Morphology and origin of dolomite in paleosols and lacustrine sequences. Examples from the Miocene of the Madrid Basin

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The northern Madrid Basin contains paleosols and lacustrine sequences composed of ordered dolomite. Smectite-rich mudstones with analcime deposited in distal alluvial fans affected by pedogenic processes. Mudstones contain dolomite and silica laminae interpreted as mineralized root-mats. Dolostones with prismatic structure constitute Stage III dolocretes. Homogeneous lacustrine dolomudstones show some features indicative of subaerial exposure. Dolomite textures include: microcrystalline dolomite, coarse crystalline mosaics located in root cavities, dolomite dumbbells and dolomite spheroids replacing clays, dolomudstones or opal. Mol values of MgCO3 of dolomites vary between 47.67% and 53.34% mol. δ18O values range between − 3.65 and − 5.51‰ VPDB. δ13C values range between − 7.19 and − 8.38‰ VPDB. Dolomicrite formed in alkaline lakes by direct precipitation and replacement of clays in soils. The rhizosphere was favorable for dolomite mosaic and dumbbells formation. Spheroidal dolomite replaced clays and opal under early phreatic diagenesis. Trioctahedral smectites, silica and analcime indicate that these pedogenic and lacustrine environments were alkaline. Smectites and/or carboxyl groups acted as a catalyst favoring the incorporation of Mg into dolomite. In these non-saline settings the kinetic barriers for dolomite precipitation lowered by high pH and Mg enrichment. Ordered dolomite formation occurred under biogenic and abiogenic conditions, during sedimentation, pedogenesis and early diagenesis.
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