F. Javier Gracia

Paleosubsidence and active subsidence due to evaporite dissolution in Spain

Evaporite formations crop out or are at shallow depth present in an extensive area of Spain. These soluble sediments occur in diverse geological domains and were deposited over a long time span, from the Triassic up to the present day. Broadly, the Mesozoic and Paleogene formations (Alpine cycle) are affected by compressional structures, wheras the Neogene (post-orogenic) sediments remain underformed. Subsidence caused by subsurface dissolution of evaporites (subjacent karst) takes place in three main types of stratigraphic settings: a) subsidence affecting evaporite-bearing Mesozoic and Tertiary successions (interstratal karst); b) subsidence in Quaternary alluvial deposits related to the exorheic evolution of present-day fluvial systems (alluvial or mantled karst); and c) subsidence in exposed evaporites (uncovered karst). These types may be represented by paleosubsidence phenomena (synsedimentary and/or postsedimentary) recognizable in the stratigraphic record, or by equivalent, currently active or modern examples which have a surface expression. Interstratal karstification of Mesozoic marine evaporites, and the consequent subsidence of overlying strata, is revealed by stratiform collapse breccias and wedge outs of the evaporites grading into unsoluble residues. In several Tertiary basins, the sediments overlying evaporites locally show synsedimentary and/or postsedimentary subsidence structures. Dissolution-induced subsidence coeval with sedimentation is accompanied by local thicknening of strata in basin-like structures with convergent dips and cumulative wedge-out systems. This sinking process controls the generation of depositional environments and lithofacies distribution. Postsedimentary subsidence produces a great variety of gravitational deformations in Tertiary supra-evaporitic units, including both ductile and brittle structures (flexures, synforms, fractures, collapse, and brecciation). Quaternary fluvial terrace deposits overlying evaporites show anomalous thickenings (>150m) caused by a dissolution-induced subsidence process in the alluvial plain, which is balanced by alluvial aggradation. The complex evolution (in time and space) of paleosubsidence leads to intricate and chaotic structures in the alluvium, which may be erroneously interpreted as pure tectonic deformations. The current subsidence and generation of sinkholes due to suballuvial karstification constitutes a geohazard which affects large, densely populated areas, and thus endangers human safety and poses limitations on development. An outstanding example can be seen in Calatayud, an important historical city where subsidence has severely damaged highly valuable monuments. Subsidence resulting from the underground karstification of evaporites has caused or influenced the generation of some important modern lacustrine basins, such as Gallocanta, Fuente de Piedra, and Banyoles Lakes. The sudden formation of sinkholes due to collapse of cave roofs is fairly frequent in some evaporite outcrops. Very harmful and spectacular subsidence activity is currently occurring in the Cardona salt diapir, where subsidence has been dramatically exacerbated by mining practices.

Morphological and evolutionary classification of sandy beaches in Cadiz coast (SW Spain)

ABSTRACT Del Río, L., Gracia, F.J. and Benavente, J., 2013. Morphological and evolutionary classification of sandy beaches in Cadiz coast (SW Spain). Sandy beaches are extremely dynamic systems, so gaining insight about decadal patterns of beach change is essential for the adequate management of risks affecting coastal zones. This work aims at improving understanding of the factors that control erosion-accretion processes and evolution of sandy beaches at the intermediate time scale. For this purpose, recent evolution of beaches along the 150 km long Atlantic coast of Cadiz (SW Spain) is investigated in relation to their morphology and dynamics. Dune toe and high water line changes are assessed based on georectified aerial photographs from 1956–2008, using GIS tools. Results show considerable spatial and temporal variability of recent shoreline changes along the study area, with mostly eroding trends along the northern sector and a predominantly stable southern sector. Important exceptions at certain points are related to the heterogeneity of the coast and the diverse natural and anthropogenic factors contributing to shoreline change in the area. A classification of the studied beaches is proposed, based on beach morphology and dynamics, thus helping to understand the way coastal morphological characteristics influence erosion-accretion trends. Rectilinear beaches are predominantly stable or accreting, whereas reef-supported beaches are mostly erosive. Z-bays generally experience erosion at one end and accretion at the opposite end, greatly influenced by local conditions. Enclosed beaches are stable where sediment budget remains unchanged, but rapidly erode or accrete if human interventions alter the balance. The classification allows identifying those beaches which are most sensitive to variations in controlling factors, such as sediment supply.

Gallocanta Saline Lake, Iberian Chain

Gallocanta Lake, covering 14.5 km 2 , is the greatest ephemeral saline lake in Europe. It is located in the Iberian Chain, NE Spain, in the bottom of a karst polje. The Gallocanta saline lake formed once Jurassic limestones were almost completely corroded, and the floor of the depression was underlain by Triassic clays and evaporites. The late Quaternary evolution of the lake can be reconstructed from the deposits underlying the lake bottom and from different levels of lacustrine terraces located in its downwind side. Different phases of flooding and desiccation can be deduced from both sources of data. The current dynamics of the lake is controlled by water-level fluctuations and wind action. Wind-driven waves and longshore currents transport sediments to the downwind zone and generate barrier islands, spits and submerged bars, with a dynamic behaviour very similar to that of marine coastal environments. Lake segmentation due to cuspate foreland growth has divided the original lake into minor ones. Segmentation is still active at present and tends to isolate a minor lacustrine body. Progressively decreasing rainfall, together with sediment supply to the lake, enhanced by extensive agricultural practices in the basin, have frequently led to lake desiccation over the last decades. Extensive polygonal soils and salt crusts cover the bottom during drying-up periods.

