REPORT OF AN ONGOING INVESTIGATION INTO THE DEVELOPMENT OF DISSOLUTION VOIDS IN PETROCALCIC MATERIALS AT MORMON MESA, NEVADA

Petrocalcic soils capping fluvial terraces near Overton, NV display stages of development from Stage IV to V. The detrital component includes siliceous and carbonate gravels that were deposited in washes that drain the nearby Mormon Mountains. Three soil profiles investigated in this study contain unique void features that appear to be the product of selective dissolution. Vertical tube-like voids are present in the calcic matrix of one of the studied profiles. In addition, all profiles contain gravels that are preferentially pitted and dissolved while adjacent (and sometimes capping) clasts and matrix materials remain unaltered.

An initial hypothesis stated that the formation of these dissolution features involved a magnesium under-saturated meteoric solution that preferentially dissolved magnesium-rich matrix and clast materials. To test this hypothesis, nine samples from the three soil profiles were analyzed for chemical composition with ICP. Three powdered samples were extracted with a Dremel tool from each profile. The sample sites included an altered part of a clast, the unaltered portion of the same clast, and material from the matrix above the altered clasts. We expected to see an elevated concentration of Mg in the matrix materials that displayed vertical voids and a decrease in Mg concentration in the altered clast materials when compared to the unaltered dolomitic limestone gravels.

Preliminary results show no elevated Mg concentrations in the matrix containing dissolution voids, nor is there a significant difference in Mg content from the altered and unaltered clast materials. However, there is notable elevated Ba in the altered sections of the clasts, which poses new questions as to the products and processes of alteration. Continued research into the formation of these unique dissolution features includes full XRD and additional ICP analyses, and examination of SEM data to test additional hypotheses on the formation of these unique dissolution features.