GEOLOGIC MAPPING OF LARGE VALLEYS IN GLACIATED REGIONS: THE USE OF LANDFORM AND LANDSCAPE SEDIMENT ASSEMBLAGES FOR MULTI-USE MAPS
E. A. Bettis III, E. R. Hajic, and D. J. Quade
The Geological Society of America
33rd Annual North-Central Section Meeting
Clarion Hotel and Convention Center, Champaign, IL, April 22-23, 1999
1999 Abstracts with Programs, v. 31, no. 5, p. A-
Large valleys of glaciated regions contain a palimpsest of landforms and underlying deposits that formed in a variety of glacial and interglacial depositional environments. Depositional units and landforms of fluvial and lacustrine environments are often extremely variable, both laterally and vertically, and occur in complex tonguing, interbedded, and stacked stratigraphic sequences. Nevertheless, there are orderly, predictable, and useful geologic units that can be identified at the quadrangle scale. However, such variability at this scale is difficult to depict with conventional mapping units.
A useful concept for overcoming some of the mapping difficulties of deposits in large valleys is that of landform sediment assemblages (LSA). LSA are associations of landforms and underlying sediment packages that have genetic and temporal relationships. LSA are defined at the finest landform scale that is reasonably mappable at 1:24,000 and differ from morphostratigraphic units in that the nature of the sediment package underlying the landform(s) is an integral part of the mapping unit's definition. LSA mapping also implicitly recognizes the horizontal and vertical variability of valley deposits and provides a convenient framework for refining mapping units as the mapping becomes more detailed. LSA can be defined at finer scales where maps with more detail than 1:24,000 quadrangles are available. Genetically related groups of LSA can be considered at smaller scales as landscape sediment assemblages.
LSA maps, in digital form, can be the source of a number of easily generated derivative maps, including detailed geologic, geomorphic, or stack-unit maps. Examples from several large midcontinent river valleys are used to illustrate the principles and applications of LSA mapping for basic geologic, environmental, land use, and cultural resources applications.