Coastal landforms and environments in the central sector of Gallocanta saline lake (Iberian Range, Spain)

Gallocanta Lake (NE Spain), with a high ecological value, is the largest and best preserved saline lake in Western Europe. The aim of this study is to map the landforms developed in the margins of the central sector of Gallocanta Lake, at adequate scale for the study of soils and habitats, which is needed for the delineation, management and protection of the wetland. Photointerpretation was combined with topographical, geological, and satellite data in a geographical information system. This study, applied in two selected areas of the central body of the lake, allowed the identification of contrasting landforms and processes in the lake margins. The southern margin, which receives most of the fluvial materials in the zone exhibits coastal progradation by means of barrier-island generation and sedimentation in the resulting coastal lagoon, a process that can be inferred from the map presented in this work and was confirmed by comparison of aerial photographs taken in different decades. With virtually no sediment supply on the margin, coastal erosion and shoreline retreat prevail. In summary, coastal landforms are a result of a complex combination of fluvial and lacustrine processes operating during high and low water-level periods.

Dunes in the Gibraltar Strait Realm

This chapter examines the origin, evolution and present state of the coastal dunes along the South Atlantic Spanish coast around the Strait of Gibraltar. Wind regime in the zone is strongly affected by the North Atlantic Oscillation (NAO). Low or negative NAO index values give rise to the prevalence of westerly humid winds, while during periods with a positive NAO index easterly, dry winds prevail. Several Holocene and historical aeolian phases have been identified in the zone, represented by different aeolian deposits, many of them containing archaeological remains. The oldest one dates to shortly after the mid-Holocene eustatic maximum. During historical times dune-building episodes were associated with a higher frequency and persistence of easterly winds. During the 20th century, and especially in the last few decades, the prevalence of positive values of the NAO index has favoured the growth and advance of mobile dunes in the vicinity of the Strait of Gibraltar. In contrast, dune ridges associated with westerly winds are much less mobile or stable and form shore-parallel ridges covered by vegetation. In the Atlantic side of the Strait, environmental health of coastal dunes is satisfactory, especially those included in the Strait of Gibraltar Natural Park, where more effective protection measures are applied. The Mediterranean side is characterized by a dense population and intense human interventions, leading to erosion problems. Climate change predictions for the next decades suggest an increase in the frequency and intensity of easterly-Levante winds in the Strait. This situation would favour the reinforcement and growth of the Levante-derived dunes around the Strait, and perhaps recovery of the small dunes existing in the Mediterranean side.

Beaches of Cadiz

The coast of Cadiz province (SW Spain) constitutes a complex system with a wide variety of hydrodynamic and geological-geomorphological conditions. Morphodynamic behaviour of beaches in this area shows more dissipative states in the Atlantic coast and more reflective states in the Mediterranean coast. Recent shoreline trends indicate a relatively stable behaviour along most part of Cadiz coast over the last decades, although there are numerous sectors with a clearly erosive trend, mainly in the Northern half of the province. The great spatial and temporal variability of shoreline changes observed is related to the heterogeneity of the coast, as well as to the diversity of factors contributing to erosion processes on each sector. Several coastal hazard hotspots appear along the study zone, mostly related to extreme meteorological events and short-term trends, namely coastal erosion and flooding. The frequency and severity of the risks derived from these hazards have increased over the last decades, mainly due to the increased anthropogenic pressure and urban development on the coast. The coast of Cadiz province (SW Spain) constitutes a complex system with a wide variety of hydrodynamic and geological-geomorphological conditions. Morphodynamic behaviour of beaches in this area shows more dissipative states in the Atlantic coast and more reflective states in the Mediterranean coast. Recent shoreline trends indicate a relatively stable behaviour along most part of Cadiz coast over the last decades, although there are numerous sectors with a clearly erosive trend, mainly in the Northern half of the province. The great spatial and temporal variability of shoreline changes observed is related to the heterogeneity of the coast, as well as to the diversity of factors contributing to erosion processes on each sector. Several coastal hazard hotspots appear along the study zone, mostly related to extreme meteorological events and short-term trends, namely coastal erosion and flooding. The frequency and severity of the risks derived from these hazards have increased over the last decades, mainly due to the increased anthropogenic pressure and urban development on the coast.

Multiple-Source Cliff Erosion in Southern Spain: Current Risk and Future Perspectives

ABSTRACT Del Río, L.; Gracia, F.J., and Benavente, J., 2016. Multiple-source cliff erosion in Southern Spain: current risk and future perspectives. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1072 - 1076. Coconut Creek (Florida), ISSN 0749-0208. This work analyses coastal erosion causes, impacts and management strategies in two sandstone cliffs in SW Spain: El Almirante (EA) and Fuente del Gallo (FG). Five sets of aerial photographs spanning the last 50 years were used, combined with detailed field inspection. Retreat rate of the cliff top in EA is 0.8–1.6 m/yr, increasing over the last decade to 4 m/yr at some points. This poses a serious risk for infrastructure located on the cliff top, and cliff recession leaves an outstanding palaeontological site exposed to wave attack. Erosion rates in FG are lower, but there is a significant risk to beach visitors due to mass movements involved in cliff retreat. Interventions to mitigate erosion impacts in EA have consisted in tetrapod deployment and concrete fencing, while in FG stabilization measures include removal of unstable blocks, soil nailing, draining tubes, seeding, a riprap and beach nourishments. However, these measures have only had a limited success, as the main causes for cliff erosion are related to marine and subaerial processes enhanced by rainfall and cliff fractures. Undermining of the cliff base by waves in FG and in eastern EA triggers rock falls, which are easily dismantled by waves, resulting in a rapidly retreating cliff face. In central EA rotational slides are responsible for cliff top recession, favoured by a plastic substratum of marls. Future projections of shoreline position indicate a likely increase in potential damage generated by erosion in both sites, leading to the need for adopting alternative mitigation measures.