Saturday, 15 July 2006

Genesis of Pisoliths and Brecciation Features in Stage VI Petrocalcic Horizons, Mormon Mesa, NV, USA.

Amy Brock and Brenda Buck. Univ of Nevada Las Vegas, Dept of Geoscience, 4505 Maryland Pkwy Box 4010, Las Vegas, NV 89154

Highly developed calcium carbonate soils hold records of past climates, vadose zone processes, and past and present landscape dynamics throughout arid and semi-arid regions of the world (including the arctic). Two important features found in highly developed petrocalcic soils that provide an indication of these dynamics are pisoliths and brecciation. Pisoliths, pisolith-like forms (a.k.a. pisoids, peloids, glaebules, ooids, ooliths, pisolites, spherules, spherical grains) and brecciation features are interpreted to result from long-term pedogenic processes and may signify an early Pleistocene to Miocene age for soils and geomorphic surfaces containing them (Machette, 1985). Pisoliths and brecciation features are used in the definition of stages V and VI petrocalcic soils (Bachman and Machette, 1977). Currently these features are only used as a descriptor in these soils. owever, it would be far more beneficial for geologists, pedologists and geomorphologists to understand the processes of their formation. The Mormon Mesa geomorphic surface is located approximately 100 km north of Las Vegas, Nevada and is unique because it is capped by a 3-4 meter thick stage VI petrocalcic horizon that has developed on Neogene sediments of the Muddy Creek Formation (Gardner, 1972; Bachman and Machette, 1977). Little detailed micromorphological work has been carried out on stages V and VI carbonate morphologies (e.g. Watts, 1980; Monger et al., 1991). The goals of this study were: (1) to identify the processes of pisolith formation, and (2) determine which features of petrocalcic soils are time-dependant and how they relate to the morphologic stage designations. This research uses scanning electron microscopy with an energy dispersive spectrometer (SEM/EDS), and x-ray diffraction (XRD), with geomorphic field mapping, and detailed physical descriptions to determine the geochemical and spatial relationships between various minerals that form pisoliths and brecciation features in petrocalcic soils at Mormon Mesa. The Mormon Mesa petrocalcic horizon displays a variety of features (many of which have been previously undescribed) such as large void pockets, enveloped pendants, root and animal burrows filled in with eolian sediment from above, ooids between lamina, vertical lamina coating brecciated fragments of the petrocalcic horizon, palygorskite clays, and lamina surrounding enveloped gravels from coarse textured parent material (Gardner, 1972; Bachmann and Machette, 1977). Identified in this study are the following processes that occur within petrocalcic horizons: (1) parent material grain or clast dissolution presumably caused by the pressure of crystallization of the pedogenic carbonate, (2) the dissolved constituents combine to result in the neoformation of secondary materials such as palygorskite, (3) erosion of the overlying unconsolidated soil exposes the petrocalcic horizon and results in physical fracturing and rotation of these fragments. Later, reburial and stabilization of the overlying soil results in recementation of these petrocalcic fragments into the upper portion of the petrocalcic horizon. These processes often include the formation of vertical and crosscutting laminae and pendant formation underneath petrocalcic fragments and (4) some pisoliths are formed via the engulfment of stage II pendants into the petrocalcic horizon. The original parent clasts that initiated the pendant formation may or may not become later dissolved through the pressure of crystallization of the engulfing pedogenic carbonate. The results of this study are important because they indicate that soils containing petrocalcic horizons and the geomorphic surfaces associated with them are not static, closed systems, but are products of a multitude of intrinsic and extrinsic processes operating at several different spatial and temporal scales. Evidence for grain dissolution and neoformation of secondary minerals indicates that these systems are open and dynamic and therefore may not be well-suited for isotopic analyses for dating or paleoclimate interpretations. Significant additional research is needed to better understand these complex processes. However, this study provides evidence that pisoliths form from pedogenic processes, therefore they can be used to differentiate pedogenic from groundwater carbonates.

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