Publication: La sedimentación salina actual en las lagunas de La Mancha: una síntesis
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1986
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Servicio de Publicaciones de la Universidad Complutense de Madrid
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Abstract
La Mancha es una región natural de más de 30.000 Km2, caracterizada por una topografía extraordinariamente plana y un clima de tipo semiárido (Cuadro 1, Fig. 2), en la que existen numerosas lagunas salinas (Fig. 1), la mayoría de las cuales, por su régimen anual, pueden ser consideradas como «playa-lakes» (Fig. 3). Desde el punto de vista hidroquímico sus salmueras están integradas por: a) aniones: SO4= y Cl-, con CO3= y CO3H— subordinados, y b) cationes: Mg2+ y Na+, con Ca2+ y K- subordinados (Cuadros II y III, Fig. 4). La alta concentración de dichas lagunas condiciona una importante sedimentación salina, que llega a producir costras anuales de más de 10 cm de espesor (Figs. 5, 6 y 8) en las que la fase sulfatada, integrada por una mineralogía muy variada, es la principalmente representada. Los minerales identificados son: a) cloruros: halita; b) sulfatos: epsomita, hexahidrita, thenardita, mirabilita, loedita, glauberita, polihalita, kieserita, loeweita, vanthoffita y yeso; e) carbonatos: calcita y dolomita (muy escasas). El grado de cristalinidad y tamaño de dichos minerales es muy variable, llegando a ser muy grandes (varios centímetros) y perfectamente enhedrales (Figs. 6 y 7). Por ello, cuando ha sido posible, además de mediante difracción de rayos X, se han estudiado en lámina delgada (Fig. 9). Se han realizado experimentos de crecimiento de cristales en el laboratorio por evaporación y descenso de la temperatura de una salmuera original, tomada en una laguna en el momento de máxima dilución. Los resultados, tanto en sus aspectos secuenciales como morfológicos, son contrastados con los datos de campo con el objeto de tener una hipótesis explicativa de los mecanismos de crecimiento que dan origen a la costra salina natural (Fig. 10). Asimismo, se investiga el efecto que las salmueras intersticiales, de naturaleza sulfatada y ricas en Mg2+ fundamentalmente, ejercen sobre los minerales arcillosos heredados de los entornos de las lagunas. Se ha comprobado que la mineralogía de arcillas en el área fuente está integrada por ilita y un interestratificado regular (14c — 14M), corrensita, mientras que, en la laguna en la que se ha realizado este estudio, la ilita sigue persistiendo, si bien con ciertos cambios (PÁg. 11), al tiempo que la corrensita ha perdido parte de sus iones Mg2- para transtormarse en una estructura a 14 Å. Además, se ha observado que los minerales de arcilla están mucho más degradados hacia el techo de la secuencia salina. Finalmente, se describen e interpretan una serie de estructuras primarias o diagenéticas tempranas, en general de carácter efímero: costras salinas, grietas de desecación, crestas de sal, «tepees», estructuras circulares, «ripples», dunas de sal, tapices algales y bioturbación (Figs. 12, 13 y 14).
