Abstract

Massive, decimeter-thick intervals formed by mosaics of subspherical calcite crystals displaying an intracrystalline fibrous microfabric have been found in Paleocene and Miocene strata in Spain. The fibrous microfabric is marked by inclusion-defined fibers that radiate from the central parts of the crystals, which can be termed pseudospherulites and not spherulites, because they are single crystals and not polycrystalline aggregates. The horizons of pseudospherulitic calcite appear in meter-scale, laterally persistent levels of crystalline limestones that exhibit complex pseudospar fabrics, they are interbedded with peritidal and/or lacustrine deposits, they grade downward into bedded dolomicrites, and they can be linked to unconformities several 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-isotope composition indicative of precipitation from meteoric water. They form xenotopic mosaics in which goethite, palygorskite, and sepiolite may be present as inclusions and in the intercrystalline matrix. Cathodoluminescence microscopy reveals peripheral growth of the pseudospherulites, which results from several phases of precipitation and even of dissolution. The first 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, and shows indications of displacive growth. The pseudospherulitic fabric is not relict and the pseudospherulites do not represent speleothems, recrystallized 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 hypothesis of a dolomicritic host is supported by the geological and diagenetic setting, by the presence of corroded dolomite inclusions within the pseudospherulites, and by the relationship between their growth and the authigenesis 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 fluctuations 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 calcite suggest some bacterial influence in the genesis of the pseudospherulitic fabric.

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