Field use of Macrofeatures for Correlating Tills and Estimating Their Ages: A Review

Abstract

Abstract Many geological sections in areas of multiple glaciation lack the organic materials necessary for absolute and relative dating of their tills, and for correlation of the tills to deposits of known age elsewhere. However, a till sheet in the field normally exhibits large-scale features, beyond those dependant upon composition, that suffice to distinguish it from most other till sheets in an area, allow its correlation with deposits of known age elsewhere, and may even, at times, indicate its approximate age. In general, the value of these features for such purposes has been underestimated or even disregarded. Examples used here are drawn from the writer's experience with the Laurentide deposits of the southwest Canadian Prairies, but only a few of the many types of features available are described. These macrofeatures derive from environmental conditions prevailing during deposition and during the post-depositional history of the till, and they remain fairly constant over much of south Alberta. Those arising partly or chiefly from post-depositional factors include oxidization and weathering, compaction, jointing, and style of breakage. Oxidization and weathering have a long record of use for estimating ages and for correlation, and so are not discussed here. Compaction is a valuable tool, for it depends mainly on the number and thicknesses of later glaciers that compressed the till; it becomes worthless, however, where older tills surface beyond the limits of younger glaciers and so are little indurated. Jointing and style of breakage normally are sufficiently distinctive for each till to permit its correlation to deposits of known age elsewhere. The prominent columnar structures displayed by many of the southern Alberta tills are of combined depositional and post-depositional origin. They are of great value for recognition and correlation of all but surface tills. On the other hand, till colours found in south Alberta are determined by both composition and post-depositional history. Other things being equal, the darker a till the greater its age. The Illinoian tills in that region are very dark brown or grey, depending upon whether they were laid down by southeast of southwest flowing ice; the Pre-classical Wisconsin tills are moderately dark brown or grey, whereas the typical Classical Wisconsin tills are buff or light yellowish brown. The oldest Illinoian till is nearly black, and it maintains this sombre colour, where not weathered, not only over a broad spectrum of bedrock types but also at depth in section, and at the surface in those places where it represents the sole glaciation. The other tills similarly retain their characteristic colours over broad areas. This allows these colour values to be used for estimating till ages. The effects of environment during deposition are demonstrated by the two “contorted” tills of southern Alberta. The contortion includes strong mixing with underlying material, along with diapirs and flame structures from the till and underlying formation shooting into and piercing each other, and in places completely enveloping segments of the other. The interfingering and mixing may be so intense that no accurate boundary can be drawn between the two units. The contortion, which is especially strong in the younger, distinguishes these two tills from all others, and so enables their correlation from section to section and to deposits of known age elsewhere. Altogether, the use of the full suite of the macrofeatures found in any area can give a fairly good indication of the relative ages and positions in the stratigraphic section of the tills there, and of their relationship to other tills found elsewhere.