A Microscopic Examination of Pedogenic Calcite in an Aridisol of Southern New Mexico

Thin section analysis of pedogenic calcite was made for a mid-Pleistocene Aridisol in southern New Mexico. The 4.5 m thick pedon was divided into eight zones of pedogenic calcite accumulation. The macromorphology of the eight zones includes Stage I filaments and coatings, Stage II and Stage III nodules and internodular fillings, and a petrocalcic horizon consisting of a Stage IV laminar zone and plugged horizon. Thin sections were made of oriented peds from each of the eight zones and revealed the following. Filaments consist of calcite crystals on roots and calcite-impregnated pore walls (hypo-coatings). Nodules contain framework grains embedded in a matrix of micritic calcite (micrite); harder nodules contain denser and more coalescent micrite than softer nodules. Internodular fillings contain calcitans and micrite that is loosely disseminated in voids. The laminar zone consists of Microcodium, framework grains packed between relatively pure micritic laminae, and sparry calcite in voids. The plugged horizon is composed of widely spaced framework grains embedded in a dense micritic matrix and calcified fungal fillaments in voids. Corroded framework grains are common in both the laminar zone and plugged horizon and reflect dissolution of the grains. Argillans in calcareous zones have been partly or wholly obliterated by calcitans.