PRELIMINARY INVESTIGATION OF POST-CEMENTATION GRAVEL DISSOLUTION IN PETROCALCIC HORIZONS, SOUTHERN NEVADA
Vertical slabs of gravel-rich petrocalcic horizon materials from two geomorphic surfaces were examined for irregular boundaries and voids at matrix-clast contacts and voids within gravel interiors. Powders of matrix materials, and portions of unaltered and altered gravels were excavated using a diamond-tipped Dremel tool for analysis with XRD. Select areas of slabs were prepared as thick sections for SEM/EDS evaluation.
Clast lithologies in all sampled horizons include predominantly dolostone with minor limestone, and rare granite and petrocalcic fragments. Clasts with significant alteration are dolostone where alteration occurs at the top of the clast. Dissolution features include less-dense portions of altered clasts or empty voids. ~1mm diameter, vertical, tubular voids extend rarely through the matrix and commonly into the upper portions of gravels. White precipitates of calcite fill few of the upper parts of tubular voids.
Initial observations suggest that dolostone gravels may undergo dissolution due to undersaturated (with respect to Mg) meteoric waters transmitted through the horizon. Future chemical and mineralogical analyses will contribute to interpretation of the chemical conditions that are required for post-induration dissolution.
Understanding the dissolution of gravels in this environment is important because older (late stage) petrocalcic soil horizons may have undergone alteration to the point that select gravel lithologies are no longer present, thus causing complications for provenance studies and parent-material characterization. Additionally, they may provide clues to the chemical evolution of the soil environment from the time of parent material deposition.