Calcite pseudospar replacive of marine acicular aragonite, and implications for aragonite cement diagenesis

Botryoidal and fan-shaped masses of polycrystalline calcite pseudospar from Permian biohermal limestones in New Mexico and Texas are interpreted as replacements of radially-divergent, acicular marine cements. The mosaic consists of composites of and individual ray-crystals, with included relicts of square-tipped, primary fibers of orthorhombic morphology. The presence of these relict fibers and relatively high Sr/MgCO (sub 3) ratios of the replacement calcites suggest the precursor cement was aragonite. As such, the interpretation of this fabric as aragonite-replacive represents an example of ancient cement neomorphism based on preserved microfabric detail as well as geochemistry and Holocene analogy. Formation of this pseudospar is believed to have occurred under a range of geochemical-diagenetic conditions in fresh-water vadose to burial environments. The microfabric elements within the pseudospar display a divergence of crystal elongation away from the substratum. However, c-axial extinction of ray- and composite crystals may or may not be coincident with crystal elongation. Such extinction patterns within the finely polycrystalline pseudospar of non-scalenohedral habit, and the presence therein of well-preserved primary fibers, make this fabric distinct from other fibrous calcites believed to be cement replacive.