Origin of the Mormon Mesa Caliche, Clark County, Nevada

Abstract

On the basis of field and laboratory data described herein, it is concluded that the Mormon Mesa caliche has formed primarily as the result of pedogenetic processes, probably under slowly aggrading, aeolian conditions. The reasons for postulating aggrading aeolian conditions are (1) the absence of A and B soil horizons, (2) the absence of laminated petrocalcic horizons, (3) the lack of quartz grain dissolution and brecciation, (4) the relatively high content of clay and silt in the calcrete (caprock) horizon, (5) the presence of aeolian sand drifts on the present mesa surface, and (6) the presence of fresh red sand in laminated CaCO 3 joint fillings. Also, the aeolian hypothesis may explain the fact that the calcrete horizon of the caliche contains a much greater amount of secondary CaCO 3 than can be accommodated in the original void space of the parent material; and, in addition, the hypothesis avoids the vexing problem, associated with degradational hypotheses, of explaining how water penetrates through the calcrete to form the underlying transition zone of friable caliche. The data further indicates that perhaps at least 25 percent of the secondary CaCO 3 in the caliche was derived from dissolved Ca ++ and HCO 3 − in rainwater, with the remainder probably furnished by aeolian carbonate dust. It is estimated that development of the caliche probably required at least 400,000 yrs and possibly as long as 2,500,000 yrs.