Cherts with moganite in continental Mg-Clay deposits: an example of "False" Magadi-type cherts, Madrid basin, Spain.

During the Miocene the Madrid basin was a closed basin in which a succession of alluvial and lake deposits were replaced by different types of crusts (mainly calcretes, dolocretes, and silcretes). In a geographically restricted area to the south, Mg-clay deposits were precipitated in ponds on the sand flats. A discontinuous nodular and stratiform Mg-rich chert appears at the top of the Mg-clay deposits and displays visual, mineralogical, and petrological characteristics that are very different from the other silcretes of the Madrid basin. As revealed by X-ray diffraction, the cherts consist mainly of quartz and moganite, which are commonly found in Magadi-type cherts. The petrological characteristics reveal that the chert consists of a groundmass crossed by several networks of quartz veins. The groundmass is formed by mosaic quartz with different sizes of crystals (usually less than 20 mm) and length-slow chalcedony. The presence of gridwork extinction patterns, the differentiation of silica flakes, and the preservation of the micromorphological characteristics of the Mg-clay deposits indicate an early replacement by quartz and moganite. The quartz networks show veins of different shapes (rectilinear, irregular, branched, and V-shaped), sizes (from a few microns to several millimeters), and textures (mainly megaquartz, microquartz, and length-slow chalcedony). The V-shaped veins, and some irregular and branched veins, are considered to be a consequence of the silicification of cracks in the host rocks. The other veins represent silica filling cracks that formed by shrinkage during the various stages of chert formation. Many macroscopic attributes (e.g., some kinds of cracks), mineralogical features (the presence of moganite), and petrological characteristics (e.g., rectilinear or gridwork orientation of quartz crystals) are similar to those of Magadi-type cherts. In spite of this, the origin and the environment of formation of this Mg-rich chert, which crystallized by silicification of Mg-clays during deposition of silcrete, are very different from those of the typical Pleistocene Magadi cherts of African lakes. A pronounced shrinkage during silicification in a zone just below the water table and the presence of the moganite are the two factors that caused these Mg-rich cherts to appear similar to Magadi-type cherts