Pseudospherulitic fibrous calcite in paleo-groundwater, unconformity-related diagenetic carbonates (Paleocene of the Ager Basin and Miocene of the Madrid Basin, Spain)

Massive, decimeter-thick intervals formed by mosaics of subspherical calcit e crystals displaying an intracrystalline fibrous microfabric have been fou nd in Paleocene and Miocene strata in Spain. The fibrous microfabric is mar ked by inclusion-defined fibers that radiate from the central parts of the crystals, which can be termed pseudospherulites and not spherulites, becaus e they are single crystals and not polycrystalline aggregates. The horizons of pseudospherulitic calcite appear in meter-scale, laterally persistent l evels of crystalline limestones that exhibit complex pseudospar fabrics, th ey are interbedded with peritidal and/or lacustrine deposits, they grade do wnward into bedded dolomicrites, and they can be linked to unconformities s everal meters higher in the section, above intervening unaltered limestones . The pseudospar zones are interpreted as early-diagenetic, paleo-groundwater alteration products after dolomicrites. The pseudospherulites are composed of low-Mg calcite with low contents of Fe and Mn, and they have a stable-i sotope composition indicative of precipitation from meteoric water. They fo rm xenotopic mosaics in which goethite, palygorskite, and sepiolite may be present as inclusions and in the intercrystalline matrix. Cathodoluminescen ce microscopy reveals peripheral growth of the pseudospherulites, which res ults from several phases of precipitation and even of dissolution. The firs t phase of crystal growth may show rhombic habits and may contain corroded dolomite inclusions, interpreted as an indication of growth at the expense of dolomicrite by a dissolution/precipitation process. The second phase of crystal growth is fibrous sensu stricto, exhibits lobate growth laminae, an d shows indications of displacive growth. The pseudospherulitic fabric is n ot relict and the pseudospherulites do not represent speleothems, recrystal lized Microcodium, or neomorphosed spherulitic carbonate, Instead, they are interpreted as split crystals that grew within dolomicritic host sediments , in part by dissolution-precipitation and in part by displacement. The hyp othesis of a dolomicritic host is supported by the geological and diageneti c setting, by the presence of corroded dolomite inclusions within the pseud ospherulites, and by the relationship between their growth and the authigen esis of sepiolite, palygorskite, and goethite, The pseudospherulites grew in the shallow subsurface, at or near the water table, from predominantly oxidizing meteoric water, and under periodically fluctuating geochemical conditions (saturation, redox) caused by fluctuatio ns of the water table during periods of subaerial exposure. The presence of fossil bacteria within the pseudospherulites, the anastomosing pattern of the inclusion-defined fibers, and the lobate growth forms of the fibrous ca lcite suggest some bacterial influence in the genesis of the pseudospheruli tic fabric.