A study of postglacial loess and paleosols in the Lethbridge area of Southern Alberta

Abstract

A study of soils developed on Holocene loess was undertaken to determine the postglacial soil-landscape evolution near Lethbridge, Alberta. The post-glacial stratigraphy of a loess mantled area along the Oldman River was reconstructed from drill cores taken along five transects. Loess thicknesses and the extent of loess mantling in this area were determined to give an indication of the paleoenvironment. This study was interfaced with the known glacial history of the area to further study the extent of glaciation in southern Alberta. Glacier Peak tephra (ca. 11.2 ka) found at an elevation of 888m along the Oldman River indicates that Glacial Lake MacLeod which covered this area immediately following deglaciation must have drained by at least 11 200 years BP. The lack of a paleosol on the glaciolacustrine surface indicates that loess mantling must have begun immediately following deglaciation. Paleosols contained within the loess deposits indicate that at least twelve soil forming intervals occurred in this area since deglaciation. The loess complex examined in this study was determined to contain two loess units: a lower, silt dominated unit and an upper, sand dominated unit. Rates of accumulation of the two units were determined from two tephra beds, Glacier Peak tephra ca. 11.2 ka and Mazama tephra ca. 6.8 ka, which were used as datum levels. The relative chronology of the postglacial soil-landscape evolution and the soil forming intervals was determined. Soils were studied in terms of soil genesis and as stratigraphic markers. Discussions focus on the implications of loess accumulation and soil forming intervals on the paleoenvironment.