DURIPANS OF IDAHO, USA - INSITU ALTERATION OF EOLIAN DUST (LOESS) TO AN OPAL-A/X-RAY AMORPHOUS PHASE

Micromorphological techniques were used to study the silicification process in a wide range of duripans from Idaho, U.S.A. Investigations indicated that, once loess was incorporated into a carbonate-rich pan, it was altered, in situ to an opal-A/X-ray amorphous phase. Loess became incorporated by at least two pathways, each of which had a distinctive micromorphic signature. The first pathway began immediately above the duripan as calcite encapsulated eolian materials to create sand-sized loess agglomerates, Incorporation was completed either as the upward-developing duripan enveloped the agglomerates or as they filled fractures in the duripans and were subsequently cemented by calcite. After encasement of the loess agglomerates within the duripan, aluminosilicate minerals were embayed by reaction with the calcareous soil solution. The most altered loess agglomerates consisted of opal-A, X-ray amorphous material, and calcite. The second pathway was initiated when fractures in the duripan were filled by eolian dust to create pedotubules. The tops of pedotubules had a fabric similar to unaltered loess (silasepic plasmic); however, the bottoms of pedotubules were invariably altered to a microgranular array of opal-A and calcite. Impetus for alteration appears to have been high pH, created by closed-system conditions, combined with increased surface free energy of minerals caused by elevated grain-to-grain pressures, which led to enhanced solubility of aluminosilicate minerals. The incorporation of loess and its subsequent alteration may be an important process in the silicification of calcareous pans.