Abstract

Petrographic determination of the degree of alteration of Holocene detrital feldspars from first-order streams indicates that a broad overlap exists in the amount of alteration on detrital feldspars from contrasting wet and dry climates. Thus the amount of alteration of detrital feldspars from ancient sandstones is probably an invalid indicator of paleoclimate. Furthermore, regardless of climate, detrital plagioclase (Ab 80-95) generally is 15-20 percent more altered than potassium feldspar. In light of this, the paleoclimatic significance of the observed relation potassium feldspar-more-weathered-than-plagioclase in ancient sandstones (Todd, 1968) needs re-evaluation. Results of this study do suggest a possible preference of limonite plus kaolinite on detrital feldspars from humid climates versus limonite plus smectite on feldspars from semi-arid climates. However, unequivocal substantiation of this difference is still lacking. Even if the compositional difference can be rigorously demonstrated, an inability to distinguish diagenetic from climate-controlled surface weathering induced alteration of feldspar will further decrease the utility of feldspar alteration relationships in paleoclimate interpretation.

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