Aránzazu Luzón

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.

QUATERNARY ALLUVIAL SINKHOLES: RECORD OF ENVIRONMENTAL CONDITIONS OF KARST DEVELOPMENT, EXAMPLES FROM THE EBRO BASIN, SPAIN

The central Ebro Basin is an exceptional region for studying karstification through time and under different environmental conditions, as sinkholes have been developing since the Early Pleistocene. Knowledge of active sinkholes is complemented with research on paleosinkholes and contemporary deposits. Sedimentological, min- eralogical, geomorphological and structural approaches permit interpretation of the natural environmental conditions that favored karst in the past and the main genetic mechanisms involved. The sedimentary features of Pleistocene terraces indicate that they were deposited by a gravel braided fluvial system characterized by higher water and sediment availability than today, probably related to meltwater flows coming from glaciated source areas, mainly in the Pyrenees. Genesis of paleosinkholes was mainly linked to this high water supply. Some of them acted as small lakes where fine sediments are exceptionally well conserved to give clues about environmental conditions. The neo- formation of palygorskite and sepiolite suggests arid to semiarid climatic conditions, in agreement with the idea of cold glacial episodes. During Pleistocene times, development of sinkholes was influenced by tectonics. Currently, the genesis and evolution of numerous sinkholes are also influenced by water supplies from human activities such as irrigation or urbanization, sharply changing the nearly steady state exhibited in the past.

Interacting tectonic faulting, karst subsidence, diapirism and continental sedimentation in Pleistocene deposits of the central Ebro Basin (Spain)

During Early, as proposed by the International commission on stratigraphy Pleistocene times, interacting fluvial and aeolian processes constructed wide alluvial plains over an evaporite-dominated Miocene substratum in the central Ebro Basin. An exceptional site where these deposits show faults, folds, diapirs, karst structures and unconformities has been studied in detail. Analysis of particular structures demonstrates the interaction by that time of tectonic faulting, diapirism, karstification and sedimentation in an area where deformation was traditionally linked to the presence of underlying evaporites, without proposing any precise mechanism. Multiple approaches (sedimentology, structural geology and geophysics) have been used in order to discriminate the origin of each type of structure as well as to understand the interaction between different processes. Numerous normal faults and fractures of variable size are consistent with the regional stress field. Pleistocene deposits are pierced by diapirs of Miocene evaporites and disrupted by karst structures with different geometries (tubular, funnel and vault), both partially controlled by tectonics. The example described is proposed as an analogue model that could successfully illustrate evolution patterns of basins of potential interest for petroleum geology where similar processes have actuated, resulting in complex stratigraphical architectures.

Correlation of sedimentary units from grain-size and mineralogic analyses as a tool for constraining trench interpretations in palaeoseismology

In palaeoseismological trench studies, precise correlation of sedimentary units between fault blocks has an unquestionable value for accurately inferring the amounts of coseismic displacement and hence for assessing seismic hazard. A methodology based on laser analysis of particle size and mineralogic composition by X-ray diffraction is proposed in order to strengthen the correlation of sedimentary units in a trench excavated across the Concud Fault (central Iberian Chain, Spain). The surveyed sediments show sharp and multiple lateral facies changes, as well as inconsistent optically stimulated luminescence (OSL) ages. The results reinforce the correlation based on field inspection of lithologic and sedimentologic features. Moreover, they allow interpretation of rejuvenation of OSL ages of samples in the upthrown fault block, which has been attributed to partial erosion of sedimentary units, as evidenced by their smaller thickness and erosive boundaries. The correlated units are then used to estimate coseismic displacements for three palaeoseismic events.

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.