Carles Roqué

Investigating gravitational grabens related to lateral spreading and evaporite dissolution subsidence by means of detailed mapping, trenching, and electrical resistivity tomography (Spanish Pyrenees)

The active lateral spread of the Peracalc Range (Spanish Pyrenees) has developed on a Cretaceous limestone sequence around 250 m thick, underlain by tectonically thickened (∼2.5 km) Triassic halite-bearing evaporites and clays. Outward expansion of the Triassic sequence by ductile deformation and probably halokinesis toward the debuttressed and unloaded front of the range has been accommodated in the overlying cap rock through the development of a striking horst and graben morphostructure. Fault scarps show anomalously high height to length ratios (aspect ratio; H max / L ) compared to the values reported for tectonic faults. This retrogressive gravitational deformation has aborted a paleodrainage, expressed as wind gaps, hanging valleys, and defeated streams. The significant vertical displacement component in this rock spread is attributed to subsidence caused by interstratal evaporite dissolution, as supported by the dissolution-induced collapse and graben structures mapped at the foot of the range. To our knowledge, the rock spread of Peracalc, covering around 4.5 km 2 and with a minimum volume of 0.9 km 3 , is the largest documented landslide of the Pyrenees. The excavation of trenches and the acquisition of electrical resistivity tomography profiles provided information on the thickness and subsurface structure of the graben fills, the age of the lateral spread (older than 45 ka), an unexpected episodic kinematic behavior of the gravitational faults, and the timing of deformation events, including slumping of lake deposits.

Origin and evolution of tufa mounds related to artesian karstic springs in Isona area (Pyrenees, NE Spain)

This work analyses a suite of relict tufa mounds generated by artesian karstic springs in Isona area (Spanish Pyrenees). Geological and geophysical data (seismics and vertical electrical resistivity soundings) indicate that the location of the discharge area in which the spring mounds formed was controlled by (1) a bulge in the axial zone of the anticline that affects the Areny-Montsec aquifer, with the consequent thinning of the overlying confining unit and (2) N-S and E-W trending extensional faults. These uncommon meteogene mounds occur in two stepped morphostratigraphic units that constitute the caprock of a mesa 9 km2 in area. The upper tufa complex is 47 m thick and has yielded several U/Th dates of >350 ka. The obtained U/Th ages for the mounds of lower tufa complex, 10 m thick, range from >350 to 214 ± 11 ka. The sedimentological analysis of the scarce exposures and electrical resistivity profiles show the same morphological and depositional components as those described in geothermal springs: (1) Cylindrical vents; (2) Pools fed by the vents and dammed by annular tufa barriers; (3) Tufa barriers (rimstones) constructed by overflowing waters through vertical accretion and progradation. These rimstones may have overhanging upflow sides. (4) Slope tufa facies with terracettes and microgours.

The impact of droughts and climate change on sinkhole occurrence. A case study from the evaporite karst of the Fluvia Valley, NE Spain

This work introduces the concept that sinkhole frequency in some karst settings increases during drought periods. This conception is tested in a sector of the Fluvia River valley in NE Spain, where subsidence phenomena is related to the karstification of folded Eocene evaporite formations. In the discharge areas, the evaporites behave as confined aquifers affected by hypogene karstification caused by aggressive artesian flows coming form an underlying carbonate aquifer. A sinkhole inventory with chronological data has been constructed, revealing temporal clusters. Those clusters show a good correlation with drought periods, as revealed by precipitation, river discharge and piezometric data. This temporal association is particularly obvious for the last and current drought starting in 1998, which is the most intense of the record period (1940-present). Climatic projections based on recent studies foresee an intensification of the droughts in this sector of NE Spain, which could be accompanied by the enhancement of the sinkhole hazard and the associated risks.

The application of GPR and ERI in combination with exposure logging and retrodeformation analysis to characterize sinkholes and reconstruct their impact on fluvial sedimentation.

