Abstract

Petrographic studies of first-cycle desert alluvium of Cenozoic age in the southwestern United States and northwestern Mexico show that the mineralogy, texture, and chemical composition of the deposits have been changed diagenetically. The mineralogy has been changed by addition of mechanically infiltrated clay, partial removal of framework grains of feldspars and ferromagnesian silicates, and precipitation of authigenic potassium feldspar, zeolite, montmorillonite, quartz, hematite, and calcite. The texture has been changed by three processes: (1) infiltration of detrital clay and formation of authigenic montmorillonite, which form interstitial clayey matrix not present in the original sediment, (2) formation of voids where framework grains have been dissolved, and (3) in situ formation of silt and other fine-grained sizes. The chemical composition has been changed by infiltration of clay minerals that are richer in aluminum and lower in alkalis and alkaline earths than the original sediment and by removal in ground water of some of the ions released by dissolution and replacement of framework grains. These changes have significantly increased the mineralogical maturity and decreased the textural maturity of the sediments diagenetically.

Four major conclusions are drawn from the studies. (1) Some or all of the diagenetic alterations observed in these deposits probably occurred in many analogous ancient first-cycle alluvial deposits at a comparable time in their history. (2) Prolonged movement of ground water through first-cycle deposits may cause unstable minerals to be removed completely, or nearly so, leaving no direct evidence that they were important original constituents of the deposits. (3) Ancient first-cycle alluvium probably rarely, if ever, has the same mineralogy, texture, or chemical composition that the sediments had when deposited. (4) The present mineralogy, texture, and chemical composition of ancient first-cycle alluvial deposits probably do not accurately reflect lithology and climate in the source area or the nature of depositional currents and other environmental factors in the depositional basin.

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