La Mancha is a naturally defined region of more than 30000 km2 which is characterized by an extremely fiat topography and a semiarid clirnate (Table 1, Fig. 2). In it are several small salme iakes or ponds (Fig. 1). Ihe majority of these can be considered as playa lakes, in view of their seasonal hydroiogic regime. hidrocbemically, the brines of the La Mancha ponds are composed predominantly of the cations magnesium and sodium, and the anions sulfate and chloride (Tables II and III, Fig. 4). The evaporative concentration of these ponds lead to precipitation of annual salme crusts which reach a thickness of more than 10 cm (Fig. 5, 6 and 8) in which sulfate, incorporated in a great variety of minerals, is the principal constituent represented. The identified minerals include haute as the sole chioride, gypsum, the magnesium sulfates epsomite, kieserite, and hexahydrite, the sodium sulfates thenardite and mirabilite, the mixed cation sulfates bloedite, glauberite, polyhalite, loeweite, and vanthoflite, plus tl-ie carbonates calcite and dolomite (both very scarce). Ihe degree of crystallinity and size of these minerais is quite variable, sometimes reaching several centimetres la maximum dimension, with a highly idiomarphic habit (PÁgs. 6 and 7). Because of this, it has been possible to study the salts in thin section (Fig. 9) as well as by X-ray diffraction. Growth experiments in the laboratory have been accomplished by evaporation and temperature decrease of an original brine, which was sample during the maximum dilution period of the pond. Ihe sequential and morphological experimental results are compared with the field data, in a attempt to docurnent the growth mechanisms that are involved in the formation of the natural salme crust (Fig. 10). Likewise, the effect of the magnesium sulfate rich interstitial brines on the detrital clay ininerais contributed from the surrounding areas was investigated. We found that illite and the regular mixed-layer chlorite-smectite, corrensite, were the principal coniponents of the clay mineralogy in the source area. In contrast, in the pond where we have done most of our study the illite remains with some changes and the corrensite has lost some magnesium in alteration to an irregular 14 A structure. Furtherrnore, we have observed that clay minerais have been significantly degraded at the top of the salino sequence. Finally, several primary or early diagenetic síructures generally having an ephemeral character have been described ami interpreted. Ihese include salme crusts, mud cracks, salt ridges, tepees, circular patterns, ripples, salt dures, algal mats and bioturbation features (Figs. 12, 13 and 14).
La Mancha is a naturally defined region of more than 30000 km2 which is characterized by an extremely fiat topography and a semiarid clirnate (Table 1, Fig. 2). In it are several small salme iakes or ponds (Fig. 1). Ihe majority of these can be considered as playa lakes, in view of their seasonal hydroiogic regime. hidrocbemically, the brines of the La Mancha ponds are composed predominantly of the cations magnesium and sodium, and the anions sulfate and chloride (Tables II and III, Fig. 4). The evaporative concentration of these ponds lead to precipitation of annual salme crusts which reach a thickness of more than 10 cm (Fig. 5, 6 and 8) in which sulfate, incorporated in a great variety of minerals, is the principal constituent represented. The identified minerals include haute as the sole chioride, gypsum, the magnesium sulfates epsomite, kieserite, and hexahydrite, the sodium sulfates thenardite and mirabilite, the mixed cation sulfates bloedite, glauberite, polyhalite, loeweite, and vanthoflite, plus tl-ie carbonates calcite and dolomite (both very scarce). Ihe degree of crystallinity and size of these minerais is quite variable, sometimes reaching several centimetres la maximum dimension, with a highly idiomarphic habit (PÁgs. 6 and 7). Because of this, it has been possible to study the salts in thin section (Fig. 9) as well as by X-ray diffraction. Growth experiments in the laboratory have been accomplished by evaporation and temperature decrease of an original brine, which was sample during the maximum dilution period of the pond. Ihe sequential and morphological experimental results are compared with the field data, in a attempt to docurnent the growth mechanisms that are involved in the formation of the natural salme crust (Fig. 10). Likewise, the effect of the magnesium sulfate rich interstitial brines on the detrital clay ininerais contributed from the surrounding areas was investigated. We found that illite and the regular mixed-layer chlorite-smectite, corrensite, were the principal coniponents of the clay mineralogy in the source area. In contrast, in the pond where we have done most of our study the illite remains with some changes and the corrensite has lost some magnesium in alteration to an irregular 14 A structure. Furtherrnore, we have observed that clay minerais have been significantly degraded at the top of the salino sequence. Finally, several primary or early diagenetic síructures generally having an ephemeral character have been described ami interpreted. Ihese include salme crusts, mud cracks, salt ridges, tepees, circular patterns, ripples, salt dures, algal mats and bioturbation features (Figs. 12, 13 and 14).