%0 Journal Article
%A Alonso Zarza, Ana María
%T Initial stages of laminar calcrete formation by roots:examples from the Neogene of central Spain
%D 1999
%@ 0037-0738
%U https://hdl.handle.net/20.500.14352/59039
%X Thin carbonate laminae formed by root activity are common within Miocene detrital deposits of the Duero and Madridbasins. The laminae are about 3 cm thick, extending several metres laterally, and displace the original detrital sediment,which ranges in grain size from fine gravel to sandy clay. The thickness, morphology, microstructure and stable isotopecompositions of the laminae indicate that they formed by the activities of roots and associated microorganisms within thesoil. The laminae are similar to those commonly recognised in thick laminar calcrete profiles. Three types of laminae arerecognised. Differences in the micromorphology of the laminae are explained as reflecting the different organisms involvedand whether calcification occurred when the root was alive or decaying. The first type occurs in a profile at Paracuellosthe Jarama, where the laminae consist of a mosaic of 20 mm calcite crystals whose arrangement indicates that calcificationtook place in the medulla of the root and probably occurred while the plant was alive. In a second type in the Villacadimaprofile, laminae comprise calcified root mats whose formation indicates the interplay of roots and fungi. Calcification of thecell-walls and intracellular spaces took place in the cortices of the roots and not in the medullas as revealed by the presenceof central pores in every calcified root. A third type of lamina is recognised in profiles at Vin˜egra de Moran˜a and consistsof mucilaginous sheets coated by needle-fibre calcite crystals. The occurrence of associated small root casts indicatesthat formation of the laminae occurred while the root was decaying and was also influenced by fungal activity. Theselaminae occur within poorly developed soils and their formation was controlled by the relationship between sedimentation,erosion and soil formation processes. Thus, the occurrence of these laminae interbedded with detrital sediments reflectsenvironments where sedimentation was relatively low and episodic, so after the detrital sediment input surfaces were stableand root mats were able to develop. Renewed sedimentation accounted for the death of the root mats and the developmentof new ones on the new surfaces. Where the sedimentation rate is lower the laminae tend to amalgamate and thicker laminarcalcrete profiles form with little or no interbedded detrital sediment.
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