Petrographic description of calcite/opal samples collected on field trip of December 5-9, 1992. Special report No. 7

Criteria for the recognition of pedogenic/supergene and nonpedogenic/hypogene deposits and their relationship to the origin of calcite/opal deposits at Yucca Mountain. Special report No. 14

“…The photographs of Hill and Schluter (1993) and drawings of Hill (1993a) are especially pertinent to the . following discussion.…”

“…Generally, progressively more complex carbonate horizons occur in soils of progressively older geomorphic surfaces (Gile et al, 1981). In petrocalcic horizons some layers may be massive and some less massive, but where the calcretes "are thick and lack any major horizontal development, bedding, or structure, they represent a very large quantity of calcium carbonate and because of their physical nature are unlikelv to be formed by ...p edogenic processes" (Goudie, 1973, p. 142;underlining ours Wahmonie travertine/gypsite mound; Hill and Schluter, 1993), a criteria which, according to Goudie (1 973), makes these of unlikely pedogenic origin. In particular, the massive deposits at Site 199 were determined by Paces et al (1993) to be of spring origin.…”

“…Pressure solution or force-of-crystallization may also contribute to the relatively pure laminar layers (Maliva and Sever, 1988;Reheis, 1988; (Hill and Schluter, 1993). The debris in these deposits has been interpreted as being talus or sand incorporated into the calcite/opal as it was being precipitated as a spring-travertine deposit (Hill, 1993a).…”

“…In multiple sets the laminae may parallel one another, or some set may truncate the other angularly (e.g., Gile, 1961 , his Figure 2A). According to Gile et al (1966) (Hill and Schluter, 1993). Layers within these bandedhaminated sequences contain various amounts of mixed calcite and opal, the darker layers containing more opal and the lighter ones more calcite.…”

“…(5) In petrocalcic deposits, sets of laminae are truncated by younger, superadjacent sets, but in the case of the Yucca Mountain deposits, the calcite/opal of the different bands seems to be intermixed ("mixed-texture") or intergrown ("invasive texture") and discrete sets of laminae do not usually appear to be truncated by later sets of laminae (see photos of Hill and Schluter, 1993).…”