FRAGIUDULTS GENESIS INVOLVING MULTIPLE PARENT MATERIALS IN THE EASTERN OZARKS OF MISSOURI

Abstract

Situated in the Missouri Ozarks (Central USA), we examined two fragipan-bearing soils developed in loess and cherty dolomite residuum (i) to characterize two Typic Fragiudult pedons formed in loess, possibly colluvial material formed from dolomitic residuum, and dolomitic residuum and (ii) to establish the role that multiple parent materials have in soil profile development. The upper and lower boundaries of the fragipan correspond to lithologic discontinuities separating (i) loess and bioturbated residuum composed of the eluvial-illuvial sequence from colluvial material forming the fragipan and (ii) the fragipan from the underlying dolomitic residuum. Clay illuviation was observed in the fragipan, inferring that weathered soil material from the upper sequum impacted fragipan expression. Iron concentrations change substantially at the parent material transitions, reflecting pedogenic processes and parent material inheritance. Silicon contents do not reflect the presence of the fragipan, and the role of Si as a binding agent responsible for fragipan brittle remains elusive. The upper sequum supports a hydroxy-Al interlayered vermicultite and kaolinite mineralogy, whereas the lower sequum has a smectite and kaolinitic clay mineralogy. It is proposed that the fragipan is a colluvial feature and that hydroconsolidation has contributed to the fragipan's properties. The upper sequum is a mixture of loess and residuum that has formed an ochric-argillic horizon sequence and has been welded to the fragipan because of clay illuviation.