This work illustrates the practicality of investigating sinkholes integrating data gathered by ground penetrating radar (GPR), electrical resistivity imaging (ERI) and trenching or direct logging of the subsidence-affected sediments in combination with retrodeformation analysis. This mutidisciplinary approach has been tested in a large paleosinkhole developed during the deposition of a Quaternary terrace on salt-bearing evaporites. The subsidence structure, exposed in an artificial excavation, is located next to Puilatos, a village that was abandoned in the 1970s due to severe subsidence damage. Detailed logging of the exposure revealed that the subsidence structure corresponds to an asymmetric sagging and collapse paleosinkhole with no clear evidence of recent activity. The sedimentological and structural relationships together with the retrodeformation analysis indicate that synsedimentary subsidence controlled channel location, the development of a palustrine environment and local changes in the channel pattern. GPR profiles were acquired using an array of systems with different antenna frequencies, including some recently developed shielded antennas with improved vertical resolution and penetration depth. Although radargrams imaged the faulted sagging structure and provided valuable data on fault throw, they did not satisfactorily image the complex architecture of the fluvial deposit. ERI showed lower resolution but higher penetration depth when compared to GPR, roughly capturing the subsidence structure and yielding information on the thickness of the high-resistivity alluvium and the nature of the underlying low-resistivity karstic residue developed on top of the halite-bearing evaporitic bedrock. Data comparison allows the assessment of the advantages and limitations of these complementary techniques, highly useful for site-specific sinkhole risk management. This article is protected by copyright. All rights reserved.

Granite caves in the north-east of the Iberian Peninsula: Artificial hypogea versus tafoni

Summary. Granite caves in the north-east of the Iberian Peninsula: Artificial hypogea versus tafoni. The origin of eight supposedly artificial caves in granite rocks located in the northeast of the Iberian Peninsula is revised in this work. Although human features are present in four of the caves, they have little morphological significance, and therefore their attribution is re-evaluated on the basis of geomorphological analysis and tafoni genesis theories. In fact, these cavities are tafoni developed in bornhardts, castle koppjes, tors and corestones.

The Olot Volcanic Field

The Olot Volcanic Field (OVF) records the youngest eruptions (0.5–0.01 Ma) of the Catalan Volcanic Zone (CVZ), associated with the post-Alpine European intraplate rift. Magma reached the surface through a fault system cross-cutting the Catalan Transversal Range, producing about 50 monogenetic volcanoes built up by strombolian and/or phreatomagmatic activity. The volcanic cones have maximum and average basal widths of 1,290 and 500 m, respectively. Maximum and mean heights are 189–100 m, respectively. Given the age of the eruptions, the primary volcanic landforms are well preserved and geomorphic features related to erosional processes are rare. Lava flows emerging from most of the volcanoes flowed along river valleys, reaching up to 10 km in length and locally generating volcanic dams.

Characterization of radon levels in soil and groundwater in the North Maladeta Fault area (Central Pyrenees) and their effects on indoor radon concentration in a thermal spa

Radon levels in the soil and groundwater in the North Maladeta Fault area (located in the Aran Valley sector, Central Pyrenees) are analysed from both geological and radiation protection perspectives. This area is characterized by the presence of two important normal faults: the North Maladeta fault (NMF) and the Tredós Fault (TF). Two primary aspects make this study interesting: (i) the NMF shows geomorphic evidence of neotectonic activity and (ii) the presence of a thermal spa, Banhs de Tredós, which exploits one of the several natural springs of the area and needs to be evaluated for radiation dosing from radon according to the European regulation on basic safety standards for protection against ionizing radiation. The average soil radon and thoron concentrations along a profile perpendicular to the two normal faults - 22 ± 3 kBq·m-3 and 34 ± 3 kBq·m-3, respectively - are not high and can be compared to the radionuclide content of the granitic rocks of the area, 25 ± 4 Bq·kg-1 for 226Ra and 38 ± 2 Bq·kg-1 for 224Ra. However, the hypothesis that the normal faults are still active is supported by the presence of anomalies in both the soil radon and thoron levels that are unlikely to be of local origin together with the presence of similar anomalies in CO2 fluxes and the fact that the highest groundwater radon values are located close to the normal faults. Additionally, groundwater 222Rn data have complemented the hydrochemistry data, enabling researchers to better distinguish between water pathways in the granitic and non-granitic aquifers. Indoor radon levels in the spa vary within a wide range, [7-1664] Bq·m-3 because the groundwater used in the treatment rooms is the primary source of radon in the air. Tap water radon levels inside the spa present an average value of 50 ± 8 kBq·m-3, which does not exceed the level stipulated by the Spanish Nuclear Safety Council (CSN) of 100 kBq·m-3 for water used for human consumption. This finding implies that even relatively low radon concentration values in water can constitute a relevant indoor radon source when the transfer from water to indoor air is efficient. The estimated effective dose range of values for a spa worker due to radon inhalation is [1-9] mSv·y-1. The use of annual averaged radon concentration values may significantly underestimate the dose in these situations; therefore, a detailed dynamic study must be performed by considering the time that the workers spend in the spa.