Micromorphology Of Pedogenic Ooids and Pisoids In Petrocalcic Soils Of The Southwestern United States.

Concentrically-zoned pedogenic carbonate, phyllosilicates, and silica form well-documented microfeatures in calcic and petrocalcic soil horizons worldwide, including those of  the Mormon Mesa, Nevada, and Jornada La Mesa (JLM), New Mexico, geomorphic surfaces. Nevertheless, the pedogenic and paleoenvironmental significance of these features is not always clear, because variations in size, structure, composition, and soil-geomorphic context have previously been used to infer distinct modes of genesis for features that appear similar. Called glaebules, ooids, pisoids, or any of a host of other terms, these features have been variably interpreted as pedogenic or non-pedogenic (i.e., lacustrine, fluvial, or groundwater), abiotic or biotic, and formed subaerially or within the soil matrix. Using optical microscopy and SEM-EDS, samples from Mormon Mesa and JLM were studied to consider a soil chemical-hydrological model of pisoid/ooid formation. In nearly all samples, concentrically alternating layers of carbonate and fibrous clay were found in many configurations, with and without a detrital grain nucleus, with and without evidence for biological activity, and ranging in diameter from 0.03 mm (nodules, ooids, and pellets) to 2 cm (pisoliths). Ooids and coalesced masses were common within the indurated matrix and in laminae of carbonate pendants. These features are interpreted to have formed solely by precipitation when thin solution films evaporated around single mineral grains, clasts, petrocalcic fragments, roots, and/or in micropores – anywhere moisture was held just prior to desiccation. Subsequent wet-dry cycles could have modified individual mineral coats, perhaps over long time-scales, with fluctuating activities of aluminum, bicarbonate, calcium, magnesium, and silicon determining the mineralogy of successive layers. Direct biological precipitation and subaerial exposure are neither required nor adequate to generate the variability of features observed, though they can produce similar features at other sites. This study has important implications for interpreting the environmental history of calcic and petrocalcic horizons, as well as for geochronology.