LITHOLOGIC DISCONTINUITIES IN SOME SOILS ON DRUMLINS - THEORY, DETECTION, AND APPLICATION

This paper discusses the importance of lithologic discontinuities in p edologic and geologic research, reviews the primary methods used to de tect them, and examines some soils in northern Michigan that exhibit v arying degrees of evidence for lithologic discontinuities. Although ma ny different parameters have been used successfully to detect disconti nuities in soils, those involving immobile and inert components offer the best likelihood of success, and these data are best reported on a clay-free basis. Parameters involving acquired (pedogenic) characteris tics or the mobile element (plasma) of soils should be avoided. Six Ty pic Eutroboralf pedons, formed on drumlins, were the primary focus of this study. Obvious-to-subtle evidence exists for lithologic discontin uities within the lower sola of these soils. Frequently, a weakly expr essed stone line exists at or near the discontinuity. In this geologic ally young landscape, the origin of the discontinuity is presumed to h ave been glaciosedimentologic rather than pedologic (i.e., formed by b ioturbation, surface creep, or eolian additions to near-surface horizo ns). Depth functions involving clay-free particle-size data, especiall y for coarser (coarse sand and fine gravel) fractions, were the most c onsistent indicators of the discontinuity. Mean particle-size data and heavy versus light minerals were also somewhat useful in discriminati ng between the two materials. This study may be the first of its kind to use measures of sand grain sphericity (e.g., mean feret diameter, c ompactness, and shape factor) to identify discontinuities in soils, al though the utility of these indices in detecting discontinuities was m ixed. The data underscore the need for multiple lines of evidence in t he detection of Lithologic discontinuities in soils and cautions that they are not all geologic/sedimentologic in origin.