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

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.