Diagenetic Palygorskite in Marginal Continental Detrital Deposits Located in the South of the Tertiary Duero Basin (Segovia, Spain)

Abstract

Abstract The genesis and evolution of clay minerals has been studied in continental detrital deposits, located in the southern edge of the Tertiary Duero Basin (Segovia, Spain). The average thickness of these materials is 40-60 m and three lithological units are distinguished, described from the lower to the upper part as: a) siliceous sands, b) sandstones and paraconglomerates, c) feldspathic sands with pebbles. In unit (a) the main clay minerals arc smectites which are dispersed within a cement formed mainly by amorphous silica and opal. It is possible to recognise a reaction edge, with smectite neoformation rims surrounding the corroded grains of quartz. Unit (b) is characterised by the presence of calcareous and siliceous cement, and fibrous aggregates of palygorskite at the interface between the carbonates and the corroded grains of quartz. The morphology and development of palygorskite aggregates changes markedly with porosity and the amount of calcareous cement. Unit (c) shows in the lower parts hy-drolysated pebbles of granite. The cement is mainly clay, with random occurrence of carbonates and siliceous materials. From the morphological, mineralopical and geochemical data, the clay mineral evolution has been established correlating with the tectonic stages of the Spanish Central Range. So unit (a) corresponds to the earliest “impulses” with deposition in small sloped bains. Orogenic activity causes erosion of carbonatic deposits and the arrival to the basin of dolomitic materials, which changes the geochemical characteristics and initiates neoformation of palygorskite in unit b. Unit c corresponds to the last orogenic impulses and represents an evolution from palygorskite and smectite in its lower areas to kaolinite and illite in the uppermost.