Micromorphology and formation of pedogenic ooids in calcic soils and petrocalcic horizons

Colin R. Robins
Annika Deurlington


Ooids, pisoids, nodules, concretions, and a host of other terms are used to describe concentrically-zoned pedogenic carbonate, phyllosilicates, silica, and other minerals in calcic and petrocalcic horizons worldwide. The pedogenic and paleoenvironmental significance of such features is not always easy to interpret because variations in size, structure, composition, and soil-geomorphic context have been used to infer distinct modes of genesis for features that appear similar. Using both optical and scanning electron microscopy, petrocalcic soil samples from the Jornada La Mesa geomorphic surface in New Mexico and the Mormon Mesa geomorphic surface in Nevada were studied to reconsider the formation of pedogenic ooids. In this paper, “ooids” are < 2 mm in diameter and are comprised of concentrically alternating carbonate and fibrous clay laminae. “Pisoids” are > 2 mm in diameter and have a range of internal morphologies. Ooids were especially common within features including clast pendant laminae, pisoids, and laminar horizons or laminar horizon caps. We propose a possible new mode of ooid genesis, suggesting that crystal growth can move ooids tiny, incremental distances over time, and that phyllosilicates are an important genetic component. Ooids are interpreted to form via: (1) mineralization during the evaporation of solutions held by surface tension around mineral grains, clasts, or petrocalcic fragments, with or without the presence of organic matter, (2) hydration and plastic behavior of pervasive, pedogenic, fibrous phyllosilicates that co-precipitate with pedogenic calcite, and (3) tiny, successive movements caused by the crystallization pressure of pedogenic carbonate and other minerals during soil solution evaporation. Spheroidal morphology is initially promoted by chemical precipitation from evaporating solutions around grains. Next, crystallization pressures from the surrounding matrix displace grains non-uniformly, promoting stochastic contacts or ‘knocking’ of ooids against one another and against the soil matrix. Over long time spans, this micrometer-scale, episodic translocation and rotation, in tandem with the plastic behavior of fibrous phyllosilicates, enhances the spherical shape of ooids. The model described here does not preclude possible biological contributions to ooid genesis, nor refute the role of other processes in producing similar features in the same soil. Finally, ooid size is limited by physical and hydrological thresholds. Pisoids, as larger features, are more complex and are more likely to involve erosion; they do not form in the same manner as ooids. This study has implications for paleoenvironmental interpretations of calcic and petrocalcic horizons, and for the selection of pedogenic features for isotopic analysis.