North American Glaciotectonic Maps

Abstract

A data base of geographic and glaciotectonic features for North America has been constructed using the ARC/INFO system at the University of Regina. The geographic base consists of multiple coverages that include basic geography, continental geology, and glaciation limits. Glaciotectonic features are classified in four main categories: basement faults, concealed structures, ice-shoved hills, and source basins. Two maps have been published from the data base: Great Plains Glaciotectonics (Aber et al. 1991) and Glaciotectonic Map of North America (Aber et al. 1995)--jump to references. Original digital data are located at the Data Access and Support Center (DASC) at the Kansas Geological Survey (KGS)--see digital data.

The continent-wide distribution of glaciotectonic features follows the three-zone model for glacial landscapes; distribution of glaciotectonic features over the continent is related in general to the presence of deformable substrata. Glaciotectonic features are particularly abundant in the northern Great Plains region, where ice sheets advanced upslope over soft substratum. Basement faults and seismic zones, induced by glacial loading and unloading, are common in regions of hard bedrock.

Contributors

The Glaciotectonic Map of North America is the result of collaboration by more than 40 people from the United States and Canada during the period 1989 to 1995. Aber et al. (1993) provided a listing of most individuals. Key persons and their roles are listed below.

James S. Aber: overall project coordinator.
David L. Ackerman: supervisor of GIS data base and map production.
John P. Bluemle: glaciotectonic compiler for north-central United States.
Julie Brigham-Grette: glaciotectonic compiler for Alaska and Yukon.
Lynda A. Dredge: glaciotectonic compiler for Arctic and eastern Canada.
David J. Sauchyn: coordinator of GIS operation.

Data Base Structure

The data base was created in vector format on ARC/INFO, a mainframe-computer geographic information system at the Spatial Analysis Laboratory of the University of Regina. The data base consists of the glaciated portions of Canada and the United States, including continental shelves. The geographic base contains several coverages, or layers, of data: basic geography, topography, geology, and glaciation limits.

Basic geography and topography are available on computer-compatible tapes (and other formats); they were obtained respectively from: World Data Bank II (U.S. Federal Computer Products Center) and U.S. National Geophysical Data Center (for more information, go to NGDC). Other coverages were manually digitized from a variety of published maps.

Classification of Glaciotectonic Phenomena

A relatively simple classification scheme was developed for glaciotectonic features. It proved difficult for contributors to use a more complicated classification scheme, due to lack of information or lack of time to evaluate each site. A simple scheme was adopted, because the nature of published descriptions and information sent by contributors varied substantially.

Many glaciotectonic features are known only from geomorphic expression with limited, or in some cases no, outcrop or test-drilling information. Conversely, detailed structural and stratigraphic information are available in some areas. For certain regions, neither contributions nor published information were submitted for the data base. In some other areas, contributors reported that nothing was known of interest. The glaciotectonic data base is thought to be relatively complete for the north-central and northeastern United States, Alaska, Canadian Prairie region, Arctic Canada and eastern Canada.

Original documentation for the glaciotectonic feature entered into the data base is given by more than 100 publications (see Aber et al. 1993). Glaciotectonic data are contained in four main classes:

Basement faults: faults or zones of seismic activity, created or reactivated by glacial loading and unloading.
Concealed (or buried) structures: folds, diapirs, faulted or brecciated bedrock, and other glaciotectonic structures that have no morphologic expression in the landscape.
Ice-shoved hills: composite ridges, cupola hills, push moraines, etc. Conspicuous morphologic expression, internal structure of imbricated thrusts, overturned folds, diapirs, or other dislocations.
Source basins: depressions from whence material was thrust into adjacent ice-shoved hills, clear geometric relationship between source depression and hill.

Map Scales and Feature Size

Scale limitations are a primary concern. Data were submitted in various formats, including: descriptive tables, topographic maps, aerial photographs, and published articles or maps. The map scale most commonly used for submitting glaciotectonic data and for digitizing was 1:250,000; larger and smaller scales ranged from 1:24,000 to 1:500,000. The largest map scale likely to be used for output in this project is 1:1,000,000; the smallest is 1:6,500,000.

In view of these scale limitations, the minimum size feature that we attempted to digitize in true shape is about 1 km². Features smaller than this were entered simply as points; source basins are also shown only as points in the approximate center of basins. This means that no information on true size or shape was entered for individual features smaller than 1 km².

Status of Glaciotectonic Mapping

All glaciotectonic, geographic, and geologic data have now been entered as various coverages into ARC/INFO. Glaciotectonic data were digitized from approximately 140 separate map sheets. These are compiled into a single continental coverage, which includes more than 300 point features (labels) and 500 area features (arcs). This coverage may, of course, be updated at any time that new or revised glaciotectonic information is contributed. This will be especially valuable for those regions that are poorly represented in the present data base, particularly the Cordilleran region.

The basic techniques were developed for combining various coverages and designing small-scale map products. A subset of geographic and glaciotectonic coverages for the northern Great Plains was selected, including North Dakota, southern Saskatchewan, and adjacent areas. The most troublesome map design problem was how to portray those areas which contain many glaciotectonic features that are too small to show in true shapes as individuals at small scale. Our solution to this problem was to use a special outline symbol to represent areas with many discrete features. This map was published at scale 1:2,500,000 by the North Dakota Geological Survey (Aber et al. 1991).

We have combined the glaciotectonic data with other geographic and geologic coverages for the continent to produce a master data base for map production. Plots were initially developed with the map region divided into four roughly equal quadrants (NW, NE, SE, SW) in a commom projection at 1:3,000,000. This format proved awkward because of its large size. Through a series of revisions, the map has been reduced to a single sheet (E-format) at a scale of 1:6,500,000. The map projection is Lambert conformal conic with standard parallels of 49° and 77°. This map and accompanying text are published by the Geological Society of America (Aber et al. 1995).

The digital data are also available in ARC/INFO export format along with ArcView project files for viewing the data files. The advantage of this approach is that digital data retain the full resolution as originally entered into the data base. Users will be able to access and manipulate data directly. These files are available for on-line FTP--see glaciotectonic digital data.

References

Aber, J.S., Bluemle, J.P., Brigham-Grette, J., Dredge, L.A., Sauchyn, D.J. and Ackerman, D.L. 1993. Glaciotectonic data base and mapping of North America. In Aber, J.S. (ed.), Glaciotectonics and mapping of glacial deposits. Canadian Plains Research Center, Canadian Plains Proceedings 25/1: 177-200. Regina, Canada.
Aber, J.S., Bluemle, J.P., Brigham-Grette, J., Dredge, L.A., Sauchyn, D.J. and Ackerman, D.L. 1995. Glaciotectonic map of North America, 1:6,500,000. Geological Society of America, Maps and Charts Series, MCH079.
Aber, J.S., Bluemle, J.P., Sauchyn, D.J. and Ackerman, D.L. 1991. Great Plains glaciotectonics. North Dakota Geological Survey, Misc. Map 31, 1:2,500,000.

GAGE homepage.
Last update: 20 May 97

For more information, contact J.S. Aber, e-mail: aberjame@esumail.emporia.edu