Manuel Pozo

Chapter 6 – Palygorskite and Sepiolite Deposits in Continental Environments. Description, Genetic Patterns and Sedimentary Settings

Abstract The most important sepiolite and palygorskite deposits and occurrences were formed in continental environments by authigenesis, through direct precipitation from solution (neoformation), or by transformation of precursor minerals, mainly detrital clays. In the sedimentary environment, transformation is mostly related to diagenesis, whereas neoformation can take place in both syngenetic (depositional) and diagenetic environments. Sepiolite and palygorskite and other Mg-clay (saponite, stevensite and kerolite) are the result of the interaction between silica- and magnesium-bearing solutions in an environment with suitable physico-chemical conditions (mostly pH and salinity), and on the activity of aluminium in solution or reactive phases in the case of palygorskite. The physico-chemical conditions of the environment control the formation of the magnesian clay mineral, also determining its fibrous or laminar nature and, consequently, its composition. These conditions occur in specific continental sedimentary environments. This contribution suggests a model for the pathways leading to sepiolite–palygorskite (and other Mg-clays) formation based on worldwide continental occurrences. Also, it establishes the sedimentological models and related lithological associations, according to the updated experimental and geological data.

The geological record of the oldest historical tsunamis in southwestern Spain

A regressive sequence was determined in the late Holocene evolution of the southwestern Donana National Park (SW Spain), based on a multidisciplinary study of sediments obtained in a drill core. In an initial phase (> cal. 2400 years BP), a shallow coastal lagoon with a partial marine connection occupied this area, with a progressive transition from subtidal to supratidal conditions. The following phase (cal. 2400-2350 years BP) is characterized by a tsunami, with the deposition of an upper chenier. This sedimentological layer is characterized by a ridge morphology, an erosive base, high bioclastic contents and the introduction of marine species toward the inner areas of this old lagoon. This tsunami may be assimilated to one of the historical tsunamis that occurred between 218 and 209 BC. A comparison with other deposits derived from these high-energy events permits to draw the differential effects produced along the western Iberian coasts. SHORT NOTES

Magnetic fingerprint of southern Portuguese speleothems and implications for paleomagnetism and environmental magnetism

Environmental magnetism of speleothems is still in its early stage of development. Here we report on our investigation of the environmental and paleomagnetic information that has been recorded in speleothems, and what are the factors that control its preservation and reliability. To address these issues, we used a multidisciplinary approach, including rock magnetism, petrography, scanning electron microscopy, stable carbon and oxygen isotope compositions, and major and trace element concentrations. We applied this to a set of samples from different stages of speleothem evolution: present-day dripwater (glass plates), a weathered stalactite, a fresh stalagmite, cave sediments, and terra rossa soils. These samples come from the Penico and Excentricas caves, located in two distinct aquifers of the Algarve region, South Portugal. Our results show that the main magnetic carriers of the speleothems under study are primary (detrital) and consist of maghemite (and magnetite?). Similarities in coercivity and temperature dependence of the studied set of samples suggest that iron oxides are inherited from the terra rossa soils that cap the cave and were transported to the speleothems by dripwater. Hence, they represent a regional environmental signature. Interestingly, a stable and probably detrital remanent magnetization could be isolated in the fresh stalagmite, whereas the weathered stalactite yielded chaotic magnetic directions and very low remanent intensities. We propose that these low intensities can be the result from (i) different remanence acquisition mechanisms between stalagmite and stalactite and/or (ii) iron dissolution by fungal activity. We also suggest that magnetic properties and color and the content in detrital elements in the fresh speleothem inform about environmental processes acting on the interface of rock (soil)-atmosphere, while oxygen isotope composition and alkaline-earth element concentrations inform about calcite-water interaction processes. These results provide a better understanding of how environmental information is recorded in speleothems and what the factors are that control the reliability of the paleomagnetic and paleo-environmental signal.

Mineralogía y geoquímica delito facies lacustres marginales en el sector de Pinto (Cuenta de Madrid)

Se estudia la mineralogia, geoquimica y evolucion sedimentaria de arcillas magnesicas pertenecientes a la Unidad Intermedia del Mioceno de la cuenca de Madrid. Se han analizado muestras correspondientes a tres unidades (Unidad Detritica, Unidad Magnesica y Unidad Carbonatica) diferenciadas en la zona de Pinto. Las arcillas magnesicas con encharcamientos palustres en los que tiene lugar la formacion de las arcillas autigenicas. La alternancia de lutitas verdes y lutitas marron-rosaceas indica fluctuaciones periodicas del margen lacustre durante episodios expansivos y retraetivos. La asociacion de minerales autigenicos observada, evolucion geoquimica y desarrollo de paleosuelos vedicos y dolocretas, sugieren condiciones climaticas semiaridas.