Riglos conical syncline, developed by the flexure of the foot wall ramp of the South Pyrenean sole thrust. The forward modelling carried out considering the geometry and kinematics (non significant rotations in the autochthonous foot wall) of this structure predicts very well the paleomagnetic observations in the field (geographic coordinates) with angular departures of only 5° (in average). The only application of the bedding correction would introduce errors (spurious rotations up to 21°, 12 in average) related to the conical geometry that would not allow the differentiation of these distinct structural units.     

Magnetometry and ground-penetrating radar surveys applied to tracing potential collectors of mining-derived pollutants in coastal sediments (Piscinas Bay,Montevecchio mining area,SW Sardinia)

Dispersion of pollutants from mining areas can result in risks to human health. The dynamic interaction of geological processes can generate complex situations that favor enrichment in toxic elements by sedimentary and diagenetic mechanisms. In order to explore the distribution of iron-rich minerals in coastal and river sediments, a geophysical campaign was performed along the outlet of the Piscinas and Naracauli Rivers that drain two abandoned mining areas in SW Sardinia. Both rivers flow into the Mediterranean Sea, a bay where fluvial, marine, and eolian processes interact. The geophysical campaign comprised magnetometry and ground-penetrating radar surveys. Magnetic properties were controlled by means of magnetic susceptibility measurements along the beach and dune areas, through sampling of surface sediments, natural outcrops and trenches, and considering different grain size intervals. Results indicate enrichment in ferromagnetic minerals at the leeside of dunes and berms. Sedimentological interpretation is supported by means of ground-penetrating radar. Due to both sedimentary and diagenetic processes, the 100- to 300-μm fraction of fine sands exhibits high susceptibility. Implied tenors of toxic elements in the magnetite may represent a serious hazard to environmental and health security. The results inferred from magnetic anomalies support the applicability of the geophysical approach in order to locate high concentrations of iron-rich particles both at surface and below ground and the usefulness of joint evaluation of magnetic susceptibility and ground-penetrating radar in order to characterize the sedimentary and geomorphology-controlled magnetite distribution.     

Thrust Ramp Geometry and Spurious Rotations of Paleomagnetic Vectors

Interaction of deformation axes during pure translation of a hanging-wall over a footwall composed by frontal and oblique ramps is carefully evaluated together with the evolution of associated paleomagnetic vectors. Four different cases are distinguished on the basis of the deflection on the paleomagnetic vectors when the bedding correction is applied during the restoration process. Two cases (frontal and oblique ramp without mutual interaction) do not produce any deflection. But two cases in the transition zone between both ramps will undergo non-coaxial axis of tilting during progressive deformation. One of them will produce spurious rotation if the bedding correction is applied. These errors will affect the oroclinal bending diagram as well as the fold test producing an apparent oroclinality and an apparent syn-folding magnetization respectively. A well-known geometry and kinematics of the thrust system is needed to properly restore the beds (and vectors) and to avoid the spurious rotations and its collateral effects in paleomagnetic investigations. A paleomagnetic study in the Pyrenean External Sierras is shown as an example. Primary Eocene vectors underwent a clockwise rotation (40° about) during the emplacement of the South Pyrenean sole thrust, however the Rasal-Gabardiella system of oblique ramps display spurious rotations ranging from –8° up to 13° if the inappropriate bedding correction is performed.