EDAFOLOGÍA, VOL 14. (1, 2, 3), PP 67-98 2007


I. Macías-García1, A. Pérez Alberti1, A. Martínez Cortizas2, C. Nieto3, X. L. Pérez Otero2,3, M. C. Monterroso2,3, M. T. García González4, R. Valle5 y F. Macías2,3

1 Dpt. Geografía. Universidad de Santiago de Compostela. 2 Dpt. Edafología y Química Agrícola. Universidad de Santiago de Compostela. 3 Laboratorio de Tecnología Ambiental. Universidad de Santiago de Compostela. 4 Instituto de Ciencias Medioambientales. CSIC. Madrid. 5 Dept Geología. ENDESA



The excavation of the lignite mine of As Pontes, almost to its base, allowed to sample and study in detail the nature and process of formation of an Oligocene-Miocene lignite deposit with up to 500 m of sedimentary material, produced in a time span of 10-11 million years. The site is characterized by alternating layers of coal (lignite) and detrital sediments of varied texture. Within the sequence the process of coal formation seems to have occurred mostly in a swampy environment, with vegetation dominated by Juncus and Thypha, that developed on hydromorphic soils, mainly Gleysols and Histosols. At a pH close to neutrality and under anoxic-suboxic conditions, Fe was reduced to ferrous state, leading to the formation of nodules and concretions of siderite and biogenic sulfides. Siderite, is clearly associated to the rhizosphere of aquatic plants in gleyic soil horizons, with segregation and mobilization of Fe, being root respiration clearly related to the formation of the mineral. These soils had surface horizons rich in organic matter (H o A) which were later fossilized by fluvial-torrential sediments derived from soils and saprolites of kaolinitic soils formed in well-drained conditions (probably lateritic soils) at the margins of the basin. Fossilization and subsequent subsidence of the hydromorphic soils led to the transformation of surface horizons into lignite and the compaction of the materials. Afterthat, the new sediments underwent pedogenesis with varying intensity and duration. In the systems at the margins of basin Plinthosols, fermonosialithic red soils highly evolved and lateritic crusts were found, which highlights the variety of soil conditions and climate evolution that occurred. Alternating conditions of biostaxy (formation of hydromorphic soils in the watershed and well drained, kaolinitic soils in the slopes) and rhesistaxy (erosion, alluvium-torrential sedimentation and fossilization of soils) may be responded to climatic change (probably between wet periods and others more arid or with intense seasonality). Two evolutionary phases have been clear identified. A initial one, most likely with marine influence, and potential mangrove soils or marsh with an abundance of neoformation of smectite clays, and a later one, clearly continental, with kaolinitic clays and Histosols and gleyic soils in the basin and kaolinitic soils in the well drained areas of the margin of the basin. More than 30 cycles of pedogenesis and fossilization have occurred in the 10-11 million years of evolution of the basin during the formation of tertiary lignites, pointing to a huge variability in the soil forming conditions. The repetition of this alternation led to a sequence of Oligocene and Miocene paleosols, clearly identifiable by soil characteristics, mineralogy of its components and the presence of charred and pyritized rhizospheres. Tectonic movements, the subsidence of the basin and the consequences of the mining operations and its modifications of the hydric and redox conditions have increased the complexity and the interpretation of the pedogenetical and geochemical evolution of the basin soils.

Key words: Paleosols, lignite, Oligocene, Miocene, siderite, Histosols, Gleisols, Plinthosols