Conceptual Mineral Genesis Models for Calcic Pendants and Petrocalcic Horizons, Nevada

Abstract

We use the very old, extant. Stage VI Mormon Mesa petrocalcic soil profile in southern Nevada to: (i) present a conceptual model summarizing known spatial and temporal relationships among authigenic calcite (or low-Mg calcite), palygorskite and/or sepiolite, and amorphous silica and (ii) adapt a second model to describe the occurrence of pedogenic barite and the effect of climate oscillations on soil mineralogy at Mormon Mesa. These conceptual models, one compiled from previous research, one built from new data in this study, directly address the importance of dust to soil genesis, especially the control of salt flux and high pH on Ba, Si, and Al mobility in alkaline soil solutions. Authigenesis of calcite and palygorskite/sepiolite creates positive feedbacks with soil solution chemistry that drives further mineralogical development. We explain the occurrence of pedogenic barite at Mormon Mesa based on higher Ba solubility and translocation into the soil profile with increased concentrations of CI and Mg in soil solutions during interpluvial climates. Development of integrated, mineral development models illustrates individual system components and/or processes that should be targeted for future chemical, biological, geochronologic, or micromorphological study, and provides a basis for comparison of calcic soil genesis from disparate sites around the world.

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

“…However, these data are not included because the carbon coat, and the pervasive, fine overgrowths of carbonate and magnesium-rich phyllosilicates rendered most EDS analyses of ooid laminae or pisoid layers into similar spectra, all containing C, Ca, Mg, Al, Si, and O in variable amounts. Additionally, EDS and X-ray diffraction (XRD) analyses of various horizons and particle-size fractions of the MM and/or JLM soils have been published by previous researchers (e.g., Monger and Daugherty, 1991a,b;Brock-Hon et al, 2012;Robins et al, 2012;Robins et al, 2014). Approximately 130 SEM images were collected as part of this study.…”

“…The goals of this paper are to revisit the genesis of concretionary microstructures in calcic and petrocalcic horizons, and to specifically evaluate the hypothesized roles of (1) soil wet-dry cycles, (2) the hydration and plastic behavior of palygorskite and sepiolite, and (3) non-uniform crystallization pressures from calcite and other pedogenic mineral growth in the formation of pedogenic ooids. With the present study, we interpret pedogenic ooids and pisoids in light of existing conceptual mineral development models for calcic soils (e.g., Bachman and Machette, 1977;Watts, 1980;Robins et al, 2012), and in light of the broader scale soil profile development models suggested for calcic soils or petrocalcic horizons by other researchers (e.g., Gile et al, 1966;Machette, 1985;Alonso-Zarza, 2003;Brock and Buck, 2009). We specifically consider the hypothesis that pedogenic mineral growth during early stage (e.g., stage II or higher) calcic soil genesis can cause tiny, stochastic, successive movements of grains or ooids in the soil with each wet-dry-crystallization cycle.…”

“…Both soils are polygenetic, and welded profiles in places also include calcic horizons with only incipient (e.g., stage I) carbonate accumulation. The genesis, profile morphology, and micromorphology of these two soils have been well-studied (Bachman and Machette, 1977;Monger and Daugherty, 1991a,b;Brock and Buck, 2009;Robins et al, 2012Robins et al, , 2014, though no previous work has specifically focused on the formation of their ooids or pisoids. The stage V (Machette, 1985;Schoeneberger et al, 2013)~780 ka to 2 Ma JLM soil is exposed in terraces, arroyos, and wide trenches excavated as part of the Desert Project (Bachman and Machette, 1977;Gile et al, 1981;Monger and Daugherty, 1991a;JER LTER, 2003).…”

“…1). Two of these profiles are located approximately 10 m apart in the south-facing median of Interstate 15, while the third, exposed in an abandoned highway roadcut, is directly comparable to the "Riverside" profile of Brock and Buck (2009) and Robins et al (2012). Oriented samples were collected using a diamondbladed Wacker BTS 1035 concrete cutting saw, a rock hammer, and a chisel.…”

“…However, carbonate crystallization cannot be considered as if occurring in a mineralogical vacuum, nor as if from a solution of calcium and bicarbonate ions alone. Phyllosilicates, sulfates, chlorides, and a variety of other mineral groups create important chemical feedbacks with soil solutions in arid soils, particularly with Ca 2+ and/or HCO 3 − ions (Watts, 1980;Verrecchia and Le Coustumer, 1996;Monger and Daugherty, 1991a;Robins et al, 2012). Calcite precipitation can also progressively change hydraulic conductivity over time, driving geomorphic feedbacks.…”